Geosystems and Geoenvironment最新文献

Neotectonics is a major factor for controlling landform development in tectonically active Chittagong Tripura Fold Belt (CTFB) region, along the western flank of the Indo-Burma ranges. Morphotectonic parameters are excellent indicators for landscape morphology evaluation to highlight past and ongoing tectonic processes in a region. To investigate the matter, different morphometric indices such as linear, areal, and relief parameters were quantified using the Digital Elevation Model (DEM) data and its attributes of Chengi River Basin (CRB) and Myinee River Basin (MRB) in CTBF. All the computations for morphometric characteristics and visualization were performed in the Geographical Information System (GIS) and statistical platforms. The CRB and MRB are identified as fifth and fourth-order basins with a total area of 503.53 km² and 267.28 km², respectively which have low drainage frequency. The Bifurcation ratio (Rb) value of both these basins is indicative of structural control. TSI controls the upper course of the rivers and the variations have been observed in year-wise Topographic Sinuosity Index (TSI) distribution. The Asymmetry Factor (AF) in both these basins indicates that CRB is tilting towards the western side, whereas the MRB is tilting towards the eastern side. Moreover, these two rivers are elongated in nature indicates tectonic activity along with Google earth (GE) seamless mosaic image-based longitudinal profiles indicates significant deformation along their courses. In addition, the moderate Hypsometric Integral (HI) value shows a convex shape in the lower portion, which might be related to the upliftment along a fault or perhaps an upliftment associated with recent folding. The overall results in both these basins indicate the presence of tectonic activity. Therefore, morphotectonic analyses using DEM, Landsat satellite data, and GE seamless data are found useful in this study and could be considered an ideal tool for any complex basin morphotectonic study.

The Upper Assam Basin is an intermontane foreland basin surrounded by gigantic mountain belts in NE India. The structural geometry of the basin is controlled by the tectonic interactions of the Himalayan orogenic belt in the north, Mishmi thrusts in the east, and the Assam-Arakan fold-and-thrust belts in the south. The basin has received significant attention not only because of its complex geological set-up but also due to its petroliferous nature for hosting significant hydrocarbon resources. The present study attempts to explore the tectonic subsidence history of the Cenozoic succession using subsurface stratigraphic details of ten (10) boreholes drilled within the upper shelf of the basin. Subsidence analysis is carried out using the backstripping technique. It is observed that the tectonic subsidence in the basin developed through four different stages. During the Paleocene-Eocene epoch, the basin witnessed slow subsidence. It increased gradually through Oligocene and attained rapid speed in the Miocene. Further, during the deposition of post-Miocene sediments (Plio-Pleistocene epoch), the tectonic subsidence in the basin remained accelerated. Subsidence curves obtained from the studied borehole depict a convex-upward profile, indicating that the basin attained a foreland configuration over time and is presently a SE dipping shelf bounded by opposite verging fold-and-thrust belts. Overall, the basin experienced tectonic subsidence of ∼2 km throughout its lifespan with an average subsidence rate of ∼30 m/Ma. This case study prominently elucidates the tectonic history of the basin, which underwent during the Cenozoic time. Our findings stress the importance of subsidence analysis through the backstripping technique as a potential approach for untangling the geohistory of sedimentary basins worldwide.

Earlier studies on Palaeoproterozoic (∼1800 Ma) alkaline (shoshonitic) rocks comprised of limited petrochemical data on the Bari syenite and other contiguous felsic rocks emplaced in anorogenic rift setting along the Son-Narmada North Fault (SNNF). Using new major and trace element data-sets, this study offers means of study of origin, source of magma, tectonic settings and geodynamic implications. The major oxide chemistry grouped Bari rocks into high alkali, but low CaO bearing peraluminous alkaline rocks. These rocks represent high abundance of HREE, Zr, Nb, Ga, Y, Eu, Ba and Sr. Primitive mantle normalized REE and trace elemental patterns correspond to A-type suite, suggesting origin of the magma mainly from the mantle. Significantly, anomalous Th/U and Rb/Cs values revealed crustal contamination of the melt, derived from partial melting of the mantle. Moreover, binary data plots between La vs. La/Sm and La vs. La/Yb are pointing towards crustal assimilation which was concomitant with the fractional crystallization of the mantle derived melt. Thus, crustal contamination coupled with the fractional crystallization of the melt mainly contributed to the formation of syenite melt. But, a high degree of partial melting of the lower crust was primarily responsible for the formation of Bari granite. The enrichment of incompatible elements in the syenite rocks suggests involvement of mantle metasomatism in their genesis. The magmatic processes related to the formation of syenite, lamprophyre, ultramafics, mafic and granite bodies were operative in the diverse magmatic realm and initiated earlier at the waning stage of the Mahakoshal orogeny and continental rifting, but magma emplaced later during Post-Mahakoshal orogeny and Pre-Vindhyan sedimentation that also in a rifted basement of the Bundelkhand craton at ∼1800 Ma during the amalgamation of the Columbian Supercontinent.

