Tancheng-Lujiang Fault Research Articles

Three-Dimensional Electrical Structure and Metallogenic Background of the Southeastern Hubei Ore Concentration Area

The Southeastern Hubei Ore Concentration Area (SHOCA) is located in the west section of the Middle and Lower Yangtze River Metallogenic Belt in China, and it is a significant copper and iron mining region in China. Here, 117 pieces of magnetotelluric array data were used to obtain a three-dimensional resistivity model of the SHOCA and to investigate the relationship between the deep electrical features and the genesis of mineral deposits. The model shows that the Qinling-Dabie Orogenic Belt exhibits high-resistivity characteristics, representing Mesozoic granites and high-pressure to ultra-high-pressure metamorphic rocks. There are several low-resistivity anomalies in the upper crust of the SHOCA, which are connected to the widespread low-resistivity anomaly in the middle-lower crust. Near the Yangxin-Changzhou Fault, there is evidence of an electrical gradient zone. The Xiangfan-Guangji Fault, located at the south margin of the Qinling-Dabie Orogenic Belt, also exhibits distinct high- and low-resistivity boundaries at the upper crust. However, the Yangtze Fault and the Tancheng-Lujiang Fault manifest as resistivity gradient zones at the lithospheric scale. These faults are connected the low-resistivity anomaly in the middle to lower crust, possibly serving as upwelling channels of deep thermal fluids, exerting control over shallow diagenesis and mineralization processes. The low-resistivity anomaly in the middle to lower crust of the SHOCA is explained as partial melting resulting from the mixing of crustal and mantle materials. These low-resistivity anomalies play a role as source components in the mineralization system, where mineral-rich hydrothermal fluids migrate upward along intra-basin faults, exerting control over the distribution of shallow mineral deposits.

Geometry and Kinematics of Northmost Yilan-Yitong Fault Zone, China: Insights from Shallow Seismic Data and Field Investigation

Detailed geological and geomorphological evidence has suggested that the Yilan-Yitong fault (YYF), one of the key branches of the Tancheng-Lujiang fault zone in northeastern China, has been an active fault since the Holocene that has extended from Liaoning Province to far-eastern Asia. However, there are no clear fault traces or late Quaternary active features northeast of Tangyuan County. In this study, we carried out shallow seismic reflection exploration, field geological investigation, and trench excavation across the YYF north of Tangyuan. The results revealed that the YYF is composed of two main branches: the west YYF branch is a late Pleistocene active fault, and the east one is a middle-to-early Pleistocene fault. In Heli Town, the west branch of YYF presents fault scarps with heights of ~0.6 m. Across the scarps, we excavated a trench, and we propose that the YYF displaced the late Pleistocene to Holocene deposits, as this was indicated by the geochronological data. The seismic reflection data and sedimentary sequence revealed that the YYF north of Tangyuan is composed of three tectonic belts: the western depression, the central bulge, and the eastern depression. Each tectonic belt is composed of several small folds formed from the end of the Paleogene to the beginning of the Neogene. After the Neogene, different subsidence and uplift events occurred in various parts of the YYF, and after the early Pleistocene, the fault showed a consistent subsidence.

General features of multi-parameter anomalies of six moderate earthquakes occurred near Zhangbei-Bohai fault in China during the past decades

The great Zhangbei-Bohai fault (ZBF) is an important seismic active zone in Northeastern China, which intersects with another great fault, namely Tancheng-Lujiang fault (TLF), in the Central Bohai Sea. Most of the previous studies only investigated the earthquakes (EQs) happened in the west section of ZBF and anomalies appeared in the land area, but few researchers focused on the EQs happened in the east section of ZBF and studied the anomalies appeared above the Bohai Sea. Recently, we have investigated the disturbances on and above the Bohai Sea associated with the Zhangbei Mb5.6 EQ in 1998, whose epicenter located in the west section of ZBF, and found that there was seismicity induced gas release from submarine TLF. To study further the multi-parameter anomalies of EQs near ZBF during past decades and their relationship with ZBF and TLF, a scrutiny strategy for seismic anomaly in the sea area was proposed in this study. After comprehensive screening of EQs occurred near ZBF from 1998 to 2022 using multi-source remote sensing data, potential seismic anomalies including the anomalies of microwave brightness temperature and sea surface roughness on Bohai Sea were found correlated with 6 of the 23 investigated EQs, and there were commonly anomalies of CO and CH4 concentration in the atmosphere. All these seismic anomalies were found developing mainly along the TLF in Bohai Sea, and started usually before shocking, appeared co-seismically and in the next few days. Regional inconsistencies among the distribution of CO and CH4 anomalies could be owning to the specific underwater reservoir in the Bohai Bay Basin. Though analyzing the focal mechanism solutions of the selected EQ cases, we found that the seismic anomaly existed only when the TLF was in a stress state of tensile relaxation.

Multiple-Level Tectonic Control of Coalbed Methane Occurrence in the Huaibei Coalfield of Anhui Province, China

The Huaibei coalfield is an important coal base and one of the hot spots of coalbed methane development in China. Therefore, a detailed understanding of gas occurrence in the Huaibei coalfield is of great significance. This paper analyzes the gas occurrence from the perspective of multiple-level tectonic control, i.e., the regional tectonic level, the coalfield tectonic level, the mining area tectonic level, and the coal mine tectonic level. This study deduces that gas occurrence in the Huaibei coalfield is characterized by multiple-level tectonic control. At the regional level, the Huaibei coalfield is located in the southeast margin of the North China plate, affected by the tectonic evolution of the North China plate and by the evolution of the Dabie–Tanlu–Sulu orogenic belt. Therefore, the regional geological tectonic is complex, leading to the high gas content and serious gas hazard. At the coalfield level, gas occurrence in the Huaibei coalfield is controlled by east–west faults, NNE faults, and the Xuzhou–Suzhou arc nappe tectonic, which results in the highest gas occurrence in the Suxian mining area, followed by the Linhuan mining area and the Suixiao mining area, while the lowest amount of gas occurs in the Guoyang mining area. At the mining area level, considering the Suxian mining area as an example, the gas occurrence is controlled by the distance from the Tancheng–Lujiang fault zone and the intensity of tectonic compression, i.e., coal mine gas in the east is the highest, followed by coal mines in the south, while coal mine gas in the west is the lowest. At the coal mine level, gas occurrence is controlled by the buried depth of the coal seam, the tensional normal fault, magmatic activity, and uplift and erosion of strata. Finally, the findings of this study may help in the prevention of gas hazard and the exploration and development of coalbed methane in the Huaibei coalfield and other coalfields of similar geological characteristics.

Very-Short-Term Variations of Sea Surface and Atmospheric Parameters Before the Ms 6.2 Zhangbei (China) Earthquake in 1998

The Ms 6.2 Zhangbei earthquake occurred at 3:53 (UTC) on 10 January 1998, with its epicenter located at about 150 km northwest of Beijing, China. Over the past 2 decades, many studies have reported that there was a positive thermal infrared (TIR) anomaly appearing along the direction from the Bohai Sea to Zhangbei within 2–3 days preceding the earthquake, which was considered to be caused by the activity of the great Zhangbei-Bohai fault (ZBF) extending across the Bohai Sea to the southeast, while neither the TIR anomaly is convinced nor the mechanism is clear. A collaborative analysis of the atmospheric disturbances several days before, during, and after the earthquake was conducted by using satellite observations and reanalysis datasets with multiple parameters, including sea surface roughness, evaporation rate, atmospheric CO concentration, atmospheric sea salt concentration, and cloud base height above the sea surface, as well as satellite infrared cloud images. Through individual analysis of the change of each parameter and synergic analysis of multiple parameters, particular atmospheric disturbances, including the formation of strip-shaped clouds on January 7 and 9, were revealed over the ZBF and another great fault named Tancheng-Lujiang fault (TLF), which extends across the Bohai Sea to the northeast. After careful investigation and attribution analysis of the spatio-temporal evolutions of the atmospheric disturbances every hour above and around the Bohai Sea from January 7 to 12, we came to the conclusion that the particular strip-shaped clouds were low-level clouds caused by the seismic activity and submarine gas release from TLF but not ZBF and was forced by particular wind field and lowering boundary layer. As an aftereffect of the gas release from TLF and the formation of the localized low-level clouds of higher brightness temperature than that of the land surface, a positive TIR anomaly, thus, appeared above TLF and by chance along ZBF, preceding the Zhangbei earthquake.

