Act 5: Adrift Between Continents
(540-420 Million Years Ago)
The Birth and Death of Oceans
​Avalonia was born off the edge of West Gondwana--a continent formed by what is now South America and West Africa. So how did Avalonia end up attached to Laurentia (the core of what is now North America)? Part of the answer has to do with the birth and death of not one but two oceans!
Toward the end of the long breakup of Rodinia (which lasted well over 150 million years), the continental masses of Laurentia/North America, Baltica/Europe and Western Gondwana began to separate from each other at a so-called "triple junction." At this triple junction, a particularly focused region of mantle convection ("mantle plume") resulted in a strong upwelling of melted crust that started to push the three continental plates away from each other (Figure below). New basaltic crust was formed between the three continental masses, forming the floor of a new ocean called the Iapetus Ocean.
Figure: Diagram about the formation of Iapetus Ocean, 600-530 Ma. Figure at right shows formation of triple junction spreading center between Laurentia, Baltica and Western Gondawana (in yellow). Over following 70 million years, the Iapetus Ocean continues to widen the space between Laurentia, Baltica and Gondwana. Source: Oriolo et al. 2017.
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The creation and expansion of the Iapetus Ocean started at roughly the same time as the conversion of subduction to transform faulting in the Avalonian terrane (circa 590 Ma). About 80 million years later a new spreading center formed between some of the magmatic arc complexes along the Gondwanan continental margin and the main Gondwanan continent. This gave rise to another ocean called the Rheic Ocean.The initiation of the Rheic Ocean between Avalonia and Gondwana could have been related to mantle upwelling that occurred after what is called slab breakoff. This phenomenon occurs when the subducting crustal slab separates from the surficial slab (often continental crust). Here the slab breakoff was simply what happened after the subduction of the final portion of the plate next to the tectonic ridge that came into contact with the Avalonian arc (see middle Figure below). This would have opened a space between mantle and crust where upwelling could occur that induced crustal spreading and rifting. As the mantle had already been feeding an oceanic ridge prior to its contact with Avalonia, it is possible that this existing area of mantle upwelling generated a new spreading center in the area of the region between Avalonia and Gondwana. As was discussed in Act 3, this was an area of extensive horizontal shear and would have been more easily penetrated by hot magma from below compared to the surrounding crust.
The figure below shows a cross section of the margin of Gondwana showing how Avalonia went from an active magmatic arc along the continental margin to a "microcontinent adrift" between continents. Over time, the Rheic Ocean expanded while the Iapetus Ocean was destroyed. The expansion of the Rheic Ocean behind Avalonia and closing of the Iapetus Ocean in front of Avalonia moved the microcontinent from the margin of Gondwana to the margin in Laurentia. This journey across oceans lasted roughly 100 million years!
Figure: Change of Avalonia's tectonic environment from active subduction at the margin of Gondwana (left) to transform faulting (middle) to passive margins at the opening of the Rheic Ocean (right). Figure from: Pollock et al. 2009.
Fellow Travelers
As the Rheic Ocean expanded, it allowed Avalonia to make a long, slow journey from the edge of one continent to another. However, it did not make this trip alone. A number of other "microcontinents" had been birthed along the coast of Western Gondwana since the time of Rodinia, and some of these also drifted away from Gondwana with the opening of the Rheic Ocean. While the Iapetus Ocean had rifted into existence in front of the wedge of terrane of the edge of West Gondwana, the later opening of the Rheic Ocean appeared to occur closer to the margin of the original Gondwanan plate (the estimated location of this new spreading ridge is shown by the black line in the figure here). As the Rheic Ocean expanded, it progressively cleaved these microcontinents from the Gondwanan continent. This is a rather simplistic picture. In fact, the rift zone formed likely consisted of a number of rifts. As a result, the rifting and faulting in these peri-Gondwanan terranes was not uniform, and the rifted terranes did not all move in synch. There is evidence that Avalonia remained geographically proximal to Ganderia and Carolinia, yet while Meguma is also shown as being adjacent to Avalonia in the period of main magmatic arc activity, it also likely moved away from Avalonia at some point, only to come back into contact at the time of its accretion to ancient North America. During their journey, it is also possible that these microcontinents made contact with Baltica, which was also moving toward Laurentia.
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Iapetus Island Arcs
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As the Rheic Ocean expanded, compressional stresses this expansion exerted on the closing Iapetus ocean basin created additional subduction zones, giving rise to additional island arcs between Avalonia and the Laurentian margin that is now the east coast of the US. These arcs have also left their imprint on Massachusetts and the east coast. The location of and the manner in which these arcs were generated is not yet clear; some of these may have been generated in a near-margin environment off the shore of Laurentia. The closing of the Iapetus led to the accretion of these arcs onto the Laurentian margin ahead of the impact of Avalonia. In the rocks of Western Massachusetts a number of distinct arc deposits have been identified : the Bronson Hill arc, the Shelburn Falls-Ammanoosuc arc, and most recently the Moretown arc. Igneous deposits that represent the formation of these arcs are dated for the Moretown at 500-475 Ma (Karabinos et al. 2017), for the Shelburne Falls-Ammanoosuc at 485-470 Ma (Karabinos et al. 1998), and for the Bronson Hill around 454-442 Ma (Tucker and Robinson 1990). The identity of the Bronson Hill arc is actively debated and has been viewed as an island arc (Moench and Aleinikoff 2002), as a portion of "failed slab" from subduction (Hildebrand and Whalen 2020), and as an arc at the margin of Ganderia (van Staal et al. 2012).
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As Avalonia approached Laurentia, the margin in front of Avalonia, which had been quiescent since its separation from Gondwana, was converted to an active subduction zone and magmatic activity started again. By this time Avalonia had undergone erosion for for over 100 million years and most of it may have been eroded down to sea level. Now however, active subduction led to new magmatic activity, leading to the building of very large volcanoes. The "root" of one of these can be seen in the Blue Hills south of Boston. Its granite pluton has been dated at about 440-450 Ma, far into the journey toward Laurentia, but not yet at the end of it. This volcanism would have generated large amounts of rock, which would have buried the older Neoproterozoic rocks deposited before Avalonia's rifting away from Gondwana.
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The exact picture of what transpired during Avalonia's transit to Laurentia is not clear, and recently new models of the tectonic activity around Avalonia and some of the other peri-Gondwanan terranes in the Rheic Ocean have been proposed to explain the data gathered so far by geologists (Domeier 2016; Waldron et al. 2018). In the years to come this picture may be further clarified.
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While simplified, the figure below shows the progression of Avalonia, Ganderia and Carolinia across the closing Iapetus Ocean to the margin of Laurentia. What followed next was a series of collisions that significantly shaped the North American continent, completed the assembly of Massachusetts and brought Avalonia to its final resting spot.
Figure: Diagrams of the journey of Avalonia from Gondwana to Laurentia from 510-440 Ma Here Ganderia is shown in dark blue and the Avalonian terrane is shown in red. a: Around 510 Ma, the Avalonian terrane is at the margin of Amazonia in Western Gondwana. The Iapetus Ocean has opened between Gondwana, Laurentia and Baltica. b: Around 480 Ma, The Rheic Ocean opens behind Avalonia, separating it from Gondwana, while a subduction zone forms at the margin of Laurentia where Iapetus oceanic crust will be consumed. c: At 440 Ma, Avalonia approaches Laurentia and may have contacted Baltica, which is also moving toward Laurentia. The continental plate of Siberia contacts Laurentia. Images from: Shellnutt et al. 2019.
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