Longmenshan Area Research Articles

The last episode of coral-stromatoporoid reef and faunal changes across a Devonian-Carboniferous Boundary section of South China

This study reports the final occurrence of coral-stromatoporoid biostromal reef and the faunal and facies change across the Devonian-Carboniferous Boundary (DCB) at Changtanzi section, Longmenshan area of South China. The biostrome occurred in the third member of the Changtanzi Formation and is uppermost Famennian in age, characterized by abundant and high diversity stromatoporoids and tabulate corals, which were subsequently greatly affected with the environmental fluctuations indicated by the end-Devonian carbon isotope excursion. Stromatoporoids and tabulate corals survived and thrived in the uppermost Famennian but their association was eventually terminated in the Lower Hangenberg Crisis (LHC) by extinction of Famennian stromatoporoids. Brachiopods and rugose corals are found to occur as early as in the Upper Hangenberg Crisis (UHC), represented by solitary rugose corals Hebukophyllum sp., Neozaphrentis sp. and Uralinia sp., as well as abundant brachiopods, gastropods, in general representing post-disaster fauna of low species level diversity. This study shows distinctive faunal and lithofacies changes during the Hangenberg Crisis, and examines the strong link between faunal and environmental changes. The result indicates that the environmental fluctuations in the Devonian-Carboniferous transitions caused sudden and significant impacts on marine ecosystems, leading to the disappearance of coral-stromatoporoid metazoan reefs.

Debris flow volume prediction model based on back propagation neural network optimized by improved whale optimization algorithm.

Debris flow is a sudden natural disaster in mountainous areas, which seriously threatens the lives and property of nearby residents. Therefore, it is necessary to predict the volume of debris flow accurately and reliably. However, the predictions of back propagation neural networks are unstable and inaccurate due to the limited dataset. In this study, the Cubic map optimizes the initial population position of the whale optimization algorithm. Meanwhile, the adaptive weight adjustment strategy optimizes the weight value in the shrink-wrapping mechanism of the whale optimization algorithm. Then, the improved whale optimization algorithm optimizes the final weights and thresholds in the back propagation neural network. Finally, to verify the performance of the final model, sixty debris flow gullies caused by earthquakes in Longmenshan area are selected as the research objects. Through correlation analysis, 4 main factors affecting the volume of debris flow are determined and inputted into the model for training and prediction. Four methods (support vector machine regression, XGBoost, back propagation neural network optimized by artificial bee colony algorithm, back propagation neural network optimized by grey wolf optimization algorithm) are used to compare the prediction performance and reliability. The results indicate that loose sediments from co-seismic landslides are the most important factor influencing the flow of debris flows in the earthquake area. The mean absolute percentage error, mean absolute error and R2 of the final model are 0.193, 29.197 × 104 m3 and 0.912, respectively. The final model is more accurate and stable when the dataset is insufficient and under complexity. This is attributed to the optimization of WOA by Cubic map and adaptive weight adjustment. In general, the model of this paper can provide reference for debris flow prevention and machine learning algorithms.

Present‐day Upper‐crustal Strain Rate Field in Southeastern Tibet and its Geodynamic Implications: Constraints from GPS Measurements with ABIC Method

