3. Microstructural and Geomechanical Constraints on Fluid Flow at the Costa Rica Convergent Margin, Ocean Drilling Program Leg 170¹

Alistair J. Bolton,^{2, 3} Paola Vannucchi,⁴ M. Ben Clennell,⁵ and Alex Maltman²

ABSTRACT

We present the results of detailed mesoscopic and microscopic observations made on sediments obtained from Sites 1040 and 1043, located at the toe of the Costa Rica convergent margin. Inferences drawn on the evolution of deformation microstructures, and in particular the role of fluids on their formation, were supplemented by geotechnical hydrogeological laboratory tests that elucidated the varying manner with which different sediments deformed both mechanically and hydrologically. Three different structural/hydrological regimes have been identified. The sedimentary prism records episodes of tectonic bulk strain in which fluid pressures fluctuate to form deformation zones of varying orientation. Shear is dominantly compactive, resulting in loss of porosity and subsequent reduction in permeability. These zones are not conducive to forming conduits, even at low effective stresses. The décollement is characterized by more brittle faulting, associated with transient periods of dilation. Such microfabrics can efficiently duct fluids along a fluid-pressure controlled fracture permeability, with an associated rise in permeability at effective stresses approaching near lithostatic in magnitude. In contrast, the underthrust section is dominantly undeformed. Deformation-induced dewatering occurs primarily in the upper lower permeability section. The results have a broad application to structural and hydrological observations made at many other convergent margins where hydromechanical coupling is intimately related.