Tan-Lu Fault Research Articles

Transformation between the Dabie Orogenic Belt and the Tan-Lu Fault zone: Insights from ENE-NE-trending gneiss belts at the tectonic node

How the Dabie Orogenic Belt (DOB) and the Tan-Lu Fault Zone (TLFZ) transformed during the early Indosinian period is the key to reveal the convergence process between the North China Block (NCB) and the South China Block (SCB). Tongcheng area in the eastern margin of northern DOB is the tectonic node that connects the WNW-trending DOB and the NE-trending TLFZ. The typical ENE-NE-trending gneiss belts penetrably developed in Tongcheng area provide an ideal natural lab to decipher the transformation between the DOB and the TLFZ. Here we conduct an integrated structural and geochronological research on the ENE-NE-trending gneiss. Zircon U–Pb dating on the ENE-NE-trending gneiss yielded metamorphic ages ranging from 255 ± 9 Ma to 203 ± 10 Ma. The weakly deformed veins which intruded the surrounding gneiss yielded two groups of ages, 825 ± 29 Ma to 713 ± 7 Ma and 125 ± 2.6 Ma (weighted mean age), which indicate the protolith age of surrounding rocks and the intrusive timing of the vein, respectively. Integrated structural, microstructural and kinematic analysis indicate that no lateral structural superposed on the gneiss or veins. Therefore, it can be speculated that the deformation and metamorphism of the gneiss should simultaneously formed during the Early Triassic, as a result of continuous tearing from the DOB to the northeastern Sulu Orogenic Belt (SOB) during subduction of the Yangtze block beneath the NCB. Formation of the tearing belts, i.e., the ENE-NE-trending gneiss belts, accomplished the tectonic transformation between the DOB and the TLFZ. They could be regarded as an embryonic form of the TLFZ, which are also apparently different from the TLFZ by characteristics of non-strike slipping.

Cenozoic extension to strike-slip transition in the Liaodong Bay Subbasin along the Tan-Lu Fault Zone, Bohai Bay Basin: New insights from stress field modelling

The NE-trending Liaodong Bay Subbasin (LDBS), a subunit of the Bohai Bay Basin (BBB), developed along the Tan-Lu Fault Zone (TLFZ) in eastern China. The LDBS is controlled by both extensional and dextral strike-slip fault systems and has an episodic evolutionary history, leading to a poor understanding of its tectonic origin. In this paper, through interpretations of seismic data and other geological and geophysical data, four best-fit finite element models were generated to reveal the tectonic development of the LDBS and the whole BBB. The deformation features predicted by the modelling results are consistent with the geological evidence. The modelling results revealed that the extension directions of the LDBS changed from WNW-ESE during the Paleocene-early Eocene, to NW-SE during the middle-late Eocene and then to NWN-SES or nearly N-S in the Oligocene and Miocene. The changes in the extension direction resulted in a strike-slip transition of the TLFZ in approximately the middle-late Eocene, from normal to dextral transtensional, and transformed the LDBS and the whole BBB from an extensional basin into a dextral transtensional basin. This transformation was likely triggered by the kinematic adjustment of the Pacific Plate from NNW to WNW in the middle-late Eocene, but was maintained and enhanced by the Indian-Asian collision during the rest of the Cenozoic.

Differential post-mineralization thermal evolution of the Jiaodong and Liaodong Areas, Eastern China: An indicator of regional tectonic activity

Post-mineralization thermal evolution is an integral part of the process of formation of ore deposits. However, as two important gold mineralization regions in eastern China, the thermal histories after ore formation of the Jiaodong area is debated while that of the Liaodong area are scarce. Given that the two areas are not only in similar tectonic settings but also have gold deposits that formed in association with the thinning and destruction of the North China Craton, a comparative study can be made. Here new zircon and apatite (U–Th)/He and apatite fission track ages are presented to reconstruct the post-mineralization geological evolution. Four periods of rapid uplift and denudation happening during the 110–100 Ma, 80–70 Ma, 42–38 Ma and 35–32 Ma are defined in Liaodong, and two such periods happening during the 100–95 Ma and 80–72 Ma in Jiaodong. The small timing difference of the rapid uplift (110–100 Ma versus 100–95 Ma) can be explained by the elevation difference of the respective samples. Hence, the evolution processes between the Jiaodong and Liaodong areas are very similar during the Cretaceous, but show marked differences during the Cenozoic. After ore-formation, the erosion amount in Liaodong and Jiaodong is estimated at ∼4.8 and ∼4.7 km, respectively. Moreover, the denudation intensity in Liaodong shows an increase from southeast to northwest. The westward convergence of the Pacific Plate and associated multi-stage activity of the Tanlu and Yalujiang faults are inferred to have induced the regional-scale rapid uplift of the Jiao-Liao areas.

