But first, two important definitions:
- Active margin: A continental margin that is adjacent to the nearest plate boundary. Active margins are usually the scene of tectonic activity like peripheral orogenies or trnasform boundaries. E.G.: the North American Pacific Northwest.
- Passive margin: A continental margin that is far from nearest plate boundary. Passive margins are usually tectonically quiet. E.G.: the North American Atlantic coast.
During the Jurassic, the Atlantic ocean began to form. Initially, the crust near the new divergent boundary was hot, so it stood out in high relief like the shores of the Red Sea today. As it cooled, the continental crust shrank and sank. Sediments began to be deposited on it. Today, its bedrock is covered with an thick apron of nearly horizontal sediments from Jurassic to recent in age. Topographic relief is low - the classic passive margin.
During the Carboniferous, continental collisions resulted in the creation of Pangea. One major suture zone was marked by the Himalaya-sized TriassicTrans-Pangean Mountains. During the Jurassic Pangea rifted apart along the zone of weakness that these mountains represent. Their western foothills became the Appalachians. Their eastern foothills, the Atlas Mountains of northwestern Africa.
Same origin as the Appalachian highlands but farther south. In this case, the continental collision was with the part of Gondwana that would become South America.
Rocks of the ancient craton of North America, formed by Pre-Phanerozoic continental collisions underlie this region. These rocks were mostly unaffected by the Phanerozoic collisions that formed Pangea. From the Paleozoic onward, this region has received sediment from the various mountains closer to the continent's periphery (Appalachians, Ouachitas, and Rockies) and has periodically been covered by shallow seas. The most recent of these occurred during the Cretaceous.
The oldest rocks of North American craton and absolute core of the continent. These, however, have been widely exposed by the action of continental glaciers during the Quaternary.
The Rocky Mountains took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States, starting with the Cretaceous - Paleogene Laramide orogeny,
about 70-40 million years ago is responsible for raising the Rocky Mountains.
Ancient Pre-Phanerozoic rocks, exposed only in the deepest canyons, make up the basement. Throughout the Paleozoic Era, the Colorado Plateau region was periodically inundated by tropical seas. Beginning in the Triassic (about 250 million years ago), deposits of marine sediment waned and terrestrial deposits dominated. These include great accumulations of dune sand hardened to form sweeping arcs in cross-bedded sandstone. The Colorado Plateau is remarkably stabile. Relatively little rock deformation has affected this high, thick crustal block within the last 600 million years or so. In contrast, the plateau is surrounded by provinces that have suffered severe deformation.
Here, the Earth's crust and upper mantle has been stretched up to 100% of its original width. Beginning in the Paleogene, the entire region was subjected to extension that thinned and cracked the crust as it was pulled apart, creating long parallel normal faults. Along these roughly north-south-trending faults, fault blocks alternately stood out or sunk down. Upstanding fault blocks were then eroded into parallel ranges of low mountains.
The Columbia Plateau province is enveloped by one of the worlds largest flood basalts. Over 500,000 square kilometers. These erupted between 17-6 million years ago during the Neogene Period. Most of the lava flooded out in the first 1.5 million years. An extraordinarily short time for such an outpouring of molten rock.
The most geologically young and tectonically active region in North America. The generally rugged, mountainous landscape of this province provides evidence of ongoing mountain-building. To a large extent, this province consists of poorly sutured microplate terrains, each with its own geologic history.
How did North America come to have these provinces? To answer this, we must make an excursion into the discipline of historical geology. This lecture offers a glimpse of in through a synopsis of the history of North America. In it, I will link extensively to two sets of paleogeographic maps:
- The Paleomaps Project by Chris Scotese
- Paleogeography Through Geologic Time by Ron Blakely of Northern Arizona University
Starting in the Cambrian, the central craton of North America appears as the ancient continent of Laurentia. As we move forward in history, pay attention to two themes:
- The evolution of the Appalachians through subduction, continental collisions, and rifting
- The evolution of the Western Cordillera through subduction and the accretion of microplate terrains.
