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Ideal morphologic representation of geologic structures using standard curves/surfaces can have far-reaching implications in estimating resources. From NW Lesser Himalaya (Uttarakhand state, India), field photographs of fully developed sigmoid-shaped brittle shear P- and ductile shear S- planes that crop out on the NW–SE (sub)vertical natural sections of rocks are matched by drawing curves using the B-spline tool in Rhinoceros software 5 SR service in 2D. These curves are advantageous to handle since the user can control their degrees, the control points are not the deciding factors, and that local modifications in shapes are permitted, unlike the Bézier curves. Sigmoid shapes are analyzed in detail using six shape parameters (lengths in between control points: L1, L2 and L3; angles in between control points: α1, α2 and α3). Good correlations between L3 vs. L1, L2 vs. L3 and L1 vs. L2 reveal the relation between wavelength (λ) and amplitude of the sigmoids that are classified into four types. Strong correlation between α2 vs. α3 and (α3−α1) vs. (α1− α2) suggest only the Type I, II and III sigmoids possess 180° rotational symmetry. Regression models (R2 values) demonstrate that the sigmoid geometries are governed by (1) pre-existing or co-evolving regional structures and (2) lithologic composition. [ABSTRACT FROM AUTHOR]

The article discusses a geological and geochemical research in Koyna Seismogenic Region in Maharashtra, India, a seismogenic area which primarily deals with the different aspects of fluid-rock interaction at shallow crustal depth. Topics covered include the fluid-rock interaction which resulted in the mineralogical changes in the granite gneiss basement, and the mineralogical and/or geochemical alterations that were recorded in the microscopic observations.

One of the major aspects of rock‐physics forward modelling is to predict seismic behaviour at an undrilled location using drilled well data. It is important to model the rock and fluid properties away from drilled wells to characterize the reservoir and investigate the root causes of different seismic responses. Using the forward modelling technique, it is possible to explain the amplitude responses of present seismic data in terms of probable rock and reservoir properties. In this context, rock‐physics modelling adds significant values in the prospect maturation process by reducing the risk of reservoir presence in exploration and appraisal phases. The synthetic amplitude variation with offset gathers from the forward model is compared with real seismic gathers to ensure the fidelity of the existing geological model. 'Prospect A' in the study area has been identified from seismic interpretation, which was deposited as slope fan sediments in Mahanadi basin, East Coast of India. The mapped prospect has shown class‐I amplitude variation with offset response in seismic without any direct hydrocarbon indicator support. The existing geological model suggests the presence of an excellent gas reservoir with proven charge access from the fetch area, moderate porosity and type of lithology within this fan prospect. But, whether the seismic response from this geological model will exhibit a class‐I amplitude variation with offset behaviour or 'dim spot' will be visible; the objective of the present study is to investigate these queries. A rock‐physics depth trend analysis has been done to envisage the possibilities of class‐I reservoir in 'Prospect A'. Forward modelling, using a combination of mechanical and chemical compaction, shows the synthetic gas gathers at 'Prospect A', which are class I in nature. The study has also depicted 2D forward modelling using lithology and fluid properties of discovery well within similar stratigraphy to predict whether 'dim spot' will be seen in seismic. The estimated change in synthetic amplitude response has been observed as ∼5% at contact, which suggests that the changes will not be visible in seismic. The study connects the existing geological model with a top‐down seismic interpretation using rock‐physics forward modelling technique to mature a deep‐water exploratory prospect. [ABSTRACT FROM AUTHOR]

The National Gas Hydrate Program Expedition 02 (NGHP-02) was designed to examine gas hydrate occurrences off the eastern margin of the Indian Peninsula. Twelve holes were logged with logging-while-drilling (LWD) tools and nine holes at five sites were cored in Area-B of NGHP-02. Five logging units were defined based on the characteristics in logging data responses. Distinctive Logging Units R1 and R2 characterized by high gamma ray log values and composed of quartz and biotite-rich medium sized sand grain units that act as gas hydrate resevoirs and correlated to strong seismic reflectors. Hydrate-bearing intervals can be identified by resistivity and sonic velocity logs and classified into pore-filling type, fracture-filling type, and a mixture of pore- and fracture-filling types based on borehole image characteristics. Pore-filling type gas hydrate is typically developed in the case where the depths of R1 and/or R2 is close to the seismic inferred bottom simulating reflector (BSR). Fracture-filling type hydrates associated the R1 section occur above the BSR associated with the anticline in Area-B. The occurrence of pore-filling and fracture-filling gas hydrate represent different processes of hydrate formation related to the regional geological structure. The stratigraphic layered pore-filling gas hydrate occurrences are generated along often dipping sand layers, which are also conduits for methane-bearing fluid migrating from below the BSR in many settings. Fracture-filling type gas hydrate is an alternative mechanism for the occurrence of gas hydrate in mudstone-rich sediments above the BSR and found associated with the anticline in Area-B. This structural constraint of the hydrate-bearing fractures in the Area-B anticline suggest that the fracture dominated reservoirs possibly formed by anticlinal flexure, and the upward flow of methane-bearing fluids along porous sedimentary layers and the subsequent formation of pore-filling gas hydrate occurrences account for the range of gas hydrate accumulations discovered during NGHP-02. • Logging units and gas hydrate zones of NGHP-02 Area-B in Krishna-Godavari Basin was established. • Gas hydrate occurrences in pore-filling, fracture-filling, and mixed fracture types were determined. • Gas hydrate occurrence is highly influenced by the anticlinal structure. [ABSTRACT FROM AUTHOR]

The purpose of this study is to better understand the tectonics of the Deccan trap, India, for which we perform field studies and geomorphic analyses. The contact between the Eastern Dharwar Craton with the South East Deccan Volcanic province around Kinwat lineament (Maharashtra, India) is NW extension of the Kaddam fault. We study this contact in the present work. The region is predominantly NE-SW strike-slip faulted along with minor folds and boudins. These structures are confined to the granitic-gneissic basement. The faults reported in this study do not match with the earlier reported—E strike-slip faults, from Gokunda region near Kinwat city. The difference in the trend of faults indicates heterogeneity in the fault regime at local-scale. Different stress-axes at Gokunda and the study area led to the development of these two differently trending (NE-SW trending faults and—E strike-slip faults near Gokunda) fault regimes. Geomorphometric analyses connote that most of the study area is moderately tectonically active and tilted. [ABSTRACT FROM AUTHOR]

Hypsometry is widely used for inferring tectonic effects and erosion status of landscapes. Tectonics, structural inhomogeneity, lithologic differences, and climatic variations lead to topographic undulations discerned into discrepancies in the values. Hypsometric index (and curve), indicative of frequency distribution of proportional elevation with the respective proportional area, is used as a tool to describe characteristics of landscape morphology, lithological variability, and degree of fluvial dissection. Many workers have used to infer comparison of rates of erosion with tectonic uplift rates. However, there are many other factors reported to influence topographic undulations other than tectonics which lead to a variety of hypsometries. Morphotectonic index, hypsometric integral, calculated using digital elevation models (DEMs) in GIS environment has been widely used for inferring tectonic effects, status of erosion, and structural controls. The present study is conducted in the North Koel River basin. This river rises in the Ranchi plateau and joins the Son River a few miles north-west of Haidarnagar, is the right bank tributary of the Son River. Along its entire course of flow, North Koel river (260 km) flows through plateau region mostly formed of metamorphic rocks. Hence, structural control seems to be the primary control on the landscape evolution of this subbasin. In this study, hypsometric integral (and curve) has been calculated for third order and upper order streams to look whether this morphotectonic index shows any sign of tectonic, structural, or lithologic control on the landscape evolution in the North Koel River basin. [ABSTRACT FROM AUTHOR]

