Links and contrasts between Paleozoic orogens along the Central Andes

ASTINI, R.A.; DÁVILA, F.M.

Mendoza

Congreso; Gondwana 12; 2005

Academia Nac. de Ciencias

After Rodinia break-up the western Gondwana margin has faced an open ocean since the Early Cambrian, resulting in permanent or quasi-permanent subduction. This characterizes a typical accretionary margin as opposed to a collisional orogen. 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 evidence for these contrasting end-member models (accretionary vs. collisional) is difficult to evaluate because many “soft-collisions” and “quiescent 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 the early orogenic stages of an active margin, seem to exert 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 of the across- and along-strike spatial variations and the evaluation of mechanisms responsible for development 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 back-arc 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 of it), 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. Major differences in the retro-arc 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). In both cases, the driving mechanism that promoted unroofing to the south (either plate reorganization or accretion of the Chilenia terrane) and basin broadening in the northern upper-plate retr-oarc region can be related 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 proto-Andean basin evolution and may also influence the modern foreland partitioning.