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Plio-Pleistocene north-south and east-west extension at the southern margin of the Tibetan Plateau

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The tectonic significance of the physiographic transition from the low-relief Tibetan plateau to the high peaks, rugged topography and deep gorges of the Himalaya is the source of much controversy. Some workers have suggested the transition may be structurally controlled

The tectonic significance of the physiographic transition from the low-relief Tibetan plateau to the high peaks, rugged topography and deep gorges of the Himalaya is the source of much controversy. Some workers have suggested the transition may be structurally controlled (e.g. Hodges et al., 2001), and indeed, the sharp change in geomorphic character across the transition strongly suggests differential uplift between the Himalayan realm and the southernmost Tibetan Plateau. Most Himalayan researchers credit the South Tibetan fault system (STFS), a family of predominantly east-west trending, low-angle normal faults with a known trace of over 2,000 km along the Himalayan crest (e.g. Burchfiel et al., 1992), with defining the southern margin of the Tibetan Plateau in the Early Miocene. Inasmuch as most mapped strands of the STFS have not been active since the Middle Miocene (e.g., Searle & Godin, 2003), modern-day control of the physiographic transition by this fault system seems unlikely. However, several workers have documented Quaternary slip on east-west striking, N-directed extensional faults, of a similar structural nature but typically at a different tectonostratigraphic level than the principal STFS strand, in several locations across the range (Nakata, 1989; Wu et al., 1998; Hurtado et al., 2001). In order to explore the nature of the physiographic transition and determine its relationship to potential Quaternary faulting, I examined three field sites: the Kali Gandaki valley in central Nepal (~28˚39'54"N; 83˚35'06"E), the Nyalam region of south-central Tibet (28°03'23.3"N, 86°03'54.08"E), and the Ama Drime Range in southernmost Tibet (87º15'-87º50'E; 27º45'-28º30'N). Research in each of these areas yielded evidence of young faulting on structures with normal-sense displacement in various forms: the structural truncation of lithostratigraphic units, distinctive fault scarps, or abrupt changes in bedrock cooling age patterns. These structures are accompanied by geomorphic changes implying structural control, particularly sharp knickpoints in rivers that drain from the Tibetan Plateau, across the range crest, and down through the southern flank of the Himalaya. Collectively, my structural, geomorphic, and thermochronometric studies confirm the existence of extensional structures near the physiographic transition that have been active more recently than 1.5 Ma in central Nepal, and over the last 3.5 Ma in south-central Tibet. The structural history of the Ama Drime Range is complex and new thermochronologic data suggest multiple phases of E-W extension from the Middle Miocene to the Holocene. Mapping in the accessible portions of the range did not yield evidence for young N-S extension, although my observations do not preclude such deformation on structures south of the study area. In contrast, the two other study areas yielded direct evidence that Quaternary faulting may be controlling the position and nature of the physiographic transition across the central Tibetan Plateau-Himalaya orogenic system.

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Date Created
2012

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Late Cenozoic-recent tectonics of the southwestern margin of the Tibetan Plateau, Ladakh, northwest India

Description

The Himalayan orogenic system is one of the youngest and most spectacular examples of a continent-continent collision on earth. Although the collision zone has been the subject of extensive research, fundamental questions remain concerning the architecture and evolution of the

The Himalayan orogenic system is one of the youngest and most spectacular examples of a continent-continent collision on earth. Although the collision zone has been the subject of extensive research, fundamental questions remain concerning the architecture and evolution of the orogen. Of particular interest are the structures surrounding the 5 km high Tibetan Plateau, as these features record both the collisional and post-collisional evolution of the orogen. In this study we examine structures along the southwestern margin of the Tibetan Plateau, including the Karakoram (KFS) and Longmu Co (LCF) faults, and the Ladakh, Pangong and Karakoram Ranges. New low-temperature thermochronology data collected from across the Ladakh, Pangong and Karakoram Ranges improved the spatial resolution of exhumation patterns adjacent to the edge of the plateau. These data show a southwest to northeast decrease in cooling ages, which is the trailing end of a wave of decreased exhumation related to changes in the overall amount of north-south shortening accommodated across the region. We also posit that north-south shortening is responsible for the orientation of the LCF in India. Previously, the southern end of the LCF was unmapped. We used ASTER remotely sensed images to create a comprehensive lithologic map of the region, which allowed us to map the LCF into India. This mapping shows that this fault has been rotated into parallelism with the Karakoram fault system as a result of N-S shortening and dextral shear on the KFS. Additionally, the orientation and sense of motion along these two systems implies that they are acting as a conjugate fault pair, allowing the eastward extrusion of the Tibet. Finally, we identify and quantify late Quaternary slip on the Tangtse strand of the KFS, which was previously believed to be inactive. Our study found that this fault strand accommodated ca. 6 mm/yr of slip over the last ca. 33-6 ka. Additionally, we speculate that slip is temporally partitioned between the two fault strands, implying that this part of the fault system is more complex than previously believed.

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Date Created
2014