Ordos Block Research Articles

Present-Day Three-Dimensional Deformation across the Ordos Block, China, Derived from InSAR, GPS, and Leveling Observations

The Ordos Block in China experiences tectonic activity and frequent earthquakes due to compression from the Tibetan Plateau and extension from the North China Block. This has prompted the construction of a high-resolution three-dimensional (3D) deformation field to better understand the region’s crustal movement. Considering the limitations of the existing geodetic observations, we used InSAR, GPS, and leveling observations to create a high-precision 3D deformation field for the Ordos Block. Spherical wavelet decomposition was used to separate tectonic and non-tectonic deformation signals. Short-wavelength non-tectonic deformation fields revealed complex surface deformation patterns caused by groundwater, oil, gas extraction, and coal mining. Long-wavelength tectonic deformation fields showed subsidence in the southern margin of the block, while the interior and northeastern margins were uplifted. By combining imaging results from the seismic velocity structure and magnetotellurics, we infer that the upwelling of deep materials beneath the northeastern margin leads to surface uplift with tensile strain rates. The crustal uplift in the area south of 38°N matches the thickening of the lower crust. The weak subsidence and eastward horizontal movement disappearing near 108°E at the southern margin support the existence of asthenosphere flow beneath the Qinling orogenic belt.

Lithospheric deformation in Mongolia revealed by anisotropic Rayleigh wave tomography: Implications for the mechanism of intracontinental orogeny

The Mongolian Plateau is situated in the eastern part of the Central Asian Orogenic Belt that was formed during the Paleozoic era. In the Cenozoic era, the plateau has been reactivated, leading to significant topographic uplift, volcanism, and seismic activity. Several destructive earthquakes, ranging from magnitudes 6 to 8, have occurred in the 20th century. But the mechanism of these neotectonics remains unclear. To better understand these processes, we determined S wave velocity and azimuthal anisotropy in the crust and upper mantle under Mongolia using Rayleigh wave dispersion curves. Our findings indicate that the lithosphere is dominated by NW-SE fast velocity directions (FVDs). However, we observed different patterns in two specific regions. The lithospheric anisotropy under the Hangay Dome is weak, suggesting that it is accompanied by significant mantle upwelling under the dome. On the other hand, the FVDs in the Gobi Desert exhibit a circular pattern that wraps around the Ordos Block, which may have been caused by past and present deformation imposed on the stable block. These results offer new insights into the lithospheric deformation and dynamics of the Mongolian Plateau.

Seismic Azimuthal Anisotropy of Northeastern Tibetan Plateau From Ambient Noise Double Beamforming Tomography: Implications for Crustal Deformation

AbstractIn this study, we obtain a three‐dimensional (3‐D) high resolution seismic anisotropy model of NE Tibet using ambient noise double beamforming (DBF) tomography with ChinArray‐Himalaya II data. DBF tomography technique allows for the simultaneous derivation of local phase velocity and azimuthal anisotropy from ambient noise cross‐correlations without performing tomographic inversion. Based on the DBF tomography results, we have constructed a high resolution 3‐D azimuthal anisotropic shear wave velocity model of NE Tibet. Our model indicates the presence of prominent low velocities and strong azimuthal anisotropy in the mid‐to lower crust beneath the Songpan‐Ganzi belt, while the mid‐to lower crust of the Qilian belt exhibits relatively weak low velocity and azimuthal anisotropy. We propose that crustal channel flow may be the primary mechanism driving crustal deformation in the Songpan‐Ganzi belt, which is in the mature part of the plateau. As the Qilian belt is at an early stage of plateau formation, it is characterized by crustal shortening with the development of a sub‐vertical foliation. Additionally, the misalignment between predicted and observed shear wave splitting reveals decoupled crust and upper mantle deformation west of the Ordos block where eastward mantle flow is deflected by the lithospheric keel of Ordos.

