UC Santa Barbara

This dissertation focuses on the dynamic relationship between evolving tectonics andsurface processes within the northwest Himalayas. By understanding how erosion at the surface has varied over space and time during the past several million years, the underlying mechanisms controlling how rock and sediment is redistributed can be identified. The studies described in Chapters 2-4 predominantly use two methods to estimate erosion rates: measurements of cosmogenic 10Be from stream sediment, which captures erosion rates over millennial timescales, and low-temperature thermochronometry, which can record erosion over timescales of millions of years. These datasets integrate erosion rates over different timescales and, when used in conjunction with each other, produce an improved understanding of the erosional history of a given landscape. In Chapters 2 and 3, we apply these methods within the Sutlej River drainage to better understand the history of large-scale drainage capture and identify mechanisms that may have facilitated drainage reorganization. Chapter 2 uses knickpoint celerity modeling, paleotopographic reconstructions, 10Be-derived denudation rates, and topographic analyses of drainage divides to quantify the magnitudes and timescales of erosion driven by capture of the Zhada Basin, an ∼23,000 km2 extensional basin in southern Tibet. This work finds that capture occurred at 735±269 ka and that capture-driven incision has removed 2010±400 km3 of sediment from the Zhada Basin since then. These findings also help identify several plausible tectonic or autogenic driving mechanisms for the capture event, which are investigated further in Chapter 3. Chapter 3 uses a new dataset of (U-Th)/He dates collected from the Sutlej River valley, along with a suite of numerical incision models, to assess whether Quaternary displacement occurred along the South Tibetan Detachment System (STDS) and how it could be mechanically connected to large-scale drainage capture throughout the Himalayas. Incision models that incorporate normal-sense slip consistent with active faulting along the STDS best correspond with the pattern of exhumation inferred from (U-Th)/He dates, suggesting that headward erosion and base level fall driven by displacement along the STDS could be a regional driver of drainage capture. Chapter 4 uses estimated denudation and exhumation rates over millennial- and million year-timescales, respectively, to interpret regional uplift patterns in the northwestern Himalaya. 10Be-derived denudation rates, inferred exhumation rates from published thermochronometric datasets, and estimated denudation mapping using ksnQ, an enhanced form of the channel steepness index, reveal the recent behavior of Lesser Himalayan duplexing, which largely influences surface topography and seismic activity. Duplexing has not migrated since at least ∼4Ma, suggesting that the Himalyan orogenic wedge may be segmented, such that cyclical development of a midcrustal ramp, and associated duplexing, may occur asynchronously along the orogen. This may explain observed patterns in exposed geology, topography, and exhumation rates in the northwest Himalayas.