Orchestration in time and space of southeast Tibetan faults, views from geology and geodesy – Tibetan-Orchestra

acquisition of observations at widely different timescales using widely different methods:1/ Cenozoic >10Ma, use thermochronology data and thermo-kinematic models to quantify the rate and timing of rock exhumation; 2/ / Holocene seismic catalog built by doing lacustrine paleosismicity; 3/ present-day, use radar satellite data of the Sentinel-1 (S1) constellation to quantify very small present-day interseismic deformation signals at unprecedented resolution

The India-Eurasia collision (~50 million years ago [Ma]) led to
the formation of the Tibetan Plateau, the world's largest and highest orogenic plateau. The formation and evolution of such a unique geological feature has been one of the main controversies in Earth Sciences for decades, especially regarding the role of faulting in the thickening of the crust. Here, we present 3D thermo-kinematic models of thermochronology data allowing to constrain the exhumation history of the Muli thrust fault, a ~250 km-long major structure of the SE Tibetan margin, linked to significant steps in surface topography and in crustal boundary at depth (Moho). We constrain a steep fault (>70°) within the upper crust, consistent with field observations, that flattens at depth (=20 km). The Muli thrust presents rapid thrusting motion (0.6 ± 0.08 km/Ma) that initiated at ~12.5 Ma, following a slower phase
(0.2 ± 0.06 km/Ma) since 50 Ma, with total rock exhumation of ~15 km. This underlines the important role of thrust faulting in the thickening of the SE Tibetan crust.

The goal is to build numerical mechanical models specifically related to our observations, such that they provide a framework for interpretation. 2-D and 3-D models will be designed to study crustal deformation of the SE Tibet (dip of the faults? level of rooting? interaction between brittle upper crust and ductile lower crust?).

Pitard, P., A. Replumaz, M.-L. Chevalier, P.-H. Leloup, M. Bai*, M.-P. Doin, C. Thieulot, X. Ou*, M. Balvay, and H. Li, Exhumation History Along the Muli Thrust—Implication for Crustal Thickening Mechanism in Eastern Tibet, GRL, 48, doi: 10.1029/2021GL093677, 2021

Ou, X., A. Replumaz, P. van der Beek, Contrasting exhumation histories and relief development within the Three Rivers Region (Southeast Tibet), Solid Earth, doi: 10.5194/se-2020-172 (https://se.copernicus.org/articles/12/563/2021/)

Despite decades of controversy, our understanding of the formation of the Tibetan Plateau remains limited. The role of competing mechanisms, such as distributed crustal thickening versus lateral propagation of thrust faulting at crustal or lithospheric scales, is still poorly understood. Indeed, in view of the 3D nature and temporal complexity of the involved deformation processes, no numerical model taking into account the role of strike-slip faults in accommodating stepwise evolution of thrust faulting, as well as the interaction between the deep crust and the surface, has yet been implemented. Therefore, it remains difficult to test the mechanical and rheological consistency, and the ability to explain observations, of end-member conceptual models at the scale of the Tibetan Plateau.
In this project, we focus on the eastern edge of the Tibetan Plateau, where the Xianshuihe strike-slip fault system (XFS) is the most seismically active fault system in Tibet over instrumental and historical periods. Since ~9 Ma, it has horizontally offset the thrust system which thickens the eastern edge of the Tibetan Plateau, the Longmenshan thrust belt (LMTB) to the northeast and the Yalong thrust belt (YTB) to the southwest. This regionally complex network of thrusts and strike-slip fault systems evolving during the collision may provide key insights into the regional orchestration between lateral propagation and crustal thickening of the plateau through time.
The goal of this project is to document and quantify the main deformation events in SE Tibet with a focus on establishing the causative relationship between successive thrust and strike-slip fault activations. We will use complementary geological, geodetic and modelling tools to provide new quantitative constraints on the formation and evolution of the Tibetan Plateau, over the Geological timescale (1 to 100 Myr, Task 1), using field, petrological, structural, geochronologic and thermochronologic studies to quantify the mechanisms of crustal thickening up to the modern value of 65 km; over the seismic cycle timescale (1 to 10.000 years, Task 2), to constrain the present-day deformation based on GPS and InSAR data, lacustrine paleoseismicity, and visco-elastic modelling;. Intermediate timescale between geologic and geodetic timescales (10 000 years to 1 Myr, Task 3), to provide schematic 2-D viscous or visco-plastic models of thrusts embedded in the crust and to study thrust activation through time and the influence of erosion. This modeling on this intermediate timescale is the most challenging part of this project and represents the main scientific and technical barrier to be lifted in order to reach our ambitious aim of including the localisation of deformation on faults in the long-term history of building the Tibetan Plateau.
The main tool of communication of the project results to multiple audiences will be a website, presenting the project and the outcomes. The most visually attractive results (especially the 3D and 4D models), the challenges encountered throughout the project, as well as field experience, will be shared with peers and the general public through social media platforms and annual events. At the end of the project, we plan a temporary exhibition on the building of the Tibetan Plateau to be held at the new scientific museum in Grenoble “les Moulins de Villancourt”.