Thick‐skinned deformation observed on deep seismic reflection profiles in western Argentina

Abstract

One of the first deep seismic reflection profiles in South America has resulted from reprocessing nearly 90 km of 96 channel Vibroseis data that were originally collected by YPF (the Argentine National Oil Company) across the Sierra de la Huerta in the Sierras Pampeanas of western Argentina. Reprocessed sections show the following features in the upper 20 km of crust: (1) prominent layering within Carboniferous(?) to Quaternary strata; and (2) a complex suite of faint reflections which are interpreted to define both extensional and compressional faults. The Mesozoic strata are bounded by one or more west‐dipping normal faults. One or two east‐dipping thrust faults shallow with depth, a geometry similar to the Rocky Mountain foreland of the western U. S. A. One thrust has a clear surface trace. Although lacking evidence of surface scarps or clear fault plane reflections, panels of constant dip domain seen in the seismic data justify the application of fault‐bend folding techniques to suggest that a second, deeper fault becomes nearly flat at 15–20 km. Cumulative Cenozoic offset on the thrusts beneath the Sierra de la Huerta is about 13 km. A well‐located earthquake hypocenter lies within the footwall block of a major thrust and is presumed to represent susidiary thrust motion along a secondary high‐angle fault. Gravity models suggest that the thrust ramps may nucleate at an anomalous dense mass in the middle or lower crust. Interpretation of lower crustal structure is hampered by the lack of clear reflections, presumably due to small‐scale and low‐amplitude impedance contrast variations of lithologic units at those depths. Consideration of recorrelation limitations and signal penetration constraints provide likely rationale for the scarcity of clear reflections in the basement rocks of the region, but low reflectance of mafic metamorphic rocks remains a possible explanation.