Deformation of the NE Tibetan Plateau revealed by velocity and azimuthal anisotropy structures

Because of the complex interaction among the Tibetan Plateau, the Alxa block and the Ordos block, the deformation mechanisms of the northeastern (NE) Tibetan Plateau have been controversial for decades. To shed light on these mechanisms, we obtain high-resolution 3-D images of the P-wave velocity and azimuthal anisotropy structures beneath the NE Tibetan Plateau and adjacent area. We invert high-quality manually picked first arrivals using an eikonal equation-based seismic traveltime tomography method. Tomographic results mainly present two features. First, low-velocity anomalies are evident in the lower crust of the Qilian block. In addition, weak azimuthal anisotropy with NW–SE-oriented fast wave directions (FVDs) is detected in the lower crust. These characteristics indicate compressional deformation of the Qilian block. Second, the lower crust and uppermost mantle below the eastern Qilian orogenic belt display high-velocity anomalies. These high-velocity anomalies are collected with the high-velocity body beneath the Alxa block. This feature may imply that the Alxa block is underthrusting beneath the eastern Qilian orogenic belt. Our tomographic results show that the 1920 Ms 8.5 Haiyuan earthquake occurred near the boundary of the high-velocity anomaly, where the FVDs change dramatically. In addition, we observe that the 1927 Ms 7.9 Gulang earthquake occurred in the normal upper crust and above the strong low-velocity anomalies. From our tomographic results, we propose that two different seismogenic structures were associated with these two great earthquakes.

Late Triassic tectonic stress field of the southwestern Ordos Basin and its tectonic implications: Insights from finite-element numerical simulations

Abstract The tectonic stress field of the southwestern Ordos Basin during the Late Triassic is controversial. The major controversy is whether the southwestern Ordos Basin was a compressional basin throughout the Late Triassic or whether it transformed from an extensional into a compressional basin during this period. We divided the Late Triassic into the early to middle and late to terminal periods. Two paleotectonic stress field simulation models of the southwestern Ordos Basin were constructed using finite-element software (ANSYS 10). Our results showed high consistency with regional geologic correlations, suggesting the credibility of the models. We found that the southwestern Ordos Basin was dominated by NE-SW extensional stress and strain during the early to middle Late Triassic, associated with strike-slip faulting along the western margin of the Ordos block. This is consistent with the development of syndepositional normal faults and was probably induced by the scissor collision from east to west between the North China craton and Yangtze block. The tectonic stress field of the southwestern Ordos Basin during the late to terminal Late Triassic mainly manifested as NE-SW compressive stress and strain. The dominant tectonic dynamics for the Ordos block during this period may have changed to northward compression of the Songpan-Ganzi and Qiangtang terranes. The southwestern Ordos Basin was characterized by compressional deformation and northeastward migration of the depocenter. The southwestern Ordos Basin transformed from an extensional basin associated with strike-slip faulting during the early to middle Late Triassic into a compressional depression basin during the late to terminal Late Triassic.

Paleoseismic evolution of the central section of the Serteng Shan fault at the northern Ordos Block (North China) since the Late Pleistocene

ABSTRACT Reliable palaeoseismological data are crucial for understanding the rupture history and seismic hazard assessment of a specific seismogenic structure. Many large earthquakes occurred around the Ordos Block of northern China. However, due to insufficient age data and event interpretations, the rupture patterns of the block are still poorly known. This study focuses on the central section of the Serteng Shan fault that is situated at the north margin of the Ordos Block. We present palaeoseismic sequence of faults using trenches combined with dating methods, including optically stimulated luminescence and 14C dating. We collected eleven optically stimulated luminescence (OSL) and seven radiocarbon samples to constrain the timing of palaeoseismic events in the excavated trench exposures along the fault strand. The results show that the studied samples were deposited since the latest Pleistocene, and the ages are, within uncertainty, in the stratigraphic order. The cause of the most recent surface rupture is still not clear. Four other older events, which occurred in 3.1–3.9 ka, 5.8−6.5 ka, 10.8–13.1 ka, and 14.2–16.5 ka, likely extended at different lengths along the Serteng Shan fault.

