Structural And Thermal Evolution Of The Himalayan Thrust Belt In Midwestern Nepal

"Spanning eight kilometers of topographic relief, the Himalayan fold-thrust belt in Nepal has accommodated more than 700 km of Cenozoic convergence between the Indian subcontinent and Asia. Rapid tectonic shortening and erosion in a monsoonal climate have exhumed greenschist to upper amphibolite facies rocks along with unmetamorphosed rocks, including a 5-6-km-thick Cenozoic foreland basin sequence. This Special Paper presents new geochronology, multisystem thermochronology, structural geology, and geological mapping of an approximately 37,000 km2 region in midwestern and western Nepal. This work informs enduring Himalayan debates, including how and where to map the Main Central thrust, the geometry of the seismically active basal Himalayan detachment, processes of tectonic shortening in the context of postcollisional India-Asia convergence, and long-term geodynamics of the orogenic wedge"--Publisher's website

The Himalaya is well known as the largest and highest mountain belt on Earth. Advances in geoscience over the past few decades have revealed a complex picture of the dynamics of this giant, opening up questions about the initial stages of Himalayan building, lateral variations in its structures, variations in tectonic forcing, tectonic-climate coupling and assessments of the natural hazards affecting this area. In this three-volume book, we present the current knowledge on the building and present-day behavior of the Himalayan range. The objective is not to be exhaustive, but to provide some key elements used by researchers to unravel the many processes acting in the Himalayan dynamics. Mountain environments are at the forefront of climate change with glacier retreat, landslides, flash floods and water availability. Understanding the delicate balance that controls the dynamics of the Himalayan giant is now, more than ever, a major challenge for the scientific community.

A synthesis of current knowledge on collisional and convergent plate boundaries worldwide Major mountain belts on Earth, such as the Alps, Himalayas, and Appalachians, have been built by compressional tectonic processes during continent-continent and arc-continent collisions. Understanding their formation and evolution is important because of the hazards associated with convergent and collisional plate boundaries, and because these mountain belts contain resources such as precious metals, rare earth elements, oil, gas, and coal. Compressional Tectonics: Plate Convergence to Mountain Building reviews our present-day knowledge of the tectonic evolution of the Alpine-Himalayan and Appalachian belts. Volume highlights include: overview of terminology relating to compressional and contractional tectonics discussion of subduction zone dynamics debates over the timing of the collision and convergence of particular subduction and suture zones examples of the different stages in the development of orogenic belts This book is one of a set of three Tectonic Processes: A Global View The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.

Himalayan foreland basin strata were recently documented to crop out in a structural window in the central portion of the Himalayan thrust belt. Presently, structural interpretations of the thrust belt do not explain in detail how these strata were incorporated into the thrust belt and why they are not widely exposed throughout the Himalaya. My research provides answers to these two issues. The structural window is located in the Lesser Himalaya of western Nepal and exposes rocks which lie structurally beneath the Main Central thrust (MCT) and Ramgarh thrust (RT) sheets. The thrust sheet consists of Proterozoic metamorphic rocks. Below the thrust sheet, footwall rocks exposed in the window are unmetamorphosed sedimentary rocks which consist of the early foreland basin strata (the Suntar and Swat formations) and the pre-foreland basin strata (the Melpani and Lakharpata formations). These unmetamorphosed rocks are present in the foreland basin beneath Siwalik group, ~100km to the south (towards the foreland). Two geologic maps were created; One covering the structural window and another covering the thrust belt from the High Himalaya to the MFT (Main Frontal Thrust). Several cross-sections were constructed from both maps. Structural reconstruction of these cross-sections reveal the following: 1) ~ 75 km-long hanging wall flat extends northward from its surface trace to the southern margin of the Lesser Himalayan duplex; 2) The geometry of the Jarjarkot klippe is narrower and structurally deeper than klippe to the west. The geometry of the northern flank of the klippe results from stacking of duplex horses, while the geometry of the southern flank results from slip over a ramp in the footwall of the MBT, 3) The early foreland basin strata in the window are modeled to have originated at the front of the thrust belt, and subsequently buried by the Ramgarh and MCT thrust sheets, and 4) Exposure of these strata results from growth of the duplex which brought them to a higher structural level than most parts of the thrust belt. This structural model explains why the only other exposure of foreland basin strata within the thrust belt is documented within a duplex on the north side of the Dadeldhura klippe. Moreover, this model predicts that foreland basin strata are likely to be exposed wherever duplexes exist.

