Bayan Har Block Research Articles

Characteristics of subsurface density variations before the 4.20 Lushan M S7.0 earthquake in the Longmenshan area: inversion results

The 4.20 Lushan MS7.0 earthquake occurred on the southwest segment of the Longmenshan fault on 20 April 2013. Some meaningful information on the preparation and occurrence of this earthquake was found based on the dynamic variation of gravity (DVG). To examine the great progress of the Lushan earthquake, we obtained the density variation (DENV) derived from the DVG using the compact gravity inversion method in this article. The inversion results reveal three main findings: (1) the DENV in the crust in the Jinshajiang fault area changed from positive in 2010–2011 to negative in 2011–2012. (2) The DENV in the Xianshuihe fault area decreased continuously from 2010 to 2012. (3) The DENV of the uppermost mantle of South China decreased in 2010–2011 and increased in 2011–2012. We propose that the flow/expansion of the middle-lower crust beneath the Bayan Har block and Moho subsidence on the southwest margin of the Chuan-Dian block may have been the major causes of the Lushan earthquake.

Postseismic deformation after 2008 Wenchuan Earthquake

Apparent postseismic deformation was observed after the 2008 Wenchuan earthquake, China. The displacement in the direction normal to the fault decays to nearly zero after 2013, but the significant dextral movement did not decay obviously during our observation of up to May 2013. It indicates that the stress paralleled to the fault in the southern part of the rupture zone was not relaxed during the coseismic slip. The unrelaxed stress transfers northward after the earthquake and continues after 2013. The postseismic deformation may be dominated by afterslip or stress readjustment, but the effects of the viscoelastic relaxation also cannot be ignored. The low velocity zone under the Bayan Har block, which is velocity strengthening, may greatly affect the postseismic deformation and makes it possible that the lower crust ductile flow and the shortening of the crust both contribute to the uplift of the Longmen Shan. The viscoelastic coefficients of the low velocity zone and the lower crust should be larger than 3×1018 Pa.s which is optimised with our observations by using the single viscoelastic relaxation model.

Multiple parameters anomalies for verifying the geosystem spheres coupling effect: a case study of the 2010 Ms7.1 Yushu earthquake in China

<p>In the research of earthquake anomaly recognition, the coupling effect of multiple geosystem spheres can be expected to reasonably interpretating the correlation between various anomalous signals before strong earthquake. Specially, the development of the Lithosphere–Atmosphere–Ionosphere (LAI) coupling model has been accepted as verified by some experimental, thermal and electromagnetic data. However, quasi-synchronous anomalies of the multiple parameters, including thermal, radon and electromagnetic data, have not been reported in a single event case for verifying the geosystem spheres coupling effect. In this paper, we firstly summarized the reported studies on the power spectrum density (PSD) in the ELF/VLF band and radon data recorded from Guza seismic station. Then, historical surface latent heat flux (SLHF) data from the NCEP/NCAR Reanalysis Project was employed for investigating anomalous change in a month before the April 14, 2010, Ms7.1 Yushu earthquake which is one of the typical intra-continental earthquakes in Tibet Plateau. The results from spatial and temporal analysis revealed that anomalous fields of PSD and SLHF data were located close to the epicenter and the ends of some active faults at Bayan Har Block and all anomalous dates converged between April 8 and 11 (6 to 3 days before the Yushu earthquake). Therefore, we suggest that the anomalies of multiple parameters before the main shock are related with the Yushu earthquake. This paper could give an ideal case study to verify the geosystem spheres coupling effect happened in a single event.</p>

The Late Pleistocene activity of the eastern part of east Kunlun fault zone and its tectonic significance

