INVESTIGADORES
DAVILA Federico Miguel
congresos y reuniones científicas
Título:
Links and contrasts between Paleozoic orogens along the Central Andes
Autor/es:
ASTINI, R. A.; FEDERICO MIGUEL DAVILA
Lugar:
Mendoza
Reunión:
Congreso; Gondwana 12; 2005
Resumen:
After Rodinia breakup the western Gondwana margin has faced an open ocean since the Early Cambrian resulting in a permanent or quasipermanent subduction. This characterizes a typical accretionary margin opposite to collisional orogens. This big picture configuration apparently results from the global plate kinematics budget, where termination of supercontinent assembly is balanced by subduction along exterior margins. Geological literature shows that bearing on these contrasted end-member models (accretionary vs. collisional) is difficult to evaluate because many “soft-collisions” and “quiescence” stages occur in a wide time-space framework. Within a largely accretionary orogen, recurrent stages of active mountain building and quiescence can be interpreted as the result of changes in plate convergence stresses. Spatially-restricted processes of transient coupling (physical interaction) along plate boundaries allow differentiation of distinct along-strike segments that, in turn, seem to control lateral crustal growth and contrasted basin formation (outboard-inboard stepping) and unroofing. Crustal fabrics, generated during early orogenic stages of an active margin, seem to exert a major control on the location of subsequent deformation episodes. Along the South American margin similarities and differences in basin development, deformation and magmatism allow understanding the across and along-strike spatial variations and evaluating mechanisms responsible for developing and preservation of sedimentary records. Comparison of two separate segments north and south of ca. 27º30’S (present coordinates) may serve in order to identify different responses in the arc and backarc regions. In the Ordovician (Ocloyic orogen) there are notable contrasts between the southern segment, where the collision of the Precordillera took place (partly spatially superposed with the Cambrian Pampean Orogen, but outboard), and the northern segment represented by the Central Andean Basin, where collisions have not been reported. Such contrasts seem true during the rest of the Paleozoic history of western Gondwana. Main differences in the retroarc stratigraphic responses can be identified across that boundary during the Siluro-Devonian. Whereas the Precordilleranic-Achalian orogeny represents renewed coupling in the southern segment, a “balanced” subduction characterizes the northern segment represented by a milder unconformity (Chanic event). Regardless the driving mechanism that promotes unroofing to the south (either plate reorganization or accretion of Chilenia terrane), basin broadening in the northern upper-plate retroarc region can relate to dynamic subsidence driven by shallow subduction. This alternative explanation for the Central Andean depozones contrasts with previous models. The two contrasted behaviours to the south and north of the 27º30’S boundary seem to have exerted control on the proto-Andean basins evolution and may also influence the modern foreland partitioning. Assuming a theoretical protracted history of subduction along accretionary margins implies development of certain recurrent features. In the southern segment one of such features is the broad silicic magmatism apparently generated during the climax to ending of the various stepping out orogenic cycles in the southern segment. Several similar “rhyolite provinces” develop atop of major unconformities of which probably the most well known is the protracted Permo-Triassic Choiyoi Group. Although less known, during the Middle Ordovician extensive silicic volcanism developed within the upper-plate at the Gondwana margin as a late-stage component of the Ocloyic orogen. This igneous complex (the Cerro Morado Group) is exposed in the Famatina belt (at ~29ºS) and extends toward the west and into the southern Puna, where it directly overlaps basement. It represents extensive magmatism after the accretion of Precordillera to the Gondwana margin. Another comparable case of a rhyolitic plateau developed atop of the Cambrian Pampean cycle and is exposed in the Sierra Norte of Córdoba, eastern Pampean Ranges. As has been suggested for the Permo-Triassic Choiyoi igneous complex, generation of extensive silicic volcanism has developed after major deformation episodes and, regardless a unique explanation, they are likely related to crustal melting. With the possible exception of the Choiyoi magmatism, the cases in the southern Central Andes seem to be related to late-stage features after “soft-collision” and consequent slab breakoff, compatible with important transient coupling in accretionary margins. These alternatives are probably also true for the Late Silurian-Devonian Precordilleranic-Achalian orogeny, although the more restricted magmatism thereafter may suggest a flat-slab stage. Alternatively to terrane accretion plate reorganization at accretionary margins may imply changes in the convergence mode and tectonic switching from advancing to retreating behaviors may lead to efficient inboard continental growth (including sedimentary basins and magmatism), whereas balanced stages may promote passive-like margins.