Highly permeable and layered Jurassic oceanic crust in the western Pacific

Abstract

Bulk permeability was determined from drillstring packer measurements in ODP Hole 801C in Jurassic oceanic crust in the western Pacific. The values average 8 × 10 −14 m 2 over 93 m of open hole, or 4 × 10 −13 m 2 if the permeable interval is confined to an 18 m thick hydrothermal zone within oceanic basement. These values are about 1–10 times higher than those reported for the upper sections of Holes 395A, 504B and 735B in 3.7, 5.9 and 11 Ma old oceanic crust respectively. The discovery that 160 Ma old oceanic crust contains a zone with extremely high permeability is not predicted by any model of ocean crustal evolution. The high permeability interval appears to be associated with the burial by off-ridge volcanism of a zone of hydrothermal precipitates and altered basalts. The generality of this result may depend on the generality of off-ridge volcanism necessary to create and preserve such zones of high permeability within the ocean crustal section. Although the crustal and permeability structures detected at Hole 801C may not fit easily into currently accepted models of crustal accretion and aging, the generality of off-ridge volcanism suggests that these structures could actually be typical of large portions of the world's oceanic crust.