Summary of the Geological History of West Avalonia
About a billion years ago, an island arc was generated at the edge of a subduction zone between two oceanic plates that lay offshore from the supercontinent of Rodinia. This island arc was located near the south pole and lay to the southeast of two cratons, Amazonia and West Africa, that were part of the Rodinian supercontinent. Around 750 million years ago (Ma), tectonic forces (forces relating to the structure of the earth's crust and the large-scale processes which take place within it) started the breakup of Rodinia. During this period, the "proto-Avalonian" island arc was pushed toward Amazonia and West Africa (a continental block called "West Gondwana"), along with a series of other island arcs and microcontinents (fragments of continents that formed in a different manner to cratons). Avalonia converged with the West Gondwanan margin (as well as one or more other microcontinents) around 650 Ma and remained in place for about the next 100 million years.
Figure at right shows the area of potential development of an oceanic island arc built up from juvenile crust off the southeast coast of Rodinia (dotted area). The orogenies from the earlier assembly of Rodinia also shown (wavy lines). The direction of motion of the cratons that made up Rodinia are shown with larger arrows. Source: Murphy et al. 2000.
​The figure below visualizesf the collection of microcontinents that may have been concentrated near the West Gondwanan margin. West Avalonia (part of which now underlies eastern Massachusetts--and Brookline) is shown in yellow near the bottom. The subduction zone at the front of is shown as a black line with teeth. The rift zone in back of West Avalonia is shown by a dark black line. Image source: Modified from Nance et al. 2008.
During this time Avalonia was situated between a subduction zone in the front and an active rift/shear zone behind. The subduction zone generated intense magmatic activity between about 630 and 590 Ma, resulting in the generation of significant igneous deposits within Avalonia. In the greater Boston area these include the Dedham Granite (609 Ma), the Westwood Granite (599 Ma), and the Lynn-Mattapan Volcanic Complex (602-593 Ma). This activity would have resulted in the building of large volcanoes behind the subduction zone. At this point Avalonia may have resembled Japan with its string of volcanoes dotting the island.
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The converging oceanic plate was subducting below Avalonia at a very high angle (almost horizontal), which would have exerted significant shear stress on the island arc and may have contributed to the development of a series of rift basins within the Avalonia terrane. One of these is preserved as the Boston Basin. Brookline is located within the Boston Basin. This basin may have formed around 600 Ma.
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This rift basin formed a deposition zone for sediment eroded from the high volcanoes and exposed igneous emplacements that surrounded it. These sedimentary deposits took for the form of relatively unsorted conglomerates, sandstones and mudstones. These sediments currently make up the Roxbury Conglomerates that account for much of the bedrock of the Boston Basin, including the majority of Brookline.
Around 590 Ma, it appears that the subduction trench in front of Avalonia came into contact with the spreading center that had been generating the oceanic basalt being subducted underneath Avalonia. Over about 40 million years, the tectonic environment in front of Avalonia shifted from active subduction to a more passive transform fault zone. This change started in the West Avalonian terrane and moved toward the East Avalonian terrane. There was a surge of volcanic activity at the close of subduction, and then magmatic activity ceased for over 100 million years. The front of Avalonia became a passive margin. The surge of volcanic activity is recorded in Boston-area rocks in the form of the Brighton Igneous Suite, and the quieter depositional environment of the passive margin preserved in a thick layer of mudstones called the Cambridge Formation.
Figure: General model for the late Neoproterozoic evolution of Avalonia. In (a), oblique subduction occurs during the interval c. 635–590 Ma during the main arc phase of Avalonian magmatism (see Act 3). A variety of volcanic basins are opened in the back arc basin, in part due to the torque to the oblique plate movement. In (b), the Avalonian arc has collided with the oceanic ridge, leading to transform movement. This movement opens new wrench basins. The entire arc front is coverted to transform faulting by about 540 Ma. Symbols: C = Cambrian; Pre C = Precambrian; A = away; T = towards. Image source: Nance et al. 2008.
