Mid-Eocene-Early Oligocene Sinistral Transcurrent Faulting in Puerto Rico Associated with Formation of the Northern Caribbean Plate Boundary Zone

Left-lateral transcurrent (wrench) faulting through the Southern Puerto Rico Fault Zone (SPRFZ) in central southern Puerto Rico is indicated by fault orientations and motion direction indicators. The SPRFZ is a west-northwest trending, 3-8 km-wide, diffuse system of faults and folds that within the study area bounds Paleocene and Eocene volcaniclastic rocks (the Eocene Belt), and is flanked by Cretaceous volcanic and volcaniclastic rocks. Structural data show that much of central southern Puerto Rico is cut by strike- and oblique-slip faults, whereas locally in the southeastern part of the Eocene Belt, rocks are pervasively thrust northeastward over Cretaceous rocks of central Puerto Rico. Structural data and the distribution of fault types support two local evolutionary scenarios: (1) different structural horizons of the transpressional SPRFZ (i.e., upper, thrust dominated horizon and lower, strike-slip dominated horizon) were juxtaposed along post-transpressional normal faults that border the Ponce Basin; and (2) sinistrai strike-slip faulting translated southwestern Puerto Rico with respect to central Puerto Rico along the transcurrent faults of the SPRFZ. The Puerto Rican portion of the Greater Antillean pre-Oligocene magmatic arc occupied a zone of transition from subduction to strike-slip deformation associated with the initial development of the Northern Caribbean Plate Boundary Zone (PBZ). Middle Eocene to early Oligocene motion through the SPRFZ occurred during initial movement along the PBZ.