Rift related magmatism during Permian time in the northern margin of Indian plate is represented by basic dykes in several Himalayan terranes including north western Pakistan. The field relations, mineralogy and whole rock geochemistry of these basic dykes reveal significant textural, mineralogical and chemical variation between two major types (a) dolerite and (b) amphibolite. Intra-plate tectonic settings for both rock types have been interpreted on the basis of low Zr/Nb ratios (< 10), K/Ba ratios (20-40) and Hf-Ta-Th and FeO-MgO-Al2O3 discrimination diagrams. The compositional zoning in plagioclase and clinopyroxene, variation in olivine compositions and major elements oxide trends indicate a vital role of fractional crystallization in the evolution of dolerites, which also show depletion in rare earth elements (REEs) and other incompatible elements compared to the amphibolites. The equilibrium partial melting models from primitive mantle using Dy/Yb, La/Yb, Sm/Yb and La/Sm ratios show that amphibolite formed by smaller degrees (< 5%) of partial melting than the dolerites (< 10%). The trace elements ratios suggest the origination of dolerites from the subcontinental lithospheric mantle with some crustal contamination. This is consistent with a petrogenetic relationship with Panjal trap magmatism, reported from Kashmir and other parts of north western India. The amphibolites, in contrast, show affinity towards Ocean Island basalts (OIB) with a relatively deep asthenospheric mantle source and minimal crustal contribution and are geochemically similar to the High-Ti mafic dykes of southern Qiangtang, Tibet. These similarities combined with Permian tectonic restoration of Gondwana indicate the coeval origin for both dykes from distinct mantle source during continental rifting related to formation of the Neotethys Ocean. [ABSTRACT FROM AUTHOR]

The Shillong Plateau (northeastern India) constitutes the only significant topography in the Himalayan foreland. Knowledge of its surface uplift history is key to understanding topographic development and unraveling tectonic-climate-topographic coupling in the eastern Himalaya. We use the sedimentary record of the Himalayan foreland basin north of the Shillong Plateau to show that the paleo- Brahmaputra river was redirected north and west by the rising plateau at 5.2-4.9 Ma. We suggest that onset of plateau uplift is a result of increased fault-slip rates in response to stresses caused by the Indian lithosphere bending beneath the Himalaya. [ABSTRACT FROM AUTHOR]

Sequential cumulative moment release data of macroearthquakes (Mw≥4.3) of seventeen seismic zones (A to Q) belonging to NE-Himalaya, Burmese-Andaman arc and West- Sunda arc are analysed by Hurst analysis, a non-parametric statistical procedure to identify clustering of low and high values in a time series. The moment release in a zone occurs in alternate positive, negative and positive sloping segments forming a wave like pattern with intervening small horizontal segment. The negative sloping segments indicate decelerated moment release pattern or temporal slackening of elastic strain release with high b-value (>0.95). The horizontal segment indicates temporal clustering of moderate magnitude events/seismic moments with moderate b-values (0.8-0.95). The positive segment is characterised by accelerated moment release within a short span of time indicating temporal clustering of larger magnitude earthquakes/seismic moments and exhibit lowest b-value (<0.7). All zones attest moderate to high Hurst K values, range 0.7-0.86. The pattern in Hurst plots, specially a reversal of trend after prolong negative slope is used for earthquake prognostication in the seismic zones. Our analysis shows that most of the zones register a notable reversal of Hurst clustering trend after a prolonged negative slope which is accompanied by a major earthquake near its end. However, South Burma region (Zone-I) and Tripura fold belt and Bangladesh Plain (Zone-K) do not show any moderate or large shock around the end of the negative sloping trend in Hurst plot. Hence, these two zones can be considered more prone to produce moderate to larger earthquakes in future. [ABSTRACT FROM AUTHOR]

On 10th December 1967, the world's largest reservoir triggered seismic (RTS) event of magnitude 6.3 shook the Koyna region, the prime site of RTS globally. Ever since, several studies have attempted to infer the seismotectonics and to comprehend the actual causative mechanism of triggered seismicity in this region. Initial studies, including those of the 1967 Koyna main shock and its aftershocks, were based on the conventional P wave polarity or the first motion approach. These studies provided the first ever understanding of a predominantly strike-slip environment in the Koyna region, concurrent with the direction of ambient stress field due to the Indian plate motion. Subsequent studies pointed to a normal faulting environment in theWarna region further south, subsequent to impoundment later in 1985. A few studies did report solutions based on composite focal mechanisms, which however, only represent the average picture of the region. More recent studies based on modelling of seismic broadband waveform data provided more accurate focal mechanisms with unprecedented location accuracies including focal depths. A catalog of 50 focal mechanism solutions is now available for the earthquakes of magnitude ∼4 and larger that occurred during the last 50 years, which has paved way for a clear understanding of the stress field and the causative model of seismogenesis in this active intra-plate seismic RTS zone in western India. Based on stress inversion using this catalog, a new tectonic model depicting a periodically varying stress field and hence faulting mechanism has been inferred. [ABSTRACT FROM AUTHOR]

We present crustal anisotropy estimates constrained by shear wave splitting (SWS) analysis using local earthquakes in the Shillong plateau and Assam valley area, North East India (NE India) region. Splitting parameters are determined using an automated cross-correlation (CC) method. We located 330 earthquakes recorded by 17 broadband seismic stations during 2001–2014 in the study area. Out of these 330 events, seismograms of 163 events are selected for the SWS analysis. Relatively small average delay times (0.039–0.084 s) indicate existence of moderate crack density in the crust below the study area. It is found that fast polarization directions vary from station to station depending on the regional stress system as well as geological conditions. The spatial pattern of crustal anisotropy in the area is controlled mostly by tectonic movement of the Indian plate towards NE. Presence of several E-W and N-S trending active faults in the area also play an important role on the observed pattern of crustal anisotropy. [ABSTRACT FROM AUTHOR]

Aftershocks of the October 8, 2005, M = 7.6 Kashmir earthquake continued for many weeks and covered a region extending over an aperture exceeding 100 km. Several hundred events were recorded well at teleseismic distances while many hundreds more are only observed at regional distances. Existing earthquake catalogs for this sequence are poor given an unfavorable distribution of stations, a complex tectonic setting, lack of local and near-regional data, and under-exploitation of the most sensitive stations. Advances in automated signal processing, improvements in seismic velocity models, and innovations in multiple event location algorithms have made it worthwhile revisiting this sequence and attempting a large-scale relocation of the aftershocks. A vast number of new phase readings have been made on permanent and temporary seismic stations both at regional and teleseismic distances and the Bayesian hierarchical multiple event location program Bayesloc was employed in multiple stages, resulting in a far more structured distribution to the seismicity. The relocated aftershocks fall mainly into two distinct clusters. One cluster lies predominantly North East of the Balaklot-Bagh Thrust and South of the Main Central Thrust, with the October 8 main shock at its most northern point. The second cluster occupies the Indus-Kohistan Seismic Zone, North of the Main Central Thrust and South of the Main Mantle Thrust. Both clusters lie North East of a NW-SE trending boundary almost parallel to the southern part of the surface rupture. An East South East trending strand of earthquakes extends from the most northerly turning point of the Main Central Thrust into the Kashmir Basin, and a scattering of events are located North of this line and South of the Main Mantle Thrust. The new location estimates result in those aftershocks with the most similar focal mechanisms being far more spatially clustered than in previously published catalogs. Mapping global CMT solutions before and after the relocation contributes to the confidence in the new epicenters and provides a clearer picture of how the source parameters vary over the aftershock region. All files required to reproduce the results using the Bayesloc program are provided. [ABSTRACT FROM AUTHOR]

