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.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

“…The remaining preserved crustal thicknesses are 18 km thickness for the Talkeetna arc (Greene et al, 2006), 15 km for the Bonanza arc (Canil et al, 2010), and about 8.3 km for the Canyon Mountain complex (Pearcy et al, 1990). The Kohistan arc is believed to be entirely preserved in crustal thickness (Miller and Christensen, 1994;Petterson, 2010). Interestingly, the estimated original crustal thicknesses of these accreted terranes are significantly larger than the average thickness of modern island arcs, most likely because of the large uncertainty and often lack of constraints in estimating the depth of crystallization.…”

“…Only a few accreted island arc terranes (i.e., Talkeetna, Bonanza, Kohistan, Canyon Mountain, and El Paxtle arcs) contain parts of all of the original crustal layers, but these accreted layers are severely thinned. Geobarometric and geologic studies suggest original crustal thicknesses of 30-35 km for the Talkeetna arc (Greene et al, 2006;Hacker et al, 2008), 24 km for the Bonanza arc (Canil et al, 2010), 45 km for the Kohistan arc (Miller and Christensen, 1994), and about 30 km for the Canyon Mountain complex (Pearcy et al, 1990). The remaining preserved crustal thicknesses are 18 km thickness for the Talkeetna arc (Greene et al, 2006), 15 km for the Bonanza arc (Canil et al, 2010), and about 8.3 km for the Canyon Mountain complex (Pearcy et al, 1990).…”

High-K ankaramitic melt inclusions and lavas in the Upper Cretaceous Eastern Srednogorie continental arc, Bulgaria: Implications for the genesis of arc shoshonites

“…Recently, such melts have been successfully obtained in melting experiments with variable Ca-rich pyroxene + olivine + amphibole assemblages at 0.5-1 GPa and temperatures of 1175-1350°C (Médard et al, 2004(Médard et al, , 2006. Such assemblages are observed as parts of exposed old arc ultramafic/ mafic cumulate complexes in the Talkeetna area, Alaska (DeBari and Coleman, 1989;DeBari and Sleep, 1991;Kelemen et al, 2003), the Kohistan Terrane of North Pakistan (Khan et al, 1993;Miller and Christensen, 1994;Burg et al, 1998;Ringuette et al, 1999;Jagoutz et al, 2007), and the Ivrea Zone in North Italy (Mehnert, 1975;Rivalenti et al, 1984;Quick et al, 1994). These complexes have been interpreted to be remnants of magma chambers emplaced at the base of the crust, close to the upper mantle-lower crust transition.…”

Geochemical and Petrological Constraints on Velocity Behavior of Lower Crustal and Upper Mantle Rocks from the Fast-Spreading Ridge at Hess Deep

“…This project was conceived based on success in investigating geochemical constraints on laboratory-measured compressionalwave velocities from other environments (e.g., Miller and Christensen, 1994). During this investigation, five primary questions regarding velocity behavior were identified: (1) Does bulk-rock chemistry, which reflects the diversity in lithologies recovered during Leg 147, demonstrate any correlative properties that can be used to interpret velocity behavior in the lower oceanic crust and upper mantle?…”