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Geochemistry, Geophysics, Geosystems

Seismic properties of the Kohistan oceanic arc root: Insights from laboratory measurements and thermodynamic modeling

Authors


Corresponding author: B. S. G. Almqvist, Institute of Geology, ETH-Z, Sonneggstrasse, 5 CH-8092 Zurich, Switzerland. (Bjarne.Almqvist@geo.uu.se)

Abstract

[1] P-wave velocities (Vp) have been measured in the laboratory and calculated using thermodynamic modeling for seven representative rock samples from the lower crust to mantle section of the Kohistan paleo-island arc. Lower crustal rocks comprise plagioclase-rich gabbro, garnet-bearing gabbro, and hornblendite; mantle rocks comprise garnetite, pyroxenite, websterite, and dunite. Measurements were performed at confining pressures up to 0.5 GPa and temperatures up to 1200°C. Vp were also calculated using rock major element chemistry with the Perple_X software package. Calculated Vp match closely the laboratory measurements. At depths representative for the arc root, Vp of upper mantle rocks vary from 7.7–8.1 km/s, whereas the lower crustal rocks have velocities between 6.9–7.5 km/s. P-wave anisotropy is small, with exceptions of sheared gabbros. Measured and calculated seismic properties are consistent with, and complement a growing database of published seismic properties from the Kohistan arc. In the light of such data, we discuss seismic imaging of present-day island arcs. Intermediate Vp (7.4–7.7 km/s) in arc roots can be explained by pyroxenites and garnet-bearing mafic rocks. Strong seismic reflectors may be related to garnetites (8.0–8.2 km/s).

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