Geomorphological indicators of shear displacements on the Urup Island (Great Kuril Ridge)

   The Urup Island, located in the southern part of the Great Kuril ridge, is an interesting object of study because its relief is very contrasting and has a significant depth of vertical dissection. It was greatly influenced by the recent tectonic processes. Judging by the geological data, the island at the present stage is located in a shear stress field with the southeastern orientation of the axis of maximum compression. It can be explained by the subduction interaction of the Pacific and Sea of Okhotsk lithosphere plates at an acute angle of about 55°. Visual and automated decoding of satellite images and digital terrain models, and the analysis of the erosion network pattern made it possible to identify geomorphologic signs of shear displacements. These include the echeloned arrangement of lineaments as separation cracks in right-lateral shear zone in the area of the Rybnaya River and on the Skvoznyakovyj isthmus, where displacements of unidentified kinematics were previously revealed. The zones, which are oriented in the north-east direction and differ in the asymmetric pattern of the drainage network, were considered: within their limits, small tributaries are located on one side of the watercourse of the highest order as megatracks of separation. In the area of the epicenter of the earthquake of 1989, which occurred under the latitudinal horizontal stretching, a linear lowering of the relief with a feathery pattern of watercourses was recorded, interpreted as a morphostructure of stretching. Similar probably stretching zones, extending mainly along the azimuth of 130–140°, were recorded on the Pacific coast of the island. Their orientation, as well as the kinematics of probable shear displacements identified from the geomorphologic data, correlates with the general ideas about the stress-strain state of the Southern Kuriles, as well as the reconstruction of the main normal stress axes of the Kastricum Peninsula performed by us from the fracture measurements. All this prove that the suggested structural-geomorphologic interpretation of the Urup Island territory is quite reasonable. In general, our new data on the configuration and kinematics of probable active shear faults and extensional morphostructures complement the existing ideas about the latest deformations of the study area.

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