T33A-2031
Lithospheric Structures Across the Longmen Shan Mountain Range From Seismologic and Gravimetric Data
The Wenchuan earthquake was somehow unexpected because it occurred in a region where the present-day deformation was not considered to be very high, as attested by GPS measurements and the lack of a well- developed foreland basin to the East of the Longmen Shan range. These observations, together with the persistence of a high topographic border are at the heart of a wide variety of mechanical models, among which several go by the existence of a low viscosity channel at mid crustal depth. Thus, bringing strong constrains on the lithospheric structure and composition in this region is a key to firm up these models. This is what motivated a sino-french team to deploy a dense seismic network between November 2005 and April 2007 in the central part of the Longmen Shan belt, very close to where the Wenchuan earthquake occurred one year after the end of this experiment. Applying the receiver function method to the teleseismic earthquakes recorded by the 36 seismic stations, we obtain a detailed and continuous image of the crustal structure from the Sichuan basin up to the Xianshuhe fault. This image reveals a crustal discontinuity at about 15km depth which may connect with the ruptured zone of the Wenchuan earthquake. Moreover, our results highlight an abrupt 20 km Moho offset between the Sichuan Basin and the Tibetan plateau and this sharp Moho offset is consistent with regional gravity data and microgravity measurements acquired along the seismological profile. These observations indicate a major contrast in strength between the Tibetan plateau and the Yangtze craton, the latest acting as a rigid block resisting the eastward displacement of the Tibetan Plateau. However, the mean crustal Poisson ratio, deduced from the analysis of the receiver functions, is low to normal beneath all the stations located on the Songpan-Ganze terrane, which suggests the absence of a thick and extensive zone of partial melt within the crust of this region.
T33A-2032
Seismic anisotropy across the Longmen Shan mountain range from a passive seismological survey.
Located between the eastern margin of the Tibetan plateau and the Yangtze craton, the Longmen Shan
mountains range is a key area for understanding mechanisms that control the deformation and the eastward
extrusion of the Tibetan plateau. This context motivated several French institutes and the University of
Chengdu to set up a seismic network across the Longmen Shan to determine the patterns of the lithospheric
deformation in this region. The profile, composed of 36 stations with a mean inter-station spacing of 10km,
was deployed in several phases from November 2005 to April 2007 and ran from the Sichuan basin, across
the Longmen Shan fold belt, the Songpan Garze terrane and up to the Xianshuhe fault.
Here, we present the first results from this experiment about the anisotropy within the lithosphere based on
shear wave splitting measurements. 41 clear SKS and SKKS phases from 23 teleseismic events were
recorded during the two periods of deployment and selected through visual inspection. We used the cross-
correlation method to calculate the splitting parameters, that is, the azimuth of the past polarization direction
and the delay time between the split phases arrivals. We show that 1) The polarization directions are
coherent in all the studied zone. They are compatible with previous observations, GPS measurements and
with the main known surface features like the Xianshuhe strike-slip fault. Strikingly, no significant change is
observed between the Longmen Shan region and the eastern part of the Yangtze craton; 2) The measured
delays are small and don't reflect important asthenospheric flow. Some measurements of S splitting near the
Beichuan-Weichuan fault system show normal anisotropy for the crust (around 0.05 to 0.1s).
Based on these results, the origin of anisotropy seems to be confined to the lithosphere and coherent
deformation of the crust and mantle lithosphere cannot be excluded.
