Brevard Fault Zone

Brevard Zone

 

The Brevard Zone is the south-eastern boundary of the Blue Ridge Belt in North Carolina and it separates the Blue Ridge and Inner Piedmont geological provinces (Fig. 1; Edelman et al., 1987; Merschat & Wiener, 1990). The Brevard zone rocks are mainly mylonitic and include thrust, strike-slip and oblique motions. A steeply dipping brittle fault of Palaeozoic age marks the change from rocks of the Blue Ridge belt to the Brevard rocks and the Chauga Belt (Merschat & Wiener, 1990). It consists of three distinct features that all dip moderately towards the southeast: 1) narrow Rosman Fault, which contains tectonic breccias and slices of buried sedimentary rocks, 2) Brevard Mylonite Zone, which consist of mylonites on either side of the Rosman Fault. It contains shear sense indicators that all uniformly indicate a dextral motion, 3) Brevard Fault Zone, located to the SE of the Rosman Fault. This one consists of stratified metamorphic rocks, which are repeated due to fault activity. The majority of the faults are believed to predate mylonitisation (Edelman et al., 1987).

The Brevard Zone is an important fault zone in order to understand the thrust tectonics and later strike slip motions during orogenesis of the Blue Ridge. Within this zone, ultramylonites and mylonites can be observed (Edelman et al., 1987), showing clear shear sense indicators, unravelling the later stages of orogenesis.

Figure 1: From Edelman et al., 1987; Merschat & Wiener, 1990

Brevard Zone Units

 

Important for understanding the evolution of the Brevard Zone is are the characteristics of four units: 1) Brevard Zone, 2) Rosman Fault, 3) Brevard Mylonite Zone, 4) Chauga Belt, that is located directly SE of the Brevard Zone.

 

Chauga Belt

 

The Chauga Belt consists of low to medium grade non-migmatitic metamorphic rocks and higher grade allochthonous rocks, which are the equivalents of the higher grade metamorphic rocks that are present in the Inner Piedmont and Blue Ridge provinces. The lower grade rocks are designated to the Chauga River and Poor Mountain Fms, from which the former only occurs to the NW of the Chauga Belt within the Brevard Zone whereas the latter one occurs to the SE of the Brevard Zone. They are overlain by the Alto allochthon and the Henderson Gneiss. The former consists of migmatised amphibolites facies rocks and is most likely thrusted over the Poor Mountain Fm. towards the NW by a sub-horizontal thrust fault. The Henderson Gneiss is a granitic gneiss that overlies the Poor Mountain and Chauga River Fms. U-Pb zircon dating indicates crystallisation occurred at 535-600 Ma (Edelman et al., 1987).

 

Brevard Fault Zone

 

The Brevard Fault zone is located directly south of the Rosman fault and to the NW of the Chauga Belt. It contains a very distinct and continuous stratigraphic succession consisting of (from top to bottom): 1) mylonitic Henderson Gneiss, 2) phyllite, 3) carbonate, 4) phyllite, 5) very fine grained quartzite and graphitic phyllite. The latter four are all part of the Chauga River Fm. The most southeastern repetition of this sequence indicates the contact of the Brevard Fault zone and the Chauga Belt. Across strike the Brevard Fault zone sequence is repeated two to four times, depending on location along strike. It is suggested that the emplacement of the Henderson Gneiss on top of the Chauga Belt is accommodated by at least part of the Brevard Fault zone. Seismic reflection indicates a shallow SE dip to a depth of 5-7 km, at which point the Brevard Fault merges with the Blue Ridge-Inner Piedmont thrust. Retrograde metamorphism is abundant within the Brevard Zone that is associated with mylonitisation and retrogression within the Chauga Belt (Edelman et al., 1987). Cook et al. (1979) claim that the throw and heave of the Brevard Fault are 8 and 30 km at least, respectively.

 

Brevard Mylonite Zone

 

The Brevard Mylonite Zone refers to the mylonitic overprint of the Brevard Fault Zone. Mylonitic deformation is characterised by grain size reduction quartzites and the Henderson Gneiss, mylonitic foliation and lineation within the Henderson Gneiss, mica augen and S-C fabrics within the Chauga Belt. To the NW of the Rosman Fault, mylonitic deformation is characterised by S-C fabrics in mica rich rocks and grain size reduction in quartz-rich rocks. The mica and hornblende grains in amphibolites are large, whereas the quartz grains in these rocks are very small. These S-C fabrics consistently indicate dextral shear. The foliation is folded with sub-horizontal fold axes (Edelman et al., 1987).