The nearly EW-trending East Kunlun fault zone is the north boundary of the Bayan Har block. The activity characteristics and the position of the eastern end of its eastward extension are of great significance to probing into the dynamic mechanism of formation of the east edge of the Tibetan Plateau, and also lay the foundation for seismic risk assessment of the fault zone. The following results are obtained by analysis based on satellite image interpretation of landforms, surface rupture survey, terrace scarp deformation survey, and terrace dating data on the eastern part of the East Kunlun fault zone: (1) the Luocha segment is a Holocene active fault, where a reverse L-shape paleoearthquake surface rupture zone of about 50 km long is located; (2) the Luocha segment is characterized by left-lateral slip movement under the compression-shear condition since the later period of the Late Pleistocene, with a rate of 7.68–9.37 mm/a and a vertical slip rate of 0.7–0.9 mm/a, which are basically in accord with the activity rate of segments on its west side. The results indicate that it is a part of eastward extension of the East Kunlun fault zone; (3) the high-speed linear horizontal slip of the nearly EW-trending East Kunlun fault zone is blocked by the South China block at east, and transforms into the vertical movement of the nearly SN-NNE trending Minjiang fault zone and the Longmenshan fault zone, and the uplift of Longmenshan and Minjiang. The area where transform of the two tectonic systems occurred confines the position of the east end; (4) Luocha segment and Maqu segment constitute the “Maqu seismic gap”, so, seismic risk at Maqu segment is higher than that at Luocha segment, which should attract more attention.

The Ms7.0 Lushan earthquake and the activity of the Longmenshan fault zone

The M s7.0 Lushan earthquake is directly related to the activity of Longmenshan fault zone. In this article, deformation monitoring data in Longmenshan and its surrounding areas were analyzed and the result shows that the activity trend of Longmenshan fault zone depends on the relative motion between Bayan Har Block and Sichuan Basin, and the main power of the movement comes from the Tibetan Plateau and the upper Yangtze craton massif of push. In recent years, the Longmenshan and its surrounding areas is one of the main seismogenic area in mainland China. In this paper, combination with seismogenic area of geological structure and crustal deformation observation data analysis results, the relationship between the earthquake and Longmenshan fault zone activity was discussed, and the key monitoring areas in the next five years were proposed.

Decomposition and Evolution of Intracontinental Strike‐Slip Faults in Eastern Tibetan Plateau

Abstract:Little attention had been paid to the intracontinental strike‐slip faults of the Tibetan Plateau. Since the discovery of the Longriba fault using re‐measured GPS data in 2003, an increasing amount of attention has been paid to this neglected fault. The local relief and transverse swath profile show that the Longriba fault is the boundary line that separates the high and flat tomography of the Tibet plateau from the high and precipitous tomography of Orogen. In addition, GPS data shows that the Longriba fault is the boundary line where the migratory direction of the Bayan Har block changed from eastward to southeastward. The GPS data shows that the Longriba fault is the boundary fault of the sub‐blocks of the eastern Bayan Har block We built three‐dimensional models containing the Longriba fault and the middle segment of the Longmenshan fault, across the Bayan Har block and the Sichuan Basin. A nonlinear finite element method was used to simulate the fault behavior and the block deformation of the Eastern Tibetan Plateau. The results show that the low resistivity and low velocity layer acts as a detachment layer, which causes the overlying blocks to move southeastward. The detachment layer also controls the vertical and horizontal deformation of the rigid Bayan Har block and leads to accumulation strain on the edge of the layer where the Longmenshan thrust is located. After a sufficient amount of strain has been accumulated on the Longmenshan fault, a large earthquake occurs, such as the 2008 Wenchuan earthquake. The strike slip activity of the Longriba fault, which is above the low resistivity and low velocity layer, partitions the lateral displacements of the Bayan Har block and adjusts the direction of motion of the Bayan Har block, from the eastward moving Ahba sub‐block in the west to southeastward moving Longmenshan sub‐block in the east. Four models with different depths to the Longriba fault were constructed: (1) a shallow fault with a depth of only 4 km, (2) a deeper fault that is half as deep as the Longmenshan fault, (3) a deep fault that is 2 km shallower than the low resistivity and low velocity layer, and (4) a fault that is as deep as the low resistivity and low velocity layer. The activity and influence of the Longriba fault with different development stage under this tectonic system were shown: in one Earthquake recurrence period, the rupture region of the fault increases with the depth of the fault, and the lateral slip partition by the fault also changes with the fault depth. It suggests that the Longriba fault is a newly generated fault that developed after the quick uplift in Late Cenozoic along this tectonic setting and gradually extended from the northwest to southeast. The calculations provide the characteristic of block deformation and fault behaviors of intra‐continental strike‐slip fault and major boundary thrust faults in the eastern margin of the Tibet plateau. Although the low resistivity and low velocity layer controls the deformation of the Bayan Har block and the uplift of the Longmenshan thrust, the partition of the Longriba fault has an important influence on the intra‐plate deformation and modern geomorphic evolution.

