from La Plata University (Argentina). He joined XR Consultants and worked as a field and structural geologist in the Subandes from 1998 to 2000. He spent two years as a visiting scientist at Cornell University working on the Subandes stratigraphy and structural geology. Now, he is a postdoctoral research associate at Colorado School of Mines.

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Magnetostratigraphy, isotopic dating, and sandstone petrography establish age limits on the depositional history of ϳ2100 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 sedi-*Second address: Magstrat, LLC, ment 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.

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