Journal of Asian Earth Sciences: X最新文献

Biogenic limestones from three sections (north, central, and south) across peninsular Italy have been analysed for major and trace elements and Nd, Pb, and Sr isotopic ratios. These data are used to monitor the evolution of the Tethys Ocean from the Triassic through to the Miocene. Limestones’ major, trace, and REE elements contents are consistent with their formation in seawater with little sign of crustal, volcanic, or hydrothermal input. V/Cr and Ce/Ce* ratios indicate their deposition in oxygenated waters. Rb-Sr-Ba discrimination diagram, consistent with the immobile trace element distribution, indicates that limestone deposition took place in either marginal or open ocean environments. Ages based on stratigraphy are in good agreement with the chronostratigraphic Sr curves implying that the Tethys ocean, throughout its history, was in contact with the open, global, ocean system. Although the isotopic values of Sr and Nd are relatively restricted, Pb is extremely variable and highly radiogenic. High Pb isotope ratios characterise limestones deposited during the rifting of the southern Tethyan ocean in the Lower Jurassic and in the Lower Cretaceous, suggesting stronger crustal inputs in small basins. The weighted average, present-day, isotope values (AIL = average Italian limestone) for the Italian limestones, excluding anomalous samples, are ⁸⁷Sr/⁸⁶Sr = 0.70785, ¹⁴³Nd/¹⁴⁴Nd = 0.51227, and ²⁰⁶Pb/²⁰⁴Pb = 18.94, ²⁰⁷Pb/²⁰⁴Pb = 15.69, ²⁰⁸Pb/²⁰⁴Pb = 38.66. These values are useful in monitoring the fate of limestones during orogenesis and the role that they may have played in magma genesis.

The surface trace of the East Zambales Fault (EZF) and its associated faults in the Lingayen Gulf have been previously mapped but no other characteristics were reported. This study utilized seismic reflection, multi-beam bathymetry, and side scan sonar to characterize the offshore EZF in terms of magnitudes of vertical displacement. Sequence stratigraphy and radiocarbon dates provided age constraints on the recurrence interval within the Holocene.

The EZF extends for ∼ 57 km into the gulf, follows a north-northwest trend, and bounds the karstic terrane (west) and fluvio-deltaic deposits (east). Sinistral motion is indicated by: 1) normal and reverse drag geometries, 2) reversal in the sense of throw with depth, 3) flower structure, and 4) right-stepping and the uplift of a pressure ridge named Pudoc Bathymetric High. The Central Lingayen Gulf Fault (CLGF), to the east of EZF, follows the same trend. The Lingayen Gulf Transverse Fault (LGTF), oriented east–west, forms a flower structure with the CLGF. The EZF, CLGF, and LGTF combined form the Lingayen Gulf Fault System, which divides the gulf into five fault blocks where uplift and subsidence locally occurred.

A paleo-delta at −60 m yielded an age of 6.8 kyBP, indicating it was formed during the first Holocene highstand. With natural compaction considered, fault-associated subsidence of 46–53 m may have occurred. The average Holocene vertical displacement is 2.1–2.2 m, which translates to a recurrence interval of 320–270 years for the fault system. The faults can likely generate earthquakes with magnitudes 7.5 (EZF), 6.7 (CLGF), and 6.6 (LGTF).

Northwestern Anatolia contains voluminous Cenozoic magmatic rocks which were emplaced during syn- to-post collisional stages of long-term crustal accretion and extensional stages since the late Paleocene. The Eocene to Late Miocene plutonic and volcanic rocks that are located throughout the Rhodopes, northern Aegean, and the western part of Western Anatolia show generally southward decreasing ages, coupled with an increasing crustal recycling component in their genesis. However, the early Eocene, ∼SW–NE-trending, mafic volcanic and the early Eocene, ∼NW-SE-trending, granitoid belts in the northeastern parts of Western Anatolia do not share these features. We present here new U-Pb zircon ages, whole-rock geochemical analyses, and Sr-Nd isotopic data from the early Eocene NW-SE-trending granitoid belt, together with age data from arc-related pyroclastics in the region, in an effort to resolve these uncertainties.

