REVISITING SHORTENING ESTIMATES ALONG THE BOLIVIAN OROCLINE: IMPLICATIONS OF THERMAL HEATING, EROSION AND CRUSTAL FLOW ON THE DEVELOPMENT OF A HIGH ELEVATION PLATEAU

In a landmark paper in 1988, Isacks cited the youthful topography of the Central Andean plateau as the primary signal of Cenozoic mountain building. The coincidence in space of two of South America's most distinguishing features, a pronounced seaward concave bend and the 4 km high Andean plateau was the impetus for a model that linked thermal heating and along strike variations in shortening to build the plateau and enhance the Bolivian orocline. Geophysical studies that defined a 60-70 km thick crust combined with new shortening estimates across the front of the orogen allowed Kley (1998, 1999) to visit the problem again. He suggested that tectonic shortening and crustal cross sectional area are only loosely correlated through the central Andes, and that other factors such as poorly constrained pre-Neogene shortening, and lower crustal flow may be necessary to explain crustal volumes.

A series of orogenic scale, balanced cross sections across the central Andes allow us to revisit the problem again. New orogen wide shortening estimates in addition to a suite of mineral cooling ages indicate that the majority of shortening in the Central Andes (~60%) is focused in the Eastern Cordillera (40-50%) and Interandean zone (15-20%) and is predominantly pre-Neogene (~40-20 Ma). Shortening in the Subandean zone accounts for 20-30% of the total shortening budget and is the locus of deformation since ~15 Ma. Shortening estimates across the Andean plateau range from 262-326 km in Peru and Bolivia to as low as 110 km in Argentina. When compared to magnitudes of shortening required to account for cross sectional area (assuming an initial crustal thickness of 40 km) in the northern portion of the Andean plateau (north of 22° S), balanced cross section estimates are sufficient or in excess of that needed. South of 22° S, shortening estimates are dramatically less (110 km) than that needed (288 km). This pronounced change in shortening is perhaps the strongest argument for both thermal heating and crustal flow as mechanisms to maintain a uniform plateau surface across a pronounced deformation gradient. While the magnitude of shortening is less north of 15° S, the percent shortening (40%) remains uniform suggesting significant climate variations have had minimal effects on the deformation history of the Andean plateau.