Over the last two decades, several competing dynamic models have been proposed to explain the kinematics of the Himalayan thrust belt. The accuracy of dynamic and kinematic models is limited by poorly documented geologic structures. With increased accessibility of the thrust belt and advances in analytical techniques, several new data sets greatly improve our understanding and provide a background to reevaluate the kinematics of the Himalayan thrust belt. In this dissertation, I integrate structural mapping, microstructural analysis, detrital and igneous zircon geochronology, low-temperature thermochronology, Nd isotopic analysis, and structural reconstructions in central Nepal to determine the evolution of the Himalayan thrust belt. Because the role and evolution of the Main Central thrust, the Ramgarh-Munsiari thrust, and the Lesser Himalayan duplex are highly debated, I emphasize these systems to provide a comprehensive structural evolution of the Himalayan thrust belt. U-Pb dating of metamorphic rims of igneous zircons and crystallization ages of cross-cutting pegmatite veins suggest that deformation on the Himalayan thrust belt started with slip on an intra-Greater Himalayan thrust active at ~20-29 Ma that emplaced the now erosionally isolated Kathmandu klippe. These ages predate the slip on the Main Central thrust. Absence of a fault contact between the Greater Himalaya and Tethyan Himalaya in the klippe suggests the South Tibetan Detachment system may have activated after the slip started on the intra-Greater Himalayan thrust. Ductile motion on the South Tibetan Detachment system may have ended prior to the activation of the Main Central thrust. This result and observations contradict the extrusion model that advocates contemporaneous activity with thrust sense shear on the Main Central thrust and normal sense shear on the South Tibetan Detachment system. In addition, there is another orogenic scale thrust, subparallel to the Main Central thrust, the Ramgarh-Munsiari thrust, that only carries lower Lesser Himalayan Paleoproterozoic rock over other Lesser Himalayan rock and accommodates a magnitude of shortening similar to the 100's km of slip on the Main Central thrust. I construct an orogenic scale balanced cross-section along the Marsyangdi River where the entire Lesser Himalayan duplex is exposed, particularly focusing on the architecture of the duplex to determine whether the duplex is forward dipping or hinterland dipping and the presence/absence of an orogenic scale, out-of-sequence thrust. I integrate quartz-feldspar deformation temperatures and zircon (U-Th)/He thermochronology and present a kinematic model that provides the structural context for geophysical, petrological, and geochronological studies in central Nepal. Collectively, this study helps to determine partitioning of strain among the various thrust sheets that account for over 2000 km of shortening in a compressional continental tectonic setting. The results suggest that deformation in the Himalaya began with the activation of an intra-Greater Himalayan thrust and successively moved south with the activation of Main Central thrust, Ramgarh-Munsairi thrust, Lesser Himalayan duplex, and finally the Subhimalayan thrust system. Although there was minor out-of-sequence thrusting in the hinterland, the bulk of the Himalaya evolved in-sequence thrusting from north to south.

