Crust and Upper Mantle Structure of the Southern Sikhote-Alin Along the Profile Spassk-Dalnyi–Zerkal’naya Bay from Magnetotelluric Sounding Data

Abstract

Magnetotelluric soundings (MTS) were conducted in a broad frequency range of 10³ kHz to 10^–3 Hz at a total of fifty sounding sites spaced 5 km apart along the profile Spassk-Dalnyi – Zerkal’naya Bay across the strike of the Sikhote-Alin Orogenic Belt (SAOB). As a result of data interpretation, the geoelectric section of the crust and upper mantle 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 20 and 50 km that is composed of different resistivity blocks bordered by low resistivity (a few tens to a few hundreds of Ohm m) subvertical and inclined zones. The abnormal low-resistivity zones associated with ore deposits are revealed in the upper crust. The upper mantle structure is nonuniform. The lowest resistivity values are noted in the depth range of 40–80 km beneath the central SAOB. It is underlain by a high-resistivity (350–450 Ohm m) layer stretching westwards from the coast and confined by a high-resistivity (about 500 Ohm m) area beneath the Khanka superterrane in the depth range of 80–120 km. The distinguished structure is associated with the ancient subduction zone. We considered the relationship between the deep structure and the resistivity values of the upper mantle and seismicity. The temperature regime in the upper mantle is evaluated. The geodynamic model of evolution of the area is proposed.