The basement in Upper Assam foreland basin exhibits a typical flexure pattern. An elongated continuous subsurface basement high has developed between Himalaya and Naga-Patkai ranges. A few prominent basement lows have developed adjacent to the foothill regions. It is noticed that the general topographic slope near the foothill regions is not only controlled by aggradation process, but also influenced by the flexured basement of the region. The basement lows have influenced the overlying topography significantly. Rivers flowing over those regions have shown unidirectional lateral migration. North bank tributaries like Subansiri, Jiadhal and Dikrang have been affected by the Subansiri basement low. Coseismic subsidence of sediments over Subansiri basement low had resulted subsidence of the North Lakhimpur-Ranga Nadi region in 1950 Assam earthquake. Some south bank tributaries like Disang and Dikhow have been affected by Nazira basement low. Topographic elevation along the Subansiri river is lower than that part of the Brahmaputra located south of Majuli. This typical topographic setting of the region makes the mouth of the Subansiri river and Majuli region highly susceptible to erosion. [ABSTRACT FROM AUTHOR]

Geological mapping and P–T path reconstructions are combined with monazite chemical age and Secondary Ion Mass Spectrometric (SIMS) U–Pb zircon age determinations to identify crustal domains with distinctive evolutionary histories in the Rengali orogen sandwiched between two Grenvillian-age metamorphic belts, i.e. the Eastern Ghats Granulite Belt (EGGB) in the south, and the amphibolite facies Gangpur Schist Belt (GSB) in the north, which in turn forms a collar along the NW/W margins of the Paleo/Mesoarchean Singhbhum Craton (SC) north of the Rengali orogen. Anatectic gneisses in the orogen core exhibit multi-phase Neoarchean/Paleoproterozoic deformation, metamorphic P–T histories and juvenile magma emplacement events. The high-grade belt is inferred to be a septum of the Bastar Craton (BC). The flanking supracrustal belt in the orogen — dominated by quartz-muscovite schists (± staurolite, kyanite, garnet pyrophyllite), inter-bedded with poorly-sorted and polymict meta-conglomerate, and meta-ultramafic/amphibolite bands — evolved along P–T paths characterized by sub-greenschist to amphibolite facies peak P–T conditions in closely-spaced samples. The supracrustal rocks and the anatectic gneisses of contrasting metamorphic P–T histories experienced D 1 , D 2 and D 3 fabric-forming events, but the high-angle obliquity between the steeply-plunging D 3 folds in the anatectic gneisses and the gently-plunging D 3 folds in the supracrustal unit suggests the two lithodemic units were tectonically accreted post-S 2 . The supracrustal belt is inferred to be a tectonic mélange formed in an accretionary wedge at the tri-junction of the Bastar Craton, the Eastern Ghats Granulite Belt and the Singhbhum Craton; the basin closure synchronous with the assembly of EGGB and the Singhbhum Craton–Gangpur Schist belt composite occurred between 510 and 610 Ma. Based on the available evidence across the facing coastlines of the Greater India landmass and the Australo–Antarctic blocks at ~ 500 Ma, it is suggested that the EGGB welded with the Greater India landmass during the Pan African along an accretion zone, of which the Rengali orogen is a part, synchronous with the final assembly of the Gondwanaland. [ABSTRACT FROM AUTHOR]

The Cauvery-Palar basin is a major peri-cratonic rift basin located along the Eastern Continental Margin of India (ECMI) that had formed during the rift-drift events associated with the breakup of eastern Gondwanaland (mainly India-Sri Lanka-East Antarctica). In the present study, we carry out an integrated analysis of the potential field data across the basin to understand the crustal structure and the associated rift tectonics. The composite-magnetic anomaly map of the basin clearly shows the onshore-to-offshore structural continuity, and presence of several high-low trends related to either intrusive rocks or the faults. The Curie depth estimated from the spectral analysis of offshore magnetic anomaly data gave rise to 23 km in the offshore Cauvery-Palar basin. The 2D gravity and magnetic crustal models indicate several crustal blocks separated by major structures or faults, and the rift-related volcanic intrusive rocks that characterize the basin. The crustal models further reveal that the crust below southeast Indian shield margin is ∼36 km thick and thins down to as much as 13-16 km in the Ocean Continent Transition (OCT) region and increases to around 19-21 km towards deep oceanic areas of the basin. The faulted Moho geometry with maximum stretching in the Cauvery basin indicates shearing or low angle rifting at the time of breakup between India-Sri Lanka and the East Antarctica. However, the additional stretching observed in the Cauvery basin region could be ascribed to the subsequent rifting of Sri Lanka from India. The abnormal thinning of crust at the OCT is interpreted as the probable zone of emplaced Proto-Oceanic Crust (POC) rocks during the breakup. The derived crustal structure along with other geophysical data further reiterates sheared nature of the southern part of the ECMI. [ABSTRACT FROM AUTHOR]

This paper provides a brief overview of the progress made towards active fault research in India. An 8 m high scarp running for more than 80 km in the Rann of Kachchh is the classical example of the surface deformation caused by the great earthquake (1819 Kachchh earthquake). Integration of geological/geomorphic and seismological data has led to the identification of 67 active faults of regional scale, 15 in the Himalaya, 17 in the adjoining foredeep with as many as 30 neotectonic faults in the stable Peninsular India. Large-scale trenching programmes coupled with radiometric dates have begun to constraint the recurrence period of earthquakes; of the order of 500–1000 years for great earthquakes in the Himalaya and 10,000 years for earthquakes of >M6 in the Peninsular India. The global positioning system (GPS) data in the stand alone manner have provided the fault parameters and length of rupture for the 2004 Andaman Sumatra earthquakes. Ground penetration radar (GPR) and interferometric synthetic aperture radar (InSAR) techniques have enabled detection of large numbers of new active faults and their geometries. Utilization of modern technologies form the central feature of the major programme launched by the Ministry of Earth Sciences, Government of India to prepare geographic information system (GIS) based active fault maps for the country. [ABSTRACT FROM PUBLISHER]

The Kashmir Basin Fault is located in the Jammu and Kashmir region of Kashmir Basin in NW Himalaya, India. It is a classic example of an out-of-sequence thrust faulting and is tectonically active as observed from multiple geological evidences. Its geomorphology, structure and lateral extent indicate significant accommodation of stress since long, which is further supported by the absence of a large earthquake in this region. It seems this fault is actively accommodating some portion of the total India-Eurasia convergence, apart from two well-recognised active structures the Medlicott-Wadia Thrust and the Main Frontal Thrust, which are referred in Vassallo et al. (Earth Planet Sci Lett 411:241-252, ). This requires its quantification and inclusion into slip distribution scheme of NW Himalaya. Therefore, it should be explored extensively because this internal out-of-sequence thrust could serve major seismic hazard in KB, repeating a situation similar to Muzaffarabad earthquake of Northern Pakistan in 2005. [ABSTRACT FROM AUTHOR]

The Manimahesh and Tal Glaciers are located in the Budhil fifth-order sub-basin of the Ravi, Himachal Himalaya, Northwestern Himalaya (India). These glaciers were analysed using high- (Corona KH-4A) to medium- (Landsat TM/ETM+/OLI, ASTER) spatial resolution satellite data between 1971 and 2013, along with extensive field measurements (2011–2014) of frontal changes. The results show that the Manimahesh and Tal Glaciers retreated by 157 ± 34 m (4 ± 1 m year–1) and 45 ± 34 m (1 ± 1 m year–1), respectively, whereas, the total area lost is estimated at 0.21 ± 0.01 km2(0.005 km2year–1) and 0.010 ± 0.003 km2(0.0002 km2year–1), respectively, between 1971 and 2013. The rate of retreat is significantly lower than that previously reported. Our field measurements (2011–2014) also suggest a retreating trend and validate the measured glacier changes using remotely sensed temporal data. [ABSTRACT FROM AUTHOR]

