Northern Margin Research Articles

Tectonic Anatomy Reveals the Chinese Altai‐Kuerti‐Dulate Arc Amalgamation in the Southern Altaids

AbstractThe Erqis tectonic belt, situated at the junction of the Chinese Altai‐East Junggar, originated through the subduction of the Ob‐Zaisan Ocean, playing a pivotal role in unraveling the tectonic evolution of the southern Altaids. Tectonic and provenance analyses discern three distinct arcs: the Chinese Altai in the north exhibits a protracted history from the late Cambrian to early Permian with a slender accretionary complex (AC) termed the Supute AC; the Kuerti intra‐oceanic arc in the middle emerged during the Silurian to Devonian coevally with a minor AC known as the Tesibahan AC; and the Dulate arc in the south predominantly evolved from the Middle Devonian to Permian, giving rise to the Fuyun AC that independently developed on its northern margin at least until ∼273 Ma. Our findings indicate the existence of multiple arcs within the Ob‐Zaisan Ocean, forming an archipelago paleogeography in the Paleo‐Asian Ocean. Provenance studies lead us to propose that cryptic sutures demarcating the Chinese Altai, Kuerti, and Dulate lie approximately along the Kuerti and Tesibahan faults, respectively, and that the oceanic branches between these arcs subducted northward beneath the Chinese Altai and Kuerti arcs and southward beneath the Dulate arc. Additionally, our work demonstrates the closure of the Ob‐Zaisan Ocean most probably postdates ∼273 Ma. Combining our data with previous research, we present a novel tectonic evolution model, elucidating several arc amalgamations with multiple subduction polarities between Chinese Altai and East Junggar throughout the late Cambrian to Permian.

Petrogenesis of Early Permian within-plate-type gabbros in the Western Qiangtang Terrane, Tibet: partial melting of lithospheric mantle metasomatized by subducted oceanic sediments on the northern margin of the Gondwana

ABSTRACT The extensively distributed Early Permian mafic rocks in the Qiangtang Terrane of the central Tibetan Plateau are the magmatic records of separation of this terrane from the Gondwana. Nevertheless, the origins and mantle source characteristics of these rocks continue to be subjects of debate. This study reports zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical and Sr-Nd-Hf isotopic data on gabbros exposed in the Zougou Region, western Qiangtang Terrane. Our zircon U-Pb isotope age suggests that the Zougou gabbros (ZGs) were formed at ca. 290 Ma, representing newly discovered Early Permian mafic rocks within the Qiangtang Terrane. The ZGs display geochemical affinities with within-plate-basalts given that they are tholeiitic and have high Zr/Y and Ti/V ratios. Compared to previously examined Early Permian mafic rocks in the western Qiangtang Terrane, the ZGs are characterized by their distinct, nonradiogenic Nd (εNd(t) = -0.54 to −2.18) isotopic signatures, which cannot be attributed to crustal contamination or assimilation. This assertion is supported by their uniform whole-rock εNd(t), εHf(t), and zircon εHf(t) values, as well as by magma mixing modelling. In conjunction with regional geology, we propose that the ZGs were derived from an isotopically enriched lithospheric mantle metasomatized by previous subduction events. Furthermore, the positive decoupling of Hf-Nd isotopes (ΔεHf(t) = +2.21 to + 2.72), negative Nb-Ta anomalies, and variable Ba/Th ratios within the ZGs indicate that the metasomatic agents were fluids released from oceanic sediments. Our study provides insights into the compositions of the lithospheric mantle beneath the Qiangtang Terrane during the Permian, which is pivotal to discriminating the role of asthenospheric and lithospheric components in the formation of the contemporaneous mafic rocks in response to Gondwana break-up.

Age and petrogenesis of the Madi intrusion in the Huashi area, northern margin of the North China Craton: Implications for magma evolution and Nb–Ta mineralization

