The Salar de Atacama Basin: a Subsiding Block within the Western Edge of the Altiplano-Puna Plateau

The internally drained Salar de Atacama (SdA) Basin, located in the proximal fore-arc between the present magmatic arc (Western Cordillera) to the east and the North Chilean Precordillera (Cordillera de Domeyko) to the west, represents a prominent morphological anomaly in the Central Andean Plateau. The basin is a post-Incaic feature that developed contemporaneously with the initial plateau uplift. Before 38 Ma,the magmatic arc was positioned in the present-day Precordillera;as a result,the Cretaceous to Eocene sequences that underlie younger SdA sediments were deposited in a proximal back-arc location, where a westward extending arm of the Salta Rift interfered with the magmatic arc. The lithospheric block underlying the SdA area has different physical properties to its surroundings. Strong, positive, isostatic residual gravity anomalies in the SdA area are caused by dense bodies that were magmatically or tectonically emplaced at depths of 10 to 15 km and 4 to 6 km, probably during Cretaceous rift processes. They contrast with gravity minima in the Precordillera and Western Cordillera. Seismic refraction profiles in the SdA lithospheric block show velocities of > 6.5 km s(-1) and 7.5 km s(-1) at depths of < 10 km and similar to 30 km, respectively, but no clear Moho. A crustal thickness of 67 km, obtained from receiver functions, is reported, however Mesozoic and Tertiary structures do not indicate crustal thickening. Seismic tomography shows v(p)/v(s) ratios that do not reveal mantle serpentinization, and high v(p) and high Q(p) values (> 2000) point to a dense and strong lithosphere down to the plate interface. It is proposed that the SdA lithospheric block which, as part of the Salta Rift system in the pre-Incaic back-arc,had been hotter than the adjacent lithosphere was not cooled sufficiently to be serpentinized after the Incaic Event. Instead, mantle rocks hydrated at 600-800 degrees C (chlorite lherzolite) fit the measured physical properties. An episode of flat or low-angle subduction from 38 to 28 Ma, that preceded the emplacement of the modern magmatic arc in the Western Cordillera, was probably an important starting condition that allowed these processes to occur. Slab resteepening caused subsidence of the heavy SdA lithospheric block and its contact with the lower plate resisted the westward advancement of the asthenospheric wedge, thus causing the eastwardly convex curvature of the volcanic front around the east side of the SdA Basin.