T33A-2033
The Mw 7.9 Wenchuan earthquake of 12 May 2008, Sichuan, China: Surface rupture and oblique right-lateral co-seismic thrusting
At 14:28 pm, on 12 May 2008, the Mw 7.9 Wenchuan earthquake struck the Longmen Shan, eastern margin of the Tibet Plateau, Sichuan, China. This event was the most catastrophic in continental China for the past 30 years. Field investigations indicate that the earthquake was due to slip on the NE-striking Longmen Shan thrusts, up to now inferred to have uplift rates of no more than 1 mm/yr and shortening rates of less than 3 mm/yr according to geodetic observations. The main surface rupture was 275 km-long, along the NE-striking Yingxiu-Beichuan fault, which dips westwards, often quite steeply near the surface ; Slip was opblique in most places, with thrust and right-slip components. Another 80 km-long rupture, displaying almost pure thrusting was observed in the foreland, along the Guanxian-Anxian fault. A small, 6 km-long, NW-striking surface rupture with oblique left-lateral thrusting to the NE, was also observed to link segments of the main rupture zones. The largest measured co-seismic offsets, along the Yingxiu-Beichuan rupture, reached up to about 10 m vertically and 12 m horizontally. The maximum vertical slip along the Guanxian-Anxian rupture zone was 4 m. The co-seismic and minimum cumulative vertical offsets of four distinct terraces at Yingxiu are 3, 10, 35 and 42 m. The smallest offset is unambiguously that of the Wenchuan event. Given the 75 ka age (Li et al., 2006) of terrace T4, the 42 m offset would imply a minimum average throw rate of 0.56 mm/yr. Fifteen km northeastwards at Gaoyuan, an 8.3 ka old fan surface (Densmore et al., 2007) is offset by 5.5 m where it is cut by the Yingxiu-Beichuan rupture. This offset includes the 2.5 m co-seismic offset of the last event and a 3 m offset of a previous event, implying a throw rate of 0.66 mm/yr. If the Wenchuan earthquake is representative of events occurring along the Yingxiu-Beichuan fault then an average recurrence interval of about 3 to 6 kyrs for great earthquakes comparable to the 12/05/2008 event can be estimated. Although preliminary these results limited to one of the active thrusts of the Longmen Shan, they indicate that seismic thrusting and crustal shortening along the eastern margin of Tibet are the primary causes of relief growth contrary as previously thought.
T33A-2034
Characteristics of the Surface Rupture Zone of the Segment North to Beichuan of the Ms 8.0 Wenchuan Earthquake, Sichuan, China
The 2008 Wenchuan earthquake measured at Ms 8.0 according to Chinese Earthquake Administration (CEA), occurred at 14:28:01(CST) on May 12, 2008 in Sichuan province of China. Field investigation on the surface ruptures between Beichuan and Qingchuan of this earthquake shows, only one surface rupture zone develops on this segment, which spreads totally along the Beichuan-Qingchuan fault, the central fault of the Longmenshan fault zone. Observation from southwest at Huangjiaba to northeast at Shikan suggests, the surface ruptures on this segment stretch continuously along the trend of the fault, with a single structure and a length of 60~90km. The surface rupture has not reached Guanzhuang, Qingchuan county. The observable rupture zone is about 62km, between Beichuan and Shikan, trending overly 20 to 55¡ã, dipping NW at an angle of 70¡ã, showing mainly thrusting with dextral strike slipping. The most distinct feature of the surface ruptures of Ms 8.0 Wenchuan earthquake displays as arching of the surface, which indicates the thrusting of the deep fault. Its horizontal motion on this segment displays as dextral strike slipping, no sinistral slipping component. The value of the vertical coseismic displacements decreases from 3m at Huangjiaba gradually to be about 1.5m at Nanba and Shikan; The amount of the dextral displacements does not change evidently, it is generally between 1.5m and 2.0m. Features of the surface rupture indicate, the causative tectonics of this Ms 8.0 Wenchuan earthquake is Yingxiu-Beichuan-Qingchuan fault, whose movement characters mainly as thrusting, with a dextral slipping component, and the thrusting direction is from west to east.
