The footwall ramp of the Subandean decollement, northernmost Argentina, from extended correlation of seismic reflection data

The link between thin-skinned foreland shortening and thickening of the lower crust beneath the hinterlands of mountain belts has seldom been observed. We have reprocessed Yacimientos Petroliferos Fiscales (YPF S.A.) seismic reflection data from the Eastern Cordillera of the Central Andes at 22 degrees S latitude using Vibroseis extended correlation; the result we tentatively interpret as an image of the crustal-scale footwall ramp of the Subandean belt decollement. Three crossing lines, extended from 5 to 15 s (45 km) provide excellent 3D control. A band of strong mid-crustal reflections between 4.5 and 6 s (-9 to -13 km) displays imbricate thrust geometry. It may be a shallow zone of decollement but is 5-10 km too shallow for the Subandean belt decollement. At 7-8 s (- 16 to - 20 km), a band of reflections dips steeply NW at about 35 degrees (migrated) and strikes parallel to the Andean structure, underlying the Camargo syncline. The dipping band correlates closely with the westward projection of the Subandean decollement. East of the dipping event, scattered horizontal reflections are present to 12 s (- 32 km). Reflections above the dipping event have geometries similar to hanging wall anticlines and may truncate against it. West of the band, reflections occur to 15 s (- 41 km). None of these lines have sufficient penetration to image Moho. The footwall ramp of the Subandean belt apparently extends to near the base of the crust and would mark the western limit of under-thrusting of undeformed craton beneath the orogen. Based on lateral variation in surface geology, existing balanced cross-sections, and the geometry imaged on the reprocessed seismic lines, we suggest that the Subandean belt decollement steepens its ramp from north to south as shortening dies out southward. At 24 degrees S, foreland structures are locally compensated and have seismicity to 25 km. (C) 2000 Elsevier Science B.V. All rights reserved.