The age data reveal that the post-collisional magmatism along the ∼ NW–SE-trending granitoid belt occurred ∼ 58–41 Ma; i.e. ∼ 30 Myr after the Pontide arc magmatism that was active from ∼ 92–74 Ma. We suggest that the ∼ SW–NE-trending mafic volcanic and the ∼ NW-SE-trending granitoid belts developed in response to break-off of two subducted slabs in the northern Neotethys. In addition, emplacement of the ∼ NW-SE trending granitoid belt may also have been influenced by a zone of weakness related to a series of NW–SE-trending dextral strike-slip shear zones lying from the Kapıdağ shear zone close to the Rhodopes in the NW to the Uludağ shear zone in the SE.

The origin of active deformation and structural evolution in large areas of Western Borneo has been highly debated, with two contrasting views involving gravitational tectonics and plate tectonics. The scarcity of field data on land has significantly hampered our understanding of the onshore structures and their relationship with those of the offshore regions. The Baram Delta province is one of the best examples in SE Asia, where debates on the origin and evolution of active tectonic versus gravitational tectonic structures are broadly associated with the complexity of faults and folds in Brunei and Sarawak (Malaysia). In this paper, we present the results of the first large-scale satellite-based structural mapping and the detailed outcrop-based structural mapping in Brunei Darussalam. The results of the satellite-based mapping reveal a major ∼NE-SW trending detachment fault in Brunei and adjacent regions, which is termed the Tutoh fault, with numerous associated secondary faults and folds. The fault acts as a detachment structure onto which several NW-SE trending faults ramp and have asymmetrical folds showing an en echelon fold-fault system. The topographic expression and the recent strike-slip faulting event on the Tutoh fault system suggest that the fault remains active, challenging the discourse that gravitational tectonics is the only cause of active deformation in Borneo.

The Kamthi Formation, in the intracratonic Pranhita-Godavari Gondwana rift basin, bore signatures of climate change from a warm humid climate in Late Permian to hot arid during the Early Triassic. Sedimentation took place mainly under fluvial conditions. The Kamthi sediments were unaffected by burial diagenesis and the source remained the same, therefore this provides an excellent opportunity to study the climatic influence on petrogenesis in an intracratonic rift basin. Subsurface palynofacies data of Kamthi Formation from the southern part of the basin established the presence of Upper Permian rock units equivalent to the Raniganj Formation, which are overlain by Lower Triassic sediments equivalent to Panchet Formation of the Damodar valley Gondwana basins, and are separated by a gradational contact. QFL and trace element compositions reveal transitional continental to craton interior provenance with dominantly felsic source areas, along with some meta-sedimentary and mafic components. Dominance of kaolinite, coal, alteration of K-feldspar, and biotite in the Raniganj equivalent sandstones attests to a humid climate. The presence of mostly fresh K-feldspar, ferruginous matrices, Fe-carbonate nodules, cutans, and microcrystalline silica cement in the Panchet equivalent sandstones mark a shift from humid to arid/semi-arid. This climatic shift is not reflected in the uniform feldspatho-quartzose to quartzose sandstone compositions and mudstone major oxide compositions. The mudstones are depleted in mobile elements, have low K₂O/Al₂O₃ (0.11–0.18), low ICV (0.35–0.72), high SiO₂/Al₂O₃ (2.3–3.4), high K₂O/Na₂O and moderately high CIA (80.12–87.05) that collectively suggest moderately intense weathering. Despite the humid climate, the Raniganj Formation equivalent rock units did not attain the highest mineralogical maturity due to proximity to relatively high-relief source areas in a fluviolacustrine environment. This relationship resulted in a lack of short-term sediment storage that favoured rapid erosion and sedimentation.

