Tan-Lu Fault Research Articles

Upper mantle temperature structure of the North China Craton

We investigate three-dimensional (3-D) temperature distribution of the upper mantle at depths of 50–200 km beneath the North China Craton (NCC) and surrounding areas using high-resolution P- and S-wave velocity models as well as mineral physics and geothermal data and methods. The thickness of thermal lithosphere is also estimated assuming its bottom has a mantle adiabatic temperature of 1300 °C. Our results show that the study area exhibits an alternate distribution pattern with three weak zones and two stable blocks. The Qilian orogen, the Shanxi rift system and the Tanlu fault system are characterized by lower seismic velocity, higher temperature, higher heat flow, and thinner thermal lithosphere. The three mechanically weak zones may be related to subductions of the Indian plate and the Pacific plate beneath the Eurasian continent, resulting in thermochemical erosion of the upwelling asthenospheric material, which have played an important role in the Cenozoic partial destruction of the lithosphere beneath the NCC. The Ordos block and part of the North China Basin have higher seismic velocity, lower temperature and thicker thermal lithosphere, suggesting that the two relatively stable blocks have suffered from less damage and still kept a remanent Archean lithospheric keel.

High-resolution 3D crustal S-wave velocity structure of the Middle-Lower Yangtze River Metallogenic Belt and implications for its deep geodynamic setting

The Middle-Lower Yangtze River Metallogenic Belt (MLYMB) is an important mineral resource region in China. High-resolution crustal models can provide crucial constraints to understand the ore-forming processes and geodynamic setting in this region. Using ambient seismic noise from 107 permanent and 82 portable stations in the MLYMB and the adjacent area, we present a new high-resolution 3D S-wave velocity model of this region. We first extract 5–50 s Rayleigh wave phase velocity dispersion data by calculating ambient noise cross-correlation functions (CFs) and then use the surface wave direct inversion method to invert the mixed path travel times for the 3D S-wave velocity structure. Checkerboard tests show that the horizontal resolution of the 3D S-wave velocity model is approximately 0.5°–1.0° and that the vertical resolution decreases with increasing noise and depth. Our high-resolution 3D S-wave velocity model reveals: (1) A V-shaped high-velocity zone (HVZ) is located in the lower crust and the uppermost mantle in the study region. The western branch of the HVZ passes through the Jianghan Basin, the Qinling-Dabie orogenic belt and the Nanxiang Basin. The eastern branch, which almost completely covers the main body of the MLYMB, is located near the Tanlu Fault. The low-velocity anomalies between the western and eastern branches are located in the area of the Qinling-Dabie orogenic belt. (2) High-velocity uplifts (HVUs) are common in the crust of the MLYMB, especially in the areas near the Tanlu Fault, the Changjiang Fault and the Yangxin-Changzhou Fault. The intensities of the HVUs gradually weaken from west to east. The V-shaped HVZ in the lower crust and uppermost mantle and the HVUs in the middle and lower crust likely represent cooled mantle intrusive rocks. During the Yanshanian period, fault systems formed in the MLYMB due to the convergence between the South China Plate and the North China Plate, the multiple-direction drifting of the Paleo-Pacific Plate and its subduction beneath the Eurasian Plate. The dehydration of the cold oceanic crust led to partial melting in the upper mantle. Temperature differences caused strong convection of the upper mantle material that underplated the lower crust and rose to near the surface along the deep fault systems. After mixing with the crustal materials, mineralization processes, such as assimilation and fractional crystallization, occurred in the MLYMB.

High‐resolution 3‐D crustal shear‐wave velocity structure in the central Tanlu fault system and implications for its geodynamic settings

High‐resolution 3‐D crustal shear‐wave velocity structure in the central Tanlu fault system and implications for its geodynamic settings

Early Cretaceous transpressional and transtensional tectonics straddling the Sulu orogenic belt, East China

