IS THE CAUCA-PATIA AND ROMERAL FAULT SYSTEM LEFT OR RIGHT LATERAL

Extrusion driven collision is controlled by strike-slip faults of oppo site senses whereas subduction related oblique convergence is accommod ated by margin parallel faults of identical senses. Northern Andes geo dynamics has been interpreted either as resulting from the obliquity o f convergence between Nazca and South American plates or from the coll ision of the Carnegie Ridge with Ecuadorian margin. The sense of the C auca-Patia and Romeral Fault System (CPRFS) has been interpreted in tw o opposite manners: right or left-lateral agreeing with oblique or col lisional patterns, respectively. We analyze arguments which favor, Sou th to 4 degrees N, a right-lateral motion along the CPRFS that is cons istent with obliquity driven geodynamics. However, North of 5 degrees N, the CPRFS is actually left-lateral. These opposite motions between the northern and southern part of the CPRFS have to be accommodated ar ound 5 degrees N by N-S stretching and/or shortening. Existing data su pport N-S shortening around 5 degrees N, this buttressing could be due to the collision between the Panama and Andean Blocks.