CRUSTAL MELT FLOW IN LARGE HOT OROGENS

Melting of lower continental crust (LCC) is an integral part of orogenesis and active orogenic plateaux are underlain by a layer of low seismic velocity and high electrical conductivity interpreted as due to melt. In migmatitic granulite-facies LCC, microstructures indicate the former presence of melt and melt extraction pathways are recorded by leucosome traces in residual rocks. Strain and anisotropy of permeability control the form of mm-cm scale melt channels, and strong anisotropy promotes high fluid focusing. In the early stage, melt accumulation is diffusion-controlled and melt accumulates around peritectic phases in low-pressure sites. A melt-bearing rock is porous when melt volume reaches the melt wetting transition, which is also where a large decrease in strength occurs, initiating a porous flow regime, in which melt flow in channels to lower pressure sites is controlled by the anisotropy of permeability. As melt flow becomes progressively more focused in the source, these channels form networks, analogous to ductile deformation band networks; they allow accumulation of melt and form the link for melt flow from grain boundaries to ascent conduits. In the transfer stage, melt moves through discordant fractures, which enable it to move shallower in the crust. Features of fracturing in formerly melt-bearing crust include blunt fracture tips, zig-zag geometry close to fracture tips and petrographic continuity between leucosome in the host and granite in the dike. Following an approach in which soil mechanics is applied to deformation of porous (melt-bearing) rock, the type of localization and ductile fracture depend upon porosity, grain packing, melt pressure, effective mean stress, differential stress, and stress path (loading vs. unloading – compactive plastic flow vs. dilatant plastic flow). In suprasolidus LCC, fracture propagation takes place by development and coalescence of melt-filled pores ahead of the fracture tip, with fracture opening involving extensive inelastic deformation and diffusive mass transfer. In large hot orogens, granites commonly occur associated with major transtensional fault systems or in extensional detachments close to the base of the upper continental crust, which was likely close to the brittle–ductile transition during emplacement; this strength peak is a natural magma trap.