Conclusiones

As previously noted, regionally curved mineral lineation patterns (Fig. 3-7) are somewhat problematic for interpreting transport kinematics. Davis (1993), Hatcher (2001), and Merschat et al. (2005) have used these regional patterns to reconstruct flow paths for IP thrust sheets. Davis (1993) suggested that buttressing of IP thrust sheets against a NE-SW trending, SE-dipping structure (potentially the BFZ) could have deflected W-advancing thrust sheets to the SW. Hatcher (2001) proposed that the initial localization of the early BFZ corresponds to a stratigraphically controlled crustal anisotropy. A zone of constricted flow, as identified by Hatcher (2001), refers to the area immediately southeast of the BFZ that is characterized by strongly aligned NE-SW oriented structures, produced by SW-directed transpressional flow (Fig. 3-7). This zone extends from the Georgia into the North Carolina IP, encompasses most of the present study area, and ultimately ends near the Sauratown Mountains window (Hatcher, 2001).

Davis (1993) and Merschat et al. (2005) used regional curved mineral lineation patterns to reconstruct and delineate patterns of Neoacadian mid-crustal flow and ductile faulting in the IP. IP mineral-shape lineations are thought to track ductile flow of plastic type-F thrust sheets. Using these patterns and orogen-parallel map-scale sheath fold axes, a model has been developed for the trajectories of IP tectonic transport, in which NW- and W-directed transport and flow were deflected to the SW by a footwall buttress beneath the Neoacadian BFZ during thrust emplacement (Hatcher, 2001; Merschat et al., 2005; Hatcher and Merschat, in press). A gradual change in the orientation of structures in a 15-20 km wide belt immediately above the BFZ is thought to result from a gradual change in displacement direction from NW to W as the BFZ is approached and to SW proximal to the BFZ. Davis (1993) suggested that this area above the BFZ, the zone of constricted alignment (Fig. 3-7), had deformed by a combination of pure and orogen-parallel simple shear, resulting from extension, ductile thrusting, and horizontal shearing.

These factors combined with the environmental conditions required for high-grade metamorphism and migmatization, ultimately produced SW-directed net displacements and lateral extrusion of ductile rock masses near the base of the IP.

No curved lineation pattern can be surmised from linear fabric patterns in this study (Fig. 3-8). The apparent lack of curved lineation patterns for the present study area may simply be a result it lying within the Neoacadian BFZ and the zone of constriction, and does not necessarily imply that these patterns are not present in the northeastern IP.

Evidence for the SW-directed ductile extrusion of IP schists and gneisses does exist in the form of SW-vergent D₂ sheath folds in both eastern and western IP terranes. Eastern IP F₂ sheath folds must have developed after eastern IP anatectic magmatism (~407 Ma), but before BCFZ emplacement (~360 Ma), as they are cored by anatectic melts restricted to the eastern IP and truncated by the BCFZ (Fig. 3-2; Plates 1 and 3). These folds were likely developed shortly after intrusion and transposition of eastern IP anatectic melts by regional inhomogeneous simple shear generated by a combination of subduction by the Carolina superterrane and SW-directed extrusion or “escape flow” by the eastern IP. These structures are the product of long-term subduction of eastern IP lithologies by the Carolina superterrane, and are successively overprinted and transposed by late Neoacadian, Alleghanian, and later fabrics.

Strongly aligned NE-SW trending mineral lineations present in the study area are parallel to SW-vergent F₂ sheath folds and NW-vergent F₃ fold hinges (Figs. 3-8 and 3-9). This evidence suggests that D₂ linear fabrics are oriented parallel to SW-vergent transport. Parallel orientations of L₂ mineral lineations and D₂ sheath fold hinges can be explained by their simultaneous development during D₂ and SW-directed crustal flow. D₄ S-C fabric intersection lineations, SE-plunging crenulation axes, and axes of SW-vergent F₄ folds are parallel and overprint earlier D₂ SW-vergent and D₃ NW-vergent folds, NE-SW oriented mineral lineations, and NE-NE-SW trending planar fabrics.

The above suggests that transport during D₂ was SW-directed, followed by NW-directed transport during D₃, and SW-directed transport during D₄. However, material transport during D₁, D₂ and D₃ may have been orthogonal to tectonic transport due to melt weakening and ductile extrusion of material to the SW. If so, this explains the strong constriction of linear fabrics proximal to the BFZ and BCFZ as a flow lineation. Intense migmatization and thermobarometric data support the conclusion that northeastern IP rocks were above minimum melt conditions during the Neoacadian. Therefore, it is plausible that strongly aligned L₂ mineral lineations in the study area represent a syn- post-Neoacadian flow lines.

Kinematic indicators display consistent shear sense throughout the study area.

