Conditions for a crustal block to be sheared off from the subducted continental lithosphere: What is an essential factor to cause features associated with collision?

摘要

Crustal blocks are detached from the subducting lithosphere at various depths. I call such phenomena shearing-off and investigate the mechanical conditions for shearing-off to occur in the continental lithosphere. The buoyancy of the crustal block does not work selectively as a driving force until it has been sheared off and I suppose that shearing-off occurs when the shear traction at the thrust, whose maximum value is calculated from the strength along the thrust, overcomes the strength of the boundary faults of the block, and determine the depth of the leading edge of the subducted crustal block when shearing-off occurs. The temperatures of the slab and at its surface are calculated by the steady state formula of Molnar and England (1990) as a function of the surface heat flow, dip angle, and convergence velocity of the subducted continental plate, as the shear stress and temperature at the ductile part of the thrust zone are consistent with the flow law. The shear stress at the brittle part is a strong function of the pore fluid pressure ratio λ. I show that shearing-off of an upper crustal block is possible only for a small value of λ such as <～0.4 when the surface heat flow value is <～70 mW/m2. I suggest that the essential factor to cause shallow offscraping of crustal blocks in collision zones, is not a buoyancy of the continental crust, but a low value of λ, which would result from a little amount of dehydration from the subducted continental plate.