Formation of the Appalachian Mountains and the East Coast of North America

Home , Acadian orogeny, Alleghanian orogeny

INTRODUCTION TO THE GEOLOGY OF THE SOUTHERN APPALACHIAN MOUNTAINS (TECHNICAL) NEPRIS SESSION ON 7 NOVEMBER 2017 FIELD TRIP TO STONE MOUNTAIN, GEORGIA Background The geology of the Atlanta area in Georgia is dominated by the Appalachian Mountain Range and the processes that created it. Stone Mountain is located near the city of Stone Mountain, a suburb of Atlanta, and is situated at the southern end of the Appalachian Mountains. The Appalachians lie just east of an early boundary of the ancestral North American continent during Precambrian times more than 1 billion years ago. The field trip to Stone Mountain will explore a diversity of igneous features exposed on its surface quantitatively with the aid of a Brunton Pocket Transit (see Appendix). The majority of rocks that are exposed at the Stone Mountain monadnock are igneous. They were formed by the intense heat that developed from the compression of rocks along the east coast of ancestral North America during the Alleghanian orogeny roughly 300 million years ago (Atkins and Joyce, 1980). They are granitic in composition, technically referred to as quartz monzonite. Common minerals seen on the mountain include quartz, potassium and orthoclase feldspar, muscovite, biotite, and tourmaline. Soil on the solid rock mountain is formed from the chemical weathering of feldspar. When feldspar weathers, one of the by products is clay minerals. Mechanical weathering is found everywhere on the mountain. The mechanical breakdown of Stone Mountain rocks is accomplished by exfoliation, jointing, freezing/thawing cycles, plant root expansion, and burrowing by mammals, worms, and insects. Exfoliation is a special kind of weathering that involves hysteresis. Hysteresis is a form of "memory effect" retained by certain substances that have been stressed. Rocks under compression for long periods can experience hysteresis. Over the course of the 300 million year existence of Stone Mountain, 10 miles of overburden has been removed by weathering and erosion (Atkins and Joyce, 1980). The release of confining pressure on the mountain with the removal of the overburden takes the form of expansion, where the mountain forms expansion cracks parallel to its surface. These cracks (joints) create sheet-like rock shapes that are found on the sides and at the base of the mountain. The explanation of the formation of the Appalachians follows that of the formation of all collisional mountain structures: plate tectonics. Plate tectonics represents a paradigm in which the earth is believed to consist of a series of concentric shells distinguished from one another by their composition. See Figure 1. The innermost shell is called the inner core and is actually a solid sphere of iron and nickel nearly 2,500 kilometers (~ 1,500 miles) in diameter. The shell that surrounds the inner core is called the outer core, composed of liquid iron and nickel, and having an outer diameter of 6,800 kilometers (~4,150 miles). Surrounding the outer core is the mantle, composed of iron/magnesium-rich silicate minerals and having the chemical composition of a dark rock called peridotite. The outer diameter of the mantle is about 12,500 kilometers (~ 7,650 miles). The mantle consists of two zones or concentric spherical shells: lower mantle and upper mantle. Over very long periods of time (millions of years) parts of the lower mantle move upward in thin cylindrical shapes called mantle plumes, while other parts move downward as slabs of material descending from the shell at the top of the upper mantle. These upward and downward movements of the mantle are responsible for some of the largest land and seafloor structures on earth. The movements are controlled by uneven distributions of heat within the earth: hotter regions tend to rise, while cooler regions tend to sink. The outermost shell of the earth's interior is called lithosphere, or sphere of rock, because of its solid character and brittle behavior. Its thickness at the top of the mantle varies from a few kilometers at mid-ocean ridges to perhaps 280 kilometers beneath the Himalaya Mountains (Pasyanos, 2008). Lithosphere is underlain by a more pliable region of the upper mantle called the asthenosphere. The asthenosphere is not rigid, but behaves as a deformable solid that can flow when subjected to stress. The asthenosphere is deformable in a manner similar to hot wax because its constituent minerals are believed to be near their melting temperature.

