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.

The Andes are an ideal setting to explore orogenic wedge evolution and the cyclical tectonic processes in Cordilleran convergent-margin systems. Paleoaltimetry data suggest that the hinterland plateau in southern Bolivia underwent rapid surface uplift at~16-9 Ma, which is predicted to have induced rapid thrust belt propagation. We integrate fission track and (U-Th)/He ages from zircon and apatite with a sequentially restored cross section to quantify the timing and rates of thrust belt propagation in southern Bolivia for the last~43 Myr. These data show that retroarc shortening in the Eastern Cordillera propagated westward from~43 to 27 Ma as the wedge grew to attain critical taper and steady state. The thrust front then advanced rapidly eastward from~25 to 17 Ma across the western Interandean zone, where a weak decollement modified the critical taper angle. The thrust front stalled for~6 Myr but resumed eastward advance into the eastern Interandean zone and Subandean zone by~11-8 Ma, which we interpret as a response to increased accretionary influx and rapid orogenic wedge expansion induced by eclogitic delamination and corresponding hinterland surface uplift at~13 Ma. Development of an orographic barrier and wetter climatic conditions resulted in relatively steady state wedge conditions from~8.5 to 1.5 Ma. Rapid wedge growth after~1.5 Ma may be attributed to mass accumulation in the orogen interior or a weakened decollement. Our data reveal space-time variations in orogenic wedge evolution consistent with models of Cordilleran cyclicity and lithospheric removal, with important additional influences of erosion, climate, and rock rheology.

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