Geochemical features and seismic imaging of the tectonic zone between the Tibetan Plateau and Ordos Block, central northern China

The Tibetan Plateau is growing by both vertical uplift and horizontal extension. It is a continuing debate how the Tibetan Plateau interacts with its surrounding plates and blocks. Due to intense tectonic activity, which produced catastrophic earthquakes, the tectonic zone between the northeast margin of the horizontal extending Tibetan Plateau and the stable Ordos Block has garnered considerable interest. This study investigated the spatial distribution of gas geochemical anomalies (e.g., high flux of CO2 in correspondence of the main faults) at regional scale together with the seismic tomography in correspondence of this tectonic zone with the aim to figure out the domain of convergent boundary between the Ordos block and Tibetan plateau, and trace the tectonic discontinuities which are able to transfer fluids through the crustal layers between the two main geological units. From northwest to southeast, obvious difference of spatial distributions of geochemical and geophysical features in the tectonic zone between the northeast margin of the Tibetan Plateau and the Ordos Block is inferred. The northeast area (Zone A) is dominated by thrust and strike-slip faults with clear velocity boundary underneath, where low crack density (ε), saturation rate (ξ) and Poisson’ ratio (σ) in the middle-lower crust coincided with the low values of heat flow and CO2 emissions, tectonic compression and regional locked-fault can be inducements. The southeast area (Zone C) is dominated by extensional tectonics with roughly E-W fast-velocity direction (FVD) of P-wave azimuthal anisotropy, where high permeability and porosity can be deduced from crustal high ε, ξ and relatively high σ anomalies, resulting in high heat flow, CO2 concentrations and fluxes at the surface, and predominantly crustal-derived gases. The intermediate area (Zone B) also dominated by thrust and strike-slip faults is an extraordinary zone, where intensely locked-fault were clearly revealed, while the predominant anisotropic FVDs in the middle crust changed obviously, more contribution of shallow gas component was detected, and CO2 flux, heat flow, and regional ε, ξ, and σ in the upper crust were higher, compared with those in Zone A, which indicated the regional crushing fragmentation underneath Zone B. The adopted multidisciplinary approach demonstrated that Zone B is the convergent boundary between the Tibetan Plateau and the Ordos Block.

Late Neoarchean plate subduction in Western North China Craton: Evidence from ca. 2.51 Ga to 2.46 Ga basement rocks in Northern Ordos Basin

The most poorly exposed and least understood in the North China Craton (NCC) lies largely hidden beneath the Ordos Basin, severely limiting the cognition of the Precambrian tectonic evolution in the western NCC and the tectonic division. Diverse orthogneiss samples collected from four boreholes drilled into the basement of the north side of Hangjinqi Fault Zone (HFZ) in the North Ordos Basin (NOB) are investigated by integrating zircon U-Pb dating, Hf-Nd isotopes, and whole-rock geochemical analyses. The TTG gneiss, quartz-monzonite gneiss, granodiorite gneiss, and amphibolite were emplaced at ca. 2.51–2.46 Ga, of which the amphibolite also recorded the metamorphic age of ca. 1.95 Ga. The TTG gneisses display low MgO, Mg#, Cr, and Ni, high HREE and Rb/Sr, enrichment of LILEs and depletion of Nb, Ta, and Ti, and positive εNd(t) (2.04–4.06) and εHf(t) (3.04–7.59) values, suggesting that they were mainly originated from partial melting of the tectonically thickened mafic island-arc crust. In contrast, the quartz monzonite gneisses and granodiorite gneisses show the geochemical features of Archean sanukitoids with higher MgO, Mg#, Cr, and Ni, and similar εNd(t) (1.34–3.82) and εHf(t) values (3.94–7.32), suggesting that they were derived from the partial melting of the mantle metasomatized by fluids and melts of the subduction. The amphibolites exhibit a mixed characteristic of E-MORB and arc-like geochemical affinities resembling the Mariana back-arc basin, which was derived from the partial melting of spinel lherzolite. The arc and arc-back lithological assemblages suggest that the north margin of the NOB might be in a geodynamic transition period from a compressional regime correlated with plate subduction to an extensional regime correlated with slab rollback during ca. 2.51–2.46 Ga. Combined with previous analytical results, the northern margin of the NOB was most likely in a long-term active continental margin setting during 2.51–1.95 Ga and involved in the collision event between the Yinshan Block and Ordos Block at ca. 1.95 Ga.

