Robin Lacassin

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.

Intraplate tectonics in Asia: A precise age for large-scale Miocene movement along the Ailao Shan-Red River shear zone, China

Tertiary left-lateral movement along the 1000 km long Ailao Shan-Red River shear zone appears to have played an important role in absorbing post-collisional northward penetration of India into Asia. Crustal strike-slip shear along this zone caused the formation of a gneiss belt, metamorphosed to amphibolite grade, including anatectic melting. Both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the Ailao Shan-Red River belt. 17 U-Pb isotope analyses were performed on small size-fractions of zircon, monazite and xenotime, extracted from two different leucogranitic layers. The two samples are located about 50 km apart in the central segment of Ailao Shan, in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation. All mineral U-Pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 Pb, originating from initial 230 Th disequilibrium in the 238 U decay series. In monazite, such disequilibrium 206 Pb reaches 20% of total radiogenic 206 Pb. The corrected U-Pb ages of monazite and xenotime lie between 22.1 and 23.9 Ma, whereas zircon yields significantly discordant U-Pb ages between 30.5 and 33.9 Ma pointing to Precambrian material in the magma source region. Inherited components could also be detected in monazite. The set of U-Pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early Miocene age of 23.0 ± 0.2Ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the Ailao Shan-Red River shear zone.

Large-scale geometry, offset and kinematic evolution of the Karakorum fault, Tibet

The total offset, lifespan and slip rate of the Karakorum fault zone (KFZ) (western Tibet) are debated. Along the southern fault half, ongoing oblique slip has exhumed dextrally sheared gneisses intruded by synkinematic leucogranites, whose age (V23 Ma, U/Pb on zircon) indicates that right-lateral motion was already in progress in the late Oligocene. Ar/Ar K-feldspar thermochronology confirms that rapid cooling started around 12 Ma, likely at the onset of the present dextral normal slip regime. Correlation of suture zones across the fault requires a total offset greater than 250 km along the active ^ northern ^ fault branch. An average long-term slip rate of 1 ? 0.3 cm/yr is inferred assuming that this offset accrued in a time span of 23^34 Ma. Southwest of the Ladakh-Karakorum Range, the large-scale boudinage of ophiolitic units suggests that an offset of several hundreds of kilometers exists along another ^ southern ^ branch of the KFZ. Towards the southeast, in the Mount Kailas region, the fault zone does not end at Gurla Mandatha, but continues eastwards, as a transpressive flower structure, along the Indus^Tsangpo suture. Our new data thus suggest that the KFZ contributed to absorb hundreds of kilometers of India^Asia convergence.

Tertiary diachronic extrusion and deformation of western Indochina: Structural and 40Ar/39Ar evidence from NW Thailand

The Wang Chao and Three Pagodas fault zones cut the western part of the Indochina block and run parallel to the Red River Fault. Evidence of intense ductile left-lateral shear is found in the Lansang gneisses, which form a 5 km wide elongated core along the Wang Chao fault zone. Dating by 40 Ar/ 39 Ar shows that such deformation probably terminated around 30.5 Ma. The Wang Chao and Three Pagodas faults offset the north striking lower Mesozoic metamorphic and magmatic belt of northern Thailand. 40 Ar/ 39 Ar results suggest that this belt suffered rapid cooling in the Tertiary, probably around 23 Ma. These results imply that the extrusion of the southwestern part of Indochina occurred in the upper Eocene-lower Oligocene. It probably induced rifting in some basins of the Gulf of Thailand and in the Malay and Mekong basins. In the Oligo-Miocene, the continuing penetration of India into Asia culminated with the extrusion of all of Indochina along the Ailao Shan - Red River fault. This occurred concurrently with the onset of E-W extension more to the south. Plotting in a geographical reference frame the diachronic time spans of movement on left-lateral faults east and southeast of Tibet implies that the northward movement of the Indian indenter successively initiated new strike-slip faults located farther and farther north along its path.

Large river offsets and Plio‐Quaternary dextral slip rate on the Red River fault (Yunnan, China)

Using multispectral SPOT images and 1/100,000 topographic data, we present an improved map of the active Red River fault zone between Midu (Yunnan, China) and Hanoi (Vietnam). The fault zone is composed of parallel strands, one of which, the Yuanjiang fault was previously undetected. There also appears to be a component of extension all along the fault zone. Such extension increases toward the SE, from Yunnan to the south China sea coast, and the vector describing the motion of south China relative to Indochina points within the N45°–135°E quadrant. We attempt to assess the Plio-Quaternary dextral slip rate on the Red River fault (RRF) by restoring large river offsets and searching for the largest, plausible one. Across much of Yunnan, the fault is perpendicular to local catchments that drain into the Red River. From precise mapping of the river courses on SPOT satellite images and on 1/100,000 topographic maps, numerous multiple offsets along the fault can be detected and reconstructed. The lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prior to the onset of dextral slip along the fault or that frequent captures have occurred. We thus try to find the best fit between series of river channels upstream and downstream from the fault by progressively restoring the dextral displacement in increments of 500 m, up to an offset of 50 km. For each increment we measure the misfits (root mean squares, RMS) between the upstream and downstream channels. The best fit and smallest RMS are obtained for an offset of 25±0.5 km that we interpret to represent the clearest, large right-lateral displacement recorded in the geomorphology along the active Red River fault. Since dextral motion is likely to have started around 5 Myr, the most probable average Plio-Quaternary slip rate on the fault is of order of 5 mm/yr. We attribute the apparent lack of seismic activity on a large stretch of the fault to millennial recurrence times between great earthquakes. Our study shows that relatively small drainage systems can keep a good record of fairly large cumulative fault offsets.

