FROM FOREARC BASIN TO INNER FOREARC MOUNTAIN BELT: FILA COSTENA, COSTA RICA

The Fila Costeña thrust belt of Costa Rica records Pliocene to recent transition from an extensive forearc basin to an inner forearc mountain belt. Along the Middle America trench, the driver for this transition is collision of the leading edge of the Cocos Ridge, an aseismic ridge generated at the Galapagos Hot spot. The thrust belt developed in a region that, from the Eocene until the late Miocene, consisted of a forearc basin developed on basement (i.e., the Terraba basin), similar to the Sandino basin offshore Nicaragua, part of an erosive segment of the Middle America forearc that did not experience the collision with the ridge and associated seamount province. Based on cross sections, the Fila Costeña thrust belt records more than 35 km shortening since deposition of Pliocene marine sediments. The structure of the range includes a duplex directly inboard of the Cocos Ridge that exposes five thrust slices. Andesite blocks with a cooling age of 3.5 Ma are observed in lahars (MacMillan et al., 2004] that are backtilted along thrust ramps on the arcward side of the Fila Costeña. The minimum total shortening combined with constraints on the maximum age of thrust initiation, indicates minimum shortening rates (~10 km/My) that are a significant percentage of the total plate convergence rate. The total shortening is also similar to the observed trench retreat inboard of the ridge and associated seamounts, or the broad embayment in the trench axis relative to northwest Costa Rica. Thus, in the absence of accretion, widespread shortening of the forearc leads to arcward retreat of the trench axis and development of positive relief in the inner forearc. Directly inboard of the ridge axis and east of the divide of the Fila Costena, the southward-facing slope of the thrust belt is dominated by landslides that remove the leading edge of the most laterally extensive thrust sheet. To the northwest where the total shortening is less, the frontal thrust steps offshore and the hanging wall is a shallow marine platform. The presence of an offshore erosional platform cutting sediments tilted along the frontal ramp of the thrust belt suggests that coastal marine erosion is able to keep pace with thrusting within the frontal thrust slice.