Ordos Block Research Articles

Contemporary kinematics of the Ordos block, North China and its adjacent rift systems constrained by dense GPS observations

The detailed kinematic pattern of the Ordos block, North China and its surrounding rift systems remains uncertain, mainly due to the low signal-to-noise ratio of the Global Positioning System (GPS) velocity data and the lack of GPS stations in this region. In this study, we have obtained a new and dense velocity field by processing GPS data primarily collected from the Crustal Motion Observation Network of China and from other GPS networks between 1998 and 2014. The GPS velocities within the Ordos block can be interpreted as counterclockwise rotation of the block about the Euler pole with respect to the Eurasia plate. Velocity profiles across the graben-bounding faults show relatively rapid right-lateral strike-slip motion along the Yinchuan graben, with a rate of 0.8–2.6mm/a from north to south. In addition, a right-lateral slip rate of 1.1–1.6mm/a is estimated along the central segment of the Shanxi rift. However, strike-slip motion is not detected along the northern and southern margins of the Ordos block. Conversely, significant extension motion is detected across the northwestern corner of the block, with a value of 1.6mm/a, and along the northern segment of the Shanxi rift, where an extensional rate of 1.3–1.7mm/a is measured. Both the Daihai and Datong basins are experiencing crustal extension. On the southwestern margin of the block, deformation across the compressional zone of the Liupanshan range is subtle; however, the far-field shorting rate is as high as 3.0mm/a, implying that this region is experiencing ongoing compression. The results reveal that present-day fault slip occurs mainly along the block bounding faults, with the exception of faults along the northern and southern margins of the block. These results provide new insights into the nature of tectonic deformation around the Ordos block, and are useful for assessing the seismic activity in this region.

Lithospheric structure across the northeastern margin of the Tibetan Plateau: Implications for the plateau's lateral growth

Variations of lithospheric structure across the northeastern Tibetan Plateau and its bounding Asian blocks, the Alxa block to the north and the Ordos block to the east, are crucial for understanding the rise and lateral growth of the Tibetan Plateau. Using waveforms from high-density seismic arrays in northeastern Tibetan Plateau and the surrounding regions, we investigated the lithospheric structure with S- and P-wave receiver functions. The results show strong and relatively simple negative velocity gradients in the depth range of mantle lithosphere (∼70–150 km) under the Ordos and Alxa blocks, similar to those under typical stable continental lithosphere. In contrast, under northeastern Tibetan Plateau including its marginal regions, the velocity gradients are weak and diffusive for the mantle lithosphere, which may be explained by elevated temperature and presence of partial melts. The changes of lithospheric structures are sharp between the Tibetan Plateau and the bounding Ordos and Alxa blocks, suggesting that these two blocks have restricted the lateral growth of the Tibetan Plateau as rigid boundaries. However, across the northeastern corner of the Tibetan Plateau to the Yinchuan rift, the lithospheric mantle structures are similar, suggesting a lateral mantle flow from the Tibetan Plateau to the gap between the Ordos and the Alxa blocks. The crustal structures along this transition show evidence of lateral growth of the Tibetan Plateau. In particular, the edge of thickened crust and evidence of Moho superposition are found between the Haiyuan Fault and the Tianjin-shan Fault, which may have replaced the Haiyuan Fault as the front boundary of the laterally growing Tibetan Plateau in its northeastern corner.

Movement characteristics and present seismic activity of Ordos Block

The present horizontal movement of the Ordos Block is the result of the movement interaction between the Ordos Block and the mainland of China. The main dynamic sources are the northeastern pushing by Qinghai–Tibet Block, and the northwestern extension of the northern margin of the block. The latest GPS velocity field observational evidences shows that the velocity field of Ordos Block and around has a significant change from 1999 to 2015, and the change is significant in the northeast margin, the northwest margin, the southwest margin, and the southeast margin of the block. The velocity of left lateral movement of the northsouthern margin of Ordos Block is much larger than the right lateral movement of its eastsouthern margin. By using the seismic data of Ordos Block since 1970, the analysis of strain accumulation, strain release, stress distribution, and stress field show that the northern margin of Ordos Block is the main active area not only in the block edge, but also in North China. The strain around Ordos Block has been accumulating continuously since 2000, and this is the longest period of strain accumulation since 1970. Some signs of strain relief appeared after Alashanzuoqi Ms5.8 earthquake in 2015. There are some areas with extremely large strain anomalies appear in the northern margin of Ordos Block.

