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The Ordos Basin has experienced a complicated tectonic evolution since the Palaeozoic. Its multi-stage evolution was closely related to the tectonic events that occurred along plate boundaries. The detrital zircon ages and crystallization age (CA)-deposition age (DA)/cumulative proportion curves obtained from Palaeozoic-Mesozoic strata from different tectonic units in and around the western Ordos Basin demonstrate that during the early Palaeozoic, the so-called Helan Aulacogen did not develop along the western Ordos Basin, the Alxa Block was an independent unit from the North China Craton, and the southern Ordos Basin was a foreland basin of the North Qinling Orogenic Belt. During the early Palaeozoic, the western Ordos Basin and its vicinity belonged to three different tectonic units (i.e. the North China Craton, the Alxa Block, and the North Qilian Orogenic Belt). At the end of the early Palaeozoic, the Alxa Block amalgamated with the Ordos Basin. From the Silurian to the Middle Devonian, the southern Alxa Block was a foreland basin of the North Qilian Orogenic Belt and underwent regional extension during the Late Devonian. During the late Palaeozoic, the western Ordos Basin and its vicinities were located in a back-arc extensional setting of the western Qinling Orogenic Belt. The southern part of the western Ordos Basin may have been a retro-arc foreland basin of the western Qinling Orogenic Belt during the Late Triassic, and the northern part of the western Ordos Basin experienced large-scale left-lateral strike-slip at the same time. The CA-DA/cumulative proportion curves can adequately explain the evolution of the western Ordos Basin during the Palaeozoic; however, the settings indicated by the CA-DA/cumulative proportion curves in intraplate evolutions are different from those proposed in other studies, which may be due to the number and distribution of samples and rapid lateral changes in sedimentary facies. [ABSTRACT FROM AUTHOR]

The Alxa Block is an important component of the North China Craton, but its metamorphic basement has been poorly studied, which hampers the understanding of the Alxa Block and the North China Craton. In this study, we conducted geochronological and geochemical studies on three TTG (tonalite–trondhjemite–granodiorite) gneisses and one granitic gneiss exposed in the Langshan area of the eastern Alxa Block to investigate their crustal evolution. The zircon U-Pb dating results revealed that the protoliths of the TTG and granitic gneisses were formed at 2836 ± 20 Ma, 2491 ± 18 Ma, 2540 ± 38 Ma, and 2763 ± 42 Ma, respectively, and were overprinted by middle–late Paleoproterozoic metamorphism (1962–1721 Ma). All gneiss samples had high Sr/Y ratios (41–274) and intermediate Mg# values (44.97–55.78), with negative Nb, Ta, and Ti anomalies and moderately to strongly fractionated REE patterns ((La/Yb)N = 10.6–107.1), slight Sr enrichment, and positive Eu anomalies, displaying features of typical high-SiO2 adakites and Archean TTGs. The magmatic zircons from the 2.84 Ga and 2.49 Ga TTG rocks had low εHf(t) values of −1.9–1.7, and −3.83–2.12 with two-stage model ages (TDMC) of 3.24–3.11 Ga and 3.10–3.01 Ga, respectively, whereas those from the 2.54 Ga TTG rock exhibited εHf(t) values ranging from −1.1 to 3.46 and TDMC from 3.0 Ga to 2.83 Ga, suggesting that the crustal materials of the basement rocks in the eastern Alxa Block were initially extracted from the depleted mantle during the late Paleoarchean to Mesoarchean era and were reworked in the late Mesoarchean and late Neoarchean era. By contrast, the Alxa Block probably had a relative younger crustal evolutionary history (<3.24 Ga) than the main North China (<3.88 Ga), Tarim (<3.9 Ga), and Yangtze (<3.8 Ga) Cratons and likely had a unique crustal evolutionary history before the early Paleoproterozoic era. [ABSTRACT FROM AUTHOR]

