The deep structure of Tibet along the Lhasa-Golmud transect – 35 years of seismic exploration in Tibet

Posted by pkm at Jan 31, 2015 11:55 AM |
James Mechie & Rainer Kind - Deutsches GeoForschungsZentrum - GFZ, Sections “Geophysical Deep Sounding“ and “Seismology“, Telegrafenberg, 14473 Potsdam, Germany

A 700 km deep seismic velocity cross-section beneath the Lhasa to Golmud transect across the Tibetan plateau is presented. This cross-section summarizes the results from almost 35 years of research along this transect in Tibet. One of the significant contributors to this transect is the INDEPTH (INternational DEep Profiling of Tibet and the Himalaya) project, which completed four phases of fieldwork across the plateau between 1992 and 2009. The project involved the participation of many scientists from several countries including China, U.S.A., Germany, Canada, U.K. and Ireland. The results along the transect published until February 2013 can be summarized as follows. Beneath the cover layer, felsic rocks rich in alpha-quartz exist down to 15-25 km depth. Beneath these depths, temperatures are probably high enough for ductile flow and partial melting to occur. The velocity increase across the boundary at 30-40 km depth marks the interface between felsic upper crust and more mafic lower crust. Crustal thickness is greatest (~74 km) south of ~31.5°N, where Indian lower crust forms the basal layer. Northwards, crustal thickness decreases to ~64 km around 33°N, before increasing to ~70 km beneath northern Tibet. Crossing the Kunlun, the crust thins to ~50 km beneath the Qaidam basin. The results from the fourth phase of the INDEPTH project indicate that the crustal thickness change from 70 km beneath the Songpan-Ganzi terrane and Kunlun mountains to ~50 km beneath the Qaidam basin is located about 100 km north of the Kunlun Fault and almost 45 km north of the North Kunlun Thrust. The Qaidam basin Moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent Songpan-Ganzi lower crust underthrusting or flowing northward beneath the Qaidam basin Moho. Thus the high Tibetan plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust. Beneath the crust, high-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong-Nujiang suture, where it downwells to 350-400 km depth. Northwards, Asian lithospheric mantle is overlain by a low-velocity, less dense, warm Tibetan plate consisting of an upper lithospheric and a lower asthenospheric part. The apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north Tibet is slower, less dense and warmer than under south Tibet, in agreement with the observed uppermost mantle velocities. This, in turn, could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau.

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