||Laurentia and the Grenville Orogeny Province
||Rifting of Rodinia ||The Juvenile Iapetus
||The Blue Ridge Mountains Today ||References
The Blue Ridge Mountains province of the larger Appalachian Mountains range
is a tectonically complex area of ancient origin. Generally speaking, the Blue Ridge Mountains
comprise the eastern edge (front range) of the larger Appalachian Mountains range and extends
from the south in northwestern Georgia to as far north as northern Maryland and south-central
Pennsylvania. The complexity of the Appalachians is primarily driven by the multitude of orogenic
processes which occured starting at over 1 Ga, which created a series of successive rifts, folds, delaminations and collision belts across a long swath of terrain.
This was in large part driven by the supercontinent cycle which focused especially upon Laurentia, starting as early as the Mesoproterozoic when that continent was
forming into a portion of the supercontient of Rodinia, at approximately 1.0 to 1.1 Ga (Torsvik, 2003).
This report will focus primarily on the formation of a subset of the present-day Blue Ridge Province, the Robertson River Igneous Suite that was created at approximately
700 Ma during the Cryogenian age as a product of the rifting of Laurentia from the supercontinent of Rodinia, following the end of that supercontinent cycle.
Figure A. Map of the modern Appalachians including the Blue Ridge Province. Map compliments of the U.S. Geological Survey, http://3dparks.wr.usgs.gov/nyc/images/fig51.jpg
Laurentia and the Grenville Orogeny Province
The original known orogenic formation that underlies the present-day Blue Ridge province was the
Grenville Province, a formation on what would be the present-day eastern edge North America, but was the western edge of Laurentia.
The Grenville Province was formed by tectonic events occurring from between 1.2 Ga to 1.0 Ga. These collisions were with continents that
likely exist now within present-day South America and Australia (Huntsman, 2007). These collisions ultimately caused
the formation of a fold belt known as the Grenville Mountains, as part of the overall process that formed the
supercontinent of Rodinia, one of the earliest known supercontinents and the predecessor of much later supercontinent Pangaea.
Of interest is the theory related to Rodinian formation known as SWEAT, which stands for Southwest U.S.-East Antarctica, which states that Antarctica collided with Laurentia near the location of present-day Arizona and New Mexico. This is based upon observation of goemagnetic and geophysical anomalies of a band of Grenville-aged rock near one Antarctic shore (Danziel, 1995). This would indicate that the overall scale of the Grenville Province was far greater than is generally assumed and would have in fact spanned multiple continents across many thousands of kilometers.
Figure B. A map showing the theorized composition of Rodinia at approximately 800 Ma. The rifting of Laurentia from Rodinia during the end of the Rodinian Supercontinent Cycle would have begun after the final collission into Antarctica and Australia to form the Grenville Orogeny (belts in green along Laurentia and possibly East Antarctica, dated 1.1 Ga). Laurentia then rifted from Rodinia forming the juvenile components of the Iapetus Ocean and causing the formation of the Robertson River Igenous Suite (750 to 700 Ma). Laurentia then moved around Africa (550 Ma) and South America (530 Ma), eventually colliding with South America (487 Ma) to form the supercontinent Pangea (260 Ma). These rifts and collisions uplifted much of the present day Appalachian orogeny including the Blue Ridge anticlinorium (Danziel, 1995). Map compliments of The U.S. Antarctic Program, http://antarcticsun.usap.gov/AntarcticSun/science/images2/rodinia_map.jpg
This formation process was not unlike the present formation of Asia overriding northern India to create the Himalaya Mountains.
This resulted in high temperatures and pressures at great depths which caused widespread regional metamorphism, resulting in large masses of mylonite along
zones of horizontal ductile shearing. Successive collisional deformation processes further compressed the Laurentian continental lithospheric material into taller
and narrower vertically-stacked stratigraphic layers (Huntsman, 2007). This resulted in thicker layers of highly metamorphic rock on top of lower-grade sedimentary rock
indicative of continental margin material. Intensive heating from both tectonic collision force as well as from decompression heat release allowed for partial melting
and plutonic intrusive activities that firmly welded this Grenville collision zone to Laurentia.
Scattered remnants of the ancient Grenville orogeny today can be seen in sections of the Blue Ridge as outcropping of elongated metamorphised plutonic rock axial with the younger Blue Ridge province or as small, isolated domelike masses in sections of the Piedmont in central Virginia.
Rifting of Rodinia
Supercontinents always act as thermal barriers, checking the release of the Earth's internal heat generated by its geodynamo, thus causing an overheat condition within the asthenosphere. (Yoshida,2013). This entire supercontinent formation via convergence and termination through rifting is referred to as the Wilson Cycle, which lasts approximately 450 million years (Erickson, 1993). Eventually the lithosphere begins to be forced upward and weaken along the new rift's fracture zones, followed by cracking in which magma is forced upward through the cracks and ultimately fragments of the supercontinent move apart as the rifts widen, while seafloor spreading continues and new oceans widen. This seafloor spreading phenomenon is being observed now for the Mesozoic-Cenozoic Supercontinent Cycle, that is still in progress.
