Ordos Block Research Articles

Ture three-dimensional fracture simulation of geology-engineering integration in low permeability and tight reservoirs in southern of Ordos Basin

In view of the characteristics of the Mesozoic oil reservoirs in the southern Ordos block such as large buried depth, strong heterogeneity, unclear understanding of oil and gas distribution, and immature prediction technology for engineering construction mechanism and effects, the fracture propagation morphology is studied. The 3D geomechanical parameters are truly 3D distributed by the integrated fracturing simulation technology of geological engineering and the research results of reservoir evaluation and rock mechanics. The spatial distribution of rock mechanics parameters of tight sandstone reservoir and interlayer is accurate. Considering the heterogeneity between layers, the distribution law of true 3D fractures in the real formation environment of high temperature and high pressure is simulated. The simulation results show that the fractures simulated by the 3d geomechanical model updated in real time are irregular and asymmetrical, and have a high degree of matching with the seismic data. This fracture model can guide the field construction and optimize the fracturing parameters of subsequent Wells to ensure that fractures occur in the most favorable position, which has high practical value and economic benefits.

Deformation of the Crust and Upper Mantle beneath the North China Craton and Its Adjacent Areas Constrained by Rayleigh Wave Phase Velocity and Azimuthal Anisotropy

The North China Craton (NCC) has experienced strong tectonic deformation and lithospheric thinning since the Cenozoic. To better constrain the geodynamic processes and mechanisms of the lithospheric deformation, we used a linear damped least squares method to invert simultaneously Rayleigh wave phase velocity and azimuthal anisotropy at periods of 10–80 s with teleseismic data recorded by 388 permanent stations in the NCC and its adjacent areas. The results reveal that the anomalies of Rayleigh wave phase velocity and azimuthal anisotropy are in good agreement with the tectonic domains in the study area. Low-phase velocities appear in the rift grabens and sedimentary basins at short periods. A rotation pattern of the fast axis direction of the Rayleigh wave together with a distinct low-velocity anomaly occurs around the Datong volcano. A NW–SE trending azimuthal anisotropy and a low-velocity anomaly at periods of 60–80 s are observed subparallel to the Zhangbo fault zone. The whole lithosphere domain of the Ordos block shows a high-phase velocity and counterclockwise rotated fast axis. The northeastern margin of the Tibetan plateau is dominated by a low-velocity and coherent NW–SE fast axis direction. We infer that the subduction of the Paleo-Pacific plate and eastward material escape of the Tibetan plateau mainly contribute to the deformation of the crust and upper mantle in the NCC.

Paleoproterozoic A1- and A2-type coexisting monzogranites in the Daqingshan Complex, Khondalite Belt, North China Craton and its tectonic implications

Paleoproterozoic granitoids, widely exposed in the Daqingshan Complex in the Khondalite Belt of North China Craton (NCC), are of great significance for understanding the magmatic evolution and tectonic setting of the blet. Zircon LA-ICP-MS U-Pb dating results of four monzogranite samples are 2385 ± 18 Ma, 2358 ± 24 Ma, 2437 ± 32 Ma, and 2338 ± 24 Ma. Zircon Lu-Hf isotopic analysis shows that the monzogranites have positive εHf(t) values of +0.5 to +4.5, with two-stage Hf model ages (TDMC) range from 2859 to 2617 Ma, indicating that these rocks were mainly produced from ancient crust through the process of assimilation-fractional crystallization. The Daqingshan monzogranites are high-K calc-alkaline to shoshonite series, metaluminous to peraluminous series, enriched in large-ion lithophile elements (LILEs, such as Rb, Ba, La, and Ce) and depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. They have low Sr/Y (3.48–35.72) and (La/Yb)N (4.79–46.50) ratio and weakly Eu negative anomalies (δEu = 0.14–1.23), indicating that they were formed in the continental magmatic arc environment. In addition, the monzogranites have high alkaline (Na2O + K2O) and SiO2 components, and high Ga/Al and HFSE values, showing the coexistence of A1- and A2-type granitoids. Combined with previous studies, our new results provide that the Khondalite Belt underwent magmatic events from 2.44 to 2.34 Ga during a long-lived arc-continental accretion process along the southern margin of the Yinshan Block. However, the Daqingshan monzogranites do not preserve records of Paleoproterozoic collisional event at 1.95–1.85 Ga in the Khondalite Belt, indicating that they did not experience subduction-related metamorphism and subsequent collision. Thus, the Daqingshan monzogranites may represent the post-collisional to late orogenic granitoids during the transition from compressive to rift tectonics following collision between the Yinshan and Ordos blocks.

