[Show abstract][Hide abstract] ABSTRACT: We measured the magnetic susceptibility of the core from the first borehole of the Wenchuan Earthquake (May 12, 2008, Mw7.9) Fault Scientific Drilling Project (WFSD-1) at 1-cm intervals. The correlations between magnetic susceptibility anomalies and fault rock occurrence are shown by a few fault zones in the WFSD-1 core. The values for the mass and ferromagnetic material magnetic susceptibility for the sample at 589.25-m depth are higher than those for the other samples. All the thermomagnetic curves display a rapid increase in slope after 380°C, and a marked peak occurs at about 510°C in the heating curves. The cooling curves are clearly higher than the heating curves. The saturation magnetization (Ms) shows a significant peak at a depth of 589.25 m, as do the mass magnetic susceptibility and the ferrimagnetic magnetic susceptibility. The mechanism principally responsible for the high magnetic susceptibility at a depth of 589.25 m might be the production of new magnetite from iron-bearing silicates (e.g., chlorite) or clays caused by frictional heating during seismic slip. Therefore, we suggest that the presence of high magnetic susceptibility fault gouges in the same country rock can be considered as an indicator of earthquakes or seismic signatures.
[Show abstract][Hide abstract] ABSTRACT: The first hole of the Wenchuan earthquake Fault Scientific Drilling (WFSD-1) was started on Nov. 6, 2008 as a rapid response to the May 12, 2008 Wenchuan earthquake (Mw 7.9), which slipped along the Longmen Shan fault on the eastern margin of the Tibetan Plateau. WFSD-1 was drilled to a depth of 1201.15 m and intersected the presumed active fault zone at a depth of 590 m (FZ590), which contains the maximum value of fracture den-sity and fresh fault gouge. Here we characterized fault rocks of FZ590 by conducting XRD analyses with cohesive and non-cohesive rock samples collected from WFSD-1 borehole cores. The results indicate that a clay anomaly zone is located in the FZ590 fault zone. The slight enrichment of smectite distributed in fresh fault gouge implies that there is a fault-related authigenic clay formation. In addition, the location of the slight enrichment of smec-tite is consistent with the plausible active slip zone determined by previous results, adding confidence to the sup-position that the principal slip zone (PSZ) of the 2008 Wenchuan earthquake is located at a depth of 589.2 m and situated at the lithological boundary of the Neoproterozoic Pengguan Complex and Triassic Xujiahe Formation. The tiny clay anomaly signal captured from borehole cores implies that low frictional heat was generated by coseismic slip, which drives only slight authigenesis processes. Thus, other dynamic weakening mechanisms, such as thermal pressurization may be involved in the fault zone of the Yingxiu–Beichuan fault during 2008 Wenchuan earthquake.
[Show abstract][Hide abstract] ABSTRACT: Primary rock magnetism analysis was performed on samples from the Jiulong outcrop across the Anxian–Guanxian fault of the 2008 Wenchuan Earthquake rupture zone. The protolith of hanging wall of this outcrop is the upper Triassic sediments, which formed the fault breccia and gouge by repeated large earthquakes. The footwall of this outcrop contains Jurassic grayish-green and dark-purple sandstones. The average magnetic susceptibility value of the gouge is slightly less than that of potential protolith. Based on the primary rock magnetism, the main magnetic carriers are Fe-sulfides for the gouge, magnetite for the fault breccia, and magnetite and hematite for the Jurassic grayish-green and dark-purple sandstones. Possibly during or after repeated large earthquakes (just like the 2008 Mw 7.9 Wenchuan Earthquake), it transformed the magnetic mineral from magnetite to Fe-sulfides by low thermal decomposition processes along the Anxian–Guanxian earthquake fault, which induces the slightly less average magnetic susceptibility values of the gouge than that of potential protolith. If this magnetic mineral changed only because of repeated large earthquake process, the heating by low velocity seismic slip friction and seismic fluid could possibly have been less than 300 °C. If this magnetic mineral of the Anxian–Guanxian earthquake fault is only induced after repeated large earthquakes, the earth surface process acts an important role for the magnetic mineral change. More other further studies should be done to verify the primary magnetic mineral phase change and discriminate the time of this magnetic mineral variation.