The Robertson River Igneous Suite was formed during this initial Laurentian rifting process that started at about 750 Ma, at the end of the Rodinian Supercontinent Cycle and the start of a new Wilson Cycle, which led for the formation of the Iapetus ocean. The suite is composed of eight granitic units that are all Late Proterozoic in age and located within the present-day Blue
Ridge anticlinorium (province) in northern and central Virginia. The suite consists primarily of a main dike batholith, with two separate intrusive bodies. It is composed primarily of extrusive rhyolite with significant alkaloid felsic granitoids, and numerous petrologically related dikes that intrude rocks of the main batholith. It is enclosed within the surrounding Grenville-age (1.0-1.1 Ga) basement complex. "The main batholith forms an elongate, narrow belt encompassing an area of more than 380 km2 (137 mi2) stretching from near Ashby Gap in northern Virginia southward nearly 120 km (72 mi) to the vicinity of Charlottesville. As such, the main batholith of the Robertson River Igneous Suite constitutes the largest body of
Late Proterozoic anorogenic intrusive rocks in the Appalachian orogen (Bartholomew and Lewis, 1984; Rankin and others, 1990).
Emplacement of the suite into Grenville-age orthogneisses and other metamorphic rocks of the Blue Ridge core zone occurred
during an extended period of magmatic activity that coincided with the initial stages of the Late Proterozoic rifting of Laurentia"
(Rankin, 1975; 1976; Tollo and Aleinikoff, 1992).
Figure C. The Battle Mountain Alkali Feldspar Granite, a formation within the Robertson River igneous suite of Central Virginia. Elevation 350 meters, dating from 704 Ma and photographed at approximately 4 kilometers distance. Photo © 2013, T.A. Woolman
The Robertson River Igneous Suite was emplaced into deformed and delaminated Grenville basement rock during the interval 735 to 700 Ma (Tollo and Aleinikoff, 1992). The presence of xenoliths as well as the observed cross-cutting relations indicate that the Robertson River magmas forcibly intruded as dikes into coarse gneisses that are now a component of the present-day Blue Ridge basement, as well as into biotite-rich granofels (Tollo, Lowe 1994).
As part of the end-stage Wilson cycle of the supercontinent Rodinia, at approximately 750 Ma during the Late Proterozoic's Cryogenian period, it is believed that a rising aesthenospheric hotspot plume heated portions of the Rodinian lithosphere that produced a bouyant uplifting process. A series of failed rifts along the central and southern Appalachians beginning at 735 Ma (Aleinikoff et al., 1995) generated an alkali A-type plutonic-volcanic formation suite (Tollo et al., 2004). This uplifting resulted in multiple ruptures along broad sections of fracture zones, leading to spreading ridge separations. This widening separation between fault basins and uplifted high relief topographies resulted in magmatic intrusions across multiple rift basin formations, creating dramatic plutonic intrusions axially with the eroding Grenville Mountains along rift-related normal faults (Huntsman, 2007). These smaller plutons of alkali-enriched granitic material rose to shallow depth underlying sections of rift basin. Concurrent basaltic volcanic magmas left a formation of basaltic dikes and sills to form a juvenile proto-oceanic crust which grew from the increasing volume of injected basaltic magma.
Figure D. Battle Mountain Alkali Feldspar Granite Formation (red square on the image right) , a unit of the Robertson River Igneous Suite of Central Virginia. Blue Ridge Basement Complex, Late Proterozoic Igneous. Composed of light-gray to gray, fine-grained to aphanitic, equigranular to sparsely porphyritic (mesoperthite phenocrysts) felsite composed of alkali feldspar, quartz, and rare plagioclase with minor biotite, aegirine, fluorite, and basnaesite; locally displays flow banding, miarolitic cavities containing quartz, and probable lithophysae; intrudes associated alkali feldspar granite; dated at 704 +/- 4 Ma (U.S.G.S. Mineral Resources On-Line Spatial Data, 1993)
The Juvenile Iapetus
Successful rifting to form the new Iapetus ocean, beginning at approximately 565 Ma (Aleinikoff et al., 1995) included deep-water distal facies of various types of clastic sediments and volcanic offshore rock deposition. This rift included new oceanic crust that began the start of the Iapetus ocean's opening. With Laurentia to the west and Baltica to the east, this ocean's formation was the driving force behind John Tuzo Wilson's theory of oceaninc formation cycles, whereby he concluded that the Atlantic ocean must have had a precursor that dated before the time of Pangea, the last supercontinent. The Iapetus, it is theorized, was derived from intracratonic basins which began to arise during this period due to early Laurentian rifting and a first sign of supercontinental breaking of Rodinia. Today, some basaltic dikes in parts of Scandanavia which date between 67 and 650 Ma and serve as evidence of the rifting events which caused the formation of the Iapetus. This oceanic formation dates to the approximate beginning of the formation of the Robertson River igneous suite along the western boundary of the Iapetus.