Holocene paleoearthquakes of the Maoergai fault, eastern Tibet

Much work about fault activity and seismic hazard assessment has been conducted on major boundary faults of eastern Tibet, whereas seismic activity on faults between subblocks is poorly understood. The Maoergai fault, ~200km northwest of and parallel to the Longmen Shan, is the eastern branch of the Longriba fault zone that divides the Bayan Har block into the Aba and Longmen Shan subblocks. Field investigation indicates that the Maoergai fault is a purely right-lateral strike–slip fault active in the Late Quaternary. Our trenches, combined with radiocarbon dating and Bayesian analysis, show that three Holocene paleoearthquakes ruptured the Maoergai fault at ~8510±420, ~7100±70, and ~5170±80calyr BP, respectively. This suggests that the fault fits a cluster model with an average recurrence interval of ~1700years and has a high potential of large earthquakes (Mw ~7.2–7.4) in the near future. Our results also indicate that an apparent fault scarp on a sloping tread provides an alternative site for paleoseismic studies to determine coseismic horizontal displacement on a strike–slip fault, and using the age of the upper terrace to determine strike–slip rate by displaced terrace riser is probably closer to the true value in streams with insufficient capacities of lateral erosion. This study provides useful information for the earthquake risk and fault behavior in eastern Tibet.

Investigation of plateau basin crustal structures and thickening mechanisms in the northeastern margin of the Tibetan plateau

This paper uses deep seismic sounding (DSS) data to contrast and analyze the crustal structures of three plateau basins (Songpan-Garze, Qaidam, Longzhong) in the northeastern margin of the Qinghai-Xizang (Tibetan) plateau, as well as two stable cratonic basins (Ordos, Sichuan) in its peripheral areas. Plateau basin crustal structures, lithological variations and crustal thickening mechanisms were investigated. The results show that, compared to the peripheral stable cratonic basins, the crystalline crusts of plateau basins in the northeastern margin are up to 10–15 km thicker, and the relative medium velocity difference is about 5% less. The medium velocity change in crustal layers of plateau basin indicates that the upper crust undergoes brittle deformation, whereas the lower crust deforms plastically with low velocity. The middle crust shows a brittle-to-plastic transition zone in this region. Thickening in the lower crust (about 5–10 km), and rheological characteristics that show low-medium velocity (relatively reduced by 7%), suggest that crustal thickening mainly takes place in lower crust in the northeastern margin of the Tibetan plateau. The crust along the northeastern margin shows evidence of wholesale block movement, and crustal shortening and thickening seem to be the main deformation features of this region. The GPS data show that the block motion modes and crustal thickening in the Tibetan plateau is closely related to the peripheral tectonic stress field and motion direction of the Indian plate. The Mani-Yushu-Xianshuihe fold belt along the boundary between the Qiangtang block and the Bayan Har block divides the different plateau thickening tectonic environments into the middle-western plateau, the northeastern margin and the southeastern plateau.

