Xiaohan Liu

The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina

The Red River Fault zone (RRF) is the major geological discontinuity that separates South China from Indochina. Today it corresponds to a great right-lateral fault, following for over 900 km the edges of four narrow (< 20 km wide) high-grade gneiss ranges that together form the Ailao Shan-Red River (ASRR) metamorphic belt: the Day Nui Con Voi in Vietnam, and the Ailao, Diancang and Xuelong Shan in Yunnan. The Ailao Shan, the longest of those ranges, is fringed to the south by a strip of low-grade schists that contain ultramafic bodies. The ASRR belt has thus commonly been viewed as a suture. A detailed study of the Ailao and Diancang Shan shows that the gneiss cores of the ranges are composed of strongly foliated and lineated mylonitic gneisses. The foliation is usually steep and the lineation nearly horizontal, both being almost parallel to the local trend of the gneissic cores. Numerous shear criteria, including asymmetric tails on porphyroclasts, C-S or C′-S structures, rolling structures, asymmetric foliation boudinage and asymmetric quartz 〈c〉 axis fabrics, indicate that the gneisses have undergone intense, progressive left-lateral shear. P-T studies show that left-lateral strain occurred under amphibolite-facies conditions (3–7 kb and 550–780°C). In both ranges high-temperature shear was coeval with emplacement of leucocratic melts. Such deformed melts yield UPb ages between 22.4 and 26.3 Ma in the Ailao Shan and between 22.4 and 24.2 Ma in the Diancang Shan, implying shear in the Lower Miocene. The mylonites in either range rapidly cooled to ≈ 300°C between 22 and 17 Ma, before the end of left-lateral motion. The similarity of deformation kinematics, P-T conditions, and crystallization ages in the aligned Ailao and Diancang Shan metamorphic cores, indicate that they represent two segments of the same Tertiary shear zone, the Ailao Shan-Red River (ASRR) shear zone. Our results thus confirm the idea that the ASRR belt was the site of major left-lateral motion, as Indochina was extruded toward the SE as a result of the India-Asia collision. The absence of metamorphic rocks within the 80 km long “Midu gap” between the gneissic cores of the two ranges results from sinistral dismemberment of the shear zone by large-scale boudinage followed by uplift and dextral offset of parts of that zone along the Quaternary Red River Fault. Additional field evidence suggests that the Xuelong Shan in northern Yunnan and the Day Nui Con Voi in Vietnam are the northward and southward extensions, respectively, of the ASRR shear zone, which therefore reaches a length of nearly 1000 km. Surface balance restoration of amphibolite boudins trails indicates layer parallel extension of more than 800% at places where strain can be measured, suggesting shear strains on the order of 30, compatible with a minimum offset of 300 km along the ASRR zone. Various geological markers have been sinistrally offset 500–1150 km by the shear zone. The seafloor-spreading kinematics in the South China Sea are consistent with that sea having formed as a pull apart basin at the southeast end of the ASRR zone, which yields a minimum left-lateral offset of 540 km on that zone. Comparison of Cretaceous magnetic poles for Indochina and South China suggests up to 1200 ± 500 km of left-lateral motion between them. Such concurrent evidence implies a Tertiary finite offset on the order of 700 ± 200 km on the ASRR zone, to which several tens of kilometers of post-Miocene right-lateral offset should probably be added. These results significantly improve our quantitative understanding of the finite deformation of Asia under the thrust of the Indian collision. While being consistent with a two-stage extrusion model, they demonstrate that the great geological discontinuity that separates Indochina from China results from Cenozoic strike-slip strain rather than more ancient suturing. Furthermore, they suggest that this narrow zone acted like a continental transform plate boundary in the Oligo-Miocene, governing much of the motion and tectonics of adjacent regions. 700 and 200 km of left-lateral offset on the ASRR shear zone and Wang Chao fault zone, respectively, would imply that the extrusion of Indochina alone accounted for 10–25% of the total shortening of the Asian continent. The geological youth and degree of exhumation of the ASRR zone make it a worldwide reference model for large-scale, high-temperature, strike-slip shear in the middle and lower crust. It is fair to say that this zone is to continental strike-slip faults what the Himalayas are to mountain ranges.