The Sulu orogenic belt (SOB) separates the North and South China blocks in East Asia and formed during Triassic continent-continent collision. However, late Mesozoic post-collisional exhumation is poorly understood due to lack of surface evidence for Paleo-Pacific subduction and associated effects. This paper interprets the tectonic history of the SOB using detrital zircon age data from Early Cretaceous sedimentary units along with previously published geochronologic and geochemical data to reconstruct sedimentological and tectonic history. Detrital zircon age distributions obtained from sedimentary units include a 2.0 Ga subpopulation that appears only in turbidite units to the southeast. This sediment probably derived from the Yangtze Block. Terrestrial facies from the Jiao-Lai basin to the northwest appear to derive from the North China Block. Geochronologic and geochemical data indicate that Early Cretaceous, post-collisional volcanism was compositionally bimodal (mafic-felsic) with associated intrusive activity that peaked at 120 Ma. Seismic images of northerly regions of the study area indicate this occurred in an extensional setting. Sedimentary facies and field structural analyses revealed an unconformity interpreted to reflect rapid uplift with NW–SE compression to the south. Given observed sinistral movement along the Tan-Lu fault, we interpret northwest and southeast regions of the SOB as experiencing transtensional and transpressional tectonics, respectively, driven by continuous subduction of the Paleo-Pacific Plate. Intrusion of the Late Yanshannian granitoids marked the final formational stage of this unique tectonic setting.

Evidence of long recurrence times and low slip rate along the 1668 Tancheng earthquake fault

Fault slip rates and recurrence intervals of large earthquakes for active faults are important parameters for better understanding active faults and assessing their seismic hazard. The Tanlu fault, located in the densely populated and economically developed area of eastern China is thought to be the source of the M 7 Anqiu earthquake in 70 BC, and M 8½ Tancheng earthquake in 1668. Understanding the recurrence interval of paleoearthquakes and fault slip rates for the Tancheng earthquake fault is critical for assessing the potential for seismic risk and assessment for the Tanlu fault. However, the only published study used several trenches and a single trench does not completely record the sequence of paleoearthquake events, leaving a large uncertainty of seismic hazard. GPS observations in North China show that there is no significant velocity gradient across the Tanlu fault, yet the long-term slip rate of the Tancheng segment of the fault is estimated to be significantly higher, up to 2 mm/a. Therefore, it is critical to conduct further research on the paleoearthquake history and slip rates of this fault segment to reconcile these contrasting results. In this study, we utilize aerial photographs and high-resolution satellite imagery, combined with field investigations of a surface rupture formed by the 1668 earthquake. The earthquake rupture is ~80 m long and ~2.5 m high, located in the Huaqiao village of northeastern Tancheng County. A gully across the rupture reveals a right lateral dislocation of approximately 36 m. We excavated two trenches on both sides of the rupture. Trench 1 reveals a positive flower structure and is consistent with a right lateral strike-slip fault with a thrust component consistent with the modern regional stress field. Trench 2 reveals a colluvial wedge formed by the latest paleoearthquake with a fine-grained deposit overlapping the colluvial wedge. The latest paleoearthquake should occur prior to deposition of the fine-grained deposit. AMS-14C radiocarbon dating was used to determine the age of the event and is approximately 12.8 +4.0/−3.7 ka, indicating a recurrence interval of large earthquakes greater than 10 ka, consistent with the results from other segments of the Tanlu fault. Combined with an average slip of ~9 m for the 1668 Tancheng earthquake, the dextral slip rate along the Tancheng earthquake fault since the end of the late Pleistocene is estimated to be 0.7 mm/a, consistent with low interseismic strain accumulation based on geodetic observations across the North China region.

New Evidence of Eocene Ingression in Grabens of the Tan-Lu Fault System (East Asia): An Important Event Level

New evidence of Eocene ingression in the area of the present-day Russian–Chinese border is provided. It was previously believed that Cenozoic grabens of the Tangcheng–Lujiang (Tan-Lu) Fault Zone, located in China and also traced in Russia, were filled predominantly by continental, rarely, lagoonal sediments. In 2015, drilling in the Huadian Basin (graben), which is part of the Tan-Lu system, recovered the Hydrobia zhuoxianensis gastropod in the black shales among the continental fauna. This finding indicates coastal marine conditions and therefore specifies the extent of the Eocene ingression. This gastropod can also be used in interregional stratigraphic correlation.

