Songpan-Ganzi Terrane Research Articles

Remelting of the Songpan-Ganze slab: Contribution to elevated magmatic oxygen fugacity and reactivation of copper in the Yulong porphyry copper deposits belt

The factors contributing to the high magmatic oxygen fugacity (fO2) of post-subduction porphyry Cu deposits (PCDs) remain controversial. The prevailing hypothesis says that this high fO2 is inherited from the lithospheric mantle modified by previous seafloor subduction. However, recent discoveries pertaining to the prevalence of anoxic oceans, particularly the well-documented Paleo-Tethyan Ocean, would challenge this hypothesis. In order to address this matter, we studied representative primitive high-Mg igneous rocks (HMIRs) from the Yulong PCDs belt in Northern Qiangtang terrane of Tibet. These alkaline rocks are emplaced in a post-subduction setting after the closure of Paleo-Tethyan Ocean. In this study, we use newly generated in-situ mineral analyses on representative samples, together with meticulously reviewed and recalculated literature data, to elucidate the mechanism by which the oxidation process operated in the mantle source of these Cenozoic rocks. Our findings suggest that these post-subduction HMIRs exhibit characteristic compositions resembling those of continental crustal rocks and carbonatites. They display elevated fO2 levels and contain high concentrations of Cu ligands (S and Cl) derived from a source with mantle-like δ18O (4.09 to 5.39‰) and moderately low ɛHf(t =37Ma) values (4.0 to 9.3). This differs from the igneous rocks from the preceding Paleo-Tethyan arc remnants that exhibit lower fO2 levels and Cu ligand concentrations, sourced from a modified mantle with higher δ18O (5.6 to 7.9‰) and ɛHf(t =37Ma) values (9.3 to 13.4). Furthermore, through the application of zircon oxybarometry and existing experimental constraints, we have successfully identified the significance of oxidized mafic basement and overlying carbonates within the Songpan-Ganze terrane as influential oxidizing agents. Through detailed examination of geochemical and seismic data, we explore that these oxidizing agents are responsible for the formation of the oxidized HMIRs and copper mineralization. Our study thus provides significant insights into the role of subducted continental materials in generating carbonated silicate melts and causing oxidation of the mantle sources in a post-subduction setting for the petrogenesis of PCDs.

Multicyclic Phanerozoic orogeny recorded in the Qaidam continent, northern Tibet: Implications for the tectonic evolution of the Tethyan orogenic system

The growth and evolution of the Eurasian continent involved the progressive closure of major ocean basins during the Phanerozoic, including the Tethyan and Paleo-Asian oceanic realms. Unraveling this complicated history requires interpreting multiple overprinted episodes of subduction-related magmatism and collisional orogeny, the products of which were later affected by the Cenozoic construction of the Himalayan-Tibetan orogen due to the India-Asia collision. In particular, the tectonic evolution of northern Tibet surrounding the Cenozoic Qaidam Basin is poorly resolved due to several phases of Phanerozoic orogeny that have been reactivated during the Cenozoic deformation. In this study, we investigated the geology of the northern Qaidam continent, which experienced Paleozoic−Mesozoic tectonic activity associated with the development of the Eastern Kunlun orogen to the south and the Qilian orogen to the north. We combined new and published field observations, geochronologic and thermochronologic ages, and geochemical data to construct regional tectonostratigraphic sections and bracket phases of Paleozoic−Mesozoic magmatism associated with oceanic subduction and continental collision. Results suggest that the Qaidam continent experienced two major phases of subduction magmatism and collision. First, a Cambrian−Ordovician magmatic arc developed in the northern Qaidam continent due to south-dipping subduction. This phase was followed by the closure of the Qilian Ocean and the collision of the North China craton and Qaidam continent, resulting in Silurian−Devonian orogeny and the development of a regional unconformity across northern Tibet. A subsequent Permian−Triassic magmatic arc developed across the northern Qaidam continent due to north-dipping subduction. This phase was followed by the closure of the Neo-Kunlun Ocean and the collision of the Songpan Ganzi terrane in the south and Qaidam continent. These interpretations are incorporated into a new and comprehensive model for the Phanerozoic formation of northern Tibet and the Eurasia continent.

