Université de Rennes1
Campus de Beaulieu
35042 Rennes Cedex
02 23 23 60 76
Deciphering the pre-Tertiary evolution of Tibet
Team members and partners
The project involves both Chinese and French scientists. It has been financed by INSU (Syster project) , by the French Ambassy in Beijing and by the Key State Laboratory for Continental Dynamics, Chinese Academy of Geological Sciences, Beijing.
Cattin Rodolphe (Univ. Montpellier 2 – Géosciences Montpellier), Huaqui Li (CAGS, Beijing), Hui Cao (CAGS, Beijing), Jolivet Marc (CNRS – Géoscience Rennes) ; Roger Françoise (CNRS – Géosciences Montpellier), Xu Zhiqin (Academician, CAGS, Beijing).
While the Tertiary evolution of the Tibetan plateau has been widely studied, the first stages of formation of this unique topographic and geologic feature are still poorly understood. The Mesozoic Indosinian orogeny that followed the closure of the Paleotethys ocean was the last major tcetonic event in south Asia prior to the Indian collision (e.g. Roger et al., 2008, 2010, 2011). It resulted in probably one of the largest ever-built mountain belt on Earth, similar to the actual Himalayan – Alpine belt. The Indosinian Orogeny set-up the crustal and lithospheric structure of South Asia and several major Tertiary structures such as the Longmen Shan range or the Kunlun range are clearly superimposed to those inherited Mesozoic lithopsheric accidents.
The aim of this ongoing project is thus to understand the importance and role of the inherited Indosinian structures on the tertiary evolution of Tibet. To reach that goal, we need to describe and date the main Indosinian structures but also to constrain the post-tectonic evolution of the Indosinian topography in order to establish the topographic and tectonic setting of Tibet prior to the India-Asia collision.
Due to its complexity, the Indosinian orogen is far from being fully understood. Recent synthesis of the data available in North Tibet (Roger et al., 2008, 2010, 2011) and Yunnan-Thailand (Sone and Metcalfe, 2008) provided a first set of geodynamic models for the development of the belt. However, these models remain localized on specific areas and do not consider the geometry and geodynamics of the whole orogen. Numerous questions remains such as the total extend and localisation of the orogen, the general chronology of the deformation, the deformation style, the occurrence of one or several belts, etc.
The Indosinian orogeny in Tibet
The Songpan-Garzê fold belt is the largest and most preserved expression of the Indosinian orogeny in Tibet. The belt is located in the eastern part of the Tibetan Plateau and west of the Sichuan basin. It is bounded by the South China, North China and Qiangtang (North Tibet) continental blocks. To the east, the Longmen-Shan thrust-nappes belt separates the Songpan-Garzê fold belt from the Sichuan basin (Zhang et al., 1984 ; Chen et al., 1995 ; Chen and Wilson, 1996 ; Yong et al., 2003). From Late Permian to Early Jurassic synchronous activity along three subduction zones (Kunlun-Anyemaqen to the north, Jinsha to the south and Yushu-Batang to the east), and subsequent convergence between the North China, South China and the Qiangtang blocks, induced the closure of the Songpan-Garzê basin, a branch of the Palaeotethys ocean (eg. Roger et al., 2004 ; 2008, 2010). The closure of the Songpan-Garzê basin, filled with a thick (5 to 15 km) sequence of Triassic flyschoid sediments, led to the growth of the Songpan Garzê accretionary prism (Zou et al., 1984 ; Sengor, 1985 ; Rao and Xu, 1987 ; Mattauer et al., 1992 ; Xu et al. 1992 ; Nie et al., 1994).
The final stages of the closure of the Songpan Garzê basin, and the last step of the Indosinian orogeny in northern Tibet are marked by slightly folded late Triassic continental sediments lying unconformably on top of the intensely folded middle Triassic marine formations (Calassou, 1994). These sequences indicate the emergence of the initially submarine accretionary wedge above sea level.
Except for very localized Tertiary and Quaternary outcrops, post-Triassic sediments are totally absent from the Songpan-Garzê - Yidun fold belt (C.I.G.M.R., 1991). However, post-Triassic deformation has been recognized within the belt. Emplacement of two granitoids around 100 Ma (Reid et al., 2005) suggests that a magmatic episode affected the Yidun block during the middle cretaceous in relation with the extension associated with the northward subduction of the Tethys underneath Asia (Reid et al., 2007). However, the plutons are close to major faults and it is possible that some later magmatic activity was related to the faults. Nonetheless it is generally accepted that potential Jurassic – Cretaceous tectonism (and especially those that could be related to the collision of the Lhasa block in Cretaceous) did not modify the general Triassic architecture of eastern Tibet (e.g. Burchfield et al., 1995 ; Roger et al., 2004 ; Harrowfield and Wilson, 2005 ; Reid et al., 2005 ; Wilson et al., 2006). The long-term cooling histories obtained on Mesozoic granites from the whole Songpan-Garzê – Yidun – Longmen Shan region and on the metamorphic series of the Danba dome, are similar and show a very slow and regular cooling pattern during the Jurassic and Cretaceous, confirming the absence of major tectonic event between ca. 150 and 30 Ma (e.g. Roger et al., 2010, 2011). This period of tectonic quiescence is also seen north of the Tibetan plateau in the Kunlun, Altyn Tagh and Tarim regions (Jolivet et al., 1999 ; 2001 ; 2003 ; Roger et al., 2003 ; Yuan et al., 2006), in the Tian Shan belt (e.g. Dumitru et al., 2001 ; Jolivet et al., 2010) and in Mongolia or Siberia (Vassallo et al., 2007 ; Jolivet et al., 2007, 2009, 2011). One fission track datum south of the Qiangtang block also indicates a similar cooling history (Wang et al., 2007). The absence of a cooling episode during the Cretaceous within eastern Tibet implies that the tectonic effects of the docking of the Lhasa block south of the Qiangtang were very low.