Postglacial denudation of western Tibetan Plateau margin outpaced by long-term exhumation

Abstract

The Indus River, one of Asia's premier rivers , drains the western Tibetan Plateau and the Nanga Parbat syntaxis. These two areas juxtapose some of the lowest and highest topographic relief and commensurate denudation rates in the Himalaya-Tibet orogen, respectively, yet the spatial pattern of denudation rates upstream of the syntaxis remains largely unclear, as does the way in which major rivers drive headward incision into the Tibetan Plateau. We report a new inventory of 10 Be-based basinwide denudation rates from 33 tributaries fl anking the Indus River along a 320 km reach across the western Tibetan Plateau margin. We fi nd that denudation rates of up to 110 mm k.y. -1 in the Ladakh and Zanskar Ranges systematically decrease eastward to 10 mm k.y. -1 toward the Tibetan Plateau. Independent results from bulk petrographic and heavy mineral analyses support this denudation gradient. Assuming that incision along the Indus exerts the base-level control on tributary denudation rates, our data show a systematic eastward decrease of landscape downwearing, reaching its minimum on the Tibetan Plateau. In contrast, denudation rates increase rapidly 150-200 km downstream of a distinct knickpoint that marks the Tibetan Plateau margin in the Indus River longitudinal profi le. We infer that any vigorous headward incision and any accompanying erosional waves into the interior of the plateau mostly concerned reaches well below this plateau margin. Moreover, reported long-term (>10 6 yr) exhumation rates from low-temperature chronometry of 0.1-0.75 mm yr -1 consistently exceed our 10 Be-derived denudation rates. With averaging time scales of 10 3 -10 4 yr for our denudation data, we report postglacial rates of downwearing in a tectonically idle landscape. To counterbalance this apparent mismatch, denudation rates must have been higher in the Quaternary during glacial-interglacial intervals.

Spatial variation in exhumation rates across Ladakh and the Karakoram: New apatite fission track data from the Eastern Karakoram, NW India

“…Munack et al . [] found that 10 Be data from the Indus valley, along the southern margin of the Ladakh Range, spanning postglacial timescales of 10 3 –10 4 years, indicate denudation rates of only 0.01–0.11 mm yr −1 . As these rates are lower than those derived from low‐temperature thermochronological data spanning greater timescales of >10 6 years (approximately 0.1–0.4 mm yr −1 ) [ Clift et al ., ; Kirstein et al ., , ; Kirstein , ; Kumar et al ., ; Sinclair and Jaffey , , this study], the authors concluded that glacial processes during previous glacial cycles must have made a significant contribution to erosional denudation and rock exhumation and cooling.…”

Recycling of Pleistocene valley fills dominates 135 ka of sediment flux, upper Indus River

“…1, Table 1) fit the picture of very slow landscape lowering in the upper Indus basin (Dortch et al, 2011;Munack et al, 2014 (Fig. 1), which we expected to denude faster following river capture ~135 ka ago.…”

“…Denudation rates along the Indus drop from >1000 mm ka -1 near the syntaxis (Burbank et al, 1996) to ~10 mm ka -1 at the western Tibetan Plateau margin . Denudation rates estimated from cosmogenic 10 Be inventories are low despite the steep alpine topography (Dortch et al, 2011;Dietsch et al, 2014;Munack et al, 2014), so that the Transhimalayan ranges of Ladakh and Zanskar host some of the oldest glacial landforms in the Himalaya-Tibet orogen Hedrick et al, 2011). Massive staircases of river-derived fill terraces (Fort et al, 1989;Clift and Giosan, 2014) located up to 400 m above present river levels, together with stacks of lake sediments and local landslide and fan deposits (Hewitt, 2002;Phartiyal et al, 2005; 2013), testify to major alternating cut-and-fill cycles in the upper Indus catchment (Blöthe et al, 2014).…”

Testing monsoonal controls on bedrock river incision in the Himalaya and Eastern Tibet with a stochastic‐threshold stream power model

“…We note that several other 10 Be data sets from the Himalaya were published after we had assembled the data set used in this study. They cover study areas in Ladakh [Munack et al, 2014;Dietsch et al, 2015], Bhutan [Portenga et al, 2015;Le Roux-Mallouf et al, 2015;Adams et al, 2016], Garhwal [Morrell et al, 2015], eastern Nepal [Olen et al, 2015], Sikkim [Abrahami et al, 2016], and Himashal Pradesh [Olen et al, 2016]. The data from Ladakh would be difficult to include in our analysis due to the lack of discharge data from this arid region.…”