Although the mineralization of rare earth elements (REEs) and rare metals is intimately associated with the extreme fractionation of granitic magmas, the metallogenic intrusions of many granite-hosted Nb–Ta deposits have undergone fluid–melt interaction. Nevertheless, the precise mechanisms by which fluid–melt interaction influences mineralization remain poorly understood. The present investigation examines the issues of the fluid–melt interaction in highly fractionated granites with Nb–Ta mineralization, utilizing data from the newfound Huashi deposit in the northern margin of the North China Craton (NNCC). The Huashi Nb–Ta–Rb–Li deposit hosted in the Madi intrusion consists of two lithologies that have evolved continuously, namely medium–fine grained granite (MGG) in the lower section and alkali-feldspar granite (AG) at the top. The ages of the MGG and AG were determined using LA–ICP–MS columbite U–Pb dating, yielding values of 182.9 ± 1.7 Ma and 184.7 ± 1.3 Ma, respectively. The Madi intrusion has high SiO2, Al2O3, and total alkali contents, along with low CaO, MgO, MnO, and TFe2O3 contents and high Al2O3 / (CaO + Na2O + K2O) (A/CNK) values, classifying it as highly peraluminous granite with a high-K calc-alkaline affinity. Additionally, the intrusion also exhibits enrichment in Rb, U, Th, and Nb alongside significant depletion in Sr, Ba, Ti, Eu, and P, with a noticeable tetrad effect of REEs. The investigation of mica and feldspar minerals in the Madi intrusion using electron probe microanalysis (EPMA) indicates that the mica is mainly zinnwaldite, while the plagioclase belongs to albite. In summary, the Madi intrusion exhibits a highly I-type fractionated granite affinity. The extreme fractionation, intense fluid–melt interaction, and hydrothermal alteration of the intrusion contribute to the formation of the Huashi deposit.

Detrital Zircons From Lower Palaeozoic Metamorphic Complexes and Silurian–Devonian Strata in the South Kitakami Belt, Northeast Japan: Implications for the Northern Extension of the Terra Australis Orogen in Northeast Gondwana

ABSTRACTTo reconstruct the detailed location of proto‐Japan around northeastern Gondwana during the early Palaeozoic, this article presents detrital zircon U–Pb age spectra for lower Palaeozoic metamorphic complexes and Silurian–Devonian strata from the western margin of the South Kitakami belt in northeast Japan. Psammitic and siliceous schists have youngest age peaks ranging from the Ordovician (479–458 Ma) to early Silurian (ca. 439 Ma). Sandstones have youngest age peaks varying from the early Silurian (ca. 435 Ma) to Late Devonian (370–360 Ma). The U–Pb age spectra exhibit major peaks at 510–500, 480–470, and 450–440 Ma. Excluding the Upper Devonian sandstones, significant age peaks occur at 650–550 and 1300–900 Ma. These data can be interpreted as a Pacific Gondwana signature, indicating a tectonic association with the Terra Australis Orogen that developed along the northeastern Gondwanan margin and Paleo‐Pacific Ocean, extending from eastern Australia (i.e., the Thomson, Lachlan, and Delamerian orogens) to Antarctica (i.e., the Ross Orogen). In northeast Asia, the Pacific Gondwana detrital zircons and 480–470 Ma zircons occur in the Bainaimiao arc belt along the northern margin of the North China Craton and in the Jiangyu continental arc in the Jilin area. The magmatic arcs of proto‐Japan and these regions were part of the Terra Australis Orogen during the Ordovician–early Silurian. A decrease in the proportion of Pacific Gondwana detrital zircons in Upper Devonian strata may be due to multistage trench retreat, such as that recognized in the Tasmanides in eastern Australia, which corresponds to the northern Terra Australis Orogen.

Remote sensing monitoring of glacier area and volume changes in glacier-fed mountainous watershed on the Northern margin of the Qinghai-Tibet Plateau under climate change.

Steady glacier runoff is related to the security and resilience of water resources in meltwater recharge basins, so the status and change of glaciers and their response to climate change in the upper reaches have received widespread concerns. Here, the spatiotemporal characteristics of glacier wastage in the Upper Reaches of Shule River Basin (URSRB) driven by climate change were analyzed based on multi-source and multi-temporal remotely sensed images. Firstly, we extracted multi-temporal glacier outlines from the Landsat time series data using Google Earth Engine (GEE) for seven different periods every approximately 5years from 1990 to 2020. The spatiotemporal analysis of URSRB glaciers demonstrates a sustained reduction in glacier area from 481.07 ± 24.24 km2 in 1990 to 384.05 ± 22.71 km2 in 2020, corresponding to a glacier shrinkage rate of - 0.67 ± 0.23%/year, characterized by considerable temporal variability. Secondly, multi-temporal DEMs derived from ASTER stereo imagery spanning from 2000 to 2020 were used to compute the glacier surface elevation changes and determine the glacier mass loss. The overall glacier surface elevation change rate was - 0.32 ± 0.14m/year, equivalent to a mass balance of - 0.28 ± 0.12m w.e./year. Lastly, to better apprehend the long-term response of URSRB glaciers to climate change, studies on climate change were carried out based on the EAR5-Land reanalysis dataset. The long-term trend of glacier wastage is attributed to the increase in summer temperature, and the negative effects of increased summer temperature on glaciers exceeded the positive effects of increased annual precipitation. In summary, glaciers in the URSRB have experienced a significant area reduction and accelerated mass loss against the backdrop of climatic warming and humidification.

