Photo credit: Blue Ridge, Gary Pfaff, 5/20/2000

Tectonics of the Blue Ridge Mountain Province

LaDorna Jo Pfaff, April 17, 2004, Emporia University, Kansas

TABLE OF CONTENTS


SCOPE OF THE REPORT

This discussion is about tectonic events that occurred in the Blue Ridge Province as seen along the Blue Ridge Parkway in North Carolina. The first part of the discussion is in chronological order. Mile-markers are noted where geologic events are illustrated. The second part is a road trip, from north to south, compiling the Mile-markers mentioned in the historical geology discussion.

INTRODUCTION

Although several orogenies effected the present-day Blue Ridge mountains in North Carolina, the majority of rocks seen along the Blue Ridge Parkway are:

    1) 1 billion year old (1 BY) granitc gneiss from the Grenville Mountain building during the Middle Proterozoic Eon

    2) Five different kinds of metamorphic sedimentary rocks from the 750 million-year-old (750 MY) Late Proterozoic Eon: Alligator Back Formation, the Ashe Metamorphic Suite, the Great Smokies Group, the Snowbird Group, and the Grandfather Mountain Formation

    3) 570 - 500 million year old (570-500 MY) Cambrian/Ordovician Period Chilohwee Group found only within the frame of the Grandfather Mountain window

Other visual indications of the tectonic activity are the faults, contacts, index minerals, and the anticline/snycline folds. Ophiolitic and ultra-mafic minerals mark suture zones in both the Late Paleozoic Alligator Back rocks and Taconic tectonics. The ophiolites represent the fragments of former ocean basins that closed. The fragments were thrust up onto continents as tectonic plates converged. Ophiolitic and ultramafic minerals in the Blue Ridge Province are very fragmented and difficult to see. In fact, they are so fragmented that many of them can only be identified by petrochemical analysis. New research suggests that most North Carolina ultramafic rocks indicate slow-spreading back-arc tectonics (Raymond, 2004, personal communication).

The Appalachian Mountains include from west to east: the Appalachian Plateau, Valley and Ridge, the Blue Ridge Province, and the Piedmont Region. The Blue Ridge Province is bounded on the west by the Great Smokies Fault near the Tennessee boundary and on the east by the Brevard Fault Zone. The mountains grew out of stacking and shuffling of thin horizontal sheets in several orogenies. The thrusting, shuffling rock sheets mixed the rocks like a deck of cards. In some places horizontally large crystalline basement rocks overthrust onto younger unmetamorphosed sedimentary strata. In the overthrust scenario the top layers of the unmetamorphosed sedimentary were scraped off and carried along westward under the crystalline basement rocks. In some cases "underthrusts" occurred. Younger unmetamorphosed sedimentary strata was carried along on top as crystalline rocks were emplaced by a thrust that took place downward - a sub-horizontal thrust. (Cook, 1980, p. 147).The layers do not stay in their original "oldest-on-the-bottom" order; the law of superposition doesn't necessarily apply.

CHRONOLOGICAL TIMELINE

[Middle Proterozoic Eon]

Middle Proterozoic Eon - 1 Billion years ago - GRENVILLE orogeny The tectonics during the early Earth's history is speculative. Current consensus is that approximately 1 billion years ago (Late Proterozoic -Precambrian) there existed a megacontinent, later named Rondinia. This megacontinent consisted of proto-Antarctica, North America, Australia, and fragments of South America. Later Rondina was split by rising magma into proto-North America (called Laurentia) and proto-Africa, (called Gondwana). The magma cooled to form granite and anorthosite. Also floating around from the break-up of the megacontinent were some continental fragments that include the proto-BLUERIDGE fragment, the proto-inner Piedmont fragment, and the proto-outer Piedmont. Cook's research suggests that the basement rocks from eastern proto-North America, the Blue Ridge rocks, and the rocks of the Piedmont came from the same mega continent that proto-NA broke off from. (Cook, 1980, p. 151) Rifting during the break-up of the megacontinent Rondinia caused deep valleys to form. Deposition of eroded material filled the rift basin between proto-North America and the Blue Ridge fragment. Sand, shale, and limestone were deposited along the shallow waters around proto- North America. Later the sands transformed into sandstone and then into quartzite, gneiss, and schist. The limestone metamorphosed into marble; and shale transformed into gneiss and schist. Blowing Rock gneiss and biotite granitic gneiss are examples of Grenville basement rocks seen on the drive.