T33A-2035
Cenozoic tectonic activity of the Longmen Shan, Eastern Tibet, and the context of the May 2008 Sichuan earthquake
The destructive May 2008 Sichuan earthquake has pointed out the seismic potential of the Longmen Shan region in Eastern Tibet. Prior to that event, this area has intrigued researchers due to the lake of active shortening, as seen by geodetic measurements, despite the existence of a major topographic escarpment between the low-elevated Sichuan basin and the Tibetan Plateau. Due to this apparent lake of significant tectonic activity, the occurrence of an earthquake of that magnitude was unexpected. The earthquake presumably occurred on the Beichuan Fault, which is one of the major structures of the area. However, the distribution of tectonic activity, in space and time, of the different faults of the central Longmen Shan is still debated and poorly understood. Putting constraints on the long term activity of those faults is particularly important to resolve the general tectonic context of the Longmen Shan and understand the signification of the May 2008 event. We present a thermochronological dataset for the Central Longmen Shan, that allows to put significant constraints on the long term faulting activity across this margin of the Tibetan Plateau. Our data shows that active exhumation started 8-10 Ma, with rates ~0.65 mm.yr-1, and that such activity was sustained at least until 3 Ma. The analysis of the spatial distribution of the ages suggests that most of the exhumation was accommodated by the Beichuan Fault (BF). Focal solutions for the May 12th event indicate a dip angle of 30-50°, which leads to a slip rate on the BF comprised between 0.4 and 1 mm.yr-1 Assuming that the average slip associated with the earthquake was ~5 m this suggests that the recurrence time for events of this magnitude may be between 5 and 12 kyr. This earthquake supports the existence of slow shortening activity in the Longmen Shan. It may be related to the exhumation phase that started at ~10 Ma and the corresponding long term rates appears to be compatible with geodetic data (< 3 mm/yr). The activity across structures such as the Beichuan Fault may be in part responsible for the topographic development of the Eastern margin of the Tibetan plateau. We show that this activity is a major characteristic of the history of this region that must be integrated in the proposed evolution models
T33A-2036
Cenozoic Normal Faulting in the Longmen Shan, Eastern Tibet
A series of normal faults that parallel the steep eastern margin of the Tibetan Plateau have been identified in the Longmen Shan region. Extension along these structures predates, or was synchronous with, regional uplift. Normal faults separate the Precambrian basement rocks of the Pengguan and Baoxing Massifs, the Kangding antiform, and the Gezong dome from Proterozoic and Paleozoic sedimentary cover rocks. The faults often parallel or reactivate Mesozoic and Cenozoic thrust faults, and may be difficult to distinguish from these shortening structures; however, kinematic analysis at both the outcrop and thin section scale reveals a normal sense of motion along the mapped structures. The faults exhibit a range of styles and both low-angle and high-angle faults are present. The timing of motion along these structures is an important issue in our understanding of the tectonic evolution of the eastern margin. Apatite (U-Th)/He data show that the normal fault on the western boundary of the Baoxing Massif is older than at least ~8 Ma. Zircon (U-Th)/He data may provide better constraints on the timing of extension in this region. Biotite and muscovite 40Ar/39Ar data suggest that mylonite within the fault zones bounding the Gezong dome and the Kangding Antiform experienced an episode of localized heating and cooling at ~30 Ma. Coupled with regional thermochronology, the data suggest that rock uplift, northeast-directed shortening and northwest-directed extension in the Danba region occurred at ~30 Ma, coincident with eastward extrusion of lithospheric fragments along the Ailao Shan and Altyn Tagh Faults. The presence of these normal faults, and the evidence for deformation 20 m.y. before regional surface uplift points to a complicated and multi-phase history of Cenozoic deformation on the eastern margin.
T33A-2037
Long-term Petrological Evolution of the Wenchuan and Beichuan Fault Zones (Longmen Shan, Sichuan)
The epicentre of the May 12 2008 (M7.9) Sichuan Earthquake seems to be near the junction between two main faults, the Wenchuan fault to the west and the Beichuan fault to the east. The rupture mainly propagated along the Beichuan fault over more than 250 km, with thrusting toward the East in the southern part becoming more dextral slip in the northern part. Displacement on the Wenchuan fault can not been excluded nevertheless. The structures of this area result from a polyphased evolution dating back from Triassic times. Particularly, intense deformation is concentrated around the Wenchuan shear zone, and was reactivated during Cenozoic times as a far effect of the Indian Asia collision. Our study was focussed on several sections across the Wenchuan and Beichuan fault zones to document their long term tectono- metamorphic evolution. Two cross sections across the Wenchuan fault were sampled, a southern one and northern one. The deformation to the south reflects southwest verging tranpressional movement, whereas strike slip movement dominated along the northern profile. The southern profile cross cut several slices in which a metamorphic gradient increasing from West to East. The largest and westermost slice consists of metabasic rocks, including metagrabbros, metabasalts and oceanic metasediments. Chemical analyses of the basic rocks show an adakitic signature. The PT estimations reveal that high pressure condition was reached for these rocks (~ 14 kbar). This slice may represented a former back arc unit having been subducted at great depth (45 km), and later exhumed along the Pengguan crystalline massif. Such gradient is not observed on the northern part of the Wenchuan fault where the temperature increases at constant depth. The age of the metamorphism is so far unknown. Hence, the metamorphic evolution of the northern Wenchuan is compatible with long term strike slip movement with little exhumation. The southern part exhumation from greater depth is evidenced, that is consistent with the transpressional structures observed in this area. For the Beichuan fault, preliminary petrological study (along a southern transect) indicates temperature conditions of the hanging wall (<350°C) lower than those from the Wenchuan fault, but with relatively high pressure (10 kbar). Temperature in the footwall reached 350°C. Preliminary petrological results suggest that the long term evolution of the Beichuan fault (at least in the south) was dominated by westward thrusting. Therefore, the complexity of the recent rupture shown by radar interferometry and field observations, is probably due to pre-existing geological structures.