Understanding the Distribution Laws of In Situ Stress in Mines Based on Global Positioning System Data and Numerical Simulation Analysis

To ensure safe exploitation of the Muduchaideng Mine, we use a numerical simulation to invert the in situ stress field based on observation data from GPS stations, investigate differences in movement velocities at different locations, and analyze the lefthanded movement in the area around the wellfield. The relationship between the difference in motion velocity and the in situ stress field was explained through theoretical analysis. The law of in situ stress distribution was studied using data collected from the surrounding mines, and a regression equation as well as the relationship between the magnitudes of the three-way stresses was obtained. The inversion results were consistent with the measured data, indicating that inversion of the in situ stress field was accurate under the conditions found in the intact Ordos block. Additionally, the influence of tectonic stress on the key layer was larger than that on the bedrock of the coal seam roof, and the energy accumulated in the key layer was higher. The stress concentration coefficient of the key layer was similar to the variation law of the coal seam thickness. Therefore, the southwest area of the wellfield with a thicker coal seam is the key area for impact ground pressure prevention and control.

The lithospheric S-wave velocity structure beneath the NE Tibetan Plateau and its surrounding craton basins

It is essential to investigate the spatial distribution of the lithosphere and asthenosphere in detail, to further obtain the understanding of the effect of plate collision and the process of orogenic movement. From the joint inversion of receiver functions and surface waves, the three-dimensional S-wave velocity structure results down to 200 km depth in the study area were obtained at 1,843 seismic stations. Analysis was performed on the sedimentary thickness, crustal thickness, lower crustal wave velocity, and lithospheric thickness. According to the crustal thickness, we evaluated the distribution of low-velocity zones in the lower crust. The results show that there are low-velocity bodies in the lower crust in the Qinling tectonic belt, but they are not connected, indicating that they may not be able to be used as a channel for material extrusion from the NE Tibetan Plateau at the crustal scale. According to the section results and the depth distribution of the lithosphere-astenosphere boundary, a relatively thick lithosphere exists below the Sichuan Basin and Ordos Basin, and the lithosphere in the east of the study area is relatively thin with a thickness of about 60–80 km, indicating that the lithosphere in the east of the study area has been severely destructed and restructured. The delamination has been observed in the lithosphere under the Songpan-Ganzi Block, showing characteristics of vertical movement of asthenosphere materials. There is a relatively thick low-velocity zone at the top of the mantle lithosphere of the NE plateau; however, it does not exist under the relatively stable Sichuan Basin and the Ordos Block. Compared with the Sichuan Basin and the Ordos Basin at both sides, the Qinling tectonic belt has a low-velocity zone at the depth of 100–160 km, which may be asthenosphere material. In combination with the polarization direction characteristics of the SKS wave, it is clearly observed that asthenospheric material movement exists in an approximate east-west direction beneath the Qinling tectonic belt. Therefore, the asthenosphere beneath the Qinling tectonic belt may serve as an important channel for material extrusion in the NE Tibetan Plateau.

