Active hazardous slow faults drive drainage evolution in the Atacama Desert

Ambrosio Vega-Ruiz, Pia Victor Victor, Ariane Binnie, Sara Pena-Castellnou, Klaus Reicherter, Luca Claude Malatesta , Jean Braun, Steven A. Binnie

The Atacama Desert of the Central Andean forearc is the driest non-polar region on Earth, where extremely low erosion rates preserve geomorphic markers over Myr timescales. Between 19°S and 20°S, subtle Quaternary east-vergent reverse faults deform the west-sloping landscape and disrupt drainage development. This setting provides a unique opportunity to investigate fault evolution, uplift thresholds for drainage reorganization, and the seismic significance of slow faulting in hyperarid landscapes. We investigated two faulted interfluves, Pampa de Tiliviche (~19.5°S) and Pampa de Soga (~19.75°S), using geomorphic mapping and cosmogenic ¹⁰Be dating of displaced alluvial surfaces, channels, and terraces. We constrained fault geometry, reconstructed fault-driven incision histories, and quantified uplift and slip rates. Faulting in interfluves started after ~2.6 and ~5 Ma, while warped terraces and riverbeds record accelerated faulting since ~330 and ~35 ka, respectively. Simplified fault geometries yield maximum long- and short-term uplift/slip rates of ~0.011 and ~0.3 mm/yr. Numerical landscape-evolution models incorporating measured rates reproduce observed geomorphic patterns and constrain the uplift rate-catchment area ratio governing drainage reorganization. Observations and modelling indicate that tectonic uplift outpaces fluvial incision in hyperarid landscapes when drainage networks with small catchment areas cannot restore channel-bed slope modified by slow uplift. Regional consistency in fault geometry, distribution, and associated microseismicity suggests these counter-slope faults form an active forearc system capable of generating M6.1–6.8 earthquakes. Our results show that extremely slow faulting can substantially reorganize drainages in hyperarid landscapes and that Quaternary faults commonly considered inactive may represent an underestimated seismic hazard.