Humber Arm Research Articles

New U-Pb zircon ages of plagiogranites from the Coastal Complex ophiolite and Twillingate batholith, Newfoundland: Evidence for the oldest and overlapping silicic magmatism in the nascent Cambrian peri-Laurentia forearc and arc terranes

The Coastal Complex (CC) and Bay of Islands Complex (BOIC) represent two supra-subduction zone (SSZ) forearc ophiolitic assemblages of different ages within the Taconic Humber Arm Allochthon of the Western Newfoundland Appalachians. To further constrain age relationships between the CC and BOIC and earliest initiation of Iapetus subduction, we acquired LA-ICP-MS U-Pb zircon ages for eleven plagiogranites sampled along strike of the CC belt. These ages combined with three prior legacy ages establish an age range of 514.3–502.7 Ma for the six CC plagiogranite plutons. The oldest precise ca. 514.3 Ma CC pluton age represents the oldest Taconic arc-related plutonic activity yet documented in Western Newfoundland and provides a minimum age for the intruded CC ophiolite basement and initiation of westerly vergent subduction within the peri-Laurentian Iapetus Ocean. This confirms the Early Cambrian CC mafic–ultramafic basement is significantly older than the juxtaposed SSZ BOIC, which formed by later rifting of the CC forearc along younger forearc spreading center in the Late Cambrian by ca. 488.3 Ma. After subduction initiation a prolonged early period CC forearc silicic magmatism lasted at least ∼ 11.6 Ma until ca. 502.7 Ma, a range consistent with age spans of magmatism in modern juvenile forearcs. We also reinvestigated the Central Newfoundland Twillingate Granite batholith thought to be one of the earliest major felsic batholiths formed within the adjacent juvenile arc massif of the Notre Dame Subzone, part of the same peri-Laurentian arc. We determined a precise 3-sample composite age of 504.3 ± 1.8 Ma that signals the formation of earliest Notre Dame arc edifice developed by Middle Cambrian. It overlaps the end-stage CC forearc plagiogranite magmatism and places a minimum mid-Cambrian age constraint on intruded ophiolitic arc tholeiitic and boninitic pillow lavas of the Sleepy Cove and correlated Lushs Bight volcanics, suggesting basement continuity with the CC.

Tectonics and foreland basin development at the leading edge of the Humber Arm Allochthon, western Newfoundland, Canadian Appalachians

The thrust front of the Northern Appalachians involves Ediacaran rift-related structures of the Laurentian margin that were re-worked in Taconic (Ordovician), Salinic (mainly Silurian) and Acadian (mainly Devonian) deformation events. Much of the thrust front is concealed under the Gulf of St. Lawrence, but Port au Port Peninsula, in western Newfoundland provides a cross-section through the Laurentian passive margin, overlying foreland basins, and the Humber Arm Allochthon. A large area of foreland basin strata is present in areas formerly mapped as Humber Arm Allochthon. Abrupt thickness changes and an abundance of fault-scarp-derived limestone conglomerate indicate that deposition was influenced by block faulting and differential subsidence in a basin undergoing active flexural extension. A coarsening upward trend in the Goose Tickle Group represents a transition from a distal to a proximal sediment source as the Humber Arm Allochthon was emplaced westwards. Goose Tickle Group separates two packages of allochthonous rocks. The West Bay Thrust Sheet, the lower package, represents the leading edge of the Taconic allochthon. Its timing of emplacement is constrained by Darriwilian 3 graptolites both above and below the thrust sheet. Its present-day configuration results from a combination of processes including thrusting, extension by gravity spreading, gravity gliding, and subsequent erosion of material that was deposited in the Goose Tickle Group. The structurally higher Lourdes Thrust Sheet is an out-of-sequence structure associated with Acadian (Devonian) orogenesis. High-angle faults show a protracted history of movement that includes early Taconic flexural extension, Acadian inversion, and later Carboniferous or Mesozoic strike-slip motion. The leading edge of the Humber Arm Allochthon was influenced by both thin-skinned and thick-skinned tectonics throughout the development of the Appalachian orogen.

