The Ouachita-Marathon-Sonora orogen is a 3000-km-long belt of deformed Paleozoic rocks bordering the southern margin of the Laurentian (North American) craton. Extending from Mississippi and Arkansas (Ouachita) southwestward through Texas (Marathon) and westward through Chihuahua and Sonora (Sonora), the orogenic system formed during a late Paleozoic collisional-subductional event. This event resulted in closure of the Rheic ocean and the development of the orogen as the southern edge of the Laurentia plate was subducted beneath a northward-advancing Gondwanan (South American) continental-margin arc. Foredeeps and foreland basins and uplifts were created on the Laurentian plate continentward of the orogen. Regional stratigraphic and structural relations indicate original physical continuity of the Ouachita-Marathon-Sonora orogenic belt along the entire southern margin of Laurentia.
In the Neoproterozoic and Early Cambrian, the supercontinent Rodinia rifted along trends later followed by the Ouachita-Marathon-Sonora orogenic belt. During the breakup of Rodinia, the promontories and embayments that developed along the south margin of Laurentia are related to a northeast-striking rift system (oceanward of the continental edge) deformed by northwest-striking synrift transform faults that offset the rift-parallel cratonal margin. Initial deposits of the rifted margin were sediments deposited in tectonic sags and basins, which opened oceanward, and in depressions adjacent to transform faults. Shelf deposition began with Middle Cambrian clastic and carbonate sediments and continued with deposition of mostly shallow-marine carbonate sediments. Offshelf deposition began with Upper Cambrian (oldest rocks recognized in thrust sheets) clastic sediments and continued into the Early Mississippian with deposition of deep-marine clastic and subordinate carbonate sediments in continental-rise and ocean-basin settings.
Deformation of the southern margin of Laurentia resulted from its late Paleozoic diachronous oblique collision with the South American part of Gondwana and development of volcanic-plutonic arc and associated fore-arc and back-arc assemblages along the northern margin of Gondwana. Westward younging of foredeep and foreland-basin depocenters and decreasing age of basin-fill sediments are in accord with the westward migratory closure of the Laurentia–Gondwana suture. Cambrian to Lower Mississippian preorogenic sediments were deposited in offshelf settings along the margins of the two continents, and Upper Mississippian to Permian synorogenic sediments were deposited in the deep-water ocean basin between the two converging continents and associated arcs, in troughs within the evolving allochthons, and in foredeeps and foreland basins on the craton margins. Ocean-basin sediments were scraped off the ocean floor and transported as allochthons formed during continental collision, 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 50–200 km above Laurentian continental-shelf and foredeep rocks. The late Paleozoic synorogenic foreland basins and uplifts developed cratonward as far north as the Transcontinental arch and Ancestral Rocky Mountains. The timing and sense of movement of these intracratonic structures have complex relationships to the collisional margin.
Deformation in all three segments of the Ouachita-Marathon-Sonora 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. This represents predominantly north to northwest contraction. The westward migratory termination of orogenesis and the related development of foredeeps along the Ouachita-Marathon-Sonora belt are consistent with oblique convergence of Gondwana (Africa and South America) with Laurentia and require some clockwise rotation of South America following initial collision in the Ouachita Mountains region. The Sonora segment of the Ouachita-Marathon-Sonora orogenic system shows that northwestern Gondwana and its associated volcanic-arc terrane once lay south of western Laurentia (south of central Sonora) and was not restricted to areas to the east of Sonora as often shown in Pangaean reconstructions. Data south of the Laurentia continental margin indicate that the Gondwanan crust was extremely variable. Ashfall tuffs in Upper Mississippian through lowermost Pennsylvanian hemipelagites and turbidites in the Ouachita Mountains and Marathon region, and rhyolite flows and ashfall tuffs (bentonites) in Permian flysch in the southern Pedregosa basin of north-central Mexico indicate explosive volcanism along the northern margin of Gondwana. Geochemistry of the upper Paleozoic extrusive Sabine Rhyolite in the flysch sequence of the Sabine uplift area indicates a continental-arc origin. Several inferred remnants of Gondwana crust and volcanic-arc rocks in the southern United States and northwestern Gulf of Mexico are collectively referred to as the Sabine block. Other inferred remnants of Gondwana crust and volcanic-arc rocks in northern Mexico include the Coahuila block in Coahuila and southeast Chihuahua, and the El Fuerte block in Sinaloa, Sonora, and adjacent Chihuahua.
Several lines of critical evidence contradict a late Paleozoic or Mesozoic megashear through Sonora. Both the Rodinia rift system and the Ouachita-Marathon-Sonora orogen represent a continuous southern continental margin westward to Baja and Alta California, across the supposed trace of the hypothetical Mojave-Sonora megashear. Although structures are present that offset the orogen and its associated rift system, their sense of offset is right-stepping rather than left-stepping as required by the megashear. Minor displacements by postorogenic faulting together with right-stepping transform faulting cumulatively represent ∼300 km of displacement in contrast to 600–1100 km of left-lateral offset postulated for the megashear. Stratigraphic facies changes around the southwest end of Laurentia do not support offset by such a megashear. Paleozoic biostratigraphic faunal groups persist throughout the length of the orogen to Baja California and are distinct from those in the Cordilleran margin. These, together with later Mesozoic faunal groups, continue across the hypothetical trace of the megashear with no offset. Finally, paleomagnetic data by other workers are not supportive of the megashear. The megashear concept is not compatible with the information presented in this chapter.