Dynamic cause of marginal lithospheric thinning and implications for craton destruction: a comparison of the North China, Superior, and Yilgarn cratons

We present a comparative tectonic analysis of the North China Craton (NCC), which has lost parts of its root, with the Yilgarn and Superior cratons, which preserve their roots. We compare the geophysical structure and tectonic histories of these cratons to search for reasons why some cratons lose their roots, while others retain them. Based on the comparison and analysis of geological, geophysical, and geochemical data, it is clear that the lithospheric thinning beneath craton margins is a common phenomenon, which may be caused by convergence between plates. However, craton destruction is not always accompanied by lithospheric thinning, except for cratons that suffered subduction and collision from multiple sides. The Western Block (also known as the Ordos Block) of the NCC, Yilgarn and Superior cratons have not experienced craton destruction; the common ground among them is that they are surrounded by weak zones (e.g., mobile belts or orogens) that sheltered the cratons from deformation, which contributes greatly to the long-term stability of the craton. Subduction polarity controlled the water released by the subducting plate, and if subducting plates dip underneath the craton, they release water that hydroweakens the overlying mantle, and makes it easy for delamination or sub-continental lithospheric mantle erosion to take place in the interior of the craton. Thus, subduction polarity during convergence events is an important element in determing whether a craton retains or loses its root.

Regional crustal deformation characteristic before 2016 Yuncheng M4.4 earthquake swarm based on CMONOC continuous GPS data

To further study regional deformation characteristic in the southeast margin of Ordos Block during the period between the Alanshan M5.8 and Yuncheng M4.4 earthquake swarm, we analyze continuous GPS sites around the study area. The time-varying strain parameters removed a linear trend deviated from the background state in varying degrees since April, 2015, and began to turn back at the end of the year 2015, meanwhile, the maximum extension strain and shear strain have the bigger variation relative to others. The GPS measurement also shows that the eastward displacement rate of the stations decreases during 2015–2016 in varying degrees compared to 2011–2015, and the variation is closely related to its geologic structural location. The differential movement between the stations is converted into regional strain accumulation due to the fault locking. Furthermore, during 2015–2016, the maximum extension rate oriented at near NS direction obviously increased, and the maximum contraction strain direction is changed from NW to EW direction, which contributes to strengthen extension and shear strain of the NE-striking faults, it's consistent with the regional background strain state of Shanxi seismic zone, this may be an important contributor to occurrence of M4.4 earthquake swarm in Yuncheng basin.

Seismic anisotropy beneath the southern Ordos block and the Qinling-Dabie orogen, China: Eastward Tibetan asthenospheric flow around the southern Ordos

SKS wave splitting analysis is performed to estimate the seismic anisotropy in the upper mantle using teleseismic data recorded by a temporary seismic array of 180 stations called SOSArray deployed in the southern Ordos block and the Qinling-Dabie orogen. The most important finding is that large delay times with NW-SE fast polarization directions in the northeastern Tibet are continuous across the boundary into the southwestern part of the Ordos block, where the SKS wave splitting results are significantly different from those in the rest of the Ordos block. Based on our SKS wave splitting results in addition to the results from previous studies, we propose an asthenospheric flow model for the eastward extrusion of the Tibetan upper mantle. The model consists of two corner flows around the southwestern corner and the southeastern corner of the Ordos block and the eastward flow along the Weihe graben and the Qinling-Dabie orogen for the escaping Tibetan upper mantle. Finally, the clockwise turning flow of the asthenosphere around the southwestern corner of Ordos block has currently extended laterally into the interior of the Ordos block, suggesting that the thick cold lithospheric root of the southwestern Ordos block there is currently being replaced with hot Tibetan asthenosphere at depths, that is, we observed an on-going process of thermal erosion of a cratonic lithosphere by lateral hot asthenospheric flow.