Evidence of detrital spinel geochemistry for depositional response to the Paleo-Tethyan closure recorded in the Songpan-Ganzi Terrane, eastern Tibetan Plateau

The Late Triassic evolution of the Songpan-Ganzi Terrane in the eastern Tibetan Plateau is crucial for understanding the closure of the eastern Paleo-Tethys Ocean. However, the strong thickening and deformation of sedimentary cover make the Songpan-Ganzi geological records complicated and even obscured, impeding the reconstruction of regional tectonic evolution. In this study, we provide new geochemical data of detrital spinels from the Upper Triassic strata in the eastern Songpan-Ganzi Terrane to reveal the provenance and the regional tectonic events involved in the eastern Paleo-Tethyan closure. Results show that detrital spinels from the Upper Triassic Xinduqiao, Zhuwo, and Zagunao formations have variable concentrations of Cr2O3 (24.36–68.56 wt%), Al2O3 (0.13–39.17 wt%), MgO (0.20–21.25 wt%), FeOT (11.15–48.49 wt%), and TiO2 (0.00–4.66 wt%). They are geochemically similar to spinels from both ophiolites and stratiform intrusions, and affinitive to the counterparts from island-arc, supra-subduction zone, or mid-ocean ridge settings. Based on comparison with the spinel compositions from adjacent tectonic zones and in combination with previously published evidence from paleogeography, paleocurrent orientation, and geochronology, we propose the detrital spinels from the Upper Triassic sequence in the eastern Songpan-Ganzi Terrane were mainly recycled from the Lower–Middle Triassic strata in the West Qinling Terrane, with minor concentrations derived from the A’nyemaqen ophiolites. This provenance inference presents a new insight into the closure time of the eastern Paleo-Tethys Ocean.

Mechanism and implications of the post-seismic deformation following the 2021 Mw 7.4 Maduo (Tibet) earthquake

SUMMARY A major earthquake shook the Chinese county of Maduo, located in the Songpan-Ganzi terrane on the Tibetan Plateau, on 21 May 2021. Here, we investigate the post-seismic deformation process of this event, with the aim to understand the fault geometry, friction behaviour and regional rheology. To keep the self-consistency between co- and post-seismic deformation models, we first constrain the fault geometry and coseismic slip model of this event, which are directly used in modelling the post-seismic deformation. The coseimsic slip model reveals that the majority of coseismic slip is confined at the middle (3–15 km) of the brittle layer, leading to significant shallow slip deficit. Secondly, we obtain the post-seismic deformation in the first 450 d following the 2021 Maduo earthquake using the GPS and InSAR displacement time-series data. Thirdly, a combined model incorporating afterslip and viscoelastic relaxation is built to explain the observed post-seismic deformation. Our results suggest that the viscoelastic relaxation effect should be considered in the observation period, in order to avoid the unphysical deep afterslip in the ductile lower crustal layer. Combined analysis on viscosities inferred from this study and previous studies suggests a weak lower crust with steady-state viscosity of 1018–1019 Pa s beneath the Songpan-Ganzi terrane, which may give rise to the distributed shear deformation and the development of subparallel secondary faults within the terrane. Besides, the inferred afterslip on uppermost patches of the middle fault segment suggests a rate-strengthening frictional behaviour that may be related to the coseismic slip deficit and rupture arrest of the Maduo earthquake.

The structure of the mantle transition zone and its implications for mantle upwelling beneath the central Tibetan plateau from triplicated P-waveforms modeling

The Tibetan plateau is considered as an ideal place for studying the continental collisions, tectonic movements and regional geodynamics. Although multiple episodes of igneous activities in Tibetan plateau have been suggested derived from mantle upwelling, their origins and corresponding geodynamic models remain vigorously debate. This study presents a high-resolution upper mantle velocity model of P-wave and its lateral variation beneath the central Tibetan plateau based on triplicated waveform modeling. The results show that, compared with IASP91 model, the 660-km discontinuity is deepened approximately 17–28 km from the Qiangtang terrane to the Beishan block. The lower mantle transition zone (MTZ) is characterized by high P-wave velocity anomalies (HVA) in range of 0.3%–0.9%. We infer that they are mainly related to the hydrous materials with low temperature in the lower MTZ. Besides, the depression of the 410-km discontinuity is about 6–15 km. A remarkable low velocity layer (LVL) is detected atop the 410-km discontinuity, exhibiting the thickness of 31–40 km and low P-wave velocity anomaly of −2.5% to −1.6%. This is against completely temperature-dominated origin of the LVL, and suggests that the material composition has also contributes. Additionally, we propose a new geodynamic model to demonstrate the asthenospheric upwelling beneath the Qiangtang and Songpan-Ganzi terranes, which suggests that the passively ascending water-enriched materials from the lower MTZ and their dehydration are responsible for the observed LVL atop the 410-km discontinuity, as well as the origin of the Cenozoic magmatic intrusion and volcanic eruption of the northern Tibet volcano in Pleistocene.