Sausar Fold Belt (SFB) in central India forms the southern part of the Central Indian Tectonic Zone (CITZ) – a crustal scale Proterozoic mobile belt dissecting the Indian craton, whose tectonothermal history and age is important for understanding the Proterozoic crustal history of the Indian craton. SFB comprises a gneissic basement (TBG: Tirodi Biotite Gneiss) overlain by a supracrustal sequence of quartzite–pelite–carbonate (SSG: Sausar Group). SSG and TBG are deformed and metamorphosed in greenschist to amphibolite facies. Two phases of granite intrusion are observed in the SSG – a syntectonic foliated granite and a post-tectonic massive granite, with clear structural relationship with the host rocks. Monazite chemical dating (U–Th–total Pb) of the foliated and massive granites yield Neoproterozoic (ca. 945–928 Ma) ages that contradict many earlier geochronological interpretations. Foliated granites and the immediately adjacent TBG show monazite grains with ca. 945 Ma mean age, interpreted as the timing of D 2 deformation and amphibolite facies metamorphism of SSG. The post tectonic granites intruded these rocks around 928 Ma, and were largely undeformed. A terminal thermal overprint is found in some monazite grain rims at ca. 785 Ma age. The younger Sausar tectonothermal events have overprinted the adjacent high-grade granulites of Ramakona–Katangi Granulite (RKG) belt, and should not be considered as parts of the same tectonothermal event representing different depth sections only. [ABSTRACT FROM AUTHOR]

The northern part of the central India tectonic zone (CITZ) is occupied by the Proterozoic Mahakoshal Belt, which is mainly comprised of granitoids and volcano-sedimentary lithounits. The granitoids (ca. 1880–1710 Ma) are exposed as small circular to elliptical-shaped, stock-like intrusive bodies, such as Nerueadamar granitoids (NG), Tumiya granitoids (TG), Jhirgadandi granitoids (JG), Dudhi granite gneiss (DG), Raspahari granitoids (RG), Katoli granitoids (KG), and Harnakachar granitoids (HG), collectively forming the granite gneissic complex (GGC). The geochemistry of biotites, host granitoids, and enclaves from these plutons has been investigated in order to understand the redox condition and likely tectonic affinity of host granitoids. The Al2O3–MgO–FeOtcontents and operated elemental substitution in biotites strongly suggest the diverse nature of host magmas such as calc-alkaline, metaluminous (I-type), peraluminous (S-type), and transitional between I- and S-types, which appear to have formed in subduction zone and syn-collisional tectonic settings. The transitional (I-S)-type granitoids inferred based on biotite compositions, however, represent both metaluminous (HG) and peraluminous (DG and KG) granitoids in terms of whole-rock molar A/CNK (Al2O3/CaO + Na2O + K2O) ratios. Ages of granitoid magmatism and its field association with contemporaneous volcano-sedimentary lithounits clearly mark the post-collisional tectonic setting, which contradicts the subduction-related tectonic setting inferred from biotites of JG and microgranular enclave (JE) hosted in JG. Whole-rock major and trace elements broadly suggest the existence of collision tectonics during the formation of granitoid plutons. The JG, KG, and DG contain a bt-Kf-mag-qtz assemblage, and their parental magmas evolved under moderate oxidizing environments (ƒO2 = −12.03 to −13.27 bars). On the other hand, RG (bt-gt-Kf-pl-qtz), NG (bt-ms-Kf-pl-qtz), and TG (bt-ms-Kf-pl-qtz) represent pure crustal-derived magmas evolved in strongly reducing conditions formed under a syn-collisional tectonic setting as evident from their mineral assemblages and biotite and whole-rock compositions. Granitoid plutons of the Mahakoshal Belt were most likely formed during amalgamation of the Columbian supercontinent. [ABSTRACT FROM AUTHOR]

Interest in hydrocarbon exploration from the Lesser Himalayan Sequence (LHS) has recently been revived amongst the petroleum geoscientists. Understanding the paleostress regime and the deformation mechanism are the two important steps to understand the structural geology of any (petroliferous) terrains. Arc-parallel shear is an integral deformation process in orogeny. The scale of such deformation features can range from micro-mm up to regional-scale. Unlike arc-orogen-perpendicular shear, different driving forces can produce arc-parallel shears. We review these mechanisms/theories from several orogens including the Himalaya, and compile 44 locations worldwide with arc-parallel shear. Due to continuous crustal shortening by the India-Eurasia collision, the squeezed rock mass at the plate interface has been building the Himalayan mountain chain. In addition, the rock mass also escapes laterally along the orogenic trend. Tectonic stress-field governs this mass flow. Field study and microstructural analysis in the northwest LHS (India) reveals arc-parallel brittle and ductile shear movement. Y- and P- brittle shear planes, and the S- and C- ductile shear planes reveal the following shears documented on the ~ NW-SE trending natural rock selections: (i) top-to-NW up, (ii) top-to-SE up, (iii) top-to-NW down, and (iv) top-to-SE down. Our paleostress analysis indicates top-to-SE down and top-to-NW down shears occurred due to stretching along ~131°–311° (D ext), whereas top-to-SE up and top-to-NW up shear fabric originated due to shortening along ~133.5°–313.5° (D compr). Previous authors considered that the arc-parallel extension generated ~15-5 Ma due to vertical thinning of the Himalaya. The NE-trending Delhi-Haridwar Ridge below the LHS plausibly acted as a barrier to the flowing mass, and piled up the rock mass in the form of NW-SE/arc-parallel compression. The NW-SE compression can be correlated with the D 3 of Hintersberger et al. (2011) during ~ 4–7 Ma. • Paleostress analysis from Lesser Himalaya in Garhwal section. • Global review of occurrence and mechanisms of arc-parallel shears. • Himalayan mechanism in the study area and applied aspects. [ABSTRACT FROM AUTHOR]

Structural studies of the Barmer Basin in Rajasthan, northwest India, demonstrate the important effect that pre-existing faults can have on the geometries of evolving fault systems at both the outcrop and basin-scale. Outcrop exposures on opposing rift margins reveal two distinct, non-coaxial extensional events. On the eastern rift margin northwest–southeast extension was accommodated on southwest- and west-striking faults that form a complex, zig-zag fault network. On the western rift margin northeast–southwest extension was accommodated on northwest-striking faults that form classical extensional geometries. Combining these outcrop studies with subsurface interpretations demonstrates that northwest–southeast extension preceded northeast–southwest extension. Structures active during the early, previously unrecognised extensional event were variably incorporated into the evolving fault systems during the second. In the study area, an inherited rift-oblique fault transferred extension from the rift margin to a mid-rift fault, rather than linking rift margin fault systems directly. The resultant rift margin accommodation structure has important implications for early sediment routing and depocentre evolution, as well as wider reaching implications for the evolution of the rift basin and West Indian Rift System. The discovery of early rifting in the Barmer Basin supports that extension along the West Indian Rift System was long-lived, multi-event, and likely resulted from far-field plate reorganisations. [ABSTRACT FROM AUTHOR]

