Middle Cambrian to Ordovician Arc - Back-arc Development on the Leading Edge of Ganderia, Newfoundland Appalachians
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Abstract This paper presents new ideas on the Early Palaeozoic geography and tectonic history of the Iapetus Ocean involved in the formation of the northern Appalachian-British Caledonide Orogen. Based on an extensive compilation of data along the length of the orogen, particularly using well-preserved relationships in Newfoundland as a template, we show that this orogen may have experienced a very complicated tectonic evolution that resembles parts of the present west and southwest Pacific Ocean in its tectonic complexities. Closure of the west and southwest Pacific Ocean by forward modelling of the oblique collision between Australia and Asia shows that transpressional flattening and non-coaxial strain during terminal collision may impose a deceptively simple linearity and zonation to the resultant orogen and, hence, may produce a linear orogen like the Appalachian-Caledonian Belt. Oceanic elements may preserve along-strike coherency for up to several thousands of kilometres, but excision and strike-slip duplication, as a result of oblique convergence and terminal collisional processes, is expected to obscure elucidation of the intricacies of their accretion and collisional processes. Applying these lessons to the northern Appalachian-Caledonian belt, we rely principally on critical relationships preserved in different parts of the orogen to constrain tectonic models of kinematically-related rock assemblages. The rift-drift transition, and opening of the Iapetus Ocean took place between c. 590–550 Ma. Opening of Iapetus was temporally and spatially related to final closure of the Brazilide Ocean and amalgamation of Gondwanaland. During the Early Ordovician, the Laurentian margin experienced obduction of young, supra-subduction-zone oceanic lithosphere along the length of the northern Appalachian-British Caledonian Belt. Remnants of this lithosphere are best preserved in western Newfoundland and are referred to as the Baie Verte Oceanic Tract. Convergence between Laurentia and the Baie Verte Oceanic Tract was probably dextrally oblique. Slab break-off and a subsequent subduction polarity reversal produced a continental magmatic arc, the Notre Dame Arc, on the edge of the composite Laurentian margin. The Notre Dame Arc was mainly active during the late Tremadoc-Caradoc interval and was flanked by a southeast- or south-facing accretionary complex, the Annieopsquotch Accretionary Tract. Southerly drift of Laurentia to intermediate latitudes of c. 20–25°S was associated with the compressive (Andean) nature of the arc and the accompanying backthrusting of the already-accreted Baie Verte Oceanic Tract further onto the Laurentian foreland. Equivalents of the Notre Dame Arc and its forearc elements in the British Isles have been preserved as independent slices in the Midland Valley and possibly the Northern Belt of the Southern Uplands. During the late Tremadoc ( c. 485 Ma), the passive margin on the eastern side of Iapetus also experienced obduction of primitive oceanic arc lithosphere. This arc is referred to as the Penobscot Arc. The eastern passive margin was built upon a Gondwanan fragment (Ganderia) that rifted off Amazonia during the Early Ordovician and probably travelled together with the Avalonian terranes as one microcontinent. The departure of Ganderia and Avalonia from Gondwana opened the Rheic Ocean. Equivalents of the Penobscot Arc may be preserved in New Brunswick and Maine, Leinster in eastern Ireland, and Anglesey in Wales. An arc-polarity reversal along the Ganderian margin after the soft Penobscot collision produced a new arc: the west-facing Popelogan-Victoria Arc, which probably formed a continuous arc system with the Bronson Hill Arc in New England. The Popelogan-Victoria Arc transgressed from a continental to an oceanic substrate from southern to northeastern Newfoundland. Rapid roll-back rifted the Popelogan-Victoria Arc away from Ganderia during the late Arenig ( c. 473 Ma) and opened a wide back-arc basin; the Tetagouche-Exploits back-arc basin. The Popelogan-Victoria Arc was accreted sinistrally oblique to the Notre Dame Arc and, by implication, Laurentia during the Late Ordovician. After accretion, the northwestward-dipping subduction zone stepped eastwards into the Tetagouche-Exploits back-arc basin. Equivalents of the Popelogan-Victoria Arc in the British Isles may be preserved as small remnants in the Longford Down Inlier in Ireland. The Longford Down Arc is not preserved in Scotland, although its presence has been inferred there on the tenuous basis of arc detritus. The suture between the Notre Dame Arc and the Popelogan-Victoria-Longford Down Arc system is the Red Indian Line in the Northern Appalachians, but in the British Isles the position is not clear. The fault-bounded Grangegeeth Arc terrane in eastern Ireland, immediately to the south of the Longford Down inlier, may be a displaced piece of the Popelogan-Victoria-Longford Down Arc system. Diachronous closure of the Tetagouche-Exploits basin during the Ashgill to the Wenlock finally caused the collision between Ganderia/Avalonia and Laurentia, whereas the Lake District Arc is related to an earlier closure of the Tornquist Sea between Baltica and Avalonia. After arrival of Avalonia at the Laurentian margin, continuous, dextral oblique convergence between Gondwana and Laurentia was accommodated by another northwest-dipping subduction zone, this time in the Rheic Ocean. The Acadian orogeny in both North America and the British Isles occurred in the Early to Mid-Devonian and is probably related to the collision of Gondwana and/or peri-Gondwanan elements (Meguma, Armorica etc.) with the northern continents.