Geochronology, geochemistry and Sr–Nd–Pb–Hf isotopes of the Paleoproterozoic mafic dykes from the Wulashan area, North China Craton: Petrogenesis and geodynamic implications

Zircon LA-MC-ICP-MS U–Pb geochronology, major and trace element geochemistry, and Sr–Nd–Pb–Hf isotope analysis were performed on the Paleoproterozoic mafic dykes from the Wulashan area to investigate their petrogenesis and tectonic implications. The mafic dykes are diabase porphyries with a dominant mineral assemblage of plagioclase and clinopyroxene. Zircon U–Pb isotopic data yield weighted mean 207Pb/206Pb ages of 1769±12 and 1770±5Ma for the NEE–EW trending and the N–S trending mafic dykes, respectively. Geochemically, these rocks belong to medium-K or high-K calc-alkaline series with K2O+Na2O contents of 3.39 to 5.28wt.%. They show low MgO contents (3.92–4.63wt.%) and Mg# values (32–37) and high contents of TiO2 (2.38–3.23wt.%) and FeOt (13.60–15.29wt.%). The dykes exhibit right-declined REE patterns, with (La/Yb)N of 4.63–9.35 and LREE/HREE of 5.02–7.92, and are characterized by enrichment in Rb, Ba, K, La, and Ce but depletion in Nb, Ta, Th, U, and Sr. Particularly, the NEE–EW trending samples display pronounced positive Ti anomalies, whereas the N–S trending samples exhibit negative P anomalies. All of the studied mafic dykes display similar Nd–Pb–Hf isotopic compositions with εNd(t) ranging from −3.41 to −1.49, (206Pb/204Pb)i of 15.0010 to 16.6626, (207Pb/204Pb)i of 15.2192 to 15.3468, (208Pb/204Pb)i of 35.0147 to 37.2389, and zircon εHf(t) values varying from −2.8 to 3.8. Geochemical modeling reveals that the NEE–EW and N–S trending mafic dykes were derived from 8% to 10% and 2%–4% partial melting of a common mantle source with ∼2% garnet. During emplacement, the parental mafic magma of the NEE–EW trending dykes experienced fractionation of clinopyroxene and accumulation of Ti–Fe oxides, whereas the N–S trending ones experienced fractionation of olivine and apatite. The selective enrichment of LILEs and LREEs, together with negative εNd(t) values and EMI-like Pb isotopic compositions, suggests that the dykes were derived from the partial melting of subcontinental lithospheric mantle metasomatized by slab-derived fluids. Furthermore, the marked depletion in Th and U and decoupled Nd–Hf isotopes indicate the incorporation of continental crustal components into the mantle prior to partial melting. Based on the geochemical features and previous regional geological data, we propose an extensional tectonic setting for the formation of the mafic dykes from the Wulashan area, possibly related to the post-orogenic setting following the collision between the Yinshan and Ordos Blocks.

Anatectic record and P–T path evolution of metapelites from the Wulashan Complex, Khondalite Belt, North China Craton

