Sedimentary and geochemical evolution of the Dras forearc basin, Indus suture, Ladakh Himalaya, India

The Nindam Formation of the Indus suture zone represents the forearc basin to the Dras-Kohistan volcanic are which was generated in an intraoceanic se tting during mid-Cretaceous time, and accreted to the margin of Eurasia dur ing Late Cretaceous time. The Nindam Formation comprises volcaniclastic sed iment and pelagic carbonate deposited in cyclic influxes of high-density, s heet-like turbidites, with locally well-defined channel morphologies, Debri s-flow deposits as much as 5 m thick that rework older material are volumet rically <5% of the sequence. A fine-grained basal section is succeeded by s everal hundred meters of cyclic sandy turbidites, and finally a more shaley top, This large-scale variation may be linked to tectonic controls on are activity, while individual sandy packages 50-100 m thick are inferred to ha ve a glacioeustatic origin, Sedimentation mirrors similar patterns observed in the modern Mariana and Tonga forearcs, The lack of major unconformities and large-scale reworking suggests that the volcaniclastic sedimentary roc ks should form a relatively complete record of geochemical change in the ar e source. The sediment trace- and rare earth element (REE) signature is similar to th at of the Dras and Chalt volcanic rocks erupted at the volcanic front durin g the intraoceanic phase of activity and is distinct from the Kardung Volca nic Group erupted during the continental are phase, following collision wit h the Eurasian margin, Nd isotopes also support an intraoceanic origin. The sedimentary rocks show either no or slight light REE enrichment, similar t o modern sediments from the Mariana are as well as the volcanic and intrusi ve core of the Dras-Kohistan Are. The overall chemistry of the Dras-Kohista n Are is thus more depleted in light REE than continental crust, Our data d o not support models that propose continental crust being formed along intr aoceanic active margins.