The Alxa Block is the westernmost part of the North China Craton (NCC), and is regarded as one of the basement components of the NCC. Its geological evolution is of great significance for the understanding of the NCC. However, the Precambrian basement of the Alxa Block is still poorly studied. In this study, we present new in situ LA‐ICP‐MS zircon U‐Pb and Lu‐Hf isotope data from the Diebusige Metamorphic Complex (DMC) which located in the eastern Alxa Block. Field and petrological studies show that the DMC consists mainly of metamorphic supracrustal rocks and minor metamorphic plutonic rocks and has experienced amphibolite‐granulite facies metamorphism. Zircon U‐Pb dating results suggested that the amphibolite sample yields a crystallization age of 2636 ± 14 Ma and metamorphic ages of 2517–2454 Ma and 1988–1952 Ma, proving the existence of exposed Archean rocks in the Langshan area and indicating that late Neoarchean to Paleoproterozoic metamorphic events existed in the Alxa Block. Two paragneiss samples show that the magmatic detrital zircons from the DMC yield 207Pb/206Pb ages ranging from 2.48 Ga to 2.10 Ga with two youngest peaks at 2.13 Ga and 2.16 Ga, respectively, and they were also overprinted by metamorphic events at 1.97–1.90 Ga and 1.89–1.79 Ga. Compilation of U‐Pb ages of magmatic detrital and metamorphic zircons suggested that the main part of the DMC may have been formed at 2.1–2.0 Ga. Zircon Lu‐Hf isotope data show that the source materials of the main part of the DMC were originated from the reworking of ancient Archean crust (3.45–2.78 Ga). The Hf isotope characteristics and the tectonothermal event records exhibit different evolution history with the Khondalite Belt and the Yinshan Block and the other basements of the Alxa Block, indicating that the Langshan was likely an independent terrain before the middle Paleoproterozoic and was subjected to the middle to late Paleoproterozoic tectonothermal events with the Khondalite Belt as a whole. [ABSTRACT FROM AUTHOR]

Detrital zircon LA-ICP-MS U–Pb ages of the Paleozoic strata in the southeastern Alxa Block indicate that the primary provenance did not change between the Devonian and the Early Carboniferous. Except for a sample from the Cambrian strata, the Early Paleozoic North Qilian Orogenic Belt may have been a major source of Paleozoic sediments in the study area. The main source of Lower–Middle Devonian sediments was the Early Paleozoic North Qilian Orogenic Belt. The source of the Upper Devonian sediments was the Alxa Block, while the North China Block was a minor contributor. The deformation of the Cambrian and Devonian Systems in the southeastern Alxa Block indicates that a strong east–west compression event occurred in the study area before the Early Carboniferous. The paleocurrents of the Upper Devonian in the southeastern Alxa Block indicate that the source was located to the north and was not the North Qilian Orogenic Belt to the south. Moreover, the deposition of the Upper Devonian in the southeastern Alxa Block was a response to a strong deformation event that occurred along the eastern boundary of the Alxa Block during the Late Devonian and Early Carboniferous. The North China Block became the primary source during the Early Carboniferous, and the Alxa Block was a minor source. A regional stratigraphic comparison also indicates that similar depositional environments were present until the Carboniferous and Permian on the Alxa Block and North China Block. All of these data indicate that amalgamation of the Alxa Block and the North China Block occurred between the Late Devonian and the Early Carboniferous. [ABSTRACT FROM AUTHOR]

Intracontinental deformation is an inseparable element of the evolutionary history of orogenic belts. It can reactivate and intensely modify the structures of previous orogenic belts due to remote effects of plate margin orogenesis. As an important part of the Phanerozoic Central Asian Orogenic Belt (CAOB), the Alxa Block and its vicinity experienced multistage intracontinental deformation during the Mesozoic and Cenozoic and became a representative region to study remote effects from plate margins. Both the previous structures of the CAOB and continuous accretion to the Eurasian continent from different directions controlled and affected the Mesozoic evolution of the Alxa Block. However, mantle loading can't pertinently explain the geological phenomena alone. In the Late Triassic, the Alxa Block was deformed by large-scale left-lateral shearing along largely northeast-southwest-trending faults and rotated anticlockwise caused by the collision between the North China Craton and Yangtze Craton and the northward movement of the North China Craton. In the Late Jurassic, the low-angle westward subduction of the Paleo-Pacific Ocean beneath the Eurasian Plate, the closure of the Mongol-Okhotsk Ocean to the north and/or the Lhasa-Qiangtang collision to the south combined to deform the Alxa Block from different directions, forming contraction structures with various strikes and invert previous extensional basins. In the Late Cretaceous, the Eurasian Plate experienced oblique collision along its southeastern margin and the subduction of the Neo-Tethys along its southern boundary, an intraplate sinistral transpressional megashear system developed along the eastern Alxa Block, and the uplift of mountain ranges along the basin margins. The multistage Mesozoic intracontinental deformation of the Alxa Block supports the view that they are driven by stresses generated at plate boundaries largely, and controlled by previous complicated orogenic structures in the upper crust. [ABSTRACT FROM AUTHOR]

