IDEAN   23403
INSTITUTO DE ESTUDIOS ANDINOS "DON PABLO GROEBER"
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
U/Pb ages of detrital zircons in synorogenic sequences of the southern central Andes (37°S) to determine age exhumation and structuration of the foreland area.
Autor/es:
SAGRIPANTI, LUCÍA; MAXIMILIANO NAIPAUER; GERMAN BOTTESI; ANDRES FOLGUERA; VICTOR A. RAMOS
Lugar:
Santiago de Chile
Reunión:
Encuentro; IGCP, Unesco. 568-Y Meetings Mendoza, Argentina-Santiago, Chile ABSTRACTS; 2010
Institución organizadora:
IGCP - UNESCO
Resumen:
The eastern slope of the Andes at 37º S was formed by the Malargüe fold and thrust belt, whose orogenic front exhumed at least two lower angular unconformities. In order to constraint the relative importance between these different constructional pulses that led to the formation of the orogenic front, a sedimentological and petrographical study was carried in the Neogene sequences of the Pampa de Carrizalito syncline eastward the Sierra de Reyes anticline plus a series of U/Pb datings in detrital zircons. The previous analyses show that in the Neogene Pampa del Carrizalito depocenter the synorogenic deposits show a typical unroofing sequence, based on the clast composition and their correlation with the stratigraphic sequence of the eastern flank of the Sierra de Reyes. The U-Pb detrital zircon data not only confirm the unroofing sequence, but validate the age of the clast provenance, as well as indicate important source changes through the synorogenic sedimentation. The fact that an entire unroofing sequence is registered in a Neogene depocenter next to the orogenic front implies that complete exhumation of the Sierra de Reyes started about this time, and therefore that previous phases are negligible in the study region. The best represented sources in U-Pb graphs correspond to Gondwanian and Andean components. The younger ones are represent with an igneous source in the basal half of the sequence by two groups, 190-180 Ma all along the sequence and 120-60 Ma disappearing upwardly. The older ones are representing at the top of the sequence where microscopic evidence of a metamorphic source appears. Different evidence suggests that a Late Cretaceous relief was uplifted and exhumed west of the described orogenic front. Detritus with Andean ages at the Pampa de Carrizalito depocenter are clearly related to this feature. However, Jurassic to Cretaceous prominent peaks are most probably related to the exhumation of Jurassic and Cretaceous granitoids, present at the western slope of the Andes. This fact implies that the Late Cretaceous relief was not a prominent feature such it is now. Cretaceous U/Pb detrital zircon ages disappear upwards, while Jurassic ones are persistent along the stratigraphic column. If the apparent dilution of Cretaceous components is related to the uplift of a mountain barrier that inhibit Andean provenance through time, Jurassic dates should also disappear. For that reason, another complementary source is needed for Jurassic components. The best candidate seems to be the Jurassic rocks exposed in the Huincul Ridge. Parallel to the Late Cretaceous source dilution increases the pre-Andean component (Famatinian, Pampean and Grenvillian zircon ages). This is interpreted as a migration of the orogenic front towards the east and the development of a broken foreland, with basement uplifts as the San Rafael basement block during the last stages of the analyzed synorogenic depocenter. This broken foreland was produced during the shallow subduction episode in the region.