Tectono-metamorphic evolution of the Tethyan Sedimentary Sequence (SE Tibet)

The Tethyan Sedimentary Sequence, one of the major tectonic units of the Himalayan belt cropping out in the inner portion of the Himalayan belt, has bee investigated in SE Tibet to unravel its tectonic and metamorphic evolution. In SE Tibet the TSS is mainly represented by a Triassic flysch (Antolin et al., 2010) deformed under very-low to low-grade metamorphic conditions (Crouzet et al., 2007, Dunkl et al. 2011) Along other sections of the belt most of the attention has been paid till now to the relations between the TSS and the STDS (Godin 2003; Carosi et al., 2007; Kellet and Godin, 2009) focusing on the relations between the development of several generations of folds and the activity of the STDS. The study area offers an almost complete transect of the TSS from the southernmost portion, close to the STDS and the inner one approaching the suture zone allowing to depict a much more complete tectonic and metamorphic evolution of the TSS unit itself. Meso and microstructural analysis highlighted that the Tethyan Sedimentary Sequence recorded at least three phases of ductile defomation, all of them associated to the development of folds and related axial plane foliations. A prominent D1 deformation is progressively overprinted by a D2 deformation approaching the Yarlung Tsangbo suture zone to the North. Structural analysis allowed to recognise two first-order different structural domains: a southern domain in which D1 is the prominent deformation and a northern domain in which the D2 overprint predominates up to transpose D1 deformation. F2 folds show a regional backward vergence (northward) with respect to the southward verging F1 folds. Finite strain data show an increase of D2-related strain moving frm South to the North. The further tectonic evolution is characterized by the development of brittle-ductile shear zones often localized on the inverted limbs of F2 folds. Kinematic indicators are mainly represented by S-C structures and point to a top-to-the-North sense of movement. A later D3 tectonic phase is associated to upright metric to decametric folds. Detailed SEM based chemical maps and EMPA profiles have been conducted on chloritoid schist and reveal how a core to rim increase of Mg, compensated by the decrease of Fe and Mn, is systematically present in chloritoid (XMg from 0.12 up to 0.17). Moreover, white mica shows a statistically change in composition as function of microstructural position: S1 white micas have a lower Si4+ (3.02-3.07 a.p.f.u.) content with respect to S2 white micas (3.09-3.15 Si4+ a.p.f.u.). It is worth to note that the new P-T-d data on polydeformed chloritoid schists point out an increase of both temperature and pressure from D1 to D2 deformation indicating prograde metamorphism related to burial during D1-D2 phases and support that F2 folds developed in a compressive tectonic framework during crustal thickening in the time span of 35-25 Ma. The integration of our new deformation and P-T data with available literature data will help to deconvolve the long lasted history of this tectonic unit, far away to be well understood.