How the Altyn Tagh fault (ATF) extends eastwards is one of the key questions in the study of the growth of the Qinghai–Tibetan Plateau. Detailed fieldwork at the easternmost part of the ATF shows that the ATF extends eastward and bypasses the Kuantan Mountain; it does not stop at the Kuantan Mountain, but connects with the northern Heishan fault in the east. The ATF does not enter the Alxa Block but extends eastward along the southern Alxa Block to the Jintanan Mountain. The Heishan fault is not a thrust fault but a sinistral strike‐slip fault with a component of thrusting and is a part of the ATF. Further to the east, the Heishan fault may connect with the Jintananshan fault. A typical strike‐slip duplex develops in the easternmost part of the ATF. The cut and deformed Quaternary sediments and displaced present gullies along the easternmost ATF indicate that it is an active fault. The local highest Mountain (i.e., the Kuantan Mountain) in the region forms in a restraining bend of the ATF due to the thrusting and uplifting. The northward growth of the Qinghai–Tibetan Plateau and the active deformation in South Mongolia are realized by sinistral strike‐slipping on a series of NE–SW‐trending faults and thrusting in restraining bends along the strike‐slip faults with the northeastward motion of blocks between these faults. [ABSTRACT FROM AUTHOR]

The ages of diabase outcropping on the southern Alxa Block and in related tectonic setting are not well constrained, but are important for exploring the tectonic evolution of the southern Alxa Block. The ages of the zircons from the diabase intruding the Middle–Late Cambrian Xiangshan Group were measured using sensitive high-resolution ion microprobe II (SHRIMPII) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and the weighted mean ages of 276.8 ± 7.9 Ma and 276.2 ± 9.7 Ma were obtained, respectively. The major elements in the diabase indicate that it may belong to the tholeiitic series. With specific features of the rare earth elements (REEs) and trace elements, potentially affected by the continental crust based on the observed strong positive Pb anomaly, the diabase was produced in an intra-plate extension environment. Similar diabase/basalts were also found along the southern margin of the Alxa Block. Combined with sedimentary, palaeo-current, mafic, and felsic rocks in the southern Alxa Block, a back-arc extension environment potentially occurred in the southern Alxa Block during the Early–Middle Permian that resulted from the convergence between the North China Block (NCB) and the Yangtze Block. [ABSTRACT FROM AUTHOR]

The tectonic affinity of the Alxa Block is important in the reconstruction of the paleogeographical evolution of China. The early Paleozoic strata (the Dahuangshan Formation and Xiangshan Group) of the southern and eastern Alxa Block have consistent rock compositions, similar depositional ages, paleocurrents, detrital zircon age distributions, and cumulative probability curves of crystallization ages for detrital zircon grains relative to the depositional ages, and were deposited in similar slope basins. All these data indicate that the early Paleozoic strata of the Alxa Block were sourced predominantly from Neoproterozoic orogenic belts in the eastern Gondwana continent, instead of the Alxa Block to the north, the North China Block to the east and the North Qilian Orogenic Belt to the south. During the early Paleozoic, the Alxa Block was an independent block with the South China Block (SCB) to the west and the North China Block (NCB) to the east and situated to the northwest of the eastern Gondwana with its long axis trending north northwest–south southeast, and it belonged to a passive continental margin dipping to the north northwest, with the eastern part of the margin located closer to the eastern Gondwana. The Hexi Corridor is part of the Alxa Block and a part of the same slope basin during the early Paleozoic. The southern boundary of the Alxa Block is now the front thrust of the North Qilian Shan. [ABSTRACT FROM AUTHOR]

