Faulting and tectonics in the foreland basin fold/thrust belt of the San Juan Province, Argentina, and a comparison to the Yakima fold/thrust belt of the northwestern United States
AuthorWhitney, Robert A
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The San Juan Province of west-central Argentina lies in a region of crustal shortening, the result of subhorizontal subduction in the Peru-Chile trench to the west. This crustal shortening is manifest as a north-south trending fold/thrust belt affecting the western edge of the South American craton and the adjacent continental slope. Shortening is accomplished on flattening thrusts, with decollement at moderate depths, controlled by bedrock lithology. Seismicity occurs in the subducted slab as Benioff zone seismicity and in the upper portions of the continental crust. A major strike-slip fault separates the foreland basin/craton shortening from the Andes uplift. The crustal shortening episode began about 9 Ma, but Quaternary activity is limited to the western edge of the craton and the eastern edge of the Paleozoic continental shelf. Historically, large earthquakes in the fold/thrust belt have not produced primary surface rupture, but secondary rupture on bending moment faults has occurred, both normal and reverse in mechanism. This phenomena indicates small secondary displacements can be indicative of very large events. The pattern of bending moment fault hypocenters complicates efforts to determine the geometry of the causative fault and to choose the correct focal plane solution. The Yakima fold/thrust belt of the northwestern U.S. is an area undergoing north-south crustal shortening similar to that in South America, although less active by about an order of magnitude. Comparison between the two regions indicates fault geometry and structure resulting from the crustal shortening are similar. Seismic risk analyses in the Columbia Plateau region have underestimated earthquake hazard by using incorrect models for the faulting and failing to consider the possibility of large magnitude events that do not have primary surface rupture.
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SubjectSan Juan Province
South American craton
Benioff zone seismicity
foreland basin\craton shortening
Paleozoic continental shelf
primary surface rupture
secondary surface rupture
bending moment faults
bending moment fault hypocenters
focal plane solution
Yakima fold/thrust belt
seismic risk analysis
large magnitude events
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