The Wulashan Complex in the Khondalite Belt, the North China Craton contains high-grade garnet-cordierite-sillimanite metapelitic rocks with anatectic record. The rocks preserve representative metamorphic assemblages of peak, post-peak near-isothermal decompression, and decompressional cooling stages. Combined with petrographic observation and P–T pseudosections of residual bulk compositions, a clockwise P–T path was inferred involving near-isothermal decompression and decompressional cooling that followed the peak metamorphism. Furthermore, based on phase equilibria modeling for the melt-reintegrated composition, a whole, clockwise P–T path with a possible pre-peak segment was inferred. In the pre-peak field of garnet+biotite+K-feldspar+plagioclase+sillimanite+quartz+magnetite+liquid, garnet was first introduced by consuming biotite, quartz, and plagioclase via the biotite dehydration melting reaction of Bt+Sil+Qz±Pl→Grt+Kfs+Melt. The peak metamorphic assemblage was interpreted to be garnet+biotite+K-feldspar+plagioclase+sillimanite+quartz+ilmenite+magnetite+liquid at 830–860°C and 9.5–11kbar. The following post-peak near-isothermal decompressional assemblage is garnet+biotite+cordierite+K-feldspar+plagioclase+quartz+magnetite+ilmenite+liquid at 840–880°C and 6.0–7.5kbar, resulting in matrix cordierite isolated from garnet due to the biotite decompressional dehydration melting reaction of Bt+Sil+Qz±Pl→Crd+Kfs±Ilm+Melt. Furthermore, subsequent decompressional cooling processes resulted in symplectites of cordierite and biotite+plagioclase mantling garnet due to the breakdown reactions: Grt+Sil+Melt→Crd+Bt+Fe-oxide, and Grt+Melt→Bt+Qz±Pl. Melt that remained in the host rocks formed thin films of plagioclase or K-feldspar distributed along plagioclase, quartz, and sillimanite grain boundaries. The results indicate that the Khondalite Belt experienced continent–continent subduction and collision followed by exhumation and cooling between the Yinshan and Ordos Blocks in the Western Block of the North China Craton, and the rocks of the Wulashan Complex were subjected to granulite-facies metamorphism accompanied by partial melting events.

Three-dimensional conductivity model of crust and uppermost mantle at the northern Trans North China Orogen: Evidence for a mantle source of Datong volcanoes

While the Eastern Block of North China Craton (NCC) had experienced significant lithospheric destruction in the Mesozoic, the Western Block of NCC and the Trans North China Orogen (TNCO) have undergone localized lithospheric modification since the Cenozoic. The northern TNCO is highlighted by the Cenozoic magmatic activities including Hannuoba basalts and Datong volcanoes and is a seismically active region. In this study 3-D electrical conductivity model of the crust and uppermost mantle is derived by the 3-D inversion technique using data from 72 broadband magnetotelluric (MT) stations. The final model shows that a 15 km thick resistive layer of about 3000 Ω m dominates the upper crust, which may represent the intact Archean and Paleoproterozoic terrains. Whereas in the mid-crust there are marked high conductivity anomalies of about 10 Ω m beneath Shanxi rifting basin, which may result from the interconnected saline fluid of 0.2% to 6% volume fraction. The most important finding is that one significant conductor extended into the mantle is located between Hannuoba field and Datong volcanoes and it connects with the mid-crust conductor beneath the Datong volcanoes. We suggest that this could be the mantle source (partial melting region) for the Quaternary volcanic activities of Datong volcanoes and the melt fraction is estimated as 6.6%. Its location inside the Western Block suggests that the volcanic activities at Datong volcanoes are irrelevant to the tectonic process to the east of TNCO. It is likely to be related to the mantle flows from the Tibetan Plateau around the Ordos block which converges at the northeastern corner of the Ordos block and local upward flow along the slope of the thinning lithosphere resulted in decompression partial melting and the melt percolated upward through the crust to feed the lava eruptions at the Datong volcanoes to the east. Finally, large crustal earthquakes in this region are generally located in resistive zones with high conductivity anomalies beneath. The fluids brought up by hot mantle material may play an important role in fault strength weakening and rupture nucleation at mid-crust depths.

