Recently active contractile deformation in the forearc of southern Peru
Highlights
► Contractile structures produce ongoing uplift in the forearc of southern Peru. ► Pleistocene river incision rates (∼0.2–0.5 mm/yr) are similar to Late Miocene rates. ► Pleistocene-aged pediment surfaces yield surface erosion rates as low as ∼0.2 m/Ma.
Introduction
While conceptually simple, models of Andean tectonics remain controversial particularly concerning the forces responsible for and the timing of range uplift. Recently, there has been renewed interest in the active tectonic and climatic processes along the Andean orogen (Allmendinger et al., 2005a, González et al., 2006, Kober et al., 2007, Schildgen et al., 2007, Schildgen et al., 2009, Jordan et al., 2010, Saillard et al., 2011). The canonical model for the western Andean margin suggests that low-relief surfaces within the Atacama Desert are ancient relict surfaces that were abandoned >7 Ma due to incision caused by periods of intense surface uplift (Tosdal et al., 1984) and are preserved by the hyperarid climate and an eastward migration of the active deformation belt through time. In this view, the Western Cordillera formed early on in the sequence of Andean uplift and today the western slope of the Altiplano is a passive monocline that produces no significant Neogene deformation (Isacks, 1988). However in the last decade, with the refinement of remote sensing and absolute dating methods, new data has called into question our present understanding of the rates, timings, styles, and locations of active deformation within the western Andean margin. Work in northern Chile highlights active structures in the forearc leading to refinement and enhanced detail of Isacks' (1988) model of the western margin. For example, Wörner et al. (2002) related contractile and extensional structures in the Precordillera to giant gravitational block rotation and oversteepening of the western margin, which formed in response to the monoclinal warping suggested by Isacks (1988). Alternatively, Muñoz and Charrier (1996), Victor et al. (2004), and Farias et al. (2005) suggest a west-vergent thrust system active between ∼30 and 6 Ma. Still other studies document more recent (Plio-Pleistocene) deformation associated with fault zones within the Coastal Cordillera and Precordillera (Armijo and Thiele, 1990, Allmendinger et al., 2005a, González et al., 2006, Audin et al., 2008) resulting in multiple models of forearc uplift through, at least in part, crustal deformation along steeply dipping faults. Others have stressed the importance of regional post-10 Ma forearc incision in response to regional uplift related to monocline growth (Schildgen et al., 2007, Schildgen et al., 2009, Hoke et al., 2007, Jordan et al., 2010) and, in the absence of evidence for abundant horizontal shortening in the western margin (Gregory-Wodzicki, 2000), have called on additional mechanisms for uplift that include subduction erosion (Hartley et al., 2000), lower-crustal ductile flow (Husson and Sempere, 2003) or lithospheric delamination (Garzione et al., 2006). One of the strongest arguments for an active forearc during the last ∼10 My comes from Allmendinger et al. (2005b) who show that the modern-day magnitude and direction of forearc rotations observed in GPS data are the same as the Miocene average of the time integrated vertical axis rotations observed in paleomagnetic data. While other studies of paleomagnetic rotations in the forearc region do not see significant post ∼15 Ma rotations (Roperch et al., 2006), some of this may be the lack of suitable units to measure as most of the forearc basin is filled with Miocene-age and older conglomerate and volcanic rocks. Taken together, these recent studies yield an emerging view of a western margin that is more active than previously recognized. In order to refine existing models, we must understand how significant and how consistently the deformation can be observed along the strike of the margin. However, this remains largely unknown. Here, we use regional mapping and geochronologic data from neomorphic features to investigate the style and magnitude of neotectonic activity along the western margin of the Altiplano in southern Peru (Fig. 1).
Section snippets
Forearc geomorphology
Since at least 3 Ma, the coastal Atacama Desert has been situated in a zone of hyperaridity that has resulted in a high degree of geomorphic surface preservation (Hartley, 2003). The low-relief surfaces of the Atacama are primarily developed on alluvial fan conglomerates of the Late Oligocene–Miocene Moquegua Fm. (and correlative units), which reach thicknesses of hundreds of meters. These surfaces have been abandoned through river incision in response to changes in regional or local base-level (
Methodology
Using Landsat and Advanced Spaceborne Thermal Emission and Reflection. (ASTER) satellite imagery and aerial photographs, we identified sites of potentially active deformation by mapping locations of abrupt changes in topography or incision and by conducting field mapping of abandoned low-relief surfaces as well as structural features. The extents and elevations of geomorphic features were measured using a theodolite and hand-held GPS.
For surface exposure dating using in situ produced cosmogenic
Erosion rates
At each of the 5 trench locations we collected 6 samples of sediment down to a maximum depth of ∼1.5–2.0 m. We collected ∼750–1000 g of sand-coarse gravel and small pebbles at each depth. As the dominant lithology is rhyolite–andesite, many trench samples did not yield enough recoverable quartz for analysis. From 2 of the trenches, SA06-T4 and SA06-T5 at site 7, we were able to measure a 10Be concentration for 5 samples. For the remaining 3 trenches, SA06-T1, SA06-T3, and SA06-TP, only 4 samples
Evidence for contractile deformation
The presence and position of a regional system of steeply dipping reverse faults located at the southwestern edge of the Precordillera (Fig. 3) is supported by a range of geophysical and geological observations (1) offset youthful geomorphic features on the southern margin of the Precordillera, (2) seismicity data reflecting contractional focal mechanism solutions, and (3) GPS data suggesting contractional strain axes of ∼050°.
Multiple flexures trending sub-parallel to the coast and Western
Conclusions
The geomorphic and structural features of southern Peru are evidence of distributed active crustal deformation along range-sub-parallel contractile and strike-slip structures. The observation that Pleistocene incision rates are comparable with Late Miocene and Early Pliocene rates supports the notion that the rates and style of surface uplift within the forearc of southern Peru has been roughly consistent since at least 9 Ma. We suggest, that in this region the steep western side of the Andean
Acknowledgments
Support for this work came from NSF Grant 03455895, the IRD, and IGPP-LLNL. Many thanks to J. Berrospi, M. Saillard, and M. Davis for dedicated field support and P. Greene and K. Hodson for laboratory support.
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