Geologic strata The Eastern
Himalayas has a more varied geomorphic history and pervasive topographic features than the Central Himalayas. In the southwest of the
Sub-Himalayas lies the
Singalila Ridge, the western end of a group of uplands in Nepal. Most of the Sub-Himalayas are in Nepal; a small portion reaches into Sikkim, India and a fragment is in the southern half of Bhutan. The region's topography, in part, has facilitated the region's rich biological diversity and ecosystem structure. The Buxa range of Indo-Bhutan is also a part of the ancient rocks of the Himalayas. The ancient folds, running mainly along an east-west axis, were worn down during a long period of
denudation lasting into
cretaceous times, possibly over a hundred million years. During this time the
carboniferous and
permian rocks disappeared from the surface, except in its north near Hatisar in Bhutan and in the long trench extending from
Jaldhaka River to
Torsa River, where
limestone and
coal deposits are preserved in discontinuous basins. Limestone deposits also appear in Bhutan on the southern flanks of the Lower Himalayas. The rocks of the highlands are mainly
sandstones of the
Devonian age, with limestones and shales of the same period in places. The core of the mountain is exposed across the centre, where
Paleozoic rocks, mainly
Cambrian and
Silurian slates and Takhstasang gneiss outcrops are visible in the northwest and northeast, the latter extending to western
Arunachal Pradesh in India. In the
Mesozoic era the whole of the worn-down plateau was under sea. In this expansive shallow sea, which covered most of
Assam and Bhutan,
chalk deposits formed from seawater tides oscillating between land and sea levels. During subsequent periods, tertiary rocks were laid down. The
Paro metamorphic belt may be found overlying Chasilakha-Soraya gneiss in some places. Silurian metamorphics in other places suggest long denudation of the surface. This was the time of
Alpine mountain and large number of "active volcanoes" formation which act as backbone of the Himalayas and much of the movement in the
palaeozoic region was probably connected with it. The Chomolhari tourmaline granites of Bhutan, stretching westwards from the Paro Chu and adds much depth below the present surface, were formed during this period of uplift, fracture and subsidence.
Tectonics The Himalayan mountain range was formed from the convergence of the India Plate and the Eurasian Plate. This convergence is still ongoing today actively uplifting the Himalayan Mountain Range. Several models have been proposed to explain the current deformation and uplift of rocks in the region. One model, published in 2019, combines P and S wave tomography with a mantle viscosity model to produce one of the most accurate models of how the Himalaya Range is deforming today. The main discrepancy between this model and actual observed GPS values lies in the Eastern Himalaya. The model says that the rocks should be deforming to the NE and E however the primary direction of deformation is to the E and the SE. ==Climate==