Seismic structure and the crustal magma plumbing system, MAR, 35°N

Laura S. Magde
University of Oregon

Friday, December 11, 1998
12:00 Noon—Pacific Forum

I will present the results of a recent tomography experiment which images the three-dimensional P wave velocity and anisotropy structure in a region 50 x 44 x 6 km³ near 35°N at the Mid-Atlantic Ridge (MAR). The best fitting 1-D velocity model is slower than the East Pacific Rise, but faster than other regions of the MAR. Our 1-D anisotropy structure is similar to that of other studies at both ridges, with ~4% anisotropy shallower than ~2 km, and no anisotropy in the lower crust. We find that seismic velocities increase and anisotropy decreases as oceanic crust matures and moves out of the axial zone. The most striking result is the three-dimensional velocity structure which images for the first time the crustal melt delivery system of a segment of the slow-spreading MAR. Near the segment center, we observe a <15 km diameter low-velocity zone (-0.4 km/s) in the lower crust (>3 km depth) which is continuous with a ~10 km wide, axis-parallel, low-velocity zone (-0.2 km/s) in the shallow crust. Three higher-amplitude low-velocity anomalies (-0.6 km/s) are observed in the shallowest crust (<2 km depth) and are located beneath seafloor volcanic features. We interpret the overall image to represent the thermal-melt signature of a magma feeding system in which highly-focused injections of magma from the mantle travel upward until they reach the brittle-ductile transition, where they then spread along the axis to supply shallow intrusive bodies and seafloor eruptions along the entire ridge segment.

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