Since the Precambrian time the Narmada-Son Lineament (NSL) is a zone of weakness and both the northern and southern portions of Narmada-Son Lineament experienced vertical block movements. It is established by the earlier workers that NSL is a sub-crustal feature which is responsible for the deposition and folding of the Vindhyans (Meso- Neoproterozoic) and Gondwanas (Permo-Carboniferous-lower Cretaceous). Recent tectonic activity in the Narmada- Son region is evidenced by high heat flow thermal activity, sulphur springs and seismically the Narmada-Son Lineament region is known to be a significantly disturbed zone. The present study area is the part of Narmada-Son Lineament zone. The drainage geomorphology of the area between Kanahar and Rihand rivers (tributaries of the tectonically controlled Son river) around Renukoot area have been studied aiming to understand active tectonics of the region, integrating detailed analysis of landforms and drainages. Various geomorphic features present in the study area such as incision of valley, aligned drainage, aligned valleys, linear valleys, offset channels, offset ridges and fault scarps demonstrate that the area is undergoing active deformation. [ABSTRACT FROM AUTHOR]

Abstract: The tectonically active Kachchh peninsula in western India lies in the southwest monsoon trajectory and hence provides a rare opportunity to decipher the temporal changes in climate–tectonics interaction in the evolution of the fluvial landforms. Reconstructions based on geomorphology, sedimentology, and geochemistry supported by optical chronology suggest that the fluvial aggradation in the region was initiated during the onset of the Indian Summer Monsoon (ISM) after the Last Glacial Maximum (LGM). The sedimentary characteristics and major elemental concentrations suggest that the sediments are dominated by fluvially reworked miliolites with subordinate contribution from the Mesozoic sandstones and shales and were deposited with the initiation of the ISM after the LGM. Temporal changes in facies architecture and major element concentrations suggest a progressive strengthening of the monsoon between 17 and 12 ka. This was succeeded by an overall strengthened ISM phase with fluctuations after 12 ka and <8 ka. Following this, a gradual decline in the ISM is inferred until around 3 ka. However, presence of the younger valley-fill sediments which are dated to ∼1 ka are ascribed to a short-lived phase of renewed strengthened ISM in the region before the onset of present day aridity. Based on the morphology of the fluvial landforms, two major events of enhanced uplift can be suggested. The geomorphic expression of the older uplift event dated to >17 ka is represented by the beveled Mesozoic bedrock surfaces which accommodated the post LGM valley-fill aggradation. The younger event of enhanced uplift which is assigned to <3 ka was responsible for the incision of the fill sediments and the Mesozoic bedrock, and the evolution of the present day fluvial landforms. The time averaged incision/uplift rate indicates that the Katrol Hill Range is uplifting at the rate of ∼4 mm per year, implying seismically active terrain. [Copyright &y& Elsevier]

Apatite fission-track analysis has been applied to the Raniganj and Panchet formations of Raniganj basin of Gondwana Supergroup to unravel its thermal and provenance history. Apatite fission track age population from both Raniganj and Panchet formations indicate partial annealing and point to a maximum temperature of around ∼100-110°C during their post depositional evolution. The sandstone of Raniganj Formation has five peak ages at 26.3, 59.3, 109.7, 173.7 and 299.9 Ma, while Panchet Formation has three peak ages at 25.4, 143.5 and 281.3 Ma. This implies that the provenance of the Raniganj Formation of late Permian and Panchet Formation of early Triassic changed obviously. According to thermotectonic evolution of the Gondwana basin, these apatites with different FT ages possibly represent different source components, although partial annealing had occurred to these apatites. Possibly all the apatites had transported from the Precambrian basement which was undergoing deformation due to Gondwana rifting initiated during Carboniferous period. Due to this, the basement was undergoing inhomogeneous thermal history which became source of sediments for Raniganj basin. Apatite FT ages of both Raniganj and Panchet formations have peak ages between 25 and 60 Ma, which perhaps recorded the cooling/uplift history during Cenozoic Alpine-Himalayan orogeny. Given a palaeo-thermal gradient of 40° C/km, it can be deduced that the Raniganj basin has uplifted about 3km at an average rate of about 0.09mm/a since 25-60 Ma. [ABSTRACT FROM AUTHOR]

This study aims for the characterization of the geological setting of 300MW Baspa II, India's largest private hydroelectric facility which was built on top of a relict rock avalanche dammed palaeo-lake (Baspa Valley, NW Himalaya). Geologically, the hydroelectric installation is located in the Higher Himalayan Crystalline, just above the active Karcham Normal Fault, which is reactivating the Early Miocene Main Central Thrust, one of the principal Himalayan faults. The area is seismically active and mass-movements are common. At ca. 8200yr BP the Baspa River was blocked behind a 142×106 m3 rock avalanche dam which created a ca. 260m deep palaeo-lake. The whole palaeo-lake was completely filled with sediments in about 3100years, making the Sangla palaeo-lake to a very rare example of a mass-movement dam with very long duration. The hydroelectric installation was built with its intake situated directly on top of the mass-movement dammed palaeo-lake of Sangla, utilizing the convex knick point in the river profile to increase the head for Baspa II for ca. 125m, compared to the reconstructed longitudinal profile of the river prior to the mass-movement, which amounts for about 18% of its design head. At least 5 levels of soft-sediment deformation have been recorded in the exposed part of the lacustrine sediments of Sangla palaeo-lake, including brecciated laminae, overturned laminae, folds, faults and deformation bands, separated by undeformed deposits. They are interpreted as seismites, indicating at least 5 earthquakes within 2495±297years strong enough to cause liquefaction. These observations extend the local seismicity record considerably into the past and indicate more and possibly stronger seismic events than might be expected from the instrumental measurements. [ABSTRACT FROM AUTHOR]

The Himalayan region has experienced a number of M8 and M5-M7.8 magnitude earthquakes in the present century. Apart from the release of strain builtup due to convergence of the Indian and Tibetan plates by seismic activity and aseismic slip, the tectonic activity in the current tectonic regime has also effected morphotectonic changes due to uplift, tilting of drainage basins, shifting or diversion of rivers and their tributary channels. Seismicity is mainly due to activity along numerous active faults, which trend parallel or transverse to the Himalayan mountain belt. In the outer Himalaya or the foothills, lying between the Himalayan Frontal Thrust (HFT) and the Main Boundary Thrust (MBT), some active faults have generated major earthquakes. The present article illustrates two such faults in the Pinjaur Dun and in the HFT zone at Kala Amb, Himachal Pradesh. Palaeoseismological study carried out at Nalagarh in Pinjaur Dun has revealed Late Pleistocene earthquakes along the Nalagarh Thrust (NT) that separates the Palaeogene rocks from the Neogene Siwaliks. The study shows evidences of at least two large magnitude earthquakes that rocked this region. The repeated reactivation of NT and HFT substantiates high seismic potential of the northwestern outer Himalaya and calls for more extensive study of palaeoearthquakes in this vastly populous mountainous region. [ABSTRACT FROM AUTHOR]

Gondwana sedimentary basins in the Indian Shield preserve a rich record of tectonic, sedimentary and volcanic processes that affected Gondwanaland. The Gondwana rocks were deposited in the linear rift basins that were formed during Permian-Cretaceous time, similar to their neighbours in Australia, Africa and Antarctica. In this study, we illustrate how Gondwana tectonics affected the Sohagpur Gondwana basin that occurs at the junction of the Mahanadi and Son-Narmada rift systems in the central India, through a high-resolution seismic reflection study along six profiles, covering the central part of the Sohagpur basin. The study reveals (1) ∼1000 m thick, gently dipping Barakar Formation, (2) thick coal seams at a depth of 350-550 m, and (3) NNW-SSE to NW-SE striking steeply dipping normal faults defining rift geometry. These results indicate that the Sohagpur basin contains a thick Lower Gondwana sedimentary succession with a high potential of coal resources and were affected by extensional tectonics. The rift structure in the study area is a syn- to post-sedimentary deformational structure that was formed arguably in response to tectonics that pervasively affected Gondwanaland. [ABSTRACT FROM AUTHOR]

