TESTING THE INFLUENCE OF EROSION ON THRUST BELT GEOMETRY AND KINEMATICS IN THE ANDES

Several fundamental problems confront regional studies attempting to assess the complex interactions between erosion and deformation in contractional orogenic belts. The Andes offer an ideal site in which to test existing models and avoid certain limitations. First, most field studies and modeling efforts have addressed opposing flanks of orogenic belts, typically the prowedge versus retrowedge sides. However, the geometry and kinematics of deformation in thrust belts is largely controlled by the preexisting stratigraphic configuration, resulting in a narrow hinterland wedge containing steeply dipping structures and a wide foreland wedge containing shallowly dipping structures. For example, despite dramatic latitudinal variations in erosion and climate, the entire length of the Andean orogen consists of an eastward tapering, east-vergent thrust belt to the east and steep forearc slope to the west. This observation suggests, contrary to some models, that erosion need not control the cross-sectional asymmetry and tectonic vergence of a mountain belt. Second, although thermochronologically determined cooling histories help define exhumation patterns in many orogens, there is an ambiguity in determining whether that exhumation is driven by thrust-induced uplift, regional isostatic uplift, or erosion. In the Andes, one of the best candidates for erosion-driven exhumation lies within the >4.5-km-high, 250-km-long range along the Altiplano-Eastern Cordillera boundary in Bolivia. Ongoing field mapping and thermochronological studies (Gillis et al., this volume) in this range, commonly referred to as the Cordillera Real, allow distinction between exhumation-related cooling synchronous with thrust slip versus exhumation-related cooling postdating thrust slip. New ⁴⁰Ar/³⁹Ar results and crosscutting structural relationships for pre-deformational (Permian-Triassic) and post-deformational (mid-Tertiary) granites indicate substantial Paleogene shortening and cooling prior to emplacement of the Quimsa Cruz batholith at ~24 Ma. Minimal or no shortening after 24 Ma requires that Neogene cooling is related to causes other than thrust slip within the range, possibly erosion, slip along a deeper mid-crustal decollement, or regional isostatic uplift.