Martin Zeckra

and 6 more

The Andean foreland is divided into morphotectonic provinces characterized by diverse deformation styles and seismogenic behavior partially stemming from distinct geological histories that preceded the current phase of subduction. The transition between the high Andes and the eastern foreland is exposed to numerous natural hazards and contains critical economic infrastructure, yet we know relatively little about regional active tectonics due to few geophysical investigations. Here we use waveforms collected during a 15-month-long seismic network deployment in the Santa Bárbara System (SBS) of northwest Argentina following the 2015 Mw 5.7 El Galpón earthquake to determine the distribution and magnitude of local earthquakes, obtain a regional 1D seismic velocity model, and improve our overall understanding of SBS neotectonics. Of the nearly 1200 recorded earthquakes, ~700 occurred in the crust with half of the moment release associated with events deeper than 25 km. The depth extent of seismicity supports the notion that the SBS upper and middle crust are homogeneous and that the lower crust is composed of granulites. These conditions likely formed during Paleozoic mountain building and Salta Rift-related Cretaceous magmatism, which dehydrated the crust. We find no clear indications that a shallow, low-angle detachment fault inferred to have been active during Cretaceous rifting exerts a strong control on modern deformation in contrast to the active décollement beneath the adjacent fold-and-thrust belt of the Subandes to the north. It remains unclear how active, inverted normal faults in the SBS shallow crust connect to the deeper zones of seismicity.

Michaël Pons

and 4 more

The non-collisional subduction margin of South America is characterized by different geometries of the subduction zone and upper-plate tectono-magmatic provinces. The localization of deformation in the southern Central Andes (29°S–39°S) has been attributed to numerous factors that combine the properties of the subducting oceanic Nazca plate and the continental South American plate. In this study, the present-day configuration of the subducting oceanic plate and the continental upper plate were integrated in a data-driven geodynamic workflow to assess their role in determining strain localization within the upper plate of the flat slab and its southward transition to a steeper segment. The model predicts two fundamental processes that drive deformation in the Andean orogen and its foreland: eastward propagation of deformation in the flat-slab segment by a combined bulldozing mechanism and pure-shear shortening that affects the broken foreland and simple-shear shortening in the fold-and-thrust belt of the orogen above the steep slab segment. The transition between the steep and subhorizontal subduction segments is characterized by a 370-km-wide area of diffuse shear, where deformation transitions from pure to simple shear, resembling the transition from thick to thin-skinned foreland deformation in the southern Sierras Pampeanas. This pattern is controlled by the change in dip geometry of the Nazca plate and the presence of mechanically weak sedimentary basins and inherited faults.