THE EXTENT AND NATURE OF RIFTING IN THE PROTO-ANDEAN MARGIN (WESTERN ARGENTINA) DURING THE MISSISSIPPIAN: IMPLICATIONS FOR TECTONIC SWITCHING

Analysis of the Paganzo Basin (western Argentina) incorporating new stratigraphic, isotopic and provenance data, provides the basis for an integrated model of evolution for the southern Central Andes from Early Carboniferous to Late Permian. Tectonic switching in the Terra Australis peri-Gondwanan accretionary orogen can be related to the following sequence of events: collision of Chilenia (~385 Ma), post-collisional rifting (360-335 Ma), reinitiation of subduction (~325 Ma), strong compression in an Andean-type setting (San Rafael Orogeny) with progressive shallowing of the subducting slab (305-270 Ma), and final slab break-off (~260 Ma).

A chronologically constrained updated stratigraphic revision of the rifting stage allows predicting a strong asymmetry with an E-dipping detachment between the two mayor coeval domains recognized within the Paganzo basin across western Argentina. Whereas in the western domain (WD: Cordillera Frontal and Western Precordillera) a thick succession of Mississippian continental to shallow-marine rocks is developed, in the eastern domain (ED: Eastern Precordillera and Sierras Pampeanas) no Mississippian sedimentary record is preserved. Early Pennsylvanian (~325 Ma) glacial rocks form a marker horizon for both domains allowing to interpret a common history. Whereas in the ED glacial rocks are recorded in deep troughs (“U” shaped valleys) carved on a regional peneplain surface atop of basement, in the WD the Nammurian glacial horizon covers the Mississippian record. Provenance and paleocurrents indicate an eastern source for glaciation, that together with geomorphologic and unroofing criteria allow interpreting development of regional epeirogenesis triggering a glaciated plateau. Whereas rifting progressed in the WD as recorded by massive alluvial-fan conglomerates giving way to shallow-marine strata and associated volcanism, the ED was being uplifted. Magmatism accompanied asymmetric rifting after initial break and crustal thinning, being represented by bimodal rhyolite-basaltic volcanism along the WD and by A-type granites with mantle signature intruding the early Paleozoic basement rocks in the ED, compatible with crustal asymmetric underplating and true rifting. This extensional episode was aborted by subduction renewal to the west.