Crustal accretion at a spreading rate end-member, the Mid-Cayman Spreading Center : insights from seismic data, gravity, and geochemistry

Abstract

Ocean crustal accretion is an important Earth process, affecting the global heat budget, geochemical cycles, and life on Earth. New crust is produced at divergent plate boundaries, or spreading centers, at a range of full plate separation rates from ultraslow spreading rates of <15 mm/yr, to fast spreading rates of > 75 mm/yr. Oceanic crustal structure, lithology, and thickness vary with spreading rate, and ultraslow-spreading centers accrete some of the thinnest and most heterogeneous crust on Earth. In order to investigate fundamental tectonic, magmatic and accretionary processes that underlie these observations, we study one of the slowest-spreading centers in the world, the Mid-Cayman Spreading Center (MCSC), situated between the North American Plate and the Caribbean Plate in the Caribbean Sea. We image the new oceanic crust formed at the MCSC by modeling wide-angle refraction seismic data, gravity data, and geochemistry of seafloor rocks. Seismic images of P-wave velocity (Vp) reveal very heterogeneous structure both parallel and perpendicular to the axial valley. A low Vp zone ~5 km beneath an off-axis massif that hosts a hydrothermal vent field likely represents fracturing associated with hydrothermal circulation, and possibly intruded magmatic sills. In order to constrain the lithology of the crust, we model amplitudes of seismic refraction data and construct gravity profiles. We find that the thickness of the igneous crust varies from ~0-6 km with a mean of ~3 km, where gabbro within a matrix of variably serpentinized mantle is distributed unevenly along the spreading center. Geochemical modeling of incompatible element concentrations in basalts provides estimates of the magmatic budget of the MCSC and two other spreading segments with faster spreading rates. The MCSC has a modeled melt thickness of ~2.8 km, with suppressed melting to ~12 km depth. These results support the hypothesis that there is increased surface cooling of the lithosphere at ultraslow-spreading rates, which reduces melt supply, and that these magmatic processes vary smoothly with spreading rate. Variability of ultraslow-spread crustal thickness can be explained by a segment-scale spatiotemporal fluctuation of lithospheric deformation and melt delivery, with an average of ~2 km of serpentinized mantle within the upper lithosphere

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