Deep structures underneath the Sihong Segment of the Tan-Lu Fault Zone, Eastern China: Interpretations of gravity anomaly and seismic profiles

The Sihong Segment of the Tan-Lu Fault Zone (SSTLFZ) is located in the Sihong county of the Jiangsu province and composed of 5 branches, which are mainly buried faults. The Tan-Lu Fault Zone is characterized by segmentation and its regional distribution, deep structures and geodynamic evolution in the Sihong area remains controversial. We provided evidence of gravity anomaly and seismic reflection profiles suggesting that the Xinyi-Xindian fault (F2) and Qiaobeizhen-Suqian fault (F5) of the SSTLFZ are segmented, which appear as thrust faults in the north and normal faults in the south. The F2, F5 and Mohe-Lingcheng fault (F3) are inferred as Quaternary faults. Based on the inversion of gravity anomaly, the tectonic structure of the bedrock consists of two grabens and one horst and the Moho discontinuity depth in the Sihong area shows an obvious uplift. We conclude that a fault-depression superimposed basin developed inner the SSTLFZ. The northern segments of the F2 and F5 were inverted due to the continuous compression since Quaternary.

Preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel based on scenario earthquake studies

In this paper we present a preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel (BSST) based on scenario earthquakes in a strike-slip stress regime. We studied two scenarios: 1) a right-lateral, strike-slip faulting earthquake event with surface wave magnitude (Ms) 7.4 occurred on the north to northeast (NNE) oriented Tan-Lu fault zone (TLFZ) with a location about 100 km away from BSST (the far field); and 2) a left-lateral strike-slip event with magnitude Ms4.1 occurred in the direct vicinity to BSST (the near field) on the northwest to west (NWW) Zhangjiakou-Penglai fault zone (Zhang-Peng Fault Zone, or ZPFZ). Selections of these two scenario events are based on the characterization of the actual seismo-tectonic settings and representation of earthquake history in the Bohai region. We studied the impacts of these two scenario events on BSST in terms of longitudinal compression, bending, and tunnel cross-section's ovaling deformation. As a preliminary study, we excluded the analysis of formation-structure interaction and focused on the ground motion parameters expressed as the maximum displacement, the peak ground velocity (PGV) and peak ground acceleration (PGA) of the free seismic wave field surrounding the tunnel walls. This simplification is acceptable for the fact that BSST is planned to be constructed in the granite bedrock formation in which the inertial contrast between the formation and the structure is minimal. Seismic hazard assessment for tunnels is completely different from the assessment for above-ground buildings. It is also substantially different from assessment for tunnels built in soil formations, for which the formation (soil)-structure interaction evaluation is critical. We use semi-analytical approach for assessing the seismic impact of the far field, large event to BSST; whereas use the two-dimensional (2D) finite difference time domain (FDTD) numerical simulation technique for assessing the response of BSST to the seismic load of the near field, smaller event. The preliminary results suggest that 1) the impact of the low frequency, far field event has very subtle impact to BSST in terms of longitudinal compression, bending, and cross-section ovaling; and 2) the impact of the high-frequency, near field event has little impact to BSST in terms of longitudinal compression and bending; while has a measurable impact to cross-section ovaling. However, this kind of deformation is within the limit of tolerance under earthquake-resistant design. Over all, the seismic risk posed by regional large earthquakes and local small events to BSST is within allowable risk threshold and manageable with the enforcement of earthquake resistant code.

Anatomy of a world-class epizonal orogenic-gold system: A holistic thermochronological analysis of the Xincheng gold deposit, Jiaodong Peninsula, eastern China