The seismically active Saurashtra horst is located within the intraplate volcanic continental margin of western India. The region is prone to moderate and low-magnitude earthquakes within the depth range of ∼ 3 to ∼ 24 km. We observed that the earthquakes in this region are associated with seismically active brittle and ductile crustal layers. To understand the dynamics of the earthquake generation process, we applied an integrated geological and geomorphological approach, supplemented by subsurface geophysical (magnetotelluric) studies. Additionally, the active surface deformation has been measured using the PSInSAR and GLA techniques. Based on the stream offset and geomorphic landform development pattern several NW-SE and NE-SW oriented strike-slip faults have been identified. The PSI-derived displacement analysis reveals that the area is deforming at the rate of ± 5 mm/yr. Furthermore, subsurface crustal heterogeneity with increasing depth has been identified using the magnetotelluric technique, which is reflected in the form of basaltic lava flows, plutonic emplacement within the granitic basement, and the presence of semi-crystallized magmatic bodies below the brittle-ductile level. Additionally, we proposed a model to depict the plutonic emplacement within the highly fractured/faulted granitic basement and their relationship to the earthquake generation process. Our model shows that crustal heterogeneity and the migration of hydrothermal fluid from the semi-crystallized magmatic body along the active fault cause earthquake nucleation processes within the brittle and ductile layers. We concluded that the upwelling magmatic fluid above the brittle-ductile transition (BDT) acted as a lubricant for the nucleation and triggering of the earthquake along the active faults. Similarly, the fractured ductile crust is weakened by fluid migration, which causes high fluid pressure in the ductile crust thereby decreasing the confining pressure and endorsing the velocity weakening in the aseismic layer, responsible for the shear instability that causes deep crustal earthquakes. More specifically, the lithological heterogeneity at brittle and ductile regimes is an important factor for the earthquake nucleation process in this part of the Indian plate.

The Umm Matierah gold prospect is located in the southeastern part of the Arabian Shield, at the northernmost tip of the Jabal Ishmas-Wadi Tathlith gold belt. Detailed mineralogical and geochemical investigations indicated that the studied metavolcanics show lithological varieties of meta-alkali basalt, meta-andesite, meta-trachyandesite, and meta-dacite. These rocks are foliated and hydrothermally altered (bleached), indicating a low-temperature propylitic alteration, affected by breccia veins and veinlets, and irregular stockwork. The Umm Matierah gold deposit is characterized by quartz-adularia-sericite-chlorite-carbonate alteration assemblage. The ore minerals of the Umm Matierah gold prospect are dominated by pyrite and arsenopyrite, with minor amounts of sphalerite. The sulfides have no preferred host rock; however, they are mainly present within the veins and veinlets and at the contacts with host rocks; they are also associated with the quartz-rich breccias. Minute gold grains are traced at the contact between the inner pitted and the outer clear zones of large pyrite crystals. Gold and sulfide enrichment do not exceed 10 vol% of the whole rock and are correlated with the thickness of extensive alteration zones that also show an ultimate association of chlorite with sulfide minerals. Compositionally, the studied rocks show 6.47 ppm average gold, are relatively rich in K, Ag, As, Sb, and W, and are relatively poor in Al, Na, Cu, Cr, Ni, Nb, Y, and Rb. The host rocks range in composition from ultrapotassic, shoshonitic, high-K calk-alkaline, to calk-alkaline end member, with transitional environmental signature from intraplate to oceanic island arc. These compositional features suggest that these rocks may have been derived from island source and subsequently slightly fractionated and contaminated during ascent and/or slightly affected by hydrothermal alteration. The host rocks display strong positive Eu, and negative Th, Nb, and Sr anomalies in keeping with the upper continental crustal pattern. There is a general enrichment of the LILEs and the LREEs relative to the MREEs. Collectively, our data, suggest that gold mineralization at Umm Matierah gold prospect, is a possible candidate for a low-sulfidation epithermal style of mineralization, spatially associated with the distal intrusion.

An ongoing question in understanding the evolution of the Himalayan-Tibetan orogeny is how much of the observed upper crustal shortening and crustal thickness is related to the Cenozoic collision between India and Asia vs earlier tectonic events along the southern margin of Asia. While the Pamir Mountains located at the western end of the orogen have been proposed to have experienced significant Cenozoic shortening, recent studies have interpreted upper crustal shortening to be primarily mid- to Late Cretaceous. To further understand the timing of upper crustal deformation in the Pamir, we investigated synorogenic clastic deposits within the footwall of the north-dipping Tanymas thrust fault along the suture between the Northern and Central Pamir terranes. Sandstones from these deposits were analyzed by detrital zircon U-Pb, zircon fission track, and muscovite ⁴⁰Ar/³⁹Ar analyses to assess the age and source of the detritus. Results show the deposits were sourced from the Northern Pamir (hanging wall of the Tanymas thrust) and provide an Early Cretaceous maximum deposition age of ∼130–120 Ma, interpreted to constrain their age and date motion on the Tanymas thrust fault as Early Cretaceous. Our results, integrated with previous studies, show Cretaceous deformation in the Pamir began in the Northern Pamir (∼140–110 Ma) before sweeping into the Southern Pamir in the mid- to Late Cretaceous (∼110–75 Ma). These results are consistent with previous interpretations of an Early Cretaceous phase of shallow- or flat-slab northward subduction followed by slab rollback and southward migration of deformation and magmatism in the mid- Cretaceous.

