McConnell et al. (2015) claimed that the absence of Middle Ordovician back-arc volcanic rocks in the Bellewstown terrane in the Irish Caledonides implies that the Tetagouche–Exploits back-arc basin never existed in Ireland. The emphasis on the absence of evidence surprised us, because ‘absence of evidence is not evidence of absence’. However, if they are correct, their interpretation has major implications for the tectonic processes responsible for the final closure of Iapetus.

The authors agree with us (van Staal et al. 1996, 1998) and others (Waldron et al. 2014) in assigning the Leinster and Bellewstown terranes to the Gander margin of Ganderia, although they disagree with a back-arc tectonic setting of the Bellewstown volcanic rocks (Winchester & van Staal 1995) and interpreted them instead as part of a Floian volcanic arc.

Setting of the Gander Zone and its Exploits subzone cover in the northern Appalachians

The northern Appalachian Gander margin comprises two pre-late Silurian sequences of predominantly sedimentary rocks. The Cambrian–Tremadocian sequence (e.g. Gander and Cookson groups) (O'Neill 1991; Fyffe et al. 2009) belongs to the Gander Zone, whereas the post-Tremadocian sequence (e.g. Davidsville & Bay d'Espoir Groups, Fig. 1) belongs to the Exploits subzone of the Dunnage Zone (Colman-Sadd 1980; Williams et al. 1988). The latter Exploits subzone sedimentary sequence overlies Gander Zone rocks in Newfoundland and New Brunswick either unconformably or conformably (Wonderley & Neuman 1984; van Staal 1987; Williams & Piasecki 1990; Colman-Sadd et al. 1992a). Unconformable relationships exist where the Gander Group was overthrust by Cambrian back-arc ophiolite (Fig. 1) during the Early Ordovician Penobscot orogeny (Colman-Sadd et al. 1992a; Zagorevski et al. 2010), whereas conformability may exist east of the Penobscot thrust front (GRUB in Fig. 1). O'Neill (1991) placed the Indian Bay Big Pond Formation (Fig. 1), comprising rocks typical of the Exploits subzone and containing late Floian–Dapingian Celtic fossils closely resembling those in the Summerford and Davidsville groups (Wonderley & Neuman 1984; Boyce et al. 1988), conformably above the Gander Group.

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Fig. 1.

Geology of Newfoundland Appalachians, modified from van Staal et al. (2014). Geological features discussed in text are indicated. The Floian–Katian, sediment-dominated Davidsville and Bay d'Espoir Groups form part of the passive margin of the Tetagouche–Exploits back-arc basin. GRUB, Gander River Ultrabasic Belt; PF, Pine Falls Formation; RIL, Red Indian Line. Salinic and Acadian in legend refer to the orogenic cycles associated with the accretion of the Gander margin of Ganderia and Avalonia respectively (van Staal et al. 2014). MN, Meelpaeg Nappe; OBSZ, Otter Brook shear zone; PAV, Port Albert volcanic unit; PF, Pine Falls Formation; RF, Reach Fault; SL, Stony Lake volcanic unit; RIL, Red Indian Line; SLD, Star Lake Dam; VLSZ, Victoria Lake shear Zone; BVBL, Baie Verte Brompton Line; CB, Cape Brule porphyry; CF, Cabot Fault; CLV, Charles Lake volcanic unit; CRF, Cape Ray Fault; DBL, Dog Bay Line; DC, Duder Complex; DF, Dover Fault; GBF, Green Bay Fault; GHF, Gunflap Hills Fault; GRUB, Gander River Ultrabasic Belt; HBF, Hermitage Bay Fault, HMT, Hungry Mountain thrust; HR, Harry's River; IIG, Indian Islands Group; LB, Loon Bay pluton; LCF, Lobster Cove Fault; LPG, La Poile Group; LRF, Lloyds River Fault; LV, Lawrenceton volcanic unit.

