Hole 896A was drilled 1 km southeast of Hole 504B during Ocean Drilling Program (ODP) Leg 148 to study the variability in the volcanic and chemical structure of the oceanic crust and to examine the effects of off-axis hydrothermal activity on the basement. The volcanic units cored from 195.1 to 469 mbsf in Hole 896A consist mainly of pillow basalts with minor massive lava flows and breccias. The dominant structures encountered in these rocks are hydrothermal veins, vein faults, and breccias. Early-stage veins are steeply dipping and filled with Fe-oxyhydroxide + clay veins whereas late-stage veins are shallow dipping and filled with fibrous carbonate and clay + carbonate. Both the fibrous and intermediate-stage, nonfibrous carbonate + clay veins show shallow to moderate dips and indicate extension in many directions locally occurring within north-south striking subvertical planes, normal to the Costa Rica Rift axis. Smectite is the dominant phyllosilicate mineral in the veins. Transformation of smectite to chlorite occurs through a mixed-layered transition zone at 440.1 mbsf, probably as a result of complex interplays among the temperature, porosity, and fluid composition conditions in the crust at this depth. Vein faults with commonly oblique to reverse senses of shearing occur in the core between 334 and 406 mbsf. Extensional faults near 335 mbsf display northwest strikes with steep to vertical dips (oriented using the paleomagnetic declination) and are oblique to the Costa Rica Rift axis. The existence of a structural break at 356 mbsf near the bottom of the lithologic Unit 30 is suggested by the presence of a major change in the downhole electrical resistivity profile. The occurrence of this break nearly coincides with an abrupt change in magnetic properties of the rocks and may represent a fault zone that has facilitated high permeability of the basement in Hole 896A. Fragmentation of cooled pillow lava rims, mass wasting on the seafloor, and hydraulic fracturing beneath the seafloor appear to be responsible for development of several types of breccias in the upper part of Layer 2.
Date of initial receipt: 17 August 1994
Date of acceptance: 22 February 1995
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