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- All Subjects: tectonics
- Creators: Heimsath, Arjun M
- Creators: Arrowsmith, Ramon J
Description
The Himalaya are the archetypal example of a continental collision belt, formed by the ongoing convergence between India and Eurasia. Boasting some of the highest and most rugged topography on Earth, there is currently no consensus on how climatic and tectonic processes have combined to shape its topographic evolution. The Kingdom of Bhutan in the eastern Himalaya provides a unique opportunity to study the interconnections among Himalayan climate, topography, erosion, and tectonics. The eastern Himalaya are remarkably different from the rest of the orogen, most strikingly due to the presence of the Shillong Plateau to the south of the Himalayan rangefront. The tectonic structures associated with the Shillong Plateau have accommodated convergence between India and Eurasia and created a natural experiment to test the possible response of the Himalaya to a reduction in local shortening. In addition, the position and orientation of the plateau topography has intercepted moisture once bound for the Himalaya and created a natural experiment to test the possible response of the range to a reduction in rainfall. I focused this study around the gently rolling landscapes found in the middle of the otherwise extremely rugged Bhutan Himalaya, with the understanding that these landscapes likely record a recent change in the evolution of the range. I have used geochronometric, thermochronometric, and cosmogenic nuclide techniques, combined with thermal-kinematic and landscape evolution models to draw three primary conclusions. 1) The cooling histories of bedrock samples from the hinterland of the Bhutan Himalaya show a protracted decrease in erosion rate from the Middle Miocene toward the Pliocene. I have attributed this change to a reduction in shortening rates across the Himalayan mountain belt, due to increased accommodation of shortening across the Shillong Plateau. 2) The low-relief landscapes of Bhutan were likely created by backtilting and surface uplift produced by an active, blind, hinterland duplex. These landscapes were formed during surface uplift, which initiated ca. 1.5 Ma and has totaled 800 m. 3) Millennial-scale erosion rates are coupled with modern rainfall rates. Non-linear relationships between topographic metrics and erosion rates, suggest a fundamental difference in the mode of river incision within the drier interior of Bhutan and the wetter foothills.
ContributorsAdams, Byron A (Author) / Whipple, Kelin X (Thesis advisor) / Hodges, Kip V (Thesis advisor) / Heimsath, Arjun M (Committee member) / Arrowsmith, Ramon (Committee member) / Hurtado, Jose M (Committee member) / Arizona State University (Publisher)
Created2014
Description
Quantifying the temporal and spatial evolution of active continental rifts contributes to our understanding of fault system evolution and seismic hazards. Rift systems also preserve robust paleoenvironmental records and are often characterized by strong climatic gradients that can be used to examine feedbacks between climate and tectonics. In this thesis, I quantify the spatial and temporal history of rift flank uplift by analyzing bedrock river channel profiles along footwall escarpments in the Malawi segment of the East Africa Rift. This work addresses questions that are widely applicable to continental rift settings: (1) Is rift-flank uplift sufficiently described by theoretical elliptical along-fault displacement patterns? (2) Do orographic climate patterns induced by rift topography affect rift-flank uplift or morphology? (3) How do uplift patterns along rift flanks vary over geologic timescales? In Malawi, 100-km-long border faults of alternating polarity bound half-graben sedimentary basins containing up to 4km of basin fill and water depths up to 700m. Orographically driven precipitation produces climatic gradients along footwall escarpments resulting in mean annual rainfall that varies spatially from 800 to 2500 mm. Temporal oscillations in climate have also resulted in lake lowstands 500 m below the modern shoreline. I examine bedrock river profiles crossing the Livingstone and Usisya Border Faults in northern Malawi using the channel steepness index (Ksn) to assess importance of these conditions on rift flank evolution. River profiles reveal a consistent transient pattern that likely preserves a temporal record of slip and erosion along the entire border fault system. These profiles and other topographic observations, along with known modern and paleoenvironmental conditions, can be used to interpret a complete history of rift flank development from the onset of rifting to present. I interpret the morphology of the upland landscape to preserve the onset of extensional faulting across a relict erosion surface. The linkages of individual faults and acceleration of slip during the development of a continuous border fault is suggested by an analysis of knickpoint elevations and Ksn. Finally, these results suggest that the modern observed climate gradient only began to significantly affect denudation patterns once a high relief rift flank was established.
ContributorsRobinson, Scott M (Author) / Heimsath, Arjun M (Thesis advisor) / Whipple, Kelin X (Thesis advisor) / Arrowsmith, Ramon J (Committee member) / Arizona State University (Publisher)
Created2014
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