Publication 86
Partial melt or aqueous fluid in the mid-crust of southern Tibet? Constraints from INDEPTH
magnetotelluric data
Li, S., M.J. Unsworth, J.R. Booker, W. Wei, H. Tan, and A.G. Jones
Abstract
The INDEPTH project has applied modern geophysical techniques to the study of the crustal structure
and tectonic evolution of the Tibetan Plateau. In the Lhasa Block, seismic reflection surveys in 1994
detected a number of bright spots at 15-20km depths that indicate zones of crustal fluids (aqueous
fluids or partial melt). Coincident magnetotelluric (MT) data collected in 1995 detected a major zone
of high electrical conductivity at the same depth as the bright spots. Using constrained inversion, the
MT data require a minimum crustal conductance of 6000 Siemens. This abnormally high electrical conductance
can be best explained by a layered model with fluids: partial melt, aqueous fluids or a combination of
partial melt and aqueous fluids. The non-uniqueness of the MT method means that a wide range of melt
fraction-thickness combinations for above models could all explain the 6000 S conductance. To distinguish
between these three models, other geophysical and geological data are required. Reflection seismic data
suggest that a high fluid content (>15%) is present at the top of the layer. The amplitude-versus-offset
data suggest that the top of this layer may be aqueous fluids rather than partial melt. Passive seismic
data imaged a 20 km thick layer of lower fluid content that is probably partial melt. Petrological studies
suggest that concentrations of aqueous fluids above 0.1% at mid-crustal depth cannot be sustained. Taken
together, these data show that the high conductivity in Southern Tibet is most likely due a relatively
thin layer of aqueous fluids (100-200 m) overlying a thicker zone of partial melt (>10 km).
Source
Geophysical Journal International, 153, 289-304, 2003.
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Alan G Jones / 10 June 2004 /
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