Massif type anorthosites at Bolangir, eastern India are emplaced at the vicinity of the proto-Indian craton-Eastern Ghats Granulite belt contact. Micro- and meso-structural evidences indicate that the emplacement of the anorthosite pluton and the adjoining granitoids was syn-tectonic with respect to the D deformation phase (950-1,000 Ma) in the host gneiss. Anisotropy of magnetic susceptibility confirms that magnetic fabrics within anorthosite were dominantly developed during D deformation. Emplacement of felsic melts in the N-S trending dilatant shear zones in the granitoids, Fe-Ti-Zr-REE rich melt bands along N-S trending shear zones and localized N-S magnetic foliation in anorthosite near the Fe-Ti-Zr-REE rich melt bands indicate change in the stress field from NNW-SSE (D) to E-W (D). Available geochronological and paleogeographic data coupled with the structural analyses of the intrusive and the host gneiss indicate that the emplacement of massif type anorthosite in the EGP is not related to the accretion of Eastern Ghats Granulite Belt over proto-Indian continent during late Neoproterozoic. [ABSTRACT FROM AUTHOR]

The 14 April 2012 earthquake of M 4.8 is the best monitored event in the Koyna region, a globally significant site of reservoir triggered seismicity in western India. Hence, investigation of this event assumes great importance, also considering its epicentral location close to that of the 1967 Koyna earthquake of M 6.3, the world's largest reservoir triggered earthquake. Inversion of P-wave amplitude data along with the first motion polarities at 30 digital seismic stations provides a well-constrained strike-slip type focal mechanism solution, similar to that of the 1967 earthquake. The mechanism is further confirmed by moment tensor inversion of 3-component waveform data recorded at the three nearest broadband stations. The depth distribution of the aftershocks clearly delineates a NNE-SSW trending fault plane dipping about 78° to the WNW and coinciding with the trend of the Donachiwada fault, as well as the left-lateral fault plane of the focal mechanism solution obtained. The precise location, focal mechanism and the seismicity distribution from our dense network indicate that the activity in the Koyna region is mainly controlled by the NNE-SSW trending Donachiwada (D) fault zone rather than the Koyna River Fault Zone (KRFZ) on the west as suggested previously. [ABSTRACT FROM AUTHOR]

Highlights: [•] We obtained the frequency-dependent shear wave quality factor Qs (f). [•] Double Spectral Ratio (DSR) method is used for computation. [•] The zone around Son–Narmada–Tapti (SONATA) obtained low Qs value. [•] Regions little away from SONATA are associated with relatively higher Qs value. [Copyright &y& Elsevier]

Highlights: [•] Two major events of enhanced tectonic uplift (incision) are dated to 20ka and 7ka. [•] Events of tectonic instability were separated by channel fill aggradation. [•] Study indicates strengthened monsoon between 9ka and 7ka. [•] Steady decline of monsoon is observed after 7ka. [•] Observations accords well with the regional climate pattern during the last 20ka. [Copyright &y& Elsevier]

The Indian subcontinent is characterized by various tectonic units viz., Himalayan collision zone in North, Indo-Burmese arc in north-east, failed rift zones in its interior in Peninsular Indian shield and Andaman Sumatra trench in south-east Indian Territory. During the last about 100 years, the country has witnessed four great and several major earthquakes. Soon after the occurrence of the first great earthquake, the Shillong earthquake ( M: 8.1) in 1897, efforts were started to assess the seismic hazard in the country. The first such attempt was made by Geological Survey of India in 1898 and since then considerable progress has been made. The current seismic zonation map prepared and published by Bureau of Indian Standards, broadly places seismic risk in different parts of the country in four major zones. However, this map is not sufficient for the assessment of area-specific seismic risks, necessitating detailed seismic zoning, that is, microzonation for earthquake disaster mitigation and management. Recently, seismic microzonation studies are being introduced in India, and the first level seismic microzonation has already been completed for selected urban centres including, Jabalpur, Guwahati, Delhi, Bangalore, Ahmadabad, Dehradun, etc. The maps prepared for these cities are being further refined on larger scales as per the requirements, and a plan has also been firmed up for taking up microzonation of 30 selected cities, which lie in seismic zones V and IV and have a population density of half a million. The paper highlights the efforts made in India so far towards seismic hazard assessment as well as the future road map for such studies. [ABSTRACT FROM AUTHOR]

Abstract: The main objective of the paper is to find the probable causes behind the shifting course of the Haora River, one of the major rivers of West Tripura and detachment of one of its major tributaries, the Katakhal River. From a recent satellite image, we observed that the River Haora has changed its course drastically near the confluence. Earlier, it used to take a sharp northward bend to meet with the River Titas immediately after crossing the Indo-Bangladesh border; but presently it is flowing westward to do so. Moreover, the Katakhal River, a right bank tributary of the River Haora, that used to flow through the northern side of the city of Agartala and meet with the River Haora at Bangladesh, is no longer a tributary of the Haora River. Now it is completely detached from the Haora River and meets with the River Titas separately. Spatiotemporal maps have been used to detect the changes. Field investigation, with the help of GPS, has been done in order to find the link between the Haora River and the Katakhal River within the Indian territory. Changing patterns of the Haora and Katakhal River confluences are also analysed, and earlier courses are identified. The shifting trends of both of these two rivers are found along the flanks of the interfluvial area because of microscale tectonic activity, i.e., upliftment of the interfluvial zone. [Copyright &y& Elsevier]

The sediments of the Indo-Myanmar Ranges of northeast India had been deposited in a subsidence basin open up by rifting nearby the Myanmar continental margin. Thickening of the crust in this belt is due to lateral compression of the basin sediments by overriding continental margin of Myanmar. From the viewpoint of the tectonic setting of the region, the variation in the degree of crustal shortening along the ranges, calculated depth to detachment and supported by petro-geochemical evidences, an evolutionary model of the Indo- Myanmar Ranges Basin has been worked out. Calculating the excess thickness of the crust, a model of the crust below the ranges is constructed using principles of differential isostasy. A narrow strip of detached continental landmass, embedded under the sediments, is predicted in this preliminary study. [ABSTRACT FROM AUTHOR]

Successive indentations of Eurasia by India have led to the Tibet-Himalaya E–W orthogonal collision belt and the SE Tibetan Plateau N–S oblique collision belt along the frontal and eastern edges of the indenter, respectively. The belts exhibit distinctive lithospheric structures and tectonic evolutions. A comprehensive compilation of available geological and geophysical data reveals two sudden tectonic transitions in the early Eocene and the earliest Miocene, respectively, of the tectonic evolution of the orthogonal belt. Synthesizing geological and geochronological data helps us to suggest a NEE–SWW trending, ~450 km-long, ~250 km-wide magmatic zone in SE Tibet, which separates the oblique collision belt (eastern and SE Tibet) into three segments of distinctive seismic structures including the mantle and crust anisotropies. The newly identified Yongping basin is located in the central part of the magmatic zone. Geochronological and thermochronological data demonstrate that (1) this basin and the magmatic zone started to form at ~48 Ma likely due to NNW–SSE lithosphere stretching according to the spatial coincidence of the concentrated mantle-sourced igneous rocks on the surface with the seismic anomalies at depth; and (2) its fills was shortened in the E–W direction since ~23 Ma. These two dates correspond to the onset of the first and second tectonic transitions of the orthogonal collision belt. As such, both the orthogonal and oblique belts share a single time framework of their tectonic evolution. By synthesizing geological and geophysical data of both collision belts, the indenting process can be divided into three stages separated by two tectonic transitions. Continent–continent collision as a piston took place exclusively during the second stage. During the other two stages, the India lithosphere underthrust beneath Eurasia. [ABSTRACT FROM AUTHOR]

The article presents abstracts on geological topics which include structural characteristics of the Qilian Shan-Nan Shan thrust belt in Tibet, China, tectonic motions of the Tibetan Plateau, and the dating of collision between the India and Eurasia plates.