Microseism noise, which occurs in the period range of 2–20 s, is the most energetic band in the earth's background spectra. In the present study, we examined the amplitude spectra and directional characteristics of microseism in the Indian Ocean. We use the data from ten openly accessible land stations located all around the Indian Ocean. The probability power spectral density was used to characterize the microseism. To characterize the microseism, we employ the frequency dependent polarization approach, which is governed by the Eigen value decomposition of the 3 × 3 spectral covariance matrix. The spatial and temporal variation of microseism was investigated in order to better understand its distribution in the Indian Ocean region, which is regarded as a global source of microseism. For some stations, we observe the splitting of double frequency microseism into short period (2–5 s) and long period (6–10 s) microseism. The polarization analysis reveals the dominant sources of the microseism are located in the Southern Ocean. We also correlated the spatio-temporal variation of significant wave heights (swh) with the power spectral densities at each station. We observe a remarkable correlation between power spectral density with the significant wave height (swh) in both spatially and temporally in secondary microseism band. We also characterize the dominant surface wave types in the microseism band. In long period band Rayleigh waves are dominant and Love waves are prominent in the short period band.

The Bundelkhand craton in India preserves important records of archean geological evolution, where several ultramafic rocks belonging to the Babina Greenstone Belt (BGB) occur as isolated and oval shaped bodies. These rocks are composed of olivine, orthopyroxene, amphiboles, and serpentine along with accessory mineral phases like chromian spinel and ilmenite. Here we present the major and trace element geochemistry of these ultramafic rocks that are characterised by low SiO₂ (45.16–49.00 wt%), high MgO (24.41–29.15 wt%) and moderate Fe₂O₃ (5.82–9.95 wt%) with high Ni (1164–1674 ppm), Cr (1532–3477 ppm) and Cu (14.7–39.5 ppm) suggesting primary magmatic nature. The rocks show low rare earth element (REE) content (ΣREE 2.1–3.5 ppm) with depleted LREE pattern and flat to slightly fractionated HREE pattern similar to abyssal peridotite signature. The Nb/Yb ratio ranges between 0.01 to 0.20 (average = 0.03), similar to that of N-MORB, suggesting magma derivation from a depleted mantle source, further substantiated by the Th/Yb vs. Ta/Yb plot. Trace elements like Ta and Pb show positive spikes, whereas La, Nb, Pr and Ce show depleted nature. The rocks generally have low platinum group elements (PGE) content (<150 ppb) except one sample where it goes up to 388 ppb. The ΣPPGE concentration is higher than ΣIPGE for all the samples and the high Pd/Ir ratio (7.55–20.98) indicating the derivation of these ultramafic rocks from low degree of partial melting. Our data suggest that the ultramafic rocks were derived from a depleted mantle source at a shallow depth with affinity towards abyssal peridotite. These rocks might represent residue after extraction of low degree melt (∼2–10%) in a mid-oceanic ridge (MOR) setting, which were captured and brought to shallow levels and subsequently exposed on the surface.