AbstractThe Earth's surface kinematics and deformation are fundamental to understanding crustal evolution. An effective research approach is to estimate regional motion field and deformation fields based on modern geodetic networks. If the discrete observed velocity field is obtained, the velocity related fields, such as dilatation rate and maximum shear strain rate, can be estimated by applying varied mathematical approaches. This study applied Akaike's Bayesian Information Criterion (ABIC) method to calculate strain rate fields constrained by GPS observations in the southeast Tibetan Plateau. Comparison with results derived from other three methods revealed that our ABIC‐derived strain rate fields were more precise. The maximum shear strain rate highlighted the Xianshuihe–Xiaojiang fault system as the main boundary for the outward migration of material in southeastern Tibet, indicating rotation of eastern Tibet material around the eastern Himalaya rather than whole extrusion along a fixed channel. Additionally, distinct dilatation rate patterns in the northeast and southwest regions of the fault system were observed. The northeast region, represented by the Longmenshan area, exhibited negative dilatational anomalies; while the southwest region, represented by the Jinsha River area north of 29°N, displayed positive dilatational anomalies. This indicates compression in the former and extension in the latter. Combined with deep geophysical observations, we believe that the upper and lower crusts of the Jinsha River area north of 29°N are in an entire expanding state, probably caused by the escape‐drag effect of material. The presence of a large, low‐viscosity region south of 29°N may not enable the entire escape of the crust, but instead result in a differential escape of the lower crust faster than the upper crust.

Analysis on multi period activity and evolution of Yingxiu-Beichuan fault in Longmenshan area

As the boundary between the Yangtze Craton and the Qinghai-Tibet Plateau, the Longmenshan tectonic belt is known for its typical fault-slip systems and strong Cenozoic activities. Due to the complexity of the structure of the orogenic belt, the tectonic evolution and formation mechanism of the Longmenshan tectonic belt have been controversial. This paper essentially focuses on the geological isotopic chronology limitation of the fault activity of the Longmenshan Central Fault (Yingxiu-Beichuan Fault). The K-Ar dating of the illite in the fault gouge sample on the fault surface reveals that the direct age of the fault would be about 111 Ma. Combined with the previous age results, the following results are obtained: The Yingxiu-Beichuan fault was a multi-stage active fault in the Mesozoic, which was active in the Late Triassic (229–216 Ma), Early Jurassic (190–171 Ma) and the end of Early Cretaceous (130–110Ma). This paper analyzes the evolution of the central fault through regional geological evolution, seismic profile analysis and structural physical simulation experiment, and the following main results are obtained: Animaqing Paleo-Tethys Ocean on the northern margin of the Songpan Garze Block and the Jinshajiang Paleo Tethys Ocean in the southern margin were closed successively in the Middle and Late Triassic, Such a fact essentially occurred under the influence of differential uplift, that is, the pre-existing Yingxiu-Beichuan fault changed from a normal fault to a reverse fault in the Late Triassic, and then reactivated. Under the influence of the gravity slip mechanism, the Yingxiu-Beichuan fault was active until it ceased in the Early Jurassic. Since then, affected by the closure of the Middle Tethys Ocean, in the Early Cretaceous, the fault developed again, and branch faults developed in front of the fault. During the Himalayan period, due to the collision and ccocnnection of the Eurasian plate and the Indian plate, the fault was reactivated.

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.

Topography effect on ambient noise tomography: a case study for the Longmen Shan area, eastern Tibetan Plateau

SUMMARY Ambient noise tomography (ANT) is a widely used method to obtain shear wave velocity structure in the crust and upper mantle. Usually, the topography is assumed to have negligible effect on the resulting models. This, however, might not be proper in regions with large topographic variation, such as plateau edges, submarine slopes and volcanic islands. In this study, we use synthetics from waveform-based numerical simulation to quantify the topography effect on ANT in the Longmen Shan area, eastern Tibetan Plateau margin. Three kinds of models are used in forward simulation to obtain theoretical waveforms, including Case1: the layered model, Case2: the layered model with topographic variation and Case3: the flattened model of Case2. The final inversion results show that the bias of ANT is negligible in the blocks with relatively flat topography, such as the interior regions of the Tibetan Plateau and the Sichuan Basin. However, for the Longmen Shan boundary zone with significant topographic variation (∼4 km), the shear wave velocity image has an obvious negative bias that can reach up to −4 per cent. The maximum depth of bias is ∼5 km, which is mirrored with the maximum topographic elevation difference of the region, and the average bias disappears as the depth decreases to the surface (0 km) or increases to three times of the maximum influence depth (∼15 km). The horizontal distribution of the tomographic bias is almost linearly related to the topographic elevation difference with a slope of −1.04 and a correlation coefficient of 0.90 at maximum influence depth. According to this first-order correction formula and the decreasing trend of average bias with depth, the topography effect on ANT can be suppressed to a certain extent.

