NORTH AMERICA/Northern Appalachians 87 Figure Tectonostratigraphic relationships pertinent to the closure of the main tract of the Iapetus Ocean, the Tetagouche Exploits back arc basin, and the oceanic crust that separated Ganderia from Avalonia and Avalonia from Meguma The Red Indian Line marks the site of the collision between the Notre Dame and Popelogan Victoria arcs The line patterns of the major structures correspond to those in Figure The arrows on the faults indicate the polarity of subduction and/or thrusting Obliquity of collision and subsequent strike slip fault reactivation is indicated by the symbols on each side of the major faults: a point represents moving towards and a cross moving away from the viewer BBCL, Baie Verte Brompton Camerons Line BVOT, Baie Verte Oceanic Tract LP, La Poile Belt volcanic rocks; M, Mascarene belt; PVA, Popelogan Victoria Arc; TEB, Tetagouche Exploits back arc basin others have close spatial associations with felsic-arc plutonic rocks that are at least as old as 565 Ma (Figure 4) The latter probably represent ensialic basement and suggest that at least some of these rocks are not truly oceanic Coeval and lithologically similar Cambrian arc or back-arc volcanic rocks and associated Neoproterozoic plutonic rocks also occur in the Gander Zone (see below), particularly in southern New Brunswick (e.g Annidale and New River Belts) and adjacent Maine (Ellsworth Belt) (Figure 1) Collectively, these rocks have been interpreted as an arc–back-arc complex, the Penobscot Complex, formed near the leading edge of the Gondwanan margin on which the arenites and shales of the Gander Zone (see below) were deposited Stitching plutons and isotopic evidence show that the Cambrian oceanic rocks of the Penobscot Complex were obducted onto the continental-margin rocks of the Gander Zone before 474 Ma, during the Penobscot Orogeny (Figure 4) The composite crust of the Penobscot Complex and the Gander Zone sedimentary rocks are disconformably overlain by younger rocks related to the Arenig–Llanvirn ensialic Popelogan–Victoria Arc/ Tetagouche–Exploits back-arc system, which was active between 478 and 454 Ma The Popelogan– Victoria Arc/Tetagouche–Exploits back-arc system has been traced from Newfoundland through central New Brunswick into Maine, mainly on the basis of a remarkably consistent and distinct lithological association of coeval plutonic, volcanic, and sedimentary rocks Correlations further to the south into Massachusetts become progressively more difficult, owing to a lack of age control and the presence of substantial cover sequences The Popelogan– Victoria Arc is probably continuous with the Bronson Hill Arc, whereas most of the products of the Tetagouche–Exploits back-arc have been buried beneath the Siluro-Devonian cover sequences of the Central Maine and Merrimack Belts (Figure 1) The Popelogan–Victoria Arc had migrated to a mid-Iapetus position by at least the late Llanvirn (around 462 Ma), according to faunal and sparse palaeomagnetic evidence, while the passive-margin side of the Tetagouche–Exploits back-arc, which was deposited on Gander-Zone rocks, was positioned further south owing to subduction-zone retreat and concurrent back-arc spreading Rifting and seafloor spreading in the Tetagouche–Exploits back-arc is necessary to account for the formation of Llanvirn back-arc oceanic crust (464–459 Ma), which is locally preserved as incomplete ophiolite complexes and large structural slices of highly tectonized back-arc oceanic basalt and gabbro in me´ lange belts in New Brunswick and Newfoundland Fossil and palaeomagnetic evidence suggest that the Tetagouche–Exploits backarc nowhere achieved a width of more than about 1000 km (largely based on the assumption that the pelagic larvae of animals such as brachiopods could traverse oceans up to this width, but no further) Gander Zone The Gander Zone (Figures and 4) is defined by a distinct monotonous sequence of Lower Cambrian to