Spatial heterogeneity of the lithospheric destruction of the North China Craton: Evidence from an extended magnetotelluric sounding profile

To study the spatial heterogeneity of the North China Craton (NCC) destruction, this paper used a magnetotelluric sounding (MT) profile that passes through almost the entire NCC from west to east. Three-dimensional inversion is used to obtain a lithospheric resistivity model of the NCC. The results show that the upper crust of the Ordos Block is characterized by high resistivity. The lower crust to the upper mantle is characterized by low resistivity. The resistivity structure below the Trans-North China Orogen (TNCO) has stratification features; The Shanxi Graben shows high-low-high-low resistivity features from the upper crust to the asthenosphere; The lithosphere of the Lüliang and Taihang uplifts show high-resistivity features, and only some local relatively low-resistivity areas appear at the crust-mantle boundary. The upper crust on both sides of the Tan-Lu Fault Zone is characterized by high resistivity, but the resistivity structures of the lower crust and the lithospheric mantle are significantly different; The lower crust and the lithospheric mantle of the Sulu Orogenic Belt on the east are characterized by high resistivity; The Luxi Uplift on the west is represented by low resistivity. We propose that the mantle low-resistivity bodies (C1 and C4) of the Western and Eastern blocks may be related to the upwelling of partial melting materials along the ancient structurally weak zones in the lithosphere. The TNCO still has a typical Archean cratonic lithosphere, and the low-resistivity body C2 may be the remnant of the subducted oceanic crust. The Tan-Lu Fault Zone is structurally weak in the Eastern Block, while its western branch is a channel for the asthenospheric upwelling. We propose that the lithosphere of the northwestern Ordos Block and the Yinchuan-Hetao area is being destructed, and the TNCO is in the initial stage of being destructed. In contrast, the lithosphere of the Eastern Block has been severely destructed. In conclusion, affected by the subduction of the paleo-Pacific plate and the collision of the Indian and Eurasian plates, the ancient structures in the NCC were reactivated in the Mesozoic and Cenozoic, resulting in the spatial heterogeneity of the NCC destruction.

Distribution pattern of the Ordovician black shale constrained by graptolite zonation in the western margin of the Ordos Block, North-West China

Distribution pattern of the Ordovician black shale constrained by graptolite zonation in the western margin of the Ordos Block, North-West China

Tectonic Significances of the Geomorphic Evolution in the Southern Alashan Block to the Outward Expansion of the Northeastern Tibetan Plateau

Fluvial landscapes are a result of lithospheric tectonic movement, climate evolution and surface processes. Existing evidence proves that regional tectonic deformation can be reflected by the evolution of fluvial landforms. The southern Alashan Block lies the closest to the northeastern Tibetan Plateau and has become the latest plateau outward expansion boundary. Yabrai Shan is located at the intersection of the Tibetan Plateau, Alashan and Ordos Blocks, thereby recording the evolution of the surface processes and tectonic activities in the northeastern Tibetan Plateau and its surrounding regions. Herein, we conducted the quantitative landform analysis and field investigations of Yabrai Shan, and the spatiotemporal distribution of geomorphological parameters indicated that Yabrai Shan is in the mature stage of geomorphological evolution controlled by tectonic activities of the Yabrai range-front fault. Based on the paleochannel reconstruction model and previous research on fault activity, two tectonic acceleration events were identified, with estimated geomorphic response times of 0.15–1.10 Ma and 1.42–2.92 Ma. Different distribution characteristics of two-phase knickpoints are caused by the change of tectonic stress in this region, revealing that the late knickpoints are the result of tectonic acceleration under the influence of the northeast expansion of the Tibetan Plateau. The transformation of Yabrai Shan from the original extensional environment affected by the Ordos Block to the compressional environment affected by the northeast expansion of the Tibetan Plateau occurred after 1.10 Ma.

Northeastward expansion of the Tibetan Plateau: Seismic anisotropy evidence from shear-wave splitting measurements

