Journal of Asian Earth Sciences: X最新文献

Himalayan rivers are considered the most sensitive of all the ecosystems to the impact of climate change. In the present investigation, hydrochemical processes controlling the meltwater chemistry of the rivers Bhagirathi, Alaknanda and Ganga in the Upper Ganga Basin, India have been studied simultaneously creating a large database for the first time. For this purpose, an extensive water quality assessment in Upper Ganga Basin has been carried out by collecting water samples from all three rivers on monthly basis from September 2016 to May 2018 and analysing these samples for hydro-chemical parameters. Hydro-chemical characteristics revealed that sulphide oxidation and carbonation- the two proton producing reactions govern the chemical weathering processes pertaining in the rivers. One of the most peculiar findings of the study is the dominance of carbonate dissolution in the whole stretch of River Alaknanda, while the dominance of sulphide oxidation in River Bhagirathi upto Dabrani revealing the continuum of Gangotri glacial processes followed by carbonate dissolution upto Haridwar. The principal component analysis further supports this weathering process in the basin.

The Koyna region in western India, characterized by recurrent seismic activity over more than five decades and confined largely within a 20 km × 30 km area and ∼10 km in depth, is a classical site to study earthquake processes. The largest earthquake in the region, the M 6.3 Koyna earthquake of December 1967, formed a ∼NNE-SSW trending surface rupture, known as Donichawadi fissure zone. The fissure zone, mapped originally over a length of ∼4 km and width of ∼200 m between Nanel and Kadoli by the Geological Survey of India (GSI) in the wake of the earthquake, comprises en-echelon fractures, near-vertical fissures, oblique or diagonal tensional cracks, mole tracks, soil lumps and laterite boulders in paddy cultivated land. We review the Donichawadi fissure zone in the light of extensive seismological data acquired in the past five decades, surface mapping studies, recent geophysical studies, soil-gas helium studies over the fissure zone during 1996–1997, chemical and noble gas isotope compositions of formation gases, and subsurface fracture data from downhole image logs in a 3 km - deep scientific borehole drilled in 2017. Integration of the datasets provide clinching evidence that the Donichawadi fissure zone is the surface manifestation of a deeper crustal fault (referred as Donichawadi fault) that has been active for more than 55 years and yield critical new insights on the geometry, distribution and possible subsurface disposition of the fissures and the associated fractures in the subsurface. We conclude that the Donichawadi fault offers a potential target for further deep probing through scientific drilling, downhole measurements and long-term monitoring to gain new insights into the genesis of triggered earthquakes in the area.

The tectonic units of Sakarya Zone (Pontides) and Tavşanlı Zone (Anatolides) are exposed in the Mihalıççık area of central Anatolia. A Cretaceous accretionary complex that forms the İzmir-Ankara-Erzincan suture zone separates the units. Within this region, the Tavşanlı Zone is made up of a coherent series and tectonically overlying late Cretaceous mélange, which has undergone blueschist facies metamorphism. Coherent series consist of strongly retrogressed schists and conformably overlying platform-type thick marbles, known as the İnönü marbles.

The İnönü marbles contain meta-bauxite lenses in the lower levels, which can be classified as karstic bauxites deposited in the karst holes within the original limestone. Considering the general stratigraphy of the Tavşanlı Zone, a middle Jurassic age can be envisaged for the bauxite formation. Geochemical data suggest that the bauxites were largely derived from redeposited argillic sediments, with minor and variable inputs from the weathering of intermediate to acidic rock. The mineral assemblage of the meta-bauxite consists of diaspore, chloritoid, muscovite, paragonite, Al-spinel (hercynite), magnetite, hematite and ilmenite. In addition, apatite, rutile, monazite, xenotime and zircon occur as accessory minerals. Limonite, goethite, akaganeite and kaolinite also formed during the latest stage of alteration. P-T conditions for the meta-bauxites are estimated as 20–27 kbar and 330–470 °C that are indicative of formation under high-P / low-T blueschist facies conditions.

The meta-bauxites are cut by veins of diaspore, muscovite-paragonite, goethite, calcite and ankerite, and include gem-quality diaspore crystals up to 6 cm long. Textural evidence indicates that the diaspore formation is related to the extraction of aluminium from the meta-bauxite body and migration of Al-rich fluids into the shear zones during the last stage of the high-P metamorphism. During exhumation of the Tavşanlı Zone, these veins were brecciated by brittle deformation and the fractured minerals were cemented by granoblastic calcite and ankerite during the latest stage of the vein formation.

