The Linville Falls Fault is a thrust fault located near the border between North Carolina and Tennessee, along the towns Linville Falls, Banner Elk, and Boone, in the Blue Ridge Province of the Appalachian Mountains.
Figure 1) Approximate location of the Linville Falls Fault in North Carolina, USA
A thrust fault is a fault which has pushed rocks of lower stratigraphic position (usually meaning older rocks) up and over higher (or younger) rocks. Seeing older rock layers on top of younger rock layers is usually a giveaway that a thrust fault has been involved. The Linville Falls Fault is a top-to-northwest thrust fault. Here one billion year old granite of the Grenville Province has been pushed over 700 million year old metamorphosed sandstone. The fault was created during the formation of the Appalachian Mountains about 300 million years ago in the Alleghenian Orogeny when Gondwana collided with Euramerica. (7)
A thrust fault is a fault which has pushed rocks of lower stratigraphic position (usually meaning older rocks) up and over higher (or younger) rocks. Seeing older rock layers on top of younger rock layers is usually a giveaway that a thrust fault has been involved. The Linville Falls Fault is a top-to-northwest thrust fault. Here one billion year old granite of the Grenville Province has been pushed over 700 million year old metamorphosed sandstone. The fault was created during the formation of the Appalachian Mountains about 300 million years ago in the Alleghenian Orogeny when Gondwana collided with Euramerica. (7)
Figure 2) A map of the geological features surrounding and including the Linville Falls Fault. All of the faults formed in the Alleghenian orogeny except the Burnsville Fault. The four thrust sheets of the Blue Ridge thrust complex are shown. (4)
During the formation of the thrust fault, collisional forces pushed a rock layer called the Blue Ridge thrust complex over 100km to its current location covering Grandfather Mountain. This rock layer forms the hanging wall of the Linville Falls fault. It is composed of alkali feldspar granite overlain by shear zone mylonites, and cranberry gneiss (also part of the Grenville Province). (8) The footwall of the fault consists of autochronous quartzite, which is part of the Grandfather Mountain formation, and quartz-sericite mylonite which was created through shearing. (4) At least 50 km of movement has occurred along the fault. (1) On one edge of the fault there is a thick ductile top-to-northwest shear zone called the Linville Falls Shear Zone, approximately one km wide near Banner Elk and consisting of mylonitic and ultramylonitic rocks with zones that have been cataclastically deformed. (4)
During the formation of the thrust fault, collisional forces pushed a rock layer called the Blue Ridge thrust complex over 100km to its current location covering Grandfather Mountain. This rock layer forms the hanging wall of the Linville Falls fault. It is composed of alkali feldspar granite overlain by shear zone mylonites, and cranberry gneiss (also part of the Grenville Province). (8) The footwall of the fault consists of autochronous quartzite, which is part of the Grandfather Mountain formation, and quartz-sericite mylonite which was created through shearing. (4) At least 50 km of movement has occurred along the fault. (1) On one edge of the fault there is a thick ductile top-to-northwest shear zone called the Linville Falls Shear Zone, approximately one km wide near Banner Elk and consisting of mylonitic and ultramylonitic rocks with zones that have been cataclastically deformed. (4)
Figure 3) Relative location of the Linville Falls shear zone (4)
The Linville Falls Fault forms the border of the Grandfather Mountain Window. (figure 2) A window is formed when a layer of rock erodes away in one location to expose a patch of different rocks underneath. (figure 4) Here, after the Blue Ridge Thrust complex was pushed over Grandfather Mountain by the fault, the peaks of the formation eroded away and in one place a gap eroded all the way through the layer, uncovering the younger rocks underneath. The other faults, such as the Brevard Fault, were formed in the same orogeny (Alleghenian) as the one that created the Linville Falls Fault. The nearby Linville Falls Gorge arose when the Linville River eroded the softer quartzite out from under the granite and caused the falls to collapse. (7)
Figure 4) A basic diagram of a window formation
The Blue Ridge Thrust complex, in the hanging wall of the Linville Falls Fault and overlying the Grandfather Mountain Window, consists of stacks of crystalline thrust sheets which can be distinguished from one another by their metamorphic history. They are (in order from structurally lowest to highest) the Pardee Point thrust sheet, the Beech Mountain thrust sheet, the Pumpkin Patch thrust sheet, and the Spruce Pine thrust sheet. (4) A zone of metamorphosed rocks near the sole of the thrust sheet displays many small isoclinal folds with northwest-trending axial planes parallel to foliation in the thrust sheet. (3)
Figure 5) A geological map showing the orientations and elevations of features surrounding the Linville Falls Fault (4)
The faults in the Blue Ridge Mountains were formed in the Alleghenian Orogeny 320 to 260 million years ago during the late Carboniferous period. The Alleghenian Orogeny is one of the six orogenies making up the formation of the Appalachian Mountains. The mountain building occurred as Pangaea was coalescing, when Africa (at the time part of the supercontinent Gondwanaland) collided with the supercontinent Euramerica. The impact compressed rocks between the two landmasses and forced them up into a mountain chain which at the time was located in the center of the supercontinent. When Pangaea broke up millions of years later, the mountain range split in two. Today we have the Appalachians in North America and the Atlas Mountains in North Africa. The buckling from the collision also created a series of sedimentary folds, perpendicular to the force, bounded by thrust faults which shortened the land by as much as 320 km. The Linville Falls Thrust Fault formed due to the compressional forces in this orogeny. (8) The folds in the Blue Ridge Thrust Sheet are thought to have formed from flattening and passive rotation of earlier folds which originally formed perpendicular to the direction of movement. (3)
Figure 6) An animation showing the break up of Pangaea and the migration of the continents to their current positions. At the beginning of the video it can be seen where North America was connected to Africa. This is where the mountains were originally.