Rotated porphyroblasts, S-C fabrics, asymmetric folds, shear bands, mica fish, rotated porphyroclasts, quartz ribbons, and deflected foliations show dominantly top-to-the-SW transport for the northeastern IP (Figs. 3-5, 3-10 and 3-20). Strongly NE-SW oriented

W K -1 65 4- O h

SW NE

1 cm

CC' S S C

ultra-mylonite sinistral

σ-grain

}

C'

Figure 3-20. Microstructural kinematic indicators from the study area. Note the consistency of shear sense indicators consisting of mica fish, S-C fabrics, σ- and δ-grains, asymetric microfolds, and stair-stepping geometries of microboudinage.

(a) Scanned thin section of mylonitic Henderson Gneiss near Whites Creek in plane-polarized light. (b) Enlarged image showing deformed microboudinage of quartzofeldspathic material. Note the top-to-the-SW stair stepping geometries.

(c) Enlarged image of SW-vergent F₄microfolds with σ-grains indicating SW shear sense. (d) Enlarged image of a sheared δ-clast indicating top-to-the-SW shear sense.

1 mm

0.5 mm 1 mm (a)

(c) (d)

(b)

W K -6 24 -O h

SW NE

σ-grain 1 cm

mica fish

C S

S

W K -1 72 0- O h

SW NE

1 cm

qtz ribbon ultra-mylonite

sinistral σ-grain σ-grain

sinistral σ-grain

}

(e)

(f)

sense indicators, however, reveal the transpressive nature of D₂, in which synkinematic mineral lineations track incremental flow paths (Worley and Wilson, 1996) and deflection of northeastern IP thrust sheets to the SW after butressing. SW-directed crustal flow can be attributed to constricted, internal, dextral plastic flow caused by buttressing of crystalline thrust sheets against the Neoacadian BFZ (Hatcher, 2001; Davis, 1993;

Merschat et al., 2005). Data from this study support the conclusions of Davis (1993) and subsequent workers; the dominant transpositional deformation event likely occurred during the Neoacadian orogeny and had a significant dextral flow component. The consistent nature of shear sense indicators qualitatively infer a dominance of simple shear in the IP during D₂ ductile deformation. During D₄, transport was likely again SW-directed, but under lower P-T conditions. A predominance of oblate planar D₂ and D₃ fabrics suggests that the main foliation is due to pervasive flattening rather than simple shearing. However, it is also possible that previously developed D₁ S-surfaces acted as slip planes during simple shear dominated D₂ and D₄ events, producing the SW-vergent overprinting fabrics observed in the study area.

Structural relationships in the Brushy Mountains provide evidence for a developmental model of the northeastern IP similar to those envisaged by previous investigators, with modifications to incorporate new data from this study. Modifications are designed to also incorporate new geochronologic data, which provide additional timing constraints, and are summarized in Table 3-1. Initially, eastern and western IP terranes underwent separate D₁ events, as evidenced by differing early deformational styles and the partitioning of anatectic melts across the BCFZ. Western IP D₁ structures indicate that some map-scale folds in Tallulah Falls Formation metasedimentary

assemblages were developed prior to TCFZ emplacement, likely during a pre- to early Neoacadian orogenic event. Eastern IP D₁ structures were developed before BCFZ emplacement, but after eastern IP anatectic magmatism. This is likely related to a pre- to early-Neoacadian subduction of the eastern IP beneath the Carolina superterrane outboard of the western IP. Subduction led to high-grade metamorphism and anatectic magmatism ~407 Ma and successive SW-directed extrusion of ductile rock masses, resulting from regional inhomogeneous simple shear, and producing macroscale sheath folds cored by anatectic melts in the eastern IP. Emplacement of the BCFZ occurred during the Neoacadian orogeny, producing ductile D₂ and D₃ structures during continued migmatization and high-grade metamorphism, which transposed and masked earlier fabrics. Parallel top-to-the-SW shear sense indicators and NE-SW oriented mineral lineations suggest that initial NW-convergence was followed by SW-directed transport

and crustal flow during D₂ transpression. Continued thrust emplacement causing uplift and cooling occurred over the course of D₃, but was NW-vergent as evidenced by fold asymmetry. Alleghanian D₄ deformation brought about differing deformational styles and structures as cooler lithotectonic assemblages underwent dextral strike-slip transport along preexisting fault and shear zones. This orogenic pulse took place over the transition from ductile to brittle deformational styles, overprints all previously developed fabrics, and is responsible for additional SW-vergent kinematic indicators in the study area. D₅ and D₆ produced brittle extensional structures that overprint all earlier structures and are likely related to Alleghanian and post-Paleozoic orogenic pulses.