Examples of the rocks of the Grenville orogeny:

Mile 284.6 - The biotite granitic gneiss (1 billion years old) is in contact with the Ashe Metamorphic Suite (750 MYA). The contact is evidence of a fault, called the Linville Falls fault. Thrust faulting has moved the older biotite granitic gneiss many miles northwestward over the younger Ashe Metamorphic Suite.

Mile 290.4 Thunder Hill Overlook, in a low outcrop across the drive is the Blowing Rock augen gneiss, It has large cream-colored feldspar crystals, some greater than 6 cm long in a fine-grained dark-colored biotite-rich matrix.

Mile 445.2 Mount Lyn Lowry Overlook, a short hike back to Mile 445.0 shows a view of the Balsam Gap Olivine deposit. The quarry deposit is in the southern valley. Dunite, emplaced during the Middle Proterozoic (1 BTA), changed to olivine. In the 1930s this quarry was the first to produce olivine in the United States. Olivine, which are heat-resistant, is used to line kilns and furnaces to bake clay bricks. Today, the Spruce Pine Mining District in North Carolina is a leading producer of olivine.

Mile 446 Woodfin Valley Overlook, across the road from the overlook is a road cut showing the highly-folded banded biotite gneiss. These rocks are the oldest on the drive. They have been deformed and metamorphosed at least 3 times:1 billion years ago, 450 million years ago, and less intensely 250 million years ago. Their highly-folded appearance hint at the tectonic stresses they have experienced.

Mile 451.2 Waterrock Knob Overlook, Roan Mountain, Mount Mitchell and Black Mountains are to the northwest. Roan Mountain is furthest in this view, along the Tennessee/North Carolina boundary. It has the oldest rocks in North Carolina. The mountain is made of middle-Proterozoic gneiss, 1.3 billion years old.

[Late Proterozoic Eon]

During the Late Proterozoic/Precambrian (750 million years ago) the megacontinent Rondinia split into proto-North America (Laurentia) and Gondwana ( Dalziel, 1995, p. 62) . The break up produced smaller fragments which included the Blue Ridge fragment, the Piedmont fragment, and the Carolina Slate fragment. The valley between proto-North America and the Blue Ridge filled with sedimentary material and volcanics. The metasedimentary rocks on the drive appear to have been deformed at least twice, once 450 million years ago, and less intensely 250 million years ago. The names given to the metamorphosed sediments are: the Alligator Back Formation, the Ashe Metamorhpic Suite, the Great Smoky Group, the Snowbird Group, and the Grandfather Mountain formation. These rocks form the backbone of the Appalachian range because they are the most resistant to erosion. (Hatcher, in Cook, 1980, p. 150) Lava changed to basalt and then into greenstone or amphibolite. Although greenstone columns of hexagonal jointing are not seen along the drive, actinolite is commonly found in the Ashe Metamorphic Suite rocks.

The Alligator Back Formation is the most common rock along the northern section of the drive. The Alligator Back rocks were named for massive ribs of gneiss that radiate from the crest of Bluff Mountain. The ribs look like an alligator. Ultramafic ophiolite fragments that have been described in the Alligator Back Suite.(Conley, 1987, pg.55-68). They indicate an ancient subduction zone. The slightly older, more resistant Ashe Metamorphic Suite starts at approximately Mile 345 and continues south for the next 95 miles. In distinguishing the two major metasedimentary rock groups, the Ashe is coarser and more granular; it has more quartz grains and amphibolite than the Alligator Back. The Alligator back has a thinly-layered pin-stripe appearance due to the gneiss and mica schist banding. Red garnets are common in the Alligator Back and Ashe Metamorphic rocks. Garnets, chlorite, and blades of kyanite are index minerals that indicate the temperature and pressure of the metamorphism. Chorite indicates low metamorphic temperatures and pressures. Garnet indicates 300-500 degrees C and pressures of 3 to 8 kilobars (equivalent to 10-30 km deep). Kyanite indicates greater temperatures and pressures: 550 - 800 degrees C and 4-14 kilobars pressure (equivalent to 12-45 km deep).

