Journal of Geodynamics最新文献

Aravalli craton of central Rajasthan comprises Mangalwar Complex and Sandmata Complex with Archean to Proterozoic basement, well known for the mineralization. Recent geological studies have also revealed that the Aravalli mountains of the Banded Gneiss Complex are composed of Paleoproterozoic granulite and amphibolite-facies. Extensive geophysical surveys comprising gravity and magnetic were conducted to assess the occurrence of potential mineral sources in the uplifted crustal blocks of the Aravalli Fold belt. The calculated Bouguer gravity anomalies trending in a NE-SW direction and three broad gravity highs (∼7–10 mGal) were observed. These gravity highs may be due to the uplifting of the basement or the presence of high-density contrast material near the subsurface. A gradient in the gravity contours at the northwestern part of the study area is due to the fault/structural contact trending along the NE-SW direction. The 2D inversion technique is used to model the gravity data perpendicular to fault/contact. The horizontal gradient reflects the distribution of structures and intrusive bodies, which will give new insight for further future mineral exploration. The spectral analysis depicts the three depth interfaces, ∼7, ∼3.1, and ∼1.1 km, representing the basement and shallow depth interfaces. Further, the subsurface ore bodies geometry was obtained through 2D modelling of gravity data incorporating the constraints from the rock sample's physical property (density). We established that the structure and lithology of the host rocks were responsible for controlling mineralization by integrating the geochemical findings with geophysical measurements. In addition, the Archean to the Proterozoic basement of the study area is undulating at a depth of ∼3–5 km.

Geophysical methods, especially the gravity method, are very helpful in ore and mineral explorations. Here, gravity modeling and interpretation for the subsurface geologic structures generally assumes either homogenous or spatially varying densities within target source rocks and surrounding structures. Therefore, the use of simple-geometric bodies helps in the validation of the subsurface ore and mineral targets. A Bat optimization algorithm is a recently developed metaheuristic algorithm that is used in various geophysical applications to explore and explain the parameters of buried ore and mineral targets. Using the Bat optimization algorithm, we were elucidating gravity anomaly profiles for ore and mineral cases. To perform global optimization, the Bat optimization algorithm is based on the echolocation behavior of bats. The global optimum solution in the Bat optimization algorithm reached the suggested minimum value of the objective function. The Bat optimization algorithm is applied to gravity data to estimate the target parameters (e.g., amplitude coefficient, depth, origin location, and geometric shape). The stability and efficiency of the introduced optimizing algorithm have been checked on two synthetic models represented in a spherical model and an infinitely horizontal cylinder model using two different kinds of noise. Furthermore, successful applications of the proposed algorithm for discovering the ore and minerals in Canada, Cuba, and India were presented. The results match well with the available geological and borehole information and other results from the published literature.

During 2008–2020, four strong earthquakes occurred in Yutian, Xinjiang Uygur Automous Region, northwest China, in particular, two M7 + and two M6 + earthquakes demonstrating the high tectonic activity of this region. We systematically use multiple electromagnetic data from satellites and ground, such as GIM TEC (Global Ionospheric Mapping Total Electron Content) published by JPL (Jet Propulsion Laboratory), and the ULF (Ultra Low Frequency) electromagnetic waves and plasma parameters onboard DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions), Swarm and CSES (China Seismo-Electromagnetic Satellite) satellites. The ionospheric perturbations were revealed frequently around the four case studies, but mostly within 10 days before, over the epicentral area, and sometimes over its conjugate region at southern hemisphere. The abnormal amplitude is quite larger in years with high solar activity than in those with low solar activity. We employ the SAMI2 model to simulate the variations from the effects of E × B under different plasma background in 2008 and 2014 to explain the great difference in different solar years. The similarity of the anomalies in this region demonstrates the higher electromagnetic and chemical emissions, implying that the electric field is possibly generated by the preparation of the seismic events in the epicentral area inducing the ionospheric disturbances above this area and its conjugate region through this coupling channel.

3D inversion of broad band MT data present variation of electrical signatures across the subducting Indian crust in Sikkim Himalaya. The vertical and horizontal geoelectric cross-sections are dominated by north-east dipping conductive zones. Two high conductivity zones (4–8 Ω m) at a depth of 5–18 km in Lesser Himalayan Domain (LHD) are explained by conductive mineral assemblage associated with abundant low saline and entrapped fluids. Another conductive feature (6–16 Ω m) in Main Himalayan Thrust Zone close to Main Himalayan Thrust ramp could have arisen from entrapment of CO₂-H₂O fluids and fluids released by metamorphic reactions. The high conductive anomaly (4–10 Ω m) at a depth of 5–16 km in Greater Himalayan Sequence (GHS) is caused by the presence of partial melts/aqueous fluids derived by present day fluid-absent melting of leucogranite source rocks. A combination of leucogranite intrusion, shear heating, and radiogenic heat production (4–17 μW/m³) are the heat sources for inferred partial melting. Though, the constrained melt fractions of 1.4–3.8% in GHS are lower than the estimation in south Tibet that might be due to the less intrusion of leucogranites. The obtained moderate viscosities of (10^4.19-10^5.49 Pa.s) from empirical relation with low melt and fluid fractions of 5–6 wt% in high conductive zone suggest viscous/ductile deformation and weakening mid-crust beneath northern Sikkim Himalaya. However, the estimated values of melt fractions and viscosities at mid-crustal depth of GHS are insufficient to develop a melt channel to flow southward between Main Central Thrust-1(MCT-1) and South Tibet Detachment (STD) envisaged by channel flow model.