Petrogenesis of Cretaceous granitoids in the Bengbu–Wuhe area, southeastern North China Craton: Implications for gold mineralization

Sinistral slip along the Tancheng–Lujiang Fault Zone (TLFZ) during the Mesozoic resulted in an offset of approximately 400 km between the Dabie and Sulu orogens. The Bengbu–Wuhe gold belt (BWGB) represents an extension of the Zhaoyuan–Yexian gold belt to the west of the TLFZ. Early Cretaceous intrusions are widely distributed throughout the BWGB. Gold deposits in the BWGB are commonly associated with intermediate acid dykes that are roughly contemporaneous with mineralization, and some orebodies are hosted in granitoids. This paper investigates granitoids within the BWGB to constrain their petrogenesis, tectonic setting, and metallogenic significance using systematic whole-rock elemental geochemistry, apatite geochemistry, zircon U–Pb dating, zircon Lu–Hf isotopes, and whole-rock Sr, Nd, and Pb isotopes. The timing of Au mineralization (134–113 Ma) in the area is consistent with the age of intrusive adakite-like rocks (130–115 Ma) and normal calc-alkaline granitoids (114–109 Ma). The adakite-like samples are characterized by pronounced enrichment in light rare earth elements (REE), depletion in heavy REE ((La/Yb)N = 30.5–66.2), an absence of Eu anomalies, high Sr contents and Sr/Y ratios, low Y and Yb contents, and fractionated REE patterns ((Dy/Yb)N = 1.22–1.69). They have relatively low Mg# values (38–42) and variable Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sr(t) = 0.7083–0.7111; εNd(t) = −22.2 to −14.9; 206Pb/204Pb(t) = 17.219–18.010; zircon εHf(t) = −21.0 to −12.3). These features indicate that the parental magmas of the adakites were derived from partial melting of thickened lower crust of the South China Plate and/or the North China Craton. The normal calc-alkaline granitoids have low Sr/Y (6.0–40.5), La/YbN (6.8–24.7), and Dy/YbN (0.90–1.28) ratios, exhibit negative Eu anomalies, and have variable Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sr(t) = 0.7082–0.7085; εNd(t) = −22.7 to −17.7; 206Pb/204Pb(t) = 16.686–16.713; zircon εHf(t) = −24.9 to −19.0), indicating they were derived from the middle to lower crust of the North China Craton. Compared with the normal calc-alkaline granitoids, the adakite-like rocks have higher zircon Ce4+/Ce3+, higher apatite SO3 and lower apatite MnO contents, lower and more variable zircon crystallization temperatures, and higher apatite molar OH/F and Cl/F ratios, indicating that the adakitic magmas were more enriched in volatiles and more highly oxidized. Taken together, field observations and geochemical data suggest that the adakite-like rocks related to the subducted South China Block are the ore-causative rocks.

Velocity structure of the upper crust and its correlation with earthquake swarms activity in Laizhou Bay and its adjacent areas, China

This study used the double-difference seismic tomography method to obtain the upper crustal velocity structure in Laizhou Bay and its adjacent areas (LBAA). We found that the horizontal and vertical distributions of the earthquakes after relocation in earthquake swarms are more concentrative than that before relocation. The lateral heterogeneity of crustal velocity structure near the surface in the LBAA is weak, and the low-velocity sedimentary caprocks are developed in most areas except for the Jiaobei uplift. The velocity structure in 5–25 km depth range is highly heterogeneity, which is closely related to different tectonic units. The local uplift tectonic units in Jiyang depression show high-velocity anomalies and should have deep crustal media properties. The high-velocity anomalies under the crust of Tancheng–Lujiang fault zone and the northern Laizhou Bay indicate the properties of upwelling mantle material media. Earthquake swarms activity in the study area are mainly concentrated in the high-velocity interlayer of the upper crust and its periphery. Compared with the relatively stable high-velocity body at the bottom crust, the location of high-velocity interlayer in the study area is more prone to earthquakes under the combined action of gravity and tectonic stress.

Lithospheric extension associated with slab rollback: Insights from early Cretaceous magmatism in the southern segment of Tan-Lu fault zone, central-eastern China

