Metamorphic evolution of the Tethyan Himalayan flysch in SE Tibet

The metamorphic conditions and the age of thermal overprint were determined in metapelites, metaarenites and metabasites of the Tethyan Himalayan Sequence (THS) in SE Tibet using Kubler Index and vitrinite reflectance data and applying thermobarometrical (Thermocalc and PERPLEX) and geochronological methods (illite/muscovite K–Ar and zircon and apatite (U– Th)/He chronology). The multiple folded thrust pile experienced a thermal overprint reaching locally peak conditions between the diagenetic stage (c. 170 8C) and the amphibolite facies (c. 600 8C at 10 kbar). Burial diagenesis and heating due to Early Cretaceous dyke emplacement triggered the growth of illite in the metapelites. Eocene collision-related peak metamorphic conditions have been reached at c. 44 Ma. During collision the different tectonic blocks of the THS were tectonically buried to different structural levels so that they experienced maximum greenschist to amphibolite facies metamorphism. Later, during Oligocene to Miocene times the entire THS underwent anchito epizonal metamorphic conditions, probably associated to continuous deformation in the flysch fold-thrust-system. This period terminated at c. 24–22 Ma. Adjacent to the north Himalayan metamorphic domes, the base of the THS was metamorphosed during Miocene times (c. 13 Ma). Post-metamorphic cooling below c. 180 8C lasted until Late Miocene and took place at different times. The northward drift of Greater India during the Cenozoic resulted in the closure of the Tethys ocean, the initiation of the India–Asia collision in the Paleocene and the subsequent uplift of the Himalayan Range (c. 55–50 Ma; e.g. Gaetani & Garzanti 1991; Patzelt et al. 1996; Najman et al. 2005). The Himalayan arc forms an active WNW–ESE asymmetric fold and thrust belt with a main southward vergence (Fig. 1a, b). The northern member of the Himalaya is the Tethyan Himalayan Sequence (THS) which is located in the highest structural position within the orogen (Le Fort 1975; Hodges 2000). For that reason the rocks forming the THS have probably better preserved the early tectonothermal evolution of the Himalayan orogen than other tectonometamorphic units, like the Greater Himalayan Sequence, made up by mid-crustal rocks which has nearly lost its pre-climax memory during metamorphism around 23–17 Ma (Guillot et al. 1993; Harrison et al. 1997; Searle & Godin 2003; Godin et al. 2006). The THS can be traced along the 2500 km of the Himalayan arc between the Nanga Parbat syntaxis in the west and the Namche Barwa syntaxis in the east (Fig. 1). The Cambrian to Eocene sequence is composed of very variable lithologies, derived from different sedimentary facies zones of the former passive continental margin of the Indian plate (e.g. Brookfield 1993; Willems et al. 1996; Garzanti 1999). Some tectonic domains are altered only diagenetically and usually low-grade metamorphism was not exceeded (e.g. Fuchs 1967; Hodges et al. 1996; Crouzet et al. 2007). The aim of this study is to constrain the postsedimentary evolution of the THS from metamorphic and geochronological data. In the study From: Gloaguen, R. & Ratschbacher, L. (eds) Growth and Collapse of the Tibetan Plateau. Geological Society, London, Special Publications, 353, 45–69. DOI: 10.1144/SP353.4 0305-8719/11/$15.00 # The Geological Society of London 2011. area (Fig. 1b) previous metamorphic and geochronological studies have been focused mainly on the Indus-Yarlung suture zone, Great Counter Thrust and Gangdese Thrust (e.g. Yin et al. 1994; Quidelleur et al. 1997; Harrison et al. 2000; Dupuis et al. 2005). However our studied profiles are distributed along valleys south of the IndusYarlung Suture where few work has been done before. Kubler Index (KI) for ‘illite crystallinity’, vitrinite reflectance data, and K–Ar dating of micron and sub-micron fractions of illite are used to constrain the degree and age of metamorphism on metapelites, slates and sandstones of the Triassic flysch, which is the dominant metasedimentary sequence of the eastern THS. Thermobarometric methods (Thermocalc and PERPLEX, Holland & Powell 1998; Connolly & Petrini 2002, respectively) were applied on metamorphosed basic dyke rocks, which experienced a greenschist to amphibolite facies overprint.