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

摘要

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 collision of India and Asia, this deposit is well placed to record the evolving uplift and erosion history of the early Himalayan orogen. Nd isotope analyses from clay extracted from shales interbedded within the dominantly alluvial sequence indicate a low negative ε_(Nd) (-1.64 to 0.72), in the basal Paleocene Chogdo Formation, slightly more negative than measured values from the Transhimalaya and Kohistan/Dras Arc. Up-section ε_(Nd) becomes more negative, as low as -10.05, indicating influence of a different, more enriched source. Ion microprobe Pb isotopic analyses of single K-feldspars help constrain this source as being either the Lhasa or Karakoram Blocks, with westward paleo-current flow favoring the former. ~(207)Pb/~(204)Pb 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 Himalayan contribution. Given the known age of rapid cooling of the High Himalaya at ～ 20 Ma, and the lack of exposure of suitable lithologies prior to that time, an age of sedimentation prior to ～ 20 Ma is inferred. The post-collisional change in paleo-flow and provenance is suggested to reflect the initiation of the Indus River during the Early Eocene. This study demonstrates the power of combined bulk sediment and single grain analyses in resolving provenance in tectonically complex settings.