Assessing Quaternary Shortening Rates at an Andean Frontal Thrust (32°30′S), Argentina

Abstract

At the latitude of 32–33°S, the orogenic front of the eastern flank of the Andes is located in the Southern Precordillera, where two major thrusts concentrate Quaternary deformation at the surface. We estimate the shortening rate for one of these, the Las Peñas thrust, for the last ~200 ka, at the site of one of the best exposures of Quaternary thrusting along the Andes. Shortening was estimated across two prominent splays of the deformation zone at the mouth of the Las Peñas River through balanced cross‐sections by using a terrace surface as a marker of cumulative deformation. Terrace ages were constrained through cosmogenic isotopes (10Be) analysis on quartz cobbles collected from the surface and three depth profiles. Results indicate a mean shortening rate across the Las Peñas thrust of 0.27 + 0.11 mm/a. Our results are lower than Holocene shortening rates (1.9–2.4 mm/a) obtained for individual splays of the Las Peñas thrust nearby. We discuss these discrepancies and implications for thrust evolution since the Late Pleistocene, in particular regarding along‐strike rate variations that highlight the complexity of the Quaternary‐active thrust deformation zone. We argue that ongoing stream incision rates largely prevail with respect to thrust activity at the study site. This observation might be connected with a recent (Holocene?) basinward shift of blind thrust activity.

Late Pleistocene to Recent Deformation in the Thick‐Skinned Fold‐and‐Thrust Belt of Northwestern Argentina (Central Calchaquí Valley, 26°S)

“…Our new GPS results for the thick‐skinned deformation belt of NWA, where Quaternary activity is ubiquitous and broadly distributed in time and space (e.g., Casa et al, 2014; García et al, 2013, 2019; Strecker et al, 2012), reveal a gradual decrease in horizontal surface velocities from west to east toward the foreland without the superposed, well‐defined, rapid velocity gradient that characterizes the thin‐skinned system to the north (Figures 1, 3, and 10). We suggest that this is a reflection of distributed, internal deformation associated with reactivated faults that penetrate the entire brittle crust, comparable to what has been proposed for the fold‐and‐thrust belts of southern Perú, northern Bolivia, and the thick‐skinned Sierras Pampeanas, based on similar GPS surface‐velocity patterns (Brooks et al, 2003; Costa et al, 2019; Horton, 1999).…”

“…1, 3, and 10). We suggest that this is a reflection of distributed, internal deformation associated with reactivated faults that penetrate the entire brittle crust, comparable to what has been proposed for the fold-and-thrust belts of southern Perú, northern Bolivia, and the thickskinned Sierras Pampeanas, based on similar GPS surface-velocity patterns (Horton, 1999;Brooks et al, 2003;Costa et al, 2019).…”

Late Quaternary Intraplate Deformation Defined by the Las Chacras Fault Zone, West‐Central Argentina

“…The maximum shortening rate that we determine is similar to that of the Las Peñas Fault in the southern Precordillera (0.27 ± 0.11 mm/yr) in the last ∼200 ka (Costa et al, 2019). However, this is an exceptionally low rate for the Precordillera, and is low even when compared to measurements on different time scales or other locations along the same fault (Costa et al, 2019;Schmidt et al, 2011a). The LCFZ's average and maximum uplift rates of ∼0.3-0.4 and ∼0.3-0.5 mm/yr, on the other hand, are comparable to uplift rates of structures in the Eastern Precordillera such as the La Laja and La Rinconada Fault (Rimando et al, 2019;Rockwell et al, 2014).…”

“…Shortening rates measured along the LFCZ are relatively low compared to shortening rates along most Quaternary-active structures in the Precordillera region to the west, which reach up to 2 mm/yr (Figure 1). The maximum shortening rate that we determine is similar to that of the Las Peñas Fault in the southern Precordillera (0.27 ± 0.11 mm/yr) in the last ∼200 ka (Costa et al, 2019). However, this is an exceptionally low rate for the Precordillera, and is low even when compared to measurements on different time scales or other locations along the same fault (Costa et al, 2019;Schmidt et al, 2011a).…”

“…Assuming zero erosion could be justified by the observed ubiquity of desert varnish, which is an indication of high degree of surface preservation (e.g., Jakica et al, 2011;Rimando et al, 2019). Furthermore, zero erosion has also been assumed in other slip rate studies employing cosmogenic dating in several sites along the Precordillera and the Sierras Pampeanas (Costa et al, 2019;Hedrick et al, 2013;Rimando et al, 2019;Schmidt et al, 2011a;Siame et al, 2002). Alluvial surfaces are more likely than not to have a component of inherited nuclide concentration (e.g., Anderson, Repka, & Dick, 1996;Blisniuk et al, 2012;Brocard et al, 2003;Lal, 1991;Repka et al, 1997).…”