Stratigraphic switching in the Proto-Andean margin (western Argentina) during the Late Paleozoic: Implications for development of contrasted sedimentation and climates

ASTINI, R.A.; MARTINA, F.; SCHMIDT, C.

Mendoza

Congreso; XVIII International Sedimentological Congress; 2010

Analysis of the late Paleozoic basins of central and western Argentina between 27º-32º SL incorporating new stratigraphic, isotopic, provenance and structural data provides the basis for an integrated model of evolution for the southern Central Andes from Late Devonian to Late Permian. Tectonic switching in the Terra Australis peri-Gondwanan accretionary orogen can be related to the following sequence of events: 1) collision of Chilenia (~385 Ma), including the deformation of previously deposited Early Devonian foreland basin successions; 2) post-collisional Mississippian rifting (370-335 Ma) associated with slab breakoff and back-stepping of subduction; 3) growth of an accretionary prism (~340-325 Ma), mainly recorded along the Chilean coast; 4) retroforeland basin development (~305-290) including a foredeep and a broken foreland with involvement of basement in deformation toward the craton; 5) strong compression in an Andean-type setting (San Rafael Orogeny) with I-type magmatism along the arc (along present Frontal Cordillera) and progressive shallowing of the subducting slab and sedimentary blanketing across a long-wavelength foreland (290-270 Ma); and 6) final slab break-off or crustal delamination (~260 Ma onwards) related to the Choiyoi magmatic province and grabens from the Cuyo-Ischigualasto basins. A chronologically constrained stratigraphic revision of the rifting stage predicts a strong asymmetry with an E-dipping detachment between two mayor coeval domains. In the basins of the western domain (WD: Cordillera Frontal and Western Precordillera) a thick succession of Mississippian continental to shallow-marine fossiliferous rocks is developed, but 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 indicating a common history across the region. In the ED this event is preserved as deep “U” shaped valleys carved into basement on a regional peneplain surface; in the WD the rocks of this glacial horizon covers the thick Mississippian record. Provenance and paleocurrents indicate an eastern source for glaciation, and suggests the development of regional epeirogenesis triggering a mayor glaciated plateau.  While the ED was being uplifted, rifting progressed in the WD and is recorded by massive alluvial-fan conglomerates associated with partly inverted normal faults giving way to shallow-marine strata. Magmatism accompanied asymmetric rifting after initial faulting and crustal thinning. This is indicated by bimodal volcanism along the WD and by A-type granites with mantle signature intruding the early Paleozoic basement rocks in the ED which is compatible with crustal asymmetric underplating. This extensional episode in the back-arc of the Gondwana margin was aborted sometime after subduction renewal to the west that initiated the Pennsylvanian foreland. The Mississippian rifting event appears to be related to an “extensional mode” within a largely convergent margin. Our model suggests that lithospheric extension in the aftermath of the Chilenia collision can explain sedimentary and magmatic distribution whithin a strong asymmetry inherited from basement fabric (east-dipping slab). Delay between Mississipian A-type and Pennsylvanian-to Early Permian I type and outboard migration of the site of magmatism is consistent with the suggested evolution. In our case-study in the Central Andes, tectonic swithching processes can be closely matched with the climate signatures recorded in the stratigraphic evolution of the Gondwana margin and may considered as an independent factor distinct from global circulation patterns. Moreover, recurrent glacial activity, major glacial expansion and progressive aridization in the study area seem to relate to topographic barrier effects and climate restrictions that do not need to correlate with other regions in Gondwana.