Provenance of the Newfoundland Appalachian foreland basins

The tectonic history of the Appalachian orogen is recorded in the adjacent foreland. Offshore seismic data suggest that an initial, Middle Ordovician foreland basin was filled by sources in Newfoundland, whereas the Late Ordovician foreland basin was loaded and filled by sources to the SW, in the Quebec segment of the orogen, where NW-vergent Taconian arc-continent collision may have continued later than in Newfoundland. We test this hypothesis using U/Pb ages of detrital zircon within foreland basin successions in western Newfoundland. Previously published results from the oldest foreland succession, the Middle Ordovician Goose Tickle Group, demonstrate ages similar to units preserved with the Humber Arm Allochthon, emplaced during Middle Ordovician Taconic orogenesis, including a predominant Paleoproterozoic peak at 1.85 Ga. In this study we investigated U/Pb geochronology of detrital zircon from younger foreland successions from Upper Ordovician to Devonian. The largest proportion of analyses within all foreland successions fall between 0.95 and 1.3 Ga, with largest peaks occurring between 1.0 and 1.1 Ga, typical of zircon derived from the Grenville Orogen. Earlier Mesoproterozoic and Paleoproterozoic ages range from 1.3 to 2.0. The abundance of Mesoproterozoic grains and conspicuous lack of 1.85 Ga Paleoproterozoic zircon in the overlying Upper Ordovician Long Point Group and latest Silurian to Early Devonian Clam Bank Formation indicates that these sediments were not derived from units within the Humber Arm Allochthon. Probability density plots of continental margin units in the Quebec/New England segment of the orogen demonstrate a similar strong Mesoproterozoic and weak Paleoproterozoic signature, suggesting derivation of the Long Point Group from the Quebec segment of the orogen. Within the mid-Paleozoic Clam Bank - Red Island Road succession, typical Gondwanan ages are absent and 1.0 Ga grains derived from the Grenville Orogen are abundant. This is consistent with underthrusting of the Gondwanan microcontinents Ganderia and Avalonia during Salinic and Acadian orogenesis. Only Mesoproterozoic zircon grains were found in the Early Devonian Red Island Road Formation, consistent with derivation from Mesoproterozoic Grenville massifs in western Newfoundland or Cape Breton Island which were exhumed during Devonian Acadian inversion.

Mélanges and disrupted rocks at the leading edge of the Humber Arm Allochthon, W. Newfoundland Appalachians: Deformation under high fluid pressure

The Humber Arm Allochthon was structurally emplaced onto the Laurentian margin in western Newfoundland during Taconian (Ordovician) and Acadian (Devonian) deformation. On Port au Port Peninsula, disrupted allochthonous rocks previously mapped as mélange and scaly shale include three mappable, variably disrupted, stratigraphic units; in addition, mixed rocks constitute mélange with much smaller area than previously mapped. At outcrop scale, a qualitative assessment of disruption distinguishes broken, but coherent stratigraphy from a more disrupted and mixed mélange unit. Within coherent regions, three generations of folds are probably related to Taconian, Acadian and Carboniferous deformation events. More disrupted regions show an average of ~24% blocks to 76% matrix with block sizes 0.5–158 cm. A new sampling technique allowed recovery of oriented mélange samples for thin-section. Multiple orientations of extensional fractures suggest approximately coaxial extension. Abundant carbonate and less common bitumen-filled veins suggest that high fluid pressure played a role in the emplacement of the Allochthon. High fluid pressure was probably also responsible for dewatering structures, sandstone dykes and partially brecciated carbonate beds. Map relationships, outcrop and thin-section scale observations lead to a reinterpreted structural history for western Newfoundland in which an early, Taconian, West Bay Thrust Sheet was rapidly emplaced onto the Laurentian margin. During emplacement, debris flows initially contributed igneous blocks to the allochthon, but the majority of fragmentation took place in an environment of horizontal tectonic extension promoted by high fluid-pressure that encouraged brittle fracture. The West Bay thrust sheet was subsequently overridden by the out-of-sequence Lourdes Thrust. Parts of the allochthon were probably re-imbricated in later events, but because of previous disruption, an organized imbricated thrust belt was not developed. At the tip of an advancing thrust wedge, a clear distinction between tectonic and surficial processes of mélange formation may not be possible.