Thermochemical structure of the North China Craton from multi-observable probabilistic inversion: Extent and causes of cratonic lithosphere modification

We present a 3D thermochemical model of the North China Craton (NCC) from the surface down to 350km by jointly inverting surface wave phase velocity data, geoid height, surface heat flow and absolute elevation with a multi-observable probabilistic inversion method. Our model reveals a thin (~65–100km) and chemically fertile lithosphere (87<Mg#<90) beneath the Eastern NCC, consistent with independent results from mantle xenoliths, and supports the idea that the Eastern NCC experienced significant lithospheric destruction and refertilization during the Phanerozoic. In contrast, beneath the Trans-North China Orogen, Inner Mongolia Suture Zone and Yinshan belt, we observe a more heterogeneous (chemically and thermally) lithosphere, indicating that these areas have been partly involved in lithospheric modification and mechanical erosion at multiple scales. A cold and chemically refractory (Mg#>90) lithospheric mantle is imaged beneath the central TNCO and Ordos Block, reaching depths >260km. This lithospheric “keel” is surrounded to the east by a high-temperature sublithospheric anomaly that originates at depths >280km. The spatial distribution of this anomaly and its correlation with the location of recent volcanism in the region suggest that the anomaly represents a deep mantle upwelling being diverted by the cratonic keel and spreading onto regions of shallow lithosphere. Our results indicate that the present-day thermochemical structure beneath the NCC is the result of a complex interaction between a large-scale return flow associated with the subduction of the Pacific slab and the shallow lithospheric structure.

Petrology, phase equilibria and monazite geochronology of granulite-facies metapelites from deep drill cores in the Ordos Block of the North China Craton

Among the various Precambrian crustal blocks in the North China Craton (NCC), the geology and evolution of the Ordos Block remain largely enigmatic due to paucity of outcrop. Here we investigate granulite-facies metapelites obtained from deep-penetrating drill holes in the Ordos Block and report petrology, calculated phase equilibria and in-situ monazite LA-ICP-MS geochronology. The rocks we studied are two samples of cordierite-bearing garnet–sillimanite–biotite metapelitic gneisses and one graphite-bearing, two-mica granitic gneiss. The peak metamorphic age from LA-ICP-MS dating of monazite in all three samples is in the range of 1930–1940Ma. The (U+Pb)–Th chemical ages through EPMA dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix. Calculated metamorphic phase diagrams for the cordierite-bearing metapelite suggest peak P–T conditions ca. 7–9kbar and 775–825°C, followed by decompression and evolution along a clockwise P–T path. Our petrologic and age data are consistent with those reported from the Khondalite Belt in the Inner Mongolia Suture Zone in the northern part of the Ordos Block, suggesting that these granulite-facies metasediments represent the largest Paleoproterozoic accretionary belt in the NCC.

Present-Day Crustal Vertical Motion Around the Ordos Block Constrained by Precise Leveling and GPS Data

Precise leveling data observed in the period of 1970–2014 around the Ordos block were collected and processed to estimate present-day crustal vertical movement. Vertical rates of 6 GPS sites were employed as a priori constraints to define the reference frame. The velocity field shows that the interior of the Ordos block moves upward at a rate of 3 mm/a as a stable block. With respect to the central Ordos, the grabens and rifts around the Ordos block are undergoing subsidence, while the northeastern and southwestern Ordos uplift at the average rates of 2 and 1 mm/a, respectively. To the southeastern margin of the Ordos block, the Weihe and southern Shanxi grabens are subsiding at the rates of 4–6 mm/a. The subsidence of the Shanxi graben indicates that the graben is experiencing extensional movement on a long timescale. To the northwestern margin of the Ordos block, the Hetao and Yinchuan rifts are subsiding at the rates of 2–3 mm/a. A 2-D buried faulting model is used to infer the normal or reverse dip-slip rates. Our solution shows that most of the normal slip rates along the faults in the grabens and rifts are ~2 mm/a.

