ACTIVE FOREARC RESPONSE TO CHANGING SUBDUCTION PARAMETERS, MIDDLE AMERICA TRENCH

Subduction of the Panama Fracture Zone along the convergent margin of Costa Rica and Panama leads to abrupt variations in subduction rate and slab dip along the Middle America Trench. As the Panama Triple Junction migrates to the southeast, the forearc-volcanic arc system of the upper (Caribbean) plate experiences an abrupt change from steep, slow (~50 km/Ma) subduction of the Nazca Plate to shallow, rapid (~90 km/Ma) subduction of the Cocos Plate. These sharp variations, combined with a well-constrained tectonic framework for the triple junction, allow an assessment of forearc evolution in response to changing subduction parameters. Northwest of the triple junction, shallower subduction of the Cocos Plate results in greater plate coupling and the formation of the Fila Costeña Thrust Belt. East of the triple junction where the slab dip steepens, the thrust belt terminates abruptly. Balanced cross-sections indicate that shortening rates are a significant portion of plate convergence and are highest near the center of the thrust belt, and decrease laterally nearest the triple junction. Right-lateral tear faults cut the thrusts of the Fila Costeña and likely accommodate this sharp gradient in upper plate shortening. Several fault-related anticlines located ~50 km east of the triple junction probably represent the ongoing propagation of the Fila Costeña to the southeast. The Talamanca Range, located west of the triple junction and northeast of the Fila Costeña, is an extinct volcanic range. Conversely, Volcán Barú, located east of the triple junction, is currently active, suggesting that volcanism probably shut-off from west to east as the triple junction passed and the Benioff zone abruptly shallowed. The forearc landscape is also dramatically altered by passage of the triple junction, with growth of a new divide related to the Fila Costeña. Consequent drainages that define the volcanic slopes of Barú become subsequent drainages with hook shapes that reflect the diversion of channels around the propagating thrust belt prior to incision of gorges into the forearc basin strata. Thus, increasing subduction rate and decreasing slab dip can together produce an increase in interplate coupling and a shift in the location of active deformation to the inner fore arc, with a dramatic impact on landscape evolution.