FOCUSED EROSION ALONG THE SUTLEJ RIVER, NW INDIAN HIMALAYA – WHY NOT A TECTONIC ANEURYSM?

The upper limits of vertical rock motions and localized mountain-building that may be caused by erosion are poorly understood. In the tectonic aneurysm model, focusing of erosion by river and/or glacial processes may generate Earth’s most extreme vertical rock motions. This is evidenced by close spatial association of huge focused-erosional systems with mountain massifs that host Earth’s most rapid crustal-scale rock exhumation, deepest gorges, and youngest exposed intrusions of crustal-melt rocks. On the other hand, the leading examples of such systems overlie sharp bends between adjoining convergent plate boundary segments, such that indentation by the folded down-going plate could force dramatic local uplift and exhumation in the overlying crust. The region where the Sutlej River crosses the High Himalayan topographic front seemingly offers an opportunity to resolve this question, because the river here has erosive power comparable to the syntaxial rivers and yet there is no tectonic aneurysm, but rather a ~10 km-amplitude river anticline. However, our extensive low temperature thermochronometric exploration (AHe, OSL feldspar) here supports an alternative explanation: the Sutlej River’s erosive power at ~>150-200 kW m-1 has only existed for less than a million years. Our new AHe dataset (n= 349) and inverse modeling thereof show an accelerated cooling (from 60-100°C to present-day temperature) at <1 Ma related to a significant exhumation episode along the Sutlej River. This was probably triggered by an increase in water discharge after the capture of a northeastern Zhada basin by the Sutlej River at ~0.8 Ma. The OSL feldspar thermochronometry of eight samples reveals a surprising result: detectable cooling from temperatures of ~80 – 65 °C since ~0.015 – 0.010 Myr. To our knowledge, this cooling can only be temporally correlated to the end of the Last Glacial Maximum, and thus likely reflects accelerated fluvial erosion in response to warming. In summary, the Sutlej River’s unusually large erosive power has been extant for less than 1 million years, explaining the lack of a tectonic aneurysm here. Unfortunately, this leaves open the question of erosive vs. tectonic control for Earth’s most dramatic localized uplift and exhumation systems.