Journal of Asian Earth Sciences最新文献

The Akkarga ultramafic massif exposed in the Trans-Uralian megazone represents the mantle section of an ophiolite assemblage that was emplaced into the upper crust during the Permian collision. Ultramafic rocks include harzburgites and subordinate dunites that have undergone complete serpentinization and host podiform chromitite bodies typical of the ophiolite complexes worldwide. Numerous lens-shaped and podiform occurrences of massive and densely disseminated chromitites are surrounded by envelopes of serpentinized dunites whereas nodular chromitites and lenticular bodies of banded disseminated ores are of subordinate importance. Three ore zones are distinguished on the massif, including the Western, Central and Eastern, but only the sites of the latter two are currently accessible for study. Chromite from chromitites of the Central Ore Zone has higher Cr# (Cr/Cr + Al) (0.81–0.83) than that of chromitites of the Eastern Ore Zone (Cr# = 0.67–0.80) and accessory chromite of peridotites (Cr# = 0.52–0.75). Chromites contain mineral inclusions, which are distributed unevenly. The most abundant are inclusions of high-Mg high-Ni olivine (Fo_94–98 and up to 1.5 wt% NiO) and calcic amphibole, while inclusions of pyroxenes and base metal sulfides are less common. Platinum group minerals (PGMs) in chromitites are represented by alloys, sulfides, and sulfoarsenides, which occur in single-mineral and composite inclusions. Ruthenium and Os disulfides typically compose the euhedral single-mineral inclusions in cores of chromite crystals, whereas the composite inclusions, mostly of irregular shapes, are dominated by Ir compounds. PGMs are regularly associated with OH-bearing silicates such as amphibole and, less frequently, chlorite. The setting, morphology and composition of the inclusions seem to support a leading role of subsolidus solid-state exsolution in the formation of primary laurite-erlichmanite mineralization in Akkarga chromitites. Subsequent hydrothermal reworking of podiform chromitites and their ultramafic hosts, which is likely related to the supra-subduction setting, led to the precipitation of more diverse interstitial assemblages, comprising base-metal sulfides, nickeline, sulfoarsenides of the Ir-subgroup platinum group elements (IPGE), REE phosphates, zircon, barite, and baddeleyite. Later granite intrusions likely provided an additional contribution of fluid-mobile incompatible elements to chromitites.

Identifying the tectonic transition from oceanic subduction to collision is crucial for tracking the final stage evolution of ancient orogenic belts. In this study, we present new geochronological and geochemical data for the Mozbaysay mafic–ultramafic complex in the Balikun area, eastern North Tianshan of the southern Central Asian Orogenic Belt. This complex had intruded into the late Carboniferous volcano-sedimentary rocks and is comprised mainly of hornblende gabbro and lherzolite. Zircon U-Pb ages of the hornblende-gabbros reveal that this complex was emplaced at ca. 305 Ma. Geochemical analyses suggest these mafic–ultramafic rocks are characterized by slight enrichment of light rare earth elements (LREEs) and relatively depleted heavy rare earth elements (HREEs), resembling enriched mid-ocean ridge basalt (E-MORB). They also exhibit restricted (⁸⁷Sr/⁸⁶Sr)_i ratios (0.702396–0.704295) and εNd(t) values (+7.0 to +9.1), indicative of a depleted mantle source with minimal crustal contamination. Incompatible element ratios (i.e., Nb/Ta, Zr/Hf, Rb/Nb, and Ba/Nb) suggest the involvement of subducted slab-derived aqueous fluids in their mantle source. These collectively indicate that the parental magmas of the Mozbaysay mafic–ultramafic rocks may have been generated by a mixed mantle source consisting of the E-MORB-like asthenospheric mantle, subcontinental lithospheric mantle (SCLM), and hydrous fluids from subducted slab. Furthermore, a slab break-off model is proposed to explain the generation of these latest Carboniferous mafic–ultramafic rocks. Integrating these findings with regional geological data, we propose that the tectonic transition from subduction (slab roll-back) to collision (slab break-off) along the Kalamaili suture zone occurred at ca. 305–300 Ma.

The growth and expansion of the southeastern Tibetan Plateau remain contentious. Positioned as one of the regions characterized by the most robust tectonic activities on the plateau, the southeastern edge provides a distinctive setting for investigating plateau uplift and landform evolution. This study focuses on the southeastern margin of the plateau in the Three Rivers Region, conducting comprehensive analyses of slope, relief, hypsometric integral (HI), and channel steepness index ($k_{\mathrm{sn}}$). We use this new dataset to highlight the more significant role of tectonic activities in shaping the landform compared to climate and lithology. By examining the spatiotemporal characteristics of long-term and short-term rock exhumation rates, derived from low-temperature thermochronology and cosmogenic nuclide ¹⁰Be analysis, we establish a correlation between erosion rates and $k_{\mathrm{sn}}$, slope, and terrain undulation. Integrating this information with geophysical evidence and GPS data, we support the model for the expansion of the southeastern edge—the steady-state terrain crustal flow model. According to this model, there is an equilibrium achieved between rock uplift and surface erosion on the southeastern edge of the Tibetan Plateau. Despite ongoing southeastward extrusion of plateau material, the overall plateau morphology remains unaltered due to intense erosion along the plateau’s edge. Consequently, the large-scale topographic expansion of the southeastern Tibetan Plateau has effectively halted.

Kırşehir Block, a large triangular continental domain exposed in Central Anatolia, is surrounded by ophiolitic melanges that mark former subduction zones associated with the closure of the northern and southern branches of the Neotethys Ocean. Notably, the Ayhan Basin, located in the inner part of the block, is one of the few locations containing a geological record from the Upper Cretaceous to the Pliocene. It provides a unique opportunity to examine the crustal deformation and the temporal and spatial stress distribution over the Central Anatolian orogen. This study encompasses a comprehensive analysis of the Ayhan Basin’s stratigraphy and structure, combined with the paleostress inversion solutions derived from over 400 fault-slip data. The findings reveal that the Ayhan Basin underwent three distinct tectonic phases: (1) ∼ NE-SW extensional phase from Late Cretaceous to Lutetian, which refers to the main basin-forming stress configuration. Importantly, this extensional phase is crucial to delineate the spatial extent of the concurrent collision in the north. (2) ∼ N-S compressional phase during the Lutetian, mainly governed by the shortening of the Kırşehir Block. This compressional phase caused the inversion of the inherited normal faults in the basin; and (3) ∼ NE-SW extensional phase active since the Late Miocene was accompanied by volcanism and uplift in the region. The Cyprus Slab’s southward retreat might be this extension’s triggering mechanism. Consequently, the temporal and spatial extent of the tectonic phases and their resultant deformations are crucial for understanding the underlying geodynamic processes and establishing their timing.

Cobalt (Co) in skarn deposits generally occurs as discrete Co minerals, or dispersed in sulfides. Recent studies have shown that magnetite from Baijian Fe-Co deposit (North China Craton) contains significant amounts of Co, but the mechanism for its enrichment was unclear. In this study, we present in situ mineral trace element and pyrite sulfur isotopic data from Baijian deposit (reserves of 112 Mt Fe at an average grade of 48.5 %): our results show that Co is mainly hosted in cobaltite, magnetite, and pyrite. Magnetite contains 47.6 ppm Co on average, accounting for most of the Co resource in the deposit. Elevated Co concentrations correspond to elevated levels of both Ti + V and Al + Mn, indicating the influence of elevated temperature. Pyrite grains from different lithologies in the deposit have variable Co contents, with those from the related diorite intrusion having the highest (averaging 9264 ppm Co). Sulfur isotopes of pyrite from the magnetite ores range from 16.6 to 19.4 ‰, similar to those from the Cixian Formation limestone and dolemite, which suggests that the sulfur was mainly derived from evaporites in those strata. Pyrite in magnetite-bearing skarn exhibits core-rim zoning, with the core having higher δ³⁴S_V-CDT values than the rim, which has values similar to those of pyrite in the diorite. A comparison of our results with other skarn Fe deposits shows that magnetite tends to have a higher Co concentration with increasing δ³⁴S_V-CDT values, suggesting that oxygen fugacity has an important influence on Co enrichment in magnetite.

