COCORP deep seismic reflection profiles suggest that a large fraction of the crust in this region is composed of tectonically thickened Paleozoic sediments (and metasediments). Reflections representing a southward-thickening wedge of layered rock on the northern portions of the survey are associated with approximately 12 km (39,000 ft) of Carboniferous flysch overlying thin, lower to middle Paleozoic shelf strata in the Frontal thrust zone. In the interior of the mountain belt, the Benton Uplift is a broad antiform, cored by crystalline basement at depths below 7 km (23,000 ft). Beneath the southern Ouachitas and the adjacent Gulf Coastal Plain, a zone of south-dipping reflections probably represents at least 14 km (46,000 ft) of tectonically thickened, lower to middle Paleozoic off-shelf strata and Carboniferous flysch. Regional Bouguer gravity data show a minimum coincident with the thickest accumulation of flysch in the Frontal thrust zone. To the south, the Benton Uplift lies on a steep gravity gradient which is continuous along most of the Ouachita trend. The Ouachita gravity signature can be modeled as a southward shallowing of the Moho (from 40 km [131,000 ft] in northern Arkansas to about 30 km [98,000 ft] just south of the Ouachitas), coincident with the tectonic thickening of the Paleozoic strata interpreted from the COCORP data. The resulting crustal section can be interpreted as the remnants of an early Paleozoic passive margin which was subducted beneath a thick accretionary wedge in Carboniferous time. The Benton Uplift is a late-stage involvement of crystalline basement in foreland thrusting as the margin entered the south-dipping subduction zone.--Modified journal abstract.