Penobscot ophiolite obduction was rapidly followed by formation of the Popelogan–Victoria arc on the Gander margin, which rifted away in Late Floian to Late Dapingian time to open the Tetagouche–Exploits back-arc basin (van Staal et al. 1998, 2016; Valverde-Vaquero et al. 2006) with resumption of sedimentation (Colman-Sadd 1980) on the passive side of the Tetagouche–Exploits back-arc basin.

Is there conclusive evidence that the Tetagouche–Exploits back-arc basin did not continue into Ireland?

McConnell et al. (2015) argued that the Tetagouche–Exploits back-arc basin did not continue into Ireland mainly because (1) no equivalent Middle Ordovician members of the Tetagouche–Exploits back-arc basin and rifted Popelogan–Victoria arc have been identified in Ireland and (2) no Gondwana-derived detrital zircons are recorded from the Southern Uplands of Scotland (Waldron et al. 2014).

We consider these two claims inconclusive for the following reasons.

1. During the c. 455 Ma arc–arc collision of the Popelogan–Victoria arc with the Red Indian Lake arc of composite Laurentia (Fig. 1), which closed the main tract of Iapetus in the Appalachians, the Popelogan–Victoria arc was on the lower plate (van Staal et al. 1998) and as a result was buried beneath a foreland basin filled by Upper Ordovician black shale and greywacke (e.g. Badger Group) (Zagorevski et al. 2008). The foreland basin became part of a forearc terrane when the trailing Tetagouche–Exploits back-arc basin started to close at c. 450 Ma by west-directed subduction beneath composite Laurentia. The degree of underthrusting of the Popelogan–Victoria arc beneath Laurentia varies, such that locally (Victoria arc in southern Newfoundland) it is not exposed (Fig. 1). The absence of the Popelogan–Victoria arc in Ireland is thus not an argument against its former existence. In fact, the Floian arc volcanic rocks in Bellewstown could represent the remnant arc left behind after intra-arc rifting led to back-arc spreading moving the active part of the arc away from the Irish Gander margin. Small volumes of Floian–Dapingian calc-alkaline rocks (mainly felsic), interpreted as remnants of the Popelogan–Victoria arc after the active part drifted away, occur on the Gander margin in Newfoundland and New Brunswick (e.g. Valverde-Vaquero et al. 2000, 2006; Fyffe et al. 2009, 2011).

2. Neither is the lack of preserved Tetagouche–Exploits back-arc basin rocks in Ireland proof of absence, because most back-arc lithosphere was probably subducted and/or underplated beneath the southern Uplands terrane during back-arc closure. Poor lithological correlation between the Bellewstown rocks and Exploits Group back-arc rocks referred to by McConnell et al. (2015) is irrelevant, because the latter formed on the active, not the sediment-dominated passive side of the Tetagouche–Exploits back-arc basin (van Staal 1994; Zagorevski et al. 2010). Also, remnants of Tetagouche–Exploits back-arc basin oceanic crust (e.g. Pine Falls Formation, Fig. 1) occur only sporadically in the northern Appalachians (Winchester et al. 1992; Valverde-Vaquero et al. 2006). Hence, we see no conclusive reason why the late Floian–Darriwilian sedimentary and volcanic rocks of the Leinster and Bellewstown terranes in Ireland could not represent remnants of a passive margin and remnant arc and hence form part of a potential Tetagouche–Exploits back-arc basin and Popelogan–Victoria arc system equivalent in Ireland. If so, remnants of the Tetagouche–Exploits back-arc basin and migrated Popelogan–Victoria arc may be buried beneath the Southern Uplands terrane. McConnell et al. (2015) rejected this option because of a lack of Gondwanan-derived detrital zircons in the Southern Uplands sedimentary rocks. However, Waldron et al. (2012) detected no Gondwana-derived zircons in its probable equivalent in Newfoundland, the Upper Ordovician–Lower Silurian Badger Group (e.g. Colman-Sadd et al. 1992b), which demonstrably overlies the various elements of the Popelogan–Victoria arc and remnants of the Tetagouche–Exploits back-arc basin.

