At least 16 km of Pliocene shortening occurred beneath the La Pilona anticline, an oil-producing structure that lies along the east flank of the Argentine Andes near the city of Mendoza. Balanced cross sections show that the anticline, which is expressed at the surface by late Cenozoic strata, overlies imbricated Paleozoic and Triassic strata that are bounded above and below by detachment surfaces with opposing vergence. Local and regional geochronologic data dictate that shortening occurred during an interval that was less than 2.7 m.y., and probably less than 1.0 m.y. Shortening rates therefore must have exceeded 0.6 cm/yr, and probably exceeded 1.6 cm/yr. Shortening rates in ancient foreland fold-and-thrust belts are generally between 0.2 and 0.5 cm/yr, but these long-term averages probably incorporate periods of tectonic inactivity as well as pulses of deformation. This study suggests that rates of motion on major structures at the deformation front may be five to ten times greater than shortening rates averaged over the entire history of a thrust belt. Dynamic models of foreland shortening must be able to account for these deformation pulses.

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