• Shear-wave splitting in and around the NE Tibetan Plateau mainly reflects anisotropy in the lithospheric mantle. • Dominant NW-SE anisotropy in the lithospheric mantle indicates NE-SW compression. • Fossil anisotropies exist in the interior Alxa Block and the western Ordos Block. The northeastern Tibetan Plateau (NETP) is a key region to clarify the nature and mechanism of the Tibetan Plateau expansion. This region includes the eastern Qilian Orogenic Belt, the Alxa Block, the Helan-Yinchuan Tectonic Belt and the Ordos Block. We make shear-wave splitting measurements using teleseismic waveforms recorded at 17 portable broadband stations that we newly deployed in the study region. Our measurements provide new information on seismic anisotropy and deformation patterns in the lithospheric mantle, which can be used to constrain the extent of the NETP northeastward expansion. The dominant fast polarization direction (FPD) is NW-SE in the eastern Qilian Orogenic Belt, the southern Alxa Block and the Helan-Yinchuan Tectonic Belt, subparallel to the prominent orientation of tectonics in the NETP. The same NW-SE FPD in the three tectonic units indicates NE-SW compression in the lithospheric mantle there caused by the NETP northeastward expansion. NE-SW FPDs are revealed in the interior Alxa Block, which may reflect fossil anisotropy probably formed by subduction and closure of the Paleo-Asian ocean. ENE-WSW FPDs exist in the western Ordos Block, reflecting fossil anisotropy probably caused by convergence of the South China and North China Cratons. Combined with the NW-SE FPDs in the southwestern margin of the Ordos Block revealed by previous studies, our results suggest that the NETP northeastward expansion has obviously affected the southern Alxa Block and the southwestern margin of the Ordos Block. However, its recent northeastward expansion mainly occurred along the Helan-Yinchuan Tectonic Belt.

Crustal structure beneath the central and western North China from receiver function analysis

The North China Craton (NCC) is one of the oldest cratons on earth. Several important tectonic transformations of Mesozoic-Cenozoic tectonic regime led to the destruction of the North China craton. The knowledge of crustal structure can provide important constraints for the formation and evolution of cratons. New maps of sediment thickness, crustal thickness (H) and vP/vS (κ) in the central and western NCC were obtained using sequential H-κ stacking. P-wave receiver functions are calculated using teleseismic waveform data recorded by 405 stations from ChinArray project. Benefiting from the densely distribution of temporary seismic stations, our results reveal details of the crustal structure in the study area. The thickness of sedimentary layer in North China ranges from 0–6.4 km, and the thickest sedimentary layer is in Ordos block and its surroundings (about 2.8–6 km); The thickness of sedimentary layer in the Mongolia fold belt and Yinshan orogenic belt is relatively thin (less than 1 km). The crustal thickness of the study area varies between 27–48 km, of which the crust of the North China Plain is about 30–33 km, the central NCC is about 33–40 km, and the Ordos block is 40–48 km thick. The average vP/vS ratios in the study area is mostly between 1.66 and 1.90, and that in the Yanshan-Taihang mountain fold belt is between 1.70 and 1.85, and that in the Ordos block is between 1.65 and 1.90, with an average value of 1.77, indicating the absence of a thick basaltic lower crust. The obvious negative correlation between crustal thickness and average vP/vS ratio within Ordos and Central Asia orogenic belt may be related to magmatic underplating during the crustal formation. There is no significant correlation between the crustal thickness and the vP/vS ratio in the Lüliang-Taihang mountain fold belt, which may be related to the multiple geological processes such as underplating and crustal extension and thinning in this area. The lack of correlation between crust thickness and topography in the central orogenic belt and the North China Basin indicates the topography of these areas are controlled not only by crustal isostatic adjustment but also by the lithospheric mantle processes.

Present-day crustal strain and major fault slip rates in North China determined using GNSS observations

