Text Box: Tectonic History of the Eastern United States

The Appalachian Mountain system, extending almost 2,000 miles from Newfoundland in Canada to central Alabama, and the adjacent coastal plain are the two major large-scale structural features of eastern United States.  Ideas about the origin of these two features have been proposed and modified for over a century, and many concepts of tectonic processes were conceived in an effort to explain the development of the Appalachians and the expansive coastal plain.  Lithospheric plate subduction and divergence, which yielded continental collisions and continental separations and accreted terranes, are now recognized as the main factors in the sequential development of the Appalachians and all of the United States east of these mountains.  

An orogeny is a large-scale deformation of the Earth's crust resulting in the formation of mountains.  Some of these orogenies have coincided with the formation of ancient supercontinents. Reconstruction of these landmasses by geologists and paleogeographers has undergone many changes; ideas, terminology, and dates of formation and breakup of the supercontinents are still being studied, debated, and refined.  Four separate primary orogenies–Grenville, Taconic, Acadian and Allegheny–have so far been identified as significantly affecting the eastern United States within the last 1.2 billion years.  

Grenville Orogeny
The earliest of these four orogenies, the Grenville (Grenvillian), is named after a town of the same name west of Montreal, Canada.  This collision of continents began about 1.2 billion years ago and created the supercontinent Rodinia that preceded by more than 700 million years the formation of the latest supercontinent–Pangaea.  Much of the crustal change occurred in Canada, west of the St. Lawrence River.  The mountains that were produced have long since eroded away, and most of the evidence of this orogeny has been obliterated by later mountain-building activity.  Although the Adirondack Mountains of New York are much younger than 1 billion years, the underlying basement rocks of these mountains have been dated as 1 billion years old and show signs of having been deformed at that time.  The identity of the landmass that collided with this portion of North America is not known precisely, but some suggest that parts of what are today Europe, Africa and even the western margin of South America became attached to eastern North America during this orogeny. 
The Rodinia supercontinent assembled by the Grenville orogeny began to break apart about 650 to 600 million years ago.  The shape of the North American continent created by this break up has been difficult for geologists to determine, particularly because the landmass was probably centered over the equator, and what is today the eastern coast of the continent possibly faced south at that time.  It has been determined with certainty, however, that the eastern edge of North America became the leading edge of future collisions with other landmasses.  This eastern edge was probably where the Appalachian Mountains, not yet formed at that time, were eventually created.  
Another result of the breakup of this early supercontinent was the creation of the proto-Atlantic Ocean (a previous version of the modern Atlantic).  This ocean has also been called the Iapetus Ocean, named for the Greek mythological god that was the father of Atlas and Prometheus.  During the formation of this ocean a large section of crust, which may have been an archipelago in the new ocean and has been named the Avalonian microcontinent, or simply Avalon, broke apart.  Part of this collided primarily with eastern Massachusetts and eastern Rhode Island about 575 million years ago.  This event, which is poorly understood, has been called the Avalonian mountain building event (and infrequently the Avalonian orogeny), and involved incomplete plate convergence and generally caused only scattered and relatively minor deformation and metamorphism.  

Taconic Orogeny
Approximately one-half billion years ago a reversal of plate motions occurred and the continents of North America, Europe and Africa were once again on a collision course.  The Iapetus Ocean between the continents began getting smaller, and the floor of the ocean was subducted beneath North America as the approaching landmasses advanced.  The subducting plate melted and magma rose to the surface in the region of the Carolinas, producing volcanoes there about 450 million years ago.  At about the same time a line of volcanic islands, an island arc complex, which had developed on the floor of the Iapetus Ocean near the eastern coast of the United States, was pushed into the coastline.  The resulting long-lasting and very complex mountain building, which consisted of three phases, and is generally considered to be the birth of the Appalachians in the United States, is called the Taconic (Taconian) orogeny for the Taconic Mountains on the Vermont-New York border.  Effects of this orogeny were greatest in the northern portion of the Appalachians, where intrusive volcanics, metamorphism, and thrust faulting accompanied the mountain building.  A section of crust between the converging continents–the Blue Ridge-Piedmont microplate–was thrust westward onto the North American continent.  Apparently the main bodies of the converging continents were not directly in touch with one another during the episode, which lasted about 40 million years, but the deformation of crustal material that was wedged and compressed between them, including the island arc complex, was sufficient to fold and fault sections to significant heights and cause the birth of the Appalachian Mountains in the United States.

