An additional 110.6 m of coring in Hole 504B on ODP Leg 148 recovered aphyric to phyric, massive diabasic rocks that are part of the lower sheeted dike complex of 5.9-m.y.-old oceanic crust. Structural studies of the drill core based on shipboard data and the analyses of geophysical downhole measurements show that fracturing was the main deformation process during the evolution of oceanic crust at Hole 504B. The existence in certain depth intervals in the core of zones of intense microfracturing suggests the occurrence of damage zones associated with faults or localized deformation zones. The majority of these microfractures have steep dips (>70°) and east-northeast strikes with subhorizontal slickenline lineations and are oblique to the orientation of the Costa Rica Rift and the modern horizontal principal stress directions. Open fractures in the core with shallow and steep dips are locally symmetric to the modern in situ stress field. Subhorizontal fractures observed in the drill core are absent on the borehole walls, as suggested by the FMS images, implying that they are drilling induced. A predominant fracture set defined by FMS images of the borehole has an average orientation of 052, 56×NW between 1900 and 2000 mbsf in the hole that is oblique to the Costa Rica Rift. The FMS and DLL-porosity logs suggest that a marked zone of subhorizontal fractures occurs at 1930 mbsf and an intense zone of vertical fractures at 2000 mbsf. Despite locally well-developed zones of fractures, fault rocks are rare in the drilled section of the hole and there is no evidence for any structural discontinuity in the core. Veins filled mainly with actinolite and chlorite seem to have formed as extension fractures and developed through syntaxial overgrowth of these vein-filling minerals on host-rock clinopyroxene along the fracture walls and/or by a succession of crack-seal increments. Steeply dipping veins striking east-southeast are commonly parallel to the dike margins and to the orientation of the Costa Rica Rift axis. Veins striking north-northeast are dike-orthogonal and might have formed as thermal contraction cracks during cooling of the dikes. Deformation in the lower sheeted dike complex at Hole 504B appears to be mainly brittle and limited to fracturing and veining.

Date of initial receipt: 18 August 1994
Date of acceptance: 13 March 1995

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