Strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens: Implications of temperature-dependent thermal diffusivity and rheology

摘要

We explore the importance of strain heating in ductile shear zones for production of leucogranites and high-temperature metamorphism in collisional orogens with temperature-dependent thermal diffusivity (D) and rock rheology. New measurements on metamorphic rocks from the Bohemian Massif show that D is -1.3 mm~2/s at 25°C and decreases exponentially to as low as 0.4 mm~2/s at >600°C. This temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant D. Using power law parameters for rheology, we show that deformation of quartzite at strain rate of 3 x 10~(-13) s~(-1) produces >100 μW/m~3 at 550°C and -7 μW/m~3 at 800°C. Deformation of stronger clinopyroxenite at this strain rate produces -8 μW/m~3 even at 1050°C. When strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle, the schist solidus is reached by -8 Myr in the vicinity of the shear zone deforming at the strain rate of 3 x 10~(-13) s~(-1). For clinopyroxenite rheology, ultrahigh temperature (UHT) metamorphic conditions are reached in -40 Myr after initiation of strain heating. Two-dimensional models that replicate the exhumation of the Greater Himalaya crystalline rocks above the Main Central thrust produce extensive partial melting and an inverted metamorphic field gradient. Occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust.