Southern Margin Research Articles

Basin–Orogen Coupling‐Driven Meso‐Cenozoic Deformation Along the Southern Margin of the Junggar Basin, NW China: Insights From Integrated Multidisciplinary Analysis

ABSTRACTThe southern margin of the Junggar Basin (SMJB) represents the typical intra‐continental basin–orogen coupling structure of the Central Asian Orogenic Belt and is a key area to study the deformation mechanisms and the geodynamic evolution processes of the North Tianshan Orogen. Herein, we compiled data from boreholes, gravity and magnetism, seismicity‐ and magnetotellurics‐derived geological profiles and field data to recover the sedimentary history of the SMJB and discuss the intra‐continental deformation driven by the basin–orogen coupling mechanism. The results show that the sedimentary and tectonic evolution of the SMJB were both profoundly controlled by the intra‐continental orogeny of the North Tianshan Orogen and its coupling with the Junggar Basin during the Meso‐Cenozoic period. Consequently, the SMJB is dominated by thick‐skinned thrust nappes accompanied with strike‐slip faulting and thrusting. The foreland thrust belt of the SMJB is characterised by three structural belts, from south to north, including the basement‐involved thrust belt, the cover‐detached foreland thrust‐fold belt and the thrusted foreland basement uplifts, respectively. Meanwhile, as indicated by the geometry of the basement thrusts, the thickness of décollements and the structure of the foreland basement, the stress field in the SMJB is obviously stronger in the west and weaker in the east. The sedimentation and deformation migrated northwards into the basin area in a stepwise process, that corresponded to the pace of the overthrusting of the North Tianshan Orogen onto the Junggar Block. Intense regional compression induced the rapid uplifting of the North Tianshan and the significant crustal shortening of the Junggar Block, driving the three structural belts to form accordingly during the Late Jurassic to the Neogene. There are at least one or two décollements within the SMJB, representing one of the main features of basin–orogen coupling structure in most cases. The décollement of some layers represents the decoupling of the sedimentary cover with the basement, which helps to accommodate the lateral crustal shortening of the SMJB by a translation into vertical uplift. As a result, the detached foreland thrust‐fold belts in the shallow level of the upper crust overthrusted upon the basement‐involved nappes of the mountain's side, forming the opposite thrust system. Coevally, the basement of the basin, in the deep level of the upper crust keeps underthrusting beneath the North Tianshan Orogen, forming a typical crocodile mouth‐like structure. In general, both the shallow and deep deformation in the SMJB have been formed by the intense intra‐continental compression during the Meso‐Cenozoic, which were driven by the basin–orogen coupling mechanism.

Late Ordovician Bentonites From the Southern Ordos Basin: Response to the Subduction of the Proto‐Tethys Ocean

ABSTRACTThe connection between the Ordovician bentonites on the southern margin of the Ordos Basin and the Early Palaeozoic volcanic rocks of the North Qinling Orogenic Belt is crucial for understanding the subduction and collisional closure of the Shangdan Ocean during the Early Palaeozoic. This paper investigates zircon U–Pb ages, geochemistry and Lu–Hf isotopic compositions of zircons in the Upper Ordovician Zhaolaoyu Formation bentonites located on the southern margin of the Ordos Basin. U–Pb dating of zircon indicates a coeval age of 453.3 ± 1.4 Ma (MSWD = 0.99), which represents the crystallisation age during the Late Ordovician Katian stage. The bentonites exhibit higher SiO2 (57.94–77.95 wt.%) and Al2O3 (9.21–14.33 wt.%), classifying them within the low‐potassium alkali basalt to medium‐potassium calc‐alkaline series. The parent rock of the bentonites is likely intermediate to felsic volcanic rocks. The rare earth element partitioning curves of the bentonites are right‐dipping, with a more pronounced negative Eu anomaly (δEu = 0.48–0.67). The zircons in the bentonites yield two‐stage model ages ranging from 546 to 956 Ma, along with εHf(t) values between 5.56 and 13.55. These results indicate that the bentonites are products of volcanic arc magma formed in a subduction–collision environment. The interbedded bentonites in the Upper Ordovician limestones of the southern margin of the Ordos Basin may be associated with the northward subduction of the Shangdan Oceanic crust, reflecting the subduction and consumption of the Proto‐Tethys Ocean along the southern margin of the North China Block.