Inversion of Taconian extensional structures during Paleozoic orogenesis in western Newfoundland

Abstract West Newfoundland was critical in developing the Wilson Cycle concept. Neoproterozoic rifting established a passive margin adjacent to the Iapetus Ocean. Ordovician (Taconian) arc–continent collision emplaced ophiolites and the thin-skinned Humber Arm Allochthon. Subsequent Devonian (Acadian) ocean closure produced basement-cutting thrust faults that control the present-day distribution of units. New mapping, and aeromagnetic and seismic interpretation, around Parsons Pond enabled the recognition of structures in poorly exposed areas. Following Cambrian to Middle Ordovician passive-margin deposition, Taconian deformation produced a flexural bulge unconformity. Subsequent extensional faults shed localized conglomerate into the foreland basin. The Humber Arm Allochthon contains a series of stacked and folded duplexes, typical of thrust belts. To the east, the Parsons Pond Thrust has transported shelf and foreland-basin units c. 8 km westwards above the allochthon. The Long Range Thrust shows major topographical expression but <1 km offset. Stratigraphic relationships indicate that most thrusts originated as normal faults, active during Neoproterozoic rifting, and subsequently during Taconian flexure. Devonian continental collision inverted the Parsons Pond and Long Range thrusts. Basement-cored fault-propagation folds in Newfoundland are structurally analogous to basement uplifts in other orogens, including the Laramide Orogen in western USA. Similar deep-seated inversion structures may extend through the northern Appalachians.

Anthropogenic modification of a fjord: Bay of Islands, Newfoundland

The majority of Newfoundland fjords are relatively pristine. However, the Bay of Islands fjord, site of Corner Brook (Newfoundland's second-largest city with a population of 20 000), shows considerable anthropogenic modification. Submarine slides have been triggered by dock and road construction. Dredge spoil and a scuttled barge litter the seafloor. A large mound of wood chips extending into the Bay of Islands is waste from a paper mill. The Bay of Islands, western Newfoundland, is a fjord bounded at its seaward end by an arcuate submarine moraine (Fig. 1a) up to 100 m thick (Josenhans et al. 1990; Josenhans & Zevenhuizen 1993; Josenhans & Lehman 1999). The moraine formed after retreat of ice that formerly extended far out on the continental shelf (Shaw et al. 2006). The deepest areas of the fjord (295 m) are immediately landward of the shallow (30 m) moraine. The fjord has three principal arms, the largest of which (Humber Arm, Fig. 1a, c), averages 100 m deep and shallows steeply at its head where the Humber River enters. Humber Arm has many of the attributes of fjords elsewhere; it hosts thick glacimarine sediments (Fig. 1b) overlain by postglacial mud (Shaw et al. 2000 …

State Sponsorship and Community Environmental Groups: The Atlantic Coastal Action Program in Newfoundland

Introduction When the Canadian federal government announced in November 2002 that it would assume part of the cost involved in building a primary sewage treatment plant in St. Johns, Newfoundland and Labrador, thereby halting the flow of untreated waste into that city's harbor, no one was happier than the members and supporters of the St. John's Harbour ACAP (SJH). ACAP is the acronym for Atlantic Coastal Action Program, an initiative of Environment Canada that provides project funding for community environmental organizations active in coastal ecosystem issues. Newfoundland (1) has two ACAPs. Besides the St. John's group, there is one in Corner Brook, the Humber Arm ACAP (HA). Although the Corner Brook organization also sought sewage treatment for its region, it has made very little headway on the issue. In this study, we examine the two local environmental groups, focussing on two distinct questions. The first asks why one group had greater success than the other. Answering this question necessarily requires looking at the structures of the two organizations, their specific objectives, and the political context in which they operated. Our second question is somewhat less obvious. Both ACAPs can be considered to be environmental groups, because both receive funding from the Canadian government. We want to know how having federal sponsorship affects their relationships with not just Ottawa but with the provincial and local authorities as well. In particular, we ask how having a link to the federal government influenced the capacity of the two to deal with wastewater issues, if it did at all. To answer these questions, the paper proceeds through a series of steps. First, it looks at the question of state sponsorship of interest groups. It then lays out the two cases that make up the study, searching for difference in structure, objectives, or relations with the governments involved that can explain why they achieved different results. The article's conclusion summarizes the findings, suggests an explanation for the results, and comments on the impact of governmental sponsorship on the two and their work. State-sponsored Groups The development of environmental in Canada is a relatively recent phenomenon. Unlike other Anglo-American countries, there were virtually no conservation or environmental prior to the 1960s. Nevertheless, in a short period of time, about two thousand environmental were established involving an estimated one to two million Canadians (Wilson, 2002; Doern and Conway, 1994). Pluralist theory views the flourishing of interest as a natural product of liberal democratic societies. Other theories focussing more on the development of new social movement have provided explanations in terms of the rise of a new middle class, the nature of late capitalism, and the development of post-materialist attitudes. Despite different theoretical perspectives, are generally seen as autonomous organizations originating in civil society. Although some researchers have noted the significance of state funding for many environmental groups, insufficient attention has been paid to the role of the state in encouraging and supporting environmental groups. In their classification of interest groups, Whittington and Van Loon (1996) make a useful distinction between groups that have been primarily responsible for their own creation and maintenance (autonomous groups) and groups that have been either created or encouraged by government itself (state-sponsored groups). As with all classification systems, there is a degree of arbitrariness in determining what is and is not a state-sponsored group; the degree of state-sponsorship may vary from those that are essentially agencies of the state to those that obtain some minimal level of support from the state. As well, the concept of state sponsorship may be considered multidimensional, including such aspects as whether government was responsible for founding or encouraging the founding of the group, the degree of financial dependence upon government, and the degree of autonomy of the group in terms of setting objectives, selecting strategies to achieve their objectives, and the choice of key personnel. …

