Eastern Margin Of Tibetan Plateau Research Articles

Asperity of the 2013 Lushan earthquake in the eastern margin of Tibetan Plateau from seismic tomography and aftershock relocation

SUMMARY The Mw 6.6 Lushan earthquake occurred on 2013 April 20 in the eastern margin of Tibetan Plateau, and was about 50km away from the 2008 Mw 7.9 Wenchuan earthquake rupture zone. By using arrival times from portable and permanent seismic stations, local earthquake tomography and aftershock relocation were conducted by using 41955 first P and 42687 first S arrivals. Low-Vp/Vs, high-Vp and Vs anomaly are found in the source region, which is well consistent with the aftershock distributions. The ruptured fault plane might be a blind thrust and is well defined by the Vp/Vs structure and aftershock seismicity. High-velocity and lowVp/Vs anomalies may suggest the mafic or ultramafic materials thrust from mid-lower crust, and define the asperity that concentrated stresses responsible for seismogenesis. In contrast, low-Vs anomalyisfoundinthegapbetweentheWenchuanandLushanearthquakes,suggesting of ductile rocks with low stress.

ESR geochronology of the Minjiang River terraces at Wenchuan, eastern margin of Tibetan Plateau, China

The Minjiang River terrace along the Longmen Shan fault zone near Wenchuan, at the eastern margin of the Tibetan Plateau, China, provides archives for tectonic activity and quaternary climate change. However, previous studies were not able to provide ages older than 100 ka due to the limitations of dating material or/and methods applied to date the fluvial sediments. In this study, we used the ESR signal of the Ti-Li center in quartz to obtain the ages of four higher terraces (T3–T6). According to the results, the terraces T3 to T6 were formed at 64±19 ka, 101±15 ka, 153±33 ka, and 423±115 ka, respectively. Combined with previous studies, these results indicate that the formations of all terraces correspond to glacial/interglacial transition periods, such as, T1-T5 being correlated to MIS2/1, MIS4/3, MIS5d/5c, and MIS6/5e respectively, while T6 probably to MIS12/11. According to these data, it is found that the average incision rate was significantly higher over the last 150 ka than that previous 100 ka (250 to 150 ka). As both tectonics and climate have affected the formation of these terraces, in addition to the overall uplifting of Tibetan Plateau, the regional uplift due to isostasy would be an additional tectonic factor in the formation of river terraces in the eastern margin of Tibetan plateau.

Constructing the Longmen Shan eastern Tibetan Plateau margin: Insights from low‐temperature thermochronology

AbstractContrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the Longmen Shan (LMS), eastern Tibetan Plateau (TP) margin. These models require different structural kinematics along the LMS, whose structural geometry is defined by three parallel NW‐dipping fault zones. From foreland (southeast) to hinterland (northwest), they are the Guanxian‐Anxian Fault, Yingxiu‐Beichuan Fault (YBF), and Wenchuan‐Maowen Fault (WMF). Newly derived and previously published low‐temperature thermochronology data from the LMS were synthesized to constrain the spatial exhumation and test previous models. The results show that (1) exhumation increases abruptly across the range‐bounding YBF, suggesting the fault being the main thrust boundary between the LMS and the Sichuan Basin to the east; (2) Younger Late Cenozoic cooling ages are found on the hinterland WMF, where a dichotomy of ages on the hanging wall versus footwall suggests Late Cenozoic thrust activity; and (3) toward the hinterland to the west, exhumation rates decrease twofold over a distance of ~30–40 km. This exhumation pattern indicates a westward decrease of tectonic uplift, providing the regional topography approached a steady state, whereby exhumation is in balance with tectonic uplift. The observed exhumation estimates support an upper crustal configuration where thrusts in the LMS merge gradually into a gentle detachment seated at a depth of ~20–30 km. Results of this study support a revised upper crustal thrusting model.