MICA COMPOSITION REFLECTING CONDITIONS OF AILLIKITE FORMATION IN ZIMA COMPLEX OF EASTERN SIBERIA

Four types of mica were identified in the macrocryst and fine porphyry aillikites from the Yarma zone of the Urik-Ija graben considering specific features of morphology, chemical composition, zonation and crystallization conditions. Mica of the first type was found in macrocryst aillikites. It is available as deformed phlogopite macrocrysts rich in TiO2 (2.5–5.7 wt. %) with varying content of Cr2O3 (from detection limit to 2.0 wt. %) and Mg# 0.87–0.89 and 0.79–0.81 in different dikes. In chemical composition this phlogopite corresponds to the secondary phlogopite from mantle xenoliths, and it was obviously captured by protoaillikite melt form the rocks of the lithosphere mantle. Mica of type 2 represents phlogopite from groundmass and infrequent phenocrysts in fine porphyry aillikites and groundmass of macrocryst aillikites. In this phlogopite, TiO2 content varies in different dikes from 0.7 to 6.0 wt. %, Mg# index varies from 0.70 to 0.90. Phlogopite crystallized within the upper crust at temperature ranging from 840 to 680 °С. Mica of type 3 represents biotite (Mg# 0.40–0.65), producing rims around phlogopite and independent grains in fine porphyry and in places in macrocryst aillikites. The rims crystallized from residual Fe-rich melt at moderately heightened fO2, the temperature of biotite formation reached 700‒760 °C. Mica of type 4 is available in macrocryst aillikites as biotite (Mg# 0.40‒0.67) with phlogopite rims. It was inferred, that this biotite crystallized in intermediate magmatic chambers in the upper crust from essentially carbonate melts, separated through crystallization differentiation from protoaillikite magmas and captured by new portions of less differentiated melts. Obtained data point to metasomatic transformation of the lithosphere mantle, preceding to protoaillikite melting, under the southern margin of the Siberian craton. Thus, we may assume existence in the upper crust of the sites composed of partially crystallized melt.

Statistical Characteristics of Snowfall on the Tibetan Plateau Affected by TCs Over the Bay of Bengal: An Observational Analysis

ABSTRACTIn this study, the characteristics of tropical cyclones (TCs) over the Bay of Bengal (BoB) that affect snowfall on the Tibetan Plateau (TP) and spatiotemporal distribution of snowfall related to BoB TCs are statistically analysed by using multi‐sources data from 1981 to 2020, with partitioning TC‐influenced snowfall by tracking cloud clusters. The results show that 141 TCs formed during the 40‐year period of 1981–2020, of which about 35% (50 TCs) impacted snowfall at 83% of meteorological stations on the TP during their northward or westward movement, and the average distance between the TC centre and the snowfall stations is 1277 km. The proportion of snowfall‐related TC frequency shows a significantly decreasing trend with a predominant cycle of 10a. The TC‐influenced snowfall frequency (SF), precipitation amount (PA) on a snowfall day and snow depth (SD) during 1981–2020 all show a non‐significant weak decreasing trend, while TC‐influenced snowfall is significantly increased in the eastern and southern edges of Xizang, western Sichuan and the southern margin of Qinghai. PA and SD in December account for more than 75% and 55% of the monthly total, respectively. The spatial pattern of PA could be objectively categorised into west‐type (24%) and southeast‐type (76%). The moisture transported by the BoB TC and a southerly jet stream formed between the trough and the western Pacific subtropical high (WPSH), the convergence of cold air and warm–moist airstream over the TP and the change in position of the south Asian high in the upper troposphere are significant factors causing the different spatial distribution. The results can provide reference for TC‐related snowfall, SD prediction and disaster assessment on the TP.

Evaluation and Application of Wellbore Stability of Deep Oil Wells in the Southern Margin of Junggar Basin

The stability of the oil well wellbore is a prerequisite for selecting the optimal completion method. In this paper, based on experimental testing and theoretical models of rock mechanics parameters in deep oil reservoirs, the in situ stress parameters of deep oil wells are accurately predicted. On this basis, a full life cycle assessment model for wellbore and perforation casing stability was established, and the effects of pressure depletion and changes in the production pressure differential on wellbore stability and casing stability were analyzed. The research results indicate that as the formation pressure decreases, the critical collapse pressure difference around the wellbore significantly decreases. The greater the production pressure difference, the more likely the wellbore is to become unstable. Under the original formation pressure coefficient, if there is no casing, the critical failure pressure difference of the wellbore wall is 55 MPa. After cementing and perforation, when the casing is uniformly stressed and the formation pressure drops to a coefficient of 0.83, the casing will not be damaged even when the wellbore is completely emptied. At this time, there is still a certain safety production pressure difference in the perforated formation. This study can effectively guide the optimization of well completion and safe development in deep oil reservoirs.

