HR: 0800h
AN: T11C-1279    [Abstracts]
TI: How Is Lower Crust Modified As A Neo-Rift Becomes A Paleo-Rift and Part Of The Craton?
AU: * Gilbert, M C
EM: mcgilbert@ou.edu
AF: University of Oklahoma, School of Geology & Geophysics, Norman, OK 73019 United States
AB: The Southern Oklahoma Aulacogen (SOA), at the southern end of Laurentia (present coordinates), if behaving as neo-rifts, such as the Rio Grande Rift, presumably possessed a rift structure in the Cambrian with a continental thickness of about 28km. Seismic data, though sparse, suggest a present thickness of the SOA is about 45km, indistinguishable from adjacent rifted Proterozoic crust. By what process do we add 15km to the original SOA crust: underplating, eclogite-gabbro transformation, or deformation? This question has bearing on how we understand and interpret all paleo-rifts now a part of continental cores. Geology of the southern Midcontinent of North America does not show evidence of significant thermal events in the Phanerozoic. This effectively rules out underplating and phase transformation as a cause of change in M-discontinuity depth. Present SOA outcrops are in the Wichita Mountains of southwestern Oklahoma, part of the easternmost Ancestral Rockies. These outcrops are in the Wichita-Amarillo crustal block uplifted about 7km in the Pennsylvanian. The Anadarko Basin to the north went down about 7km. Large Pennsylvanian thrust faults in the upper brittle crust are documented. Thus it appears that compressive deformation may be able to account for the change in crustal thickness from neo-rift type to paleo-rift and craton type. However, the accommodation made in the lower crust may be more dramatic than deformation in the upper crust because shortening, and thickening of the order of 2X, is probably required. Comparisons with other paleo-rifts in North America, such as the Middle Proterozoic Midcontinent Rift and the NeoProterozoic Reelfoot Rift, show that their crustal thicknesses now also match their previously rifted margins. Can the same sequence, as seems to be the case with the SOA, apply to other paleo-rifts?
DE: 9350 North America
DE: 8109 Continental tectonics--extensional (0905)
DE: 8110 Continental tectonics--general (0905)
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting