Tectonic evolution of the Southern Central and Northern Patagonian Andes (34º30´- 43º30´S)

FOLGUERA A.; ROJAS VERA E.; SPAGNUOLO M.; GARCÍA MORABITO E.; ORTS, D.L.; BOTTESI G.; RAMOS V.A.

Göttingen, Alemania

Simposio; International Lateinamerika-Kolloquium 2009; 2009

A big controversy still exists in relation to the temporal definition of main contractionalphases that had led to the construction of the Southern Central and Northern PatagonianAndes since the early works of Groeber (1929): How many phases have coexisted spatially? What is the real extent for each phase and therefore their relative importance? What is the relationship between the phases of Andean construction and the behavior of the volcanic arc through time as indicative of potential geometry of the subducted slab? Have these contractional stages been followed by gravitational disequilibria? The western sector of the Andes at these latitudes between the Central Valley in Chile and the drainage divide area is formed by an inverted Late Oligocene to Early Miocene basin (25-18 Ma) named the Cura Mallín basin. Two main phases have led to the closure of this basin ca. 17-18 Ma constrained by fission track data at the western slope of the Andes and ca. 12-9 Ma respectively (Burns, 2002). Synorogenic deposits associated with this early phase of contraction have by-passed a highly erodedCretaceous to Eocene fold and thrust belt, being deposited at the Río Grande basin off the Late Miocene orogenic front since 18 Ma (Silvestro and Atencio, 2008). Older contractional deformations in the area have defined two orogenic fronts constrained by unconformities: i) a Late Cretaceous orogenic front (100-65 Ma) east of the Cura Mallín basin, evidenced by an unconformity between Paleogene successions and the Cretaceous deposits, which marks the maximum advance of latest Cretaceous to Paleocene marine-Atlantic transgression in the area, constrained by fission track ages at the Cordillera del Viento (Burns, 2002) and the Chihuidos uplift (Zamora Valcarce et al., 2008). This orogenic phase has been associated with synorogenic deposits corresponding to the Neuquén Group, locally containing synorogenic unconformities,presently cannibalized by Neogene structures at the inner part of the fold and thrust beltand buried beneath Neogene sequences at the Río Grande basin. These sequences contain a first input of Late Cretaceous detrital zircons through the entire Mesozoic column that indicates incipient exhumation of the arc located to the west (Ramos et al., 2008). And ii) a Late Eocene orogenic front, evidenced by an unconformity between Oligocene and Eocene strata, developed east of the Río Grande Neogene foreland basin, associated with synorogenic sequences of the Pircala Formation. Late Cretaceous to Late Eocene deformations can be taken as two end limits of a first wave of contractional deformation affecting the western border of the continent at these latitudes. After that an extensional event created the Cura Mallín Basin superimposed to the Late Cretaceous thrust fan and the Palaoco Basin at the Late Eocene orogenic front. These extensional structures were finally inverted around 17-18 Ma. Both contractional phases ofdeformation correlate spatially and temporally with two eastward arc expansions that covered parts of the eastern slope of the Andes at these latitudes.During last years increasing evidence about the inception of shallow subduction regimes has been revealed as decisive factors in constructing Andean relief. These are revealed by i) extreme outward expansion of arc-related sequences followed by ii) periods of crustal thickening, and iii) subsequent location of the arc-front nearer to the trench, coexisting with iv) extensional collapse. Extensional depocenters created at stage iv), controlled the emplacement of both mantle derived materials and crustal melts. Particularly, initial stages in Andean deformation along the western margin of the Southern Central and Northern Patagonian Andes (36º-43º30´S) are associated with previous eastward-arc expansions. Those can be associated with two discrete shallowsubduction regimes: Eastward expansion of Late Cretaceous to Eocene (75-45 Ma) arcrelatedsequences (36º-43º30´S) that describe two separate lobules centered at 36º-39ºS and 40º-43º30´S respectively, where arc migration was maximum with values around 400 km. Both correlate with Late Oligocene to Early Miocene intraplate volcanic plateaus at the outer retroarc area. Younger arc related rocks were emplaced more than 550 kilometers away from thetrench in the eastern slope of the Andes, during Late Miocene times (20-4 Ma) from 34º30´to 37º45´S. At the retroarc zone basement blocks cannibalized the foreland basin in Late Miocene times associated with the Malargüe fold and thrust belt to the west, whose main phase of contraction has been constrained in 20-10 Ma. This indicates a genetic relationship between the arc expansion, uplift of the Andes, sedimentation in the foreland basin, and the breaking of the foreland area. This stage changed to an extensional regime since Latest Miocene-Early Pliocene times. Extensional troughs were developed in the area that previously recorded arc expansion until late Quaternary, controlling the emplacement of crustal melts and poorly differentiated mantle products. Presently this area is associated with crustal attenuation as well as anomaloussublithospheric heating inferred by teleseismic, tomographic and gravimetric analysis.