Act 6: Accretion to Laurussia
(420-380 Million Years Ago)
​A Series of Unfortunate Events
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As the Iapetus Ocean closed, pushed in part by the expansion of the Rheic Ocean, the island arcs created off the margin of Laurentia and the peri-Gondwanan terranes of Avalonia, Ganderia and Carolina closed in on Laurentia. During their approach the oceanic crust between Laurentia and the island arcs was being subducted below the Laurentian margin as well as beneath the oceanic arcs and microcontinents. But when oceanic crust between the continental masses and island arcs runs out, island arcs do not easily subduct beneath continents. They are both more massive and less dense due to the presence of more granitic crust and the wedges of sediment deposited from erosion. As a result, these collisions fuse the smaller terranes onto the larger, resulting in an orogeny, whose magnitude in part depends on the relative size of the colliding masses.
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The closure of the Iapetus did not result in simply the collision of Avalonia and Laurentia. It was instead a series of collisions that took place over a period of almost 60 million years during which mutliple island arcs were plastered onto the Laurentian margin followed by the impact of the peri-Gondwanan terranes. This series of collisions added hundreds of miles of new crust to the edge of Laurentia. At the beginning of this series of unfortunate events, the continental margin was along the border of Massachusetts and New York. By the end, it extended out to the continental shelf off the current coast of Massachusetts. During this same period, Baltica made contact with Laurentia along the area of present Greenland, instigating yet another orogeny.
It is hard for us to imagine the scale and duration of this series of collisions. The rate of movement between the crustal blocks was slow--just a matter of a few inches a year, but the weight of these were massive, with even the smaller microcontinents being trillions of tons or more. The incredible energy of this series of collisions was released over almost 40 million years. Rock and sediments from both sides were crushed together and folded. Some of the crust was pushed down and melted and deformed under great pressures and, in some cases, high temperature. Some of the crust melted deep beneath the earth and new igneous deposits were formed from magma rising into the collision zone. In addition, the collisions created a mountain chains (orogenies) as some of the rock was pushed up. However, as with a collision between a bus and a compact car, the smaller object is usually the worse for wear. Not being as dense as the solid granite of the continent and not as massive, the series of island arcs and micrcontinents were plastered along the continental edge and fused to it. Subsequent collisions further compressed and modified the terranes in front. The result of this activity was a series of bands of rock of the original terranes distributed along the Laurentian margin. The Figure below shows the location of these accreted terrranes in New England. The original continental margin is shown in blues and pinks (pink regions show the roots of Grenville orogeny from the initial assembly of Rodinia over a billion years ago). The green areas are the oceanic arcs accreted to the margin, the red areas the portions of Ganderia, and the butterscotch color the Avalonian terrane. The table at bottom right shows the approximate timing for the collisional events/orogenies. The accretion of Avalonia started around 420 Ma.
Following its impact, yet another peri-Gondwanan terrane, Meguma, followed. Some investigators consider the impact of the two terranes to be part of the Acadian orogeny, but others identify a separate ("Neo-Acadian") orogeny due to Meguman accretion. Meguma rocks are found in Nova Scotia but not in New England. Some scientists have hypothesized that the suture between the Avalonian and Meguman terranes is marked by the Nauset magnetic anomaly running on a NE-SW axis and passing just southeast of Harwich on Cape Cod, but there is no definite evidence as of yet. Because all bedrock on Cape Cod is buried under a thick layer of glacial deposits, the evidence for Meguma’s presence next to Avalonian terrane in the state is based on data from limited drill cores. Some investigators posit that Meguma and Avalon had been in contact before Avalonia was separated from West Gondwana, separated during the transit to Laurentia, and then re-accreted on the Laurentian margin (Shellnut et al. 2019).
Figure used under Creative Commons Attribution-Non-Commercial 3.0 Unported
Avalonia Comes to Rest
Around 420 Ma the Avalonian terrane made contact with the Laurentian margin. By that time, Laurentia and Baltica had also joined together, along with the Siberian craton to form a supercontinent called Laurussia. Avalonia collided at the juncture of Laurentia and Baltica. The majority of West Avalonia was driven into the Laurentian craton (mainly in what is now the Northeast US and Maritime Canada), while East Avalonia was largely attached to Baltica. The suture between Avalon and previous Gondwanan terranes can be found at the Clinton—Newbury and Bloody Bluff faults. During this time, the microcontinent of Carolina, trailing to the south of West Avalonia, collided further down the east coast of Laurentia. Carolina makes up portions of the eastern seaboard from Georgia to Virginia. The Figure below shows the orientation of these microcontinents just prior to contact with Laurussia. ​ The location of New England in the island chain is shown by the red dot. The heavy black lines trace Tornquist suture zone, where the join between Avalonia and Baltica can be seen in the bedrock.
Early Silurian reconstruction of the Rheic Ocean immediately prior to the closure of Iapetus by way of subduction beneath Laurentia (toothed red line). Stippled areas denote inferred regions of thinned and/or anomalous thickness of continental and arc crust with Cadomia placed adjacent to Gondwana. Rheic ridge-transform system purely schematic. Heavy black lines trace Tornquist suture zone. Image adapted from: Nance et al. 2010.
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With this event, Avalonia became a lost microcontinent, merged into the margin of the ancient North American continent. However, it was yet to be further modified through another "big bang" collision event and then buried under the sediments generated by that event for almost 250 million years.