John Wilson further noted that the current Atlantic had begun to open at approximately the same site where its precursor ocean had closed. That site became to be known as the Iapetus Suture, which is the site of several major faults within a fracture zone created by the collision of Laurentia, Avolonia, Baltica and potentially other continents. These continental landmasses collided through the subduction of oceanic crust material which slowly diminished the ocean by 420 Ma (Meert, Torsvik, 2003).
The Blue Ridge Mountains Today
The modern-day Blue Ridge Mountains are a physiographic province withing the larger, overall Appalachian Mountains range. This province consists of differentiated northern and southern physiographic regions, which divide near the Roanoke River gap (USGS, 2014). The Blue Ridge anticlinorium is located in the eastern United States, starting at its southernmost portion in northwestern Georgia, then ending northward in central Pennsylvania. The Blue Ridge contain the highest peaks in eastern North America below New Hampshire. Over 100 of the peaks exceed 1500 meters, and the highest peak in the province is Mt. Mitchell in North Carolina which is 2037 meters high.
The basement complex rocks of the Blue Ridge Province are "dominated by compositionally diverse felsic
gniesses and granitoids, including abundant opx-bearing charnockites. Collectively, these rocks
preserve an extended record of igneous and metamorphic processes associated with Grenvillian
orogenesis" (Tollo, 2004). The initial protolithic magmatic material was emplaced at approximately
1.1 to 1.0 Ga according to U-Pb zircon dating from multiple sites in Northern Virginia,
followed by Mesoproterozoic compression and uplift (Tollo, 2004), resulting in the present-day
Blue Ridge anticlinorium that consists of igneous and metamorphic rocks.
Aleinkoff, J.N., Zartman, R.E., Walter, M., Rankin, D.W., Lyttle, P.T. and Burton, W.C. (1995), U-Pb ages of metahyolites of the Catoctin and Mt. Rogers Formations, central and southern Appalachians: Evidence of two pulses of Iapetan rifting: American Journal of Science, v. 295, p. 428-454.
Danziel, I. (1995). Earth Before Pangea, Scientific American vol. 272/1, p. 58-63.
Erickson, J. (1993). Craters, Caverns and Canyons Delving Beneath the Earths Surface (The Changing Earth Series). Published by Facts on File. p. 34-37.
Meert, J.G. & Torsvik, T.H. (2003). The making and unmaking of a supercontinent: Rodinia revisited, Tectonophysics 375, pp. 261288.
Tollo, R. (2004). PETROLOGY AND GEOCHRONOLOGY OF GRENVILLE-AGE MAGMATISM, BLUE RIDGE PROVINCE, NORTHERN VIRGINIA. Northeastern Section (39th Annual) and Southeastern Section (53rd Annual) Joint Meeting of the GSA (March 2527, 2004). Retrieved March 24, 2014 from https://gsa.confex.com/gsa/2004NE/finalprogram/abstract_69016.htm
Tollo, R.P., Aleinikoff, J.N., Bartholomew, M.J., Rankin, D.W., 2004, Neoproterozoic A-type granitoids of the central and southern Appalachians: Intraplate magmatism associated with episodic rifting of the Rodinian supercontinent: Precambrian Research, v. 128, p.3-38
Tollo, R., Lowe, T. (1994) GEOLOGIC MAP OF THE ROBERTSON RIVER IGNEOUS SUITE, BLUE RIDGE PROVINCE, NORTHERN AND CENTRAL VIRGINIA, U.S. Geological Survey, retireved from http://pubs.usgs.gov/mf/1994/2229/report.pdf on April 15, 2014.
Torsvik, T.H. (May 30, 2003). The Rodinia Jigsaw Puzzle, Science, issue 300, p. 1379-1381.
U.S. Antarctic Program, http://antarcticsun.usap.gov/AntarcticSun/science/images2/rodinia_map.jpg, accessed April 21, 2014.
U.S. Geological Survey, Appalachian Map http://3dparks.wr.usgs.gov/nyc/images/fig51.jpg
U.S. Geological Survey, Mineral Resources On-Line Spatial Data, Robertson River Igneous Suite; Battle Mountain Alkali Feldspar Granite - felsite (1993). Retrieved April 4, 2014 from http://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=VAZrbf%3B0
U.S. Geological Survey, (August 8, 2005). The Appalachains (Map). Retrieved March 27, 2014 from http://3dparks.wr.usgs.gov/nyc/images/fig51.jpg
U.S. Geological Survey, Physiographic divisions of the conterminous U. S., U.S. Geological Survey. Retrieved 12 April 2014. http://water.usgs.gov/GIS/metadata/usgswrd/XML/physio.xml
Yoshida, M. (2013). Mantle convection modeling of the supercontinent cycle: Introversion, extroversion, or a combination? (June 2, 2013). Retrieved April 1, 2014 from http://www.sciencedirect.com/science/article/pii/S1674987113000844
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