TALK: PALEOGENE BROKEN FORELAND AND SYNOROGENIC RIFTING IN CENTRAL PATAGONIA

GUIDO GIANNI; CARLOS DAL MOLIN; CESAR NAVARRETE; ANDRÉS ECHAURREN

San Juan

Congreso; Reunión Científica de AAGG; 2014

Asociación Argentina de Geofísicos y Geodestas

In Central Patagonia an intraplate orogen defined as the Patagonian broken foreland has developed since late Early Cretaceous times (Fig.1). The core region is characterized by the San Bernardo fold and thrust belt (FTB) developed ~450 km away from the trench (Peroni et al., 1995) (Fig.1).The most recognized age of uplift of this Patagonian broken foreland is middle Miocene (Bilmes et al., 2013, and references therein). However, recent description in surface as well as subsurface growth strata of the late Early Cretaceous to Late Cretaceous Chubut Group, revealed a much earlier uplift history of this belt (Gianni et al., 2014). Additionally, this fold and thrust belt interferes at its central part with an E-W rift of the San Jorge gulf Basin (Figari et al., 1999) (Fig.1). This basin, aided by its transversal disposition to the main Andean trend, responded to the general E-W contraction reactivating extensionally through a roughly N-S axis previous lower Cretaceous structures. Thus, Gianni et al., (2014) suggested that the San Jorge Gulf Basin underwent a synorogenic foreland rifting stage in late Early Cretaceous to Late Cretaceous times where the Chubut Group deposited synextensionally. Continued contraction in Danian times as depicted by recent description of marine synorogenic strata (Echaurren et al., 2014, this meeting) can account for further synorogenic rift reactivation during the early Cenozoic. However, Foix et al., (2013) pointed out that basin reactivation may have also taken place later during the late Paleocene as evidenced by a synextensional control on the deposition of the Rio Chico Group. Here, we inspect outcrops of the Rio Chico Group at key sites inside the San Bernardo FTB and analyze 3-D seismic lines in the subsurface of the neighbor Rio Mayo Basin. We found robust evidence of synorogenic deposition of continental beds of the Rio Chico Group associated to growth of different contractional structures. This contraction is related to a renewed uplift of the San Bernardo FTB in late Paleocene, following Cretaceous uplift. Our results are a complement to previous proposal implying that regional compression in early Cenozoic acted in concert with extension in the eastern San Jorge Gulf area, resulting in a new synorogenic foreland rifting episode. The origin of such a strong compression enough to break the foreland area in Late Cretaceous to Paleocene times could have been related to collision of the Farallon/Antarctica midocean rigde and attached young lithosphere that interacted with the South American margin while it drifted to the west. Subduction of this oceanic feature may have helped the slab to maintain a shallow configuration as depicted by arc expansion initially during the Cretaceous and following as an arc gap in Paleocene times.