Magnetostratigraphic Constraints on the Development of Paired Fold-Thrust Belts/Foreland Basins in the Argentine Andes

REYNOLDS, JAMES H., Norwich University, Northfield, VT, JOHN F. DAMANTI, Miami University, Miami, OH, and TERESA E. JORDAN, Cornell University, Ithaca, NY

Development of a paired fold thrust-thrust belt/foreland basin is correlated to the flattening of the subducting Nazca plate between 28-33 degrees S. Magnetostratigraphic studies in Neogene basin-filling continental strata determine local basin subsidence rates and provide relatively precise chronostratigraphic correlation between different depositional environments. Temporal constraints on cross-cutting structures and synorogenic sediment compositions reveal the pattern of Neogene paleogeographic evolution in the Andean foreland. The western side is dominated by the Precordillera fold-thrust belt, which impinges on block uplifts of the Sierras Pampeanas to the east.

The data demonstrate that most existing lithostratigraphic units are diachronous and require new tectonic interpretations. Increases in sediment accumulation rates closely correspond to changes in provenance and indicate that the Frontal Cordillera, on the Chile-Argentina border, was a positive topographic province by 18 Ma. The Precordillera evolved from about 16 Ma to the present as thrusting migrated from west to east. Published ages from intercalated airfall tuffs constrain some sedimentary sections in the eastern Sierras Pampeanas

where the earliest uplift occurred since 10 Ma. The youngest uplifts are on the west side close to continuing thrusting in the Precordillera.

Not all fold-thrust belt/foreland basin pairs are associated with flat subduction, suggesting that tectonic controls exceeding the scale of individual plate segments may be important. The hydrocarbon-producing Subandean fold-thrust belt/foreland basin, located in the area of "steep" subduction that underlies northern Argentina and Bolivia (18-24 degrees S), is also believed to have evolved since middle Miocene time. Recently initiated magnetostratigraphic studies in the Subandean foreland basin will attempt to temporally constrain the Neogene tectonic evolution for comparison with the southern region.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)