Constraints to the timing of Neo-Tethys closure determined from the Indus Group molasse, Ladakh Himalaya, NW India

The Indus Group is a Tertiary aged sequence composed of marine and terrestrial sedimentary rocks which were deposited in an evolving late-forearc to intermontane basin setting during the closure of Neo-Tethys and onset of India-Asia collision (Brookfield and Andrews-Speed 1984, Van Haver 1984, Searle 1990, Sinclair and Jaffey 2001, Clift et al. 2002). Clift et al. (2002) have constrained the age of collision by determining the lowermost stratigraphic point in the Indus Group that contains detritus from both Indian and Asian plates, and also by identifying where the Asian plate derived Indus Group unconformably overlies Indian plate margin sediments. The Chogdo Formation, dated by an overlying limestone at older than 50.5 Ma (Green et al. 2008) is identified by Clift et al. (2001), to be the oldest unit of the Indus Group to contain detritus from both the Indian and Asian plates, and to stratigraphically overly Lamayuru Group Indian slope turbidites and Jurutze forearc basin rocks, thereby constraining the timing of ocean closure at prior to 50.5 Ma. However, despite its importance, these previous evaluations of the Indus Group have been hampered by poor stratigraphic knowledge and uncertain lateral correlations, largely due to the relatively complex deformation of the rocks and poor biostratigraphic control. We use a combination of geological mapping, biostratigraphy, facies analysis, petrography, bulk rock geochemistry, and isotopic characterisation of single detrital grains to 1) create an accurate and more widely representative stratigraphy for the Indus Group, 2) determine the nature of the contacts which separate the overlying Indus Group from underlying Indian and Asian plate formations and 3) determine the provenance of the Group, in particular the stratigraphic level within the Indus Group at which both Indian and Asian plate detrital minerals occur together, in order to constrain the time of collision and discover which geological terranes where exhumed and actively eroded during the early stages of the Himalayan orogeny. Our Initial analyses indicate that: 1) the Chogdo Formation may not be as widely occurring as previously interpreted, partly due to obscured tectonic contacts and problems with lateral correlations along strike; 2) there is no apparent location where the Chogdo overlies Indian Plate sediments; 3) there is no confirmed evidence to suggest that the Chogdo Formation contains Indian Plate detritus. Reassessment of constraints to the timing of Neo-Tethys closure as determined from the Chogdo Formation is required.