The eastern China region underwent an intense lithosphere thinning event during Mesozoic but the mechanism for this process remains controversial. Here we investigate the Early Cretaceous Guandian intrusive complex in southern segment of the Tancheng-Lujiang (Tan-Lu) fault zone, central-eastern China, to gain insights into lithospheric extension and thinning. We present zircon UPb ages and LuHf isotope data, together with whole rock major and trace elements, and SrNd isotopes. The major rock types in this igneous complex are high-K calc-alkaline granodiorites, diorites and lamprophyre dikes. LA-ICP-MS zircon UPb dating yielded emplacement ages of 128 ± 2 Ma, 131 ± 2 Ma and 130 ± 1 Ma, for lamprophyre dikes, diorites and granodiorites respectively. These ages coincide with the commencement of lithospheric extension and thinning in eastern China. The elemental and isotopic characteristics of the lamprophyre dikes suggest that they were derived from partial melting of amphibole-bearing lithospheric mantle that was metasomatized by subducted continental crust and slab-derived fluids. Petrographic evidence, as well as geochemical and SrNd isotope characters suggest that the dioritic and granodioritic magmas were likely derived through magma mixing between partial melts of ancient lower crust and enriched lithospheric mantle. In conjunction with the coeval regional magmatism, mineralization and tectonic setting, we correlate the genesis of Guandian complex with crust-mantle interaction, and the extensional tectonic regime is correlated to slab rollback of the Paleo-Pacific Plate.

Liquefaction structures induced by the M5.7 earthquake on May 28, 2018 in Songyuan, Jilin Province, NE China and research implication

An earthquake of magnitude M5.7 occurred in Yamutu village, Songyuan City, Jilin Province, NE China (45°16′12″N/124°42′35″E) on May 28, 2018, with a focal depth of 13 km. The epicenter is located at the intersection of the Fuyu/Songyuan-Zhaodong Fault, Second Songhua River Fault and Fuyu North Fault which lies northwest of Tancheng-Lujiang Fault (Tan-Lu Fault). The earthquake-induced widespread liquefaction structures and ground surface fissures within 3 km from the epicenter, caused serious disasters to the local surroundings. The visible liquefied structures include sand volcanoes, liquefied sand mounds, sand dikes and sand sills. Sand volcanoes can be divided into sand volcano with a crater, sand volcano without a crater and water volcano (no sand). Other soft-sediment deformation structures (SSDS) induced by the earthquake include deformation lamination, load and flame structures, deformation folds, dish structures, convolute bedding and water-escape structures. The formation process of the sand volcanoes comprises three stages: (1) building up excess pore-fluid pressure in the liquefied layer, (2) cracking of the low-permeable overlying layer, and (3) mixture of sand-water venting out of the ground surface. During the upward movement, the liquefied sand is injected into the low-permeable layer to form sand veins, sand sills and various types of deformation structures. Vertical distribution of seismic liquefaction structure can be divided into four zones: the thoroughly liquefied zone, the lower liquefied zone with SSDS, the upper liquefied zone with SSDS, and the ground surface liquefied zone. The liquefaction occurred at a burial depth of 2–5 m, and the thickness of liquefied sand is 2 m. NE-SW (35°–215°) trending compressive stress is possibly the seismogenic trigger of the Songyuan M5.7 earthquake that caused the fault (Fuyu/Songyuan-Zhaodong Fault) to reactivate. The study of the Songyuan seismic liquefaction structures gives insight into the prediction of modern earthquakes and disaster-prone areas. Meanwhile it provides abundant basic material for studying earthquake-induced SSDS in both ancient and modern sediments. The research is obviously of great significance to reveal that the northern Tan-Lu Fault has entered a stage of active seismic activity since the twenty-first century.

Generation of leucogranites via fractional crystallization: A case study of the Jurassic Bengbu granite in the southeastern North China Craton

The magmatic processes that form leucogranites are closely related to plate collision. As the products of crustal anatexis, leucogranites provide clues to intracrustal differentiation and the regional tectonic evolution. The Bengbu area is located along the southeastern margin of the North China Craton (NCC), 150 km north of the Dabie Orogen, and is bounded by the Tancheng–Lujiang Fault Zone to the east. Late Jurassic leucogranites are widely distributed in the Bengbu area. We collected samples of the Bengbu leucogranite (BLG) and analyzed data on whole-rock geochemistry, zircon U–Pb geochronology, zircon in situ Lu–Hf isotopes, quartz in situ O isotopes, and total bulk Sr–Nd–Pb isotopes. Zircon U–Pb dating yielded formation ages for these leucogranites of ∼160 Ma. The samples are characterized by high SiO2 (72.4 wt.%−76.9 wt.%), alkali (7.3 wt.%−9.4 wt.%), and Al2O3 (13.4 wt.%−15.5 wt.%) contents. In addition to two samples exhibiting the rare-earth element tetrad effect, all rocks have high differentiation index values (95–99), extremely low iron–magnesium–titanium oxide contents (total Fe2O3 < 1.4 wt.%, MgO < 0.3 wt.%, TiO2 < 0.2 wt.%), and low Zr/Hf (20–50) and Nb/Ta (5–25) ratios. These data suggest that the BLG is a highly fractionated granite. Furthermore, the samples contain Neoproterozoic and Triassic zircons, have low Ti-in-zircon temperatures (682–697 °C), low Rb contents (<192 ppm), high Sr contents (48–791 ppm), low Rb/Sr ratios, low radiogenic Pb isotopic compositions (206Pb/204Pb(t) = 17.169–17.297), and highly enriched Sr–Nd–Hf isotopic compositions (87Sr/86Sr(t) = 0.7082–0.7104, εNd(t) = −16.6 to −12.3, and zircon εHf(t) = −14.5 to −21.7). Taken together, the geochronological and geochemical results suggest that the BLG was formed by high-degree fractional crystallization after hydrous partial melting of the North Dabie granitic gneisses, which have been subducted beneath the Bengbu area by Triassic continental subduction. Following collision between the South China Block and the NCC, Late Jurassic granitoids in the eastern NCC, including the BLG, were generated in the resulting post-orogenic extensional environment and were also influenced by ongoing subduction and rollback of the Izanagi Plate.

