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- All Subjects: Geochronology
- Creators: Semken, Steven
Description
Understanding the evolution of the Himalayan-Tibetan orogen is important because of its purported effects on global geodynamics, geochemistry and climate. It is surprising that the timing of initiation of this canonical collisional orogen is poorly constrained, with estimates ranging from Late Cretaceous to Early Oligocene. This study focuses on the Ladakh region in the northwestern Indian Himalaya, where early workers suggested that sedimentary deposits of the Indus Basin molasse sequence, located in the suture zone, preserve a record of the early evolution of orogenesis, including initial collision between India and Eurasia. Recent studies have challenged this interpretation, but resolution of the issue has been hampered by poor accessibility, paucity of robust depositional age constraints, and disputed provenance of many units in the succession. To achieve a better understanding of the stratigraphy of the Indus Basin, multispectral remote sensing image analysis resulted in a new geologic map that is consistent with field observations and previously published datasets, but suggests a substantial revision and simplification of the commonly assumed stratigraphic architecture of the basin. This stratigraphic framework guided a series of new provenance studies, wherein detrital U-Pb geochronology, 40Ar/39Ar and (U-Th)/He thermochronology, and trace-element geochemistry not only discount the hypothesis that collision began in the Early Oligocene, but also demonstrate that both Indian and Eurasian detritus arrived in the basin prior to deposition of the last marine limestone, constraining the age of collision to older than Early Eocene. Detrital (U-Th)/He thermochronology further elucidates the thermal history of the basin. Thus, we constrain backthrusting, thought to be an important mechanism by which Miocene convergence was accommodated, to between 11-7 Ma. Finally, an unprecedented conventional (U-Th)/He thermochronologic dataset was generated from a modern river sand to assess steady state assumptions of the source region. Using these data, the question of the minimum number of dates required for robust interpretation was critically evaluated. The application of a newly developed (U-Th)/He UV-laser-microprobe thermochronologic technique confirmed the results of the conventional dataset. This technique improves the practical utility of detrital mineral (U-Th)/He thermochronology, and will facilitate future studies of this type.
ContributorsTripathy, Alka (Author) / Hodges, Kip V (Thesis advisor) / Semken, Steven (Committee member) / Van Soest, Matthijs C (Committee member) / Whipple, Kelin X (Committee member) / Christensen, Philip R. (Philip Russel) (Committee member) / Arizona State University (Publisher)
Created2011
Description
The collision of India and Eurasia constructed the Himalayan Mountains. Questions remain regarding how subsequent exhumation by climatic and tectonic processes shaped the landscape throughout the Late Cenozoic to create the complex architecture observed today. The Mount Everest region underwent tectonic denudation by extension and bestrides one of the world’s most significant rain shadows. Also, glacial and fluvial processes eroded the Everest massif over shorter timescales. In this work, I review new bedrock and detrital thermochronological and geochronological data and both one- and two-dimensional thermal-mechanical modeling that provides insights on the age range and rates of tectonic and erosional processes in this region.
A strand of the South Tibetan detachment system (STDS), a series of prominent normal-sense structures that dip to the north and strike along the Himalayan spine, is exposed in the Rongbuk valley near Everest. Using thermochronometric techniques, thermal-kinematic modeling, and published (U-Th)/Pb geochronology, I show exhumation rates were high (~3-4 mm/a) from at least 20 to 13 Ma because of slip on the STDS. Subsequently, exhumation rates dropped drastically to ≤ 0.5 mm/a and remain low today. However, thermochronometric datasets and thermal-kinematic modeling results from Nepal south of Everest reveal a sharp transition in cooling ages and exhumation rates across a major knickpoint in the river profile, corresponding to the modern-day Himalayan rainfall transition. To the north of this transition, exhumation histories are similar to those in Tibet. Conversely, < 3 km south of the transition, exhumation rates were relatively low until the Pliocene, when they increased to ~4 mm/a before slowing at ~3 Ma. Such contrasting exhumation histories over a short distance suggest that bedrock exhumation rates correlate with modern precipitation patterns in deep time, however, there are competing interpretations regarding this correlation.
My work also provides insights regarding how processes of glacial erosion act in a glacio-fluvial valley north of Everest. Integrated laser ablation U/Pb and (U-Th)/He dating of detrital zircon from fluvial and moraine sediments reveal sourcing from distinctive areas of the catchment. In general, the glacial advances eroded material from lower elevations, while the glacial outwash system carries material from higher elevations.
A strand of the South Tibetan detachment system (STDS), a series of prominent normal-sense structures that dip to the north and strike along the Himalayan spine, is exposed in the Rongbuk valley near Everest. Using thermochronometric techniques, thermal-kinematic modeling, and published (U-Th)/Pb geochronology, I show exhumation rates were high (~3-4 mm/a) from at least 20 to 13 Ma because of slip on the STDS. Subsequently, exhumation rates dropped drastically to ≤ 0.5 mm/a and remain low today. However, thermochronometric datasets and thermal-kinematic modeling results from Nepal south of Everest reveal a sharp transition in cooling ages and exhumation rates across a major knickpoint in the river profile, corresponding to the modern-day Himalayan rainfall transition. To the north of this transition, exhumation histories are similar to those in Tibet. Conversely, < 3 km south of the transition, exhumation rates were relatively low until the Pliocene, when they increased to ~4 mm/a before slowing at ~3 Ma. Such contrasting exhumation histories over a short distance suggest that bedrock exhumation rates correlate with modern precipitation patterns in deep time, however, there are competing interpretations regarding this correlation.
My work also provides insights regarding how processes of glacial erosion act in a glacio-fluvial valley north of Everest. Integrated laser ablation U/Pb and (U-Th)/He dating of detrital zircon from fluvial and moraine sediments reveal sourcing from distinctive areas of the catchment. In general, the glacial advances eroded material from lower elevations, while the glacial outwash system carries material from higher elevations.
ContributorsSchultz, Mary Hannah (Author) / Hodges, Kip V (Thesis advisor) / Whipple, Kelin X (Committee member) / Semken, Steven (Committee member) / Heimsath, Arjun M (Committee member) / Till, Christy (Committee member) / Arizona State University (Publisher)
Created2017