Basin Closure Research Articles

Multicyclic Phanerozoic orogeny recorded in the Qaidam continent, northern Tibet: Implications for the tectonic evolution of the Tethyan orogenic system

The growth and evolution of the Eurasian continent involved the progressive closure of major ocean basins during the Phanerozoic, including the Tethyan and Paleo-Asian oceanic realms. Unraveling this complicated history requires interpreting multiple overprinted episodes of subduction-related magmatism and collisional orogeny, the products of which were later affected by the Cenozoic construction of the Himalayan-Tibetan orogen due to the India-Asia collision. In particular, the tectonic evolution of northern Tibet surrounding the Cenozoic Qaidam Basin is poorly resolved due to several phases of Phanerozoic orogeny that have been reactivated during the Cenozoic deformation. In this study, we investigated the geology of the northern Qaidam continent, which experienced Paleozoic−Mesozoic tectonic activity associated with the development of the Eastern Kunlun orogen to the south and the Qilian orogen to the north. We combined new and published field observations, geochronologic and thermochronologic ages, and geochemical data to construct regional tectonostratigraphic sections and bracket phases of Paleozoic−Mesozoic magmatism associated with oceanic subduction and continental collision. Results suggest that the Qaidam continent experienced two major phases of subduction magmatism and collision. First, a Cambrian−Ordovician magmatic arc developed in the northern Qaidam continent due to south-dipping subduction. This phase was followed by the closure of the Qilian Ocean and the collision of the North China craton and Qaidam continent, resulting in Silurian−Devonian orogeny and the development of a regional unconformity across northern Tibet. A subsequent Permian−Triassic magmatic arc developed across the northern Qaidam continent due to north-dipping subduction. This phase was followed by the closure of the Neo-Kunlun Ocean and the collision of the Songpan Ganzi terrane in the south and Qaidam continent. These interpretations are incorporated into a new and comprehensive model for the Phanerozoic formation of northern Tibet and the Eurasia continent.

How and when did Paleozoic Maizuru back-arc basin close? Implications on the East Asian continental margin tectonics

The paleo-continental reconstruction of Asia during the Permian–Triassic boundary indicates major tectonic readjustments between multiple continental blocks in the north of North China Craton (NCC) due to complex plate subduction system(s). One of the major consequences was the closure of the Paleo-Asian Ocean. In a similar time frame, Proto-Japan was also evolving through plate-subduction with the opening of the Maizuru back-arc basin. The spectacular linear near-continuous belt of the Maizuru Terrain in the Inner Zone of Southwest Japan holds the rock record of the opening of the back-arc basin, sediment-fill in this basin culminating into closure-related pulsative debris flow deposits (i.e. the Tonoshiki Formation). However, the exact deformation mechanism(s), paleo-stress regime, their mesoscopic to microscopic rock record, and exact timing hold the keys to connecting the closure event of the Maizuru back-arc basin and the possible tectonics, in particular the closure of the PAO in Asia. With this aim, we report here the micro- to meso-structural geological records of rock units of the Maizuru Terrane which indicate evidence of (1) pre-depositional hydrofracturing, (2) extensive syn-lithification fracture-vein formation due to (3) broad WNW-ESE to E-W compressional paleostress regime using the fracture-vein analysis, (4) deformational e-twin development with the dominance of Type II twin indicating a maximum temperature of 300 °C with no evidence of overprinting. This brittle deformation affected all the lithounits of the Maizuru back-arc basin but not the overlying units deposited after the basin closure. New U-Pb detrital zircon age data of the rocks belonging to the Maizuru Group and also the overlying Fukumoto Formation fixes the age of the above-mentioned deformation event at ∼ 250 Ma. All these data help to assess the possible paleo-position of Proto-Japan and a clear tectonic scenario between the NCC, the Central Asian Orogenic Belt, and the Khanka-Jiamusi Massif in East Asia.

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.

