Cathaysia Research Articles

The Joining of North and South China During the Permian: Coherent Metamorphic Evidence From East Asia Orogenesis

AbstractThe joining of the North and South China blocks marks the formation of the united East Asian continent, which is an integral part of Pangea. The amalgamation of these two continental blocks also resulted in the formation of the world‐class ultrahigh pressure orogenic belt, namely the Dabie–Sulu orogenic belt. Figuring out the time of the initial collision between the two continents is critical for resolving questions such as the duration and the geodynamic processes of the orogeny. A Triassic joining time has been suggested by the geochronology of the eclogite facies rocks in this orogenic belt. However, paleomagnetic and paleontological studies suggested a Permian docking time for these continental blocks. In this paper, we present new metamorphic ages of rocks from northern Dabie and the Permo–Triassic intracontinental orogen of South China, which are all closely associated with this continental collisional event. New age dating results, as well as a synthesis of recent studies on metamorphic rocks occurring in southeastern North China and the Cathaysia Block of South China, show that the onset of the collisional orogenesis along the eastern part of the orogenic belt dates back to the Middle Permian (270–252 Ma). Considering these data and other geological records, we provide a new tectonic model for the major continents of East Asia, in which we show that the initial collision between North and South China occurred in the east during the Middle Permian, and then propagated westward. Continental collisions forming mountains and closing seaways in East Asia thus apparently occurred before the end‐Permian mass extinction, suggesting these paleogeographic changes might have also preconditioned and facilitated the end‐Permian biospheric crisis in the region.

The mechanism responsible for the geochemical diversity of the Putuoshan volcanic-plutonic complex and its implications for the late Mesozoic crustal evolution of the Cathaysia Block, Southeast China

Essential mechanisms for the geochemical diversity observed in a silicic igneous rock association, which is of paramount importance for understanding the differentiation and growth of continental crust, remain highly controversial. Here the issues are explored by comparing the Late Cretaceous intrusions (minor-phase miarolitic alkali-feldspar granite and main-phase alkali-feldspar granite) and coeval volcanic rocks (rhyolite and rhyodacite) within the Putuoshan (PTS) volcanic-plutonic complex in the coastal area of southeast (SE) China. Zircon UPb ages indicate that the rhyolite, rhyodacite, minor-phase granite and the previously studied main-phase granite are nearly contemporaneous (ca. 98–94 Ma), but the coexisting dacites are obviously earlier at ca. 160 Ma. The dacites show adakitic geochemical characteristics and extremely enriched NdHf isotopic compositions with Paleoproterozoic model ages, suggesting their derivation from remelting of Paleoproterozoic basement rocks in a thickened lower crust. In contrast, the coexisting Late Cretaceous volcanic and intrusive rocks have comparable and relatively depleted NdHf isotopic compositions. Our data, combined with previous studies suggest that parental magmas of these Late Cretaceous magmatic rocks were derived from partial melting of juvenile crust with varying degrees of involvement of mantle-derived melts, and the observed compositional variation are mainly controlled by crystal-melt segregation and rejuvenation of crystal mush facilitated by recharge of hot mafic magmas in the shallow magma reservoir. The Jurassic and Late Cretaceous magmatic rocks within the PTS complex were generated in an advancing subduction compressive regime and a retreating subduction extensional regime of the paleo-Pacific plate, respectively. The gradually increasing tectonic extension during the Cretaceous due to slab rollback and the induced underplating of depleted mantle-derived magmas not only gave rise to the continental rejuvenation of SE China but also are the main driving mechanism of this Late Cretaceous long-lived transcrustal magma system.