Xincheng is a world-class orogenic-gold deposit hosted by the Early Cretaceous Guojialing granitoid in the Jiaodong Peninsula, eastern China. A zircon U–Pb age of 126±1.4Ma, together with previous data, constrain the emplacement of the Guojialing intrusion to 132–123Ma. The granitoid underwent subsolidus ductile deformation at >500°C following its intrusion. The small difference in age between the youngest zircon U–Pb age of unaltered granitoid (~123Ma) and the ca. 120Ma 40Ar/39Ar ages of sericite, associated with breccias and gold mineralization within it indicate initial rapid cooling from magmatic temperatures to those prevalent during brittle deformation and associated gold mineralization at ~220–300°C. Evidence of a direct association between granitic magmatism and gold mineralization, such as at least localized near-magmatic depositional temperatures and metal zoning evident in undoubted intrusion-related gold deposits, is absent. The 40Ar/39Ar age of ~120Ma coincides with the mineralization age of many other orogenic-gold deposits along the Jiaojia Fault. Sixteen zircon fission-track (ZFT) ages across the ore and alteration zones range from 112.9±3.4 to 99.1±2.7Ma. The long period of cooling to the ~100Ma ZFT closure temperatures recorded here suggests that ambient temperatures for hydrothermal alteration systems lasted to ~100Ma, possibly because of their focus at Xincheng within the young Guojialing granitoid as it cooled more slowly below approximately 300°C to 220°C. However, the restricted number of auriferous ore stages, combined with the presence of cross-cutting gold-free quartz-carbonate veins, indicate that gold itself was only deposited over a restricted time interval at ~120Ma, consistent with studies of orogenic gold deposits elsewhere. This highlights the complex interplay between magmatism, deformation and the longevity of hydrothermal systems that cause genetic controversies. Based on apatite fission-track (AFT) ages, the Xincheng gold deposit was then uplifted and exhumed to near the surface of the crust at 15Ma, probably due to movement on the crustal-scale Tan-Lu Fault. Recognition of such exhumation histories along gold belts has conceptual exploration significance in terms of the probability of discovery of additional exposed or sub-surface gold ore bodies as discovery is as much a function of preservation as formation of the deposits.

Uppermost mantle velocity and anisotropy structure beneath the North China Craton and its adjacent regions

The North China Craton has experienced significant tectonic reactivation and destruction, and it is a natural place to study structural heterogeneities and their influences on the lithospheric evolution. In order to have a better understanding of the lithosphere modification of the North China Craton, we collected as many as possible bulletin and exceptionally high-quality handpicked Pn arrival times, yielding a well-distributed dataset with high lateral resolution, to conduct a new uppermost mantle velocity model of the North China Craton. The present tomographic results show distinct lateral heterogeneities in both velocity and anisotropy, which are well correlated with the major active tectonic units. Obvious high Pn velocity anomalies with E-W fast directions are imaged beneath the Ordos Block, suggesting the Ordos Block lithosphere may remain a strong Archean craton. The high-velocity North China Basin may indicate a thick detached lithospheric structure, which could have been preserved during multiple tectonic episodes. Prominent low Pn velocity anomalies are observed beneath the central North China Craton and along the Tanlu fault. These low velocity anomalies suggest complex lithospheric deformation due to the upwelling of hot material triggered by the northwestward subduction of the Pacific plate. The Pn fast directions beneath the central North China Craton are generally parallel to the strike of mountains, suggesting that the mountains may be formed in response to the deformation of the uppermost mantle velocity structure. The distribution of large earthquakes and the pattern of velocity anomalies may imply that the uppermost mantle velocity structure could increase seismicity and impact the corresponding active tectonic faults beneath the North China Craton. These observations provide new geophysical insights on the lithospheric deformation and mantle dynamic processes of the North China Craton.

Formation conditions and accumulation characteristics of Bozhong 19-6 large condensate gas field in offshore Bohai Bay Basin

Based on the study of natural gas resource, low buried hill trap formation mechanism, high quality reservoir control factors and natural gas preservation conditions, the formation conditions and reservoir accumulation characteristics of Bozhong 19-6 large condensate gas field were summarized. Large gas generation potential of multiple sets of thick humic-sapropelic source rocks in high maturity stage in Bozhong depression was the basis of large gas field formation. The multi-stage tectonic evolution since Indosinian period formed large-scale buried hill traps. The Tanlu fault activity formed multi-type reservoirs, and buried hill metamorphic rock of Archean and sand-conglomerate of Kongdian Formation were high-quality reservoirs. Thick overpressure lacustrine mudstone and weak neotectonic movement provided good preservation conditions. Bozhong 19-6 gas reservoir was a condensate gas reservoir with very high condensate oil content, and the gas origin was humic-sapropelic and kerogen-cracking gas, and the gas field had large gas thickness, high gas column characteristics and the accumulation process was first oil and then gas. The buried hill reservoir was a massive reservoir and the Kongdian reservoir was a stratified reservoir. The gas field had multi-channel hydrocarbon intense charge from overpressure source rocks, atmospheric-weak overpressure reservoir favorable for accumulation, thick overpressure mudstone caprock favorable for preservation, and natural gas ultra-late rapid accumulation model.

