The Hindu Kush seismic zone as a paradigm for the creation of ultrahigh-pressure diamond- and coesite-bearing continental rocks

Coesite eclogites and diamond-bearing ultrahigh-pressure (UHP) metamorphic rocks along ancient plate boundaries were mostly derived from quartz- and c arbonate-bearing rocks originally formed close to the earth's surface. Thei r mineral assemblages and PT conditions require that they were subducted to depths of 90-130 km (27-40 kbar) and then brought back to the surface, sti ll retaining evidence of their UHP formation. The geological record shows t hat continental-derived UHP rocks can be formed by subduction of thinned co ntinental-margin crust beneath ophiolites (e.g., Oman ophiolite, west Himal ayan ophiolites) or beneath island arcs (e.g., Kohistan Arc, Pakistan) as w ell as in continent-continent collision zones (e.g., Dabie Shan-Sulu Belt, Kazakhstan, western Norway, Alps). We present a model, based on the geometr y of the seismically active Hindu Kush continental subduction zone and its restoration, assuming present-day plate motions, which explains how surfici al graphite-rich shales and carbonates deposited along the northwest Indian plate margin were dragged down to these depths, anchored by the eclogitize d leading edge of the thinned Indian plate crust. We suggest that coesite e clogite and diamond-bearing UHP metamorphism is occurring today at depth al ong the Hindu Kush seismic continental subduction zone.