Seismic origin of the Atacama Desert boulder fields A. Matmon a, ⁎, J. Quade b , C. Placzek c , D. Fink d , ASTER Team e , M. Arnold, G. Aumaître, D. Bourlès, K. Keddadouche, A. Copeland b , J.W. Neilson b a Institute of Earth Sciences, Hebrew University, Jerusalem, 91904, Israel b Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c Centre for Tropical Environmental and Sustainability Science (TESS) and School of Earth and Environmental Sciences, James Cook University, Townsville, Australia d Australian Nuclear Science and Technology Organization, PMB1, Menai, NSW 2234, Australia e CEREGE, UMR 6635 CNRS-Aix-Marseille University, BP 80, 13 545 Aix en Provence Cedex 4, France abstract article info Article history: Received 11 June 2014 Received in revised form 10 November 2014 Accepted 14 November 2014 Available online 24 November 2014 Keywords: Boulder fields Atacama Desert Seismicity Cosmogenic isotopes Extensive fields of large boulders are common around the base of hills in the Atacama Desert. How these boulders are transported from nearby hillslopes is unclear given the lack of rainfall of the region. Here we document the central role of seismicity, not runoff, in transporting and smoothing N 1 ton boulders all across the hyperarid core of the Atacama Desert. The generally granitoid boulders emerge as corestones on hillslopes at an erosion rate of 0.1–1 m Ma -1 . Thereafter, physical and cosmogenic isotopic evidence suggests that boulders slide and bounce rather than roll down hills and onto adjacent flats. In the transport process, the largest boulders are split and the smaller ones are weathered to grus, narrowing average boulder mass to ~2 tons (b 1m 3 ). At the base of hills, the boulders bunch together and rub during the frequent earthquakes in the region, producing distinctive smoothing around boulder mid-sections, and silt moats around the boulder bases. Our measurements show a strong correlation between boulder field density and rubbing, and only when the density exceeds 60–70% does rubbing become common. Except for slow removal by rubbing, the boulders seem to undergo no further erosion while in the flats. Exposure times for some boulders are N 12 Ma, making them among the oldest continuously exposed features on the Earth. Boulder rubbing is just one geologic feature among many in the Atacama that un- derscore the role that seismicity probably plays in shaping landscapes of the waterless worlds of the solar system. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Large boulder fields comprised of hundreds to thousands of boulders are common around the base of hills in the western Atacama Desert. Streams of large boulders originating from bedrock outcrops are observed along slopes. The boulders cluster at the base of the slopes to form dense fields in which boulders are tightly packed and in many cases touch each other. Some Atacama boulder fields are located in broad valleys which provide space for some boulders to be dispersed across the flats. Other fields are located in valleys within the mountain range; such valleys are often entirely covered with boulders. The western Atacama Desert boulders fields are a unique geomor- phic feature (Quade et al., 2012). Their origin, the way they develop over time, and the geomorphic processes that operate on them are not fully understood. Although conspicuous across the Atacama Desert, these ancient features have not been a primary research target aimed to understand the evolution of boulder fields. No systematic surface- exposure dating of boulder fields has been carried out. Previous surface-exposure dating studies in the Atacama Desert focused on bedrock, sediment, and clasts on alluvial fans as primary targets (e.g. Dunai et al., 2005; Kober et al., 2007; Nishiizumi et al., 2005; Owen et al., 2013). Cosmogenic isotope concentrations that were measured in samples collected from boulders in the framework of these studies were always very high. For example, a multi-cosmogenic nuclide study (Placzek et al., 2010) showed that concentrations of 10 Be and 26 Al in bedrock and sediments in the Atacama, although high and suggestive of very slow erosion rates, are significantly lower than the concentrations of these nuclides in boulders. The extremely high cosmogenic nuclide concentrations, which reach saturation in some boulders, suggest that they are not significantly affected by weathering, erosion, and transport processes that operate presently on most exposed bedrock and sediment in the Atacama Desert. The pronounced contrast between the active surface processes that shape the Atacama landscape (Amundson et al., 2012; Evenstar et al., 2009; Ewing et al., 2006; Placzek et al., 2010; Wang et al., 2014) and the extreme stability of the boulders suggests that boulder fields formed and have been developing differently from the rest of the landscape. Quade et al. (2012) observed shaking and rubbing boulders during an earthquake and suggested the association between the evolution of the Atacama boulder fields and seismic activity. Such a suggestion seems reasonable when considering the proximity of the Atacama Geomorphology 231 (2015) 28–39 ⁎ Corresponding author. Tel.: +972 2 6586703; fax: +972 2 5662581. E-mail address: arimatmon@mail.huji.ac.il (A. Matmon). http://dx.doi.org/10.1016/j.geomorph.2014.11.008 0169-555X/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph