Philippe Hervé LELOUP is a structural geologist working on continental deformation. After a PhD supervised by P. Tapponnier at the IPG Paris and a post-doc in UCLA with T.M. Harrison he is now the head of the surface and lithosphere group at the Laboratoire de Géologie de Lyon (France). He studies the large shear zones and fault zones of Asia (Ailao Shan - Red River, Karakorum, MCT, STD, Xianshuihe ...) since nearly 30 years in order to discuss the continental crust rheology. He is especially interested in constraining the ductile shear zone kinematics and strain rates by combining structural geology, geochronology, and petrology. He also works on how to constrain fault rates at geological time scales (several Ma) in order to compare them with those measured at shorter time-scale.
In eastern Tibet, the left-lateral strike-slip Yushu-Xianshuihe-Xiaojiang fault system (YXX-FS) is ~1200 km long, veering from N100° to N175° broadly following a small circle whose pole is located in the eastern Himalayan syntaxis. Several competing models have been proposed to explain the geological evolution of eastern Tibet in general, and of the YXX-FS in particular: fault following slip-lines in a plastic media, book-shelf fault in a large right-lateral shear zone, or fault bounding a lower channel flow veering around the syntaxis. In this contribution we document the timing of onset of the YXX-FS, its propagation through time, its rate at various time-scales; and discuss how these relate to the deformation models.
The YXX-FS comprises four fault segments from east (Tibet) to west (Yunnan): Yushu-Ganzi, Xianshuihe, Anninghe, and Zemuhe–Xiaojiang. It is one of the most tectonically active intra-continental fault system in China, if not in the world, along which more than 20 M>6.5 earthquakes occurred since 1700. Slip-rates of 3.5 to 30 mm/yr along the YXX-FS have been suggested by matching geological offsets of ~60-100 km with initiation ages of ~2 to 17 Ma. Late Quaternary rates deduced from morphological offsets, Insar, paleoseismology and GPS also show large uncertainties, between 3 and 15 mm/yr.
The timing of initiation of the Yushu-Ganzi segment has been constrained at ~12.6±1 Ma and its total offset to 76 - 90 km (Wang et al., 2009) yielding a rate of 6.6+0.8-0.7 mm/yr. By measuring the offsets of moraine crests and fan edges across the fault using LiDAR and kinematic GPS, and dating their surfaces using 10Be, we determined slip-rates of 7+1.1-1.0 mm/yr, 3 – 11.2 mm/yr and 8.5+0.8-0.7 mm/yr at three different sites. This suggests a constant rate of 6-8 mm/yr along the fault segment since ~13Ma.
The timing of initiation of the Xianshuihe segment was thought to be prior to 12.8±1.4 Ma (Roger et al., 1995), but new field studies and geochronological ages suggest that the fault initiated later. Using thermochronological data and thermokinematic modeling, we show that rapid exhumation started along the fault in the north of the Gongga batholith at ~9 Ma and slowed down at ~4 Ma. Since then, very fast exhumation takes place in the South Gongga batholith where the Gongga Shan (7556 m) stands. This exhumation pattern is explained by the switch of motion between different fault strands within a restraining bend. This constrains the fault onset at ~9Ma, and together with an offset of ~60 km, yields a slip rate of ~7 mm/yr similar to that along the Yushu-Ganzi segment.
These data suggest that the YXX-FS (a) initiated at ~13 Ma, after a major faults reorganization following the end of Indochina extrusion at ~17 Ma, (b) propagated from west to east and (c) has a constant slip rate of 6-8 mm/yr. This allows reconstructing SE Tibet structural evolution and to discuss the various model proposed.