Topographically-controlled Deglacial History of the Humber River Basin, Western Newfoundland

The Humber River in western Newfoundland flows through a large interior basin, that influenced Late Wisconsinan ice flow from major dispersal centres to the north, in the Long Range Mountains, and to the east in The Topsails. An early southward ice flow from a source to the north covered coastal areas in the western part of the basin. Subsequent regional ice flow was southwestward to northwestward from The Topsails, while south to southwestward flowing ice from the Long Range Mountains occupied the upper Humber River valley. This flow was confluent with ice from The Topsails and moved northwestward toward Bonne Bay. Regional deglaciation began about 13 ka from the inner coast. Ice occupying the Deer Lake valley dammed glacial Lake Howley in the adjacent Grand Lake and Sandy Lake basins to an elevation up to 85 m above present lake levels, which were controlled by drainage through a western outlet feeding into St. George’s Bay. The lake was lowered by exposure of the South Brook valley outlet, and finally drained catastrophically through a spillway at Junction Brook. Marine limit at the coast was 60 m asl. Inland deltas at the head of Deer Lake and fine-grained sediment exposed in the Deer Lake valley show inundation below 45 m present elevation. This produced a narrow embayment extending at least 50 km inland from the modern coast and is named here as ‘Jukes Arm’. Dated marine macrofossils in the Humber Arm and lower Humber River valley, indicate the deltas at the head of Deer Lake formed about 12.5 ka.

Chronic stress in cunner, Tautogolabrus adspersus, exposed to municipal and industrial effluents

Chronic changes were measured in fish downstream from a pulp and paper mill located in the Humber Arm, Newfoundland, Canada. Cunner (Tautogolabrus adspersus) downstream of the mill were longer and heavier at an earlier age than reference fish. Condition factor was similar among sites. Hepatosomatic indices and ethoxyresorufin- O-deethylase activity were higher in fish near the mill. Gondal condition suggested that gametogenesis might have been delayed in these same fish. Although splenosomatic indices were greater in exposed fish, splenic hemosiderin was not significantly different from reference values. Gill and liver changes in exposed cunner were significant. Gill parasites showed no site difference; however, the prevalence and abundance of gastrointestinal acanthocephalans were significantly greater in fish from impact sites. These results suggest that effluent from this mill may be responsible for observed biological effects; however, further study is needed to determine whether gradients in nutrient loading and untreated sewage discharge may also contribute to fish health in the Humber Arm.

Development of a folded thrust stack: Humber Arm Allochthon, Bay of Islands, Newfoundland Appalachians

In the Humber Arm area of the western Newfoundland Appalachians, four distinct stratigraphic successions derived from the Laurentian continental margin are exposed. Each succession is believed to be characteristic of a separate thrust sheet. The platform sheet represents the ancient Laurentian shelf and its foreland basin cover; the Watsons Brook sheet is characterized by a succession including shelf-margin carbonates overlying foreland basin clastics; the Corner Brook sheet comprises continental slope and rise clastic and carbonate sedimentary rocks of the Humber Arm Supergroup; and the Woods Island sheet includes clastics of the Blow Me Down Brook formation that overlie mafic volcanics. Sheets are subdivided by thrusts into tectonic slices. Disrupted units and mélange, with scaly S1 foliation, are found along the boundaries of some slices. Thrust sheets and related structures have been deformed by F2 folds with axial planar S2 cleavage. S1 scaly foliations are transposed into parallelism with S2. There is a transition in the style of F2 folds across the area, from upright and subhorizontal in the west to overturned folds with west-dipping axial planes and steeply raking or reclined fold hinges in the east. Strongly curved fold hinges may reflect later shearing along the S2 surfaces, producing sheath-like fold geometries. Shear zones close to the east edge of the outcrop of the Watsons Brook sheet display kinematic indicators indicating both D2 reverse-sense and D3 normal-sense dip-slip shears. Subsequent events produced L4 and L5 crenulation lineations on the S2 surfaces. At minimum, several tens of kilometres of shortening affected the part of the margin preserved in the Humber Arm area; true shortening and transport amounts may have been much larger.

Post-Taconian shortening, inversion and strike slip in the Stephenville area, western Newfoundland Appalachians

Deformed terrigenous and carbonate sedimentary rocks representing the early Paleozoic Laurentian continental margin form a series of elongate, fault-bounded blocks that plunge north beneath the Humber Arm Allochthon in the Stephenville area, west Newfoundland Appalachians. The continental shelf succession was folded and thrust-faulted after emplacement of the Humber Arm Allochthon. In the west of the area, Table Mountain is cut by a "pop-up" structure bounded by downward-converging reverse faults. Structures at the east margin of Table Mountain indicate both dextral and reverse slip. The Phillips Brook Structure, farther east, contains multiple, fault-bounded carbonate slices; one slice, carried by the West Blanche Brook fault, is thrust over the Humber Arm Allochthon. The western edge of the Indian Head massif, consisting of Grenville basement, is also a thrust contact. These reverse faults and thrusts, which cut the carbonate succession and postdate emplacement of the Humber Arm Allochthon, are in turn overprinted by structures formed during dextral strike-slip motion. Unconformable Early Carboniferous cover postdates most of the deformation. Cross-sections indicate shortening of a few kilometres, and basement was clearly involved in the deformation. The Port au Port Peninsula, immediately to the west, records a history of Acadian inversion of Taconian basins. The prevalence of pop-up structures and mappable variations within the stratigraphic units forming the top of the carbonate succession indicate that a complex geometry of Taconian horsts and grabens was inverted during post-Taconian (?Acadian) shortening and dextral strike-slip motion. These relationships suggest a variety of attractive targets in petroleum exploration.

The nature and origin of asymmetric arrays of shear surfaces in fault zones

Abstract Mid- to upper-crustal fault zones often possess arrays of shear surfaces whose traces on sections perpendicular to the fault surface and parallel to the ac -plane conform with one or more of the ‘Riedel shear’ orientations. These shear surfaces often are oblique to the transport plane, however, so arrays exhibit monoclinic rather than orthorhombic symmetry. In a mudstone-dominated mélange in Humber Arm Supergroup strata in the Bay of Islands, Newfoundland, and in serpentinites from the base of the Bay of Islands complex, shear surfaces have orientations inclined to major fault-zone boundaries and an inferred ac -plane for macroscopic fault-related deformation. Deformation in these zones exhibits an overall monoclinic symmetry. The 3D, asymmetric character of shear surface fabrics suggests that a factor other than stress or the symmetric strain rate tensor controlled their formation. The velocity gradient tensor in a steady, non-coaxial shearing flow possesses a symmetry consistent with monoclinic fabrics. Shear surfaces in asymmetric arrays may initiate with predictable orientations relative to the velocity gradient tensor and then rotate toward flow apophyses, which identify stable positions in steady, 3D flows.