Late Holocene activity and historical earthquakes of the Qiongxi thrust fault system in the southern Longmen Shan fold-and-thrust belt, eastern Tibetan Plateau

The 12 May 2008 Wenchuan earthquake (Mw 7.9) generated a 285-km-long surface rupture zone along the Longmen Shan fold-and-thrust belt (LSFTB) on the eastern margin of Tibetan Plateau. The Wenchuan earthquake did not rupture into the southwestern Longmen Shan, along which there is no evidence for large paleo- or historical earthquakes. Seismic reflection profiles and field investigations reveal that the 50-km-long Qiongxi thrust fault (QTF) of the southern LSFTB is currently active. The QTF consists of three west-dipping ramp segments overlain by fault-bend folds rooted in a regional detachment that transfers shortening from the mountain belt into the Sichuan basin. Trench investigations, coupled with interpretations of seismic reflection profiles and radiocarbon results, show that a recent surface-rupturing earthquake occurred on the QTF during the Late Ming to Qing Dynasty, between AD 1600 and 1800. In addition, seismic reflection profile and topographic analysis indicate the presence of a subtle topographic, produced by kink-band migration folding above a fault bend at about 5km depth. These findings confirm that the QTF is a significant seismic hazard, and that it should be incorporated into current regional seismic hazard models for the densely populated Sichuan basin.

Late Cretaceous Tectonic Change of the Eastern Margin of the Tibetan Plateau – Results from Multisystem Thermochronology

Abstract Partially due to lack of structural and sedimentary records to constrain the Jurassic-to-Cretaceous evolution, there was a missing process here in the eastern margin of Tibetan Plateau as it changed from the Paleo-Tethyan to Neo-Tethyan regime. Based on the analysis of 125 thermochronology ages (U/Pb, 40Ar/39A, 87Rb/86Sr, FT, U-Th/He) of igneous rocks from the eastern margin of Tibet, we propose a multisystem thermochronology approach to restore the cooling and emplacement of granites and decipher the missing process. Our integrated study suggests that a key Late Cretaceous (about 100Ma) tectonic change from the Paleo-Tethyan to Neo-Tethyan regime took place there. In the Late Triassic period, the initial emplacement of granite in the Songpan-Ganzi Fold Belt (SGFB) was characterized by a decrease in emplacement age and depth from west to east, and from north to south. Subsequently, all were followed by a very long period of slow cooling, which was followed by a rapid emplacement of about 100Ma. The intensive granite emplacement took place all over except northeastern SGFB, with a decrease in emplacement depth from west to east, which was linked with the far-field effect of Lhasa-Qiangtang collision. After this episode, the cooling history of granite in SGFB had a rapid emplacement on the subsurface under the control of the Neo-Tethyan regime. This process has control of the post Late Cretaceous regional magmatic activity and tectonics, as well as the sedimentary response in Sichuan and Xichang basin.

Variations in rupture speed, slip amplitude and slip direction during the 2008 Mw 7.9 Wenchuan Earthquake

Theoretical and observational evidence implies that variable rupture velocity may be associated with high fault slip and high moment release rates during earthquakes. We investigate this relationship for the 2008 Mw 7.9 Wenchuan China, Earthquake, which appears to have experienced a highly variable moment release rate. We used an empirical Green's function (EGF) deconvolution analysis of teleseismic waveforms to retrieve the primary rupture characters for this event. Based on field observations and the deconvolved source—time function from thrust and strike–slip EGFs, we divided the ruptured fault into three segments, allowing us to determine the spatial slip distribution with different average rupture velocities on each segment. We deployed a grid search analysis, in which we integrated the teleseismic waveform inversion and forward modelling of the regional surface wave to determine the optimum rupture speed in each fault segment. Our result shows that the 2008 Wenchuan Earthquake had slip amplitude, direction and rupture velocity that were highly spatially variable. The earthquake initially ruptured with nearly pure thrust motion and a slow rupture velocity of 1.7 km s—1. Then, the rupture speed increased up to 3.1—3.3 km s—1 and produced the largest slip on the second segment, where two parallel faults ruptured simultaneously. Rupture velocity then slowed down to 2.5—2.9 km s—1 on the final segment, which underwent primarily strike–slip motion. In total, the fault extended 300 km with an average rupture speed in the range of 2.6—2.9 km s—1. The high rupture speed (close to the shear wave velocity) in the second segment may be related to the simultaneous rupture of the two parallel faults on well–established pre–existing structures. The spatial distribution of rupture velocity and slip are likely related to the strongly rotational and 3–D deformation in the eastern Tibetan Plateau margin, which leads to the heterogeneous stress field of the Longmen Shan region.

