Zhiqin Xu

Mesozoic and Cenozoic tectonics of the northern edge of the Tibetan plateau: fission-track constraints

Abstract Fission-track analysis on zircons and apatites yields new information about the timing of deformation of the northern Tibetan plateau. Ages on zircons, ranging from 221±22 to 96±4 Ma are indicative of a general late Triassic–early Jurassic cooling probably driven by the collision between the Qiantang and Kunlun blocks. Mid-Jurassic slow cooling is recorded also in the apatites in regions not affected by later Cenozoic deformation. This Jurassic denudation was followed by a period of sedimentation during the Cretaceous, except along the Altyn Tagh fault (ATF) zone, and in some restricted areas of the western and eastern Qilian Shan. This long and relatively quiet period ended at about 40±10 Ma along the major Altyn Tagh and Kunlun strike-slip fault zones, which were activated by the India–Asia collision. This first movement along lithospheric faults resulted in the eastward extrusion of the Tibet plateau, which was followed, in late Oligocene–Miocene times, by a major compression event, initiating the formation of the high relief of north Tibet. A final compressional event took place at 9–5 Ma and is well correlated with high sedimentation rates in the basins of this region. This compression induced continental subduction in the Kunlun ranges, the Altun Shan belt, and possibly the Qilian Shan belt.

Rapid slip along the central Altyn Tagh Fault: Morphochronologic evidence from Cherchen He and Sulamu Tagh

consistent with the d 18 O record from the Guliya ice cap in the West Kunlun; the features of interest were all formed by glacial and fluvial processes subsequent to marine isotope stage 5e, with the youngest features having formed during the early Holocene Optimum. This ‘‘near-field,’’ morphochronological slip rate is averaged over many earthquake cycles and is hence little affected by interseismic strain. It is kinematically consistent with other, somewhat lower, geomorphic slip rate measurements to the east. The average rate, and lower bounds obtained from alternate interpretational models, 18.4 mm/yr, cannot be reconciled with the most rece geodetic measurements (� 7 mm/yr), suggesting that interseismic strain and interactions with adjacent faults may lead to disparate geologic and geodetic rate estimates. This late Pleistocene-Holocene, morphochronologic rate would imply that, at this longitude, the Altyn Tagh Fault, on the north edge of Tibet, might absorb almost as much of India’s convergence relative to Siberia as the Himalayan Main Frontal Thrust does on the southern edge of the plateau. INDEX TERMS: 1035 Geochemistry: Geochronology; 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 8102 Tectonophysics: Continental contractional orogenic belts; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: slip rates, cosmogenic dating, Indo-Asian collision

Ophiolites of the Kunlun Mountains, China and their tectonic implications

Abstract Three ophiolite belts, ranging in age from Cambrian to Triassic, provide valuable data on the tectonic evolution of the Kunlun Mountains which lie along the northern margin of the Tibetan Plateau. All of the ophiolites contain dismembered but nearly complete assemblages of peridotite, mafic and ultramafic cumulates, pillow and massive lavas and radiolarian cherts, (1) The oldest belt contains ophiolites of Cambrian age, but the initial spreading of the ocean basin most likely began in the Late Proterozoic. These ophiolites outcrop in the central part of the eastern Kunlun, extend for at least a few hundred kilometers, and are believed to reflect a small ocean basin. Closure of this ocean led the accretion of south Kunlun block to the north Kunlun block and Tarim craton. Ophiolitic lavas in this belt have island are affinities. (2) A second belt of early Carboniferous ophiolites extends nearly 600 km along the northern margin of the western Kunlun. Lavas in this belt are basalt, basaltic andesite and andesite, all of which have compositions characteristic of volcanic arcs. This ocean basin possibly developed on the basis of the early Paleozoic oceanic basin. Closure of this ocean basin by southward subduction in the early Permian produced a suite of calc-alkaline volcanic rocks in what is now the central part of the western Kunlun. (3) The third belt extends nearly 1200 km along the southern margin of the Eastern Kunlun and contains numerous ophiolites of Early Permian to Middle Triassic age. These ophiolites are highly tectonized, containing volcanic rocks with the geochemical characteristics of mid-ocean ridges, oceanic islands and volcanic arcs. This belt is tentatively interpreted as the suture zone between Gondwana and Eurasia.

Timing of granite emplacement and cooling in the Songpan–Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications

Abstract New U–Pb and Rb–Sr geochronology on syn- and post-orogenic granites provide constraints on the timing of major tectonic events in the Songpan–Garze fold belt, west Sichuan, China. The Ma Nai granite was probably syn-kinematic with the main deformation and yields an age of 197±6 Ma that is interpreted as an upper age limit of the Indosinian event. Zircons and apatites from the post-kinematic Rilonguan granite also yield Jurassic ages (195±6 and 181±4 Ma). The post-orogenic Markam massif gives two ages of 188±1 and 153±3 Ma. Both granites are undeformed and cut structures in the Triassic sedimentary rocks. These results demonstrate that the major deformation and decollement tectonics in the Songpan–Garze fold belt occurred prior to the Early Jurassic. The wide range of ages obtained for post-kinematic granites (from Early Jurassic to Late Jurassic) suggests that, locally, magmatic activity persisted for a long time (at least 50 Ma) after the Indosinian compressional tectonism. No Tertiary ages have been obtained, suggesting that these granites were not affected strongly by the India–Asia collision.