Large river chronology along the Jinshaan Gorge on the Yellow River and its implications for initialization

The ancient fluvial terrace is one of the keys to understanding how a large river develops in long-term landform evolution. However, surface erosion usually dissects the once extensive terrace into discontinuous segments and makes it difficult to explore the evolution of a large river worldwide. Fortunately, an exception is found in the Jinshaan Gorge of the Yellow River (JGYR), which preserves many Late Miocene-Late Quaternary fluvial terraces and thus can help understand the origin and evolution of a large river. Detailed investigation of the high terraces along the JGYR indicates a broad valley that was later superposed by a V-shaped valley due to rapid incision. To constrain the timeframe in which the broad valley persisted, as well as the onset time of the V-shaped valley formation, we select ten typical fluvial terrace profiles, apply terrestrial cosmogenic nuclide (TCN) 26Al/10Be burial dating to yield the ages of terrace deposits, and correct the results by considering both inherited and postburial nuclide production during slow red clay and loess deposition. Integrated with previous studies of magnetostratigraphic ages, our chronological results show that the lateral erosion and valley widening of the infant JGYR formed a broad valley during ca. 8–3.7 Ma; subsequent downward incision began ca. 3.7 Ma, accelerated to form a V-shaped valley and incising meanders into the initial broad valley since ca. 2.5 Ma. Moreover, most terrace ages plot in either periodic or short-lived arid conditions during ca. 8–3.7 Ma, while a distinctive downward incision has occurred under strengthened oscillating arid conditions since ca. 2.5 Ma, suggesting that climate aridification favours terrace aggradation, and tectonic uplift mainly dominated incision, which was consistent with the onset of the extension of the Shanxi Graben System ca. 8 Ma and the enhanced uplift of the Ordos Block since ca. 3.7 Ma.

Mesozoic exhumation and ca. 10 Ma reactivation of the southern Yin Shan, North China, revealed by low-temperature thermochronology

The thermochronological history of orogens along plate margins provides unique constraints on regional tectonic evolution. Given existing thermochronological data that identify late Mesozoic–Cenozoic multiphase cooling events, we perform new apatite fission-track and (U-Th)/He low-temperature thermochronology on Wula Shan and Dahong Shan rocks to reveal the first stages of cooling along the southern Yin Shan orogenic belt, northern margin of the North China Block (NCB). Our new thermal history modeling suggests that cooling of the southern Yin Shan began in the Late Triassic (ca. 230 Ma), with rapid cooling at ca. 210–178 Ma and ca. 10~ Ma in the Wula Shan and ca. 200–160 Ma in the southern Dahong Shan. Late Triassic-Early Jurassic cooling stages are related to uplift and exhumation of the northern NCB, which given the tectonic setting of the late Indosinian event, are mainly interpreted as signatures of Mongol–Okhotsk Ocean subduction. Subsequently, enhanced plate convergence of East Asia with closure of the Mongol–Okhotsk Ocean led to intense thrust-nappe and intracontinental orogeny during the early Yanshanian. Importantly, our new finding of ca. 10 Ma rapid cooling of the Wula Shan at a rate of ~3.5 °C/Ma corresponds to coeval uplift and synrift basin sedimentation in the Ordos Block periphery, western NCB, which could mark the response to craton-scale rotational deformation. The completion of this low-temperature thermochronological database allows us to reappraise the Mesozoic exhumation and ca.10 Ma reactivation history and its implications for the southern Yin Shan.