The lithospheric structure of the on-shore Potiguar Basin has been investigated through velocity-depth profiles developed from the joint inversion of receiver functions and surface-wave dispersion at 16 seismic stations in and around the basin. The Potiguar Basin is an aborted rift basin that formed during the opening of the South Atlantic Ocean in the Lower Cretaceous, and is characterized by an unusual surface heat-flow with values as high as 101 mW/m². Our results reveal: (i) A relatively thin crust of ∼30 km below the on-shore Potiguar Basin and a relatively thicker crust of ∼32 km around the basin; (ii) the existence of an anomalous uppermost mantle of ∼4.3 km/s at 30–40 km depth under most seismic stations; and (iii) the presence of a negative velocity gradient centered at ∼125 km depth, which probably represents a shallow Lithosphere Asthenosphere Boundary (LAB). We argue that the anomalous uppermost mantle is associated with magmatic intrusions just below the Moho, deeper than previously postulated from independent heat-flow studies, and that those intrusions result from heating by an active, hot sublithospheric mantle under the basin that keeps the lithosphere thin. We further argue that heating from the magmatic intrusions, along with direct heating from the sublithospheric mantle, may explain the unusually elevated heat flow observed at the surface.

Metamorphic crustal formations of the Aktyuz block (SE part of the Chu-Kendyktas terrane; SW segment of the Central Asian Orogenic Belt) include garnet-bearing orthogneisses and gneissic granites of the Aktyuz Complex, garnet-bearing ortho- and paragneisses of the Kemin Complex and paragneisses with schists of the Kokdzhon Complex. The gneisses of the Aktyuz and Kemin Complexes associated with intensively altered eclogites, are referred to the retrogressed felsic granulites, which likely experienced high-pressure re-equilibration and dehydration melting under eclogite facies conditions. The eclogite-bearing garnet-mica gneisses of the Aktyuz Complex contain zircons with magmatic cores, overgrown by the rims with the low Th/U ratios of 0.005–0.05. The obtained age clusters of ca. 844 Ma and ca. 490 Ma likely characterize two stages of the rocks’ evolution in the late Neoproterozoic (emplacement of the gneisses’ protoliths) and in the latest Cambrian (high-pressure metamorphism of the gneisses’ protoliths). The garnet-epidote gneissic granites of the Aktyuz Complex and garnet-bearing chloritized orthogneisses of the Kemin Complex yielded late Neoproterozoic (Tonian) protoliths’ crystallization ages of 820–805 Ma, but these rocks do not show any evidence of the later re-equilibration and apparently avoided high-pressure metamorphism. Thus, the protoliths of the late Neoproterozoic orthogneisses represented by anorogenic granitoids, comprised Precambrian basement of the Aktyuz block in the Chu-Kendyktas terrane, and some part of the felsic rocks was involved into Early Palaeozoic subduction processes. Detrital zircons from the metasedimentary formations of the Kokdzhon and Kemin Complexes of the Aktyuz block display the main age peaks at 600, 800, 1000 Ma and weaker peaks at ∼1.5 and 2.5 Ga. The protoliths of the rocks were terrigenous lithologies, which are believed to have been formed after eroded felsic complexes of mostly Ediacaran, late Neoproterozoic, Mesoproterozoic and Palaeoproterozoic-to-Neoarchean ages, and accumulated during the Cambrian. The rocks likely made up sedimentary cover of the Chu-Kendyktas terrane and constituted the sand-siltstone-shale series. The presence of varisized rims of 495–471 Ma in the detrital zircons of the metasedimentary formations of the Kokdzhon and Kemin Complexes is consistent with the near-peak-to-retrograde stages of the latest Cambrian-Middle Ordovician metamorphic evolution of the rocks. The age estimates obtained for the crustal complexes of the Aktyuz block correlate well with those of the similar complexes known from the adjacent Issyk-Kul (North Tien Shan) terrane (Makbal Complex) and Zheltau terrane (Southern Kazakhstan; Koyandy Complex) in the SW part of the Central Asian Orogenic Belt.

