New data on the formation of the enigmatic zebra stripes in the Atacama Desert – towards unravelling key mechanisms and time scales of geomorphic processes under extreme hyperaridity

The most enigmatic hillslope forms in the hyperarid core of the Atacama Desert are the so-called zebra stripes, covering slopes between the coastal range and the pre-Andean cordilleras. They are defined as contour-parallel, lateral bands of loose angular gravels resting on hillslopes between 4° and 30°. While a previous study argued that they represent fossil evidence of overland flow, recent investigations documented a remarkable form-concordance with deposits from experiments on segregation-induced granular fingering, suggesting that dry granular flow may play a role in their formation. Here, we add new data on key mechanisms and time scales of geomorphic processes related to zebra stripe formation based on UAV-derived aerial photos, geomorphological and sedimentological investigations, different monitoring techniques, as well as terrestrial cosmogenic nuclide concentration measurements and OSL rock surface dating. OSL results show long exposure, no overturning, and inactivity at the northern, and Pleistocene/Holocene activity and clast overturning at the southern study site. 10Be and 21Ne measurements at this southern active site suggest downslope increasing TCN concentrations and clast transport on Pleistocene time scales and faster transport rates on steep slopes. While no stripe activity was detected during a Mw 5.4 earthquake, the monitoring results document clast overturning by freeze-thaw processes during a period of snow and frost in July 2019. This suggests a significant role of freeze-thaw processes in zebra stripe formation due to selective, long-term and subtle clast overturning and the evolution of the typical sorting pattern, potentially allowing for using zebra stripes as an indicator of paleo-climatic conditions.