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Tectonic Evolution of the Easternmost Himalayan Collisional System


The Cenozoic India-Asia collision generated the Tibetan Plateau and the Himalayan collisional system, the latter consisting of the convergence-perpendicular Himalayan orogen and the convergence-parallel Eastern and Western Flanking Belts located along the margins of India. Studying the evolution of each of these tectonic domains is critical to understanding the collision process and differentiating the end-member models of indenter-induced continental deformation. Despite this importance, there is a notable lack of geologic investigations on the development of the flanking belts in comparison to the extensive research of the Tibetan Plateau and east-trending Himalayan orogen. To address this problem, the research of this dissertation is focused on the Mesozoic-Cenozoic tectonic evolution of the northernmost segment of the Eastern Flanking Belt, the northern Indo-Burma Ranges, which are located directly east to southeast of the eastern Himalayan syntaxis. In the following chapters, I integrate the results of geologic field mapping, balanced cross section construction and restoration, U-Pb zircon geochronology, whole-rock geochemistry, thermobarometry, and (U-Th)/He zircon thermochronology to examine the litho-structural framework of the northern Indo-Burma Ranges and tectonic relationships in time and space with the adjacent eastern Himalayan orogen, the southern Tibetan Plateau, and the Eastern Flanking Belt.

The research of this dissertation shows that the study area exposes a southwest- to west-directed Cenozoic thrust belt cored by a hinterland-dipping duplex system. Thrust faults sole into a northeast- to east-dipping d�collement, which extends to >30 km depth. Southwestward forward propagation of the thrust belt in the foreland was coeval with out-of-sequence thrusting in the hinterland. This structural framework combined with the observed southward deflection in the trends of ductile stretching lineations within shear zones (northeast-trending in the north and east-trending in the south) suggest deformation around the eastern Himalayan syntaxis is best approximated by models of clockwise lithospheric flow accommodated by distributed thrusting.

Major lithologic units involved in the northern Indo-Burma thrust belt from south to north include the easternmost continuations of the Tertiary Sub-Himalayan Sequence, Proterozoic-Cambrian Lesser Himalayan Sequence, and Indus-Yarlung suture zone of the Himalayan orogen and the Mesozoic northern Gangdese batholith belt and Mesoproterozoic basement of the Lhasa terrane. However, several Himalayan-Tibetan lithologic units are missing, including the Paleoproterozoic-Ordovician Greater Himalayan Crystalline Complex, Proterozoic-Eocene Tethyan Himalayan Sequence, Mesozoic-Cenozoic Xigaze forearc basin, and Cenozoic igneous rocks of the southern Gangdese batholith. Research suggests that these units were present in the study area at the onset of the Cenozoic India-Asia collision and their present-day absence is related to an eastward increase in post-collisional crustal shortening and continental underthrusting along the Himalayan collisional system. This interpretation is supported by a Cenozoic shortening strain estimate of ~81% (>156 km) across the northern Indo-Burma Ranges and a dramatic southward decrease in the width of the collisional system from ~200 km across the Himalayan orogen to ~5 km across the study area.

Active deformation across the northern Indo-Burma Ranges and adjacent southeastern Tibetan Plateau is characterized by right-slip transpression partitioned between the range-bounding, oblique-slip Mishmi thrust in the southwest and right-slip Puqu and Parlung faults of Jiali fault zone in the northeast. The leading Mishmi thrust is kinematically-linked with the ~1000-km-long, right-slip Sagaing fault to the south via a previously-unmapped, southwest-trending restraining bend. This structural relationship of the Eastern Flanking Belt provides a key example of the spatial transition from transpressional deformation near the corner of an indenter to discrete right-slip motion along the side of an indenter during continental collision.

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