Tectonophysics 04/2014; s 619–620:58–69. · 2.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Wenchuan earthquake Fault Scientific Drilling project (WFSD) started right after the 2008 Mw 7.9 Wenchuan earthquake to investigate its faulting mechanism. Hole 1 (WFSD-1) reached the Yingxiu─Beichuan fault (YBF), and core samples were recovered from 32 to 1201.15 m-depth. Core investigation and a suite of geophysical downhole logs (including P-wave velocity, natural gamma ray, self-potential, resistivity, density, porosity, temperature, magnetic susceptibility and ultrasound borehole images) were acquired in WFSD-1. Integrated studies of cores and logs facilitate qualitative and quantitative comparison of the structures and physical properties of rocks. Logging data revealed that the geothermal gradient of the volcanic Pengguan complex (above 585.75 m) is 1.85 °C/100 m, while that of the sedimentary Xujiahe Formation (below 585.75 m) is 2.15 °C/100 m. In general, natural gamma ray, resistivity, density, porosity, P-wave velocity and magnetic susceptibility primarily depend on the rock lithology. All major fault zones are characterized by high magnetic susceptibility, low density and high porosity, with mostly low resistivity, high natural gamma ray and sound wave velocity. The high magnetic susceptibility values most likely result from the transformation of magnetic minerals by frictional heating due to the earthquake. The YBF exposed in WFSD-1 can be subdivided into five different parts based on different logging responses, each of them corresponding to certain fault-rocks. The high gamma radiation, porosity and P-wave velocity, as well as low resistivity and temperature anomalies indicate that the Wenchuan earthquake fault zone is located at 585.75─594.5 m-depth, with an average inclination and dip angle of N305° and 71°, respectively. The fact that the fracture directions in the hanging wall and footwall are different suggests that their stress field direction is completely different, implying that the upper Pengguan complex may not be local.
Tectonophysics 04/2014; s 619–620:86–100. · 2.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Wenchuan earthquake Gouge graphitization and dynamic fault weakening during the 2008 Mw 7.9 ABSTRACT The Longmenshan fault that ruptured during the 2008 Mw 7.9 Wenchuan (China) earthquake was drilled to a depth of 1200 m, and fault rocks including those in the 2008 earthquake slip zone were recovered at a depth of 575–595 m. We report laboratory strength measurements and microstructural observations from samples of slip zone fault rocks at deformation conditions expected for coseismic slip at borehole depths. Results indicate that the Longmenshan fault at this locality is extremely weak at seismic slip rates. In situ synchrotron X-ray diffraction analysis indicates that graphite was formed along localized slip zones in the experimental products, similar to the occur-rence of graphite in the natural principal slip zone of the 2008 Wench-uan rupture. We surmise that graphitization occurred due to frictional heating of carbonaceous minerals. Because graphitization was associ-ated with strong dynamic weakening in the experiments, we further infer that the Longmenshan fault was extremely weak at borehole depths during the 2008 Wenchuan earthquake, and that enrichment of graphite along localized slip zones could be used as an indicator of transient frictional heating during seismic slip in the upper crust. INTRODUCTION The northeast-southwest–trending Longmenshan thrust belt borders the western margin of the Sichuan basin in China (Fig. 1A). On 12 May 2008, the main Longmenshan fault (Beichuan-Yingxiu rupture zone) rup-tured in a Mw 7.9 earthquake near Wenchuan in southeast Tibet, produc-ing a 240-km-long surface rupture zone (Xu et al., 2009) (Fig. 1A). The typical surface slip along the rupture trace was 3–4 