Act 7: Burial and Recovery
(380 Million Years Ago to Present)
Burial: A Supercontinental Event
​Historically, the last supercontinent to form was Pangea, which began to come together about 300 million years ago and was fully formed by 270 million years ago. Previously, Laurentia, Baltica and Siberia had assembled into Laurussia at the closing of the Iapetus Ocean (ca. 430-420 Ma), while the "eastern" and "western" parts of Gondwana had come together about 500 Ma. Following the closure of the Iapetus Ocean, the Rheic Ocean began closure, bringing Western Gondwana and Laurussia onto a collision course. The series of collisions between Iapetan island arcs and peri-Gondwanan microcontinents were merely appetizers compared to this collision, which unleased a massive mountain building event on the Laurentian continental plate: the Alleghanian orogeny. This mountain building event lasted almost 40 million years and affected a zone almost 2000 miles long from eastern Canada to the southern US, giving rise to both the modern Appalachian and Alleghany Mountains. When completed, the Appalachians towered to the height of the modern Himalayas. The massive forces unleashed in this collision further metamorphosed rock that had already been created or modified in earlier orogenies at the Laurentian margin. While some rock was piled high in mountains, basement rock was also driven deep into the earth, thickening the crust at the location of impact. Regional magmatism resulted in the intrusion of plutons, mainly in the southern US. The closing of the South American portion of Gondwana onto Laurentia also generated the Ouachita orogeny that gave rise to the Ouachita mountains of the southern United States. This same impact with Baltica generated the Variscan orogen. The figure below shows the journey of Avalonia from Gondwana and its final position of Avalonia in the midst of Pangea.
Paleozoic reconstructions (modified from Scotese, 1997; Cocks and Torsvik, 2002; Stampfli and Borel, 2002) at 540 Ma, 460 Ma, 370 Ma and 280 Ma showing the evolution of the Iapetus and Rheic Oceans between Gondwana and Laurentia- Baltica. A-C = Avalonia-Carolinia (from Murphy and Nance, 2008). Image source: Nance et al. 2012. Cited sources for Figure: Scotese CR. Continental Drift. 1997;7th ed. PALEOMAP Project, Arlington, Texas. 79 p. Cocks LRM, Torsvik TH. Earth geography from 500 to 400 million years ago: a faunal and palaeomagnetic review. Journal of the Geological Society 2002;159,:631-644. Stampfli GM, Borel GD. A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrones. Earth and Planetary Science Letters 2002;196, 17e33. Murphy JB, Nance RD. The Pangea conundrum. Geology 2008;36:703e706.
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​When the Avalonian terrane was pinned between these colliding supercontinents, its rock strata were modified by this impact. It may be that, at that time, portions of the Cambridge argillite, a fine mudstone which had been deposited at the passive margin of Avalonia, were thrust over the clastic turbidites generated from erosion of the island arc and now part of the Boston Basin. This juxtaposition of argillite and turbidite deposits is not visible at the surface, but can be seen in samples taken from subway and water tunnels dug beneath Boston.
Erosion of the highlands created by the multiple collisions and orogenies along the Laurentian margin buried the older deposits of Avalonia under miles of sediments shed both from volcanoes built by subduction during Avalonia's journey to Laurentia (e.g., the volcanoes of the Blue Hills) as well as from mountains that were pushed up during the collision between Avalonia and Ganderia, between Avalonia and Meguma, and finally, from the creation of the towering Appalachian mountains in the final assembly of Pangea. Since that time, the region containing the rocks of Avalonia have been largely shaped by erosion.
The erosion of the volcanoes and mountains of the region formed massive sedimentary wedges that were further eroded by rivers and streams carrying sediments toward the sea. The majority of this eroded material was carried south. The magnitude of the sediments deposited from erosion in the Avalonian region of Massachusetts can be seen from the sedimentary rocks found in the Narraganset Basin located south of Boston and stretching into Rhode Island. The Narragansett Basin was formed by rifting that occurred at the time Avalonia collided with Laurentia. Subsequently, the basin was filled with sediments from erosion of the Avalonian highlands and areas of uplift generated by the localized orogenies. The sediments in the Narragansett Basin are 12,000 feet deep!
A Long Uncovering
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Following the repeated accretion of terranes to ancient North America, capped by the formation of Pangea, subsequent events significantly altered the eastern margin of Laurentia. 80 million years after the amalgamation of Pangea, mantle convection started beneath the margins between Laurussia and Gondwana. The hot upwelling magma from this convection thinned the crust at the suface, and rift zones began to appear along this margin, accompanied by the intrusion of magma. A whole series of rift basins, reminiscent of the East African rift basin in modern Africa, formed during the early Mesozoic era, about 220 Ma. These rift basins stretch from Canada to North Carolina (figure right). One of these located in Massachusetts, the Hartford-Deerfield basin, forms the Connecticut River valley. During the Mesozoic, the basin was the site of a series of playa-like lakes frequented by dinosaurs that left footprints first discovered in the early part of the 1800s. These types of footprints can also be seen at the well-known Dinosaur Park in Connecticut. While these type of rift basins are often the precursor for formation of new ocean crust, the rifting ultimately ceased. Instead, a new rift zone appeared between Laurussia and Gondwana around 170 Ma. This rift zone gradually expanded over the next 40 million years, and by 130 Ma the modern Atlantic Ocean had formed. This ocean basin is still expanding about an inch per year. The new rift also split Laurentia from Baltica, eventually giving birth to North America and Europe. The rifting between Laurentia and Baltica split the Avalonian terrane. As a result, rocks with similar characteristics and histories are found on both sides of the Atlantic.
Image source: https://en.wikipedia.org/w/index.php?curid=22607209
As a result of the formation of the Atlantic Ocean, the eastern coast of North America became a passive continental margin. With no tectonic activity, the major geological activity along the margin since that time has been erosion. From their towering heights at formation, the Appalachians (the Berkshires in Massachusetts) have largely been reduced to rolling hills. Similarly, hundreds of millions of years of rock deposits that lay above Avalonia were stripped away by erosive forces. The final stage of the work of erosion was accomplished by glaciation that started about 30-40,000 years ago and began to retreat around 18,000 years, with the final stages of melting between 12,000 and 10,000 years ago. The glaciers generated by the so-called Laurentide ice sheet were miles deep and scoured the remaining rock like bulldozers. As these glaciers melted back, they left some of the ancient deposits of Avalonia now exposed at the surface for us to see.