2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 2
Presentation Time: 8:30 AM


HARRY, Dennis L. and HUERTA, Audrey D., Department of Geological Sciences, Univ of Alabama - Tuscaloosa, Box 870338, Tuscaloosa, AL 35487-0338, dharry@wgs.geo.ua.edu

The spatial association between continental extension and prior orogenic events is typically considered within the context of a "Wilson Cycle", where continental breakup and collision occur repeatedly within the same structural province. A classic example is the U.S. Atlantic margin (USAM), where Mesozoic continental breakup follows the trend of the late Paleozoic Appalachian fold and thrust belt (AFTB). This association of extensional tectonism with older orogenic provinces is usually attributed to weakening of the lithosphere by faulting in the brittle upper crust and to the presence of a crustal root beneath the orogen, which increases the thickness of relatively weak ductile felsic crust at the expense of stronger ultramafic mantle in the lithosphere. Finite element models show that this concept accounts for major elements of extension on the USAM, including early (Triassic) extension within the AFTB, later continental breakup further to the east, and dismemberment of the orogen onto opposing conjugate rift margins. The Gulf of Mexico (GOM) is similar to the USAM in that the locus of continental breakup trends sub-parallel to the middle Paleozoic Ouachita fold and thrust belt (OFTB). However, crustal-scale transects derived from gravity modeling, seismic, and well data show that extension in the central GOM is restricted to regions south of the OFTB. Unlike the AFTB, the OFTB appears to have acted as a strong barrier to extension rather than a zone of weakness. We attribute this to differences in the collissional events associated with formation of the two orogens. The AFTB resulted from continent-continent collision, creating a thick crustal root (and zone of weakness) that persists today under the south-central Appalachian mountains. In contrast, the OFTB is usually associated with an arc-continent collision. The new crustal transects presented here and previous deep seismic profiling and gravity modeling suggest that the subduction complex is preserved in a nearly pristine state beneath the OFTB. The orogen appears to be underlain by a fragment of remnant Precambrian oceanic crust and relatively shallow mantle, both of which are relatively strong in comparison to continental and/or arc crust. This created a zone of inherited strength beneath the OFTB that acted as a barrier to Mesozoic extension.