Rheological Model of the Crust Beneath Southern Sikhote-Alin: Evidence from Gravity Data

The analysis of the spatial distributions of the density gradients characteristic of spherical sources of gravity anomalies provided grounds for the assessment of the rheological properties of the crust beneath the southern Sikhote-Alin mountainous system, which includes two layers with elevated rigidity and two layers of lowered viscosity. The direct proportional correlation between the rigidity and the density of the modeled mediums is confirmed by solving the direct linear problem of the gravitational potential. The rigid medium is characteristic of the lower crystalline layer of the Amur Plate, the top of which gently dips toward the Sea of Japan coast, and clinoform slabs of the oceanic crust, which underlie the Taukhe and Kema terranes and thrust over the lower layer of the continental crust. The viscous medium corresponds to the subcrustal zone of partial melting beneath the East Sikhote-Alin volcanic belt and to the accretionary complexes of the Samarka and Zhuravlevka terranes, which are involved into shear deformations. The Late Paleozoic and Early Mesozoic geodynamic history of the Sikhote-Alin crust was characterized by the interaction between the continental and oceanic rigid crustal wedges, which determined the directed accretion of the terrigenous turbiditic and island-arc sequences of the marginal sea to the Amur Plate.