Vol 10, No 4 (2016)

Developing continuous chronologies of paleoenvironmental change in northern areas of the Far East using ¹⁴C can be problematic because of the low organic content in lake sediments. However, Holocene age-models can be supplemented by widespread tephra deposits reported in the Magadan region. The best documented of these tephras has been correlated to the KO tephra from southern Kamchatka dated to 7600 BP. Although a key chronostratigraphic marker, no detailed compendium of the distribution of this tephra and its associated ¹⁴C dates has been available from sites in the northern Far East. We provide such a summary. Known locally as the Elikchan tephra, lake cores indicate an ash fall that extended ~1800 km north of the Kamchatkan caldera with a ~500 km wide trajectory in the Magadan region. Other Holocene tephras preserved in lake sediments have poorer age control and possibly date to ~2500 BP, ~2700 BP and ~6000 BP. These ashes seem to be restricted to coastal or near-coastal sites. A single record of a ~25,000 BP tephra has also been documented ~100 km to the northeast of Magadan.

The seismic anisotropy of the mantle is studied based on the data of S and ScS waves from earthquakes occurred in the mantle transition zone over the period of 2007–2013 and recorded by seismic stations in the continental margin of Asia, on Sakhalin Island, and in the southern part of the Kamchatka Peninsula. The measurements of the azimuths of polarization of the fast S and ScS waves in the continental margin of Asia show that they are predominantly oriented in the E–SE directions. Based on the distribution of the shear wave splitting parameters, the symmetry of the medium can be described in terms of a transversely isotropic model with a horizontal symmetry axis and may correspond to horizontal flow in the upper mantle beneath the Amur Plate. The fast azimuths of polarization of ScS wave, which were determined to be of N–NE directions in the northern area of Sakhalin Island and in the continental part of Asia, may correspond to an inclined flow under the conditions of oblique subduction and complex geometry of the downgoing Pacific Plate. In the south of the Kamchatka Peninsula, the S- and ScS-wave azimuths of polarization from the M 8.4 Sea of Okhotsk earthquake are determined to be oriented along the direction of the Pacific Plate motion. The fast-S-wave azimuths of polarization from the aftershocks of the Sea of Okhotsk earthquake and from other large events of 2008–2009 are determined to be nearly parallel to the motion trend of the Pacific Plate, but orthogonal to it for the events of 2008–2009. On the basis of the distribution of azimuths of polarization of the fast S waves, the symmetry of the medium can be described in terms of a transversely isotropic model with the symmetry axis inclined orthogonally to the plane of downgoing plate and oriented westward orthogonally to the trench strike.

The first data on the whole-rock chemical composition of Jurassic–Lower Cretaceous sedimentary rocks cropping out in the Soloni–Umal’ta river interfluve (Bureya sedimentary basin) are used for revealing the distribution of their rock-forming elements. It is shown that the clastic material originated mostly from acid igneous rocks, while their intermediate varieties, as well as quartz-rich sedimentary and metamorphic rocks, played a subordinate role. It is assumed that the bulk of the clastic material was transported from the west and southwest (Bureya massif) and a smaller share from the east. The most significant differences between the Lower–Middle Jurassic and Upper Jurassic–Lower Cretaceous rocks mark a break in sedimentation.