Geophysical evidence for dewatering and deformation processes in the ODP Leg 170 area offshore Costa Rica

Abstract

We use a combination of borehole data from Ocean Drilling Program (ODP) Leg 170 and multichannel seismic reflection (MCS) data to quantify thickness changes in underthrust sediments away from the boreholes. Sediments thrust beneath the upper plate at convergent margins may be more rapidly loaded than in any other environment. Depending on the porosity and permeability of the available fluid pathways, these sediments can compact and dewater very rapidly, as observed in this area offshore the Nicoya Peninsula, Costa Rica. Rapid thinning and dewatering was previously interpreted in this area from MCS data, but the lack of velocity data in this deep-water environment caused ambiguity in the estimates of thickness change. We employ a non-linear inversion technique using detailed density data, primarily logs and some laboratory measurements and coincident MCS data to create 1D synthetic seismograms and detailed velocity functions at three ODP drill sites. Because only a small part of one hole was logged with a sonic tool and the laboratory measurements significantly underestimate in situ velocities, these results provide the most accurate estimate of the velocity profiles. We used these velocity functions to depth-migrate seven MCS lines in the vicinity of the trench and lower slope spanning a distance of ∼9 km along strike. Analysis of the depth-migrated images shows that there is significant variation along strike in how the underthrust section compacts, which appears to be related to the distribution of normal faults on the Cocos Plate. We interpret that preferentially rapid dewatering in the upper part of the underthrust section may lead to deformation below the original decollement and detachments at deeper stratigraphic levels.