Françoise Roger

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.

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.

Miocene emplacement and deformation of the Konga Shan granite (Xianshui He fault zone, west Sichuan, China) : Geodynamic implications

The presently active sinistral Xianshui He strike-slip fault (XSH) is a lithospheric scale strike-slip fault in the eastern Himalaya. In the study area this fault affects the eastern edge of the Konga Shan granitic massif, where it has caused both brittle and ductile deformation. A RbSr isochron and Nd and Pb isotope study of three samples, and a UPb zircon study of a single sample, were completed on the granite. UPb data indicate a granite emplacement age of 12.8 ± 1.4 Ma. The RbSr isochrons show that the granite emplacement and the deformational event were synchronous, at around 12-10 Ma (minimum age for deformation). The Nd and Pb isotope compositions of whole rocks and K-feldspars indicate the involvement of Proterozoic continental crust, which is confirmed by UPb systematics indicating inherited zircons. Sinistral faulting along the XSH began at the latest at 12 Ma and marks the extrusion toward the east of the West Sichuan and South China blocks, following their extrusion along the Red River fault zone between 50 and 21 Ma.

Evidence for Mesozoic shear along the western Kunlun and Altyn-Tagh fault, northern Tibet (China)

The strike-slip faults of north Tibet accommodate part of the Cenozoic convergence between India and Asia. Along the Karakax valley south of Yecheng and near the Xidatan trough south of Golmud, the active traces of the Altyn-Tagh and Kunlun faults follow narrow belts of metamorphic rocks. The deformation recorded in those mylonites is sinistral strike-slip. Rb/Sr and 40Ar/39Ar ages of deformation from syntectonic fabrics formed at 350–400°C 120 Ma. Argon loss suggests that deformation was associated to a 250–300°C thermal pulse that lasted 5–20 Ma after the onset of movement. Unroofing occurred much later, around 25 Ma when sudden cooling suggests a component of thrusting or more likely normal faulting. The Cretaceous shear may be related to collision between the Qiantang and the Lhasa blocks. The Karakax and Xidatan shear zones may have formed a unique, continuous boundary in the Cretaceous, which was later reused by the Tertiary strike-slip faults, leading to potentially calculable offsets along the Altyn-Tagh fault.

Mesozoic-Cenozoic tectonothermal evolution of the eastern part of the Tibetan Plateau (Songpan-Garzê, Longmen Shan area): insights from thermochronological data and simple thermal modelling

Abstract We present a synthesis of the tectonic and thermochronological evolution of the Eastern Tibet since the Triassic. The long-term cooling histories obtained on magmatic and metamorphic rocks of the South Songpan-Garzê, Kunlun and Yidun blocks are similar showing a very slow and regular cooling during Late Jurassic and Cretaceous, confirming the suspected lack of major tectonic events between c. 150 and 30 Ma. The exhumation linked to the Tertiary growth of the Tibetan Plateau initiated around 30 Ma and concentrates at the vicinity of the major tectonic structures. Exhumation rates increased again from about 7 Ma in the Longmen Shan. To interpret this very slow cooling rate between Late Jurassic and Early Cenozoic from granites of this area, we use a simple 1D thermal model that takes into account the thermal properties of both sediments and crust. The results suggest that: (1) high temperature (500 °C) can be kept over a long period of time; (2) during Cretaceous, cooling is mostly controlled by the thermal properties of sediments of continental origin; and (3) the initial Late Triassic rapid cooling rate was caused by the large thermal contrast between the granite body and the sedimentary rocks rather than by a high exhumation rate.

Géochronologie U-Pb sur zircons et géochimie (Pb, Sr et Nd) du socle de la chaîne de Songpan-Garze (Chine)

The basement of the Songpan-Garze fold belt has been dated at 829 ± 9 Ma (2σ) using the U-Pb method on zircon fractions. This age is interpreted as the emplacement age, and together with Pb, Sr and Nd isotope data, suggests that this belt was part of the Yangtse craton (South China block). The emplacement of these calc-alkaline Upper Proterozoic granitoids may be linked to a south-dipping subduction under the South China block during the Jinnigian orogeny (1000-850 Ma).

Histoire de l'exhumation de l'Altun Shan: indications sur l'âge de la subduction du bloc du Tarim sous le système de l'Altyn Tagh (Nord Tibet)

Abstract The Altun Shan is a tectonic block to the south of the Tarim basin, North Tibet, bounded to the south by the Altyn Tagh fault. U/Pb crystallisation ages on zircons are 441 ± 9 Ma. 40 Ar/ 39 Ar analysis on muscovite yields an age of 383 ± 7 Ma. Both indicate a cooling rate of 7.3 ± 0.6 °C·Ma −1 from 441 to 383 Ma. Further cooling, probably due to exhumation, began at this time and continued at a rate of 0.8 ± 0.3 °C·Ma −1 From three apatite fission-track analyses, cooling rate increased to 8 ± 2 °C·Ma −1 at 10 Ma.