Tectonics of Hess Deep: a synthesis of drilling results from Leg 147

We present a synthesis of the tectonics of the Hess Deep rift valley, equatorial East Pacific Rise (EPR), as deduced from structural, paleomagnetic, petrological, geochemical, and borehole geophysical data acquired during Ocean Drilling Program (ODP) Leg 147. These data show that the crustal section and underlying shallow mantle lithologies now exposed in Hess Deep were formed by seafloor spreading at the north-south-trending EPR, were then transported eastward away from the EPR ridge axis, and became influenced by extensional tectonism caused by amagmatic rifting in advance of the westward-propagating Cocos-Nazca spreading center. Seawater influx into layer 3, the plutonic portion of the oceanic crust, commenced soon after crystallization, with pervasive influx of water along randomly oriented microfracture networks and grain boundaries, mostly at temperatures of 600°-750°C. This permeability was probably created by tensile brittle failure upon subsolidus cooling and thermal contraction of the gabbro, and must have been established within several tens of thousands of years after axial magma emplacement and within a few kilometers of the ridge axis. When the upper plutonic section had cooled to a temperature of ~450°C, probably a few tens of kilometers from the EPR axis and tens to a couple of hundred thousand years after axial magmatism, it became influenced by the effects of Cocos-Nazca rifting, and a dense array of east-west tensile fractures developed. Widespread access of seawater to sub-Moho levels and consequent onset of serpentinization occurred at this time. The ODP drill sites are located on an intra-rift horst block in the north of Hess Deep, and previous workers had speculated that serpentinite diapirism was responsible for the differential uplift of this block. However, textural evidence from the drill cores suggests that serpentinization of the shallow mantle there was a predominantly static phenomenon; hence, exhumation of the lower crust and shallow mantle appears to have taken place by block faulting rather than incoherent diapirism. Uplift of the intra-rift ridge was accommodated on normal Cataclastic shear zones, which contain low-temperature mineral assemblages (150°-250°C), implying that uplift occurred at a relatively late stage. Paleomagnetic data from the drillsites, fully restored to geographical coordinates, show that the intra-rift ridge has suffered significant tectonic rotation, with both a northward component of tilt and a counter-clockwise vertical-axis component of rotation. We do not have sufficient evidence to constrain unequivocally the axis or axes of rotation, but argue that compound rotations are likely in such a complex tectonic setting. In conjunction with other geophysical evidence from the Hess Deep area, we tentatively conclude that a counter-clockwise vertical-axis tectonic rotation occurred at a very early stage, perhaps in the overlap basin of the duelling propagators at 2°N on the EPR. At a later stage, and associated with the opening of the Hess Deep rift, northward tilting appears to have occurred (about a subhorizontal axis), as a result of rotational normal faulting above a southdipping detachment surface. Later, post-detachment normal faulting was probably responsible for the isolation and differential uplift of the intra-rift ridge