Zircon U-Pb Age and Hf-O Isotope Evidence of the Taihua Complex on the Southern Margin of the North China Craton: Implications for Its Magmatic Process During the Early Paleoproterozoic

The Xiaoqinling is the largest terrane of the Taihua Complex and records Paleoproterozoic silicic magmatism, which is one of the most potential areas for understanding secular changes in tectonic processes during the Tectono-Magmatic Lull (TML). In this study, we use zircon U–Pb dating, Lu–Hf and O isotopic analysis to constrain the age, provenance, and magmatic processes of the protoliths for the Paleoproterozoic granitic gneiss, tonalitic gneisses, and migmatitized tonalitic gneiss from the Xiaoqinling area. The isotopic composition of zircon provides a key proxy for querying the crustal record. Integration of our new U–Pb and Hf–O isotope data with available published data from other regions of the North China Craton (NCC), allows us to better assess the secular changes of the North China Craton during the magmatic lull at ∼2.3 Ga. This study shows that the protolith of the tonalitic gneisses in the study area have crust-like Hf–O isotopic compositions with slightly negative to positive εHf(t) (−3.0− +2.1) and low to moderately elevated δ18O values (2.71–7.87‰), indicating a mixed origin of continental crust assimilated by mantle-derived magmas.

Late Triassic to Middle Jurassic magmatism in the Eastern Pontides (NE Türkiye): Evidence for divergent double subduction of Paleo-Tethys lithosphere

The Meso-Cenozoic geodynamic evolution of northern Türkiye remains a subject of debate primarily due to a lack of systematic geological, geochemical, geochronological, and geophysical investigations. This paper presents comprehensive geochronological and geochemical data on the Late Triassic andesite porphyry, andesite breccia and quartz diorite porphyry, and the Middle Jurassic diabase, and amphibole-poor and amphibole-rich andesite porphyry from Çevrepınar Igneous Complex (Gümüşhane) in the southern part of the Eastern Pontides Orogenic Belt (EPOB), a well-preserved continental arc in the Alpine-Himalayan orogen. Zircon U–Pb geochronology indicates crystallization ages of ∼208–202 Ma (Rhaetian) for the Late Triassic rocks and ∼175–172 Ma (Aalenian) for the Middle Jurassic rocks. Whole-rock geochemical and Sr-Nd-Pb isotopic data, and zircon εHf(t) data indicate that both the Late Triassic and the Middle Jurassic rocks originated by low-degree melting of a spinel lherzolite lithospheric mantle source modified by subduction-related fluids and/or melts. Based on the new and published data, we suggest that the Late Triassic to Middle Jurassic arc magmatism in EPOB occurred as a result of southward subduction of the Paleo-Tethys oceanic lithosphere beneath the northern margin of Gondwana-Land. Late Triassic to Jurassic arc magmatism and basin evolution occurred synchronously in the northern and southern peripheries of the present-day Eastern Black Sea Basin, indicating divergent double subduction of the Paleo-Tethys oceanic lithosphere beneath the northern margin of Gondwana and southern margin of Laurasia during the Early Mesozoic.

Genesis of the Haopinggou breccia-hosted Au deposit, western Henan Province, China

