Thrust sheets within the Grandfather Mountain Window, Southern Appalachians,
U.S.A.
Jerry Bowling and Paul Winberry
Summary
A structural window is formed where erosion removes overlying thrust sheets
so that the underlying thrust sheets are visible at the surface.
Grandfather Mountain window is the largest such structural window in the
Blue Ridge Thrust Complex of the Southern Appalachians (Figure 1). The
window measures approximately 45 miles long and 20 miles wide. Grandfather
Mountain has been studied for many years and is a favorite stopping place
for geology field trips. Relationships between thrust sheets within
the window and those juxtaposed on the outside are, at best, difficult
to determine. Information from the main thrust sheets within the
window has been used to amend the calculation of shortening in the southern
Appalachians. The purpose of these stops will be to examine the contact
between two of these thrust sheets and perhaps understand the difficulty
in pinpointing exactly how many thrust sheets lie within the Grandfather
Mountain Window.
Figure 1. Location of Grandfather Mountain window.
Geologic Setting
The Blue Ridge Thrust Complex is a stack of crystalline thrust sheets separated
by mylonitic zones. Grandfather Mountain window contains basement
and clastic rocks and is surrounded by granitic and migmatitic rocks (Figure
2). Rocks within the window are thought to be the footwall of a duplex
system of thrust sheets (Figure 3), however even the rocks within the window
are thought to have originated elsewhere (i.e. allochtonous) (Rodgers,
1970).
The two main thrust sheets in the area are the Beech Mountain Thrust
Sheet and the Tablerock Thrust sheet. The Beech Mountain Thrust Sheet
lies immediately adjacent to the Grandfather Mountain Formation as part
of the Western Blue Ridge Complex. The Beech Mountain Thrust
Sheet predominantly consists of granitic and biotite gneisses (Goldberg
and Dallmeyer, 1997). The Tablerock thrust sheet is the main thrust
sheet within the GFM window and lies in the southern corner. The
southern section is more riddled with faults (e.g. Bryant and Reed, 1970)
than the northern section. Therefore, the possibility exists that
more thrust sheets lie within the window.
The Linville Falls Thrust outlines Grandfather Mountain Window from
the rest of the Blue Ridge Complex. On the northern and western sides,
the Linville Falls Fault dips at a low-angle to the north and west.
However, the southeast side is characterized by a steeply dipping fault
which gives way to the neighboring zone of intensely sheared and retrograded
rocks called the Brevard Zone. Information gleaned from mapping the
thrust sheets adds at least 40 km of displacement to the apparent basement
shortening (Rodges, 1970).
Figure 2. General map of the Grandfather Mountain Window and vicinity.
Modified from Bryant and Reed (1970).
Figure 3. A representative cross-section through the Grandfather Mountain
Window area. From Boyer and Elliot (1982).
History
Grandfather Mountain Window and the neighboring area have a complex history
of deformation events. The basement rocks within the window date
to around 1.1 Ga. Overlying the basement are clastics of Cambrian
age . These basement and clastic rocks were ductily deformed and
metamorphosed during metamorphic events that occurred between 450 and 350
Ma. Thrusting events followed in the Late Devonian and lasted until
the Late Triassic. These overlying thrust sheets were later removed
by erosion to form the window we see today.
Exposed Rocks
Rocks within the grandfather mountain window are usually thought to represent
the shady valley thrust sheet. The basement rocks dating back to
1.1 Ga. Two dominant formations considered to be basement
are the Blowing Rock Gneiss and the Wilson Creek Gneiss. These rocks
are part of what is known as the Grenville Basement. The clastic
section includes rocks of the Grandfather Mountain Formation, the Chilhowee
Group and the Shady Dolomite. The GFM formation consists of at least
20,000 feet of sandstones, mudstones, and conglomerates but also contains
basalts, rhyolites, and tuffaceous rocks. The Chilhowee Group is
predominantly quartzite of Lower Cambrian age. The Cranberry Gneiss formation
frames most of the Grandfather mountain window and is part of the Beach
Mountain Thrust Sheet. Rocks within the Table Rock Thrust Sheet are
locally overturned and were isoclinally folded before emplacement.
Location
Stop 1: Linville Falls
Location: Linville Falls Park exit on the Blue Ridge Parkway near Linville
Falls N.C (Figure 4). There is a visitor contact station in the Linville
Falls area via an access road at milepost 316.4. Trail maps, publications
and area information are available here. A picnic area is also located
here.
Stop Description: The type locality of the Linville Falls Fault. At
this locality will see Cranberry Gniess which are Grenville basement rocks
in the hanging wall. These rocks have been retrogressively metamorphosed
to greenschist facies. The footwall at this location is composed
of the Erwin Quartzite. Upstream of the first overlook the fault is well
exposed and a .5 meter mylonite zone can be observed (Figure 5). NE trending
open folds and NW trending lineations can also be observed at the first
overlook. The lineations are taken represent the transport direction.
Figure 4. Linville Falls.
Figure 5. Linville Falls Fault.
Stop 2: Wisemans View
Location: Take highway 183 out of Linville Falls, and turn onto Kistler
Memorial Highway (a dirt road). Wiseman’s view is at the end.
Stop Description: Allows for a general overview of the Grandfather mountain
window. The overlook is located on the Tablerock Thrust Sheet, Erwin Quartzite.
Linville Falls Fault is in the valley to the west. The Table Rock thrust
sheet is the cliff face below the overlook. Table Rock Mountain,
a klippe of the Table Rock thrust sheet, can also be seen from this view.
References
Adams, Mark G. and Charles H. Trupe, 1997, Paleozoic Structure, Metamorphism,
and Tectonics of the Blue Ridge of Western North Carolina, Ed. Kevin G.
Stewart. Carolina Geological Society, 1997 Field Trip Guidebook, p. 49-66.
Goldberg, S.A., and R.D. Dallmeyer, 1997, Chronology of Paleozoic metamorphism
and deformation in the Blue Ridge Thrust Complex, North Carolina and Tennessee,
American Journal of Science, v. 297, p. 488-526.
Kulander, Byron R. and Stuart L. Dean, 1986, Structure and Tectonics
of Central and Southern Appalachian Valley and Ridge and Plateau Provinces,
West Virginian and Virginia, v. 70, p. 1674 ? 1684.
Boyer, S. E. and D. Elliott, 1982, Thrust Systems: American Association
of Petroleum Geologists Bulletin, v. 66, p. 1196-1230.
Bryant, B., and J.C. Reed, Jr., 1970, Geology of the Grandfather Mountain
window and vicinity, North Carolina and Tennessee: U.S. Geological Survey
Professional Paper 615, 190 p.
Rodgers, J., 1970, The Tectonics of the Appalachians, John Wiley &
Sons, 271 p.