INITIATION OF THE HIMALAYAN OROGEN AS AN EARLY PALEOZOIC THIN-SKINNED THRUST BELT

Geologic relations in Nepal and northern India suggest that many aspects of the Himalayan orogen are the result of both early Paleozoic and Tertiary tectonism. The main expressions of early Paleozoic tectonism, as recognized by ourselves and many other workers, are as follows. (1) A thick sequence of Neoproterozoic clastic strata and Cambrian-Ordovician carbonates was imbricated along north-dipping thrust faults. From south to north, in central Nepal, the clastic\carbonate panels include the Raduwa Schist\Bhainsedobhan Marble, Kulikhani Schist\Markhu marble, Formation I\Formation II, and Sanctuary Formation\Annapurna and Nilgiri Formations. The Paleozoic thrust between Greater Himalayan rocks and Tethyan strata has been reactivated as the South Tibetan detachment fault during mid-Cenozoic time. (2) Granitic sills with 510-470 Ma ages were emplaced along the thrusts during and following motion. These syn- to post-tectonic sills include the Palung and related granites, the Sheopuri granitic gneiss, Formation III augen gneiss, and perhaps the Kangmar granite. (3) Greater Himalayan rocks were metamorphosed to garnet grade in the crystalline thrust sheets and to kyanite and sillimanite grade in the Greater Himalaya (as recorded by early Paleozoic U-Th-Pb monazite ages and by cross-cutting relations with Cambro-Ordovician granites). (4) Ordovician foreland-basin strata record uplift and erosion of the orogen. These strata include the Jurikhet conglomerate and Chisapani quartzite, "boulder beds" in the Tistung and Damgad Formations, and coarse, immature sandstone overlying the North Face Quartzite. Analogous but much thicker Middle Ordovician conglomerates in Tethyan strata of northern India (Zanskar area) have been described by Garzanti et al. (1986).

We suggest that this tectonism occurred within a south-vergent, southward-propagating, thin-skinned thrust belt that was active during Late Cambrian(?) through Middle Ordovician time. This tectonism may have occurred along or outboard of India's northern margin, with large-scale emplacement of the orogen over Lesser Himalayan strata during early Paleozoic and/or Tertiary time. Resolving the relative proportions of early Paleozoic versus Tertiary tectonism remains a fundamental challenge in understanding the Himalayan orogen.