Estimating Moho basement and faults using gravity inversion in Yushu-earthquake area, China

A gravity survey was conducted one month after the 2010 Yushu earthquake in the epicenter area. The cross-fault survey line was 500 km long, from Langqian county to Qingshuihe county, in a transition zone between Bayan Har block and Qiangtang block, in an area of high elevation, large undulating terrain, and complex geological features. An interpretation of the data was carried out together with other kinds of data, such as seismic exploration and magnetic exploration. The result shows that gravity is sensitive to fault boundary; the geologic structure of the region is complex at middle and upper depths, and the density profile reveals an eastward-pushing fault movement.

Seismic relocation, focal mechanism and crustal seismic anisotropy associated with the 2010 Yushu MS7.1 earthquake and its aftershocks

The 2010 Yushu MS7.1 earthquake occurred in Ganzi-Yushu fault, which is the south boundary of Bayan Har block. In this study, by using double difference algorithm, the locations of mainshock (33.13°N, 96.59°E, focal depth 10.22 km) and more than 600 aftershocks were obtained. The focal mechanisms of the mainshock and some aftershocks with MS>3.5 were estimated by jointly using broadband velocity waveforms from Global Seismic Network (GSN) and Qinghai Seismic Network as well. The focal mechanisms and relocation show that the strike of the fault plane is about 125° (WNW-ESE), and the mainshock is left-laterally strikeslip. The parameters of shear-wave splitting were obtained at seismic stations of YUS and L6304 by systematic analysis method of shear-wave splitting (SAM) method. Based on the parameters of shear-wave splitting and focal mechanism, the characteristics of stress field in seismic source zone were analyzed. The directions of polarization at stations YUS and L6304 are different. It is concluded that after the mainshock and the MS6.3 aftershock on April 14, the stress-field was changed.

Tectonic Block Attribution of the Wenchuan and Yushu Earthquakes Distinguished by Focal Mechanism Types

AbstractThe November 14, 2001 Mw7.8 Kunlun Mountain Pass earthquake, the March 21, 2008 Mw7.2 Yutian earthquake and the May 12, 2008 Mw7.9 Wenchuan earthquake all occurred in the middle of Tibet Plateau and formed a new seismicity group. The epicenters of the Kunlun Mountain Pass earthquake and Wenchuan earthquake are located at the northern and eastern boundary of the Bayan Har Block, respectively and the Yutian earthquake at the western end of it. GPS observational data indicate that this block moves to east. The rupture characteristics of the three large earthquakes in different parts of the block suggest the mechanical consistency of the Bayan Har block motion. The intensity and frequency of Yutian aftershocks significantly increased before the main shock and the stronger aftershocks of the Wenchuan earthquake, which proves that there is an internal relation between the three large earthquakes. The April 14, 2010 Mw6.9 Yushu earthquake, the November 19, 1996 Mw6.9 Karakorum Mountain earthquake and the November 08, 1997 Mw 7.5 Mani earthquake all occurred on the southern boundary of the Bayan Har block, and their focal mechanisms are consistently left‐lateral strike slip, which were caused by the eastward motion of the Qiangtang block and formed a new strong earthquake group. The results show that the block has the character of rigid body motion which is the common origin of the two large earthquake groups.

The Mechanism and Dynamics of the Generation and Occurrence for Wenchuan MS8.0 earthquake

AbstractOn 12 May 2008, an MS8.0 earthquake occurred in Wenchuan, Sichuan Province. Its causative fault is the Yingxiu–Beichuan fault. Analysis of the velocity field before the earthquake discovers that the shortening rate along the direction of N103°E from west Bayan Har block (BHB) to the Longmen Shan fault zone (LFZ) is 13.0 mm/a, the right lateral slipping rate of LFZ is 1.1 mm/a, and the faults are locked. The coseismic displacement field shows that the earthquake maybe was caused by the thrust of BHB in direction of SE and the underthrust of Sichuan Basin (SB) in the direction at NW simultaneously. Analysis of the strain field shows that the principal compressive and the principal tensile strain rates in the epicenter area are –30.840×10–9/a and 13.956×10–9/a, respectively before the earthquake, and the principal compressive strain axis is in a direction of N105.4°E which is consistent with the direction of N103°E of the principal compressive stress axis from focal mechanism results. The earthquake fault trend and slipping direction obtained by the mechanism proposed in the paper are also consistent with the results of the focal mechanism and earthquake field investigations. The interactions of the Indian, the Philippine Sea and the Pacific plates with the Eurasian plate are long‐term acting forces responsible for the Wenchuan great earthquake occurrence. The Sumatra‐Andaman huge earthquake on December 2004 promoted occurrence of the Wenchuan earthquake.

