The Deep Seismic Structure of the Pacific Tectonic Belt along the 8-DV Reference Profile

This article presents the results of deep seismic studies over the Sikhote-Alin fragment of the 8-DV reference geophysical profile running across the large fold structures of the Pacific tectonic belt, that is, the Sikhote-Alin Orogenic Belt with the superimposed Middle Amur sedimentary basin and the East Sikhote-Alin volcanoplutonic belt. A deep seismic section of the Earth’s crust and upper mantle with the distribution of longitudinal wave velocities is constructed. In the upper part of the section, P-wave velocities vary from 4.0–5.0 km/s within the Middle Amur sedimentary basin and the eastern part of the Eastern Sikhote-Alin volcanoplutonic belt to 5.5–5.6 km/s in the central part of the Sikhote-Alin Orogenic Belt. It was established that the thick Paleozoic sedimentary complex within the Middle Amur sedimentary basin is faulted and folded in a complex manner. Crystalline rocks with a compressional wave velocity of 6.0–6.2 km/s occur at depths from 8 to13 km in different parts of the Middle Amur sedimentary basin and at depths of 4–6 km within the Sikhote-Alin orogenic complex. Compressional wave velocities of the crust in the Sikhote-Alin fragment of the 8-DV profile are between 6.3 and 6.35 km/s. The crust varies in thickness from 32–34 km in the northern part of the fragment to 36–37 km in its central and eastern parts. Along the Moho, P-wave velocities are 8.1–8.3 km/s; a number of areas with reduced V_p of 7.8 to 8.0 km/s were identified in the junction zones of large crustal blocks. P-wave data were used to identify anomalies which were correlated with structural-tectonic zones, deep fault zones, and seismicity. The thick Middle Amur sedimentary basin was distinguished along the Central Sikhote-Alin deep fault. This basin is filled by Paleozoic low-velocity deposits and is displaced along the fault plane by 4–5 km at its junction with the Sikhote-Alin Orogenic Belt. Zones of increased heterogeneity in the upper crust projected onto the area within the contours of the Middle Amur sedimentary basin are characterized by increased seismicity.