at the close of the paleozoic, eastern north america was attached to what is now
Geologic provinces of Northward America
Prototype
The most geologically immature and tectonically active region in N America. The generally rugged, mountainous landscape of this province provides show of ongoing mountain-edifice. To a large extent, this province consists of poorly sutured microplate terrains, each with its own geologic history.
How did North America come up to accept these provinces? To answer this, nosotros must make an excursion into the subject area of historical geology. This lecture offers a glimpse of in through a synopsis of the history of N America. In it, I will link extensively to two sets of paleogeographic maps:
- The Paleomaps Project by Chris Scotese
- Paleogeography Through Geologic Fourth dimension by Ron Blakely of Northern Arizona University
Starting in the Cambrian, the cardinal craton of Northward America appears as the ancient continent of Laurentia. As we move forrard in history, pay attention to ii 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. Nosotros therefore take up the story after the breakup of Pannotia in the latest Proterozoic. (650 m.a.)
In the latest Proterozoic, the key craton of North America, called Laurentia, was fully assembled. At this bespeak nosotros don't call information technology "North America" because, strictly speaking, North American was formed past the rifting of Pangea, which occurred after. The rocks of Laurentia, all the same, 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, information technology was surrounded on all sides by passive margins - continental margins that are far from any plate purlieus. Additionally, ocean levels were historically high, so much of the continent was submerged as shallow body of water.
Ordovician - Taconic Orogeny During the Heart Ordovician , still, Laurentia was passively carried toward an oceanic-oceanic convergent purlieus. Existence a large continent, it couldn't exist subducted. Instead, the subduction zone was shut downwards and its associated volcanic arc was welded onto Laurentia's (modern) east coast in the Taconic Orogeny, the starting time of three collisions that would create the Appalachians. For the next 250 million years, what's now the east declension would be an agile margins.
By the eye Paleozoic, the due east coast was an active margin, fringed by a new active subduction zone. During the Silurian , a minor continent chosen Avalonia rode the oceanic lithosphere into this subduction zone and in the early on Devonian , was welded to Laurentia as a microplate terrain. Further n, the craton of Europe, called Baltica, collided with Greenland (which was office 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 afterwards, in the Late Devonian, Laurentia-Baltica was carried into a subduction zone along its mod western edge and collided with the associated volcanic arc. Now the w declension was also an active margin. This consequence, the Antler Orogeny, was similar a western version of the Taconic Orogeny, and signaled the kickoff of the slow accretions of many microplate terraines to Laurentia's western margin.
During the Carboniferous, Laurentia-Baltica collided with Gondwana (a giant continent containing the makings of Africa, South America, Antarctica, Republic of india, Commonwealth of australia, and New Zealand.) The portion of Gondwana destined to get 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 upwardly in this new orogeny and extensively folded.
While this was happening, other continents, including Kazakhstan and Siberia, were colliding with Baltica and with 1 another. The result was Pangea - the last supercontinent, which contained every major landmass except the two continents destined to become North and South Communist china. Pangea was hour-glass shaped with 2 major regions:
- Laurasia in the n
- Gondwana in the south
By the Late Triassic, Pangea was beginning to rift apart forth the suture of the Trans-Pangean Mountains. Remains of rift valley lakes are preserved in the sandstones of the Newark Supergroup. By the Early on Jurassic (right), actual bounding main-floor crust was forming between Laurasia and Gondwana. This rifting marks the birth of North America, containing the onetime Laurentia at its centre; and of the Atlantic Ocean. Ironically, the high mountains of the central Trans-Pangean Mountains (the region closest to the new body of water bowl) now eroded abroad, leaving the erstwhile foothills as sorry 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 due west, an active margin was present, and Laurentia experienced a series of collisions with volcanic arcs and small-scale continents, as microplate terrains continued to accrete.
Plate tectonic processes were running at full speed every bit the Atlantic continued to lengthen and widen. During periods of vigorous plate tectonic activity, the ocean floors, being warmer than usual, actually stand up up somewhat higher topographically. This, in turn, pushes bounding main h2o onto the continents. The Cretaceous was such a fourth dimension. 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 internet effect was that bounding main 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 subconscious past thick limestones accumulated during this interval.
Elsewhere in the globe, Africa and India rifted away from Gondwana. Still, North America remained firmly attached to Laurasia in the east and west.
Laramide Orogeny: In the w, ii interesting processes were occurring:
- The west coast remained an agile margin, with a subducting plate - the Farallon Plate - feeding a volcanic-magmatic range - the Sierran arc. (Today'southward Sierras are the uplifted roots of this Mesozoic range.)
- Compression caused past 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 edifice of the Rockies is known equally the Laramide Orogeny.
Finally, during the early Cenozoic, eastern N America rifted from what nosotros now telephone call Eurasia, opening the gateway between the Altantic and Arctic oceans. In the w, the ii continents remain connected.
Starting around 40 million years ago, the western edge of Due 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 purlieus. Today's Juan de Fuca and Cocos Plates are last sad remnants of the Farallon Plate. As a result of this transformation, big 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 about 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 past numerous parallel normal faults, forming a series of parallel mount ranges and valleys.
Notation: The normal fault-bound blocks created in this way are known as:
- Horsts: The upthrown blocks, which are weathered into mount ranges.
- Grabens: Downthrown blocks buried beneath valley sediments.
A final finishing bear upon during the late Neogene was the evolution of the Isthmus of Panama, an island arc coalescing to connect N and South America within the last 5 million years. This even had momentous implications:
- Ocean circulation was drastically contradistinct when tropical waters of the Atlantic and Pacific were separated.
- The exchange of indigenous Northward and S American land animals, the Great American Interchange, resulted in the extinction of many Due south American forms.
Cardinal concepts and vocabulary:
- Continental margins
- Passive margin
- Active margin
Source: https://www.geol.umd.edu/~jmerck/geol100/lectures/25.html
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