Ongoing India–Asia convergence controlled differential growth of the eastern Tibetan Plateau

The eastern Tibetan Plateau is known for its unique topography and active tectonics, serves as a crucial region for investigating geodynamics and lithosphere–atmosphere interaction. Despite its importance, the formation process and geodynamics of this region remain controversial. We integrated new and reported thermochronological data, and paleo-crustal thickness reconstruction to explore the tectonic deformation, climate–tectonic coupling, and geodynamics of the area. New apatite fission track data, collected from the upstream areas of major rivers in the eastern Tibetan Plateau, revealed multiple rapid river incision phases during the Eocene and Neogene–Quaternary. Apatite (U-Th)/He data obtained from a borehole near the Gaoligong shear zone indicates that the Yingjiang fault experienced two rapid exhumation phases, which initiated at ∼22 Ma and ∼18 Ma, respectively. Thermo-kinematic modeling (Pecube), based on multiple thermochronometers, predicted three phases of thrusting along the Longmenshan fault, which were initiated at ∼34 Ma, ∼17 Ma and ∼14 Ma, respectively. During the late Eocene period, Sr/Y and (La/Yb)n ratios suggest that the eastern Qiangtang and Songpan–Ganzi terranes underwent considerable uplift, followed by outward uplift in the collisional front zone and the southeastern margin of the plateau during the late Oligocene. Paleo-crustal thickness reconstruction also indicate another ∼1 km surface uplift during the Neogene–Quaternary. Our findings, when combined with reported thermochronological data, suggest that during the Neogene–Quaternary, the collisional front zone and the eastern margin of the plateau underwent greater exhumation compared to the eastern Qiangtang and Songpan–Ganzi terranes. The temporally and spatially varied exhumation shaped the landscape of the eastern Tibetan Plateau, driven mainly by precipitation-related erosion and promoted by fold-thrust and strike-slip thrust. This was controlled primarily by eastward lithosphere/asthenosphere extrusion associated with ongoing India–Asia convergence.

Origins and evolution of two types of Late Triassic granitic magmas in the Caolong-Xiangkariwa area of central-eastern Songpan-Ganze terrane, northern Tibet: Implications for pegmatite lithium mineralization

Pegmatite-type lithium (Li) deposits are an important source of the low-carbon energy metal, which is generally considered to be formed by the high degrees of differentiation of strongly peraluminous granitic magma. However, the question of whether the parental rocks of these peraluminous magmas are igneous or sedimentary has been widely debated. Recent studies have identified a world-class pegmatite-type Li deposit belt (up to 2800 km long) closely associated with the Late Triassic granitoids and (meta)sedimentary rocks along the West Kunlun and Hoh Xil-Songpan-Ganze terranes in northern Tibet. This study presents a comprehensive petrological, geochronological, and geochemical study on the two types of Late Triassic granitoids in the Caolong-Xiangkariwa area of the Yushu region of central-eastern Hoh Xil-Songpan-Ganze terrane in northern Tibet: type A includes the Tongtianhe-Zhenqin-Zhaduo-Zhaya-Xiangkariwa (TZX) diorites and granodiorites (218−212 Ma), and type B includes the Caolong two-mica and muscovite granites (213−205 Ma), as well as ore-barren, beryl-bearing, and spodumene-bearing pegmatite dikes (209−196 Ma). The TZX diorites and granodiorites contain variable amounts of amphibolite and biotite. They are metaluminous to slightly peraluminous with variable SiO2 (54.4−71.3 wt%), MgO (0.72−7.26 wt%) contents, and Mg# values (40−70). Some samples with low SiO2 (54.4−58.5 wt%) contents have geochemical features similar to high-Mg andesites: e.g., high MgO contents (5.24−7.26 wt%) and Mg# values (61−70). The TZX diorites and granodiorites exhibit less enriched (86Sr/87Sr)i (0.7080−0.7118) and εNd(t) (−4.8 to −8.0). By contrast, the Caolong two-mica and muscovite granites are characterized by the occurrence of abundant muscovite (plus tourmaline and garnet) and are strongly peraluminous. The Caolong granites and pegmatites are geochemically characterized by high SiO2 contents (68.8−79.8 wt%) and low MgO contents (0.01−0.10 wt%) and Mg# values (4−23). They have εNd(t) values (−8.8 to −11.4; granites: −8.8 to −10.8; pegmatites: −9.2 to −11.4) that are more enriched than the TZX diorites and granodiorites but similar to those of Songpan-Ganze (meta)sedimentary rocks (−7.4 to −12.9), and the Caolong two-mica granites have high zircon δ18O values (11.6−12.1). In addition, decreasing K/Rb ratios correspond to increasing Cs contents in K-feldspar and muscovite from the Caolong granites and pegmatites. Taking into account the Late Triassic granitoids and (meta)sedimentary rocks in the Hoh Xil-Songpan-Ganze terrane, we suggest that the TZX diorites and granodiorites were most probably formed by the partial melting of metasomatized lithospheric mantle that subsequently underwent extensive fractional crystallization. Conversely, the Caolong two-mica and muscovite granites were likely generated purely by the partial melting of (meta)sedimentary rocks rather than via the evolution of dioritic-granodioritic magmas. In summary, there were two kinds of granitic magmas with different sources in the Caolong-Xiangkariwa area. Finally, the Caolong pegmatites were most likely formed by the extreme differentiation of two-mica granitic magmas rather than the dioritic-granodioritic magmas. Therefore, the evolution of magmas derived from (meta)sedimentary rocks were most likely to have formed the Li-rich pegmatites. Moreover, these (meta)sedimentary rocks representing the ultimate Li source must have undergone strong chemical weathering, resulting in significant Li enrichment.