Crustal and upper mantle structure beneath SE China from joint analysis of receiver functions and Rayleigh-wave dispersion

SUMMARY Various geodynamic models, such as a mantle plume, lithospheric delamination and the subduction of palaeo-Pacific Plate, have been proposed to account for the Mesozoic magmatic activity and related mineralization in SE China. However, previous seismic velocity models are insufficient in resolution to constrain the accompanying crust–mantle interactions, which limits our understanding of the deep geodynamic processes responsible for the Mesozoic magmatism in SE China. In this study, we construct a new 3-D shear wave velocity model of the crust and upper mantle beneath SE China using joint inversion of receiver functions and Rayleigh-wave dispersion. We also determine the crustal thicknesses and Vp/Vs ratios by H–k stacking of receiver functions. Compared to the western Cathaysia Block, the coastal volcanic-intrusive belt (CVIB) southeast of the Zhenghe-Dapu Fault shows relatively high-velocity anomalies in the lower crust and high Vp/Vs ratios, indicating significant underplating of mafic material beneath the CVIB. Furthermore, it is noticeable that the asthenospheric low-velocity anomalies are stronger beneath the CVIB than beneath the interior of SE China, suggesting a southeastward increase in the intensity of asthenospheric upwelling and partial melting, which was likely responsible for the strong volcanism and the inferred mafic underplating beneath the CVIB. Our observations provide new evidence and more details for the northwestward subduction and subsequent rollback of the palaeo-Pacific Plate causing the Late Mesozoic magmatism in SE China.

Two episodes of late Mesoproterozoic magmatism in the southwestern Yangtze Block, China, and their association with Grenville-aged orogenic cycles

We report three sets of newly identified late Mesoproterozoic mafic intrusions in the southwestern Yangtze Block, China. Secondary ion mass spectrometry (SIMS) baddeleyite and zircon U-Pb analyses yield crystallization ages of ca. 1.15 Ga for the Boka dolerites, ca. 1.07 Ga for the Lanniping gabbros, and ca. 1.06 Ga for the Tong’an mafic dikes. These are tholeiitic to alkaline rocks with enriched mid-ocean-ridge basalt (E-MORB)−like trace-element patterns and superchondritic εNd(t) values, indicating a partial melting origin from the asthenospheric mantle. These results, along with existing data, define two episodes of late Mesoproterozoic, rift-related mafic magmatism in the southwestern Yangtze Block: an early phase at 1.17−1.14 Ga and a late phase at 1.07−1.02 Ga. Initiation of the two episodes of extensional events in the southwestern Yangtze Block coincided with high-grade Grenville-aged metamorphism in Hainan Island and possible early collision of Cathaysia with the southwestern Yangtze Block, indicating an impactogen model. Therefore, we interpret the observed two episodes to have resulted from local collision between the southwestern Yangtze Block and the Cathaysia Block (including Hainan Island), which was possibly a part of western Laurentia during the assembly of Rodinia.

Azimuthal Anisotropy of Receiver Functions in the Central South China Block and its Tectonic Implications

By the H-к stacking of the receiver functions and the splitting of the Pms phases, using seismic data from the Regional Seismic Network and the Huanan Seismic Array, a high-resolution temporary seismic array deployed for 2 years in the study area. This study revealed the strong lateral heterogeneity in crustal structures in the central South China block. Crustal thickness reduces from northwest to southeast, with significant differences across the boundary of sub-blocks. The average crustal Vp/Vs ratio gradually increases from west to east, leading to high values in the coastal region, which suggests that the subduction of the Pacific plate has possibly caused the underplating of magma or the upwelling of upper mantle material. The crustal azimuthal anisotropy of the Dabie orogen and the Jiangnan orogen is generally consistent with the strike of the tectonic belt as well as with the orientation of the absolute plate movement. We suggest that the crustal azimuthal anisotropy of the orogen is related to the extension and deformation of the lithosphere. The anisotropy in the crust is close related to crustal deformation. The orientation in the crust and the upper mantle in the Cathaysia block are generally consistent with the orientation of the absolute plate motion, indicating that the azimuthal anisotropy of the Cathaysia block is related to lithospheric deformation and the under-invasion of upper mantle material.

