Deep structure, genesis, and seismic activation of the Bureya orogen, Russian Far East

Abstract

The Bureya orogen is a special object among the geodynamic factors determining the high seismicity of the Lower Amur region. Its location and deep structure are studied on the basis of comprehensive geophysical and tectonic data. This orogen is a low-density lithospheric domain expressed by an intensive negative gravity anomaly and Moho sunken down to 40 km depth. Within the limits of this lithospheric structure, contemporary uplifting takes place to form a meridional dome peaking at more than 2000 m altitude. The position of the orogen in the regional structure gives us grounds to think that the Bureya orogen formed in the Paleogene, at the finishing stage of tectonic block movement along the Pacific margin represented by the NE-trending strike-slip faults of the Tang Lu Fault Zone. Compression was concentrated at the triple junction between the Central Asian, Mongolian–Okhotian, and Sikhote Alin tectonic belts. The meridional orientation of the Bureya orogen is associated with the parallel elongated Cenozoic depressions in the region. The united morphotectonic system may have formed resulting from lithospheric folding under horizontal shortening in the Paleocene–Eocene. The wavelength of the Lower Amurian fold system is 250 km, which is consistent with the theoretical estimates and examples of lithospheric folds in other regions. The contemporary activation of the Bureya orogen began in the Miocene, under the effect of the Amurian Plate front moving in the northeastern direction. As a result of shortening, the meridional cluster of weak (M ≥ 2.0) earthquakes formed along the western boundary of the orogenic dome. The most intensive deformations caused another type of seismicity associated with the activation-related uplift of the mentioned orogen. As a result, the so-called Bureya seismic zone formed above the apex of the dome, and it is here that the strongest regional earthquakes (M ≥ 4.5) occur.