Constraints on the tectonic evolution of the Santa Cruz Patagonian Southern Andes from anisotropy of magnetic susceptibility and structural data (46°-48°SL).

MIGUEL E RAMOS; SILVANA GEUNA; INÉS ARAMENDÍA; MARCO MAFFIONE; GREGORIO BOIXART

Santiago de Chile

Simposio; Primer simposio de tectónica Latinoamericana; 2016

The Andean orogeny in the Eastern Southern Patagonian Cordillera reflects the consequences of the Mesozoic and Cenozoic subduction history. Particularly, the area located between the Buenos Aires- Carrera and Cochrane- Posadas Lakes (46° -48°SL) offer an interesting sample of the interaction between initiation of the Patagonian fold and thrust belt in latest Cretaceous and early Tertiary times with the development of the Austral Basin as a foreland basin. Although several previous works outline the main events in the tectonic evolution of this region of the Andes, the timing of deformation and the mechanism are not yet well constrained. In this study, we infer the nature and orientation of the paleostress field at the Southern Patagonian Andes during the Early Cretaceous (Aptian-Albian) and Miocene by using anisotropy of magnetic susceptibility (AMS) data. The paleomagnetic data is complemented by a structural analysis based on field evidence. In order to undertake this work, 7 Lower Cretaceous and 8 Upper Miocene sites within the fold and thrust belt were sampled. The stratigraphic record between these two periods is very poor and does not meet the conditions for an AMS study. Interestingly a marked N-S directed maximum principal stress (s1), was obtained for the trangresive successions of the Aptiano-Albiano corresponding to the Río Mayer and Río Belgrano Formations. The setting of the paleostress orientation with an orthogonal distribution respect to the E-W Andean vergence, could be related to the stress partitioning coused by the old ridges subduction estimated by recent reconstructions. Otherwise, some authors have displayed, for a regional tectonic model, the drifting of the Antarctic Peninsula from the west side of Patagonian margin. This terrene motion along with the Weddell sea opening could be linked with a N-S directed compresional stress suffered by the Patagonian Andes. For the Miocene, synextensional growth stratas were recognized in the Jeinimeni Formation. The normal faults limit the progressive unconformities wedges, inferring a depositation coetaneous with extensional tectonics. The AMS study carried out on these strata proves that the pulses of deformation and depositation are coetaneous, and can be linked to a late Miocene regional extensional event. On the other hand, the growth strata could be representing a rapid sedimentation process in these transgressive successions resulting in collapse structure.