Constraints on the stratigraphic age of metasedimentary rocks from the Larsemann Hills, East Antarctica: possible implications for Neoproterozoic tectonics

In this study we investigate crystallization ages of detrital zircon from paragneiss from the Larsemann Hills, East Antarctica, using the single zircon PbPb thermal evaporation technique. The 207Pb/206Pb ages of eleven detrital zircons range from 1200 ± 6(σ) Ma to 766 ± 12(σ) Ma. Although these ages must be regarded as minimum crystallization ages, investigation of the U content and damage state of the internal crystal structure using fission track analysis suggests that they may approach crystallization ages. This is confirmed by zircons from a granitic orthogneiss (W20401), which show consistent radiogenic Pb ratios released during the final evaporation steps. The 207Pb/206Pb age of this orthogneiss is 940 ± 6(σ) Ma, similar to its whole-rock RbSr isochron age of 1024 ± 45(σ) Ma (IR = 0.7241 ± 40). Its SmNd model age (TDM) is about 2.2 Ga. Further Sm Nd model ages (TDM) include 2.1 Ga for the paragneiss, and 1.6–1.7 Ga for a 0.55 Ga granite. The presented field and isotopic chronological data suggest that: (1) the 1-Ga event represents emplacement of the orthogneiss protolith, but does not necessarily involve high-grade metamorphism; (2) deposition of the paragneiss protoliths occurred during the Neoproterozoic, which suggests that accretion of the East Antarctic craton had not been completed until the early Palaeozoic; (3) deformation and granulite facies metamorphism in the region can be attributed to the ∼ 0.5-Ga (‘Pan-African’) event.

Tertiary deformation and metamorphism SE of Tibet: The folded Tiger-leap décollement of NW Yunnan, China

The Yulong-Haba Xue Shan range, in the northwestern part of Yunnan (China), is a large N-S antiform that folds the Paleozoic series of the Yangzi platform. The upper Yangzi River (Jinsha Jiang) has cut a 3500 m-deep valley (Hu Tiao gorge) across this antiform, thus exposing folded, bedding-parallel, ductile shear zones (decollements), with transport toward the SSW (in the present geographical coordinates). The large finite shear strain implies tens of kilometers of transport, pointing to the regional significance of these decollements. Rb/Sr radiometric dating of phlogopites that crystallized in marbles within the foliation planes yields the age of the metamorphic and deformation event (35.9 ± 0.3 (2σ) Ma). The age derives from an internal Rb-Sr isochron, made on different size fractions of the same mineral, which provides a novel demonstration of the feasibility of such plots. Transport on the decollement and related shortening occurred prior to, or at the onset of, extrusion of Indochina along the Ailao Shan-Red River shear zone, ≈80 km west of the Yulong Shan. The 39Ar/40Ar age spectra of K-feldspar from the core of the Yulong Shan suggest uplift by antiformal folding around 17 Ma, as Indochinas extrusion came to an end. We infer that other large-scale Cenozoic decollements such as that exhumed in the Yulong Shan underlie some of the vast, folded areas that surround the eastern Himalayan syntaxis. Transport on such decollements, first toward the south and then toward the east, and folding above them, might have occurred during two principal shortening phases, whose ages bracket Indochinas escape toward the SE.

