Late Quaternary evolution of the Kumkol Basin at the northeastern margin of the Tibetan Plateau revealed by tectonic geomorphology and the analysis of in situ cosmogenic nuclides

Abstract

The Tibetan Plateau was formed by the collision between the Indian and Eurasian plates, and it continues to grow laterally while keeping its elevation constant. The mechanism of the lateral expansion at the northeastern margin of the plateau, however, is highly debated owing to the scarcity of tectonic research. We conducted tectonic geomorphological analyses in the Kumkol Basin, where lateral growth is ongoing. This basin encloses a large active anticlinorium, the Kumkol Anticlinorium (KA), which would provide a quantitative constraint on the lateral growth of the plateau if the tectonically deformed landforms were dated. Our geomorphological mapping revealed six terrace steps, designated T1 to T6 in descending order, along the Pitileke River. In situ cosmogenic radionuclide concentrations in subsurface and surface samples from T1, T2, T3, T4, and T6 showed their formation ages to be 252 ± 24 ka, 140–160 ka, 103 ± 23 ka, 98 ± 11 ka, and 0–23 ka, respectively. Thus, T1 formed during marine isotope stage (MIS) 8, T2 during MIS 6, and T3 and T4 during MIS 5. Our results are quantitative evidence that degradation–aggradation cycles in the basin are correlated with global climatic fluctuations. To estimate the geometry of faults contributing to the evolution of the anticlinorium, we used a dislocation fault model and the overall surface deformation across the KA. The results suggest that the KA has been produced principally by slip on a detachment fault at 15 to 10 km depth, which is within or just above the brittle–ductile transition zone. We then determined the maximum uplift rate to be ~1 mm/yr and the horizontal shortening rate to be 2.5–3.2 mm/yr from the formation ages of the terraces and their deformation. These results suggest that there is a large-scale detachment fault below a basement-involved fold-and-thrust belt and that the concentration of stress at the plateau margin likely facilitated the development of crustal-scale deformation underlying the Kumkol Basin, and this deformation then accelerated the lateral growth of the Tibetan Plateau.