Low δ18O zircons, U-Pb dating, and the age of the Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China

Zircons from metamorphosed granites exposed near Qinglongshan have δ18OVSMOW values of −7 to 0‰ in both grain rims and cores. The concordant 238U/206Pb ages of zircon cores are 684 to 754 Ma with rims at 221 Ma. Discordant 238U/206Pb ages range from 242 to 632 Ma. Results demonstrate a Neoproterozoic age for the origin of the Qinglongshan oxygen and hydrogen isotope anomaly. The low δ18O values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate. Groundwater circulation was driven by heat from cooling granitic magma. The geologic age of the hydrothermal system correlates with that of the Nantuo tillite in the Sinian strata of the South China block, suggesting that Qinglongshan’s cold climate may be a manifestation of Neoproterozoic “snowball Earth.”

Petrology and geochronology of eclogites from the western segment of the Altyn Tagh, northwestern China

Abstract The eclogites from the western segment of the Altyn Tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses. The petrographic features and reaction textures testify to three main metamorphic stages: (1) the peak eclogite facies stage (P>15 kbar, T=665–880°C), (2) the granulitic facies stage (P=11–14 kbar, T=670–800°C) and (3) the amphibolite facies stage (P=6.3–9.5 kbar, T=619–738°C) during decompression. The trace element, rare earth element and Sm–Nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type (T-type) mid-ocean ridge basalt (MORB). The well preserved eclogite was selected for Sm–Nd and U–Pb isotopic dating. The Sm–Nd isotopic data yield a whole rock–garnet–omphacite isochron of 500±10 Ma age. The U–Pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia, giving a weighted mean age of 503.9±5.3 Ma. The two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites, and reflects the existence of a mountain root related to continental subduction during Early Paleozoic. Moreover, based on the comparison with similar rocks in the northern margin of the Qaidam basin, a displacement of 350–400 km for the Altyn Tagh fault have been estimated.

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P–T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.

Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China

The Dulan eclogite–gneiss region is located in the eastern part of the North Qaidam eclogite belt, NW China. Widespread evidence demonstrates that this region is a typical ultrahigh-pressure (UHP) metamorphic terrane. Eclogites occur as lenses or layers in both granitic and pelitic gneisses. Two distinguished sub-belts can be recognized and differ in mineralogy, petrology and geochemistry. The North Dulan Belt (NDB) has tholeiitic protoliths with high TiO2 and lower Al2O3 and MgO contents. REE patterns and trace element contents resemble those of N-type and E-type MORB. In contrast, eclogites in the South Dulan Belt (SDB) are of island arc protoliths with low TiO2, high Al2O3 and show LREE-enriched and HFSE-depleted patterns. Sm–Nd isotope analyses give isochron ages of 458–497 Ma for eclogite-facies metamorphism for the two sub-belts. The ages are similar to those of Yuka and Altun eclogites in the western extension of the North Qaidam-Altun eclogite belt. The Dulan UHP metamorphic terrane, together with several other recently recognized eclogite-bearing terrenes within the North Qaidam-Altun HP-UHP belt, constitute the key to the understanding of the tectonic evolution of the northern Tibetan Plateau. The entire UHP belt extends for more than 1000 km from the Dulan UHP terrane in the southeast to the Altun eclogite–gneiss terrane in the west. This super-belt marks an early Paleozoic continental collision zone between the Qaidam Massif and the Qilian Massif.

New U-Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault

[1] 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 (similar to 35-32 Ma); (3) an Oligo-Miocene magmatic event (similar to 25-22 Ma), at least partly synkinematic to the right-lateral deformation; and (4) a period of metamorphism metasomatism (similar to 22-14 Ma) interpreted as thermal and fluid advection in the shear zone. The Labhar Kangri granite located similar to 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 similar 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.

Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)

The western part of the Tanghe Nan Shan range southwest of Subei (western Gansu, China) is presently growing on thrust ramps splaying from the left-lateral Altyn Tagh Fault. Late Cenozoic thrusting has folded and sliced Oligocene-Miocence red beds into an imbricate wedge, capped by warped and uplifted Quaternary terraces that form a 2- to 5-km-wide ledge, north of the steeply faceted range front. Seismic scarps 1.5 to 4.5 m high cut young fans along the outer thrusts. Carbon 14 dating of organic remains collected on strath terraces constrains the chronology of deposition and incision by the streams. Most of the fans and terraces in the southern part of the Subei basin appear to have been emplaced after the last glacial maximum, particularly during the early Holocene optimum (9–5 ka). Measurements of the shapes of the warped terraces constrain minimum and maximum throws on the outer thrusts. The minimum vertical throw of 34 ± 2 m of a surface dated at 8411 ± 530 years B.P. at one site provides a minimum vertical uplift rate of 4.1±0.5 mm/yr. The maximum possible uplift (115±15 m) of the oldest terrace surface, whose probable age is 15 to 18 ka, places an upper bound on the uplift rate of 7±2 mm/yr. The thrust geometry at depth and the cumulative shortening (10–20 km) deduced from balancing sections logged across the imbricate thrust wedge are consistent with a shortening rate of about 5 mm/yr and an onset of thrusting at about 4±2 Ma. Such a shortening rate implies a significant northward decrease in slip rate along the Altyn Tagh Fault. The recent growth of the western Qilian mountain ranges thus appears to be intimately coupled with sinistral motion on the Altyn Tagh Fault and the extrusion of Tibet.