River evolution in the Atacama Desert: comparisons of cosmogenic nuclide and molecular clock dating approaches

The Atacama Desert in northern Chile is considered to be the driest, oldest desert on Earth. None-the-less, even the hyperarid core of this desert is host to several deeply incised, fluvially active river canyons. The history of these drainages is tied to the tectonic and climatic evolution of the region, however, the timing and hence the mechanisms driving canyon formation are poorly constrained. We have used cosmogenic 10Be exposure ages to define the Quaternary incision history for the southern tributary of the Tiliviche River, which exits to the Pacific approximately 19.5°S. Ages from flights of fluvial fill-cut terraces at two separate locations are consistent and suggest this branch of the river formed shortly after 2 Myr ago by relatively slow fluvial incision. This slow incision persisted until around 300-400 kyr ago, whereupon rates of downcutting increased by around an order of magnitude. In this contribution, we discuss how the timing of the Tiliviche River incision rate increase may relate to tectonic mechanisms, both local faulting and with regards to proposed increases in regional scale crustal uplift. We include exposure ages from several proximal sites and suggest linkages between these events and the Tiliviche River evolution. Lastly, our project aims to compare the results from cosmogenic nuclide dating with fluvial histories derived from the timing of amphipod speciation. Our initial results show that there is much potential in combining cosmogenic nuclide approaches with molecular clock dating and implies river evolution in this area is controlled by a common, regional-scale process.