[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: 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: 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: 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: 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: 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.
[show abstract][hide abstract] ABSTRACT: The Ama Drime range located at the transition between the high Himalayan range and south Tibet is a N-S active horst that offsets the South Tibetan Detachment System (STDS). Within the horst, a paragneissic unit, possibly attributed to the upper Himalayan crystalline series, overly the lower Himalayan crystalline series Ama Drime orthogneissic unit containing large metabasite layers and pods that have experienced pressure >=1.4 GPa. Combining structural analysis with new and published pressure-temperature (P-T) estimates as well as U-Th/Pb, 39Ar/40Ar and (U-Th)/He ages, the P-T-deformation-time (P-T-D-t) paths of the main units within and on both sides of the horst are reconstructed. They imply that N-S normal faults initiated prior to 11 Ma and have accounted for a total exhumation
[show abstract][hide abstract] ABSTRACT: A combined geochronological and paleomagnetic investigation has been performed on Paleocene volcanic sequences in the Lhasa block near the localities of Mendui (30.1°N/90.9°E). A total of 15 sites have been sampled from rhyolitic tuffs. Stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions. The tilt-corrected mean direction is D/I=359.0°/26.1° with α95=9.2° and N=14 sites, corresponding to a paleopole at 73.6°N, 274.3°E with A95=7.3°. Positive fold tests suggest a primary origin for the characteristic remanence. In order to provide a more accurate pole, we propose to combine site-mean directions from this study and Achache et al.'s (1984). The combined average palaeomagnetic direction from early Paleocene volcanic rocks is D=356.6°, I=25.9°, κ=21.7, α95=6.8° after tilt correction, N=22 sites, corresponding to a pole at 73.2°N, 282.4°E with A95=5.4°. The paleomagnetic results yield a paleolatitude of 13.6±5.4°N for the southern margin of Eurasia at ∼55Ma. Compared with expected paleomagnetic directions from the stable India and Eurasia blocks, significant crustal shortening of 1400±600km and 2000±550km respectively may have occurred between the southern margin of Eurasia and the stable India, and within Eurasia since the collision of India and Eurasia.
[show abstract][hide abstract] ABSTRACT: In the high Himalayan range, near the transition to the Tibetan plateau,
at least two generations of syn-convergence normal faults have been
described. The oldest one corresponds to a major orogen parallel (~E-W)
normal fault termed the South Tibet Detachment (STD) while the younger
one corresponds to orogen perpendicular (~N-S) active normal faults
similar to those observed thorough South and Central Tibet. The timing
of activation and end of each of these fault systems has major bearings
on the mechanical models of geodynamic evolution of the India-Asia
convergence zone. Just North of the highest stretch of the Himalayan
range, comprised between the Chomolongma and KanchenJunga summits, rocks
of the Himalayan crystalline slab are exhumed in the 6500 m high Ama
Drime range. This range is a horst bounded on both sides by preeminent
N-S active faults: the Kartha fault to the west and the Dinggye fault to
the east. Both of these faults show steep (dip ~35-65°) brittle
fault planes and quartzitic cataclasites. In the footwall of both faults
outcrop shallower (dip ~30-45°) ductile mylonites showing evidences
for normal shear sense. These N-S faults clearly offset the STD. In the
core of the range outcrop orthogneiss and migmatites embedding
amphibolite layers that belong to the Lower Himalayan Crystalline series
and that have been buried to a depth of ~60 km (1.7 GPa). Our study that
combines structural and petrographic analysis of the mylonites with
U-Pb, Ar/Ar and U/He geochronology, indicates that the Dinggye shear
zone has been activated prior to 11 Ma ago and that rocks rapidly cooled
below ~300°C at ~10Ma. Data from the Kharta shear zone are more
dispersed but are also compatible with exhumation starting ~12 Ma ago.
