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  • 1. Almqvist, Bjarne S. G.
    et al.
    Bender, Hagen
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bergman, Amanda
    Stockholm University, Faculty of Science, Department of Geological Sciences. Ramböll Sverige AB, Sweden.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Magnetic properties of pseudotachylytes from western Jämtland, central Swedish Caledonides2020In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 11, no 3, p. 807-828Article in journal (Refereed)
    Abstract [en]

    Fault kinematics can provide information on the relationship and assembly of tectonic units in an orogen. Magnetic fabric studies of faults where pseudotachylytes form have recently been used to determine direction and sense of seismic slip in prehistoric earthquakes. Here we apply this methodology to study magnetic fabrics of pseudotachylytes in field structures of the Koli Nappe Complex (central Swedish Caledonides), with the aim to determine fault kinematics and decipher the role of seismic faulting in the assembly of the Caledonian nappe pile. Because the pseudotachylyte veins are thin, we focused on small (ca. 0.2 to 0.03 cm(3)) samples for measuring the anisotropy of magnetic susceptibility. The small sample size challenges conventional use of magnetic anisotropy and results acquired from such small specimens demand cautious interpretation. Importantly, we find that magnetic fabric results show inverse proportionality among specimen size, degree of magnetic anisotropy and mean magnetic susceptibility, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. In general, however, it is shown that the principal axes of magnetic susceptibility correspond to the orientation of foliation and lineation, where the maximum susceptibility (k(1)) is parallel to the mineral lineation, and the minimum susceptibility (k(3)) is dominantly oriented normal to schistosity. Furthermore, the studied pseudotachylytes develop distinct magnetic properties. Pristine pseudotachylytes preserve a signal of ferrimagnetic magnetite that likely formed during faulting. In contrast, portions of the pseudotachylytes have altered, with a tendency of magnetite to break down to form chlorite. Despite magnetite breakdown, the altered pseudotachylyte mean magnetic susceptibility is nearly twice that of altered pseudotachylyte, likely originating from the Fe-rich chlorite, as implied by temperature-dependent susceptibility measurements and thin-section observations. Analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust, but these data yield no kinematic information on the direction and sense of seismic slip. Additionally, the combined structural field and magnetic fabric data suggest that seismic faulting was postdated by brittle E-W extensional deformation along steep normal faults. Although the objective of finding kinematic indicators for the faulting was not fully achieved, we believe that the results from this study may help guide future studies of magnetic anisotropy with small specimens (< 1 cm(3)), as well as in the interpretation of magnetic properties of pseudotachylytes.

  • 2. Bauer, F. U.
    et al.
    Glasmacher, U. A.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Grobe, R. W.
    Mambo, V. S.
    Starz, M.
    Long-term cooling history of the Albertine Rift: new evidence from the western rift shoulder, DR Congo2016In: International journal of earth sciences, ISSN 1437-3254, E-ISSN 1437-3262, Vol. 105, no 6, p. 1707-1728Article in journal (Refereed)
    Abstract [en]

    To determine the long-term landscape evolution of the Albertine Rift in East Africa, low-temperature thermochronology was applied and the cooling history constrained using thermal history modelling. Acquired results reveal (1) old cooling ages, with predominantly Devonian to Carboniferous apatite fission-track ages, Ordovician to Silurian zircon (U-Th)/He ages and Jurassic to Cretaceous apatite (U-Th-Sm)/He ages; (2) protracted cooling histories of the western rift shoulder with major phases of exhumation in mid-Palaeozoic and Palaeogene to Neogene times; (3) low Palaeozoic and Neogene erosion rates. This indicates a long residence time of the analysed samples in the uppermost crust, with the current landscape surface at a near-surface position for hundreds of million years. Apatite He cooling ages and thermal history models indicate moderate reheating in Jurassic to Cretaceous times. Together with the cooling age distribution, a possible Albertine high with a distinct relief can be inferred that might have been a source area for the Congo Basin.

  • 3. Bauer, Friederike U.
    et al.
    Glasmacher, Ulrich A.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Karl, Markus
    Schumann, Andreas
    Nagudi, Betty
    Tracing the exhumation history of the Rwenzori Mountains, Albertine Rift, Uganda, using low-temperature thermochronology2013In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 599, p. 8-28Article in journal (Refereed)
    Abstract [en]

    The Rwenzori Mtns form a striking feature within the Albertine Rift of the East African Rift System. They are made up of a dissected Precambrian metamorphic basement block reaching heights of more than 5 km. Applying low-temperature therrnochronology a complex exhumation history becomes evident, where rock and surface uplift can be traced from Palaeozoic to Neogene times. Fission-track and (U-Th-Sm)/He cooling ages and derived cooling histories allow distinguishing different blocks in the Rwenzori Mtns. In the central part a northern and a southern block are separated by a putative NW-SE trending fault; with the northern block showing distinctly younger apatite fission-track ages (similar to 130 Ma) than the southern block (similar to 300 Ma). Cooling ages in both blocks do not vary significantly with elevation, despite considerable differences in elevation. Thermal history modelling reflects protracted cooling histories. Modelled t-T paths show decoupled blocks that were relocated separately along distinct fault planes, which reactivated pre-existing structures, inherited from Palaeozoic folding and thrusting. Initial cooling affected the Rwenzori area in Silurian to Devonian times, followed by Mesozoic and Cainozoic cooling events. Pre-Neogene evolution seems to be triggered by tectonic processes like the opening of the Indian Ocean and the south Atlantic. From thermochronological data the formation of a Mesozoic Albertine high is conceivable. In Cainozoic times the area was affected by rifting, resulting in differentiated surface uplift. Along the western flank of the Rwenzori Mtns, surface uplift was more pronounced. This is also reflected in their recent topography, formed by accelerated rock uplift in the near past (Pliocene to Pleistocene). Erosion could not compensate for this most recent uplift, resulting in apatite He ages of Oligocene to Miocene age or even older.

  • 4.
    Bender, Hagen
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Almqvist, Bjarne S. G.
    Bergman, Amanda
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rock magnetic properties of pseudotachylytes, Jämtland, SwedenManuscript (preprint) (Other academic)
  • 5.
    Bender, Hagen
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Absolute timing of Caledonian orogenic wedge assembly, Central Sweden, constrained by Rb-Sr multi-mineral isochron data2019In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 344, p. 339-359Article in journal (Refereed)
    Abstract [en]

    Multi-mineral Rb-Sr isochron ages for mylonites of the Caledonian nappe pile (Jamtland, Sweden) constrain the orogenic evolution of the central Scandinavian Caledonides. An age of 437.8 +/- 3.9 Ma from a metamorphic leucosome in the central nappe stack dates crystallization of migmatized gneiss. Thirteen isotopic ages derived from mylonites across all structural levels are between 434.6 +/- 6.9 Ma and 426.3 +/- 3.1 Ma. These results, complemented by a dense network of kinematic field data, are interpreted to reflect the timing of protracted top-to-the-ESE general shear under decreasing amphibolite- to upper-greenschist-facies conditions across all nappes. Three samples, derived from the top, middle and bottom of the nappe pile, yield additional deformation ages between 416.0 +/- 3.6 Ma and 410.1 +/- 3.0 Ma. These ages are interpreted to represent post-assembly imbrication of the nappe stack under decreasing metamorphic conditions. The new tectonochronologic data are consistent with a tectonic model that explains nappe stacking in Jamtland by three stages of ESE-directed in- and out-of-sequence thrusting. In the first stage, postdating earlier subduction-exhumation of the high-grade metamorphic Seve Nappe Complex, the latter was emplaced in-sequence in the lower/mid-crust above nappes in its footwall (foreland) at c. 430 Ma. This event is related to the collision of Baltica with an island arc. Concurrently at c. 430 Ma, determined by paleogeography and structurally unrelated to the first stage, second-stage out-of-sequence thrusting emplaced the Kali Nappe Complex atop the developing nappe stack during final Baltica-Laurentia collision. Eventually, imbrication and exhumation of the composite Jamtland nappe stack in the uppermost middle crust during the third stage at c. 414 Ma was caused by continuous underthrusting of Baltica beneath Laurentia. This three-stage thrust model reflects the complex interactions of Baltica-arc-Laurentia collisions that were accommodated by two merging subduction zones.

  • 6.
    Bender, Hagen
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stephens, Michael B.
    Absolute timing of Caledonian orogenic wedge assembly, central Sweden, constrained by Rb-Sr multi-mineral isochron dataIn: Lithos, ISSN 0024-4937, E-ISSN 1872-6143Article in journal (Refereed)
  • 7.
    Bender, Hagen
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Almqvist, Bjarne S. G.
    Grasemann, Bernhard
    Stephens, Michael B.
    Metamorphic Zonation by Out-of-Sequence Thrusting at Back-Stepping Subduction Zones: Sequential Accretion of the Caledonian Internides, Central Sweden2018In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 37, no 10, p. 3545-3576Article in journal (Refereed)
    Abstract [en]

    Exhumation of the high-grade metamorphic Seve Nappe Complex and its emplacement between lower-grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jamtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top-to-the-ESE, foreland-directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation-parallel boudinage, and abundant top-to-the-ESE and rare, scattered top-to-the-WNW shear-sense indicators imply foreland-directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in- and out-of-sequence thrusts that generated the metamorphic zonation. Our model envisions in-sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out-of-sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high-grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.

  • 8. Bolhar, R.
    et al.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ireland, T. R.
    Zircon in amphibolites from Naxos, Aegean Sea, Greece: origin, significance and tectonic setting2017In: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 35, no 4, p. 413-434Article in journal (Refereed)
    Abstract [en]

    We report U-Pb zircon ages of c. 700-550Ma, 262-220Ma, 47-38Ma and 15-14Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700-550Ma and 262-220Ma are assigned to inherited zircon. Available geochemical data suggest that the 15-14Ma metamorphic rims grew insitu in the amphibolites, corresponding to a high-grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40Ma zircon rims also grew insitu, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late-Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40Ma zircon rims would have grown insitu, and (ii) emplacement in the Miocene as a result mafic underplating during large-scale extension. In this case, only the 15-14Ma metamorphic outer rims would have formed insitu in the amphibolitic host rocks.