In the previous lecture, we introduced the topics of Rodinia and Pannotia, the supercontinents that preceded Pangea. We therefore take up the story after the breakup of Pannotia in the latest Proterozoic. (650 m.a.)
In the latest Proterozoic, the central craton of North America, called Laurentia, was fully assembled. At this point we don't call it "North America" because, strictly speaking, North American was formed by the rifting of Pangea, which occurred later. The rocks of Laurentia, however, were destined eventually to become part of North America.
Cambrian - Passive margins
As Laurentia rifted away from Pannotia it assumed a positon near the equator. By the Cambrian Period, it was surrounded on all sides by passive margins - continental margins that are far from any plate boundary. Additionally, sea levels were historically high, so much of the continent was submerged as shallow sea.
Ordovician - Taconic Orogeny
During the Middle Ordovician , however, Laurentia was passively carried toward an oceanic-oceanic convergent boundary. Being a large continent, it couldn't be subducted. Instead, the subduction zone was shut down and its associated volcanic arc was welded onto Laurentia's (modern) east coast in the Taconic Orogeny, the first of three collisions that would create the Appalachians. For the next 250 million years, what's now the east coast would be an active margins.
Silurian - Devonian - Acadian Orogeny in the East, Antlers Orogeny in the West
By the middle Paleozoic, the east coast was an active margin, fringed by a new active subduction zone. During the Silurian , a small continent called Avalonia rode the oceanic lithosphere into this subduction zone and in the early Devonian , was welded to Laurentia as a microplate terrain. Farther north, the craton of Europe, called Baltica, collided with Greenland (which was part of Laurentia.) Collectively, this constitutes the Acadian Orogeny, the second in the Appalachian-building series. Thick deposits of sediment eroded from these mountains were deposited to the west, in in low-lying flood plains and shallow seas where the Appalachians now stand.
Slightly later, in the Late Devonian, Laurentia-Baltica was carried into a subduction zone along its modern western border and collided with the associated volcanic arc. Now the west coast was also an active margin. This event, the Antler Orogeny, was like a western version of the Taconic Orogeny, and signaled the beginning of the slow accretions of many microplate terraines to Laurentia's western margin.
Carboniferous - Alleghenian Orogeny
During the Carboniferous, Laurentia-Baltica collided with Gondwana (a giant continent containing the makings of Africa, South America, Antarctica, India, Australia, and New Zealand.) The portion of Gondwana destined to become Africa struck Laurentia's eastern margin, while the portion destined to become South America struck the southeast margin. Collectively, this is called the Alleghenian Orogeny. The huge mountain range created is called the Trans-Pangean Mountains. The sediments that had eroded from Taconic and Acadian mountains and deposited to the west were caught up in this new orogeny and extensively folded.
While this was happening, other continents, including Kazakhstan and Siberia, were colliding with Baltica and with one another. The result was Pangea - the last supercontinent, which contained every major landmass except the two continents destined to become North and South China. Pangea was hour-glass shaped with two major regions:
- Laurasia in the north
- Gondwana in the south
Early Mesozoic - Accretion and rifting
By the Late Triassic, Pangea was beginning to rift apart along the suture of the Trans-Pangean Mountains. Remains of rift valley lakes are preserved in the sandstones of the Newark Supergroup. By the Early Jurassic (right), actual sea-floor crust was forming between Laurasia and Gondwana. This rifting marks the birth of North America, containing the former Laurentia at its center; and of the Atlantic Ocean. Ironically, the high mountains of the central Trans-Pangean Mountains (the region closest to the new ocean basin) now eroded away, leaving the former foothills as sad remnants. These are the Appalachians, the Ouachitas of Oklahoma and Arkansas, and the Atlas of Morocco and Mauretania. Florida, formerly a chunk of Gondwana, remained stuck to eastern North America. From this time onward, eastern North America has been a passive margin.
In the west, an active margin was present, and Laurentia experienced a series of collisions with volcanic arcs and small continents, as microplate terrains continued to accrete.