Initiation Timing of the Jiamusi–Yitong Fault Zone in NE China

The NE–striking Jiamusi–Yitong fault zone (JYFZ) is the most important branch in the northern segment of the Tancheng–Lujiang fault zone. The precise shearing time of its large–scale sinistral strike–slip has yet to determined and must be constrained. Detailed field investigations and comprehensive analyses show that strike–slip faults or ductile shear belts exist as origination structures along the western region of Yitong Graben. The strike of the shear belts trend to the NE–SW with steep mylonitic foliation. The zircon U–Pb dating result for the granite was 264.1±1 Ma in the ductile shear belt of the JYFZ. The microstructural observation (rotated feldspar porphyroclasts, S–C fabrics, and quartz c–axis fabrics, etc.) demonstrated the sinistral shearing of the ductile shear zones. Moreover, the recrystallized quartz types show a transitional stage of the subgrain rotation toward the recrystallization of the grain boundary migration (SR–GBM). Therefore, we suggest that the metamorphic grade of the shear zone in the ductile shear zones should have reached high greenschist facies conditions, and the deformation temperatures should approximately 450–500°C, which is obviously higher than the blocking temperature of muscovite (300–400°C). Hence, the 40Ar /39Ar isochron age of muscovite from ductile shear zones should be a cooling age (162.7±1 Ma). We infer that the sinistral strike–slipping event at the JYFZ occurred in the late Jurassic period, and it was further inferred from the ages of the main geological events in this region that the second sinistral strike–slip age of the Tancheng–Lujiang fault zone occurred during the period of tectonic movements in the Circum–Pacific tectonic domain. This discovery also indicates the age of the Tancheng–Lujiang fault zone that stretches to northeastern China. The initiation of the JYFZ in the late Jurassic is related to the speed and direction of oblique subduction of the west Pacific Plate under the Eurasian continent and is responsible for collision during the Jurassic period.

Zircon U-Pb ages of the cetaceous sedimentary rocks in the Laiyang Basin, eastern China and their tectonic implications

The Laiyang Basin is located in the northern margin of the Sulu high-pressure and ultrahigh-pressure (HP-UHP) metamorphic belt. The sediment source and depositional time of the Laiyang Basin can provide important insights into the exhumation mechanism of the Sulu Orogen and the formation of the Tancheng-Lujiang (Tan-Lu) Fault, but have not been well constrained. In this study, seven samples were collected from typical sections of the Laiyang and Qingshan groups in the southern margin of the Laiyang Basin in the Sulu Orogen. LA-ICPMS U-Pb dating of detrital zircon from these samples yielded ages from 3145 Ma to 115 Ma and can be divided into four groups: (1) 3145–1728 Ma, which indicate a provenance from the Precambrian basement rocks of the North China Craton; (2) 908–522 Ma, which are consistent with the ages of the protolith of the Sulu HP-UHP rocks; (3) 240–220 Ma, which are identical to the ages of the HP-UHP metamorphism. This suggests that the HP-UHP rocks of the Sulu Orogen had been exposed to the surface and contributed to the deposition of the Laiyang Group by the Early Cretaceous; (4) 148–115 Ma (weighted mean age: 125 ± 0.5 Ma; MSWD = 1.2), which, as the main group, suggests that the major source of the sediments in the Laiyang Group was from Early Cretaceous igneous rocks. These young ages, combined with the ages of 119 ± 1 (MSWD = 0.7) Ma determined from the overlain Qingshan Group, confine the depositional age of the Laiyang Group to 119–125 Ma. Further comparison of detrital zircon U-Pb age spectrum between the Laiyang Basin and the Hefei Basin indicates that these two basins were developed independently, rather than developed from single basin and then truncated by the Tan-Lu Fault as suggested by previous studies. Based on the formation age of the Laiyang Basin and other age constraints, we further suggest that the Tan-Lu Fault was formed during the Late Triassic to Early Jurassic. The Mesozoic geological settings in the Sulu Orogen, including the sediments of the Laiyang Basin, Early Cretaceous magmatic intrusive and detachment activities of the Wulian Fault, imply that the Cretaceous exhumation of the Sulu HP-UHP belt was consistent with a metamorphic core complex model.

