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.If the subducted continental lithosphere is a craton (i.e., thick, cold lithosphere) then breakoff will be delayed and focused toward the oceanic side of the passive margin. In contrast, if the continental lithosphere is young and thin, breakoff will be accelerated and focused toward the continental side of the passive margin. Breakoff could also occur in oceanic lithosphere if the oceanic lithosphere is very young. We also investigate the case where the driving extensional body force is reduced when buoyant continental crust peels off from the downgoing plate and underplates the overriding plate or returns toward the surface. We find that even when all continental crust is removed, slab breakoff can still occur, but occurs deeper where the continental lithosphere is hot.We briefly discuss how slab breakoff relates to the exhumation of UHP rocks, and its prediction for their lower temperature bound P-T paths. If the rise of slivers along the interplate thrust has been the process that exhumed UHP rocks, the P-T paths observed in UHP metamorphic belts suggest that i) continental collisions proceed with convergence velocities much smaller than 50 mm/a, ii) the slivers rise sufficiently quickly that they do not attain equilibrium with the interplate thrust, or iii) shear heating is significant.
Weitere Kapitel dieses Buchs durch Wischen aufrufen
- Thermal Controls on Slab Breakoff and the Rise of High-Pressure Rocks During Continental Collisions
J. Huw Davies
Friedhelm von Blanckenburg
- Springer Netherlands
- Chapter 4