Geometry, kinematics, and magnitude of extension across the Thakkhola Graben, Central Nepal Himalaya

The Thakkhola Graben has been a region of geologic inquiry for many decades. Although it is widely viewed to be in a class of structures that are important in accommodating the three-dimensional strain within the Himalayan thrust wedge, we still lack a detailed understanding of the total finite strain accommodated by graben-bounding faults, as well as their shape and cross-cutting relationships with structures deeper in the thrust wedge. Using geologic mapping and structural analysis, we show that a suite of pre-extensional shortening structures is offset by normal-oblique faults bounding the Thakkhola Graben that we use to define a piercing line. We calculate these faults to have accommodated 8.7 kilometers of vertical thinning, 7.2 kilometers of arc-perpendicular shear, and only 2.2 kilometers of arc-parallel extension. The magnitude of arc-parallel extension is quite low compared to extensional structures to the west in the Gurla Mandhata-Humla region. The cross-cutting relationships established in this study and timing constraints determined by previous works are consistent with a structural history of crustal thickening leading to foreland propagation of the locus of arc-perpendicular shortening contemporaneous with hinterland extension.

Palynological evidence for the Neogene environment analysis of the Thakkhola Graben, Nepal

The Thakkhola Graben, a north-south graben, lies in central Nepal Himalayas consisting of Neogene sediments. The presence of pollens in these sediments provides a unique natural laboratory to understand the paleoenvironment during the formation of the graben. This study provides a detailed description of the pollen collected from the Tetang and Thakkhola formations with the help of the Light Microscope and Scanning Electron Microscope. A variety of pollen assemblages from the Thakkhola Graben explains that the sediments contain dominant alpine trees with some steppe vegetation. Presence of evergreen subtropical and temperate deciduous broad-leaved forest, needle-leaved element, and high altitude taxa show a mixed pollen assemblage in fluvial-lacustrine sediments of the graben. The presence of evergreen subtropical and temperate deciduous broadleaf forest (Quercus, Betula, Juglans, Alnus), needle-leaved element (Pinus, Tsuga) and high altitude taxa (Picea, Abies) with Artemisia, Chenopodiaceae, Poaceae, Rosaceae show mixed pollen assemblages indicating warm and humid paleoclimate. The coniferous pollen indicates the altitude was higher during the deposition time and the presence of Betula, Quercus, and Juglans suggests temperate forest. Domination of Artemisia and Chenopodiaceae shows a strong influence of the Himalayan topographic barrier during the sediment deposition.

Granites from Mustang and Surrounding Regions (Central Nepal)

Two Himalayan leucogranite plutons of northern central Nepal have been recognized, mapped with the help of satellite imagery, sampled and analyzed. The 1600 km Dolpo-Mugu may be the largest High Himalaya leucogranite, and the 325 km2 Mustang leucogranite is a North Himalaya circular pluton cut in half by the Thakkhola graben. Both are very similar to the Manaslu granite. They have provided much of the filling material for the graben after a high rate of denudation.

A contribution to the geology of the Annapurna and Nilgiri Himals, Nepal

SummaryWithin the Tethyan Zone of Central West Nepal, the Nilgiri Limestone is the dominant unit. The significance of the separation of the Larjung Formation is questioned in the light of new outcrops examined, and the Nilgiri Sandstone is described. Recumbent fold structures are correlated between Annapurna, Nilgiri and Dhaulagiri. Between these mountains the Thakkhola graben faults are continued southwards, and estimates are made of the magnitude of their throws.