Southern Tan-Lu Fault Research Articles

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.

Lateral variation in Moho depth around the southern Tanlu fault zone and its adjacent area

We estimated Moho depth beneath the southern Tanlu fault zone and its adjacent area using common-conversion-point (CCP) stacking of receiver functions, which were computed from teleseismic records of the CEArray. Our estimated Moho depth matches well with 2-D profiles derived from active-source deep seismic reflection surveys, suggesting that the calculated the Moho depth map is likely accurate beyond the 2-D profiles. Overall, the estimated Moho depth map showed a high spatial correlation with tectonic provinces, i.e., Moho topographic boundaries are in good agreement with geological boundaries. Beneath the Dabie orogenic belt and the mountainous areas in southern Anhui Province, the Moho lies relatively deep, and there is an obvious difference in Moho depth between the two sides of this segment of the Tanlu fault. We further selected four depth profiles with dense instrumentation to show Moho depth changes across different tectonic blocks in the study area. We saw two step-like changes in Moho depth beneath the Xiangfan-Guangji and Gushi-Feizhong, which run parallel along the WNW-ESE direction and delineate the southern and northern bounds of the northern Dabie orogenic belt, which is likely the suture zone between the North China Block and South China Block. Crust beneath the northeast corner of the study area is significantly thinner than other areas, which is consistent with the crustal detachment model proposed for suturing between the North and South China blocks in the region east to the Tanlu fault.

Geochronology and Petrogenesis of Granitoid Intrusions in the Feidong District, Southern Tan–Lu Fault Zone, China

AbstractThe Feidong district is located in the southern segment of the Tan–Lu fault zone that separates the South China Block (NCB) from the North China Craton (NCC). We report zircon U‐Pb geochronology and Hf isotope data, as well as whole‐rock geochemistry for Xishanyi granodiorite and Jianshan granite in the Feidong district. Zircon U‐Pb dating results show that the emplacement ages of the Xishanyi and Jianshan intrusions are 124 ± 3 Ma and 130 ± 1 Ma respectively, coeval with magmatic events linked to large‐scale lithospheric thinning in eastern China. The whole‐rock geochemistry of the Xishanyi and Jianshan intrusions demonstrate that they are peraluminous, high potassium calc‐alkaline I‐type granites with adakitic characteristics. Both intrusions underwent weak crustal assimilation during emplacement. The in situ zircon ∊Hf(t) values of the Xishanyi granodiorites range from –26.4 to –21.8, with TDM2 model ages of 2552 to 2841 Ma. The in situ zircon ∊Hf(t) values of the Jianshan granite are from –27.5 to –23.0 with TDM2 model ages of 2632 to 2904 Ma. The peak age of inherited zircon grains from the Xishanyi granodiorite and the Jianshan granite were ∼2.07 Ga and ∼1.94 Ga, respectively. After compared with the regional magmatism, we suggest that both the Xishanyi and Jianshan granitoid intrusions were derived from partial melting of the NCC lower crust.

Tectonic mechanism and evolution of eastern China during the Early Cretaceous: a view from magmatism in the middle to Southern Tan-Lu fault zone

ABSTRACT Mesozoic magmatic rocks are widely distributed in and around the Tan-Lu fault zone (TLF) and show great significance related to the destruction processes of the lithosphere and the evolution of magma sources. This study aims to decipher the petrogenesis of the Early Cretaceous magmatism along the TLF, in order to address the tectonic mechanism for the North China Craton (NCC) lithospheric destruction. This contribution combines investigations of geochemistry, SHRIMP zircon U-Pb ages and O-Hf isotopes of the intermediate to felsic rocks from middle-southern TLF with previous geochronological and geochemical characteristics of other rocks from the study area. Four different rocks are identified: high Mg# adakitic rocks (HMA), low Mg# adakitic rocks (LMA), normal intermediate-felsic rocks (NIF) and A-type granitoids. The emplacement time of the LMA rocks in the Dabie area (143 Ma) were earlier than in the Sulu area (130 Ma). The HMA rocks were formed at the same time (~135 Ma) in the above two areas but the magmatism lasted longer in Jiaodong area, which was near the subduction zone of the Paleo-pacific plate. The HMA rocks are concluded to be derived from the melting of a delaminated thickened lower crust and including reaction with overlying mantle. After 135 Ma, the magmatism in this area exhibited an eastward-younging trend, consistent with plate retreat in the Early Cretaceous. Plutons that were emplaced during 135–125 Ma have higher Ba/Th, Ba/La and Ce/Pb, suggesting magma source for these rocks was metasomatized by hydrous fluids from the subducted plate. At ~125 Ma, A-type granitoids began to be formed. The TLF acted as a favourable channel for transporting mantle material and fluids which made the lithosphere in this area more likely to be destroyed and thinned.