Joint‐Rupture Pattern and Newly Generated Structure of Fault Intersections on the Northern Margin of the Linhe Basin, Northwestern Ordos Block, China

AbstractThe activity of major active faults around the Ordos Block is of great interest to seismologists, as at least fourM ≥ 8 earthquakes have been recorded. However, the Linhe Basin, which has the thickest Cenozoic sediments, has no record of large earthquakes. Does this basin have the structural conditions required for large earthquakes? The northern boundary fault of the Linhe Basin is composed of the NE‐striking Langshan piedmont fault (LPF), E‐W‐striking western section of the Seertengshan piedmont fault (WSPF), and NW‐striking eastern section of the Seertengshan piedmont fault (ESPF). Based on large‐scale active fault mapping, this article analyzes data from 23 trenches, using an unmanned aerial vehicle to measure the faulted landform, and combines these data with Quaternary dating methods to acquire the paleoearthquake sequences of the LPF, WSPF, and ESPF. Furthermore, this article explores their rupture modes and discusses the structural evolution of two intersection points. Through paleoseismic comparison, seven trenches revealed historical earthquakes from 7 BC, with a common magnitude ofM8.1. The trenches also revealed seven paleoseismic events since the Holocene, which conformed to the periodic model with a period of 1.37 ± 0.11 ka. The elapsed time since the latest event (2.0 ka) has exceeded the earthquake recurrence period; thus, the area is currently at risk of anM7.4–8.0 earthquake. The NE‐striking LPF and E‐W‐striking WSPF are connected by two left‐stepping small fault segments. The E‐W‐striking WSPF and NW‐striking ESPF are connected by a large triangular relay ramp.

Paleoproterozoic polyphase deformation in the Helanshan Complex: Structural and geochronological constraints on the tectonic evolution of the Khondalite Belt, North China Craton

The Khondalite Belt is widely regarded as a Paleoproterozoic collisional orogen between the Yinshan and Ordos Blocks in the Western Block of the North China Craton. However, the deformation history of the Khondalite Belt is poorly understood, and many aspects of the tectonic processes involved in the collision remain enigmatic. In this paper, we carried out detailed field-based structural analyses integrated with geochronological studies in the Helanshan Complex, one of the multiply deformed high-grade gneiss terrains in the Khondalite Belt. Three major phases of deformation (D1-D3) were identified in the Helanshan Complex. D1 mainly produced S(SW)-verging recumbent to overturned intrafolial isoclinal folds F1, originally sub-horizontal penetrative transposition foliations S1 and SSW-plunging mineral lineations L1. D1 deformational fabrics occurred coevally with the granulite-facies peak metamorphism. D2 dominantly generated steeply SE(E)-plunging asymmetrical upright open folds F2 that successively refolded the previous D1 structures. D3 gave rise to a series of NE- to E-trending ductile shear zones, featured by top-to-the-(N)NW oblique thrusting with a component of sinistral strike-slip movement. LA-ICP-MS U-Pb zircon dating results of seven deformation-related samples reveal that D1 occurred at some time from 1954 Ma to 1911 Ma, most likely at 1954–1949 Ma. D2 successively happened in the period of 1911–1870 Ma, and D3 had broadly taken place at ∼ 1870 Ma. The D1-D3 deformation sequences recorded tectonic processes including the initial top-to-the-SSW thrusting and crustal thickening, subsequently followed by exhumation of the khondalites in the Paleoproterozoic. Combined with previous studies, these new structural and geochronological data indicate that the Khondalite Belt underwent a long-term (>100 Myr) orogenic history and developed polyphase deformation, high-grade metamorphism as well as magmatism in response to the NNE-SSW-directed oblique collision between the Yinshan and Ordos Blocks at ∼ 1.95 Ga.

The lithospheric effective elastic thickness and its tectonic implications in the Ordos Block and adjacent areas

The Ordos Block, located at the western edge of the North China Block, is a stable rigid block. It is characterized by inactive tectonic activity and rare earthquakes, while the south‐west margin of this block and adjacent areas present the opposite features. There is still considerable ambiguity with regard to the lithospheric mechanical strength in these areas. The effective elastic thickness (Te), widely used for comprehending the mechanism of cratonic deformation and lithospheric dynamics, is a proxy of the lithospheric strength. This study provided detailed two‐dimensional lithospheric Te spatial variations in the Ordos and adjacent areas by using coherence analysis of the topography and Bouguer anomaly. The result showed that the higher Te values in the Ordos Block varied from 80 to 100 km and implied that it is capable of strong rigidity. The lower Te values were distributed over the Qinling Block (20–45 km), the Qilian Block and Haiyuan arcuate tectonic region (20–40 km), as well as the Baryan‐Har Block (15–30 km). By comparing the Te values with the geophysical results, we believe that the higher Te value in the Ordos Block was supported not only by the crust, but also by the upper mantle. In contrast, the mechanical strength of the south‐west margin, especially the Baryan‐Har Block and western Qinling Block, was mainly borne by the brittle upper crust, but the tectonic activity and brittle failure likely resulted in a lower Te. Furthermore, we found that a large number of shallow earthquakes were located above low‐velocity and low‐resistivity layers, most of which probably occurred in the brittle upper crust with small mechanical strength.