The Aravalli Craton of the Indian shield constitutes heterogeneous basement lithologies (Banded Gneissic Complex; BGC), and among them, the granitoids are the most voluminous lithology. The BGC comprises two lithotectonic units, viz., BGC-I and BGC-II. The BGC-II has been further classified as amphibolite facies Mangalwar and granulite facies Sandmata Complexes. In the present study, the gneisses of the Mangalwar Complex are geochemically categorized into (i) low-and high-pressure sodic gneisses and (ii) potassic gneisses. The sodic gneisses are metaluminous and characterized by high Sr/Y and La_N/Yb_N ratios; and exhibit subduction-related negative anomalies of Nb and Ti. The ε_Nd (t = 2992 Ma) ranges from +2.3 to +3.1, with an average Nd-depleted mantle model age (T_DM) of 3.06 Ga. The whole-rock Sm-Nd isochron age is ∼3.0 Ga (2992 ± 340 Ma). Genetically, the sodic gneisses originated from the melting of an enriched precursor (oceanic plateau) in an arc environment. These gneisses show strong correlations with the gneisses from BGC-I depicting similar geochemical signatures. In contrast, the potassic gneisses are characterized by slightly higher SiO₂ along with high K₂O and high large-ion lithophile elements and negative Eu anomalies along with negative ε_Nd (t = 1.7 Ga) (−13.2 to −3.9), higher initial ⁸⁷Sr/⁸⁶Sr isotopic ratios and average T_DM = 2.87 Ga. These geochemical features of the potassic gneisses indicate that they were derived from the reworking of the pre-existing TTG-like (sodic gneisses) crust during the Paleoproterozoic Era.

Baihetan reservoir, the second-largest hydropower station in China, is located at the east boundary of the Sichuan-Yunnan block, one of the most seismic active zones in continental China. Many studies have shown that the potential risk of reservoir-induced seismicity will ramp up when the crustal stress field is remarkably perturbed following its impoundment. In this work, based on the fully-coupled poroelastic theory, we built models with different parameters to quantitatively analyze the displacement and stress field changes caused by the Baihetan reservoir impoundment. The preliminary results show that the maximum subsidence reaches about 0.3 m, and the stress change is about 0.4 MPa at 5 km after five years of impoundment. Most strikingly, we find a significant increase of Coulomb Failure Stress Change (ΔCFS) on the fault planes at the vicinities of the surrounding Xiaojiang Fault, Daliangshan Fault, and Lianfeng Fault, suggesting high reservoir induced seismicity risks. However, the occurrence of induced/triggered earthquakes is not only related to the regional tectonic loading, but also the rock strength. Moreover, the reservoir water level rise rate will lead to different spatial–temporal patterns of the induced micro-seismicity, however, this difference will gradually disappear as the reservoir continues to operate, and the large earthquake is mainly affected by geological tectonics.

The vast swathe of area in the Trans-Aravalli region between the western part of the South Delhi Fold Belt and eastern part of the Barmer basin, north-western India exhibits limited geological information due to thick sand cover of the Thar Desert. The present study is to unravel the hidden architecture of the above area using recently acquired high-resolution aero-magnetic data. Here, our interpreted results brought out a buried ring structure in the northern part and NW-SE trending constellation of linear magnetic highs in the southern part. Considering the regional geological milieu, our study correlates the buried ring structure with a possible remnant volcanic cone of the Neoproterozoic silicic Malani Large Igneous Province. Secondly, the significant NW-SE trending linear magnetic highs in the south possibly represents a humongous dyke swarm activity, facilitated by the intricate network of fractures. The orientation of these magnetic linear are conspicuously similar with the Deccan linked Sarnu-Dandali dykes and regional Barmer-Cambay rift system. The northwestern portion of study area displays high magnetic and gravity responses of long wavelength features indicating crustal heterogeneity. The study for the first time documents two concealed structures, which may hold a link of Neoproterozoic Malani and Mesozoic Deccan magmatic pulses.

