A THEORY FOR BUCKLING OF THE MANTLE LITHOSPHERE AND MOHO DURING COMPRESSIVE DETACHMENTS IN CONTINENTS

Current knowledge of rock mechanics suggests a strength minimum in the lowermost continental crust, along which simple shear could occur if a strong force drove the mantle lithosphere. We examine the change of shape of the Moho during this process, assuming plane strain, a depth-dependent, effectively-Newtonian crustal viscosity, and small-amplitude sinusoidal waves. Under the assumption that effective crustal viscosity is ηminexp[k(c-z)/c], waves will propagate at speeds relative to the surface of between Vm/k and Vm, where c is crustal thickness, z is depth, and Vm is mantle lithosphere velocity. If the mantle lithosphere is in relative horizontal tension ("pulled"), then all wavelengths are damped. If it is in relative horizontal compression ("pushed"), waves with wavelengths less than a critical value will grow exponentially. The wavelength of the fastest-growing wave varies in proportion to: the crustal thickness, c; the amount of horizontal compression I to a power ≤ 1 2; the maximum viscous compliance (1/ηmin) to a power ≤ 1 6; and the viscous flexural rigidity of the mantle lithosphere D to a power ≤ 1 6. For reasonable parameter values the wavelength that eventually dominates will be in the range 80-300 km. This wave propagates relative to the surface at 70-100% of Vm, and for some parameter combinations is almost fixed in the mantle lithosphere reference frame. Thus this is best understood as a buckling process. After deformation ends, the rise in effective viscosities would tend to retard the decay of Moho topography. Slow isostatic equilibration could allow these waves to be expressed as parallel surface structures. © 1990.