Date

2016

Document Type

Thesis

Degree

Master of Science

Department

Earth and Environmental Sciences

First Adviser

Zeitler, Peter K.

Other advisers/committee members

Pazzaglia, Frank J.; Meltzer, Anne S.

Abstract

The India-Eurasia collision has affected many regions throughout the Asian continent. The Gobi Altai, located in south-central Mongolia has been transpressionally reactivated in response to these far-field stresses, forming localized regions of high topography. New apatite U-Th/He ages along the mountain blocks Ikh Bogd and Baga Bogd in the Gobi Altai constrain the onset of exhumation and faulting. These ages identify the apparent beginning of Cenozoic deformation as compared to a previous thermochronology study that used apatite fission-track length modeling to suggest a young cooling event at 5±3 Ma (Vassallo et al., 2007). Single-grain analyses of a suite of samples from Ikh Bogd show a systematic relationship between age and depth below a paleo-low relief surface that caps the massif. Oldest ages are found at the highest elevations and show more dispersion, which is expected for slow cooling rates that are inferred. Conversely, low elevation samples give the youngest and most tightly constrained ages, as expected for fast cooling. Mean sample ages from replicate analyses demonstrate the onset of Cenozoic cooling at ~35 Ma after an extended period of slow cooling and exhibit a well-defined exhumed helium partial retention zone. The data imply intracontinental reactivation began earlier than previous modeling suggested and has been active since the Oligocene. This estimate is well-supported by Miocene to recent age basin sediments which indicates high, eroding topography was present at this time.

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