In this section we review several studies that have addressed the geomorphic evolution of the syntaxial region in the Pleistocene in the wider evolution of the Himalaya–Tibet region, showing evidence for surficial erosive processes that likely enhanced the Pleistocene exhumation of the domal pop-up.
Along the southern margin of the Tibetan plateau, Montgomery et al. (2004) and Korup and Montgomery (2008) observed evidence for repeated Holocene and Pleistocene glacial damming of the Yarlung Tsangpo and its tributaries Yigong Tsangpo and Parlung Tsangpo upstream of the eastern syntaxis. According to Korup and Montgomery (2008) the equilibrium line altitude of glaciers in the region (ELA, where annual accumulation equals annual ablation) was depressed enough with respect to present-day during both the
Pleistocene and Holocene glacial maxima to allow the development of moraine dams capable of blocking the three river courses in the area. Upstream of the glacial dams, river incision into bedrock was dramatically impeded by impoundment during glacial occupancy, with further upstream interglacial aggradation, infill of meltwater lakes with sediment and burial of
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valley floors under glacio-fluvial sediments long after the dams were breached and the glacier melted off. Conversely, the river below the dam continued to incise bedrock efficiently, thus contributing to maintenance of a steep river profile below the existing knickzone. Repeated glacier-controlled damming upstream and fluvial bedrock incision downstream may have retarded or prevented the migration of the knickpoints upstream.
The occurrence of hundreds of breached dams upstream of the Yarlung Tsangpo Gorge suggests outburst floods as a frequently-occurring process in the area, with extreme events as those reported by Lang et al. (2013). These floods, especially when extreme, would result in lateral river erosion that in turn would trigger landslide erosion of threshold
hillslopes (Larsen and Montgomery, 2012). The spatial association between highest landslide erosion rates and maxima in unit stream power occurring on the Yarlung Tsangpo knickzone and its tributaries Po Tsangpo and Parlung suggests that river incision drives erosion on threshold hillslopes by both the vertical incision mechanism and lateral erosion mechanism (driving landslide erosion of threshold hillsopes) in the eastern syntaxial region (Larsen and Montgomery, 2012).
In the north-western Himalaya Brozović et al. (1997) found that the distribution of slope minima over the highly elevated area extending from northern Karakoram to the western syntaxis follows trends in the altitude of the snowline, taken as a proxy for regional ELA. Overall, mean regional elevation, hypsometry and slope distributions appear to be controlled by the extent of glaciation regardless of rock uplift rates as estimated from thermochronometry. Any (tectonically or isostatically induced) surface uplift will tend to cause an increase in the glacier-covered area that in turn will increase erosion rates, thus establishing a negative feedback mechanism returning the landscape towards its previous hypsometry. The high peaks of the region (e.g., Nanga Parbat, 8125 m) represent remnants of topography that surface processes were not able to remove. This mechanism of glacial
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orogenic topography, has been suggested for other glaciated and uplifting orogens of the planet (the St. Elias Orogen, Spotila et al., 2004; the Cascade Range, Mitchell and
Montgomery, 2006; the Andes, Montgomery et al., 2001). Its exceptional erosional efficiency has been numerically modelled and confirmed at the global scale by Egholm et al. (2009).
Another climate-dependent factor that has been suggested to promote focused
denudation at the southern edge of the Tibetan Plateau is orographic precipitation. As such, it was included in the thermo-mechanical model of Beaumont et al. (2001) in which erosion is the dynamic link between channel flow in the middle to lower crust and ductile extrusion of the high grade metamorphic rocks of the GH. Spatial coincidence between focused rapid exhumation of the steep southern Himalayan front since at least the Pliocene (as indicated by young mineral cooling ages) and orographic precipitation has been observed in northwest India along a ~120 km wide NE-SW Himalayan transect spanning the Sutlej region (Thiede et al., 2004; 2005). The monsoonal precipitation controlled by topography is here interpreted as exerting a strong control on erosional processes (landsliding, debris-flow activity, mass removal by streams), whilst fluvial erosion unloading appears to be focused on high
mountainous areas. Furthermore, a direct link between monsoon-driven sustained erosion at the foot of the GH and Pliocene-Holocene sustained out-of-sequence thrusting has been suggested in Nepal south of the MCT (Wobus et al., 2003; Hodges et al., 2004). Other studies (e.g., Burbank et al., 2003; Adlakha et al., 2013; Godard et al., 2014) based on fission-track and cosmogenic radionuclide dating challenge this coupling between spatial gradients in precipitation and variations in long-term denudation in the Himalayas.
In the modern syntaxes the water budget is dominated by moisture influxes by the Westerlies (western syntaxis) and the East Asian Monsoon (eastern syntaxis), whilst in the Central Himalaya precipitation is dominated by the Indian summer monsoon (Burbank et al. 2012). Above ~4 km altitude across the Himalaya, 40% of the annual precipitation arrives as snowfall, with snow accumulation steadily increasing from ~ 2 km to 4.5–5 km altitude,
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before levelling off and then dramatically decreasing above ~ 6 km. Hence, whilst the Central Himalaya receives > 70% of its annual precipitation during the Indian summer monsoon, in the highly elevated syntaxial regions, snowfall, and subsequent melting in the Himalayan rain shadow makes a far larger contribution to annual discharge (65 and 35 % in the western and eastern syntaxis, respectively) than in Central Himalaya.
The mean ELA position in northwest Himalaya during the Quaternary has been estimated to be ~ 400 to 500 m lower than at present (and up to 1000 m lower during the Last Glacial Maximum, ca. 20k yrs. ago, Brozović et al., 1997). Similarly, if modern glaciers in the eastern syntaxial region are advanced to the position of tens of large moraines that in the Holocene blocked or constricted the Yarlung Tsangpo and its tributaries, the resulting ELA is depressed by 470 ± 170 m (1s; Korup and Montgomery, 2008). It is estimated that such a glacial advance impounded between 30% and 55% of the length of the Yigong and Parlung Tsangpo during the Holocene, and commensurately higher for the greater ELA depression (inferred to be ~ 1km) during earlier Pleistocene advances (Korup and Montgomery, 2008, and references therein). This supports the inference that surface processes operating in the eastern syntaxial region during the Pleistocene–Holocene were influenced by climatic conditions, with glacial processes dominating over and directly influencing fluvial and hillslope erosive processes.
In summary we argue that glacial erosion, supported by frequent mass wasting in landslides, with detritus carried downstream by a major river with a steep gradient, likely enhanced focused erosion in the eastern syntaxial region to keep pace with the rapid rock uplift caused fundamentally by buckling of the crust, the surface expression of which is the syntaxial antiform with its ~20 km amplitude domal pop-up centred on the Namche Barwa– Gyala Peri massif.
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As reported by Wang et al. (2014), ponding of sediment upstream (west) of the syntaxial dome began prior to 2.0–2.5 Myr ago and was caused by the rise of the bedrock floor of the ancestral canyon on the west flank of the uplifting syntaxial (pop-up) dome. The resulting marked change of the pre-existing river gradient on the west side of the dome appears to have created a ‘paleo-knickpoint’ at the eastern termination of the ponded sediment. This ‘paleo-knickpoint’ may have migrated along the river depending upon the pace of uplift and erosion but it is possible that it was relatively near the position of the current knickpoint, as both are logically on the west upstream flank of the domal uplift.