Examples of Alligator Back Rocks on the Blue Ridge Parkway:

Mile 217.3 Cumberland Knob - The picnic area and knob are supported by the Alligator Back thinly foliated schist. There are outcrops of schist along the hiking trails.

Mile 230.1 Little Glade Millpond Overlook - Muscovite and biotite mica form "pinstripes" in the Alligator Back rock outcrops across the parkway.

Mile 242.3 Alligator Back Overlook - on the southeast side of the overlook and on the upper half of the trail to Bluff Mountain are excellent examples of Alligator Back finely-laminated gneiss and schist.

Examples of Ashe Metamorphic Suite :

Mile 281.9 Contact between the Alligator Back Formation and the Ashe Metamorphic Suite. The chief visual differences are that the Alligator Back rocks are more thinly layered. The Ashe rocks are more granular and in thicker layers, and contain more amphibolite and quartz.

Mile 352.4 - Bald Knob Ridge Overlook - opposite the overlook is a good exposure of Ashe metagraywacke and mica schist. The Ashe Metamorphic Suite rocks are folded into large anticlines and synclines created by extreme compressional forces during the Taconic (550-450 MYA) and Acadian (400-350 MYA) orogenies.

Mile 364.1 Craggy Dome Overlook, Ashe Metamorphic Suite with index minerals: red garnets and blue-gray blades of kyanite.

Mile 323.0 - Bear Den Overlook From the western side of the overlook, Mount Mitchell is the highest peak near the southern end. It is made up of the Ashe Metamorphic rocks, highly resistant meta greywacke. Mount Mitchell is the highest mountain in North Carolina (2037 meters).

Lesser amounts of the other Late Proterozoic metasedimentary rocks (Great Smoky, Snowbird, and Grandfather Mountain rocks) are found on the drive.

Examples of Grandfather Mountain formation:

Mile 292.8 - marks the contact between the Grandfather Mountain Formation and the Blowing Rock gneiss. The Grandfather Mountain Formation shows the primary sedimenatyr and volcanic layers even though it has been metamorphosed into fine-grained arkose, shale, and meta-volcanic rock. The Late Proterozoic sediments and volcanics were deposited into the deep rift along the edge of the continent, and were metamorphosed by the Taconic orogeny in the Ordovician Period.

Mile 302.1 Wilson Creek Valley Overlook, The green rocks at the head of the creek seen kbelow are metadiabase which indicates volcanism. The magma that intruded the Grandfather Mountain sediments changed into a metadiabase. The green color is from the minerals actinolite and chlorite.

Examples of the Great Smoky Group and Snowbird Group:

Mile 450.2 Yellow Face Overlook Great Smoky Group is a rock unit that consists of metagraywacke, schist, and slate. However, it contains no metamorphosed basalt, amphibolite. The absence of amphibolite distinguishes if from the Ashe and Alligator Back rocks. The Great Smoky Group commonly has rusty brown and greenish-yellow stains on its surface caused by the sulfide mineral, pyrrhotite. Pyrrhotite decomposes to a weak sulfuric acid which enters the streams making them more acidic. Garnet crystals in the muscovite schist and micaceaous metagraywacke are common. The sediments that became the Great Smoky Group were deposited into large rift troughs closer to the edge of the continent than the Ashe and Alligator Back.

Mile 467.4 - Ballhoot Scar Overlook At this overlook Great Smoky rocks are exposed at the northern end, and the older Snowbird Group at the southern end. The Snowbird Group is a buff-colored cross-bedded feldspathic quartzite, called Longarm Quartzite. The fault between the two groups is called the Greenbrier fault, a Taconic thrust. The younger Great Smoky Group has been thrust over the older Longarm Quartzite. Usually thrust faults move older rocks over younger rocks. Some geologists think the Greenbrier fault formed over an unconformity between the two rock groups; others think it may have been a sub-horizontal thrust.