m, with a maximum slip of ~9.5 m near the town of Hongkou in Wenchuan. The coseismic slip velocity was estimated to be in the range of 1–3 m/s (Xu et al., 2010). To investigate the processes associated with large earthquakes, the Wenchuan Earthquake Fault Scientifi c Drilling-1 Project (WFSD-1) was initiated less than one year after the mainshock, with the intention of penetrating the Longmenshan fault at depth (Li et al., 2013). A borehole was drilled to a depth of 1200 m, ~40 km northeast of the epicentral area (Fig. 1A). At 570 m depth, the borehole passed from the Pengguan com-plex (diorite, porphyrite, volcanic rocks, and pyroclastics) into the Xujiahe Formation (coal-bearing sandstone), broadly marking the location of the Longmenshan fault zone (Fig. 1B). Detailed structural analysis of recov-ered borehole materials indicated that the Longmenshan fault is a broad zone of damage between 575 and 595 m depth (mainly within the Xujiahe Formation) containing fault breccias, cataclasites, and fault gouges. At a depth of 590 m, toward the bottom of the fault zone, a 54-cm-thick zone of black fault gouge was recognized, containing highly re-fl ective slip surfaces with slickenlines (Fig. 1C). In one location, the black gouge zone is cut by an ~200-µm-thick ultrafi ne-grained slip zone (Fig. 1D). Based on microstructural observations, it was suggested that this is the principal slip zone (PSZ) that ruptured in this location during the 2008 Wenchuan earthquake (Li et al., 2013). A distinguishing characteris-tic of the thin PSZ is the presence of graphite-rich layers (containing >60 wt% graphite) (Fig. 1D) that were not identifi ed in the surrounding black gouge zones or elsewhere in the fault zone (Si et al., 2012). In this paper, we report mechanical and microstructural data from rotary-shear friction experiments that applied seismic slip pulses on fault gouges of WFSD-1. By in situ synchrotron X-ray diffraction (XRD) analysis, we investi-gated the processes within gouges deformed under earthquake-like conditions.
[Show abstract][Hide abstract] ABSTRACT: Fault zones record a series of faulting events that have occurred under different physical conditions during their evolution. Therefore, it is essential to understand the internal structures of fault zones in order to better understand the mechanical behavior of faults. The internal structure of the Wenchuan earthquake fault zone that prevailed at the Bajiaomiao outcrop and in the WFSD-1drilling cores, located along the southern segment of the Yingxiu─Beichuan surface rupture in the Hongkou area, is described in details in this paper. Based on field surveys, X-ray diffraction analysis, microstructure and analysis of the drilling cores, an ~ 240 m-wide fault zone was confirmed as the Yingxiu─Beichuan fault zone (YBF) at the Bajiaomiao outcrop, corresponding to the ~ 100 m fault zone in the WFSD-1 drilling cores. Fault rocks, including fault breccia, fault gouge and cataclasite were identified in both the outcrop and drilling cores, while pseudotachylyte was only present at the outcrop. Two different types of gouge veins, formed by thermal pressurization and fluidization respectively, are observed in this area. The YBF possesses the characteristics of a multiple core model, and consists of 5 different fault rock units. From top to bottom, these are cataclasite zone, black fault gouge─breccia zone, gray fault breccia zone, dark-gray fault breccia zone and black fault gouge─breccia zone. Outcrop investigation and drilling core research show that the slip zone of the Wenchuan earthquake does not completely follow the ancient fault slip zone. The Wenchuan earthquake fault is a high angle thrust fault which crosses the YBF obliquely. The multi-layered fault rocks displayed in the research area might indicate that the YBF comes from the long-term fault activity and evolution over the last ~ 15─10 Ma.