The lower temperature thermo-chronometers [(U-Th)/He apatite] confirm
apparent exhumation rates of about 1 mm/yr in the last 5 Myr for the
whole range (Jessup et al., 2008). Total exhumation linked with the
Kharta and Dinggye Faults and shear zones is on the order of 2 to 4 kbar
(7 to 15 km), and could have taken place in two separate phases, the
youngest one starting ~5 Ma ago. East of the Ama Drime, in the hanging
wall of the Dinggye fault, the STD separates Paleozoic Tethyan series at
the top from High Himalayan Crystalline (HHC) micaschists and
leucogranites at the bottom. The STD dips ~5-15° to the North.
Immediately below the STD, the HHC is highly deformed in the STD shear
zone, lineations trend NE and the shear senses indicate top to the N
motion. P-T paths constrained by garnet isopleths in the HHC micashists
show decompression and cooling from ~5 kbar (~18 km) and ~650°C,
after an initial heating phase. U/Pb dating of Monazite and zircons in
both deformed and undeformed leucogranites suggest that ductile
deformation lasted until at least ~16 Ma but ended prior to ~15Ma in the
STD shear zone a few meters below the detachment. Ar/Ar micas ages span
between ~15 and 13 Ma indicating rapid cooling down to below ~300°C
at that time. These data are interpreted as reflecting ~4 kbar (~15km)
of exhumation along the STD and STD shear zone prior to ~13 Ma. Such
timing for the end of motion along the STD system fits particularly well
with the ~12Ma timing of initiation of the crosscutting Kharta and
Dinggye faults. We thus propose that the local direction of extension
switched from N-S to E-W at ~12.5 Ma in the Ama Drime area. It is not
yet clear if such switch occurred synchronously at the scale of the
orogen (i.e. from the Thakkhola to the Yadong grabbens). Such switch
should be taken into account in mechanical models of the Himalaya -
[show abstract][hide abstract] ABSTRACT: Radiometric age dating of detrital zircons is highly advantageous for analysis of the depositional environment and to identify
source areas. Aiming at the uplift and denudation of the surrounding ranges, LA ICP-MS U-Pb analysis has been performed on
zircon grains from a conglomerate collected at the Lower Pliocene of Mazartagh, which is in the center of the Tarim basin,
Xinjiang (新疆), China. A wide range of ages mainly falling into three groups was yielded: 200–500, 800–1 100, and 1 800–2 000
Ma. Zircon features principally indicate magmatic origin. According to the comparison between the analyzed zircons with those
from surrounding orogenic belts, the younger grains are mainly related to the west while the older ones are to the regions
more eastward. The variations might imply the W-E propagation tectonic activation and uplift of the surrounding orogenic belts.
The west segment uplifted and was denuded firstly, driven by the approximate W-E height difference, upon the denudation, transportation
and deposition, acting as the source of young zircons analyzed. With the eastward spreading of tectonic movement, the segment
more to the east rose, the meridional relief increased rapidly and began to control the flow direction, then more zircons
joined in the Pliocene in Mazartagh. It is difficult to definitely explain the source of grains with similar values to that
from the Altyn Mountain region, more detailed data and chronological ages with higher precision will be helpful for making
more credible conclusion.
Journal of Earth Science 01/2009; 20(2):401-416. · 0.40 Impact Factor
[show abstract][hide abstract] ABSTRACT: Zircons and monazites from 6 samples of the North Ayilari dextral shear zone (NAsz), part of the Karakorum fault zone (KFZ), have been dated with the U-Th-Pb method, using both ID-TIMS and SIMS techniques. The ages reveal (1) inheritance from several events spanning a long period between the late Archean and the Jurassic; (2) an Eocene-Oligocene magmatic event (∼35–32 Ma); (3) an Oligo-Miocene magmatic event (∼25–22 Ma), at least partly synkinematic to the right-lateral deformation; and (4) a period of metamorphism metasomatism (∼22–14 Ma) interpreted as thermal and fluid advection in the shear zone. The Labhar Kangri granite located ∼375 km farther Southeast along the KFZ is dated at 21.1 ± 0.3 Ma. Such occurrence of several Oligo-Miocene granites along the KFZ, some of which show evidence for synkinematic emplacement, suggests that the fault zone played an important role in the genesis and /or collection of crustal melts. We discuss several scenarios for the onset and propagation of the KFZ, and offset estimates based on the main sutures zones. Our preferred scenario is an Oligo-Miocene initiation of the fault close to the NA range, and propagation along most of its length prior to ∼19 Ma. In its southern half, the averaged long-term fault-rate of the KFZ is greater than 8 to 10 mm/a, in good agreement with some shorter-term estimates based on the Indus river course, or Quaternary moraines and geodesy. Our results show the KFZ cannot be considered as a small transient fault but played a major role in the collision history.
