EVIDENCE FOR LATE MIOCENE UPLIFT BY LONG-WAVELENGTH ROTATION OF WESTERN FLANK OF ALTIPLANO SEGMENT OF CENTRAL ANDES 20^O30' - 21^O30'S, CHILE

Extensive preservation of Miocene units across the western slope of the hyper arid Central Andes of Northern Chile provides an excellent opportunity to elucidate the morphologic evolution of the Altiplano plateau. Seismic reflection data between the latitudes of 20^o30' and 21^o30'S, imaging upper Cenozoic strata, reveal the tectonic evolution of the N-S trending Pampa del Tamarugal, an endorheic nonmarine forearc basin, in relation to the adjacent western Andean slope. Published studies north of 20^o30'emphasize that a series of early and middle Miocene west-vergent reverse faults in the valley and western slope, expressed as west-facing monoclines, uplifted the western slope. In striking contrast, the seismic data in our study area show a complex amalgamation of synthetic and antithetic reverse faults of early and middle Miocene age, most with no surface expression, in the basin. Upper Miocene alluvial deposits, whose base is ~7 Ma, dip ~2.6^oW over a width of ~10 km. These strata onlap in the eastern basin the tilted top surface of the upper member of the Altos de Pica Formation, which in our study area is comprised of sheetwash fanglomerates at least as young as 13 Ma and probably ~10 Ma. The eastern 20 km width of this surface, between its eastern onlap on Mesozoic rocks and a western monoclinal fold axis, has an average modern slope of ~5^oW, which decreases westward across a fold axis. Assuming an original depositional slope of 1^o for alluvial facies, the eastern fringe of the basin was rotated 1.6^o since 7 Ma and 2.4^o between ~10 Ma and 7 Ma. In contrast, between 24 and ~10 Ma, which encompasses the time of known transpressional deformation along the Precordilleran fault system, only 1^o of rotation is estimated. Based solely on these Miocene surfaces, westward rotation east of the monoclinal axis accommodated at least 840 m of uplift between ~10 and 7 Ma, and ~560 m of uplift younger than 7 Ma. Of the 3000 m of modern relief between the Altiplano and Pampa del Tamarugal, a minimum of 50 percent of the relief along the western slope has formed since ~10 Ma at this location. There is no evidence that the Oligocene to late Miocene reverse faults within the Pampa del Tamarugal contributed significantly to Andean uplift. Instead, uplift occurred largely through a mechanism controlled by lower or middle crustal flow or whole-crustal warping.