Tracing patterns of erosion and drainage in the Paleogene Himalaya throughion probe Pb isotope analysis of detrital K-feldspars in the Indus Molasse, India

The Indus Molasse is a pre- and syn-tectonic sedimentary sequence situated in the Indus Suture Zone of the western Himalaya. Spanning in time the coll ision of India and Asia, this deposit is well placed to record the evolving uplift and erosion history of the early Himalayan orogen. Nd isotope analy ses from day extracted from shales interbedded within the dominantly alluvi al sequence indicate a low negative E-Nid (-1.64 to 0.72). in the basal Pal eocene Chogdo Formation, slightly more negative than measured values from t he Transhimalaya and Kohistan/Dras Are. Up-section E-Nd becomes more negati ve, as low as -10.05. indicating influence of a different, more enriched so urce. Ion microprobe Pb isotopic analyses of single K-feldspars help constr ain this sourer as being either the Lhasa or Karakoram Blocks, with westwar d paleo-current flow favoring the former. Pb-207/(204)b ratios are too low to be consistent with known Indian plate sources. a conclusion supported by the lack of muscovite or garnet that would be indicative of a High Himalay an contribution. Given the known age of rapid cooling of the High Himalaya at similar to 20 Ma, and the lack of exposure of suitable lithologies prior to that time, an age of sedimentation prior to similar to 20 Ma is inferre d. The post-collisional change in paleo-flow and provenance is suggested to reflect the initiation of the Indus River during the Early Eocene. This st udy demonstrates the power of combined bulk sediment and single grain analy ses in resolving provenance in tectonically complex settings. (C) 2001 Else vier Science B.V. All rights reserved.