The paradox of success: Water resources closure in Axarquia (southern Spain)

Axarquia is a semi-arid region in southern Spain that in the past 25 years has experienced significant population growth, along with an economic boom driven by an increasing influx of tourists to Costa del Sol and the expansion of irrigated export-oriented subtropical crops. The combination of these factors has led to a chronic structural scarcity condition that has been intensified by the occurrence of a long and extreme drought. As a result, its only reservoir has reached historically low levels and the piezometric levels in its main aquifer have decreased significantly, suggesting that groundwater reserves are being overexploited. The water crisis is impacting citizens (urban supply), farmers (losses of yields and crops), and the environment (decreasing water reserves). The authorities have responded through supply-side measures such as incorporating reclaimed wastewater in the system and planning the deployment of desalination infrastructure in the region, but demand control and proper governance are required to guarantee sustainability. Consequently, in this case study we apply the European Environment Agency's DPSIR (driving forces, pressures, state, impact, and response model of intervention) framework to understand the basin closure process in Axarquia and assess the main actions that have been undertaken by public and private sector stakeholders to address the challenges faced by the region. Our results provide a valuable reference case to support the analysis of similar closure events, the early identification of potential crisis conditions, and the design of potential solutions in water scarce regions in the European Union, the Mediterranean, and elsewhere.

Detrital isotopic record of a retreating accretionary orogen: An example from the Patagonian Andes

Abstract U-Pb zircon geochronology and isotopic records have played an influential role in our understanding of convergent margin dynamics. Orogenic cyclicity models link tectonic regimes with magmatic isotopic signatures in advancing orogens, relating compressional regimes with evolved signatures and extension with juvenile signatures; however, such frameworks may not apply for retreating orogens, which commonly produce substantial crustal heterogeneities during backarc rifting and ocean spreading. We explore the Mesozoic to Cenozoic Patagonian Andes tectonic evolution, combining U-Pb zircon ages, bulk rock εNd, and new detrital zircon εHf from the retroarc basin to understand the associated magmatic arc evolution during retreat and advance of the margin. Our results reveal a protracted phase of isotopically juvenile magmatism between 150 and 80 Ma, which began during backarc extension and persisted long after the margin switched to a contractional regime. We propose that the prolonged juvenile isotopic trend started mainly due to trenchward migration of the arc during backarc extension (150–120 Ma) and persisted due to partial melting of underthrusted juvenile attenuated and oceanic crust during backarc basin closure (120–80 Ma). This interpretation implies that tectonic stress alone does not predict isotopic trends, and factors like assimilation or the composition of underthrusted crust are important controls on magmatic isotopic composition, especially in retreating and transitional orogens.

Late Paleozoic magmatism and foreland deformation associated with opening and closing of marginal basins in the North Patagonian Andes

Remnants of coeval Devonian oceanic and continental foreland rocks are preserved in the basement of the North Patagonian Andes. Our previous studies of the igneous rocks here have shown that the primitive oceanic and continental igneous rocks are coeval and belong to a marginal basin that opened and closed over 50 Myr. A structural study and four new U–Pb SHRIMP ages and zircon Hf–O determinations allowed the identification of three metamorphic episodes, the first one of which, M 1 (D 1 -S 1 ), is coeval with Andean-type granite intrusion in the foreland (405–380 Ma). This activity was simultaneous with the development of an oceanic ridge and a marginal basin, at the outer edge of which a primitive granitic oceanic arc formed (380–385 Ma; zircon δ 18 O 3.6–5.2‰). The extinction of this ridge initiated under-thrusting of the oceanic crust below the continent and an important mid- to high-grade metamorphic event, M 2 (D 2 -S 2 ) at 375–360 Ma, its peak dated by metamorphic zircon rims in migmatite at 365 ± 3 Ma. Basin closure occurred after the intrusion of S-type granites (357 ± 2 Ma; zircon δ 18 O 7.4–10.4‰) in the foreland and the accretion of gabbros, cumulate gabbros and trondhjemites at the proto-Pacific margin. Compression prevailed for 20 Myr in the foreland, causing a mylonitic medium-grade M 3 (D 3 -S 3 ) event. Supplementary material: Datasets are available at https://doi.org/10.6084/m9.figshare.c.6989873

Exhumed Serpentinites and Their Tectonic Significance in Non‐Collisional Orogens

AbstractExhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid‐rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non‐collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non‐collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in‐situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ11B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ11B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic‐Early Cretaceous, and potentially Late Cretaceous‐Paleocene high‐pressure (HP)–low‐temperature metamorphic sequences. Whole‐rock trace element data and in‐situ B isotopes favor serpentinization by a crust‐derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid‐Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back‐arc basins. Subsequent compressional phases trigger short‐lived subduction in the back‐arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non‐collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back‐arc basins.

