FORE ARC RESPONSE TO MIGRATION OF THE COCOS, NAZCA, PANAMA TRIPLE JUNCTION IN CENTRAL AMERICA

Temporal and spatial variations in shortening, uplift, and landscape evolution along the Pacific coast of Panama and Costa Rica reflect abrupt changes in slab dip and shortening rate that occur at the triple junction between the Cocos Plate, Nazca Plate and Panama Block. There is a three-fold increase in convergence rate and an abrupt shallowing of slab dip across the subducting Panama Fracture Zone. These dramatic changes in the subduction parameters migrate southeast with the triple junction at ~50 km/ Ma. In Costa Rica, shallow subduction of the Cocos Ridge resulted in extinction of the volcanic arc and development of a fold and thrust belt within the fore arc basin which consists of a series of 2-5 thrust slices that form a duplex, with a basal décollement near the basement-cover contact. Shortening is in excess 40 km with the greatest shortening, the most thrust faults, and the highest topography inboard of the axis of the Cocos Ridge. Shortening rates are on the order of several 10's of mm/yr, a large portion of the 90 mm/yr plate convergence rate, consistent with significant plate coupling. The amount of shortening and number of thrusts decreases toward the onland projection of the subducting PFZ. With the exception of a single thrust along the base of the slope of Volcan Baru, the thrust belt terminates in the upper plate directly inboard of the triple junction, suggesting that the thrust belt propagates with the migration of the triple junction allowing for a time for space substitution in the structural and landscape evolution.

Rivers draining Volcan Baru are straight, consequent streams on the active lahar fans. Growth of the thrust belt to the southeast results in backtilting and erosion of the lahar fans, development of a divide within the thrust belt, and shifting of subsequent rivers around the more resistant, uplifting Tertiary strata. Uplifted fluvial terraces contain volcanic clasts from the arc demonstrating subsequent growth of the divide. Marine terraces along the frontal thrust indicate uplift rates of ~0.3 mm/yr in gently dipping strata increasing to 1.5 mm/yr where the thrust ramp steepens, indicating that coastal uplift patterns reflect subsurface thrust geometry. Regional analyses of dated marine deposits indicate spatial variations in uplift that reflect bathymetric features on the subducting Cocos plate.