Boron isotopes in tourmaline from drill core of the Jiajika granitic pegmatite type lithium deposit: Insights for granitic magma evolution and lithium enrichment

The Jiajika pegmatite-type Li deposit located in the southeastern Songpan-Ganze terrane of the eastern Tibetan Plateau is one of the largest hard-rock Li deposit in China, and its ore-genetic mechanism is still controversial. To understand the Li concentration of potentially economic levels in the Jiajika pegmatite-type Li deposit, the association between Li enrichment and granitic magma evolution is investigated using B isotope in tourmaline grains along the entire 3211-m-depth of the JSD-1 drill core in this work. Tourmalines hosted in granite, pegmatite, barrovian-type metamorphic rock and calc-magnesium-silicate metamorphic rock are being plotted in the fields of Li-poor granitoids and associated pegmatites and aplites on the Al-Fe-Mg and Ca-Fe-Mg ternary diagrams. These characteristics, along with their high Fe/Mg ratios and high Al cation numbers in Y position, are indicative of a magmatic origin. The B isotope (δ11B) of tourmalines in granite and pegmatite in Jiajika granitic pegmatite type Li deposits vary from −9.49 to −7.06‰, in line with the range observed in 90 % of worldwide granites and pegmatites. In comparison of the measured data of δ11B vs. Li to the modeled curves indicates that the differentiation process of granitic magma follows an equilibrium crystallization model better than a fractional crystallization model. The compiled evidence proves that magma differentiation accompanied by fluid exsolution facilitates the enrichment of Li during granitic magmatic evolution. The formation of the domal structure of granitic sheets due to the decompressing effect of magma ascent favors massive fluid exsolution at the late-stage granitic magma, which contributes to the deposition of spodumene dominated ore body at the shallower depth of 5–150 m.

Geologic scenario from granitic sheet to Li-rich pegmatite uncovered by Scientific Drilling at the Jiajika lithium deposit in eastern Tibetan Plateau

The Jiajika pegmatite-type lithium deposit emplaced in Upper Triassic turbidites is located in the southeastern Songpan-Ganze terrane of the eastern Tibetan Plateau and is the largest hard-rock lithium deposit in China. The Jiajika Scientific Drilling Project (JSD) consists of the JSD-1 borehole, which is 3211 m deep, and two 1000 m boreholes, JSD-2 and JSD-3. The JSD-1 borehole is composed of the upper pelitic metasediment unit (0–900 m) and the lower are calc-silicate unit (900–3211 m). Pelitic metasediments at the depth of 0–400 m experienced Barrovian-Buchan metamorphism, while metasediments at the depth of 0–900 m were associated with domal deformation (D2). The calc-silicate unit below 900 m only experienced earlier orogenic deformation (D1). The mineralization of Li-Be and Nb-Ta occurs at a depth of 0–1200 m and 0–1800 m, respectively, while the Rb enrichment occurs throughout the whole borehole. Analyses of K-Fe-Ba-B-Li-Rb isotopes suggest that the Jiajika lithium deposit resulted from high-degree fractionation of the parent granitic magma and strong fluid immiscibility. The pegmatites may have crystallized from volatile-rich melts during two-stage magmatic-hydrothermal events at 205–210 Ma and 192–193 Ma. The JSD-1, JSD-2 and JSD-3 indicate that S-type granite related to the Jiajika lithium deposit was emplaced as multi-layer domal sheets, rather than as a deeply rooted pluton. The domal granitic sheets are surrounded by pegmatites and meta-sedimentary rocks. Therefore, we propose a “domal granitic sheet” model to interpret the geologic scenario from granite sheet to Li-rich pegmatite in the Jiajika ore field. The Upper Triassic turbidites in the Songpan-Ganze terrane underwent significant orogenic decollement at ∼ 230 Ma, followed by deep anatexis, basement doming, magma rising and emplacement of granitic sheets. Finally, the rare-metal-rich pegmatites intruded under domal decompression condition, forming a multi-layer architecture intercrossed by the granitic sheets and the meta-sediments.

