Longmen Shan Thrust Belt Research Articles

Structural and geochronological constraints on the Mesozoic‐Cenozoic tectonic evolution of the Longmen Shan thrust belt, eastern Tibetan Plateau

The Longmen Shan thrust belt in the eastern margin of the Tibetan Plateau underwent deformation of D1 associated with the Late Triassic collision between the North and South China Blocks, followed by deformation of D2 and D3 related to the eastward growth of the Tibetan Plateau. The D1 is marked by moderately tight folds (f1), spaced cleavage (S1), mineral lineation (L1), multistage reactivated faults, and top‐to‐the south thrusting. This deformation initiated before 237 Ma and ended at 208 Ma. The 193–159 Ma D2 deformation is top‐to‐the‐southeast directed and normal ductile in nature and was associated with the formation of half‐graben basins. The Mesozoic deformation of the Longmen Shan belt was related to the subduction of the Paleo‐Tethys oceanic lithosphere, followed by the South China Block underneath the Songpan‐Ganze Terrane. The D1 was produced in the pro‐side of the overriding Songpan‐Ganze plate, most probably driven by slab corner flow above the retreating South China Block. The D2 and extensional Songpan‐Ganze turbidite basin were produced by roll‐back and migration of the overriding plate above the retreating subduction zone. This model for the deformation from D1 to D2 is similar to a Mediterranean‐style model. The D3 is characterized by SE‐ward thrusting with stepwise migration of rapid denudation and dextral strike‐slip faulting during the Late Cretaceous to present. Therefore, the Longmen Shan thrust belt involved Mesozoic compressional and extensional events that were overprinted by the Late Cretaceous to Cenozoic compressional event.

Structural superimposition and conjunction and its effects on hydrocarbon accumulation in the Western Sichuan Depression

Abstract Records of unconformities and lithologic facies and analysis of paleo-structure show that at least eleven tectonic boundaries exist in the Meso-Cenozoic continental strata in the Western Sichuan Depression. Affected by the Longmenshan thrust belt on the west side, Micangshan structural belt on the north side and Chuandian structural belt on the southwest side, NE-trending, sub-EW-trending and sub-NS-trending structures were developed correspondingly in the Western Sichuan Depression. The NE-trending structures were developed in varying degrees in most tectonic movements, while the sub-EW-trending and sub-NS-trending structures were formed in the Yanshanian and Himalayan, respectively. The three groups of structures were superimposed by one another in multi-stage tectonic movements, producing mainly five types of structural complexes, i.e. coaxial superimposition, orthogonal superimposition, oblique superimposition, multi-direction superimposition, and structural conjunction. Structural controls on hydrocarbon accumulation in the Western Sichuan Depression can be expressed as the following three aspects, i.e. timing, location and modification. Hydrocarbon distribution and the formation of primary gas pools were mostly controlled by early structures (in the Indosinian and Yanshanian), whereas late structures (mainly in the Himalayan) only had modification effects on early gas pools, resulting in structural displacement, reservoir improvement, the destruction of primary gas pools and the formation of secondary oil pools. Accurate reconstruction of the Indosinian-Yanshanian paleo-structures and fracture behaviors during the multi-stage and multi-direction structural superimposition are two important aspects that need to be paid more attention to in the future hydrocarbon exploration in this region.

Seismic slip recorded by fluidized ultracataclastic veins formed in a coseismic shear zone during the 2008 Mw 7.9 Wenchuan earthquake

Dark, aphanitic material occurs as a thin ( w 7.9 Wenchuan, China, earthquake. It also forms injection veins in a >5-cm-wide coseismic shear zone within the Longmen Shan thrust belt. Powder X-ray diffraction data and microstructural analysis indicate that the veins contain little or no glassy material, and are composed of fine-grained fragments set in an ultrafine-grained matrix, all sourced from the country rock of interbedded siltstone and mudstone. On the basis of X-ray diffraction and mesostructural and microstructural features, it is concluded that these veins are ultracataclastic veins generated by coseismic comminution with little melting, accompanied by rapid fluidization in the coseismic shear zone during the 2008 Wenchuan earthquake. The findings support earlier suggestions that (1) ultracataclastic veins that resemble pseudotachylyte veins in appearance can be generated by crushing and rapid injection accompanied by fluidization of fine-grained material during seismic faulting, with little or no melting; and (2) ultracataclastic veins formed in this way may record seismic slip events in seismogenic fault zones, acting as earthquake fossils in much the same way as pseudotachylyte veins.

