DEFORMATION STRUCTURES AND IMPLICATIONS FOR FLUID FLOW AT COSTA RICA CONVERGENT MARGIN, ODP SITES 1040 AND 1043.

During ODP Leg 170, five sites were drilled and sampled off the Costa Rica Pacific margin. Two of them, site 1040 and 1043, yielded material from a wedge of deformed sediments, the main decollement zone, and the underthrusting sedimentary sequence of the subducting Cocos plate. Detailed mesoscopic and microscopic analyses of the deformation features characteristic of each domain help to define four different structural/hydrologic regimes. Unexpectedly, the wedge of deformed hemipelagic sediments does not represent an accretionary wedge, because little or no transfer of material from the subducting plate has occurred. The deformed sedimentary wedge records periods of tectonic bulk strain, in which fluid pressure intermittently rises and induces fracturing, alternating with periods of gravitational bulk strain. The last stage of this alternating strain regime is represented by the growth of thick, sub-horizontal shear zones. The sediments caught up in the decollement zone show distinctly different deformation features, reflecting different mechanical behaviour and a different response to fluid how, but again transient pulses of increased pore pressure are interpreted as the driving mechanism of dewatering. The underthrust pelagic and hemipelagic section is also characterised by dewatering features, but only in the uppermost part. The basal sediments of this section; in contact with underlying gabbro intrusions, have a completely different structural history and hydrologic regime. Structures here are interpreted as related to near ridge processes, and the hydrologic system is not linked to the upper sediment-dewatering regime, but rather seems to have been fed by a seawater source.