Magnetostratigraphy of Neogene Andean foreland-basin strata, lat 33 degrees S, Mendoza Province, Argentina

Neogene nonmarine strata in the southernmost part of the Precordillera thru st belt of the Andes of Argentina accumulated during the Miocene and Plioce ne in response to eastward advance of thrust sheets located to the west. To link the sequence of deformational events with the sedimentary record, fiv e stratigraphic sections in the Mendoza province were dated using magnetic polarity stratigraphy. calibrated with Ar-40-Ar-39 dates from interbedded t ephras, These data establish age control on tectonic activity, volcanism, a nd deposition. The existence of a backare foreland basin that developed prior to or ca, 16 Ma is recorded hy the oldest strata, the re. 15.7-12.2 Ma (Baksi time scal e) Marino Formation. Deposition of this unit is linked to a major phase of thrust activity in the central part of the Aconcagua fold-and-thrust belt, in the principal Cordillera. Initiation of deposition coincides with an eas tward shift of the subduction-related magmatic foci to the Argentine Princi pal Cordillera. The overlying ca, 11.7-9.0 Ma La Pilona Formation is though t to record the initiation of exhumation of the Frontal Cordillera to the n orthwest of the study region. An extensive ash-rich unit, the 8.9 (locally 9.5?) to 8.7 Ma Tobas Angostura Formation, is correlated with the latest an d probably the largest extrusive episode in the Principal Cordillera. Chron ologic data con-strain the timing of oil generation in the Cuyo basin and d evelopment of prospective closures. The new data also require that land mam malian fossils found in the Rio de los Pozos Formation are late Miocene, ra ther than Pliocene, as previously believed.