Hairpin river loops and slip-sense inversion on southeast Asian strike-slip faults

In the Golden Triangle region of southeast Asia (northern Thailand, Laos and Burma, southern Yunnan), the Mekong, Salween, and neighboring rivers show hairpin geometries where they cross active strike-slip faults. Restoration of young, left-lateral offsets of these rivers leaves residual right-lateral bends of many kilometers. We interpret these hairpins as evidence of late Cenozoic slip-sense inversion on these faults, about 5 to 20 Ma. Near the Red River fault, stress field and slip-sense inversion occurred ca. 5 Ma. This implies that the present course of these large rivers has existed for at least several million years. Pliocene–Quaternary slip rates, possibly on the order of 1 mm/yr, are inferred on each of the strike-slip faults of the Golden Triangle.

Rupture process of the Mw = 7.9 2015 Gorkha earthquake (Nepal): Insights into Himalayan megathrust segmentation

We investigate the rupture process of the 25 April 2015 Gorkha earthquake (Mw = 7.9) using a kinematic joint inversion of teleseismic waves, strong motion data, high-rate GPS, static GPS, and synthetic aperture radar (SAR) data. The rupture is found to be simple in terms of coseismic slip and even more in terms of rupture velocity, as both inversion results and a complementing back projection analysis show that the main slip patch broke unilaterally at a steady velocity of 3.1–3.3 km/s. This feature likely contributes to the moderate peak ground acceleration (0.2 g) observed in Kathmandu. The ~15 km deep rupture occurs along the base of the coupled portion of the Main Himalayan Thrust and does not break the area ranging from Kathmandu to the front. The limitation in length and width of the rupture cannot be identified in the preearthquake interseismic coupling distribution and is therefore discussed in light of the structural architecture of the megathrust.

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.

Bounds on strain in large Tertiary shear zones of SE Asia from boudinage restoration

We have used surface-balanced restoration of stretched, boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the Oligo-Miocene Red River-Ailao Shan shear zone (Yunnan, China) and of the Wang Chao shear zone (Thailand). The layer-parallel extension values thus obtained range between 250 and 870%. We discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones. Assuming simple shear, these values imply minimum total and late shear strains of, respectively, 33 ± 6 and 7 ± 3 at several sites along the Red River-Ailao Shan shear zone. For the Wang Chao shear zone a minimum shear strain of 7 ± 4 is deduced. Assuming homogeneous shear would imply that minimum strike-slip displacements along these two left-lateral shear zones, which have been interpreted to result from the India-Asia collision, have been of the order of 330 ± 60 km (Red River-Ailao Shan) and 35 ± 20 km (Wang Chao).

Discussion on the role of the Red River shear zone, Yunnan and Vietnam, in the continental extrusion of SE Asia Journal, Vol. 163, 2006, 1025–1036

P. H. Leloup, P. Tapponnier & R. Lacassin write: In his recent paper, Searle (2006) acknowledges that the 1000 km long Ailao Shan–Red River shear zone is a large Miocene left-lateral shear zone, but speculates that left-lateral slip started after 21 Ma and claims that the total finite offset remains unknown. From this he concludes that continental extrusion was only a relatively minor tectonic factor during the India–Asia collision, as long argued by other workers (e.g. England & Houseman 1986; Cobbold & Davy 1988; Dewey et al . 1989; Houseman & England 1993). We summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts. Timing of left-lateral shear along the Ailao Shan–Red River shear zone . The Ailao Shan–Red River shear zone is composed mostly of high-grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left-lateral shear parallel to the belt (e.g. Tapponnier et al . 1986, 1990; Leloup et al . 1993, 1995, 2001). The crystallization of the granitoids has been dated between 22 and 35 Ma (Fig. 1d–f), for example by Scharer et al . (1990, 1994) and Zhang & Scharer (1999), leading those workers to propose that left-lateral shear started at least c . 35 Ma ago. In contrast, Searle (2006) claims that all the deformed granitoids found within the shear zone predate left-lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation, rather than a lower limit, thus suggesting a maximum age of 21 Ma for this deformation. A clear understanding of the P – T history and in situ deformation history of the intrusions, as well as of their relationships with surrounding paragneisses, is fundamental for interpreting correctly the geochronological …