Tectonophysics 01/2014; s 619–620:101–114. · 2.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: On 12 May 2008, the Wenchuan earthquake (Mw 7.9) produced complicated thrust-type co-seismic surface rupture zones, which encompass the dextral-slip thrust of the Yingxiu–Beichuan fault, the approximately pure thrust of the Guanxian–Anxian fault, and the sinistral-slip thrust of the Xiaoyudong rupture zone located between the former two. In order to understand the faulting mechanism, we discuss the rupture process by examining the segmentation and kinematics of the surface rupture zones, together with the co-seismic fault striations at various sites. Based on the two along-strike main displacement peaks (6–6.5 m and 11–12 m) and on the different geometric and kinematic patterns for the southern and northern segments of the surface rupture zones, we find that the Wenchuan earthquake might have consisted of two rupture stages, which is in agreement with seismic wave inversion results. By comparing the kinematics of fault striations occurring in the Bajiaomiao and Beichuan areas, it suggests that during the first stage, thrusting along both the Yingxiu–Beichuan fault and Guanxian–Anxian fault produced the ~ 80–100 km-long Yingxiu–Qingping surface rupture segment and the ~ 80 km-long Guanxian–Anxian surface rupture zone, respectively. Then, faulting was triggered along the Yingxiu–Beichuan fault by the first rupture process, yielding the second rupture stage, which was characterized by dextral strike-slip (or dextral oblique thrusting). Due to the overlap between the two rupture stages, the southern segment (Yingxiu–Qingping) of the Yingxiu–Beichuan rupture zone comprises two different processes while the northern segment (Gaochuan–Beichuan–Shikan) only suggests one rupture phase.
Tectonophysics 01/2014; s 619–620:13–28. · 2.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The inverse/right-lateral Yingxiu–Beichuan fault ruptured during the Wenchuan earthquake for ~ 270 km. We investigated the northeastern segment of that fault by analysing 3 newly acquired and 3 older seismic lines that cross the fault. We combine this analysis with that of the surface ruptures and of the regional geology in order to discuss the geometry, the style and the amount of motion of the fault. We conclude that the Yingxiu–Beichuan fault is very steep (~ 70°) at the surface and has still a dip > 45° at 6 km depth. The fault offsets all previous structures and particularly the Tangwangzhai nappes from which a vertical offset of 3 to 6 km is evaluated. The right-lateral component of motion along the Yingxiu–Beichuan fault appears to be larger at the surface than at depth and increases only slightly towards the NE. We suggest that the Yingxiu–Beichuan fault is continuous at depth between its northern and southern segments.
Tectonophysics 01/2014; s 619–620:159–170. · 2.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Motivated by an interest in investigating large earthquake mechanisms,
the Wenchuan earthquake Fault Scientific Drilling project (WFSD) has
been launched on November 4, 2008, only 178 days after the Wenchuan
earthquake struck. Large earthquakes have a signi?cant influence on the
rock magnetic records in fault slip zones. The first borehole (WFSD-1)
was drilled through 1201.15 m including Pengguan complex rocks of about
800 Ma and alternating sandstones and siltstones of Triassic age at the
southern segment of the Yingxiu-Beichuan fault (N31° 8'59.36",
E103° 41'28.71"). WFSD-1 shed light to the existence of at least 12
fault zones. The Principal Slip Zone (PSZ) of the Wenchuan earthquake
has been identified at a depth of 589.17 m to 589.28 m (FZ590). To
understand the high magnetic susceptibility in FZ590, we sampled 6
specimens every 10 cm down from 589.05 m-depth to 589.55 m-depth. The
amount of sample is typically about 3-5 g of powder due to the limited
and valuable material available. A series of rock magnetic
investigations were made, such as mass magnetic susceptibility,
high-temperature magnetic susceptibility, magnetic hysteresis loops. The
mass and ferromagnetic materials magnetic susceptibility from the 589.25
m-depth sample shows a higher peak than from other samples, while the
paramagnetic materials magnetic susceptibility shows a decrease from
589.05 to 589.55 m-depth. The k-T curves of the selected samples all
display a rapid slope increase after 380°and a marked peak occurs at
about 510°in the heating curves. The magnetic susceptibility reaches
zero at about 585°. Every cooling curve shows a clear hump between
580° and 380°, which is clearly higher than the heating curves.