Heat flow, heat production, thermal structure and its tectonic implication of the southern Tan-Lu Fault Zone, East–Central China

We present new heat flow and heat production data to constrain the lithospheric thermal structure along the southern Tan-Lu Fault Zone (STLFZ), a continuous active deep crustal fault zone in East Asia, and use these thermal data to infer how the thermal structure of the STLFZ influences regional focal depths. We report five new high-quality heat flow measurements derived from temperature–depth profiles at five well sites and detailed thermal conductivity measurements of 13 outcrop samples and 115 dry core samples from five wells along the STLFZ. We analyze these new data in combination with published data to derive a thermal profile along the STLFZ and map the 350 °C isotherm along the STLFZ, which correlates well with the maximum depth of recorded seismicity. The results indicate that: (1) the thermal gradient along the STLFZ increase from 22.2 °C km–1 in the south to 34.8 °C km–1 in the north; (2) heat flow values are in the 44.0–75.4 mW m–2 range along the STLFZ, with a mean value of 59.9 ± 10.5 mW m–2; (3) the highest heat flow value is located in Lu-Zong Basin, with extremely high radioactive heat production of 15–20 μW m–3 in the upper crust that may be related to local ore bodies; and (4) the bottom boundary of the shallow-earthquake seismic zone in the STLFZ coincides with the 350 °C isotherm.

Late Cenozoic tectonic evolution of the southern segment of the Tan-Lu fault zone, Eastern China

The knowledge of the geometry, kinematics, assemblage and evolution of the Tan-Lu fault zone in late Cenozoic times remains scarce, despite its importance to understanding regional geodynamics and relevant mineralization and seismicity. To address the above issues, we provide new results of morphotectonic analysis, geological mapping, boreholes, seismic reflection and geochronology in the southern Tan-Lu fault zone at Sihong, Jiangsu Province, Eastern China. Field mapping in line with drilling and seismic reflection data suggest that the east-dipping Jiji-Wangji fault (F4) and west-dipping Shanzuokou-Sihong fault (F1), as the boundaries of the Tan-Lu fault zone, have been likely reactivated in a normal sense since the middle Neogene, and continued by the early-middle Pleistocene. These two faults predominantly controlled a graben that was filled by fluvial deposits in the fault zone. Additionally, the east-dipping Xinyi-Xindian fault (F2) and west-dipping Mohe-Lingcheng fault (F3) within the Tan-Lu fault zone had similar kinematics but behaved less active during the same time interval. The assemblage of the faults F1-F4 defined a horst-graben system in the Neogene-early Quaternary. During the late Pleistocene, intense deformation of the Tan-Lu fault zone concentrated on two splay faults, namely the fault F2 and Qiaobeizhen-Suqian fault (F5), while fault F4 likely ceased. These two active faults (F2 and F5) were interpreted as reverse dextral strike slip structures under a NE-oriented compressional stress field, in which the other two faults F1 and F3 changed the normal sense to reverse. The activities of faults F1 and F5 caused uplift of the Chonggang Hill and controlled sedimentary depocenters between these two faults as revealed by boreholes. Optically stimulated luminescence data place age control on the activities of the reactivated fault F2 between 105.6 ± 1.1 ka and 46.9 ± 5.9 ka and a newly developed fault F5 between 135.6 ± 10.6 ka to 23.96 ± 1.69 ka. Based on these results, we reconstructed the late Cenozoic tectonic evolution of the Tan-Lu fault zone, in which a dramatic kinematic shift probably occurred by the middle-late Pleistocene, possibly resulted from the far-field effect of the Indo-Asian convergence.