Crust (outer sphere) State: solid Lower Mantle Continental thickness: ~30-90 km State: solid Oceanic thickness: ~2 to ~10 km Depth: 670 km to 2,891 km Composition: ultramafic Continental composition: granitic 3 Oceanic composition: basaltic Density: 4.4 to 5.6 g/cm Continental density: 2.7 g/cm3 Oceanic density: 2.9 g/cm3

Lithosphere State: solid Depth: surface to ~300 km Continental thickness: ~30 to ~300 km Oceanic thickness: ~2 km to 50-100 km Composition: crust and rigid upper mantle Density: ~3.4 g/cm3

Upper Mantle (below crust) Outer Core State: solid State: liquid Depth: ~2 km to 650-670 km Depth: 2,891 to 5,150 km Upper zone: rigid upper mantle Composition: Fe, Ni, S, O 3 Middle zone: asthenosphere (weak) Density: 9.9 to 12.2 g/cm Lower zone: transition zone Composition: ultramafic silicate Density (upper to lower): 3.4 to 5.6 g/cm3 Inner Core State: solid Depth: 5,150 to 6,371 km Composition: Fe, Ni, S Figure 1. Internal structure of the earth. Density: 12.8 to 13.1 g/cm3 Copyright © 2017 by Randal L. N. Mandock 1 28 October 2017 According to plate tectonics, the surface of the earth represents a jigsaw puzzle of lithospheric blocks, or plates, interlocked with one another, but able to move horizontally and vertically. Some plates are composed of a combination of continental and oceanic lithosphere, whereas others are entirely oceanic in nature. The upper zone of a lithospheric plate is called the crust. The crust makes up the surface bedrock sometimes seen in rock outcrops. The crust is underlain by a rigid layer of "frozen" asthenosphere. This zone of the upper mantle may be thought of as frozen to the base of the crust, and as such, it forms a single lithospheric structure with the crust. Figure 2. Simple map showing very approximate boundaries of some of the major tectonic plates. (After Archi4102, 2013)

A tectonic plate is thus a block of lithosphere that consists of rigid upper mantle frozen onto the base of the crustal rocks above. There are two types of earth crust: continental and oceanic. Continental crust is granitic in mineral composition, being made of light-colored minerals rich in silicon (in the form of silica, SiO2) and aluminum. Oceanic crust is basaltic, consisting of a lower silicon content but higher in iron and magnesium. Continental lithosphere is less dense (less weight per unit volume) than oceanic lithosphere because the crustal part is composed of less dense granitic rock. Oceanic lithosphere is more dense because its crustal part is composed of more dense basaltic rock. There are three types of plate boundaries: convergent, divergent, and transform. A convergent plate boundary exists where plates are converging with one another as in a collision. See Figure 3. Where the convergence brings continental lithosphere into collision with oceanic lithosphere, the less dense continental lithosphere overrides the more dense oceanic lithosphere. The overridden oceanic lithosphere can sink (subduct) slowly into the mantle as the continental lithosphere rides above it, or it can sink rapidly as an oceanic lithospheric slab at a high angle with respect to the earth's surface. The slab's age, temperature, and resultant density determine whether it will sink slowly or rapidly and will also determine its angle of descent. The older, colder, more dense slabs sink most rapidly and at the steepest angles. The younger, warmer, less dense slabs can sink at very shallow angles of a few degrees from the earth's surface. As a slab sinks beneath continental lithosphere at an oceanic- continental convergence zone, for example, it releases water and other volatiles (e.g., CO2) that act as a catalyst to melting. The asthenosphere above the descending slab partially melts because of this catalyst. Being less dense than the solid that generated it, the partially-melted asthenosphere rises to the base of the crust as a hot plume predominantly basaltic in composition. See 2 in Figure 3. Figure 3. Cross section of a convergent subduction zone and a young divergent boundary in a back-arc basin. (Stern, 2006)

These plumes of magma heat the crust above and can penetrate it, melt it, mix with it, and produce volcanoes. The