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The Exploits Subzone of the Newfoundland Appalachians comprises remnants of Cambro-Ordovician peri-Gondwanan arc and back-arc complexes that formed within the Iapetus Ocean. The Exploits Subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the Iapetus tract: the Penobscot orogeny (c. 480 Ma), which juxtaposed the Penobscot Arc (c. 513 --486 Ma) with the Gander margin, and c. 450 Ma collision of the Victoria Arc (c. 473 --454 Ma) with the Annieopsquotch Accretionary Tract that juxtaposed the peri-Laurentian and peri-Gondwanan elements along the Red Indian Line. The newly recognized Pats Pond Group forms a temporal equivalent to other Lower Ordovician intra-oceanic complexes of the Penobscot Arc. The Pats Pond Group (c. 487 Ma) has a geochemical stratigraphy that is consistent with rifting of a volcanic arc. An ensialic setting is indicated by low εNd values (εNd 0.3 to -0.5) near the stratigraphic base and its abundant zircon inheritance (c. 560 Ma and 0.9 --1.2 Ga). The spatial distribution of Tremadocian arc --back-arc complexes indicates that the Penobscot arc is best explained in terms of a single east-dipping subduction zone. This model is favored over west dipping models, in that it explains the distribution of the Penobscot arc elements, continental arc magmatism, and the obduction of back-arc Penobscot ophiolites without requiring subduction of the Gander margin or subduction reversal. The newly recognized Wigwam Brook Group (c. 454 Ma) disconformably overlies the Pats Pond Group and records the youngest known phase of ensialic arc volcanism (εNd --4.1) in the Victoria Arc, which is also related to east-dipping subduction. Thus the Penobscot and the overlying Victoria Arc are reinterpreted in terms of a single, relatively long-lived east-dipping subduction zone beneath the peri-Gondwanan microcontinent of Ganderia. The cessation of arc volcanism towards the top of the Wigwam Brook Group and the subsequent syn-tectonic sedimentation in the Badger Group constrain the arrival of the leading edge of Ganderia with the ensialic arc complexes to the Laurentian margin to c. 454 Ma.
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The Proterozoic and Phanerozoic metallogenic and tectonic evolution of the Russian Far East, Alaska, and the Canadian Cordillera is recorded in the cratons, craton margins, and orogenic collages of the Circum-North Pacific mountain belts that separate the North Pacific from the eastern North Asian and western North American Cratons. The collages consist of tectonostratigraphic terfanes and contained metallogenic belts, which are composed of fragments of igneous arcs, accretionary-wedge and subduction-zone complexes, passive continental margins, and cratons. The terranes are overlapped by continental-margin-arc and sedimentary-basin assemblages and contained metallogenic belts. The metallogenic and geologic history of terranes, overlap assemblages, cratons, and craton margins has been complicated by postaccretion dismemberment and translation during strike-slip faulting that occurred subparallel to continental margins. Seven processes overlapping in time were responsible for most of metallogenic and geologic complexities of the region (1) In the Early and Middle Proterozoic, marine sedimentary basins developed on major cratons and were the loci for iron-stone (Superior Fe) deposits and sediment-hosted Cu deposits that occur along both the North Asia Craton and North American Craton Margin. (2) In the Late Proterozoic, Late Devonian, and Early Carboniferous, major periods of rifting occurred along the ancestral margins of present-day Northeast Asia and northwestern North America. The rifting resulted in fragmentation of each continent, and formation of cratonal and passive continental-margin terranes that eventually migrated and accreted to other sites along the evolving margins of the original or adjacent continents. The rifting also resulted in formation of various massive-sulfide metallogenic belts. (3) From about the late Paleozoic through the mid-Cretaceous, a succession of island arcs and contained igneous-arc-related metallogenic belts and tectonically paired subduction zones formed near continental margins. (4) From about mainly the mid-Cretaceous through the present, a succession of continental-margin igneous arcs (some extending offshore into island arcs) and contained metallogenic belts, and tectonically paired subduction zones formed along the continental margins. (5) From about the Jurassic to the present, oblique convergence and rotations caused orogen-parallel sinistral, and then dextral displacements within the plate margins of the Northeast Asian and North American Cratons. The oblique convergences and rotations resulted in the fragmentation, displacement, and duplication of formerly more continuous arcs, subduction zones, passive continental margins, and contained metallogenic belts. These fragments were subsequently accreted along the margins of the expanding continental margins. (6) From the Early Jurassic through Tertiary, movement of the upper continental plates toward subduction zones resulted in strong plate coupling and accretion of the former island arcs, subduction zones, and contained metallogenic belts to continental margins.
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