Moho geometry beneath the eastern margin of the Andes, northwest Argentina, and its implications to the effective elastic thickness of the Andean foreland

Abstract

The seismically constrained shape of the Moho beneath the eastern margin of the northwest Argentine Andes (24°S) is interpreted in the context of regional isostasy to constrain the flexural rigidity of the lithosphere in this actively deforming orogen. Seismic travel time residuals from regional intermediate depth earthquakes are inverted for three‐dimensional variations in the Moho depth beneath a portable seismic network spanning the eastern margin of the Puna plateau and the Santa Barbara foreland system. Beneath the easternmost Santa Barbara ranges, the Moho is relatively flat and lies at 40–45 km depth. Farther west, beneath the eastern margin of the Puna, the Moho dips 10–15° to the west before flattening out at a depth of 60–65 km beneath the plateau. The Moho inversion model and coincident gravity data constrain the flexural rigidity of the foreland lithosphere to be between 1021 and 1022 m. The corresponding effective elastic thickness of 6–12 km is a factor of 2 to 4 less than that estimated in previous published studies for the Bolivian Altiplano and Subandean ranges farther to the north. These results indicate that in contrast to the Bolivian Andes, the foreland lithosphere in northwest Argentina does not provide significant flexural support to the plateau margin, and the eastern margin of the Andes is nearly in a state of local isostatic compensation. The north to south decrease in the effective elastic thickness of the Andean lithosphere supports published seismic wave attenuation studies which indicate that south of 23°S the lithospheric thickness decreases beneath the plateau and foreland.