CRYPTIC STRIKE-SLIP FAULTING SHUTTING DOWN ISLAND ARC MAGMATISM: THE ISTHMUS OF PANAMA

New geochronological, geochemical, and paleomagnetic data in the Isthmus of Panama were integrated to characterize the chronology and style of arc-continent collision. The dataset we collected revealed the presence of a large, left-lateral strike-slip fault that was active between 38 and 28 Ma, offsetting the Panama arc more than 100 km left-laterally, shutting down magmatism on the eastern part of the arc, driving vertical-axis rotations and oroclinal bending, and exhuming the roots of the arc. Renewed magmatism only 10 Ma after initiation of strike-slip faulting, intra-arc basin formation and sedimentation concealed most of the evidence for this fault. New and published radiometric data, confirm the presence of a Late Cretaceous to Eocene arc from western Colombia to western Panama. This arc is composed of basaltic/pelagic sequences, and plutonic rocks that can be discriminated into three groups according to geochemical affinities: 1) a Late Cretaceous oceanic plateau that is possibly part of the CLIP and forms the arc basement; 2) a Late Cretaceous protoarc emplaced on top of the oceanic plateau shortly after subduction initiation; and 3) a Late Cretaceous to Eocene arc that emplaced after the protoarc. This geochemical/stratigraphic succession is similar to already documented sequences in south Costa Rica and west Panama, also supporting the continuity of an arc from eastern to western Panama. Paleomagnetic analyses in the same sequences reveal large (70.9º ± 6.7º) counterclockwise vertical-axis rotations west of the Rio Gatun Fault and moderate ones (between 40º ± 4.1º and 56.2º ± 11.1º) east of it. Younger, overlapping Oligocene-Miocene sequences show no significant vertical-axis rotations, and similarly, an Oligocene and younger arc shows little deformation and cross-cuts the older arc. This once continuous arc was used as a strain marker to restore Isthmus deformation, and show that, unlike other arc-continent collisions, this is a long-lived collisional process, active today and that started during late Paleogene times, propagating more than 500 km from the collision zone, and was partitioned between discrete faulting and vertical-axis block rotation.