Fault interaction along the Central Andean thrust front: The Las Peñas thrust, Cerro Salinas thrust and the Montecito Anticline

The region in west-central Argentina between the thin-skinned Precordillera and the thick-skinned Sierras Pampeanas structural domain is among the most active zones of thrust tectonics in the world. We quantify the rates of deformation on the east-vergent Las Peñas thrust (LPT), and the west-vergent Cerro Salinas thrust (CST). The Montecito anticline (MA) is located at their intersection. We mapped three key locations, collected stratigraphic logs from the MA, dated three ashes using U-Pb in zircon and dated 10 terraces using cosmogenic Be-10 depth profiles. Five terrace levels are present where the Rio Las Peñas crosses the LPT, up to 45 m above the modern river. Cosmogenic dating of the uppermost terrace (T1) yields and age of 123.8 +26.5/-12.3 ka. A reconstruction of this surface using a blind thrust rupture scenario indicates 73 +/- 7 m horizontal shortening and 34 +/- 3 m vertical displacement. Shortening across the structure is therefore 0.59 +0.10/-0.13 mm/yr with a vertical uplift rate of 0.27 +0.05/-0.06 mm/a. Previous work indicates higher rates to the south on the order of 2 mm/yr (Schmidt et al., 2011). Lower terraces give ages of 38.0 +11/-6.2 ka (T2) and 1.5 +5.0/-0.6 ka (T4). Three terrace levels are preserved near the center of the CST. The middle surface (T2) is folded across the axis of the structure and yields an age of 112.5 +33/-14.4 ka. Given 22.9 m surface uplift, this indicates a vertical uplift rate of 0.20 +0.05/-0.06 mm/yr, similar to the rate on the LPT. The upper terrace (T1) yields a younger age (97.1 +29.8/-12.4 ka); the T1 and T2 ages overlap within uncertainty, indicating rapid river incision at the time of their formation. An intercalated ash within the Neogene strata gives an age of 16.2 +/- 0.2. Previous work indicates long-term shortening rates of 0.8 mm/yr (Verges et al., 2007) and that the CST initiated after 8.5 Ma. The lowermost unit exposed in the MA is the Los Pozos Fm., with no indication of syn-depositional deformation. An intercalated ash from the top of this formation yields an age of 5.76 +/- 0.09 Ma. Internal unconformities are present within the overlying transitional unit and the Mogotes Fm., indicating deformation post-dates 5.8 Ma in the MA. An ash within the Mogotes Fm. is 1.52 +/- 0.06 Ma. Slip is modeled as 3.5 km reverse slip across an east-dipping dislocation with a 45 degree dip. This suggests horizontal shortening and vertical uplift of 0.42 mm/yr since the onset of deformation. Uplifted terraces near the center of the MA are 4.7 +0.8/-0.3 ka (T2) and 1.9 +3.4/-1.9 ka (T3), 6 and 4.6 m above the modern river, respectively. This suggests recent vertical uplift or incision rates of 1.3-2.4 mm/yr. These data suggest that deformation in the MA is comparable to that at the LPT and CST. Deformation in the MA could be accelerating, but alternatively, river incision could be accelerating due to climate change.