Petrogenesis and tectonic significance of Paleoproterozoic metavolcanic rocks in the Khondalite Belt, North China Craton

As a typical continent–continent collision belt, the Khondalite Belt (KB) of the North China Craton (NCC) represents a Paleoproterozoic orogenic belt that underwent subduction and collision. Although many high-grade metamorphic rocks have been well studied, the low to medium grade metamorphic rocks well preserved in KB are less studied, which hinders a better understanding of the initiation of Paleoproterozoic subduction and collision processes. The Erdaowa Group is a representative of such a metamorphic rock series. Zircon U-Pb dating of the metavolcanic rocks in the Erdaowa Group yields the eruption ages of 2465 ± 24 Ma and 2350 ± 34 Ma, and the metamorphic ages of 1933 ± 32 Ma and 1844 ± 30 Ma. The geochemical results show that the metavolcanic rocks of the Erdaowa Group belong to tholeiitic basalts series. The enrichment of large ion lithophile elements (LILEs) such as Rb and Ba, and the depletion of high field strength elements (HFSEs) such as Ta, Nb and Ti indicate that the metavolcanic rocks have arc geochemical affinities and may have been formed at an active continental margin associated with subduction. Zircon Lu-Hf isotopic analysis shows that εHf(t) values are −0.15 – + 8.72, and the single-stage depleted mantle model age is 2448 – 2707 Ma, indicating that the primary magma originated from partial melting of the depleted lithospheric mantle and experienced slight crustal contamination. The results further prove that the low and medium grade rocks were directly involved in the subduction and collision between the Yinshan Block and the Ordos Block.

Variations of crustal thickness and average Vp/Vs ratio beneath the Shanxi Rift, North China, from receiver functions

The Shanxi Rift located in the central part of the North China Craton (NCC) as a boundary between the Ordos block and the Huabei basin. The Shanxi graben system is a Cenozoic rift and originated from back-arc spreading related to westward subduction of the western Pacific and far field effects caused by northward subduction of the Indian plate. It has also had strong earthquake activity in China since the Quaternary. To investigate the tectonic evolution and tectonic setting of strong earthquakes in the Shanxi Rift, we apply the receiver function H-kappa stacking method to determine the crustal thickness and average Vp/Vs ratio in the area. The results show that the thickness of the crust increases from approximately 30 km in the Huabei basin to approximately 47 km in the Yinshan Mountains with a close correlation between the Moho depth and topography. The Yuncheng, Linfen and Taiyuan grabens have varying degrees of crustal thinning. The crustal average Vp/Vs ratio in the Shanxi Rift has significant heterogeneity; the high Vp/Vs ratio (~ 1.85) are found in the Datong and Yuncheng grabens, and Vp/Vs ratio of the Taiyuan and Linfen grabens is approximately 1.75 which close to the global average value ~ 1.782. Combining the observations in this study with previous research, we suggest that the grabens in the Shanxi Rift experienced extensional deformation from south to north and that the possibility of strong earthquakes in the central part of the Shanxi seismic belt is greater than that on the northern and southern sides.

Crustal thickness andVp/Vsvariation beneath continental China revealed by receiver function analysis

