-C—022. AR00C0030

140 Actas X Congreso Latinoamericano de Geologfa y VI Congreso Nacional de Geologfa EconGmica, vol. II MAGNETOSTRAT1GRAPHY AND "AR -39AR DATING OF THE NEOGENE SYNOROGENIC STRATA OF NORTHERN MENDOZA, ARGENTINA:TECTONIC IMPLICATIONS

Maria Veronica Irigoyen1, Victor A. Ramos2 and Richard L.Brown3

'SEGEMAR-IGRM. Julio A. Roca 651, piso 10,1322 Buenos Aires. Argentina. 2Universidad de Buenos Aires, Pabellon 2, Ciudad Universitaria, 1428 Buenos Aires.Argentina department of Earth Sciences, Carleton University, Ottawa, Ontario, KIS SB6

Keywords: magnetostratigraphy, ^Ar -39Ar dating, Neogene, strata, Mendoza, tectonics, Andes

Abstract: At the southernmost extension of the Paleozoic age. Exhumation and displacement of the Precordillera fold-and-thrust belt, 33* S latitud, about Frontal Cordillera overlapped in time with deformation 4000 m of Cenozoic foreland-basin strata record the in the Principal Cordillera in which the latest phase of eastern migration of the Andean thrust front since the deformation is known to occur at ~8.0 Ma (e.g., Neogene. These deposits accumulated in response to Ramos 1996). exhumation and deformation of the western Principal The extensive ash-rich unit, the 8.9-8.7 Ma and Frontal cordilleras. The detrital clastic strata Tobas Angostura Fm., is correlated with the latest cropping out in the La Pilona-Tupungato region, extrusive episode that included pyroclastic eruptions in comprise five formational units that record fluvial, the Principal Cordillera. An unconformity separates the eolian and alluvial deposition in an arid setting. To link Rio de los Pozos Fm., probably at least as young as the sequence of deformational events in the western ~7.2 Ma, from the youngest Mogotes Fm. This unit, mountain belts with the sedimentary record, all units whose deposition spans -3-1 Ma, includes the large- except the youngest have been dated using magnetic boulder conglomerates that today lie at the foot of the polarity stratigraphy calibrated with ^Ar - 39Ar dates cordon del Plata immediately to the west of the study on interbedded tephras. A precise chronology of these site. Therefore, deposition of this unit is linked with deposits in conjunction with a multiple data set that deformation in the flanking Frontal Cordillera. include rates of sedimentation in the foreland, a Neogene strata are deformed into fault-propagation provenance study on these rocks, and facies and folds and emergent east-verging thrusts which, in some textural patterns, provide the basis for documenting cases, also display a sinistral strike-slip component. details of tectonic activity, volcanism and deposition. The timing of deformation of the Tertiary sequence is The oldest Marino Fm., whose deposition constrained by an ash-tuff layer unconformably spans ~15.7-12.2 Ma, is interpreted to record two overlying these deposits and dated at 0.39 ± 0.03 Ma. phases of thrusting in the Principal Cordillera (Irigoyen 1997). The earlier phase is tied to deformation of the REFERENCIAS Mesozoic andesitic volcanic complex cropping out in the western part of the Aconcagua fold-and-thrust belt. Irigoyen, M. V., 1997. Magnetic polarity stratigraphy The second phase of thrusting is linked with and geochronological constraints on the sequence of deformation of the Mesozoic marine sequences of the thrusting in the Principal and Frontal Cordilleras central part of the Aconcagua fold-and-thrust belt. and the Precordillera of the Argentine Central Andes The overlying La Pilona Fm., whose (33° S latitude)[Ph.D.thesis]: 392 pp. Carleton deposition spans -11.7-9.0 Ma, is thought to record University, Ottawa, Canada. the initiation of exhumation of the Frontal Cordillera. Ramos, V. A., 1996. Evolution tectonica de la Alta Provenance and paleocurrent data are consistent with Cordillera de Mendoza. En: Geologia de la regidn clast derivation from northwestern highland sources del Aconcagua, provincia de San Juan y Mendoza. (i.e., cordon and cuchillas del Tigre) which provided Subsecretaria de Mineria de la Nation. Direction volcanic detritus from Hie Choiyoi Group and low- Nacional del Servicio Geologico, Anales 24, grade metamorphic and sedimentary rocks of lower Capitulo 16:447-460. Buenos Aires.