Channel flow of the lower crust and its relation to large-scale tectonic geomorphology of the eastern Tibetan Plateau

The Tibetan Plateau is a large-scale tectonic geomorphologic unit formed by the interactions of plates. It has been commonly believed that convective removal of the thickened Tibetan lithosphere, or lateral flow of the lower crust beneath the Tibetan plateau plays a crucial role in the formation of the large-scale tectonic geomorphologic features. Recent geological and geophysical observations have provided important evidence in support of the lower crustal channel flow model. However, it remains unclear as how the geometry of lower crustal channel and the lateral variation of crustal rheology within the lower crust channel may have affected spatio-temporal evolution of the tectonic geomorphologic unit of the Tibetan Plateau. Here, we use numerical methods to explore the mechanical relations between the lower crustal channel flow and the tectonic geomorphologic formation around the eastern Tibetan plateau, by deriving a series of governing equations from fluid mechanics theory. From numerous tests, our results show that the viscosity of the channeled lower crust is about (1−5)×1018 to (1−4)×1020 Pa s (Pa·s) beneath the margin of the eastern Tibetan Plateau, and increases to about 1022 Pa s beneath the Sichuan Basin and the southern region of Yunnan Province. Numerical tests also indicate that if channel flows of the lower crust exist, the horizontal propagation and the vertical uplifting rate of the eastern Tibetan Plateau margin could be accelerated with the time. Thus, the present results could be useful to constrain the rheological structure of the crust beneath the eastern Tibetan plateau, and to understand the possible mechanics of rapid uplift of the eastern Tibetan Plateau margin, especially since its occurrence at 8Ma as revealed by numerous geological observations.

Temporal and Spatial Features of Isostasy Anomaly Using Gravitational Admittance Model at Eastern Margin of Tibetan Plateau

AbstractThe eastern margin of Tibetan plateau belongs to north‐south seismic belt. There is a distinct variation characteristic of gradient zone between positive and negative anomalies on satellite anomaly figure of free air gravity. Along the anomaly zone there are many deep faults. The eastern margin is also a boundary region of tectonic units. In this paper, we used the gravitational admittance theory and method to compute the background isostasy anomaly of gravitational admittance model in this region. Through comparing the isostasy anomaly characteristics of Airy model and elastic plate model, and analyzing the regional seismic activity, we evaluated the results and optimized the parameters used for calculation. Based on this, we finished the comparison and analysis of isostasy anomaly response characteristics along three profiles across the eastern margin of Tibetan plateau, and taking this background isostasy anomaly as the reference field, further explained the temporal and spatial features of mobile gravity measurement dataset of 1998~2005. The result shows that the isostatic anomaly derived with the gravitational admittance model is applicable to the region of tectonic boundary and can indicate the state of non‐equilibrium of upper crust.

