Possible chemical modification of oceanic lithosphere by hotspot magmatism: Seismic evidence from the junction of Ninetyeast Ridge and the Sumatra-Andaman arc

Tomograms of the upper mantle, which represent perturbations in P-wave, shear wave speed, and bulk-sound speed, image the Indo-Australian plate subducting beneath the Eurasian plate along the Sumatra-Andaman arc. The detailed images of the subducting slab are well constrained by the large number of intermediate depth earthquakes and moderate seismic station coverage along the entire Indonesian-Sumatra-Andaman arc. The steeply dipping slab is coherent, defined by seismicity and higher velocity perturbations, beneath the entire arc, but at approximately 9.5-12.5 °N and at 60-160 km depth, the slab is characterized by a low P-wave signal, with only a small decrease in the S-wave speed, and a modest decrease in bulk-sound speed. At this position along the arc the inactive Ninetyeast Ridge, suggested to be a plume-fed spreading ridge, is being subducted beneath the Andaman Islands. The negative P-wave and bulk-sound speed anomalies in the slab, along with only a small decrease in S-wave speed cannot be explained by thermal variations alone and hence suggest a possible change in composition. We find that the seismic anomalies in this region of the slab are best fit by orthopyroxene-rich zones within the peridotitic lithospheric mantle. If correct, we speculate that these pyroxene-rich lithologies formed by the interaction of upwelling magmas with preexisting oceanic lithospheric mantle beneath the Ninetyeast Ridge before subduction. Our observations represent one of few seismic evidences for extensive chemical modification of lithospheric mantle.