C. S. Dubey

Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the eastern Himalaya

Despite being the largest active collisional orogen on Earth, the growth mechanism of the Himalaya remains uncertain. Current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the Himalayan and Tibetan crust during Cenozoic India-Asia collision. A major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the Himalayas from 91°E to 97°E, which makes up about one-quarter of the mountain belt. To address this issue, we conducted detailed field mapping, U-Pb zircon age dating, and 40Ar/39Ar thermochronology along two geologic traverses at longitudes of 92°E and 94°E across the eastern Himalaya. Our dating indicates the region experienced magmatic events at 1745–1760 Ma, 825–878 Ma, 480–520 Ma, and 28–20 Ma. The first three events also occurred in the northeastern Indian craton, while the last is unique to the Himalaya. Correlation of magmatic events and age-equivalent lithologic units suggests that the eastern segment of the Himalaya was constructed in situ by basement-involved thrusting, which is inconsistent with the hypothesis of high-grade Himalaya rocks derived from Tibet via channel flow. The Main Central thrust in the eastern Himalaya forms the roof of a major thrust duplex; its northern part was initiated at ca. 13 Ma, while the southern part was initiated at ca. 10 Ma, as indicated by 40Ar/39Ar thermochronometry. Crustal thickening of the Main Central thrust hanging wall was expressed by discrete ductile thrusting between 12 Ma and 7 Ma, overlapping in time with motion on the Main Central thrust below. Restoration of two possible geologic cross sections from one of our geologic traverses, where one assumes the existence of pre-Cenozoic deformation below the Himalaya and the other assumes flat-lying strata prior to the India-Asia collision, leads to estimated shortening of 775 km (∼76% strain) and 515 km (∼70% strain), respectively. We favor the presence of significant basement topography below the eastern Himalaya based on projections of early Paleozoic structures from the Shillong Plateau (i.e., the Central Shillong thrust) located ∼50 km south of our study area. Since northeastern India and possibly the eastern Himalaya both experienced early Paleozoic contraction, the estimated shortening from this study may have resulted from a combined effect of early Paleozoic and Cenozoic deformation.

Geologic correlation of the Himalayan orogen and Indian craton: Part 1. Structural geology, U-Pb zircon geochronology, and tectonic evolution of the Shillong Plateau and its neighboring regions in NE India

The Himalayan orogen has experienced intense Cenozoic deformation and widespread metamorphism, making it diffi cult to track its initial architecture and the subsequent deformation path during the Cenozoic India-Asia collision. To address this issue, we conducted structural mapping and U-Pb zircon geochronology across the Shillong Plateau, Mikir Hills, and Brahmaputra River Valley of northeastern India, located 30‐100 km south of the eastern Himalaya. Our work reveals three episodes of igneous activity at ca. 1600 Ma, ca. 1100 Ma, and ca. 500 Ma, and three ductile-deformation events at ca. 1100 Ma, 520‐500 Ma, and during the Cretaceous. The fi rst two events were contractional, possibly induced by assembly of Rodinia and Eastern Gondwana, while the last event was extensional, possibly related to breakup of Gondwana. Because of its prox imity to the Himalaya, the occurrence of 500 Ma contractional deformation in northeastern India implies that any attempt to determine the magnitude of Cenozoic deformation across the Himalayan orogen using Proterozoic strata as marker beds must fi rst remove the effect of early Paleozoic deformation. The lithostratigraphy of the Shillong Plateau established by this study and its correlation to the Himalayan units imply that the Greater Himalayan Crystalline Complex may be a tectonic mixture of Indian crystalline basement, its Proterozoic-Cambrian cover sequence, and an early Paleozoic arc. Although the Shillong Plateau may be regarded as a rigid block in the Cenozoic, our work demonstrates that distributed active left-slip faulting dominates its interior, consistent with earthquake focal mechanisms and global positioning system velocity fi elds across the region.