Paleomagnetism of the 550 Ma Skinner Cove volcanics of western Newfoundland and the opening of the Iapetus Ocean

The Skinner Cove Formation of western Newfoundland is an alkali volcanic suite transported in the Ordovician onto Laurentia's Iapetus Ocean margin. It has previously yielded a U–Pb zircon age of 550.5 +3 −2 Ma, has no penetrative deformation and has only been affected by zeolite facies metamorphism. At 10 sites, flows and dykes of the Skinner Cove Formation exhibit a stable characteristic `A' remanence carried by magnetite. This `A' remanence is shown to be primary by an intraformational conglomerate test using 20 trachybasalt clasts (whereas 11 alkali basalt clasts were overprinted, probably because they originally contained titanomagnetite from which rutile exsolved soon after burial by overlying flows). After tilt-correction, the mean `A' remanence direction is southeast and down ( D=144°, I=32°; α 95=11°, k=21, N=10 sites). The paleolatitude calculated from the ten `A' site virtual geomagnetic poles is 19°S ± 9°, which likely represents the paleolatitude of Laurentia's Iapetan margin at ∼550 Ma. Evidence that the Skinner Cove Formation originated at this margin includes its structural position within the Humber Arm Allochthon, its within-plate trace element geochemistry, and its similarity in age to other alkali magmatism that more certainly represents Laurentia's Iapetan margin. Comparison with other Laurentian paleomagnetic data implies that Laurentia drifted very rapidly northward from the south polar region to the equator between ∼570 and 550 Ma. We suggest that the start of this rapid northward drift at ∼570 Ma marked the onset of Iapetus sea-floor spreading between Laurentia and West Gondwana. Further well-constrained paleomagnetic data of ∼570 to ∼550 Ma age are needed from Laurentia to test its high rate of northward drift, and from West Gondwanan cratons to test that they remained relatively stationary.

Structural and tectonic evolution of the Humber Zone, western Newfoundland 2. A regional model for Acadian thrust tectonics

The Humber Zone of the western Newfoundland Appalachians represents a Cambrian‐Ordovician passive continental margin which was deformed in Taconian (mid‐Ordovician) and Acadian (Silurian‐Devonian) orogenic events. A deformation front is imaged in seismic reflection data offshore of western Newfoundland. Structures associated with this deformation front are exposed on Port au Port peninsula, where Silurian rocks are strongly deformed but Mississippian strata are flat lying, indicating that latest thrusting was Acadian. A gravity low in the Gulf of St. Lawrence corresponds to a sediment‐filled Acadian foreland basin. Previous models suggest that the on‐land shelf succession is autochthonous to parautochthonous. However, two Lithoprobe seismic reflection transects show sub‐horizontal reflections between 2 and 5 s two‐way travel time, which extend up to 85 km east of the thrust front. These are interpreted as autochthonous platform and basement. In this model, shallower reflectors and outcropping units include both allochthonous platform and basement, comprising the Acadian Port au Port allochthon. The Taconian Humber Arm allochthon was carried westward as a high structural slice during thrusting of this allochthon. No major structural discontinuity exists between Grenville age crystalline rocks of the Long Range massif and platform rocks interpreted as allochthonous in the northern seismic line. A monocline at the southern extremity of the Long Range probably represents an oblique or lateral hanging wall ramp above the basal detachment. Within the Long Range thrust zone at the western margin of the massif the Long Range thrust shows only a few kilometers of displacement. However, the Parsons Pond thrust, which we interpret to run offshore at Green Point, juxtaposes contrasting successions with different structural and thermal histories; it probably carries a much larger amount of the total displacement. The basal decollement of the Port au Port allochthon is therefore interpreted to pass beneath the southern part of the Long Range massif.

Basin development and inversion at the Appalachian structural front, Port au Port Peninsula, western Newfoundland Appalachians

In the Humber Zone of the Newfoundland Appalachians, Cambro-Ordovician shelf and foreland basin successions are affected by Middle Ordovician (Taconian orogeny) and Devonian (Acadian orogeny) deformation. On Port au Port Peninsula the presence of the Late Ordovician to Late Silurian Long Point – Clam Bank succession allows these episodes to be separated. The Taconian foreland basin stratigraphy on Port au Port Peninsula is highly variable. On the west coast, platform carbonates are overlain by megaconglomerates of the Cape Cormorant Formation, which record progressive exposure of 1 km of the platform succession. The conglomerates are restricted to a narrow zone, consistent with derivation from a fault scarp originally immediately west of the outcrops (in palinspastic restoration). Farther east, at Victors Brook, the Cape Cormorant Formation is absent, but the overlying, almost undeformed Goose Tickle Group contains conglomerate derived both from the upper part of the platform succession and from the Taconian Humber Arm Allochthon. Southeast of Victors Brook, the top of the platform is overlain directly by scaly shales and mélange of the Humber Arm Allochthon, which includes deformed equivalents of the foreland basin succession. The distribution of conglomeratic units, the presence and configuration of faults, and the preservation of the Goose Tickle Group in the Victors Brook area imply that a fault-bounded basin developed in advance of the Humber Arm Allochthon during the Taconian orogeny. This basin is interpreted to have resulted from flexural extension of North American lithosphere. The close spatial coincidence between later Acadian structures and the Taconian basin boundaries implies that the basin-bounding faults were reactivated as thrusts and reverse faults, and that the basin underwent inversion during Acadian thrusting. The western basin-bounding fault, modified by the development of a "short cut" thrust, developed into the present-day Round Head thrust.