Sources:
(1) CAROLINA GEOLOGICAL SOCIETY 1997 FIELD TRIP AND ANNUAL MEETING ROAD LOG AND STOP DESCRIPTIONS
http://carolinageologicalsociety.org/CGS/1990s_files/gb%201997.pdf#page=27
(2) Significance of lineation and minor folds near major thrust faults in the southern Appalachians and the British and Norwegian Caledonides Bruce Bryant and John C. Reed Jr.
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(3) PALEOZOIC STRUCTURAL EVOLUTION OF THE BLUE RIDGE THRUST COMPLEX, WESTERN NORTH CAROLINA Kevin G. Stewart, Mark G. Adams , and Charles H.Trupe
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(4) STRUCTURAL RELATIONSHIPS IN THE LINVILLE FALLS SHEAR ZONE, BLUE RIDGE THRUST COMPLEX, NORTHWESTERN NORTH CAROLINA Charles H. Trupe
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(5) Geology of the Linville Falls Quadrangle North Carolina GEOLOGICAL SURVEY BULLETIN 1161-B
http://pubs.usgs.gov/bul/1161b/report.pdf
(6) Linville Gorge
http://www.unc.edu/courses/2008spring/geog/006d/001/Class_Readings/Class%20%2313%20-%20April%2021%20(WNC%20-%20Team%20Presentations%20&%20Papers)/Linville%20Gorge_Class%20Paper.pdf
(7) USGS Valley and Ridge Province
http://web.archive.org/web/20110722154205/http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm
(8) CONDITIONS AND TIMING OF METAMORPHISM IN THE BLUE RIDGE THRUST COMPLEX, NORTHWESTERN NORTH CAROLINA AND EASTERN TENNESSEE Mark G. Adams and Charles H. Trupe http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(9) Image of window: http://upload.wikimedia.org/wikipedia/commons/b/b1/Thrust_system_en.jpg
(10) Video of Pangaea: https://www.youtube.com/watch?v=WaUk94AdXPA
(1) CAROLINA GEOLOGICAL SOCIETY 1997 FIELD TRIP AND ANNUAL MEETING ROAD LOG AND STOP DESCRIPTIONS
http://carolinageologicalsociety.org/CGS/1990s_files/gb%201997.pdf#page=27
(2) Significance of lineation and minor folds near major thrust faults in the southern Appalachians and the British and Norwegian Caledonides Bruce Bryant and John C. Reed Jr.
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(3) PALEOZOIC STRUCTURAL EVOLUTION OF THE BLUE RIDGE THRUST COMPLEX, WESTERN NORTH CAROLINA Kevin G. Stewart, Mark G. Adams , and Charles H.Trupe
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(4) STRUCTURAL RELATIONSHIPS IN THE LINVILLE FALLS SHEAR ZONE, BLUE RIDGE THRUST COMPLEX, NORTHWESTERN NORTH CAROLINA Charles H. Trupe
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(5) Geology of the Linville Falls Quadrangle North Carolina GEOLOGICAL SURVEY BULLETIN 1161-B
http://pubs.usgs.gov/bul/1161b/report.pdf
(6) Linville Gorge
http://www.unc.edu/courses/2008spring/geog/006d/001/Class_Readings/Class%20%2313%20-%20April%2021%20(WNC%20-%20Team%20Presentations%20&%20Papers)/Linville%20Gorge_Class%20Paper.pdf
(7) USGS Valley and Ridge Province
http://web.archive.org/web/20110722154205/http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm
(8) CONDITIONS AND TIMING OF METAMORPHISM IN THE BLUE RIDGE THRUST COMPLEX, NORTHWESTERN NORTH CAROLINA AND EASTERN TENNESSEE Mark G. Adams and Charles H. Trupe http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805
(9) Image of window: http://upload.wikimedia.org/wikipedia/commons/b/b1/Thrust_system_en.jpg
(10) Video of Pangaea: https://www.youtube.com/watch?v=WaUk94AdXPA