Kinematic Analysis of Structures from Serpentinite in Southwestern Puerto Rico: Implications for Early Tertiary Transpression along the Boundary Between the Caribbean and North America Plates

Serpentinite in southwestern Puerto Rico records part of the complex tectonic history along the boundary between the Caribbean and North America plates. Kinematic analysis of more than 1,800 measurements of shear zones and brittle faults collected at more than 300 stations within the Monte del Estado and Río Guanajibo serpentinite bodies reveal southwest-directed contraction partitioned among thrust and strike-slip faults. Two predominant fault groups are distinguished. One group is characterized by NW-SE striking thrust faults and E-W striking left-lateral faults, and a second group characterized by NW-SE striking right-lateral faults and E-W thrust faults. Shortening directions compatible with these trends are NE-SW and N-S. Shear planes from each group are cross-cutting suggesting that the structures are concurrent. Alternatively, N-S directed shortening may be older and the related faults may have been rotated or may record a change in the orientation of the principal stress axis leading to development of new faults and reactivation of older shear planes. Contractional structures in the serpentinite have similar senses and orientations to those in the Eocene Cerrillos Belt comprising, from SE to NW, formations including Jacaguas Group, Monserrate, Anón, and Río Culebrinas. Therefore we correlate the shortening in the serpentinite to Early Tertiary transpression recorded in the Cerrillos Belt formations, thereby expanding the region in which contraction, uplift, and left-lateral shearing took place. Major mapped faults such as Cordillera and Cerro Goden, accommodated most of the left-lateral movement. Thrust faults in the serpentinite accommodated most of the shortening. These early Tertiary structures obscure shear planes associated with the postulated Early Cretaceous uplift, suggested by a nonconformity beneath Late Cretaceous strata. Normal fault zones in the serpentinite generally have orientations similar to normal faults that cut Miocene to Pliocene carbonates.