THE GEOLOGY OF THE BREVARD ZONE AND ADJACENT TERRANES IN ALABAMA

[摘要] Detailed and reconnaissance field and gravity studies indicate that the Brevard zone in Alabama consists of a lithologically-distinctive, multiply-deformed polymetamorphosed sequence of Cambrian(?)-age metasedimentary and metaigneous rocks bounded on the northwest and southeast by high-angle reverse faults that flatten with depth. It is likely that this sequence underlies the Inner Piedmont at depths (LESSTHEQ) 10km, and crops out to the southeast as the parautochthonous cover overlying Grenville basement in the Pine Mountain window. Along strike, Brevard zone lithic types (i.e., the Jacksons Gap group) are dominated by mica schists and phyllites to the northeast, and by quartzites to the southwest. This probably represents a change in original lithofacies and, when coupled with relevant structural data, indicates a possible southeastern clastic source. The Jacksons Gap group and immediately adjacent rock sequences have been subjected to at least four phases of contractional deformation, and possibly one phase of minor transpressional deformation. All deformational phases are regionally similar with respect to style, orientation, and relative timing, but vary spatially in intensity. D(,1) and D(,2) were ductile events that produced tight to isoclinal folds, and amphibolite- and greenschist-facies mineral assemblages, respectively. The Inner Piedmont allochthon was probably emplaced during D(,2). D(,3) produced most map-scale folds in the Brevard and a locally well-developed axial plane foliation. D(,4) was a cross-folding event that may be related to minor strike slip, and D(,5) was a brittle thrusting event confined mainly to the northwest-bounding fault of the Brevard, the Abanda. Based on reasonable geometric assumptions (e.g., the strike change of the Brevard south of Jacksons Gap is the up-plunge expression of a southeast-dipping fault ramp), it is likely that the Abanda fault soles in the major detachment beneath the Alabama Piedmont, and that the southeast-bounding fault of the Brevard, the Katy Creek, soles southeast of the Pine Mountain window. A minimum thrust-related shortening of 18 km on the Abanda, and 47 km on the Katy Creek could produce the observed geometries. These assertions are compatible with two-dimensional modeling of both newly-collected and recently-published gravity data. . . . (Author;;s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI

[发布机构] Rice University