Research articleUsing spatial patterns of fluvial incision to constrain continental-scale uplift in the Andes
Introduction
Fluvial geomorphological archives are being increasingly used to reconstruct climate and deformation (e.g. Macklin et al., 2002; Bridgland et al., 2012; Stokes et al., 2017; Evenstar et al., 2018; Stokes et al., 2018). These archives comprise dominantly erosional (e.g strath terrace) through to depositional records (e.g. distributive fluvial systems) and can be regionally extensive (e.g. pediment systems). The common theme is that these types of geomorphic archive have low angle surface expressions which can be reconstructed to enable surface deformation to be quantified (e.g. Gesch, 2014; Demoulin et al., 2017; Stokes et al., 2018). Typically the majority of these studies concentrate on a single pediment surface over small basin scale or focus on trunk drainage or tributaries along a single river system (e.g. Anton et al., 2014, Martins et al., 2017, Evenstar et al., 2018). Whilst correlation of fluvial system behaviour has been attempted across regional scales (>100's km) to examine climate controls (e.g. Macklin et al., 2002), flooding records (e.g. Benito et al., 2000) and uplift rates (Litchfield and Berryman, 2006) this has typically been restricted to time-scales younger than the Pliocene. The few studies that look at regional scale deformation over longer timescales tend to utilise modelling of profiles or terrace patterns due to limited preservation of the fluvial system (e.g. Demoulin et al., 2007; Boulton et al., 2014). Where appropriate records are well preserved (e.g. in arid landscapes), availability of high-resolution satellite data sets facilitate regional geomorphic reconstruction over much wider spatial and temporal scales than was previously possible.
The Atacama is the most arid and potentially, oldest desert region in the world (Hartley et al., 2005; Dunai et al., 2005). The long term sustained arid climate leads to preservation of ancient landscapes, some of which are dated back to the Late Oligocene (Dunai et al., 2005; Evenstar et al., 2009; Evenstar et al., 2017). Several of these surfaces are identified as being generated following abandonment of fluvial incision events associated with fluctuating climate and long-term uplift of the Andean mountain chain (Evenstar et al., 2017). Within hyper-arid fluvial systems, as a function of their low geomorphic erosional efficiency, even modest vertical crustal uplift is likely to be more directly expressed as surface uplift, and thus more likely to be recorded within fluvial archives (e.g. Demoulin et al., 2017). In the central Andes (Atacama Desert) the sustained dominance of extreme aridity is expressed by the presence of near linear to convex hypsometric curves that are typical of areas experiencing ineffective fluvial incision (Montgomery and Brandon, 2002). This incision, in response to tectonic uplift, is documented in the fluvial archives, and along with the past climate (e.g. Dunai et al., 2005; Evenstar et al., 2009; Evenstar et al., 2017), these fluvial archives are increasingly well understood (Victor et al., 2004; Farías et al., 2005; Jordan et al., 2014; van Zalinge et al., 2017) and can be exploited to examine continental scale uplift. These approaches have documented vastly different rates of fluvial incision in northern Chile from 100 m/m.y. (Hoke et al., 2007) to 10 m/m.y (Cooper et al., 2016) since the Middle Miocene leading to different models for timing of Andean uplift.
Here, we utilise freely available satellite remotely sensed data, together with longer term geological and geomorphological erosional and depositional records to reconstruct and quantify the last 11 Myr of pediment and fluvial landscape erosion along the western side of the Central Andes orogen, a classic subduction zone setting. The low erosional efficiency of this landscape means that the surface uplift is preserved in excellent detail. The present study seeks to establish a continental scale view of the fluvial incision and how this varies parallel to the Andean Mountain chain. Using this we aim to establish the main controlling factors on incision rates in order to better understand their use in constraining continental scale uplift within the Central Andes.
Section snippets
Study area: landscape components and depositional record
The study region runs along the western margin of the Central Andean mountain range from 18°00'S to 20°15′S (Fig. 1a). This region consists of several different morphotectonic provinces; the Coastal Cordillera, Longitudinal Valley, Precordillera and Western Cordillera (Fig. 1a). The Coastal Cordillera runs along the coast of the Pacific Ocean and is characterized by the eroded Jurassic magmatic arc. The Coastal Cordillera is absent around the border between Chile and Peru (18°30'S) but
Study area climate
The past climate of the region is reasonably well constrained (Fig. 2). The climate has been arid to semi-arid since at least the late Jurassic (Hartley et al., 2005), arid since the late Oligocene and predominantly hyperarid since the mid Miocene (Evenstar et al., 2009; Jordan et al., 2014; Evenstar et al., 2017; Rech et al., 2019). Since the Oligocene, short intervals (< 1 myr) of slightly more humid conditions have been recorded (Evenstar et al., 2009, Evenstar et al., 2015, Evenstar et al.,
Reconstructing regional fluvial palaeo-surfaces
Relict paleo-surfaces of the PPS combined with the elevation data in the Longitudinal Valley were used to identify and extrapolate geomorphic surfaces (pediments and valley bottoms) throughout the past ca. 11 Myr. Four regional elevation profiles (A-D) were constructed across the PPS, one parallel to the drainage (A-East to West) and three perpendicular to the drainage (B, C and D-North to south) (Fig. 3). The landscape profiles were constructed using DEM data from Shuttle Radar Topography
Results
Profile A runs 45 km from east to west at 18°45′S in Fig. 3 and highlights the general relationship of the surfaces within the region. AS4 is predominantly preserved in the west of the Longitudinal Valley and shows an average depositional slope of 5° to the west. Cutting into AS4 within the centre of the Longitudinal Valley, DS3 incises 90 m and shows a similar depositional slope angle. To the east of DS3, DS2 cuts 90 m into and erodes DS3, with a higher slope angle of 10° to the west. Fig. 4
Discussion
The geomorphic archive, using both pediplain and fluvial surfaces, is reconstructed from Northernmost Chile and combined with age constraints to calculate fluvial incision rates over a continental scale (>250 km) and back to the Miocene. These data demonstrate three spatially marked changes in incision rates across the region, from north to south. These distinctly different regions have been termed Sectors 1–3. Sector 1 has the highest rates of fluvial incision through time (ca. 200 to
Conclusions
Within the Atacama Desert, Northern Chile, slow long-term erosion creates exceptional preservation of geomorphic archives (fluvial and pediment surfaces) allowing the long-term reconstruction of fluvial archives on a continental scale. In this study, fluvial profiles are constructed over a wide region (>250 km) along the western margin of the Andes (18°00'S to 20°15′S) and over a time frame from Miocene to Present day. The results reveal that incision patterns reconstructed over a wide area
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was funded by BHP and the University of Brighton Rising Stars. The authors are grateful to Marit Van Zalinge and Masie Mather for discussions on the manuscript. The authors thank two anonymous reviewer for their comments on improving the manuscript.
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