Late Quaternary paleoseismicity and seismic potential of the Yilan-Yitong Fault Zone in NE China

The Yilan-Yitong Fault Zone (YYFZ), which is composed of two nearly parallel branches with a spacing of 5–30 km and a length of ∼1100 km, is considered to be the key branch of the Tancheng-Lujiang Fault Zone (TLFZ) in NE China. It was traditionally believed that the YYFZ experienced weak activity or was inactive during the Late Quaternary, without the capability to generate strong earthquakes (M ≥ 7), based on the absence of typical outcrops and large historical or instrumental earthquakes (M > 6). However, our paleoseismic study shows that the YYFZ is the primary seismotectonic structure (M ≥ 7) that poses significant earthquake threats to NE China. The synthesis of data collected from geologic investigations, geomorphic mapping, trench logging and the dating of samples indicates that the YYFZ is an active structure that has undergone segmented strong tectonic deformation since the Late Quaternary with a characteristic assemblage of landforms, including linear scarps and troughs, offset or deflected streams, linear sag ponds, small horsts and grabens. The latest ruptures of the YYFZ migrated from previous boundary faults into the basin interior, forming a left-stepping en echelon pattern in plain view, and the kinematics of these events in the Late Quaternary were dominated by reverse dextral slipping. Multi-segment cluster faulting might have occurred during three cluster periods, i.e., ∼34750–35812 a BP, ∼21700–22640 a BP, and ∼4000 a BP-present, which implies that the recurrence interval of large earthquakes along the YYFZ may be as long as tens of thousands of years.

Moho imaging based on receiver function analysis with teleseismic wavefield reconstruction: Application to South China

A detailed and reliable Moho structure can improve our understanding of the tectonic evolution of crust and mantle. Most researchers directly interpolate the imaging results of receiver functions (RFs) from the sparse stations to obtain a continuous Moho image. However, the interpolation process is usually not robust because of the sparse and irregular geometry of seismic networks. In light of the approximate plane-wave feature of the teleseismic wavefront, we introduce the Radial Basis Function (RBF) technique, a high-precision interpolator with strong adaptability, into teleseismic wavefield reconstruction. Then, we use the reconstructed wavefield to image the Moho structure beneath South China with RF analysis. Compared to the conventional H-κ stacking methods, the Moho that is obtained through this reconstructed wavefield exhibits more details and shows greater consistency to the tectonic provinces. The Moho depths beneath South China gradually shallow from ~50km at the Yangtze Block (YB) in the northwest to ~25km to the southwest of the Cathaysia Block (CB). The Vp/Vs ratio at the YB is ~1.80, decreasing to ~1.57 at the Jiangnan Orogen, and then increasing to ~1.70 at the CB. Obviously varying Moho depths and Vp/Vs ratios in both the Ganjiang Fault (GF) and Tancheng-Lujiang Fault (TLF) regions is observed, which indicates a great similarity in the deep tectonic environment of the GF and TLF. The mantle corner flow under South China is suspected to invade into the interior of the Yangtze Craton. Teleseismic wavefield reconstruction, as a pre-processing technique, can effectively improve the imaging quality of RF analysis.