Risk hotspots and influencing factors identification of heavy metal(loid)s in agricultural soils using spatial bivariate analysis and random forest

Heavy metal(loid)s (HMs) in agricultural soils not only affect soil function and crop security, but also pose health risks to residents. However, previous concerns have typically focused on only one aspect, neglecting the other. This lack of a comprehensive approach challenges the identification of hotspots and the prioritization of factors for effective management. To address this gap, a novel method incorporating spatial bivariate analysis with random forest was proposed to identify high-risk hotspots and the key influencing factors. A large-scale dataset containing 2995 soil samples and soil HMs (As, Cd, Cr, Cu, Mn, Ni, Pb, Sb, and Zn) was obtained from across Henan province, central China. Spatial bivariate analysis of both health risk and ecological risks revealed risk hotspots. Positive matrix factorization model was initially used to investigate potential sources. Twenty-two environmental variables were selected and input into random forest to further identify the key influencing factors impacting soil accumulation. Results of local Moran's I index indicated high-high HM clusters at the western and northern margins of the province. Hotspots of high ecological and health risk were primarily observed in Xuchang and Nanyang due to the widespread township enterprises with outdated pollution control measures. As concentration and exposure frequency dominated the non-carcinogenic and carcinogenic risks. Anthropogenic activities, particularly vehicular traffic (contributing ∼37.8 % of the total heavy metals accumulation), were the dominant sources of HMs in agricultural soils. Random forest modeling indicated that soil type and PM2.5 concentrations were the most influencing natural and anthropogenic variables, respectively. Based on the above findings, control measures on traffic source should be formulated and implemented provincially; in Xuchang and Nanyang, scattered township enterprises with outdated pollution control measures should be integrated and upgraded to avoid further pollution from these sources.

Tectono-thermal evolution from the proximal to hyperextended domains of the northern South China Sea margin since initial rifting

The Pearl River Mouth Basin (PRMB) is located on the northern margin of the South China Sea (SCS). Heat flow measurements indicate that the PRMB is a typical ‘hot basin' characterized by a high background heat flow. However, the tectono-thermal evolution of the PRMB and the mechanisms involved are still controversial due to the different input parameters used in tectono-thermal models, small datasets and the limited area of the basin studied. We used tectono-thermal modelling with a multi-stage finite stretching model, constrained by extensive well data, to systematically analyse the tectono-thermal evolution of the PRMB since the onset of rifting. The study area was expanded relative to previous studies to cover both the proximal and hyperextended regions of the northern SCS margin, providing a more comprehensive understanding of its tectono-thermal history. Numerous wells and seismic data covering the entire basin were used, along with appropriate input parameters, to address the gaps in previous studies and to enhance the accuracy of the results. Our findings indicate that the PRMB underwent two phases of heating, primarily due to thinning caused by lithospheric extension during rifting. By the end of rifting, the Northern Depression Zone and the Kaiping Sag had reached peak heat flow, while the Panyu Lower Uplift and Baiyun Sag saw their highest heat flow since the initial rifting. The PRMB experienced thermal decay during the post-rift stage, but a significant reheating event occurred from 23.03 to 13.82 Ma, likely due to northwards lower crustal flow from the Baiyun Sag. The Panyu Lower Uplift and the Baiyun Sag had reached their peak heat flow by 13.82 Ma and the heat flow generally increased by 1–3 mW m −2 in the other studied areas during this stage. The basin's thermal decay slowed at 5.33 Ma and localized heating events linked to magmatic activities in the southern Dongsha Uplift were observed. In general, the proximal domain of the northern SCS margin reached peak heat flow by the end of rifting, whereas the hyperextended domain reached peak heat flow by 13.82 Ma. The heat flow in the hyperextended domain was generally higher than that in the proximal domain as a result of the thinner crust of the hyperextended domain caused by intense multiple lithospheric detachment–extensional thinning processes. During the rifting stage, the lithospheric extensional thinning process was the most important factor influencing the tectono-thermal evolution of the northern SCS margin, while lower crustal flow and magmatism were the most important factors during the post-rift stage.