We present an overall analysis of the recent seismic activity occurred in the Adriatic Sea region, a strongly debated sector of the Mediterranean area, where several authors have proposed different models of plate configuration and kinematics. In the past, seismic investigations of this marine area have been strongly hampered by non-optimal network geometries, but data quality increase and recent methodological improvements lay the groundwork to attempt more accurate analyses including proper evaluations of result reliability. On these grounds, we investigated the seismic activity of the last decades by means of new hypocenter locations, waveform inversion focal mechanisms and seismogenic stress fields. We used the Bayloc non-linear probabilistic algorithm to compute hypocenter locations for the most relevant seismic sequences by carefully evaluating location quality and seismolineaments reliability. We also provided an updated database of waveform inversion focal mechanisms including original solutions estimated by applying the waveform inversion method Cut And Paste and data available from official catalogs. Then, focal mechanism solutions have been used to estimate seismogenic stress fields through different inversion algorithms. Seismic results indicate a relevant degree of fragmentation and different patterns of deformation in the Central Adriatic region. In particular, our analyses depicted two NW-SE oriented, adjacent volumes: (i) a pure compressive domain with NNE-trending axis of maximum compression characterizes the northeastern volume where the seismic activity occurs on W-to-NW oriented seismic sources; (ii) a transpressive domain with NW-trending axis of maximum compression characterizes the southwestern sector where thrust faulting preferentially occurs on ENE-to-NE oriented planes and strike-slip faulting on E-W ones. Joint evaluation of seismic findings of the present study and kinematic models proposed in the literature indicates just in the Central Adriatic region the presence of a broad deformation zone, accommodating a still evolving fragmentation of the Adriatic domain in two blocks rotating in opposite directions. On these grounds, the obtained results not only furnish new seismological evidence supporting the "two-blocks model" proposed by previous authors, but they also provide additional constraints, useful for better understanding and modeling the seismotectonic processes occurring in the Adriatic region.

Data availability

Data used in the present study were collected from catalogs and bibliographic sources indicated in detail in the article. Waveform inversions performed in this study used data available in the database EIDA, http://orfeus-eu.org/webdc3/ (accessed February 2022)

Stress patterns are reconstructed for large fault zones and the respective fault-bounded blocks in Western Transbaikalia (southeast of Lake Baikal). The reconstruction is based on analysis of structural measurements combined with slickenside data and earthquake mechanisms. The combination of several methods for stress inversion provides high quality of the results. The inferred diversity of local stress tensors is analyzed in terms of the hierarchy of crustal stresses unevenly distributed in space and time. The tectonic stress fields at the local, subregional, and regional levels result from changes in the magnitude of principal stresses and their respective switch during the evolution of fault zones. The reported regional paleostress reconstructions for rocks of different ages not only have confirmed the sequence of events in the Mesozoic-Cenozoic history of Western Transbaikalia but revealed additionally the previously unknown stress regime of strike-slip which existed there after the closure of the Mongolia-Okhotsk ocean. The heterogeneous patterns of local stress tensors indicate the absence of a single crustal stress field in the region.

We interpret the crustal and upper mantle structure along ∼2500 km long seismic profiles in the northeastern part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP) and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination dripping. An intra-lithospheric discontinuity (ILD) at ∼75 km depth with positive velocity contrast is probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long wave train of scattered phases of up to 4 s duration are observed in the 800–1300 km offset range, which are modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially molten or close to the solidus, which explains both low velocity and the heterogeneous structure.

Ediacaran syn-tectonic plutonic rocks (amphibole gabbros, quartz diorites/tonalites to biotite- and muscovite-bearing granites) of the Angra Fria Magmatic Complex (Kaoko Belt, north-western Namibia) belong to two compositionally similar, magnesian, transitional tholeiitic–calc-alkaline suites, the Older (∼625–620 Ma) and the Younger (∼585–575 Ma). Both have counterparts in the broadly contemporaneous Florianópolis Batholith (southern Brazil), from which they were separated during the Cretaceous opening of the southern Atlantic. In the Angra Fria Magmatic Complex, the only unequivocal mantle contributions are identified in mingling zones of the Younger Suite and hybrid mafic–intermediate dykes of uncertain age. Previously published Hf-in-zircon isotopic data, together with new whole-rock geochemical and Sr–Nd isotopic signatures, underline an important role of crustal anatexis of a material with late Palaeoproterozoic to early Mesoproterozoic mean crustal residence (1.9–1.5 Ga). This interval resembles some of the published Nd model ages for Tonian ‘Adamastor Rift’-related felsic magmatic rocks in the Namibian Coastal and Uruguayan Punta del Este terranes. In detail, the Older Suite probably originated mainly by fluid-present melting of metabasalts and metatonalites, followed by (near) closed-system fractional crystallization (with or without accumulation) of amphibole ± plagioclase. For the Younger Suite, the principal process was the dehydration melting of relatively felsic lower crustal protoliths (metagreywackes or intermediate–acid orthogneisses >> metapelites), leaving garnet in the residue. Based on the geological context, the conspicuous enrichment of hydrous-fluid-mobile large ion lithophile over the conservative high field strength elements is not interpreted through a classic model of oceanic plate subduction, devolatilization, and fluxed-melting of the overriding mantle wedge. Instead, it is thought to reflect high-grade metamorphism of deeply buried continental crust and attendant water-fluxed melting of the overlying crustal lithologies, connected with inversion of the Tonian ‘Adamastor Rift’.