The Dras arc Complex: lithofacies and reconstruction of a Late Cretaceous oceanic volcanic arc in the Indus Suture Zone, Ladakh Himalaya

Abstract

The purpose of this paper is to give an integrated description and interpretation of mainly volcaniclastic sediments related to excellently exposed oceanic volcanic arc successions in the Ladakh Himalayas. The mainly Late Cretaceous (Aptian—Paleocene?) Dras arc Complex in the Indus Suture Zone (N. India) is reconstructed as an oceanic arc, passing southwards into a proximal to distal forearc apron. The arc complex comprises three structural units. From west to east these are the Suru unit, the Naktul unit and the Nindam Formation. The Suru unit and the Naktul unit are unconformably underlain by dissected Late Jurassic? oceanic crust and mantle. The Suru unit preserves the interior of the arc and is divided into Dras 1 and Dras 2 sub-units. The Dras 1 Sub-unit, of mid-Late Cretaceous age, was intruded by arc plutonics, deformed, then unconformably overlain by the poorly dated Dras 2 Sub-unit (Lower Tertiary). The Dras 1 Sub-unit comprises arc extrusives, volcaniclastic and tuffaceous sedimentary rocks, and mainly redeposited shallow-water limestones. The Dras 2 Sub-unit is dominated by coarse volcaniclastics and lava flows, passing up into rhythmically layered acidic extrusives, with interbedded turbiditic siltstones and siliceous pelagic limestones. Further east, the Naktul unit is mainly clastic, with large volumes of massive volcaniclastic talus, thick-bedded debris flows, volcaniclastic turbidites and reworked shallow-water carbonates. Pillowed extrusives and ribbon radiolarites are present, mainly low in the succession in some areas, while pelagic carbonates are abundant near the top. The Naktul unit is interpreted as a proximal forearc apron. The Nindam Formation in the east is dominated by deep-water volcaniclastic turbidites, tuffaceous sediments and pelagic carbonates, with subordinate debris flows and is interpreted as a distal deep-water forearc succession. Cyclical alternations of mainly volcaniclastics and pelagic carbonates in the Nindam Formation are thought to reflect relative sea-level changes, of tectono-magmatic and/or global eustatic origin. The oceanic Dras arc Complex formed within Neotethys in the mid-Late Cretaceous above an assumed northward-dipping subduction zone. Around 79 Ma (Campanian), the oceanic arc underwent collisional deformation, possibly related to accretion to the Trans-Himalaya (Ladakh Block) and/or the southern margin of Eurasia, to form an Andean-type active continental margin. The Suru unit in the west (Dras 1 Sub-unit) was deformed, then overlain by the mainly acidic Dras 2 Sub-unit. However, there is no direct evidence that the Nindam Formation in the east was deformed until the Early Eocene when final collision of India and Eurasia took place, with southward thrusting of the Dras arc Complex. This was followed by partial erosion during deposition of the “Indus molasse”, and then back-thrusting and folding related to uplift of the High Himalayas in the later Tertiary.