Within subduction zones, one tectonic plate dives beneath another and recycles material back into the mantle. Stresses associated with subduction zones produce some of the highest mountains and largest earthquakes around the Earth. The Nazca plate subducts into the mantle beneath the western portion of South America. In central Argentina, it subducts at a very low angle and remains at shallow depths for 100's of kilometers before sinking into the mantle; this is the most extreme example of present day shallow subduction in the world. The Sierras de Cordoba in the eastern Sierras Pampeanas of central Argentina mark the location where the Nazca slab eventually descends into the mantle. The collocation of this mountain range with the change in slab dip suggests a connection, but the means by which stresses may be transferred from the slab to the overriding plate to uplift the mountains remain elusive. This project is focused on identifying the mechanisms through which forces could be transferred from the descending Nazca plate up though the mantle wedge and into the South American lithosphere. A great deal of active crustal deformation and seismcity occurs in the Sierras de Cordoba region; this study will help characterize the seismic hazards in the vicinity of Cordoba through improved understanding of lithospheric dynamics that will come from this study. The involvement of researchers and students from Argentina, and our resulting collaboration on data analysis, interpretation, and development of new ideas and theories, make the international component of this project one of its major strengths. The funding of this research reflects the prominence of its international component as the Geophysics program, within the Division of Earth Science, supports this project with a contribution from the Office of International Science and Engineering.
There is much debate about the mechanics, dynamics, and structure of both the overriding plate and subducting slab related to flat-slab subduction. This project is undertaking a deployment of digital seismograph systems to investigate the deep structure of the eastern Sierras Pampeanas mountain range in central Argentina in order to determine details of their origin. Active basement uplifts within the eastern Sierras Pampeanas, which overlie the shallowly subducting Nazca plate, offer an ideal region to investigate the influence of flat slabs on surface deformation. The scientific goals of this experiment include determining the mechanism and mode by which the overriding South American lithosphere deforms away from its margin and understanding why the subduction angle of the downgoing Nazca plate steepens after traversing 100's of km at a shallow angle. Additionally, the fate of water, and other volatiles, within the slab as it descends into the mantle will be investigated to determine how devolatization may contribute to deformation within the upper plate or influence the coupling between shallowly subducting slabs and the overriding plate. Such knowledge will be applicable to understanding past (Jurrassic to Createcous) deformation in regions of the western United States such as Utah, Colorado, and Wyoming.
The eastern Sierras Pampeanas are located >600 km east of the Andean Cordillera in central Argentina and have been interpreted to be a response to shortening related to flat-slab subduction of the Nazca plate. Uplift of the ranges has been broadly documented to occur during Neogene time, but many questions remained regarding the timing and style of deformation, and the subsurface structural configuration. Range-bounding faults in the eastern Sierras Pampeanas thrust late Proterozoic to Cambrian schist and gneiss over poorly dated Pliocene to Pleistocene alluvial strata. The timing of fault displacement and age of footwall strata suggest that deformation may have been active at least by Pliocene time. Seismometers deployed as part of this project across the range provided data for locating seismicity and calculating receiver functions. These observations indicate that the Moho lies at a depth of 37 km and that a midcrustal discontinuity appears to correspond to a detachment zone between 15 and 20 km depth that aligns with a plane of seismicity. Incorporating these observations with previous observations from the Sierras Pampeanas led to our tectonic model, where the craton acts as a rigid backstop to eastward propagating stresses from the shallowly subducting slab. Deformation then propagates back to the west via westward-verging faults along a midcrustal detachment. Initial results from our project were based on employing a double difference earthquake location method to clarify the hypocentral distribution of local seismicity within the Sierra de Cordoba and to define zones of both clustered and diffuse seismicity. Overall, seismicity in this region during the study period is constrained to the upper ∼ 25 km of the crust and is mostly concentrated along minor structures and not along the major range-bounding faults. Focal mechanism solutions from the nine clearest events show mainly normal deformation with an oblique component and E–W oriented P -axes that likely result from shortening along structures with oblique orientations to the direction of plate convergence. The present pattern of deformation was then be compared to earlier episodes of deformation within the Sierra de Cordoba by investigating the stratigraphy surround large faults in the area. Based on this type of geologic investigation we were able to learn more about the timing of displacement along faults within the eastern Sierras Pampeanas. Though absolute dating approaches have not been widely used in the mountains of central Argentina the relative dating appears to indicate that young sediments are being overthrust by basement rocks of Proterozoic to Cambrian age. Crustal thickness in the Sierras Pampeans follows boundaries between the terranes that accreted together to form South America. The pattern of crustal structures in the Sierras Pampeanas were greatly refined by combining data together from the ESP array that we operated and the neighboring SIEMBRA array opperated by collaborators at the University of Arizona. Variations in seismic wave speeds across the crust of the Sierras Pampeanas identified by inverting surface wave arrivals largely relate to changes in lithology. At greater depths, it appears that subduction-related hydration plays a significant role in controlling shear wave velocities within the upper mantle. In areas of flat-slab subduction, slab velocities increase from west to east while velocities in the overlying lithosphere decrease, which may result from fluids exiting the slab and hydrating the overlying mantle.The orientation of anisotropy in the mantle beneath the Sierra de Córdoba consistently aligns in an east-west orientation. These seismic wave speeds implies that anisotropy measured in the subduction zone mantle may be related to deformation in the mantle wedge rather than mantle flow below the slab, as previously thought.