Crustal structure and deformation in southeastern China revealed by receiver functions

• A V-shaped zone with high Vp/Vs ratio and thin crust observed by receiver functions. • Most stations located near the V-shaped zone exhibit significant crustal anisotropy. • Hot fingers grow from the mantle corner flow and result in Moho uplift in SE China. • Mantle corner flow might be a main reason for crustal thinning in the V-shaped zone. The subduction of the Paleo-Pacific and Philippine plates generates large-scale magmatism and crustal thinning in southeastern (SE) China. However, the dynamic process for crustal thinning in this region is still not well understood. In this study, a large number of receiver function data were used to investigate the crustal thickness and Vp/Vs ratio beneath SE China by the H-κ stacking technique. We also apply a comprehensive analysis method on the Moho Ps converted wave in radial and transverse receiver functions to calculate crustal seismic anisotropy with fast polarization direction and splitting time. The measured results show that the high Vp/Vs ratio (∼1.85) was mainly spread around two areas, forming a “V-shaped” zone. The western branch of the “V-shaped” zone was NS-oriented with a thin crustal thickness of ∼29.1 km beneath the Nanxiang-Jianghan basin. The eastern branch extended along the Tanlu fault with a thin crustal thickness of ∼28.8 km. Moreover, the measured crustal seismic anisotropy also shows that most stations located near the “V-shaped” zone exhibit significant crustal anisotropy with splitting times of 0.20–0.72 s. The fast directions of these stations are parallel to the direction of the V-shaped extension. Combining the geological data, we propose that the “hot fingers” may grow from mantle corner flow in the southern part of the V-shaped zone associated with the collision of the Pacific and Philippine plates. We deduce that the V-shaped crustal thinning and Moho uplift in SE China might be attributed to this mantle corner flow.

Genesis of High-Mg Adakites in the Southeastern Margin of North China Craton: Geochemical and U-Pb Geochronological Perspectives

In the eastern North China Craton (NCC), Mesozoic tectonics was dominated by the Paleo-Pacific subduction rollback and the Tanlu crustal-scale fault movement. The regional transtension had generated extensive adakitic magmatism, some Cu-Au ore-forming but others not. To establish the geodynamic setting and any metallogenic link for the adakites from the southeastern NCC margin, we analyzed the ore-barren adakitic rocks from underground mines in the Huaibei-Linhuan coalfield (where surface igneous outcrops are scarce), and compared their ages and geochemistry with other mineralized and ore-barren adakites across Eastern China. Zircon U-Pb dating reveals two magmatic episodes in the Huaibei-Linhuan coalfield: 1) early-Early Cretaceous (ca. 130–129 Ma) (quartz-)diorite and granodiorite, and 2) late-Early Cretaceous (ca. 115.8 and 105.8 Ma) microgabbro and dolerite. Whole-rock geochemistry indicates that the (quartz-)diorite and granodiorite are high-Mg adakitic, featured by low K2O/Na2O (avg. 0.33), high Sr/La (avg. 44.3), and lack of correlation between SiO2 (fractionation index) and Sr/Y (avg. 56.55) and MREE/HREE (avg. 1.09), resembling typical adakites derived from oceanic-slab partial melting. Geochronological correlation with the regional tectonic events suggests that the slab-melting may have been caused by the Paleo-Pacific subduction rollback. Further extension and crustal thinning in the late-Early Cretaceous along the southern Tanlu fault may have formed the gabbro-dolerite in the coalfield. Geochemical comparison suggests that parental magma of the Huaibei-Linhuan adakites may have had similar water content [similar zircon 10,000*(Eu/Eu*)/Y and Eu/Eu* ratios] to typical porphyry Cu-Au ore-forming magmas, yet the former may have been considerably more reduced (lower zircon Ce/Nd and whole-rock V/Sc ratios). We considered that the assimilation of Carboniferous-Permian coal seams in the area may have further lowered the magma fO2 and thus its potential to form Cu-Au mineralization.