Origin of the Narenwula quartz vein-type W deposit, Inner Mongolia, China: Insights from geochemistry of hosting granites, fluid inclusions, H-O-S isotopes, and wolframite trace elements

Wolframite that coexists with quartz is common hydrothermal mineral in tungsten deposits, and its geochemical features can be used to constrain the tungsten mineralization. Here we show that the Narenwula deposit is a typical quartz vein-type W deposit on the northern margin of the North China Craton, Northeast China, and temporally and spatially associated with Early Cretaceous granitoids, provides an opportunity to decipher the W mineralization process. All the orebodies are composed of a series of northeast- trending wolframite-polymetallic sulfides bearing quartz veins accompanied by intense alterations, mainly hosted in the Early Cretaceous monzogranite and porphyraceous monzogranite. Three paragenetic stages were identified: (I) quartz-wolframite, (II) quartz-wolframite-sulfide, and (III) quartz-fluorite-calcite. LA−ICP−MS trace element analyses revealed that both wolframite (I) and (II) have similar trace elements, REEN patterns, and overall consistent Nb/Ta and Y/Ho ratios, indicate a single fluid source and evolution in the W mineralization of the Narenwula deposit. Fluid inclusions microthermometry of quartz and wolframite, coupled with H-O isotopes suggest that the early ore-forming fluids of the Narenwula deposit are mainly magmatic water with relatively moderate-to-high temperature and salinity, while the late ore-forming fluids are mixed with meteoric water, with medium-to low temperature and low salinity. The δ34S values of pyrite (4.63–6.65 ‰, average = 5.44 ‰) further support a magmatic origin for the sulfides. The metallogenic mechanisms at the Narenwula deposit include fluid boiling and fluid-rock interactions were the two major factors controlling the deposition of wolframite and sulfide, while simple cooling and fluid mixing may have also promoted wolframite precipitation.

New Discovery of Natural Zeolite-Rich Tuff on the Northern Margin of the Los Frailes Caldera: A Study to Determine Its Performance as a Supplementary Cementitious Material.

The release of Neogene volcanism in the southeastern part of the Iberian Peninsula produced a series of volcanic structures in the form of stratovolcanoes and calderas; however, other materials also accumulated such as large amounts of pyroclastic materials such as cinerites, ashes, and lapilli, which were later altered to form deposits of zeolites and bentonites. This work has focused on an area located on the northern flank of the San José-Los Escullos zeolite deposit, the only one of its kind with industrial capacity in Spain. The main objective of this research is to characterize the zeolite (SZ) of this new area from the mineral, chemical, and technical points of view and establish its possible use as a natural pozzolan. In the first stage, a study of the mineralogical and chemical composition of the selected samples was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), and thermogravimetric analysis (TGA); in the second stage, chemical-qualitative and pozzolanicity technical tests were carried out at 8 and 15 days. In addition, a chemical analysis was performed using XRF on the specimens of mortars made with a standardized mixture of Portland cement (PC: 75%) and natural zeolite (SZ: 25%) at the ages of 7, 28, and 90 days. The results of the mineralogical analyses indicated that the samples are made up mainly of mordenite and subordinately by smectite, plagioclase, quartz, halloysite, illite, and muscovite. Qualitative chemical assays indicated a high percentage of reactive silica and reactive CaO and also negligible contents of insoluble residues. The results of the pozzolanicity test indicate that all the samples analyzed behave like natural pozzolans of good quality, increasing their pozzolanic reactivity from 8 to 15 days of testing. Chemical analyses of PC/SZ composite mortar specimens showed how a significant part of SiO2 and Al2O3 are released by zeolite while it absorbs a large part of the SO3 contained in the cement. The results presented in this research could be of great practical and scientific importance as they indicate the continuation of zeolitic mineralization beyond the limits of the San José-Los Escullos deposit, which would result in an increase in geological reserves and the extension of the useful life of the deposit, which is of vital importance to the local mining industry.

Extensive restoration of the entire habitat complex is key to the successful recovery of threatened species: The case of the natterjack toad Epidalea calamita at the northern range margin

Increasing concerns about biodiversity loss have prompted international efforts to restore degraded habitats. Unfortunately, wildlife habitat restoration often fails due to inadequate planning, inappropriate scale, or lack of providing key elements of habitats. We investigated the effectiveness of habitat restoration of a threatened amphibian species, the natterjack toad Epidalea calamita, in Estonia, comparing two different restoration approaches – partial habitat restoration and extensive habitat restoration over 16 years. Our goal was to investigate the impact of these two different approaches on the populations, to find out which habitat components play a key role in restoration and which measures are of primary importance in the restoration of habitats for small amphibian populations. We used GIS analysis to identify changes in the area of the habitat complex and breeding sites in seven natterjack toad locations following the restoration activities. To assess population response to habitat restoration, we used the census of the natterjack toad egg-strings as a proxy to the population size. The average number of egg-strings increased by 1.2-fold after partial habitat restoration and 3.5-fold when comparing partial habitat restoration with extensive restoration. Partial habitat restoration, which consisted of improving the condition of extant habitat, had no significant effect on population size (number of egg-strings), while extensive habitat restoration, which included improving and enlarging the entire habitat complex (both terrestrial and aquatic habitats as well as migration matrix between these components) led to population growth. The habitat features that positively influenced population growth were the area of entire habitat complex and the number and size of the breeding sites. In terms of habitat restoration and conservation measures, the number of natterjack toad populations was positively affected by restoration of the entire habitat complex (terrestrial habitats and water bodies), which was supplemented by the supportive rearing of tadpoles.

