The Sikkim-Darjeeling Himalaya: Landforms, Evolutionary History and Present-Day Processes

Abstract

The Sikkim-Darjeeling Himalaya contains the stretch of tectonically active Eastern Himalaya. It is composed of three main tectonic units: Higher Himalaya, Lower Himalaya and Siwaliks separated by thrusts, but joined by great fluvial system of the Tista River. The Higher Himalaya with relief up to 2,000–4,000 m was uplifted by about 2,000 m in the Quaternary rising above the snowline. Its mountain massifs previously had fluvial relief which later had been totally transformed by glacial and cryonival processes. The Lower Himalaya dissected 1,000–2,000 m locally with remains of mature relief fragments are continuously in the forest belt. The Siwaliks in this part of the Himalayan Range are reduced to a narrow belt, which blende with Lower Himalaya. Along the Frontal Fault it rises above 1,000 m directly over the alluvial plains of the Sub-Himalayan foredeep, which is still active and split into blocks of various tectonic tendency. Steep edge of the Himalaya is exposed to high monsoon rainfalls and is afflicted by heavy downpours and continuous rains. Following deforestation this marginal part and also the southern Sikkim area have been affected by large-scale mass movement processes and floods. Only the Tista and several large rivers are transporting heavy sediment loads down to the Brahmaputra River. The smaller streams dissecting the mountain edge form a belt composed of fans.