Some remarks on the stability of blueschists and related high P-low T assemblages in continental orogens

Abstract

The thermal evolution of the crust in continental orogens is sensitive to the deformation in the mantle part of the lithosphere, a point obscured in models that apply a boundary condition of constant basal heat flux within the mantle lithosphere. We show that for part of the plausible range of conductivities and heat source distributions within the continental lithosphere the maximum or potential temperatures in thickened continental crust typical of collision zones do not necessarily exceed the stability field of the high P-low T facies such as blueschists provided the mantle lithosphere is also thickened during the deformation. The appropriate range of parameters includes conductivities towards the upper limit of the plausible range ( k > ∼ 2.2 W m −1 K −1) together with low crustal heat production rates with distributions concentrating heat production in the upper crust, and relatively thin initial lithosphere. These observations suggest that at least some of the high P-low T assemblages found within continental orogens, for example in the Dalradian of the Scottish SW Highlands, may reflect the thermal response to whole lithospheric thickening and do not necessarily reflect thermal regimes attendent with subduction or unusually rapid excavation.