UNDERSTANDING GRADIENTS IN THE DIFFERENTIAL STRESS DRIVING FLOW: IMPLICATIONS FOR THE CRUSTAL ESCAPE FLOW MODEL IN THE SOUTHERN APPALACHIAN INNER PIEDMONT

The southern Appalachian Inner Piedmont (IP) has been interpreted to represent a relict crustal escape flow system that was active during the Neoacadian (370-340 Ma) orogeny. Critical to the support of this hypothesis is the identification of both the high-temperature, rheologically weak “channel,” with crustal flow driven by relatively low differential stress, and the rheologically strong “buttress,” where deformation was driven by relatively high differential stress. Paleopiezometric analyses from southern Appalachian quartz mylonites, quartzites, and quartz-bearing pelitic rocks of the eastern Blue Ridge (EBR; buttress) and IP (channel) allow gradients of differential stress driving flow to be examined. Samples located along a transect in the EBR northwest of the Brevard fault zone (BFZ) are used to define gradients in differential stress and deformation temperatures in the proposed buttress. Preliminary results for these samples suggest that deformation temperature increased, and differential stress decreased during deformation approaching the BFZ from the northwest. Mechanisms of quartz recrystallization shift from minor grain boundary bulging and dominant subgrain rotation (SGR) in samples located away from the BFZ to dominant SGR and minor grain boundary migration (GBM) in samples in the immediate BFZ footwall. Recrystallized grain sizes in the EBR are, in almost all cases, less than 150 microns on the major axis of ellipses used to approximate grain size. In the proposed crustal channel, quartz-rich samples collected along a transect south of Rosman, NC in the Brevard and Brindle Creek thrust sheets of the IP show GBM and SGR as the dominant mechanisms of deformation, as well as a general increase in grain size into the “core” of the proposed crustal channel. Recrystallized grain sizes in the Piedmont away from the BFZ commonly exceed 500 microns. These preliminary results suggest increasing deformation temperatures and decreasing differential stresses from close to the BFZ into the Piedmont, which is consistent with increased cooling of the proposed channel proximal to the colder, stronger Blue Ridge. Ongoing piezometric analyses, combined with quartz c-axis thermometry and thermochronology, may provide additional evidence to better refine the hypothesized buttress-channel relationship.