Paleomagnetic data from northeast Asia confirm that the Korea/North China and southern Sikhote Alin blocks rotated in opposite directions from the Berriasian to the Campanian (145–72 Ma). The Songliao Basin evolved between these rotated blocks with synrift sequences dating from the Tithonian or the Berriasian/Hauterivian. Geologic maps and tomographic images demonstrate a curvilinear subduction zone with associated accretionary wedge/magmatic arc stretched from Sikhote Alin, to Japan and southeast Korea near the Cretaceous Tertiary boundary. Arc-related volcanism migrated over 1000 km southeastwards across northeast Asia to the Japan Sea and Sikhote Alin coast from ∼ 140 – 70 Ma. This suggests that opposite microplate rotations resulted from Pacificward retreat of a curved subduction zone from Early to Late Cretaceous. Toroidal or radial flows in the mantle wedge exerting basal drag on the over-riding microplates is a likely driving mechanism. Anisotropic tomography suggesting fossil curved mantle flows which match the forces required to produce opposite rotations and the distribution of crustal thickness and Vp/vs ratios under the Songliao Basin support this mechanism. A major petroliferous basin in China may therefore be the result of double saloon door tectonics occurring during the Cretaceous behind a contemporaneous continental arc.

The Spiti region, renowned as the Museum of Indian Geology, is a world-famous sedimentary succession containing well-exposed sequences from Neoproterozoic to Cretaceous age. In this study, Triassic siliciclastic sedimentary rocks of the Lilang Supergroup were chosen to understand weathering history, provenance, paleoclimate, and depositional conditions using a geochemical and isotopic approach. Triassic shales show more or less similar compositions with substantial enrichment in CaO compared to PAAS (Post Archean shales from Australia), which may be attributed to the association with limestones in the region. However, the sandstones display significant depletion in the trace element concentrations signifying the effect of quartz dilution. The relative depletion of mobile elements (Rb, Ba) as against immobile elements (Zr, Nb, Hf) can be noticed in the trace element spider diagram of the shales. The Triassic sedimentary rocks are characterized by enriched LREE and depleted HREE patterns with pronounced negative Eu anomalies. The Chemical Index of Alteration (CIA; 56–86) indicates low to intense chemical weathering in the source area. The unusual decrease in CIA and other weathering indices in the stratigraphically up section is attributed to changes in climate and environmental conditions during the deposition of sediments in the Triassic period. Detangling the signatures is crucial to understanding the mass extinction crisis, particularly the role of anoxia in these events. Triassic black shales represent suboxic to anoxic depositional conditions in the redox-sensitive elemental binary diagrams. The carbon isotope data of the present study is very well supported by the Total Organic Carbon (TOC), which infers that the oceanic biological system tried to recover from the depletion of biological life. The εNd and ⁸⁷Sr/⁸⁶Sr systematics record a shift in source terrains from the Early to Late Triassic period. The Early Triassic samples show much older depleted mantle model ages (T_DM = 1.94–1.98 Ga) compared to Late Triassic sediments (T_DM = 1.76–1.91 Ga). Similar interpretations can be drawn from Th/Sc ratios (from ∼ 6 to ∼ 0.05) and (La/Yb) _N ratios (from ∼ 32 to ∼ 5), which record an increase in these ratios from Early Triassic to Late Triassic formations of the Spiti sedimentary rocks. Overall, trace elemental ratios and radiogenic isotopic signatures of the Triassic rocks of the Spiti region point towards Pan African granitic origin with minor impressions from the juvenile mafic-rich sources, such as Panjal Traps, the African craton, and Arabian-Nubian shield.