Structural characteristics of the Yilan–Yitong and Dunhua–Mishan faults as northern extensions of the Tancheng–Lujiang Fault Zone: New deep seismic reflection results

The Tancheng–Lujiang Fault Zone (TLFZ) can be subdivided into three segments that exhibit sharp contrasts in their deep structures. A deep seismic reflection profile (length ~600km) across the north part of the TLFZ, which provides new constraints on the structural styles of the northern TLFZ, was recently completed by the Chinese Sinoprobe Project. Here, the TLFZ branches into the Yilan–Yitong Fault (YYF) to the west and the Dunhua–Mishan Fault (DMF) to the east. The YYF developed as an internal fault in the Songnen–Zhangguangcai massif, while the DMF serves as the tectonic boundary between the Nadanhada terrane and the Khanka massif. Both faults developed large-scale flower structures, with that of the YYF being negative and that of the DMF being positive with reverse faults. The Moho in the profile is at a depth of 25–39km and is offset by the faults. The north part of the TLFZ extends into the upper mantle as thin shear zones with the reflectors truncated in the middle/lower crust. This feature differs from most crustal-scale strike-slip faults that distribute over a discrete shear zone in the lower crust, such as the San Andreas Fault.

Paleomagnetism and U-Pb geochronology of the late Cretaceous Chisulryoung Volcanic Formation, Korea: tectonic evolution of the Korean Peninsula

Abstract Late Cretaceous Chisulryoung Volcanic Formation (CVF) in southeastern Korea contains four ash-flow ignimbrite units (A1, A2, A3, and A4) and three intervening volcano-sedimentary layers (S1, S2, and S3). Reliable U-Pb ages obtained for zircons from the base and top of the CVF were 72.8 ± 1.7 Ma and 67.7 ± 2.1 Ma, respectively. Paleomagnetic analysis on pyroclastic units yielded mean magnetic directions and virtual geomagnetic poles (VGPs) as D/I = 19.1°/49.2° (α 95 = 4.2°, k = 76.5) and VGP = 73.1°N/232.1°E (A 95 = 3.7°, N = 3) for A1, D/I = 24.9°/52.9° (α 95 = 5.9°, k = 61.7) and VGP = 69.4°N/217.3°E (A 95 = 5.6°, N = 11) for A3, and D/I = 10.9°/50.1° (α 95 = 5.6°, k = 38.6) and VGP = 79.8°N/242.4°E (A 95 = 5.0°, N = 18) for A4. Our best estimates of the paleopoles for A1, A3, and A4 are in remarkable agreement with the reference apparent polar wander path of China in late Cretaceous to early Paleogene, confirming that Korea has been rigidly attached to China (by implication to Eurasia) at least since the Cretaceous. The compiled paleomagnetic data of the Korean Peninsula suggest that the mode of clockwise rotations weakened since the mid-Jurassic. Such interesting variation of vertical rotations in the Korean Peninsula might result from the strike-slip motions of major faults developed in East Asia (the Tancheng-Lujiang fault to the northwest and the Korea-Taiwan strait fault to the southeast), near-field tectonic forcing of the subducting Pacific Plate beneath the Eurasian Plate, and far-field expressions of the India-Asia collision.

Zircon U–Pb geochronological, geochemical, and Sr–Nd isotope data for Early Cretaceous mafic dykes in the Tancheng–Lujiang Fault area of the Shandong Province, China: Constraints on the timing of magmatism and magma genesis

The timing and source of magmatism that formed Early Cretaceous dolerite dykes in the Tancheng–Lujiang (Tan–Lu) Fault area of the southeastern North China Craton was determined using geochronological, geochemical, and whole-rock Sr–Nd isotopic data. Laser ablation–inductively coupled plasma–mass (LA–ICP–MS) spectrometry U–Pb analysis of zircon yielded consistent ages of 129.6±0.7, 126.8±0.7, 125.5±0.7, 124.9±0.9, 126.4±0.7, and 125.5±0.7Ma for six samples of the mafic dykes within the NCC. The K2O+Na2O concentrations (5.02–5.21wt.%) of the dykes indicate they are alkaline and these dykes have K2O concentrations (2.35–2.48wt.%) that indicate they are shoshonitic. These dolerites are also characterized by high and wide ranging (La/Yb)N (14.5–36.0), have slightly negative Eu anomalies (δEu=0.70–0.91) and positive Ba, U, K, and Pb anomalies, and are depleted in the high field strength elements (Nb, Ta, P, and Ti). In addition, these mafic dykes are characterized by high radiogenic Sr [(87Sr/86Sr)i=0.7099–0.7100] and negative εNd (t) values (−14.4 to −13.7). These data suggest that the magmas that formed the dykes were derived through the partial melting (12.0–15.0%) of an enriched region of the mantle that was hybridized during interaction with subducted sedimentary rocks from the Yangtze Craton. The parental magmas then fractionated olivine and Fe–Ti oxides during ascent and underwent negligible crustal contamination during magma emplacement. These mafic magmas were finally emplaced as dyke swarms associated with lithospheric extension.

