Paul Tapponnier

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.

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.

SEISMIC HAZARD IN THE MARMARA SEA REGION FOLLOWING THE 17 AUGUST 1999 IZMIT EARTHQUAKE

On 17 August 1999, a destructive magnitude 7.4 earthquake occurred 100 km east of Istanbul, near the city of Izmit, on the North Anatolian fault. This 1,600-km-long plate boundary slips at an average rate of 2–3 cm yr-1 (refs 3,4,5), and historically has been the site of many devastating earthquakes. This century alone it has ruptured over 900 km of its length. Models of earthquake-induced stress change combined with active fault maps had been used to forecast that the epicentral area of the 1999 Izmit event was indeed a likely location for the occurrence of a large earthquake. Here we show that the 1999 event itself significantly modifies the stress distribution resulting from previous fault interactions. Our new stress models take into account all events in the region with magnitudes greater than 6 having occurred since 1700 (ref. 7) as well as secular interseismic stress change, constrained by GPS data. These models provide a consistent picture of the long term spatio–temporal behaviour of the North Anatolian fault and indicate that two events of magnitude equal to, or greater than, the Izmit earthquake are likely to occur within the next decades beneath the Marmara Sea, south of Istanbul.

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.

Surface features associated with transform faults: A comparison between observed examples and an experimental model

Abstract In several tectonic provinces where active ridge segments are offset, transform faults are expected but not observed. This paper discusses the evolution of the surface expression of some transform faults with the help of a few geological examples and a simple experimental clay model in which the importance of en-echelon fault systems is assessed. We conclude that the azimuth of observed fault traces may not coincide with the direction of movement, but be oblique to it. Thus we must be cautious when using a fieldobserved fault direction to infer a transform-fault direction for use in plate-tectonics models. This study also suggests the scale at which the assumption of rigid plates fails.

Confrontation of mantle seismic anisotropy with two extreme models of strain, in central Asia

Although most authors agree that convergence is accommodated by a large shortening in Asia, two radically different modes of shortening have been put forward. In one approach, the mechanism of shortening is considered to be homogeneous, producing diffuse deformation in the crust and the mantle. In the second one, motions are localized along faults which are supposed to extend into deep lithospheric shear zones, inducing a heterogeneous deformation pattern. Seismic anisotropy is one observable manifestation of large scale deformation. Large scale seismic anisotropy field are inferred from the two extreme competing models. 3D-mapping of anisotropy, inferred from surface waves, reveals a better coherence on average with filtered heterogeneous strain models between 100 and 200 km depths. It suggests a strong coupling between surface and deep motions down to at least 200 km. At larger depths, the coherence with homogeneous strain models suggests that mantle flow becomes more homogeneous.

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.

Signature morphologique de l'activité de la faille des Cévennes (Languedoc, France)

Abstract The Tertiary Cevennes faul! presents typical geomorphic characters of active faults. Its trace is sharp on SPOT images and air photographs. It is segmented (15–30 km) and has locally multiple strands. Along the main fault trace, abandoned valleys and terrace risers of the Herault and Gardon rivers are offset sinistrally by several hundreds of metres. This suggests that the fault is an active Quaternary fault whose left slip-rate may range between 0.1 and 2 mm·yr1.