Solid Earth Sciences最新文献

Two 500-m-deep holes are proposed to be drilled at the Kane oceanic core complex (OCC), one aimed at a serpentinized peridotite massif and the other at a gabbroic body. Ocean drilling is vital to validate lithological interpretation derived from seismic structures. Utilizing a long marine streamer, seismic imaging can effectively delineate the dominant lithologies within the OCCs. This study applied a suite of techniques, including downward-continued multi-channel seismic data, full waveform inversion, and reverse time migration, to obtain detailed fine-scale shallow structures beneath the Kane OCC. Through the downward continuation method, refracted seismic data at near offsets were utilized, thus doubling the resolution at shallow depths. Compared to previous findings, our results greatly enhance the understanding of shallow subseafloor structures, revealing a more precise morphology of the gabbroic bodies and well-imaged very shallow low velocities caused by seawater percolation in the shallow fissure due to footwall rotation extension. We present reference drilling hole velocities and link them to the likely composition, considering potential alteration processes such as serpentinization of mantle peridotite. Our investigation suggests that the lower crustal melt fluxes of the Kane OCC represent a transitional phase from low to medium accretion, providing valuable insights for future scientific ocean drilling projects in this region.

In order to ascertain the governing mechanisms, sources, and speciation of ionic species, identify hydrochemical facies, and assess their appropriateness for agricultural use, groundwaters from several portions of the semi-arid basement complex of north–central Nigeria were analyzed based on chemistry. Basement aquifers made of gneiss-migmatites, metasediments, and granitoids store groundwater. Standard analytical techniques were used to analyze the fluids for main cations, anions, and physical characteristics. The results showed that the waters were slightly acidic (with a mean pH of 5.38, below the permissible range of 6.5–8.5), and that the predominant cations and anions were Na⁺ >Ca²⁺ >K⁺ >Mg²⁺ and Cl⁻ > NO₃^- > HCO₃^- > SO₄²⁻, respectively. Analysis of the ions' stoichiometric ratios reveals that alkali elements predominate, making up about 55.3% of the ions and being connected to silicate weathering. Based on ionic ratio calculations, it was determined that ion exchange was a key factor controlling water chemistry. Ionic species cross plots show that silicate weathering (sodic and calcic plagioclase) predominates. Hydrochemical facies, Gibbs plots, and principal component (correlation, cluster and factor) studies all show that ionic elements are geogenic, essentially coming from the weathering of silicates with ion exchanges. Based on predicted saturation indices, hydrochemical modeling by the computer program VISUAL-MINTEQ reveals that the majority of main ions occur in free mobile states with associated mineral species, all at undersaturated levels. Based on measurements of the sodium adsorption ratio (SAR), sodium percentage (% Na), and chloro-alkaline indices (CAI), the waters have been evaluated for their suitability for agricultural use.

Geochemistry was performed on clastic core sediments from the Tiko Mangroves estuary, South western region of Cameroon, to categorise the rock source composition, tectonic setting, past weathering intensity of the source area in relation to past climate and sediment maturity in relation to sedimentary cycle. Plots of La/Co, La/Sc, Cr/Th, discriminant function (DF1&2), TiO₂ vs Al₂O₃ and TiO₂ vs Zr point to an acid (felsic) and mixed (intermediate) rock source composition for the Tiko sediments. The acid composition portrayed by the sediments is also confirmed by their LREE (Light rare earth elements) abundance, and a negative Europium anomaly on chondrite normalisation, while the intermediate (mix) composition reflects the multiple sources of the sediments (Douala Basin and basaltic debris from Mount Cameroon). Binary plots Discriminant function (A-P) M, and Discriminant function (A-P) MT signpost active tectonic domain for the studied sediments, that resulted from the tectonothermal of the Pan African orogenic history and eruptive activity of the mount Cameroon. The weathering indexes denoted as CIX (chemical index of weathering) and PIX (plagioclase index of weathering) for the Tiko sediments advocate an intense source area weathering in a humid hot climate. The PIX advocate a high-level plagioclase lixiviation. The low values of ICV (index compositional variation) less than 1 (<1) couple with correlation plots Zr/Sc vs Th/Sc, and Zr vs (La/Yb)_N confirms that Tiko mangrove sediments are matured recycled sediments with compositional variations. This is the first comprehensive provenance study of mangrove ecosystem sediments in Cameroon.

The thermal springs of the study area are situated in North-eastern Arunachal Himalayas, India along Subansiri and Siang River valleys with surface temperature ranging between 20 and 57 °C. The pH of thermal springs varies from 7.69 to 9.31, indicating near neutral to alkaline nature of thermal and non-thermal waters. The major geochemical processes influencing hydrochemistry are demonstrated using conventional graphical plots, geochemical modelling by PHREEQC and multivariate statistical analysis. The thermal waters of Chetu and Taksing in Subansiri valley are primarily Na–Cl and Na–HCO₃ type, while, thermal water of Yangte in Siang valley is also mixed water-type and others of Ca–Mg–HCO₃ type. The geochemically distinct type of waters is obvious from dendrogram derived from hierarchical cluster analysis. Quartz geothermometer predicts reservoir temperatures of thermal springs of 88 ± 13 °C; while, Na–K Giggenbach geothermometer predicts 182 °C and 176 °C for Chetu and Taksing hot springs. Thermal waters are immature and highly prone to mixing with meteoric waters as evident from enthalpy-chloride modelling. Evaporite dissolution, silicate weathering and ion exchange processes are found to contribute to total ion budget in geothermal waters. The saturation indices studies depict oversaturation of all thermal waters with calcite and dolomite. Considering all geochemical features, a conceptual hydrological model resembling geomorphology and origin of thermal springs of North-Eastern Arunachal Himalaya has been proposed. The thermal waters of Subansiri valley display very high Sr and F⁻ content which prohibit them from drinking and utilization purposes. High concentration of toxic elements is addressed to geogenic causes over anthropogenic contributions due to lesser accessibilities at hot spring spots.

