Abstract

Serpentinite emplacement mechanisms along convergent plate boundaries are complex and not well understood. This investigation combined structural and stratigraphic data from serpentinite in Puerto Rico to suggest an emplacement mechanism along the North American–Caribbean plate boundary. Northward-dipping thrust faults and foliations along the southern margins of the Monte del Estado and Río Guanajibo serpentinite bodies in southwestern Puerto Rico indicate southwestward thrust movement of serpentinite. Structural analysis of faults, folds, and foliation recorded within the serpentinite bodies and analysis of contact relationships with volcano-sedimentary rocks lead to the interpretation that thrust movements record contractional deformations occurring during the Maastrichtian–Paleocene and late Eocene–early Oligocene intervals. The conclusion that serpentinite has been most recently emplaced above thrust faults during transpression between the Caribbean–North America plates is in contrast to the earlier hypothesis that serpentinite was emplaced in response to diapirism. Late Eocene transpression reactivated older faults and also displaced the serpentinite. Although the timing of the serpentinite emplacement in southwestern Puerto Rico is similar to emplacement events in Cuba and Hispaniola, there are differences in the origin, mechanism, and direction of emplacement of the serpentinite masses. The heterogeneity in the serpentinite emplacement mechanism and transport direction evokes reconsideration of the deformation history along the Caribbean–North American plate boundary.

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