Zebra stripes in the Atacama Desert revisited – Granular fingering as a mechanism for zebra stripe formation?

Geomorphology 344 (2019) 46–59

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Zebra stripes in the Atacama Desert revisited – Granular fingering as a mechanism for zebra stripe formation? Simon Matthias May a,⁎, Dirk Hoffmeister a, Dennis Wolf a, Olaf Bubenzer b a b

University of Cologne, Institute of Geography, 50923 Cologne, Germany Heidelberg University, Institute of Geography, Heidelberg Centre for the Environment, 69120 Heidelberg, Germany

a r t i c l e

i n f o

Article history: Received 7 May 2019 Received in revised form 24 July 2019 Accepted 24 July 2019 Available online 26 July 2019 Keywords: Zebra stripes Atacama Desert Granular fingering Seismic shaking

a b s t r a c t Although a number of studies have pointed out the remarkable slowness of Earth surface processes in the Atacama Desert, process mechanisms under such extremely limited water availability are poorly understood, and process rates remain unknown. This paper revisits the discussion on the formation of the prominent Atacama-specific hillslope zebra (stone) stripes, previously interpreted to result from palaeo-overland flow (Owen et al., 2013). Compared to this study, our data document different stripe characteristics with regard to stripe form and orientation as well as sorting- and bedding-patterns of stripe-confining surface gravel units. We found a remarkable form-concordance between zebra stripes and deposits from experiments on segregation-induced granular fingering. Hence, we propose a combination of seismic shaking and instantaneous dry granular free surface flows as the key mechanism for zebra stripe formation. Our findings underline the potential significance of seismicity in shaping Atacama landscapes, which bear important analogies to extraterrestrial surfaces. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The hyperarid core of the Atacama Desert, northern Chile, stretches between 22° and 25°S and is among the driest places on Earth. While this hyperaridity is thought to have established not later than midMiocene times (Dunai et al., 2005; Rech et al., 2010), even the driest parts of the Atacama exhibit erosion rates comparable to other deserts (0.2–0.4 m/Myr), and cosmogenic nuclide concentrations seem to indicate some limited Pleistocene modification of the landscape (Placzek et al., 2010). Overland flow or flash floods following rare precipitation events occur episodically (Wilcox et al., 2016), but geomorphic activity due to salt-driven shrink-swell processes, dust deposition, or seismic shaking has significantly contributed to the formation of the characteristic landscape (Abele, 1990; Ewing et al., 2006; Quade et al., 2012). The central Atacama thus “provides a glimpse into processes that operate in a relatively waterless world” (Quade et al., 2012: 854), including extraterrestrial surfaces such as on the Moon or Mars. Perhaps the most enigmatic hillslope forms in the Atacama are the so-called zebra (stone) stripes, covering slopes in the central desert between the coastal range and the pre-Andean cordilleras (Fig. 1). First recognized in the early 20th century (Mortensen, 1927), the study of Owen et al. (2013) hitherto represents the only detailed investigation on zebra stripes. Zebra stripes are defined as contour-parallel, thin ⁎ Corresponding author. E-mail address: [email protected] (S.M. May).

https://doi.org/10.1016/j.geomorph.2019.07.014 0169-555X/© 2019 Elsevier B.V. All rights reserved.

lateral bands of rather angular gravels resting loosely on hillslopes between 4° and 25°, underlain by fine-grained gypsum-rich soil. They are characterised by grain sorting and specific wavelengths (i.e. stripe spacing), and seem to be unique to the Atacama Desert. A regional analyses on the relationship between bedrock type and zebra stripe occurrence based on 1:100.000 geologic maps by Owen et al. (2013) suggests that, among a number of lithologies, particularly the presence of schist, gabbro, and certain monzogranites favours zebra stripe occurrence, whereas other monzogranites and especially diorites showed lowest zebra stripe coverages. However, these authors suggested that zebra stripes are found in a distinct zone east of the coastal range (21°40′–25°10′S) (Fig. 1) and proposed three possible mechanisms for the formation of the Atacama zebra stripes – overland flow, salt shrink-swell processes, and seismic shaking. They assumed that each proposed mechanism could produce zebra stripes with distinct characteristics, allowing for the deduction of the correct process from field analyses. Based on sorting and wavelength patterns as well as their regional distribution, Owen et al. (2013) suggested that zebra stripes represent fossil evidence of overland flow, rather than resulting from saltdriven shrink-swell processes or seismic shaking. However, recent investigations emphasise the significance of seismicity in the formation of the Atacama boulder fields and in geomorphic activity in general, thereby challenging the water-related evolution of zebra stripes as well (Matmon et al., 2015). Here we add new data on zebra stripe characteristics based on Unmanned Aerial Vehicle- (UAV) derived high-resolution orthophotos,

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Fig. 1. (a) Overview of the Atacama Desert, N Chile, between c. 21° and 26° S. The section includes the study areas of Owen et al. (2013; red box, OR, Oficina Rosario; 24°22′54.61″S, 69°56′23.50″W) and this study (yellow boxes, PP and PR), as well as approximate locations of boulder fields investigated by Matmon et al. (2015; green boxes). Further locations with zebra stripes are exemplarily depicted (orange boxes; 1 – 21°48′36.08″S, 69°53′22.62″W; 2 – 22°27′42.07″S, 70° 6′21.32″W; 3 – 23°24′25.87″S, 69°26′16.49″W; 4 – 23°28′42.45″S, 70° 6′22.66″W; 5 – 25°28′31.07″S, 70° 0′20.36″W). Note that zebra stripe sites of this study and sites 1, 2 and 5 are located outside of the zebra stripe zone (white dotted line, 2) proposed by Owen et al. (2013). Isohyetal lines (orange) of average precipitation in mm/a (Houston, 2006), the transition from winter to summer rain regimes (grey dashed line, 3), and the Atacama Fault System (AFS, Lavenu et al., 2000) are denoted (map based on SRTM (NASA) data). (b) Section of the Paposo fault showing topographical conditions and locations of study sites PR and PP. Red and blue areas refer to hillslope surfaces with and without zebra stripes, respectively; based on these areas, morphometric analyses of the broader study area were carried out (cf. Fig. 9).

geomorphological surveys and sediment sampling. Although we focus on features along the Paposo fault segment (a section of the Atacama Fault System (AFS); Figs. 1,2), zebra stripes decorate numerous hillslopes in the central Atacama (Fig. 1). Therefore, our study improves the understanding of zebra stripes' origin and the evolution of hyperarid hillslopes in general.

2. Physical setting 2.1. Past and present climate The hyperarid core of the Atacama (~22°–25°S) is characterised by an average rainfall of b10 mm/year (Houston and Hartley, 2003). Its driest parts are located between the coastal ranges and the pre-Andean cordilleras, including the central depression. Precipitation generally increases north-, east- and southwards due to convective rainfall from tropical easterlies in the Andes, and the increased influence of southern westerlies in austral winter, respectively (De Porras et al., 2017). The western slopes of the coastal cordillera are frequently affected by marine fog until ~1100 m above present mean sea level (asl) (Larrain et al., 2002; Ewing et al., 2006), which may locally extend farther inland upon topographic connection to the coast (Cereceda et al., 2002). However, severe precipitation events may reach hyperarid parts of the Atacama, as shown by recent flooding events in the Salado River basin in March 2015, at Alto Patache in August 2015, or in the Iquique area in January 2019 (Wilcox et al., 2016; Orellana et al., 2017).

