Act 3: Main Volcanic Arc
(630-590 Million Years Ago)
The breakup of Rodinia been accompanied by the accretion of proto-Avalonia to the margin of Amazonia and subsequently to the creation of Avalonia through continental margin volcanism. During the Rodinian breakup, a smaller supercontinent, called Gondwana, began to form. It eventually included cratons that now make up South America, Africa, India, Australia and Antarctica. West Gondwana (the West African and Amazonian cratons) now collided with other cratons of the modern African continent--East Africa and Congo craton, during the period of about 630-600 Ma, enlarging West Gondwana (an event recorded by the West Gondwana Orogen). About 610–600 Ma, major rifting occurred that separated Baltica, Laurentia and Amazonia, leading to the formation of an ancient ocean called the Iapetus Ocean. When continental plates join together, there is a joint between them called a suture. When the continents separate later under the influence of magmatic convection from the mantle, they often separate at these joints. This process of joining and separating continental plates from the expansion and contraction of ocean basins in the middle is called the Wilson cycle, and is a central tenet of plate tectonics. The rifting that gave birth to the Iaepetus Ocean occurred along sutures formed at the creation of Rodinia.
​The growth of Avalonia during this period continued due to subduction of oceanic crust at its front, but another key tectonic force also came into play: the creation of a series of back arc basins behind the main arc. In some magmatic arc environments, convection in the mantle below related to the descent of the subducting plate can causes release of additional heat behind the arc that begins to thin the crust. A new area of magma upwelling can form in this area, resulting in crustal tearing (called rifting) and the release of more dense, basaltic magma. This magma can be erupted at the surface or under water, or could be intruded into the underlying rock. Unlike the magmatism of the volcanic arc described in Act 2, where melted basalt mixes with the granitic crustal rock, this magma undergoes little intermixing with the granitic crust. With the heating, rifting and eruptions of lava, these area also subside, forming a what is called a back-arc basin; this may be partially filled with ocean waters (figure left).
Image source: Zyzzy2.
​The tectonic environment of Avalonia during this period was an active subduction zone at the front of the arc and an expanding rift zone at the rear. The continued subduction of the sea floor at the front of the arc would have provided igneous melts with a more granitic character that fed the growing volcanic mountains, while the rifts produced in the back arc area would introduce more basaltic material from below. In addition, there is evidence that the back arc basin area was affected by significant shear stress due to the fact that the descending oceanic crust was subducting with significant lateral movement relative to the Avalonian plate. This lateral movement introduced significant shear stress at both the surface and deeper down. The result of this deep shear stress contributed to the initial formation of the Burlington mylonites, a mile-thick layer of rock that stretches from Weston to Lynnfield and Danvers. Some portions of the Burlington mylonite are transected by granites deposited around 630 Ma, so the mylonite must have formed earlier. These mylonites indicate a horizontal motion consistent with this kind of lateral shear. At the surface, both the convection-induced rifting and the shear stress would have resulted in the formation of rift basins and what are called pull-apart basins that result from lateral faulting and crustal movement. The Boston Basin has been interpreted to have been created as a rift or pull-apart successor basin (Hon et al. 1987; Nance 1991; Rast and Skehan 1990) that developed in a back-arc (Galli and Bailey 1986) or intra- arc (Socci and Smith 1987) environment near or at the end of the main phase of Avalonian magmatism.
The Boston Basin is surrounded by igneous deposits that mainly preceded its formation. Source rock from these deposits can be found in the basin's mixed sedimentary rock, showing that the basin was collecting the eroding sediment from the surrounding igneous arc deposits. The long-period of subuction under Avalonia would have built a series of high volcanoes--like those on the United States Northwest coast--that would have provided an abundant source of eroded sediment. Many of the Boston Basin deposits feature conglomerates with large cobbles that suggest that the location of the deposits were not far from the source (larger clasts are not easily carried long distances). The sedimentary deposits of the southern part--including the conglomerates--are mainly turbidites with the majority showing evidence of underwater deposition. Turbidites are deposits that result from the gravity flow of sediments down a slope, typically underwater. The sediments may have been deposited upslope by streams and rivers. Instability from cumulative sediment overload on the slope, or triggered by earthquakes (which would be a recurring reality in an active back arc basin), would result in a turbid flow of the sediment downslope (think the underwater equivalent of a mudslide). You can see recreations of turbidite flows in this video. The environment of deposition for Boston Basin sediments is summarized in more detail here.
The sedimentary rock is interspersed with both erupted (volcanic) and intruded (plutonic) basaltic deposits, which appear to have occurred after turbidite deposition. These igneous deposits speak to the continued tectonic activity of the time. The argillite of the northern half of the basin likely occurred after these deposits and appear to have been made during a time when tectonic activity had ceased and Avalonia became part of what is known as a passive margin.
The hypothetical location of Avalonia at the end of the breakup of Rodinia (635-590 Ma) is shown in the Figure left. As described in Act 2, Avalonia was not the only microcontinent or terrane generated along the edge of West Gondwana--the figure on the left shows some of the others in white. The red dot shows the Brookline area of West Avalonia.The triple junction forming between Laurentia, Baltica and West Gondwana in the upper left is where the final breakup of Rodinia occurred. The subduction zone feeding Avalonian magmatism is shown by the black line; teeth show that the oceanic plate is moving under Rodinia. This figure shows the signfiicant lateral movement of the subducting plate, nearly transverse to the Amazonian margin.
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Image source: Nance et al. 2012