The hysteresis loops show the character of closed at about 0.3 T and the
low-coercivity phases. The hysteresis parameters are plotted in a Mr/Ms
versus Hcr/Hc diagram, except the 589. 55 m-depth sample, which could
not be determined due to a very weak expression. All the samples display
typical Pseudo-Single Domain (PSD) field. Rock magnetic data from a
small amount of samples provide valuable information on the core PSZ.
The primary ferromagnetic minerals in this segment are magnetite with
the PSD grain size, which suggests that the grain size cannot be the
main reason for the high magnetic susceptibility at the PSZ. The
dominant mechanism responsible for the 589.25 m-depth high magnetic
susceptibility might be the production of new magnetite from
iron-containing silicates or clays (e.g. chlorite) caused by frictional
heating during earthquakes. Keywords Wenchuan Earthquake,
Yingxiu-Beichuan Fault, Slip Zone, Magnetic Properties
[Show abstract][Hide abstract] ABSTRACT: Scientific drilling in active faults after a large earthquake is ideal to study earthquake mechanisms. The Wenchuan earthquake Fault Scientific Drilling project (WFSD) is an extremely rapid response to the 2008 Ms 8.0 Wenchuan earthquake, which happened along the Longmenshan fault, eastern margin of the Tibetan Plateau. In order to better understand the fault mechanism and the physical and chemical characteristics of the rocks, the WFSD project will eventually drill 5 boreholes along the two main faults. This paper focuses on the first hole (WFSD-1), which started just 178 days after the earthquake, down to a final depth of 1201.15 m. Petrological and structural analyses of the cores allowed the identification of fault-related rocks in the Yingxiu–Beichuan fault (fault gouge, cataclasite, and fault breccia), and the Principle Slip Zone (PSZ) location of the Wenchuan earthquake was determined. We found 12 fault zones in the entire core profile, with at least 10, including the Yingxiu–Beichuan fault zone, with a multiple cores structure and minimum width of ~ 100 m. The co-seismic slip plane of the Wenchuan earthquake at depth (corresponding to the Yingxiu–Beichuan fault zone at the outcrop), as well as its PSZ, was expected to be located at the bottom of the fault zone (at 759 m-depth). Instead, it was found at ~590 m-depth with 1 cm-wide fresh fault gouge, as determined by logging data such as temperature, natural gamma ray, p-wave velocity and resistivity, combined with the fresh appearance, magnetic susceptibility, and microstructure of the gouge. The Wenchuan earthquake slip plane has a dip angle of ~ 65°, showing the high-angle thrust feature. The distribution of fault gouge with several meters thick, the location of the Wenchuan earthquake's PSZ and the thickness of fresh gouge all imply a correlation between the width of the fault zone and the number of seismic events.
[Show abstract][Hide abstract] ABSTRACT: Twenty sites were drilled in the late Cretaceous Shexing Formation for palaeomagnetic studies in the Lhasa terrane near the locality of Maxiang (29.9°N/90.7°E). The stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions. The tilt-corrected mean direction is D/I=350.8°/32.1° with α95=8.1° and N=20 sites, corresponding to a paleopole at 75.0°N, 306.7°E with A95=6.8°. Positive fold tests indicate a primary origin for the characteristic remanence. Based on previous Cretaceous data mainly from the Takena Formation and Paleocene data from the Linzizong volcanic rocks near the city of Lhasa, the latitude of the southern margin of Asia is located at about 15°N, and yields a stable position of the Lhasa terrane during Cretaceous and Paleocene. Compared with expected paleomagnetic directions from the stable India and Eurasia blocks, the collision palaeolatitude further implies the total latitudinal convergence was accommodated by 1700±800km (16.2±7.6°) between southern Tibet and stable Eurasia and 1500±830km (14.4±7.9°) between southern Tibet and stable India since the collision of India and Eurasia. A collision age between c. 54 and 47Ma was determined using the results for the southern margin of Eurasia according to our new data and the extent of ‘Greater India’.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t This contribution provides new constraints on the timing of Tibetan glacial recessions recorded by the abandonment of moraines. We present cosmogenic radionuclide 10 Be inventories at 17 sites in southern and western Tibet (32 crests, 249 samples) and infer the range of permissible emplacement ages based on these analyses. Individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for 10 Be abundance. We consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages: 1) Erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age; and 2) Erosion progressively exposes large boulders with little prior exposure, and so the oldest sample age records the minimum moraine emplacement age. We found that depending on the scenario chosen, the moraine emplacement age can vary by > 50% for w100 ka-old samples. We consider two scaling models for estimating the production rates of 10 Be in Tibet, which has an important, although lesser, effect on inferred moraine ages. While the data presented herein effectively increase the database of sample exposure ages from Tibet by w20%, we find that uncertainties related to the interpretation of the 10 Be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies. A reanalysis of published data from moraines throughout Tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the Tibetan moraine age database. Thus, when viewed in a similar way with other samples collected from this area, uncertainties within moraine exposure ages obscure attribution of Tibetan glacial advances to temperature minima or precipitation maxima. Our work suggests that more reliable chronologies of Tibetan glaciations will come from improvements in production rate models for this portion of the world, as well as a better understanding of the processes that form and modify these geomorphic surfaces.