SUMMARYUsing data from 3837 seismic stations deployed in or around continental China, we construct high-resolution models of crustal thickness (H) and seismic compressional and shear velocity ratio (Vp/Vs or κ) in continental China by analysis of 150 543 receiver functions. We group the receiver functions in cells with a spatial resolution of 0.25° × 0.25° in the North–South China Seismic Belt and parts of the North China Craton, and of 0.5° × 0.5° in other regions, classify the receiver functions based on their characteristics, and develop a modified H–κ stacking method to construct models in the regions where the receiver functions are significantly affected by sedimentary basins and by Moho architecture. The inferred crustal thickness model displays an eastward thinning trend from the thickest crust (>80 km) beneath the Qiangtang Block to the thinnest crust (<26 km) beneath the southern part of the Cathaysia Block. Crustal thickness is 26–50 km in several major basins and 26–55 km in the Precambrian cratonic blocks. The inferred Vp/Vs model in the crystalline crust displays moderate-to-high values (1.75–1.85) in the southeastern margin of the Tibetan Plateau, the Tengchong volcanic field, the Emeishan large igneous province, the north-central areas of the Bohaiwan and Songliao basins, the western margin of the Taikang Hefei Basin and the southeastern margin of the Cathaysia Block. Lower values (≤1.72) characterize the major regions of the Cathaysia Block and the Jiangnan Orogenic Belt, and the hinterlands of the Ordos Block and Sichuan Basin. We discuss possible tectonic processes, secular crustal evolution and crustal compositions that are consistent with our inferred crustal thickness and Vp/Vs structure in continental China. This study establishes a framework of seismic data sharing for future studies in the seismological community in one of the first steps of developing a China Seismological Reference Model.

Crustal Anisotropy Beneath the Trans-North China Orogen and its Adjacent Areas From Receiver Functions

As an important segment of the North China Craton, the Trans-North China Orogen (TNCO) has experienced strong tectonic deformation and magmatic activities since the Cenozoic and is characterized by significant seismicity. To understand the mechanism of the crustal deformation and seismic hazards, we determined the crustal thickness (H), Vp/Vs ratio (κ) and crustal anisotropy (the fast polarization direction φ and splitting time τ) beneath the TNCO and its adjacent areas by analyzing receiver function data recorded by a dense seismic array. The (H, κ) and (φ, τ) at a total of 309 stations were measured, respectively. The Moho depth varies from ∼30 km beneath the western margin of the Bohai bay basin to the maximum value of ∼48 km beneath the northern Lüliang Mountain, which shows the positive and negative correlations with the elevation and the Bouguer anomaly. The average φ is roughly parallel to the strikes of the faults, grabens and Mountains in this study area, whereas a rotating distribution is shown around the Datong-Hannuoba volcanic regions. Based on the φ measured from the Moho Ps and SKS/SKKS phases, we propose that the crustal deformation and seismic hazards beneath the TNCO could be due to the counterclockwise rotation of the Ordos block driven by the far-field effects of the India-Eurasian collision.

Asthenospheric Flow Channel From Northeastern Tibet Imaged by Seismic Tomography Between Ordos Block and Yangtze Craton

AbstractThe collision between India and Eurasia forming the Tibetan Plateau since the early Cenozoic had led to the crustal and upper‐mantle (asthenospheric) extrusion from the plateau to the east. However, the Ordos and Yangtze cratonic keels next to the northeastern and eastern Tibetan Plateau obstruct this eastward asthenospheric extrusion. With a dense seismic array, for the first time, we imaged a narrow low P‐wave and S‐wave velocity anomaly characterized with higher Vp/Vs between these two Archean cratons at depths ranging from 150 to 300 km using finite‐frequency tomography. We interpret this low‐velocity anomaly as a channel of the eastward asthenospheric flow from the Tibetan Plateau. We provide solid seismic observations for a pathway of the Tibetan asthenospheric extrusion due to the ongoing collision of the Indian and Eurasian continents. This asthenospheric extrusion from the Tibetan Plateau could influence the lithospheric reworking in the central and eastern North China Craton.

Upper mantle seismic anisotropy beneath the western and central North China Craton

We estimate over 2,000 shear wave splitting parameters from 84 teleseismic earthquakes recorded at 465 temporary broadband stations distributed over the western and central North China Craton to constrain lithospheric deformation and mantle flow. In the interior of the Ordos Block, the W-E trending fast polarization directions with small delay times are indicative of stable terrane, where a thick Archean cratonic keel has been imaged. The dominant W-E fast orientations with relatively large delay times observed in southern Trans-North China Orogen show a systematic directional deviation of ~30° from the absolute plate motion (APM) direction and are explained by a combined effect of APM-induced mantle fabric and a mantle flow around the southeast edges of the Ordos Block. The prominent rift-orthogonal fast orientations beneath rifts to the northern Ordos Block imply the existence of active mantle flow. Also, it is inferred that a mantle plume is responsible for generating the parabolic fast direction pattern beneath the Datong Volcano.

