The free mantle surface-new possibilities to reveal subcrustal inhomogeneities from the structure of the earth's crust

Abstract

If crustal material (including water in oceanic areas) is compressed to the mean density of the subcrustal layer, the surface of the body can be considered as a free mantle surface (FM)). Joint analysis of FMS and of gravity anomalies may reveal density inhomogeneities of the material between the Moho and the surface of isostatic compensation in the astenosphere, and may more precisely define the position of the material providing isostatic compensation of crustal inhomogeneities. The depth of FMS can be determined by using seismic refraction data and an empirical velocity-density relationship. 300 seismic refraction determinations of the Pacific Ocean crust structure and deep seismic sounding profiles crossing different tectonic units of the USSR territory have been employed for the FMS depth determination. It is shown that the subcrustal layer under oceanic basins is very homogenous (FMS depth 4.6 ± 0.1 km). Shallow depths to FMS (2.6 km) is a characteristic feature of the mid-oceanic rise indicating a low density of the subcrustal material. Very significant inhomogeneities were revealed in deep-trench island-arc marginal-basin systems. The relationship between the FMS depth and age of tectonic stabilization was found for the continental areas. The density of the subcrustal layer (or thickness of the lithosphere) is higher under areas with the more ancient crust. The role of the subcrustal layer inhomogeneities in isostatic compensation has been elucidated.