What collided with India at ~50 Ma? Constraints from the sedimentary record in the NW Himalaya, Ladakh.

Abstract:

The timing of India-Asia collision is critical to the understanding of crustal deformation processes. In the NW Himalaya, a number of workers have proposed a ~55-50 Ma age for collision along the Indus suture zone which separates India from the Kohistan-Ladakh Intraoceanic Island arc (KLA) to the north. This is based on a number of factors including the age of youngest marine sediments in the suture (e.g. Green et al. 2008), age of eclogites indicative of onset of Indian continental subduction (e.g. de Sigoyer et al. 2000), and first evidence of detritus from north of the suture zone deposited on the Indian plate (e.g. Clift et al. 2002). Such evidence can be interpreted as documenting the age of India-Asia collision if one takes the KLA to have collided with the Asian plate prior to its collision with India (e.g. Petterson 2010 and refs therein). However, an increasing number of workers propose that the KLA collided with Asia subsequent to its earlier collision with India, dated variously at 85 Ma (Chatterjee et al. 2013), 61 Ma (Khan et al. 2009) and 50 Ma (Bouilhol et al. 2013). If correct, then the previous constraints to dating the collision as outlined above have in fact been dating the timing of India-arc collision, rather than the final ocean closure and terminal collision of India+arc with Asia as previously believed. This, plus the questioning of earlier provenance work of Clift et al. (2002) regarding the validity of their data for constraining the time when detritus from north of the suture first arrived on the Indian plate (Henderson et al. 2011) suggests that the time is right for a reappraisal of this topic. But which method to use? A provenance study now brings with it a requirement to distinguish between detritus from the KLA and Southern margin of Asia. Recently, Bouilhol et al (2013) undertook a detailed study of the KLA, using temporal and spatial variation of zircon U-Pb and Hf as well as Sr-Nd bulk analyses, to document the arc’s collision with India at 50 Ma and its subsequent collision with Asia at 40 Ma. Such variation should be reflected in the detrital record of material eroded from the arc. We use zircon U-Pb and Hf analyses from Palaeogene sediments deposited in and adjacent to the Indus suture in Ladakh, to further explore the interpretations presented in that research.