Thermal Controls on Slab Breakoff and the Rise of High-Pressure Rocks During Continental Collisions

Abstract

We investigate the thermal controls on slab breakoff—the detachment of oceanic lithosphere from continental lithosphere during continental collision. We show that the depth of slab breakoff is sensitive to the thickness of the lithosphere and to the temperature of the convecting mantle, but comparatively insensitive to the distribution of radioactivity and shear heating. This is because lithospheric strength is largely controlled by the temperature of the uppermost mantle (well characterized by Moho temperature), and is largely insensitive to near-surface crustal temperatures (which is what foreland surface heat flow reflects). The insensitivity to subducting crustal temperatures allows us to confidently predict that slab breakoff will also be insensitive to erosion or accretion at the slab-wedge interface. It has been shown elsewhere that slab breakoff is also sensitive to the subduction velocity, and relatively insensitive to the angle of dip and critical strain rate. If the mantle was hotter in earlier Earth history, then we could expect shallower slab breakoff and hence less ultrahigh-pressure (UHP) metamorphism and greater amounts of collisional magmatism in the Archean.