TECTONIC ACCRETION AT THE OUACHITA MARGIN OF LAURENTIA

The Alabama-Oklahoma transform and Ouachita rift frame the Ouachita embayment of southern Laurentia. The PASSCAL (wide-angle reflection/refraction) velocity model shows abrupt transition (~25 km wide) from continental crust to thin transitional or oceanic crust along the Alabama-Oklahoma transform. A gravity model is consistent with an upper-plate Ouachita rift margin indicated by synrift and early passive-margin thermal history. Late Cambrian through Early Mississippian strata include passive-margin carbonate-shelf and off-shelf deep-water facies. The Arkoma foreland basin records diachronous (Mississippian to Pennsylvanian) closure of a remnant ocean basin in the Ouachita embayment. Laurentian continental crust and passive-margin cover remained in the footwall of the highly allochthonous, internally disharmonic Ouachita allochthon of off-shelf passive-margin facies and very thick synorogenic turbidites (Ouachita facies).

The leading part of the Ouachita allochthon is thrust over the continental margin and passive-margin shelf edge onto the proximal synorogenic clastic wedge in the Arkoma basin. South of the edge of continental crust, the Ouachita orogen consists of a very thick, tectonically thickened accretionary prism of Ouachita facies sedimentary rocks. The detached sedimentary wedge overlies transitional to oceanic crust and is overlain, at an angular unconformity, by post-orogenic Pennsylvanian (Desmoinesian) to Permian (Guadalupian) strata beneath the Mesozoic-Cenozoic cover of the Gulf Coastal Plain. Accretion of the prism of continental-slope deposits (both passive-margin and synorogenic) indicates a soft collision where neither a continent, a microcontinent, nor an arc collided with Laurentian crust. More than 100 km south of the edge of continental crust beneath the Ouachita thrust belt, the velocity model, gravity model, and drill samples combine to indicate a volcanic arc and continental crust of unknown heritage. On the east, the Black Warrior foreland basin reflects accretion of the arc onto the Alabama-Oklahoma transform margin of southern Laurentia. Farther south and east, the Suwannee-Wiggins suture bounds accreted African crust. On the west, the Waco uplift basement rocks may constitute a microcontinent caught within the Ouachita accretionary prism.