Paleomagnetism of Permian red beds in Punta del Viento, eastern flank of the Sierra de Umango, La Rioja, Argentina: the Paganzo basin revisited

GEUNA, S.E.; ESCOSTEGUY, L.D.; LIMARINO, C.O.

Perugia, Italia

Congreso; XXIV General Assembly of the International Union of Geodesy and Geophysics (IUGG); 2007

A preliminary paleomagnetic study was performed on Permian red beds of Patquía/La Colina (PLC) Formation, the youngest unit forming the infill of the Upper Paleozoic back-arc Paganzo basin in central-western Argentina. Eight sampling sites were obtained in  medium and fine-grained red sandstones cropping out in Punta del Viento (29o7´S, 68o33´W). We found a pattern of variation in magnetic properties along the sequence which condenses in an only place the properties observed in many different, well known sections of Paganzo basin. This allowed us to construct an integrated column for PLC Formation, where all sites are characterized by hematite as the only magnetic carrier, and a reversed-polarity magnetic remanence. The column integrates as follows: 1) A lower section with higher intensity of natural remanent magnetization (NRM ~20-40 mA/m), which coincides with the lower member of the PLC Formation, composed of fluvial deposits with some intercalations of alkaline basalts, dated to the earliest Permian (~295 Ma). Paleomagnetic studies of this section have been reported from Las Mellizas, Huaco and Lower Los Colorados. 2) An upper section with lower NRM (~1-8 mA/m), corresponding to the upper member of the PLC Formation, characterized by aeolian rocks and ephemeral lake deposits marking the aridization of the region. Stratigraphic relations and palynological assemblages point to a Middle-Late Permian age, which makes these deposits coeval with ignimbrites, rhyolites and dacites of Choiyoi Group, being deposited at that time in the arc-related basins to the west. However, the all-reversed character of the upper section constrains its age to older than 265 Ma, the end of the Permo-Carboniferous Reversed Superchron (PCRS). The upper section of PLC Formation has been paleomagnetically studied in Upper Los Colorados, Paganzo town, Cerro Colorado and Chancaní. Paleomagnetic poles reported for both sections are clearly different. The lower section provided a pole position coincident with Late Carboniferous-Early Permian poles for Gondwana, while the upper section poles are departed from this position. We have found evidence to discard local rotations about vertical axis and remagnetizations to explain the departure, and we therefore preferred to interpret the Middle Permian poles as due to apparent polar wander (APW) in a ~30o counter clockwise rotation of the region, after deposition of the Early Permian lower section. The minimum age of 265 Ma for the upper section forces the apparent polar wander (APW) to be rapid. The period of rapid APW would coincide with reported block rotations in the arc-related basins, and with widespread remagnetization in Precordillera, all of which indicates a significant tectonic event. Permo-Triassic Choiyoi volcanics in the arc, and aeolian Middle Permian sediments of PLC upper member in the back-arc Paganzo basin, both postdate the referred tectonic event, which is therefore tentatively placed on 280 Ma.