  • 9. Craddock, John P.
    et al.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pfiffner, O. Adrian
    Deformation of the European Plate (58-0 Ma): Evidence from Calcite Twinning Strains2022In: Geosciences, E-ISSN 2076-3263, Vol. 12, no 6, article id 254Article in journal (Refereed)
    Abstract [en]

    We present a data set of calcite twinning strain results (n = 209 samples; 9919 measured calcite twins) from the internal Alpine nappes northwestward across the Alps and Alpine foreland to the older extensional margin along the Atlantic coast in Ireland. Along the coast of Northern Ireland, Cretaceous chalks and Tertiary basalts are cross-cut by calcite veins and offset by calcite-filled normal and strike-slip faults. Both Irish sample suites (n = 16 with four U-Pb vein calcite ages between 70–42 Ma) record a sub-horizontal SW-NE shortening strain with vertical extension and no strain overprint. This sub-horizontal shortening is parallel to the margin of the opening of the Atlantic Ocean (~58 Ma), and this penetrative fabric is only observed ~100 km inboard of the margin to the southeast. The younger, collisional Alpine orogen (~40 Ma) imparted a stress–strain regime dominated by SE-NW sub-horizontal shortening ~1200 km northwest from the Alps preserved in Mesozoic limestones and calcite veins (n = 32) in France, Germany and Britain. This layer-parallel shortening strain (−3.4%, 5% negative expected values) is preserved across the foreland in the plane of Alpine thrust shortening (SE-NW) along with numerous outcrop-scale contractional structures (i.e., folds, thrust faults). Calcite veins were observed in the Alpine foreland in numerous orientations and include both a SE-NW layer-parallel shortening fabric (n = 11) and a sub-vertical NE-SW vein-parallel shortening fabric (n = 4). Alpine foreland strains are compared with twinning strains from the frontal Jura Mountains (n = 9; layer-parallel shortening), the Molasse basin (n = 26; layer-parallel and layer-normal shortening), Pre-Alp nappes (n = 39; layer-parallel and layer-normal shortening), Helvetic and Penninic nappes (Penninic klippe; n = 46; layer-parallel and layer-normal shortening plus four striated U-Pb calcite vein ages ~24 Ma) and calcsilicates from the internal Tauern window (n = 4; layer-normal shortening). We provide a chronology of the stress–strain history of the European plate from 58 Ma through the Alpine orogen.

  • 10. Davidson, Jonathan Robert Joseph
    et al.
    Fairley, Jerry
    Nicol, Andrew
    Gravley, Darren
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Canterbury, New Zealand.
    The origin of radon anomalies along normal faults in an active rift and geothermal area2016In: Geosphere, E-ISSN 1553-040X, Vol. 12, no 5, p. 1656-1669Article in journal (Refereed)
    Abstract [en]

    Radon anomalies are widely reported in the vicinity of active faults, where they are often inferred to result from upward migration of fluids along fault zones. We examine the up-fault flow hypothesis by measuring radon (Rn-220 and Rn-222) in soil gas above two active normal fault zones within the central Taupo rift, New Zealand. In agreement with previous investigations, we find that the average concentrations of both radon isotopes are generally higher near mapped faults, although in some cases we find that the difference with background populations is not significant. Soil samples recovered from 1 m depth indicate that some of the radon anomalies along faults may reflect local changes in soil types. The Rn-220 isotope emanation measured from extracted soil samples shows a linear correlation with the field concentration measurements (R-2 = 0.90, p value = 3 x 10(-6)), whereas Rn-222 emanation shows no linear correlation (R-2 = 0.17, p value = 0.17). The soil gas isotopes measured show a significant linear correlation of Rn-220 and Rn-222 concentrations (R-2 = 0.44-0.55, p value <10(-5)) near faults. This correlation suggests a constant radon isotopic ratio is emitted from the soils tested, and this finding is supported by emission data measured on extracted soil samples. The distribution of Rn-222 concentration compared to Rn-220 can be explained by small-scale diffusion for >90% of the soil gas measurements, showing that a majority of radon anomalies along faults are not necessarily caused by advection of gases along fault planes and can be explained by an increase in radon soil emanation. However, diffusion cannot account for all of the observed patterns in the data, and in some specific locations along faults, Rn-222 concentrations are most likely produced by advective flow of subsurface gases, suggesting channelized gas flow in portions of some faults.

  • 11. Duffy, Brendan
    et al.
    Quigley, Mark
    Harris, Ron
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Canterbury.
    Arc-parallel extrusion of the Timor sector of the Banda arc-continent collision2013In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 32, no 3, p. 641-660Article in journal (Refereed)
    Abstract [en]

    Structural studies of synorogenic basins in Timor using field and remote sensing techniques provide new structural and geomorphic evidence for syn-collisional extension in the converging plate boundary zone between the Australian Plate and Banda Arc. Fault mapping and kinematic analysis at scales ranging from outcrop (<1m(2)) to the dimensions of the active orogen in East Timor (similar to 100km(2)) identify a predominance of NW-SE oriented dextral-normal faults and NE-SW oriented sinistral-normal faults that collectively bound large (5-20km(2)) bedrock massifs throughout the island. These fault systems intersect at non-Andersonian conjugate angles of approximately 120 degrees and accommodate an estimated 20km of NE-directed extension across the Timor orogen based on reconstructions of fault-dismembered massifs. Major orogen-parallel ENE-oriented faults on the northern and southern sides of Timor exhibit normal-sinistral and normal-dextral kinematics, respectively. The overall pattern of deformation is one of lateral crustal extrusion sub-parallel to the Banda Arc. Stratigraphic relationships suggest that extrusion began prior to 5.5Ma, before pronounced rapid uplift of the orogen. We link this to progressive coupling of the fore-arc to an underthrust plateau on the Australian Plate and subduction of its ocean crust. Our results enable us to track the structural evolution of the upper crust during dramatic plate-boundary reorganizations accompanying the transition from subduction to collision. The deformation structures that we document suggest that both upper and lower plate deformation during incipient island arc-continent collision was largely controlled by the geometry and topography of the lower plate.

  • 12. Gessner, Klaus
    et al.
    Gallardo, Luis A.
    Markwitz, Vanessa
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thomson, Stuart N.
    What caused the denudation of the Menderes Massif: Review of crustal evolution, lithosphere structure, and dynamic topography in southwest Turkey2013In: Gondwana Research, ISSN 1342-937X, E-ISSN 1878-0571, Vol. 24, no 1, p. 243-274Article, review/survey (Refereed)
    Abstract [en]

    The deformation of Earth's lithosphere in orogenic belts is largely forced externally by the sinking slab, but can also be driven by internal delamination processes caused by mechanical instabilities. Here we present an integrated analysis of geophysical and geological data to show how these processes can act contemporaneously and in close proximity to each other, along a lithosphere scale discontinuity that defines the lateral boundary between the Hellenide and Anatolide segments of the Tethyan orogen in western Turkey. The Hellenides and Anatolides have experienced similar rates of convergence, but display remarkable differences in the structure of Earth's crust and lithospheric mantle across the Aegean coast of the Anatolian peninsula. We review the tectonics of southwest Turkey in the light of new and published data on crustal structure, cooling history, topography evolution, gravity, Moho topography, earthquake distribution and seismic tomography. Geological data constrain that one of Earth's largest metamorphic core complexes, the Menderes Massif, experienced early Miocene tectonic denudation and surface uplift in the footwall of a north-directed extensional detachment system, followed by late Miocene to recent fragmentation by E-W and NW-SE trending graben systems. Gravity data, earthquake locations and seismic velocity anomalies highlight a north-south oriented boundary in the upper mantle between a fast slab below the Aegean and a slow asthenospheric region below western Turkey. Based on the interpretation of geological and geophysical data we propose that the tectonic denudation of the Menderes Massif and the delamination of its subcontinental lithospheric mantle reflect the late Oligocene/early Miocene onset of transtension along a lithosphere scale shear zone, the West Anatolia Transfer Zone (WATZ). We argue that the WATZ localised along the boundary of the Adriatic and Anatolian lithospheric domains in the Miocene, when southward rollback of the Aegean slab started to affect the central Aegean-Menderes portion of the Tethyan orogen. Transtension across the West Anatolia Transfer Zone affected the entire Menderes Massif in the Early Miocene. The current crustal expression of this boundary is a NNE-trending, distributed brittle deformation zone that localised at the western margin of the denuded massif. Here, sinistral transtension accommodates the continuing velocity difference between relatively slow removal of lithospheric mantle below western Anatolia and trench retreat in the rapidly extending Aegean Sea region. Our review highlights the significance of lateral variations of the lower plate in subduction-collision systems for evolving structure and surface processes in orogenic belts, particularly in relation to the formation of continental plateaux and metamorphic core complexes.

  • 13. Glodny, Johannes
    et al.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Cycladic Blueschist Unit of the Hellenic subduction orogen: Protracted high-pressure metamorphism, decompression and reimbrication of a diachronous nappe stack2022In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 224, article id 103883Article in journal (Refereed)
    Abstract [en]

    We discuss the assembly and tectonochronology of the Cycladic Blueschist Unit (CBU) in the central Aegean Sea region, Greece, as a composite, subduction-related, high-pressure (blueschist to eclogite facies) stack of nappes. Our work is based on a review of geochronologic data from the Cycladic islands of Tinos, Syros and Sifnos. In addition, we present a large set of 34 new, internally consistent Rb-Sr multimineral isochron age data from these islands that can be linked to distinct metamorphic stages and episodes of ductile deformation. The reviewed and new ages provide remarkably similar results indicating that the age data and their interpretation is robust. We find that three distinct, significantly diachronous, CBU high-pressure nappes were sequentially accreted between ~55 and ~30 Ma, with a fourth ~24-22 Ma old high-pressure belt, the Basal Unit, below the CBU. Each CBU nappe was exhumed soon after its high-pressure imprint and experienced greenschist-facies overprinting with partial metamorphic reworking ~10 Ma after high-pressure accretion. At the time the structurally higher high-pressure nappe was exhumed, an underlying nappe was accreted and metamorphosed at high-pressure conditions. Our work suggests that the high-pressure nappes were considerably exhumed as rather coherent units during sustained lithospheric shortening in a subduction channel by return flow. The early Oligocene underthrusting of the External Hellenides to the south of the Cyclades, and tectonically below the CBU, is interpreted to have reduced the taper of the Hellenic subduction wedge, which caused significant shortening across the Cyclades region and an important phase of out-of-sequence thrusting that emplaced the non-high-pressure upper units above the CBU. Our analysis shows that this shortening event also caused significant reimbrication within the CBU along the recently discovered Trans Cycladic Thrust.