Structural records of the Late Cretaceous–Cenozoic extension in Eastern China and the kinematics of the Southern Tan-Lu and Qinling Fault Zone (Anhui and Shaanxi provinces, PR China)

Our study of the Late Cretaceous–Cenozoic extension in Eastern China is a kinematic analysis of faults (≈1500 striated fault data, 175 stress solutions at 90 sites) that has permitted us to separate stress fields belonging to six tectonic events. A WNW–ESE extension (1) has affected Eastern China from the Campanian to the Late Palaeocene–Early Ypresian. Under this extension, the Qinling (QLFZ) and Southern Tan-Lu (STLFZ) Fault Zones have been dextral transtensional fault zones. A first transpressional event (2) (NNE–SSW shortening) has occurred during the Late Ypresian–Lower Lutetian; the QLFZ and the STLFZ have been dextral transpressional fault zones. A NE–SW extension (3) has affected the Weihe graben during the Palaeogene; after a second transpressional event (4), the NE–SW extension (5) has been rejuvenated during the Late Miocene and the QLFZ has been a normal fault zone again and the STLFZ a sinistral transtensional fault zone; synrift subsidence has occurred in the Weihe graben. The second transpressional event (4) (WNW–ESE shortening) has occurred during the Late Oligocene–Early Miocene, during a period of uplifting and exhumation. At that time, the STLFZ and the QLFZ have become sinistral transpressional fault zones. A period of extension (6) has taken place during the Late Pliocene–Quaternary; the extensional direction has trended ≈NNW–SSE; during this event, the QLFZ has been a sinistral transtensional fault zone and the STLFZ a dextral transtensional fault zone. These results are compared with those previously published in order to evaluate the regional significance of these tectonic events.

Generation of Early Cretaceous high-Mg adakitic host and enclaves by magma mixing, Dabie orogen, Eastern China

Early Cretaceous granodioritic host rocks with mafic microgranular enclaves (MME) in the Dabie orogen provide important insights into magma mixing between melts from delaminated eclogitic lower continental crust and mantle-derived magma. The Meichuan host granodiorites (132±1Ma) from the eastern margin of the Dabie orogen, adjacent to the Tan–Lu fault, are high-Mg adakitic rocks containing 63.5–69.6wt.% SiO2 and 2.2–4.5wt.% MgO with high Mg# values (Mg#=molar 100×Mg/(Mg+Fe)) from 50.4 to 63.0. Whole-rock geochemical features include high Cr (56.0–281.3ppm), Ni (49.3–140.0ppm), and Sr (745–1373ppm) concentrations, low Y (7.6–12.4ppm) and heavy rare earth elements (HREE; e.g., Yb: 0.64–1.14ppm), resulting in high Sr/Y (67.2–130.5) and (La/Yb)N (22.2–68.7) ratios. The host granodiorites also have moderately enriched initial 87Sr/86Sr ratios (0.7056–0.7059), low εNd(t=130Ma) values (−18.8 to −15.0), enrichment of large ion lithophile elements, and depletion of high field strength elements. These geochemical features indicate that the host granodiorites were derived from partial melting of delaminated lower continental crust and subsequently hybridized by mantle melts. The MMEs have a wide range SiO2 contents from 47.7 to 61.0wt.%. MMEs with low SiO2 contents (<51.0wt.%) have extremely high MgO, Ni, Cr, Y, and Yb, and low Sr contents, resulting in high Mg# values and low Sr/Y and (La/Yb)N ratios, similar to Early Cretaceous mafic–ultramafic rocks in the Dabie orogen. MMEs with high-SiO2 (>51.0wt.%) have relatively low MgO (but Mg# values are still more than 50, 55.2–61.3), Y, and Yb, and high Sr concentrations, resulting in high Sr/Y and (La/Yb)N ratios, analogous to high-Mg adakitic rocks. Evidence from fieldwork and geochemistry clearly points to magma mixing during magma evolution, e.g., the contact relationships between coeval MMEs and their host; excellent linear trends on Harker diagrams from low-SiO2 MMEs via high-SiO2 MMEs to granodioritic host without interruption. Thus, the Meichuan granodioritic host and MMEs can be interpreted as a mixture between a mantle-derived magma having high Mg# values and low Sr/Y and (La/Yb)N ratios, and a delaminated eclogitic lower continental crust-derived adakitic melt, e.g., low Mg# values and high Sr/Y and (La/Yb)N ratios. The distribution of the late Mesozoic high-Mg adakites in eastern China generally forms a high-Mg adakite belt along the southern Tan–Lu fault. Our results suggest that the large strike–slip motion of the Tan–Lu fault might have triggered the foundering of the over-thickened lithosphere which caused orogenic collapse and subsequent crust–mantle interaction. Generation of the Meichuan granodioritic host and MMEs requires both delamination of eclogitic lower crust and magma mixing in the mantle.