Development of a mid-Neoproterozoic arched rift in eastern South China as a response to the Rodinia breakup

The initial time (before or after ∼ 820 Ma) and geodynamic mechanism (bottom-up rising mantle plume or top-down deep subduction) of Rodinia breakup remain unresolved. This issue has impacted our current understanding of the tectonic evolution of South China. Some scholars believe that separation of the South China Craton (SCC) from the Rodinia supercontinent and contemporary SCC rifting began at ca. 860 Ma and was induced by a subduction-related extension event. In addition, other researchers consider that the SCC rifting occurred after ca. 820 Ma. We show that renewed Rodinia breakup within or along the margin of South China occurred ca. 820–740 Ma, which is supported by early studies and our newly obtained U–Pb zircon ages of 755.4 ± 1.0 and 801.6 ± 5.2 Ma from basalts and mafic intrusive bodies in an arched (arc-shaped) belt. Major elemental variations and calculated datasets for the rift-related igneous rocks suggest a mantle plume origin for the dated rocks, which indicates that the SCC rifting was driven by a rising mantle plume and formed several rifts. This arched belt, including < 820 Ma sedimentary and igneous rocks within it, may well have been the eastern part of the original Nanhua Rift, i.e., the prototype of the Nanhua Rift should be a “T” shape. Owing to a barren magmatic area at the “T” junction, the Nanhua Rift must have split into the Nanhua Rift and arched rift. We suggest that the first stage of rifting event during 860–825 Ma may occurred in the interior Yangtze or Cathaysia Block and was associated with a subduction-related extension event.

Yangtze and Cathaysia blocks of South China: Their separate positions in Gondwana until early Paleozoic juxtaposition

The position of South China in Gondwana remains controversial. In all previous models, the Yangtze and Cathaysia blocks, the two major components of South China, were amalgamated by the Tonian, and South China was considered to be a single coherent block in Gondwana. Based on a summary and critical reevaluation of available paleomagnetic, paleontological, stratigraphic, and detrital zircon data, we propose that the Yangtze and Cathaysia blocks were not juxtaposed until the early Paleozoic and were located at two separate parts of the northern margin of Gondwana before that time. The Yangtze block was located to the northwest of India in the Ediacaran and early Cambrian. Cathaysia was close to or a part of northern Australia in the Ediacaran, when Australia and India were separated by the Kuunga Ocean, and amalgamated with the eastern margin of India in the late Ediacaran to early Cambrian when the ocean closed. Our model implies that the early Paleozoic orogenesis in South China could have been related to two distinct events, the late Ediacaran to early Cambrian collision of Cathaysia/Australia with India (the Kuunga orogeny) and the Late Ordovician amalgamation of Cathaysia with Yangtze. It also implies that the Yangtze and Cathaysia blocks likely occupied two separate positions in Rodinia.

Structural and geochronological constraints for the Mesozoic nappe structures of the Taqian–Fuchun thrust zone, eastern Jiangnan orogenic belt, South China

The NE‐trending Jiangnan orogenic belt has experienced multi‐tectonic episodes since the Mesozoic separates the Yangtze Block in the north‐west from the Cathaysia Block in the south‐east. The Taqian–Fuchun thrust zone in north‐east Jiangxi Province is located in the eastern Jiangnan orogenic belt. It is considered as an essential unit to understand the geometry, kinematics and dynamics of the Mesozoic nappe structures in the eastern Jiangnan orogenic belt and an ideal region to reveal the Mesozoic tectonic evolution in South China. Here, we present new detailed structural data coupled with zircon U–Pb and apatite fission‐track dating studies, establishing a four‐stage deformation history and describing the geometry, kinematics, ages and dynamic background. The Middle–Late Triassic NW–SE compression event (D1, 243–226 Ma) is mainly characterized by regional angular unconformity between the Lower and Upper Triassic and asymmetric folding of the underlying strata. The associated regional NW–SE shortening was related to the intercontinental collision. The Middle–Late Jurassic NW–SE compression event (D2, 175–160 Ma) is characterized by the angular unconformity between the Middle Jurassic and Cretaceous and the SE‐vergent imbricate nappe structures. The shortening has been associated with the NW‐ward subduction of the Paleo‐Pacific Plate. The Late Jurassic–Cretaceous NW–SE extension event (D3, 160–67 Ma) includes the transition stage from compression to extension (160–145 Ma) and the intensive extension stage (145–67 Ma). The former is characterized by the development of the extensional detachment structures and intermediate‐acid magmatism, while the latter is characterized by the development of Cretaceous extensional sedimentary basins and A‐type granites. This extension was correlated to the slab roll‐back of the Paleo‐Pacific Plate. The Paleocene–Eocene compression event (D4, 67–35 Ma) is manifested by the thrusting of metamorphic rocks in the Shuangqiaoshan Group to the Cretaceous red beds. This shortening possibly originated from the collision between the India and Eurasian Plates. This study provides new constraints on the Mesozoic structural style and establish a reliable geochronological framework of the Mesozoic tectonic evolutions of the eastern Jiangnan orogenic belt.

