JCJC SIMI 5-6 - JCJC - SIMI 5-6 - Environnement, Terre et Espace

The Longriba fault: the key to unravel the tectonics of the Eastern Tibetan Margin? – LONGRIBA

The Longriba fault: the key to unravel the tectonics of the Eastern Tibetan Margin

The paradox of a high Tibetan plateau associated with low convergence rate has led to an underestimation of the seismic hazard in Sichuan. To solve this paradox we need to evaluate the geological heritage that prevail the India-Asia collision. We focus our study on the Longriba fault system located 150 km west of the front, which accommodates a large part of the movement between the Tibetan plateau and the south China craton. This fault system may represent the border of South China block.

Longirba Fault system: Inheritance and Activity

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 is to characterize 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 is to estimate the activation age of the Longriba fault with geomorphic analyses and dating (cosmogenic, OSL), and to comparre it to the age of the uplift of the Tibetan plateau in this area. We expect that the uplift of the Tibetan plateau occurred before the 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 the Paleo-Tethys ocean.
The third aim of the proposal 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.

We have obtain results for the long and short terms studies :
- Long term : The thickening of the Tibetan plateau has been done following four stages of deformation. Three of them are related to the Upper Triassic Songpan Garze wedge formation by thin skinned deformation. The last one is due to the Cenozoic thick skin reactivation.
- The erosional pattern on the eastern Tibetan border, has been precised with isotope cosmogenic data on river sands. These data confirms the proposition of regressive erosion from the front of the plateau toward the center.
- The Be10 analyses of river sands in the Longmen Shan before and after the may 12th 2008 earthquake,also allow to show the impact of the earthquake on the erosion. Just after the earthquake the Be10 value fall down due to huge landslides that have renew the surface of the belt. Three years after the earthquake the Be10 value increase again, suggesting less landslides.

-The next step for the long term study is to date the different stages of thickening in the eastern border of the Tibetan plateau. We plan to do it with 39Ar/40Ar method insitu (with lazer ablation) on metamorphic phengites, and with U/Pb dating of metamorphic monazites and allanites.
-With Spot and pleiade images we plan to do the very precise mapping of the Longriba fault system. The mapping will allow the characterization of the long term displacement along the Maoergai and Longriqu faults.
-Vertical profiles have been sampled in granite outcropping from both part of the Maoergai fault. We will study the thermochronology on apatites and zircons of this sample. This will give us information on the uplift related to the Maoegai fault.
- Many other granites of the eastern Tibetan plateau, have been sample to characterize the pattern of uplift at the scale of this border. This study is associated to the cosmogenic study of river sands, that give the erosional pattern across the same surface.
- The geochemical signature of the granites will also documented the nature of the basement from both part of the Longriba fault system.
- INSAR study has begun on 2 tracks of 800 km long across the Tibetan border, to study the activity across the Longriba fault system and its links with other actives faults. Each track contents 36 ENVISAT images from 2003 to 2010.

1. de Sigoyer et al., Journal of Asian Earth Science accepted
2. de Sigoyer et al., Journal of Metamorphic Geology submitted

Conferences
1. Godard et al. dec 2012 AGU
2. Doin et al., juin 2013 ESA-NRSCC Dragon Symposium, Palerme
3. Godard juil. 2012 congrès Chengdu University of Techonology
4. de Sigoyer juil 2012 congrès Chengdu University of Techonology
5. de Sigoyer août 2013 congrès Chengdu Chinese Earthquake Center
6. Ansberque et al., sept 2013 Congrès Int. Assos. Geormorphology Paris
7. Godard et al., nov 2013, EAGER, Lab Int assos Taiwan-France
8. Ansberque et al. dec 2013 AGU
9. de Sigoyer et al., dec 2013 AGU

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 prior to the devastating earthquake, May 12th 2008, Mw 7.9. 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 appears to accommodate a large part of the present-day relative movement (6-8 mm/yr) between the Songpan block (Bayar Han block) and the south China block (Thatcher, 2005, 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 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 the 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 (Pullen et al., 2008).
The third aim of the 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 also produce scenarios for the deformation history of the Songpan Garze area and for the thickening of the eastern plateau margin since Triassic time.

Project coordination

De Sigoyer Julia (Organisme de recherche)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ENS CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR PARIS B

Help of the ANR 345,000 euros
Beginning and duration of the scientific project: August 2011 - 48 Months

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