The active fault belts in eastern Tibet margin inferred using magnetotellurics

A magnetotelluric (MT) sounding has been carried out in the eastern margin of the Tibetan plateau. The survey line is about 145 km long, trending in NEE direction and crossing the Daliangshan block in the eastern edge of the Tibetan plateau. The field measurements acquired effective data of 68 sites. Through data processing and a 2-D inversion with consideration of topography, a 2-D electrical structure model of crust and upper mantle was constructed. The structure reveals that there is a deep electrical boundary between the Daliangshan block in the west and Sichuan block in the east. West to the boundary, the crust has a relatively low resistivity with respect to the east and can be divided into three layers, the middle layer has low-resistivity with a minimum of 3-10 W·m, presumably associated with partial melt and/or salty fluids. Beneath the intersection area of the Anninghe fault, the Xianshuihe fault and the Longmenshan fault, which the MT profile crosses, the faults are separated into upper and lower sections. The upper section exhibits a nearly vertical low-resistivity zone in the upper crust, and the lower section manifests an electrical boundary in the lower crust and upper mantle. Other faults in the Daliangshan block are either nearly vertical low-resistivity zones or electrical boundaries. It is suggested that the formation of the low-resistivity layer in the middle crust is associated with the southeastward motion of the eastern margin of the Tibetan plateau, clockwise rotation of the Chuandian (Sichuan-Yunnan) block, and the westward obstruction from the Sichuan block in Huanan terrain. Seismicity, including the M 8.0 Wenchuan earthquake in the study area, is discussed. Eastern margin of Tibetan plateau. Active fault. Electrical structure. Channel flow. Wenchuan Earthquake.

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.

Thrust-controlled, sediment-hosted, Himalayan Zn–Pb–Cu–Ag deposits in the Lanping foreland fold belt, eastern margin of Tibetan Plateau

Cenozoic economic Zn–Pb–Cu–Ag ore deposits occur in the Lanping foreland fold belt (Lanping basin) of the Eastern Indo-Asian collision zone (EIACZ), eastern margin of the Tibetan plateau. They are sediment-hosted and controlled by two Cenozoic thrust–nappe systems developed in the western and eastern segments of the Tertiary basin, respectively, where thrusting was towards the centre of the basin in an approximately E–W direction and juxtaposes Triassic–Cretaceous over Tertiary strata. In the western system, Cu deposits occur as veins in the root zone, hosted by ~ NS-striking faults in Jurassic intense folds. Zn deposits occur in its front zone, controlled by second-order faults of major thrust faults and thrust-related strike-slip faults. In the eastern system, Zn–Pb–Cu–Ag deposits are located in the front zone, controlled by an E-dipping thrust fault and its second-order faults. The giant Jinding Zn–Pb deposit is controlled by a trap structure related to regional thrusting. Homogenization temperatures of fluid inclusions from these deposits are predominately < 200 °C, with salinities of 1.6 to 27.7 wt.% NaCl equiv. Ore sulfides have Pb isotopes similar to the rocks in the basin and δ 34S CDT values ranging from − 30.4‰ to 11.2‰. H–O isotopes of fluid inclusions are similar to those of basinal brines, and C–O isotopes of hydrothermal carbonate minerals are partly close to those of carbonates and sedimentary organisms. These data suggest that the ore-forming fluids and materials mainly derive from the fluids and various rocks of the basin, respectively. The sediment-hosted Zn–Pb–Cu–Ag deposits in the Lanping foreland fold belt share some similarities with MVT-, SST-, SSC-, and SEDEX-type deposits in that there is a lack of any igneous affinity and instead must relate to activities of basinal fluids. The deposits show, however, a set of unique features: (1) they were formed in a strongly deformed foreland basin within a continent collision orogen and (2) closely related to regional thrust–nappe structure; (3) they are fault-controlled, commonly without strong selection for type of host rocks; and (4) contain a variety of metals including Zn, Pb, Cu, Ag, and minor Sr, Co, etc. It is suggested that the Zn–Pb–Cu–Ag deposits controlled by the thrust systems in the Lanping foreland belt are a new sub-type within the family of sediment-hosted base metal deposits.

