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Geological and Geophysical Evidence for Deep Subduction of Continental Crust Beneath the Pamir

By
V. S. Burtman
V. S. Burtman
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Peter Molnar
Peter Molnar
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Published:
January 01, 1993

Geological and geophysical observations imply Cenozoic subduction of intact Eurasian continental lithosphere, approximately 300 km in downdip length and including relatively thin (20–25 km) continental crust, beneath the Pamir. An inclined seismic zone dips at about 45° south-southeastward to a depth of 150 to 200 km beneath the Pamir and projects to the surface near the northern margin of the Pamir. The downdip length of the seismic zone of about 300 km implies a comparable amount of subduction of lithosphere in late Cenozoic time. The seismicity and tectonically most active part of the Pamir and its surroundings follows the northern margin of the Pamir. Quaternary offsets on faults and repeated geodetic observations suggest that roughly half of India’s present 44-mm/a convergence with Eurasia is absorbed by localized crustal shortening and underthrusting at this zone.

Three kinds of geologic observations suggest that the outer (northern) margin of the Pamir has been thrust at least 300 km over southern Eurasia. A Paleozoic suture between Eurasia and continental fragments originally from Gondwanaland crosses northern Afghanistan and follows the Kunlun in northern Tibet. Between Afghanistan and Tibet it is deflected roughly 300 km northward around the Northern Pamir. Large systematic anomalies in paleomagnetic declinations measured in Cretaceous and Paleogene sedimentary rock on the northern and western margins of the Pamir can be explained by bending of this margin and by 300 to perhaps 700 km of northward displacement of the Pamir with respect to the region farther west and north. East-west-trending facies boundaries in Cretaceous and Paleogene sedimentary rock in the Tadjik Depression are abruptly truncated at the western edge of the Pamir. Their eastward continuations, which follow the northern margin of the Pamir, lie a minimum of 200 km farther north. In addition, crustal shortening within the outer zone of the Pamir appears to exceed 100 km. Together this displacement of 200 km and the internal shortening of about 100 km also imply that the Northern Pamir has been displaced northward 300 km or more with respect to the rest of Eurasia farther north.

If continental crust has been subducted into the asthenosphere beneath the Northern Pamir, some other process must account for the present thick crust beneath the Pamir. Geologic mapping of major thrust and strike-slip faults within the Pamir indicates that crustal shortening, largely by thrust faulting, has absorbed more than 300 km of north-south convergence. Hence the southern part of the Pamir has been displaced northward this additional amount with respect to Eurasia. Crustal thickening within the Pamir, from normal thickness to roughly 70 km, can account for about 300 ± 100 km of convergence and therefore can account for the Cenozoic shortening within the Pamir. In contrast, the crust presently beneath the Tadjik Depression, west of the Pamir, includes a relatively thick sequence of Mesozoic and Cenozoic deposits (10–15 km) overlying a relatively thin crystalline basement (20–25 km). The sedimentary cover the Tadjik Depression has undergone folding and faulting in the Cenozoic era; the basement of most of the depression appears to have remained intact.

The geography and estimates of amounts of convergence that can be accounted for in different ways imply that the intermediate-depth earthquakes beneath the Pamir occur within the lithosphere that lay east of the Tadjik Depression before it was overridden by the Pamir. We assume that the thin crystalline basement that presently underlies the Tadjik Depression characterizes that subducted lithosphere. Its sedimentary cover was detached, and some of it now crops out along the northern margin of the Pamir, but the continental lithosphere with its 20 to 25 km of crystalline crustal basement has been subducted intact to a depth of 200 km or more. The present rate of convergence across the Pamir suggests that this process continues unabated.

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GSA Special Papers

Geological and Geophysical Evidence for Deep Subduction of Continental Crust Beneath the Pamir

V. S. Burtman
V. S. Burtman
Search for other works by this author on:
Peter Molnar
Peter Molnar
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Geological Society of America
Volume
281
ISBN print:
9780813722818
Publication date:
January 01, 1993

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