Pn anisotropic tomography and mantle dynamics beneath China

We present a new high-resolution Pn anisotropic tomographic model of the uppermost mantle beneath China inferred from 52,061 Pn arrival-time data manually picked from seismograms recorded at provincial seismic stations in China and temporary stations in Tibet and the Tienshan orogenic belt. Significant features well correlated with surface geology are revealed and provide new insights into the deep dynamics beneath China. Prominent high Pn velocities are visible under the stable cratonic blocks (e.g., the Tarim, Junngar, and Sichuan basins, and the Ordos block), whereas remarkable low Pn velocities are observed in the tectonically active areas (e.g., Pamir, the Tienshan orogenic belt, central Tibet and the Qilian fold belt). A distinct N-S trending low Pn velocity zone around 86°E is revealed under the rift running from the Himalayan block through the Lhasa block to the Qiangtang block, which indicates the hot material upwelling due to the breaking-off of the subducting Indian slab. Two N-S trending low Pn velocity belts with an approximate N-S Pn fast direction along the faults around the Chuan-Dian diamond block suggest that these faults may serve as channels of mantle flow from Tibet. The fast Pn direction changes from N-S in the north across 27°N to E-W in the south, which may reflect different types of mantle deformation. The anisotropy in the south could be caused by the asthenospheric flow resulted from the eastward subduction of the Indian plate down to the mantle transition zone beneath the Burma arc. Across the Talas-Fergana fault in the Tienshan orogenic belt, an obvious difference in velocity and anisotropy is revealed. To the west, high Pn velocities and an arc-shaped fast Pn direction are observed, implying the Indo-Asian collision, whereas to the east low Pn velocities and a range-parallel Pn fast direction are imaged, reflecting the northward underthrusting of the Tarim lithosphere and the southward underthrusting of the Kazakh lithosphere. In most parts of eastern China, pronounced low Pn velocities and a complex anisotropy pattern are observed, implying the re-orientation of the olivine arrangement in the thin lithosphere due to the westward subduction of the Pacific plate.

Uppermost mantle P wavespeed structure beneath eastern China and its surroundings

Pn travel-time tomography provides a way of improving structural information on the uppermost mantle across eastern China exploiting recent developments of dense seismic networks with well recorded seismic events. We used waveforms from 2009 at Chinese stations, supplemented by bulletin arrival times. An initial P wave model was constructed using the crustal model from CRUST1.0 coupled to a P wave model in the mantle derived from the SL2013sv model to capture the broad-scale features. This starting model enables us to compensate for the large contrasts in crustal thickness across the region. All events were relocated using the initial 3-D P model, and after relocation, consistent patterns of travel-time residuals are obtained. We extract Pn as the first arrival in the distance range 1.8∘ to 12∘. We use the FMTOMO (Fast Marching TOMOgraphy) approach to invert the travel-time results to generate a P wavespeed structure with a resolution of 2∘×2∘ down to 75km. There are considerable variations in Pn wavespeed in the uppermost mantle across the region. The central portion of the North China craton is imaged with particularly slow P wavespeeds, whilst most of the neighbouring Ordos block is fast. Fast P wavespeeds extend through much of the uppermost mantle beneath eastern Central Asia Orogen, northeast China and beneath the Korean peninsula. In the south, the Sichuan Block and the western Yangtze craton show rather fast P wavespeeds. The Tanlu fault system appears to cut through the crust into the mantle with marked slow P wavespeed at its southern end.

High-resolution crustal structure of the Yinchuan basin revealed by deep seismic reflection profiling: implications for deep processes of basin

The Yinchuan basin, located on the western margin of the Ordos block, has the characteristics of an active continental rift. A NW-striking deep seismic reflection profile across the center of Yinchuan basin precisely revealed the fine structure of the crust. The images showed that the crust in the Yinchuan basin was characterized by vertical stratifications along a detachment located at a two-way travel time (TWT) of 8.0 s. The most outstanding feature of this seismic profile was the almost flat Mohorovicic discontinuity (Moho) and a high-reflection zone in the lower crust. This sub-horizontal Moho conflicts with the general assumption of an uplifted Moho under sedimentary basins and continental rifts, and may indicate the action of different processes at depth during the evolution of sedimentary basins or rifts. We present a possible interpretation of these deep processes and the sub-horizontal Moho. The high-reflection zone, which consists of sheets of high-density, mantle-derived materials, may have compensated for crustal thinning in the Yinchuan basin, leading to the formation of a sub-horizontal Moho. These high-density materials may have been emplaced by underplating with mantle-sourced magma.

