STRUCTURAL ANATOMY OF THE MUNSIARI AND VAIKRITA THRUST ZONES IN THE GARHWAL HIMALAYA AND THE PROTOLITH GEOCHRONOLOGY

Abstract

The Himalayas, the world’s loftiest and youngest orogenic belt, epitomize an active convergent tectonic system. During the Early Eocene, continent-continent collision between the Indian and Asian Plates developed a series of regional fault systems, segmenting the Himalayas into different litho-tectonic units. Moving northward from the Indo-Gangetic Plains, these units include the Sub-Himalayan Sequence, Lesser Himalayan Sequence (LHS), Greater Himalayan Sequence (GHS), and Tethyan Himalayan Sequence. From south to north, these are separated by the Main Frontal Thrust, Main Boundary Thrust, Main Central Thrust (MCT), and South Tibetan Detachment System, respectively. This thesis focuses on the MCT, which serves as the terrane boundary between the LHS and GHS. The MCT is a key yet debated thrust system due to its spatio-temporal variations, multiplicity in nomenclature, and diverse identification criteria. In the Garhwal Himalayas, the Munsiari Thrust (MT) and the Vaikrita Thrust (VT) have been interchangeably referred to as the MCT in the south and north, respectively. However, the geochronologic and isotopic lines of evidence distinguish the MT as an intra-LHS thrust and the VT as the MCT. The MT separates the late Paleoproterozoic Berinag and Munsiari groups within the LHS. In contrast, the VT juxtaposes the Neoproterozoic Vaikrita Group of the GHS against the Munsiari Group of the LHS; thus, the VT corresponds to the MCT in this thesis. The Berinag Group consists of ubiquitous volcano-sedimentary sequences comprising quartzite interlayered with chlorite schist, slate, carbonate, and amphibolite. The Munsiari Group comprises a mylonitized package of augen gneiss, psammitic gneiss, garnet-kyanite mica schist, meta-volcanic, and calc-silicate gneiss. The Vaikrita Group primarily contains sillimanite-kyanite-garnet schist/gneiss, augen gneiss, psammitic gneiss, calc-silicate gneiss, amphibolite, pegmatite dyke, and tourmaline-garnet leucogranite. Despite extensive research, significant knowledge gaps persist in understanding the structural architecture, kinematic evolution, and paleo-tectonic framework of the MT and VT shear zones. Specifically, the existing field identification criteria for the VT are inconsistent and ambiguous. Furthermore, the chronological evolution of these shear zones and their relationship to pre-Himalayan tectonism are poorly constrained. As both thrusts are indispensable in deciphering the geodynamic evolution of the Himalayas, this research aims to address these issues using an integrated approach involving shear zone architecture, large-scale mapping, ii | P a g e structural analysis, and U-Pb zircon geochronology. The study area includes the Alaknanda, Dhauliganga, and Bhagirathi river valleys of the Garhwal Himalayas, which provide accessible and complete sections of the LHS and GHS. Structural analysis along these valleys reveals that the MT and VT run parallel to the mylonite foliations (Sm1/2) in the footwall and hanging wall rocks, which share similar fabric orientations. The modal Sm1/2 consistently dips ~35–40° northeast, and stretching/mineral lineations (Ln) plunge down-dip across these thrusts. Therefore, no significant structural discordance exists across the moderately northeast dipping (~35–45°) thrusts. The MT is a ~3.0–3.2 km, and the VT is a ~1.2–8.5 km thick shear zone, comprising tectonic mosaics of mappable high-strain cores and low- to moderate-strain damage zones across the three valleys. In the Alaknanda Valley, the MT shear zone comprises the Gulabkoti and Helang–Lyari–Paini cores with the Gulabkoti–Helang damage zone. In the Bhagirathi Valley, the Sainj core flanked by the Maneri–Sainj and Sainj–Saura damage zones constitute the MT shear zone. The VT shear zone features the Sailang core and associated Paini–Sailang and Sailang–Joshimath damage zones in the Alaknanda Valley; the Tapovan core and Tapovan damage zone in the Dhauliganga Valley; and the Helgu and Bhangeli cores with the Bhangeli–Gangnani damage zone in the Bhagirathi Valley. The shear zone cores are characterized by ductilely sheared mylonite/phyllonite, L-S tectonite, and sheath folds. The damage zones enveloping the cores comprise brittle and brittle-ductile structures such as en-echelon fractures, sigmoidal veins, and fault gouge. A decrease in quartz grain size and area, S-C fabric dihedral angle, and an increase in the strain ratio (Rs) indicate a progressive increase in strain from damage zones to cores. Geological mapping reveals that the Berinag and Vaikrita Group rocks, sharing a common top-to-the-SW progressive ductile shearing, are tectonically interleaved with the Munsiari Group rocks within the MT and VT shear zones. At least three successive fold groups (F1, F2, and F3) and two axial plane mylonite foliations (Sm1 and Sm2) were developed, and subsequently, the early structures were transposed during the repeated shearing events. The transposed fold limbs are now preserved as the mappable tectonic imbricates and horses within the shear zone cores. The MT consistently outcrops as a sharp tectonic dislocation that defines a distinct litho-tectonic boundary in all three investigated valleys. In contrast, the VT exhibits varied rheological characteristics across these valleys. It manifests as a ductile shear zone in the Bhagirathi Valley but as a brittle-fault zone in the Dhauliganga Valley. The VT lacks any distinct surface expression in the Alaknanda Valley. In areas where the VT cannot be identified on the outcrop, such as the Alaknanda Valley, it is routinely marked by the `kyanite-in isograd´ criterion. This criterion is iii | P a g e based on the assumption that the first appearance of Barrovian kyanite marks the beginning of the GHS and, thus, marks the VT. This thesis reports the undoubted occurrences of the Barrovian kyanite within the LHS rocks and, hence, negates the tenability of the `kyanite-in isograd´ criterion for delineating the VT. It postulates delineating the VT by identifying a high-strain anomalous zone in the strain profile, from the footwall to the hanging wall. In the Alaknanda Valley, despite post-deformational strain recovery, the Sailang core, accommodating a higher Rs value of 3.14 than the strain in the surrounding damage zones, represents the VT. U-Pb zircon geochronology elucidates the chronological framework of the shear zone rocks. Previously published maximum depositional ages (MDAs) of the Berinag Group, based on various statistical methods such as the youngest single grain age and youngest graphical peak age, vary between ~1.95–1.75 Ga and fail to reflect the unique true depositional age (TDA). This study constrains the TDA of the Berinag Group quartzite to 1879 ± 16 Ma, derived from the crystallization age of interlayered meta-volcanic chlorite schist, which penecontemporaneously occurs with the quartzite beds. Amongst others, the youngest grain cluster at 2σ (YGC2σ) method best estimates the MDA of the Berinag quartzite at 1871.3 ± 5.3 Ma, closely aligning with its TDA. Additionally, the TDA is consistent with the average crystallization age of 1858 ± 15 Ma for all orthogneiss reported in the proximal Munsiari Group and its equivalents. This thesis infers that the active northern Indian margin experienced a rapid and synchronous episode of mafic volcanism and sedimentation in a rifted back-arc basin setup during ~1.87–1.86 Ga, which is now represented by the volcano-sedimentary sequences of the Berinag Group. At the same time, felsic plutonism occurred in the proximal northern magmatic arc, now represented by the Munsiari Group. Similar magmatic and detrital ages, along with a high aspect ratio (~2.9) and low roundness (~0.37) of zircons in the quartzite, reveal the detritus in the basin was predominantly derived from a nearby northern magmatic arc. In the lower reaches of the Dhauliganga Valley, the Vaikrita Group rocks in the proximal hanging wall damage zone of the VT exhibit complex intrusive relationships. Psammitic gneiss is intruded by megacrystic granite gneiss and aplite gneiss, which are further intruded by leucogranite. These rocks display moderately northeast-dipping Sm1/2 with scattered northeast-plunging Ln, along with evidence of top-to-the-SW ductile shearing along the VT. U-Pb zircon geochronology reveals that the Neoproterozoic psammitic gneiss records the MDA at 1093 ± 2 Ma. The megacrystic granite gneiss and leucogranite preserve the early Paleozoic magmatic/metamorphic ages at 507 ± 2.7 Ma, 483 ± 9.5 Ma, and 482 ± 1.4 Ma, alongside inherited Neoproterozoic zircon cores. The leucogranite and aplite gneiss also reveal the Tertiary iv | P a g e tectonic imprints at 25 ± 1.5 Ma, 21 ± 0.97 Ma, and 16 ± 0.91 Ma, along with Cambro-Ordovician and Neoproterozoic inherited zircon cores. The Vaikrita Group rocks in the VT shear zone, thus, reflect the imprints of the Cambro–Ordovician Kurgiakh Orogeny, overprinted by Late Oligocene to Middle Miocene Himalayan Orogeny. Combined with the structural evidence, these findings suggest that the VT originated as an early Paleozoic proto-tectonic boundary and was remobilized during the Himalayan tectonism. This thesis provides valuable insights into the structural and geochronological evolution of the Munsiari and Vaikrita/Main Central thrusts in the Garhwal Himalayas. It highlights the significance of the shear zone framework approach utilizing the core-damage zone architecture and identifying the high-strain anomalies in understanding the tectonic and kinematic evolution of thrusts. The geochronological results constrain the ages of the pre-Himalayan and Himalayan tectonic events. This integrated approach can serve as a model for investigating the geodynamic evolution of the other parts of the Himalayas and other orogenic systems worldwide.