Lower crustal attenuation in northeastern Tibetan Plateau from ML amplitude

This study investigated the crustal attenuation structures of Sg and Lg waves of the northeastern Tibetan Plateau. We collected ML amplitude data recorded at 168 permanent stations between 1985 and 2016 and 11 temporary broadband stations between 2014 and 2016. Detailed Q0 variation maps of Sg and Lg waves were obtained by applying ML amplitude tomography. The average Q0 values of the Sg and Lg wave were 440 and 220, respectively. Relatively high attenuation anomalies of both waves appeared in the central and eastern regions of the Bayan Har Block and the east edge of the Qiangtang Block, which may be related to partial melting, high geotemperature, and strong tectonic processes. High attenuation anomalies were also found in the Qilian Orogenic Belt and Hetao Graben, which may be related to their active tectonic behavior and densely distributed faults. The relatively low attenuation anomalies of both waves were revealed in the Alax and Ordos blocks, Qaidam, Tarim, Qinghai Lake, and Gonghe basins, which can be explained by the tectonically stable properties and ancient composition of geological elements. These results indicate that the path between the highly attenuated lower crust of the Bayan Har Block and the Qilian Orogenic Belt is obstructed by three adjacent low attenuated areas (i.e., the Qilian, Qinghai Lake, and Gonghe basins); thus, it appears unlikely that a crustal flow channel from the interior of the Tibetan Plateau to the Qilian Orogenic Belt will form.

Deep electrical structure and dynamic mechanism of the Yinchuan Graben on the western margin of the Ordos Block

Deep electrical structure and dynamic mechanism of the Yinchuan Graben on the western margin of the Ordos Block

The Crustal Anisotropy of West Ordos Block and Its Geodynamic Implications

AbstractBased on the teleseismic records of the dense broadband seismic array, the crustal anisotropy parameters of the west Ordos block and its adjacent regions were determined with P wave receiver functions. The results indicate that the dominant direction of the fast wave is NEE‐SWW, N‐S in Yingshan block and north Ordos block, N‐S in the south Ordos block, and E‐W in the Qinling–Dabie orogen belt. The fast wave direction of crustal anisotropy in the north Ordos block is distinctly different from that in the south Ordos block, both well related to the major geological events, such as the Paleoproterozoic continent‐to‐continent collision between the Ordos Block and the Yinshan Block, the collision between the Western Block and Eastern Block of the North China Craton and the material migration from the Tibetan plateau in Cenozoic. Our results indicate that the crustal anisotropy in the Ordos block preserves the traces of ancient tectonic movements. Combining the results of previous geophysical studies, we presented a crust‐mantle interaction model to explain the geophysical observations. The most prominent features of the model are the horizontal eastward expansion of the mantle material in the southern Ordos and the vertical upwelling of the mantle material in the northern Ordos. The different modes of movement of the mantle material led to the deep contrasting structures of north and south Ordos, including the crustal anisotropy. The mantle upwelling also implies that the north Ordos block might be currently experiencing craton destruction.

Petrogenesis and tectonic implications of TTG granitoids from the Daqingshan Complex of the Khondalite Belt, North China Craton

Located in the Western Block of the North China Craton, the Khondalite Belt is one of the three Paleoproterozoic tectonic belts that were linked to the final assembly of the craton. At present, a popular model is that the Khondalite Belt was formed by the collision between the Yinshan and Ordos blocks at ∼1.95 Ga. However, the initiation of oceanic subduction and its related arc magmatism and accretionary process before the collisional event were poorly constrained. The Daqingshan Complex is located in the middle East part of the Khondalite Belt, and contains highly deformed and metamorphosed rock assemblages, and thus represents a key area to decipher the above issue. In this study, we carried out petrological, geochemical and geochronological analysis on the TTG granitoids of the Daqingshan Complex. Zircon U-Pb results from three typical TTG samples yielded upper intercept ages of 2545 ± 50 Ma, 2484 ± 68 Ma and 2452 ± 32 Ma, indicating that the TTG granitoids were emplaced in the late Neoarchean. Metamorphic zircons from two samples gave ²⁰⁷Pb/²⁰⁶Pb weighted mean ages of 1892 ± 53 Ma and 1906 ± 27 Ma, respectively, recording the timing of a continent-to-continent collisional event. Thirteen TTG granitoid samples are geochemically low-, medium- and high-K calc-alkaline, with metaluminous to peraluminous trends and are enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, La, Ce, Nd, and depleted in high field strength elements (HFSEs) such as Nb and Ta. Chondrite-normalized rare earth element (REE) patterns show fractionation with (La/Yb) _N ratios ranging from 8.20 to 27.47, with weak Eu negative anomalies (δEu = 0.50 – 0.98). In addition, TTG granitoids of the Daqingshan Complex belong to I-type granitoids, and their igneous protoliths were intimately related to a subduction-related magmatic arc environment. New results of this study reveal that the initial oceanic lithosphere subduction operated since ∼2.55 Ga along the southern margin of the Yinshan Block, and generated the coeval arc-related TTG granitoids. Closure of the ocean led to the final collision between the Yinshan and Ordos blocks and the amalgamation of the Western Block at 1.95 to 1.85 Ga.

