Seismic velocities from the Kohistan Volcanic Arc, northern Pakistan

Abstract

The rocks bounded by the Main Mantle Thrust and the Northern Suture in northern Pakistan constitute an exhumed section through a Cretaceous volcanic arc. Samples have been collected from all the principal lithological groups of the arc, and P and S-wave seismic velocities have been measured in the laboratory with the prime objective of comparing the velocities with those determined by seismic refraction experiments on modern volcanic arcs. Velocities were measured at up to 0.7 GPa and consideration has been given to the effect of confining pressure, pore pressure and temperature and to the deformation and metamorphism involved in the Himalayan collision. The reconstructed velocity section through the arc shows a distinct ‘upper crust’ comprising granitic-dioritic intrusions, metasediments and volcanics with a P-wave velocity of 6.2–6.4 km S −1 depending on the parameters used. Beneath, the ‘lower crust’ of amphibolitcs and pyroxene granulite has a velocity mainly about 6.4–6.7 kms −1 , though the garnet granulitcs extend to 7.8 km S −1 . The more mature arc with a higher proportion of granitic rocks would show a slightly lower upper crustal velocity than the younger arc. Velocity inversions might be expected with a thermal gradient of as little as 10–15 ° C km −1 , depending on the pore pressure. In general, the proposed velocity structure is comparable with that of volcanic arcs and with many other sections of the continental crust.