Comparison of crustal thickening budget and shortening estimates in southern Peru (12-14 S): Implications for mass balance and rotations in the "Bolivian orocline"

Abstract

Two fundamental questions of interest with regard to the Andean Plateau are the mass balance of material needed to create and sustain a 3-4-km-high plateau and the relationship between the plateau and the bending of the Bolivian orocline. The link be- tween these two questions is the distribution of shortening through the Central Andes. Is crustal shortening suffi cient to support an isostatically compensated 60-70-km-thick crust? Is differential shortening between the hinge of the orocline and its limbs suffi cient to account for both physiographic curvature and measured rotations? Three-dimensional (3-D) models of deformation within curved orogens require, in addition to paleomagnetic data, viable two-dimensional estimates of dis- placements. To this end, we present new map data and use those data to derive estimates of shortening across the northern margin of the Andean Plateau. The cross-section extent, from the eastern edge of the volcanic arc to the foreland basin, is approximately one half of the physiographic width of the Andean Plateau in Peru. Cross-sectional shortening estimates in southern Peru (12-14°S) provide a preferred estimate of 123 km or 40% short- ening, with a minimum estimate of 58 km or 25% shortening and maximum estimate of 333 km or 65% shortening. The largest con- trolling factor on the magnitude of shorten- ing is where basement becomes involved in thrust belt deformation. Using our preferred shortening estimate, the differential shorten- ing between our cross section on the northern margin of the Bolivian orocline (12-14°S) and cross sections across the axis of the Bolivian orocline (17-18°S) is 177 km, corresponding to 23° of rotation. The differential shortening for the young (younger than 15 Ma) defor- mation of the Altiplano and Subandean re- gion between 12-14°S and 17-18°S is 88 km, correlating to 11.5° of rotation. While dif- ferential shortening can account for young (younger than 15 Ma) vertical-axis rotations, it cannot completely account for early 45- 35 Ma rotations (37° ± 15°). This suggests that forearc rotations refl ect both bending due to differential shortening, ~12°, as well as block rotations (~25°-30°), which facilitated defor- mation in a tightening core and transported material toward the center of the orocline. The preferred estimate of shortening is well short of the required 240-300 km of shorten- ing needed to account for a 60-70-km-thick crust under the entire plateau. This suggests that for an isostatically equilibrated crust, either (1) there is a signifi cant amount of shortening (~100-150 km) in the western half of the plateau that is hidden by the volcanic arc; or (2) crustal material is being added to the Peruvian section of the Andean Plateau, most likely through lower-crustal fl ow.