Lithostratigraphy, Fracturing, and Fluid Flow in the Upper Oceanic Crust

Abstract

At the time of reentry into Hole 504B during Leg 140 a major temperature inversion and associated temperature gradient minimum was recorded between 283 and 288 mbsf, together with a significant minimum at 325 mbsf. Neither of these temperature anomalies was observed at the time of reentry during Leg 137, only a few months previously. The temperature inversion is located slightly below the top of the basement; the minimum at 325 mbsf is located slightly below Unit 2D. At both depth intervals horizontal fracturing is extensive, and it is suggested that fluid could have entered the borehole, or been trapped within the fractured rock, at these two locations. In the pillow lava section (274^05 mbsf, corresponding to seismic Layer 2A) horizontal fracturing dominates in clearly defined zones, and here the variation in temperature gradient could result from fluid flow or differential recovery following inflow during Leg 137. Thus the temperature variations in the basement are apparently favored by the presence of the horizontal fracturing. The pillow lava sequence (405-550 mbsf corresponding to a part of seismic Layer 2B) is characterized by zones of either vertical or horizontal fracturing. The stepwise behavior of the temperature gradient suggests a correlation to the horizontal fractured zones, and some fluid flow or differential recovery of the Leg 137 inflow is present.