Three-dimensional magnetotelluric imaging of the Eastern Qinhang Belt between the Yangtze Block and Cathaysia Block: Implications for lithospheric architecture and associated metallogenesis

The Eastern Qinhang Belt represents the suture zone between the Yangtze and Cathaysia Blocks and controls the world-class Dexing porphyry copper (Cu) deposits. A three-dimensional three-dimensional structural interpretation based on magnetotelluric array data covering the Eastern Qinhang Belt was determined to provide new information on the lithospheric architecture of this mineral system. The regional electrical structure of the Eastern Qinhang Belt is consistent with the pattern of the Appalachian-style multi-terrane Wilson cycle, with the main electrical gradients corresponding to pre-existing lithospheric faults. It is possible that the central domain between the Huaiyu Terrane and Western Cathaysia Block preserved the slivers of the mafic lower crust generated by the modification of Neoproterozoic subduction. Partial melting of this juvenile mafic lower crust was triggered by the extension from the rollback of the Paleo-Pacific Plate during the Mid-Jurassic. The Northeast Jiangxi Fault represents the conduit for the rise of ore-bearing magmas capable of producing porphyry Cu mineralization.

Low‐Temperature Thermochronological Perspective on Geodynamic Evolution of the Cathaysia Block Since Early Mesozoic

AbstractMesozoic intrusive rocks are extensively outcropped in the Cathaysia Block (CB), indicating that they underwent significant exhumation after being formed. However, tectonothermal evolution of the CB during Mesozoic–Cenozoic times is still poorly constrained and associated geodynamic mechanisms driving the regional exhumation remain elusive. Toward this end, we present first zircon and apatite (U‐Th)/He data of eight Mesozoic granitic plutons distributed across the intracontinental CB. Our new dating results are integrated with a compilation of regional low‐temperature thermochronological data to determine the CB evolution in a tectonic and topographic evolution framework. Zircon and apatite (U‐Th)/He central ages of the eight granitoids range from 146 to 30 and 82 to 31 Ma, respectively, implying a long‐lasting exhumation of the intracontinental CB. Inverse thermal modeling of the thermochronological data for the eight plutons indicates that the intracontinental CB underwent three exhumation phases at 150–110, 110–85, and 66–38 Ma, of which the former two exhumation phases were prolonged and significant. A compilation of regional thermochronological data reveals a propagating locus of fast exhumation phase from the intracontinental CB to the seaward epicontinental CB over time. Combined with other geological evidence, we infer that primary exhumation events of the CB resulted from changing subduction processes of the Paleo‐Pacific Plate, which include slab break‐off and foundering in the Late Jurassic, progressive slab retreat in the Early Cretaceous, and normal subduction in the Late Cretaceous, with minor exhumation events presumably triggered by the Paleogene opening of the South China Sea Basin.