Tectonic evolution of the Triassic Songpan‐Ganzi basin as constrained by a synthesis of multi‐proxy provenance data

AbstractThe triangular Songpan‐Ganzi flysch terrane exposes a Triassic turbidite sequence with an average thickness of ca. 8 km. The sediments may have been accumulated in a remnant Paleo‐Tethyan ocean bounded by the converging North China, South China, and the Qiangtang terrane from three sides, or a back‐arc basin with an oceanic basement created during the Triassic closure of the Paleo‐Tethyan ocean. To differentiate the two competing models, we systematically reviewed the available provenance data that include U–Pb detrital zircon ages at the basin scale, paleocurrent directions, sandstone petrography, and heavy‐mineral assemblages from the Triassic Songpan‐Ganzi basin samples. We use the Kolmogorov–Smirnov tests to differentiate competing hypotheses for detrital‐zircon provenance interpretations and DZmix modelling to quantify relative contributions of detrital zircon from all potential source areas for the Triassic Songpan‐Ganzi deposits. The most important result of this work is that the Songpan‐Ganzi basin had a stable and locally derived source system: the western, central and eastern sub‐basins were mainly sourced from the north whereas the easternmost and southeastern sub‐basins were mainly sourced from westernmost South China (i.e., the Longmen Shan area) and the Qiangtang terrane. The stability of the source areas around the Songpan‐Ganzi basin throughout the Triassic is most compatible with the remnant ocean model that predicts a long‐lived marine basin with a pre‐Triassic oceanic/continental basement trapped between converging continental blocks during the Triassic.

The reservoir characteristics research of Devonian Jinbaoshi Formation, Northwest Sichuan Basin, SW China

A great breakthrough was achieved on the Middle Devonian's oil-gas exploration in the Longmenshan area in northwestern Sichuan. However, only the Guanwushan Formation was researched in detail. The underlying formation - Jinbaoshi Formation as a set of the potential well reservoir and its reservoir characteristics have not been discussed. Based on the section's fine description, thin section analysis, and core borehole logging, combined with petrophysical analysis, the reservoir characteristics, controlling factors, and evolution process of the Jinbaoshi Formation reservoir in the Longmenshan area are deeply studied in this paper. The results showed that the Jinbaoshi Formation has developed a set of sand-shale interbedding deposit of terrigenous clastic and carbonate rocks. The reservoir lithology is mainly quartz sandstone and calcareous quartz sandstone. The quartz sandstone reservoir has good physical properties, but the calcareous quartz sandstone has the characteristics of strong heterogeneity, low porosity, and permeability. Residual intergranular pores are mainly developed in the reservoir space. And the development of intercrystalline pores and intergranular dissolved pores can also be seen, but the development frequency is low, and the scale is small. Among them, the Devonian Jinbaoshi Formation quartz sandstone's average porosity is about 8%, and the highest is 18.43%. The main diagenesis types of the Jinbaoshi Formation are compaction, recrystalization, and dissolution. Compaction is the main reason for the destruction of original pores, and dissolution is the main driving force for later porosity and permeability improvement. Rock fragments, calcareous silty clasts, and bioclastic grains are the main dissolution objects. The formation of intergranular dissolved pores significantly improves the physical properties of the reservoir. Therefore, the reservoir development of the Jinbaoshi Formation is mainly controlled both by favourable sedimentary facies and diagenesis. Quartz sandstone and calcareous sandstone of the Jinbaoshi Formation were controlled by early sedimentary facies in the syngenetic rock stage and developed more early intergranular pores; in the early diagenesis stage, compaction-pressure solution and cementation resulted in residual intergranular pores. In the middle-late diagenesis stage, the calcareous cement is dissolved, and more pores were formed.

