Abstract
A net of mainly reversed seismic refraction profiles has been measured in the years 1987 and 1989 in northern Chile, northern Argentina and southern Bolivia to investigate the crustal structure beneath the Andes from the coastal range to the Andean foreland. Regular blasts of the Chuquicamata copper mine, sea shots in the Pacific Ocean and land shots in Chile, Argentina and Bolivia were used as energy sources. One- and two-dimensional model calculations were applied to the data. Strong west-east as well as north-south variations in the crustal structure are observed which allow one to distinguish mainly three different crustal blocks.
The forearc block (Coastal Cordillera to Precordillera) increases in thickness from 40 km at the coast to 70 km under the Precordillera. A discontinuity between 20 km (west) and 35 km (east) depth is interpreted as the base of the Jurassic crust. The Moho of the oceanic crust is proven at 40 km depth under the Coastal Cordillera. The depth range from 20–30 (west) to 35–70 km (east) characterized by alternation of high and low velocities is interpreted as a mixture of continental crustal and mantle material and Nazca plate derivates. The average velocities of 6.6 km/s beneath the Coastal Cordillera to 6.2 km/s (Precordillera) suggest a rigid behaviour for this forearc block.
For the Western Cordillera and the western Altipiano the data indicate a strong, vertically structured, about 70-km-thick crust. Only at the southern W-E profile are clear signals observed from the lower crust and uppermost mantle. At the other related profiles signals from this depth range are missing. These data indicate a weakened crust with, in part, strongly reduced velocities, also suggesting partially melted zones. The backarc crust (eastern part of the Altipiano, the Eastern Cordillera and the Subandean Ranges) shows a rigid behaviour. Here, continental crust is overthrusted over its foreland. The data evidence the base of the crust in a rather shallow position at 25–30 km depth beneath the Eastern Cordillera. The underlying upper and middle crusts are characterized by a low velocity zone. Total crustal thickness decreases from 70 km under the Altipiano to 40 km at the Andean foreland, already representing shield conditions.
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Wigger, P.J. et al. (1994). Variation in the Crustal Structure of the Southern Central Andes Deduced from Seismic Refraction Investigations. In: Reutter, KJ., Scheuber, E., Wigger, P.J. (eds) Tectonics of the Southern Central Andes. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-77353-2_2
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