[Late Cambrian and Ordovician Period]

During this time (550 - 450 MYA) proto-North America and the Blue Ridge fragment were joined in the TACONIC orogeny. To the east was an eastward dipping subduction zone, and further east was the Carolina Slate belt fragment. The ancient suture (subduction zone) is marked by ophiolites. Peridoties from the upper mantle have been transformed into greenish serpentineite. The resulting volcanic arc produced by the subducution zone erupted in the Carolina slate fragment. Sediments from 1) the western flank of the Blue Ridge and 2) erosion of proto-North American transformed into the sandstones and shales of the present Valley and Ridge Province. The Murphy Marble Belt, presently west of the Blue Ridge Province, probably formed during this time. The Murphy Marble Belt is an long, lens-shaped mass of metamorphic marble, up to 4.8 km wide, that extends from Georgia through North Carolina. This marble was used for the statue in the Lincoln Memorial. At approximately this time, the Ordovician Period (500-440 MYA), there was a shift of sediments from the east. During the Taconic orogeny, 10 - 20 kilometer- thick thrust sheets, both overthrust and sub-horizontal, moved westward onto the continent. Many faults were produced during the Taconic orogeny: Holland, Hayesville, and Greenbreir faults are seen on the drive. Grandfather mountain window bridges two time periods; the younger rocks, the Cambrian-aged Chilhowee Group, are exposed in a window framed by older Blowing Rock gneiss. The Grandfather window was produced by the older rock being thrust over the younger rock. Erosion through the older rock finally exposed the underlying younger rock. The Chilhowee Group seen in the window is comprised of two rock types: 1) quartzite on the Linville Mountain and 2) dolomite in the valley. The quartzite is a white fine-grained feldspathic rock which is resistant to erosion. In some places, iron oxide stains the rock making the quartzite difficult to identify. The dolomite has dissolved forming a valley. The Chilhowee Group is not seen elsewhere in western North Carolina.

Views of Grandfather Mountain Window:

Mile 320.8 Chestoa View, view of Grandfather Mountain Window. Walk down the path to the observation area. The overhanging cliff is the Alligator Back Formation. The rocks within the window are younger Cambrian rocks: highly-resistant white Chilhowee Quartzite supporting Linville Mountain and highly-soluble dolomite in the valley; the outer window frame is the Middle Proterozoic Blowing Rock gneiss. Beneath the Cambrian Chilhowee quartzite is the Late Protoerozoic Grandfather Mountain biotie granitc gneiss.

Photomosaic from Chestoa View Overlook. The Linville Falls fault, marked in blue, is the boundary that surrounds the frame of Proterozoic Blowing Rock gneiss. Reference: Carter, 1999, p. 25

Examples of Chillowee Formation and other examples of Taconic orogeny on the North Carolina Blue Ridge Parkway:

Mile 315.0 Looking north is a field of boulders made of meta-arkose Grandfather Mountain Formation. South of the boulder field, about 80 meters, are outcrops of white, fine-grained feldspathic quartzite of the Chilhowee group.

Mile 316.14 Linville Falls Visitor Center take the easy half-mile hike on the Erwin's View Trail to the Upper Falls. Descending the steps to the Upper Falls Overlook is the fault between the older biotite granitic gneiss and the younger Cambrian-age Chilhowee Group. The rocks show parallel folds spaced less than a meter apart. The folds form perpendicular to the direction of the thrust fault. The fault moved the overlying biotite granitic gneiss many miles northwest away from the falls. The Linville River flows over a capstone of gneiss. Directly underneath is Chilhowee metasandstone. The fault is marked by very-fine-grained mylonitic banded rocks. Mylonitic textures denote extreme granulation and sheering of rocks that have been pulverized during overthrusts of rocks.

