Geoelectric section of the crust and upper mantle of the northern Sikhote-Alin from magnetotelluric sounding data

Abstract

Magnetotelluric soundings (MTS) were conducted in a broad frequency range of 10 kHz to 0.001 Hz at a total of fifty-seven sounding sites of the profile spaced 5 km apart and intersecting the northern Sikhote-Alin across the strike. The analysis of the obtained magnetotelluric parameters has been made which shows three-dimensional geoelectric nonuniformities in the lower crust and upper mantle. The MTS curve interpretation was carried out in the framework of a three-dimensional model. As a result of the inverse problem solution, the geoelectric section has been constructed down to 150 km depth. The section distinguishes the crust with a resistivity higher than 1000 Ohm m and variable thickness between 30 and 40 km which is consistent with deep seismic sounding (DSS) data. The crust is subdivided into four blocks by deep faults, and each block is characterized by a set of parameters. The data support the existence of the Vostochny deep fault in the study area, whereas, on the contrary, the deep roots for the Central Sikhote-Alin fault have not been established. The upper mantle structure is nonuniform; three low-resistivity zones are identified that coincide with the boundaries of crustal blocks. In the revealed zones, an increase in the resistivity is noted from the continent to the Tatar Strait coast. A high-resistivity layer of 300–400 Ohm m was observed in the coastal area, which was steeply dipping from the crustal base down to 120 km depth and extended beneath the continent. Based on a set of geological and geophysical data, the ancient subducting plate is suggested in this area, and the evolutionary model of the region is proposed starting from the Late Cretaceous. The most probable mechanism of conductivity within the upper mantle is determined from petrological and petrophysical data. The low resistivity values are linked to dry peridotite mantle melting.