Edad de la sedimentación y deformación de la Formación Tunas en las Sierras Australes de la provincia de Buenos Aires (37°-39°S - 61°-63°W).

TOMEZZOLI R.N.

Revista de la Asociación Geológica Argentina

Asociación Geológica Argentina

Año: 1999 vol. 54 p. 220 - 228

Abstract: The Sierras Australes constitutes a curved orogenic thrust and fold belt with a Northwest - Southeast strike, exposed in the Southwest part of Buenos Aires province (Argentina). Implications of recent paleomagnetic results on these rocks are analized. The Tunas Formation has a broad areal extension in the region and it is the youngest unit of the Carboniferous-Permian Pillahuincó Group. The paleofloristic and paleofaunistic associations attest a Permian age for these rocks. Recent paleomagnetic date indicate that the Tunas Formation acquired its magnetization both during and before folding. While the syntectonic magnetizations were isolated in the area of the Sierra de las Tunas (northern and western) the pretectonic magnetizations were found in the Sierra de Pillahuincó (southern and eastern). The syntectonic poles were grouped into a mean one called TUNAS paleomagnetic pole. This pole corresponds to the Early Permian section of the South America APWP, and indicates that folding in the Sierras Australes occurred during those times, very soon after or practically contemporaneously with deposition of the Tunas sequence. The Early Permian deformation may represent the activity of the San Rafaelic orogenic phase in the region. Pretectonic magnetizations isolated in the Sierra de Pillahuincó, yielded the the Pillahuincó paleomagnetic pole. This pole corresponds to the late Early Permian section of the South American APWP. The presence of younger and different kind of magnetizations than in the Sierra de las Tunas put in evidence a migration of the orogenic front towards the East-Northeast, with a gradual attenuation of the deformation.