Episodic bimodal magmatism at an active continental margin due to Paleo-Pacific Plate subduction: A case study from the southern segment of the Tan-Lu Fault Zone, eastern China

Volcanic rocks in the southern part of the Tan-Lu Fault Zone (TLFZ), eastern China, provide key evidence for the connection between spatially and temporally related pulses of Cretaceous magmatism, activity on the TLFZ, and the dynamics of the Paleo-Pacific Plate. We identify two stages of bimodal volcanism in the Chaohu–Lujiang segment, southern TLFZ, based on lithological assemblages and precise isotopic ages. The Early Cretaceous (125–121 Ma) volcanic rocks are fractionated aluminous A-type rhyolites and basalts, whereas the Late Cretaceous volcanic rocks (97–83 Ma) range in composition from porphyritic andesites and trachydacites to rhyolites. The Early Cretaceous rhyolites display strongly negative Nd-Hf isotopic compositions (εNd(t) = −16.3 to −16.1, εHf(t) = −24.1 to −17.9). The most likely source rocks of the fractionated aluminous A-type rhyolites are metaigneous rocks of the Feidong Complex. The Early Cretaceous potassic basalts (K2O/Na2O = 1.5–3.4, εNd(t) = −9.0) exhibit more enriched isotopic compositions than the Late Cretaceous sodic porphyritic andesites (K2O/Na2O = 0.6–0.8, εNd(t) = −5.6, εHf(t) = −15.1 to −6.8), indicating derivation from heterogeneously enriched lithospheric mantle. This heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures. Modeling of selected elements reveals that the Late Cretaceous trachydacites (εNd(t) = −12.2 to −8.2, εHf(t) = −16.5 to −6.4) originated by mixing between coeval mafic and felsic magmas. The most negative (εNd(t) = −18.8 to −17.1, εHf(t) = −20.5 to −4.7) isotopic compositions, along with Neoproterozoic inherited zircons within the Late Cretaceous rhyolites, indicate derivation from lower crust of the South China Block. We suggest that the Early Cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting Izanagi slab. However, the TLFZ became a focus of Late Cretaceous bimodal volcanism due to weak far-field extension caused by the slow northward motion of the Paleo-Pacific Plate. Moreover, the high velocity and low angle of subduction of the Izanagi slab were the main driving mechanisms behind an Early Cretaceous transpressional event that separated the two stages of bimodal volcanism.

Correlation of lithospheric “de‐rooting” of the Sulu‐Dabie Orogen to tectonic‐sedimentary process of the Hefei Basin: Constraints from Mesozoic coupling of basin and orogen

The lithospheric “de‐rooting” of the Sulu‐Dabie Orogen (SDO) is an important tectonic mechanism for post‐orogenic tectono‐magmatism, which influenced the sedimentation, evolution, and development of oil–gas accumulation in the Hefei Basin (HB). Combined with seismic, geological, and sedimentary data of the HB and the SDO, this paper studies the coupling of the HB and the SDO since the Late Mesozoic. While the North China Block (NCB) indented south‐eastwards under the Greater South China Block (GSCB) since the Mesozoic, the SDO underwent three intense tectonic stages, i.e., subduction, exhumation, and uplift. The upwelling of deep mantle material causes the exhumation of the ultrahigh‐pressure metamorphic (UHPM) rocks to the upper crust or surface, forming the UHPM. Influenced by the indentation of the NCB, the “de‐rooting” of the SDO lithosphere, caused the delamination and thinning of the lithosphere, accompanied by the continuous flexural subsidence in the HB. The basin underwent at least four stages of tectonic evolution, involving sequentially: the flexural basin, the foreland basin, the strike‐slip‐related basin, and the faulted graben development periods. The tectonic evolution of the HB was closely coupled with the SDO orogeny. The “de‐rooting” of the SDO lithosphere resulted in a strong uplift and erosion of the orogen, providing abundant source of material for the HB and causing the successively northwards migration of basin depocenters at its early stage. Finally, influenced by the subduction retreat of the (Palaeo‐) Pacific Plate and the strike‐slip movement of the Tanlu Fault, the depocenters migrated to the east.