This study focuses on the ability of spinel crystals in ultramafic xenoliths, like zircon, to store the primary composition and structure when exposed to magma fluid flows or the thermal heating of ultramafic fragments in magma flows during volcanic eruptions. Mantle ultramafic rock xenoliths from explosives of the Avacha Volcano in Kamchatka have different facies of their metasomatic changes above the magma chamber. An alternative hypothesis of their source is from fragments of layered mafic intrusions containing spinels, which store initial petrogenetic records of the original magma melt. Experiments were conducted to prove this idea using a Budker Institute of Nuclear Physics (INP SB RAS) unit. Large scales of change in the composition and structure of spinel crystals were caused by the hot fluids from melted xenolith sites.

A new machine learning model, named, EEWPEnsembleStack has been developed for predicting the magnitude of the earthquake from a few seconds of recorded ground motion after the arrival of the P phase. The testing and training dataset consists of 2360 and 591 strong-motion records from central Japan recorded by the Kyoshin Network. Eight parameters that are well correlated with the magnitude have been used for training and testing of the model. Feature ablation study using several models shows that a minimum mean absolute error of 0.42 has been obtained for the case when the model has been trained by using all parameters rather than by a single parameter. The model ablation study indicates that among all individually trained single models, the minimum error has been obtained for a Decision Tree regression model. However, the error is minimized when all machine learning models have been together utilized in the EEWPEnsembleStack model for the training purposes. The EEWPEnsembleStack model has been used to predict a 6.3 magnitude earthquake by using its 21 records from various stations that lie within 50 to 150 km epicentral distance. The predicted magnitude from the developed model using weighted magnitude prediction is obtained as 6.4, which is close to the actual magnitude. The comparison of the predicted magnitude of this earthquake from the developed model with that predicted by using popular ${\tau }_c$ and $P_d$ methods clearly indicates the suitability of the developed machine learning model over other conventional models.

A swarm type of seismic activity started near Khankotda and Matwa villages in the Jamnagar district in the middle of September 2019. 76 earthquakes belonging to this activity were well recorded and located by the network operated by the Institute of Seismological Research. The general trend of the 2019 swarm earthquakes is NW-SE, in conformity with the strike of the local lineaments and dykes. This swarm is not associated with any major faults in the region (the region is devoid of any major mapped faults), the closest being the ENE-WSW oriented North Kathiawar Fault, ∼60 km north of it. The epicentres of the 2019 swarm are in the proximity to the swarms that occurred in 2006 and 2007. Akin to the previous swarms in this region, the current one also appears to be triggered following the heavy rainfall. The focal mechanisms of 11 M > 3 earthquakes in this swarm are determined. The mechanisms reveal a reverse regime with ESE-WNW strike. We also used the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique to map the surface deformation in the affected area. An uplift of ∼1.1 to 5.3 mm is noticed to the north of an ESE-WNW trending local lineament in the vicinity of the swarms with subtle fluctuations (ups and downs) during September to December 2019, which gradually stabilised. The seismicity pattern, focal mechanisms and surface deformation lead us to interpret this lineament as the seismogenic fault, which generated this swarm activity.

Assam region is mainly formed by the deposit and erosion process of the Brahmaputra River. The frequency of seismic events results in landform deformation, which highly influences the drainage basin pattern and causes drainage anomalies, having a subsequent effect on the flood distribution pattern. In the present study, morphometric parameters and geomorphic indices for the Assam region are derived from SRTM DEM data of 30 m resolution using GIS to characterize the tectonic activity, which in turn influences the drainage pattern. The indices are classified into three tectonic activity classes, and the average of the classes is combined to generate the indices of relative active tectonics (IRAT). The four classes of IRAT are defined for the study area as (i) very high; Class 1 (1.57–1.80), (ii) high; Class 2 (1.81–2.06), (iii) moderate; Class 3 (2.07–2.26), and (iv) low; Class 4 (2.27–2.30). Class 1 corresponds to basins 1 and 6. Basins 2, 4, and 5 fall under Class 2. Class 3 consists of basins 7, 8, and 9, and Class 4 comprises basins 3 and 10. Results show that most of the study area lies in very high to moderate active tectonic zones and the identified zones are consistent with significant faults and thrusts present in the basins. The combined approach of GIS-based morphometric and geomorphic study allows for identifying deformed landforms resulting from active tectonics. The results can also be employed for the development of watershed management and sustainable land use planning.