FORELAND SHORTENING AND CRUSTAL BALANCING IN THE ANDES AT 30-DEGREES-S LATITUDE

Excellent surface exposures, known Benioff zone geometry, a dynamic morphology, and the availability of industry seismic reflection data all make the Andes at 30°S an excellent transect for investigating crustal‐scale balanced sections. 150–170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland. The thin‐skinned, east‐verging Precordillera of western Argentina accounts for 60–75% of the total shortening and formed mostly since major volcanism ceased at ∼10 Ma. Industry seismic reflection data show that the décollement of the Precordillera belt is located anomalously deep at ∼15 km. The belt is dominated by fault propagation folds and contains several prominent out‐of‐sequence thrust faults. Seismic stratigraphie analysis shows that Miocene strata in the Iglesia Valley, located between the Precordillera and the crest of the Andes, accumulated in a piggy‐back basin. Onlap relations on the western side indicate that the High Cordillera was uplifted as a major fault bend fold over a buried ramp. Thrusting in the two western belts, both in the High Cordillera of Chile, formed during the waning stages of arc volcanism, 11–16 Ma. and account for 25–40% of the shortening. The observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses, indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted Nazca Plate. Our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography, requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the Andes. This non‐unique model provides a simple explanation of the first order morphology of the Andes at this latitude. Copyright 1990 by the American Geophysical Union.