Effects of Lithospheric Properties on Crustal Strain at Both Ends of Longmen Shan Orogenic Belt: Based on Numerical Simulation

Strain partitioning and accommodation are fundamental constraints to evaluate tectonic models of orogenic plateaus. The uplift mechanism issue of the eastern Tibetan Plateau has remained a long-term focus since the last century, namely, the steep uplift of the Longmen Shan area. Several tectonic models have been proposed to describe the uplift process of the central Longmen Shan area along the eastern margin of the Tibetan Plateau. Such as upper crustal shortening, mid-crustal channel flow, and whole-crust shearing. However, these models are typically examined through vertical differences. Geophysical, geological, remote sensing and geochemistry observation data indicate that there occur not only vertical differences but also obvious horizontal differences along the Longmen Shan orogenic belt. Based on the finite element method, we employed two-dimensional profiles crossed northern and southern Longmen Shan fault to reconstruct the uplift process of the Longmen Shan orogenic belt. The mechanical properties of the lithosphere south of the Longmen Shan orogenic belt were slightly less favorable than those of the lithosphere north of the Longmen Shan orogenic belt. For the better fitting result in the southern part viscosity of lower crust is less than 1021 Pa∙s and in the northern part viscosity of lower crust is around 1022 Pa∙s. The uplift processes in the upper and lower crust of the Longmen Shan orogenic belt were partially decoupled. The deformation of lithosphere in the northern Longmen Shan orogenic belt is smaller than it in southern Longmen Shan orogenic belt. Due to that the rigid Ruoergai block maybe resists the formation of a weak layer or enters of weak materials to the northern Longmen Shan block, resulting in the observed difference in lithospheric properties between the northern and southern Longmen Shan blocks.

Predicting Change in Adaptation Strategies of Households to Geological Hazards in the Longmenshan Area, China Using Machine Learning and GIS

Hydrological changes combined with earthquakes easily trigger secondary disasters, including geological hazards. The secondary hazard of precipitation is the main disaster type in the Longmenshan Area (China). The 2008 Wenchuan earthquake caused more than 60,000 landslides, severely affecting rural households. This study aimed to answer two questions: (1) How did households adapt to the landslide-prone post-earthquake environment? (2) How will the households’ adaptation strategies change if landslide frequency changes? Different post-disaster adaptation strategies of households in Longmenshan Town, Sichuan, China were identified through a questionnaire survey and then clustered into groups based on similarity using a K-means algorithm. Afterward, a gradient boosting decision tree (GBDT) was used to predict change in adaptation strategies if there was a change in the frequency of landslides. The results show that there are three types of landslide adaptation strategies in the study area: (1) autonomous adaptation; (2) policy-dependent adaptation; and (3) hybrid adaptation, which is a mixture of the first two types. If the frequency of landslides is increased, then around 5% of households previously under the autonomous adaptation type would be converted to policy-dependent and hybrid adaptation types. If the frequency of landslides is reduced, then around 5% of households with policy-dependent adaptation strategies would be converted to the autonomous adaptation type. This exploratory study provides a glimpse of how machine learning can be utilized to predict how adaptation strategies would be modified if hazard frequency changed. A follow-up long-term study in Longmenshan Town is needed to confirm whether the predictions are indeed correct.