The article presents abstracts on geological topics which include the Indo-Asian Collision, orogenic evolution in the Andes, and the inner structure of mud volcanoes.

Abstract: Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone. [Copyright &y& Elsevier]

Abstract: Tectonic history of the Himalaya is punctuated by successive development of the faults that run along the boundaries between different lithotectonic terrains. The Main Boundary Fault, defining the southern limit of the Lesser Himalayan terrain, is tectonically most active. A review of published literature reveals that the nature and age of reactivation events on the Main Boundary Fault is one of the poorly understood aspects of the Himalayan orogen. By systematic outcrop mapping of the seismites, this study identifies a Late Miocene-Early Pliocene reactivation on the Main Boundary Thrust in southeast Kumaun Himalaya. Relatively friable and cohesionless Neogene sedimentary sequences host abundant soft-sediment deformation structures in the vicinity of the Main Boundary Thrust. Among a large variety of structures, deformed cross-beds, liquefaction pockets, slump folds, convolute laminations, sand dykes, mushroom structures, fluid escape structures, flame and load structures and synsedimentary faults are common. The morphological attributes, the structural association and the distribution pattern of the soft-sediment deformation structures with respect to the Main Boundary Fault strongly suggest their development by seismically triggered liquefaction and fluidization. Available magnetostratigraphic age data imply that the seismites were developed during a Late Miocene-Early Pliocene slip on the Main Boundary Thrust. The hypocenter of the main seismic event may lie on the Main Boundary Thrust or to the north of the study area on an unknown fault or the Basal Detachment Thrust. [Copyright &y& Elsevier]

The plate boundary between India and Sunda plates across the Indo-Burmese arc (IBA) region is probably the most neglected domain as far as the plate motion, crustal deformation and earthquake occurrence processes are concerned. Because of the limited or no geodetic measurements across the IBA region, debate continues on the most appropriate plate boundary model for the region. Subduction along this boundary occurred in geological past, but whether it is still active is a debatable issue. It is believed that the predominantly northward India-Sunda relative plate motion of about 36 mm/year is partitioned between the Indo-Burmese wedge (IBW) and the Sagaing Fault (SF). However, it is not clear how relative plate motion between India and Sunda plates is accommodated across the IBA region - whether localized, partitioned or distributed, and in particular what is the slip rate and mode of slip accommodation across faults in the region? In such cases, Global Positioning System (GPS) measurements of crustal deformation have proved to be the best and probably the only tool. Our detailed seismo-tectonic study, crustal deformation study using high precision GPS measurements of eight years, strain rate estimates, field studies, analytical and finite element modelling of GPS data from the IBW region in North East India provide evidence for present-day active deformation front (or the plate boundary fault) between the India and Burma plates. On the basis of our extensive studies, it is now suggested that the Churachandpur-Mao Fault (CMF), a geologically older thrust fault, accommodates motion of about 16 mm/year through dextral strike-slip manner. The motion across the CMF constitutes about 43% of the relative plate motion of 36 mm/year between the India and Sunda plates. The remaining motion is accommodated at SF. On the basis of modelling, which suggests low friction along the CMF, absence of low-magnitude seismicity along the CMF, lack of historic and great and major earthquakes on the CMF and regions around it, and field studies, it is proposed that the motion across the CMF occurs predominantly in an aseismic manner. Such behaviour of the CMF significantly lowers the seismic hazard in the region. [ABSTRACT FROM AUTHOR]

Abstract: The Southern Granulite Terrain (SGT) of southern India comprises a collage of Archean and Neoproterozoic high-grade metamorphic terrains, where the regional tectonic and high-grade metamorphic events culminated at ca.2.52Ga in the northern granulite blocks and ca.0.55Ga in the Neoproterozoic Madurai and Trivandrum Blocks. These domains were presumed to be separated either by the crustal-scale Cauvery shear zone system (CSZ) or the less known Karur–Kambam–Painavu–Trichur shear zone (KKPT). The Neoproterozoic granulite domains comprise mainly the Madurai Block (MB) to the north and Trivandrum Block (TB) to the south separated by the Achenkovil shear zone (ASZ) system. The SGT has been central to several studies focusing on the amalgamation of Gondwana supercontinent. A favored view has been that the Neoproterozoic terrains of the SGT represent segments of orogen(s) that accreted to an Archean continental margin during the early Cambrian although whether the tectonic regime was collisional, accretionary or intracratonic remains enigmatic. This article presents a large data set of major, trace element, Rb–Sr and Sm–Nd isotopic compositions focusing on Sm–Nd model ages (TDM) signifying the crustal residence age of charnockitic orthogneisses from both Archean and Neoproterozoic domains of the SGT, especially from the data deficient MB and TB domains. We present a regional Sm–Nd model age map for the SGT that delineates four distinct isotopic provinces with ages in the range of ∼3.5–3.2Ga, 3.2–2.8Ga, 2.8–2.0Ga and 2.0–1.2Ga. Clearly, the KKPT shear zone separates two isotopic provinces, extending the Archean terranes further south across the CSZ. The Archean and Neoproterozoic charnockites show distinct compositional characteristics indicating contrasting conditions of magma genesis and tectonic setting. Most importantly, an overwhelming majority of the Neoproterozoic charnockite gneisses show Sm–Nd model ages significantly older than their crystallization age constrained by zircon dating. The charnockite gneisses bear a strong affinity to calc-alkaline magmas and compositions consistent with mixing of Neoproterozoic arc magmas and older crustal components prompting a close analogy with magmatism at convergent margin-collisional tectonic settings. [Copyright &y& Elsevier]

Abstract: We present possible tectonic models for two crustal domains of the Proterozoic Eastern Ghats Belt, India based on recent petrological, geophysical and geochronological data. Although both the domains presently expose deep crustal sections, they evolved in two distinct time segments of the Precambrian through accretion–collision processes. This is why we argue that no unique model can explain the complexities of the belt. The southern part of the belt evolved through subduction-dominated accretionary processes encompassing India, east Antarctica, Australia and Laurentia as part of supercontinent Columbia during the span of ca. 1.90–1.60Ga. To the contrary, the central domain witnessed a prolonged accretion–collision history initiated at ca. 1.50Ga and culminated at ca. 0.90Ga with the formation of supercontinent Rodinia. The latter united cratonic India with east Antarctica as a separate continent Enderbia that existed until about ca. 0.50Ga. The pre-1.50Ga history of this domain is ambiguous at the present state of knowledge. The northern domain has a much younger (ca. 0.90–0.50Ga) tectonothermal history which is unrelated to either of the studied domains. The present models explain the reported petrotectonic processes including the ultra high temperature metamorphism in both the domains. The episodic growth of the Eastern Ghats Belt matches with Proterozoic supercontinent cycles. [Copyright &y& Elsevier]

