Publication 94
Precise temperature estimation in the Tibetan crust from seismic detection of the alpha-beta
quartz transition
Mechie, J., S.V. Sobolev, L. Ratschbacher, A. Yu. Babeyko, G. Bock, A.G. Jones, K.D. Nelson,
K.D. Solon, L.D. Brown, and W. Zhao
Abstract
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 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 and middle
continental crust composed of quartz-rich felsic rocks, the a-b 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 finding
implies that a temperature of 700 °C is achieved at a depth of 18 km under the southern
Qiangtang block, which agrees 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 geologic
processes rather than being a lithologic boundary.
Source
Geology, 7, 601-604, 2004.
MTNet Home Page
Alan's Home Page
Alan G Jones / 30 June 2004 /
alan-at-cp.dias.ie