Kinematics of late Quaternary slip along the Yabrai fault: Implications for Cenozoic tectonics across the Gobi Alashan block, China

The Yabrai range-front fault accommodates deformation within the middle Gobi Alashan block between the Tibetan Plateau and the Ordos block. As such, it provides the opportunity to examine the transition between contractional deformation associated with the growth of the Tibetan Plateau and extensional deformation across North China. Geomorphic mapping of the active fault trace and trench investigations reveal that the Yabrai range-front fault is composed of three segments of varying fault strike, but for which the sense of motion, scarp height, and slip history appear to be kinematically compatible along the fault. Displaced Holocene and late Pleistocene alluvial deposits indicate that the southwestern segment is characterized by oblique-normal displacement with a minor sinistral component, whereas the middle segment appears to exhibit nearly dip-slip normal displacement. In contrast, slip along the northeastern segment appears to be primarily sinistral strike-slip with a minor reverse component. Geomorphically fresh fault scarps are developed within late Pleistocene–Holocene alluvial fans and terraces along the southwestern and northeastern segments, whereas the middle segment of the fault defines the bedrock-alluvial contact along the range front. The 10Be exposure ages of displaced alluvial fans along the southwestern segment yield a throw rate of ∼0.1 mm/yr over late Pleistocene time. Lateral slip rates along the northeastern fault segment range between 0.23 ± 0.02 and 0.78 ± 0.12 mm/yr. Regionally, the orientation and sense of motion along the Yabrai range-front fault are consistent with NE-SW shortening, and we suggest that recent activity along this fault system reflects incipient deformation of the foreland at the northeastern margin of the Tibetan Plateau.

Dextral strike-slip of Sanguankou-Niushoushan fault zone and extension of arc tectonic belt in the northeastern margin of the Tibet Plateau

The kinematic characteristics of the Sanguankou-Niushoushan fault (SGK-NSSF) are of great significance to the understanding of the extension of the arc tectonic belt in the northeastern margin of the Tibet Plateau. Using field surveys and various data collection methods, including large-scale geological mapping, measurement of typical topographies, and dating of sedimentary strata, it was determined that the SGK-NSSF exhibits obvious dextral strike-slip characteristics and thus is not a sinistral strike-slip fault, as believed by previous researchers. The results of this study show that the geological boundaries for the Paleozoic, Mesozoic, and Cenozoic eras were all dextrally dislocated by the fault, with the faulted displacements being similar. The maximum strike-slip displacement of the fault, after elimination of topographic effects, was found to be 961±6 m. The Sanguankou fault at the northern section exhibits obvious characteristics of more recent activities, with a series of small gullies having undergone synchronized dextral writhing after traversing the fault. The average horizontal slip rate of the fault since the late Quaternary was determined to be approximately 0.35 mm/a. The pre-existing fold structures formed during the late Pliocene were dislocated by the fault and became ex situ, indicating that dextral strike-slip of the fault could not have occurred prior to the late Pliocene. The maximum displacements and average slip rates were used to estimate the onset time of the dextral strike-slip activities of the fault as being after 2.7 Ma. In this study, the understanding of previous researchers concerning the extension in the northeastern margin of the Tibet Plateau was combined with analyses of the successive relationships between fold deformations and fault activities. This led to the finding that the extension in the northeastern margin of the Tibet Plateau reached the vicinity of the SGK-NSSF during the late Pliocene (∼2.7 Ma), causing regional uplift and fold deformations of the strata there. During the early Quaternary, the northeastern compression of the Tibet Plateau and the counterclockwise rotation of the Ordos block collectively resulted in the dextral strike-slip activities of the SGK-NSSF. This then formed the foremost margin of the arc tectonic belt extension in the northeastern margin of the Tibet Plateau.

