Act 4: Rifting in Gondwana
(590-540 Million Years Ago)
During this period, the subduction that had fed the volcanic fore-arc of Avalonia for almost 120 million years came to an end. The Avalonian arc came into contact with the ridge of the spreading center that had been generating the basaltic crust feeding its volcanoes. Instead of sliding under Avalonia, the two plates now began to move past each other horizontally, in what is called transform faulting. This phenomenon can be seen in the southern California area, where the San Andreas fault marks the boundary of two plate margins sliding past one another. This change led to a cessation of the magmatic activity that fed Avalonia's volcanoes. However, this transform faulting caused stress on both sides of the rift and resulted in additional rifting along the transform boundary, with the formation of so-called wrench basins and the continued deposition of basaltic magma in the tears created by this rifting activity. In addition, erosion into these surrounding basins continued from the arc. The figure below illustrates the change from subduction to transform faulting.
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.
​
The evidence of the Avalonian rocks shows that contact with the spreading center and the shift from subduction to transform faulting did not occur at the same time along the fore arc. As the diagram below shows, subduction ended earlier in West Avalonia (US and Canadian portion) than in East Avalonia (European portion). Murphy et al. (1999) estimated that the transition from arc-related to wrench-related tectonic activity was occurred around 590 Ma in New England, in the interval between 610–560 Ma in southern New Brunswick, at 605 Ma in mainland Nova Scotia, between 575 Ma and 560 Ma in Cape Breton Island, at 575 Ma in Newfoundland, in the interval of 570–560 Ma in Britain and 570–540 Ma in France. After transition from subduction to transform faulting, the volume of magmatism was relatively small. However, this rifting activity did result in the extrusion of oceanic basalts, which can be seen in several locations in New England and Eastern Canada. During this time, Avalonia (along with some of the other microcontinents along the margin of Gondwana, began to move eastward away from the South American mass and toward Western Africa (figure below).
Figure: Transverse rifting southeast of Avalonia about 600-550 mya. Blue line shows the fault line and arrows show relative movement of the crustal blocks. Avalonia shown in yellow. South pole view with circles at 30 and 60 degrees south of the equator. Image adapted from: Willner et al. 2013.
Over the next 100-150 million years, the formerly active margin of Avalonia became relatively inactive. Evidence of the conversion of Avalonia from active island arc to quiescent marine platform can be found in the thick wedge of sediment that forms the Cambridge argillite. These deposits were first dated to a maximum age of 570 Ma based uranium-lead dating of zircons found in an ash bed deposited at the base of the formation and located in Somerville (Thompson and Bowring 2000). In addition, the fossil Aspidella, a Neoproterozoic organism typically found in deep water fine sediments, and which was first identified in Cambridge argillite during tunnel construction near Harvard Square, has been dated to between 610-550 Ma. Recent excellent work by Thompson and Crowley (2020) has provided a better interpretation of this deposit. Their work includes an important revision of the dating of the ash bed deposit from the previous estimate of 570 Ma to about 550 Ma, suggesting that the Cambridge Formation likely represents a shelf formation deposited following the transition of this portion of Avalonia from an island arc at the edge of an active subduction zone to one with a quiescent transform margin.
​
In this Act the active volcanism of Avalonia came to an end and tectonic changes began to position the microntinent for its next act: a trans-oceanic journey to Laurentia.