The Banyo Syenitic Pluton (BSP) is located on the south western extension of the Mayo Nolti Shear Zone (MNSZ). It is a NNE-SSW elliptical pluton. On the petrographic view, the BSP displays two rock types namely: hornblende-pyroxene-quartz syenite (HPQS) and hornblende-biotite granite (HBG), intruded in a plutono-metamorphic basement rock consists of biotite granite (BG) and hornblende-biotite gneiss (HBGn). Structural investigations indicate that the study area recorded three deformation phases: D₁, D₂ and D₃. D₁ is a flattening phase characterized by WNW-ESE to NNW-SSE (N110°E to N160°E) trending metamorphic foliation (S₁) with moderate (50°–60°) dips toward NNE to ENE. D₂ trends N–S and is characterized by crenulation cleavages (S₂), N–S sinistral shear and coeval S₂ foliation in hornblende-biotite gneiss. The emplacement of the HPQS and (HBG) took place during this phase considering the N–S global trend of the entire pluton and the NNE-SSW shape of the HPQS in the one hand and sinistral shear deformation microstructures display by plagioclase and K-feldspar crystals in HPQS. D₃ displays NE–SW (N30E to N45E) trend in HBGn and HPQS. Magnetic data indicate an inward-dipping NNE-SSW concentric pattern around station N32. The NNE-SSW elliptic shape and concentric magnetic foliation trajectories displayed by the BSP indicate its synkinematic emplacement during the D₂ N–S sinistral activation of the MNSZ with the feeder zone (station N32) located on the north eastern border. The sinistral activation is related to the Saharan Metacraton convergence over the Cameroon northern margin. This emplacement was disturbed by an overprinting E–W to NNW-SSE dextral syn-D₃ shear phase probably due to the dominant convergence (during its late stage) of the West African Craton over the Cameroon western border. Structural field data and magnetic fabrics infer that the Banyo syenitic pluton was emplaced in a N–S to NNE-SSW oriented fracture initiated during the transcurrent strike-slip MNSZ. The BSP is intruded in HBGn basement rock whose deformation ages are bracketed between 600 Ma (for the early syn-D₁ deformation structures) and 550 (for the late syn-D₃ deformation structures). The location of the BSP on the N–S to NNE-SSW sinistral MNSZ, between the GGSZ to the north and the RLSZ to the south, displaying similar structural features and N–S syn-to late kinematic syenitic plutons respectively dated at 593 Ma and 590 Ma help in dating the BSP pluton at 593 - 590 Ma. This age range dates several synkinematic emplacements of Pan-African younger syenite and granitoids along N–S and NE–SW shear zones during the western Gondwana post-collisional history.

The Kongur-Muztaghata-Maeryang terrane in NE Pamir is considered to be the western extension of the Songpan-Ganze terrane located in the northern Tibetan Plateau. The Kongur-Muztaghata gneiss dome (KMGD) is situated in the north while the Maeryang gneiss dome (MYGD) is in the south. The KMGD comprises Triassic granites and granitic gneiss in the core and Early Paleozoic-Triassic sediments in the mantle that underwent Barrovian-type and Buchan-type metamorphisms. Based on geochemical and geochronological data, the Kongur-Muztaghata magmatic arc was formed around ∼252–204 Ma due to northward subduction of the Paleo-Tethys Jinsha oceanic slab. The collision of the Kongur-Muztaghata magmatic arc and the Qiangtang terrane occurred subsequently. Previous research suggested that the KMGD was formed in the Miocene (21–8 Ma). However, our new in-situ monazite U–Pb data for the mantled metasediment shows that the KMGD was initially formed at ∼198 Ma.

The MYGD is comprised of an Early Paleozoic-Triassic metasediment mantle and a Cambrian anatexis complex core that underwent Barrovian-Buchan metamorphisms. Our new structural, geochemical, and geochronological data suggest that the protolith of the Maeryang orthogneiss was formed around ∼519-513 Ma, with the surrounding Early Paleozoic metavolcanic rocks erupted at ∼519-508 Ma. Together, they formed the Early Cambrian magmatic complex. In-situ U–Pb dating of monazites and zircon metamorphic rims for the Triassic metamorphic rocks in the mantle indicate that the Barrovian-Buchan metamorphism in the MYGD occurred around ∼206-187 Ma, likely caused by anatexis in the deep crust of the gneiss dome core. Thus, we propose that the KMGD and MYGD underwent a two-stage exhumation: the initial uplift during the Late Triassic-Early Jurassic thermo-tectonic event associated with the Cimmerian orogeny and the late rapid exhumation since the Miocene driven by the collision between the Eurasian and Indian plates.

It is widely recognized that in geothermal fields, meteoric water infiltrates deep into the subsurface of the earth and then travels through cracks and fractures, returning to the surface as it becomes heated. The patterns of fluid flow are primarily determined by the interaction between forces driven by gravity and pressure gradients. The ultimate forms of fluid flow patterns are primarily determined by the anisotropies of permeability associated with fault zones. In this study, a series of numerical simulations utilizing the finite volume approach were conducted to investigate the effects of fault zone architecture on fluid flow patterns and temperature distributions. Four distinct types of fault zone architecture were created in the simulations, including localized barrier, combined conduit-barrier, localized conduit, and distributed conduit. The results revealed that fault zone architecture has only a minor effect on fluid flow velocities and temperature distributions, except in cases along faults with very high permeabilities. The simulations suggest that this type of 2-D numerical modeling can be easily applied and utilized in other faulted geothermal systems.