Eyewitness Accounts of Surface Thrusting and Folding during the 2008 Mw 7.9 Wenchuan Earthquake, China

Research Article| November 01, 2010 Eyewitness Accounts of Surface Thrusting and Folding during the 2008 Mw 7.9 Wenchuan Earthquake, China Aiming Lin; Aiming Lin 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Search for other works by this author on: GSW Google Scholar Zhikun Ren; Zhikun Ren 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Search for other works by this author on: GSW Google Scholar Gang Rao Gang Rao 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Search for other works by this author on: GSW Google Scholar Author and Article Information Aiming Lin 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Zhikun Ren 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Gang Rao 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2010 by the Seismological Society of America Seismological Research Letters (2010) 81 (6): 884–891. https://doi.org/10.1785/gssrl.81.6.884 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Aiming Lin, Zhikun Ren, Gang Rao; Eyewitness Accounts of Surface Thrusting and Folding during the 2008 Mw 7.9 Wenchuan Earthquake, China. Seismological Research Letters 2010;; 81 (6): 884–891. doi: https://doi.org/10.1785/gssrl.81.6.884 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The Mw 7.9 Wenchuan earthquake, which occurred on 12 May 2008 in the Sichuan region of China (Figure 1), resulted in significant and widespread damage throughout a large area of central and western China. Official estimates of casualties released by the Chinese government include some 70,000 confirmed deaths, plus approximately 374,000 injured and 18,000 missing. Field investigations and seismic data show that the earthquake was triggered by active faults of the Longmen Shan thrust belt, located on the marginal zone between the high Tibetan plateau and the Sichuan basin. A 285-km-long surface rupture zone produced by the... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Surface Rupture of the 2008 Wenchuan, China, Earthquake in the Qingping Stepover Determined from Geomorphologic Surveying and Excavation, and Its Tectonic Implications

The Wenchuan Mw 7.9 earthquake of 12 May 2008 caused the rupture of the Longmen Shan thrust belt, which bounds the eastern margin of the Tibetan plateau, and generated a very complex surface rupture. The Beichuan-Yingxiu fault (BYF )w as themainseismogenicfaultandformedtwodistinctivelydifferentsurfacerupturezones separated by the Qingping and Gaochuan stepovers. Real-time kinematic (RTK) sur- veying of alluvial terrace sequences indicates that terraces T1-T3 and river floodplain T0 have the same vertical displacement associated with the Wenchuan earthquake. Trench excavation and optical stimulated luminescence (OSL) dating of alluvial depos- its indicate that only the deformation of the Wenchuan earthquake was recorded in the Qingping stepover since at least ∼20 ka. Other deformations since ∼20 ka probably occurred in other places or did not reach the surface. This can be meaningful to analyze the completeness of paleoearthquakes in trench excavation on thrust faults. The width of the Qingping stepover is ∼1 km and it is not strong enough to form a barrier to arrest the propagation of rupture in the 2008 Wenchuan earthquake. The Gaochuan stepover is ∼7 km wide and it is a barrier to separate the two surface rupture zones associated with the Wenchuan earthquake along the BYF. The empirical rela- tionship of rupture displacement and stepover dimensions on strike-slip faults is prob- ably suitable for the large thrust fault zone with a strike-slip component. This may be beneficial to the study of cascade rupture sections and seismic hazard assessment on large thrust faults.