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Situated at ~24°40′S, our study area is located in the southern part of the central desert receiving 5–10 mm/year average rainfall (Figs. 1,2). At Paranal observatory (2600 m asl), situated some 5 km (PR; Paranal) and 20 km (PP; Paposo) from the sites of this study, temperatures range between 25 °C during austral summer (daytime) and 5 °C during austral winter (night) (Fig. 3). Although ~1500 m higher than the typical upper limit of marine fog, relative humidity at 2 m above ground surface may frequently reach 100% in austral summer. All study sites have elevations of ~2000–2100 m asl, i.e. lie ~600 m lower than the Paranal observatory; temperatures are thus assumed to be slightly higher at the study sites. Hyperarid conditions in the Atacama prevail since the mid-Miocene (Dunai et al., 2005; Rech et al., 2010), although intermittent pluvial periods are suggested (Jordan et al., 2014), and wetter periods caused widespread lake formation in the Atacama (e.g., Gaupp et al., 1999; Sáez et al., 2012; Kirk-Lawlor et al., 2013), potentially until the late Pleistocene (Jordan et al., 2018; Ritter et al., 2018). A number of geological records such as lake (Grosjean et al., 2001) and rodent midden pollen records (Latorre et al., 2002; De Porras et al., 2017), palaeosols (Veit, 1996) or fluvial and geoarchaeological evidence (Vargas et al., 2006; Nester et al., 2007; Latorre et al., 2013;) also document climatic fluctuations since the late Pleistocene. While this evidence mainly stems from the eastern margins of the central (absolute) desert, where Andean precipitation resulted in fluctuations of alluvial activity as well as wetland and vegetation composition and extent, suitable geological archives for the reconstruction of palaeoclimatic changes in the central desert, disconnected to Andean discharge, are scarce. In this regard, Owen et al. (2013) concluded that zebra stripe formation resulted from palaeo-overland flow during wetter climatic conditions in the past, although the timing of these wetter periods remains unsolved. 2.2. Geologic setting and geomorphic processes The study sites are located along the Paposo fault segment, a section of the 1000 km-long Atacama Fault System (Figs. 1,2), the dominant structure of the northern Chilean forearc (Loveless and Pritchard, 2008). The local geology at the two study sites PR and PP is dominated by three main geologic units comprising Jurassic quartz diorites and monzodiorites (Izcuña, Jki), andesitic and andesitic-basaltic lavas with porphyritic sections [La Negra formation, Jln(a)] and medium- to finegrained sandstones [Caleta Coloso formation, JKicc(b)] (Fig. 2). While these bedrock units constitute the basement of most of the zebra stripe-covered hillslopes, some zebra stripes seem to be related to monzonitic diorites, quartz diorites or gabbros of the Paranal Plutonic Complex [Jsp(d)] as well. Site PP is located at the junction of the southern extension of the NNW-stretching Caleta Coloso fault and the NNEstretching Paposo fault, the latter constituting the eastern boundary of site PR (cf. Álvarez et al., 2016; Domagala et al., 2016). In the Antofagasta area, fault structures seem to be extensional in most cases (Delouis et al., 1998; González et al., 2003; Loveless and Pritchard, 2008), although Loveless et al. (2006) and Loveless (2007) report on minor reverse movements superimposed on some faults. At least a local reactivation of the Paposo fault segment was observed during the Mw = 8.1 Antofagasta megathrust earthquake on July 30th 1995 (Ruegg et al., 1996; Delouis et al., 1998; González et al., 2006), although the occurrence of extensive coseismic offset and faulting during this event has been challenged (Loveless and Pritchard, 2008). On Quaternary time scales, evidence for younger fault activity in that region was provided by González et al. (2006) based on fault scarps affecting cosmogenic nuclide-dated alluvial fan sediments of 424 ± 151 ka. Geomorphic processes under hyperaridity are suggested to be of remarkable slowness, as evidenced by the age of surfaces and landforms in the Atacama (Dunai et al., 2005; Clarke, 2006) and other deserts (Matmon et al., 2009). With 0.2–0.4 m/Ma, erosion rates in the central Atacama are comparable to those of other deserts (Placzek et al., 2010) as well. While a variety of landforms in the Atacama thus may

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Fig. 2. Geological and topographical setting of study sites PP1–2 (24°26′38.08″S, 70°19′26.64″W) and PP3 (24°26′36.97″S, 70°19′54.52″W). (a) Geological units (generalized) along the Paposo fault section shown in Fig. 1b, including areas with (red) and without (blue) zebra stripes used for morphometric analyses (Fig. 9); note that zebra striping is found along the entire fault line, but morphometric analyses were only performed in the direct vicinity of sites PP1–3 (based on local geological maps, sheets Punta Posallaves y Sierra Vicuña Mackenna and Blanco Encalada y Pampa Remiendos; Álvarez et al., 2016; Domagala et al., 2016). Zebra-striped hillslopes mainly occur on the geological units Jln, Jkicc, and Jki. (b) Topography based on TanDEM-X WorldDEM™ in the surroundings of sites PP1–3. (c) Slope angles based on TanDEM-X WorldDEM™ in the surroundings of sites PP1–3. (d) Spot 7 satellite image of the broader study area including sites PP1–3. Striped/non-striped areas are depicted in red/blue (©Airbus DS (2017)). Cross sections A-A′ and B-B′ of hillslopes at PP3 and PP1 are depicted in (a) and (b).

represent fossil evidence of past geomorphic activity (cf., Owen et al., 2013), cosmogenic nuclide concentrations seem to indicate a Pleistocene modification of the landscape (Placzek et al., 2010). Overland flow or flash flood activity generated by rare precipitation events may episodically occur even in the driest parts of the Atacama (Wilcox et al., 2016), and may result in substantial geomorphic impact on longer (e.g., millennial) time scales. In the central desert, thick atmospherically derived salt and dust deposits (Michalski et al., 2004; Ewing et al., 2006) typically mask hillslopes, resulting in rather smooth slope morphologies. As a consequence, Atacama soils contain large amounts of salts such as nitrate, iodate and the common sulphates gypsum and/or anhydrite (Ewing et al.,

2006). Coupled with limited in-soil chemical transformation and loss, this leads to salt-dominated soils and a remarkable volumetric expansion under long-term hyperaridity. Most salt-rich soils are characterised by a cm- to m-scale prismatic structure, which is explained by shrinking and swelling due to hydration and dehydration of soil material (haloturbation; Ewing et al., 2006; Owen et al., 2013). Polygonal structures and patterned ground with distinct clast sorting are common for most (sub-) horizontal surfaces in the Atacama, though present on numerous hillslopes as well. Salt-driven solifluction-type processes are thus thought to be responsible for various hillslope forms in the central desert (Abele, 1990). With the transition from hyperarid to semiarid environments, hillslope denudation rates increase as a power law function

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Fig. 3. Climate data as recorded at Paranal meteorological station, Paranal observatory (~2600 m asl; data available at http://archive.eso.org/wdb/wdb/asm/meteo_paranal/ form). Air temperature at ground level (°C) and relative humidity at 2 m (%) between Jan 01, 2015 and Dec 31, 2018. Blue line – relative humidity at 2 m, 24 h running mean; black line – air temperature at ground, 24 h running mean; grey lines – 1-min humidity and temperature data (min/max).

with (southwards) increasing precipitation, ranging between ~1 m/My (hyperarid) to ~40 m/My (semi-arid). This is thought to correspond with a shift in soil formation and transport processes from abiotic (salt shrink-swell) to biotic (bioturbation) (Owen et al., 2011) and a related higher slope mobility. In addition, Quade et al. (2012) and Matmon et al. (2015) documented a seismic origin of large boulder fields in the central Atacama, which are suggested to bounce and slide downslope, and to bunch and rub each other at the base of the slopes during frequent earthquakes. According to their studies, seismicity-driven processes are actively shaping hillslopes in the driest parts of the Atacama, which is thereby challenging the general perception of stagnant Earth surface processes. The significance of seismicity in the Atacama is expressed in recurrence rates of 3–4 N M5 earthquakes per year, and return periods of 80–130 years for M8 earthquakes (Barrientos et al., 2004).

Fig. 5. (a) Zebra stripes at site PP3 (24°26′36.97″S, 70°19′54.52″W). Note WNW-trending seismic crack at the foot of the slope (dashed line). (b) Close-up of stripes shown in (a) with characteristic frontal bulges; tablet for scale in lower left. (c) Clast cover and salt weathering of large bedrock clasts upslope of site PP1.

3. Methods 3.1. UAV-based orthophotos In this study, imagery was captured by an Unmanned Aerial Vehicle (UAV; type: DJI Phantom 4, rotary-wing quadrocopter) with a 12

megapixel FC330 camera and a 20 mm full frame equivalent lens, fixed by a shock-absorbent gimbal, and set to capture images every 10 s. Flights at all sites were manually conducted between 10 am and 12 pm local time on a cloud-free day in March 2018 in a line-based pattern at two different heights, flying slower than 2.5 ms−1. Camera was

Fig. 4. (a) DEM and orthophoto of zebra stripes at site PP3. Rill erosion and channel incision commences at ~2140 m asl (eastern slope section). Orientation of stripe axes is mainly contouroblique. (b) UAV-derived slope values for a slope section at ~2130 m asl illustrating distinct downslope boundaries of stripe fronts, indicated by steeper slopes. Note WNW-trending seismic crack at the foot of the slope (dashed line). White lines = contour lines in m asl.