This change of tectonic environment at the front of Avalonia was matched by different activity in the rifted and sheared region between Avalonia and West Gondwana. A new spreading center appeared in this area, which began to separate Avalonia and some of the other peri-Gondwanan terranes from the Gondwanan margin. Around 480 Ma this spreading center generated a new ocean behind Avalonia called the Rheic Ocean. Tectonic forces that created the Rheic Ocean began to move Avalonia ever so gradually away from West Gondwana. Over the next 100 million years, Avalonia completed a transit that started off the coast of West Gondwana near the south pole and would end with its colliding with Laurentia (ancient North America) and Baltica (ancient Northwestern Europe) thousands of miles away.
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Avalonia made the journey toward Laurentia and Baltica in the company of other microcontinents including Ganderia (which impacted Laurentia before Avalonia), Meguma (which collided with Avalonia after it had accreted to Laurentia and Baltica), and Carolina, which was accreted to Laurentia south of Avalonia.
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.
The end of Avalonia came in several stages between about 440 Ma and 380 Ma. The driving force sealing its fate was the continued opening of the Rheic Ocean, which was pushing close the Iapetus Ocean and moving Avalonia toward Laurentia and Baltica. At some point in its journey, a new subduction zone was created in front of Avalonia, which began to build volcanic mountains to replace those that had undoubtedly eroded to sea level during the 150 million years that had elapsed since the end of the previous active subduction. One legacy of this subduction is the Blue Hills south of Boston, which are the remains of a magmatic chamber that fed a series of volcanoes that formed around 440 Ma. Around 420 Ma, the now revitalized Avalonian island arc first impacted Laurentia and Baltica, pushing up high mountains in what is called the Acadian orogeny. This slow-mo collision lasted for almost 40 million years until about 380 Ma. In the Massachusetts area, West Avalonia was driven under another accreted terrane called Ganderia. The suture of this collision is found in Massachusetts in the Bloody Bluff Fault. Avalonia was now joined to two continental masses.
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.
From about 380 Ma to 335 Ma, the Rheic Ocean began to close, resulting in the assembly of the last supercontinent, Pangea. The assembly of Pangea brought the African portion of Gondwana into contact with Laurussia (the assembled continental blocks of Laurentia and Baltica). The collision of Africa with Laurussia (and South America into the southern part of Laurentia) marked the final assembly of Pangea and unleased a massive mountain building event on the Laurentian continent called the Alleghanian orogeny. This mountain building event lasted almost 90 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, including Avalonian terrane. The Avalonian rock that had been earlier modified in collision with Laurentia was now further modified in the crush between Laurentia and Gondwana.
After being crushed between Gondwana and Laurentia, West Avalonia was subsequently buried under miles of sediment eroded from the high mountains created by the formation of Pangea. The Narragansett Basin, which opened to the south of the Boston Basin, contains over 12,000 feet of such sediments! Then, about 180 Ma, Pangea began breaking up. Early rifting along the east coast of what is now the US was followed by the generation of what is now the Mid-Atlantic Ridge, resulting in the rifting apart of what is now Europe, North America, Africa and South America. The separation of Europe and North America pulled apart the buried microcontinent of Avalonia, with most of East Avalonia buried in different regions of northwest Europe and West Avalonia attached to North America, stretching from New England through Maritime Canada. The birth of the Atlantic Ocean also led to a new period of erosion on the eastern seaboard of North America. Over the next 180 million years, the Appalachian and other regional mountains continued to erode, and miles of rock overlying ancient Avalonia was carried by rivers south and east into the Atlantic. Finally, over the last two million years, erosion was aided by glaciers spawned during the ice age. In its last stage, miles-thick glaciers flowed over the New England landscape from about 34,000 to about 12,000 years ago. These glaciers were the last step in revealing the ancient rocks of Avalonia on the east coast, including in the Boston area and Brookline.