Late Oligocene Formation of the Pearl River Triggered by the Opening of the South China Sea

AbstractThe Pearl River is one of the largest rivers entering the South China Sea, yet its initiation time remains debated, a topic we address using Pb isotopes in detrital K‐feldspar. Based on these Pb data, Eocene and Early Oligocene sandstones from the northern South China Sea are interpreted to have been supplied with sediment by proximal rivers draining the Cathaysia Block. In contrast, the Late Oligocene and Miocene sandstones are mainly derived from the western Pearl tributaries (e.g., Hongshui River), suggesting that the Pearl River had formed by the Late Oligocene. Detrital zircon data from the Beibuwan Basin previously suggested that the western tributaries flowed into this basin before being captured by the paleo‐Pearl River. These lines of evidence suggest that progressive headward erosion of the eastern Pearl River and late Oligocene integration of this large fluvial system can be linked to contemporaneous sea‐floor spreading of the South China Sea.

Insights into the Crustal Evolution and Tungsten Mineralization of the West Cathaysia Block: Constraints from the Inherited Zircons from the Mesozoic Dengfuxian and Paleozoic Tanghu Plutons, South China

The formation and evolution of the ancient continental crust are crucial issues in solid-earth geology which are commonly associated with global tectonic events and the formation of economically valuable magmatic-hydrothermal ore deposits. The Cathaysia Block, one of the ancient continents in Southeast Asia, can be subdivided into two parts: the West Cathaysia Block and the East Cathaysia Block. Unlike the East Cathaysia Block, no Precambrian rocks are exposed in the West Cathaysia Block, constraining further understanding of the formation and evolution of this block. In this study, a total of four hundred and thirty-three zircon U-Pb dating analyses and two hundred and eighteen Lu-Hf isotopic analyses on zircon grains from the Jurassic Dengfuxian granites and Ordovician Tanghu granites, Nanling Range, were carried out. LA-ICP-MS zircon U-Pb dating yields mean average 206Pb/238U ages of 152.6 ± 2.2 Ma (MSWD = 1.6) and 442.4 ± 1.7 Ma (MSWD = 3.8), which are regarded as the rock-forming age for the Jurassic Dengfuxian granites and Ordovician Tanghu granites, respectively. The 207Pb/206Pb ages of the inherited zircons from the Jurassic Dengfuxian granites and Ordovician Tanghu granites range from 522 Ma to 3395 Ma, hosting two major peaks at the 0.9–1.0 Ga and 2.4–2.5 Ga. In contrast to the East Cathaysia Block, the West Cathaysia Block lacks the age peak of 1.8–1.9 Ga, indicating that the West Cathaysia Block was not influenced by the assembly of the Columbia supercontinent in the Paleo-Proterozoic. In combination with the Lu-Hf isotopes, we proposed that the crust evolution of the West Cathaysia Block in Archean is dominated by juvenile crustal growth events, and dominated by the crustal reworking since the Proterozoic. The long duration of crustal reworking in the West Cathaysia Block resulted in the enrichment of lithophile elements (e.g., W, Sn, Nb, and Ta) in the crust of that region. Therefore, the Jurassic granites in the Nanling Range, which are mainly derived from the partial melting of Proterozoic basement rocks, became associated with large-scale tungsten polymetallic mineralization.

Extremely weak Lg attenuation reveals ancient continental relicts in the South China block

Whether the South China block (SCB) occupied an external or internal position in the Rodinia supercontinent remains controversial. The external model favors an accretion orogenic amalgamation setting, whereas the internal model assumes postcollisional extensional rifting in the Neoproterozoic. Recent geochronological data suggested that the SCB was assembled from several ancient nuclei or microcontinental fragments with various origins and geological histories during the early Neoproterozoic, and supported the external position model for the SCB in Rodinia. However, whether these ancient continental nuclei are hidden in the crust requires direct constraints such as those from seismic tomography. In general, these early ancient continental relicts preserved in the present-day crust tend to be characterized by high strength, high velocity, and weak attenuation. Compared with velocity, seismic attenuation tends to be more sensitive to ancient rigid continental fragments. Here, we constructed a high-resolution broadband crustal Lg attenuation model for the SCB and explored the existence and scale of the ancient continental fragments potentially hidden in the deep crust. Four extremely weak attenuation anomalies in the SCB indicate potential locations of these ancient continental relicts in the crust; one is in the Sichuan basin and the other three are beneath the Cathaysia block. The three ancient continental relicts in the Cathaysia block, which are closely corresponding to early surface lithological records, provide seismic evidence supporting an external position for the SCB in the Rodinia supercontinent.

