Unravelling the preservation of fault scarps in the Atacama Fault System, northern Chile: Insights from cosmogenic nuclides and topographic analysis

Fault scarps are commonly used as temporary markers in palaeoseismological analysis, with their preservation controlled by the balance between slip and erosion rates. The northern Chile forearc, located in the core of the Atacama Desert, represents one of the driest settings on Earth, where mean annual rainfall is near zero and erosion rates are exceptionally low. Here, we investigate an anomalous case in the Atacama Fault System (AFS), northern Chile, where free faces have persisted for tens of thousands of years. To examine this balance, we focus on three branches of the AFS (Naguayán, Cerro Fortuna, and Salar del Carmen Faults), combining geomorphological mapping, ¹⁰Be-derived erosion rates, and sedimentological evidence. Our results indicate that the free faces have remained preserved for more than 14 ka despite a slow fault slip rate of ∼0.07 mm yr⁻¹ and have retreated at an average rate of only ∼0.03 m ka⁻¹. Scarp degradation is predominantly controlled by slow, diffusive processes, with the diffusion coefficient for the Salar del Carmen fault scarp calculated as k = 0.063 ± 0.018 m² ka⁻¹. Localized channel incision across fault scarps has been triggered by sporadic high-intensity rainfall events, as evidenced by low catchment-mean erosion rates (∼0.5–2.9 m Ma⁻¹) estimated from ¹⁰Be concentrations. The remarkable preservation of free faces is attributed to two main factors: extensive gypsic soil development and the prevalence of long-term hyperaridity. Minimal but significant moisture input from coastal fog promotes gypsic soil formation, enhancing moisture retention and driving gypsum cementation that stabilizes alluvial surfaces and prevents free face collapse. Sporadic rain events retain erosion rates in active catchments that are lower than the fault slip rate, allowing for scarp preservation. We propose a conceptual model in which the combined effects of (i) a hyperarid climate, (ii) sporadic but geomorphically effective hydrologic events, (iii) low tectonic slip rates, and (iv) salt-induced cementation act together to inhibit scarp degradation. These findings challenge conventional degradation and underscore the importance of evaluating surface processes in palaeoseismological interpretations, improving our understanding of fault activity, landscape evolution, and seismic hazard in hyperarid regions.