The Red River Fault (RRF) zone is situated on the southeastern boundary of the Tibetan Plateau. To further explore the geodynamic model of the region and the collision mechanism of the Eurasian continent, the electrical structure model of the lithosphere in the RRF was obtained through 3-D inversion of MT data. The preferred model indicated that fault zones were primarily distinguished by low resistivity anomalies, with some exhibiting high resistivity characteristics. It was hypothesized that the low resistivity anomalies may be attributed to the presence of saline fluids or metal sulfide within the fault zones, on the other hand, the high resistivity characteristics suggested inactivation due to fault cooling. Rapid changes in the electrical structure occurred on both sides of the fault zones, causing the upper and middle crust to be divided into distinct secondary tectonic units. Based on the effective viscosity calculation, we analyzed the crust-mantle rheology of the RRF area. We assumed that the RRF area was composed of three geodynamic models: the “crustal flow” model was distributed from the northern part of the Lancang River Fault (LCRF) zones to the southern part of the RRF zones. The “ductile continuum rheology” model was distributed from the north of the RRF to the western part of the Xiaojiang Fault (XJF) zones. The “mantle upwelling” model was distributed in the southeast part of the RRF.

Physics-based simulations have been extensively employed to generate synthetic earthquake catalogs over the past decade. In this kind of simulation, primary known faults are typically modeled, while smaller and intermediate faults are often neglected. As a result, the location of earthquakes in the simulation is confined to the modeled faults. Furthermore, due to the elimination of off-fault seismicity, a complete match of the frequency-magnitude distribution of the synthetic catalog with observed data is not achieved. This paper presents an approach to include off-fault seismicity (or background seismicity) within the simulation. First, background earthquakes are separated from the primary faults’ earthquakes, and a fault section is assigned to each event. By employing a scaling relationship, the dimensions of these fault sections are determined according to the magnitude of the corresponding event. The fault section’s slip rates are estimated based on the observed frequency-magnitude distribution. Combining the background fault model with the primary fault model results in a comprehensive model that accounts for all stress interactions between fault elements within the simulator in actual locations. As a result, a broader range of frequency-magnitude distribution in the synthetic catalog can be matched to the observations. A case study using one of the available simulators, Virtual Quake, on a sample area is presented. The results demonstrate that eliminating off-fault seismicity could lead to an underestimated assessment of the earthquake probabilities for the whole region and a biased estimation of earthquake rates in individual faults.

Southeast (SE) Tibet has become a focal point of scientific interest, with an ongoing debate about the presence of crustal flow channels in the region. We built a high-resolution crustal velocity and density model of SE Tibet from joint inversion of surface wave dispersion and gravity anomaly data. We used a 3D joint inversion method to invert for the crustal S-wave velocity and density structure. The joint inversion enhances both model resolution and robustness in comparison to traditional travel-time inversion of surface waves. A notable high velocity anomaly is evident in the crust of the Panzhihua area, potentially indicating an association with the Emeishan Large Igneous Province. We found obviously low velocity and low density anomalies in the middle-lower crust, suggesting that the crust of SE Tibet is ductile and may promote crustal flow. Additionally, there is an obvious low velocity and low density anomaly beneath the Tengchong volcano, suggesting the possible presence of a magma chamber beneath this region. This low-V anomaly beneath Tengchong volcano may be connected to the crustal low velocity channel in SE Tibet.

The restraining and releasing bend region, encompassing the South Qianning, Kangding, and North Moxi segments, occupies a distinctive position within the Xianshuihe fault zone and plays a pivotal role in the analysis of seismic hazards. Our study focused on paleoearthquake research on the Qianning segment of the Xianshuihe fault zone through the integration of tectonic geomorphology, trench excavations, and radiocarbon dating methodologies. Three events, designated as ET1-ET3, have been found, occurring at 635–519, 823–672, and 3515–1482 yr B.P., respectively. A chronological framework for earthquake events has been established since the Holocene. The coefficient of variation (CoV) (0.75 reveals a weakly periodic recurrence model governing the activity of the Qianning segment. Moreover, both the Qianning and Kangding segments exhibit heightened susceptibility to cascading ruptures within the context of a single earthquake. The integration of shallow and deep data reveals a noteworthy transformation in the fault structure, transitioning from a solitary deep structure within the Qianning segment to a flower structure in the Kangding segment. This structural evolution is attributed to the migration of activity from the Yalahe Fault to the Selaha Fault and Zheduotang Fault, resulting in the short-cutting process within the Xianshuihe Fault Zone.