THE STRUCTURE AND STRATIGRAPHY OF THE ALMORA THRUST FOOTWALL AND THE IMPLICATIONS FOR THE KINEMATIC EVOLUTION OF THE HIMALAYAN THRUST BELT

The Almora-Dadeldhura klippe is a Greater Himalayan thrust sheet erosionally isolated within Lesser Himalayan rocks in the Himalayan thrust belt of western Nepal and the Kumaun-Garhwal region of NW India. This klippe is bounded at the base by the Almora thrust in NW India, which was passively folded into a synform by deformation in the footwall. Exposed Lesser Himalayan rocks to the south and north of the klippe are difficult to correlate and controversial. These rocks are divided into Paleoproterozoic Inner (1900-1600 Ma) and Neoproterozoic-Cambrian Outer (800-500Ma) Lesser Himalayan rocks. South of the klippe, only Outer Lesser Himalayan rocks have been mapped. In addition, the Tons thrust, a Lesser Himalayan intraformational thrust, divides the northern, older Inner Lesser Himalayan rocks and southern, younger Outer Lesser Himalayan rocks in the Garhwal region, where the overlying Almora klippe Greater Himalayan rocks are eroded away. This study tests whether the rocks in the footwall of the Almora thrust are Inner or Outer Lesser Himalayan rocks, and also tests the presence of the Tons thrust north of the Almora klippe.

Field mapping accompanied by U-Pb zircon analyses, Lu-Hf analyses of zircons, and REE analyses illuminate the stratigraphic and structural relationships. Our data from south of the Almora klippe shows a quartzite with a detrital zircon age of 1.85 Ga with a strongly negative ɛHf value, and a U-Pb age of 1.9 Ga in a granite with an enriched LREE signature. These data indicate that Inner Lesser Himalayan rocks are present and that the previous mapping is incorrect. These Inner Lesser Himalayan rocks are likely carried by the Ramgarh-Munsiari thrust, which moved Paleoproterozoic rock far to the south along strike in the thrust belt, and is passively folded under the klippe. The Tons thrust is not present north of the Almora klippe because no Outer Lesser Himalayan rocks are present. Instead, this unit yielded a Paleoproterozoic (~1.6 Ga) detrital zircon age with a strongly negative ɛHf value. In addition, only Paleoproterozoic rocks are present in the Almora thrust footwall with no older cratonic basement (~2.5 Ga) rocks. Thus, the thrust belt does not contain Indian cratonic basement rock as previously suggested.