 

Rosman Fault

 

The Rosman Fault marks the sharp tectonic contact between the Chauga Belt and the Blue Ridge belt within the Brevard Zone. It contains relatively unmetamorphosed slices of carbonate and shale. It is interpreted as being a thrust fault, since these carbonate and shale slices are believed to have come from the sedimentary rocks underlying the Blue Ridge-Inner Piedmont thrust. This fault is characterised by complex folds, bedding disruption and brecciation. The Rosman Fault is younger than the Brevard Fault, as is suggested by brecciated mylonitic rocks along the fault. Brittle structures are present to the SE of the Rosman Fault, though it is unknown if these structures are related to the Rosman Fault or not (Edelman et al., 1987).

 

Evolution of the Brevard Zone

 

The Brevard zone has had two deformation stages, the first one being ductile, the second one being brittle. These two stages occurred during the Cambrian and middle Palaeozoic (Hatcher, 1978). However, Edelman et al. (1987) claim that the Brevard Zone is an Carboniferous strike-slip fault, which was first formed by the Alleghanian orogeny, that had been reactivated in the late-Carboniferous by a later Alleghanian thrust fault and thrust splay. The orientation of the zone is controlled by reworked pre-Alleghanian nappes (Edelman et al., 1987). Cook et al. (1979) argue that the shallow dip of the Brevard Fault indicate an offset of a normal fault that later acted as a ramp to accommodate thrust motions. During the late Alleghanian orogeny the Rosman Fault was initiated, reactivating the Brevard mylonites. The Rosman Fault formed as a splay off the Blue Ridge-Inner Piedmont thrust (Edelman et al., 1987).

It functioned as the early root zone for the faults in the Blue Ridge. It has been reactivated several times and the final brittle stage caused it to be thrust over the Blue Ridge as backlimb fault. The Brevard Zone is the sole of the Inner Piedmont nappe (Cook et al., 1979; Hatcher, 1978). The Brevard Zone is observed to be a very linear fault that splays at its ends, has a moderate dip along its length, is stratigraphically controlled over some of its extent, resides in different rock units along its length, though within the same lithologies with different names and finally, it has experienced a few early ductile and a few late brittle deformation phases.

 

Brevard Zone localities

 

The localities of the Brevard Zone outcrops (Fig. 1) are located about 10 km north of the town of Toccoa along the Panther Creek Road, which can be reached from US 184 coming from Toccoa. From the US184 turn left at Yonah Dam Road, after which you’ll reach the Panther Creek Road (these are on the border between Georgia and North Carolina). The second one is about 15 km NW of Walhalla in North Carolina. This one can be reached via the US28 coming from Walhalla (go in NW direction) and go left at State Road S37-193. Then go left again at State Road S37-290. The third location is about 2 km west of Rosman along the US64. This location is just south of the junction with the US215, though probably along the 215 there are also outcrops of the Brevard Zone.

 

34°40’16.86″N, 83°20’53.81″W

35° 8’29.87″N, 82°50’23.57″W

34°47’40.27″N, 83°11’45.57″W

 

References

 

Cook, F.E., Albaugh, D.S., Brown, L.D., Kaufman, S., Oliver, J.E., Hatcher, R.D., Jr., 1979. Thin-skinned tectonics in the crystalline southern Appalachians; COCORP seismic reflection profiling of the Blue Ridge and Piedmont. Geology 7, 563-567.

Edelman, S.H., Liu, A., Hatcher, R.D., Jr., 1987. The Brevard Zone in South Carolina and Adjacent Areas: An Alleghanian Orogen-Scale Dextral Shear Zone Reactivated as a Thrust Fault. The Journal of Geology 95, 793-806.

Hatcher, R.D., Jr., 1978. Tectonics of the western Piedmont and Blue Ridge, southern Appalachians: review and speculation. American Journal of Science 278, 276-304.

Merschat, C.E., Wiener, L.S., 1990. Geology of Grenville-age Basement and Younger Cover Rocks in the West Central Blue Ridge, North Carolina. Carolina Geological Society, Guidebook for 1990 Annual Meeting, pp. 1-27.