An Early Paleoproterozoic back-arc system along the southern margin of the Yinshan Block: Evidence from a newly-defined bimodal volcanic sequence in the Daqingshan Complex, Khondalite Belt

As one of the 2.1 to 1.9 Ga orogenic belts that welded the Columbia supercontinent, the Khondalite Belt in the North China Craton is a typical continent-continent collisional orogen that formed through the collision between the Yinshan and Ordos Blocks. Previous studies mostly focused on the collisional event in the Khondalite Belt but paid little attention to how the subduction system operated before the final closure of the ocean. To address this issue, we identified a series of interlayered meta-mafic and felsic rock assemblages in the Daqingshan Complex and implemented geochemical and geochronological analyses. Petrological and geochemical studies revealed that these rocks are bimodal and include plagioclase amphibolite (Group 1) and biotite plagiogneiss (Group 2). Geochemically, Group 1 samples show tholeiitic affinity, whereas Group 2 samples belong to the high-K calc-alkaline series. Geochemical data indicate that the protolith magma of Group 1 was most likely derived from the partial melting of lithospheric mantle with minor crustal contamination, whereas Group 2 rocks represent highly differentiated magma derived from the partial melting of ancient crustal materials. All the samples show depletion of HFSEs and enrichment of LILEs, indicative of a subduction-related magmatic arc environment. Zircon U-Pb dating results show that the protoliths of Group 1 samples yield crystallization ages of ∼2.47 Ga and metamorphic ages of 1.95 to 1.85 Ga, whereas the protoliths of Group 2 samples yield crystallization ages of ∼2.40 Ga and metamorphic ages of ∼1.85 Ga. Our new results and available geochemical, petrological, and isotopic data demonstrate that the bimodal volcanic sequence of the Daqingshan Complex was developed in a 2.47 to 2.40 Ga back-arc system along the southern margin of Yinshan Block. Subsequent collision between the Ordos and Yinshan Blocks resulted in the formation of the Khondalite Belt and final amalgamation of the Western Block between 1.95 and 1.85 Ga.

The paleoearthquakes of the Langshan Piedmont fault, North China since late Pleistocene

The Langshan Piedmont fault (LPF) is an active Holocene normal fault that borders Langshan Mountain and the Hetao Graben in the northwest of Ordos Block of China. In this study, paleoseismic trenching was undertaken at three sites and 11 fault outcrops are well exposed. The results show that eight paleoearthquake events occurred on the LPF since 43 ka BP, and six paleoearthquake events are complete since 14 ka BP, the latest event is 2300–2400 a BP; the LPF has the characteristic of quasi-periodic recurrence and the average recurrence interval is 2350 years. Therefore, there is a high probability that the LPF will have another earthquake. The vertical activity rate of the LPF is ~1.12 ​mm/yr since Holocene, and ~0.77 ​mm/yr since 14 ka BP. In this study, it is considered that the LPF is mainly characterized by simultaneous rupture of the whole segment. The vertical displacement of the paleoearthquake events is 0.8–2.5 ​m and the corresponding maximum paleoearthquake magnitude is M 7.4. The seismic hazard of Langshan area should not be ignored. The buildings in Langshan area, especially Wulatehouqi and Qingshan town, should be able to withstand a magnitude 8 earthquake.