Mile 323.0 Bear Den Overlook rocks show a migmatic appearance. Migmatites indicate melting and flowing of the rocks. The stresses caused during the Taconic orogeny caused the migmatic marble-cake formation.

Mile 330.9 Museum of North Carolina Minerals exhibits the Murphy marble, among many other minerals and rocks.

Major Taconic faults:

Mile 440.8 Holland fault- The fault contact here is between muscovite-biotite gneiss of the Ashe Metamorphic Suite and earlier biotite gneiss of the Grenville basement rocks. During the Taconic orogeny, the Ashe rocks were shoved westward over the older Grenville basement rocks. Both rock units were metamorphosed during the thrusting.

Mile 446.2 Hayesville fault - The fault contact here is between older Grenville biotite gneiss and the younger metagraywacke of the Great Smoky Group. The Great Smoky rocks were deposited close to the edge of the continent. Since the rocks contain no amphibolite, the sediments were more distant from volcanoes than Ashe and Alligator Back rocks.

Mile 468.7 Greenbrier fault - 1 mile after the Raven Fork Overlook marks the fault contact between the younger Snowbird Group and the older biotite granitic gneiss of the Grenville basement rocks. The gneiss shows very-fine-grained mylonitic texture which indicated great stress during faulting.

[Devonian Period]

During this time (400 to 350 million years ago) in the ACADIAN orogeny, the Carolina Slate belt joined with North America continent with strong overthrust sheets. The Blue Ridge fragment was already attached to North America. Stone Mountain is an igneous plug that formed and crystallized into granodiorite during this period. The plug was an intrusive stock originally 390 meters deep in the earth. During the later Alleghenian orogeny and erosion of overlying rocks, Stone mountain surfaced. Kings Mountain, to the east near Charlotte, North Carolina, is evidence of the Acadian orogeny . Since most of the mountain building in the Acadian orogeny took place further eastward, it can not be seen from the Blue Ridge drive.

Mile 232.5 Stone Mountain Overlook View of Stone Mountain. The domed surface of Stone Mountain shows concentric exfoliation rings when the grandiorite expanded as the overlying rock was removed.

[Pennsylvanian and Permian Period]

300 million years ago to 250 million years ago, during the Pennsylvanian and Permian Periods, proto Africa and/or South America (called Gondwanaland) collided with the east coast of proto-North America in the ALLEGHENIAN orogeny. The megacontinent, Pangea, was formed. [Note: some references call the Alleghenian orogeny by another name, the Appalachian orogeny.] This collision took place with more sub-horizontal undertrusts sheets, as opposed to the overthrusting that was prevalent in the Taconic orogeny. The Brevard fault broke though the surface 300 to 250 million years ago bringing carbonate rocks from below. Seismic data suggests that a possible SUB-horizontal thrust fault splayed and broke off forming the Brevard fault. The carbonate rocks in the fault zone may have been scraped from the underlying sedimentary layers as the thrust splayed away (Cook, 1980, p.147). From the Blue Ridge Drive the Brevard fault is seen as a steep escarpment that migrates northwest. The Brevard fault separates the Blue Ridge Province from the eastern Piedmont Province.

Examples of the Brevard fault zone:

Mile 218.6 Fox Hunters Paradise Take a short walk from the upper parking lot to the crest of High Piney Spur. To the east is a steep slope which marks the Brevard fault zone.

Mile 289.9 Yadkin Valley Overlook The view shows the Yadkin River. At the foot of the mountains the river abruptly changes direction from southeasterly to northeasterly following the trend of the Brevard Fault zone.

Mile 413.2 Pounding Mill Overlook to the southwest is a broad farming valley. The Brevard Fault Zone underlies the valley. The fault is major geologic structure that trends southwest to northeast along the eastern edge of the Blue Ridge Province throughout most of North Carolina.

TOUR: MILE MARKERS (NORTH TO SOUTH) CROSS-REFERENCED TO TIMELINE


REFERENCES:

LINK TO GLOBAL TECTONICS HOMEPAGE

University of Kansas, Emporia, Advisor: James S. Aber, Ph.D. http://academic.emporia.edu/aberjame/tectonic/