High-Mg adakitic rocks in the Dabie orogen, central China: Implications for foundering mechanism of lower continental crust

The late Mesozoic high-Mg adakitic rocks from Eastern and Central China provide important insight into the foundering mechanism of the over-thickened lower continental crust. The Chituling high-Mg adakites (131 ± 3 Ma, SHRIMP zircon U–Pb age) from the eastern margin of the Southern Dabie ultrahigh pressure metamorphic zone, adjacent to the Tan–Lu fault, have high Al 2O 3 and Sr contents, high Sr/Y, La/Yb, and Mg# (44–63), but low Y and Yb contents. The samples also have moderately enriched 87Sr/ 86Sr i (0.70691–0.70775), very low ε Nd( T) (− 20.7 to − 24.9), unradiogenic Pb isotopes, enrichment of large ion lithophile elements, and depletion of high field strength elements. These geochemical features indicate that they did not result from melting of young oceanic crust, assimilation and fractional crystallization, or magma mixing. Instead, they were derived from partial melting of delaminated lower continental crust, with subsequent reaction with surrounding mantle peridotites during ascent to crustal depths. The reactions between the adakitic melt and peridotites also generated the enriched mantle source of the post-collisional basaltic rocks in the Dabie orogen. Distribution of the late Mesozoic high-Mg adakites in eastern and central China generally forms a high-Mg adakite belt along the southern Tan–Lu fault. Therefore, we propose that the large strike-slip motion of the Tan–Lu fault in eastern and central China due to the western subduction of the Pacific plate in the early Cretaceous might trigger the foundering of some fragments of the over-thickened lithosphere near the Tan–Lu fault, which caused mantle upwelling and partial melting of the thickened lower crust in the Dabie orogen and eastern boundary of the North China Craton. This further weakened the gravitationally instable lithosphere, consequently resulting in delamination and foundering of the mountain root underneath the Dabie orogen, which could be an important foundering mechanism of lower continental crust.

Structural and stratigraphical constraints on the kinematics history of the Southern Tan–Lu Fault Zone during the Mesozoic Anhui Province, China

The Tan–Lu Fault Zone (TLFZ) extends in a NNE–SSW direction for more than 2000 km in Eastern China. It has been considered either as a major sinistral strike-slip fault, as a suture zone or as a normal fault. We have conducted a structural analysis of the southern segment of this fault zone (STLFZ) in the Anhui Province. The ages (Triassic to Palaeocene) of the formations affected by the faults have been re-appraised taking into account recent stratigraphical studies to better constraint the ages of the successive stages of the kinematics of the STLFZ. Subsequently, the kinematics of the faults is presented in terms of strain/stress fields by inversion of the striated fault set data. Finally, the data are discussed in the light of the results obtained by previous workers. We propose the following history of the STLFZ kinematics during the Mesozoic. At the time of collision, a ∼ NNE orientated Tan–Lu margin probably connected two margins located north of the Dabie and Sulu collision belts. During the Middle–Late Triassic, the SCB has been obliquely subducted below the NCB along this margin which has acted as a compressional transfer zone between the Dabie and Sulu continental subduction zones. The STLFZ has been initiated during the Early Jurassic and has acted as a sinistral transform fault during the Jurassic, following which the NCB/SCB collision stopped. A ∼ NW-trending extension related to metamorphic domes was active during the basal Early Cretaceous (∼ 135–130 Ma); it has been followed by a NW–SE compression and a NE–SW tension during the middle–late Early Cretaceous (∼ 127 to ∼ 105 Ma, possibly ∼ 95 Ma); at that time the TLFZ was a sinistral transcurrent fault within the eastern part of the Asian continent. During the Late Cretaceous–Palaeocene, the STLFZ was a normal fault zone under a WNW–ESE tension.