Fault-Scarp Features and Cascading-Rupture Model for the Mw 7.9 Wenchuan Earthquake, Eastern Tibetan Plateau, China

On 12 May 2008, the M w 7.9 Wenchuan earthquake ruptured two northeast-striking imbricated reverse faults and one northwest-striking reverse fault along the middle Longmen Shan thrust belt, at the eastern margin of the Tibetan plateau. The morphology of the coseismic scarp varies drastically along strike. We distinguish eight different categories: (1) simple thrust scarp, (2) hanging-wall collapse scarp, (3) simple pressure ridge, (4) dextral pressure ridge, (5) fault-related fold scarp, (6) backthrust pressure ridge, (7) local normal fault scarp, and (8) pavement suprathrust scarp. The coseismic surface ruptures confirm that the Wenchuan earthquake is dominated by reverse faulting with some right-lateral component that varies from site to site. The surface rupture can be divided into two parts, the Yingxiu segment and the Beichuan segment. When the earthquake is split into two subevents accordingly, they correspond to an M w 7.8 event and an M w 7.6 event, respectively. These two segments can in turn be divided into four second-order subsegments, which are equivalent to four subevents of M w 7.5, M w 7.7, M w 7.0, and M w 7.5, respectively. The segmentation of the rupture is consistent with a cascading-rupture pattern, responsible for the total 110 s of the earthquake rupture. In addition to surface ruptures, the focal mechanisms determined for the aftershocks recorded by the local seismic network are used to constrain the fault geometry of the subsegments. They show that the dip of the fault responsible for the earthquake varies along strike, and the fault tends to flatten at depth. In addition, the fault plane gets steeper northward, enabling the fault to accommodate a larger strike-slip component along a lateral ramp. This major earthquake confirms that crustal shortening could be the main driving force for the growth of high topography along the eastern margin of the Tibetan Plateau and that lower crustal flow is not required.

Co-seismic landslides induced by the 2008 Wenchuan magnitude 8.0 Earthquake, as revealed by ALOS PRISM and AVNIR2 imagery data

The magnitude 8.0 Wenchuan Earthquake occurred on 12 May 2008 in Sichuan Province, China, resulting in numerous landslides along the marginal zone between the Tibetan Plateau and Sichuan Basin. We used Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) and Advanced Visible and Near Infrared Radiometer type 2 (AVNIR 2) data from the Advanced Land Observing Satellite (ALOS) and Landsat Enhanced Thematic Mapper (ETM) images acquired before and after the earthquake to identify co-seismic landslides. Interpretations of remote sensing images, combined with fieldwork, reveal that co-seismic landslides are mainly restricted to a corridor of <18 km in width about the co-seismic surface rupture zone along a length of >285 km along pre-existing active faults of the Longmenshan Thrust Belt. The landslides mainly occurred upon steep slopes (dips of 30–75°). The distribution and topographic features of co-seismic landslides indicate a close relationship with seismic slip along the co-seismic surface rupture zone. The locations of landslides are controlled by the tectonic topography developed along pre-existing active faults of the Longmenshan Thrust Belt. Our results demonstrate that remote sensing techniques provide a powerful tool for identifying co-seismic landslides produced in intermountain regions by strong earthquakes.

Structural analysis of the coseismic shear zone of the 2008 M w 7.9 Wenchuan earthquake, China

Field investigations reveal that the surface rupture of the 2008 M w 7.9 Wenchuan earthquake, China, occurred along a pre-existing shear zone in the Longmen Shan Thrust Belt. Structural analyses of the coseismic fault zone and fault rocks show that i) the main coseismic shear zone consists of a fault core that includes a narrow fault gouge zone of <15 cm in width (generally 1–2 cm) and a fault breccia zone of <∼3 m in width, and a wide damage zone of >5 m in width that is composed of cataclastic rocks including fractures and subsidiary faults; ii) the foliations developed in the fault core and damage zones indicate a dominantly thrust slip sense, consistent with that indicated by the coseismic surface rupture; and iii) coseismic slip was largely localized to within a narrow fault gouge zone of <2–3 mm in width. The structural characteristics of the coseismic shear zone and cataclastic rocks indicate that the location of coseismic slip zone associated with the 2008 Wenchuan earthquake was controlled by a pre-existing shear zone and that the main active fault of the Longmen Shan Thrust Belt has moved as a thrust since the formation of cataclastic rocks along the fault during the late Miocene or early Pliocene.