Stress development in heterogeneous lithosphere: insights into earthquake initiation in the Tan-Lu Fault Zone

The Tan-Lu Fault Zone (TLFZ), East China, is the site of several historical M 6.5–8.0 earthquakes and remains seismically active today. Although differential kinematic and geomorphic features in the upper crust can well divide the TLFZ into several segments, many questions remain regarding the spatial distribution of the middle-large earthquakes along the active faults and what triggers earthquakes in the seismic zone. Namely, what is the relationship, if any, between the observed seismicity and the fault segmentation along the TLFZ? Here we present a three-dimensional viscoelastic finite element model to simulate the stress distribution along the TLFZ and better understand the relationship between the regional stress field and observed seismicity, with a particular emphasis on how the heterogeneous lithosphere influences earthquake initiation at different depths and along the various segments of the TLFZ. The global crustal model Crust 1.0 and structural information from regional and areal studies are utilized to infer the lithospheric viscosity structure and provide an important input to the numerical model. The model results are well supported by the geological evidence, in terms of both the observed stress field and distribution. Our results suggest ~ENE-oriented maximum principal compressive stress in East China, which are consistent with the local fault-plane solutions. The calculated maximum shear strain and rupture values at different crustal depths are consistent with the distribution of the large earthquakes, which suggest that these larger earthquakes are initiated by stress localization in the heterogeneous lithosphere along the TLFZ. Cross-sections of our calculated maximum shear strain and rupture values along the TLFZ support the initiation of large earthquakes in the middle crust, particularly along the Bohai Bozhong and Yishu segments. Furthermore, the “soft” low-velocity zone (fluid system) in the lower crust along the TLFZ provides the potential for earthquake stratification and initiation in the middle crust along the TLFZ. Our results provide strong evidence that the stress localization of the heterogeneous lithosphere in different segments of the TLFZ and the lower crustal fluid system initiate different magnitude of earthquakes in each seismic zone.

Shallow Crustal Structure of the Tanlu Fault Zone Near Chao Lake in Eastern China by Direct Surface Wave Tomography from Local Dense Array Ambient Noise Analysis

Ambient noise tomography (ANT) has been used successfully to image shallow earth structure. Here we perform ANT on a local dense seismic array around the Tanlu fault zone (TFZ) to the southeast of Hefei City, Anhui Province in eastern China. The array consists of 53 stations with average spacing close to 5 km. Cross-correlations of vertical-component ambient noise data of different station pairs are computed in 1-h segments and stacked over 1 month from 17 March to 26 April 2017. Clear fundamental-mode Rayleigh waves are observed between 0.2 and 5 s period. We then use the direct surface-wave tomographic method with period-dependent ray tracing to invert group and phase dispersion travel-time data simultaneously for three-dimensional (3D) shear-wave velocity (Vs) structure. The Vs model shows clear correlation with the known geologic features. The TFZ is associated with a high-velocity anomaly zone in the shallow crust, corresponding to metamorphic rocks due to magma intrusion. Low-velocity anomaly zones are mainly located to the west of the TFZ, caused by thick sedimentary layers in the Hefei Basin. Our study shows that, with ambient noise data recorded on a dense array and an advanced surface-wave inversion method, we can image detailed structure around the fault zone.

