The Spatial And Temporal Evolution Of The Argentine Precordillera At 30Xc2Xbas: Shortening Over A Shallow Slab

The Argentine Precordillera is a foreland fold-and-thrust-belt in western Argentina that overlies the central Chilean flat slab region of the subducting Nazca plate. The Precordillera has accommodated shortening over the past ~20 million years; over this time, the Nazca slab evolved from a relatively steep subduction angle to horizontal subduction. Because the shortening in the Precordillera spans the shallowing of the slab, changes in the deformation patterns can provide insight into the relationship between the down-doing slab and the over-riding plate at zones of shallow subduction. In this thesis, I present field-based structural data, cross sections of the Precordillera, and estimates of shortening magnitude and rates for the region in an effort to characterize the impact of a shallowing slab on deformation at the surface. ! To calculate shortening magnitudes for cross sections that have rigorous uncertainty estimates, I developed an algorithm that propagates known input uncertainties through an area balancing calculation and yields both Gaussian and maximum uncertainty estimates. The area balancing method is complementary to the line-length balancing method and allows one to include known initial uncertainties, such as the uncertainty on stratigraphic thicknesses and the location of the decollement, neither of which are included in the "minimum shortening" estimates often cited in line-length balanced shortening calculations. ! I constructed two cross sectional profiles through the Precordillera to determine shortening magnitude and rates since 20 Ma. Calculations for the profiles yield ~115 ± 44 (100) km of shortening in the Precordillera, where the uncertainty values are both Gaussian and maximum respectively. This shortening magnitude agrees with other published values of shortening for the Precordillera (72 - 136 km) determined via linelength balancing, and the variation in published values falls within the calculated uncertainty estimates. Variations in shortening rate throughout the Precordillera correlate temporally with changes in the geometry of the Nazca slab. Prior to the shallowing of the slab, the Precordillera accommodated 2 - 3% of the total plate convergence; after the slab began to shallow, shortening in the Precordillera accommodated 10 - 12% of the convergence rate