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 basin in the southern portion of Baltica. The Southern Oklahoma aulacogen (SOA), 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 to New Mexico. 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 ~15 km, and the deepest of these sedimentary layers has been interpreted as rift fill. In addition, the Wichita uplift is underlain by very high velocity and density mafic material even at upper crustal depths of <10 km. The Dniepr-Donets Basin (DDB) 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 break-up and ocean crust formation. The main feature of the DDB is a Late Devonian rift basin overlain by a substantial (but variable) post-rift sedimentary sequence that records several extensional or transtensional and at least one moderate compressional reactivation. The width of the rift zone varies from 150 km in the Pripyat Trough, 60-70 in the Dniepr, and 140-160 km in the southeastern segment. Recent deep seismic surveys in the Donets segment of the basin resolve the geometry of the sedimentary basin, indicating an asymmetric form with a steeper basement surface in the south than in the north and a total sedimentary thickness of about 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 along the profile. Thus, the major differences between these two major rifts are the degree of inversion and the nature of the magmatic modification of the crust.