A Possible Role for Collision-Induced Mantle Extrusion as Evidenced from Arc-Forearc Rollback in Western Pakistan

Several models have been invoked to explain geodynamic responses to major continental collisions. For example, thin viscous models have emphasized crustal thickening and shortening while ‘extrusion tectonics' attempts to accommodate crustal shortening by regional-scale transform faults. More recently, it has been suggested that during and after the collision of thick, continental plates, ductile asthenosphere has been increasingly displaced laterally from the locus of plate convergence. These hypotheses have been tested and, to varying degrees, corroborated in the eastern Himalayas and Tibet, eastern and southeast Asia, and relatively ‘unconstrained' Pacific basins to the east. Our current work focuses on the more complex region associated with the northwestern collision syntaxis where numerous accreted micro-continents represent significant potential obstacles to mantle extrusion. In this region, the main collision was complicated by the inferred microplate collage, with progressive closure of diverse neo-Tethyan basins mostly post-dating those associated with the northeastern collision syntaxis. As elsewhere, the large number of ophiolites in western Pakistan appear to represent fragments of intra-oceanic forearc complexes, as evidenced by their characteristic sedimentary sequences, complexes of oceanic basement underlain by MORB-like metamorphic ‘soles' and superposed by boninitic and calcalkaline ‘proto-arcs' and the presence of high temperature, hydrothermal deposits, serpentinite diapers, and mud volcanoes. The Cenozoic Makran complex of Pakistan and Iran represents a unique, subparallel sequence of southward-convex lithologic associations – presumably formed during rapid episodes of subduction initiation, basin opening, and subsequent closure - while, interestingly, Early Quaternary volcanism of Afghanistan, and Pleistocene calcalkaline activity in Koh-i-Sultan appear some 600 km north of ongoing subduction to the south of Makran is consistent with the combined effects of west- and southwestward mantle extrusion. According to this hypothesis, episodes of southward rollback, with progressive slab steepening, are accompanied by magma formation over extensive depth ranges including deep-seated carbonates and calc-alkaline magmas of shallower provenance.