LATERAL GROWTH OF AN ACTIVE FORE ARC THRUST BELT IN RESPONSE TO TRIPLE JUNCTION MIGRATION, COSTA RICA

The Fila Costena along the Middle America fore arc of Costa Rica is an active thrust belt that telescopes the Tertiary fore arc basin and propagates southeast along the margin in response to lateral migration of the Cocos-Nazca-Caribbean triple junction. The north-trending Panama Fracture zone (PFZ) subducts beneath the Caribbean plate and moves parallel to the southeast-trending Middle America trench at approximately 5 cm/yr. After passage of the triple junction, the overriding Caribbean plate is subjected to a large increase in convergence rate and subducting seafloor roughness as well as a decrease in slab dip. From southeast to northwest across the onland projection of the subducting PFZ, there is an abrupt increase in upper plate shortening, an increase in the elevation of the Talamanca Range (the extinct arc), and a cessation of active volcanism. Thrust faults in the Fila Costena are southwest-directed and thicken the Tertiary fore arc basin from the Golfo de Nicoya in the northwest (the triple junction location in the late Miocene) to the Panama border (at the initiation point of upper plate shortening). In general, the decollement depth and total shortening are greatest inboard of the subducting Cocos Ridge, a broad bathymetric high oriented parallel to the relative plate motion vector. In this region, new mapping depicts a duplex at the front of the thrust belt with imbricates rooted at the contact between Eocene limestones and the oceanic basement of the Caribbean plate. The roof thrust wraps around the southeastern edge of the duplex where the shortening decreases dramatically. Given age constraints on the initiation of thrusting (post-Pliocene) and the substitution of space for time related to migration of the triple junction, the total shortening from balanced cross sections of the thrust belt (20-30 km) near the onland projection of the PFZ indicates that thrust slip rates represent a significant percentage (10-40%) of the total plate convergence between the Cocos and Caribbean plates. Thus, the onset of rapid, shallow subduction has resulted in partial locking of the outer fore arc, with an arcward shift in the focus of plate convergence to an upper plate shortcut that cuts the fore-arc basin. The demise of the fore arc basin may be a fundamental process along convergent margins that experience a history of shallow, rough crust subduction.