A Study on Geophysical Features of Deep Structures of the Hefei Basin and the Southern Tanlu Fault Zone

AbstractBased on detailed processing of the gravity, magnetic, electric and seismic profile data and by applying synthetic geophysical inversion technique and interpretation, this paper studies the deep structures and tectonic styles of the Hefei Basin and the southern Tanlu fault zone. In the Hefei basin, the thrust faults occurred in the strata below Indo‐China movement unconformity, while rifting system developed in the upper strata above the unconformity. Tectonic reversion in the Hefei basin happened in the late Jurassic. Rifting basin was developed in the early Cretaceous, but a reversion occurred at the latest of the early Cretaceous, and led to thrust and nappe structures. The Tanlu fault zone that bounds the Heifei basin on the east displays a style of normal faulting in the shallow part and reverse faulting in the deep part, which records the change in tectonic regimes during the Mesozoic in East China.

Cretaceous−Cenozoic history of the southern Tan-Lu fault zone: apatite fission-track and structural constraints from the Dabie Shan (eastern China)

Apatite fission-track (AFT) and structural data outline the Late Cretaceous−Cenozoic history of the southern Tan-Lu fault zone (TLFZ), one of Asia's major faults, the Triassic–Jurassic Dabie orogen, Earth's largest track of ultrahigh-pressure rock exposure, and its foreland, the Yangtze foreland fold-thrust belt. The fission-track analyses utilized the independent ( φ-), Z- and ξ-methods for age determination, which yielded within error identical ages. Ages from Triassic–Jurassic syn-orogenic foreland sediments are younger than their depositional age and thus were reset. A group of ages records rapid cooling following shallow emplacement of granitoids of the widespread latest Jurassic−Early Cretaceous “Yanshanian” magmatism. Most ages are 90 to 55 Ma and document cooling following reheating at 110–90 Ma, the time when the basement units of the Dabie Shan were last at >200 °C. This cooling coincides with rifting marked by the Late Cretaceous−Eocene red-bed deposition in eastern China. During this period, the Dabie basements units exhumed in the footwall of the Tan-Lu fault with the Qianshan basin in the hanging wall; the associated stress field is transtensional (NW-trending principal extension direction). The youngest fission-track ages and temperature–time path modeling point to enhanced cooling in the footwall of the Tan-Lu and associated faults at 45±10 Ma. The related stress field is transtensional, with the principal extension direction changing trend from NW to W. It may be the far-field expression of the India–Asia collision superposed on the back-arc extension setting in eastern China. A regional unconformity at ∼25 Ma marks an upper bound for the inversion of the Late Cretaceous−Eocene rift structures. During the Neogene, further subsidence in the eastern China basins was accommodated by sub-horizontal NE–SW extension, and followed by the presently active NW–SE extension. The Tan-Lu fault along the eastern edge of the Dabie Shan had normal and then sinistral-transpressive motion during the Late Cretaceous−Eocene. Its motion changed during the Neogene from sinistral transtensive to normal and then to its present dextral transtensive activity.

Kinematic History and Changes in the Tectonic Stress Regime during the Cenozoic along the Qinling and Southern Tanlu Fault Zones

AbstractSince the mid‐late Eocene, North China has been subjected to extensional stress, resulting in the formation and development of basins. The dynamic origin of this crustal extension has long been an issue of debate. This paper presents the results of kinematic analyses of faults obtained from two seperated areas in North China. In the Weihe graben situated on the southernmost margin of the Ordos block, analyses of fault kinematics were coupled with an analysis of the basin's subsidence history. Three successive extensional tectonic phases accompaning the basin's formation and development have been distinguished. The Palaeogene extension was oriented in a WNW‐ESE direction; the Neogene extension in a NE‐SW direction and the Pliocene‐Quaternary extension in a NW‐SE direction. Such changes have also been recorded by fault kinematics along the southern Tanlu fault zone. This has been demonstrated by three successive sets of fault striations indicating normal dip slip resulting from NW‐SE extension, then left‐lateral slip with a normal component resulting from NE‐SW extension, and right‐lateral slip with a minor normal component, respectively. The kinematic history of faults and their chronological evolution indicate changes in continental dynamics acting in North China over Cenozoic time.