T33A-2038
Thermal Mechanical Evolution of the Eastern Tibetan Plateau
The long term thermo-mechanical history of the Tibetan plateau is crucial to understand the present active
deformation along its eastern margin as highlighted by the may 12 2008 Wenchuan Earthquake. Geophysical
data indicate a 20 km Moho vertical offset underneath the Longmen Shan. This crustal thickening and the
Tibetan plateau development result from tectonic accretion during the Mesozoic and Cenozoic.A key question
is the relative impact of these events on the tectonic evolution of the eastern margin of the tibetan plateau.
We propose a new structural map including published and new field observations and an augmented P-T-t
study on the following structural units of the region :
-Triassic turbidites are affected by intense upright folding and reverse faults associated with low-grade
metamorphism reaching up to 450C .Turbidites are intruded by granitic plutons dated from 230 to 160 Ma.
-Crystalline Proterozoic rocks exposed in the LongmenShan,(Penguan massif) and in the Gezong and
Gongcai domes. Metasediments associated with the Gezong and Gongcai domes reach 500C around 260
Ma (Ar Ar on amphibole (Zhou et al 2008)). Cooling of these domes occurred between 159 and 50 Ma. The
turbidites are thrusted over these rocks.
-Amphibolite facies metasediments and metabasites, associated with migmatites and granites are exhumed in
the Danba metamorphic dome and juxtaposed to the turbidites along a low-angle shear zone. Our data on
metabasites show a peak metamorphism at 800C and 5kbar around 220 Ma. Two high grade metamorphic
events are identified by Huang et al (2003a, 2003b) at 200 and 165 Ma (Indosinian). A 65 Ma U-Pb on
monazite associated with sillimanite is obtain by Wallis et al (2003), but the signification of this age is
understand. The metamorphic record of rocks of the Gezong dome and of the Danba area is different. These
rocks probably represent different structural levels although the nature of the contact between them has not
been identified.
Perspectives for this work is to test, through the exhumation rate, the validity of the extreme views for the
thickening and surrection of the eastern margin : the rigid one, where the thickening is the result of pile of
crustal scale and the viscous one, where a ductile level flows and is stopped at the Yantze craton.
Huang M. H., Buick I. S. and Hou L. W., 2003a. Tectonometamorphic evolution of the eastern Tibet plateau :
evidence from the Central Songpan Garze Orogenic Belt, Western China. Journal of petrology, vol 44, 2, pp
255-278
Huang M., Maas R., Buick I. S. and Williams I. S., 2003b. Crustal response to continental collisions between
the Tibet, South China and North China Blocks : geochronological constraints from the Songpan Garze
Orogenic Belt, Western China. Journal of metamorphic geology, 21, pp 223-240
Wallis S. Tsujimori T., Aoya M., Kawakami T., Terada K., Suzuki K., Hyodo H. 2003. Cenozoic and Mesozoic
metamorphism in the LongmenShan orogen : Implications for geodynamic models of eastern Tibet. Geology,
vol 31, 9, pp 745-748.
Zhou, M.-F., D.-P. Yan, et al. 2008. Structural and geochronological constraints on the tectono-thermal
evolution of the Danba domal terrane, eastern margin of the Tibetan plateau. Journal of Asian Earth
Sciences 33(5-6): 414-427.
T33A-2039
Did a single large river system once drain the margin of SE Tibet and flow into the South China Sea?
