Understanding the Structural Evolution in the Central Bolivian Andes: An Integrated Flexural, Thermo-Kinematic, and Landscape Modelling Approach

The central Bolivian Andes are an ideal location to examine the influence of variations in climate and stratigraphic architecture on the structural evolution of the mountain range due to large climatic gradients and significant variations in sedimentary basin geometries along strike. The style of deformation in the central Andes is interpreted as thin-skinned faulting and folding of Paleozoic cover rocks over basement thrust sheets, as shown in balanced cross-sections. The region near 18°S is of particular interest due to the narrow extent of Subandes (SA), a poorly defined Interandean zone, and the highest precipitation gradients in the region. The preserved basin history contained in the Altiplano (AP) and Eastern Cordillera argues for an early fold-and-thrust belt located in the now-Western Cordillera, and the subsequent propagation of shortening to the east around 50 Ma. Flexural modeling of balanced cross-sections highlights the need for multiple basement thrust sheets with 35-97 km of displacement. Dynamic subsidence in the foreland and support in the hinterland is required to reproduce SA foreland basin depth, limit the AP sedimentation, and facilitate AP uplift it to its current elevation. Thermokinematic modeling is compatible with initiation of deformation at 50-45 Ma with faster velocities from 8 Ma to present during SA deformation. Out-of-sequence (OOS) thrusting at the westernmost limit of the SA is required to match young measured apatite fission-track (1.8-8.1 Ma) and zircon helium (8.1 Ma) ages. This varies from similar modeling in northern Bolivia, where largely in-sequence deformation reproduces the measured cooling ages (Rak et al., 2017). OOS faulting is supported by high Ksn values, indicative of uplift, and necessary for landscape evolution models (using a modified version of Cascade) to reproduce the steep topographic gradients documented in the region. OOS faulting was likely promoted by the abrupt eastern edge of the Paleozoic basin rocks, which limited forward propagation of structures, and/or increased erosion due to focused precipitation. Our results highlight the importance of incorporating detailed structural modeling in differentiating the geometry, kinematics, and timing of deformation to reproduce the thermochronologic ages, basin histories, and modern day landscape.