CRETACEOUS AND PALEOGENE CONTRIBUTIONS TO ANDEAN OROGENSIS ON THE WESTERN MARGIN OF THE CENTRAL ANDEAN PLATEAU, PRECORDILLERA, CHILE, 20°30'-22°30'S

The Central Andean Plateau is largely a Neogene feature. Estimating the relative role of different processes in its construction, such as crustal shortening, magmatic addition and delamination, is sensitive to the initial conditions at the beginning of the Neogene. The geology of the Chilean Precordillera records various shortening events that predate the Neogene and so need to be considered in models concerning formation of the plateau.

The style, location and timing of shortening in the Chilean Precordillera, 20°30'-22°30'S, was strongly controlled by the location of Jurassic and Early Cretaceous extension during formation of the Tarapacá ensialic back-arc basin. Facies and stratal thickness variations indicate an important basin-bounding normal fault(s) separated the basin center, located in the western part of the Precordillera (Sierra de Moreno) and Pampa Tamarugal, from the eastern shoulder of the basin, located in the eastern part of the Precordillera (Sierra del Medio). The first shortening event, in the early Late Cretaceous, deformed the basin fill and produced rock uplift and exhumation sufficient to strip the 3-4 km of basin fill and expose Paleozoic basement to erosion in the Late Cretaceous. This was followed by late Late Cretaceous extension and volcanism (Cerro Empexa and Quebrada Mala Formations). A second shortening event occurred in the earliest Paleocene and gently folded the latest Cretaceous volcanic-sedimentary succession. Conformable Jurassic to Middle Eocene successions in the Sierra del Medio demonstrate that the eastern shoulder of the basin escaped Late Cretaceous and Paleocene deformation; the basin-bounding fault apparently reactivating various times and accommodating the differences in strain. Migration of arc magmatism to the Sierra del Medio in the Early and Middle Eocene (Icanche Fm.) thermally weakened the eastern borderland and set the stage for the important Middle Eocene-Early Oligocene Incaic shortening event. Reverse faulting of basement against supracrustal rocks, the denudation history recorded in syntectonic basins, published thermochronology and REE chemistry of syntectonic plutons indicate kilometers of rock uplift and exhumation during the Incaic event, and crustal thicknesses >40-45 km by the Early Oligocene for the Precordillera region.