Crustal-thickness variations in the central Andes

Abstract

We estimated the crustal thickness along an east-west transect across the Andes at lat 20؇S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70-74 km under the Western Cordillera and the Eastern Cordillera thin to 32-38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20؇S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16؇S, 55-60 km) to south (20؇S, 70-74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustalthickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton.

Possible orogeny-parallel lower crustal flow and thickening in the Central Andes

“…These results are in accordance with geophysical observations of the velocity-density distribution at 21°-24°S (e.g. Beck & Zandt, 1996, 2002Yuan et al, 2001Yuan et al, , 2002: more specific information indicates that the brittle-ductile transition zone would occur at 14 to 20 km (low velocity zone), while a change to a more mafic composition would occur at 50 to 55 km depth (higher velocity).…”

More constraints to determine the seismic structure beneath the Central Andes at 21°S using teleseismic tomography analysis

“…Other projects carried out in the region also provided important results concerning lithospheric structure, crustal-thickness variations and lateral variations in the upper mantle (e.g. Beck et al, 1996;Dorbath and Granet, 1996;Myers et al, 1998;Dorbath and Masson, 2000;Beck and Zandt, 2002). Beneath the plateau, Myers et al, (1998) found low velocities on the eastern side towards the Eastern Cordillera suggesting lithospheric delamination.…”

“…The crustal thickness has been investigated by several authors (e.g. Schmitz et al, 1993;Wigger et al, 1994;Zandt et al, 1996;Beck et al, 1996) and reaches ~70-75 km under the Altiplano. Crustal shortening occurring mainly in the Eastern Cordillera (EC) and Subandean region is likely to be responsible for the thickened crust (e.g.…”

Introductory Chapter: An Introduction to the Seismic and Sequence Stratigraphy and to the Integrated Stratigraphy: Concepts and Meanings

“…This variation is accompanied by signiicant variations in elevation and gravimetric anomalies [52]. Signiicant variations in the crustal thickness of Andes have been previously pointed out by geological studies [53]. Moreover, the lithospheric lexure of the central Andes and the corresponding bending of stratigraphic sequences have been investigated in detail [54].…”