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

Magnetostratigraphy, isotopic dating, and sandstone petrography establish a ge limits on the depositional history of similar to 2100 m of foreland basi n strata in the Neogene Metan Subgroup of northwest Argentina. The strata w ere deposited:between ca. 15.1 and 9.7 Ma in the eastern Sistema de Santa B arbara. The region is positioned above the Cretaceous Salta rift basin, in the Transition Zone between modern relatively steep and flat subducting seg ments of the Nazca plate. Formations within the subgroup are shown to be diachronous over a 60 km dis tance; the younger ages are in the east, Changes in paleocurrent how direct ions and the lithic clast component of sandstones collected from the Arroyo Gonzalez section suggest that basal fluvial strata were derived from the c raton 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 sedim ent accumulation rates increased dramatically, These changes correlate with contemporaneous tectonism in the west. A local increase in basin accommoda tion 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 t hrusting rapidly-migrated eastward, The eastern Cordillera Oriental ranges began to rise between 25 degrees and 26 degreesS ca, 10 Ma. As thrusting mi grated eastward, low-energy depositional environments mere overwhelmed ca, 13.7 Ma, Above an erosional unconformity that removed strata to an age of c a, 9.7 Ma, basal strata from the overlying Jujuy Subgroup were deposited be ginning after 9 Ma. Sandstones from Rio Yacones suggest that the Cordillera Oriental uplift con tinued for several million years longer between 24 degrees and 25 degreesS, Uplift of the Sistema de Santa Barbara, in the distal portion of the forel and, did not begin until after ca, 9 Ma.