Indus Molasse Research Articles

The Indus-Tsangpo suture zone in Ladakh, northwest Himalaya: Further palaeomagnetic data and implications

A total of 81 samples (244 specimens) from Upper Cretaceous Indus Molasse and Middle to Upper Cretaceous Dras Flyschoids of the Indus-Tsangpo suture zone in Ladakh (northwest Himalaya) has been studied by thermal demagnetization methods. Both formations showed a characteristic magnetization component indicative for equatorial to low northern palaeolatitudes of acquisition. Similar palaeolatitudes have been obtained before from secondary magnetization components of Early Tertiary age in the Ladakh Intrusives and in the Tibetan Sedimentary Series of central Nepal. The present characteristic components are interpreted likewise as secondary magnetizations which stabilized between 50 and 60 m.y. ago, during Greater India's collision with Asia's southern margin. The Dras Flyschoids show another magnetic component which, in case of primary origin, indicates acquisition at a low southern palaeolatitude. If correct, this interpretation supports recent suggestions for Late Cretaceous obduction of an island arc on Greater India's northern margin.

Middle Paleozoic to Cenozoic geology and tectonic evolution of the northwestern Himalaya

Previously published data and data collected by the authors from the northwestern Himalaya are summarized and assessed. The account focuses on Kohistan and the Karakorum Range in Pakistan, and on Ladakh and the Zanskar Range in India. Five palaeotectonic units are recognized. From south to north these are: 1. (1) Palaeozoic-Eocene continental shelf sequence on the northern margin of the Indian continent; 2. (2) a Triassic-Jurassic continental slope sequence: the Lamayuru unit of the Indus zone and of the Spongtang klippe; 3. (3) the Cretaceous-Tertiary Kohistan-Ladakh magmatic arc, which includes the Kokistan sequence, the Ladakh batholith, the Dras unit, the Spongtang klippe, the Indus molasse unit and the Shyok zone; 4. (4) the Karakorum batholith: a late Tertiary zone of granitoid magmatism and metamorphism; 5. (5) a PalaeozoicMesozoic continental shelf and slope sequence deposited on the southern margin of an Asian continent. The following sequence of events is tentatively proposed: 1. (1) Permian-Carboniferous continental rifting; 2. (2) Triassic-Jurassic collapse of continental shelf, and formation of deep (oceanic?) basin; 3. (3) Cretaceous formation of Kohistan-Ladakh magmatic arc, in response to subduction from the south as ocean closed; 4. (4) late Cretaceous-Eocene emplacement of Kohistan arc and of thrust sheets of Indus zone (magmatic arc units and Lamayuru unit) southwards onto Indian continental margin, as India approached and collided with magmatic arc; 5. (5) silicic phases of Ladakh batholith (Palaeocene-Eocene) and Karakorum batholith (Miocene) may be related to subduction of oceanic basin between magmatic arc and Asian margin. Alternatively, they are late- and post-collisional products; 6. (6) the Shyok zone, between the Ladakh and Karakorum batholiths, was emplaced to the south in Oligocene or Miocene time; 7. (7) final collision between India, the magmatic arc and Asia in Oligocene-Miocene time was followed by continued crustal shortening which disrupted thrust units.