Magnetostratigraphy and Paleomagnetism of Early Neogene Synorogenic Strata: Basement Partitioning and Minor Block Rotation in Argentine Broken Foreland

ZAMBRANO, O; RAPALINI, A.E.; D¨¢VILA, F.M.;; ASTINI, R.A.;; SPAGNUOLO, C.M

INTERNATIONAL JOURNAL OF EARTH SCIENCES

SPRINGER

Año: 2009 p. 1 - 34

1437-3254

Magnetostratigraphic and paleomagnetic studies on early Andean synorogenic strata (Del Crest¨®n Fm.), in the Famatina Belt (28.7¡ãS, 67.5¡ãW) allow bracketing new ages and calculating sedimentation rates within the broken foreland of west Argentina. . The Del Crest¨®n Fm represents the first record of broken foreland sedimentation within the southern Central Andean belt and the earliest retroarc volcanic rocks, exposed several hundred kilometres from the trench; which was interpreted as a premature flat-slab stage, previous to the Mio-Pliocene to present stage, broadly described in the literature. Twenty-five out of forty-nine sites collected along the succession presented a primary remanence, as determined through positive fold and reversal tests. Correlation of the local magnetic polarity section with the global polarity timescale indicates that the sedimentation of Del Crest¨®n Fm started at ~16.7 Ma and continued until ~14.5 Ma. The youngest strata is represented by conglomerates bearing abundant Lower Paleozoic granite boulders indicating unroofing of the crystalline basement within the NW Sierras Pampeanas. This result supports the hypothesis of an early broken-foreland stage at these latitudes of the Andes, with involvement of the basement in deformation and coeval retroarc volcanism, common attributes of flat-subduction regimes. A mean site paleomagnetic direction of Dec: 6.3¡ã, Inc: -43.6¡ã (¦Á95: 8.0¡ã, N=24) indicates that the central part of the43.6¡ã (¦Á95: 8.0¡ã, N=24) indicates that the central part of the Famatina Belt within the Sierras Pampeanas did not undergo large vertical axes rotations since the Miocene. A similar interpretation was proposed in our previous studies on Paleozoic successions in the same area. This is in conflict, at least locally, with previous models in the Andes, which suggested a widespread rotation along the Sierras Pampeanas and Puna.