Active thrusting in the inner forearc of an erosive convergent margin, Pacific coast, Costa Rica

Structural and geomorphic analyses of the Fila Costeña thrust belt in southwest Costa Rica indicate active thrusting within the inner forearc. The Fila Costeña exposes three major thrust faults that imbricate the late Tertiary forearc basin sequence of the Térraba basin. The frontal thrust of the Fila Costeña marks the boundary between an uplifting inner forearc and a subsiding outer forearc, with only local uplift astride the indenting Cocos Ridge. On the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low‐angle décollement horizon near the contact between the basement and the cover sequence of the Térraba basin. This décollement lies at a depth of ∼4 km. The minimum shortening recorded by restoration of fault‐related folds is 17 km, or 45%. Observations of late Tertiary marine sediments, tilted and faulted late Quaternary fluvial terraces, and raised Holocene marine terraces indicate that Fila Costeña uplift was likely initiated in the Quaternary and is ongoing. Given that the coastal mountains are separated from the Talamanca Range by a valley, the décollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal‐scale ramp system, a structural geometry that has resulted in uplift of the Talamanca Range, the highest peaks in Central America. The dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough, subducting crust.