Largely on the basis of geomorphological features it has been proposed that in the Cenozoic a large single river system once drained the SE margin of the Tibetan plateau and flowed southwards into the South China Sea. This palaeodrainage model proposes that river capture and drainage reversal accompanied Neogene uplift of the SE margin of Tibet leading to a progressive reduction in drainage area to eventually form a drainage similar in catchment area to that of the modern Red River. To test this model we examined detrital zircon U-Pb ages from Cenozoic sediments deposited in the South China Sea Yinggehai Basin, offshore Red River delta, and presumed palaeo-Red River deposits deposited in the Jianchuan Basin near the modern day river headwaters in China. If a large palaeo river system once drained the southeast margin of Tibet we would expect to find a wide age range diagnostic of a large drainage area encompassing at least four major tectonic terranes that welded together in the Mesozoic. We would also expect samples from both basins to show a similarity in zircon age distributions through time. Samples from the Yinggehai Basin that span the Oligocene and mid Miocene (circa 25-13 Ma), the time at which a large drainage system may have existed, contain a consistently identical provenance with the bulk of zircon sources recording circa 250 Ma and 100-110 Ma ages. Zircon typology indicates a tholeitic/alkaline I- type granite terrain for the Cretaceous ages. Zircon U-Pb data from Oligocene sediments collected from the onshore Jianchuan Basin also record a narrow range of ages and are also dominated by 250 Ma sources although by contrast Cretaceous grains are virtually absent. Importantly, both basins exhibit a very narrow range of detrital zircon ages suggestive of a restricted drainage. According to the large single drainage model a progressive reduction in drainage area initiated in the middle to late Miocene as east Tibet experienced surface uplift. We do find evidence for a major change in the marine basin sediment provenance between circa 13 and 3 Ma when the range of zircon ages dramatically expands to include Palaeozoic and Proterozoic sources accompanied by loss of the Cretaceous ages. These changes coincide with shifts in sediment accumulation in the South China Sea Basins. So what might have caused this? The young and restricted age range of zircons from Oligocene and early Miocene samples are hard to reconcile with the existence of a large regional scale drainage system. Based on the provenance data we envisage a relatively small palaeodrainage system with restricted and localized erosion linked to strike-slip deformation. The Late Miocene to Pliocene change in South China Sea basin provenance can be linked regional uplift and a phase of incision by the Red River in the Pliocene rather than any major drainage re-organization connected to uplift along the eastern margin of Tibet.
T33A-2040
The history of the Keriya River: Implications for the evolution of drainage- geomorphology- tectonics- climate of the West Kunlun Mts, Tibet
During passed 2008, two great earthquakes, more than 7 magnitude, occurred beneath the steep margins of the Tibetan plateau. One is Wenchuan in its eastern margin; another is West Kunlun in its northeastern margin. Both of earthquakes have something to do with the magnitude 8.1 earthquake in the Kunlun Mts on 14 Nov. 2001. Therefore they may some relationship on kinematics. The great earthquakes are main deformational events, which contribute, step-by-step, long-term deformation and tectonic geomorphology of these steep margins of the Tibetan plateau. But we have little knowledge about their long-term deformation. A river is a basic element of geomorphology, which forms during evolution of geomorphology. And the Tibetan plateau is the giant tectonic geomorphology in the world resulted from continental collision. A river of the Tibetan plateau documents changes of its drainage, geomorphology, tectonics and climate. Therefore it can tell the evolution of the Tibetan plateau. Up to now we know little about history of a river in the world. In this presentation we demonstrate the history of the Keriya river with systematic measurements of geomorphology, geomorphic markers, sedimentary and Paleo-current, AFT ages and structural geology of the northwestern margin of the Tibetan. The key issue is the age of a geomorphic marker. We make this issue sure by the Ar-Ar dating of the basalt on the top of the Xiyu conglomerate, which gave a mean weighted age at 1.1¡À0.1Ma. The Kashi group in Oligocene in the southwest Tarim basin is shallow sea to lagoon facies, the Wuqia group is delta to plain fluvial facies, the Atushi formation is sediments of fluvial to fan margin facies, while Xiyu conglomerate is upper fan to middle fan alluvial-diluvial deposition. More than 5000 paleo-current data we obtained show that dramatic change occurred from SE and SEE direction in the Wuqia age to NW and NNW direction, the modern drainage direction in the Atushi age, which implies that the great geomorphology change occurred. Analyzing of the AFT of the Yuetangnengdailiya granite in steep slope of west Kunlun Mts gives the ages arranging from 2.9¡À0.5¡«0.9¡À0.3Ma. And different rocks have the characteristiation thrust nappe structures, also reflecting rapid cooling since middle Pliocenc of the upper the younger and the lower the older, presenting backward propage. The history of the Keriya river that we observe is younger than 1.1 Ma. The history of the Keriya river that we can know by analyses is younger than the Xiyu age. The history of the Keriya River that we can infer to the Maximum is not more than the Atushi age in Pliocene. During the Wuqia age in Miocene, no influence from the present Tibetan plateau can be found on the sediments, which demonstrate that the steep margin and its plateau occurred from Pliocene and the evolution of drainage, geomorphology, tectonics and climate since then.