Indian Geological Sequences: Salient Features and Major Events focuses on the first ever differentiation of the entire Indian record into a five tier hierarchical succession of geological sequences – five giga to 12 mega to 25 1st order to 68 2nd order to over 100 3rd order ones as its basic skeleton. Using the developed grid of sequence timelines, the diverse inter-disciplinary geological manifestations on the broad tectono-stratigraphically homogeneous supra-region of GTM (Gondwanian Tethyan Margin from Arabia to Australia) have been mutually integrated towards chronicling of events with precision never ever realized earlier. The giga-sequences GS-I to GS-IV comprising eight mega-sequences MS-I to MS-VIII deal with the Precambrian in brief. GS-V (¬ 635 ma onward) MS-IX (¬ 635-444 ma) and MS-XII (¬ 61.6 onward) include orogenies while MS-X (¬ 444-259 ma) and MS-XI (¬ 259-61.6 ma) are dominated by dismemberment tectonics. The prime focus is on the correlation of events across scores of sedimentary basins from outcrop to subsurface, onshore to offshore, marine to non-marine, shallow to deep water, plant to animal, micro to macro-fossils, and Proto-Paleo-Neotethys to Indian Ocean. Among the major Phanerozoic events time precised are the ¬ 500 ma Acantha Zone mega MFS accretion of the then alien TH to the Indian margin, and the ¬ 50 ma P8 Zone mega MFS impingement of India on Asia while the important dismemberment events include the ¬ 159 ma late Middle Oxfordian Orientalis Zone Schilli Subzone 1st order MFS initiation of the Indian Ocean which culminated in steps with oceanic separation of Sri Lanka from Antarctica at the ¬ 107 ma early Middle Albian Dentatus Zone mega MFS, ¬ 90 ma 1st order SB initiation of separation of Madagascar so also of Mascaranes basin, NER, Central Indian basin, Wharton basin, separation of Greater Seychelles from India at the ¬ 64.5 ma intra Danian 2nd order Quadratus Zone MFS and ¬ 24 ma Complanata Zone MFS thrusting due south of Greater Himalaya upon Lesser Himalaya. Indian Geological Sequences: Salient Features and Major Events is a valuable reference for researchers and scientists in the field of Earth Sciences. Relates multi-dimensional geological events of one region to another in a vast supra-region through precisely dated sequence timelines Links macro- and micro-evolutionary advent and extinction events to macro- and micro-geological events Includes multidisciplinary data sources, from sedimentological, geochemical, and geophysical records

The construction of orogenic systems involves a complex interaction between numerous factors, including, but not limited to, the lithology and rheology of the rocks involved in deformation, the pressure and temperature conditions that they were buried to and exhumed from, and the structural style of their deformation and emplacement. Therefore, the relative influence and interplay of each of these factors provides critical information for analysis of how an orogenic belt has evolved through time. It follows that to fully understand the complex deformation mechanisms that contribute to strain accommodation in orogenic systems, no single data set is sufficient. Instead, an integrated series of complementary data sets, such as those that describe metamorphic (i.e., peak and deformation temperatures, peak pressure, and mineral assemblage) and structural conditions (i.e., structural mapping, finite strain data, fabric analysis, shear sense data) are necessary. In addition, to understand the lateral variability within an orogenic system, data sets distributed along-strike must be compared to provide a more complete picture of how a mountain belt evolves in space and time. As a classic locality for understanding the dynamics of continental convergence, the Himalayan-Tibetan orogen provides a natural laboratory to evaluate the interplay between metamorphic and structural processes, and how they affect the overall thermal and structural architecture of a mountain belt.The Himalayan-Tibetan orogenic system has formed from the collision and continued convergence of the Indian and Asian continental plates. Initial collision occurred at ~50-55 Ma, and the Himalayan thrust belt, which makes up the southern portion of the orogen (Fig. 1), began accommodating shortening at ~25 Ma (e.g., Yin and Harrison, 2000; Yin, 2006; Najman et al., 2006, Najman et al., 2012). Despite being studied for decades, several key debates exist regarding the basic geometric framework and kinematic development of the Himalayan thrust belt. Additionally, the >2,000 km length of the thrust belt suggests that lateral variability is inevitable. This work will utilize a suite of complementary analytical and structural techniques in order to evaluate the geometric, kinematic, and thermal framework of important segments of the Himalayan thrust belt in the kingdom of Bhutan.