Diverse magmatism along the northern margin of Tarim during the Ediacaran: Transition from Rodinian dispersing to Gondwana assemblage

The Ediacaran igneous rocks and sedimentary sequences play a key role for our understanding the transition from dispersing of the Rodinian continents to the assemblage of the Gondwana. On the other hand, some continental massifs at the marginal area of the Rodinia and Gondwana could preserve solid evidences for deciphering the geodynamic background from the break-up of Rodinia to the assemblage of Gondwana. In this contribution, we report field observations, petrography, ages and systematic geochemistry of the diverse Ediacaran igneous activities in the northern margin of the Tarim Block, which was favored as a marginal continental massif in the Rodinia configuration by most geologists. The Ediacaran igneous rocks along the northern margin of Tarim include the Baicheng and Yangxia granites, Tiemenguan syenite, Kurle mafic dykes and the basalt layers in the Zamoketi Formation (northeastern Tarim) and the Sugetbrak Formation (northwestern Tarim). Precise zircon UPb dating revealed that these distinct rocks were broadly coeval with crystallization/eruption ages of 630–600 Ma. Bulk-rock elemental and SrNd isotope compositions as well as zircon LuHf isotope compositions demonstrate that the Baicheng and Yangxia granites were derived from Neoarchean mafic lower crust and underwent intensive fractionation before their emplacements. The Tiemenguan syenites have potassic andesitic compositions, in combination with their pronounced enriched NdHf isotopes and extremely high Zr saturation temperatures (∼900 °C), we argue that their possible shoshonitic primary magmas were derived from previously metasomatized subcontinental lithospheric mantle source (amphibole-bearing) and then, coupling with assimilation and fractional crystallization (AFC) effects in magma chamber and/or en route to the emplacement space. This can account for their intriguing geochemical features. The ca. 615 Ma Zamoketi and Sugetbrak basalts and the slightly younger porphyrite dykes (ca. 580 Ma) cutting the Zamoketi basalt layer, show typical OIB-like geochemical signatures, they were most likely derived from a depleted asthenospheric mantle source with variable fractionation of olivine and pyroxene and negligible crustal contamination before eruption. On the whole, the geochemistry of the diverse Ediacaran igneous rocks unambiguously demonstrates that they were genetically related to an extensional environment. In combination with the significant passive continental features of the Ediacaran-Cambrian sedimentary sequences, we consider that these Ediacaran igneous rocks were the latest phase of igneous activity related to the dispersing of the Tarim at the margin of the Rodinia supercontinent.The dispersing of the Rodinian supra-continent was diachronic with the assemblage of the Gondwana. Late Tonian to Ediacaran dispersing of the Tarim from northern margin of Australia induced the opening of the Proto-Tethys Ocean. Then the initiation of the southward subduction of the Proto-Tethys Ocean at ca. 530 Ma (Fortunian) led to the Tarim assemblage to Gondwana at ca. 440 Ma.

Earthquake detection probabilities and completeness magnitude in the northern margin of the Ordos Block

Earthquake detection probabilities and completeness magnitude in the northern margin of the Ordos Block

Ediacaran to Jurassic geodynamic evolution of the Alborz Mountains, north Iran: geochronological data from the Gasht Metamorphic Complex

