Abstract
Exhumation of ultra-high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy. However, direct observation of ongoing subduction of continental crust is rare. The Pamir is regarded as a possible place of active continental subduction because of the intermediate-depth seismicity, crustal xenoliths and estimates of crustal shortening versus convergence rates. Here we present for the first time receiver function images from a passive-source seismic array traversing the Tien Shan and the Pamir plateau showing southward subduction of Eurasian continental crust. In the eastern Pamir, we observe a southerly dipping 10–15km thick low-velocity zone (LVZ) that extends from 50km depth near the base of the crust to more than 150km depth with a dip angle increasing to subvertical. While the upper- and mid-crustal material seems to be shortened and incorporated into the Pamir, the lower Eurasian crust detaches and subducts. In its deeper part (>80km) the LVZ envelopes the intermediate-depth earthquakes. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust.
Highlights
The Pamir, situated north of the western Himalayan syntaxis, is thought to consist of the same collage of continental terranes as Tibet, that were progressively accreted to Eurasia prior to the Indian-Eurasian collision around 50 million years ago (Schwab et al, 2004)
In the Pamir, the total amount of crustal shortening that occurred during the Cenozoic is even higher than in Tibet, since a similar amount of total convergence has been accommodated over a much smaller distance
While low velocity layers hosting earthquakes at intermediate depths in subduction settings are a common feature in oceanic subduction zones (Bostock, 2012; Ferris et al, 2003), they are rare in intra-continental collisional settings
Summary
We operated a modern seismic array in the Pamir, a key area for Indo-Asian collision. We constructed receiver function cross sections traversing Tien Shan and Pamir. We used a modified CCP stacking to image strongly dipping interfaces. We observed subduction of Eurasian continental crust beneath the Pamir. A south-dipping low-velocity zone coincides with the intermediate-depth seismicity. Negmatullaev d a Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, 14473, Potsdam, Germany b
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