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

Abstract

The Tethyan Sedimentary Sequence, one of the major tectonic units of the Himalayan belt cropping out in the inner portion of the chain, has been investigated in SE Tibet to unravel its tectonic and metamorphic evolution.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 Tsangpo 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 to the North. It is worth to note that 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 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.