Geodynamics of the Sichuan earthquake region in May 12, 2008

A disastrous earthquake with a magnitude of M 7.8‐ 7.9 occurred 93 km northwest of the administrative center Chengdu, Sichuan Province, China, on May 12, 2008. The hypocenter depth was about 30 km. Aftershocks with magnitudes up to M 6.1‐6.4 went on until the first half of June. Their epicenters were confined to the north-northwest‐trending zone extending for 315 km on each side of the site of the main event (http:// earthquake.usgs.gov/eqcenter/page). About 70 000 people were killed, tens of settlements were destroyed, and the total damage reached billions of dollars. The study of recent geodynamics of the earthquake region and adjacent territories allows us to suggest some considerations on the causes of increased seismicity in this part of Central Asia. The analysis of mechanisms in epicenters and GPS data revealed the direction of dislocations along faults bounding crustal blocks in the region, as well as the nature of stresses that emerged in these conditions. An increased liberation of energy comparable with its quantity in subduction and collision zones has been established for a series of interblock zones. Along with data on the deep structure of blocks and their boundaries, this provides a way of distinguishing risk zones in terms of seismicity in Central Asia. Vectors of horizontal crustal displacements based on the ITRF-system space geodesy investigations (http:// itrf.ensg.ign.fr/ITRF_solutions/2005/ITRF2005.php) and data on active faults [1] revealed in Central Asia. Vectors of horizontal crustal displacements in Central Asia revealed a series of blocks bounded by disjunctive dislocations. The blocks behave independently in the present-day geokinetic field [2‐4]. The May 12 event took place in the zone of the Longmen Shan major fault [5] representing a boundary between the Southeast China and Bayanhar blocks (Fig. 1). Such boundaries are, as a rule, expressed not by individual dislocations but wide interblock zones, within which an intense crushing of host rock takes place and a fair quantity of seismic energy is relieved. The position of hypocenters mainly at a depth of 20‐40 km corroborates the relatively shallow penetration of interblock zones into the lithosphere. The cited computations of seismic energy relieved along boundaries of the majority of blocks in Central and Eastern Asia, the Bayanhar block included, exhibit a very high seismic activity of these structures [6].

Rupture behavior and deformation localization of the Kunlunshan earthquake (M w 7.8) and their tectonic implications

Earthquake surface rupture is the result of transformation from crustal elastic strain accumulation to permanent tectonic deformation. The surface rupture zone produced by the 2001 Kunlunshan earthquake (Mw7.8) on the Kusaihu segment of the Kunlun fault extends over 426 km. It consists of three relatively independent surface rupture sections: the western strike-slip section, the middle transtensional section and the eastern strike-slip section. Hence this implies that the Kunlunshan earthquake is composed of three earthquake rupturing events, i.e. the Mw=6.8, Mw=6.2 and Mw⩽=7.8 events, respectively. The Mw=7.8 earthquake, along the eastern section, is the main shock of the Kunlunshan earthquake, further decomposed into four rupturing subevents. Field measurements indicate that the width of a single surface break on different sections ranges from several meters to 15 m, with a maximum value of less than 30 m. The width of the surface rupture zone that consists of en echelon breaks depends on its geometric structures, especially the stepover width of the secondary surface rupture zones in en echelon, displaying a basic feature of deformation localization. Consistency between the Quaternary geologic slip rate, the GPS-monitored strain rate and the localization of the surface ruptures of the 2001 Kunlunshan earthquake may indicate that the tectonic deformation between the Bayan Har block and Qilian-Qaidam block in the northern Tibetan Plateau is characterized by strike-slip faulting along the limited width of the Kunlun fault, while the blocks themselves on both sides of the Kunlun fault are characterized by block motion. The localization of earthquake surface rupture zone is of great significance to determine the width of the fault-surface-rupture hazard zone, along which direct destruction will be caused by co-seismic surface rupturing along a strike-slip fault, that should be considered before the major engineering project, residental buildings and life line construction.