Study on the crustal velocity structure of the Dunhua-Mishan fault based on the dense seismic array by ambient noise tomography

The Dunhua-Mishan fault is a component of the Tanlu fault zone, and it is also a major regional fault structure in Liaoning area. The study area is located in Fushun area of the Dunhua-Mishan fault. In order to study the crustal velocity structure in this region, we used 60 sets of three-component short-period seismographs and lay out two temporary profiles in Fushun, Liaoning from Nov. 3 to 25, 2019. Each temporary profiles is about 12km long that is composed of 30 seismometers, which nearly vertically cross the Dunhua-Mishan fault where the survey lines are 6km apart. The installation stations spacing distance from 200 to 400m use the external GPS time service to record frequency range of 0.2 to 150Hz in the instrument. The crustal structure characteristics of the fault zone are studied by ambient noise cross-correlation method. In order to efficiently extract high and low frequency surface wave dispersion signals at the same time, we adopt the Extended Range Phase Shift (ERPS) method. Finally, we use ERPS method to process the collected data and invert the S-wave velocity structure. We initially obtained the S-wave velocity structure of the upper crust below 5 km in the area, which matches well with the local geological data. The S-wave velocity structure has significant lateral inhomogeneity of the Dunhua-Mishan fault in Fushun area. The S-wave velocity is obviously lower on the Dunhua-Mishan fault, while with the existence of ancient mixed granite the velocity on both sides of the fault is obviously higher.

Late Cretaceous to Early Cenozoic extension in the Lower Yangtze region (East China) driven by Izanagi-Pacific plate subduction

The intracontinental response to plate-boundary and mantle processes, such as subduction or continental collisions, is an important topic in the Earth Sciences. Cretaceous to Cenozoic extension across East China is related to the interaction between the Asian continental lithosphere and western Pacific oceanic subduction system, and the associated intra-plate structures and basins provide robust temporal and spatial records of regional continental deformation. Although extension may primarily be driven by slab rollback and/or back-arc opening, mid-ocean-ridge subduction may have played a significant role in this process. To explore how the western Pacific subduction system impacts intracontinental deformation across East China, we use onshore-offshore geological records, geophysical survey observations, and time-dependent Pacific plate models to document the temporal and spatial variation of continental deformation in the Lower Yangtze region adjacent to the Yellow Sea. Based on our compilation, we argue that mantle-wedge convection, with coupled subduction of the trench-parallel Izanagi (Paleo-Pacific)-Pacific mid-ocean ridge, exerts a first-order control on extensional deformation in East China. Beneath eastern China, the mantle wedge history consists of two phases: (1) initial development of the mantle wedge as the Cretaceous Izanagi slab transitioned from flat-slab to steep subduction; and (2) mature mantle wedge convection during Cenozoic subduction of the Pacific plate. Relatively steep subduction of the progressively younging Izanagi plate during the Late Cretaceous-Paleocene resulted in strong mantle convection and coupled upper plate extension in eastern China. The collision of the Izanagi-Pacific plate ridge system with the trench resulted in a magmatic gap and short-lived uplift of eastern China. Following this, subduction of the rapidly aging Pacific plate resulted in a larger, but relatively weaker, mantle convection cell that drove weaker upper plate extension. The vigor of wedge mantle convection was modulated by the age of the Pacific subducting slab, which directly influenced the formation and evolution of intracontinental extension and sedimentary basins across eastern China. The pre-existing Triassic Tan-Lu fault zone has reactivated since the Late Eocene (ca. 40 Ma) due to westward subduction of the Pacific slab to partition strain between extensional dextral faulting in the Bohai Bay and Lower Yangtze basins, thereby diversifying Cenozoic lithospheric extensional deformation in eastern China. Several major Cenozoic extensional basins spatially correlate with the Triassic Sulu orogenic belt, which we interpret to suggest that these basins are underlain by the older and fragmented orogenic basement that acted as mechanically weak zones for localized high-magnitude extension. We present a comprehensive tectonic model that relates intracontinental deformation to subduction zone dynamics, particularly emphasizing the importance of the age of the subducting slab, thus providing a coherent geologic process to explain the deformation kinematics in eastern China and the diversity of observed Mesozoic-Cenozoic basin formation.