The Alborz Mountains in north Iran underwent several tectono-metamorphic events during opening and closure of the Palaeotethys and Neotethys Oceans. These events are recorded by rare and discontinuously exposed metamorphic rocks, such as the HP-LT Asalem-Shanderman Complex and the Gasht Metamorphic Complex (GMC), that are considered to have been metamorphosed during the closure of the Palaeotethys Ocean. The GMC comprises poorly exposed metasediments and amphibolites metamorphosed under greenschist- to amphibolite-facies conditions, along with smaller volumes of granites. Different dating methods were applied to selected samples of the GMC basement to constrain the geological evolution of this part of the Alborz Mountains. A metagranite yielded two LA-ICP-MS U–Pb zircon ages of 638.4 ± 4.1 Ma and 590.3 ± 4.8 Ma that possibly date protolith crystallisation and later deformation and metamorphism, respectively, and a granite yielded a late Ediacaran 551 ± 2.5 Ma U–Pb zircon crystallisation age. A northern provenance from the basement to the South Caspian Basin can neither be established nor ruled out because no age data are available for this unit. Derivation of the GMC from Turan Block basement is unlikely, as this has a different crustal makeup and is probably composed of Paleoproterozoic and early Neoproterozoic material. The zircon ages are similar to published ages from the Arabian-Nubian Shield, indicating that this part of the Alborz basement may have belonged to the northern margin of Gondwana in the Neoproterozoic before rifting and drifting away along with other Iranian blocks (the Cimmerian terranes) during opening of the Neotethys Ocean. Chemical Th-U-total Pb ages for metamorphic monazites from two metapelite samples yielded a very large range of spot ages, of which c. 80% falls between 200 and 250 Ma, that do not allow to distinguish between Eo-Cimmerian and Main Cimmerian events in the GMC. However, they may indicate that the amphibolite-facies peak metamorphism of the GMC occurred sometime in the Triassic, in any case much later than the Carboniferous metamorphism in the neighbouring Asalem-Shanderman Metamorphic Complex to the north. Peak-metamorphic amphibole from amphibolite, retrograde white mica and foliation-defining biotite from metapelites and magmatic white mica from granite yielded much younger 175.1 ± 0.5 to 177.0 ± 0.4 Ma 40Ar/39Ar plateau ages. The Toarcian 40Ar/39Ar ages for minerals with different nominal closure temperatures reflect very rapid cooling of GMC basement below the Shemshak Group due to extension-triggered uplift. This late Toarcian to Aalenian extension event can be correlated with the regional Mid-Cimmerian unconformity of mid-Bajocian age (c. 170 Ma) that resulted from the tectonic movements causing rapid uplift and erosion. Extension probably started in the western Alborz Mountains in the Toarcian, migrated eastward, and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin. It was probably triggered by the onset of the subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent.

The Fremont Frontier: Living at the Margins of Maize Farming

Abstract The Fremont provide an important case study to examine the resilience of ancient farmers to climatic downturns, because they lived at the far northern margin of intensive maize agriculture in the American West, where the constraints on maize production are made abundantly clear. Using a tree-ring and simulation-based reconstruction of average annual precipitation and maize growing degree days, along with cost-distance to perennial streams, we model spatial variability in Fremont site density in the eastern Great Basin. The results of our analysis have implications for defining the ecological envelope in which farming is a viable strategy across this arid region and can be used to predict where and why maize farming strategies might evolve and eventually collapse as climate changes over time.

Opening and Closure of the Sulu Sea: Revealed by Its Peripheral Subduction and Collision Processes

The Sulu Sea is a small marginal sea in the western Pacific, but it is a very complex and tectonically active region, situated amidst the convergence of the Eurasian, Pacific, and India-Australian plates. Deciphering its geodynamic evolution is crucial, but our understanding of its opening, closure, and tectonic history remains inadequate. The main aim of this study was to systematically study the opening and subsequent closure of the Sulu Sea though discerning tectonic unconformities, structural features, and subduction-collision tectonic zones around margins of the sea. The interpreted sections and gravity anomaly data indicate that the NE Sulu Sea has undergone Neogene extension and contraction due to subduction and collision along the northern margins of the Sulu Sea, whereas the SE Sulu Sea gradually extended from northwest to southeast during the Middle Miocene and has subsequently subducted since the Middle Miocene along the southeastern margins of the Sulu Sea. Several subduction and collision boundaries with different characteristics were developed including continent-continent collision, arc-continent collision, and ocean-arc subduction. The different margins of the Sulu Sea showed distinct asynchronous subduction and collision processes. The northern margins of the Sulu Sea can be divided into three subduction-collision tectonic zones from west to east: the Sabah-Nansha block collision has occurred in NE Borneo since the Early Miocene, followed by the SW Palawan-Cagayan arc collision in SW Palawan Island since the Middle Miocene, and the NW Palawan-Mindoro arc collision since the Late Miocene with further oblique subduction of the Philippine Sea Plate. The southeastern margins can also be divided into two subduction tectonic zones from south to east: the SE Sulu Sea has subducted southward beneath the Celebes Sea since the Middle Miocene, followed by the southeastward subduction beneath the Philippine Sea Plate since the Pliocene. Since the Miocene, the interactions among the Australia-India, the Philippine Sea, and the Eurasian plates have formed the circum-Sulu Sea subduction-collisional margins characterized by microplate collisions, deep-sea trough development, and thick sediments filling in the orogenic foreland. This study is significant for gaining insights into the opening and closure of marginal seas and the dynamics of multiple microplates in Southeast Asia.