Late Mesozoic - Sea level highstand and the Laramide Orogeny
Plate tectonic processes were running at full speed as the Atlantic continued to lengthen and widen. During periods of vigorous plate tectonic activity, the ocean floors, being warmer than usual, actually stand up somewhat higher topographically. This, in turn, pushes sea water onto the continents. The Cretaceous was such a time. In addition, superplumes, unusually large and abundant mantle plumes, caused widespread volcanic eruptions in the West Pacific. Finally, the Cretaceous was an ice free world. The net effect was that sea level stood at its highest since the early Paleozoic. At this time, North America was bisected by a shallow Central Cretaceous Seaway. The Proterozoic bedrocks of much of the continent today are hidden by thick limestones accumulated during this interval.
Elsewhere in the world, Africa and India rifted away from Gondwana. Still, North America remained firmly attached to Laurasia in the east and west.
Laramide Orogeny: In the west, two interesting processes were occurring:
- The west coast remained an active margin, with a subducting plate - the Farallon Plate - feeding a volcanic-magmatic range - the Sierran arc. (Today's Sierras are the uplifted roots of this Mesozoic range.)
- Compression caused by this subduction resulted in the first uplift of the Rocky Mountains. This is weird, because the rockies were a considerable distance inland from the western margin. Apparantly, the Farallon plate subducted at an unusually shallow angle. The building of the Rockies is known as the Laramide Orogeny.
Cenozoic - Basin and Range extension
Finally, during the early Cenozoic, eastern North America rifted from what we now call Eurasia, opening the gateway between the Altantic and Arctic oceans. In the west, the two continents remain connected.
Starting around 40 million years ago, the western edge of North America gradually encountered and overrode the divergent boundary separating the Pacific Plate from the Farallon Plate. As this happened, its western margin changed from a convergent to a transform boundary. Today's Juan de Fuca and Cocos Plates are last sad remnants of the Farallon Plate. As a result of this transformation, large regions of western North America, which had been compressed for over 100 m.y. by the shallowly subducting Farallon slab were suddenly released from compression and "relaxed," widening to nearly twice their original width. This region is called the Basin and Range and the widening event is the "Basin and Range extension." Its widening is marked by numerous parallel normal faults, forming a series of parallel mountain ranges and valleys.
Note: The normal fault-bound blocks created in this way are known as:
- Horsts: The upthrown blocks, which are weathered into mountain ranges.
- Grabens: Downthrown blocks buried beneath valley sediments.
Latest Neogene - Isthmus of Panama
A final finishing touch during the late Neogene was the evolution of the Isthmus of Panama, an island arc coalescing to connect North and South America within the last five million years. This even had momentous implications:
- Ocean circulation was drastically altered when tropical waters of the Atlantic and Pacific were separated.
- The exchange of indigenous North and South American land animals, the Great American Interchange, resulted in the extinction of many South American forms.
Key concepts and vocabulary:
- Continental margins
- Passive margin
- Active margin
- Atlantic Plain province
- Appalachian highlands
- Ouachita-Ozark Interior highlands
- Interior plains
- Laurentian Upland province
- Rocky Mountains
- Colorado Plateau
- Basin and Range
- Columbia Plateau
- Pacific Mountain System
- Historical Geology
- Cambrian - time of passive margins for Laurentia
- Ordovician - Taconic Orogeny in (future) east
- Silurian - Devonian
- Acadian Orogeny in (future) east
- Devonian - Antler Orogeny in (future) west - Active margins all around Laurentia
- Alleghenian Orogeny in (future) east
- Pangea's hourglass shape - Laurasia in north, Gondwana in south
- Trans-pangean mountains
- Early Mesozoic Era (Triassic and Early Jurassic)
- Accretion of microplate terrains to future) wes
- Rifting of Pangea in east to form early North Atlantic
- Late Mesozoic Era (Cretaceous)
- Sea-level highstands, Central Cretaceous Seaway of North America
- Laramide Orogeny yields Rocky Mts.
- Subduction of Farallon Plate
- Early Cenozoic Era (Paleogene)
- North America rifts form Eurasia in east
- Basin and Range Extension
- Horsts and grabens
- Late Cenozoic Era (Neogene)
- Isthmus of Panama