Fault detection by reflected surface waves based on ambient noise interferometry

Detecting subsurface fault structure is important for evaluating potential earthquake risks associated with active faults. In this study, we propose a new method to detect faults using reflected surface waves observed in ambient noise cross correlation functions. Ambient noise tomography using direct surface waves obtained from ambient noise interferometry has been widely used to characterize active fault zones. In cases where a strong velocity contrast exists across the fault interface, fault-reflected surface waves are expected. We test this idea using a linear array deployed in the Suqian segment of Tanlu fault zone in Eastern China. The fault-reflected surface waves can be clearly seen in the cross-correlation functions of the ambient noise data, and the spatial position of the fault on the surface is close to the stations where the reflected signals first appear. Potentially reflected surface waves could also be used to infer the dip angle, fault zone thickness and the degree of velocity contrast across the fault by comparing synthetic and observed waveforms.

Thermal Maturation Regime Revisited in the Dongying Depression, Bohai Bay Basin, East China

To the accurate reconstruction of the hydrocarbon generation history in the Dongying Depression, Bohai Bay Basin, East China, core samples of the Eocene Shahejie Formation from 3 shale oil boreholes were analyzed using organic petrology and organic geochemistry methods. The shales are enriched in organic matter with good to excellent hydrocarbon generation potential. The maturity indicated by measured vitrinite reflectance (%Ro) falls in the range of 0.5–0.9% and increases with burial depth in each well. Changes in biomarker and aromatic hydrocarbon isomer distributions and biomarker concentrations are also unequivocally correlated with the thermal maturity of the source rocks. Maturity/depth relationships for hopanes, steranes, and aromatic hydrocarbons, constructed from core data indicate different well locations, have different thermal regimes. A systematic variability of maturity with geographical position along the depression has been illustrated, which is a dependence on the distance to the Tanlu Fault. Higher thermal gradient at the southern side of the Dongying Depression results in the same maturity level at shallower depth compared to the northern side. The significant regional thermal regime change from south to north in the Dongying Depression may exert an important impact on the timing of hydrocarbon maturation and expulsion at different locations. Different exploration strategies should be employed accordingly.

Kinematic Characteristics of the Jiangsu Segment of the Anqiu–Juxian Fault in the Tanlu Fault Zone, Eastern China

Abstract The Tancheng–Lujiang (Tanlu) fault zone is the most active fault zone in eastern China. In this zone, the Anqiu–Juxian fault represents the most recently active fault and has the clearest surface traces and the highest seismic risk. This study comprehensively analyzes the kinematic characteristics of the Jiangsu segment of the Anqiu–Juxian fault using field geological surveys, trenches, shallow seismic reflection surveys, combined borehole section exploration, and middepth seismic reflection surveys. The results show that the Jiangsu segment of the Anqiu–Juxian fault features a single branch in the bedrock outcrop area, with reverse strike-slip motion near North Maling Mountain and Chonggang Mountain and normal strike-slip motion near South Maling Mountain. The sedimentary zone features two normal strike-slip faults (east and western branches), which represent the synsedimentary boundaries of a half-graben rift basin. The kinematic process is represented by rotational movement along the strike-slip fault with a curved path. The resulting tensile and compressive stresses are accommodated by dip-slip movement at both ends of the strike-slip fault. The activity of the Jiangsu segment of the Anqiu-Juxian fault can be divided into two periods. The first period of activity occurred before the later part of the Late Pleistocene, when movement along this curved segment occurred, forming the western branch of the Xinyi segment and the eastern branch of the Suqian segment. The second period of activity started in the later part of the Late Pleistocene and continues today. It is characterized by activity on the western branch of the Xinyi segment and the western branch of the Suqian segment of the Jiangsu segment, while the eastern branch of the Xinyi segment and the eastern branch of the Suqian segment became inactive and can be considered Late Pleistocene faults. The maximum vertical slip rate of the Jiangsu segment of the Anqiu–Juxian fault since the Pleistocene has been 0.28 mm/a. The Jiangsu segment of the Anqiu–Juxian fault formed via dextral strike-slip faulting, mainly due to the southward movement of the region to the east of the fault.