Orbital forcing of late Early Devonian storm events on a mixed carbonate-siliciclastic shelf, Longmenshan area, Sichuan Province, China

AbstractThe Yangmaba Formation of latest Early Devonian age in the Longmenshan area of Sichuan Province, China, is a shelfal facies that consists of four types of carbonate-siliciclastic deposits: clay-rich, siliciclastic sand-rich, carbonate-dominated, and hybrid mixed. Storm deposits vary in their thickness, composition, and abundance-per-meter within these mixed carbonate-siliciclastic deposits. Meter-scale statistics of the relative storm frequency (events-per-meter) and magnitude (bed thickness) were compiled in two coeval sections through each of the hybrid facies in the lower Yangmaba Formation, which spans approximately the entire conodont Polygnathus patulus Zone of the uppermost Emsian Stage. Low-pass filtering, spectral analysis, and Acycle software interpretation of these tempestite statistics yield 3.5 main oscillations in each section, with an average wavelength of ~23 m. These long-wavelength trends are semi-coincident with interpreted long-term variations in sea level, where shallower depths allowed a greater influence by storms. Superimposed on the long-wavelength cycles are medium-wavelength cycles of 5.5–6.3 m. The estimated ~1.5 m.y. time-span of this conodont zone of the lower Yangmaba Formation and the approximate 1:4 ratios of these wavelengths indicate that frequency and intensity of major storms and the recording of tempestites in the sedimentary record were modulated by ~100- and 95-k.y.-short-eccentricity orbital-climate oscillations superimposed on a main 405-k.y.-long-eccentricity cycle. These eccentricity climate cycles governed storm intensity and regional sea level on this margin of the tropical Yangtze Platform of South China.

Geometry-preserving full-waveform tomography and its application in the Longmen Shan area

Geometry-preserving full-waveform tomography and its application in the Longmen Shan area

Spatiotemporal Evolution of Earthquakes in Longmenshan Fault and Adjacent Area, before and after the 2008 Wenchuan Earthquake

Seismicity sequence following a main earthquake usually contains much meaningful information for unveiling the focal mechanism and predicting the reoccurrence interval of large earthquakes. The spatiotemporal evolution of earthquakes before and after the 2008 Wenchuan earthquake (Ms 8.0) is analysed comprehensively in this study. The frequency-magnitude relation of the 3493 earthquake events retrieved from the database of the International Seismological Centre indicates that the adopted catalogue is complete for magnitudes ≥Ms 3.4. The seismicity during the 10 years before the Wenchuan earthquake remained stable, including the magnitudes and focal depths. However, seismicity attenuated sharply in the year following the Wenchuan earthquake, and the magnitude of earthquakes before the Wenchuan earthquake decreased gradually. The area of the seismogenic zone of the 2008 Wenchuan earthquake was smaller than the earthquake stricken area. The earthquakes that occurred in the Longmenshan fault area and adjacent area in the study period were mainly shallow earthquakes. The focal depths of earthquakes in the study area became stable gradually after the Wenchuan earthquake, mainly within the range from 10 to 16 km. The earthquakes in the study area were mainly distributed with an along-dip distance of 0–20 km, and the seismicity was distributed uniformly along the fault strike.

Weak and vanishing upper mantle discontinuities generated by large-scale lithospheric delamination in the Longmenshan area, China

A large amount of high-quality teleseismic data is used for common conversion point (CCP) stacking of receiver functions in the Longmenshan area and near-by region. The results show that a large-scale high-velocity anomaly (LHVA) or lithospheric delamination can modify the structure of upper mantle discontinuities or weaken the phase boundary of olivine, which is a very important finding and can be used to assess stagnant slabs in the mantle transition zone globally. The deepening region of the 660 km discontinuity beneath the Songpan-Ganzi terrane identified in this study might be generated by the LHVA.

Weak or vanishing mantle discontinuity induced by a large-scal lithospheric delamination in the Longmenshan area, China

Weak or vanishing mantle discontinuity induced by a large-scal lithospheric delamination in the Longmenshan area, China

The iron oolitic deposits of the lower Devonian Yangmaba formation in the Longmenshan area, Sichuan Basin

