ALLUVIAL FAN FORMATION IN COLD AND WARM DESERTS: CASE STUDIES FROM MONGOLIA AND CHILE (Invited Presentation)

Alluvial fan (AF) formation is sensitive to environmental change. Thus, AFs provide essential archives for reconstructing Quaternary paleoenvironmental conditions, particularly climate, hydrology, and tectonics. Their formation occurs in different environments characterized by distinct relationships between sediment production and transport capacity. However, mesoscale to macroscale AF formation (radius <a few km to >20 km) differs between cold deserts and warm deserts. In Mongolia, AFs and bajadas are among the most common fluvial systems and landform features and are often associated with steep mountain fronts, closely related to and often modified by active tectonic uplift and faulting. Autocyclicity plays an important role in the development of these AF systems, which adapt to changing sediment sources and deposition patterns, in concert with changing channel slopes. Pleistocene periglacial sediment production (frost weathering) appears to be the most important factor in AF formation as it provides an abundant source of sediment during periods of climatic instability, in turn demonstrating the importance of climatic and weathering cycles. The development of AFs can be linked to climate variability on glacial-interglacial timescales or expressed in late Quaternary pulses of increased humidity, weathering, and sediment flux. In northern Chile, coastal AFs are dominated by debris-flows and hyperconcentrated flows deposits. Their formation can be mainly associated with extreme weather events during the late Pleistocene. AFs located in the central and hyperarid Coastal Cordillera of the Atacama Desert are sheet-flow dominated and mainly associated with semi-arid conditions during the mid-Pleistocene. Due to prolonged aridity since the Miocene, these AF systems did not experience such autocylicity as evident for AFs in cold desert environments. In addition, large-scale AFs (radius >20 km) in the mountain front of the Andes can be associated with Cenozoic tectonics and active uplift. The latter are usually the oldest AFs, dating back to the Miocene and Pliocene and are mostly inactive.