  • 14.
    Hansman, Reuben J.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Albert, Richard
    Gerdes, Axel
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite2018In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 46, no 3, p. 207-210Article in journal (Refereed)
    Abstract [en]

    Direct dating of brittle structures is challenging, especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events. We report 22 calcite U-Pb ages from tectonites and carbonate host rocks that date 3 diagenetic and 6 brittle deformation events. Results show that U-Pb dating of calcite fibers from these structures is compatible with overprinting relationships. Ages indicate that diagenesis occurred between 147 +/- 6 Ma and 103 +/- 34 Ma, and was followed by top-to-the-south, layer-parallel shearing due to ophiolite obduction at 84 +/- 5 Ma (2 sigma errors). Sheared top-to-the- northeast, layer-parallel veins were dated as 64 +/- 4 Ma and are interpreted to have developed during postobduction exhumation. After this event, a series of strike-slip structures, which crosscut and reactivated older faults due to northwest-southeast horizontal shortening, were dated as 55 +/- 22 Ma and 43 +/- 6 Ma. Eight ages from strike-slip faults and thrusts resulting from northeast-southwest shortening range from 40.6 +/- 0.5 Ma to 16.1 +/- 0.2 Ma. The youngest ages are from minor overprinting fibers ranging in age between 7.5 +/- 0.9 Ma and 1.6 +/- 0.6 Ma. Our results show that U-Pb dating of calcite fibers can be successfully used to constrain a complicated succession of brittle deformation structures that encompasses two orogenies and an intervening extension period.

  • 15.
    Hansman, Reuben J.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jabal Hafit anticline (UAE and Oman) formed by decollement folding followed by trishear fault-propagation folding2018In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 117, p. 168-185Article in journal (Refereed)
    Abstract [en]

    Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of an orogen. The Jabal Hafit anticline, in the foreland of the Al Hajar Mountains, extends through Oman and the United Arab Emirates and is a relatively simple and well exposed structure. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and demonstrates that the anticlines geometry can be reproduced by a two-phase model: (1) flexural slip folding, followed by (2) trishear fault-propagation folding. The trishear mode occurred due to a west-dipping backthrust that propagated from the decollement, to accommodate WSW-directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene to early -middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains.

  • 16.
    Hansman, Reuben J.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Oligocene–Miocene trishear fault-propagation folding of the Jabal Hafit Anticline, supported by a three-dimensional geological model; and assessing structure-from-motion (SfM) photogrammetry of unmanned aerial vehicle (UAV) photographs for mappingManuscript (preprint) (Other academic)
    Abstract [en]

    Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of a field area. Adopting new technologies such as unmanned aerial vehicle (UAV) acquired photographs processed by structure-from-motion (SfM) software to create terrain models will enhance structural mapping. The Jabal Hafit anticline which extends through Oman and the United Arab Emirates is a relatively simple and well exposed structure, located in the foreland of the Al Hajar Mountains. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and shows that its geometry can be reproduced by a trishear fault-propagation fold. This fold formed above a west-dipping thrust that accommodated WSW–directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene through to the early–middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains. The anticline was also used as a test site for carrying out UAV-SfM mapping and shows that this is an invaluable tool for the field geologist.   

  • 17.
    Hansman, Reuben J.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. eden.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Workflow: From photo-based 3-D reconstruction of remotely piloted aircraft images to a 3-D geological model2019In: Geosphere, E-ISSN 1553-040X, Vol. 15, no 4, p. 1393-1408Article in journal (Refereed)
    Abstract [en]

    Geological field mapping is a vital first step in understanding geological processes. During the 20th century, mapping was revolutionized through advances in remote sensing technology. With the recent availability of low-cost remotely piloted aircraft (RPA), field geologists now routinely carry out aerial imaging without the need to use satellite, helicopter, or airplane systems. RPA photographs are processed by photo-based three-dimensional (3-D) reconstruction software, which uses structure-from-motion and multi-view stereo algorithms to create an ultra-high-resolution, 3-D point cloud of a region or target outcrop. These point clouds are analyzed to extract the orientation of geological structures and strata, and are also used to create digital elevation models and photorealistic 3-D models. However, this technique has only recently been used for structural mapping. Here, we outline a workflow starting with RPA data acquisition, followed by photo-based 3-D reconstruction, and ending with a 3-D geological model. The Jabal Hafit anticline in the United Arab Emirates was selected to demonstrate this workflow. At this anticline, outcrop exposure is excellent and the terrain is challenging to navigate due to areas of high relief. This makes for an ideal RPA mapping site and provides a good indication of how practical this method may be for the field geologist. Results confirm that RPA photo-based 3-D reconstruction mapping is an accurate and cost-efficient remote sensing method for geological mapping.

  • 18.
    Hansman, Reuben J.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Chinese Academy of Sciences, China.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Scharf, Andreas
    Glodny, Johannes
    Wan, Bo
    Structural architecture and Late Cretaceous exhumation history of the Saih Hatat Dome (Oman), a review based on existing data and semi-restorable cross-sections2021In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 217, article id 103595Article, review/survey (Refereed)
    Abstract [en]

    The Saih Hatat Dome in the Al Hajar Mountains provides an outstanding opportunity to study subduction/exhumation processes coeval with obduction of the Semail Ophiolite. The exceptionally good outcrop conditions offer a unique opportunity to constrain the geometry of this subduction/obduction complex. In this review, the metamorphic, structural, and tectonic evolution of the Oman high-pressure complex in the Saih Hatat Dome is discussed. New structural cross-sections are developed and are used to interpret a geometrically feasible tectonic model for the Saih Hatat Dome. Our review highlights the importance of two major tectonic boundaries: (1) The As Sheik Shear Zone which separates the high pressure rocks of the As Sifah Unit (1.7–2.3 GPa and 510–550 °C) from the overlying Hulw Unit (1.0–1.2 GPa and 250–300 °C), and was active at ~79–76 Ma; and (2) the Upper Plate–Lower Plate Discontinuity, which forms a major surface in the landscape and developed by ~76–74 Ma, cutting through structures of the HP rocks in the lower plate (footwall). This discontinuity is associated with a pronounced strain gradient, notably in its upper plate (hanging wall), and separates rocks that have markedly different deformation geometry. The Upper Plate–Lower Plate Discontinuity initiated with a modest dip angle, making it a neutral structure in terms of crustal shortening vs extension. As a result, there is no discernable break in P-T conditions across it. The upper plate is dominated by the Saih Hatat Fold Nappe, forming between ~76 and 70 Ma. Subsequently, the upper plate has been dissected by a number of NNE-dipping thrusts at ~70–66 Ma, followed by normal faults at <~66 Ma. Our review and tectonic model indicate that the Oman high-pressure rocks were exhumed in a contractional tectonic setting that was possibly driven by forced return flow assisted by buoyancy forces. During this exhumation, when the rocks reached the greenschist-facies middle crust the Upper Plate–Lower Plate Discontinuity formed, as a shallow, south-dipping backthrust. Final exhumation of the high-P rocks was achieved by late normal faults after ~66 Ma.

  • 19.
    Hansman, Reuben J.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thomson, Stuart N.
    den Brok, Bas
    Stübner, Konstanze
    Late Eocene Uplift of the Al Hajar Mountains, Oman, Supported by Stratigraphy and Low-Temperature Thermochronology2017In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 36, no 12, p. 3081-3109Article in journal (Refereed)
    Abstract [en]

    Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U-Th)/He (AHe), 15 fission track (AFT), and four zircon (U-Th)/He (ZHe) sample ages. These data show differential cooling between the two major structural culminations of the mountains. In the 3km high Jabal Akhdar culmination AHe single-grain ages range between 392 Ma and 101 Ma (2 sigma errors), AFT ages range from 518 Ma to 324 Ma, and ZHe single-grain ages range from 62 +/- 3Ma to 39 +/- 2 Ma. In the 2 km high Saih Hatat culmination AHe ages range from 26 +/- 4 to 12 +/- 4 Ma, AFT ages from 73 +/- 19Ma to 57 +/- 8 Ma, and ZHe single-grain ages from 81 +/- 4 Ma to 58 +/- 3 Ma. Thermal modeling demonstrates that cooling associated with uplift and erosion initiated at 40 Ma, indicating that uplift occurred 30 Myr after ophiolite obduction and at least 10 Myr before the Zagros collision. Therefore, this uplift cannot be related to either event. We propose that crustal thickening supporting the topography of the Al Hajar Mountains was caused by a slowdown of Makran subduction and that north Oman took up the residual fraction of N-S convergence between Arabia and Eurasia.

  • 20. Karabacak, Volkan
    et al.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Uysal, I. Tonguç
    The off-fault deformation on the North Anatolian Fault zone and assessment of slip rate from carbonate veins2020In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 795, article id 228633Article in journal (Refereed)
    Abstract [en]

    Discrepancies in earthquake characteristics between geodetically determined and geologically observed slip rates are prominent for the eastern part of the North Anatolian Fault (NAF) zone. We investigated the spatial distribution of deformation across a NAF fault segment near Erzincan, NE Turkey, through field observations of physiographic features along the principle fault and a carbonate-filled fissure system. We propose that distributed deformation across the principal fault in a stretching zone caused sigmoidal simple-shear rotation of the carbonate-filled fissure system. Displacement data of that sigmoidal fissure using isotopic age data of the carbonate fill indicates an off-fault deformation rate of 15.90 +/- 3.19 mm per year for the Holocene, which corresponds to at least 50% of the geodetically estimated annual slip across the fault. It is critical to take the distributed deformation into account for determining earthquake characteristics of active fault systems.

  • 21.
    Lang, Karl A.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Tübingen, Germany.
    Glotzbach, Christoph
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kamp, Peter J. J.
    Ehlers, Todd A.
    Linking orogeny and orography in the Southern Alps of New Zealand: New observations from detrital fission-track thermochronology of the Waiho-1 borehole2020In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 552, article id 116586Article in journal (Refereed)
    Abstract [en]

    Numerical modeling coupling erosion with crustal deformation predicts that development of an orographic rain shadow may explain the asymmetric exhumation of convergent plate boundary orogens. This prediction is consistent with observations from the Southern Alps of New Zealand, where bedrock thermochronology indicates crustal exhumation has been concentrated along the wet, windward side of the mountains. While the spatial correspondence of exhumation and precipitation patterns is compelling, a robust evaluation of the link between orographic and orogenic processes further requires a temporal comparison between exhumation history, plate reconstructions and paleoclimatic records. We present a detailed exhumation history of the Southern Alps from detrital apatite and zircon fission-track thermochronology of the Waiho-1 borehole, a 3.6 km-thick sequence of proximal foreland basin sediments. Inverse thermal modeling of a 2062-grain dataset predicts two periods of rapid exhumation in the Early and Late Miocene. Rapid exhumation in the Early Miocene was synchronous with the development of transpression along the Alpine Fault and a ca. 20-22 Ma pulse of exhumation may reflect cooling during inversion of preexisting extensional basins. Rapid exhumation in the Late Miocene was not synchronous with a discrete change in plate convergence but increased exhumation rates after 7.4 Ma may instead reflect localization of plate boundary deformation along the Alpine Fault as orographic precipitation concentrated exhumation in the Alpine Fault hanging wall. We propose that, similar to prior interpretations from bedrock thermochronology, detrital thermochronology of the Southern Alps foreland basin is consistent with numerical model predictions linking asymmetric exhumation of the orogen to the growth of an orographic rain shadow.

