Projet LONGRIBA : Comprendre la tectonique de la marge Est du plateau Tibétain

***Faille de Longriba : la clef pour comprendre la tectonique de la marge Est du plateau Tibétain ?

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PI : De Sigoyer, Julia, Laboratoire de Géologie, Ecole Normale Supérieure, Paris

Equipes ISTerre impliquées :
– Cycle sismique et déformations transitoires
)]

ABSTRACT

The paradox of a high eastern Tibetan plateau associated with very low convergence rate (Shen et al., 2005 ; Gan et al., 2007) has led to an underestimation of the seismic hazard in the Longmen Shan area (China) prior to the May 12th 2008, Mw 7.9 earthquake. A crucial problem in our understanding of the seismic hazard in Eastern Tibet is the difficulty of reconciling long-term geological studies (several millions years), which suggest strong thickening of the Tibetan margin and the Longmen Shan belt, and short-term geophysical studies (seconds to decades) that show low convergence rate across this area. Compounding this difficulty is the complex structural geology of the margin, which comprises a large number of poorly-understood faults that interact in as-yet-unknown ways. In this project we will investigate this paradox directly, focusing our study on the Longriba fault system located 150 km west of the Longmen Shan front. This fault is a key structure that is rarely taken into account in conceptual frameworks for the geodynamics and seismo-tectonics of this area, although it seams to accommodate a large part of the present-day relative movement (6-8 mm/yr) between the Songpan block (Bayan Har block) and the south China block (Thatcher, 2007, Shen et al 2005). It seems also to partition the movement between dextral strike-slip movement along the fault itself and residual E-W convergence that is taken on the Longmen Shan. The interaction between the Longriba fault and the faults in the Longmen Shan is critical, because its estimated Quaternary slip rate is an order of magnitude higher than that of the Longmen Shan faults ; it may thus account for a large part of the present-day deformation, and must be accounted for in any regional assessment of seismic hazard.
The first aim of this proposal (Task 2) is to characterize and quantify the recent activity of the Longriba fault system by geodetic, neotectonic, and strain rate modelling analyses. The potential interactions between the Longriba system and the southern Xianshui He and East Kunlun faults will be also studied, to understand its role in a regional context.

The second aim of this proposal (Task 3) is to estimate the activation age of the Longriba fault with geomorphic analyses and dating (cosmogenic, OSL), and to use low-temperature thermochronology to understand the potential role of Longriba fault in accommodating uplift of the eastern Tibetan plateau in this area,. We expect that the uplift of the Tibetan plateau occurred before the (very recent) activation of the Longriba fault, implying that the pre-Longriba tectonic framework could was very different from that at present, allowing more convergence across the eastern Tibetan margin and Longmen Shan belt than today.
Recent geochemical studies on the eastern Songpan Garze Triassic granitoids suggest that the Longriba fault could be located in a place of slab retreat and slab break-off at the boundary between the Yangtze craton margin and a Paleozoic ocean.

The third aim of this proposal (Task 4) is to test this hypothesis, with geochemical tracer of the origin of these granites (major, trace and isotopic elements) and geochronological (U-Th-Pb system) data to understand the importance of geological inheritance to the present day tectonic setting. Via high-precision petrostructural study and dating (U-Th-Pb La-ICPMS and 39Ar-40Ar) we will produce scenarios for the deformation history of the Songpan Garze area and for the thickening of the eastern plateau margin since Triassic time.

Longriba Fault :

Regional GPS data of Shen et al. (2005, 2009) show displacement of eastern Tibet toward the NE at 6 to 8 mm/yr, relative to a fixed Sichuan Basin. To examine how strain is partitioned, Shen et al. 2009 devised a block-motion model constrained using regional pre-earthquake GPS velocities (Fig. 3). For the region between the East Kunlun fault to the north and the Xianshuihe fault to the south, most of the relative motion between the eastern Tibetan plateau and Sichuan basin (4-5 mm/yr) appears to be accommodated along the dextral Longriba fault system, located 150 km northwest of and parallel to the Longmen Shan faults. The remaining part (<2 mm/yr) has to be absorbed by tectonic structures located between the two domains, in the SW Longmen Shan block and at the Longmen Shan front. We stress that these inferred slip rates are 5-10 times greater than those of the faults in the Longmen Shan described above.

The Longriba fault is a system of NE-trending parallel dextral and thrust faults (Xu et al., 2008) in the eastern Bayan Har block (that represents most of the Songpan Garze unit see the insert map of the Tibet). The northeastern segment of the Longriba fault consists of two subparallel N54±5°E trending branch faults about 30 km apart, with Quaternary offset landforms related to the fault segments. The northern branch fault, also termed the Longriba fault, has a relatively large reverse component, while the southern branch fault, the Maoergai fault, is inferred to be a pure right-lateral strike slip fault (Fig.1, Fig. 4). A swarm of seismicity has been observed along the southern part of the Longriba faults but no other studies were published on active deformation of Longriba fault in this area.

The previous regional geodetic studies have delivered a better understanding for the active tectonic framework of the area, showing that most of the deformation is taken up on quasi-unknown Longriba fault system. This partitioned system may explain why the present-day convergent rates are so low across the Longmen Shan belt. However, the lack of information on the structure and history of the Longriba fault, coupled with the recent demonstration that the greater Longmen Shan region continues to be tectonically active, leads to the conclusion that there is an urgent need for a better documentation of this fault system.