Structural and tectonic evolution of the Humber Zone, western Newfoundland, 1. Implications of balanced cross sections through the Appalachian structural front, Port Au Port Peninsula

The Appalachian structural front in western Newfoundland, which is principally a submarine feature that comes ashore on Port au Port Peninsula, is interpreted to be a structural triangle zone, or tectonic wedge. Rocks within the triangle zone, which are therefore transported, include the Taconian (Middle Ordovician) Humber Arm Allochthon, Taconian foreland clastic sediments, and the Cambro‐Ordovician platform succession. The transported clastic sediments and platformal rocks, as well as structurally involved Grenville crystalline basement exposed east of the peninsula, compose the Acadian (Siluro‐Devonian) Port au Port Allochthon, which carried the Humber Arm Allochthon as a high structural slice; Acadian transport of tens of kilometers westward is suggested. This interpretation contrasts markedly with the traditional and widely accepted interpretation of the Taconian and Acadian orogenies in western Newfoundland. Here we reinforce our earlier arguments for an allochthonous interpretation by presenting a series of six closely spaced, balanced cross sections across the Port au Port Peninsula. These sections illustrate our interpretation of complex and noncylindrical structures within the triangle zone, as well as the envisioned structural linkage between exposures on Port au Port Peninsula and the geometry of the triangle zone interpreted from offshore seismic data 23 km along strike to the northeast. The southeast vergent Tea Cove thrust, which formed an early upper detachment to the triangle zone, is offset and overturned by the north‐northwest vergent Round Head thrust. The Round Head thrust is interpreted to flatten into a second southeast vergent thrust, the Red Brook detachment, which formed a late upper detachment to the triangle zone. Some map‐scale structures are interpreted as fault‐bend folds above oblique ramps on the Red Brook detachment. Allochthonous crystalline basement within the triangle zone, beneath its attached platformal cover, is interpreted as a result of thrust reactivation of a preexisting basement‐cutting normal fault, which may have formed during rifting of the early Paleozoic Iapetus Ocean. This interpretation is suggested by the stratigraphy and geometry of spatially restricted coarse conglomeratic units on Port au Port Peninsula, and is supported by the restored cross sections.

Mid-Paleozoic thrusting at the Appalachian deformation front: Port au Port Peninsula, western Newfoundland

Structures exposed on Port au Port Peninsula in western Newfoundland record the nature of the Appalachian deformation front, which forms the western boundary of the Humber tectono-stratigraphic zone. The major structures affect the Late Ordovician to Late Silurian Long Point – Clam Bank succession, but not the unconformably overlying Carboniferous rocks; they are probably of Devonian age.At the west coast of the peninsula, Long Point and Clam Bank strata are affected by both east-vergent and west-vergent structures. The basal surface of the succession is interpreted as an east-vergent thrust, forming the upper detachment of a "triangle zone," and correlates with a similarly located contact seen in offshore multichannel seismic profiles. Within the succession, east-vergent deformation zones locally duplicate the stratigraphy. West-vergent structures, including a map-scale overturned fold north of Round Head mountain, are probably younger.Farther south, Middle Ordovician foreland basin sediments are also affected by east-vergent thrusts, which have been variably rotated by west-vergent folds. In the underlying Cambrian–Ordovician platform carbonate succession, east-vergent thrusts duplicate the stratigraphy.These structures are related to telescoping of the carbonate platform and the overlying Humber Arm Allochthon during Devonian westward wedging of the structural triangle zone beneath the Long Point – Clam Bank succession. The platform succession must therefore be allochthonous, and the Humber Arm Allochthon has been transported to the west of its Late Ordovician position.

