LITHOSPHERIC STRUCTURE OF THE SOUTHERN OKLAHOMA AULACOGEN AND SURROUNDING REGION AS DETERMINED FROM BROADBAND SEISMOLOGY AND GRAVITY

The Southern Oklahoma Aulacogen (SOA) is easily recognizable on Bouguer gravity anomaly maps as perhaps the second highest amplitude gravity anomaly in North America. The SOA lies in the Granite Rhyolite province of Oklahoma and is characterized by large magnitude basement faults that were active during Cambrian rifting and were reactivated during the late Paleozoic (313-285 Ma) during the Ouachita orogeny. The SOA was originally considered to be a failed rift of a triple junction associated with the Cambrian-aged opening of the Iapetan Ocean. This model is supported by the three-armed pattern of gravity highs at the junction of the SOA with the Ouachita orogen, the age of the bimodal series of gabbroic and rhyolitic rocks (that are clearly mantle derived), and the interpretation of a thick succession of clastic metasedimentary rock as rift-fill. The metasedimentary rocks, however, have been found to be much older than the SOA faulting and volcanism. More recent investigations favor models that describe the SOA as a system of leaky transform faults that are roughly parallel to the Alabama-Oklahoma transform fault, which frames part of the Iapetan margin of southern Laurentia. This study will use Earthscope broadband seismic and gravity data to resolve the nature of the SOA and to determine the depth (into the mantle) to which features related to the formation of the SOA are preserved.

Seismic receiver functions (RF) from the Earthscope data have documented an abrupt change in crustal structure across the SOA. The RF analysis shows a mid-crustal boundary at a depth of about 15 km south of the SOA; the boundary dips north toward the SOA. North of the SOA, the midcrustal boundary appears to be 5 km shallower and flat. The Moho appears to be 45 km deep to the south of the SOA but appears to be at a depth of about 38 km to the north. Additional processing is underway to map out variation in the Vp/Vs ratio throughout the region and into the mantle. The gravity data are dominated by a linear maximum over the SOA that is related mainly to 12 km of gabbros and rhyolites as imaged by a wide-angle seismic reflection/refraction survey. To model the deeper crustal structure, a 2D model will be constructed using the RF analysis. The combined gravity and seismic models will be interpreted to determine how the SOA was formed.