Geological Society of America Special Paper 295 1995 Structure and neotectonics of an oblique-subduction margin, southwestern Panama Radim A. Kolarsky* Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Paul Mann Institute for Geophysics, University of Texas at Austin, 8701 Mopac Boulevard, Austin, Texas 78759 ABSTRACT We present on- and offshore structural data from the Nazca-Panama plate boundary zone in the Gulf of Chiriqui and surrounding onshore areas of southwest Panama. Major offshore structures interpreted on multichannel seismic profiles from the Gulf of Chiriqui include Cebaco basin complex, a series of northeast- striking, Plio-Pleistocene half-grabens, and Montuosa basin, an asymmetric Plio- Pleistocene sag basin associated with a major strike-slip fault. We interpret Cebaco basin complex as a pull-apart basin between two major, left-lateral strike-slip faults that accommodate oblique motion between the Nazca plate and the mainland of southwestern Panama. Interpretation of regional seismic stratigraphic data indi- cates that the Plio-Pleistocene extensional phase that produced the Cebaco basin complex extended the area by about 7%. We studied outcrop-scale, conjugate strike-slip fault systems exposed on land- masses surrounding the Gulf of Chiriqui in order to place kinematic and age con- straints on large-scale faults mapped on seismic profiles. Fault systems deforming Eocene to Lower Miocene sedimentary rocks on Coiba Island and the Azuero and Sona Peninsulas suggest an approximately northwest-southeast orientation of maximum ex- tensional strain in an area that encompasses the offshore Cebaco basin complex. We propose three possible models to explain the observed pattern of strike-slip deformation observed in the Gulf of Chiriqui: (1) Neogene oblique subduction of the Nazca plate beneath Panama produces left-lateral strike-slip faulting and related northwest-oriented extension within the forearc (Gulf of Chiriqui) (2) Plio- Pleistocene shallow subduction/collision between the Cocos ridge and Costa Rica produces southwestward motion or "escape" of a Gulf of Chiriqui block that is de- tached from the rest of Panama by left-lateral strike-slip faults, and (3) Neogene bending of the Panama island arc following collision with the South American conti- nent is accommodated in part by strike-slip motion and underthrusting along the southwest margin of Panama. Observed deformation may be a composite effect of more than one of these tectonic mechanisms. •Present address: Eastern E&P Region, Texaco USA, P.O. Box 60252, New Orleans, Louisiana 70160. Kolarsky, R. A., and Mann, P., 1995, Structure and neotectonics of an oblique-subduction margin, southwestern Panama,zyxwvutsrqponmlkihgfedcbaVU in Mann, P., ed., Geologic and Tectonic Development of the Caribbean Plate Boundary in Southern Central America: Boulder, Colorado, Geological Society of America Special Paper 295. 131 Downloaded from http://pubs.geoscienceworld.org/gsa/books/book/441/chapter-pdf/967017/spe295-0131.pdf by University of Houston user on 16 April 2023