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.     

Metallogeny of deep structures of the Earth’s crust in Southern Sikhote-Alin: Evidence from gravity data

The spatial relations between ore deposits in Sikhote-Alin and deep density inhomogeneities of the Earth??s crust down to a depth of 30 km have been examined. The ore areas and regions show a discrete correlation with the anomalies of the normalized density of the equivalent spherical sources of gravity anomalies at depths of 1 to 2, 4 to 5, 10?C12, and 24 km presumably provoked by magmatic bodies of different compositions. The depth of the magmatic bodies with the intermediate-to-basic composition of the initial magmas and the southeastward-decreasing vertical range of their correlation with the ore regions depend on their structural position. In the case of magmatic bodies of felsic and mixed compositions, the metallogenic specialization of the corresponding ore-magmatic systems is correlated with their inferred vertical range. Tin ore systems are characterized by a smaller vertical range (5?C10 km) of the correlation with density inhomogeneities, whereas tin-tungsten-gold ore systems are marked by a wider range (20?C25 km). Tin-lead-zinc systems occupy an intermediate position (12?C20 km). The ore-controlling role of the boundaries between the lithostructural complexes of the Earth??s crust and the central-type structures in the distribution of deep sources of ore mineralization is shown.     

A viscous layer at the crust-mantle boundary in the Far East of Russia

Geophysical indications, spatial parameters, and localization of the crustal and subcrustal viscous layers in the Far East of Russia are considered. The low-viscosity zones and layers are correlated equally distinctly with the Jurassic-Cretaceous plutonic and Cretaceous, Early Cenozoic, and Recent volcanic belts in tectonic domains of different age and geographic position, testifying to the long-term (up to 100–120 Ma) existence of viscous layers permanently fed by mantle-derived energy and mass flows through a deep fault zone and central-type structures of plume nature. The widespread low-viscosity zones in the lower crust and the uppermost mantle provided in the past and are providing now favorable conditions for lateral displacements of overlying tectonic sheets and fast relaxation of elastic stress in the Earth’s crust, provoking earthquakes.     

Rheological Gravity Model of the Crust in the Sinegorsk Ore Region (Southwest of Primorsky Krai)

The spatial correlation of the South Sinegorsk basin with the Paleozoic magma chamber at the depth of 5–7 km is revealed as a result of interpreting the statistical gravitation model representing the rheological properties of the crust in the Sinegorsk ore region to the depth of 30 km. The intrusion center of the volcanic formation is approached at the midst of the basin to the depths of 1–2 km, where a local volcanotectonic elevation is formed with uranium mineralization associated with the eastern flank of the latter. The western flank of this volcanic center occupying the area of the Monastyrishche volcanic depression, overlapped mainly by Cenozoic cover, is assumed to be highly promising. The distribution of density contrasts in the middle and lower layers of the crust indicates the absence of any actual relationships between the uppercrust igneous structures and the deep-seated upper-mantle formations. It is concluded that the top layer of the crust to the depth of 10 km was separated during the Late Paleozoic–Mesozoic time from the mantle “root” of the South Sinegorsk structure and displaced southwestwards synchronously to overthrust and strike–slip dislocations in the west of Sikhote Alin. The occurrence of this root is proposed because of the presence of Paleozoic andesibasalts, basite–hyperbasites, peridotites, and basalts over the entire area considered.