MINERAL ZONING OF THE QUATERNARY LAVAS OF THE KURILE ISLAND ARC

Geologic mapping in the Bolivian Andes and balanced cross-section construction permit the determination of bounds on the amount of crustal shortening that has occurred in the mountain belt. Assumptions are carefully selected in the cross-section interpretations so that a precise minimum is calculated, as well as larger, more plausible estimates. The minimum bound on crustal shortening within the Cordillera Oriental and Subandean Zone is 210 km. Relaxation of specific assumptions yields estimates of 325 and 670 km; independent estimates cluster in the range of 300 to 350 km. The estimates are used to evaluate the contribution of crustal shortening to the present crustal thickness in the central Andes, and, by inference, that of magmatic addition. The minimum estimate of crustal shortening accounts for at least two-thirds of the present cross-sectional area, whether the entire crust across the width of the mountain belt is considered, or just the sedimentary wedge within the Cordillera Oriental and Subandean Zone. Magmatic addition is volumetrically less important in thickening the crust. Consideration of the deformation in the Altiplano indicates that crustal shortening has been an important process there as well. The balance between magmatic and crustal shortening in creating the thickened crust also may be affected by other processes. Tectonic erosion may augment the thickening, suggesting that magmatic addition would make an even smaller volumetric contribution. Strike-slip faulting or delamination (for shortening estimates greater than 300 km) may thin the crust laterally or vertically, respectively; these processes either allow greater amounts of magmatic addition or accommodate larger amounts of shortening. The shortening that has occurred across the mountain belt has been driven neither by magmatic intrusion nor by continental collision; it has occurred in response to subduction of only oceanic lithosphere.