  • 22.
    Linnros, Henrik
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hansman, Reuben
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The 3D geometry of the Naxos detachment fault and the three-dimensional tectonic architecture of the Naxos metamorphic core complex, Aegean Sea, Greece2019In: International journal of earth sciences, ISSN 1437-3254, E-ISSN 1437-3262, Vol. 108, no 1, p. 287-300Article in journal (Refereed)
    Abstract [en]

    We reconstruct the three-dimensional (3D) geometry of the Naxos detachment fault and quantify E-W shortening associated with strong N-S extensional deformation. In addition, 3D reconstruction of the detachment indicates how it interacted with the underlying metamorphic sequence, as well as a set of concentric metamorphic isograds that formed during extensional deformation. For doing so, we used the software MOVE (TM) (Midland Valley Ltd.) to develop a 3D model of the Naxos metamorphic core complex (NCC) and the Naxos detachment fault. The model is constrained by structural data, metamorphic isograds, and fission-track ages from the footwall of the NCC. Our analysis shows that greater minimum amounts of E-W shortening correlate with higher metamorphic grade, ranging from 6 to 10% outside the migmatite dome and up to 17% within the migmatite dome. The metamorphic isograds around the migmatite dome are less intensely folded by E-W shortening than the lithologic layering. We conclude that this is because the isograd surfaces froze-in after folding had already started. Our model indicates that the isograds are cut by the brittle detachment, demonstrating that the final stages of top-to-the-NNE extension outlasted the formation of the metamorphic dome and the isograds. Despite some limitations of the 3D model, the outlined geometric reconstruction of a major extensional fault system in the central Aegean highlights: (1) the interplay between large-scale extension and temperature-dependant shortening perpendicular to the extension direction, (2) the evolution of metamorphism and migmatization, (3) the formation of metamorphic isograds, and (4) granitoid intrusions during extension and exhumation.

  • 23. Long, Kun
    et al.
    Zhang, Zhenyu
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Faulkner, Daniel
    Gamage, Ranjith P.
    Microcracks development and porosity evolution in sandstone, Sichuan basin, China: an experimental approach2021In: Bulletin of Engineering Geology and the Environment, ISSN 1435-9529, E-ISSN 1435-9537, Vol. 80, no 10, p. 7717-7729Article in journal (Refereed)
    Abstract [en]

    The process of fault rupture is closely related to the weakening of fault rock. The weakening process of fault rock from Sichuan basin was investigated when subjected to different loading levels and subsequently unloaded under uniaxial compression. With nuclear magnetic resonance, the transverse relaxation time spectra (T2) distribution and porosity characteristics of the sandstone were quantified; meanwhile, optical microscopic experiments were performed to explore the microcrack growth patterns. According to the growth curve of the normalized porosity, the degradation process of sandstone was divided into three phases. During the first phase, the normalized porosity grew at a small accelerating rate. The intergranular microcracks grew slightly, and there was no distinct change in the number of transgranular and transgranular-intergranular microcracks during this phase, suggesting the weakening was mainly caused by the initiation of new microcracks. In the second phase, the growth rate of the normalized porosity increased linearly as a response to the percentage of failure. At this stage, although intergranular microcracks still dominate, there was a significant increase in the density of transgranular-intergranular microcracks and transgranular microcracks. Furthermore, microcracks developed in parallel with principal stress direction with the increase of the microcrack number and length. In the third phase, the normalized porosity increases at a large accelerating rate. Microcracks continued to grow along the major principal stress direction and penetrate each other. It is noteworthy that the crack density of fault sandstone increased exponentially with load level, and the crack density increased sharply at about 75% of peak strength, suggesting the initiation of fault rupturing. Such results contributed to improve the understanding of porosity change and cracking development of rocks in fault zones, which is pertinent to shear fracture nucleation.

  • 24.
    Peillod, Alexandre
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Majka, Jarosław
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Drüppel, Kirsten
    Patten, Clifford
    Karlsson, Andreas
    Włodek, Adam
    Tehler, Elof
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Differences in decompression of a high-pressure unit: A case study from the Cycladic Blueschist Unit on Naxos Island, Greece2021In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 386–387, article id 106043Article in journal (Refereed)
    Abstract [en]

    Determining the tectonic evolution and thermal structure of a tectonic unit that experiences a subduction-related pressure temperature (P-T) loop is challenging. Within a single unit, P-T conditions can vary from top to bottom which can only be revealed by detailed petrological work. We present micropetrological data from the middle section of the Cycladic Blueschist Unit (CBU) in Naxos, Greece, which indicates a different P-T loop than that for the top of the sequence. Using Zr-in-rutile and Ti-in-biotite thermometry coupled with quartz-in-garnet elastic barometry and phase equilibrium thermodynamic modeling, we identify a prograde path from 15.4 +/- 0.8 kbar to 19.9 = 0.6 kbar and from 496 +/- 16 degrees C to 572 +/- 7 degrees C (2a uncertainty), equilibration during decompression at 8.3 +/- 15 kbar and 519 +/- 12 degrees C followed by near-isobaric heating to 92 +/- 0.8 kbar and 550 +/- 10 degrees C (or even 584 +/- 19 degrees C), and a final greenschist-facies equilibration stage at 3.8 +/- 0.3 kbar and 520 +/- 4 degrees C. We compare these P-T estimates with published data from the top and also the bottom of the CBU section and find that the bottom half of the CBU on Naxos records higher peak high-pressure (HP) of about 4 kbar than the top of the unit, defining the thickness of the CBU section on Naxos to about 15 km in the Eocene. We determine that crustal thickening of up to similar to 15% occurs in the upper half of the CBU section during exhumation of the HP rocks in an extrusion wedge in a convergence setting. At about 30 Ma, the bottom half of the CBU was finally thrust onto the radiogenic Cycladic basement. Subsequently this bottom half of the CBU section underwent isobaric heating of 9-96 degrees C between c. 32-28 and 23-21 Ma. Isobaric heating occurred below the upper CBU section that thickened during decompression and commenced when HP metamorphism in the Cyclades ended. This suggests that thermal relaxation following tectonic accretion in the Cyclades controlled heating of the evolving Cycladic orogen during a tectonically quiescent period before lithospheric extension commenced by 23-20.5 Ma.

  • 25.
    Peillod, Alexandre
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    An Eocene/Oligocene blueschist-/greenschist facies P-T loop from the Cycladic Blueschist Unit on Naxos Island, Greece: Deformation-related re-equilibration vs. thermal relaxation2017In: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 35, no 7, p. 805-830Article in journal (Refereed)
    Abstract [en]

    Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P-T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 +/- 16 to 619 +/- 32 degrees C and 15.5 +/- 0.5 to 16.3 +/- 0.9kbar from a subsequent greenschist facies overprint at 384 +/- 30 degrees C and 3.8 +/- 1.1kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist facies assemblages between 40.5 +/- 1.0 and 38.3 +/- 0.5Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32Ma, with one sample showing rejuvenation at c. 27Ma. We conclude that the high-P rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 +/- 4.6km/Ma, completing a full blueschist-/greenschist facies metamorphic cycle soon after subduction within c. 8Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic re-equilibration, and is unrelated to the strong high-T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.

  • 26.
    Peillod, Alexandre
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Linnros, Henrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hansman, Reuben J.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The role of ductile flow of the lower crust in controlling heat advection in the footwall of the Naxos extensional fault system (Aegean Sea, Greece)In: Article in journal (Refereed)
    Abstract [en]

    Geothermobarometric and structural work indicates that metamorphism in the footwall of the migmatitic Naxos core complex is strongly coupled to heat advection in the lower crust. Using the average pressure-temperature (P-T) method of THERMOCALC for geothermobarometric calculations coupled with detailed textural work, we constructed P-T paths for nine samples that have different spatial relationships to the Naxos extensional fault system and a migmatite dome in the center of the Naxos core complex. All nine samples show early near isothermal decompression. The late, cooling segment of the P-T paths shows systematic spatial differences in temperature gradients relative to the distance of the samples below the Naxos extensional detachment (representing the top of the Naxos extensional fault system). The differences in late thermal gradients correlate with finite strain, a proxy for the intensity of ductile flow. High finite strain correlates with high thermal gradients that do not change with distance below the Naxos detachment, whereas low finite strain correlates with downward decreasing thermal gradients. The difference in late thermal gradients and finite strain define a thermal asymmetry controlled by ductile flow, which we interpret to show that ductile flow controlled advection of heat and thus high-grade metamorphism on Naxos. We conclude that advective heating is an important driver of metamorphism in extensional settings.

  • 27.
    Peillod, Alexandre
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tehler, Elof
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Quo vadis Zeus: is there a Zas shear zone on Naxos Island, Aegean Sea, Greece? A review of metamorphic history and new kinematic data2021In: Journal of the Geological Society, ISSN 0016-7649, E-ISSN 2041-479X, Vol. 178, no 5Article, review/survey (Refereed)
    Abstract [en]

    Detecting zones of considerable early-orogenic displacement in rather monotonous rock sequences which have undergone a late and pervasive tectonometamorphic overprint is challenging. It has been proposed that the alleged Zas shear zone in the passive margin sequence of the Cycladic Blueschist Unit on Naxos Island, Greece, separates amphibolite facies, non-high-pressure rocks (Koronos Unit) below the shear zone from Eocene high-pressure rocks (Zas Unit) above the shear zone. We review existing pressure-temperature data from the Koronos Unit and present new kinematic data from the anticipated Zas shear zone to evaluate the tectonic significance of this recently proposed structure. This has implications for unravelling the subduction history of rock units from tectonometamorphic datasets. Common to all pressure-temperature data from the Koronos Unit is a well-defined amphibolite facies equilibration stage at 8-11 kbar and 600-700 degrees C, followed by initial near-isothermal to slightly prograde decompression and subsequent pronounced cooling. This segment of the high-temperature pressure-temperature loop is associated with top-to-the-NNE extensional deformation in the footwall of the Miocene Naxos-Paros detachment. Little is known about the metamorphism preceding the amphibolite facies overprint in the Koronos Unit. Our review shows that it is likely that the rocks experienced a prior high-pressure metamorphic overprint that is typical of rocks of the Cycladic Blueschist Unit. Our kinematic data show that the Zas shear zone contains variably deformed rocks with dominantly top-to-the-NNE shear sense indicators that developed under greenschist facies metamorphism in the footwall of the Naxos-Paros detachment. No significant offset can be detected across the Zas shear zone and the geology on either side of it does not support large-scale movement across the shear zone. We discuss a model in which the Zas shear zone is considered to be a minor zone of deformed schist near the biotite-in isograd of Miocene high-temperature metamorphism. We conclude that there is no need to tectonically subdivide the passive margin sequence of the Cycladic Blueschist Unit on Naxos.