The October 6, 2008 Mw 6.3 magnitude Damxung earthquake, Yadong-Gulu rift, Tibet, and implications for present-day crustal deformation within Tibet

A Mw 6.3 magnitude earthquake occurred on October 6, 2008 in southern Damxung County within the N–S trending Yangyi graben, which forms the northern section of the Yadong-Gulu rift of south-central Tibet. The earthquake had a maximum intensity of IX at the village of Yangyi (also Yangying) (29°43.3′N; 90°23.6′E) and resulted in 10 deaths and 60 injured in this sparsely populated region. Field observations and focal mechanism solutions show normal fault movement occurred along the NNE-trending western boundary fault of the Yangyi graben, in agreement with the felt epicenter, pattern of the isoseismal contours, and distribution of aftershocks. The earthquake and its tectonic relations were studied in detail to provide data on the seismic hazard to the nearby city of Lhasa. The Damxung earthquake is one of the prominent events along normal and strike-slip faults that occurred widely about Tibet before and after the 2008 Mw 7.9 magnitude Wenchuan earthquake. Analysis of these recent M ⩾ 5.0 earthquake sequences demonstrate a kinematic relation between the normal, strike-slip, and reverse causative fault movements across the region. These earthquakes are found to be linked and the result of eastward extrusion of two large structural blocks of central Tibet. The reverse and oblique-slip surface faulting along the Longmenshan thrust belt at the eastern margin of the Tibetan Plateau causing the Wenchuan earthquake, was the result of eastward directed compression and crustal shortening due to the extrusion. Prior to it, east–west extensional deformation indicated by normal and strike-slip faulting events across central Tibet, had led to a build up of the compression to the east. The subsequent renewal of extensional deformational events in central Tibet appears related to some drag effect due to the crustal shortening of the Wenchuan event. Unraveling the kinematical relation between these earthquake swarms is a very helpful approach for understanding the migration of strong earthquakes across Tibet.

Structural interpretation of the coseismic faults of the Wenchuan earthquake: Three‐dimensional modeling of the Longmen Shan fold‐and‐thrust belt

The 2008 Mw 7.9 Wenchuan earthquake is a result of ongoing India‐Tibet collision and reflects the growth of the Longmen Shan fold‐and‐thrust belt. In this paper, we construct a 3‐D structural model of the geometry of the coseismic faults and related structures of the Wenchuan earthquake by integrating geological investigations, relocated aftershocks, and seismic reflection profiles. In the 3‐D structural model, the differences between the southern and northern segments of the rupture are highlighted. The structural transition zone between the two segments contains a major geometric segment boundary, reflecting differences in the structural configuration of the thrust ramp and the tectonic evolution of the fault system, which appears to have localized significant damage from Anxian to Beichuan. Within the northern segment, we identify a transverse structure across which the Beichuan fault plunges under the Tangwangzhai syncline. This boundary corresponds to a marked change in the nature of the surface rupture and is illuminated by a microearthquake sequence perpendicular to the Longmen Shan thrust belt. In the southern segment, our investigations confirm that uplift due to active faulting and folding is largely responsible for the areas of steepest topography. On the basis of this association, the southwestern segment of the Longmen Shan, south of the Wenchuan earthquake, is likely active and presents a significant earthquake hazard, despite the lack of historical earthquakes in this region. This study illustrates the importance of building 3‐D models to study active faulting and folding, as well as to assess earthquake hazard.

Evidence for a Tang-Song Dynasty great earthquake along the Longmen Shan Thrust Belt prior to the 2008 M w 7.9 Wenchuan earthquake, China

The magnitude (M w) 7.9 Wenchuan earthquake occurred on 12 May 2008 in the Longmen Shan region of China, the transition zone between the Tibetan Plateau and the Sichuan Basin, resulting in widespread damage throughout central and western China. The steep, high-relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic uplift and denudation accompanied by folding and thrusting, yet no large thrust earthquakes are known prior to the 2008 M w 7.9 Wenchuan earthquake. Field and excavation investigations reveal that a great historical earthquake occurred in the Sichuan region that ruptured a >200-km-long thrust fault within the Longmen Shan Thrust Belt, China, which also triggered the 2008 M w 7.9 Wenchuan earthquake. The average co-seismic slip amount produced by this historical earthquake is estimated to be 2–3 m, comparable with that caused by the 2008 Wenchuan earthquake. Paleoseismic and archaeological evidence and radiocarbon dating results show that the penultimate great earthquake occurred in the Sichuan region during the late Tang-Song Dynasty, between AD 800 and 1000, suggesting a recurrence interval of ~1,000–1,200 years for Wenchuan-magnitude (M = ~8) earthquakes in the late Holocene within the Longmen Shan Thrust Belt. This finding is in contrast with previous estimates of 2,000–10,000 years for the recurrence interval of large earthquakes within the Longmen Shan Thrust Belt, as obtained from long-term slip rates based on the Global Positioning System and geological data, thereby necessitating substantial modifications to existing seismic-hazard models for the densely populated region at the eastern marginal zone of the Tibetan Plateau.