Oolitic ironstones are widely distributed in South China, particularly in Middle and Upper Devonian strata, and are referred to as Ningxiang-type iron deposits. The hybrid siliciclastic-carbonate deposits of the Lower Devonian Yangmaba Formation iron-enriched interval (dominated by hematite ooids and chamosite ooids) are recognized in the Shigouli section of the Longmenshan area, Sichuan Basin. The Yangmaba Formation ironstones differ from other Devonian Ningxiang-type oolitic ironstones: The Ninxiang-type ironstone deposits are commonly interpreted to have been deposited in a narrow coastal zone or inland restricted bays and lagoons with clastic and carbonate sedimentary sequences; by contrast, the studied oolitic ironstones are interpreted to have been deposited in a shoreface zone to inner shelf based upon field and petrographic investigations. Our study shows that this oolitic ironstone consists of dominantly hematite ooids and chamosite ooids. Major element and trace element discrimination diagrams, PAAS (Post-Archaean average Australian sedimentary rock) normalized REE (Rear earth element) patterns, and positive Ce/Ce* anomalies show evidence of a hydrogenous origin. The weathering of adjacent terrestrial rocks from the formation of the palaeo-land surface is interpreted to be the main Fe source for the oolitic ironstones.Environmental energy and redox conditions are the important factors controlling ironstone formation and distribution of the hybrid deposits of the Yangmaba Formation in the Shigouli section of the Longmenshan area. Oolitic ironstones formed in oxidizing conditions of the upper shoreface zone include the carbonate concentric ooids with slightly hematite ooids occurring in higher energy upper oxidizing shoreface subzone, moderately hematite concentric ooids deposited in middle oxidizing shoreface subzone of fluctuant environmental energy, and hematite ooids present in lower oxidizing shoreface subzone of lower-energy conditions or weakly agitated environment. The chamosite concentric ooids and superficial ooids formed in reducing conditions of the lower shoreface and upper offshore transition. The former occurred in upper reducing shoreface subzone near the fair weather wave base, and the chamosite superficial ooids were deposited in lower reducing shoreface subzone and upper offshore transition that was affected by episodic high energy events.

Estimation of the Rupture History of the 2008 Mw8.0 Wenchuan Earthquake by Joint Inversion of Teleseismic and Strong‐Motion Records

In order to reproduce the rupture history of the 2008 Mw8.0 Wenchuan earthquake, the teleseismic and strong‐motion records are adopted. Based on a multiple‐segment, variable‐slip model, the finite fault inversion method is utilized to recover the rupture process. The results are as follows: (1) the rupture duration of the Wenchuan earthquake is about 100 s, and the released seismic moment is 1.24 × 1021 N·m, equal to the moment magnitude Mw8.0. There are 5 asperities on the fault plane, indicating that the earthquake is composed of at least 5 subevents. (2) The slip is mainly distributed on the Beichuan fault, indicating that the Beichuan fault is the main rupture fault. On the southern part of the Beichuan fault, the dislocation underside the Longmenshan area and Hongkou‐Yingxiu near‐surface area is dominated by thrust, and the maximum slip is 11.8 m. Slip between the Yuejiashan and Qingping area is dominated by thrust. On the northern part of the Beichuan fault, the area under Beichuan is dominated by thrust, the slip under Nanba is thrust and strike, near Qingchuan, the slip turns into the strike slip, and the maximum slip is 13.1 m. The dislocation under Bailu is also dominated by thrust, with maximum slip 8.9 m. (3) The rupture of the Wenchuan earthquake is mainly a unilateral rupture to the northeast. The rupture started at the low dip angle part of the Beichuan fault, and after 3 s, it propagated to the Pengguan fault. After 10 s, the largest asperity under Longmenshan in the south section of the Beichuan fault began to break, lasting for about 24 s. Then, the Xiaoyudong fault was triggered by the Pengguan fault, and the bilateral rupture of the high dip angle part of the Beichuan fault started at about 6 s. South section of the Beichuan fault began to break at about 35 s, and at 43 s, 63 s, and 80 s, the rupture extended to Beichuan, Nanba, and Qingchuan areas.