[show abstract][hide abstract] ABSTRACT: The Karakorum Fault zone (KFZ) plays an important role in understanding the formation, evolvement and deformation of the Tibetan
Plateau. The high-T dextral shearing metamorphic rocks, e.g., mylonites or mylonitized gneisses-granites, locally crop out along the southeastern
part of the KFZ in the Ayila Ri’gyü Range area. The SHRIMP U-Pb dating of the syn-kinematic crystallized zircons indicates
that the initial age of the KFZ is ∼27 Ma, ∼10 Ma older than previous results. The extensive high-T dextral shearing along the KFZ started at least at 27–20 Ma, accompanied by the syn-kinematic emplacement of leuco-granites.
Deformation and concomitant fluid circulation during shearing most likely occurred as early as at 25–13 Ma. The KFZ probably
grew from southeast to northwest along the fault as a result of continuous convergence between the India plate and Eurasia
Chinese Science Bulletin 01/2007; 52(8):1089-1100. · 1.32 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Central Orogenic Belt (COB) of China is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt. Here we report discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the COB. This discovery demonstrates that the country rocks of continental affinity shared in the ultra-high-pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes, thereby establishing the existence of a UHPM belt extending more than 4000 km. Geochronological dating yields Early Palaeozoic ages in the west and Early Mesozoic ages in the east, recording two separate continental collisions overprinted within the COB. Occurrence of UHP metamorphism during recurrent continental collision here and in the Alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.
Terra Nova 10/2003; 15(6):370 - 379. · 2.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these
rocks are the product of the syntectonic anatexis in the process of the left-lateral strike-slip shear, and are the result
of the ductile transpression. There are two types of zircon sorted from the mylonite formed with synshear anatexis. Among
them, one is the anatectic long columnar zircon and another is the residual metamorphic sub-rounded columnar zircon. Two groups
of age for single zircon measured by ion microprobe (SHRIMP) are obtained: one is 461–547 Ma for the sub-rounded columnar
residual metamorphic zircon, and the other is 239–244 Ma for the long-columnar anatectic zircon. This type of zircon is directionally
spread in rock, and the long axis direction of its crystal is identical to that of stretching lineation, representing the
direction of tectonic stress in the process of the strike-slip. 40Ar-39Ar age of the directional growth hornblende in the same mylonite sample measured is 223–226 Ma. The above geological and chronological
data prove that the syntectonic anatexis occurred during Indo-Chinese Epoch, showing that the strong strike-slip movement
occurred in the Altyn Tagh fault zone at least as early as Indo-Chinese Epoch.
Chinese Science Bulletin 01/2002; 47(1):28-33. · 1.32 Impact Factor
[show abstract][hide abstract] ABSTRACT: Zircon SHRIMP dating from Qaidamshan granite shows that the granite age is 446 Ma, similar to that of eclogite in the UHP
belt. We think that both granite and eclogite may be formed at different stages during tectonic evolution of this area. Together
with other studies we suggested that the collision of ocean and continent plates may occur at the early Caledonian in this
area, forming the eclogite, and the collision of continent and continent plates at the late Caledonian, forming the Qaidamshan
granite with the zircon SHRIMP age of 446 Ma.
Chinese Science Bulletin 01/2002; 47(5):418-422. · 1.32 Impact Factor