How and when the Paleo-Asian Ocean was finally closed still is a hot topic open to debate. Here we presented geochronological (laser ablation inductively coupled plasma mass spectrometry, LA–ICP–MS, and 40Ar/39Ar), geochemical, and Sr–Nd–Hf isotopic data for the granitoids from the NE Alxa block in the southern margin of the Central Asian Orogen Belt, northern China, in an attempt to constrain the closure of the Paleo-Asian oceanic branch in this region. These granitoids were dated at 268–255 Ma. They show distinct adakitic signatures of high Sr/Y (19–100) and (La/Yb) N (13–50) ratios, and low Y (3.98–16.50 ppm) and Yb (0.24–1.46 ppm) concentrations. They are also characterized by relatively high K 2 O/Na 2 O ratios (0.7–1.37), and low MgO (0.22–0.75 wt%), Cr (1.41–9.96 ppm), and Ni (0.36–6.31 ppm) contents and variable Mg# values (26–50). In addition, they display negative ε Hf (t) (−16.46 to −7.48) and ε Nd (t) (−14.2 to −9.83) and high (87Sr/86Sr) i (0.7072–0.7196) and Th (7.33–14.10 ppm), indicating that they were most likely generated by partial melting of thickened continental lower crust up to ~59 km thick. This thickened crust was built on the early Permian transitional Alxa crust (~32 km thick), likely caused by the rifting related to the upwelling of mantle plume. Consequently, the NE Alxa block could have witnessed a tectonic transition during the mid-Permian from rifting to arc–continent collision and resultant crustal thickening following the closure of the Paleo-Asian oceanic branch. The subsequent upwelling of asthenosphere in response to the delamination of the orogen during the middle–late Permian provided thermal source for the adakitic magmatism. • 268–255 Ma granitoids in NE Alxa are reported for geochemistry & Sr-Nd-Hf isotopes. • They show distinct adakitic signatures (Sr/Y = 27–100; (La/Yb) N = 13–50). • They were derived from thickened lower crust due to closure of Paleo-Asian ocean branch. • Alxa underwent a cycle of thinning to thickening to delamination from 270 to 240 Ma. [ABSTRACT FROM AUTHOR]

As an important tectonic unit in the middle part of the southern Central Asian Orogeny, the Alxa Block was affected by multiple deformational events occurring in the Central Asian tectonic regime during the Paleozoic and Mesozoic. The record of deformation in the Alxa Block represents the entire evolutionary process from a continental margin to an intraplate setting. In the Langshan region of the northeastern Alxa Block, four important Paleozoic‐early Mesozoic deformation events have been distinguished: (1) nearly north‐south‐striking ductile thrusting along the eastern Alxa Block in the Late Devonian; (2) nearly east‐west‐trending brittle top‐to‐the‐south thrusting in the Mid‐Late Permian; (3) nearly east‐west‐trending initially ductile then brittle dextral transtension with 30–40 km of displacement in the Late Permian; and (4) northeast‐trending ductile sinistral shearing with 120–125 km of displacement in the Mid‐Late Triassic. The ductile thrusting in the Late Devonian may have resulted from the interaction between the North China Craton and the Alxa Black, which was a peri‐eastern Gondwana block. Parallel east‐west‐trending thrusts and related folds formed in the Neoproterozoic Langshan Group due to the closure of the Paleo‐Asian Ocean in the Mid‐Late Permian. The Late Permian east‐west‐trending and dextrally ductile transtension may have resulted from intraplate adjustment after the Central Asian Orogeny and the formation of the Ural Orogenic Belt to the west; this shear zone may have been part of a huge ductile tectonic belt developed along the whole southern Central Asian Orogenic Belt (CAOB). During the Early Triassic, the subduction of oceanic crust along the southern central CAOB terminated then the eastern Alxa Block was affected by the collision between the North China Craton and the Yangtze Craton to the south. Since the late Mesozoic, the Langshan region has experienced another five important deformation events, which were all far‐field tectonic effects related to plate margin tectonic activities (e.g., the closure of the Mongolian‐Okhotsk Ocean, the collision between the Qiangtang and Lhasa blocks and the India‐Eurasia collision). The large‐scale brittle and ductile strike‐slip faults in the northeastern Alxa Block all formed in intraplate settings since the late Paleozoic in response to the collisional orogenies occurring along the plate margins or the intraplate adjustment following the closure of the Paleo‐Asian Ocean. [ABSTRACT FROM AUTHOR]

