The cause of loss of lithospheric rigidity in areas far from plate tectonic activity

Significant losses of lithospheric strength are generally considered to be almost entirely associated with abnormal heating or steep lithospheric bend ing and/or stretching near to active plate boundaries. Several areas-the we stern Greater Caucasus, the North Crimean basin, the Carpathian foredeep, t he Peri-Caspian basin and the Trans-Caspian areas-are shown to have steep b asement slopes, usually comprising a difference in height of several kilome tres over lateral distances of only 20-30 km, corresponding to very low, si milar to 3-5 km, effective elastic thicknesses of the lithosphere. Each of these areas is shown to have undergone rapid steepening of the basement slo pe, usually within 1-2 Myr but in up to 10 Myr in some areas. At such times , these localities were far from active plate boundaries and in positions w here bending forces could not have been transmitted to them from far-distan t plate activities. Surface and/or subsurface loading can similarly be excl uded as mechanisms for such steepening, and there is no apparent outflow of crustal materials into adjacent regions. It is suggested that such rapid s ubsidence far from plate tectonic activity is caused by rapid increases in the local density of the lithosphere. This could occur as a result of, for example, a gabbro-eclogite transformation in the lower crust, catalysed by the infiltration of volatiles from the asthenosphere. The resultant contrac tion of the mafic rocks would be non-uniform in space and produce high devi atory stresses, reducing the viscosity in the lower crust to similar to 10( 23) Pa s. This would result in the rapid subsidence of the top of this laye r, accompanied by steep ductile bending of the overlying upper crust. Such steer downwarping of the basement would be accompanied by a similar steepen ing of the underlying weakened mantle. The formation of such steep slopes t hus indicates a weakening of the entire lithospheric layer, most probably d ue to the infiltration of volatiles from the asthenosphere, and unrelated t o coeval plate tectonic activity.