RESOLVING GEOLOGICAL AND GEOPHYSICAL EVIDENCE FOR A REACTIVATED CAMBRIAN PLATE BOUNDARY BENEATH THE OUACHITA OROGEN: THE ALABAMA-OKLAHOMA TRANSFORM FAULT

The Alabama-Oklahoma transform fault has been inferred from geologic and seismic data to be a lithosphere-scale structure buried beneath the Ouachita orogenic belt. Models for opening of the Iapetus ocean and extraction of the Argentine Precordillera (a terrane presently in South America) from Laurentian (North American) lithosphere in the Cambrian require the presence of this 800 km-long sub-vertical strike-slip plate boundary. Incipient (incomplete) collision of the Sabine continental-margin arc terrane with this boundary during the Ouachita orogeny resulted in the transform fault being completely overridden by a thick accretionary wedge in the Pennsylvanian. The inferred location of the transform fault also coincides with the occurrence of diamond-bearing Cretaceous lamproites, which may have been preferentially emplaced into seismically anisotropic lithospheric mantle of during hotspot-related magmatism. Crustal and mantle xenoliths from the lamproites appear to have sampled an intensely reactivated zone of Archean to Mesozoic continental lithosphere that remains poorly understood.

Recent mapping of the lithosphere-asthenosphere boundary reveals a pronounced seismic velocity gradient between “stable” and “unstable” continental lithosphere that may coincide with the Alabama-Oklahoma transform fault and a zone of “tectonic inheritance.” The fault represents an excellent future target for EarthScope’s Flexible Array. Mapping of mantle anisotropy may detect changes in the orientation of the fast direction across the transform or preserved within the boundary zone. Magnetotelluric (MT) data may constrain differences in conductivity of the sub-continental lithospheric mantle that might be attributed to distinctly different mantle north and south of the transform. Existing geophysical evidence for the existence of the Alabama-Oklahoma transform fault is not convincing, so the region is an excellent future target for EarthScope’s Flexible Array. Achieving a better understanding the region offers an opportunity to gain new insights into the role of structural reactivation and the control of anisotropy in the mantle during a Wilson cycle of continental rifting, passive margin evolution, and rift inheritance that culminated with terrane accretion during the Ouachita orogeny.