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Fig. 6. Subsurface stratigraphy in trenches at sites PR (a; trench T1, cf. Fig. 12), PP1 (b; trench T2; with locations of XRD samples shown in Fig. 7) and PP3 (c; trench T3), which is generally similar to the findings of Owen et al. (2013). Solid gypsum with in situ weathered angular bedrock clasts was found at the base (cf. Fig. 7), covered by gypsum-rich strata of powdery, compact, or bladed texture, and subsequent units of angular clasts in a light brown silty to clayey matrix with a vesicular horizon (d) in the uppermost 1–2 cm. Stripe-confining surface clast units are 5–10 cm thick.

set to shutter speed priority (1/1000) with ISO-100. Missions resulted in a high overlap of N9 images per point. A total of 217–225 images were used for subsequent image processing, which was conducted with AgiSoft Photoscan Professional (v. 1.4.2). Images were aligned using the direct GPS measurements of the UAV recorded for each image. Processing in ultra-high quality for the dense point cloud generation resulted in a mean ground pixel resolution of 1.12 cm to 1.25 cm for the DEM and the orthophotos (Fig. 4). All data is exported in WGS_1984_UTM_Zone_19S (EPSG: 32719) with a grid size of 0.011 m and 0.013 m. The estimated positional error is 1.5 m in the horizontal and 1.9 m in the vertical direction for sites PP1 and 2, and 1.2 m and 0.9 m for site PP3. Orthophotos and DEM can be accessed online (Hoffmeister, 2018a, 2018b). 3.2. Geomorphological survey, sediment sampling and grain size analyses Field work included geomorphological surveys, sediment sampling, and measurements of stripe morphology (stripe width; granular finger width and length; slope angle) using a measuring tape and a laser range finder and was carried out in March 2018 and March 2019. We investigated zebra stripes at four different hillslope sites along a 20 km-long stretch of the Paposo fault segment (PP1, PP2, PP3 and PR; Figs. 2, 4–5). For sites PP1–3, we calculated a fingering index by dividing the granular finger length by granular finger width in order to illustrate basic relationships between fingering patterns and local slope angles (Table S1). Small trenches (T1–3) were dug at all sites (PP1, PP2, PP3 and PR; Figs. 4, 6, 11), photographically documented and sampled at an interval of 5–10 cm depending on stratigraphical alternations and material characteristics, i.e., a sampling interval representative of the stratigraphy. Trenches are located in the lower slope sections, where stripe morphology was distinctive and individual stripes were clearly separated by clast-free areas (Figs. 4–5). Grain size and X-ray diffraction (XRD) analyses (n = 4; Fig. 7) were carried out in the Laboratory for Physical Geography, University of Cologne. XRD analyses were conducted for samples of T2 (site PP1) in order to exemplarily characterise the mineralogical composition of the subsurface stratigraphy by a powder X-ray diffractometer (Siemens D 5000; Siemens AG, Munich, Germany) with 4 s per point step size and 0.05° steps. A fixed 1° divergence slit and

anti-scatter was used at diffraction angles from 5 to 75° 2 h. The Cu Ka radiation source was operated at 40 keV and 40 mA. The data were analysed with the Diffrac-Plus Eva software package (Bruker AXS; Bruker Corporation, Billerica, MA, USA). Granulometry of samples from surface clast units (Fig. 8) was determined on 1–2 kg of dried sediment using a Camsizer (Retsch Technology) for size fractions between 30 μm and 3 cm. 3.3. Remotely-sensed morphometric measurements and sediment characteristics To document morphological differences between zebra stripecovered slopes and slopes without zebra stripes in the broader study area, we performed regional morphometric analyses in the surroundings of sites PP and PR. Stripe areas and areas without stripe cover were defined based on the visual interpretation of Spot 7 satellite images (Fig. 2, captured on 26th of July 2017) and field experience. Slope, curvature and aspect of these slope areas were calculated based on TanDEM-X data using ArcGIS 10.5 software (Figs. 2, 9). The TanDEM-X WorldDEM™ dataset was made available for this study in 2017 by the DLR via a science grant (cf. Acknowledgments). Based on UAV data, we additionally carried out local morphometric measurements for specific slope sections of sites PP1–3, i.e. upper, middle and lower slope sections, in order to characterise stripe patterns in relation to local topographical characteristics (Fig. 10). Stripe width, stripe length and the obliqueness of stripes with respect to contour lines was measured using ArcGIS 10.5 software. Finally, slope-wide grain size characteristics, i.e. in the form of a grain size index (GSI), were calculated from the UAV-derived orthophotos based on the method proposed by Xiao et al. (2006) (Fig. 11). 4. Results 4.1. General setting and stratigraphical findings The subsurface stratigraphy at all sites is composed of solid gypsum with angular bedrock clasts at the base (Fig. 6). According to the XRD analyses of samples of T2, these basal sections are free of siliciclastic components and only contain Gypsum and Niter (Fig. 7). Subsequently,

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to the underlying silt-rich vesicular horizon. Clast coverage gradually decreases upslope towards clast-free interspaces. In contrast, stripe fronts show an abrupt transition to downslope interspaces, i.e. clast coverage abruptly changes from 100% (stripe, surface clast unit) to b10% (clast-free interspace), related to a distinct though small micro-topographical step and higher slope angle (Fig. 4b). The stratigraphical pattern consisting of a gypsum and niterdominated basal unit and a slightly incrusted vesicular horizon is generally present in the entire study area, including hillslopes without zebra stripe patterns (Fig. 2). In addition, most zebra stripe-free slopes are covered by surface clasts, and some are characterised by patterned ground structures as well, but these slopes lack the distinct zebra stripe pattern described above. However, since vesicular structures are typically related to the presence of surface clasts and the trapping of air below these clasts (e.g., Dietze et al., 2016), they are less distinct in the clast-free or clast-reduced interspaces.

4.2. Geomorphological observations and granulometry

Fig. 7. Results of XRD measurements of samples PP1/2, 1/3, 1/5 and 1/6 (cf. Fig. 6) at site PP1. Lowermost trench sections mainly contain gypsum and niter, whereas gypsum, quartz and muscovite were found in the subsequent silty units, below the vesicular horizon.

the basal units are covered by gypsum-rich strata of different texture and subsequent units of few angular clasts in a light brown silty to clayey matrix, where quartz and muscovite were found in addition to the aforementioned minerals (Figs. 6–7). A vesicular horizon comprises the uppermost 2 cm of this layer, with numerous vesicles visible below clasts (Fig. 6d). The stripe-confining gravel layers overlying the fine-grained soil surface consist of mostly uncemented and loosely bedded angular clasts (Fig. 6). Surface stripes in this study did not conform to subsurface structures, nor did they agree with subsurface gypsum layer characteristics. The surface gravel units at all study sites are multigrain and 5–10 cm thick (Figs. 6, 13), unstratified and lack a prominent internal structure. Clast coverages of middle and downslope stripe sections are 100%, and surface clast units are almost free of silt (Figs. 5, 8, 12–13). Only few clasts (b10%) were weakly cemented

Orientation of the main axes of larger stripes is contour-oblique (Figs. 4,11–12), i.e., western and/or northern stripe sections typically stretch farther downslope compared to eastern/southern sections, rather than contour-parallel as observed by Owen et al. (2013). This contour obliqueness seems to vary between different slope sections (Fig. 10), as observed for sites PP1–2, where stripes are characterised by angles of 62–79° with respect to contour lines and form rather long, almost slope-parallel stone stripes in the upper slope sections. At this site, a successive decrease of this obliqueness is observed in the middle and lower slope sections. Thereby, lower slope angles tend to be related to higher contour obliqueness (cf. Fig. 10). While no significant difference of the contour obliqueness is observed at PP3 for the different slope sections, stripe width and length at this site clearly increase in middle and lower slope sections, in contrast to site PP1–2 (Fig. 10). However, smaller stripes in the interspaces between larger stripes appear to be contour-parallel at many places. The UAV-derived orthophotos (Figs. 4, 11–12) revealed a garlandlike appearance of the stripes. At all sites, stripe fronts show a distinct boundary to clast-free interspaces indicated by high slope values (Fig. 4b), and consist of multiple bulge-type and downslope-facing lobes of 0.5–2.0 m width, with coarser clasts laterally confining the frontal lobes (Figs. 12, 13). Individual lobes at all sites show a distinct lateral and downslope coarsening trend, which is manifested in the grain size analyses of a sequence of surface samples from trenches T2 and T3 at sites PP1 and 3 (Figs. 4, 8, 10). The downslope coarsening trend in individual stripes is clearly visible in the grain size index data based on UAV as well and can be observed along the entire slope of study sites PP1–3 (Fig. 11). While some zebra stripes in the middle slope sections seem to be coarser (e.g., D at site PP3; Fig. 11a; B at site PP2; Fig. 11b) than others, the grain size index data suggests rather similar grain size spectra along the entire investigated slope sections. No distinct slope-wide downslope coarsening or fining pattern of zebra stripes is thus observed. However, the frontal lobes at sites PP1–3 are between 55 and 260 cm wide, and lobe lengths are between 50 and 570 cm (Table S1). As expected, the fingering index (lobe length/lobe width) weakly correlates (Rcorr = 0.445) with slope angle, i.e., higher values of the fingering index (longer frontal lobes) tend to occur on steeper slopes (Fig. 14). Since the data suggested a non-linear (exponential) relationship, an exponential regression model was estimated. It shows a weak but statistically significant relationship between fingering index and slope (p = 0.001, coef. = 1.663). Although trend-inconsistent data was particularly found at PP1, where low fingering index values were found on steeper slopes as well (Table S1, Fig. 14), the model explains 20% of the variance in the empirical data (R2 = 0.199).

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Fig. 8. Zebra stripe characteristics at PP1 and PP3. (a, b) Location and grain size distribution of surface samples 1–5 at trench T3 at site PP3 (cf. Figs. 2, 4, 11), illustrating downslope coarsening; dark green: very coarse gravel – dark orange: fine sand. Stripe is ~3 m wide. (c) Clast coverage is 100% for middle and downslope stripe sections and frontal bulges are separated by coarse lateral margins; note downslope increase of mean grain size. (d) Angularity/irregularity of surface clasts at stripe fronts (site PP3). (e) Stripes at site PP1 illustrating downslope coarsening of grain size and spacing of 3–4 m between individual stripes. Note contour-oblique orientation of stripes. (f, g) Location and grain size distribution of surface samples 1–3 at trench T2 at site PP1 (cf. Figs. 2, 11), illustrating lateral/downslope coarsening of frontal bulges; note downslope/lateral increase of mean grain size.

5. Discussion 5.1. Zebra stripe occurrence and characteristics along the Paposo fault No systematic regional analysis of the relationship between zebra stripe occurrence and bedrock characteristics was performed in this study. However, zebra-striped hillslopes at study sites PP and PR have mainly developed on quartz diorites and monzodiorites (Izcuña formation, Jki), andesitic and andesitic-basaltic rocks [La Negra formation, Jln (a)] and medium- to fine-grained sandstones (Caleta Coloso formation, JKicc) (Fig. 2). This is partly in contrast to the results of Owen et al. (2013: 169), who stated that “monzogranite and diorite were generally the least likely to have zebra stripes”. Therefore, it can be deduced that

there is no clear relationship between zebra stripe occurrence and bedrock type and specific characteristics (e.g., composition, grain size, foliation, or age). In conclusion, it can be assumed that different bedrock characteristics such as texture, weathering patterns, and, consequently, clast size distribution or clast shape, may control zebra stripe occurrence, potentially in combination with local slope characteristics. However, the stripe-confining surface gravel units of the stripes investigated in this study rest on top of a thin fine-grained vesicular horizon (Av) that is typically associated with dust accreting desert pavements (McFadden et al., 1998; Anderson et al., 2002; Dietze et al., 2012) and that creates a rather flat, low-friction surface. As documented by the stratigraphic findings at sites PP1, PP3 and PR and exemplified by the XRD measurements at site PP1 (Figs. 6–7), the Av horizon has

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~10–15 cm below surface. This subsurface stratigraphy is also similar to other, previously investigated zebra stripe sites in the region (Owen et al., 2013). While it serves to account for both the presence of gravel-sized clasts and their downslope movement after exhumation, it does not provide an explanation of the transport mechanisms that are responsible for the formation of the zebra stripes.

Fig. 9. Results of morphometric analyses for zebra stripe-covered hillslopes and hillslopes without zebra striping (cf. Figs. 1,2). While no significant differences are documented with respect to slope aspect, zebra-striping tends to occur on hillslopes with slightly convex plan curvatures, with slightly concave profile curvatures, and generally steeper slopes.

developed in gypsum-rich strata containing sharp-edged, angular bedrock fragments. The presence of these bedrock fragments points to the combined action of bedrock shattering (Fig. 5c) and clast exhumation by salt weathering and haloturbation processes that are intensified by the occurrence of fog (Goudie, 2004). A sulphate-indurated horizon as typical for the central Atacama constitutes the base of all profiles from

Fig. 10. Results of morphometric analyses for sites PP1–3, including stripe width, length, contour obliqueness, and slope angle. At site PP1–2, a successive decrease of stripe obliqueness with respect to contour lines is observed from upper to lower slope sections. At PP3, stripe width and length increase in middle and lower slope sections.

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Fig. 11. UAV orthophoto-derived grain size index data of hillslopes at sites PP3 (a) and PP1–2 (b). Coarse (dark red) and fine (dark blue) index values refer to gravel-sized material (e.g., at the downslope boundary of stripes/frontal lobes) and silt-dominated clast-free/clast-reduced surfaces (e.g., in the interspaces between stripes), respectively. Cross sections are depicted for 6 individual zebra stripes from the two sites PP3 and PP1, clearly showing the relative difference between stripe-defining surface gravel units (high values, coarse material) and clast-free/ clast-reduced interspaces (low values, fine material). While the downslope coarsening pattern of individual stripes is clearly visible, no distinct fining or coarsening trend was found along the entire slope.

5.2. Zebra stripes as evidence for paleo-overland flow? In summary, zebra stripe-covered hillslopes of this study (i) tend to be laterally convex (positive plan curvature) and exhibit a rather concave downslope profile (negative profile curvature), and individual zebra stripes are characterised by (ii) stripe fronts with multiple bulge-type and 0.5–2.0 m wide lobes, in which (iii) a distinct lateral and downslope coarsening trend is observed. These frontal lobes (iv) tend to be longer with increasing slope angle. Aeolian transport by strong and unidirectional winds is capable of generating large gravel-dominated ripples on planar surfaces (e.g., Milana, 2009; Bridges et al., 2015; Lämmel et al., 2018), but the different internal architecture (i.e., no distinct stratification, no cross-

bedding), the distinct sorting pattern with multiple bulges, and the absence of rounded clasts exclude a wind-induced formation of the zebra stripes (Fig. 8a-c). On the other hand, periglacial processes such as frost shattering (providing the source of coarse material), freeze-thaw cycles and frost creep can form elongate, slope-parallel and sorted stone stripes that convey coarse material downslope (e.g., Kessler and Werner, 2003). Even though cooler climatic conditions cannot be excluded for the study area during the Pleistocene, recent frost-related processes are unlikely to occur (Fig. 3). In analogy to periglacial and frost-driven processes, slope-parallel stripes may theoretically arise from recurrent haloturbation processes. The marked slopeperpendicular or slope-oblique orientation of most of the zebra stripes, however, is in stark contrast to periglacial and haloturbation-related

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Fig. 12. (a) Detail of zebra stripes close to Paranal Observatory (PR; 24°38′57.29″S, 70°21′35.56″W; photo) with numerous frontal lobes. (b–d) UAV-based orthophotos at PP1 and PP2 (24°26′38.08″S, 70°19′26.64″W), and PP3 (24°26′36.97″S, 70°19′54.52″W). Frontal bulges show the characteristic sorting by granular fingering (cf. Fig. 5). Granular fingers appear longer at site PP2 (d) with slightly increased slope angle; light blue lines – contour lines from UAV data. (e) Temporal evolution of a granular flow and granular fingers observed in experiments (modified from Pouliquen and Vallance, 1999, with permission of AIP Publishing). Note characteristic sorting pattern. (f) Granular fingering observed in experiments with different granular mixtures (modified from Valderrama et al., 2018). Note restricted fingering with increased portions of coarse particles. The orientation of the flow front axes is contour-oblique.

forms, clearly excluding both frost- or salt-driven processes as the main process responsible for stripe formation. Downslope transport and sorting of coarse, sand to gravel-sized sediment can result from unconcentrated overland flow (e.g., Dietze et al., 2013). It usually results from the exceedance of soil infiltration capacities during intense and typically storm-related rain events on steep hillslopes characterised by reduced (or absent) vegetation cover and low soil infiltration capacities (Horton, 1945; Parsons, 2004). In fact, narrow, widely spaced, transverse-to-slope bands of fine gravel and coarse sand were interpreted to represent relict sheet flow bedforms (i.e., megaripples and/or large antidunes) on gently inclined alluvial fans in the Mojave Desert (Wells and Dohrenwend, 1985). In addition, a previous interpretation of zebra stripes in the Atacama favoured unconcentrated overland flow as the key formative process as well (Owen et al., 2013), considering the stripes an essentially selfstabilizing form produced by shallow sheet flow and coarse grain interactions on a smooth bed, where coarsest gravels initially accumulate in slope-perpendicular stone lines and subsequently function as a barrier for smaller grains (Owen et al., 2013). However, runoff from overland flow is commonly laminar in the upper slope sections and restricted to a maximum length of 90 m (e.g., NRCS, 2010) before giving way to turbulent flow and the formation of channel networks that are restricted to lower slope

sections, where a drainage area threshold is reached (Schumm, 1956) and shear stresses exceed the resistance of the soil surface. Clear changes in downslope stripe morphology and distribution would be expected as a result but have not been observed at our investigated sites (Figs. 10–11). Instead, zebra stripes uniformly cover the entire slope from just below the drainage divide to the foot of most slopes (Figs. 4,11). A possible explanation for this absence of downslope morphological changes could be run-on-runoff processes, during which precipitation-generated runoff develops on bare soil slope sections with low infiltration capacities. When flowing downslope onto typically vegetation-covered slope sections with higher infiltration capacities (i.e., run-on)(e.g., Corradini et al., 1998), flow velocities and transport capacities decrease. This self-stabilizing process is responsible for the generation of banded vegetation patterns and associated landforms in semiarid environments (tiger stripes, brousse tigreé; Dunkerley and Brown, 1999; Valentin et al., 1999; Pelletier et al., 2012; Sherratt, 2015), and may potentially serve as an explanation for the formation of zebra stripes in the Atacama. Given the lack of both vegetation and observable overland flow under modern climatic conditions, active stripe formation due to run-on-runoff processes requires more humid climatic conditions in the past (Owen et al., 2013). In general, the tendency of zebra stripes to occur on laterally convex slopes and the sorting pattern present in the zebra stripes investigated

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Fig. 13. Schematic drawing of zebra stripes investigated in this study. (a) Oblique view, (b) cross section (after stripes at site PP3).

here may point to clast transport by overland flow to some degree. However, none of the above-mentioned flow processes are capable of explaining the characteristic frontal bulges observed in the stripes of this study (Figs. 8,12), that exhibit a distinct lateral and downslope coarsening trend and a general (though weak) relationship between lobe lengths and slope angle, but that are clearly absent in ripple- or antidune morphology. In addition, discrepancies exist between the interpretation of zebra stripe formation by overland flow on the one hand, and the rather uniform morphology and distribution of the stripes in downslope directions, the typically observed downslope extent of unconcentrated overland flow, as well as the existence of incised channels in the middle to lower slope sections on the other. In contrast, the bulge-type morphology of the zebra stripes may be caused by slumping processes, provoked by water saturation of surface gravel deposits and simultaneous reduction of the internal friction. In the hyperarid setting of the Atacama, however, moistening of surface gravels may occur due to fog- or rainfall-related infiltration independent from any type of surface runoff (e.g., Davis et al., 2010). In combination, this suggests that alternative processes need to be considered to explain all morphological and sedimentary aspects of zebra stripe formation process. 5.3. Segregation-induced granular fingering – a mechanism for zebra stripe formation? Stratification, particle segregation and sorting of mixed particles in granular systems is a rather general process (Krengel et al., 2013). Experimental and modelling studies on particle behaviour in granular systems is important in the context of industrial processing of loose materials or the understanding of landslide and avalanche processes (Pouliquen et al., 1997). However, these studies document that segregation-induced fingering in waterless granular free-surface flows produces specific forms and sorting patterns (Pouliquen et al., 1997; Pouliquen and Vallance, 1999) (Fig. 12e,f), resembling those of the frontal bulges of the Paposo stripes. During segregation-induced fingering,

Fig. 14. Relationship between slope angle and fingering index (i.e., finger length/finger width) based on data from sites PP1, PP2 and PP3 (Table S1). The fingering index weakly correlates (Rcorr = 0.445) with slope angle, i.e. higher values of the fingering index (longer frontal lobes) tend to occur on steeper slopes. Although with n = 55, the data suggest a non-linear (exponential) relationship. Inconsistencies (low index values on steep slopes) are particularly observed at site PP1, which may be explained by the combined effect of grain size characteristics and slope. Rcorr = coefficient of correlation; coef = regression coefficient; R2 = coefficient of determination. Since a non-linear behaviour was expected, linear regression analysis was carried out using the natural logarithm of the dataset (i.e., exponential regression) with slope values in m/m; the plot shows the original (non-logarithmic) data including the exponential fit.

large particles in mixed granular material segregate to the surface due to kinetic sieving and squeeze expulsion, and are preferentially sheared to the flow front. Resulting from greater frictional forces, a number of finger-like channels emerge due to frontal instabilities, a process that is amplified by reinjection and recirculation of coarser grains (Pouliquen et al., 1997; Gray et al., 2015; Valderrama et al., 2018) (Fig. 12e,f). During the laboratory experiments, these finger instabilities occurred at slope angles of ~25–35° and after adding N5% of large irregular-shaped particles (crushed fruit stones; Pouliquen et al., 1997) to a rather homogeneous mass of smaller spherical glass beads. The studies show that polydispersity and the presence of coarse and irregular particles are necessary preconditions to form finger instabilities at the flow front. Particle segregation and granular fingering seem to represent the crucial mechanism in ridge formation from polydisperse volcanic debris avalanche deposits as well (Valderrama et al., 2018). The coarse particle size and the ratio of coarse and fine material controls different fingering patterns and the characteristics of ridges (Valderrama et al., 2018), whereas particle angularity has only second-order effects on fingering patterns (Pouliquen et al., 1997). Coarser particles and/or a high proportion of coarse material tend to form poorly developed, short fingers, constrained by coarse particles at the flow front. This results in a series of lobes forming slope-perpendicular ridges, where coarser margins confine individual frontal lobes (Fig. 12f). Likewise, a variety of similar forms decorate hillslopes of lunar craters, generally exemplifying particle segregation with coarse-grained finger margins under the absence of liquid or gas (Kokelaar et al., 2017). However, comparisons between laboratory experiments and natural systems require careful interpretation. Due to different flow velocities

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and inertia, slope angles necessary for the generation of slow granular flows in natural systems may be lower than in simplified models, flow dynamics may be changed by topographic irregularities, and natural materials may behave differently to the materials used in the experiments (Valderrama et al., 2018), which may explain the discrepancy between average slope angles of fingered zebra stripes along the Paposo fault and the slope angles related to granular fingering during laboratory experiments. Nevertheless, while critical grain sizes or slope thresholds may be different in natural systems, general interpretations on granular fingering processes based on the mentioned experiments are considered valid. In addition to particle shape and size distribution, finger length (fingering index) seems to be positively though weakly correlated with slope angle, according to our field measurements (Fig. 14). The only weak and non-linear relationship may be related to the inhomogeneous conditions (e.g., micro-topographical differences, the presence of large clasts impeding downslope movement of smaller clasts). In addition, the restricted fingering on steeper slopes at site PP3 may be explained by slightly coarser grain sizes and/or the different ratio of coarse and fine material compared to stripes at PP1 (Fig. 8c,g). In combination, granular fingering experiments (Pouliquen et al., 1997; Valderrama et al., 2018) and the morphological and sedimentological characteristics resulting from dry granular flow processes exhibit important similarities with the zebra stripes of this study, e.g., the polylobate form with quasi slope-perpendicular or slopeoblique stripe fronts, the frontal sorting and granular fingering, and the coarse and irregular (Fig. 8d) initial sedimentology (i.e., 80% gravel, 20% sand) with all size fractions between very fine sand and very coarse gravel present. Dry granular flow therefore offers an alternative explanation for zebra stripe formation in the Atacama, raising questions regarding their causes and triggers. 5.4. Remaining challenges – origin and timing of zebra stripe formation Compared to other hyperarid deserts in southwestern N America, southern Iran and Central Asia, the Atacama seems to provide most favourable conditions to the formation of zebra stripes, i.e. (i) the virtual absence of sand-dominated processes; (ii) the presence of favourable rock types (Owen et al., 2013); (iii) the abundance of steep (N10°) hillslopes below 2500 m (reduced freeze-thaw processes); (iv) extensive salt weathering and haloturbation processes providing the source of irregular and angular gravels; (v) the characteristic plain, gypsum-rich soil surface related to the long-term accumulation of atmospheric salt and dust; and (vi) hyperaridity over longer (at least Quaternary) time scales (e.g., Reich and Bao, 2018). Consequently, the hyperarid core of the Atacama Desert hosts all physical preconditions required to facilitate dry granular flow processes on steep hillslopes. While a number of processes may lead to the perturbation or destabilization of dry unconsolidated material and the initiation of dry granular flow, e.g., undercutting or oversteepening of hillslopes, volcanic eruptions or earthquakes, seismicity in the Atacama is exceptionally high, with return periods of 80– 130 years for M8, and 3 years for M5 events (Barrientos et al., 2004), and seismic shaking has been suggested to play a key role in the triggering and evolution of boulder fields in the Atacama (Quade et al., 2012; Matmon et al., 2015). Against this background, seismically induced dry granular flow events, facilitated by the coincidence of intense seismic activity and hyperaridity, are assumed to represent a potential alternative mechanism for zebra stripe formation. No absolute ages of stripe activity are available, due to challenges using common dating techniques such as optically stimulated luminescence (no burial) or cosmogenic nuclides (inheritance; Owen et al., 2013). Cosmogenic nuclide dating resulted in zebra stripes exposure ages of ~106 years, and soil sulphate concentrations suggest

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final exhumation of clasts from the prismatic soil horizons at N103 years at the site investigated by Owen et al. (2013). These age estimates encompass a rather long period and do not give chronological evidence of stripe activity/inactivity. However, stripes of the study presented here are thicker and better sorted, contain a variety of grain size fractions, and appear better preserved compared to the stripes at Oficina Rosario (Owen et al., 2013) (Fig. S1), which may be explained by a higher degree of degradation/erosion of the latter site. Likewise, annual dust accumulation amounts to 2–4 g m−2 y−1 (Ewing et al., 2006), and inactive stone stripes are assumed to accumulate dust over long periods, as observed in marble dust samplers over a period of a few years (Ewing et al., 2006). Contrastingly, the loose bedding of the rather dust-free surface gravel units suggests a comparably recent activity of the Paposo stripes. This is in agreement with the neotectonic behaviour of the Atacama Fault System and recent motions along the Paposo and adjacent fault segments, e.g., during the Mw = 8.1 Antofagasta earthquake on July 30 th 1995 (Ruegg et al., 1996; Delouis et al., 1998; González et al., 2006) and awaits further testing.

6. Conclusions In this study, we investigated zebra stripes along the Paposo fault (Atacama Desert), a section of the Atacama Fault System in northern Chile. In the broader study area, zebra stripe-covered hillslopes tend to be laterally convex and vertically concave, and are indicated by higher slope angles compared to areas without zebra stripes. At all study sites, the longest axes of zebra stripes are oblique to the contour lines with varying angles (~10–80°). Zebra stripe fronts in the study area exhibit multiple bulge-type and 0.5–2.0 m wide lobes, with individual lobes showing a distinct lateral and downslope coarsening trend. Based on their distinct morphology, we suggest instantaneous dry granular flow and poorly developed granular fingering as the key mechanism for zebra stripe formation, as observed in experiments on particle behaviour in dry granular systems. Stripe morphology and stratigraphy excludes water- and wind-related ripple- or dune-type formation, and shallow sheet flow of unusual length or run-on-runoff processes would be required if their formation resulted from overland flow processes. However, our findings suggest that water is not a necessary precondition for zebra stripe formation, although we cannot entirely rule out a water-related origin. The hyperarid core of the Atacama Desert seems to host all physical preconditions required to facilitate dry granular flow processes on hillslopes as an alternative mechanism for zebra stripe formation, and most favourable conditions to the formation of zebra stripes, i.e. including long-term hyperaridity, favourable rock types, steep hillslopes, extensive salt weathering and haloturbation, plain soil surfaces, and a remarkably high seismicity, are found in the central Atacama. In addition to the rubbing boulders, which are assumed to have experienced ∼40,000–70,000 h of earthquake-driven erosion during the last 1.3 Ma (Quade et al., 2012), and which seem to occur in a similar distribution area (Fig. 1), zebra stripes may thus represent another unique geomorphic feature in the Atacama, originating from seismic shaking. Against this background, our findings provide new and additional data for understanding zebra-stripe origin and formation, in turn complicating their geomorphic interpretation or the inference of paleoclimatic fluctuations. Despite the possibility of undiscovered zebra stripes in other hyperarid deserts, the concurrence of longterm hyperaridity and high seismicity, combined with geological and topographical preconditions conducive to zebra stripe formation, may well be specific to the Atacama, ultimately explaining their uniqueness and the abundance and preservation of seismic landforms in this driest part of the Earth.

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Acknowledgements Funding by Deutsche Forschungsgemeinschaft (DFG, project number 268236062, SFB 1211, subprojects C3, Z2 and Z3, http://sfb1211. uni-koeln.de/) is gratefully acknowledged. Marie Gröbner, Lennart Meine and Florian Steininger assisted in field and lab work. The research was kindly supported by Eduardo Campos (Universidad Católica del Norte, Facultad de Ingeniería y Ciencias Geológicas, Antofagasta/Chile). TanDEM-X WorldDEM™ data was provided by a DLR science grant in 2017. Appendix A. Supplementary data Drone-derived datasets (orthophotos and DEM) of this study are available at https://www.crc1211db.uni-koeln.de/site/index.php (DOIs: https://doi.org/10.5880/CRC1211DB.17, https://doi.org/10. 5880/CRC1211DB.18). Supplementary material (Fig. S1 and Table S1) are available online at https://doi.org/10.1016/j.geomorph.2019.07.014. References Abele, G., 1990. Salzkrusten, salzbedingte Solifluktion und Steinsalzkarst in der nordchilenisch-peruanischen Wüste. Mainzer Geographische Studien 34, 23–46. Álvarez, J., Jorquera, R., Miralles, C., Padel, M., Martínez, P., 2016. Cartas Punta Posallaves y Sierra Vicuña Mackenna, Región de Antofagasta. Serie Geología Básica 183-184, mapa escala 1:100.000. Servicio Nacional de Geología y Minería, Santiago. Anderson, K., Wells, S., Graham, R., 2002. Pedogenesis of vesicular horizons, Cima Volcanic Field, Mojave Desert, California. Soil Sci. Soc. Am. J. 66, 878–887. https://doi.org/ 10.2136/sssaj2002.0878. Barrientos, S., Vera, E., Alvarado, P., Monfret, T., 2004. Crustal seismicity in central Chile. J. S. Am. Earth Sci. 16, 759–768. https://doi.org/10.1016/j.jsames.2003.12.001. Bridges, N.T., Spagnuolo, M.G., de Silva, S.L., Zimbelman, J.R., Neely, E.M., 2015. Formation of gravel-mantled megaripples on Earth and Mars: insights from the Argentinean Puna and wind tunnel experiments. Aeolian Res. 17, 49–60. https://doi.org/ 10.1016/j.aeolia.2015.01.007. Cereceda, P., Osses, P., Larrain, H., Farıas, M., Lagos, M., Pinto, R., Schemenauer, R.S., 2002. Advective, orographic and radiation fog in the Tarapacá region, Chile. Atmos. Res. 64 (1–4), 261–271. https://doi.org/10.1016/S0169-8095(02)00097-2. Clarke, J.D., 2006. Antiquity of aridity in the Chilean Atacama Desert. Geomorphology 73 (1–2), 101–114. https://doi.org/10.1016/j.geomorph.2005.06.008. Corradini, C., Morbidelli, R., Melone, F., 1998. On the interaction between infiltration and Hortonian runoff. J. Hydrol. 204 (1–4), 52–67. https://doi.org/10.1016/S0022-1694 (97)00100-5. Davis, W.L., de Pater, I., McKay, C.P., 2010. Rain infiltration and crust formation in the extreme arid zone of the Atacama Desert. Chile. Planet. Space Sci. 58 (4), 616–622. https://doi.org/10.1016/j.pss.2009.08.011. De Porras, M.E., Maldonado, A., De Pol-Holz, R., Latorre, C., Betancourt, J.L., 2017. Late Quaternary environmental dynamics in the Atacama Desert reconstructed from rodent midden pollen records. J. Quaternary Sci. 32 (6), 665–684. https://doi.org/10.1002/ jqs.298. Delouis, B., Philip, H., Dorbath, L., Cisternas, A., 1998. Recent crustal deformation in the Antofagasta region (northern Chile) and the subduction process. Geophys. J. Int. 132 (2), 302–338. https://doi.org/10.1046/j.1365-246x.1998.00439.x. Dietze, M., Bartel, S., Lindner, M., Kleber, A., 2012. Formation mechanisms and control factors of vesicular soil structure. Catena 99, 83–96. https://doi.org/10.1016/j.catena.2012.06.011. Dietze, M., Groth, J., Kleber, A., 2013. Alignment of stone-pavement clasts by unconcentrated overland flow – implications of numerical and physical modelling. Earth Surf. Proc. Land. 38 (11), 1234–1243. https://doi.org/10.1002/esp.3365. Dietze, M., Dietze, E., Lomax, J., Fuchs, M., Kleber, A., Wells, S.G., 2016. Environmental history recorded in aeolian deposits under stone pavements, Mojave Desert, USA. Quat. Res. 85 (1), 4–16. https://doi.org/10.1016/j.yqres.2015.11.007. Domagala, J.P., Escribano, J.A., de la Crus, R.S., Saldías, J.M., Jorquera, R.B., 2016. Cartas Blanco Encalada y Pampa Remiendos, Región de Antofagasta. Serie Geología Básica 187-188, mapa escala 1:100.000. Servicio Nacional de Geología y Minería, Santiago. Dunai, T.J., Gonzáles López, G.A., Juez-Larre, J., 2005. Oligocene –Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms. Geology 33, 321–324. https://doi.org/10.1130/G21184.1. Dunkerley, D.L., Brown, K.J., 1999. Banded vegetation near Broken Hill, Australia: significance of surface roughness and soil physical properties. Catena 37 (1–2), 75–88. https://doi.org/10.1016/S0341-8162(98)00056-3. Ewing, S.A., Sutter, B., Owen, J., Nishiizumi, K., Sharp, W., Cliff, S.S., Perry, K., Dietrich, W., McKay, C.P., Amundson, R., 2006. A threshold in soil formation at Earth's arid — hyperarid transition. Geochim. Cosmochim. Ac. 70, 5293–5322. https://doi.org/ 10.1016/j.gca.2006.08.020. Gaupp, R., Kött, A., Wörner, G., 1999. Palaeoclimatic implications of Mio–Pliocene sedimentation in the high-altitude intra-arc Lauca Basin of northern Chile. Palaeogeogr. Palaeocl. 151 (1–3), 79–100. https://doi.org/10.1016/S0031-0182 (99)00017-6.