Hot mantle upwelling and Mesozoic mineralization in Southeast China

Our study region is located in the Cathaysia block (CB), Southeast China. The most significant feature of the CB is the widespread Mesozoic magmatism and poly-metallic mineralization. However, there is still no consensus on their genesis and related tectonic processes. In this study, we determine high-resolution (0.3° in the horizontal direction) tomographic images of Vp and Vs and Vp/Vs ratio in the crust and upper mantle using 6773 P-wave and 4526 S-wave travel-time data of teleseismic events recorded by a broadband linear seismic array. The array was deployed by our team in the middle part of the CB, consisting of 81 portable stations with an interval of 5–8 km. Our tomographic results reveal two obvious low-velocity (low-V) zones with high Vp/Vs ratio (high-σ) beneath the Nanling ore belt and the Wuyishan terrane, which may reflect magmatic channels formed by partial melting that may provide heat for metal mineralization. Two high-V anomalies with low-σ are located beneath the western Wuyishan terrane and the eastern CB, which may represent refractory lithosphere or remnants of delaminated lithospheric material caused by underplating of the magma due to the subduction of the paleo-Pacific plate and its following rollback. There exists an anomaly with low-Vp, high-Vs and low-σ at depths of 150–300 km beneath the western Wuyishan terrane, which may be related to the reserved trace of magmatic conduits. On the basis of the tomographic results, we propose a geodynamic model to explain the deep mechanism of mineralization beneath the CB.

Provenance of the Early Permian bauxitic claystone in Huayingshan region, Sichuan Basin, South China: Constraints from U–Pb ages and trace elements of detrital zircons

The Early Permian bauxite in the Sichuan Basin is an important part of karst-type bauxite deposits in South China Block and its provenance is poorly understood. Here we present results of mineralogy, whole-rock geochemistry, detrital zircon U–Pb ages and trace element compositions of the bauxitic claystone from the Lower Permian Dazhuyuan Formation and those of the siltstone from the underlying Lower Silurian Hanjiadian Formation in Huayingshan region, Sichuan Basin, South China Block to constrain the provenance of the bauxitic claystone. Detrital zircons from bauxitic claystone and siltstone have similar age spectra, and the ages of bauxitic claystone are classified into five groups of 2600–2200 Ma, 1800–1600 Ma, 1200–900 Ma, 900–650 Ma, and 650–400 Ma respectively. Those of the siltstone are classified into four groups of 2600–2200 Ma, 1200–900 Ma, 900–650 Ma, and 650–400 Ma respectively. We propose that the bauxitic claystone in the study region has mixed sources: the siltstone of the Lower Silurian Hanjiadian Formation probably constitutes the dominant source, and the Late Paleoproterozoic magmatic rocks along the western margin of the Yangtze Block are probably the secondary source. Moreover, the source materials of the Hanjiadian Formation siltstone are mainly sourced from Grenville-aged magmatic rocks in the Cathaysia Block and Neoproterozoic magmatic rocks in the Jiangnan orogen, with minor contributions from the underlying clastic sediments in the Cathaysia Block and Pan-African magmatic rocks in the orogen in eastern Gondwana.

Early Palaeozoic intracontinental orogeny in South China Block: Insights from migmatites and potassic mafic rocks in southern Guangdong