Pan-African events in Prydz Bay, East Antarctica, and their implications for East Gondwana tectonics

Abstract Decompression, anatexis and the clockwise granulite-grade P-T evolution of high-grade rocks of Prydz Bay reflect late collisional extension that occurred c. 530 Ma in the Prydz Belt. Rapid cooling of the mid-crust high-grade terranes of the Prydz Belt was achieved during c. 517-486 Ma by tectonically driven exhumation along dextral ductile shear zones in which late tectonic partialmelt bodies were emplaced. Instead of a model of polyphase metamorphism and deformation for the basement-and-cover sequences in Prydz Bay, we apply an accretionary one, i.e. an accretionary wedge with allochthonous blocks, to interpret the tectonic history in the Late Neoproterozoic of the Prydz Belt. SHRIMP U-Pb dates and Nd isotopic data available for both Prydz Bay and the Grove Mountains are used to explain amalgamation of the high-grade terranes in the Prydz Belt. This demonstrates that the assembly of the East Antarctic Craton was completed in the Pan-African event, and the East Antarctic Craton is a Pan-African-age collage rather than a keystone of East Gondwana during the Neoproterozoic.

Pyroxene exsolution in mafic granulites from the Grove Mountains, East Antarctica constraints on Pan-African metamorphic conditions

Exsolution textures were observed in clinopyroxenes in mafic granulites from the Grove Mountains, East Antarctica. The first generation of exsolution occurs only in subcalcic augite megacrysts, in which coarse orthopyroxene lamellae exsolved parallel to the (100) plane. Electron microprobe reintegration of such augites yields a pre-metamorphic igneous temperature of ca. 970°C. The second generation of exsolution develops in both igneous and metamorphic clinopyroxenes. In metamorphic clinopyroxenes, two sets of very narrow pigeonite lamellae, subsequently inverted to orthopyroxene, exsolved parallel to (100) and (001) planes, respectively with 0° and 113° deviation to c -axis of the host. A peak metamorphic temperature of ca. 850°C was obtained from the reintegrated compositions of exsolved clinopyroxene, and an exsolution temperature of 740–770°C from the orientations of exsolved lamellae. The preservation of augite megacrysts and appearance of equilibrium assemblages suggests that mafic granulites from the Grove Mountains were formed during a single episode of high-grade metamorphism. Temperature estimates for original pyroxene crystallization and pigeonite exsolution from clinopyroxene host provide a good constraint on the peak conditions for this metamorphic event. In contrast with the Prydz Bay area, it is proposed that the Pan-African metamorphism in East Antarctica had reached a higher temperature of ca. 850°C, and later underwent a slow cooling process, which probably resulted from the magmatic underplating of the lower crust.

Grove Mountains (GRV) 99027: A new Martian meteorite

We report the petrography, mineral chemistry and oxygen isotopic composition of GRV 99027, a new Martian meteorite recently collected during the 16th Chinese Antarctic Research Expedition. This meteorite consists of two textural regions. The interstitial region is characterized by the presence of plagioclase and phosphate, and higher FeO contents of olivine and orthopyroxene, in comparison with the poikilitic region. All of the observations are similar to the three known Martian lherzolites. We classify GRV 99027 as the fourth sample of Martian lherzolite.

Fluctuation history of the interior East Antarctic Ice Sheet since mid-Pliocene

Abstract Cosmogenic 10Be and 26Al measurements from bedrock exposures in East Antarctica provide indications of how long the rock surface has been free from glacial cover. Samples from the crests of Zakharoff Ridge and Mount Harding, two typical nunataks in the Grove Mountains, show minimum 10Be ages of 2.00 ± 0.22 and 2.30 ± 0.26 Ma, respectively. These ages suggest that the crests were above the ice sheet at least since the Plio–Pleistocene boundary. Adopting a ‘reasonable’ erosion rate of 5–10 cm Ma-1 increases the exposure ages of these two samples to extend into the mid-Pliocene. The bedrock exposure ages steadily decrease with decreasing elevation on the two nunataks, which indicates ~200 m decrease of the ice sheet in the Grove Mountains since mid-Pliocene time. Seven higher elevation samples exhibit a simple exposure history, which indicates that the ice sheet in the Grove Mountains decreased only ~100 m over a period as long as 1–2 Ma. This suggests that the East Antarctic Ice Sheet (EAIS) was relatively stable during the Pliocene warm interval. Five lower elevation samples suggest a complex exposure history, and indicate that the maximum subsequent increase of the EAIS was only 100 m higher than the present ice surface. Considering the uncertainties, their total initial exposure and subsequent burial time could be later than mid-Pliocene, which may not conflict with the stable mid-Pliocene scenario.