Breccia-hosted Au deposits within the North China Craton (NCC) exhibit a complex genesis, with some breccias being genetically linked to Au mineralization and others unrelated. Understanding the origin of these breccias is crucial for elucidating the genesis of associated ore deposits and guiding exploration strategies. The Haopinggou deposit in the Xiong’ershan district, on the southern margin of the NCC, comprises quartz-carbonate vein-hosted Ag–Pb–Zn–Au deposits and related breccia pipe in metamorphic rocks of the late Archean to early Paleoproterozoic Taihua Group. The origin of the Haopinggou breccia is crucial to understand the Xiayu Ag-Pb-Zn-Au mineralization (5640 t Ag). In this study, we evaluate the genetic links between breccia and Ag-Pb-Zn-Au mineralization, with new insights obtained from the Haopinggou deposit characteristics, breccia characteristics, breccia-hosted Au mineralization, and isotopic data, which refine our understanding of Haopinggou Ag-Pb-Zn-Au hydrothermal systems and exploration models in discordant breccia pipes. Recent underground tunnel surveys and drill-core logging reveal a close spatial link between the breccia and hidden granite porphyry. Juvenile magmatic clasts (granite porphyry) and cement (K-feldspar and quartz crystallized from granite porphyry) are present in the breccia pipe above a granite porphyry apophysis, indicating that the breccia was formed by phreatomagmatic brecciation caused by emplacement of the granite porphyry. Fluidization at the cm- to 100-m-scale reveals that the movement direction of clasts inside the breccia pipe was from northwest to southeast after crypto-explosion, creating a northwest-trending breccia zone. Anhedral monazite (131.2 ± 7.6 Ma) encapsulated in pyrite from the breccia constrains the upper limit of brecciation. The δ34S values of pyrite in rock powder and hydrothermal cements range from 3.8 ‰ to 5.3 ‰ (mean: 4.8 ‰) and 2.7 ‰ to 7.5 ‰ (mean: 5.2 ‰), respectively, with δ34S-sphalerite in hydrothermal cement ranging from 5.7 ‰ to 6.1 ‰ (mean: 5.9 ‰) and δ34S-chalcopyrite ranging from − 0.9 ‰ to 5.6 ‰ (mean: 2.9 ‰). The S isotope characteristics of vein-hosted and breccia-hosted mineralization zones are consistent, with complete alignment of the sulfide assemblages and alteration features. These pieces of evidence collectively indicate that both vein- and breccia-hosted mineralization zones are products of magmatic activity. The metallogenic model for the Haopinggou breccia pipe underscores its unique formation mechanism, driven by pre-existing northwest-striking structural weaknesses and fluidization, indicative of a predominant northwest–southeast clast movement, and suggesting the potential for deep-seated porphyry-type Au mineralization.

Mississippian foraminifera from the Lueyang Formation in Guozhen area, South Qinling Orogen, China

The Mianlue Suture Belt, located in the southern part of the Qinling Orogenic Belt, is a significant tectonic zone in China that formed by the collision of the South China Block with the North China Block during Triassic. The Lueyang Formation, distributed along the southern margin of the Mianlue Suture Belt, is one of the major components of the Late Paleozoic marine strata of the Mianlue Belt. Unfortunately, only a few published paleontological studies are available. This hinders the understanding of the age and sequence of this formation and the evolution of the Mianlue Ocean. Herein, we report, for the first time, at least 51 species and 25 genera of foraminifera from the Lueyang Formation at the Zhuyuanba section. The Zhuyuanba foraminiferal fauna is dominated by Eostaffella, Archaediscus, Paraarchaediscus, Omphalotis, Bradyina, Janischewskina, Endothyra and Mediocris, and possesses some stratigraphically important taxa such as Bradyina cribrostomata, Janischewskina spp. and Paraarchaediscus stilus. Two foraminiferal zones are recognized in the Lueyang Formation, ascendingly the Visean Paraarchaediscus stilus Zone and Serpukhovian Bradyina cribrostomata Zone. So, the age of the Lueyang Formation at the Zhuyuanba section is considered from Visean to Serpukhovian. This paper confirms for the first time that the Lueyang Formation contains the Visean and Serpukhovian strata, providing new paleontological evidence for understanding the evolution of the Mianlue Ocean during the Late Paleozoic.

Net primary productivity of Betula platyphylla woodland and the response of current-year shoots to vegetation degradation in Mongolian forest steppe

At the southern margin of forest steppe vegetation in Mongolia, Betula platyphylla woodlands occur on north-facing slopes and hill tops, where congeneric shrub Betula fusca often coexists. Woodland degradation results in B. fusca scrub with scattered B. platyphylla trees. We studied structure and productivity of a B. platyphylla woodland and the current-year shoot allometry in the Hustai National Park of central Mongolia. For the B. platyphylla woodland examined, stand basal area and estimated aboveground biomass for stems ≥ 3 cm diameter were 3.9 m2 ha–1 and 13.3 Mg dry mass ha–1, respectively. Leaf area index was 0.93 indicating open canopy layer. Aboveground net primary productivity as the sum of coarse wood production and current-year shoot mass amounted to 1.89 Mg dry mass ha–1 year–1, which was high relative to small aboveground biomass. Analysis of current-year shoot allometry demonstrated that B. platyphylla had more xeromorphic shoots in the scrub, with smaller specific leaf area, lower mass ratio of leaf laminae to supporting parts, and smaller lamina mass against sum of petiole basal area, than in the woodland. Our results indicate that current-year shoot allometry characterizes tree response to woodland degradation at the dry margin of forest steppe.