A study of the deep electrical structure of the northern segment of the Tan-Lu fault zone, NE China

We studied a magnetotelluric transect from Tongliao, Inner Mongolia, to Baishan, Jilin, that crosses the northern segment of the Tan-Lu fault zone. Based on the profiling data, we calculated and analyzed the two-dimensional (2D) deviation degree and the tectonic strike, inverted the data using the 2D nonlinear conjugate gradient inversion method, and obtained an electrical structure model of the crust and upper mantle along the transect. Our analysis reveals that the western portion of the transect is within and at the margin of a typical sedimentary basin, and a high-conductivity layer is present in the lower crust. In comparison, the eastern portion of the transect is in the Jihei fold zone and has a relatively stable electrical structure, but its resistivity is one to two orders of magnitude higher than that of the western portion. A distinct electrical anomaly exists between the western and eastern sides. The Yilan-Yitong Fault of the northern segment of the Tan-Lu fault zone dips to the northwest, has strike-slip characteristics, extends to a depth of 40–50 km, and induces upwelling of upper mantle materials. The Dunhua-Mishan Fault of the northern segment of the Tan-Lu fault zone has a clear electrical gradient, and similar to the Yilan-Yitong Fault, it also dips to the northwest. However, the Dunhua-Mishan Fault is mostly fractured, extends to a depth of approximately 20 km and is slightly smaller than the Yilan-Yitong Fault. The inferred distribution of deep resistivity values indicates that the lithosphere is generally thinner at the center of the transect and thicker at the edges: the lithosphere is approximately 150 km thick on the western end, thins eastward to approximately 60 km thick near the Tan-Lu fault zone and then gradually thickens eastward to 100 km thick.

Early Paleogene strike‐slip transition of the Tan–Lu Fault Zone across the southeast Bohai Bay Basin: Constraints from fault characteristics in its adjacent basins

The Tan–Lu Fault Zone (TLFZ) is the largest continental‐scale strike‐slip fault zone in East China. It experienced a complex Meso‐Cenozoic deformation and controlled the development and evolution of the Bohai Bay Basin (BBB). Though its Mesozoic sinistral and Cenozoic dextral motions have been well documented, its strike‐slip transition history and mechanism from sinistral to dextral motions in the Early Paleogene remain poorly understood. To investigate these issues, we made a thorough analysis on the fault geometry and kinematics as well as the response of depocentre in the Early Paleogene of the sags adjacent to the TLFZ across the southeast BBB. The results show that the WNW‐/NW‐, E‐W‐, and ENE‐/NE‐trending extensional faults developed in the Early Paleogene of those sags share almost the same geometrical and kinematic features. Such features indicate that the TLFZ is sinistral in Ek3‐Ek2 depositional stage, extension dominated in Ek1 depositional stage, and dextral in Es4L depositional stage. Such strike‐slip transition of the TLFZ is a comprehensive effect of the plate events around the Eurasian Plate. The NNW‐directed subduction of the Pacific to the Eurasian plates triggers the sinistral motion of the TLFZ during 65–55 Ma, but this sinistral motion is terminated by the far‐field effect of the NNE‐directed India–Eurasia collision initiating at 55 Ma. Then, the kinematic adjustment of the Pacific Plate from NNW to WNW breaks the stress balance exerted on the TLFZ by the subduction of the Pacific to Eurasian plates and the India–Eurasia collision and makes it turn into dextral motion gradually during 48–42 Ma.

Location and sinistral displacement of the eastern Liaoyuan Accretionary Belt along the Tan–Lu Fault Zone, NE China

The NE- to NNE-striking, continental-scale Tan–Lu Fault Zone (TLFZ) in NE China is subdivided into the ∼900 km long Yilan–Yitong Fault (YYF) in the west and the ∼1000 km long Dunhua–Mishan Fault (DMF) in the east. Both faults record sinistral displacement during the earliest Cretaceous, leading to an obvious offset of the boundary between the Liaoyuan Accretionary Belt (LAB) and the North China Craton (NCC). Previous studies have demonstrated that the LAB in the south of the eastern Central Asian Orogenic Belt (CAOB) extends eastwards into an area between the YYF and the DMF. The boundaries between the LAB and NCC indicate a 35 km sinistral displacement upon the YYF. However, it remains unclear whether the LAB extends to the east of the DMF, and the amount of sinistral displacement on the DMF is poorly constrained. Here we present lithological observations and U–Pb and Hf isotopic data from zircon to constrain the age and position of the LAB to the east of the DMF. The data indicate an eastward extension of the LAB across the DMF, suggesting the continuous presence of the LAB east of the TLFZ in NE China. The U–Pb ages, εHf(t) values, and two-stage model ages of zircons from the study area constrain the boundary between the LAB and NCC to an extension of the NE-dipping Chifeng–Kaiyuan Fault to the east of the DMF. The geological boundaries on both sides of the DMF indicate a 170 km sinistral displacement along the fault. Thus, the TLFZ has a total sinistral displacement of 205 km in NE China. The eastern branch of the TLFZ records significantly more movement due to its proximity to the Pacific Ocean, where subduction of oceanic crust might have caused sinistral faulting along the TLFZ.

