Impact of Climate and Tectonics on the Morphodynamics of Alluvial Piedmonts, Implications for Sediment Transfer and the Stratigraphic Record

Abstract

Alluvial piedmonts encircle most of the Earth mountain ranges. The erosion product of these mountains must cross the piedmont domain before reaching the basins where they can enter the sedimentary record. The flux of sediment transfers environmental signals, e.g. tectonics and climate, from source to sink, their preservation is critical for the sedimentary record. Alluvial piedmonts are very reactive to external and internal forcing often incise by tens to hundreds of vertical meters over a few thousand years only to subsequently aggrade by roughly the same amount. In my thesis, I set to study the morphodynamics of alluvial piedmonts in two areas, Death Valley, California, and the Eastern Tian Shan. In Death Valley I show that cycles of aggradation and incision repeatedly bury active fault scarps and that leads to the accumulation of tectonic slip until the next incision episode when a tall waterfall is released in the catchment. This process links the release of tectonic offset to climatic periods and it also accounts for many unexplained 10 to 30 m waterfalls in the Death Valley area. In the Eastern Tian Shan, I establish that the northern alluvial piedmont, that is incised by 100 to 300 m in the Holocene, undergoes repeated cycles of aggradation and incision driven by fluctuations in Westerlies moisture and insolation at a 21 kyr period without the need for monsoonal moisture to drive the system. The varying extent of glacial overprint in the Eastern Tian Shan is responsible for the morphological contrast between the incised north and aggraded south piedmonts. I produce more terrace abandonment ages that show that all rivers of the northern piedmont incised since the last deglaciation but did so out of sync because of the geometry of the upstream glaciated valleys, and internal feedbacks with alluvial valley walls during river entrenchment. With new luminescence ages sampled in the stratigraphy I establish that sediments from the last 0.5 Myr are remobilized in the modern sediment flux by the alluvial incision. I develop a numerical tool that predicts locally the probability of sediment mixing based on the essential length- and timescales of the individual rivers of an alluvial piedmont.