Groundwater Basin Openness and Sustainability

AbstractGroundwater pumping from wells, together with water uses such as agricultural irrigation have been converting formerly open groundwater basins into closed systems that accumulate total dissolved solids (TDS). This process of anthropogenic basin closure and salinization (ABCSal) would appear to pose a threat to groundwater sustainability that is at least as formidable as groundwater overdraft and contamination from the surface, yet has been little explored. Models of groundwater flow and solute transport herein show that groundwater basin openness itself should be considered a primary determinant of sustainability. Results show that groundwater basin closure is a threshold condition that sets the aquifer system on a path of increasing salinity that can only be halted by opening the basin. Further, the magnitude of groundwater pumping and degree of basin closure significantly influence the spatial distribution of salinity. In open basins, salinity approaches dynamic equilibrium over long‐term conditions. Stratification of higher‐TDS groundwater overlying lower‐TDS groundwater occurs below farmlands whose irrigation‐supplying wells are impacted by irrigation return flow from upstream farmlands, and act to redistribute relatively saline groundwater to the land surface. More intensive pumping leads to groundwater basin closure and more vertically‐oriented groundwater flow toward pumping wells. TDS retainment in the basin and repeated well capture, re‐distribution as irrigation water, and evapoconcentration lead to progressive salinization. Regardless of basin closure status, fresh recharge protects nearby downstream portions of the basin from salinization, indicating that managing or limiting the spread of contaminated groundwater may be achieved via managed aquifer recharge of good quality water.

High-pressure metamorphism of Precambrian continental crust in the southwestern part of the Central Asian Orogenic Belt (South Kazakhstan and North Tien Shan) and tectonic implications for the evolution of the Palaeo-Asian Ocean

AbstractIn this study, we investigated the high-pressure (HP) metamorphism of the Precambrian continental crust exposed in the Zheltau terrane in South Kazakhstan (Koyandy complex) and the Chu-Kendyktas terrane in the North Tien Shan of Kyrgyzstan (Aktyuz, Kemin and Kokdzhon complexes) within the SW part of the Central Asian Orogenic Belt. HP quartz–feldspar lithologies of the Koyandy complex consist of migmatized kyanite-bearing garnet–mica paragneisses, garnet–kyanite paragneisses and their derivatives associated with eclogites. Paragneisses demonstrate prograde evolution involving mica dehydration melting and producing magnesium-rich garnet, kyanite and K-feldspar at the near-peak to retrograde stages at pressures of 15–18.5 kbar and temperatures of 800–870°C. The widespread growth of micas in these rocks reflects lower stages of retrogression at P = 10–12 kbar and T = 720–770°C. The age distributions of the cores of detrital zircon grains from the paragneisses indicate a predominance of Neoproterozoic and minor occurrence of Mesoproterozoic and Palaeoproterozoic sources of their protoliths. The ages of ∼487–485 Ma obtained from the zircon rims of the paragneisses reflect the timing of their HP metamorphic re-equilibration. These age clusters are consistent with the age estimates obtained from the rims of zircons in the eclogite-bearing garnet gneisses of the adjacent Aktyuz complex in the North Tien Shan. The P–T paths and zircon ages obtained from the high-grade quartz–feldspar gneisses of the Zheltau and Chu-Kendyktas terranes are thus interpreted to indicate involvement of the crustal material derived from the Precambrian basement (magmatic zircons aged ca. 844 Ma) and its Ediacaran–Cambrian sedimentary cover (detrital zircons with maxima at 1 Ga and 800–600 Ma) in the latest Cambrian subduction processes induced by the closure of the oceanic basins assigned to the Palaeo-Asian Ocean.