Discovery of the Longriba fault zone in eastern Bayan Har block, China and its tectonic implication

Re-measured GPS data have recently revealed that a broad NE trending dextral shear zone exists in the eastern Bayan Har block about 200 km northwest of the Longmenshan thrust on the eastern margin of the Qinghai-Tibet Plateau. The strain rate along this shear zone may reach up to 4–6 mm/a. Our interpretation of satellite images and field observations indicate that this dextral shear zone corresponds to a newly generated NE trending Longriba fault zone that has been ignored before. The northeast segment of the Longriba fault zone consists of two subparallel N54°±5°E trending branch faults about 30 km apart, and late Quaternary offset landforms are well developed along the strands of these two branch faults. The northern branch fault, the Longriqu fault, has relatively large reverse component, while the southern branch fault, the Maoergai fault, is a pure right-lateral strike slip fault. According to vector synthesizing principle, the average right-lateral strike slip rate along the Longriba fault zone in the late Quaternary is calculated to be 5.4±2.0 mm/a, the vertical slip rate to be 0.7 mm/a, and the rate of crustal shortening to be 0.55 mm/a. The discovery of the Longriba fault zone may provide a new insight into the tectonics and dynamics of the eastern margin of the Qinghai-Tibet Plateau. Taken the Longriba fault zone as a boundary, the Bayan Har block is divided into two sub-blocks: the Ahba sub-block in the west and the Longmenshan sub-block in the east. The shortening and uplifting of the Longmenshan sub-block as a whole reflects that both the Longmenshan thrust and Longriba fault zone are subordinated to a back propagated nappe tectonic system that was formed during the southeastward motion of the Bayan Har block owing to intense resistance of the South China block. This nappe tectonic system has become a boundary tectonic type of an active block supporting crustal deformation along the eastern margin of the Qinghai-Tibet Plateau from late Cenozoic till now. The Longriba fault zone is just an active fault zone newly-generated in late Quaternary along this tectonic system.

Study on Crustal Composition and Geodynamics Using Seismic Velocities in the Northeastern Margin of the Tibetan Plateau

AbstractBased on P‐wave, S‐wave velocity structures and Poisson's ratio of the Maqin‐Lanzhou‐Jingbian seismic refraction/wide angle reflection profile, which crosses the northern Bayan Har block, the Qinling block, the Qilian block, the Haiyuan arcuate tectonic region, and the Ordos block, the crustal composition of the northeastern margin of the Tibetan plateau is studied and the geodynamic issues are discussed. At first, a set of feasible methods are systemically summed up, with which in situ P‐wave velocities observed by deep seismic sounding can be corrected to the velocity under the special condition of temperature and pressure in the laboratory. It is the key to the methods to get temperatures in the crust using heat flow. The second, in situ P‐wave velocities of the profile are corrected to the velocities under a standard pressure of 600 MPa and room temperature, the corrected velocities, combined with Poisson's ratio, are compared with laboratory measurements of ultrasonic velocities, and the crustal petrologic composition is determined in the study area. Results show that the average corrected velocity in the northeastern margin of the Tibetan plateau is 6.43 km/s, and the bulk crustal composition is felsic like the composition of the upper crust. In the Bayan Har block and southern part of the Qinling block, there is a lack of Vp > 6.9 km/s mafic rock layer in the lowest crust, instead there are alternating felsic and intermediate layers in the lower crust, and the bulk composition of the lower crust is felsic. In other blocks, there is a 2~10 km thick mafic layer of Vp > 6.9 km/s and the bulk composition is intermediate in the lower crust. At last, on the basis of the crustal structure and composition in the northeastern margin of the Tibetan plateau, the opinion that crustal thickening occurred mainly in the lower crust is supported, and it is proposed that delamination possibly occurred in the lower crust of the Bayan Har block and southern Qinling block in the recent geologic time, which accelerated the uplift of the Tibetan plateau.