Indosinian magmatism in NE Vietnam: Petrogenesis and geodynamic implications of Triassic mafic suites from the Song Hien region

The Song Hien region is located at the southern margin of the South China Block and includes Early–Middle Triassic mafic suites. The petrogenesis and tectonic context of these rocks are important to comprehending one of the most challenging periods in the tectonic evolution of northern Vietnam related to the South China–Indochina collision. In the present study, we provide the first systematic examination of whole-rock geochemical and SrNd isotopic compositions of mafic suites from the NE Vietnam. The studied Early–Middle Triassic suites are dominated by basalts. The rocks are enriched in LILEs, U, and Th and depleted in Nb and Ta and show a wide range of geochemical and SrNd isotopic compositions ((La/Yb)CN = 0.4–4.6; Dy/Dy⁎ = 0.7–1.3; (87Sr/86Sr)i = 0.7053–0.7125; εNd(t) = (+3.7)–(−8.0)). They all originated from shallow melting of a subduction-modified lithospheric mantle, and the compositional diversity of these basalts is attributed mainly to geochemical and Nd-isotopic heterogeneity in the mantle source. The geochemical and SrNd isotopic features of the studied mafic suites indicate a negligible contribution of the asthenospheric mantle to magma generation and differ significantly from those of the Emeishan plume-related basalts. A model in which Early–Middle Triassic mafic magmatic activity in the NE Vietnam region is the result of decompression melting driven by convective thinning of the South China mantle lithosphere during the 250–240 Ma period is discussed in the study. This period corresponds to the South China–Indochina continental collision. In the NE Vietman region, the Early–Middle Triassic syn-collisional magmatism has an important metallogenic context, controlling NiCu sulfide and Sn(Cu) mineralizations.

APPLICATION OF BENZOCARBAZOLE MOLECULAR MIGRATION MARKERS IN RECONSTRUCTING RESERVOIR FILLING AT THE SOLVEIG FIELD, NORWEGIAN NORTH SEA

Benzocarbazole (BC) migration tracers were used to investigate the complex filling of reservoir segments at the Solveig field in the Norwegian North Sea. The study suggests that the benzocarbazole ratio [a]/([a]+[c]) of crude oils and extracts decreases with inferred increasing migration distance. The complex filling history of the Solveig field is evident from the observation of variable degrees of palaeo biodegradation associated with two palaeo oil‐water contacts in residual oil zones below non‐ to moderately biodegraded live oil columns. Live oil properties also vary significantly across the field. Benzocarbazole ratios (BCRs) obtained from oils and reservoir core extracts appear not to be affected by biodegradation and indicate a migration and filling trend from NW to SE. The BCR values were set by the initial phase of filling and do not show any overprint effects as a result of later and more mature oil charges.BCRs from both oils and extracts of reservoir cores, particularly those composed of clean sands, helped to reconstruct migration processes in the Solveig field. Migration is construed to have first filled reservoir segment D in the NW of the field and to have continued further east towards segment C, and then via segment B and finally into segment A. Migration then continued along the southern margin of the Haugaland High to a well location to the east of the Solveig field. A fractionation effect for benzocarbazoles derived from oils versus those from extracts was noted and was attributed to differential partitioning behavior. Nevertheless, spatial trends for oil‐ and extract‐derived BCRs were congruent. This allowed the generation of spatially more highly‐resolved benzocarbazole datasets for migration assessment by combining data from both samples types (oil and reservoir extracts) if partitioning is accounted for.

Catastrophic outburst floods along the middle Yarlung Tsangpo River: Responses to coupled fault and glacial activity on the Southern Tibetan Plateau