Upwelling processes variability and water circulation along the northern margin of the Gulf of Cadiz

The present study investigates the contributions of upwelling mechanisms (coastal upwelling from boundary divergence and Ekman pumping from spatially variable wind) at the northern margin of the Gulf of Cadiz (NMGoC) based on high-resolution wind. The effects of the spatiotemporal upwelling mechanisms variability are then explored using current observations along with sea surface temperature and sea level anomaly. Upwelling favourable conditions occur throughout the year along the NMGoC, with the strongest intensity near Cape São Vicente due to a persistent positive wind stress curl. In winter, the surface water divergence is restricted to the coastal boundary due to strong coastal upwelling events that result in a cross-shore sea-level gradient close to the coast. Towards the summer, the gradient increases and extends further offshore (over the slope) at the western region due to the intensification of the Ekman pumping. Therefore, the seasonal and spatial variability of coastal upwelling and Ekman pumping contributes significantly to the offshore position of the geostrophic Gulf of Cadiz Current over the western shelf slope. Furthermore, a permanent sea level depression corresponds to the location of the strongest Ekman pumping, near Cape São Vicente. The dynamic adjustment of this feature may drive the cyclonic cell and alongshore poleward currents often observed in the area.

Source and age of the Ngaye banded iron formations, Adamawa Yadé Domain, Central Cameroon: Constraints from whole-rock geochemistry and U-Pb zircon geochronology

The Ngaye inlier of the Adamawa Yadé Domain is located in the Central African Fold Belt, adjacent to the Nyong Complex at the northern margin of the Congo craton, in central Cameroon. This area consists of metamorphosed granite-greenstone associations comprising amphibole- and pyroxene-rich banded iron formations (BIFs), amphibolites, and migmatitic gneisses. This study reports detailed petrographic, whole-rock geochemical, and LA-ICP-MS U-Pb zircon data for the Ngaye BIFs and associated amphibolites to better constrain their source, tectonic setting, and age. Amphibole-rich BIFs exhibit high REE-Y content and uncommon LREE-enriched patterns. In contrast, pyroxene-rich BIFs display combined low REE-Y content, seawater-like patterns, and positive Eu anomalies, suggesting a mixture of seawater and hydrothermal fluids during their deposition. The geochemical data of associated amphibolites suggest a back-arc setting for the Ngaye metavolcanosedimentary sequence, similar to that of the Nyong Complex. LA-ICP-MS U-Pb zircon dating of amphibole-rich BIFs indicate a maximum depositional age of ca. 2186 Ma and subsequent metamorphism at ca. 2038 Ma, overlapping with that of metavolcanosedimentary sequences from the Nyong Complex. Neoproterozoic age of ca. 598 Ma obtained for these BIFs is interpreted as the Pan-African metamorphic/hydrothermal imprint. This finding suggests that the post-depositional fluid overprint was probably related to the regional Pan-African tectono-thermal event.

The Early Neoproterozoic Andean-Type Orogenic and Within-Plate Magmatic Events in the Northern Margin of the Yangtze Craton during the Convergence of the Rodinia Supercontinent

Although considered a crucial component of the Rodinia supercontinent, it remains uncertain how the Yangtze craton relates to the accretion and breakup of Rodinia. Here, the Huanglingmiao granitic complex (HGC), an intermediate-acid rock series that intruded on the southern Kongling terrane of the northern Yangtze craton margin, is investigated to help resolve this conundrum. Our analysis indicates that these rocks consist of tonalite, trondhjemite, granodiorite, oligoporphyritic granodiorite, porphyric biotite granodiorite, and fine- to medium-grained granodiorite dyke compositions. Collectively, this assemblage is further subdivided into two categories by their temporal, spatial, and geochemical features into early TTG-like and later granitic–dioritic units, which are composed of tonalite, trondhjemite, granodiorite, porphyritic granodiorite, and the fine- to medium-grained granodiorite dykes, respectively. Zircon U-Pb dating yields ages of 865~850 Ma for the TTG-like rocks, 844~825 Ma for the porphyritic granodiorites, and ~800 Ma for the granodiorite dykes. Combined with geochemical evidence, the data suggest that the early- and late-series rocks were formed by a partial melting of Mesoproterozoic and Paleoproterozoic crustal materials, respectively, suggesting that the vertical layering of the crust controlled the composition of the independent units. In addition, isotopic evidence points to different sources for the various rocks in the Kongling terrane and that mantle-derived materials influenced the early-series lithologies. Combined with previous studies on the northern margin of the Yangtze craton, it is inferred that the early-series rocks formed in an active continental margin environment, while the late-series rocks display within-plate boundary formation characteristics. The multiple magmatic activities revealed by this study record sequential partial melting with tectonic transition characteristics from an Andean-type to within-plate magmatism in the northern margin of the Yangtze craton. Taken together, these observations point to a strong association between these rocks, convergence, and incorporation of the northern Yangtze craton margin into the Rodinia supercontinent during the Tonian Period.

