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

Abstract

Significant losses of lithospheric strength are generally considered to be almost entirely associated with abnormal heating or steep lithospheric bending and/or stretching near to active plate boundaries. Several areas—the western Greater Caucasus, the North Crimean basin, the Carpathian foredeep, the Peri‐Caspian basin and the Trans‐Caspian areas—are shown to have steep basement slopes, usually comprising a difference in height of several kilometres over lateral distances of only 20–30 km, corresponding to very low, ∼3–5 km, effective elastic thicknesses of the lithosphere. Each of these areas is shown to have undergone rapid steepening of the basement slope, 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 where bending forces could not have been transmitted to them from far‐distant plate activities. Surface and/or subsurface loading can similarly be excluded as mechanisms for such steepening, and there is no apparent outflow of crustal materials into adjacent regions. It is suggested that such rapid subsidence 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 contraction of the mafic rocks would be non‐uniform in space and produce high deviatory stresses, reducing the viscosity in the lower crust to ∼ 1023 Pa s. This would result in the rapid subsidence of the top of this layer, accompanied by steep ductile bending of the overlying upper crust. Such steep downwarping of the basement would be accompanied by a similar steepening of the underlying weakened mantle. The formation of such steep slopes thus indicates a weakening of the entire lithospheric layer, most probably due to the infiltration of volatiles from the asthenosphere, and unrelated to coeval plate tectonic activity.