  • 28.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    THE EAST AFRICAN RIFT SYSTEM2014In: Austrian Journal of Earth Sciences, E-ISSN 2072-7151, Vol. 107, no 1, p. 132-146Article in journal (Refereed)
    Abstract [en]

    The East African Rift System is one of the most outstanding and significant rift systems on Earth and transects the high-elevation Ethiopian and East African plateaux. Rifting putatively developed as a result of mantle plume activity that initiated below East Africa. The rift is traditionally interpreted to be composed of two distinct segments: an older, volcanically active Eastern Branch and a younger, much less volcanic Western Branch. Rift-related volcanism commenced in the Eocene and a major phase of flood basalt volcanism occurred in Ethiopia by 31-30 Ma. Rift development in the Eastern Branch has a distinct northward progression with a juvenile rifting stage in northern Tanzania, well advanced rifting in Kenya and the transition of continental rifting to incipient sea-floor spreading in Ethiopia and Afar. The Western Branch in general has not yet progressed to an advanced rifting stage and rift basin architecture retains a pristine geometry. The onset of topographic uplift in the East African Rift System is poorly dated but has certainly preceded graben development. It is widely believed that topography has been caused by plume activity. The uplift of the East African Plateau might be connected to African Cenozoic climate change and faunal and human evolution.

  • 29.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bernet, Matthias
    Tulloch, Andy
    Kinematic, finite strain and vorticity analysis of the Sisters Shear Zone, Stewart Island, New Zealand2015In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 73, p. 114-129Article in journal (Refereed)
    Abstract [en]

    The Sisters Shear Zone (SSZ) on Stewart Island, New Zealand, is a greenschist-facies extensional shear zone active prior to and possibly during the development of the Pacific Antarctica spreading ridge at similar to 76 Ma. We report quantitative kinematic and rotation data as well as apatite fission-track (AFT) ages from the SSZ. Early kinematic indicators associated with the NNE-trending stretching lineation formed under upper greenschist-facies metamorphism and show alternating top-to-the-NNW and top-to-the-SSE senses of shear. During progressive exhumation lowermost greenschist-facies and brittle-ductile kinematic indicators depict a more uniform top-to-the-SSE sense of shear in the topmost SSZ just below the detachment plane. Deformed metagranites in the SSZ allow the reconstruction of deformation and flow parameters. The mean kinematic vorticity number (W-m) ranges from 0.10 to 0.89; smaller numbers prevail in the deeper parts of the shear zone with a higher degree of simple shear deformation in the upper parts of the shear zone (deeper and upper parts relate to present geometry). High finite strain intensity correlates with low Wm and high Wm numbers near the detachment correlate with relatively weak strain intensity. Finite strain shows oblate geometries. Overall, our data indicate vertical and possibly temporal variations in deformation of the SSZ. Most AFT ages cluster around 85-75 Ma. We interpret the AFT ages to reflect the final stages of continental break-up just before and possibly during the initiation of sea-floor spreading between New Zealand and Antarctica.

  • 30.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bolhar, Robert
    Tilting, uplift, volcanism and disintegration of the South German block2020In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 795, article id 228611Article in journal (Refereed)
    Abstract [en]

    We present a large set of fault-slip data together with six U-Pb ages on calcite vein fillings constraining the kinematics and timing of near-surface (uppermost 2-3 km of crust) faulting of the uplifted South German block in the northern foreland of the NW-ward advancing Alps. The structural data confirm earlier studies showing Miocene subhorizontal NE-directed extension with shortening mainly being subvertical but also subhorizontally NW oriented. The U-Pb ages are between 15.3 +/- 0.2 and 2.3 +/- 1.1 Ma (2 sigma analytical uncertainty) and date kinematically coordinated normal and strike-slip faulting in the northern Alpine foreland. The faults in the South German block have limited strike lengths indicating that they do not cut deeply into the crust. However, the faults appear to control volcanic centers, the melts of which originated at the base of the lithosphere. We suggest that the Miocene faults could tap the mantle because they reactivated formerly deepseated faults. The ages for near-surface faulting are coeval with eruption of the Urach and Hegau volcanics. Published data suggest that Alpine shortening induced large-wavelength lithospheric folding in the foreland since 18-16 Ma. Volcanism in the South German block focussed along a lithospheric antiform which is, in part, superimposed on the foreland bulge of the Alps. We propose a close link between progressive southward tilting, uplift and faulting of the South German block due to lithospheric-scale folding and upward flow of the asthenosphere, flexure of the lithosphere, deep-seated volcanism and the NW-ward advancing Alps.

  • 31.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fassoulas, Charalampos
    Uysal, I. Tonguç
    Bolhar, Robert
    Tong, Kui
    Todd, Andrew
    Nappe Imbrication Within the Phyllite-Quartzite Unit of West Crete: Implications for Sustained High-Pressure Metamorphism in the Hellenide Subduction Orogen, Greece2022In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 41, no 11, article id e2022TC007430Article in journal (Refereed)
    Abstract [en]

    We report four K-Ar fault-gouge and eight U-Pb calcite ages from the high-pressure Phyllite-Quartzite Unit (PQ) and the overlying, strongly thinned non-high-pressure Tripolitza and Pindos units of western Crete, Greece. We relate consistent 26–21 Ma fault-gouge ages to a discrete top-to-the-S, brittle-ductile, contractional shear zone (Intra-PQ Thrust) that formed during high-pressure conditions within the Phyllite-Quartzite Unit. The Intra-PQ Thrust separates the Phyllite-Quartzite Unit into an upper and a lower unit and is associated with gypsum deposits and high finite strain. Below the Intra-PQ Thrust, U-Pb calcite ages between about 16 and 13 Ma are also associated with top-to-the-S kinematic indicators and, in part, developed during aragonite stability. The brittle-ductile contact zone to the overlying Tripolitza and Pindos units yielded U-Pb calcite ages of 13 to 12 Ma. Our data imply that the upper Phyllite-Quartzite Unit underwent high-pressure metamorphism at the commonly envisaged time of 24–21 Ma. However, the lower Phyllite-Quartzite Unit started to be underthrust at 26–21 Ma and was high-pressure metamorphosed at 16–13 Ma, as suggested by the U-Pb calcite ages of aragonite-bearing samples and published zircon fission track ages. We discuss a tectonic model of successive underthrusting, high-pressure metamorphism and subsequent exhumation of both Phyllite-Quartzite units in extrusion wedges until 12–11 Ma, after which Crete underwent crustal extension. Our work suggests that there might be more than one Cretan Detachment. It also has implications for sustained underthrusting and exhumation of continental crust in subduction zones.

  • 32.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gerdes, Axel
    Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc2016In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 35, no 6, p. 1367-1391Article in journal (Refereed)
    Abstract [en]

    We report fault slip data from exhumed fault surfaces along the NNE trending Alpenrhein valley and its intersection with the WNW striking Bodensee Graben near the Alpine thrust front of the Central Alps in the Swiss-Austrian-German border region. This conjugated graben system straddles the boundary between the Alps and its foreland and allows comparing the kinematics of graben formation between the two different tectonic domains. Our data show sinistral transtension along the Alpenrhein Graben and dextral transtension along the Bodensee Graben. Both transtensional graben systems resulted from the same kinematic regime of NW directed shortening and NE oriented extension. The graben faults are not older than NW striking tear faults associated with NW directed Oligocene nappe emplacement in the Helvetics starting at 35-30 Ma and ending by 25-20 Ma. Compatible with this are six U-Pb ages of calcite fibers from four samples yielding consistent ages ranging from 25.3 +/- 5.6 Ma to 21.8 +/- 3.4 Ma (2 sigma errors). Earthquake data since 1996 show that kinematic directions persisted until the Recent. Our data broadly fit with the kinematic evolution of the Oberrhein Graben, which shows significant E-W extension in the Oligocene. We suggest that Oligocene extension in the Alps and its foreland resulted from the increased curvature of the Western Alps arc and associated moderate tangential stretching in the internal parts of the Central Alps. We discuss a tectonic model of eastward rollback of the west Mediterranean subduction zone associated with counterclockwise rotation of Adria, the latter of which aided the formation of the Western Alps arc.

  • 33.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gessner, Klaus
    Thomson, Stuart
    Variations in fault-slip data and cooling history reveal corridor of heterogeneous backarc extension in the eastern Aegean Sea region2017In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 700, p. 108-130Article in journal (Refereed)
    Abstract [en]

    We report fault-slip data across the boundary between the highly extended and largely submerged crust underlying the Aegean Sea from Samos in the north to eastern Crete in the south, and the much less extended and emergent crust of western Anatolia. We identify three brittle deformation increments, a late Miocene (mainly Pliocene) to Recent crustal stretching increment, an intermittent early to late Miocene shortening increment concurrent with extension and magmatism, and a Miocene extensional event. The youngest increment documents late Miocene to Recent NNE extension over large areas, but can locally also be oriented SE (Amorgos and Astipalea Islands), and ESE (eastern Crete) suggesting overall oblate strain geometry. The intermittent Miocene (similar to 245 Ma) fault-slip records suggest overall prolate strain geometry, where NNE stretching is accompanied by E-W shortening. The older extension event is mainly NNE directed but on Samos Island extension is E-W, probably reflecting local extension in a sinistral wrench corridor in the early/mid Miocene. Overall it seems that since the early Miocene NNE-trending extension is the dominant regime in the eastern Aegean with an intermittent component of short-lived E-W shortening. The existence of a corridor of heterogeneous crustal deformation which is spatially associated with uncharacteristically old fission track ages - and the apparent change in strain geometry in time challenge concepts that propose that the eastern Aegean Sea and western Anatolia have been deformed as a continuous tectonic domain since the Miocene. We propose that the regional variation in extensional strain geometry resulted from a sinistral wrench component that was superimposed on the regional 'background' NNE extension by translation across a diffuse plate boundary. We conclude that the eastern shoreline of the Aegean Sea is controlled by a Miocene to Recent sinistral wrench corridor that accommodated movement between different lithospheric domains.

