RHEOLOGY OF PARTIALLY MOLTEN CRUST

Partial melting of crust is an integral part of orogenic events. Active orogenic plateaux (Tibet, Altiplano) are underlain by a layer of low seismic velocity and high electrical conductivity, interpreted as a layer of partial melt. Field observations, experimental work, and numerical simulations are combined to study the relationship between melt fraction and rheology. The rheology of crustal partial melt has been approached dominantly from the point of view of the rheologically critical melt percentage (RCMP), evaluated at about 30%. However, Rosenberg and Handy (2005, J. Met. Geol. 23, 19-28) have reinterpreted experimental data and shown that at a melt fraction of ~7%, melt interconnectivity is established, resulting in a substantial strength drop compared to solidus conditions. The second threshold is reached at higher melt fraction (RCMP) when the solid framework is disrupted, but this transition does not result in as large a strength drop. These new results may help us understand the rheology and structural development of partially molten crust observed in exhumed migmatite terrains. The Shuswap metamorphic core complex displays an extended upper crust directly underlain by a high-grade metamorphic core. This core has a flat foliation, shows evidence of partial melting reactions, and contains leucosomes. Foliation steepens around migmatite domes cored by diatexite migmatites in which the melt fraction was large and the solid fraction was disrupted. In light of experimental data, a long-lived discontinuity, now represented by the detachment zone, separates the upper crust from the underlying partially molten crust that contained moderate melt fraction (over 7% threshold) and was able to flow rapidly. The diatexite dome developed in response mainly to a density change associated with increasing melt fraction (equivalent to the RCMP); once established, the buoyant rise of dome rocks may have enhanced a positive feedback between melting and upward flow. Therefore, relatively modest amounts of melt are sufficient to produce a substantial strength drop in the crust; a larger melt fraction results in gravitational instabilities and diapiric flow.