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.