Post-seismic deformation of the 2008 Wenchuan earthquake reveals a misaligned rheological boundary in the lower crust along the eastern Tibetan Plateau margin

SUMMARY Along the margins of orogenic plateaus, the viscous Earth structure and fault geometries play a primary role in controlling the tectonic evolution and earthquake generation. After the 2008 Mw 7.9 Wenchuan earthquake, the long-standing debate regarding the tectonics producing and maintaining prominent topography across the Longmen Shan reignited. Post-seismic deformation, representing the surface strain history in response to lithospheric stress perturbations, provides important insights into the lithospheric rheology and active structures. Here, we construct a new 3-D post-seismic deformation model for the Wenchuan earthquake, invoking viscoelastic relaxation and afterslip. Our best-fitting model indicates that the steady-state viscosities of the lower crust in the region to the immediately west of the Songpan-Ganzi terrane and beneath the Songpan-Ganzi terrane are estimated to be 4.0 × 1018 and 1.0 × 1018 Pa s, respectively. Our results, combining geophysical and geodetic observations and model analyses, highlight the prevalent parallelism between the rheological and structural boundaries of the lower crust, which diverge northward away from the trend of the Longmen Shan fault at ∼20°. This diverging rheological structure and the partially coupled upper and lower crust have broad implications for the stress build-up, strain partitioning and deformation styles along the eastern Tibetan Plateau margin.

Geochemistry of the Lancang River (Upper Mekong River) overbank sediments: Implications for provenance, weathering and sedimentary characteristics

Large rivers originating from the Tibetan Plateau play a crucial role in sediment production and transportation and thus influence the ocean composition and global climate. This study focuses on the geochemical characteristics, including major, trace and rare earth elements of overbank sediments from the Lancang River (Upper Mekong), which drains southwest China and is one of the largest international rivers in Asia. We characterize sediment maturity and recycling, weathering and provenance in a typical, tectonically active cold-humid climate region. Lancang River (LR) sediments with an average chemical index of alteration (CIA) of 73 ± 6 and an average index of compositional variability (ICV) of 1.1 ± 0.6 suggest an intermediate mineralogy and chemical maturity corresponding to moderate weathering in a slightly warmer climate. The provenance of LR sediments is characterized by felsic source rocks, similar to the upper continental crust (UCC), followed by recycled sedimentary rocks, which were possibly derived from the Yidun, North Qiangtang (NQ) and Songpan-Ganzi (SGZ) terranes at upstream locations when the Lingcang basalt (LCB) provided mafic rocks for LR sediments. However, recycled materials have limited impacts on sediment maturity, as reflected by the degree of weathering. Highly physical weathering (erosion) has created a massive source for LR sediments due to active tectonics, while the impacts of climate on weathering processes have increased. Meanwhile, the overbank sediment in large rivers could be a good sampling medium for evaluating terrestrial weathering, provenance and sediment characteristics using geochemical evidence.

Genesis and exhumation of the Kongur-Muztaghata and Maeryang gneiss domes in NE Pamir since the Mesozoic

The Kongur-Muztaghata-Maeryang terrane in NE Pamir is considered to be the western extension of the Songpan-Ganze terrane located in the northern Tibetan Plateau. The Kongur-Muztaghata gneiss dome (KMGD) is situated in the north while the Maeryang gneiss dome (MYGD) is in the south. The KMGD comprises Triassic granites and granitic gneiss in the core and Early Paleozoic-Triassic sediments in the mantle that underwent Barrovian-type and Buchan-type metamorphisms. Based on geochemical and geochronological data, the Kongur-Muztaghata magmatic arc was formed around ∼252–204 Ma due to northward subduction of the Paleo-Tethys Jinsha oceanic slab. The collision of the Kongur-Muztaghata magmatic arc and the Qiangtang terrane occurred subsequently. Previous research suggested that the KMGD was formed in the Miocene (21–8 Ma). However, our new in-situ monazite U–Pb data for the mantled metasediment shows that the KMGD was initially formed at ∼198 Ma.The MYGD is comprised of an Early Paleozoic-Triassic metasediment mantle and a Cambrian anatexis complex core that underwent Barrovian-Buchan metamorphisms. Our new structural, geochemical, and geochronological data suggest that the protolith of the Maeryang orthogneiss was formed around ∼519-513 Ma, with the surrounding Early Paleozoic metavolcanic rocks erupted at ∼519-508 Ma. Together, they formed the Early Cambrian magmatic complex. In-situ U–Pb dating of monazites and zircon metamorphic rims for the Triassic metamorphic rocks in the mantle indicate that the Barrovian-Buchan metamorphism in the MYGD occurred around ∼206-187 Ma, likely caused by anatexis in the deep crust of the gneiss dome core. Thus, we propose that the KMGD and MYGD underwent a two-stage exhumation: the initial uplift during the Late Triassic-Early Jurassic thermo-tectonic event associated with the Cimmerian orogeny and the late rapid exhumation since the Miocene driven by the collision between the Eurasian and Indian plates.

