Mechanisms for basement shortening in the Andean foreland fold belt of southern South America

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Abstract
Summary: A mechanism for basement shortening underneath allochthonous foreland fold and thrust belts is reverse faulting along inherited normal faults. Normal faulting of basement rocks beneath much of southernmost South America indicates that up to 30–40% extension occurred before sedimentation in a subsiding foredeep. The basement is divided into blocks of similar dimensions to those in the Basin and range of Nevada, U.S.A. Post-depositional reverse motion along some of these high-angle block boundaries under the foreland can account for the 20–40 km of shortening observed in the cover rocks, without showing upthrust wedges of basement rocks. The folds in the cover rocks of the foreland fold-belt are asymmetric, open to closed with over-steepened to overturned limbs which are frequently truncated by high-angle reverse faults. These faults attain a shallower dip with depth toward a décollement plane above a basal volcanic section. Basement involvement in thrusts (0.5–1.5 km vertical displacement) is suggested where basal volcanics are involved in thrust wedges, as revealed by drilling and seismic reflection data. Block faults have been observed beneath many forelands and have always been assumed to be pre-orogenic and passive with respect to deformation of the cover. The block dimensions, fault angles, vertical displacements and other parameters from the southern Andes are inserted to calculate pre-depositional extension and syn-orogenic shortening that can occur without producing obvious basement upthrusting. The results are compared with data from the Appalachians and Canadian Rockies.