  • 34.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gessner, Klaus
    Thomson, Stuart N.
    South Menderes Monocline: Low-temperature thermochronology constrains role of crustal extension in structural evolution of southwest Turkey2017In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 712, p. 455-463Article in journal (Refereed)
    Abstract [en]

    We report apatite and zircon fission-track data across the contact zones between the Menderes nappes, the Cycladic blueschist unit, and the Oren nappe, in the Anatolide belt of southwest Turkey. These data resolve previous debate on the deformation history of these Cretaceous to Eocene nappe contacts, including whether they were reactivated during late Oligocene to Miocene crustal extension. Apatite fission-track ages range from 18 to 28 Ma in the Menderes nappes, and 31 to 42 Ma in the Oren nappe. Zircon fission-track ages are 29 to 31 Ma in the Menderes nappes, 30 to 33 Ma in the Cycladic blueschist unit, and between 93 Ma and 129 Ma in the Oren nappe. The data reveal that the tectonic contacts within the Menderes nappes, and with the overlying Cycladic blueschist unit have been sealed since similar to 35-30 Ma. In the Oren nappe, zircon fission-track ages are distinctly older (with mixed single grain ages between similar to 300 and 75 Ma) reflecting partial to full resetting during late Cretaceous high-pressure metamorphism, The contact between the Cycladic blueschist unit and the Oren nappe shows no significant tectonic movement in the brittle crust after similar to 70 Ma. These data reveal that the nappe boundaries in the southern Menderes Massif have undergone no significant differential offset relative to the Earth's surface since the Oligocene and therefore were not reactivated during late Oligocene to Miocene continental extension. We interpret the steeply dipping portion of the nappe pile in the southern Menderes Massif as a tilted crustal section, which we name the 'South Menderes Monocline'. We speculate this monocline formed by differential uplift in the Miocene, either at the hinge zone of a plateau, or by unloading of the Simav detachment footwall.

  • 35.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Geometry and Kinematics of Bivergent Extension in the Southern Cycladic Archipelago: Constraining an Extensional Hinge Zone on Sikinos Island, Aegean Sea, Greece2021In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 40, no 6, article id e2020TC006641Article in journal (Refereed)
    Abstract [en]

    We report the results of a field study on Sikinos Island in the Aegean extensional province of Greece and propose a hinge zone controlling incipient bivergent extension in the southern Cyclades. A first deformation event led to top-S thrusting of the Cycladic Blueschist Unit (CBU) onto the Cycladic basement in the Oligocene. The mean kinematic vorticity number (Wm) during this event is between 0.56 and 0.63 in the CBU, and 0.72 to 0.84 in the basement, indicating general-shear deformation with about equal components of pure and simple shear. The strain geometry was close to plane strain. Subsequent lower-greenschist-facies extensional shearing was also by general-shear deformation; however, the pure-shear component was distinctly greater (Wm = 0.3–0.41). The degree of subvertical pure-shear flattening increases structurally upward and explains alternating top-N and top-S shear senses over large parts of the island. Along with an increased coaxial deformation component, the strain geometry became oblate. Published quantitative data from nearby Ios Island are similar and both data sets define an extensional hinge zone between top-N extensional deformation across large parts of the central and northern Cyclades and top-S extensional deformation at the southern and western fringe of the archipelago. This extensional hinge zone is an important large-scale structure forming early in the history of lithospheric extension due to southward retreat of the Hellenic slab.

  • 36.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    The importance of tangential motion in the Central Alps: Kinematic analysis and Rb-Sr dating of mylonitic rocks from the Pennine nappes in the eastern Central Alps2021In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 218, article id 103644Article in journal (Refereed)
    Abstract [en]

    We discuss the important role of tangential stretching in the Central Alps highlighting that the Alps are a truly three-dimensional orogen. A review of pressure-temperature (P-T) conditions allows us to define three major breaks in metamorphic pressure (P-breaks) across the nappe pile. We constrain the kinematics of mylonites associated with these P-breaks and report eight new RbSr multi-mineral isochrons that define the age of the mylonites. Kinematic data and RbSr geochronology indicate that the timing of top-SE normal shearing in the Avers-Turba mylonite zone (≥45–34 Ma; P-break1) and in a zone of distributed top-E normal shearing (>35–30 Ma) above the (ultra)high-P Adula nappe of the distal, thinned European margin (P-breaks 2 and 3). The ages from both sets of mylonite are grading into each other at about 35–34 Ma. All ages appear to postdate high-P metamorphism in the Pennine nappes but are, in part, coeval with and predate (ultra)high-P metamorphism in the underlying Adula nappe of the distal European (Helvetic) margin. The oldest RbSr age of 42.3 ± 2.5 Ma (2σ uncertainties) dates the waning stage of mylonitization and indicates that the Avers-Turba mylonite zone started to operate ≥45 Ma, and predated the exhumation of the Adula nappe by ~10 Ma. We suggest that the motion in the Avers-Turba mylonite zone overlapped with backfolding of the Schams nappe in an extrusion wedge. When this process started, the Adula nappe was still being underthrust/underplated. The ages for distributed top-E normal shearing are contemporaneous with thrusting of the Adula nappe on top of distinctly lower-P Helvetic nappes. When the Adula nappe was being thrust onto non-high-P nappes the necessary reduction of its overburden was largely accomplished by distributed top-E normal faulting above the Adula nappe, and possibly still ongoing top-SE normal shearing. We discuss that major normal shearing occurred during lithospheric shortening. Our data indicate considerable tangential (out-of-plane) movements during overall top-N/NW thrust propagation in the Central Alps and have implications for current tectonic models of the Alps.

  • 37.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Hansman, Reuben
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Scharf, Andreas
    Mattern, Frank
    Callegari, Ivan
    van Hinsbergen, Douwe J. J.
    Willner, Arne
    Hong, Yangbaihe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Samail subduction zone dilemma: Geochronology of high-pressure rocks from the Saih Hatat window, Oman, reveals juxtaposition of two subduction zones with contrasting thermal histories2024In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 250, article id 104711Article, review/survey (Refereed)
    Abstract [en]

    The Samail Ophiolite in the Oman Mountains formed at a Cretaceous subduction zone that was part of a wider Neo-Tethys plate-boundary system. The original configuration and evolution of this plate-boundary system is hidden in a structurally and metamorphically complex nappe stack below the Samail Ophiolite. Previous work provided evidence for high-temperature metamorphism high in the nappe pile (in the metamorphic sole of the Samail Ophiolite), and high-pressure metamorphism in the deepest part of the nappe pile (Saih Hatat window), possibly reflecting a downward younging, progressive accretion history at the Samail subduction zone. However, there is evidence that the two subduction-related metamorphic events are disparate, but temporally overlapping during the mid-Cretaceous. We present the first geochronologic dataset across the entire high-pressure nappe stack below the Samail Ophiolite, and the shear zones between the high-pressure nappes. Our 22 new Rb-Sr multimineral isochron ages from the Saih Hatat window, along with independent new field mapping and kinematic reconstructions, constrain the timing and geometry of tectonometamorphic events. Our work indicates the existence of a highpressure metamorphic event in the nappes below the ophiolite that was synchronous with the hightemperature conditions in the metamorphic sole. We argue that the thermal conditions of these synchronous metamorphic events can only be explained through the existence of two Cretaceous subduction zones/segments that underwent distinctly different thermal histories during subduction infancy. We infer that these two subduction zones initially formed at two perpendicular subduction segments at the Arabian margin and subsequently rotated relative to each other and, as a consequence, their records became juxtaposed: (1) The hightemperature metamorphic sole and the Samail Ophiolite both formed above the structurally higher, outboard, 'hot' and rotating Samail subduction zone and, (2) the high-pressure nappes developed within the structurally lower, inboard, 'cold' Ruwi subduction zone. We conclude that the formation and evolution of both subduction zones were likely controlled by the density structure of the mafic-rock-rich Arabian rifted margin and outermost Arabian Platform, and the subsequent arrival of the buoyant, largely mafic-rock-free, full-thickness Arabian lithosphere, which eventually halted subduction at the southern margin of Neo-Tethys.

  • 38.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Peillod, Alexandre
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The timing of high-temperature conditions an d ductile shearing in the footwall of the Naxos metamorphic core complex, Aegean Sea, GreeceIn: Article in journal (Refereed)
    Abstract [en]

    We present eight Rb-Sr multi-mineral isochron ages showing that high-temperature metamorphic conditions and partial melting during top-to-the-NNE extensional shearing in the footwall of the Naxos metamorphic core complex lasted until about 14-12 Ma. One migmatite sample yielded an age of 14.34 ± 0.20 Ma (2σ uncertainties) for crystallization of migmatization related melt pockets. Four pegmatite samples, which are in part associated with partial melting of their host rocks, provided overlapping ages ranging from 13.81 to 12.23 Ma (age range includes 2σ uncertainties). Additional three samples of amphibolite-facies schist supplied Rb-Sr ages of around 14 Ma. Samples showing fluid- and/or deformation-assisted white mica and biotite reworking gave Rb-Sr mineral apparent ages of 11.1 ± 2.7, 10.16 ± 0.24, 9.7 ± 0.7 and 9.6 ± 0.15 Ma. These ages are interpreted to be associated with late stages of extensional shearing under greenschist-facies metamorphic conditions. Together with published U-Pb zircon ages of migmatite, and S- and I-type granite crystallization, the data indicate that presence of melt in the Naxos migmatite dome lasted for at least 7 Ma (from ~18 to ~11 Ma). This demonstrates that the thermal anomaly resulting from and aiding extensional deformation was a long-lived and not a transient event. We conclude that melt-assisted deformation facilitated large-scale displacement on the Naxos extensional fault system by drastically weakening the extending crust for long periods of time.

  • 39.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Peillod, Alexandre
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The timing of high-temperature conditions and ductile shearing in the footwall of the Naxos extensional fault system, Aegean Sea, Greece2018In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 745, p. 366-381Article in journal (Refereed)
    Abstract [en]

    We present eight Rb-Sr multi-mineral isochron ages showing that high-temperature metamorphic conditions and partial melting during top-to-the-NNE extensional shearing in the footwall of the Naxos extensional fault system (i.e. Naxos metamorphic core complex) lasted until about 14-12 Ma. One migmatite sample yielded an age of 14.34 +/- 0.2 Ma (2 sigma uncertainty) for crystallization of migmatization-related melt pockets. Four pegmatite samples, which are in part associated with partial melting of their host rocks, provided overlapping ages ranging from 13.81 to 12.23 Ma (age range includes 2 sigma uncertainty). Additional three samples of amphibolite-facies schist supplied Rb-Sr ages of around 14 Ma. Samples showing fluid- and/or deformation-assisted white mica and biotite reworking gave Rb-Sr mineral apparent ages of 11.1 +/- 2.7, 10.16 +/- 0.24, 9.7 +/- 0.7 and 9.6 +/- 0.15 Ma. These ages are interpreted to be associated with late stages of extensional shearing under greenschist-facies metamorphic conditions. Together with published U-Pb zircon ages of migmatite, and S- and I-type granite crystallization, the data indicate that the presence of melt in the footwall of the Naxos extensional fault system lasted for at least 7 Ma (from similar to 18 to similar to 11 Ma). This demonstrates that high temperatures and crustal melting resulting from and aiding extensional deformation was a long-lived and not a transient event. We conclude that melt-assisted deformation facilitated large-scale displacement on the Naxos extensional fault system by drastically weakening the extending crust for long periods of time.