Acadian Orogeny
The next episode of mountain building to affect the Appalachians occurred mostly in the Devonian Period, about 400 to 380 or 360 million years ago, although the event may have been initiated in the Silurian.  This was the Acadian orogeny, named after a French region in Nova Scotia, Canada.  The process began when the continents that had separated after the Taconic orogeny once again began to be pushed together because of the development of converging plate motion.  The Iapetus Ocean narrowed for millions of years until the northern portion of the ocean closed completely as northwest Africa, a portion of Europe, and perhaps part of the now-fragmented Avalon mass collided with eastern North America.  Island arcs and microplates between the continents were again squeezed and pushed onto the continent, but this orogeny was generally more intense than the previous Taconic orogeny because apparently now a full continent-continent collision took place.  This was the most severe of all orogenies that affected the Appalachians in the northern section of the continent, and effects were most notable from West Virginia to Newfoundland.  Mountains at least the height of the present-day Rocky Mountains were produced by thrust faulting and folding.  

Allegheny Orogeny
Separation of North America from Europe and Africa, and the concomitant widening of the Iapetus Ocean following the Acadian orogeny set the stage for the final collision and culminating deformational event in the formation of the Appalachians.  This episode of mountain building began with continental collision about 330 million years ago, continued to produce folding and faulting for at least 60 million years, and ceased about 260-250 million years ago.  Now called the Allegheny (or Alleghany or Alleghanian) orogeny, it was previously commonly referred to as the Appalachian orogeny, and still is by some researchers.  However, the term Appalachian used in this context can be somewhat misleading, perhaps implying that this last orogeny caused all of the Appalachian Mountains, not acknowledging effects of previous orogenies.  
Although apparently Africa and Europe collided with the northern portion of North America first, the southern Appalachians were affected to a greater degree than the northern section.  The Blue Ridge portion of crust, particularly in western North Carolina and eastern Tennessee, was thrust at least 90 miles to the west, and many igneous bodies intruded; there was intense folding in the Ridge and Valley region of western Virginia and eastern Tennessee; and thrust faulting occurred as far west in Tennessee as the Cumberland Plateau–nearly to the central portion of the state.  Farther south, the northern part of South America (Colombia and Venezuela) collided with North America at this time, raising the Ouachita Mountains in Arkansas and Oklahoma, and the Marathon Mountains of Texas. 
Pangaea, the Atlantic Ocean and Terranes
The assembly of the continents created by the Allegheny orogeny created the Pangaea supercontinent, and completely closed the Iapetus Ocean about 300 million years ago, near the end of the Paleozoic Era.  In the early part of the Mesozoic Era, about 200 million years ago during the Triassic Period, approximately 100 million years after Pangaea developed, plate motion again reversed, and the breakup of Pangaea began.  The subsequent thinning and rifting of the lithosphere is in evidence today as grabens, called Triassic Basins, that occur throughout the Piedmont Plateau province.  Intrusion of dikes and sills and the extrusion of lavas accompanied the rifting, which is referred to as the Palisadian disturbance.  This marked the beginning of the modern Atlantic Ocean, which continues to grow wider.   
Each of the four separate primary orogenies–Grenville, Taconic, Acadian and Allegheny–that affected the eastern United States within the last 1.2 billion years and involved continental and/or microcontinental collisions, brought crustal material that had originated in diverse locations in contact with the North American continent.  Large fragments of this crustal material that remained attached to the continent are called terranes.  The term terrane has been used in various ways, but currently it is most commonly used to refer to such crustal material that has accreted (become attached) to a continent but was originally part of some other landmass; the term accreted terrane is also frequently used.  Less commonly used are exotic terrane and suspect terrane.  
The Appalachians and eastern coastal plain consist of several terranes, the number and boundaries of which are still being deciphered.  The Carolina terrane, one of the largest, consists of the eastern halves of Virginia, North and South Carolina as well as southeastern Georgia.  It was added to North America perhaps during Ordovician/Silurian times.  Coastal New England is part of the Avalon terrane (Avalonia), which despite being named for the Avalon Peninsula in Newfoundland, Canada was derived from ancient rocks of western Europe.  Much of the Appalachians south of Pennsylvania are part of the Piedmont terrane, which includes central Virginia, the western halves of North and South Carolina and northern Georgia.  The Suwannee terrane, which mostly consists of the northern half of Florida, was probably originally a part of African Gondwanaland and accreted during the Carboniferous/Permian periods.
Text by Jim Henry (Copyright 2007) Illustrations by Miller Wylie and Mark Abolins