• A strain model and an elastic block model of North China were established. • The western Ordos and the northern North China exhibited larger strain rates. • The NNE- and NWW-trending faults showed dextral and sinistral slip respectively. • The fault deformation in North China was controlled by neighboring blocks. North China is characterized by numerous faults, where many earthquakes were recorded. However, the fault slip and crustal deformation mechanism underlying the effects of neighboring block motions remain poorly understood. In this study, we first constructed a strain model using global navigation satellite system (GNSS) velocities to analyze the relationship between seismicity and strain. We then constructed a block model based on the fault distribution and GNSS observations to obtain fault slip. The strain and fault slip rates were then used to determine the crustal deformation mechanism in North China. The strain model suggests the western border of the Ordos Block and the northern North China Plain had larger maximum shear strain rates. These areas are located in a transition between positive and negative dilatation rates, which indicate intense crustal deformation. Based on the block model, we found the NNE-trending faults were dominated by dextral slip. The northern and southern borders of the Ordos Block indicate left-lateral motion (0.4–2.5 mm/a), while its western and eastern margins exhibited right-lateral slip (1.0–3.0 mm/a). The periphery of the Ordos Block exhibited normal slip (0.2–3.2 mm/a), except at the southwestern tip. These results suggest that the eastward extrusion of the Tibetan Plateau caused left-lateral shear in North China, which led to sinistral slip of NNE-trending faults. The northward expansion of the northeastern Tibetan Plateau also contributes to sinistral slip along the northern and southern margins of the Ordos Block, as well as dextral slip along its western and eastern margins.

Crustal structure beneath the northeastern margin of the Tibetan Plateau constrained by receiver functions and gravity data

The northeastern margin of the Tibetan Plateau is a key area for studies of the crustal thickening and outward expansion of the plateau and its interactions with surrounding blocks. Detailed crustal structure information can provide important constraints for understanding the tectonic evolutionary process in this region. We obtained the crustal thicknesses and average Vp/Vs ratios via joint estimation from teleseismic P-wave receiver functions and the complete Bouguer gravity anomaly. Our results show that the crustal thickness gradually decreases northward and eastward from ∼65 km beneath the Qilian Tectonic Belt to ∼40 km within the Alxa and Ordos blocks. A low Vp/Vs ratio (1.71–1.78) is found in the northeast Tibetan Plateau, which is indicative of its felsic crustal composition. A maximum 9.3 km deviation from the Airy isostatic crustal thickness is revealed beneath the Songpan–Garze Terrane and the West Qinling Orogen. The correlation between the elevations and corresponding crustal thicknesses within these two terranes is weak, suggesting that contributions of the isostatic compensation come not only from the crust but also from the lithospheric mantle. We found that the crustal thicknesses below the northeast Tibetan plateau are anti-correlated with the average crustal Vp/Vs ratios and densities, which is in favor of the previous hypothesis of upper-crust thickening or lower-crust delamination of the coherent thickening crust.

Late Quaternary faulting and paleoearthquakes along the northern section of the Bayanhaote fault and their implications for regional seismotectonics

Analysis of the late Quaternary activity and paleoseismicity of the Bayanhaote fault is critical because it is part of the frontier of the propagating Tibetan Plateau and the boundary between the Alxa and Ordos blocks, and the development of a regional seismotectonic model surrounding the Helan Mountains is crucial. We studied offset landforms and paleoearthquakes along the northern section of the Bayanhaote fault and found that it deformed dextrally, with a slight reverse slip to the east. A channel and a deluvial edge on the alluvial terrace, aged between 56.28 ± 4.04 ka and 82.2 ± 5.78 ka, are right-laterally offset and a sag pond formed on the east side of the fault scarp. We calculated a dextral slip rate of 1.0–2.4 mm/a. Three surface-rupturing paleoearthquakes were discovered in the reversely offset strata in a trench south of Sumutu village. We infer that these three earthquakes might be a portion of surface-rupturing earthquakes by comparing them with documented paleoearthquake data along the southern half of the Bayanhaote fault. We established a regional seismotectonic model around the Helan Mountains using our new and published geological and geophysical data. The seismic risk along the dextral Bayanhaote fault to the west of the Helan Mountains is also substantial. The dextral Bayanhaote fault west of the Helan Mountains and the normal fault system in the east constitute the active boundary belt between the Alxa and Ordos blocks.

Timing of Paleoearthquakes and Seismic Hazard of the Zhuozishan West Piedmont Fault in the Northwestern Ordos Block

The Zhuozishan West Piedmont Fault (ZWPF) is an active normal fault in the northwestern corner of the Ordos Block. Studying the recurrence characteristics of paleoearthquakes is useful for understanding the regional seismic hazard potential. In this study, paleoseismic trench and optically stimulated luminescence (OSL) dating methods are used to determine the temporal sequence of paleoearthquakes. Severn earthquakes were constrained to be E7 (73.4 ± 12.9 ka to 75.3 ± 10.6 ka), E6, (63.7 ± 7.4 ka to 64.4 ± 8.2 ka), E5 (53.2 ± 7.0 ka to 58.7 ± 6.5 ka), E4 (42.6 ± 6.3 ka to 47.7 ± 5.8 ka), E3 (31.0–3.4 ka to 31.3 ± 3.4 ka), E2 (26.1 ± 2.7 ka to 28.1 ± 3.8 ka) and E1 (15.8 ± 1.9 ka to 20.2 ± 2.6 ka). The recurrence interval of strong earthquakes in the ZWPF is roughly constrained to be 8.1–14 ka, with magnitudes of Mw 7.0–7.4 that ruptured the entire fault. Based on the timing of the latest earthquake event, the elapsed time approached or exceeded the recurrence interval revealed from paleoearthquakes. Hence, we suggest that the northwestern corner of the Ordos Block has a significantly high seismic hazard potential and that the occurrence of a strong earthquake should not be underestimated.

Entrenchment of the Yellow River since the late Miocene under changing tectonics and climate

The distinctive right-angled bend of the Yellow River around the Ordos Block, a large Permo-Mesozoic continental block in East Asia, has attracted much attention from geomorphologists and geologists concerning its formation and evolution for more than a century. The Jinshaan Gorge, an over 600 km long entrenched reach of the Yellow River, currently connecting the Hetao Graben to the north and the Weihe Graben to the south, is considered to have developed in very recent geological time. However, the timing and processes of drainage integration between the two grabens and formation of the Jinshaan Gorge as a deeply incised form are still under debate, owing to limited data and various interpretations of them. In order to better understand the geomorphological evolution of the Yellow River, we investigated the sedimentology, magnetostratigraphy, and detrital zircon U-Pb dating in the upper Neogene and Quaternary deposits along the Jinshaan Gorge. We further integrated the new data with the sedimentary record of the adjacent basins in the northeastern Tibetan Plateau (NTP) and the basins around the Ordos Block, as well as the incision history of the Yellow River inferred from its terrace sequence. We show that the upper Neogene alluvial sediments preserved in the broad Tangxian valley along the Jinshaan Gorge are a mixture of materials from the Lvliang Shan and the Ordos Block. Importantly, they share almost the same provenance and zircon age spectra with the Pleistocene and Holocene Yellow River deposits. The broad Tangxian valley commenced incision during the Late Pliocene, followed by the formation of two Early Pleistocene terraces dated to ~2.2 Ma and ~1.2 Ma. The incision of the Yellow River and the accumulation in the basins were characterized by steady and slow rates before the Late Pleistocene, but an acceleration thereafter.We propose a three-stage evolutionary model for the development of the Yellow River, controlled by changing tectonics and climate during the Neogene. Before the late Miocene, an endorheic river system was developing in the Longzhong Basin. From the late Miocene to Early Pliocene, the Yellow River developed the current drainage system from the Longzhong Basin to the lower reach, constrained by the topographic pattern of the Ordos Block and its periphery, in turn controlled by coeval tectonic movements. Since the Late Pliocene, the Yellow River has undergone rapid incision with integration of the upper reach in the NTP and entrenchment in the upper and middle reaches, which were controlled by the continuously rising NTP and a deteriorating climate. The Tangxian valley developed and was filled between the end of the Miocene and Pliocene, approximately 8 to 3 Ma, and it was abandoned in the Late Pliocene in response to important climatic variations. The next acceleration in both incision and subsidence around the Ordos Block in the Late Pleistocene was also coeval with strong climatic oscillations, suggesting a possible link between climate and tectonics in the Neogene entrenchment history of the Yellow River.