Bay of Islands and Little Port complexes, revisited: age, geochemical and isotopic evidence confirm suprasubduction-zone origin

The Bay of Islands Complex of the Humber Arm allochthon, west Newfoundland, contains the best-exposed ophiolite in the Appalachian Orogen. Associated structural slices, the Little Port and Skinner Cove complexes, also contain rocks formed in an oceanic domain, although their relationship to the Bay of Islands Complex remains controversial.To constrain the origin of the complexes and obtain a better understanding of the geology of the Humber Arm allochthon, we have undertaken an integrated geochronological, geochemical, and isotopic study. A U/Pb zircon age of [Formula: see text] Ma for the Little Port Complex and a zircon and baddeleyite age of 484 ± 5 Ma for the Bay of Islands Complex have been obtained. Geochemical and isotopic data on trondhjemitic rocks from the two complexes indicate that petrogenetic models for these rocks must account for fundamental differences in source materials and mineralogy during differentiation. The Little Port Complex trondhjemites are characterized by initial εNd of −1 to +1, whereas those in the Bay of Islands have εNd of +6.5. Geochemical signatures in mafic and felsic volcanics of the complexes are diverse, and show a complete gradation between arc and non-arc.The Bay of Islands and Little Port complexes are not related by any form of a major mid-ocean-ridge – transform-fault model. An alternative model to explain the relationships between the two complexes interprets the Little Port as arc-related and the Bay of Islands as a suprasubduction-zone ophiolite.

Tectonic history of the central Humber Zone, western Newfoundland Appalachians: post-Taconian deformation in the Old Man's Pond area

In the Humber Zone of the west Newfoundland Appalachians, the Middle Ordovician Taconian orogeny led to emplacement of the Humber Arm and other allochthons above a rift and shelf succession of late Precambrian to Early Ordovician age. Later deformation reversed this stacking in the Old Man's Pond area, where shelf sediments are thrust over the Old Man's Pond Group. East-vergent folds were subsequently developed in association with a regional, west-dipping cleavage that overprints the Taconian structures and intensifies eastward. Later, a major episode of shearing along normal-sense, northwest-dipping shear zones juxtaposed the Old Man's Pond Group against metamorphosed rift-related sediments and volcanics of the Hughes Lake Slice. This extensional episode may be related to intrusive events in adjacent central Newfoundland. The shear zones are overprinted by "cross folds," west-vergent folds, crenulation cleavage, and thrusts. These late structures are inferred to be related to westward transport of the rift–shelf succession and the Grenville-age basement rocks of the Long Range massif above a detachment surface, probably in Devonian time. The structural history is difficult to reconcile with a single, Devonian, "Acadian" orogeny, but is consistent with published isotopic data from central Newfoundland that suggest continued protracted tectonic activity in the Humber Zone in Silurian time.

The distribution and possible biostratigraphic significance of the ichnogenus Oldhamia in the shales of the Blow Me Down Brook Formation, western Newfoundland

The Blow Me Down Brook Formation is a siliciclastic unit in the Humber Arm allochthon of western Newfoundland that was deposited on the ancient North American continental slope and rise during the early stages of the development of the Iapetus Ocean. This unit consists primarily of sand-rich turbidites that appear to be barren of body fossils. The shales, however, contain trace fossil assemblages dominated by the ichnogenus Oldhamia and also include Planolites, Gordia, and other indistinguishable simple forms. Oldhamia is considered as an index fossil for the Early Cambrian, and the validity of this age assignment is reviewed.Oldhamia traces have been found in the Blow Me Down Brook Formation at 27 localities along the length of the allochthon and occur beneath each of the four ophiolitic massifs. Together, the assemblages from this unit contain five Oldhamia ichnospecies including O. radiata, O. antiqua, O. smithi, O. flabellata, and the newly reported O. curvata. Different ichnospecies are dominant in the Oldhamia assemblages at the 27 localities and these differences appear to be systematic. In the northernmost localities of the Blow Me Down Brook Formation simple radial forms predominate, including O radiata and O. smithi, whereas in the southernmost localities dendritic forms such as O. flabellata are common. This north to south distribution from radial to dendritic forms may represent an evolutionary trend of feeding optimization.