The immense outburst floods that have occurred on the Tibetan Plateau during the Late Quaternary are closely linked to tectonic and climatic factors. These floods likely induced very rapid, short-term geomorphic impacts on the evolution of mountain drainage systems and patterns of sedimentary movement. In this study, four glacially-dammed lake outburst flood events that occurred along the middle reaches of the Yarlung Tsangpo River since the Middle Pleistocene were reconstructed by combining comprehensive geomorphic, stratigraphic and geochronologic investigations. The most recent outburst flood sequence occurred at ∼10.5 ka, with a peak discharge of ∼5 × 105 m3/s. The tilted uplift of the Cona Normal Fault has resulted in localized topographic lift and the formation of river knickpoints, contributing to the development and stabilization of glacial dams. River damming and outburst events have also been influenced by glacial-interglacial climate fluctuations since the Middle Pleistocene. The focused erosion and extensive mobilization of sediment by these low-frequency, high-energy floods have resulted in a repeated pattern of material transport and deposition from the Tibetan Plateau interior to its exterior. Coupled with glacial activity, the primary factor impacting the sustained stability of knickpoints on the Yarlung Tsangpo River along the Tibetan plateau's southern margin has been differential rock uplift, which results in a distinct geomorphic pattern characterized by knickpoints, glacial dams and alternating wide valleys and deep gorges.

Paleo-redox conditions during the demise of a carbonate platform in the Tethyan ocean: Evidence from phosphatized and metals (Mn and Fe) rich hardgrounds

Phosphatized Mn and Fe rich hardgrounds and condensed pelagic deposits in carbonate platform successions are precious archives of abrupt climate and environmental changes (redox conditions and phosphorous availability) in the past shallow-water marine environment. While numerous examples have been documented in the Cretaceous successions of the Northern Tethys, the scarcity of similar descriptions from the southern margins suggests differences in sedimentary processes or preservation conditions.In this work we study three phosphatized Mn and Fe rich hardgrounds and pelagic condensed deposits that mark the repetitive demise of the Panormide carbonate platform developed in the Southern Tethyan margin during the Cretaceous. The integration of SEM-EDS, PXRD, and Micro-Raman spectroscopy data shows that these hardgrounds consist of fine-grained Fe (goethite and hematite) and Mn (birnessite and/or vernadite) oxides dispersed in a calcite and apatite matrix. Micro-Raman spectroscopy shows the presence of oxidized Mn species: Mn3+ and Mn4+. The oxidation of Mn2+ → Mn3+/4+ and/or Fe2+ → Fe3+ occurred at the sediment-seawater interface under oxic conditions (where both Mn and Fe oxidize) or suboxic conditions (where only Fe oxidizes). The paleoenvironmental perturbations that triggered the formation of both hardgrounds and condensed pelagic deposits were likely related to pCO2 cycle, upwelling of P-Mn-Fe-rich water masses, eutrophication and phosphatization related to the Cretaceous climate oscillations during the main Oceanic Anoxic Events. These perturbations were likely enhanced by tectonic activity. Moreover, we show that the formation of the phosphatized metals-rich hardgrounds and the recovery of shallow-water sedimentation occurred after long-term periods (6–12 Ma). Thus, the Panormide serves as a remarkable example of resilience amidst significant climatic changes.

Coupled role of climate and tectonics in the deposition of the late Quaternary sedimentary sequence in the southern margin of the central Ganga Plain, India

ABSTRACT The Ganga Plain’s sedimentation is primarily controlled by Himalayan tectonics, variations in monsoon intensity, and glacier size. However, the significance of intrabasinal tectonics, which is an interplay of the Himalaya tectonics and basement tectonic configuration of the Ganga Plain, in comprehending the late Quaternary Ganga Plain sedimentation remains unclear. In this study, the sediment provenance and extent of weathering experienced by the sediments are studied using the mineralogical and geochemical (major, trace, and rare earth elements) composition of a sedimentary cliff sequence exposed at the Ramnagar locality near the cratonic peripheral bulge in the southern Ganga Plain. In the Ramnagar cliff sediment section, two sediment packages, designated SP-A and SP-B, are identified based on mineralogy, texture, and geochemistry. These packages of sediments show mild chemical weathering and distinct geochemical compositions. The lower part (SP-A) of the Ramnagar cliff section is characterized by higher contributions from mafic sources (pyroxene, feldspar, and mica as dominant minerals), while the upper part (SP-B) is predominately derived from felsic sources (mica and feldspar as dominant minerals), which are supplied via the peninsular and the Himalayan rivers, respectively. The southern part of the central Ganga Plain shows evidence of a shift in the sediment provenance from mafic to felsic source rock at 40 ka. This study demonstrates how weathering and sedimentation are controlled by both regional tectonics and climate in the southern margin of the central Ganga Plain.