Coseismic vertical deformation of the MS8.0 Wenchuan earthquake from repeated levelings and its constraint on listric fault geometry

The devastating MS8.0 Wenchuan earthquake ruptured two large parallel thrust faults along the middle segment of the Longmenshan thrust belt. Preseismic and postseismic leveling data indicated the hanging wall of the Yingxiu-Beichuan-Nanba thrust fault mainly presented coseismic uplift with respect to the reference point at Pingwu county town, and the observed maximum uplift of 4.7 m is located at Beichuan county (Qushan town) which is about 100 m west of the fault scarp. The foot wall of the Yingxiu-Beichuan-Nanba thrust fault mainly showed subsidence with maximum subsidence of 0.6 m near the rupture. By employing a listric dislocation model, we found that the fault geometry model of exponential dip angle δ=88°×[1−exp(−9/h)] with depth of 18 km and uniform thrust-slip of 5.6 m could fit the observed coseismic vertical deformation very well, which verifies the listric thrust model of the Longmenshan orogenic zone.

Co-seismic ground-shortening structures produced by the 2008 Mw 7.9 Wenchuan earthquake, China

We observed ground-shortening structures produced by the 2008 M w 7.9 Wenchuan earthquake along the 285-km-long Wenchuan co-seismic surface rupture zone. The structures are characterized by co-seismic thrusting and folding structures that include thrust fault scarps, flexure-slip folds, and mole tracks. Fold structures are generally developed within alluvial deposits, river channel deposits, and sealed road surfaces, mainly restricted to within a < 100-m-wide corridor (generally < 50 m) about the co-seismic thrusting fault scarp along the co-seismic surface rupture zone. The amount of co-seismic horizontal ground shortening accommodated by co-seismic thrusting and folding ranges from 0.1 up to 7.3 m (generally 1–3 m), consistent with GPS observations. Our results, combined with geological and GPS data, confirm that present-day shortening strain upon the eastern marginal zone of the Tibetan Plateau is mainly released by seismic thrusting–folding along active faults within the Longmen Shan Thrust Belt.

Structural model of 2008 Mw 7.9 Wenchuan earthquake in the rejuvenated Longmen Shan thrust belt, China

The 12 May 2008 Wenchuan earthquake ( M w = 7.9) of China occurred in the Longmen Shan thrust belt and is a consequence of the ongoing uplift process of the eastern margin of Tibetan plateau. The coseismic surface ruptures coincide with the pre-existing thrust fault trace. Destruction of buildings and the location of seismically induced landslides are located in the hanging wall of the reactivated thrust faults. In order to detect any indication of coseismic deformation linked to subsurface structures, we interpret the subsurface fault and fold geometries using petroleum seismic reflection profiles, as well as coseismic surface ruptures and seismicity. Our data suggest that the deformation can be divided into two different structural segments related to the regional coseismic deformation during the Wenchuan earthquake. In the southern segment (the Yingxiu–Hongkou–Hanwang segment), two coseismic surface rupture zones coincide with two pre-existing thrust faults (the Yingxiu–Beichuan and Pengguan faults) in the seismic profiles and both coseismic active thrusts become incorporated into the deep main detachment. This through-going thrust fault connecting directly from the hypocenter to the surface break, but cannot be easily extended to the northern segment along the Yingxiu–Beichuan thrust fault zone. In contrast, only one shallow coseismic active thrust fault with oblique-slip occurs as the active passive roof of imbricate thrust sheets in the northern segment (Beichuan–Qingchuan segment). Our results show ramp-flat geometry and the wedge characteristics at the blind front of the rejuvenated thrust belt. We emphasize that there is a potential 15–17 km-deep main detachment associated with this large earthquake in the Longmen Shan belt, and infer that some active displacements along the 6–9 km depth shallow detachment have propagated into the Sichuan basin since late Cenozoic.

