INSIGHTS INTO THE TERTIARY TECTONIC EVOLUTION OF WESTERN PUERTO RICO FROM PALEOSTRESS STUDIES

Puerto Rico, which is part of the Puerto Rico-Virgin Islands Microplate (PRVIM), is a Cretaceous to early Tertiary island-arc terrane that has a complex tectonic history. Previous studies of western Puerto Rico have produced conflicting models of its Tertiary tectonics. New paleostress data have been obtained for Eocene, Oligocene, and Miocene rocks using fault slip data and the FMSI technique to help constrain the Tertiary tectonics. The derived paleostress in the northern Oligocene-Miocene carbonates suggest one phase of N-S tension, and a younger phase of E-W tension. Analysis of faults in the Oligocene-Miocene southern sedimentary rocks indicates one phase of NW-SE tension. Eocene volcaniclastic rocks of the Cerrillos Belt are highly deformed by normal, thrust, and strike-slip faulting, as well as macroscale folding. Fault slip analysis in the western part of this belt suggests a NNE-SSW compressional phase, and an additional NNE-SSW tensional phase.

These results indicate that in the late Eocene-early Oligocene strain was partitioned between NE-SW shortening and left-lateral slip along faults in the Great Southern Puerto Rico Fault Zone. We suggest that this deformation resulted from transpression in the North America-Caribbean Plate Boundary Zone. Post-mid Miocene N-S tension was generated when convergence between the Caribbean and North American plates caused arching of the PRVIM (van Gestel and others, 1998), thus locally extending the outer arc of this structure. The younger E-W tension is associated with the opening of the Mona Rift, and results from the separation of Puerto Rico and Hispaniola as indicated by GPS geodesy. The lack of faults resulting from E-W tension in the southern rocks may be because southwestern Puerto Rico is decoupled from northwestern Puerto Rico and the motion between the two areas is accommodated by strike-slip reactivation of earlier formed W-trending faults.