Metodologías

The IP is a large NE-SW trending belt of high-grade para- and orthogneisses extending from eastern Alabama to northern North Carolina (Fig. 3-1). It is bounded to the northwest and southeast by large faults (Fig. 3-1). Based on recent detailed field studies from the South Mountains and Brushy Mountains in North Carolina (e.g., Bream, 1999; Giorgis, 1999; Hill, 1999; Williams, 2000; Bier, 2001; Kalbas, 2003;

Merschat, 2003), the IP has been subdivided into eastern and western components. The Brindle Creek thrust, originally described by Giorgis (1999), was recognized by Bream et al. (2001) as the southern Appalachian Neoacadian (360-340 Ma) terrane boundary separating the eastern IP Cat Square terrane from the western IP (eastern Tugaloo terrane) (Fig. 3-1).

Western IP units lie entirely between the BFZ to the northwest and the BCFZ to the southeast (Fig. 3-1). Lithologies consist of late-Neoproterozoic to

early-Paleozoic paragneisses along with Ordovician-Silurian(?) meta-igneous, -volcanic, and

Study Area

PineMountainfault

SequatchieValley fault

EBR

EBR

Hayesville fault

Figure 3-1. Generalized tectonic map of the southern Appalachians. Red box represents the location of present study area. Yellow box represents the location of previous detailed geologic mapping in the Brushy Mountains from Kalbas (2003 and Merschat (2003).

Tectonic map of the southern Appalachians (modified from Hatcher, 2002) shows the major lithotectonic units. CS–Cat Square terrane. EBR–Eastern Blue Ridge.

WBR–Western Blue Ridge. GMW–Grandfather Mountain window. PMW–Pine Mountain window. SMW–Sauratown Mountains window. BFZ- Brevard fault zone. BCFZ-Brindle Creek fault zone. AA–Alto allochthon. HG-Henderson Gneiss. SRA–Smith River

allochthon. WS–Winston-Salem. Hk–Hickory.

Hatcher and Hooper, 1992; Davis, 1993; Hatcher, 1993). Micro-, meso-, and map-scale structures display consistent northeast-southwest alignment across the belt.

Eastern IP units are bounded by the BCFZ to the northwest and the Central Piedmont Suture to the southeast (Fig. 3-1). The Brindle Creek thrust sheet is the only lithotectonic package recognized within the eastern IP. It is composed of Silurian-Devonian metasedimentary and Silurian-Devonian-Mississippian metaigneous lithologies.

Several internal shear zones occur within the Brindle Creek thrust sheet. Changes in the orientations of structures from strong NE-SW alignment proximal to the BCFZ, to E-W, to N-S further to the southeast, exist across the terrane.

The Moravian Falls and Taylorsville 7.5-minute quadrangles are located in the central and northeastern Brushy Mountains, ~65 km NNE of Hickory and ~50 km W of Winston-Salem, NC (Fig. 3-1). Moderate relief and good exposure permit detailed examination of structurally complex fabrics. Large roadcuts, rock quarries, clear cut ridges, creeks, and exfoliation surfaces provide fresh exposures for detailed study.

Recent property development has provided new road access to remote areas. Access to many foot traverses is provided by hunting trails, logging roads, and creek beds.

The structural geology and tectonic history of the study area make it an ideal location for studying a structurally complex area containing multiple ductile and successive brittle deformational events. This area contains a segment of the BCFZ and terrane boundary, the northeastern part of the PSSZ, and a portion of the Neoacadian BFZ to the northwest, including the MSFZ, and the TCFZ (Fig. 3-2; Plate 1). The presence of four map-scale shear zones within a single 7.5-minute quadrangle provides an excellent opportunity to study several Paleozoic ductile fault zones. The close proximity of the BCFZ and the Neoacadian BFZ allows for analyses of the multi-phase interaction between these crustal-scale structures, which is unique to this study. The characteristics of and relationships between these large-scale shear zones provide insight into the developmental history and workings of polyphase northeastern IP ductile deformation during successive crystalline thrust sheet emplacement.

METHODS

A detailed geologic map of the Moravian Falls quadrangle and the northwestern portion of the Taylorsville quadrangle were constructed during the course of the 2003

Brindle

Poplar

Springs Whites

Tumblebug Creek fault

Creek Ordovician Brooks Crossroads granitoid

Obc

Lenoir Quarry migmatite/

Ordovician Poor Mtn Amphibolite?

lqm

Lower Tallulah Falls/Ashe Formation (Cambrian?)

ltf

Upper Tallulah Falls/Ashe Formation (Cambrian?)

utf

Devonian Walker Top Granite

Dwt

Devonian mylonitic Walker Top Granite

Dmwt

Neoacadian (~366 Ma) thrust fault (teeth located on hanging wall) Early Neoacadian thrust fault (teeth located on hanging wall)

Oblique-slip fault (arrows indicate shear sense)

Possible sheath fold axis (arrows point in direction of shear sense)

Sample collected for SHRIMP geochronologic analysis

Lithologic contact

MN GN MILS40 15'MILS

UTM GRID AND 1970 MAGNETIC NORTH DECLINATION AT CENTER OF SHEET

533

(a)

Figure 3-2. (a) Simplified geologic map of the Moravian Falls and a portion of the Taylorsville 7.5-minute quadrangles, Wilkes and Alexander Counties, North Carolina.

EXPLANATION

WESTERN INNER PIEDMONT Ordovician Brooks Crossroads granitoid

Obc

Lenoir Quarry migmatite/

Ordovician Poor Mtn Amphibolite?

lqm

Lower Tallulah Falls/Ashe Formation (Cambrian?)

ltf

Upper Tallulah Falls/Ashe Formation (Cambrian?)

utf

Devonian Walker Top Granite

Dwt

Devonian mylonitic Walker Top Granite

Dmwt

Neoacadian (~360 Ma) thrust fault Lithologic Contact

base maps. Structural and lithologic data were recorded at >1,700 stations (Plate 2).

Orientations of structural fabrics were measured with a Brunton^TM pocket transit. All structural and lithologic data, interpreted contacts, observed contacts, and float lithologies were manually plotted in the field before compiling a spreadsheet database in Microsoft Excel^TM (Appendix B) and a digital geologic map in Adobe Illustrator^TM (Plate 1). All data were then georeferenced using MAPublisher^TM software for Adobe Illustrator^TM. Scaled field sketches were constructed, digital photographs were taken, and samples were collected for selected outcrops and cataloged by station number (Plate 2).

Structural analyses of all data were conducted on completion of all fieldwork.

Coeval structures were initially grouped by their relative timing based on field

observations, crosscutting relationships, and orientations. Structural domains were then identified from characteristic homogeneous and heterogeneous planar fabric orientations and separated on form-line maps. Planar and linear fabric orientations were grouped into respective domains and plotted on fabric diagrams constructed with Geoorient^TM software (Holcombe, 2005). Fabric data were contoured using the percent per percent area method.

Oriented samples were collected from selected outcrops for microstructural analyses. Microstructures and petrofabrics were examined and interpreted for oriented standard (2.4 x 4.6 cm) and oversized (5.08 x 7.62 cm) thin sections.