STRUCTURAL TRANSECTS ACROSS THE ISTHMUS OF PANAMA: OROCLINE OR SUBDUCTION-RELATED GEOMETRY?

Detailed geologic mapping and new geochronology and paleomagnetic data from two transects crossing most of the Panama Isthmus show constrasting structural styles for the Panama area. Such contrasting styles can be explained in terms of the oblique collision of the Panama block with northwestern South America, and solution of ensuing deformation in an oroclinal belt that has its western termination in the Canal area. Along the Canal area we have found pervasive extensional deformation coeval with lower to middle Miocene shalllow marine to continental sedimentation and volcanic and subvolcanic activity. Transtensional deformation formed the small Canal basin at the westernmost tip of the upper portion of an ophiolitic belt that extends east to the Colombian border. Sedimentation in the Canal basin took place in small compartments where syntectonic sedimentation and volcanoclastic input were abundant. Middle Miocene volcanic activity was restricted to the Canal area and put an end to sedimentation. East of the canal area, we have mapped a pillow-basalt, chert, diabase sequence intruded by Paleocene-Eocene arc plutonic rocks and covered by a Oligocene and younger, nearly undeformed sedimentary sequence from shallow carbonate platform to continental clastic and volcanoclastic deposits. The ophiolite sequence is thought to be Upper Cretaceous and contains a pre-Oligocene deformational fabric. Preliminary paleomagnetic data indicates that these rocks formed south of the equator and arrived to its present position by Oligocene times. These assemblages contain the record of the Paleogene/Neogene evolution of the arc at the trailing edge of the Caribbean plate as it drifted between the Americas, collided with northwestern South America and accommodated deformation thus shutting the volcanic arc and defining an oroclinal belt.