Increased shortening during periods of slab transition: evidence from the Argentine Precordillera at ~30° S

While the identification of active flat slab regions has become relatively straightforward based on the location and distribution of earthquakes in the subducting slab, the link between deformation in the overriding plate and the geometry of the down-going slab remains more elusive. To investigate the relationship between the two plates, we compare structural data from the Argentine Precordillera at ~30° S, above the modern Chilean flat slab, to geophysical data related to the Nazca plate. This comparison reveals a correlation between a significant increase in the shortening and reactivation on the main thrusts in the mountain belt and the inferred timing of shallowing of the Nazca slab. Structural data suggest that Tertiary shortening occurred in three main phases: > 20 Ma, ~ 11 - 4 Ma, and ~ 3 - 0 Ma. During the earliest period of motion, the western portion of the Precordillera was the locus of deformation and shortened 30 - 40 km before 20 Ma. At this time, the Nazca slab had yet to begin shallowing, and the aseismic Juan Fernandez ridge, which is often invoked to explain the flat slab, was far north of the region. From ~ 11 - 4 Ma, when the Nazca slab shallowed as the location of the Juan Fernandez ridge stabilized beneath the Precordillera, motion on the main thrusts in the central Precordillera contributed 50 - 60 km of shortening. Additionally, there is evidence of activity and reactivation on all the major thrusts in the central and western Precordillera during this period. The most recent phase of motion, ~ 3 - 0 Ma, corresponds to reactivation on the central Precordillera thrusts as well as the formation of the eastern Precordillera. Shortening from reactivation in the central Precordillera is less than 10 km, and shortening in the eastern Precordillera varies along strike from 7 to 17 km. It is likely that the Nazca slab reached its current orientation of nearly horizontal by ~ 3 Ma, when motion transferred from the thin-skinned thrusts of the western and central Precordillera to the thick-skinned uplifts of the Sierras Pampeanas and the associated eastern Precordillera. That shortening nearly doubled during the shallowing of the slab, combined with the significant decrease in shortening after the slab reached its current geometry, suggests that the majority of the deformation in the Precordillera is related to the period of transition in the slab geometry. Because connecting structures at the surface to the geometry of the subducted slab is especially challenging in regions where the slab is no longer horizontal, this correlation may indicate transitional periods in other mountain belts. If the correlation between increased shortening and shallowing slab geometry is robust, we may interpret observations of increased shortening and reactivation as evidence of significant changes in the subducted slab in regions where direct inference of slab geometry is unknown.