U-Pb zircon geochronology of major lithologic units in the eastern Himalaya: Implications for the origin and assembly of Himalayan rocks

Models for the origin and deformation of Himalayan rocks are dependent upon geometric and age relationships between major units. We present field mapping and U-Pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern Himalaya, revealing that: (1) the South Tibet detachment along the Bhutan-China border is a top-to-the-north ductile shear zone; (2) Late Triassic and Early Cretaceous sedimentary samples from the northern Indian margin show a similar age range of detrital zircons from ca. 3500 Ma to ca. 200 Ma, but the Late Triassic rocks are distinguished by a significant age cluster between ca. 280 and ca. 220 Ma and a well-defined age peak at ca. 570 Ma, (3) an augen gneiss in the South Tibet detachment shear zone in southeast Tibet has a Cambrian–Ordovician crystallization age, (4) Main Central thrust hanging-wall paragneiss and footwall quartzites from the far western Arunachal Himalaya share similar provenance and Late Proterozoic maximum depositional ages, and (5) Main Central thrust footwall metagraywacke from the central western Arunachal Himalaya has a Paleoproterozoic maximum depositional age, indicated by a single prominent age peak of ca. 1780 Ma. Recent work in the eastern Himalaya demonstrates that in the early-middle Miocene, the Himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south. A proposed subsequent (middle Miocene) brittle low-angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here. The ca. 280–220 Ma detrital zircons of the Late Triassic strata are derived from an arc developed along the northern margin of the Lhasa terrane. Detritus from this arc was deposited on the northern margin of India during India-Lhasa rifting. Along-strike heterogeneity in Main Central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western Arunachal. Nonetheless, the Late Proterozoic rocks in the Main Central thrust hanging wall and footwall in far western Arunachal can be correlated to each other, and to previously analyzed rocks in the South Tibet detachment hanging wall to the west and in the Indian craton to the south. These findings are synthesized in a reconstruction showing Late Triassic India-Lhasa rifting and Cenozoic eastern Himalayan construction via in situ thrusting of basement and cover sequences along the north Indian margin.

Visualization of 3-D digital elevation model for landslide assessment and prediction in mountainous terrain: A case study of Chandmari landslide, Sikkim, eastern Himalayas

Techniques for recognizing and mapping of landslides are complex in mountainous terrains. Most of the methods applied to landslide identification and prediction involves assignment of different contributing factors in landslide hazard zonation; however, it is difficult to observe the main causes of landslides. 3-D digital elevation modeling capabilities and Guided Visual Program (GVP) module of Datamine Software is utilized to successfully enumerate the various contributing factors for causing the landslide in Sikkim, Eastern Himalaya in the case study of Chandmari Landslide. A landslide warning system is discussed for the sitespecific Chandmari landslide area.

GIS-based morpho-tectonic studies of Alaknanda river basin: a precursor for hazard zonation

Abstract Alaknanda river basin is considered to be tectonically active where damaging earthquakes and landslides have occurred. The whole basin was divided into 8 sub-basins to carry out morphometric analyses, hypsometric integral (HI) analysis and valley floor width to valley height ratio (Vf) factor. The sub-basins 2 and 3 show that they are highly active, because of the higher values of bifurcation ratio, stream frequency asymmetric factor, and lower values of form factor, elongation ratio and circulatory ratio. In these areas, HI values are very low indicating that the landscape is highly eroded, deeply dissected and tectonically active. The result obtained from Vf was similar which classified both these basins as highly active. Morphometric analysis, HI and Vf analyses along with structural map of study area are used to prepare morpho-tectonic map classifying the whole area into very high, high, moderate and low zones of tectonic activity. This map clearly indicates that the areas near MCT II (Munsiari Thrust), MCT III (Ramgarh Thrust) and North Almora Thrust are tectonically very active which fall in sub-basins 2, 3, 4 and parts of 5. Various locations such as Chamoli, Birahi, Pipalkoti, Rudraprayag, etc. are situated in these zones where many earthquakes and landslides occur every year. Moreover, the data plotted for earthquakes and landslides occurrences are consistent with morpho-tectonic map and can be used as a precursor for demarcation of natural hazard vulnerable zones.

