TECTONIC HISTORY OF THE SOUTHERN MARGIN OF NORTH AMERICA

The Ouachita–Marathon–Sonora orogen (OMS) is a 3000-km-long belt of deformed Paleozoic rocks bordering the southern margin of the North American craton (Laurentia). Extending from Mississippi and Arkansas (Ouachita) southwestward through Texas (Marathon) and westward through Chihuahua and Sonora, Mexico (Sonora), the orogenic system formed during a late Paleozoic collisional-subductional event, resulting in closure of the Rheic ocean as the Laurentian plate was subducted beneath a northward-advancing South American (Gondwanan) continental-margin arc.

In the Neoproterozoic and Early Cambrian, the supercontinent Rodinia rifted along trends later followed by the OMS orogenic belt. During the breakup of Rodinia, the configuration of the southern margin of Laurentia was related to a northeast-striking rift system deformed by northwest-striking synrift transform faults that offset the rift-parallel cratonal margin.

In the late Paleozoic, deformation of southern Laurentia resulted from its diachronous oblique collision with Gondwana and development of volcanic-plutonic arc and associated fore-arc and back-arc assemblages. Westward younging of foredeep and foreland-basin depocenters and decreasing age of basin-fill sediments reflect westward migratory closure of the Laurentia–Gondwana suture. Ocean-basin sediments were scraped off the ocean floor and transported as allochthons, in advance of synchronous Pennsylvanian and Permian foredeeps and foreland basins and uplifts. These allochthons were part of a large accretionary wedge formed above a south-dipping subduction zone and thrust northwestward above Laurentian continental-shelf and foredeep rocks for a distance of 50–200 km.

Deformation in all three segments of the OMS orogenic belt began in mid-Mississippian time and ended diachronously in the Late Pennsylvanian in the Ouachita Mountains, Early Permian in the Marathon region, and Late Permian in Sonora. The westward migratory termination of orogenesis and the related development of foredeeps along the OMS belt are consistent with oblique convergence of Gondwana with Laurentia, and require some clockwise rotation of South America following initial collision in the Ouachita Mountains region.