Seismic signature and geochemistry of an island arc: A multidisciplinary study of the Kohistan accreted terrane, northern Pakistan

Abstract

Systematic sampling and mapping in the Kohistan accreted arc terrane of northern Pakistan has provided a sample suite representing the lithologic diversity of the section from its base along the Main Mantle Thrust upward through several stacked intrusions and their metamorphosed equivalents into the Kohistan batholith. A new lithologic column for the terrane has been developed during the course of this study using geothermobarometry based on elemental exchange reactions calculated from electron microprobe analyses of mineral assemblages. The pressures calculated from the chemical analyses of various mineral assemblages are constrained by recently published activity‐composition models and an internally consistent thermodynamic data base. Compressional and shear wave velocities and densities have been measured on samples that represent the diverse lithologies in this section and have been correlated with the new lithologic column. This data set has been augmented with published compositions and values for the uppermost part of the arc, not sampled in this study. Compressional wave velocities range from 4.3 km s−1 in supracrustal volcanogenic sediments to 7.5 km s−1 in lower crustal mafic cumulates. Underlying ultramafic rocks with velocities of 8.0 to 8.4 km s−1 define a sharp seismic Moho. The strong inflection in the velocity profile to greater than 8.0 km s−1 is due to the transition from plagioclase‐bearing to plagioclase‐free cumulate rocks in a continuous ultramafic‐mafic intrusion. The base of the crust lies at least four kilometers below this transition. Correlation of these laboratory‐measured values and the new lithologic column has allowed development of a velocity profile comparable to profiles developed through more conventional field seismic methods. Geochemical indices of fractionation exhibit a reasonable correlation with Compressional and shear wave velocities. An estimate of mean crustal Vp is remarkably similar to models for other cordilleran terranes at 6.7 plus or minus 0.05 km s−1. An estimate of the bulk chemical composition of the Kohistan terrane does not compare favorably with most published assessments of bulk continental crustal chemical composition, but is significantly more mafic than the latter. The striking resemblance of our reconstructed velocity profile to models generated from field seismic studies not only addresses the veracity of these models, but suggests that the Kohistan arc is a superbly well‐preserved analog for other arc terranes.