Monazite ages from carbonatites and high-grade assemblages along the Kambam Fault (Southern Granulite Terrane, South India)

Abstract Monazite ages from carbonatites and high-grade assemblages exposed along a significant lineament within the Southern Granulite Terrane of India termed the Kambam fault were obtained in thin section (in situ) using an ion microprobe. X-ray maps for Ce and Th were acquired in larger monazites to decipher the significance of the ages of individual spots within grains. The Kambam carbonatite contains large (millimeter-sized) apatite rimmed by ~10 μm thick bands of monazite. Monazite commonly appears as a lower-Th, late-stage mineral in carbonatites, and bands surrounding apatite are interpreted as products of metasomatism, rather than exsolution. The age of a Kambam carbonatite monazite band is 715 ± 42 Ma (Th-Pb, ± 1σ), but monazite filling cracks within the apatite is ~300 m.y. younger (405 ± 5 Ma). The younger monazite grains are in contact with quartz, a mineral thought to be an indicator of subsolidus alteration in carbonatites. The age of the monazite rim is similar to ages of several carbonatites located 50-400 km further north, and chemical analyses show that this sample displays chemical trends similar to the other complexes (e.g., Y/Ho, Ce/Pb, REE, and HFSE patterns). The mid-Neoproterozoic event is recorded in garnet-bearing assemblages ~20 km west of the Kambam fault (733 ± 15 Ma) and garnet-bearing enclaves within Southern Granulite Terrane charnockites (701 ± 26 Ma; 786 ± 84 Ma). The results show that monazite can crystallize during metasomatism and be useful in deciphering fluid processes occurring at deeper crustal levels. The Kambam fault, which records over 300 million years of monazite growth, should be considered a major boundary in reconstructions of Gondwana.

Geo-spatial Technology for Landslide Hazard Zonation and Prediction

Similar to other geo hazards, landslides cannot be avoided in mountainous terrain. It is the most common natural hazard in the mountain regions and can result in enormous damage to both property and life every year. Better understanding of the hazard will help people to live in harmony with the pristine nature. Since India has 15% of its land area prone to landslides, preparation of landslide susceptibility zo‐ nation (LSZ) maps for these areas is of utmost importance. These susceptibility zo‐ nation maps will give the areas that are prone to landslides and the safe areas, which in-turn help the administrators for safer planning and future development activities. There are various methods for the preparation of LSZ maps such as based on Fuzzy logic, Artificial Neural Network, Discriminant Analysis, Direct Mapping, Regression Analysis, Neuro-Fuzzy approach and other techniques. These different approaches apply different rating system and the weights, which are area and fac‐ tors dependent. Therefore, these weights and ratings play a vital role in the prepa‐ ration of susceptibility maps using any of the approach. However, one technique that gives very high accuracy in certain might not be applicable to other parts of the world due to change in various factors, weights and ratings. Hence, only one meth‐ od cannot be suggested to be applied in any other terrain. Therefore, an under‐ standing of these approaches, factors and weights needs to be enhanced so that their execution in Geographic Information System (GIS) environment could give better results and yield actual ground like scenarios for landslide susceptibility mapping. Hence, the available and applicable approaches are discussed in this chapter along with detailed account of the literature survey in the areas of LSZ mapping. Also a case study of Garhwal area where Support Vector Machine (SVM) technique is used for preparing LSZ is also given. These LSZ maps will also be an important input for preparing the risk assessment of LSZ.

Geochemical and Geochronological Data from Charnockites and Anorthosites from India's Kodaikanal-Palani Massif, Southern Granulite Terrain, India