Geophysical evidence on segmentation of the Tancheng-Lujiang fault and its implications on the lithosphere evolution in East China

The north–south trending Tancheng-Lujiang (Tanlu) fault belt extends from northeast China to the Dabie–Sulu orogenic belt, for a length of more than 3000km. This fault belt probably has close links with the lithosphere evolution, seismic activity and mineral resource concentration in East China. Surface geological mapping and studies on sedimentation and basin formation have indicated segmentation at the southern, middle and northern domains of the fault. Here we employ geophysical constraints to evaluate these fault segments. Unlike previous geophysical studies focused on laterally varying crust/mantle seismic velocity structure across the fault, in this study we have integrated a variety of geophysical data sets, such as crustal P-wave velocity, earthquake occurrence and released seismic energy, seismogenic layer thickness, surface heat flow and geothermal field, to understand the deep structure and strength of the lithosphere along the Tanlu segmented fault belt. The results demonstrate remarkable crustal-scale north-to-south segmentation this major fault. The geophysical evidence and some geochemical constraints suggest that the Tanlu fault belt probably served as a channel for melt and fluid percolation, and exerted a significant control on the lithosphere evolution in East China.

North and South China suturing in the east end: What happened in Korean Peninsula?

The history of basin formation and crustal deformation of the Korean Peninsula is here critically reviewed to show it is an important tectonic architecture of the collision between North and South China in Early Mesozoic. To synthesize our current geologic and paleomagnetic knowledge, we propose a new crustal detachment and extrusion model for the continental collision and rotation between the North and South China Blocks (NCB and SCB) east of the Tancheng–Lujiang fault. We emphasize that the Yellow Sea Transform Fault (YSTF) played a key role in accommodating the collisional history of the two blocks in the east end. The YSTF allowed the Korean Peninsula exempted from being directly involved in the NCB–SCB collision. During the Early Triassic collision phase of the two blocks, a large piece of the upper crust of the northeastern part of the SCB was detached and slumped over the Korean Peninsula and fragmentally deposited in the metamorphosed Okcheon Trough. In the course of the exhumation of the Sulu ultra-high-pressure (UHP) metamorphic belt in the Middle Triassic, the northern part of the SCB extruded eastward toward Korea. A general time correlation of the Songnim orogeny of Korea and the early exhumation of the Sulu UHP metamorphic belt suggests their genetic relation. The exhumation of the Sulu UHP metamorphic belt triggered the mid Triassic extrusion tectonics, which in turn facilitated the clockwise rotation of the SCB. The eastward extrusion and clockwise rotation of the SCB (relative to NCB) exerted a large amount of eastward compression to the Korean Peninsula and led to the Middle–Late Triassic Songnim orogeny. Geologic and paleomagnetic information synthesized in this paper has revived and reinforced the belief for the single Sino-Korean Craton which comprises Korea and north China since the Precambrian time.

Late Mesozoic–Cenozoic décollement structure and its deep geological background in western Shandong, China

Data from seismic reflection profiles, drilling, stratigraphy, structural deformation studies and physical rock properties reveal the existence of décol1ement structures in both shallow and deep levels in western Shandong, China. The most outstanding shallow décol1ement structures occur along the regional unconformity surface between the Cambrian and Archean, and the disconformity surface between the Carboniferous and Ordovician. The décol1ement structure surface manifests as a fault zone with cataclastic rocks and asymmetrical folds. Some of the cataclastic rocks underwent dynamic metamorphism and hydrothermal alteration, including silicification, marbleization and specularite mineralization. Above the décol1ement structure, the bottom of the Cambrian might be missing or overprinted because of décollement. The striations, asymmetrical folds and boudinage structures indicate the direction of the main décollement to the NNW and NNE. A deep level décollement structure occurs at a depth of 12–22 km and up to 30 km distance to the south. The early Cretaceous and Eocene are two main periods of activity, with the Cretaceous décollement probably initiated by mantle upwelling derived from subduction and collision of the Yangtze Plate with the North China Plate along the Tancheng-Lujiang Fault in the late Triassic–early Jurassic. This circumstance implies a multidirection of subduction and collision of these two plates in the early late Mesozoic.