Sedimentary and geochemical evolution of the Dras forearc basin, Indus suture, Ladakh Himalaya, India
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The Nindam Formation of the Indus suture zone represents the forearc basin to the Dras-Kohistan volcanic are which was generated in an intraoceanic setting during mid-Cretaceous time, and accreted to the margin of Eurasia during Late Cretaceous time. The Nindam Formation comprises volcaniclastic sediment and pelagic carbonate deposited in cyclic influxes of high-density, sheet-like turbidites, with locally well-defined channel morphologies, Debris-flow deposits as much as 5 m thick that rework older material are volumetrically <5% of the sequence. A fine-grained basal section is succeeded by several 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 have 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 rocks should form a relatively complete record of geochemical change in the are source. The sediment trace- and rare earth element (REE) signature is similar to that of the Dras and Chalt volcanic rocks erupted at the volcanic front during the intraoceanic phase of activity and is distinct from the Kardung Volcanic Group erupted during the continental are phase, following collision with the Eurasian margin, Nd isotopes also support an intraoceanic origin. The sedimentary rocks show either no or slight light REE enrichment, similar to modern sediments from the Mariana are as well as the volcanic and intrusive core of the Dras-Kohistan Are. The overall chemistry of the Dras-Kohistan Are is thus more depleted in light REE than continental crust, Our data do not support models that propose continental crust being formed along intraoceanic active margins.