Three dimensional crustal P‐wave structure beneath the central south segment of the Tanlu Fault Zone determined by local earthquake travel‐time tomography

AbstractThe Tanlu Fault Zone in eastern China stretches more than 2,400 km and is a significant, active, feature in the tectonics of East Asia. Here, we show the results of local earthquake travel‐time tomographical approach based on a comprehensive and systematic set of seismic data collected from 2008 to 2018 along the Central South Segment of the Tanlu Fault Zone. According to the model, the uppermost 0–10 km of the crust shows a close relationship between seismic velocity and the lithological properties of surface geological units. The middle crust at 10–20 km depths exhibits a strong connection between its high‐low velocity transitions and sites of augmented seismic activity. The top mantle at 34 km depths has lower velocities in the western region in comparison to the east, implying that the seismic Moho is deeper in the western area. These characteristics may reflect spatial variations in the destruction of the North China Craton.

A multi-scale 3-D crust velocity model in the Hefei-Chao Lake area around the southern segment of Tanlu Fault Zone

Regional high-precision velocity models of the crust are an important foundation for examining seismic activity, seismogenic environments, and disaster distribution characteristics. The Hefei-Chao Lake area contains the main geological units of Hefei Basin, with thick sediments and the Chao Lake depression. Several major concealed faults of the southern NNE-trending Tanlu Fault Zone cross this area. To further explore the underground distribution characteristics of the faults and their tectonic evolutionary relationship with adjacent tectonic units, this study used ambient noise data recorded by a seismic array deployed in Hefei City and Chao Lake, constructing a 3-D velocity model at the depth of 1–8 km. Then a multi-scale high-resolution 3-D velocity model of this area was constructed by this new upper crustal velocity model with the previous middle and lower crustal model. The new model reveals that a high-velocity belt is highly consistent with the strike of the Tanlu Fault Zone, and a low-velocity sedimentary characteristic is consistent with the Hefei Basin and Chao Lake depression. The distribution morphology of high and low velocity bodies shows that the sedimentary pattern of Hefei-Chao Lake area is closely related to the tectonic evolution of the Tanlu Fault Zone since the Mesozoic. This study also identifies multiple low-velocity anomalies in the southeastern Hefei City. We speculate that strong ground motion during the 2009 Feidong earthquake (magnitude of 3.5) was related to amplification by the thick sediments in the Hefei Basin. We also discuss further applications of multi-scale high-resolution models of the shallow layer to strong ground motion simulations in cities and for earthquake disaster assessments.

Production Logging, Openhole-Log Interpretation Help Discover New Oil Reserves

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30644, “Discovery of New Oil Reserves by Combining Production Logging With Openhole-Log Interpretation in Low-Resistivity Pay,” by Xinlei Shi, Peichun Wang, and Jinxiu Xu, CNOOC, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4–7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. In this paper, the authors examine the evaluation of a low-resistivity-pay siliciclastic reservoir in Bohai Bay, China. A significant amount of irreducible water is bound to the rock surface, dramatically lowering the resistivity of the pay zone. The authors explore a theory that the low resistivity is caused by bound water trapped in clay minerals, using production logging to provide the ground truth of reservoir fluids in the low-resistivity pay and improve the petrophysics model. With the improved model, production predictions were made for offset wells based on their openhole logs. The production histories of these wells are highly consistent with the authors’ predictions. Introduction LD oil field is in the eastern Bohai Sea, China, structurally in the transition zone between the Liaohe depression of the Tanlu fault and the Bozhong depression and at the dip end of the Bodong low uplift extending to the northeast. The main oil reservoirs are developed in the Guantao and Dongying formations. Reservoir depth ranges from approximately 1022.1 to 2585.8 m. Reservoir lithology is mainly sandstone and gravelly sandstone. The porosity distribution range of the Guantao formation is 24 to 30%. Permeability distribution range is 333 to 3333 md belonging to medium-high-porosity and - permeability reservoirs. The porosity distribution range of the Dongying formation is from 6 to 12%, and the permeability distribution range is from 3 to 33 md belonging to medium-low- porosity and -permeability reservoirs.