S-wave crustal and upper mantle’s velocity structure in the eastern Tibetan Plateau — Deep environment of lower crustal flow

A teleseismic profile consisting of 26 stations was deployed along 30°N latitude in the eastern Tibetan Plateau. By use of the inversion of P-wave receiver function, the S-wave velocity structures at depth from surface to 80 km beneath the profile have been determined. The inversion results reveal that there is significant lateral variation of the crustal structure between the tectonic blocks on the profile. From Linzhi north of the eastern Himalayan Syntaxis, the crust is gradually thickened in NE direction; the crustal thickness reaches to the maximum value (∼72 km) at the Bangong-Nujiang suture, and then decreased to 65 km in the Qiangtang block, to 57–64 km in the Bayan Har block, and to 40–45 km in the Sichuan Basin. The eastern segment of the teleseismic profile (to the east of Batang) coincides geographically with the Zhubalong-Zizhong deep seismic sounding profile carried out in 2000, and the S-wave velocity structure determined from receiver functions is consistent with the P-wave velocity structure obtained by deep seismic sounding in respect of the depths of Moho and major crustal interfaces. In the Qiangtang and the Bayan Har blocks, the lower velocity layer is widespread in the lower crust (at depth of 30–60 km) along the profile, while there is a normal velocity distribution in lower crust in the Sichuan Basin. On an average, the crustal velocity ratio (Poisson ratio) in tectonic blocks on the profile is 1.73 (σ = 0.247) in the Lhasa block, 1.78 (σ = 0.269) in the Banggong-Nujiang suture, 1.80 (σ = 0.275) in the Qiangtang block, 1.86 (σ = 0.294) in the Bayan Har blocks, and 1.77 (σ = 0.265) in the Yangtze block, respectively. The Qiangtang and the Bayan Har blocks are characterized by lower S-wave velocity anomaly in lower crust, complicated Moho transition, and higher crustal Poisson ratio, indicating that there is a hot and weak medium in lower crust. These are considered as the deep environment of lower crustal flow in the eastern Tibetan Plateau. Flowage of the ductile material in lower crust may be attributable to the variation of the gravitational potential energy in upper crust from higher on the plateau to lower off plateau.

Crustal thickening and variations in architecture from the Qaidam basin to the Qang Tang (North–Central Tibetan Plateau) from wide-angle reflection seismology

