Seismic Attenuation Structure and the Fine-Scale Structure of a Seismic Zone at the Mid-Atlantic Ridge, 35 degrees N

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The magma plumbing system at slow-spreading ridges is poorly understood, especially in comparison with out understanding of ridge structure and processes at faster spreading ridges. How magma is stored in and transported through the crust, and how this magmatic system intereacts with faulting, are two crucial, but unknown, aspects of crustal accretion at slow-spreading ridges. Recent seismic tomography experiments at the Mid-Atlantic Ridge at 35 degrees N have imaged an intriguing distribution of low seismic P wave velocities beneath the inner valley of a magmatically-robust spreading segment. As the shallower low-velocity regions lie directly beneath seafloor volcanic features, the tomographic images may indicate the presence of high temperatures and partial melt associated with a crustal magma plumbing system. However, the low-velocity regions may instead be due to increased porosity, an alternative interpretation that is supported by an observed increase in Vp/Vs with depth. Microearthquake hypocenters at the center of the same segment devine an along-axis trend within a narrow depth range (3-4 km) beneath the inner valley floor. Most of the focal mechanisms are consistent with extensional faulting, but strike-slip and other anomalous mechanisms are also present. The hypocenters appear to be associated with a western valley-bounding fault. At the same time, the earthquakes are located near a change in character of the low-velocity anomalies from a centrally-located, pipe-like body below 4 km depth to an along-axis region of anomalously low velocities in the upper crust.