Reconstruction of geologic history of Indus suture zone of Ladakh, Indian Himalayas, by high resolution fossil and radiometric ages

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Main results of this project are summarized as follows.1. Late Triassic to Early Cretaceous radiolarians are found from chert clasts in the Hemis Conglomerate (syn-collisional Molasse sequence) along the Indus suture zone. Since the chert is considered to have been part of the ocean-plate cover sediments, these fossil evidence shows the pre-Late Triassic opening of the Ceno-Tethys (pre-Late Triassic rifting of Karakoram block from the northern margin of Gondwana) in Ladakh Himalayas.2. Shelf-facies marine formations along the Shyok suture zone are subdivided into the Middle Jurassic (Callovian) ammonoids-bearing Soltak Formation and Middle Cretaceous Shyok Formation with Albian Orbitolina. These fossils strongly indicate that the ocean along the Shyok suture was closed in the post-Middle Cretaceous time.3. Plutonic rocks of the Ladakh arc which were formed by the subduction of oceanic plate of Ceno-Tethys are calc-alkaline on the basis of their mineralogy and geochemistry, and have K-Ar ages of 50-51 Ma. Granite and diorite with calc-alkaline signature and 64-66 K-Ar ages are also distributed on the northern margin of the arc. These lines of evidence clearly show that the arc magmatism had continued at least for sixteen million years during 50-66 Ma before the collision between Eurasia and India.4. Mylonites along the Karakoram fault have vertical lineation and extensional deformation structures together with the right-lateral strike-slip shear indicators designating same sense as the present-day activity. These structures show a long and complicated deformation history more than fifteen million years until now. Aspect ratios of dynamically recrystallized quartz grains suggest that the width of shear zone has narrowed with the decreasing temperature of the fault zone.