62 NORTH AMERICA/Ouachitas Figure Tectonic elements of the Ouachita Orogen Abbreviations (AM Arbuckle Mountains, Atf Appalachian Tectonic Front, Bbu Broken Bow Uplift, Bu Benton Uplift, CBp Central Basin Platform, Db Delaware Basin, Dru Devils River Uplift, Tp Texarkana Platform, Su Sabine Uplift, WM Wichita Mountains) (After Viele and Thomas, 1989.) interested reader should consult these The seminal work of Flawn and his colleagues in 1961 provided a complete review of the mountain belt and has been a cornerstone for all subsequent work 28 years later, the Geological Society of America published a thorough update of the Ouachita Orogen This discussion is synthesized from these sources Cambrian Continental Margin The trace of the Ouachita Orogen is irregular (Figure 1) and this spatial distribution is directly related to the Late Proterozoic–Early Cambrian rifting of the North American Craton From central Alabama and the junction with the Appalachian Mountains, the fold and thrust belt extends WNW across the Mississippi embayment into Arkansas and Oklahoma There the trend shifts to the SSW, extending from Oklahoma into central Texas At that point the trend shifts westerly continuing into West Texas, only to turn south again near the Marathon Uplift The mountain belt extends a few hundred kilometers into Mexico before it is obscured by a Mesozoic tectonic overprint This orthogonal pattern of recesses and salients reflects the rifting geometry along the southern margin of North America during the Late Precambrian and Early Cambrian The spreading ridge for this rifting is inferred to be subparallel to the east coast of North America and to the south-trending margin in Texas (Figure 1) The connection between Alabama and Oklahoma and between central and West Texas are considered to have been transform boundaries The Southern Oklahoma aulacogen is coeval with this rifting event and continues the WNW trend of the transform boundary into the craton Additional Late Precambrian intracratonic rifting has been documented in the Mississippi Valley; the grabens trend NNE from the Ouachita Front through eastern Arkansas and western Tennessee Igneous rocks exposed in the southern Oklahoma aulacogen reveal a 570–525 Ma bimodal volcanic package consistent with the rift model Approximately km of Cambrian clastic units in the Mississippi Valley graben indicate additional regional extension However, outside these rift zones, including the Ouachita Orogen, there is little evidence of this rift sequence