Theses and Dissertations at Montana State University (MSU)
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Item The kinematics and dynamics of rifting in south-central Tibet(Montana State University - Bozeman, College of Letters & Science, 2023) Reynolds, Elizabeth Aislin Nicole; Chairperson, Graduate Committee: Andrew K. Laskowski; This is a manuscript style paper that includes co-authored chapters.Southern Tibet is a unique location to study complex interactions between continental collision and extension, or stretching, of the Earth's crust which forms linear structures called rifts. The study of rifts is important because the rocks they expose can record thermal changes in the Earth's crust related to large-scale processes such as shifts in tectonic plates which occur over long timescales and are difficult to observe. Rifts also interact with topography, can influence river systems, and cause changes in rainfall distribution across a landscape by forming topographic drainage divides. Despite their importance, the kinematics and dynamics of rifting, or processes that occur during rift formation and evolution, are not well understood. This study uses field and radiometric dating techniques to investigate the shape, orientation, and timing of extension in southern Tibet by testing kinematic models for two classes of rifts: (1) Tibetan rifts which are defined as rifts that are >150 km in length and crosscut the Lhasa Terrane, and (2) Gangdese rifts that are defined as rifts <50 km long that are isolated within the high topography of the Gangdese Range. Evaluation of rift age across the Tangra Yumco rift and three Gangdese rifts suggests the TYC rift formed through the linkage of smaller normal fault segments into larger and longer structures over time, while Gangdese rifts may have relatively constant lengths. Additionally, interactions between rifts and contractional structures have likely influenced the evolution of topography and drainage patterns in southern Tibet for at least the past sixteen million years. To further investigate structural interactions, a broader compilation of thermochronology ages expands results to include another Tibetan rift, the Lunggar rift. Trends in the data reveal all samples from Gangdese rifts and Tibetan rifts that spatially overlap the Gangdese Range yield ages between ~28-16 Ma, whereas samples north of the Gangdese Range yield ages between ~12-4 Ma. I interpret these results to reflect Gangdese rift initiation at ~28 Ma in conjunction with, and perhaps balancing, uplift driven by the India-Asia collision, while young ages North of the Gangdese Range (~12-4 Ma) reflect extension along Tibetan rifts.Item Record of crustal thickening and synconvergent extension from the Dajiamang Tso Rift, southern Tibet(Montana State University - Bozeman, College of Letters & Science, 2021) Burke, William Brian; Chairperson, Graduate Committee: Andrew K. Laskowski; Andrew K. Laskowski, Devon A. Orme, Kurt E. Sundell, Michael H. Taylor, Xudong Guo and Lin Ding were co-authors of the article, 'Record of crustal thickening and synconvergent extension from the Dajiamang Tso Rift, southern Tibet' submitted to the journal 'MDPI geosciences -- special volumes' which is contained within this thesis.Gangdese Rifts such as the Dajiamang Tso Rift of south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present quantitative crustal thickness estimates calculated from Trace/Rare Earth Element zircon data paired with U-Pb geochronology and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso Rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70-42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter Thrust are ~28 Ma while Gangdese arc samples in the footwalls of the Dajiamang Tso Rift are 16-8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Asia collision onset (100-70 Ma). Following collision onset, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Modern crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E-W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by underthrusting Indian lithosphere balanced by upper plate extension.