Deep Background of Wenchuan Earthquake and the Upper Crust Structure beneath the Longmen Shan and Adjacent Areas

Abstract:By analyzing the deep seismic sounding profiles across the Longmen Shan, this paper focuses on the study of the relationship between the upper crust structure of the Longmen Shan area and the Wenchuan earthquake. The Longmen Shan thrust belt marks not only the topographical change, but also the lateral velocity variation between the eastern Tibetan Plateau and the Sichuan Basin. A low‐velocity layer has consistently been found in the crust beneath the eastern edge of the Tibetan Plateau, and ends beneath the western Sichuan Basin. The low‐velocity layer at a depth of ∼20 km beneath the eastern edge of the Tibetan Plateau has been considered as the deep condition for favoring energy accumulation that formed the great Wenchuan earthquake.

Parameters of Coseismic Reverse‐ and Oblique‐Slip Surface Ruptures of the 2008 Wenchuan Earthquake, Eastern Tibetan Plateau

Abstract:On May 12th, 2008, the Mw7.9 Wenchuan earthquake ruptured the Beichuan, Pengguan and Xiaoyudong faults simultaneously along the middle segment of the Longmenshan thrust belt at the eastern margin of the Tibetan plateau. Field investigations constrain the surface rupture pattern, length and offsets related to the Wenchuan earthquake. The Beichuan fault has a NE‐trending right‐lateral reverse rupture with a total length of 240 km. Reassessment yields a maximum vertical offset of 6.5 ± 0.5 m and a maximum right‐lateral offset of 4.9 ± 0.5 m for its northern segment, which are the largest offsets found; the maximum vertical offset is 6.2 ± 0.5 m for its southern segment. The Pengguan fault has a NE‐trending pure reverse rupture about 72 km long with a maximum vertical offset of about 3.5 m. The Xiaoyudong fault has a NW‐striking left‐lateral reverse rupture about 7 km long between the Beichuan and Pengguan faults, with a maximum vertical offset of 3.4 m and left‐lateral offset of 3.5 m. This pattern of multiple co‐seismic surface ruptures is among the most complicated of recent great earthquakes and presents a much larger danger than if they ruptured individually. The rupture length is the longest for reverse faulting events ever reported.

Coseismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China

The Mw 7.9 Wenchuan, China, earthquake ruptured two large thrust faults along the Long-menshan thrust belt at the eastern margin of the Tibetan Plateau. This earthquake generated a 240-km-long surface rupture zone along the Beichuan fault and an additional 72-km-long surface rupture zone along the Pengguan fault. Maximum vertical and horizontal offsets of 6.5 m and 4.9 m, respectively, were measured along the Beichuan fault. A maximum vertical offset of 3.5 m was measured along the Pengguan fault. Coseismic surface ruptures, integrated with aftershocks and industry seismic profiles, show that two imbricate structures have ruptured simultaneously, resulting in the largest continental thrust event ever documented. Large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region, as everywhere along the edge of Tibetan Plateau.

Ambient noise Rayleigh wave tomography in western Sichuan and eastern Tibet

We perform Rayleigh wave tomography in western Sichuan and eastern Tibet by applying the ambient noise method to broadband data recorded at 49 stations from the Sichuan digital seismic network. Cross-correlations of vertical-component ambient noise data are computed in one-hour segments and stacked over 7 months from June to December 2007. Then Rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple-filter analysis method. Group velocity maps at periods from 8 to 44 s were inverted. The resulting group velocity maps generally show good correlations with tectonic features. The Sichuan Basin is clearly resolved with lower group velocities at the short periods (8 to 20 s) due to thick sedimentary layers, and a prominent low-velocity zone exhibits in the middle crust beneath the eastern margin of the Tibetan Plateau. The Longmen Shan thrust belt marks not only the rapid lateral velocity variation between the topographically high Tibetan Plateau and low-elevation Sichuan Basin, but also the transition from the thick crust beneath the Tibetan Plateau to the normal continental crust beneath the Sichuan Basin.