Wide-angle reflection–refraction profiles at crustal scale have been obtained along a 700 km transect of northeastern Tibet from six shotpoints. A combination of sampling by broadside fan-recordings and in-line profiling allows us to image increases in Moho depth towards the centre of the plateau. These occur with different styles across the two block boundaries between North Kun Lun, Bayan Har and Qang Tang, as shown also by the changes resolved in the internal architecture and layer velocities in the crust. Clear S-waves are observed: together with P-waves they bring rare constraints on the composition of crustal units, which reveals an abnormally felsic average composition for the whole crust and its lower half along the whole length of the transect. Across the North Kun Lun–Qaidam block, variations of crustal thickness are detected, with Moho depth between 55 and 65 km, and an inverse correlation with basement topography. In the north of the Bayan Har block, a strong heterogeneity in the middle of the 70 km thick crust is shown. As suggested from synthetic seismograms modelling, it may result from thin layer inclusion marking the northward tectonic superposition of the Bayan Har crust on the North Kun Lun crust. As imaged by a fan profile, the corresponding upper part could have been transported northwards to thicken also the upper crust of the present North Kun Lun block. In the north of Qang Tang, a model is obtained with a 75-km thick crust and a lower half of the crust with average velocities characteristic of felsic composition. From Bayan Har to Qang Tang, variations of deep architecture are documented across the Jinsha suture and Xianshuihe fault from recording in-line and broadside to several shotpoints. Together with the felsic composition of the lower part of the crust, a Moho offset suggests the imbrication of the Bayan Har crust in the Qang Tang lithosphere between a south-dipping Moho above and a north-dipping Moho below. A model of indentation of the Qang Tang by an originally thinner Bayan Har crust and lithosphere, with part of the Qang Tang brought to greater depth would also be consistent also with the Moho topography across the transition from the Qang Tang to Bayan Har, as imaged by fan profile.

Relation between electricity structure of the crust and deformation of crustal blocks on the northeastern margin of Qinghai-Tibet Plateau

Study on the electricity structure along a magnetotelluric (MT) sounding profile on the northeastern margin of Qinghai-Tibet Plateau indicates that four crustal blocks can be de-termined from southwest to northeast, namely Bayan Har block (BH), Qin-Qi block (QQ), Hai-yuan block (HY) or the North-South seismotectonic belt and Ordos block (OD). The BH, QQ and OD blocks display a similar electricity structure of the crust. The upper crust represents a high- resistivity layer and the upper part of lower crust represents a low-resistivity layer with the resis-tivity increasing gradually with depth from the lower part of lower crust to the upper mantle. The electricity structure of the crust in these three blocks is similar to that in the complete blocks on the southern and eastern margins of the Qinghai-Tibet Plateau and belongs to normal electricity layering of the crust in slightly deformed or complete intracontinental blocks. The crust in HY block as a boundary zone has been significantly deformed, hence its electricity layering was de-stroyed and the structure was complex and the block became a recent tectonically active and great seismo-active region. The contact belts between the blocks on the northeastern margin of Qinghai-Tibet Plateau exhibit both upthrusting outward and strike-slip movement different from those on the southern and eastern margins of the plateau. The genesis of the low-resistivity layer in the crust is analyzed and the thickness of the lithosphere is estimated in the paper.

Electrical Conductivity Structure of the Crust and Upper Mantle Along the Profile Maqên-Lanzhou-Jingbian in the Northeastern Margin of the Qinghai-Tibet Plateau

Magnetotelluric (MT) measurements have been carried out at 62 sites along the profile Maqen-Lanzhou-Jingbian in the junction zone of the northeastern margin of the Qinghai-Tibet Plateau and the Ordos block. The Robust technique was used in data processing and the Bahr's technique was used for impedance tensor decomposition to remove 3-D effect. The apparent resistivity, impedance phase curves, 2-D skewness and regional tectonic strike were analyzed. The RRI 2-D inversion technique was used for data inversion and interpretation. The results are as follows. (1) The target area is separated by Maqen, Lanzhou and Majiatan-Dashuikeng fault zones into four electric conductivity blocks, namely the Bayan Har block (I), Qinling-Qilian block (II), boundary zone (III) and Ordos block (IV). (2) The crust of blocks I, II, and IV has similar characteristics of electric conductivity structure: a high resistivity layer in the upper crust, a low resistivity layer in the upper part of lower crust, and the resistivity increasing gradually with depth from the lower crust to the upper mantle. The electric conductivity of block 3 is poorly layered and complex in structure, where the recent tectonic movement is more intense. (3) The Maqen, Haiyuan and Luoshan-Yunwushan fault zones are steep fault zones extending through the crust, while the fault zone on the northern margin of the Qinling Range is confined inside the crust.