Leg 147

The Hess Deep Rift Valley

Hess Deep is the deepest part of a westward-propagating rift valley that is opening up the eastern flank of the equatorial East Pacific Rise (EPR) in advance of the westward-propagating Cocos-Nazca spreading center. Leg 147 represents the first leg in a long-term program of proposed coring aimed at characterizing the lower crust and upper mantle generated at such fast-spreading ridges and, using offset drilling, achieved the first recovery of these lower crustal and upper mantle rocks. Two sites were investigated during Leg 147 (Sites 894 and 895).

Lithological and geochemical trends indicate that the rocks recovered from the crest of the central intra-rift ridge, north of the deepest part of Hess Deep, crystallized from a MORB-like tholeiite and represent a sequence of high-level gabbros that crystallized from the roof of a magma chamber downward, similar to the upper plutonic sections of many ophiolites. The contact between finer, foliated gabbro and vein-like patches of coarse-grained gabbronorite that postdate the gabbro, and the virtual absence of orthopyroxene within the gabbro, suggest that melt segregations were locally filter-pressed from the foliated host and that accumulation and flow of interstitial melts is a significant process within the upper part of magma chambers.

Drilling on the south slope of the intra-rift ridge recovered harzburgite (residual mantle peridotites), dunite (the residue of melting formed by greater melt extraction or cumulus products of melt crystallization), and less abundant gabbro, olivine gabbro, and troctolite (possibly formed by melt migration and impregnation). The harzburgite-dunite-gabbro association is similar to the transition zone in ophiolite complexes and the relative abundance of dunite suggests that the cored sections are close to the petrologic Moho. The refractory nature of the peridotites suggests a very high degree of partial melting beneath the EPR or that the mantle was initially more refractory. The ultramafic lithologies are affected by extensive alteration dominated by serpentinization. The moderate-to-pervasive metamorphism and associated vein formation in the ultramafic and associated mafic rocks reflects extensive interaction with seawater-derived fluids.

NRM intensities in both gabbros and peridotites suggest that the lower crust and mantle contribute to seafloor magnetic anomalies. In the peridotites, magnetization is related to the presence of secondary magnetite, and therefore depends on the degree of serpentinization. Paleomagnetic data for both sites indicate that the crustal blocks have been tectonically rotated by ~35-40 deg. Since magnetization is related to magnetite in the ultramafic rocks, this rotation postdated serpentinization.