Journal of Asian Earth Sciences最新文献

Alkaline lake is one of the most productive ecosystems on Earth, commonly characterized by massive organic matter accumulation. However, the primary biological precursors of organic matters and the controls on their accumulation in the ancient alkaline lakes remain poorly understood. Here we use petrology and organic geochemistry analysis of the Upper Paleozoic Fengcheng Formation of the Halaalate area in the Junggar Basin, NW China, to study the biological diversity and controls of water salinity on primary productivity in the ancient alkaline lakes. Two depocenters have been identified in the Halaalate area: a proximal depocenter close to the boundary mountains and a distal depocenter far away from source areas. The results show that water salinity was much larger for the first and second members of Fengcheng Formation (FC1 and FC2) compared to the third member (FC3), and the distal depocenter had more saline lake water than the proximal depocenter. Abundant primary producers have been identified to be flourishing in the low-salinity alkaline lakes, such as cyanobacteria, dinoflagellates and green algae, whereas only a special haloalkaliphilic green alga can survive in the hypersaline alkaline lakes. Therefore, the low-salinity alkaline lakes are characterized by a higher primary productivity and can deposit mudstones containing richer organic matters compared to the high-salinity ones. This study suggests that water salinity is the major factor controlling the biomass and biodiversity of ancient alkaline lakes and mudstones deposited in the low-salinity alkaline lakes are more promising for oil exploration.

An M_W 7.4 earthquake struck Maduo County, Qinghai, China on 22 May 2021. To better understand the seismogenic structure of this region, we collect local earthquake arrival time data at the MAD station in the China Earthquake Networks Center observational bulletins during 1 June to 20 September 2021, and manually pick P and S wave arrival times from high-quality seismograms recorded at 34 recently deployed MaduoArray portable seismic stations. Using these arrival times, we relocate the Maduo earthquake sequence using earthquake association, absolute location and relative location methods. Our results show that the 2021 Maduo earthquake sequence occurred along the Kunlun Mountain Pass-Jiangcuo fault zone in the NWW-SEE direction and the aftershocks are located on both sides of the mainshock, showing characteristics of bilateral rupture. Vertical cross-sections of the aftershock distribution illustrate a nearly vertical shape of the seismogenic fault that tilts toward the northeast and southwest in different sections, reflecting a complex geometry of the fault plane. There is a horsetail bifurcation phenomenon at the eastern end of the fault zone. A sparse area of aftershocks appears at about 30 km east of the mainshock epicenter, which may be associated with a uniform fault friction and sufficient release of rupture energy caused by super-shear rupture of the mainshock. Taking into account many geophysical results including seismic tomography and magnetotelluric soundings, we speculate that the occurrence of the Maduo earthquake could be affected by crustal fluids in the fault zone. The fluids may ascend from the lower crustal flow beneath northeastern Tibet.

Rodingite, a metasomatic rock type related to the serpentinisation of ultramafic rocks, occurs as dykes or lenses in serpentinite of the ophiolitic mélange. The formation age, protolith and metamorphic context of the rodingites are crucial for evaluating the hydrothermal activity of the ancient ocean floor and the tectonic history of the ophiolite. This study presents particular research on metamorphic petrology, geochemistry and zircon U–Pb chronology of rodingites and their associated mafic–ultramafic rocks in the Baixingtu ophiolite, the middle segment of the Erenhot-Hegenshan ophiolite belt (EHOB), southeastern Central Asian Orogenic Belt. The mean metamorphic ages of rodingites are 345.8 ± 3.8 Ma, 339.9 ± 4.8 Ma, and 344.5 ± 9.2 Ma. According to the chlorite thermometer, the final mineral assemblages of rodingites formed at temperatures ranging from 114.99 °C to 351.10 °C. The high oxygen fugacity of nascent clinopyroxenes and the negative anomaly of Ce in adjacent serpentinites (δCe = 0.34–0.77) prove that rodingitisation occurs in shallow oceanic crust by the reaction of seawater with ultramafic rocks to produce Ca-rich fluids. Accordingly, the Baixingtu ophiolite was produced by an ocean floor metamorphism, whose rodingitisation occurred shortly after the formation of the oceanic crust. Combined with other ophiolite data from the EHOB, the Hegenshan Ocean was constantly generating new oceanic crust in the Early Carboniferous.

The objective of this study is to systematically document the depositional architecture and evolution of the carbonate platform margin in the Lower Cambrian Xiaoerblak Formation of the northwestern Tarim Basin. This study uses the following approaches: (1) seismic reflector identification; (2) lithofacies and paleoenvironmental interpretation based on Paleozoic outcrops; and (3) thin section examination. Identification of seismic reflectors and determination of lithofacies associations in Paleozoic outcrops reveal two phases of architectural evolution of the Xiaoerblak Formation platform margin. Phase 1 corresponds to the Lower Xiaoerblak Formation, characterized by discontinuous to semicontinuous moderate- to high-amplitude reflectors, revealing a uniform, gentle ramp platform margin thinning toward the basin. The dolomudstone and laminate lithofacies associations in the outcrops show a middle-ramp low-energy depositional environment. Phase 2 corresponds to the Upper Xiaoerblak Formation. The seismic stratigraphic units display upwardly convex irregular reflectors, indicating the development of a rimmed carbonate platform margin system. The lithofacies associations reveal reef-shoal interbedding, suggesting a high-energy marginal marine environment. The tectonic and paleomorphic evolution of the Tarim Basin, along with Paleozoic outcrop features, suggest that paleomorphic inheritance from the Neoproterozoic created a homogeneous, broad, low-angle shelf. This, combined with the continuous sea-level fall in the early Cambrian and the tropical environment, provided an ideal depositional environment for carbonate platform development in the Xiaoerblak Formation.