Evidence of crustal ‘channel flow’ in the eastern margin of Tibetan Plateau from MT measurements

Magnetotelluric (MT) survey has been carried out in the eastern margin of the Tibetan Plateau and its neighboring Shimian-Leshan area, Sichuan Province. Analysis of this MT data reveals that the electric structure of the Tibetan Plateau differ much from that of the Sichuan block. In general, the electric resistivity of crust beneath the Sichuan block in the east is larger than that of the eastern margin of the Tibetan Plateau in the west. The crust of the plateau is divided into upper, middle, and lower layers. The middle crust is a low resistivity layer with minimum down to 3–10 Ωm about 10–15 km thick. It presumably contains partial melt and/or salt-bearing fluids with low viscosity, prone to deform and flow, producing a “channel flow” under the southeastward squeeze of the eastern Tibetan Plateau. This low-resistivity layer makes the upper crust decoupled mechanically from the lower crust. In the brittle upper crust, faults are dominated by left-lateral strike-slip and thrust motions, leading to surface rising and shallow earthquakes. The low-resistivity layer also cut the Xianshuihe-Anninghe fault zone into two sections vertically. In this region, the thicknesses of upper, middle, and lower crust vary laterally, producing a transitional zone in the eastern margin of the Tibetan Plateau characterized by thicker crust and higher elevation in the west and thinner crust and lower elevation in the east.

Constraining the stepwise migration of the eastern Tibetan Plateau margin by apatite fission track thermochronology

Granites sampled from Garze-Litang thrust, Longmen Shan thrust, Garze and Litang strike-slip faults in the eastern Tibetan Plateau have been analyzed with apatite fission track thermochronological method in this study. The measured fission track apparent ages, combined with the simulated annealing modeling of the thermal history, have been used to reconstruct the thermal evolutionary histories of the samples and interpret the active history of the thrusts and faults in these areas. Thermal history modeling shows that earlier tectonic cooling occurred in the Garze-Litang thrust in Miocene (∼20–16 Ma) whereas the later cooling occurred mainly in the Longmen Shan thrust since ∼5 Ma. Our study suggests that the margin of eastern Tibetan Plateau was extended by stages: through strike-slip faults deformations and related thrusts, the upper crust formed the Garze-Litang margin in the Miocene epoch and then moved to the Longmen Shan margin since ∼5 Ma. During this process, the deformations of different phases in the eastern Tibetan Plateau were absorbed by the thrusts within them and consequently the tectonic events of long-distance slip and extrusion up to hundreds of kilometers have not been found.

Tectonic evolution of the Mesozoic Songpan-Ganze orogenic belt in the eastern margin of Tibetan plateau: Constraints from structural analyses and 40Ar– 39Ar dating

Tectonic evolution of the Mesozoic Songpan-Ganze orogenic belt in the eastern margin of Tibetan plateau: Constraints from structural analyses and 40Ar– 39Ar dating

Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau

Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity (Ū), viscosity (µ) and channel thickness (h), uniquely through the term µŪ/h2. These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 1018 Pa s flowing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr−1.

Crustal P-Wave Velocity Structure from Altyn Tagh to Longmen Mountains along the Taiwan-Altay Geoscience Transect

Based upon the seismic experiments along Geoscience Transect from the Altyn Tagh to the Longmen Mountains, the crustal P-wave velocity structure was derived to outline the characteristics of the crust in the eastern margin of Tibetan Plateau. The section shows a few significant features. The crustal thickness varies dramatically, and is consistent with the tectonic settings. The Moho boundary abruptly drops to 73km depth beneath the southern Altyn Tagh from 50km in the Tarim basin, then rises again to about 58km depth beneath the Qaidam basin. The Moho drops again to about 70km underneath the Songpan-Garzê Terrane, then rises stepwise to 60km near Longmen Mountains. To further southeast, the crust thins to 52km beneath the Sichuan basin in the southeast of Longmen Mountains. In the north of the Kunlun Fault, a low-velocity zone,which may be a layer of melted rocks due to high temperature and pressure in the depth,exists in the the bottom of the middle crust.The two depressions of the Moho correlate with the Qilian and Songpan-Garzê Terranes, implying that these two mountains have deep roots. According to our results, it is deduced that the thick crust of the northeastern Tibetan Plateau probably is a result of east-west and northwest-southeast crustal shortening since the Mesozoic during the collision of the Asian and Indian plates.