Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Abstract (EDOC: 7022)
In the deep crust, temperature, which is among the key parameters controlling lithospheric dynamics, is inferred by extrapolation from the surface using several assumptions that may well fail in regions of active tectonics and fluid migration. In the rare case that temperatures of 700°C or higher are exceeded in the upper/middle continental crust composed of quartz-rich felsic rocks, the α-β quartz transition (ABQT) will occur, generating a measurable seismic signature and offering the possibility for precisely estimating temperature from the known ABQT phase diagram. Here it is shown that all expected seismic features of the ABQT are met by the boundary between the upper and middle crust below the INDEPTH III profile in central Tibet. This implies that a temperature of 700°C is achieved at a depth of 18 km under the southern Qiangtang block, that agrees well with the depth to the top of a high electrical conductivity anomaly, likely representing partially melted crust. To the south in the northern Lhasa block, the ABQT lies at 32 km depth, corresponding to a temperature of 800°C. It thus appears that this seismic boundary representing the ABQT is the result of (recent) geological processes rather than being a lithological boundary.
(2004): Precise Temperature Estimation in the Central Tibetan Crust From Identification of the α-β Quartz Transition by Project INDEPTH Seismic Profiling. CGU-AGU-SEG-EEGS 2004 Joint Assembly (Montreal 2004), T21A-04.