Tectonics and river processes : three case studies in South and East Asia

Abstract

The thesis focuses on the interactions between tectonics, climate, and river erosion processes occurring in the Sutlej River valley and Namcha Barwa syntaxis in NW and NE Himalaya, respectively, and the Kuqa fold-thrust belt in northwestern China. In the Sutlej River valley, rapid rock exhumation, high relief, and kilometer-scale deformation patterns are spatially correlated along the river course. Rather than tectonics, proposed models for the rapid growth are explained by climatically-modulated erosion and focused river erosion processes based on long-term observations. However, over glacial-interglacial timescales, exhumation variabilities, and the primary driver remain unexplored. This study uses K-feldspar multi-optically stimulated luminescence thermochronological analyses of rock samples from the Sutlej River valley and analyses of relief and river networks to address these questions. Results show that the exhumation patterns are related to the Sutlej River knickzone development. The western and eastern areas of the knickzone record slow to moderate cooling with exhumation rates of 1–5 mm/yr between 0.2–0.1 Ma, which is related to the weakening phase of the river drainage reorganisation. Whereas the central area experienced rapid cooling initiated by deglaciation processes at 15 ka, followed by a very rapid cooling enhanced by the early Holocene monsoon intensification at 10 ka. In the northeastern Himalaya range, the predictions for the structural evolution of the Namcha Barwa syntaxis are based on long-term timescale records, but it is unclear whether the hypotheses are consistent with its present-day growth. The syntaxis may have developed either by focused erosion processes, operating at the scale of the Yarlung-Parlung River gorge based on the “tectonic aneurysm” model, or by tectonic processes via crustal-scale folding or through the indentation of the upper plate by the folded subducting plate based on the tectonics processes model. The geometry and kinematics of present-day active structures were determined using inversion modelling of the InSAR dataset of the 17 November 2017 Mw 6.5 Mainling earthquake, which ruptured the center of the syntaxis. The syntaxial growth's short-term timescale topographic, river, and geomorphic markers were estimated. The main findings are consistent with tectonic processes as the primary control of the deformation focus. However, focused erosion at the Yarlong-Tsangpo gorge continues to perturb present-day regional tectonics. In the Kuqa fold-thrust belt, key aspects of its structural development are unresolved. The fold-thrust belt may have developed through forward-propagating salt-cored detachment folds, which locally nucleated late thrust faults. Alternatively, detachment folds may have grown interspersed with either (i) long-lived pre-kinematic salt diapirs that evolved into local salt thrusts and/or (ii) source-fed thrust faults and associated folds. These hypotheses were tested by examining geomorphological records of lateral ridge propagation in the Kuqa fold-thrust belt. Major findings indicate that the frontal Kuqa fold-thrust belt developed either by detachment folding with local late thrust fault formation or via such structures interspersed with source-fed thrust faults. These studies demonstrate that tectonics and river erosion processes modulated by climatic shifts prominently operate at different scales during mountain building in a continent-continent collision system.