Seismic hazard assessments for the Ordos Block and its periphery in China

Seismic hazards in the Ordos Block and its periphery were estimated based on 500 years of intensity observations. Firstly, historical intensity observations were collected, the completeness of the earthquake catalog was tested, and aftershocks were deleted. Secondly, the intensity data were digitized and placed in a geographic information system (GIS). Finally, the digitized intensity data were analyzed to determine the frequency–intensity relationship (i.e., seismic hazard curve) for each cell. The seismic hazards were quantified by the intensity and return period in a site or an area. Assuming a Poisson distribution for the earthquake occurrence in time, a probability of I≥VII, VIII, or IX in 50 years was quantified. We also estimated the corresponding intensities with 10% probability of exceedance in 50 years. The results show that the periphery of Ordos Block faces significant seismic hazards. Our study also suggests that the current designed peak ground acceleration or intensity for Ordos Block may not be adequate.

Two episodes of structural fractures and their stress field modeling in the Ordos Block, northern China

The importance of the Ordos Block, which is surrounded by different Chinese continental blocks, is well documented, but the development of the structural fractures and the stress fields within the Late Mesozoic and Cenozoic eras in this stable block (dips of the Mesozoic and Cenozoic strata are less than 3°) have been poorly studied. In this paper, two dominant groups of structural fractures with NW to EW and NNE to ENE trends are identified through field measurements and imaging log observations. The maximum principal compressive stress magnitudes and stress trajectories are calculated employing 2D finite element models (2D-FEM). Based on the displacement fields, the rotation of the Ordos Block and comparisons between the measured and the calculated stresses, it can be deduced that there are two episodes of fracture formation in the Ordos Block. The calculated orientations of maximum compressive stress in the Late Mesozoic and the Cenozoic eras are found to be WNW and NE respectively, which imply that the NW to EW trending structural fractures were developed in a Late Mesozoic stress field whereas the NNE to ENE ones were developed in a Cenozoic stress field in the block. The change in stress fields may have resulted in the change in tectonic setting from the northwestward subduction of the Izanagi Plate in the Late Mesozoic to the collision between the Indian Plate and the Eurasian Plate in the Cenozoic. The change in the Mesozoic and Cenozoic stress fields is of great significance to the further fracture prediction in fractured reservoirs, basin analyses in the Ordos Basin and research on the geodynamics of the North China Craton.

Application of the revised Ti-in-zircon thermometer and SIMS zircon U-Pb dating of high-pressure pelitic granulites from the Qianlishan-Helanshan Complex of the Khondalite Belt, North China Craton

The Ti-in-zircon thermometer is a newly established, single-mineral trace element thermometer, which has been successfully used to estimate the peak and post-peak temperatures of high-grade metamorphic rocks. Here the revised Ti-in-zircon thermometer and in situ SIMS U-Pb dating are applied to detrital and metamorphic zircons of pelitic granulites from the Qianlishan-Helanshan Complex. The SIMS U-Pb results show that detrital zircons yield 207Pb/206Pb ages scattering from 2021 to 2449Ma with a peak age of 2030Ma, possibly sourced from felsic to intermediate igneous rocks at a relatively low temperature magmatic environment. Most metamorphic zircons yield ages of 1953±15Ma, 1954±20Ma and 1948±18Ma with Ti contents of 3.9–82.6ppm, corresponding to metamorphic temperatures of 664–992°C. Minor metamorphic zircons give 207Pb/206Pb ages of 1923±9Ma and Ti contents of 5.8–16.3ppm with temperatures of 697–794°C. These define metamorphic ages and temperature conditions at peak and post-peak metamorphic stages of pelitic granulites, respectively. Combined with previous studies, our new data establish a linking between zircon U-Pb ages and metamorphic thermal evolution. This demonstrates a ∼1.95Ga peak temperature granulite-facies metamorphism and subsequent near-isothermal decompression process at ∼1.92Ga, which is considered to record a ∼1.95Ga continent-continent collisional event between the Yinshan and Ordos blocks, followed by post-orogenic exhumation at ∼1.92Ga.