Abstract: Studies in Proterozoic sedimentary basins are generally fraught with continuous and conformable hypothesis. Unlike many such undeformed and unmetamorphosed Proterozoic sedimentary basins around the Globe, the Mesoproterozoic Singhora basin, a so called “proto-basin” for the Chhattisgarh basin of central India, registers signatures of pervasive deformation documented in both field and anisotropic magnetic susceptibility (AMS) based study. While three constituent Formations of the Singhora Group viz. Saraipali, Bhalukona and Chuipali record outcrop-scale deformation in presence of folds having plane non-cylindrical to non-plane cylindrical geometry, the signatures of deformation within the coarse granular sandstones and conglomerates of the basal Rehtikhol Formation are shown principally from angular relation between the magnetic foliation and bedding plane under AMS study. Systematic formation-wise structural analysis reveals that the basin, as a whole, is deformed in the form of a regional-scale non-plane non-cylindrical fold; whereas non-planarity can be visualized in the regional scale, non-cylindricity is more prominent in the outcrop-scale. Penetrative fabric is only observed in the south-eastern corner of the basin. Compression in two directions is inferred, one being broadly N−S and the other E−W, with resultant overall compressional direction of NW−SE. Additionally, a narrow high strain zone is developed at the southern boundary of the basin in contact with gneissic basement, evident from the sheared metabasite band. Basic rocks (sheared and massive), metamorphosed at greenschist facies, present at the basin margin, and unaltered basaltic intrusives within the basin allowed us to constrain the broad time frame of deformation. Despite variation in mineralogy and major element geochemistry between the metamorphosed and unaltered variety, the intrusives are identified as cogenetic from the similarity in REE pattern and trace element character. Considering the ∼1420Ma emplacement age of the intrusive into the Saraipali Formation, the deformation age is constrained as <1420Ma. Signatures of compressional deformation traced within all formations of the Singhora Group suggest that the Group is pervasively deformed and thereby indicates an unconformable relationship with its overlying undeformed Chandarpur Group of sediments. The present study warrants necessity of further work in the area to establish causal relationship, if any, between the operative tectonics within the frontal thrust belt in the immediate eastern proximity of the basin and the compressional deformation event/s recorded within the basin. [Copyright &y& Elsevier]

Bhopalpatnam Granulite Belt which occur along SW margin of Bastar Craton and NE shoulder of Pranhita-Godavari Rift comprise of charnockite (enderbitic variety), garnet-sillimanite-biotite gneiss, quartzo-feldspathic gneiss and corundum bearing aluminous gneiss. High La/Yb ratio, low Eu anomaly (Eu/Eu*=1.0), high LREE/HREE ratio with uniform REE pattern, high La/Sc ratio (0.53-6.43), high Th/Sc ratio (0.03-2.56), low Ni (5.52-20.95), low Cr (31.05-117.05) and uniform Zr/Hf distribution pattern indicate a Proterozoic character. Distribution pattern of KO, NaO and CaO in ternary diagram show quartz-monzonite-granodiorite trend for the bulk rocks indicating that the bulk rock composition is close to TTG of early Archaean, which might have supplied the sediments for the rocks of Bhopalpatnam Granulite Belt. Geochemical and mineralogical evidence indicate an argillaceous protolith for garnet - sillimanite - biotite gneiss and corundum bearing aluminous gneiss, whereas an arkosic protolith for quartzo-feldspathic gneiss. The geochemical signatures also suggest an active continental margin setting for the rocks of Bhopalpatnam Granulite Belt with prominent Nb and Ta anomaly favouring a subduction environment between Bastar Craton and East Dharwar Craton. This is in conformity with the finding of the earlier workers suggesting a clockwise P-T path based on the combined fluid inclusion and mineral phase equilibria. The LILE geochemistry of charnockite suggests a bi-phase evolution. High LREE/HREE ratio portrays a highly evolved nature of the charnockitic melt generated through partial melting of the continental crust at the final stage of the granulite facies metamorphism during collision between Bastar and East Dharwar Cratons. [ABSTRACT FROM AUTHOR]

The Sonia Sandstone of Proterozoic Jodhpur Group, Marwar Supergroup, exposed around the Sursagar dam area of Jodhpur town, Rajasthan exposes two varieties of nodular features, often spectacular in shape and size. On the basis of mode of occurrence (intra- or interbed) and stratal involvement (single or multiple) the features are classified as Type I and II. From granulometric and microscopic (optical and scanning electron) studies carried out on sandstones from the nodules and their host sandstones, geochemical analysis (SEM-EDAX) of intragranular cement present within Type I nodules, and appreciation of control of associated fracture system within Type II nodules, it is proposed that the two types of nodules vary in their formative mechanism and stage of formation. While Type I nodules are identified as product of processes operative at the early diagenetic, pre-lithification stage, the Type II nodules are undoubtedly the result of post-lithification origin triggered by formation of fracture system. Here we propose generation of vapour pressure (not exceeding the overlying hydrostatic pressure) by decay of thin, laterally impersistent organic mat as the causal factor for intrabed nodule (Type I) formation, which forced rarefication of local grain packing vis-a-vis early diagenetic silica cementation. The study warrants necessity of more studies on nodules to understand possible roles of organic matter and bed-transgressive fracture systems in their formation, going beyond the generalised secondary mineralisation hypothesis. [ABSTRACT FROM AUTHOR]

Abstract: The tectonics of seismically active Sikkim Himalaya, as inferred by numerous seismological studies, is distinct from the conventional thrust tectonics proposed for the Himalayan collision belt. Here, focal mechanisms of several moderate magnitude earthquakes and composite fault plane solutions of microearthquakes have revealed strike–slip motion along faults transverse to the northward convergence direction of the Indian plate. In the present study, we analyze broadband magnetotelluric data of 12 sites located along an approximately N–S profile cutting across major geological sub-domains of Sikkim to test whether magnetotelluric strikes also support such transverse tectonic nature of the region. We have performed strike analysis of the data by two decomposition approaches as well as by phase tensor method. The study has revealed local variations in the strike directions within the region consistent with the geological and tectonic setup and the presence of transverse tectonic features in the region of Main Central Thrust Zone (MCTZ) where major axis of phase ellipses align in NNW–SSE to NW–SE direction. This trend coincides with the one obtained by microseismic data recorded after the September 18, 2011 earthquake (Mw 6.9). Magnetotelluric strike analysis thus supports the presence of NNW-to-NW trending transverse tectonic zone in MCTZ. [Copyright &y& Elsevier]

Abstract: The Himalayan orogeny, a result of the collision of India and Eurasia, provides direct evidence of strain accommodation and large-scale rheological behavior of the continental lithosphere. Knowledge of the timing of the India–Eurasia collision is essential to understand the physical processes involved in collisional systems. Here we present a geochronological and multi-isotopic study on rocks from the upper crust of the Kohistan Paleo-Island Arc that formed in the equatorial part of the Neo-Tethys Ocean. In situ U–Pb geochronology and Hf isotopes in zircon, and whole-rock Nd and Sr isotopic data of plutonic rocks from the Kohistan-Ladakh Batholith, are used to construct a continuous record of the isotopic evolution of the source region of these granitoids that are related to both the subduction of the oceanic lithosphere and subsequent arc–continent collisions. We demonstrate that profound changes in the source region of these rocks correspond to collisional events. Our dataset constrains that the Kohistan–Ladakh Island Arc initially collided along the Indus suture zone with India at 50.2±1.5Ma, an age generally attributed to the final India–Eurasia collision for the entire Himalayan belt. In the western Himalaya, the final collision between the assembled India/Arc and Eurasia however, occurred ∼10Ma later at 40.4±1.3Ma along the so-called Shyok suture zone. We present evidence indicating that a similar dual collision scenario can be extended to the east and conclude that a final India/Arc–Eurasia collision at ∼40Ma integrates crucial aspects of the magmatic, tectonic, and sedimentary record of the whole Himalayan mountain belt. [Copyright &y& Elsevier]