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

RAMOS, MIGUEL E.; MAFFIONE MARCO; RAMOS, VICTOR A.; GEUNA, SILVANA; BOIXART GREGORIO; ARAMENDIA INES; GHIGLIONE MATÍAS

Santiago

Simposio; Primer Simposio Latinoamericano de Tectónica; 2016

Asociación Latinoamericana de Tectónica

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 example 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 weighted by field structural evidences. 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 Aptian-Albian 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 caused by subduction of old ridges identified in recent reconstructions. Some authors have displayed, in their regional tectonic model, the break-up of the Antarctic Peninsula from the west side of Patagonian margin. The Antarctic Peninsula detachment 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 strata were recognized in the Jeinimeni Formation. The normal faults limit the progressive unconformities wedges, inferring a coeval sedimentation with extensional tectonics. The AMS study carried out on these strata proves that the pulses of deformation and sedimentation are coeval, and can be linked to a late Miocene regional extensional event. On the other hand, the growth strata could be representing a rapid sedimentation in these transgressive successions related to a structural collapse.