10Be dating reveal a one-million-years records of landslide activities in the Central Western Andes

The western flank of the Central Andes shows a high concentration of giant landslides (Strasser and Schlunegger, 2005; Audin & Bechir 2006; Pinto et al., 2008; Matther et al., 2014; Crosta et al., 2014, Margirier et al., 2015; Zerathe et al., 2017; Delgado et al., 2020) related to specific characteristics such as a strong local relief (canyons, structural-flexures, etc.), strong and recurrent seismo-tectonic activities, and atypical climate combining long-term hyper-aridity and punctual extreme precipitation events. In this context, ongoing studies inventorying more than one-thousand giant paleo-landslides in this region underline their spatial clustering that is controlled by coupled conditioning factors including high topographical gradients and specific lithology (Delgado et al., 2020).

The purpose of this study is to constrain now the kinematics of landsliding and ultimately to get time-frequency law of the gravitational slope destabilizations of this Andean region. For this, we focus on the Locumba valley (south Peru) where more than 30 giant landslides are clustered and distributed in two main typologies (rockslide and rock-avalanche). We applied cosmogenic nuclide dating to 8 paleo-landslides, sampling 52 boulders. We used alternatively ¹⁰Be/quartz or ¹⁰Be/feldspar depending on the available lithology.

Our dataset opens an unprecedented opportunity for paleolandslides studies and reconstructions. Indeed, the exposure-ages obtained range from the Holocene to the Pleistocene, the oldest ages reaching one-million years. This new temporal-scale allows to address and discuss triggering processes in the context of seismo-tectonic activities and Quaternary climate changes. Exposure-ages distribution shows several time-frequency peaks suggesting that gravitational destabilizations are episodic phenomena with time recurrence on the range of ~100 ka. Additionally, our time-constraints indicate that most of the current landscapes along this Western Andean flank are older than one-million years. Especially, fluvial incision and valley deepening processes are currently very low as testified by relicts of landslide dams and associated lacustrine sediments of hundred’s thousand years old that are preserved along the main canyons and still not fully re-incised.