The Yidun Terrane in the eastern Tibetan plateau is separated from the Songpan-Ganzi Terrane by the Ganzi-Litang suture zone to the east and Qiangtang Block by the Jinsha suture zone to the west. Both suture zones are marked by eastern Paleo-Tethyan ophiolites. The western part of the Yidun Terrane, the Zhongza Massif, is dominated by Paleozoic sedimentary sequences. In the eastern part, Triassic subduction-related plutonic rocks and volcanic-flysch successions of the Yidun Group are important elements for understanding the evolution of eastern Paleo-Tethys and amalgamation of East Asia. The Yidun Group includes the Lieyi, Qugasi, Tumugou and Lanashan Formations from base upwards. Two major depocenters for the Yidun Group can be recognized. Sedimentary detritus supplying for the northern depocenter were dominantly sourced from the Zhongza Massif and received recycling components in a passive margin setting. In the southern depocenter, the Qugasi Formation accumulated materials from the Zhongza Massif, whereas the Tumugou Formation received additional materials from locally distributed Triassic arc rocks and crystalline basement rocks, which indicates transition from a passive margin to a magmatic arc setting. In the southern Yidun Terrane, (quartz-) dioritic hypabyssal intrusions are spatially associated with andesites and dacites and have zircon U-Pb ages from ~230 to 215 Ma. They have adakitic geochemical features and are divided into the ~230-215 Ma high silica (HSA) and ~215 Ma low silica (LSA) adakitic rocks. The HSA formed from subducted slab melts with limited interaction with the overlying mantle wedge, whereas the LSA were generated from slab melts with more extensive interaction with mantle due to slab break-off at ~215 Ma. In the northern Yidun Terrane, granitic plutons and volcanic rocks occur in two parallel N-S belts. The ~228 Ma volcanic rocks in the Xiangcheng region are adakites generated from slab melts, whereas the ~231-230 Ma volcanic rocks in the Changtai region, including basalts, andesites, dacites and rhyolites, formed in a back-arc setting. The Changtai basalts were produced by low degrees of partial melting of an OIB-like mantle source with minor involvement of subducted slab components. The Changtai andesites/dacites represent evolved members from the basaltic magmas through an AFC process, whereas the rhyolites formed from anatexis of a garnet-bearing crustal source. These volcanic rocks are 4-6 mys older than arc granitic rocks in the northern Yidun Terrane. The spatio-temporal framework of all the subdution-related igneous rocks suggests initiation of subduction of the Ganzi-Litang oceanic lithophere under the southern Yidun Terrane at ~230 Ma, resulting in the adakitic magmatism in the Shangri-La and Xiangcheng regions and the back-arc magmatism in the Changtai region. Subsequently at ~224 Ma, the subduction extended to the northern Yidun Terrane, leading to the formation of the arc granitic plutons. From south to north, the Yidun Terrane was sequentially amalgamated with the Songpan-Ganzi Terrane during the Late Triassic. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

Thesis (Master of Science)--Miami University, Dept. of Geology, 2004. / Title from first page of PDF document. Document formatted into pages; contains [1], v, 76 p. : ill. Includes bibliographical references (p. 42-50).

New applications of the optically stimulated luminescence (OSL) dating were carried out with the aim of understanding late Quaternary activities for the Tibetan Plateau. This included studying the slip rate of the Altyn Tagh Fault, northeast Tibetan Plateau, and revealing the environmental changes derived from large inland lake’s evolution, central south Tibet. Two deflected streams across the Altyn Tagh Fault close to Aksay (39°24.572’N, 94°16.012’E) were investigated. Geomorphological analysis suggests that loess covering deflected stream banks has recorded past faulting events. A conceptual model is proposed illustrating the relationship. OSL dating of sixteen loess samples at both streams support the model, suggesting the loess is deposited episodically after fault strikes and subsequent channel wall refreshment. The age and offset indicate a slip rate of 11 ±2 mm/yr for this part of the Altyn Tagh Fault. Another river section near Aksay was also investigated for the slip rate information. Two risers between three terraces are clearly offset; OSL dating of loess covering terrace surfaces yielded terrace ages. Using the upper-terrace age to represent riser displacement duration, the rate is estimated to be 12 ±1 mm/yr. The result suggests that using upper terrace is more suitable in this region. Notably, though, the slow rate is at odds with proposals that assume high-speed extrusion (~23 mm/year) of the Tibetan Plateau being accommodated by the Altyn Tagh Fault. Palaeo-shorelines around the third largest lake in Tibet, Zhari Namco, were for the first time systematically investigated using OSL dating. Twenty-two sediment samples from eleven shorelines indicate that the water level has dropped ~128 m and the lake has undergone stepwise shrinkage since 8.2 ka. Digital elevation model calculation indicates the lake has shrunk from 4605 km2 in size at 8.2 ka to 996 km2 at present, which is equivalent to ~300 km3 of water. This implies a significant reduction in precipitation over the past 8.2 ka, a result of weakening Indian Monsoon or a shift of monsoon circulation path. The result is consistent with other lake-core, ice-core climate proxies and solar insolation changes, implying the dominance of a weakening Indian Monsoon over central Tibet in the Holocene. Using the elevation of the highest shoreline of the four largest lakes in Tibet, the early Holocene Pan-lake hypothesis is proposed for the central Tibet. In addition to these applications of OSL dating, technical studies on sensitivity changes and residual doses have been carried out for potassium rich feldspar (K-feldspar). Recent development of infrared stimulated luminescence (IRSL) signals from K-feldspar has shown great potential for extending the datable range for OSL dating. Sensitivity changes and residual doses of post-IR IRSL and multi-elevated -temperature post-IR IRSL protocols for K-feldspar were studied. A sensitivity decrease is observed after adopting a high temperature IRSL. IRSL signals stimulated at high temperature are found to contain large residual doses. The residual dose rises with stimulation time, suggesting that the initial part of IRSL signals contains more easy-to-bleach signals comparing with the later part. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy