Variation in vorticity of flow during exhumation of lower crustal rocks (Neoproterozoic Ambaji granulite, NW India)

The exhumation of the Neoproterozoic Ambaji granulite in the Aravalli-Delhi mobile belt, NW India, took place along NNW-SSE trending D2-shear zones. The shear zones evolved from a high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low temperature (450 °C) retrograde sinistral top-to-NW shearing event at the brittle-ductile-transition (BDT). The vorticity of flow (Wm) along the shear zones is estimated with the Rigid Grain Net and strain ratio/orientation techniques. The Wm estimates of 0.32–0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with Wm estimates of 0.64–0.87 and ~1.0 implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing. We interpreted that the high temperature event happened in a compressive tectonic regime which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during exhumation of granulite. This tectonic evolution is comparable with that of the Himalayas

Extension driven brittle exhumation of the lower-middle crustal rocks, a paleostress reconstruction of the Neoproterozoic Ambaji Granulite, NW India

The exhumation of the deep crustal rocks through brittle crust by extensional tectonics is recognized in orogens of all ages. Paleostress analysis are generally used to understand the brittle exhumation process. In this study, we reconstructed the paleostress of faults of the Neoproterozoic Ambaji Granulite, South Delhi Terrane of the Aravalli-Delhi Mobile Belt, NW India by analyzing the fault slip data in Win Tensor program. Several NE-SW and WNW-ESE faults have been mapped in the area and found to be normal faults with a few strike-slip faults. The strike-slip faults are pre-kinematic to normal faults. Tensor solutions for 237 fault slip data points estimate WNW-ESE extension for strike slip faults and NW-SE direction extension for normal faults. From these results, we interpret that the NE-SW striking, orogen-parallel normal faults were produced from a NW-SE directed extensional stress and are primarily responsible for brittle exhumation of the granulite through crustal extension and thinning at 764–650 Ma. This is comparable to earlier studies on brittle exhumation along the Southern Tibet detachment in the Higher Himalayas. On a more regional scale, our results are in agreement with the extensional tectonics that affected the entire Aravalli-Delhi Mobile Belt and adjoining continents of the Neoproterozoic-Cambrian Gondwanaland Supercontinent

Structural controls on bedrock weathering in crystalline basement terranes and its implications on groundwater resources

Abstract Crystalline basement rock aquifers underlie more than 20% of the earth’s surface. However, owing to an inadequate understanding of geological structures, it is challenging to locate the groundwater resources in crystalline hard rock terranes. In these terranes, faults, fractures, and shear zones play an important role in bedrock weathering and ultimately groundwater storage. This study integrates important geological structures with 2D high-resolution subsurface resistivity images in understanding the factors that influenced bedrock weathering and groundwater. The results reveal the variability of weathered zone depth in different structural zones (Zone-I to Zone-IV). This is due to the presence of foliations, fractures, and faults. A thicker weathered zone develops when a fracture/fault overprints a pre-existing planar pervasive structure like foliations (Zone-II) as compared to zones only with faults/fractures (Zone-III). Further, the transmissivity of boreholes also shows relatively higher in Zone-II than Zone-III, which implies a good pact between different structural features and possible groundwater storage. The study also demonstrates the role of paleostress and different tectonic structures influencing the depth of the “Critical Zone”. While the geology may vary for different structural terranes, the approach presented in this paper can be readily adopted in mapping bedrock weathering and groundwater resources in crystalline basement terranes globally

Recharge and Geochemical Evolution of Groundwater in Fractured Basement Aquifers (NW India): Insights from Environmental Isotopes (δ18O, δ2H, and 3H) and Hydrogeochemical Studies

Considering water as a limiting factor for socio-economic development, especially in arid/semi-arid regions, both scientific communities and policymakers are interested in groundwater recharge-related data. India is fast moving toward a crisis of groundwater due to intense abstraction and contamination. There is a lack of understanding regarding the occurrence, movement, and behaviors of groundwater in a fractured basement terrane. Therefore, integrated environmental isotopes (δ18O, δ2H, and 3H) and hydrogeochemical studies have been used to understand the recharge processes and geochemical evolution of groundwater in the fractured basement terranes of Gujarat, NW India. Our results show that the relative abundance of major cations and anions in the study basin are Ca2+ > Na+ > Mg2+ > K+ and HCO3− > Cl− > SO42− > NO3−, respectively. This suggests that the chemical weathering of silicate minerals influences the groundwater chemistry in the aquifer system. A change in hydrochemical facies from Ca-HCO3 to Na-Mg-Ca-Cl. HCO3 has been identified from the recharge to discharge areas. Along the groundwater flow direction, the presence of chemical constituents with different concentrations demonstrates that the various geochemical mechanisms are responsible for this geochemical evolution. Furthermore, the chemical composition of groundwater also reflects that the groundwater has interacted with distinct rock types (granites/granulites). The stable isotopes (δ18O and δ2H) of groundwater reveal that the local precipitation is the main source of recharge. However, the groundwater recharge is affected by the evaporation process due to different geological conditions irrespective of topographical differences in the study area. The tritium (3H) content of groundwater suggests that the aquifer is mainly recharged by modern rainfall events. Thus, in semi-arid regions, the geology, weathering, and geologic structures have a significant role in bringing chemical changes in groundwater and smoothening the recharge process. The findings of this study will prove vital for the decision-makers or policymakers to take appropriate measures to design water budgets as well as water management plans more sustainably

Implication of Dynamic Recrystallization Mechanism for the Exhumation of Lower Crustal Rocks:A Case Study in the Shear Zones of the Ambaji Granulite, NW India

Shear zones are important channels for the exhumation of lower crustal rocks. The Ambaji granulite of the Aravalli-Delhi mobile belt (ADMB) has been exhumed along several shear zones, and earlier studies have shown a two-stage exhumation process during a continuous compressional tectonic event, consisting of an initial phase of vertical flow that brought the granulites to the brittleductile transition zone and a successive phase during which the granulite underwent a lateral flow. In this contribution, we studied the microtectonics of granulites by analyzing the dynamic recrystallization behaviour of quartz, while the granulite was passing through the vertical flow regime to the horizontal flow regime. We show that the dynamic recrystallization process assists the flow pattern at different levels of exhumation. The vertical flow is dominated by grain boundary migration (GBM), registering high temperatures for recrystallization between 490 and 600°C and low flow stresses of 12-15 MPa. The horizontal flow at the brittle-ductile transition (BDT) is characterized by bulging (BLG) and subgrain rotation (SGR), which occurred at low temperatures of 390-490°C and high flow stresses of 18-26 MPa. Strain rates are between 1:20 × 10−12 and 7:26 × 10−14/s. For the ductile exhumation of the granulite, we suggest that at depths of ~22 km, the granulite exhumed in a vertical direction facilitated by GBM. Once the granulite reached the BDT, at ~16 km depth, the material flowed laterally assisted by BLG and SGR. Once an exhuming body reaches the BDT, the deformation mechanism changes to BLG-SGR, and the only direction in which the material can move further is in the horizontal plan

A review on deformation structures of different terranes in the Precambrian Aravalli-Delhi Mobile Belt (ADMB), NW India:Tectonic implications and global correlation

The Aravalli-Delhi Mobile Belt (ADMB) in the northwestern part of the Indian Shield represents the final stage of a complex tectonic evolution witnessed by the recognition of three distinct orogenies that have resulted in northwestward accretion of the terranes belonging to Archaean to Neoproterozoic ages. In this contribution, a review of the deformation structures of different terranes is discussed with their tectonic implications and global correlation with other supercontinent assemblies. In the west, the NE-SW trending Neoproterozoic South Delhi terrane is marked by coaxial folding between DF1 and DF2 along the NE-SW axis and cross folded by DF3 folds in the NW-SE axis. Several meso- to large-scale DF2 thrusts and DF4 fractures occur in the belt, that acted as channels for the exhumation of granulite and basement gneisses. Excess shortening led to orogen parallel extension and lateral escape of the material that reactivated the DF2 thrusts as strike-slip faults. Based on the ages of syn-DF1 granite gneisses, DF4 fractures, the South Delhi orogeny has been constrained between 0.87 and 0.6 Ga. The Paleoproterozoic North Delhi Terrane is marked by a coaxial folding between NF1 and NF2 folds and later cross-folded by NW-SE trending NF3 folds, producing dome- and basin-structures. Age of syntectonic granite and late-stage metamorphism constrain the north Delhi orogeny between 1.8 and 0.96 Ga. The Paleoproterozoic Aravalli Terrane is divided into a shallow-marine eastern and deeper marine western part by the Rakhabdev suture zone. The entire assemblage of terranes was folded by NE-SW isoclinal and recumbent AF1 folds which, with progressive deformation, were reoriented with a E-W axial trend. The AF2 is upright and NE-SW trending. The AF3 folds are E-W to NW-SE trending and have produced type 1 and type 2 interference patterns, with AF2 and AF1 respectively. Age of syn-AF1 migmatisation in the northern part and syn-AF3 granites in the south constrain the Aravalli orogeny between 1.7 and 0.96 Ga, coeval with the North Delhi orogeny. The granulite and charnockite were tectonically emplaced within the Sandmata Complex during the Aravalli orogeny. The Archean Bhilwara terrane, produced from the Bhilwara orogeny, marks the stabilisation of the crust in NW India by the intrusion of Berach and equivalent granites at 2.6 Ga. The terrane is divided into the Sandmata and Mangalwar complexes that consist of migmatite gneisses with slivers of greenstone. Several Neoarchean to Paleoproterozoic volcano-sedimentary schist belts were tectonically interlaced within the Mangalwar Complex. The migmatitic rocks of the terrane show flow folding in various directions while the schist belts are characterized by extremely appressed NE-SW trending reclined folds (BF1 and BF2), inverted BF2 folds, E-W open BF3 folds, and multiple strike-slip shear zones and thrusts. The ADMB exhibits a syntaxial bend in the eastern part attributed to indentation tectonics by Berach granite during syn-South Delhi orogeny. The Aravalli orogeny can be correlatable with Nuna orogeny, whereas the South Delhi orogeny can be correlated with the Pan-African orogeny that gave rise to Columbia and Gondwanaland Supercontinent assembly. The Grenville orogeny has experienced thermal rejuvenation in the Aravalli and Bhilwara terranes