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Basement plays an important role in Andean deformation in central western Argentina. Shortening is controlled by moderate-angle basement-rooted thrusts, primarily eastwardly directed. Four glide horizons in the sedimentary cover locally modify the style of deformation. A variety of structures result, including broad open folds, overturned folds, imbricate thrusting, and decollement-type faults.
Two fault zones, active chiefly during a Miocene compressional event, exert primary control on the structure. The western-most is a zone of east-verging imbricate thrusts, closely paralleling the hinge line between thick Jurassic sediments to the west and thinner deposits to the east. This zone probably represents reactivation of basin-opening normal faults, which were active during back-arc extension in the late Triassic and early Jurassic. Balanced cross sections indicate marked basement shortening along this belt. The eastern fault zone lifts basement upward and eastward with a throw of 2 to 7 km (1.25 to 4.3 mi). Where these fault zones are widely separated, they divide the foothills into three structural belts: a realm west of both faults consisting of broad basement-cored folds ith minor thrusting, a central zone with a variety of structures deriving from both fault zones, an an eastern region with basically minor structures obscured by Quaternary alluvium. Where these two faults zones closely approach each other or merge, the marginal belts remain virtually unchanged. However, the central zone, which is updip from the thickest basinal sediments and encompasses a region of complex stratigraphy, is more intensely deformed and becomes a structurally elevated band, paralleling the faults.
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