Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

Abstract

A complete “in situ” section of upper oceanic crust, from extrusive lavas, through dikes into gabbros has been recently drilled for the first time in 15 Ma old crust created at the superfast spreading East Pacific Rise (EPR). The upper igneous basement cored at Ocean Drilling Program (ODP)–Integrated Ocean Drilling Program (IODP) Holes 1256C and 1256D consists of sheet flows and massive flows, including a ponded lava flow unit (“lava pond”) interpreted as a thick lava flow delivered off‐axis and accumulated in a topographic depression. Fine‐scale structural analysis shows that the lava pond is much less fractured than the deeper lavas, probably as the effect of structure spacing that is larger than in the underlying lava flows. In Hole 1256D, structures are mainly subhorizontal in the lava pond and become steeper downhole, toward the sheeted dike complex. This reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth, inducing mostly subvertical eruptive fractures and fracture network, and the distance of the site from active rifting at the EPR. Contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns, as obtained by shipboard and downhole logging data. Electrical resistivity, compressional velocity, and bulk density are generally higher, whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole. The occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and, consequently, its permeability regime which, in turn, controls the geometry and distribution of hydrothermal circulation and basalt alteration.