Metamorphic history of amphibolite facies metapelites from the Southern margin of the Limpopo Belt: In situ U-Pb dating of Zircon, monazite and rutile

The Southern Marginal Zone (SMZ) comprises, in part, Archean granulite facies metapelitic rocks deposited at ∼ 2733 Ma and metamorphosed to granulite facies at ∼ 2713 Ma of the Limpopo Belt in direct contact with the Kaapvaal craton. The SMZ also contains amphibolite facies metapelites within a fragment that is in direct contact with the thrust-faulted tectonic boundary with the Kaapvaal craton. These metapelites are proposed to represent former granulites that were comprehensively rehydrated under amphibolite facies. However, no previous study has directly investigated the conditions of metamorphism or timing of the proposed higher-grade events in these retrograde rocks. Neither has the timing of retrogression in the retrogressed metapelites been well constrained. Here, detailed petrographic analysis, mineral composition and in situ U-Pb dating of zircon, monazite and rutile are presented for the amphibolite facies metapelites. These rocks are characterized by assemblages consisting of garnet, orthoamphibole, biotite, quartz, plagioclase, rutile, kyanite and graphite. Although there is pervasive retrogression to orthoamphiboles (anthophyllite to gedrite series), rare relics of orthopyroxene are preserved. The compositions of the first garnet (Grt 1) generation suggests granulite facies peak metamorphic conditions of 860 ± 10 °C and 11 ± 0.4 kbar. Metamorphic zircon grains and overgrowth rims and monazite, included in Grt 1, indicate that peak metamorphic conditions occurred between 2714 ± 7 and 2713 ± 4 Ma. Dating of rutile inclusions in Grt 1 from granulite yield a Concordia age of 2677 ± 6 Ma. This indicates post-granulite facies cooling and shows that later retrograde metamorphism did not reach temperatures high enough to reset rutile inclusions in Grt 1 garnet. Orthoamphibole and second garnet with kyanite inclusions (Grt 2) compositions suggest that retrogression occurred under amphibolite facies conditions of 600 ± 10 °C and 6.0 ± 0.5 kbar. Rutile included in Grt 2 and as discrete grains equilibrated with the retrograde textures record a range of Paleoproterozoic ages (2212 ± 19, 2148 ± 12 and 2012 ± 10 Ma). Thus, the metamorphic history of the amphibolite facies metapelites of the SMZ is hotter than thought and similar to their Archean granulite facies counterparts, except for younger Paleoproterozoic ages, likely indicating a pervasive retrogression to amphibolite facies conditions.

Genesis of honeycomb buildups in the Permian Zechstein Group, Southern North Sea

Seismic interpretation has revealed a hitherto unreported honeycomb pattern of carbonate buildups within the Orchard Platform (Southern North Sea). The Z2 Stassfurt Halite Fm. onlaps the southern margin of the Orchard Platform and is also found infilling Z2 intra-platform lagoons to form salt lakes. Post Z2 evaporation, the deeper Z3 water column drowned the Orchard Platform inhibiting the platform recovery attempted by the Z3 Plattendolomit Fm. The palaeobathymetric variability of the drowned Orchard Platform was sufficient to bring parts of the seafloor into the photic zone allowing for the sporadic growth of the Z3 Plattendolomit Fm. However, the palaeobathymetric lows remained beneath the photic zone ensuring an incomplete regeneration of the Orchard Platform with the creation of a high-frequency network of intra-platform lagoons which mimic the polygonal texture of a honeycomb. Whilst previously accepted as collapse structures or karst systems, this study correlates the development of the honeycomb buildups to variations in seafloor palaeobathymetry which in turn mimic the structural lineaments of the Zechstein subcrop. Syn-depositional instability in the Zechstein subcrop caused the topsets of the Z2 salt lakes to become warped. The warped halite provided seed points for Z3 Plattendolomit Fm. growth which allowed for linear ridges of carbonate to traverse the Z2 salt lakes and eventually connect with the honeycomb buildups. Deposition in the Mesozoic lead to loading of the Zechstein. Halite-filled Z3 lagoons accommodated this loading, which caused a pinching effect on the Z3 honeycomb buildups. The sedimentological understanding provided by this study not only de-risks frontier exploration but also provides insight into carbonate growth in restricted platform recovery scenarios.