Cenozoic tectonosedimentary evolution of the Linxia Basin, northeast Tibetan Plateau based on an analysis of detrital zircon provenance

The Linxia Basin is an important region for understanding the tectonism and uplift history in the northeast Tibetan Plateau, and also contains rich mammalian faunas of evolutionary significance. In this paper, we conduct a provenance study of Cenozoic sediments, relying primarily on U-Pb dating of detrital zircons, to investigate the exhumation history of surrounding terranes and to gain a better understanding of the relationship between tectonism, basin development and paleoenvironment.Samples were collected from the Maogou section in the central Linxia Basin, which has been recently dated by magnetostratigraphy. Detrital zircon populations show five UPb age groups of 65–350 Ma, 350–600 Ma, 600–1600 Ma, 1600–2200 Ma and 2200–2600 Ma; however, UPb age patterns varied up-section, indicating different sources at different times.Sediments from the lower part of the section (>27.8 Ma) were sourced from multiple regions, including the distal East Kunlun Shan, Qilian Shan, and Songpan–Ganzi terranes, and as well as the proximal West Qinling region. Contributions from the Songpan–Ganzi terrane increased considerably during the late Oligocene (27.8–23 Ma), as a result of strong tectonic movements of the northeastern Tibetan Plateau. During the early Miocene (23–17.2 Ma), the northern sources of the Laji Shan–Maxian Shan region supplied more sediments to the basin, indicating the uplift and exhumation of these mountain ranges. Until the middle Miocene (17.2–12.8 Ma), the Songpan-Ganzi terrane and the northern sources dominated the basin sediments, while the northern sources and the Jishi Shan region became increasingly important sources at around 11.6 Ma. This study explores the tectonosedimentary evolution of the Linxia Basin in the northeastern Tibetan Plateau, providing a reference for further research of the paleoclimate and mammalian evolution in this critical region.

Three-dimensional electrical structure and seismogenic environment of the crust–mantle in the Lushan earthquake region, China

The Longmenshan (LMS) southern section is the front of the eastwards material migration from the Qinghai–Tibet Plateau. To understand whether the 2022 Ms6.1 Lushan earthquake was a strong aftershock of the 2013 Ms7.0 Lushan earthquake and why the aftershocks of the two earthquakes were concentrated relative to the 2008 Wenchuan earthquake, a three-dimensional electrical structural model of the Lushan earthquake region is established based on magnetotelluric (MT) array data. The model shows high-conductor layers (HCL) in the Songpan–Ganzi terrane (SGT) middle crust, Sichuan Basin (SCB) shallow surface, western Sichuan foreland depression (WSCFD) and LMS Moho. The analysis suggests that the HCL in the study area at a depth of 10 km is caused by saline fluids in the fracture zone or porous sedimentary rock voids, while the high-conductor body in the middle and lower crust and upper mantle is caused by partial melting of rocks. The Lushan earthquake area is subject to compressional deformation horizontally as the eastwards migration of Qinghai–Tibet Plateau material is blocked by the SCB and vertically by the upwards extension of high-conductor plastic material at depth. The aftershocks of the Lushan earthquake area are mainly confined within the Baoxing–Zhonglin–Daxing–Lushan high-conductor region, which has noticeable electrical structural differences compared to the overall high-resistivity structure of the LMS southern section; these differences explain why the aftershocks of the two earthquakes did not expand in the northeast and southwest directions of LMS. Because the 2022 and 2013 Lushan earthquakes both occurred in the same high-conductor region in the deep part of the Dachuan–ShuangShi fault to the Danyi blind fault, the 2022 Lushan earthquake was a strong aftershock of the 2013 Lushan earthquake in terms of electrical structural characteristics.

A study of site response in the Longmen Shan and adjacent regions and site response models for the Sichuan Basin