Seismic attribute and well-log analysis for channel characterization in the upper San Andres and Grayburg formations of the Midland Basin, Texas

The Permian Basin is one of the most prolific, and currently one of the most active, oil and gas basins in the USA. The Lower Permian strata in the Permian Basin have produced more than 14 billion barrels of oil (BBO), making it the largest volume of hydrocarbon in the basin. Sedimentation in the Midland Basin during late Leonardian through early Guadalupian (ca. 272–269 Ma) resulted in progradation of shelf edge and ultimately closure of the basin by Middle Permian time. We analyzed a merged seismic survey covering parts of the Permian Basin (i.e., Central Basin Platform and Midland Basin) in Andrews, Ector, and Midland Counties, Texas. The seismic survey and well logs show the presence of gently dipping (ca. 1°) clinoforms in the Upper San Andres and Grayburg Formations on the eastern edge of the Central Basin Platform and western Midland Basin. The seismic attributes, curvature, and spectral decomposition identify low sinuosity slope channels oriented north-south, but such channels do not appear beyond the slope. The shelf edge shifts from north to south during deposition of the Upper San Andres and Grayburg Formations. We identify five basinward shifts noted by the migration of the shelf edge toward the basin center and the presence of channel features along the depositional slope. The petrophysical analysis indicates that channels cut into carbonate rocks and are filled by siliciclastic sediments; this interpretation is supported by the most negative curvature anomalies along the channel axes caused by the differential compaction between the carbonate and siliciclastic rocks. There are a few channels with a northwest-southeast strike, which matches the direction of the Concho Lineament observed by satellite data. Such observations are consistent with previous interpretations of the northern Midland Basin closure during Middle-Late Permian time.

Relict basin closure of the Paleo-Asian Ocean: New insights from geochronological and geochemical analysis of the Yangjiagou Formation, NE China

Relict basin closure of the Paleo-Asian Ocean: New insights from geochronological and geochemical analysis of the Yangjiagou Formation, NE China

Provenance of the conglomerate and siliciclastic rocks from the Gadag Greenstone Belt, Western Dharwar Craton, India: Implications for understanding Neoarchean basin margin sedimentation

Petrographic, whole‐rock geochemical and geochronological analysis of polymictic conglomerate, associated siltstone and greywacke in the Neoarchean Gadag Greenstone Belt in the Western Dharwar Craton, India, provides new insights into sedimentary provenance and tectonic setting. The polymictic conglomerate contains granitic, gneissic, felsic and mafic volcanic, quartzite and banded ferruginous chert clasts within a quartzo‐feldspathic matrix. Framework grains in the siltstone and greywacke are quartz, feldspar with rock fragments, embedded in a chlorite‐dominant matrix. The chemical index of alteration values (~70) and A‐CN‐K plot for the rocks suggest a low‐to‐moderate degree of weathering. Rare earth element (REE) patterns show moderate fractionation (LaN/YbN ~ 12.31) with a weak negative europium anomaly (0.82). Enrichment in transition elements (Ni, Cr, Co, Sc) and depletion in high‐field‐strength elements, as well as relatively low La/Co (0.62), moderate to high Th/Yb (3.06), La/Th (5.16), La/Yb (17.5), and La/Sc (~2.37) ratios, suggest a mixed felsic‐mafic provenance. U–Pb dating of detrital zircon from the conglomerate and greywacke revealed major age peaks at ca. (3.2, 2.9, 2.8, 2.6, 2.5 Ga) and a maximum age of deposition of 2,508 ± 26 Ma, 2,493 ± 20 Ma, respectively. The above data suggest low to moderately weathered proximal sources for both conglomerate and greywacke, indicating that these two rock types were not deposited simultaneously, but rather separately. The association of these conglomerates and greywacke with continental arc‐related volcanics indicates the closing of an arc at the time of sedimentation. Complex provenance consisting of recycled and magmatic arc during basin closure confirm the notion of modern style plate tectonics operating during Neoarchaean.

