DEFORMATION, PRECIPITATION AND THE WIDTH OF THE CENTRAL ANDEAN PLATEAU, BOLIVIA

Analog and numerical models predict a coupling between climate and tectonics whereby variations in erosion modify the deformation and topography of orogens. A testable prediction from modeling studies is the change in width of mountain ranges as a result of increased precipitation. Unfortunately, few observational tests of these models exist. Here we evaluate the effect of climate on a critically tapered orogen, the Central Andes, using sequentially restored, balanced cross sections through wet (15-16ºS) and dry (21°S) regions of the orogen. Orogen growth or propagation causes the elongation of a fold-thrust belt in the direction of horizontal shortening. Two of the largest factors influencing thrust belt propagation are (1) the ratio of rock strength to decollement strength and (2) the magnitude of erosion. In the Central Andes, tectonics, basin geometry and style of deformation are remarkably similar along strike allowing us to use variations in propagation (or changes in percent shortening) to evaluate whether along strike changes in width and morphology are consistent with more rigorous erosion in the north. Results indicate similar percent shortening along the northern (40%) and southern (37%) sections suggesting, to a first order, a wetter climate has not limited the width (propagation) in the north. However, we can use comparisons of early (45-25 Ma) and recent (~19 or 8-0 Ma) shortening to test the longevity (~10⁷ yrs) of modern climate gradients. Early deformation in the Eastern Cordillera and Interandean zones produced 45% shortening of both sections indicating that N-S variations in erosion had no impact on the development of the fold-thrust belt from 45-25 Ma. Recent deformation produced 41% (N) versus 32% (S) shortening in the Subandean zone (SAZ) which implies a coupling between climate and tectonics since ~19 or 8-0 Ma. The ~40 km difference in the width between the northern and southern SAZ can be directly compared to the modern change in precipitation (a difference of ~2 m/yr). Using values appropriate to northern and southern Bolivia, analytical models predict a 35%-60% reduction in orogen width due to a 2-fold increase in precipitation. Our calculations suggest that the 2-3 fold increase in precipitation in the north limited SAZ growth (propagation) by ~30%, a value near the lower bounds of analytical models.