Shortening and structural architecture of the Andean fold-thrust belt of southern Bolivia (21°S): Implications for kinematic development and crustal thickening of the central Andes

Abstract

Reliable crustal shortening estimates for the central Andes (South America) are a critical component in validating models of Cordilleran processes. In southern Bolivia, insight into crustal shortening and the kinematic development of the Andean thrust belt are limited by the lack of a unified structural evaluation across the entire width of the retroarc region. To address these shortcomings, we (1) estimate crustal shortening by integrating new geologic mapping with published geophysical data to construct a balanced cross section across the Subandean zone (SAZ), Interandean zone (IAZ), and Eastern Cordillera (EC) at 21°S; (2) develop a kinematic model for the retroarc thrust belt; and (3) estimate crustal budgets and average crustal thicknesses over the region. We estimate 337 ± 69 km (36% ± 7%) of total shortening (SAZ, 82 km; IAZ, 70 km; EC, 120 km; Altiplano, 65 km). The thrust belt developed from late Eocene time to the present by tectonic wedging and eastward emplacement of two ~10-12-km-thick basement thrust sheets that distribute slip into overlying sedimentary rocks. Our range of crustal shortening values can account for 90%-118% of the current retroarc crustal area. Assuming an initial crustal thickness of 35 km, the EC and Altiplano did not achieve modern crustal thicknesses (~65 km) until the present. However, assuming a 40-km-thick initial crust, the EC and Altiplano attained the critical thickness for either eclogitic phase changes or lower crustal flow (>45-50 km) by ca. 27-25 Ma, modern thicknesses by ca. 10 Ma, and currently exceed geophysically observed thicknesses by ~2.5-14.5 km; this suggests crustal losses significant enough to have affected hinterland surface elevation.

Thermochronologic Evidence for Late Eocene Andean Mountain Building at 30°S

“…PrC: Precordillera; PR: Pampean Range; SBS: Santa Bárbara System. (right) Magnitude of horizontal shortening estimated for the different latitudes, after McQuarrie (), Anderson et al (), Allmendinger et al (), Allmendinger and Judge (), Cristallini and Ramos (), Cegarra and Ramos (), Giambiagi et al (), and Mescua et al ().…”

“…Based on these observations, we propose the existence of two continuous Eocene belts: a western one that comprises, from north to south, the Western Cordillera (north of 27°S), the Valeriano Fault zone (28°-29°S), and the Vicuña-Rivadavia to Guanta area (30°S); whereas the eastern belt includes the Eastern Cordillera (in the north) and the Colangüil range (at 30°S). The After Hong et al (2007) and Oncken et al (2006) between 23°and 27°S, Rossel et al (2016) (2002), Anderson et al (2017), Allmendinger et al (1990), Allmendinger and Judge (2014), Cristallini and Ramos (2000), Cegarra and Ramos (1996), Giambiagi et al (2015), and Mescua et al (2014).…”

Tectonic Regimes of the Central and Southern Andes: Responses to Variations in Plate Coupling During Subduction

“…Upper‐crustal retroarc structures have accommodated orogen‐perpendicular (east‐west) shortening, with significant along‐strike (north‐south) variations (Figure ). Estimates of total crustal shortening, although hindered by magmatic overprinting and volcanic/sedimentary cover rocks in the arc and hinterland, attain a maximum of 300–350 km in the central Andes (at 15–25°S), 20–50 km at the transition from the central to southern Andes (33–35°S), and <20 km in the southern Andes of northern Patagonia (35–45°S) (Allmendinger, ; Allmendinger et al, ; Anderson et al, ; Eichelberger et al, ; Kley et al, ; Kley & Monaldi, ; McQuarrie, ; McQuarrie et al, ; Oncken et al, ; Perez et al, ; Roeder, ; Roeder & Chamberlain, ; Sánchez et al, ; Sheffels, ; Turienzo et al, ; von Gosen, ; Zapata & Allmendinger, ). Synorogenic sedimentary basins are preserved on both orogenic flanks, including forearc basins controlled by diverse structures and retroarc hinterland and foreland basins mostly associated with shortening‐induced topographic loading and lithospheric flexure (Horton, , , ; Horton & DeCelles, ; Jordan, ; Jordan et al, ; Watts et al, ).…”

“…In retroarc regions, the Altipano‐Puna plateau is supported by a 60–70 km thick crust produced by considerable shortening. Total east‐west shortening at 23°S is difficult to estimate due to the volcanic and sedimentary cover of the Puna plateau, but is roughly inferred to be 100–200 km on the basis of balanced cross sections at 21–25°S (Anderson et al, ; Baby et al, ; Cladouhus et al, ; Coutand et al, ; Grier et al, ; Kley, ; Kley et al, ; Kley & Monaldi, , ; Pearson et al, ). The eastern flank of the orogen is represented by a wide fold‐thrust belt consisting of a bivergent, basement‐involved zone (Eastern Cordillera) and a presently deforming, thin‐skinned frontal segment (Subandean and Santa Bárbara zones) (Allmendinger et al, ; DeCelles et al, ; Kley & Monaldi, ; Siks & Horton, ).…”

Foreland Basin Record of Uplift and Exhumation of the Eastern Cordillera, Northwest Argentina

“…Our findings do not support the suggestion that deformation began earlier in this area of Argentina during the Eocene (e.g., Fosdick et al, ). We also note the contrast between the deformation record in Argentina and that from farther north in Bolivia, where the onset of deformation is earlier and overall shortening magnitudes much greater (Anderson et al, ; Calle et al, ; McQuarrie, ).…”