PALEOPROTEROZOIC GRAPHITIZATION IN THE CLÁUDIO SHEAR ZONE (SOUTHERN SÃO FRANCISCO CRATON - BRAZIL) INVESTIGATED BY RAMAN SPECTROSCOPY AND X-RAY DIFFRACTION

The southern São Francisco craton hosts some of the largest graphite mines in Brazil, located in the city of Itapecerica. This paper addresses graphite crystals that occur within the regional structure known as the Cláudio shear zone in terms of thermometry. Raman spectroscopy and X-ray diffraction data showed that, on average, graphite crystals of the Cláudio shear zone have high crystallinity and are similar to those from mines in Itapecerica. The average temperature found (702 ºC) in this study is close to the samples from Itapecerica. However, this temperature is still lower than the expected (~900 ºC) from previous studies. The hypothesis for this would be that the graphite devolved high crystallinity, but was modified during the evolution of the metamorphism, or during the post-collisional stage fluid percolation. Thus, the modified crystallinity parameters lead to the relatively low-temperature values found. The proximity and similarity of geological framework suggest correlation between the precursor basins that formed the Itapecerica and the Cláudio shear zone graphite occurrences. From a global perspective, the closure of precursor basins and formation of the Cláudio shear zone are part of the collisions of Archean and Paleoproterozoic crustal blocks that formed the Columbia supercontinent.

First finding of continental deep subduction in the Sesia Zone of the Western Alps and implications for subduction dynamics.

Continental deep subduction after the closure of large oceanic basins is commonly ascribed to the gravitational pull of the subducting oceanic slab. However, it is not clear how continental lithosphere adjacent to small oceanic basins was subducted to mantle depths. The Sesia Zone in the Western Alps provides an excellent target for exploration of subduction dynamics in such a tectonic setting. Here we report the first finding of coesite in a jadeite-bearing orthogneiss from the Sesia Zone, providing the first evidence for deep subduction of the continental crust to mantle depths for ultrahigh-pressure (UHP) metamorphism in this zone. Three coesite inclusions were identified by laser Raman spectroscopy in two garnet grains. Based on zircon U-Pb dating and trace element analysis, the UHP metamorphic age was constrained to be 76.0±1.0 Ma. The phase equilibrium modeling yields peak metamorphic pressures of 2.8-3.3GPa, demonstrating the continental deep subduction to mantle depths of >80km. The subducted continental crust was a rifted hyperextended continental margin, which was converted to the passive continental margin during seafloor spreading and then deeply subducted during the oblique convergence between the Adria microplate and Eurasian plate in the Late Cretaceous. Because the slab pull could only play a limited role in closing small oceanic basins for continental collision, the distal push of either continental breakup or seafloor spreading is suggested as the major driving force for the deep subduction of continental crust in the Western Alps. Therefore, deep subduction of the continental crust bordering small oceanic basins would have been induced by the far-field stress of compression, whereas that bordering large oceanic basins was spontaneous due to the oceanic slab pull. This provides a new insight into the geodynamic mechanism of continental deep subduction.

The tempo of back-arc basin evolution: Insights from the early Paleozoic Proto-Tethyan North Qilian orogenic belt, northeastern Tibet

Back-arc basins are pivotal sites for the generation and consumption of oceanic lithosphere in mature intra-oceanic convergent plate margins. However, the processes and mechanisms by which back-arc oceanic basins generate and terminate remain enigmatic. Here we document the life cycle of an extinct Paleozoic back-arc from an ophiolite-accretionary complex in the Proto-Tethyan North Qilian orogenic belt, northeastern Tibet, which records early spreading and subsequent closure through two episodes of subduction. Zircon U–Pb ages of gabbro and plagiogranite indicate two episodes of oceanic crust formation at ca. 480 Ma and ca. 452–448 Ma. Low-Ti gabbro, basalt, and plagiogranite (ca. 480 Ma) from the accretionary complex have back-arc geochemical signatures, whereas ultramafic rocks, gabbro, basalt, and plagiogranite suite from the ophiolite sheet (ca. 452–448 Ma) exhibit forearc characteristics. These data, together with recent structural and metamorphic evidence, indicate that the ophiolitic rocks were derived from both back-arc and forearc settings, and record the magmatic response to early spreading of the North Qilian back-arc basin and subsequent subduction initiation. Early subduction and slab rollback of the North Qilian oceanic lithosphere led to formation of the North Qilian arc and spreading of the back-arc, whereas later subduction initiation of the back-arc oceanic crust resulted in its termination and closure. These findings indicate that the subduction of back-arc oceanic crust is a primary mechanism in the closure of back-arc basins and thereby highlights the complex evolutionary tempo of back-arc basins in Earth history.

