Hydration of mantle peridotite, occurs in active tectonic settings, such as mid-oceanic ridges, magma-poor continental rifts, subduction zones, and transform faults and fracture zones. Hydration of mantle peridotite can produce talc and/or anthophyllite above 400-600C and the serpentine minerals: antigorite, lizardite and chrysotile at temperatures below 400-600C. These reactions are coupled with volume gains of 1% and >15%, respectively. Both reactions are also accompanied by considerable mechanical weakening. This is seen from the coefficients of internal fraction of peridotite, lizardite and talc, which are 0.75, 0.35 and 0.2, respectively.
This study evaluates the hypothesis that, because of the accompanying volume gain and mechanical weakening, peridotite hydration reactions, occurring at the landward termination of transform fracture zones, provide a contributory driving force for elevating, or sustaining the elevation of, passive continental margins of the North Atlantic region during the Cenozoic.
Based on a time-averaged propagation velocity of the serpentinization front of 0.2-20 cm/yr., which was estimated by Skelton et al. (2005) for serpentinization of exhumed mantle at the Iberia margin, we calculate a maximum time-averaged uplift rate of 0.03 – 3 cm/yr. or 1 km of uplift in 0.03 – 3 million yrs. This result (which assumes a volume expansion of 15.5% and that this volume expansion is entirely translated into vertical motion) is broadly consistent with observations from the passive continental margins of the North Atlantic region. Serpentinisation might thus be more effective than other metamorphic reactions (e.g. granulite to amphibolite, eclogite to amphibolite) as a driving force for elevation of passive continental margins. This hypothesis gains further support from the spatial coincidence between most of the uplifted segments of the margin with the landward termination of transform fracture zones. However, critical shortfalls of this model are that (1) extensive peridotite hydration is unlikely at depths exceeding 10-20km because temperatures exceeding 400-600C will result in the production of talc and/or anthophyllite, not the serpentine minerals, and therefore the accompanying volume expansion is unlikely to exceed 1% and (2) the timing of uplift requires that pulses of extensive peridotite hydration occurred along inactive segments of transform fracture zones. We must therefore conclude that the volume expansion caused by peridotite hydration was probably insufficient to account for widespread elevation of the passive continental margins of the North Atlantic region. However, we suggest that mechanical weakening, which accompanies peridotite hydration, even at depths exceeding 10-20 km, might promote and/or focus tectonic motion. This could, for example, enable landward “flow” of partly hydrated peridotite, which would sustain elevation of a passive continental margin and explain the common observation of pairing with offshore subsidence.
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