Electrical Resistivity, Crustal Melting and Strain Localization beneath the Himalayan-Tibetan Belt
Abstract
The construction of mountain belts resulting from continental collisions involves crustal thickening, regional deformation and high-grade metamorphism. As observed in many ancient orogenic terrains, elevated temperatures locally induce melting and strain weakening, which may profoundly affect the rheology of the continental crust, its deformation regime and hence the development of these orogens. The Himalaya-Tibetan system is an active collisional belt allowing us to probe the three-dimensional thermo-mechanical distribution of an archetypal continent-continent orogen. Several magnetotelluric and seismic field campaigns (INDEPTH, Hi-CLIMB and HIMPROBE projects) have identified low resistivity layers with well-constrained tops at a depth of 10-15 km in southern Tibet (10 Ω.m) and 20-25 km in northwestern Himalaya (3 Ω.m), coincident with low velocity zones. Such electrical anomalies have been interpreted as evidence of high concentrations of interconnected fluids. Despite numerous field and numerical studies on the Himalayan range, identification and quantification of hydrous melting and potential strain weakening of the crust remains unaddressed by the appropriate experimental studies. Laboratory resistivity and viscosity measurements were performed in a gas-medium deformation apparatus (Paterson press), which was coupled to an impedance spectrometer. These measurements were conducted on natural muscovite-bearing metapelitic samples containing ca. 2 wt% water. Dehydration-melting of muscovite crystals leads to the production of hydrous melts. Electrical measurements show that with increasing temperature, the electrical resistivity decreases dramatically from 1830 Ω.m at 550°C to 8 Ω.m at 850°C. Image and chemical analyses of the partially molten samples after experiments indicate 23 vol.% hydrous melts, which are leucogranitic in composition. Our electrical measurements thus show that geophysical data clearly pinpoint ca. 25 vol.% partial melting at 750-800°C and 18-24 km depth beneath southern Tibet (10 Ω.m) and crystal-free leucogranite magma bodies at 800°C and 12 km depth beneath northwestern Himalaya (3 Ω.m). Presence of partial melting and fully molten bodies has serious impact on the rheological behavior of the middle crust. Our viscosity measurements show that ca. 25 vol.% partial melting reduces the viscosity, η, to 1011.5 Pa.s, whereas pure melt bodies have been shown to present lower η, i.e. 104 Pa.s. These viscosity values are several orders of magnitude lower than previous estimations used in thermo-mechanical models of crustal channel flow and ductile extrusion (η = 1018-1019 Pa.s). Our quantitative results represent a benchmark for conceptual models on the development and sustainment of mountain belts relying on an internally consistent set of petrological-geophysical-rheological constraints.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T51F2672H
- Keywords:
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- 3619 MINERALOGY AND PETROLOGY / Magma genesis and partial melting;
- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology;
- 3914 MINERAL PHYSICS / Electrical properties;
- 8031 STRUCTURAL GEOLOGY / Rheology: crust and lithosphere