González, G., Cembrano, J., Carrizo, D., Macci, A., Schneider, H., 2003. The link between forearc tectonics and Pliocene-Quaternary deformation of the Coastal Cordillera, northern Chile. J. S. Am. Earth Sci. 16, 321–342. https://doi.org/10.1016/S0895-9811 (03)00100-7. González, L., Dunai, T., Carrizo, D., Allmendinger, R., 2006. Young displacements on the Atacama Fault System, northern Chile from field observations and cosmogenic 21Ne concentrations. Tectonics 25, 1–15. https://doi.org/10.1029/2005TC001846 TC3006. Goudie, A.S., 2004. Salt heave or haloturbation. In: Goudie, A.S. (Ed.), Encyclopedia of Geomorphology. Routledge, London, pp. 892–893. Gray, J.M.N.T., Gajjara, P., Kokelaar, P., 2015. Particle-size segregation in dense granular avalanches. C. R. Phys. 16, 73–85. https://doi.org/10.1016/j.crhy.2015.1001.1004. Grosjean, M., Van Leeuwen, J.F.N., van der Knaap, W.O., Geyh, M.A., Ammann, B., Tanner, W., Messerli, B., Núñez, L.A., Valero-Garcés, B.L., Veit, H., 2001. A 22,000 14C year BP sediment and pollen record of climate change from Laguna Miscanti (23 S), northern Chile. Global Planet. Change 28 (1–4), 35–51. https://doi.org/10.1016/S0921-8181 (00)00063-1. Hoffmeister, D., 2018a. Orthophoto and digital elevation model of zebra stripe area PP1, Paposo Fault, Chile. CRC1211 Database (CRC1211DB). https://doi.org/10.5880/ CRC1211DB.17. Hoffmeister, D., 2018b. Orthophoto and digital elevation model of zebra stripe area PP3, Paposo Fault, Chile. CRC1211 Database (CRC1211DB). https://doi.org/10.5880/ CRC1211DB.18. Horton, R.E., 1945. Erosional development of streams and their drainage basins; hydrophysical approach to quantitative morphology. Bull. Geol. Soc. Am. 56, 275–370. https://doi.org/10.1130/0016-7606(1945)56[275:EDOSAT]2.0.CO;2. Houston, J., 2006. Variability of precipitation in the Atacama Desert: its causes and hydrological impact. Int. J. Climatol. 26, 2181–2198. https://doi.org/10.1002/joc.1359. Houston, J., Hartley, A.J., 2003. The Central Andean west-slope rainshadow and its potential contribution to the origin of hyper-aridity in the Atacama Desert. Int. J. Climatol. 23, 1453–1464. https://doi.org/10.1002/joc.938. Jordan, T.E., Kirk-Lawlor, N., Jordan, T.E., Kirk-Lawlor, N.E., Blanco, N.P., Rech, J.A., Cosentino, N.J., 2014. Landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 Ma, Atacama Desert, Chile. GSA Bull. 126 (7–8), 1016–1046. https://doi.org/10.1130/B30978.1. Jordan, T., Blanco, N., Quezada, A., Jensen, A., Vásquez, P., Sepúlveda, F., 2018. Comment on paper by Ritter et al. (2018), Evidence for multiple Plio-Pleistocene lake episodes in the hyperarid Atacama Desert, published in Quaternary Geochronology: v. 44, p. 1– 12. Quat. Geochronol. 47, 163–169. Kessler, M.A., Werner, B.T., 2003. Self-organization of sorted patterned ground. Science 299 (5605), 380–383. https://doi.org/10.1126/science.1077309. Kirk-Lawlor, N.E., Jordan, T.E., Rech, J.A., Lehmann, S.B., 2013. Late Miocene to Early Pliocene paleohydrology and landscape evolution of Northern Chile, 19 to 20 S. Palaeogeogr. Palaeocl. 387, 76–90. https://doi.org/10.1016/j. palaeo.2013.07.011. Kokelaar, B.P., Bahia, R.S., Joy, K.H., Viroulet, S., Gray, J.M.N.T., 2017. Granular avalanches on the Moon: mass-wasting conditions, processes, and features. J. Geophys. Res. Planet 122, 1893–1925. https://doi.org/10.1002/2017JE005320. Krengel, D., Strobl, S., Sack, A., Heckel, M., Pöschel, T., 2013. Pattern formation in a horizontally shaken granular submonolayer. Granul. Matter 15, 377–387. https://doi. org/10.1007/s10035-013-0411-2. Lämmel, M., Meiwald, A., Yizhaq, H., Tsoar, H., Katra, I., Kroy, K., 2018. Aeolian sand sorting and megaripple formation. Nat. Phys. 14 (7), 759–765. https://doi.org/10.1038/ s41567-018-0106-z. Larrain, H., Velasquez, F., Cereceda, P., Espejo, R., Pinto, R., Osses, P., Schemenauer, R.S., 2002. Fog measurements at the site Falda Verde north of Chanaral compared with other fog stations of Chile. Atmos. Res. 64, 273–284. https://doi.org/10.1016/S01698095(02)00098-4. Latorre, C., Betancourt, J.L., Raylander, K.A., Quade, J., 2002. Vegetation invasions into Absolut Desert: a 45,000-yr rodent midden record from the Calama-Salar de Atacama Basins, northern Chile (22–24°S). Geol. Soc. Am. Bull. 114, 349–366. https://doi.org/ 10.1130/0016-7606(2002)114b0349:VIIADAN2.0.CO;2. Latorre, C., Santoro, C.M., Ugalde, P.C., Gayo, E.M., Osorio, D., Salas-Egaña, C., De Pol-Holz, R., Joly, D., Rech, J.A., 2013. Late Pleistocene human occupation of the hyperarid core in the Atacama Desert, northern Chile. Quaternary Sci. Rev. 77, 19–30. https://doi.org/ 10.1016/j.quascirev.2013.06.008. Lavenu, A., Thiele, R., Machette, M.N., Dart, R.L., Bradley, L-N., Haller, K.M., 2000. Maps and Database of Quaternary Faults in Bolivia and Chile: USGS Open-File Report 00-283. https://pubs.usgs.gov/of/2000/ofr-00-0283/. Loveless, J.P., 2007. Extensional tectonics in a convergent margin setting: deformation of the northern Chilean forearc. Ph.D. thesis. Cornell Univ., Ithaca, N.Y. Loveless, J.P., Pritchard, M.E., 2008. Motion on upper-plate faults during subduction zone earthquakes: case of the Atacama Fault System, northern Chile. Geochem. Geophy. Geosy. 9, Q12017. https://doi.org/10.1029/2008GC002155. Loveless, J.P., Pritchard, M.E., Allmendinger, R.W., 2006. Motion of upper plate faults during subduction zone earthquakes: the curious case of the Atacama Fault in northern Chile. Seismol. Res. Lett. 77 (2), 291. Matmon, A., Simhai, O., Amit, R., Haviv, I., Porat, N., McDonald, E., Benedetti, L., Finkel, R., 2009. Desert pavement–coated surfaces in extreme deserts present the longest-lived landforms on Earth. Geol. Soc. Am. Bull. 121, 688–697. https://doi.org/10.1130/B26422.1. Matmon, A., Quade, J., Placzek, C., Fink, D., Arnold, M., Aumaitre, G., Neilson, J.W., 2015. Seismic origin of the Atacama Desert boulder fields. Geomorphology 231, 28–39. https://doi.org/10.1016/j.geomorph.2014.11.008. McFadden, L.D., McDonald, E.V., Wells, S.G., Anderson, K., Quade, J., Forman, S.L., 1998. The vesicular layer and carbonate collars of desert soils and pavements: formation, age and relation to climate change. Geomorphology 24 (2–3), 101–145. https://doi.org/ 10.1016/S0169-555X(97)00095-0.

