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Origin of Serpentinization Patterns Beneath the S‐Reflector Detachment Fault in the Galicia Margin, Offshore Spain

Serpentinization of exhumed mantle peridotite is thought to influence the mechanics and evolution of hyperextended rifted margins. The Galicia margin is an example of where this may have occurred. New constraints on the 3‐D seismic velocities in the upper mantle beneath the Galicia S‐reflector detachment fault suggest heterogeneously distributed serpentinization (0–90%). To the first order, areas with high degrees of serpentinization correspond with the intersections of crust‐cutting normal faults and the S‐reflector detachment, suggesting that these faults served as conduits transporting fluids to the upper mantle. The role of temperature on alteration was also investigated, as it is a key condition for serpentinization. Estimated paleotemperatures ranged from 55 to 140 °C along the S‐reflector detachment. This temperature range may have played a role in the heterogeneity of alteration. These broader patterns of both high and low mantle alteration correspond to areas of thick and thin overburden immediately postrift, respectively. Several areas that are estimated to lie within the favorable paleotemperature range, and that have crust‐cutting fault intersections, exhibit unusually low degrees of serpentinization. This is possibly due to locally reduced fault permeability. The preservation of relationships between the degree of serpentinization and the occurrence of crust‐cutting fault intersections and postrift overburden thickness implies that volumetrically significant amounts of mantle serpentinization occurred after movement ceased on the detachment. Serpentinization‐related volume increases may have caused postrift uplift of the detachment fault surface. Thus, the current morphology of this detachment surface may be due largely to heterogenous postkinematic processes, rather than primary fault‐related processes.