ABSTRACT The early Palaeozoic orogen in South China is considered as one of the typical examples of intraplate orogens in the world. However, the nature of the orogen, for example, the onset time and the timing of tectonic transition from syn-orogenic compression to post-orogenic extension, are still controversial. Here, we present new zircon U–Pb age and Hf isotope data, and whole-rock geochemical and Sr–Nd isotopic data for migmatites and potassic mafic rocks in southern Guangdong, from the Cathaysia Block, South China. Migmatites consist of metatexite and diatexite, including various amounts of palaeosome, melanosome, and leucosome. Leucosomes from metatexite and diatexite/migmatite granite samples yield consistent U–Pb ages of 451.5 ± 2.0 Ma to 458 ± 16 Ma, respectively. Zircon U–Pb dating from two potassic plutons yield consistent crystallization ages of 451–446 Ma. Our new results of field investigation, mineralogy and microstructures, geochronology, and geochemical compositions demonstrate that the protoliths of migmatites are metasedimentary rocks, while metatexites and diatexites are the products of variable degrees of partial melting of the protolith. The potassic rocks have SiO2 contents ranging from 52.5 to 60.2 wt.%, high K2O/Na2O ratios (0.7–1.3), with MgO contents of 3.0–6.2 wt.% and Mg number of 42–57. They have enriched Sr–Nd–Hf isotopic compositions with (87Sr/86Sr)i ratios of 0.7085–0.7151, ɛNd(t) values of −9.9 to −6.2, and εHf (t) values of −10.5 to −5.3, indicating that they were generated from an enriched mantle source modified by ancient oceanic subducted sediments. Regional crustal anatexis occurred at about 451 Ma, and the timescale from crustal anatexis to melt homogenization and accumulation spans over an interval of ~15 Myr. The association of migmatites and the potassic rocks may form in post-orogenic extension setting. Thus, we suggest that the tectonic transition from syn-orogenic compression to post-orogenic extension occurred by the Late Ordovician (ca. 451–446 Ma) for the early Palaeozoic orogen in South China.

Defining the Yangtze–Cathaysia suture zone in South China using ambient noise tomography

SUMMARYThe South China Craton formed after collision between the Yangtze and Cathaysia Blocks. The suture zone between these two blocks is the crux of understanding the geological evolution of South China. Due to the relative paucity of high-resolution subsurface data, the location of the suture zone in the southwestern region remains uncertain. To find the suture zone, we created a high-resolution model of the crust and upper mantle structure using 27 months of continuous waveform data acquired from 457 stations in South China. The model uses extracted phase velocity dispersion curves of Rayleigh waves at 4–40 s from ambient noise cross-correlation functions, to perform a direct inversion for an azimuthal anisotropic VSV model. In the crust a prominent belt of strong azimuthal anisotropy exists between the Yangtze and Cathaysia Blocks. The fast polarization axes in this belt correspond to the NE-trending orogen between the two blocks. In the deeper crust of the Cathaysia Block we image multiple areas of diffuse low-velocity anomalies. As defined by the margins of the strongly anisotropic belt, the Yangtze–Cathaysia suture zone is delineated by lines that traverse: Shaoxing–Jiangshan–Pingxiang–Longsheng–Hechi and Shitai–Jiujiang–Yueyang–Dayong–Jishou. Further analysis of the seismic data links deep low velocities with widespread Late Mesozoic magmatism in the Cathaysia Block. The deep low velocities are likely a result of dehydration of the subducting Pacific Plate causing mantle upwelling and underplating.

Characteristics of azimuthal anisotropy in SE Tibetan plateau and its relationship with the background of block structure