Structures and Kinematics of the Huanghua Depression in Bohai Bay Basin, East China: Implications for the Formation Mechanism of a Transtensional Basin

Abstract The Bohai Bay Basin in East Asia is a rift basin created by Cenozoic subduction of the oceanic Pacific plate beneath the Asia continent. Many prior studies suggest that the basin was initially formed in the Paleocene with the development of several NNE-trending extensional grabens, but subsequently impacted by right-lateral shear along these existing NNE-trending structures in the middle Eocene, transforming the Bohai Bay Basin into a transtensional basin and producing EW-trending grabens in the Bozhong and the northeastern Huanghua depressions. However, how this transformation occurred remains to be fully understood. Based on seismic and drilling data, we herein investigated the fault structures, basin architecture, and evolutionary stages of the Huanghua Depression in the central-west Bohai Bay Basin to examine the strain partitioning and evolution mechanism during the Paleogene syn-rifting stage. The results reveal that the Huanghua Depression is composed of three structurally distinctive zones, namely, a dextral transtensional, a NW-SE extensional, and a N-S extensional zones from southwest to northeast, which are separated from each other by two transfer zones. The NW-SE extensional zone is interpreted as a horsetail structure on the northern termination of the dextral transtensional zone. This dextral transtensional zone and the Tan-Lu Fault zone to the east served as strike-slip boundaries within which EW-trending depressions such as the northeastern Huanghua and Bozhong depressions formed in the middle Eocene.

An Early Cretaceous gold metallogenesis in the Wuhe area, Eastern Anhui province: Constraints from geology, fluid inclusion, H-O isotope and geochronology on the Hekou gold deposit

The Wuhe area in eastern Anhui province, hosting 23 metric tons of gold, has been recognized as a new gold ore cluster in the North China Craton (NCC) since the Hekou gold deposit was discovered in 2017. Its metallogenic setting and ore-forming process remain unclear. In this contribution, we perform a combined geology, cathodoluminescence, fluid inclusion, H-O isotope and pyrite Rb-Sr geochronologic studies on the Hekou gold deposit of the Wuhe area. In order to perform detailed fluid inclusion petrography study, we used scanning electron microscope-cathodoluminescence (SEM-CL) on a well preserved sulfides-bearing quartz grain from the auriferous quartz vein. In this quartz grain, generation A barren quartz in the core zone (QA, CL-white), minor sulfides bearing generation B (QB, CL-light gray) in intermediate zone and sulfides rich generation C (QC, CL-dark gray) in the distal zone and fractures were revealed. Only in QC, the higher carbonic proportion (50–80 vol%) inclusions (CC) coexist with the two-phase liquid-rich aqueous fluid inclusions (LC) and sulfides, meanwhile the CC and the LC show a similar Th of 258–313 °C and 249–308 °C, but different salinities of 1.4–5.3 wt% NaCl equivalent and 6.0–10.9 wt% NaCl equivalent, respectively. It suggests that dominating fluid immiscibility recorded in QC is responsible for the prime deposition of gold and sulfide minerals. The δ18O (-4.1 to −6.5‰) and δD (-67.1 to −81.3‰) values of fluid inclusions in quartz suggest that ore-forming fluids were mainly derived from magmatic water which could related to mantle source. Gold-bearing pyrites yield an Rb-Sr isochron age of 128.1 ± 2.7 Ma, which is 8 m.y. earlier than the peak value of gold mineralization ages (ca. 120 Ma) at Jiaodong in response to the Early Cretaceous lithospheric thinning of the NCC and the reactivation of the Tan-Lu Fault zone (TLFZ). The Wuhe area shares similar characteristics of geology, fluid inclusions, H-O isotopes, and ore-forming ages with the Jiaodong gold ore cluster, highlighting the exploration potential in this region.

Chronological and geochemical variations of the late Mesozoic granitoids in the Taihang Mountains and middle-southern Tan-Lu Fault: Implications for lithosphere destruction of the North China Craton