As an important tectonic unit in the middle part of the southern Central Asian Orogeny, the Alxa Block was affected by multiple deformational events occurring in the Central Asian tectonic regime during the Paleozoic and Mesozoic. The record of deformation in the Alxa Block represents the entire evolutionary process from a continental margin to an intraplate setting. In the Langshan region of the northeastern Alxa Block, four important Paleozoic‐early Mesozoic deformation events have been distinguished: (1) nearly north‐south‐striking ductile thrusting along the eastern Alxa Block in the Late Devonian; (2) nearly east‐west‐trending brittle top‐to‐the‐south thrusting in the Mid‐Late Permian; (3) nearly east‐west‐trending initially ductile then brittle dextral transtension with 30–40 km of displacement in the Late Permian; and (4) northeast‐trending ductile sinistral shearing with 120–125 km of displacement in the Mid‐Late Triassic. The ductile thrusting in the Late Devonian may have resulted from the interaction between the North China Craton and the Alxa Black, which was a peri‐eastern Gondwana block. Parallel east‐west‐trending thrusts and related folds formed in the Neoproterozoic Langshan Group due to the closure of the Paleo‐Asian Ocean in the Mid‐Late Permian. The Late Permian east‐west‐trending and dextrally ductile transtension may have resulted from intraplate adjustment after the Central Asian Orogeny and the formation of the Ural Orogenic Belt to the west; this shear zone may have been part of a huge ductile tectonic belt developed along the whole southern Central Asian Orogenic Belt (CAOB). During the Early Triassic, the subduction of oceanic crust along the southern central CAOB terminated then the eastern Alxa Block was affected by the collision between the North China Craton and the Yangtze Craton to the south. Since the late Mesozoic, the Langshan region has experienced another five important deformation events, which were all far‐field tectonic effects related to plate margin tectonic activities (e.g., the closure of the Mongolian‐Okhotsk Ocean, the collision between the Qiangtang and Lhasa blocks and the India‐Eurasia collision). The large‐scale brittle and ductile strike‐slip faults in the northeastern Alxa Block all formed in intraplate settings since the late Paleozoic in response to the collisional orogenies occurring along the plate margins or the intraplate adjustment following the closure of the Paleo‐Asian Ocean. [ABSTRACT FROM AUTHOR]

• Late Cenozoic fault systems partly bound and transect the Alxa Block north of Tibet. • Fault orientations and kinematics suggest inherited structures are reactivated. • Sinistral transpressional faults converge westwards towards the Altyn Tagh Fault. • The regional fault network produces diverse orogen types and bulk eastward extrusion. • Crustal thinning in the Ejina Basin may be a response to Alxa Block extrusion. In this study, new structural field data and geological observations are coupled with remote sensing analysis and compilation of previous published work to establish a new kinematic framework for the neotectonic reactivation of the Alxa block, China. The Alxa block occupies a key position in Central Asia north and east of the contractional and sinistrally transpressive deforming zones of the Qilianshan-Hexi Corridor-easternmost Altyn Tagh Fault and Beishan deforming belts, south of the sinistrally transpressive southernmost Gobi Altai, and west of – or overlapping with - Ordos block extensional and transtensional graben systems. In addition, the Alxa block is internally transected by important fault systems (Bayan Nuru, Bayanwula, Chahanbulage) that record previously unrecognized Late Cenozoic activity. Active mountain building processes in the Alxa block show remarkable variation from large extensional horst blocks (Helanshan, SW Langshan, SW Yabrai) to localized uplifts along sinistral wrench-belts (Bigesitai) to dominantly contractional (Longshoushan) and sinistral transpressional ranges (Helishan). The triangular wedge-shape of the Alxa block likely pre-disposed it to eastward extrusion in response to NE-directed India-Eurasia collision-derived compressional stresses from the south, and E-W tensional forces driven by Pacific margin rollback to the east. A combination of inherited regional lithospheric structure and the modern east-directed geodetically derived crustal velocity field led to: 1) crustal thinning and subsidence of the Ejina basin to the west in the wake of the eastward extruding Alxa block, and 2) distributed sinistral transtension and crustal extension within the Alxa block interior, and in the Hetao and Yinchuan grabens to the east. [ABSTRACT FROM AUTHOR]