Abstract

A new structural model is presented for the Cenozoic part of the main range east of Santiago, a chain made up of thick Miocene ignimbrites and andesitic lavas overlying primitive Oligocene volcanoclastics deposited on a thin continental crust. Because the latter (Coya-Machali formation) locally show tight folding close to the margins of the Miocene volcanics, they had previously been described as deformed prior to construction of the are. Our field observations, analysed along two profiles (at 33 degrees and 34 degrees 15'S), however, support the conclusion hinted at by ongoing geochronological and geochemical studies: deformation of both formations is synchronous and related to Late Miocene inversion of the Coya-Machali Intra-Arc Basin. Uplift of the rigid are rocks and part of its volcanoclastic basement took place mainly along two northward shallowing boundary structures that seem to share a lower decollement: the eastern Fierro out-of-sequence thrust and a series of western back-thrusts that resemble triangle zones. Seismic data obtained from El Teniente porphyry copper mine (34 degrees S) rockbursts and two gravimetric profiles at the western end of the southern profile are compatible with a decollement located at that locality, 2.3 km under the surface. (C) 1999 Elsevier Science B.V. All rights reserved.