[Show abstract][Hide abstract] ABSTRACT: This contribution provides new constraints on the timing of Tibetan glacial recessions recorded by the abandonment of moraines. We present cosmogenic radionuclide 10Be inventories at 17 sites in southern and western Tibet (32 crests, 249 samples) and infer the range of permissible emplacement ages based on these analyses. Individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for 10Be abundance. We consider two scenarios to interpret the age of
glacial recession leading to the moraine surface formation from these sample exposure ages: 1) Erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age; and 2) Erosion progressively exposes large boulders with little prior exposure, and so the oldest sample age records the minimum moraine emplacement age.We found that depending on the scenario chosen, the moraine emplacement age can vary by > 50% forw100 ka-old samples. We consider two scaling models for estimating the production rates of 10Be in Tibet, which has an important, although lesser, effect on inferred moraine ages. While the data presented herein effectively increase the database of sample exposure ages from Tibet by w20%, we find that uncertainties related to the interpretation of the 10Be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies. A reanalysis of published
data from moraines throughout Tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the Tibetan moraine age database. Thus, when viewed in a similar way with other samples collected from this area, uncertainties within moraine exposure ages obscure attribution of Tibetan glacial advances to temperature minima or precipitation maxima. Our work suggests that more reliable chronologies of Tibetan glaciations will come from improvements in production rate models for this portion of the world, as well as a better understanding of the processes
that form and modify these geomorphic surfaces.
[Show abstract][Hide abstract] ABSTRACT: The Yumu Shan, located on the northernmost margin of the North Qilian Shan, contributes key information for deciphering the deformation and uplift of the NE Tibetan Plateau. This paper presents magnetostratigraphic results from Late Cenozoic sediments in the Yumu section of the northwestern Yumu Shan. The upper Shulehe Fm., Yumen Conglomerates Fm., and Jiuquan Gravels Fm., were deposited before ∼5.23 Ma, during the periods ∼3.58 to perhaps ∼1.77 Ma, and after ∼0.8 Ma, respectively. Four significant tectonic episodes occurred at ∼5.89 Ma, ∼3.58 Ma, during ∼2.88–2.58 Ma, and ∼0.9–0.8 Ma, based on sedimentation, magnetic susceptibility and angular unconformities. Being synchronous with tectonic activity throughout the NE Tibetan Plateau, these tectonic ages do not support the northward growth model for the NE Tibetan Plateau during the Late Cenozoic.
Quaternary International - QUATERN INT. 01/2011; 236(1):13-20.