New constraints on the timing of partial melting and deformation along the Nyalam section (central Himalaya): implications for extrusion models

Abstract New structural, U–Th/Pb and Ar/Ar data along the Nyalam section constrain the timing of partial melting, crystallization and deformation in the Greater Himalayan Sequence. Prograde metamorphism was followed by the onset of partial melting at c. 30 Ma. In the central Greater Himalayan Sequence, in situ melts crystallized between 24 and 18 Ma. Subsequent cooling was very fast (c. 200 °C Ma−1) and coeval with the emplacement of undeformed dykes that lasted until c. 15 Ma. In the upper Greater Himalayan Sequence, fast cooling continued until c. 13 Ma. Combined with published P–T and thermochronological data from the Langtang and Dudh Kosi valleys, these data imply that: (a) the partial melt zone thinned over time; (b) the end of melting preceded the end of motion on the Main Central Thrust and the South Tibetan Detachment by 6 and 2 Ma, respectively; (c) the South Tibetan Detachment possibly initiated at c. 25 Ma, probably reactivating a pre-existing thrust; and (d) the present-day topography has been established for <6 Ma and focused erosion on the present-day southern slopes of the Himalaya was not active at the time of the exhumation of the Greater Himalayan Sequence. These observations suggest that the Main Central Thrust/South Tibetan Detachment systems are not passive structures induced by focused erosion, as has been suggested previously by some lower crustal channel flow models. Supplementary material: U/Pb, geochemistry and Ar/Ar data, the Ar/Ar analytical procedure, field pictures, T11N26, T11N27, T11N31, T11N53A and T11N40 quartz ⟨C⟩ axis CPOs are available at http://www.geolsoc.org.uk/SUP18835

Fractal analysis applied to cataclastic rocks

Abstract Cataclastic rocks can be characterized by their fractal dimension, which is dependent on the mode of fragmentation. A mode of fragmentation may be defined in terms of base “ B ” and the number of fragile elements “ N f ”. Fractal dimension of a cataclastic rock can be obtained by finding the slope of the least square fit straight line of particle size-frequency, area-frequency and volume-frequency distribution in a log-log coordinate system. The relationships are D = −slope (size frequency), D = −2slope (area frequency) and D = −3slope (volume frequency) respectively. In the analyses particle shape does not affect fractal dimension, and neither does the diameter measured for a cataclastic rock.

Provenance of the Upper Cretaceous to Lower Tertiary Sedimentary Relicts in the Renbu Mélange Zone, within the Indus-Yarlung Suture Zone

The Upper Cretaceous to Lower Tertiary sediments in the Indus-Yarlung suture zone provide critical records on the history of accretion along the southern margin of Asia prior to, and during, the India-Asia collision. In this article, we report field and petrographic observations, in situ detrital zircon U-Pb ages, Lu-Hf isotopic analyses, and Cr-spinel electron microprobe data from the Upper Cretaceous to Lower Tertiary sedimentary rocks of the Renbu mélange zone in east Xigaze, southern Tibet. The Renbu mélange zone consists of two serpentinite mélange subzones separated by a mud-matrix mélange. Using similarities found in the compositions of detrital Cr-spinels and detrital zircon U-Pb ages, we propose to correlate the northern Renbu mélange subzone with the upper part of the Xigaze forearc basin. Detrital zircons from sandstones in the southern Renbu mélange subzone indicate an influx of Cretaceous–early Cenozoic zircon grains with juvenile Hf isotopic compositions, suggesting a provenance from the Lhasa terrane, especially the Gangdese arc. Compared to the mélange in the western Xigaze, our new results show that much younger sediments were deposited on the top of the accretionary wedge, with the youngest U-Pb age cluster including six single grains in the range of 64–53 Ma and a large peak at 67 Ma. Our results support the idea that the foreland basin developed along the Indus-Yarlung suture during the India-Asia collision.