Fractures, Porosity, and Stress in the Dolerites of Hole 504B, Costa Rica Rift

Abstract

Downhole measurements were recorded within the complete basement section of Hole 504B during ODP Leg 148. Because of poor core recovery, these data are essential for understanding the structural context of this deep borehole. A much better understanding of the tectonic, thermal, hydrological, and alteration regimes of the sheeted dike complex is required to evaluate whether further penetration is within reach of future drilling legs.The set of geophysical measurements from Hole 504B has been analyzed, and 4500 traces of fractures from FMS images were mapped over the bottom 167 m of the hole. Steep to near-vertical structures dominate and are oriented mainly in a 15°N strike direction at the base of the hole, that of maximum horizontal stress direction. From electrical resistivity, three porosity estimates were computed (1) on the basis of the Archie formula, (2) taking into account the presence of clays and the associated contribution of counter-ions to total rock conductivity, and (3) from the difference between two electrical resistivity measurements with different depths of investigations. Whereas the different estimates are in reasonable agreement and compare favorably with measurements made on core, the differences are indicative of the origin and structure of the inferred high porosity zones. Although subvertical conductive structures dominate throughout the sheeted dikes, many might be related to drilling. Subhorizontal structures are more numerous in the upper part. A correlation is obtained between changes in thermal gradient recorded at the end of Leg 140 and relatively high porosity sections, probably associated with fractures and relatively high permeability.In all, this integrated description of the fractured sheeted dike complex of Hole 504B provides a more dynamic understanding of the penetrated structure. Many of the steep features coincide in azimuthal direction with that of borehole enlargements, which might arise for failures generated in tension. These borehole enlargements developing in the direction of tensile failure confirm earlier determinations of SH max orientation. It is therefore likely that borehole enlargements and near-vertical fracturing originate from specific cooling of the hole.

Velocity structure of upper ocean crust at Ocean Drilling Program Site 1256

“…Porosity may explain the change in velocity gradient at Hole 504B where the hole clearly penetrated well below the layer 2/3 boundary [ Salisbury et al , 1985]. In Hole 504B (Figure 15), porosity computed from resistivity increases down to 1500 mbsf [ Pezard et al , 1996]. Lab porosity decreases down to 1650 mbsf.…”

“…Red line shows VSP velocities from Swift et al [1998a]. Light blue line is porosity computed from resistivity [ Pezard et al , 1996] and black circles are porosity measured in the shipboard laboratory.…”

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

“…The deep and shallow laterologs differ not only in depth of penetration but also in overall current geometry: the shallow measurement path is predominantly vertical and the deep path is more horizontal. Pezard and Anderson [1989] and later Pezard et al [1996] used this distinction to identify basalt intervals at Hole 504B with dominantly horizontal ( R deep < R shallow ) or vertical ( R deep > R shallow ) fractures, and a similar approach has been used at nearby Hole 896A [ de Larouzière et al , 1996]. Comparison of the deep and shallow laterologs for Hole 801C tholeiites shows generally very close agreement [ Shipboard Scientific Party , 2000a].…”

“…Surface conduction on clays can have a significant effect on basalt resistivity, particularly at elevated temperatures [ Olhoeft , 1981]. Consequently, calculations of porosity from resistivity for Hole 504B are based on log‐based estimates of both pore fluid and clay conduction [ Pezard , 1990; Pezard et al , 1996]. At Sites 768 and 770, in contrast, clay conduction was detectable but small enough to neglect without substantially biasing porosity determination [ Jarrard and Schaar , 1991].…”

Influence of normal faults and basement topography on ridge-flank hydrothermal fluid circulation

“…Some faults are traced through the sediments and up to the seafloor itself, suggesting recent extensional movements and continuous tectonic activity. Detailed downhole measurements, borehole televiewer images and core analysis (Pezard et al 1996, 1997) confirm the presence of a north‐dipping fault zone at 800 mbsf in Hole 504B, with a mean dip of 45° towards the ridge axis and subvertical dips at the seafloor.…”