Copyright © 2017 by Randal L. N. Mandock 2 28 October 2017 volcanoes can form a line or an arc on the earth's surface called a continental volcanic arc. New continental crust is created by continental volcanic arcs. On the other hand, when two oceanic plates converge at an oceanic-oceanic convergence zone, the denser one subducts beneath the less dense one. The subducting plate releases volatiles into the asthenosphere above, which in turn generate magma. The basaltic magma rises toward the oceanic crust above, penetrates it, and produces volcanic islands that form a line or an arc. This string of volcanoes is called a volcanic island arc. Collisional mountain ranges form when two continental plates converge at a continental- continental convergence zone. Since neither of the continental plates is dense enough to subduct very deeply into the mantle, one may partially override the leading edge of the other and create a mountain range where the crust below is thicker than before the collision. A divergent plate boundary exists where plates pull apart from one another. See 1 in Figure 3. The tensional stress reduces confining pressure in the divergence zone, allowing the asthenosphere to partially melt and produce magma. The magma erupts, solidifies, and creates new oceanic crust. A transform plate boundary exists where plates slip or grind past one another along a transform fault. A transform fault is a vertical fracture in the lithosphere where two plates slip past one another in a purely horizontal motion. All of these tectonic movements have been involved in the formation of the Appalachian Mountains and the east coast of North America.

Rodinia (1.3-0.75 Billion Years Ago) A brief geologic history of the Appalachians begins with the convergence of proto-continents called cratons into an early supercontinent named Rodinia. See Figure 4. The assembly of all or nearly all the cratons and crustal blocks in existence at the time took place between 1.3-0.9 billion years ago (Ga). The supercontinent persisted for about 150 million years after the last of the continental blocks came together. A mantle superplume developed about 850 million years ago (Ma) under the northern part of Rodinia. The plume was likely caused by the rapid sinking of cold and dense mantle slabs that had accumulated below the asthenosphere along Rodinia's outer margin and by a build up of heat in the mantle below the supercontinent due to its behavior as a thermal insulator. The plume followed the drifting supercontinent and produced interior rifting between 825-740 Ma (Li et al., 2008). Cratons began to separate from the supercontinent in stages beginning about 750 Ma (Torsvik, 2003, 2008). The northern part of modern South America, known as the Amazonian craton, broke away from the southeastern margin of Rodinia's central craton, Laurentia, about 570 Ma (Fichter and Baedke, 2000). This rifting of what would later become the east coast of North America was the birth of the first ancestral Atlantic Ocean, the Iapetus Ocean. The rifting did not leave the east coast of the Laurentian craton with a smooth coastal outline, but rather jagged. The jagged regions that pointed toward the Iapetus Ocean in the east are called promontories, and they were high in elevation and resistant to erosion.

Figure 4. Rodinia. Green lines represent active plate margins. Red lines represent zones of mountain building. (After Li et al., 2008)

Grenville Orogeny and the Building of the East Coast of North America (1.3-1.0 Billion Years Ago) The Grenville orogeny was a long belt of mountain building that extended from Labrador in northeastern Canada south through the Appalachians and west across Texas and northern Mexico (Tollo et al., 2004). During the Grenville orogeny three cratons converged with the eastern and southeastern margins of Laurentia as Rodinia was being assembled: Amazonia, Rio de la Plata, and Kalahari (Figure 4). The first craton to collide with Laurentia was Rio de la Plata at about 1.1 Ga. The next was Kalahari at about 1.05 Ga. The third was Amazonia at about 1 Ga. As explained above, roughly 200-300 million years later continental rifts developed between Laurentia and each of the three cratons. The rifts grew larger until by 600 Ma Kalahari became completely separated from Laurentia, and Amazonia did the same at 570 Ma. Rio de la Plata did not break away until 550 Ma (Li et al., 2008). The Kalahari craton eventually became southern Africa as the continents reassembled over time. Rio de la Plata is now located in South America, making up Uruguay, southern Brazil, east central Argentina, and southern Paraguay.