The Cambrian collision of the Yangtze Block with Gondwana: Evidence from provenance analyses

The Neoproterozoic to Paleozoic Proto-Tethys Ocean has preserved critical records of Earth’s evolutionary history. The Lower Cambrian detrital strata along the northern margin of the Yangtze Block provide valuable information for gaining insight into the evolution of the Proto-Tethys Ocean, which is important in understanding the collision between the Yangtze Block and the Gondwana assembly. Here, we present comprehensive U-Pb ages and trace elements of zircons from Lower Cambrian detrital strata in the Yangtze Block. Our results revealed prominent age peaks at 600−500 Ma and 1100−800 Ma for detrital zircons in the Cambrian strata. The trace element characteristics of the detrital zircon indicate that the source rocks were mainly intermediate-felsic magmatic suites with minor input from mafic sources. These results suggest that the detrital materials were not only supplied from the interior of the Yangtze Block but also from magmatic sources in the internal orogenic belt of Gondwana. Furthermore, the kernel density estimate plots of detrital zircon U-Pb ages indicate a stable source-sink system in the Yangtze Block during the Early Cambrian. However, our data indicate that the crustal thickness of the Yangtze Block increased significantly since ca. 526 Ma, and the cumulative characteristics of the detrital zircons indicate that the rocks were deposited in a compressional tectonic environment after ca. 526 Ma, in contrast to those during the late Neoproterozoic, which were generally deposited in an extensional tectonic environment. This transition indicates that the initial collision between the Yangtze Block and the Gondwana supercontinent might have occurred at ca. 526 Ma, marking the rapid aggregation of the northern domain of the Gondwana assembly and potentially serving as an important indicator of the closure of the Yangtze-associated Proto-Tethys Ocean. Our findings provide key insights into late Neoproterozoic−early Paleozoic ocean-land evolution.

Magmatic and sedimentological arguments for an Ediacaran active margin in the Bayankhongor Zone in western Mongolia, Central Asian Orogenic Belt

Geochronological and geochemical investigation of the magmatic and sedimentary rocks from the south-eastern tip of the Bayankhongor Zone (central Mongolia) constrains the Neoproterozoic evolution of the northern margin of the Baidrag Block. There, ultramafic to felsic igneous rocks of the Khan-Uul Massif intruded the volcano-sedimentary sequence of the Ulziit Gol Unit. U–Pb zircon ages show that both the plutonic and volcanic rocks were coeval products of the same Ediacaran (598–564 Ma) magmatism. Most of the samples have low contents of high field strength elements typical of arc-related magmas; however, some mafic rock samples display N-MORB-like chemistry. Magmatic rocks in the Khan-Uul Massif and Ulziit Gol Unit yielded exclusively positive initial εHf in zircon values (+4.9 to +13.8), implying derivation from depleted-mantle sources. Furthermore, heterogeneity of the whole-rock initial εNd values (–4.0 to +2.1) documents that fractional crystallization was accompanied by variable crustal contamination. The detrital zircon age patterns of the sandstone and tuffites in the Ulziit Gol Unit indicate that the sequence was filled dominantly by Ediacaran magmatic detritus derived from the Khan-Uul Massif and its volcanic equivalents, while the older cratonic material from the Baidrag basement represented only a subordinate component. Combined magmatic and sedimentary records imply that both the Khan-Uul Massif and the Ulziit Gol Unit may have formed in the same back-arc basin environment. This challenges the previous view that the entire Bayankhongor Zone was ophiolitic in nature. Distribution of coeval Ediacaran supra-subduction systems on the scale of the Mongolian Collage indicates the closure of multiple oceanic basins rimming the Siberian Continent. Following Rodinia break-up, this peri-Siberian realm was presumably formed by sub-parallel continental ribbons and oceanic basins. It is proposed that the amalgamation of these blocks and closure of intervening oceans reflected the Ediacaran advancing mode of the Palaeo-Pacific subduction.