Tectonic geomorphology and Quaternary fault slip rates in the Tsambagarav Massif, Mongolian Altai

초록

The NNW-striking Mongolian Altai is a Late Cenozoic dextral strike-slip deformation belt affected by the far-field stress caused by the collision between India and Eurasia. The Tsambagarav Massif is bounded by faults, has a triangular plan shape, and attains the highest elevation of the belt. This massif is adjacent to the central segment of the Hovd fault, which is the largest fault in the Mongolian Altai. Here, we use Be-10 surface exposure dating, radiocarbon dating, and paleoseismic trenching to reveal deformation patterns and Quaternary slip rates for two range-bounding faults: the Tsambagarav dextral reverse fault and the Lower thrust fault. The Lower thrust exhibits thrusting motion along the low-angle structural fabric of the basement rock and cuts Quaternary alluvial fan deposits, vertically offsetting their surfaces by 3.9-5.1 m, with fault scarps extending several kilometres along-strike. The Tsambagarav dextral reverse fault is characterized by strike-slip motion along the high-angle structural fabric of basement rock. Various surface deformation features indicate recent dextral strike-slip, including displacements of Late Quaternary moraine by similar to 50 m. Dividing the measured displacements by Be-10 surface exposure ages of alluvial fan surfaces (for the Lower thrust) and moraine (for the Tsambagarav dextral reverse fault) gives a reverse slip rate of 0.3-1.7 mm/yr for the Lower thrust and a horizontal slip rate of 1.7-2.6 mm/yr for the dextral reverse fault. Radiocarbon ages of animal bones in a pond deposit accumulated along an associated tensional crack indicate that the last earthquake that caused a surface rupture occurred between 1924 and 1708 cal yr BP. Our study suggests that strain partitioning has occurred on the faults of the Tsambagarav Massif.

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