Copyright © 2017 by Randal L. N. Mandock 3 28 October 2017 Taconic Orogeny (450-440 Million Years Ago) The mountains of the east coast of Laurentia (ancestral North America) created by the Grenville orogeny eroded away until the next convergence called the Taconic orogeny built them up again. This orogeny took place during the period 450-440 Ma. Vastly different tectonic events occurred in the northern and southern Appalachians during the Taconic orogeny. In the northern Appalachians the "Volcanic Arc" shown in Figure 5 is a crude representation of a suite of terranes (tectonic fragments of crustal material) that converged on Laurentia (labeled "continental crust" in Figure 5). As shown in Figure 5, Laurentia was a plate that was likely composed of continental and oceanic lithospheric parts. The oceanic part is shown to be subducting beneath the "Volcanic Arc." The suite of terranes likely consisted of at least three volcanic arcs and perhaps two microcontinents that approached Laurentia either as a single unit or as a sequence of collisions (Fichter and Baedke, 2000). About 540 Ma the oceanic lithosphere of the Laurentian plate began to subduct. As it subducted, the terranes were drawn into a collision or series of collisions approximately 450 Ma that forced them to be thrust over this plate. See Figure 5. The overthrusting of the terranes onto the continent produced enough heat and stress to change the rocks from sedimentary and igneous into metamorphic. Events much more complicated took place in the southern Appalachians during the Taconic orogeny. Referring to Figure 6, the oceanic lithosphere beneath the Iapetus Ocean is shown as a slab on the right being subducted beneath the oceanic portion of the Laurentian plate. The subducting slab produced the following effects: 1. A volcanic island arc. (Note that the volcano shown in Figure 6 was likely becoming inactive because it was no longer located above the zone of magma-generating dehydrating slab fluids.) 2. An extensional back-arc basin with an underwater divergent zone Figure 5. Northern part of the Taconic penetrating the continental slope-rise area. Orogeny of eastern North America. 3. The rising mid-ocean ridge type of magma emplaced basalt in Courtesy of USGS. (Topinka, 2007) the divergence zone (asthenosphere upwelling) and provided the heat needed to melt the slope-rise sediments that produced the granitoids in the back-arc oceanic crust. 4. The rollback velocity vr of the Iapetus lithosphere exceeded the convergence velocity vo of the overriding Laurentian plate. This caused the subduction zone to retreat towards the Iapetus Ocean and away from Laurentia. 5. The tension in the back-arc basin was produced both by the corner flow induced by the subducting slab and by vr > vo.

Figure 6. Illustration of the Taconic Orogeny in the south. (After Holm-Denoma and Das, 2010)

Copyright © 2017 by Randal L. N. Mandock 4 28 October 2017 Acadian Orogeny (375-325 Million Years Ago) The next significant tectonic event was the Acadian orogeny. The mountain building associated with the Acadian orogeny began about 375 million years ago in northern Laurentia (modern-day Canada), migrated southward, and reached today's Alabama by about 350 million years ago (Ettensohn, 1985).

Figure 7. The Appalachian Orogeny (Taconic Through Alleghanian Orogenies). Courtesy of USGS. (USGS, 2006)

Copyright © 2017 by Randal L. N. Mandock 5 28 October 2017 The primary tectonic event during the Acadian orogeny was the collision between Avalonia and Laurentia. See Figure 7. Avalonia was a microcontinent believed to have originated from one or more continental volcanic arcs, volcanic island arcs, or both. The arcs were created between 730-570 million years ago from igneous activity produced by subduction of an oceanic slab (Murphy et al., 2001; Strachan, 2000). Avalonia was either part of, or it was accreted to, the southern supercontinent named Gondwana (or Gondwanaland). Gondwana was a supercontinent that formed from the southern cratons that broke apart from Rodinia. It was the land mass that included the ancestral African continent. After the final break up of Rodinia, Avalonia rifted from Gondwana during the opening of the Iapetus Ocean. It drifted northward and eventually collided with Laurentia. Avalonia incorporated itself into Laurentia as an exotic (foreign) terrane by way of an oblique collision along a strike-slip fault that ran roughly parallel to the Laurentian coast. The fault plane of a strike-slip fault is vertical, and the movement along each side of the fault is horizontal in opposite directions. Imagine a scissors with the handle in Newfoundland (Canada) and the blades pointing southwestward toward the east coast of Laurentia. When the blades are opened to about 30 apart the blade on the left will represent the east coast of Laurentia, and the blade on the right will represent Avalonia. As Avalonia slowly slipped northward along the strike-slip fault, the scissors slowly closed. As they closed, Avalonia first collided with the northernmost promontory (St. Lawrence), then the next promontory to the southwest (New York), then the next to the southwest (Virginia), and finally to the last even further to the southwest (Alabama).