S.M. May et al. / Geomorphology 344 (2019) 46–59 Michalski, G., Bohlke, J.K., Thiemens, M., 2004. Long term atmospheric deposition as the source of nitrate and other salts in the Atacama Desert, Chile: New evidence from mass‐independent oxygen isotopic compositions. Geochim. Cosmochim. Acta 68 (20), 4023–4038. Milana, J.P., 2009. Largest wind ripples on Earth? Geology 37, 343–346. https://doi.org/ 10.1130/G25382A.1. Mortensen, H., 1927. Der Formenschatz der nordchilenischen Wüste. Abhandlungen der Gesellschaft der Wissenschaften in Göttingen, Berlin 191 p. Nester, P.L., Gayó, E., Latorre, C., Jordan, T.E., Blanco, N., 2007. Perennial stream discharge in the hyperarid Atacama Desert of northern Chile during the latest Pleistocene. P. Natl. A. Sci. 104, 19724–19729. https://doi.org/10.1073/pnas.0705373104. NRCS (USDA Natural Resources Conservation Service), 2010. National Engineering Handbook: Part 630 – Hydrology (chapter 15). Orellana, H., García, J.L., Ramírez, C., Zanetta, N., 2017. El aluvión del 9 agosto 2015 en Alto Patache, región de Tarapacá, Desierto de Atacama 1. Rev. Geogr. Norte GD. 68, 65–89. https://doi.org/10.4067/S0718-34022017000300065. Owen, J.J., Amundson, R., Dietrich, W.E., Nishiizumi, K., Sutter, B., Chong, G., 2011. The sensitivity of hillslope bedrock erosion to precipitation. Earth Surf. Process. Landf. 36 (1), 117–135. https://doi.org/10.1002/esp.2083. Owen, J.J., Dietrich, W.E., Nishizumi, K., Chong, G., Amundson, R., 2013. Zebra stripes in the Atacama Desert; fossil evidence of overland flow. Geomorphology 182, 157–172. https://doi.org/10.1016/j.geomorph.2012.11.006. Parsons, A.J., 2004. Overland flow. In: Goudie, A. (Ed.), Encyclopedia of Geomorphology. Routledge, London, pp. 737–739. Pelletier, J.D., DeLong, S.B., Orem, C.A., Becerra, P., Compton, K., Gressett, K., Lyons-Baral, J., McGuire, L.A., Molaro, J.L., Spinler, J.C., 2012. How do vegetation bands form in dry lands? Insights from numerical modeling and field studies in southern Nevada, USA. J. Geophys. Res.-Earth 117 (F4), F04026. https://doi.org/10.1029/2012JF002465. Placzek, C., Matmon, A., Granger, D.E., Quade, J., Niedermann, S., 2010. Evidence for active landscape evolution in the hyperarid Atacama from multiple terrestrial cosmogenic nuclides. Earth Planet. Sc. Lett. 295, 12–20. https://doi.org/10.1016/j. epsl.2010.03.006. Pouliquen, O., Vallance, J.W., 1999. Segregation induced instabilities of granular fronts. Chaos 9, 621–630. https://doi.org/10.1063/1.166435. Pouliquen, O., Delour, J., Savage, S.B., 1997. Fingering in granular flows. Nature 386, 816–817. https://doi.org/10.1038/386816a0. Quade, J., Reiners, P., Placzek, C.J., Matmon, A., Pepper, M., Ojha, L., Murray, K., 2012. Seismicity and the strange rubbing boulders of the Atacama Desert, northern Chile. Geology 40, 851–854. https://doi.org/10.1130/G33162.1. Rech, J.A., Currie, B.S., Shullenberger, E.D., Dunagan, S.P., Jordan, T.E., Blanco, N., Tomlinson, A.J., Rowe, H.D., Houston, J., 2010. Evidence for the development of the Andean rain shadow from a Neogene isotopic record in the Atacama Desert, Chile. Earth Planet. Sc. Lett. 292, 371–382. https://doi.org/10.1016/j.epsl.2010.02.004.

59

Reich, M., Bao, H., 2018. Nitrate deposits of the Atacama Desert: a marker of long-term hyperaridity. Elements 14, 251–256. https://doi.org/10.2138/gselements.14.4.251. Ritter, B., Binnie, S.A., Stuart, F.M., Wennrich, V., Dunai, T.J., 2018. Evidence for multiple Plio-Pleistocene lake episodes in the hyperarid Atacama Desert. Quat. Geochronol. 44, 1–12. https://doi.org/10.1016/j.quageo.2017.11.002. Ruegg, J.C., Ruegg, J.C., Campos, J., Armijo, R., Barrientos, S., Briole, P., Thiele, R., Arancibia, M., Cafiuta, J., Duquesnoy, T., Chang, M., Lazo, D., Lyon-Caen, H., Ortlieb, L., Rossignol, J.C., Serrurier, L., 1996. The Mw= 8.1 Antofagasta (North Chile) earthquake of July 30, 1995: first results from teleseismic and geodetic data. Geophys. Res. Lett. 23, 917–920. https://doi.org/10.1029/96GL01026. Sáez, A., Cabrera, L., Garcés, M., van den Bogaard, P., Jensen, A., Gimeno, D., 2012. The stratigraphic record of changing hyperaridity in the Atacama desert over the last 10 Ma. Earth Planet. Sc. Lett. 355, 32–38. https://doi.org/10.1016/j.epsl.2012.08.029. Schumm, S.A., 1956. Evolution of drainage systems and slopes in badlands at Perth Amboy, New Jersey. Geol. Soc. Am. Bull. 67, 597–646. https://doi.org/10.1130/00167606(1956)67[597:EODSAS]2.0.CO;2. Sherratt, J.A., 2015. Using wavelength and slope to infer the historical origin of semiarid vegetation bands. P. Natl. A. Sci. 112 (14), 4202–4207. https://doi.org/10.1073/ pnas.1420171112. Valderrama, P., Roche, O., Samaniego, P., van Wyk des Vries, B., Araujo, G., 2018. Granular fingering as a mechanism for ridge formation in debris avalanche deposits: Laboratory experiments and implications for Tutupaca volcano. Peru. J. Volcanol. Geoth. Res. 34, 409–418. https://doi.org/10.1016/j.jvolgeores.2017.12.004. Valentin, C., d'Herbès, J.M., Poesen, J., 1999. Soil and water components of banded vegetation patterns. Catena 37 (1–2), 1–24. https://doi.org/10.1016/S03418162(99)00053-3. Vargas, G., Rutllant, J., Ortlieb, L., 2006. ENSO climate teleconnections and mechanisms for Holocene debris flows along the hyperarid coast of western South America (17–24 S). Earth Planet. Sci. Lett. 249, 467–483. https://doi.org/10.1016/j.epsl.2006.07.022. Veit, H., 1996. Southern Westerlies during the Holocene deduced from geomorphological and pedological studies in the Norte Chico, Northern Chile (27–33 S). Palaeogeogr. Palaeocl. 123 (1–4), 107–119. https://doi.org/10.1016/0031-0182(95)00118-2. Wells, S.G., Dohrenwend, J.C., 1985. Relict sheetflood bed forms on late Quaternary alluvial-fan surfaces in the southwestern United States. Geology 13, 512–516. https://doi.org/10.1130/0091-7613(1985)13b512:RSBFOLN2.0.CO;2. Wilcox, A.C., Escauriaza, C., Agredano, R., Mignot, E., Zuazo, V., Otárola, S., Castro, L., Gironás, J., Cienfuegos, R., Mao, L., 2016. An integrated analysis of the March 2015 Atacama floods. Geophys. Res. Lett. 43, 8035–8043. https://doi.org/10.1002/ 2016GL069751. Xiao, J., Shen, Y., Tateishi, R., Bayaer, W., 2006. Development of topsoil grain size index for monitoring desertification in arid land using remote sensing. Int. J. Remote Sens. 27 (12), 2411–2422.