Magnetostratigraphy and paleomagnetism of early and middle Miocene synorogenic strata: basement partitioning and minor block rotation in Argentine broken foreland

Abstract

Magnetostratigraphic and paleomagnetic studies on early Andean synorogenic strata (Del Creston Fm.), in the Famatina Belt (28.7°S, 67.5°W) clarify details of chronology that permit calculation of sedimentation rates within the broken foreland of west Argentina. The Del Creston Fm represents the first record of broken foreland sedimentation within the southern Central Andean belt and the earliest retroarc volcanic rocks exposed several hundred kilometers from the trench. Twenty-five out of 49 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 time scale indicates that the sedimentation of Del Creston Fm started at ~16.7 Ma and continued until ~14.5 Ma. The youngest strata are 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°, Inc: −43.6° (α95: 8.0°, N = 24) confirm our earlier intrepretation that the central part of the Famatina Belt within the Sierras Pampeanas did not undergo large vertical axes rotations since the Middle Miocene.