Dynamic mechanisms controlling the topography of Longmenshan area

The Longmenshan fault, which defines the eastern edge of the Tibetan Plateau, is one of the steepest margins of the plateau with a sharp elevation drop of about 4 km over a distance less than 100 km across the Longmenshan fault. The mechanism which is responsible for controlling and maintaining the elevation difference is highly debated. Using multiple observations including seismic velocity model, Moho depth, effective elastic thickness of the lithosphere, we conducted a quantitative study for elucidating the contributions from crust and lithospheric mantle by an integrated analysis of lithospheric isostasy and flexure. It is shown that the topography of the Longmenshan fault is supported by both lithospheric isostasy and flexure statically, and lower crustal channel flow and mantle convection dynamically. Different mechanisms have different weights for contribution to the topography of the Songpan-Ganzi block and the Sichuan Basin. The static and dynamic support contribute roughly the same to the topographic difference of ~4 km between the two sides of the Longmenshan fault. The static topographic difference of ~2 km is mainly resulted from the lithospheric isostasy, while the dynamic one of ~2 km is contributed by the uprising of the accumulated material in the lower crust beneath the Songpan-Ganzi block and the downward drag force caused by the upper mantle convection under the Sichuan Basin. It is thus suggested that the lower crustal flow and upper mantle convection are dynamic forces which should be taken into account in the studies on the dynamics in the Longmenshan and surrounding regions.

Lithospheric delamination and upwelling asthenosphere in the Longmenshan area: insight from teleseismic P-wave tomography

We apply teleseismic P-wave tomography to reconstruct the velocity structure of the Longmenshan area. Our results show possible large-scale delamination beneath the Songpan-Ganzi and Qiangtang terranes, which induced upwelling asthenosphere. Upwelling asthenosphere might have led to lower crust heating, facilitating eastward extrusion of the Songpan Ganzi terrane resulting in localized deformation and uplift along the Longmenshan orogenic belt. We suggest that the eastward extrusion of the Songpan-Ganzi terrane against the rigid lithospheric root of the Sichuan Basin results in stress accumulation and release, leading to large earthquakes in the Longmenshan area.

The stress evolution before the 2013 M7.0 Lushan earthquake in the Longmen Shan area since 1900

Strong earthquakes can disturb the regional stress field. The variation in the static Coulomb stress changes induced by earthquakes can explain the spatial distribution of certain aftershocks, and cascade earthquake sequences. However, the coseismic Coulomb stress change is not the only stress perturbation, interseismic and postseismic viscoelastic relaxation can also affect the evolution of the stress field. This study focuses on how stress perturbation from four strong earthquakes (the 1933 M7.5 Diexi, 1955 M7.5 Kangding, 1976 M7.2 Songpan and 2008 M8.0 Wenchuan) occurred around the Longmen Shan area in western China affect the occurrence of the 2013 Lushan M7.0 earthquake. In addition, we examine the stress evolution along the Longmen Shan fault zone and its adjacent faults to assess the regional seismic hazard. We construct a 3D viscoelastic finite element model that includes the fault structure, inhomogeneous medium, tectonic stress, gravity to simulate the evolutionary stress field when using observed Global Positioning System (GPS) vectors as the boundary conditions. The simulated results show that the coseismic Coulomb stress change induced by the Kangding, Songpan, and Wenchuan earthquakes is positive at the hypocenter of Lushan earthquake. In particular, the coseismic Coulomb stress change imparted by the Wenchuan earthquake exceeds the earthquake-triggering threshold of 0.01MPa. Additionally, the tectonic stress at the hypocenter of the Lushan earthquake increases with time, indicating the stress cumulates gradually at this location. We also compare the distribution of the earthquakes (from 1933 to 2013) with the effective and Coulomb evolutionary stress fields. Almost 70% of the earthquakes in the research region are located in the effective and Coulomb stress-enhanced areas. Our study helps to better understand the origin of Lushan earthquake and provide a theroetical basis for assesing regional seismic hazard.