[Show abstract][Hide abstract] ABSTRACT: On 20 March 2008, at 22:33:01 UTC, a Ms ≈ 7.3 earthquake struck the Pingding Range, south of the Ashikule basin and northeast of the Guliya ice cap in the Western Kunlun Mountains (Xinjiang, China). The earthquake (Ashikule earthquake) produced a >=32 km-long zone of fresh scarps and cracks, mostly along the large normal fault bounding the west side of the range, above 5400m asl. (West Pingding fault). Based on geometry, geomorphology and co-seismic deformation, the surface rupture can be divided into three sub-segments. The central, N 170°E-trending segment is simplest, with fairly linear, continuous breaks cutting the base of faceted spurs, about one third of the total relief upslope. The more complex northern segment, which veers eastwards away from the range-front to cut through bedrock, is mostly composed of right-stepping cracks striking NE-SW to N-S. To the north, it dies out into an "en-echelon", left-lateral zone of open fissures in the flat, gravel flood-plain of the Yulong Kash before reaching the active, NNE-striking faults and aligned volcanic vents that splay southward from the Achikule branch of the Altyn Tagh fault. Past a sharp 30°-W bend near 35°27'N, the NNE-trending, southern segment of the rupture slices across large glacier tongues with flanking lateral moraines, and snow-covered, till-mantled slopes, before disappearing southwards beneath the eastern Guliya ice-cap. Along the central segment, the main scarp-faces dip steeply (60 to 85°W), exposing coarse, poorly consolidated glacial till or debris-flows. Large, boulder-filled, free-faced cracks reach widths of up to ≈ 8-10 m, probably due to inner-wall collapse. Total station profiles show surface throws ranging between ≈ 1 and ≈ 5m, a few of them cumulative. Many profiles, however, show little surface-slope offset overall, implying that slip beneath the coarse Quaternary deposits may have occurred on a plane dipping little more than ≈ 30° westwards, roughly parallel to the topographic gradient. The geometry of the scarps and rupture is consistent with local 80-90°E trending extension, in keeping with the slip-vector direction (83-84°E) on the west-dipping focal mechanisms' nodal planes. Together with the eruption of one Achikule basaltic vent in 1951, the occurrence of this large earthquake on the West Pingding normal fault confirms the existence of a young, important active link between the Altyn Tagh and Gozha faults. This left-lateral, transtensional fault zone probably takes up a significant fraction of the eastward extrusion of central Tibet relative to the Tarim, reducing the amount and rate of sinistral shear between the latter and the Aksaichin-Tianshuihai block along the Karakax fault, main other branch of the Atyn Tagh system west of 83°E.
[Show abstract][Hide abstract] ABSTRACT: In response to the collision of India with Asia, thick molasse deposits were shed off the high evolving edge of the northern Tibetan Plateau, providing great potential for understanding the relationships between mountain building, rock denudation, and sediment deposition. A precise knowledge of the initial accumulation of the molasse deposits is important for understanding uplift history along the northern margin of Tibet. We have carried out a detailed magnetostratigraphic study of Neogene redbeds sediments at Jianglisai section in the southern margin of the Tarim basin. A total of 700 paleomagnetic samples (with an interval of about 2m) were collected from the Wuqia Formation, Artux Formation and Xiyu Formation in the Jianglisai section. Stepwise thermal demagnetization was used to isolate the magnetic components in these samples. A higher temperature component (HTC) was separated between 300°C and 680°C from most of samples. Positive fold and reversal tests suggest a primary origin for the HTC. A magnetic polarity sequence is established and then compared with the geomagnetic polarity timescale. Detailed magnetostratigraphic studies of the molasse deposits constrain the base of the Xiyu conglomerate here at ~2.15 Ma. Together with other magnetostratigraphic studies carried out around the Tianshan and the southern Tarim basin, our study unambiguously demonstrates that the onset of deposition of the Xiyu conglomerates is diachronous, and the Xiyu Formation should not be considered as a chronostratigraphic marker related to any particular tectonic or climatic event. Detailed magnetostratigraphic studies also show that the abrupt increase in sedimentation rate in the studied section began at about 8Ma. Together with the synthesis of chronologic and structural results, we attribute the accumulation of the sediments to the tectonic uplift along the northern edge of the Tibetan Plateau due to the collision of the Indo-Asia.