The southeastern Tibetan Plateau, which includes the Tibetan Plateau, Yangtze Block, and Cathaysia Block, is geotectonically situated in the compound part of the Tethys-Himalayan tectonic domain and the Peninsular Pacific tectonic domain. It is one of the critical regions for studying the scientific problems of plateau material lateral escape, lithosphere deformation, geotectonic properties of blocks, and deep dynamics. In this study, we use ambient noise data recorded by 401 broadband stations to obtain high-resolution short-period (T = 4–32 s) Rayleigh wave azimuthally anisotropic phase velocity maps. These could provide fresh clues for an in-depth understanding of the crust-mantle velocity structure, deformation mechanism, and geotectonic evolution in the southeastern Tibetan Plateau. Within the Simao block, the strikes of the faults and the orientations of the principal compressive stress of the stress field both generally coincide with the fast-wave polarization direction (FPD). The FPD near the Lancangjiang fault zone in the west is in the NE-SW direction, near the Wuliangshan fault zone in the center is near the NS direction, and near the Red River fault zone in the east is the NW-SE direction. We estimate that the compressive stress in the southwest direction of the Tibetan Plateau material has a controlling effect on the crustal deformation of the Simao block, which is likewise blocked by the Lincang granite belt, resulting in strong tectonic deformation. The FPD of the crust in the middle Red River fault zone is NS direction, significantly different from the fault strike. Combining with the seismic activity and GPS results, the depth of 8 km below the surface of the middle Red River fault is completely locked, and we conclude that the anisotropy of the upper crust of the middle part of the Red River fault zone is related to the action of the regional tectonic stress field. Taking into account geochemical and thermochemical results, we speculate that the complex tectonic stress at the junction of the blocks leads to prominent regional characteristics of the FPDs of azimuthal anisotropy in the crust, suggesting that the Shizong-Mile fault zone may be the western boundary between the Yangtze block and the Cathaysia block.

The Western Segment of the Precambrian Suture Between the Yangtze and Cathaysia Blocks: Constraints From Magnetotelluric Data in Southwest China

AbstractThe western segment of the suture zone between the Yangtze and Cathaysia blocks, which is the most important tectonic boundary related to the formation and rifting of south China, is enigmatic and not fully understood due to the sporadic exposure of Precambrian strata and ophiolites. Three‐dimensional electrical resistivity models derived from inversion of magnetotelluric data identified a lithospheric‐scale conductive zone extending northeastwards beneath the Youjiang basin, which was interpreted as the western segment of the suture zone. The high conductivity and coincident high magnetic anomalies closely match the location of Carlin‐type gold deposits, which can be explained by fluids and gold‐bearing sulfide minerals in a fossil suture zone. Inconsistent with the southeast‐dip resolved at the eastern segment of the suture zone (the Jiangshan‐Shaoxing fault) in the earlier study, the slightly north‐dipping geometry at the western suture zone implies the reactivation by northward subduction and closure of the Paleo‐Tethys Ocean.

Origin and implication of two newly identified peraluminous A-type granites in the early Paleozoic orogeny, Southeast Asia

Abstract: The amalgamation of the Yangtze and the Cathaysia Block in Neoproterozoic time led to the formation of the South China Block (SCB) and generated the Jiangnan Orogen with the occurrences of juvenile magmatic rocks. After this orogeny, a typical collisional orogen formed during the early Paleozoic period in Southeast Asia which is mainly distributed in the Wuyi-Nanling-Yunkai area in the SCB. However, the transitional time from syn-collisional compression to post-collisional extension is debatable. Here, we present new data on zircon U-Pb zircon ages, Lu-Hf isotopes, and geochemistry for the Guzhang and Shadi granites from the Nanling area, South China. Both plutons have similar zircon 238U/206Pb ages of ca. 430 Ma. Petrographic and geochemical characteristics (e.g., FeOt/(FeOt+MgO) = 0.82–0.95) indicate that both granites are peraluminous A-type, with high Ga/Al ratios (2.43–2.91) as well as high concentrations of Zr, Nb, Ce, Y (sum values from 327 to 527 ppm), and formation temperature (820°C–845°C). Shadi granite exhibit high positive εHf(t) values (clustering within 0 to +6) while Guzhang granite show relatively lower εHf(t) values (−8.7 to −2.9). Their mildly negative to positive zircon εHf(t) values are higher than that of many coeval granites and can be derived from anhydrous melting of tonalitic genesis in the middle crustal depth, with the Shadi pluton having more orthometamorphite in the source. The ages and Hf isotopic compositions of inherited zircons (εHf(t = 960 Ma) = 9.2, εHf(t = 950 Ma) = 7.3) suggest that the Neoproterozoic juvenile magmatic rocks in the Jiangnan Orogen were a significant source for these granites. We interpret these A-type granites derived at the post-collisional stage. Their occurrence indicates that the geological setting of this Paleozoic orogen shifted from compression to extension no later than 430 Ma.