PALEOZOIC FLAT-SLAB SUBDUCTION AND BROKEN FORELAND SCENARIOS PRIOR TO THE ANDES MOUNTAIN BUILDING

The western Gondwana margin has faced an open ocean since the Early Cambrian, resulting in quasi-permanent subduction. A protracted history of subduction along accretionary margins implies development of certain recurrent features. Various transient coupling mechanisms generate discrete orogenic features. Soft-collisions, associated to the accretion of minor crustal blocks, are one mechanism for strong coupling. Convergence obliquity and steepness of subduction are other two important mechanisms influencing stresses and subsidence patterns, promoting across- and along-strike segmentation. Changes in the convergence mode, and tectonic switching from advancing (contractile) to retreating (extensional) behaviors may lead to inboard continental growth, whereas balanced stages may promote passive-like margins. In the Central Andes two separate segments north and south of ca. 27º30'S may serve in order to identify different responses in arc and retroarc regions. N and S of this boundary, mechanisms responsible for development and preservation of sedimentary records seem to have been largely different. Whereas during the Ordovician (Ocloyic Orogeny) the collision of the Precordillera terrane took place in the southern segment, in the northern segment (Central Andean Basin) collisions have not been demonstrated. Such contrasts seem true during the rest of the Paleozoic history of western Gondwana and clearly impacted on foreland development. While during the Silurian and Devonian the Precordilleranic-Achalian Orogeny represents renewed coupling in the southern segment, a “balanced” subduction characterized the northern segment. Although accretion of the Chilenia terrane have been pointed as the possible coupling mechanism to the south, basin broadening to the north can be better related to foreland dynamic subsidence driven by shallow subduction. Such a mechanism contrasted with thick-skinned deformation in the southern segment. Scarcity of volcanism may also be related to this flat-slab stage. During the Late Paleozoic a broken foreland is now strongly supported by paleogeographic studies indicating dynamic topography as a suitable basin driving mechanism. Thus, traditional widespread unconformities can be due to regional tilting.