The geometry of the central Andean backarc crust: Joint interpretation of cross-section balancing and seismic refraction data

Detailed cross section balancing and the interpretation of seismic refraction data in the Andean backarc at 21°S latitude allow the development of a crustal balancing model for the backarc region. The thrust belt developed on the eastern flank of the Andean orogen can be divided into three major units: the Subandean fold and thrust belt incorporating Paleozoic to Cenozoic sediments, the Interandean with tightly folded Silurian to Triassic strata, and the eastern margin of the Eastern Cordillera with predominantly Ordovician anchimetamorphic sediments, structured by large open folds. The structure of the thrust belt is dominated by two large “blind” thrusts. eismic velocities in the upper to middle crust of the Eastern Cordillera with an underlying thick low velocity zone, derived from seismic refraction investigations, are interpreted as lower crustal material overthrust to the east over the crust of the Brazilian shield. The minimum shortening between the Eastern Cordillera and the foreland since Upper Cretaceous is about 145 km, as derived from cross section balancing. The crustal thickening, derived from the seismic refraction data, can be explained only from the foreland to the Eastern Cordillera by tectonic shortening. At least two main thrusts, cutting the whole backarc crust, are necessary to fit the high velocity bodies in the Eastern Cordillera to the geological observations. Further to the west, the thick continental crust, as derived from seismic refraction and gravimetric data, cannot be explained by tectonic shortening alone.