Petrography and scanning electron microscopy (SEM) investigations aided by energy dispersive X-ray (EDX) analysis and quantitative measurement of reservoir properties were used to extensively examine the physical and diagenetic characteristics of the Joyan Member Sandstone of Jaisalmer Formation. The Joyan Member Sandstone is fine- to medium-grained, moderate to well sorted, sublitharenite to litharenite. Mechanical compaction, precipitation of calcareous, ferruginous and silica cements, clay minerals, dissolution and alteration of unstable clastic grains such as feldspar and rock fragments, and grain fracturing are the identified diagenetic features. Feldspar and rock fragments underwent significant changes to kaolinite and chlorite while silica cement primarily originated from the dissolution and alteration of these grains and pressure solution. Mechanical compaction and the authigenic cements like calcareous, ferruginous, and silica reduced primary porosity, while secondary porosity was created by dissolution of clastic grains and cements. Compaction reduced porosity from an anticipated original 40% to around 13.4%. Porosity was reduced by cementation to 20.8%. Cementation reduced the porosity of the Joyan Member Sandstone somewhat more than compaction. Calcareous cementation played a major role in the porosity evolution of Joyan Member Sandstone. During early burial, the early calcareous cement occupied most of the pore spaces, leading to a significant reduction in porosity. However, incomplete filling or scattered patches of calcareous cement helped to preserve some primary porosity. In addition to calcareous cement, clay minerals like kaolinite and chlorite also acted as pore-filling and pore-lining cements. Kaolinite had a booklet-like or lamellar pattern contributing to minor porosity loss through pore-occlusion, while pore lining chlorite helped to retain porosity by preventing syntaxial silica overgrowth. Extensive dissolution of calcareous cement significantly increased the secondary porosity. Diagenesis affects reservoir quality by reducing initial porosity through cementation and compaction, and then increasing it through dissolution of early calcareous cement and unstable grains. The diagenesis of the studied sandstone is closely linked to its potential as a reservoir.

Subduction zones play a critical role in the global carbon cycle by regulating carbon exchange between the Earth's surface and interior. Processes that are known to release carbon from the slab, including metamorphic decarbonation and carbonate dissolution, cannot explain the high CO₂ flux in magmatic arcs. Slab melting is the least considered mechanism for carbon mobilization at subarc depths based on the high solidus temperatures of carbonated lithologies, which were experimentally determined under dry or H₂O-absent conditions. Subducted sediments are major carbon carriers, however, their melting behaviour with excess H₂O remains largely unexplored. Here, we perform fluid-present melting, high-pressure experiments at 750–1100 °C and 2.5–4 GPa using starting compositions similar to global average subducted sediments to determine the solidus, melting relations and carbonate stability fields. The onset of melting is between 750 and 800 °C at 2.5 GPa and between 850 and 900 °C at 4 GPa. Dolomite melts out on or close to the solidus, whereas crystalline aragonite persists >150 °C above the solidus. Flux melting of carbonated sediment at moderately hot subduction zones is examined to be feasible in the framework of the previously constructed dehydration history of the underlying serpentinites, providing a pathway to transfer carbon from the slab to the subarc mantle. However, complete breakdown of refractory aragonite requires at least 50 °C higher than that predicted for the hottest slab P‒T paths. Thus, even in the presence of H₂O, partial subducted carbon may survive the melting event occurring at shallow regions and reach considerable mantle depths.

Subduction mélange, with its distinctive block-in-matrix structure, is documented in exhumed fossil subduction zones worldwide. Rocks from these terranes preserve features that record tectonic processes from the time that the rocks were at the subduction interface. Careful study of these features allows for connections to be made with tectonic processes occurring in active subduction zones. The Catalina Schist mélange has served as an exhumed analog in such studies as it records abundant evidence for tectonic processes occurring at the subduction interface. Focusing of fluids in mélange matrix at the subduction interface is documented, and this fluid-rich environment may have contributed to seismic activity. Deformation and tectonic mixing juxtaposed disparate materials (mafic, ultramafic, and sedimentary rocks) over length-scales of 10s of km along the interface, occurring in concert with metasomatism and mass transport by fluids to create mineralogically, chemically, and rheologically distinct compositions. These processes may have impacted seismic behavior and plate geodynamics along with influencing the chemistry of arc magmas that form above the subduction interface. Evidence suggests that the duration of tectonic formation of mélange may be variable from one locality to another, with relatively small differences in peak ages of blocks of <10 Myrs recorded in the amphibolite facies rocks of the Catalina Schist.