  • 40.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Scharf, Andreas
    Hansman, Reuben
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Some Like It Cold: The “Conundrum of Samail” Revisited2023In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 42, no 1, article id e2022TC007531Article in journal (Refereed)
    Abstract [en]

    Most tectonic models consider that the “Samail subduction zone” was the only subduction zone at the mid-Cretaceous convergent Arabian margin. We report four new Rb-Sr multimineral isochron ages from high-pressure (HP) rocks and a major shear zone of the uppermost Ruwi-Yiti Unit of the Saih Hatat window in the Oman Mountains of NE Arabia. These ages demand a reassessment of the intraoceanic suprasubduction-zone evolution that formed the Samail Ophiolite and its metamorphic sole in the Samail subduction zone. Our new ages constrain waning HP metamorphism of the Ruwi subunit at ∼99-96 Ma and associated deformation in the Yenkit shear zone between ∼104 and 93 Ma. Our ages for late stages of deformation and HP metamorphism (thermal gradients of ∼8–10°C km−1) overlap with published ages of ∼105-102 Ma for Samail-subduction-zone prograde-to-peak metamorphism (thermal gradients of ∼20–25°C km−1), subsequent decompressional partial melting of the metamorphic sole and suprasubduction-zone crystallization of the Samail Ophiolite (thermal gradients of ∼30°C km−1) between ∼100 and 93 Ma. Thermal considerations demand that two subduction zones existed at the mid-Cretaceous Arabian margin. High-pressure metamorphism of the Ruwi-Yiti rocks occurred in a mature, thermally equilibrated “Ruwi subduction zone” that formed at ∼110 Ma. Initiation of the infant, thermally unequilibrated and, thus, immature, outboard intraoceanic Samail subduction zone occurred at ∼105 Ma. The Samail Ophiolite and its metamorphic sole were then thrust over the exhuming Ruwi-Yiti HP rocks and onto the Arabian margin after ∼92 Ma, while the bulk of the Saih Hatat HP rocks below the Ruwi-Yiti Unit started to be underthrust in the Ruwi subduction zone.

  • 41.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hampton, Sam
    Faulting in Banks Peninsula: tectonic setting and structural controls for late Miocene intraplate volcanism, New Zealand2012In: Journal of the Geological Society, ISSN 0016-7649, E-ISSN 2041-479X, Vol. 169, no 6, p. 773-785Article in journal (Refereed)
    Abstract [en]

    An analysis of faulting in the late Miocene volcanic rocks of Banks Peninsula, South Island, New Zealand, shows that the formation of the volcanic edifice was largely controlled by NE-SW-striking dextral-oblique strike-slip faults. The data show a variable component of west-east-or NW-SE-oriented shortening and north-south or NE-SW extension. Synvolcanic faults reactivated Cretaceous normal faults and are interpreted to have formed a local pull-apart basin that controlled volcanism. Further east, the geometry of Akaroa Harbour is controlled by a north-south-striking oblique reverse fault. Limited fault-slip data collected from sub-recent loess deposits are not significantly different from the data collected in the volcanic rocks and appear to show that the kinematic field did not change significantly over the last c. 10 Ma. The overall kinematic field causing the recent series of earthquakes in the greater Christchurch region is also not fundamentally different from the one that controlled the eruption of the volcanic rocks. We conclude that the inherited Cretaceous faults controlled the development of the late Miocene volcanism on Banks Peninsula and largely provided a major anisotropy along which the recent faults ruptured.

  • 42.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mortimer, Nick
    Butz, Christoph
    Bernet, Matthias
    Extensional deformation along the Footwall Fault below the Hyde-Macraes Shear Zone, Otago Schist, New Zealand2018In: New Zealand Journal of Geology and Geophysics, ISSN 0028-8306, E-ISSN 1175-8791, Vol. 61, no 2, p. 219-236Article in journal (Refereed)
    Abstract [en]

    We describe ductile-to-brittle structures across the Footwall Fault directly below the Hyde-Macraes Shear Zone in the Otago Schist. These indicate that pervasive deformation along the Footwall Fault was due to northeast-southwest crustal extension. Structures below the Footwall Fault are ductile top-to-the-northeast extensional shear bands that formed in a penetrative lower greenschist facies foliation. With progressive deformation, shear bands formed at higher angles to the foliation, culminating in the shear bands being cut by low- and high-angle normal faults. In the hanging wall, extensional structures are much less pervasive and comprise a few high-angle normal faults. This sharp contrast in structural evolution of footwall and hanging wall resembles the tectonic evolution of major extensional shear zones worldwide. We propose that mid Cretaceous normal movement on the low-angle Footwall Fault accompanied the formation of the steeper Waihemo Fault further north. On a regional scale, much of the boundary between the Otago Schist core and its northern flank is cut by mid Cretaceous normal faults. Zircon fission-track ages from the footwall range from 79 +/- 13.7 to 46.1 +/- 6.2Ma (2 sigma uncertainties) and young in the direction of hanging wall transport. One sample from the hanging wall yielded a zircon fission-track age of 76 +/- 12.9Ma. The fission-track ages are hard to interpret as zircon across the fault zone was affected by partial or full annealing. In part, the ages confirm mid Cretaceous movement but may also be interpreted to suggest additional reactivation of the Footwall Fault at slow inferred slip rates.

  • 43.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mortimer, Nick
    Deckert, Hagen
    Critical-wedge theory and the Mesozoic accretionary wedge of New Zealand2019In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 122, p. 1-10Article in journal (Refereed)
    Abstract [en]

    We apply principles of critical-wedge theory to explain the Jurassic and Early Cretaceous tectonic development of the Mesozoic accretionary wedge of New Zealand, one of the best-exposed subduction complexes on Earth, by summarizing factors that can lead to drastic changes in wedge balance. The wedge has a Carboniferous to Cretaceous (similar to 330-110 Ma) stratigraphic record, with an early Cretaceous (similar to 145-130 Ma) hiatus in elastic deposition across the Esk Head Melange near the front of the wedge. In the rear of the wedge, the age of moderately high-pressure metamorphism of the Otago Schist, which represents the forearc-high of the wedge, occurred near the end of this stratigraphic hiatus. The Otago Schist has a subhorizontal foliation indicative of underplating as the mode of accretion. The outer (frontal) part of the wedge has a subvertical foliation indicative of frontal accretion. We propose that in the Jurassic and earliest Cretaceous the wedge grew by frontal accretion. By similar to 135 Ma the wedge became too long and was subcritical. This resulted in tectonic erosion of the front of the wedge. The tectonically eroded material was underthrust/underplated beneath the rear of the wedge to form the subhorizontally foliated Otago Schist. Colliding seamounts and/or oceanic plateaux (preserved in the Esk Head Melange) may have aided wedge instability. The proposed wedge instability and increased horizontal shortening was largely coeval with major changes in the chemistry of subduction-related arc magmatism and a phase of shortening and high-pressure metamorphism in the arc. Our model provides a holistic and coherent interpretation explaining the major tectonic features of the long-lived New Zealand part of the east Gondwana subduction zone.

  • 44.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pantazides, Hermes
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Uplift of the Troodos Massif, Cyprus2019In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 38, no 8, p. 3124-3139Article in journal (Refereed)
    Abstract [en]

    We constrain the fault pattern and the kinematics of faulting that facilitated the uplift of the Troodos Massif in Cyprus in the eastern Mediterranean. The fault pattern consists of E-W striking reverse faults, N-S striking normal faults, and NW striking dextral and NE striking sinistral strike/oblique-slip faults. Fault-slip analysis reveals that this overall pattern resulted from subhorizontal NNW directed shortening and coeval, subhorizontal ENE directed extension. Dated sediments affected by faulting reveal that the N-S striking normal faults are, at least in part, younger than 2.14 Ma. This suggests that the entire fault pattern, or at least a large part of it, resulted or was reworked during post-2.14-Ma deformation. Published work further shows that the uplift of the Troodos Massif was controlled by underthrusting of the Eratosthenes Seamount, which entered the Cyprus subduction zone by about 2 Ma. The alleged E-W extent of the underthrust segment of the seamount approximately matches the size of the high-topography area of the Troodos Mountains. We interpret this geometry to have caused spatially limited crustal thickening underneath the high-topography area by subhorizontal N-S shortening in front of and above the underthrusting seamount and coeval, subhorizontal E-W extension at its flanks, and that the uplift of the high-topography area occurred in the footwall of the N-S striking normal faults. This model is broadly in line with seismicity and may also explain why serpentinized mantle rocks and dense gabbro are now exposed on top of the Troodos Massif.

  • 45.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pantazides, Hermes
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Forced Return Flow Deep in the Subduction Channel, Syros, Greece2020In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 39, no 1, article id e2019TC005768Article in journal (Refereed)
    Abstract [en]

    We present the results of a detailed structural study in the Cycladic Blueschist Unit at Fabrika on Syros Island, Greece, and discuss their significance for tectonic processes at the subduction interface. Some samples record top-to-the-west shear reflecting prograde (burial), peak high-pressure (HP) and initial decompression (exhumation) conditions. Other nearby samples record top-to-the-east shear during HP metamorphism and exhumation. Some rocks re-equilibrated at greenschist-facies conditions and record top-to-the-west shear. Greenschist-facies top-to-the-west shear is also found at the base of non-HP upper units above the Fabrika HP sequence. We interpret the HP structures to reflect forced return flow and incipient formation of an extrusion wedge in the subduction channel. The HP top-to-the-west structures resulted from thrusting along the base of the wedge and started to form during burial before the rocks reached their deepest point. The HP top-to-the-east structures reflect deformation near the top of the developing extrusion wedge. After considerable exhumation during ongoing subduction, out-of-sequence, top-to-the-west thrusts emplaced the non-HP upper units above the exhuming extrusion wedge 10 Myr after the wedge initially formed. Our work suggests that the HP rocks were considerably exhumed during sustained lithospheric shortening in the subduction channel by forced return flow. Because return flow is controlled by the velocity of the subducting slab, it may explain why HP rocks can be exhumed at subduction rates. On the regional scale we find that four distinct HP belts were sequentially accreted and exhumed between 50 and 20 Ma suggesting continuous subduction-channel return flow in the Hellenic subduction zone.