In the Late Mesozoic, the North China Craton (NCC) underwent significant lithospheric thinning and destruction, especially in the eastern part, but the mechanism and timing related to this process are still contentious. The Taihang Mountains (TH) are located in the western part of the eastern NCC and the Tan-Lu Fault (TLF) is in the eastern part, which are two essential magmatic areas that reveal deep processes of magma origin. We investigated the spatial-temporal distribution of igneous rocks from these two areas to constrain the tectonic setting and magmatic sources. SHRIMP zircon U-Pb ages of the granitoids within the Fangshan pluton in northern TH area range from 136 to 128 Ma. Their <i>ε</i>_Hf(t) values and δ¹⁸O values show ranges of −27.7 to −18.5 and 6.68 to 7.26 permil, respectively. Hence, we conclude that the rocks were formed by mixing between underplating magma and the melts from the lower crust. The O-Hf isotopic compositions of six granitoid samples from the Yunmengshan complex in northern TH are also reported. In combination with previous studies, we propose that the geochemical characteristics of the magmatic rocks from the TH area had insignificant changes during late Mesozoic time, but the rocks from the TLF area varied greatly. The difference between those two areas may reflect the diverse impact of the Paleo-Pacific subduction process. The high Mg# adakitic rocks (HMA) from TLF area have higher Mg# values than the HMA rocks from TH area. Our conclusion is that the HMA rocks in the TLF area were mainly formed by delaminated lower crust interacting with mantle materials and that the Paleo-Pacific subduction had limited impact on TH magmas. Based on chronology and geochemical characteristics, we recognize three stages: 1) ∼166 to 140 Ma, multi-directional compression resulted in crustal shortening and thickening in the NCC, accompanied by regional partial melting of the crust and underplating of mafic magmas, 2) 140 to 125 Ma, the TLF underwent left-lateral strike-slip movement. Subsequent delamination of the lower crust around the fault and the NCC evolved into an extensional tectonic environment, 3) after 125 Ma, a large-scale extension of the NCC occurred likely due to stress relaxation after delamination. The TLF acted as a favorable channel for transporting mantle material and fluids, which implies that the large-scale fault zone was a key factor of the NCC lithosphere destruction.

Dynamical significance of the Tanlu Fault Zone in the destruction of the North China Craton: The evidence provided by the three-dimensional Magnetotelluric array study

Due to the subduction of the Paleo-Pacific Plate below the East Asian continent, the North China Craton (NCC) is considered to have been thinned in the Mesozoic, with the lithosphere of its east portion significantly thinned. As the eastern boundary of the NCC, The Tanlu Fault Zone (TLFZ) is one of the largest strike-slip fault systems in eastern Asia. Tanlu is believed to have recorded the destabilization of the NCC. Magnetotelluric array data collected in the TLFZ and adjacent areas are obtained to image the lithospheric three-dimensional resistivity structure and to discuss the structural constraints on this event. According to the resistivity model from the three-dimensional inversion, the crust of the Luxi Uplift (LXU) to the west of the TLFZ and the crust of the Sulu Orogenic Belt (SOB) to the east of the TLFZ exhibits high resistivity (>300 Ωm). Lithospheric mantle of the SOB also exhibits highly resistive characteristics. Conversely, the LXU mantle showed distinct low-resistivity characteristics (C3). Low-resistivity anomalies were found corresponding to both branches of the TLFZ. The eastern branch is a lithospheric low-resistivity belt, while the western branch formed a significant mantle resistivity boundary between the low-resistivity LXU and the high-resistivity SOB. Thus, we considered that molten materials may have upwelled along the weak area of the western branch, inducing a significant lithospheric thinning beneath the LXU. On the other hand, the lithosphere of the SOB has not been significantly thinned according to its high resistivity. The significant contrast of the resistivity may indicate that the western branch of the TLFZ is the boundary between the NCC and the SOB, with the sinistral strike-slip occurring along it.

Quantitative prediction of multiperiod fracture distributions in the Cambrian-Ordovician buried hill within the Futai Oilfield, Jiyang Depression, East China

In the Futai carbonate oilfield of the Jiyang Depression, Bohai Bay Basin, oil enrichment is significantly related to fracture development during the oil production process. In this research, by methods of core observation and image logging interpretation, the tectonic fractures’ characteristics were distinguished. Numerically, through the elastoplastic mechanics method of finite element (FE) simulation, the three-dimensional (3-D) paleotectonic stress field (two key fracture-generating periods) and the current stress field in the Futai buried hill were quantitatively simulated. The equations between fracture parameters and tectonic stresses were deduced to predict the fracture distribution and development in various periods. Simultaneously, through the mechanical superposition algorithm, the fracture parameters under the present conditions were simulated by modifying the current stress field from preexisting fractures. The results indicated that: (1) The Yanshanian maximum principal stress ranged from −85 MPa to −435 MPa in the NWN-SES (344°) (343°–164°) direction. (2) The Himalayan minimum principal stress ranged from 5 to 30 MPa in the NWN–SES direction. (3) The fracture formation during the Yanshanian and Himalayan orogeny was primarily influenced by the regional stress field in the Jiyang Depression or the secondary stress field derived from the Tanlu fault zone, followed by the lithology. The current fracture porosity was primarily controlled by fold, followed by the large faults oriented by N–S direction and then by the secondary faults oriented by E-W direction. With successful calculations, the developed 3-D FE geomechanical modeling and fracture parameter calculation method holds great promise for characterizing fractures in other carbonate reservoirs.

