Act 2: Early Magmatic Arc
(750-660 Million Years Ago)
Breaking Up is Hard to Do
The tectonic forces in the mantle that bring plates together into supercontinents like Rodinia eventually will force them apart. About 400 million years after initial assembly, around 750 Ma, the supercontinent of Rodinia began to break up (figure right). Some scientists hypothesize that the breakup was instigated by a large mantle plume beneath the crust (Li et al. 1999). The breakup was a slow affair, taking over 150 million years to complete. The era during which Rodinia formed and began to break up is called the Proterozoic ("earlier life"), and the specific period when the early magmatic arc of Avalonia formed falls within the Neoproterozoic (1 Ga - 540 Ma). During this period, only single and multi-cellular life could be found in the oceans. There are active debates about what occurred during the breakup of Rodinia. As mentioned in Act 1, some geologists identify that another supercontinent, Pannotia, was formed around 600 Ma. Another active discussion is whether the breakup of the Rodinian supercontinent led to the development of one or more global ice ages. It seems apparent that widespread glaciation occurred at several periods during the breakup of Rodinia. The issue of debate was how widespread this was. One view, which is that ice covered virtually the whole earth, is often referred to as "snowball earth."
Figure: A reconstruction of Rodinia at 750 Ma. The initial rifting between Laurentia, East Antarctica/Australia, and South China (attributed by some to a mantle superplume resulting in a triple junction) is shown. The Avalonian arc terrane off the coast of Amazonia is shown in the lower right. Orange regions mark the orogenic belts ("Grenville orogen") related to the formation of Rodinia . Figure from Torsvik 2003.
As Rodinia rifted apart, subduction continued under the Avalonian terrane for almost 110 million years. During this period, the Avalonian island arc moved toward the continental margin of West Gondwana (cratons now part of West Africa and South America), and was progresively squeezed by the outward motion of West Gondwana to the west and the subduction of an oceanic plate on the eastern margin. There are a number of igneous deposits between 760-650 Ma in both the North American and European remnants of Avalonia that provide evidence for subduction-fueled magmatism during this time. While evidence of magmatic arc activity has not been decisively identified in Massachusetts (although the 700 Ma-old Fishbrook gneiss has been suggested as a highly-metamorphosed remnant of this volcanism (Olzewski et al.1980), a number of igneous emplacements in the Avalonian terrane in Newfoundland and Nova Scotia support this hypothesis. Equivalent rocks can be found in Wales and Britain, as well as in Europe in isolated deposits bounded by fault boundaries.
​​Eventually, Avalonia was pushed into the margin of Gondwana. The right figure shows a diagram illustrating this process. Between 670-650 Ma, there is evidence of metamorphism related to the compression of Avalonia against the Gondwanan margin. In one scenario (see for example Henderson, et al. 2016), the Avalonian terrane was accreted to the Amazonian (South American) margin. This accretion was in reality a super-slow-motion collision between the island arc crust and the continental crust. When such collisions occur, they result in deformation of some parts of the rock in the "collision zone," creating metamorphic rock due to increased heat and pressure of the collision, and also result in mountain building from compressed crust pushing upward to the sky as well as downward into the mantle. Metamorphic rock formations in the Malvern Hills in the United Kingdom (Strachan et al. 1996) may also be evidence of the orogenic activity related to this accretion event. However, this accretion appeared to have been more of a "soft dock" than the significant orogenic changes sometimes found with accretion of island arcs onto continental margins. This may be because Avalonia was not simply pushed into the Gondwanan continental edge, but rather squeezed against other terranes similar to Avalonia that also bordered West Gondwana.
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The Siblings of Avalonia
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While the formation of the larger cratons ended around 1.2 Ga, the tectonic processes that generated them have continued to create smaller terranes over the last billion years. These terranes are typically in the form of island arcs, which arise when one oceanic plate is subducted below another. Island arcs can be found in the oceans all over the world, but in some areas island arcs are more concentrated. One good example is in Southeast Asia (see figure at right), where multiple small plates are found and a number of island arcs (the Phillipines, Indonesia, Burma, Sumatra, and Java) are located near larger continental masses (China and Australia). This region may be suggestive of the tectonic environment in which Avalonia was situated at the start of the breakup of Rodinia.
Figure shows a diagram of the approach of Avalonia to the margin of Gondwana. Top: island arc is offshore of contiental margin. Both have active subduction zones. As basaltic crust descends below the margin of the other plate, a sediment wedge accumulates in the subduction trench (light green). Descending slab undergoes partial melting 100-150 miles down, with magma rising through and mixing with the crust. Magma is sometimes released through volcanoes in both the island arc and continental margin. Middle: As the island arc closes on the margin, the wedge sediments from the continental trench are pushed up and deformed. Bottom: At contact, wedge sediments and island arc sediments are pushed up into mountains and metamorphosed by pressure and heat. Some portions of the basaltic slab may also be pushed up over the sediment and altered. This is called obduction. The slab beneath continent may break off due to stress of collision and no longer feeds active volcanism along the margin. Erosion fills the valleys between the dormant volcanoes and the upthrusted wedge and ocean deposits. Subduction continues at the island arc margin.
​Image source: Modified from Nance et al. 2008.
Over the last decades, geologists have identified many "peri-Gondwanan" terranes. The list includes Bohemia (now part of eastern Europe), Cadomia (now embedded in Western Europe), Carolinia (now part of southeastern US), Chortis (now part of Mexico), Florida (the core of the Florida peninsula), Ganderia (like Avalonia distributed between New England, Maritime Canada and Western Europe), Iberia (now part of Spain), Yucatan Maya (now part of Mexico), Meguma (now part of Nova Scotia), and Oaxaquia (now part of Mexico).
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These other terranes did not all form the same way Avalonia did. Based on study of the isotopic differences within their basement rock, the peri-Gondwanan terranes can be grouped into four broad categories based on their initial environment of formation: (1) those that evolved upon juvenile crust about a billion years ago (like Avalonia—see Act 1); (2) those that evolved upon ancient continental crust of the West African craton (like Florida, see figure right); (3) those that evolved upon ancient continental crust of the Amazon craton (like Ganderia, see figure right); and (4) cratonic terranes that were part of Gondwana and not produced as magmatic arcs (like Oaxaquia see figure right). These four types suggest the diversity of tectonic activity that took place off the Gondwanan margin for almost 400 million years. Complex plate motions between the major continental cratons and these micrcontinents may have resulted in their concentration at the edge of Western Gondwana. The figure on the right shows a reconstruction with the approximate location of many of these terranes at the final breakup of Rodinia, between 640-590 Ma. The active subduction zone off the margin of Gondwana is shown as a black line with teeth. The generation of the triple junction spreading center between Laurentia, Baltica, and West Gondwana is also shown.
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By about 650 Ma, Avalonia was fully docked along the Gondwanan margin. In the geologic record for the terrane, there appears to have been about a 15 million-year pause in volcanism, which then gave way to the next Act in Avalonia's development.