Late-Quaternary geomorphic scenario due to changing depositional regimes in the Tangtse Valley, Trans-Himalaya, NW India

Highlights

• Landscape evolution in last 48 ka in Tangtse valley, Ladakh

• Fluvial depositional environment at 48 ka and ~ 30–21 ka-period of aridity

• Lacustrine environment between ~ 9.6 and 5.1 ka indicative of wet and warm conditions

• Incision depth of ~ 130 m between 22 ± 4 ka and ~ 9.6 ka

• A lake with ~ > 50 km extent, sixth basin of the Pangong Tso

Abstract

Records from the Tangtse Valley in the Trans-Himalaya reveal depositional history since 48 ka, with fluvial aggradation followed by incision, lacustrine sediment fill, and later incision. Varied sedimentary architecture with fluvial episodes intervened by lacustrine pulses, flood events, colluvial and glacial activity are preserved. The valley is located west of the Pangong Tso/Bangong Co, one of the largest lakes in Tibet which has served as a spillway, flooding and damming the entire Tangtse Valley, resulting in the formation of a lake. Today Pangong Tso consists of five basins separated by shallow sills and is fed by snow melt. Documentation based on ¹⁴C and OSL chronologies of the sediment sections throughout the valley reveals evidence of a sixth basin of Pangong Tso toward west, occupying the present day Tangtse Valley between 9.6–5.1 ka. This event coincides with periods of high lake levels in Tibet, China as well as intensified monsoon periods over the Indian subcontinent. A fluvial regime around 48 ka and 30–21 ka with comparatively arid conditions and dry phases interspersed by flooding is documented. The valley has been incised to depths of 40–50 m in the upper part and to 130 m in the lower part. The incision rate ranges from 0.3 to 1.2 mm yr^− 1 in the upper part and reaches as high as 10.8 mm yr^− 1 in the lower valley. Much of the incision took place between 22 and 9.6 ka although repeated sediment fill-incision cycles in the valley from 30–22 ka, 22–9.6 ka, 9.6–5.1 ka, 5.1 ka, and even to present time were observed.

Introduction

The southwestern part of the Tibetan plateau is a cold, arid and high-altitude desert, much like its central counterpart. However, unlike the plateau area, mountainous river valleys are seen, occupied by the Indus River and its tributaries Shyok, Tangtse and Nubra, flowing along sutures/faults viz. the Indus Suture Zone (ISZ), the Shyok Suture Zone (SSZ) and the Karakorum Fault (KF). In general, the regional and global climate has been profoundly influenced by the uplift of Himalaya and Tibet (Wake and Mayewski, 1996) which in turn is responsible for shaping the geomorphology of this region. The Ladakh region in Trans-Himalaya is situated on the southwestern edge of the Tibetan Plateau and provides an ideal research site to study geomorphic and causative climatic, lithologic, and tectonic changes. It being a high-altitude and a cold desert, enormous amount of unconsolidated sediment (generated due to frost action/physical weathering) lies loose on the high-angled slopes and the valley floor.

The Westerlies are the dominant source of precipitation, in the form of snow, to the region in winter months (October–April). A secondary source of precipitation is the summer monsoon (July–September) from the south (Mayewski et al., 1980, Goudie et al., 1984). On one hand, in cases of abrupt precipitation the loose sediment over the slopes slides to the river bed and blocks it, which forms water ponds behind this debris, with the debris serving as a dam. On the other hand, due to the proximity of the region to the KF, tectonic changes, violent seismic shocks and active deformation develop as a result of continued post-collision convergence of the Indian plate and Asian landmasses (Molnar et al., 1987, Valdiya, 1988). The KF shows slip rates of ~ 30 mm yr^− 1 on the basis of Holocene offsets of geomorphic features (Avouac and Tapponnier, 1993, Matte et al., 1996) and the current rates estimated from the GPS measurements are 3.4 ± 5 mm yr^− 1 (Jade et al., 2004). The aggradation phases and incision rates in these valleys vary widely. For the Indus valley catchment (along the ISZ), which today is less tectonically active than KF, incision rates are of 2.5 mm yr^− 1 (fill) and 0.02–1 mm yr^− 1 (Dortch et al., 2013, Blöthe et al., 2014). The Trans-Himalayan and Karakoram relief indicate active orogeny during the Quaternary (Gansser, 1964, Gansser, 1983, Gee, 1989), which continues to date (Phartiyal and Sharma, 2009, Hintersberger et al., 2010, Hintersberger et al., 2011, Hewitt, 2009, Hewitt, 2011, Korup et al., 2010). Active tectonics result in the generation of enormous amounts of debris generated by reactivation of thrusts and faults leading to slope failure and drainage blocking, and the subsequent formation of lakes and landslide dams. There are several records of such climatically and tectonically formed lakes from the Indus catchment (Burgisser et al., 1982, Cronin, 1982, Shroder, 1993, Owen, 1988, Cronin, 1989, Fort et al., 1989, Kotlia et al., 1997a, Kotlia et al., 1997b, Kotlia et al., 1998, Phartiyal et al., 2005, Phartiyal and Sharma, 2009, Sangode et al., 2011, Phartiyal et al., 2013, Blöthe et al., 2014, Nag and Phartiyal, 2014). These lakes may drain or breach, leaving behind a sedimentary record of exposed lacustrine and associated fluvial and colluvial sediments distributed along the river valleys that can be analyzed to reveal palaeoclimate, tectonics and earth surface processes.

In the Trans-Himalaya range, east of the Tangtse River Valley, the present day lake, Pangong Tso (Tso = lake), has high, distinct palaeo strand lines and provides past information about the wider extent of the lake area. This lake lies between India and China and occupies a long submerged valley of ~ 145 km. Today the lake is a chain of five basins separated by shallow sills and evolves as a series of lakes with connecting rivers (Hutchinson, 1937). The river have been dammed to the west by a ridge either formed by tectonic activity associated with the right lateral strike-slip Karakoram fault (KF) (Huang et al., 1989) or by the Last Glacial Maximum moraine deposit (Norin, 1946), a flood event draining the Pangong Tso waters through the Tangtse Valley to Shyok River reported at 11 ka (Dortch et al., 2011). The distribution of lacustrine facies and the associated fluvial deposits of this important Tangtse Valley lying west of the Pangong Tso have been studied in order to reconstruct geomorphological evolution and depositional regimes of the area over the last 48 ka.