  • 46.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Uysal, I. Tonguc
    Yuce, Galip
    Unal-Imer, Ezgi
    Italiano, Francesco
    Imer, Ali
    Zhao, Jian-xin
    Recent mantle degassing recorded by carbonic spring deposits along sinistral strike-slip faults, south-central Australia2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 454, p. 304-318Article in journal (Refereed)
    Abstract [en]

    The interior of the Australian continent shows evidence for late Quaternary to Recent fault-controlled mantle He-3 and CO2 degassing. A series of interconnected NW-striking sinistral faults, the Norwest fault zone (NFZ), in south-central Australia are associated with travertine mounds, the latter show a regular spacing of 50-70 km. U-series ages on 26 samples range from 354 +/- 7 to 1.19 +/- 0.02 ka (2 sigma errors) and suggest a clustering every similar to 3-4 ka since similar to 26 ka. Geochemical data demonstrate a remarkable mantle to-groundwater connection. Isotopic data indicate that the groundwater is circulating to depths >3 km and interacting with Neoproterozoic/Cambrian basement and mantle volatiles. He-3/He-4 isotope ratios show that the He comes in part from the mantle. This demonstrates that the NFZ cuts through the entire crust and provides pathways for mantle degassing. Scaling relationships suggest that the series of sinistral faults that make up the NFZ are interconnected at depths and have a significant strike length of 60-70 km or more. The NFZ occurs where a major compositional boundary and a significant heat flow anomaly occurs, and a major step in lithospheric thickness has been mapped. We discuss a tectonic model in which recent stress field, heat flow and lithospheric structure in central Australia reactivated a set of steeply dipping Neoproterozoic faults, which may now be growing into a crustal/lithospheric-scale structure.

  • 47.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Uysal, I. Tongue
    Glodny, Johannes
    Cox, Simon C.
    Littl, Tim
    Thomson, Stuart N.
    Stübner, Konstanze
    Bozkaya, Ömer
    Fault-gouge dating in the Southern Alps, New Zealand2017In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 717, p. 321-338Article in journal (Refereed)
    Abstract [en]

    We report two Ar-40/Ar-39 illite ages from fault gouge directly above the current trace of the Alpine Fault in New Zealand at Gaunt Creek (1.36 +/- 0.27 Ma) and Harold Creek (1.18 +/- 0.47 Ma), and one Ar-40/Ar-39 illite age from fault gouge from the Two Thumbs Fault on the east side of the Southern Alps. Metamorphic muscovite clasts inherited into the Alpine Fault gouge yielded Ar-40/Ar-39 ages of 2.04 +/- 0.3 Ma at Gaunt Creek and 11.46 +/- 0.47 Ma at Harold Creek. We also report Rb-Sr muscovite-based multimineral ages of Alpine Schist mylonite adjacent to the dated fault gouge at Harold Creek (13.1 +/- 43 Ma) and Gaunt Creek (8.9 +/- 3.2 Ma). Ar-40/Ar-39 muscovite ages from the Gaunt Creek mylonite yielded plateau ages of 1.47 +/- 0.08 Ma and 1.57 +/- 0.15 Ma. Finally, we report zircon fission track (0.79 +/- 0.11 and 0.81 +/- 0.17 Ma) and zircon (U-Th)/He ages (0.35 +/- 0.03 and 0.4 +/- 0.06 Ma) from Harold Creek.& para;& para;We interpret the fault gouge ages to date growth of newly formed illite during gouge formation at temperatures of similar to 300-350 degrees C towards the base of the seismogenic zone. Simple backcalculation using current uplift/exhumation and convergence rates, and dip angles of 45-60 degrees at the Alpine Fault support that interpretation. We infer that the fault gouge ages record faulting and gouge formation as the rocks passed very rapidly through the brittle-ductile transition zone on their way to the surface. Rb-Sr and Ar-40/Ar-39 ages on muscovite from Alpine Schist mylonite date muscovite growth at similar to 11 Ma together with a younger phase of cooling/shearing at similar to 1.5-2 Ma. Our ages from the Alpine Schist indicate extremely rapid cooling exceeding 200 degrees C/Ma. The fault gouge age from the Two Thumbs Fault is significantly too old to have formed as part of the late Neogene/Quaternary Southern Alps evolution.

  • 48.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Uysal, Ibrahim Tonguc
    Tong, Kui
    Todd, Andrew
    K-Ar fault-gouge dating in the Lower Buller gorge constrains the formation of the Paparoa Trough, West Coast, New Zealand2021In: New Zealand Journal of Geology and Geophysics, ISSN 0028-8306, E-ISSN 1175-8791, Vol. 64, no 1, p. 49-61Article in journal (Refereed)
    Abstract [en]

    K-Ar dating of fault gouge from the intersection of the WNW-striking Ohika Detachment of the Paparoa Metamorphic Core Complex and a NNE-striking high-angle normal fault (Ohikaiti Fault) yielded an age of 103 +/- 3.5 Ma (1 sigma uncertainty) for metamorphic white mica and an age of 35.2 +/- 2.3 Ma for fault-gouge formation. The K-Ar four-point isochron is well defined and the upper-intercept age of 103 +/- 3.5 Ma is inferred to reflect growth of metamorphic white mica in the footwall of the Paparoa Metamorphic Core Complex. We relate the fault gouge at the Ohikaiti Fault to the formation of the Paparoa Trough in the latest Eocene and the deposition of the Rapahoe Group. The NNE-striking Ohikaiti Fault either reactivated a favourably oriented segment of the Ohika Detachment or cut the detachment and displaced it when the Paparoa Trough formed. We discuss a model of relatively modest deformation in the Paparoa Trough as part of the Challenger Rift System because the northern West Coast region was close to the pole of plate rotation in the late Eocene and Oligocene.

  • 49.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Yngwe, Frej
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    "To Be, or Not to Be, That Is the Question" - The Cretan Extensional Detachment, Greece2018In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 37, no 9, p. 3069-3084Article in journal (Refereed)
    Abstract [en]

    We present new structural data from the contact zone between the high-pressure Phyllite-Quartzite unit (PQ) and the overlying non-high-pressure Tripolitza unit in east-central Crete for testing propositions that this contact represents the top-to-the-north Cretan extensional detachment. Detailed mapping in the Neapoli valley shows that the PQ/Tripolitza contact is associated with a shallowly dipping penetrative foliation and associated south-southwest (SSW)-trending stretching lineation in the PQ. Kinematic indicators yielded a consistent top-to-the-SSW sense of shear. The Tripolitza unit above was brittlely deformed, and top-to-the-SSW thrust faults sole out in the contact zone. Undoing the effects of subsequent large-wavelength, low-amplitude folding about west-northwest-trending axes of the contact indicates that the top-to-the-SSW kinematics resulted from crustal shortening. Therefore, the PQ/Tripolitza contact in the Neapoli valley is an out-of-sequence thrust, the Tripolitza thrust. We discuss a tectonic model in which the high-pressure PQ was first exhumed in a shortening-related extrusion wedge between about 22 and 17Ma. The top of the exhuming extrusion wedge was then cut off by out-of-sequence thrusts in the upper crust at 17-11Ma during sustained N-S shortening. Subsequently, extensional graben in Crete formed after 11Ma. The Aegean Sea basin is a superb example of large-scale horizontal extension in the overriding plate above a subduction zone. A matter of debate is how much this extensional deformation aided the return of once deep-seated rocks back to the Earth's surface. On the island of Crete, the alleged Cretan detachment has been proposed to be a large-scale extensional structure. However, several colleagues came to very different conclusions. We decided to help in answering the question of whether or not the Cretan detachment is a shortening or extensional structure by conducting a detailed field study along a well exposed and structurally simple segment of the Cretan detachment. Our results show that the proposed Cretan extensional detachment is actually a shortening structure.

  • 50. Rosenbaum, Gideon
    et al.
    Menegon, Luca
    Glodny, Johannes
    Vasconcelos, Paulo
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Massironi, Matteo
    Thiede, David
    Nasipuri, Pritam
    Dating deformation in the Gran Paradiso Massif (NW Italian Alps): Implications for the exhumation of high-pressure rocks in a collisional belt2012In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 144, p. 130-144Article in journal (Refereed)
    Abstract [en]

    The Gran Paradiso massif, situated in the internal part of the Western Italian Alps, records a complex tectono-metamorphic history involving high-pressure metamorphism and subsequent exhumation during retrograde metamorphism. The exact timing of deformation and, consequently, the geodynamic evolution of this part of the Western Alps is still debated and is addressed here by the application of Rb/Sr geochronology, Ar-40/Ar-39 step heating and Ar-40/Ar-39 total fusion dating techniques. Geochronological results are presented from shear zone samples in the core of the Gran Paradiso massif (Piantonetto Valley). and in the area closer to the contact with the overlying Piedmont ophiolitic domain (south and southwest of Pont Valsavarenche). The shear zones operated during crustal thinning and exhumation of the Gran Paradiso massif. Ar-40/Ar-39 step heating results from shear zones in the Piantonetto Valley show acceptable plateau ages that are interpreted to represent two events of mica growth. Similar ages, and an additional younger age cluster, are recognised in the Ar-40/Ar-39 total fusion analyses, indicating that specific cleavage domains operated at 39.2 +/- 0.2, 36.5 +/- 0.6 and 33.3 +/- 0.4 Ma. P-T pseudosections show a progressive decrease in metamorphic conditions during deformation, suggesting that the age of incipient exhumation and the related deformation in the Piantonetto Valley is equal to or older than 39.2 +/- 02 Ma. In the Pont area, the last increments of deformation in a top-to-W shear zone postdate 36.6 +/- 0.6 Ma (Rb/Sr mineral data), whereas the present-day top-to-W contact of the Gran Paradiso massif with the overlying Piedmont domain is dated at 41.2 +/- 1.1 Ma (Rb/Sr multi-mineral isochron age). We propose a model that considers exhumation of the Gran Paradiso nappe at 41-34 Ma. During this period, the nappe was coupled with the Zermatt-Saas zone, forming an extruding wedge. The kinematics associated with this wedge involved top-to-W shearing within the Gran Paradiso nappe (e.g. Pont area shear zones) and top-to-E shearing at the top of the extruding wedge (e.g. Orco shear zone). Subsequent deformation (after similar to 34 Ma) was characterised by coaxial strain involving orogenic-scale backfolding and backthrusting.

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