The Kodaikanal–Palani Massif is an important component of India’s Southern Granulite Terrain; understanding the tectonic history of its rocks lends considerable insight into its role within South India. The massif is located south of the Palghat Cauvery Shear Zone (PCSZ). Compilations of available geochronologic and geochemical information from charnockites north and south of the PCSZ show these rocks largely differ in age, with northern samples recording Archaean crystallization events, whereas those to the south yielding Cambro-Ordovician and Neoproterozoic ages. The Kodaikanal–Palani charnockitic rocks contain monazite grains that fall within the Cambro-Ordovician timescale. The Oddanchatram anorthosite, located along the northern boundary of the Kodaikanal–Palani Massif, contains zircon grains that record mid-Neoproterozoic to Cambro-Ordovician crystallization ages. This anorthosite differs in texture and composition depending on location, that may be the result of its multi-stage metamorphic and/or intrusion history. Charnockitic rocks north and south of the PCSZ also differ geochemically. For example, north of the PCSZ, these rocks become more calcic with increasing SiO2 contents, whereas those to the south become alkali-calcic. Southern charnockitic rocks tend to have higher K2O/TiO2, Zr/SiO2, Rb/Sr, Ba and Rb contents, but lower Sr/Ba ratios. Using available geochemical data, we find more charnockitic rocks south of the PCSZ record zircon saturation temperatures between 800°C and 900°C than those to the north. Although samples of charnockitic rocks within the Kodaikanal–Palani Massif yield similar monazite ages, the rocks differ in their whole rock geochemistry and zircon and monazite saturation temperatures depending on location. The geochemical data from these rocks suggest that charnockitic rocks within the Kodaikanal–Palani Massif possibly experienced different mechanisms of generation and/or metamorphic histories.

Liquefaction potential evaluation for subsurface soil layers of Delhi region

The study area Delhi is second most populous city and third largest urban area in the world. Though the area lies in seismic high damage risk zone, number of high rise building and construction of mega structure at several sites of the city increase rapidly. In this study field Standard Penetration Test (SPT) values of soil collected from 750 boreholes data were analyzed to identify liquefiable sub-surface soil layers. Finally, liquefaction susceptible sub-surface maps of the region at various depth (20 m, 15 m, 12 m, 9 m, 6 m and 3 m) from ground level is prepared. The outcome of this study will be useful input for preliminary foundation and designing of earthquake resistant high rise building and seismic microzonation studies of Delhi.

Present activity and seismogenic potential of Himalayan sub-parallel thrust faults in Delhi: inferences from remote sensing, GPR, gravity data and seismicity

The National Capital Territory (NCT) of Delhi and its environs have been jolted by earthquakes from a far-field seismic source in the Himalayas. Thus the seismo-tectonic activity in this region can be related to thrust faults sub-parallel to the Himalayan thrusts system trending NW-SE. In this present research work various techniques like Remote Sensing (RS), GPR (Ground-Penetrating Radar) and Bouguer gravity anomaly analysis were executed in the Delhi region to identify seismogenic faults sub-parallel to Himalayan thrust systems. The straightening of the Yamuna River and other drainages, separation of the North Delhi ridge from the Central Delhi ridge, shifting of the North Delhi ridge in the north-western direction and linear alignment of vegetation helped in delineating the probable NW-SE faults using satellite image and DEM (Digital Elevation Model) data. These NW-SE faults/thrusts were also identified by GPR (Ground-Penetrating Radar) surveys, using 200 MHz and 100 MHz antennas, carried out at 8 areas. Nearly 200 GPR profiles were taken and the best results were obtained near the Timarpur, Vasant Vihar, Mehrauli and Faridabad areas. The radar profiles were processed using low- and high-pass Finite Impulse Response (FIR) filters, for noise removal and Automatic Gain Control (AGC) for amplitude correction to enhance the data. The Bouguer gravity anomaly analysis confirms the presence of two NW-SE trending faults viz. the Yamuna-Timarpur-Sonepat fault passing from the northern portion and the Faridabad-Mehrauli-Rohtak fault in the southern portion cutting across the city, thus implying the in-between area as a graben. Moreover the fault plane solutions of the majority of the seismic events show orientation in a NW-SE direction along the Delhi Sargodha ridge (DSR). These faults are sub-parallel to Himalayan thrust systems and have reverse fault characteristics. Hence, the possibility of finding more hidden faults beneath the Indo-Gangetic alluvium, sub-parallel to the regional strike of the Himalayan fault system cannot be ruled out.