Geodynamics of decratonization and related magmatism and mineralization in the North China Craton

The North China Craton (NCC) experienced strong destruction (i.e., decratonization) during the Mesozoic, which triggered intensive magmatism, tectonism and thermal events and formed large-scale gold and other metal deposits in the eastern part of the craton. However, how the decratonization controls the formation and distribution of large-scale of gold and other metal deposits is not very clear. Based on a large number of published data and new results, this paper systematically summarizes all the data for the rock assemblages, chronology, geochemistry and petrogenesis of Mesozoic magmatic rocks, as well as for the mineralizing ages of gold and other metal deposits and the evolution of the Mesozoic basins in the eastern NCC. The results are used to restore the extensional rates of Mesozoic to Cenozoic basins and the strike-slip distance of the Tanlu Fault, to ascertain the location of the Paleo-Pacific plate subduction zones during the Mesozoic to Cenozoic, and to reconstruct the temporal and spatial distribution of Mesozoic gold and other metal deposits and magmatic rocks in the eastern NCC. It is obtained that the magmatism and mineralization in the eastern NCC westward migrate from east to west during the Early to Middle Jurassic, but they eastward migrate from west to east during the Early Cretaceous. The metallogenesis of these deposits is genetically related to magmatism, and the magmas provided some ore-forming materials and fluids for the generation of metal deposits. The geodynamic mechanism of decratonization and related magmatism and mineralization is proposed, i.e., the westward low-angle subduction of the Paleo-Pacific slab beneath the NCC formed continental magmatic arc with plenty of porphyry Cu-Mo-Au deposits in the Jurassic, similar to the Andean continental arc in South America. The mantle wedge was metasomatized by the fluids/melts derived from the subducting slab, laying a material foundation for hydrothermal mineralization in the Early Cretaceous. While the rollback of the subducting slab with gradually increasing subduction angle and the retreat of the subduction zones during the Early Cretaceous induced strong destruction of the craton and the formation of extensive magmatic rocks and large-scale gold and other metal deposits.

Crustal azimuthal anisotropy in the Jiaodong Peninsula: Evidence for the suture between the North China Craton and South China Block

The Jiaodong Peninsula is an ideal region for studying the geodynamics of collision between the North China Craton (NCC) and South China Block (SCB). It is comprised of the Jiaobei massif and the Northern Sulu UHP massif which are separated by the Wulian suture zone (WSZ). Continent-continent collision is necessarily accompanied with crustal deformation. Consequently a detailed crustal deformation pattern would offer observational constraints for understanding the process of the collision. In this study, we measured the crustal azimuthal anisotropy beneath a broadband seismic profile of 20 stations together with 9 permanent stations in the Jiaodong Peninsula. Sinusoidal moveout of Pms phases from radial receiver functions and azimuth-weighted stacking of transverse receiver functions were jointly applied to ensure the reliability of results. In the WSZ, the fast directions are roughly parallel with the NE oriented fault strike, suggesting the Jiaodong Peninsula may preserve fossilized anisotropy in the crust induced by the Triassic collision of the NCC and SCB although have undergone subsequent lithosphere destruction and thinning since the late Mesozoic. We therefore support that the WSZ represents the NCC-SCB suture east of Tanlu fault. In the Jiaobei massif, the crustal azimuthal anisotropy is controlled by both the nearly E-W local stress and NNE-strike detachment faults. The anisotropy beneath the Northern Sulu massif is quite weak with delay times all less than 0.20 s, probably related to the high-density UHP metamorphic rocks which are difficult to generate strong anisotropy inside.