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1 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
2 United Kingdom Nirex Ltd., Curie Avenue, Harwell, Didcot, Oxon OX11 0RH, United Kingdom
The Nindam Formation of the Indus suture zone represents the forearc basin to the Dras-Kohistan volcanic arc, 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 arc 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 arc 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 arc 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 arc, as well as the volcanic and intrusive core of the Dras-Kohistan Arc. The overall chemistry of the Dras-Kohistan Arc 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.
Key Words: Himalaya • volcaniclastics • volcanic arcs • trace elements • isotope geochemistry • Indus suture zone
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