Deep-sea drilling by the Ocean Drilling Program (ODP) of several active convergent margins in the last two decades has increased the level of understanding of how such geologically active regions evolve, for example Barbados, Legs 110 and 156 (Mascle, Moore, et al., 1988; Shipley, Ogawa, Blum, et al., 1995) and 171A (Moore, Klaus, et al., 1998); Nankai, Leg 131 (Taira, Hill, Firth, et al., 1991); Peru, Leg 141 (Behrmann, Lewis, Musgrave, et al., 1992); Cascadia, Leg 146 (Westbrook, Carson, Musgrave, et al., 1994; and, more recently, Costa Rica, Leg 170 (Kimura, Silver, Blum, et al., 1997). Highly saturated, weakly bonded sediments respond to tectonic compression by loss of porosity (Bray and Karig, 1985), with subsequent release of large quantities of fluid. The resultant fluid flow can be responsible for the transport of chemically exotic fluids into oceanic waters and may exert a significant control on biological and geochemical fluxes in the oceans (Kastner et al., 1991).
Of particular importance to structural studies are processes occurring within the décollement, where fluids exist at near lithostatic pressures (e.g., Bangs et al., 1990; Moore et al., 1995; Screaton et al., 1995; Fisher et al., 1996). Reduced effective stress along the décollement facilitates decoupling between overriding prism sediments and underthrust material (Housen et al., 1996), in a similar manner to that proposed for the formation of ancient fold and thrust belts (Hubbert and Ruby, 1959). The coexistence of extensive overpressure, implying retarded fluid flow, with sites of accelerated fluid flow represents a paradox and has been modeled as transient episodes of fluid flow (Bekins et al., 1995) superimposed on intervals of retarded flow and pressure buildup (Carson and Screaton, 1998).
Weak sediments deform in a variety of ways, and often the resultant microfabrics provide clues to the behavior of fluids during deformation. This paper describes observations made on sediments retrieved during ODP Leg 170 drilling of the Costa Rica convergent margin. Inferences drawn from structural relationships were augmented by laboratory permeability and deformational experiments, which were designed to examine the extent to which the fluid transmissibility of different microfabrics responds to fluctuations in fluid pressure. Quantitative assessment of structures and qualitative analysis of sediment hydrology provide a sound foundation for further work, such as geochemical interpretation and theoretical flow modeling. Leg 170 drilling of the Costa Rica margin retrieved good quality cores and successfully penetrated the décollement, providing excellent opportunity for expanding understanding of the processes associated with plate convergence, where a comparison can be made with more extensively studied margins such as North Barbados Ridge (Mascle, Moore, et al., 1988; Shipley, Ogawa, Blum, et al., 1995).
Subduction of the Cocos plate beneath the Caribbean plate gives rise to the Costa Rica convergent margin. Plate convergence is at a rate of approximately 8-9 cm/yr (Demets et al., 1990) and in a direction approximately perpendicular to the Middle America Trench. On the subducting plate, Pliocene-Pleistocene claystones and silty claystones overlie siliceous nannofossil chalk and calcareous diatomites. The absence of trench deposits makes the margin ideal for studies concerning both compactional behavior of unlithified sediments during underthrusting and mass balance and recycling calculations. One of the primary objectives of the leg was to assess the extent to which sediments have been accreted to the continental margin, the mechanisms by which sediments reacted to convergence, and the associated flux of fluids within the area.
Five sites were drilled during Leg 170 (Fig. F1): one reference site on the subducting Cocos plate (Site 1039); two sites through the lower toe slope sediments (henceforth referred to as the sedimentary prism); the décollement, the underthrust section, and oceanic crust (Sites 1040 and Site 1043); and two sites midslope (Sites 1041 and 1042). Here we focus on structures and fluid flow within the toe of the sedimentary prism (Sites 1040 and 1043), allowing comparisons between the underthrust region of these two sites with the stratigraphically equivalent reference site.
Below, the paper consists of five sections. The first section introduces the experimental methodology used for conducting the permeability tests and observing the microstructures. The second section presents the results of this microstructural examination. Where appropriate, different structures are interpreted with regard to fluid behavior during deformation. Subdivisions are made depending on the specific locality where the sediments were retrieved. The third section presents the results of permeability testing. Observed hydrological behavior is correlated with the inferences drawn from microstructural examination. The fourth section amalgamates the results in a model that may explain the observed patterns of fluid flow and deformation. Finally, the fifth section concludes with the pertinent observations.