Co-seismic thrusting rupture and slip distribution produced by the 2008 Mw 7.9 Wenchuan earthquake, China

Field investigations reveal that the M w 7.9 Wenchuan (China) earthquake of 12 May 2008 produced a 285-km-long surface rupture zone, with dominantly thrusting slip accompanied by a right-lateral component along the central-northern segments of the zone, and left-lateral component along the southern segment, along the Longmen Shan Thrust Belt, eastern margin of the Tibetan Plateau. The co-seismic ruptures mainly occurred along the pre-existing Yingxiu–Beichuan, Guanxian–Anxian, and Qingchuan faults, which are the main faults of the Longmen Shan Thrust Belt. The displacements measured in the field are approximately 0.5–6.5 m in the vertical (typically 1–3 m), accompanied by an average left-lateral component of < 2 m along the 50-km-long southernmost segment of the rupture zone and an average right-lateral component of < 1 m along the 150-km-long central-northern segments. The maximum thrust slip amount is estimated to be ~ 10 m, accompanied by 9 m of shortening across the rupture zone; this finding is consistent with estimates based on seismic data. The rupture length and maximum vertical displacement are the largest among all intracontinental thrust-type earthquakes reported to date. Our findings demonstrate that i) the Wenchuan earthquake occurred upon pre-existing active faults of the Longmen Shan Thrust Belt, thereby controlling the spatial distribution of co-seismic surface rupture and displacement, and the rupture processes of the earthquake; ii) the long rupture length and large thrusting slip resulted from compressive stress associated with eastward extrusion of the Tibet Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate; and iii) present-day shortening strain upon the eastern margin of the Tibetan Plateau is mostly released by seismic slip along thrust faults within the Longmen Shan Thrust Belt.

U-Pb dating, geochemistry, and tectonic implications of the Songpan-Ganzi block and the Longmen Shan, China

The Songpan-Ganzi triangular tectonic block is surrounded by the East Kunlun-Qaidam block in the north, the Qiangtang block in the southwest, and the Longmen Shan in the southeast. The Longmen Shan is the eastern margin of the Tibetan Plateau in Central Asia. The tectonic affinity and relationship of these regions with the Yangtze and North China blocks have been rigorously debated. In this paper, we present our recent studies on the geochemistry of the Precambrian to Triassic clastic sedimentary rocks from this area. Detrital zircons were extracted from the sedimentary rocks for U-Pb dating by SHRIMP and LA-ICP-MS. Nd depleted mantle model ages indicate that these provenances have an affinity to the Yangtze craton and Qinling Shan, in contrast to those of the North China craton. All strata, from Cambrian to late Triassic, contain detrital zircon U-Pb ages of 1400 to 600 Ma, with most falling between 900-720 Ma. The Songpan-Ganzi block was the most important part of the Northern Tethys in the Triassic period, in addition to being the largest flysch basin in the world. The Longmen Shan thrust belt comprises both the telescoped, unmetamorphosed, shallow water carbonate-dominated passive margin of the Yangtze craton and its crystalline basement. The western and northern margins of the Yangtze craton, as well as the Qinling Shan, were the sources of sediments in the Songpan-Ganzi block and the Longmen Shan. The oldest dated detrital zircons indicate that the Yangtze craton started forming during the period 2800-2200 Ma. Because the basement of the Yangtze craton experienced multi-episodic intracrustal processes, the oldest basement components account for only a small portion of the provenance components. The detrital zircon ages of 2200-1400 Ma indicate a major contribution of the Qinling Shan and/or the northwestern margin of the Yangtze craton to the clastic sediments in the Songpan-Ganzi block. The presence of detrital zircon of Neoproterozoic to Triassic ages suggests derivation from the Qinling Shan and the western and northern margin of the Yangtze craton. Triassic tectonic movement during this time was at its most intensive and extensive, and led to exhumation of basement in the Yangtze craton and Qinling Shan, along with nascent crustal components and their erosion to the East Tethys.