Tectonic evolution of the Gander margin in the northern Appalachians and Irish Caledonides.

There are remarkable similarities in tectonic evolution between the Irish and North American Gander margins. Both were subjected to an Early Ordovician orogenic event: Penobscot in the Appalachians and Monian in the British Isles (Colman-Sadd et al. 1992a; Tietzsch-Tyler 1996), and both accreted nearly coevally to composite Laurentia during Silurian closure of the Iapetus Ocean sensu lato, while situated on the lower plate (van Staal et al. 1998, 2008; McConnell et al. 2015). Hence, considering the upper plate setting with a south-dipping Benioff zone demanded by the Floian–Katian Leinster–Bellewstown volcanic rocks in Ireland (McConnell 2000; McConnell et al. 2015), the questions we pose are as follows.

1. Why and what tectonic mechanism caused the north-facing arc systems to shut off and caused the change in subduction polarity from south- to solely north-dipping after the Katian to explain Silurian accretion of Ganderia to the Southern Uplands terrane?

2. What caused Monian orogenesis and how was north-facing Ordovician suprasubduction-zone magmatism subsequently established on Irish Ganderia?

No tectonic mechanisms have been provided by McConnell et al. (2015) to explain why and how these tectonic processes took place, other than calling upon non-cylindricity in the Iapetus Ocean.

Events equivalent to the evolution of the Penobscot and Popelogan–Victoria arc and Tetagouche–Exploits back-arc basin suprasubduction-zone systems in the Irish Caledonides could provide a solution to these problems, even if supporting field evidence is poorly preserved or lacking. If so, modifications and/or additional tectonic processes are needed to explain the Middle to Late Ordovician phase of the Leinster arc–back-arc magmatism. Potentially correlative magmatism occurs sporadically on the Newfoundland Gander margin (Valverde-Vaquero et al. 2006).

A non-cylindrical Late Ordovician–Silurian closure of Iapetus between the Newfoundland and Irish sectors is possible, but regardless of how this was accomplished tectonically, the nearly coeval shut-off of north-facing arc-magmatism and subsequent transfer of Ganderia from an upper to a lower plate setting in both the northern Appalachians and Irish Caledonides indicate causative tectonic linkage between these two segments. A major reduction in back-arc extension towards the NE, such that the Popelogan–Victoria arc continued into a north-facing Leinster–Lake District arc–back-arc system (McConnell et al. 2015), could explain the lack of an exposed oceanic Tetagouche–Exploits back-arc basin equivalent in Ireland and Britain. However, the lifespan (476–457 Ma) and uninterrupted nature of the Popelogan–Victoria arc and Tetagouche–Exploits back-arc basin magmatism differs significantly from the more episodic Leinster–Lake District arc–back-arc system with its Late Ordovician climax. Nevertheless, the two arc–back-arc systems terminated nearly coevally, roughly coincident with Sandbian–Katian accretion of the Popelogan–Victoria arc to composite Laurentia. Final closure of Iapetus was accomplished by north-dipping subduction in both sectors, which was accommodated by subduction of Tetagouche–Exploits back-arc basin oceanic lithosphere in the northern Appalachians and normal Iapetus lithosphere in the Irish Caledonides. So, why did south-dipping subduction also terminate in the Katian in the Caledonides?

Absence of critical elements in some segments may be important, but does not imply that tectonic processes between these segments were fundamentally different. The observable geological record is patchy in any ancient mountain belt. Nevertheless, we concede that matching the tectonic causes of the Leinster–Lake District arc–back-arc-like magmatism with those responsible for the Appalachian Popelogan–Victoria arc and Tetagouche–Exploits back-arc basin system is not straightforward. A diachronous ridge–trench collision along the length of the southern margin of Iapetus in the northern Appalachian (Zagorevski et al. 2012) and Irish–British Caledonian segment could explain the Late Ordovician shutdown of the Popelogan–Victoria arc and the Leinster–Lake District arc–back-arc system, and termination of south-dipping subduction before their accretion to Laurentia.