Global back-arc extension due to trench-parallel mid-ocean ridge subduction

Subduction of young seafloors straddling mid-ocean ridges (MOR) is an inevitable consequence of plate tectonics. Surprisingly, this process correlates globally with prolonged back-arc extension when the MOR is largely trench-parallel. We investigate the underlying mechanism by analyzing the East China Sea Basin (ECSB) whose Cenozoic tectonic history consists of three syn-rift stages with the rift center progressively migrating oceanward. Global geodynamic models satisfying the past subduction history and present-day mantle structures successfully reproduce the lithospheric stress states of the evolving ECSB. We show that segmented removal of the Mesozoic Izanagi slab due to subduction of the young seafloor initiated Paleocene rifting within the western ECSB. Detachment of the former slab facilitated a strong landward mantle wind driven by the large pressure gradient across the slab. The resulting mantle traction pushed the thickened upper plate landward while entraining the young seafloors behind to slowly subduct, a process causing long-lasting Eocene extension of the central ECSB. The waning mantle wind after 30 Ma reduced basal traction and upper plate extension. A final phase of ECSB extension since the late Miocene formed the Okinawa Trough, when the subducting plate became old enough to trigger slab retreat. A similar dynamic scenario is also predicted in other circum-Pacific margins. We conclude that this enduring back-arc extension during MOR subduction represents an important mechanism for continental evolution during the closure of major ocean basins.

Spatial and temporal cycle variations in the Eocene Lacustrine Green River Formation, Piceance Creek Basin, Colorado

Lithological cycles in the lacustrine Green River Formation of the Piceance Creek Basin, Colorado, vary spatially and temporally, and are challenging to correlate across the basin. In the center of the basin, lacustrine deposits contain bedded evaporites and kerogen-rich carbonate mudstone, whereas basin margin deposits contain alluvial sandstone, kerogen-poor mudstone, stromatolite, and mud cracks. Because coarsening upward cycles identified at the basin center differ significantly from cycles at the basin margin, exactly how cycle boundaries extend from one location in the basin to another is commonly unclear. In this investigation, eleven lithofacies are identified based on rock composition, sedimentary textures, physical sedimentary structures, biogenic structures, and chemical features. Lithofacies are grouped into four facies associations: (1) Alluvial Facies Association; (2) Shoreline Facies Association; (3) Shallow Lake Facies Association; and (4) Deep Lake Facies Association. Lithological cycles in the Green River Formation vary temporally and spatially, with each stratigraphic section preserving a record of deposition in a particular area of the basin as the Piceance lake evolved. To address the temporal and spatial variation of the cycles, multiple “type” cycles are described for each location in the study area. Facies associations and temporal stacking of cycles suggest that the Piceance lake was initially a deep, freshwater lake, following the Long Point transgression, as lake level rose above the Douglas Creek Arch to fill both the Uinta and Piceance Creek basins. Lake levels then stabilized but later began a period of increased fluctuation. Following basin closure, the Piceance lake became hypersaline under evaporative concentration conditions, precipitating nahcolite and halite at the basin center, with exposure of mudflats at the basin margin. The lake would remain this way until the Mahogany transgression deepened and diluted the lake. Together, facies associations and “type” cycles described in the Green River Formation lacustrine deposits reveal a record of lateral and vertical facies changes that occurred in the Piceance lake during its evolution.

