Partial melt or aqueous fluid in the mid-crust of Southern Tibet? Constraints from INDEPTH magnetotelluric data

Shenghui Li*, Martyn J. Unsworth, John R. Booker, Wenbo Wei, Handong Tan, Alan G. Jones

*Corresponding author for this work

Research output: Contribution to journalArticle

166 Citations (Scopus)

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 15-20 km 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 brightspots. Using constrained inversion, the MT data require a minimum crustal conductance of 6000 S. 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 the 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 per cent) 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 per cent at mid-crustal depth cannot be sustained. Taken together, these data show that the high conductivity in Southern Tibet is most probably the result of a relatively thin layer of aqueous fluids (100-200 m) overlying a thicker zone of partial melt (> 10 km).

Original languageEnglish
Pages (from-to)289-304
Number of pages16
JournalGeophysical Journal International
Volume153
Issue number2
DOIs
Publication statusPublished - May 2003
Externally publishedYes

Keywords

  • Electrical conductance
  • Electrical conductivity
  • Magnetotellurics (MT)
  • Partial melt
  • Saline fluid
  • Seismic bright-spots
  • Tibetan Plateau

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