Braza, Mary
(2023)
Geometry, Kinematics, and Exhumation Pathways in the Himalaya: Thermokinematic Models for Western Nepal and Arunachal Pradesh, India.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Exhumation pathways of rocks in the Himalayan orogen are constrained using thermokinematic forward models of balanced cross-sections in Nepal and Arunachal Pradesh, India. Integration of balanced cross-section geometries, kinematic order of faults, and measured datasets including geologic mapping, basin accumulation data, peak temperature estimates, and cooling ages requires more complicated exhumation pathways than simple in-sequence (forelandward-propagating) deformation proposed for the Himalaya. In both Nepal and Arunachal Pradesh, shortening is initially accommodated on large displacement, low angle faults like the Main Central thrust, before transitioning to formation of the Lesser Himalayan duplex when the strata in front of the active ramp cools below ~375°C. Fast shortening rates (~20-45 mm/yr) are required during large displacement, low angle faulting at ~25-12 Ma. A distinct slowdown in shortening rates is observed after ~10 Ma, though the specific rates, detailed timing, and drivers of the slowdown differ along strike. Thermokinematic models highlight variations in the exhumation pathways that result from the different geometries and kinematics of each transect. Sections in western Nepal and Arunachal Pradesh require a complex kinematic sequence involving early foreland propagation of Lesser Himalayan faults to reproduce older (~15 Ma) zircon fission track and (U-Th)/He cooling ages in the more frontal portions of the fold-thrust belt and young (~5-10 Ma) muscovite 40Ar/39Ar and zircon fission track cooling ages in the hinterland. Reproducing these young cooling ages in the hinterland also requires out-of-sequence thrusting on formerly active, large displacement faults like the Main Central and Ramgarh-Munsiari thrusts to produce young (<5 Ma) uplift and exhumation in the north. However, the magnitude, timing, and faults that accommodate the out-of-sequence motion differ for each transect. Thermokinematic forward models for far western Nepal, western Nepal, and Arunachal Pradesh, India demonstrate that while components of deformation are uniform across the orogen, distinct along-strike variations in the size and position of active ramps, kinematic order of faults, and timing of fault motion, and thus the exhumation pathways, are evident. These differences are required to reproduce the local surface geology, basin accumulation, peak temperature conditions, and cooling ages.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
6 September 2023 |
| Date Type: |
Publication |
| Defense Date: |
25 May 2023 |
| Approval Date: |
6 September 2023 |
| Submission Date: |
1 August 2023 |
| Access Restriction: |
1 year -- Restrict access to University of Pittsburgh for a period of 1 year. |
| Number of Pages: |
322 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Dietrich School of Arts and Sciences > Geology and Environmental Science |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Himalaya, Nepal, northeast India, thermal-kinematic modeling, thermochronology, balanced cross-sections, continental collision |
| Date Deposited: |
06 Sep 2023 12:36 |
| Last Modified: |
06 Sep 2023 12:36 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/45210 |
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