Alleghanian Orogeny (320-250 Million Years Ago) The last mountain-building event to enlarge the east coast of North America and push the Appalachians as high as the modern Himalayas was the Alleghanian orogeny. It was in this orogeny that the northern supercontinent of Laurasia (formed by the coalescence of Laurentia with most of the other land masses of the Northern Hemisphere) converged with Gondawana to produce the last supercontinent, Pangaea. See the sequence of events in Figure 7. During the Pennsylvanian Period (~323 to ~290 million years ago; Swezey, 2002) the African coast of Gondwana collided with what became the modern east coast of North America as the oceanic lithosphere beneath the Rheic Ocean (referred to in Figure 7 as the "Eastern Iapetus Ocean") subducted under Gondwana. The convergence of Gondwana with Laurasia became the final closing of the Iapetus Ocean and its successor the Rheic Ocean. The convergence zone stretched from Newfoundland to the Marathon Mountain area of Texas. As happened before, by the end of the Mesozoic Era (65 Ma) the Appalachians had eroded to an almost flat plain known as a peneplain (USGS, 2017).

Miocene Epeirogenic Uplift (16-12 Million Years Ago) If the Appalachians eroded to an almost flat plain by 65 million years ago, and if there have been no orogenies along the east coast of North America since then, why are the Appalachians a mountain range today? During the Middle Miocene epoch (16-12 Ma) sedimentation rates and grain sizes delivered from the Appalachians to offshore basins in the Atlantic Ocean and the Gulf of Mexico began to increase (Gallen et al., 2013). This increase has been a consequence of the increase in relief of >150% that the southern Appalachians have been undergoing since the Miocene. An ongoing increase in relief implies that the topography of the southern Appalachians is currently in a state of adjustment to a new regional base level (the minimum elevation above sea level to which regional mountain streams erode their beds), and is therefore not in dynamic equilibrium. One explanation for such a change in base Figure 8. Effects of erosion on Appalachian geomorphology: level is tectonic: delamination of the rounded mountain tops, incised by streams. (After Flynn, 2008) subcrustal lithosphere beneath the Appalachian mountains. Detachment of the dense lithospheric roots at the lower boundary layer of a shortened and thickened crust would cause uplift due to the inflow of hot lower-density asthenosphere to replace the detached roots (Elkins- Tanton, 2005; Kay and Kay, 1993). Based on seismic evidence, this is hypothesized to have happened to the Appalachian peneplain.

Copyright © 2017 by Randal L. N. Mandock 6 28 October 2017 Conclusions Although the rounded mountain tops and generally gentle slopes found in many Appalachian locations are suggestive of an old mountain range, the ongoing increase in relief of the Appalachians indicates continuing rejuvenation (represented in this case by a rising above sea level) and youth. Uplift (i.e., being pushed higher above sea level) in the Appalachians differs from the rise of mountain ranges in continental extensional geologic provinces such as the Basin and Range of the western U.S. and Mexico (Parsons, 1995). Uplift in the Basin and Range produced jagged peaks and higher current elevations (~13,000-foot maximum) than in the modern Appalachians (~6.700-foot maximum). Regions of continental compressional tectonics such as the Himalayas likewise produce sheer cliffs, jagged peaks, and the loftiest elevations (29,035-foot maximum). The geomorphic (shape of the earth's surface) differences among these distinct orographic settings are due to the forces acting on each of their lithospheric environments. In contrast to the tensional stresses (forces) that resulted in formation of the Basin and Range Province, and the compressional stresses that are still growing the Himalayas, the force that is rejuvenating the Appalachians is buoyancy. The southern Appalachian Mountains are being uplifted today by tectonic buoyancy processes in the upper mantle (e.g., see Mandock, 2016). This uplift is moderated by continuous erosion. The present geomorphology of the southern Appalachians is a byproduct both of the Alleghanian collision between Laurasia and Gondwana (the North American and African parts of the northern and southern supercontinents) and the erosion of the Appalachian region as it undergoes continual uplift. The strength and resistance to geological weathering and erosion of the rocks that predated the breakup of Pangaea (which began about 200 million years ago) influence the geomorphology by controlling to a certain extent the paths of erosional streams as they descend towards their base level. More resistant rock structures form topographic highs, and less resistant rocks erode to form topographic lows. In summary, the explanation for why the Appalachians are a mountain range today reduces to the ongoing battle between the force of buoyant tectonic uplift and the lowering of the mountain tops by the forces of erosion.

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Figure A.1. Brunton Pocket Transit