We investigated the regional attenuation and site responses in the Sichuan Basin and adjacent Songpan-Ganze terrane of the Tibetan Plateau using seismic data recorded at 41 stations from regional earthquakes occurring between January 2009 and October 2020. Fourier amplitude spectra of Lg waves were computed and binned into 18 frequency bins with center frequencies ranging from 0.1 Hz to 20.4 Hz. The quality factor is estimated as Q(f)=313f0.74 for the Sichuan Basin and Q(f)=568f0.34 for the Songpan-Ganze terrane, reflecting significant differences in the crustal structure beneath these two regions. Relative to the Songpan-Ganze terrane, site responses in the Sichuan Basin are characterized by strong amplification effects at frequencies lower than 6 Hz and obvious attenuation at higher frequencies (>10 Hz). κ0 of stations in the Sichuan Basin show clearly geographical dependence with an average value of 0.045 s, whereas stations in the Songpan-Ganze terrane generally have smaller κ0 values with an average value of 0.028 s. In particular, site response and κ0 of stations in the Sichuan Basin are found to be dependent on the geographically variable thickness of the sedimentary deposits (sediment thickness). These units are comprised of sedimentary rock and semi-consolidated sediments, with a maximum thickness reaching approximately 10 km. Site response terms in the Sichuan Basin derived from the Lg Fourier spectra exhibit consistent patterns versus sediment thickness as frequency increases. We developed site response models as functions of sediment thickness for stations in the Sichuan Basin. The site response model derived from Lg site terms is consistent with that based on site response terms from coda amplitude spectra and horizontal to vertical (H/V) spectral ratios. The models were then incorporated in the stochastic method of ground motion predictions in the Sichuan Basin for six earthquakes occurring between October 2020 and June 2022. Residual analysis suggests that incorporating the site response models as functions of sediment thickness can improve the ground motion prediction model for the Sichuan Basin from moderate earthquakes.

Two Phases of Crustal Shortening in Northeastern Tibet as a Result of a Stronger Qaidam Lithosphere During the Cenozoic India–Asia Collision

AbstractAlthough two phases of Cenozoic crustal deformation in the Qilian Shan thrust belt of northeastern Tibet has been documented, their dynamic causes remain unclear. To address this issue, we investigate whether the mechanical strength of the Qaidam Basin, which is located between the Eastern Kunlun Range and the Qilian Shan, was a controlling factor for the observed deformation history by performing 2‐D thermal–mechanical simulations. Our models consider the division of the Tibetan terranes (i.e., Qilian arc, Kunlun‐Qaidam, Songpan‐Ganzi, Qiangtang, and Lhasa, from north to south) bounded by suture zones. Simulation results show that weak suture zones in central Tibet can lead to peeling off and sinking of the mantle lithosphere beneath the Qiangtang and Songpan–Ganzi terranes. After lithospheric delamination, varying north‐south width, thickness and strength of the Qaidam crust creates three end‐member model results: (a) the mantle lithosphere of the Lhasa terrane delaminates while the mantle lithosphere beneath Eastern Kunlun‐Qaidam terrane and the Qilian Shan thrust belt (KQQ) remains undeformed, (b) the Lhasa mantle lithosphere moves northward while the KQQ mantle lithosphere subducts southward, and (c) the Lhasa mantle lithosphere moves northward while lithospheric thickening occurs north of the Qaidam Basin. Our simulations show that pre‐existing weaknesses in northeastern Tibet can be activated and deformed shortly after the onset of collision, and a second wave of deformation sweeps across northeastern Tibet after lithospheric delamination in northern Tibet.

The Late Triassic Longmenshan lateral foreland thrusting: New insights from geological evidence and 3-D particle discrete-element simulation

This work proposes a new lateral foreland thrusting model based on geological evidence and 3-D particle discrete-element simulation to explain the Longmenshan southeastward thrusting during the closure of the Songpan-Ganze basin. The Late Triassic NE−SW compression caused by the northward movement of the Qiangtang Block and the resulting differential shortening within the wedge-shaped Songpan-Ganze terrane produced southeastward topographic gradient. The thick sedimentary pile, driven by the horizontal tectonic force and the deviatoric stress generated from gravitational effect, and decoupled from the subducting basement by the low-strength décollement, was laterally extruded and resulted in the southeastward Longmenshan thrusting. Therefore, the Longmenshan thrust belt is a lateral foreland thrust belt of the Songpan-Ganze terrane. For the first time, 3-D particle discrete-element simulation was used for the geological study of the Longmenshan area, and it clearly reproduces the dynamical process of the Longmenshan southeastward thrusting and well predicts the Xiaojin Arcuate Zone. The particle discrete-element simulation results verify the new model and reveal that two key factors facilitate the lateral foreland thrusting: the wedge-shaped geometry that produces differential shortening and lateral topographic gradient, and the low-strength décollement that decouples the extruded sedimentary pile from the basement. The lateral foreland thrust belt, which is unique in its tectonic location and dynamic behavior, is a new kind of foreland thrust belt that is different from the pro-foreland and retro-foreland thrust belts, and it provides new insight into the tectonic evolution of collisional orogens.

