HR: 11:10h
AN: T31E-10
TI: Is the conductive Tibetan crust due to partial melt or aqueous fluids?
AU: * Li, S
EM: shenghui@geophys.washington.edu
AF: University of Washington, Geophysics Program, Seattle, WA 98195 United States
AU: Unsworth, M
EM: unsworth@geophys.washington.edu
AF: University of Washington, Geophysics Program, Seattle, WA 98195 United States
AU: Booker, J
EM: booker@geophys.washington.edu
AF: University of Washington, Geophysics Program, Seattle, WA 98195 United States
AU: Wei, W
EM: wei@CUGB.EDU.CN
AF: China University of Geosciences, Department of Applied Geophysics, Beijing, 100083 China
AU: Tan, H
EM: thd@CUGB.EDU.CN
AF: China University of Geosciences, Department of Applied Geophysics, Beijing, 100083 China
AU: Jones, A
EM: ajones@cg.nrcan.gc.ca
AF: Geological Survey of Canada, 615 Booth Street, Ottawa, ON K1A0Y3 Canada
AB: Magnetotelluric (MT) fieldwork in Tibet has shown an extensive high conductivity zone in the mid-crust. In the same region, INDEPTH seismic data reported a series of bright spots. A fluid phase is clearly responsible for both these observations. However the nature of the fluid is still unresolved. The two leading candidates are partial melt and saline aqueous fluids, but conclusive evidence to distinguish them is still lacking. To clarify this situation, MT data have been analysed to further constrain the nature of the fluid. Initial inversions showed that the conductance (the product of a layers thickness and conductivity) of the mid-crustal conductor varies with latitude. From the Zangbo suture to the Damshung graben the conductance is $\sim$ 10,000 S. This value decreases in the northern part of the Lhasa block to around $\sim$ 4000 S and is approximately constant across the Bangong suture into the Qiangtang terrane. In the Northern Qiantang the conductance rises again and reaches a value of $\sim$ 10000 S at 34$^\circ$ N near Toutouhe. Can we use these observations to discriminate between aqueous fluids and partial melt as a cause of the high conductivity? While MT is able to accurately resolve the conductance of this layer, it cannot alone separate the thickness and conductivity of the layer. However additional information is available from laboratory studies of the conductivity of both partial melts and water-saturated rocks. The conductivity of a rock is dominated by the conductivity of the pore-fluid and its porosity. For each type of fluid we have estimated the maximum possible pore fluid conductivity, consistent with the known tectonic and thermal setting. We have used this information to infer the range of porosities required to explain the observed conductances. This analysis suggests that shallow ($\sim 15$ km) conductive zones near Damshung cannot be explained on the basis of just partial melt. However deeper zones of high conductivity to the north may be due to melt.
DE: 8110 Continental tectonics--general (0905)
SC: T
MN: 1999 AGU Fall Meeting