Normal fault populations across the Costa Rica margin, NW of the Osa Penninsula, and implications for upper plate stress

Abstract:

The upper plate of the Costa Rica subduction zone is dissected by hundreds of normal faults that cut through the uppermost 500 – 1000m of slope sediment cover sequences within the upper slope and outer shelf of the Costa Rica margin. These faults are typically spaced 10s to 100s of m apart and have only small offsets of < 30 m. We have mapped more than 100 of these faults by tracing them along the inline, crossline and horizontal slices through a 3D seismic reflection data volume. These data were acquired on the R/V Langseth in 2011 using a 3,300 in³airgun source and a 6,000 m streamer with inline and crossline spacing of 18.75 m and 12.5 m respectively. Two primary populations of normal faults have conjugate sets that can be mapped across the upper slope and the outer shelf. The first set strikes slightly west of north and rotates systematically from 345° at the northern edge of the survey, which is located midway between the shelf break and the shoreline, to an orientation of 325° beneath the upper slope. We interpret this set as a result of WNW-ESE extension, which has been hypothesized to be a result of the collision between the Cocos Ridge (~75 km to the SE) and the Costa Rica margin (La Femina, et al., 2009). The second set of normal faults strikes east with an azimuth of 85°. This set of faults has a consistent orientation across the shelf and upper slope. This orientation is roughly parallel to the orientation of the subducting basement ridges (78°), which form horst and graben structures on the down going plate. These faults may be a result of uplift and extension as subducting ridges pass underneath. However, beneath the upper slope additional fault populations emerge with dominant azimuths primarily oriented at 5° and 65°. These faults appear to mark the localized extension caused by growth and uplift in the underlying margin wedge. Further, recent movement on shallow thrust faults indicates active shortening of the upper plate. We speculate that the contrast in the variability of fault populations beneath the upper slope and shelf is a result of a change in the competence of upper plate that allows more stress localization within the margin wedge beneath the slope than beneath the shelf.

LaFemina, et al. (2009), Fore-arc motion and Cocos Ridge collision in Central America, Geochem. Geophys. Geosyst., 10, Q05S14.