Post-Collisional Silica-Undersaturated Bamaoqiongzong Volcanic Rocks from Northern Qiangtang: Indicators of the Mantle Heterogeneity and Geodynamic Evolution of Central Tibet

AbstractThe formation of post-collisional mantle-derived rocks in the Tibetan Plateau has been linked to the deep geodynamic processes that cause surface uplift. Co-existing silica-oversaturated to silica-undersaturated mantle-derived rocks have been identified in the northern Qiangtang Terrane (NQT). However, the origins of silica-undersaturated magmas are controversial, and the mechanisms responsible for variable silica activity in the mantle-derived rocks are unclear. Here, we present 40Ar/39Ar chronology, mineral chemistry, and whole-rock geochemical data for the Bamaoqiongzong (BMQZ) volcanic rocks of the NQT. The BMQZ volcanic rocks consist of olivine leucitites, trachybasalts, and phonolites and were erupted at ca. 29 Ma. All samples are unsaturated in silica and characterized by enrichment in light rare earth elements and large-ion lithophile elements, depletion in high-field-strength elements, and the presence of negative Nb–Ta–Ti anomalies and positive Pb anomalies. All samples show limited variation in (87Sr/86Sr)i (0.7079–0.7085) and εNd(t) values (−6.9 to −5.3). The geochemical compositions of the BMQZ volcanic rocks indicate that they were produced by partial melting of carbonated phlogopite–lherzolite within the lithospheric mantle. The formation of the olivine leucitites-trachybasalts-phonolites suite was controlled by fractional crystallization and magma mixing in a magmatic plumbing system. This plumbing system included several independent reservoirs and conduits within the crust. The enriched mantle sources of the BMQZ volcanic rocks were formed by the addition of carbonate-rich melts released from the southward-subducted Songpan–Ganzi Terrane after the Late Cretaceous. Our new results, together with published data, reveal systematic variations in geochemical compositions between silica-undersaturated and silica-oversaturated rocks in the NQT, which are ascribed to variations in the nature of the subducted continental materials added during intracontinental subduction. Carbonate-rich melts that were formed by the breakdown of carbonate minerals helped to generate the mantle sources of silica-undersaturated rocks, whereas silicate melts produced by the partial melting of sediment diapirs contributed to the generation of the silica-oversaturated rocks. On the basis of published numerical modelling of continental subduction and crustal deformation records in the NQT, we suggest that intracontinental subduction and lithospheric thinning together contributed to the generation of post-collisional mantle-derived rocks in the Tibetan Plateau.

Imaging the Northeastern Crustal Boundary of the Tibetan Plateau With Radial Anisotropy

AbstractWe obtained a 3‐D crustal radial anisotropy model beneath northeastern (NE) Tibet by joint inversion of Rayleigh and Love dispersion curves from ambient noise tomography. Positive crustal radial anisotropy and significant low velocity are dominant beneath the Songpan‐Ganzi Terrane (SGT), indicating the presence of crustal channel flow. The Qilian orogen is characterized by negative radial anisotropy, which could be caused by folding and thrusting due to crustal shortening. This difference in radial anisotropy suggests that crustal shortening deformation may have occurred at a relatively early stage of the plateau evolution (the Qilian orogen) and crustal channel flow at the later stage (the SGT). The crustal radial anisotropy clearly defines the NE crustal boundary of the expanding Tibetan Plateau, which is roughly situated along the north Qilian frontal thrust from 97° to 103.5°E in the north and turns north‐south, passing the Zhuanglanghe Fault and terminating at the Kunlun Fault in the south.

Late Permian High-Ti and Low-Ti Basalts in the Songpan–Ganzi Terrane: Continental Breakup of the Western Margin of the South China Block

Although the Emeishan Large Igneous Province (ELIP) has been thoroughly researched, the role of the ELIP in the tectonics of the Songpan–Ganzi extensional basin in the eastern Tibetan Plateau has long been argued without any corroborated and robust evidence. We have investigated the basalt succession of the Dashibao Formation along Xindianzi and Xuecheng sections in the southeast margin of the Songpan–Ganzi Terrane (SGT). New SIMS zircon U-Pb ages and geochemical features of the Dashibao Formation are reported in this paper. Zircons of basalt sample XDZ02-1 yielded a weighted mean age of 259.1 ± 1.66 Ma, which is in alignment with the period when the main eruption of the ELIP occurred. Zircons from two tuff samples XDZ05-1 and XC05-2, overlying the basalt succession, were dated at 251.8 ± 1.57 Ma and 251.5 ± 0.27 Ma, respectively. These new dating results revealed a ~10 Ma eruption for the Dashibao basalts. The Dashibao basalts are geochemically classified into alkaline basalts (Group 1) and tholeiitic basalts (Group 2), which are a part of the Emeishan basalt. We thus propose that the Dashibao basalts erupted in a continental rift setting, located at the margin of the ELIP. The temporal and spatial coincidence of the Dashibao basalt, ELIP, and continental rifting in the western margin of the South China Block suggest that the continental breakup is a response to the Permian mantle plume that triggered the separation of the SGT in the eastern Tibetan Plateau.