We present new U-Pb detrital zircon ages, depositional history and tectonic model for the Liuqu Conglomerate (LQC) in southern Tibet that represents a critical geochronometer for the collision history of the Tibetan-Himalayan Orogenic Belt. LQC is a ∼5 km–thick, late Mesozoic–Cenozoic molasse deposit occurring strictly within the Yarlung Zangbo Suture Zone (YZSZ) and is tectonically overlain to the north by the Cretaceous Xigaze ophiolite and to the south by the Mesozoic Tethyan Himalaya sequence. It consists of matrix- and clast-supported conglomerates with sandstone intercalations, and its matrix includes poorly to moderately sorted sandstone and mudstone. New U–Pb detrital zircon dating of LQC sandstones has revealed a youngest zircon age of 307 ± 13 Ma and an oldest zircon age of 3362 ± 51 Ma. The age spectrum of zircons displays a prominent peak of ∼935 Ma, two large peaks at ∼516 Ma and 1474 Ma, and two small clusters of ∼2429 Ma and ∼2772 Ma that point to East Gondwana as the likely provenance for the LQC depocenter. The LQC represents fluvial deposits of an axial river system, which developed in an orogen-parallel, transtensional accommodation space within the YZSZ, after the collision of the Late Jurassic-Early Cretaceous Trans–Tethyan arc–trench system with the northern edge of India in the latest Cretaceous. The Indian subcontinent with the accreted Tethyan ophiolites and the intra–suture LQC depocenter arrived at and collided with the active margin of Eurasia during the latest Oligocene (∼23 Ma). The LQC depocenter started receiving clastic material and zircons for the first time from the Gangdese Magmatic Belt and the Xigaze forearc basin to the north by ∼20 Ma. The ensuing continent–continent collision resulted in significant crustal uplift across the collision zone, and in the inversion and rapid exhumation of the LQC strata by the early–Middle Miocene. The depositional and exhumation history of the fluvial LQC formation within the YZSZ involved two discrete collision events during the evolution of the Tibetan-Himalayan Orogenic system.

Ophiolitic mélanges, units that contain components of an ophiolite suite, provide crucial information on earth history and orogenic evolution. In this paper, four ophiolitic mélanges are characterized, including the Baijiantan-Yeyagou, Hebukesair, Zhaheba and Hongguleleng mélanges in the Junggar region (NW China), southern Altaids. Detailed geological mapping, structural, geochronological, and geochemical analyses constrain ages, geochemical affinities and relationships of magmatic with clastic rocks. MORB-/OIB-type (meta)gabbros and plagiogranite are the oldest mélange components, slightly older or coeval with associated chert and MORB-/OIB-type basalt; these rocks collectively constitute an ophiolite suite. Ophiolitic rocks predate associated clastic sedimentary rocks (conglomerate and turbidite) by ∼90-25 My, and associated SSZ-type hornblende gabbro, basaltic andesite, diabase, diorite and rhyolite by ∼78-37 My, except in the Hongguleleng mélange, where ophiolitic rocks predate latter units by ∼9-25 My. The ophiolitics are repeated by imbricate thrusts and duplexes, and folded. Ophiolitic blocks in mélanges locally preserve similar structures. Such blocks commonly have MORB and/or OIB geochemical affinities. Significantly older ages of MORB/OIB igneous rocks compared to ages of associated clastic sedimentary/SSZ-type igneous rocks shows that the former rocks formed as part of the crust of a large ocean, far from a convergent margin. The far-traveled oceanic crustal slices were imbricated and disrupted into block-in-matrix structures during accretion and incorporation into a subduction complex. SSZ-type magmatic rocks locally intrude into and extrude onto clastic rocks, demonstrating that a mélange contains multi-stage magmatic rocks. Folds, tilted structures and shear band cleavages are locally cross-cut by dikes, and these rocks are themselves have been dismembered into blocks. An intruded conglomerate in the Hongguleleng mélange contains pebbles of gabbro and basaltic andesite, the latter of which overlies sandstone. Superimposed folds in clastic rocks and chert record the polydeformation of the mélanges. Determination of the complex relationships of multi-stage magmatism and deformation illuminates the tectonic history of ophiolitic mélanges in the Junggar region. This history includes formation and subduction-accretion of the crust of a large ocean and post-subduction intracontinental deformation.