Back-arc basin closure at the East Asian margin during Permo-Triassic boundary: Evidence from geochemistry and U-Pb zircon data of sedimentary breccia from Maizuru Terrane, Southwest Japan

The paleo Maizuru back-arc basin, which was located at the East Asian continental margin opened in Early Permian times and closed during the Permo-Triassic (P-T) boundary. The back-arc basin sediments are exposed in the present-day Maizuru Terrane located in Southwest Japan and hold the key to understand the tectonic evolution of both this back-arc basin and that of the East Asian continental margin. The opening of the Maizuru back-arc basin was accompanied by the deposition of a massive black mudstone-dominated formation, presently recognized as the Maizuru Group. The closure of the back-arc basin which was accompanied by the deposition of coarse-grained materials such as the Tonoshiki breccia made up of differently-sized extremely angular clasts implies that the P-T boundary was a tectonically active period in the Maizuru Terrane. Petrographical and geochemical studies revealed that Tonoshiki breccia can be divided into two main types, a first type mainly made up of mafic to intermediate rock clast fragments (Type I) and a second type mostly consisting of felsic rock clast materials (Type II). Both types commonly contain massive black mudstone clasts that were derived from the underlying Maizuru Group. The U-Pb detrital zircon geochronology of those two types of breccia suggests that they were deposited during two phases of the tectonic activity-induced debris flow that occurred at ca. 259 Ma and ca. 251 Ma for the first and second type, respectively. Type I was sourced from the Southern zone, whereas Type II was sourced mostly from the Northern zone of the Maizuru Terrane. This switch of provenance testifies to the intense tectonic activity that reigned during the P-T boundary. The closure of the Maizuru back-arc basin was induced by the subduction of the basement crust of the basin under the East-Asian continent during the P-T boundary.

Causes and consequences of the Macta basin closure, Algeria

ABSTRACT The Macta River basin in Algeria is under pressure. A water accounting of the basin demonstrates the severity of the crisis, with a net water depletion rate estimated at 93–142%, depending on the assumptions made. This reflects the overexploitation of the aquifers whose annual depletion is estimated at between 86 and 126 Mm3. This paper first discusses the causes of basin overbuilding and the over-allocation of water, and then analyses the economic, social and environmental consequences. It calls for a stricter water accounting of river basins in Algeria as the 2030 Agenda for Sustainable Development and its SDG 6 are implemented.

Horizontal Force Required for Subduction Initiation at Passive Margins With Constraints From Slab Detachment

Subduction zones are essential drivers of plate tectonics. However, the processes causing subduction zone initiation (SZI), involving the formation of a new plate boundary, and the forces required remain disputed. Here, we focus on horizontally forced SZI at passive margins and quantify the horizontal force required for SZI with two-dimensional petrological-thermomechanical numerical models. The initial configuration, involving two passive margins bounding a 400 km wide basin with exhumed mantle, is calculated by a numerical rifting simulation to guarantee thermomechanical feasibility and isostatic equilibrium. SZI occurs at one passive margin during convergence of the basin-margin system. SZI is caused by thermal softening due to local shear heating. A local temperature increase of only ca. 50°C is sufficient to cause SZI. Corresponding simulations without shear heating do not show SZI, showing unequivocally that thermal softening controls SZI. We systematically limit the lithospheric deviatoric stress to determine the minimum force required for SZI. A minimum force (per unit length) of ca. 14 TN m−1 is required for SZI; a force magnitude that agrees with independent estimates from mantle convection models. However, for the associated stress limit of 200 MPa the lithosphere is so weak that slab detachment (SD) already occurs at the onset of basin closure in the horizontal, non-subducted region of the subducting plate. Only for stress limits >=300 MPa (yielding a force of ca. 15 TN m−1) subduction of continental crust occurs and SD occurs in the subducted slab. The force required for SZI is a proxy for the effective compressive lithospheric strength. This strength also controls subsequent SD due to tensile slab pull, which increases during subduction, also since our models include the olivine–wadsleyite phase change. Our simulations show a causal link betweeen SZI and subsequent SD: If forces required for SZI are smaller, then the lithosphere is weaker and then SD occurs at shallower levels and corresponding slabs are shorter. Concerning the European Alps, our results imply that if there was no SD since the Europe–Adria collision some 30 Myr ago (as proposed by some studies), then the subducted lithosphere must be considerably strong.