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Middle Miocene tectonic development of the Transition Zone, Salta Province, northwest Argentina: Magnetic stratigraphy from the Metán Subgroup, Sierra de González

¹ Division of Environmental Studies, Mathematics, and Natural Sciences, Brevard College, Brevard, North Carolina 28712, USA, and Magstrat, LLC, P.O. Box 300, Webster, North Carolina 28788, USA
² XR Exploracionistas y Servicios Regionales, s.r.l., Manzana N—Casa 14-1° Etapa, B° General Belgrano, Salta 4400, Argentina
³ Department of Earth and Environmental Sciences, Lehigh University, 30 Williams Drive, Bethlehem, Pennsylvania 18015, USA
⁴ Department of Geosciences and Natural Resources Management, Western Carolina University, Cullowhee, North Carolina 28723, USA
⁵ U.S. Geological Survey, M.S. 926A, National Center, Reston, Virginia 20192, USA

Magnetostratigraphy, isotopic dating, and sandstone petrography establish age limits on the depositional history of 2100 m of foreland basin strata in the Neogene Metán Subgroup of northwest Argentina. The strata were deposited between ca. 15.1 and 9.7 Ma in the eastern Sistema de Santa Bárbara. The region is positioned above the Cretaceous Salta rift basin, in the Transition Zone between modern relatively steep and flat subducting segments of the Nazca plate.

Formations within the subgroup are shown to be diachronous over a 60 km distance; the younger ages are in the east. Changes in paleocurrent flow directions and the lithic clast component of sandstones collected from the Arroyo González section suggest that basal fluvial strata were derived from the craton to the east beginning in middle Miocene time, just prior to 15.1 Ma. By ca. 14.5 Ma, the paleocurrent flowed from a source in the west and sediment accumulation rates increased dramatically. These changes correlate with contemporaneous tectonism in the west. A local increase in basin accommodation may be partly related to a zone of weakness near the eastern boundary of the Salta rift.

Uplift in the western Cordillera Oriental apparently began by 13.7 Ma and thrusting rapidly migrated eastward. The eastern Cordillera Oriental ranges began to rise between 25° and 26°S ca. 10 Ma. As thrusting migrated eastward, low-energy depositional environments were overwhelmed ca. 13.7 Ma. Above an erosional unconformity that removed strata to an age of ca. 9.7 Ma, basal strata from the overlying Jujuy Subgroup were deposited beginning after 9 Ma.

Sandstones from Río Yacones suggest that the Cordillera Oriental uplift continued for several million years longer between 24° and 25°S. Uplift of the Sistema de Santa Bárbara, in the distal portion of the foreland, did not begin until after ca. 9 Ma.

Key Words: Argentine Andes • magnetostratigraphy • Neogene • Salta Argentina • tectonics • transition zones

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