2. Morphology and Alteration of the Upper Oceanic Crust from In-situ Electrical Experiments in Dsdp/odp Hole 504b

During ODP Leg 111, the drillship JOIDES Resolution returned to DSDP Hole 504B, on the south flank of the Costa Rica Rift. The main objective of Leg 111 was the study of the physical and chemical structure of young oceanic crust in the deepest hole yet drilled into the oceanic crust. In addition to the drilling of an additional 212.3 m into the massive units of Layer 2C, Leg 111 provided the opportunity to conduct an extensive suite of in-situ experiments, in­ cluding the recording of a continuous electrical resistivity profile, from which total and fracture porosity estimates were computed. This allowed us to discriminate the large-scale layers of the crust, to isolate individual lithologic units (such as pillows and massive flows), and to describe the overall morphology of the upper crust. In the extrusive part of the crust, certain massive flows (> 10 m thick) appear to be permeability barriers that subse­ quently constrain fluid circulation. We interpret the flows of Unit 2D as the upper limit of an underpressured aquifer located within Layer 2A and Unit 27 as the boundary between low-temperature, seawater alteration facies of basalt and higher temperature alteration phases. This relationship between morphology and hydrological regime, and therefore al­ teration of the basaltic basement, might be viewed as related to the accretion process of the upper oceanic crust. The presence of fractures can be attributed in most cases to thermal stresses from the cooling of a nearby unit or to tectonic features such as the inferred listric fault at about 4300 m below rig floor (825 m below seafloor). The porosity estimate obtained from the analysis of in-situ measurements is (1) reduced in accounting for clay conduction via cation exchange mechanisms, (2) constant over long sections of the drill hole, and (3) observed to decrease by steps with increasing depth. The extremely low values of porosity computed in the dikes indicate a slow regime of hydrothermal circulation deep into the crust and correlate with the low values of in-situ permeability deduced from packer experiments.