STRUCTURAL AND PETROLOGICAL INSIGHTS AND CONTRASTS BETWEEN THE WICHITA AND ARBUCKLE SEGMENTS OF THE SOUTHERN OKLAHOMA AULACOGEN AS REVEALED BY DEEP DRILLING INTO RIFT-RELATED ROCKS

The early Cambrian Southern Oklahoma Aulacogen (SOA) is a failed-arm continental rift zone that influenced crustal evolution in southern Laurentia for at least 200 million years. This extensive bimodal igneous province contains >250,000 km³ of volcanic and plutonic rocks and strikes inland ~600 km from the Laurentian continental margin. Drill cuttings from 28 deep drilling penetrations from south-central to far-western Oklahoma provide access to 37.8 km of uncorrelated igneous section and highlight petrological and tectonic contrasts from east to west.

In the Arbuckle Mountains area of south-central Oklahoma, voluminous rhyolitic and basaltic lavas are intercalated in the subsurface; pyroclastic rocks occur only in minor amounts and, in the basaltic intervals, show evidence of phreatomagmatic processes. The abundant gabbros found in the Wichita Mountains are lacking, but granite and diabase sills intruding the volcanic rocks are common. In the Wichitas, only limited amounts of basalt have so far been documented within the rhyolite sequence. Rhyolites and granites in both regions have within-plate, A-type geochemical signatures. Basaltic rocks from the Arbuckles have tholeiitic to transitional compositions and trace-element and Nd and Sr isotopic contents similar to ocean-island and flood basalts in rift environments.

Pennsylvanian tectonic inversion of the SOA during the assembly of Pangea reactivated rift-bounding normal faults into regional thrust faults, the Frontal Fault Zone in the west and the Washita Valley Fault Zone in the east. Uplift began in the Wichitas as a single 200 km long block at ~320 Ma but the main phase of Arbuckle uplift and exposure of rift-related igneous rocks did not occur until ~300 Ma. Left lateral movement occurred along the entire uplift although the magnitude is debated. A far-west, ultra-deep well drilled into the intermediate part of the Frontal Fault Zone encountered down-dropped blocks bounded by normal faults with over 4.9 km of offset and ultimately penetrated amphibolite-facies metamorphic rocks. In the Arbuckles, well control constrains west to east changes of dip of the reactivated south-plunging Washita Valley Fault Zone, steepening from 55^o to >80^o over a distance of 35 km.