MIGRATION OF DEFORMATION AS A RESULT OF CLIMATE VARIATIONS: THE ANDES

The Eastern and Western Cordillera of the Central Andes differ considerably in both climate and deformation style. Climatically, the eastern slopes are tropical, receiving greater than 3 meters of rain per year, while the western slopes can go decades without recorded rainfall. These climatic differences are matched by strikingly different tectonic settings. The Eastern Cordillera and Sub-Andes deform by active shortening and thrusting and the Western Cordillera and Escarpment deform by long-wavelength warping. Here we show that the modern distribution of deformation in the Central Andes can be a result of enhanced orographic precipitation pattern beginning ca. 7-10 Ma. Reduced erosion on the western side would have steepened the orogen, forcing deformation to shift to the east where high precipitation amounts would have enhanced erosion. We support this hypothesis with a suite of geochemical and geomorphologic data as well as numerical modeling studies.

Erosion rates as measured by cosmogenic nuclides are < 0.01 mm yr^-1 in the west and more than an order of magnitude higher, > 0.2 mm yr^-1, in the east. Stream profiles from the Western Escarpment are indicative of slow knickzone retreat in the absence of modern tectonic forcing while streams on the Eastern Escarpment are the product of strong climate-tectonic feedbacks. These data support the results of both critical taper calculations and fault-scale friction calculations. The reduction in erosion at 7-10 Ma results in the steepening of the topographic ramp, shifting the Western Escarpment into a super-critical state and increasing the friction on internal faults. Such steepening is therefore accompanied by a termination of internal deformation with strain accommodated along a basal detachment. At the same time, high erosion rates on the Eastern Escarpment promote advection of crustal material and enhanced internal deformation in a sub-critical state.