Permo-Triassic source-to-sink systems of the Turpan basin: insights into three tectonic evolutionary episodes and their implications for the formation rhythm of the Central Asian Orogenic Belt

ABSTRACT The Tianshan Orogenic Belt underwent a series of complex tectonic changes from the Late Palaeozoic to the Cenozoic, providing key insights into the evolution of the Central Asian Orogenic Belt (CAOB). The continuous Permo-Triassic sedimentary records of the Turpan Basin are among the best archives of the tectonic evolution of the Tianshan Orogenic Belt. However, the evolution of the Turpan Basin during this period is poorly understood. Using detrital zircon U-Pb chronology and geochemistry, we reconstructed the source-to-sink system changes and the basinal tectonic evolution in the Permo-Triassic, thus providing new evidence of its tectonic evolution. Our results show that the Early-Middle Permian source-to-sink systems of the Turpan Basin were mainly proximal and constrained by horst and graben landforms due to internal extension formed in the back-arc setting during accretionary orogenesis in Central Asia. In the Late Permian-Early Triassic, exotic sources, including the North Tianshan and Yili Block-Central Tianshan, dominated the source-to-sink system, which suggests that rifts were infilled and expanded across the entire thermal sag basin. In the Middle-Late Triassic, the sediment sources diversified, with external and internal provenances revealing that a foreland basin formed with thrusting at the southern margin, and the intraplate-associated Bogda forebulge experienced initial uplift, signifying that the CAOB gradually changed from an accretionary to an extrusive orogeny. The three evolutionary episodes of the Turpan Basin implicated the long-lasting effect of the orogenic process from the final closure of Palaeo-Asian Ocean to the following build-up of the CAOB.

The Miocene Acceleration of Strike-Slip Deformation in the Northern Tian Shan, Central Asia

Abstract The Tian Shan has been deformed during the Cenozoic as a far-field response to the India–Eurasia collision. This contribution reports new apatite fission-track (AFT) and apatite (U-Th)/He data from the northern Tian Shan, including areas along the southern margin of the Junggar Basin and adjacent to the western segment of the Bolokenu-Aqikekuduk Fault (BAF). Thermal history modeling of the presented data suggests that the Cenozoic exhumation along the basin margin started >20 Ma, and the oblique-slip BAF could have been active since at least ~10 Ma with a total exhumation magnitude of ~2 km during the late Cenozoic. Since then, the mean exhumation rate on southern wall of the BAF is comparable to the Quaternary uplift rate previously measured on the same segment of the fault. To examine the extent of the Miocene acceleration of the transpressional deformation pattern, an exhumation model since 50 Ma was established by inverse modeling >1000 published and new AFT and (U-Th)/He ages from the Tian Shan and the northeastern Tibetan Plateau. The model suggests that after the initial unroofing, a significant acceleration in exhumation occurred during 15–10 Ma, in regions including the interiors of northern and southwestern Tian Shan near major strike-slip structures. This change coincided with the northward propagation of the Pamir and the eastward expansion of the northeastern Tibetan Plateau along the west and south margins of the Tarim Basin, respectively.

Landscape positioning of Neolithic mustatil stone structures along the margins of the Nefud Desert, Saudi Arabia

Monumental rectangular stone structures called mustatils are an important emerging feature of the Holocene archaeological record of northwestern Arabia. To date, few have been excavated, with available radiocarbon dates suggesting an age range of ca. 5400–4200 BC. Here we present a rigorous spatial analysis to identify the patterning and landscape context of 169 mustatils in the southern and western margins of the Nefud Desert. This included: (1) a systematic survey of satellite imagery to identify mustatils; (2) viewshed analysis to examine location and landscape visibility; (3) a point process model to understand how diverse environmental and landscape variables affect mustatil locations; (4) mark correlation function to assess spatial patterning of mustatils based on their size. Results indicate that mustatil locations are determined most by proximity to water (likely locations of enhanced surface water occurrence under the enhanced humidity of the Mid-Holocene), on east facing slopes, close to rocky areas, at elevations between 880 and 950 masl, and on or near topographic ridges (positive topographic position index). Viewshed analysis showed that mustatils are preferentially located in areas that have good views, but not the best that are available, indicating complex landscape positioning that balances a range of topographic and behavioural factors. Using a rank permutation method with size (length) of mustatils as a proxy for labour mobilization we show that mustatils within clusters are not arranged hierarchically based on size, and were likely built by non-stratified groups of people. Our analyses show that people were choosing multiple factors when deciding where to build mustatils, and that the distribution of mustatils may relate to different groups of people resulting in the construction of complex ritual landscapes. As one of the earliest examples of large-scale monumental stone structure construction in global prehistory, understanding mustatils can enlighten us on human-environment interaction during the Neolithic.