The Southern Oklahoma and Dniepr-Donets Aulacogens: A Comparative Analysis

The classic failed continental rift or aulacogen is one that intersects a rifted continental margin at a high angle. Based on recent geological and geophysical studies, we have revisited a classic analogy that was drawn between two major intracratonic rifts, the Southern Oklahoma aulacogen in the southern portion of Laurentia and the Dniepr-Donets aulacogen in the southern portion of Baltica. The Southern Oklahoma aulacogen, also known as the Wichita aulacogen, consists of a linear alignment of extensively inverted rift structures that begins at the rifted margin of Laurentia in northeast Texas and extends northwestward at least to southern Colorado. Deep seismic profiles have revealed the upper crustal structure of this feature, and gravity data provide a regional context for interpreting these results. Velocities low enough to indicate the presence of sedimentary rocks extend to a depth of similar to 15 km, and the deepest of these sedimentary layers has been interpreted as rift fill. In addition, the main inversion structure of the Southern Oklahoma aulacogen (Wichita uplift) is underlain by very high-velocity and dense mafic material even at upper crustal depths of similar to 5 km. The Dniepr-Donets aulacogen has been cited as a type example of an aulacogen and is clearly a "failed rift" in the sense that it did not itself lead to continental breakup and ocean crust formation. The main feature of the aulacogen is a Late Devonian rift basin overlain by a substantial (but variable) postrift sedimentary sequence that records several extensional or transtensional events and at least one moderate compressional reactivation. Recent deep seismic reflection and refraction surveys resolve the geometry of the sedimentary succession in the Donets segment of the basin, indicating an asymmetric form with a steeper basement surface in the south than in the north and a total sedimentary thickness of similar to 20 km. A thick (>10 km) high-velocity (>6.9 km/s) lower crustal body lies beneath the rift basin itself and is offset slightly to the north compared to the main basin depocenter. The Moho displays only slight topography around a depth of 40 km although, based on older deep seismic data, it shallows somewhat under the rift axis in the Dniepr segment to the northwest. Thus, major differences between these two major rifts are the nature of the magmatic modification of the crust and degree of inversion. Both the age of initial rifting and subsequent inversion in the Dniepr-Donets aulacogen are redefined compared to what was thought at the time the original analogy was made with the Southern Oklahoma aulacogen.