This study aims to constrain the paleolatitude of the southern margin of the Lhasa terrane during the late Paleocene and refine the constraints on intracontinental shortening within Asia resulting from the India–Asia collision. An integrated paleomagnetic and petrographic study was conducted on the upper Paleocene Jialazi Formation in the Xigaze forearc basin, southern Tibet. The limestone in the Jialazi Formation was demonstrated to reliably preserve primary remanence. Combined with previously published data, the tilt-corrected mean direction was Ds = 166.6° and Is = -38.2° with α₉₅ = 4.1 (n = 118), corresponding to a paleomagnetic pole at 75.3°N, 323.4°E, with an A₉₅ of 3.7°. Consequently, the paleolatitude of the Xigaze forearc basin from 56–59 Ma was estimated at ∼ 21.5°N for the reference point at 29.8°N, 84.9°E. Compared with data from the western Lhasa terrane, the Xigaze forearc basin and the Linzhou Basin, these findings suggest that the southern margin of the Lhasa terrane had an east–west orientation during the Late Cretaceous–early Eocene. A comparison with the apparent polar wander paths for Asia indicates that the intracontinental shortening between the Lhasa terrane and stable Asian interior has been 890 ± 470 km since the late Paleocene.

International Ocean Discovery Program (IODP) Expedition 350 drilled Site U1437 in a submarine volcano-bounded basin situated in the modern Izu-Bonin rear-arc, NW Pacific Ocean. Subaqueous volcaniclastic sediments and rocks with a maximum deposition age of 15.4 ± 0.8 Ma were recovered from 0 to 1,806.5 m below seafloor (mbsf). In order to document post-depositional processes in such geological setting, we describe variations in bulk and clay mineralogy over the entire volcaniclastic succession. Four alteration stages (1, 2, 3 and 4) were identified through the occurrence and development of diagenetic background mineral assemblages (smectite ± Na-Ca zeolites ± illite) that were further superimposed by hydrothermal alteration. Stages 1 and 2 are characterized by diagenetic reactions linked with low fluid/rock interactions that enabled glass devitrification and subsequent lithification under burial conditions. Stages 3 and 4 are characterized by moderate to pervasive alteration processes that are well developed in coarser-grained rocks, and that may be induced by thermal pulses associated with fluid inputs. Below 1,460 mbsf, infilling and replacement textures overprinted the background alteration and can be directly linked with the development of two hydrothermal mineral assemblages: (i) ordered C/S (chlorite-smectite mixed-layers) ± chlorite ± albite, and (ii) calcite ± chalcedony ± anhydrite ± laumontite. Both assemblages evidence relatively low-temperature (up to 225 °C) hydrothermal activity that affected subaqueous volcaniclastic rocks at Site U1437. These assemblages are comparable with propylitc alteration facies present in ore-bearing hydrothermal systems. The preferential development of alteration mineral assemblages in high-permeability, coarse-grained lithofacies, reflects the significant influence of the physical properties of volcaniclastic rocks with depth on chemical kinetics, in comparison with those imposed by the local geothermal gradient.

Deciphering the interactions between tectonic and exhumation processes in the Tanggula Mountains (central-northern Tibetan Plateau) can provide insights into the processes of the Tibetan plateau uplift and its geomorphic evolution. In this study, we present new detrital apatite fission track (AFT) data from Cenozoic sediments of the Tuotuohe Basin (northeastern part of the Qiangtang terrane) and its periphery (including the Tanggula Mountains), with the aim to reconstruct the cooling history of the Tanggula Mountains during the Cretaceous and the Cenozoic era. Our results show that the provenance of detrital material evolved in the Tuotuohe Basin and highlight that previously deposited sediments were recycled into the Tuotuohe Basin at ∼ 27.5 Ma. The data further outline that the Tanggula Mountains and the Tuotuohe Basin experienced three major phases of tectonic uplift and exhumation: 122–106, 65–54, and 44–35 Ma. These exhumation-induced cooling phases might be related with three phases of primary tectonic activity, i.e., the collision between the Qiangtang and Lhasa terranes (central part of the Tibetan Plateau) that started during the Early Cretaceous, the collision of the Indian and Eurasian plates in the Early Cenozoic and finally, the “hard collision (the Indian and Eurasian continents)” that occurred during the Early Eocene–Oligocene.

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.