The recent tectonic stress field of the Amur region

The Amur region (Priamurie) is located in the NE part of the Amur lithospheric plate and its surrounding territories. Seismic activity is moderate in Priamurie, and the regional earthquakes, including the strongest ones, occur mainly in three seismic belts: Stanovoi (the zone of influence of the eastern flank of the Stanovoi fault), Yankan-Tukuringra-Soktakhan (the eastern flank of the Mongolia-Okhotsk lineament), and Turan-Selemzhinsky (from the Lesser Khingan to the north). The Sakhalin Branch of FRC GS RAS Catalogue of focal mechanisms of 57 regional earthquakes provide the data for a more precise estimation of the parameters of the crustal stress state in the study area. The Cataclastic Analysis Method (CAM) developed by Yu.L. Rebetsky (stage 1) was used to estimate the orientations of the main axes of the stress tensor and the Lode – Nadai coefficient. The analysis shows that the Upper Priamurie is dominated by shearing and compression with shearing. The Amur plate moves relative to the Aldan-Stanovoi block along the South Tukuringra and North Tukuringa faults to the east. Vertical shearing is predominant along the Dzheltulak fault and the western segment of the North Tukuringra fault. The NNE-trending compression takes place in the area located east of the quiescence zone of the Dzhagda ridge. Along the Mongolia-Okhotsk fault system, near the Sea of Okhotsk, the direction of compression changes to the northward one. The tectonic stress field along the Tanlu fault zone is inhomogeneous and comprises the alternating zones of horizontal compression and stretching with varying directions of the main stress axes. To the east of the band characterized by the maximum seismic activity, compression changes its direction to the southeast- and eastward. Probably, the impact of the oceanic subduction on the northern part of the Japan-Korean block begins to manifest itself in this part of the Amur region. The tectonic stress field reconstructed from the seismological data is consistent with the measurements of the modern crustal movements. The results of our study can prove useful for clarifying the tectonics of the region.

Study on the relationship between multi-stage strike-slip mechanism and basin evolution in Fangzheng fault depression

Fangzheng fault depression is controlled by the northern of the Tan-Lu fault zone. It undergoes multi-stage strike-slip, extrusion modification, and erosion of the thermal uplift, forming a tectonic pattern of uplifts connected with sags. Through the regional dynamic analysis, the study of the activity law of the western Pacific plate has clarified the formation and transformation of the regional tectonic stress field. Under the background of the multi-stage of the strike-slip mechanism in the northern part of the Tan-lu fault, the Fangzheng fault depression has a characteristic of the “left-lateral strike-slip pull-apart basin, right-lateral strike-slip extrusion transformation.” According to the difference of the strike-slip, the Fangzheng fault depression has divided into two parts: the East fault depression and the West fault depression. The seismic data, seismic attribute analysis, and geological modeling techniques have applied to analyze the two fault depressions, the East fault depression has actively controlled by the strike-slip activity, and the structure is complex. The seismic data quality is poor; the structure of the West Fault Depression is the opposite and structural characteristics of asymmetrical difference strike-slip in the East and West fault depressions. Interpretation of seismic sections through a slippery background, the strike-slip attributes of the whole fault depression from south to north are segmented, and the strike-slip mechanism from east to west is different. Under the control of the multi-stage strike-slip mechanism, the Fangzheng fault depression is divided into six stages of strike-slip evolution, corresponding to the six different stages of the strike-slip control basin, the formation process of the asymmetric difference strike-slip fault basin is clarified, which provides a reference for the study of the strike-slip pull-apart basin with multi-stage structure.