Four-dimensional upper crustal construction at fast-spreading mid-ocean ridges: A perspective from an upper crustal cross-section at the Hess Deep Rift

Abstract

The faulted walls of the Hess Deep Rift in the equatorial Pacific provide one of the few significant tectonic windows into the oceanic crust created at a fast-spreading mid-ocean ridge. The 5000-m-deep rift graben exposes a slightly dismembered section of oceanic crust created at the East Pacific Rise ∼1 Ma. The upper portion of the North Wall of the rift features a laterally extensive exposure of upper crustal rock units similar to those found in many ophiolite complexes. For tens of kilometers along the rift wall, basaltic lavas (with sparse dikes) grade down into a spectacular sheeted dike complex that is underlain locally by a heterogeneous assemblage of gabbroic rocks. The geometry of lava flows, dikes and fault zones that developed during magmatic accretion provide constraints on the cross-axis (temporal) evolution of the upper crust in this area. Outcrop-scale digital imagery from submersible vehicles provides a well-constrained geological framework for samples collected from these upper crustal rock units, and the geochemistry of lava and dike samples provides additional temporal constraints on crustal accretion and the role of along-axis magma transport during accretion. Integrated results of geological and geochemical investigations suggest a four-dimensional model for upper crustal construction whereby tectonic and magmatic processes act in concert to construct the upper oceanic crust within a few kilometers (few tens of thousands of years) of a fast-spreading ridge axis. In this model, a sub-axial melt lens, where it exists, is recharged from below with distinct parental melts, and limited along-axis mixing preserves geochemical segmentation of the melt lens. Dikes emanate from distinct portions of the melt lens and transport magmas vertically, as well as significant distances laterally along-axis, erupting lavas in the special case where magmas ascend through dikes to higher levels in the crust and intersect the seafloor. Differences in the phenocryst contents and compositions of lavas and dikes suggests that density filtering of buoyant magmas, among other factors including magma overpressure and orientations of local stresses, plays a role in the eruption of magma on the surface. During spreading and magmatic construction, subaxial subsidence accommodates thickening of the basaltic lava unit. Sub-axial subsidence, faulting, and block rotation result in an upper crust with inward-dipping (toward the axis) lava flows and outward-dipping (away from the axis) dikes. Substantial differences in upper crustal geology may result from segment-scale variations in which vertical magma delivery dominates in areas of robust magma supply (likely over sites of recently recharged melt lenses at segment centers) but lateral magma transport dominates in areas of relatively lower magma supply (possibly toward segment ends).