Volcanic flows of the Late Ordovician Dunn Point Formation contain a generally univectorial magnetization which passes the fold test. Its steep direction (D = 335°, I = −61°, k = 79, α95 = 4.2°, paleopole at 2°S, 136°E) is unlike any known for North America or for the Avalon terrane for post‐Ordovician time, so that a primary age of the magnetization appears to be very likely. The paleolatitude (42°S) for the Avalon terrane derived from this result is much higher than that predicted for the area on the basis of the cratonic North American apparent polar wander path, and a substantial post‐Ordovician displacement (>3500 km) of Avalon with respect to the craton can be deduced. In all likelihood the Avalon terrane did not collide with North America until Middle Devonian time. This collision produced the Acadian orogeny. High Ordovician paleolatitudes have also been obtained for northern Africa and for several localities in Hercynian Europe (the Armorica plate) and it is inferred that these areas may have drifted together until Late Ordovician time.
Paleomagnetic results have been obtained for three new sites in steeply inclined subarea1 volcanic flows of the Dunn Point Formation from the Avalon terrane of Nova Scotia. Demagnetization analysis reveals a characteristic magnetization, carried by hematite, which is similar to that reported in previous studies of this unit. A new and improved fold test for the characteristic component, combining paleomagnetic and structural data from the present study with paleomagnetic data for vertically dipping flows (and laterites) studied previously by the authors, is significantly positive. The new fold test reconfirms the prefolding age of the characteristic component. A primary age of magnetization is inferred, based on field evidence for early oxidation of the flows. The overall mean direction for the characteristic component of D = 344O, I = -60°, k = 68.6, ass = 4.1" (paleopole at 2ON, 130'E) corresponds to a Late Ordovician-earliest Silurian paleolatitude for Avalon of 41°S. The latter is much higher than that predicted for the area on the basis of the cratonic North America apparent polar wander path, implying a substantial post-Ordovician displacement (> 1700 km) of Avalon with respect to the craton. The Dunn Point result supports previous paleogeographic reconstructions that show Avalon together with the continental blocks of Hercynian Europe and northwestern Gondwana.
The data on the paleomagnetics of the volcanic rocks provided by Wellensiek et al. [ 1990] (hereinafter referred to as Wellensiek et al.) are welcome contributions to ongoing studies of the tectonic history of Maine and the northern Appalachians. Deficiencies in the presentation of important aspects of the geology of the region and of the particular area of the study by Wellenseik et al. motivate this comment, better to inform readers of the context of the paleomagnetic findings. REGIONAL SETTING 1. The Lunksoos "terrane" (formerly Weeksboro-Lunksoos anticlinorium [Osberg et al., 1985], also Lunksoos anticlinorium [Boone and Boudette, 1989]) is not part of the Kearsarge-Central Maine synclinorium. The latter is a broad Acadian structural feature that preserves a Silurian-Devonian trough. The northwestern boundary of this trough is the hinge zone defined by Moench [Moench and Pankiwskyj, 1988] in the Rangely area, northwestern Maine that may correspond with the deeply buried eastern margin of Grenvillian crust [Stewart, 1989]. This hinge zone is manifested along the SE flank of the Lunksoos anticlinorium by unnamed conglomerate of Silurian age (map unit S g of Neuman [ 1967]). 2. Of the several anticlinoria ("terranes" of Wellensiek et al.) in northern Maine the Lunksoos is Copyright 1991 by the American Geophysical Union. Paper number 91TC00059. 0278-7404/91/91 TC-00059 $02.00 the only one that preserves a non-North American Early Ordovician fauna, the Celtic brachiopod assemblage of the Shin Brook Formation [Neuman, 1984]. The Miramichi anticlinorium that extends across New Brunswick to the east also has Lower Ordovician fossiliferous rocks that contain such a fauna, but none of these is known to occur in Maine in the southwestern end of this anticlinorium. The few identifiable Ordovician shelly fossils elsewhere in northern Maine are all younger than those of the Shin Brook Formation, and they are assemblages of "Scoto-Appalachian" provincial affinities [Neuman, 1984]. 3. The statement that the "Stacyville volcanics" overlie the Shin Brook Formation is incorrect. No such relations are known to exist in the Lunksoos anticlinorium where only undated "metadiabase" overlies the Shin Brook. Such a relationship might be inferred, however, from known or estimated ages within the Lunksoos anticlinorium and from apparently unfaulted sequences of similar rocks of the same age span of the Miramichi anticlinorium in the Hayesville area, New Brunswick [Irrinki, 1980]. 4. The presence of the Katahdin Granite, the largest granitic body in Maine, should have been shown on Figure 1 of Wellensiek et al.. Its eastern margin is as close as 3 km SW of the three sample sites, wmc, wv3, and wv4. GEOLOGIC SETI'ING OF THE "STACYVILLE VOLCANICS" IN THE SHIN POND AREA 1. The source of the map (Figure 1 of Wellensiek et al.) is not attributed; apparently it was taken from an enlargement of a part of the 1985 state bedrock geologic map [Osberg et al., 1985] that was
Paleomagnetic and tectonostratigraphic data for the northern Appalachians record Silurian closure of a major ocean, the Iapetus Ocean, that was bordered by the Laurentian craton and the Avalonian microcontinent. In Ordovician times this ocean consisted of at least two basins (Iapetus I and II) and extended from a paleolatitude of 10 to 20°S (Laurentian margin) to ca. 50°S (Avalonian margin); Gondwana was located yet farther south. Paleomagnetic data from the Middle Ordovician Robert's Arm, Chanceport, and Summerford groups in north-central Newfoundland, which represent intraoceanic arcs and ocean islands, yield paleolatitudes of 30 to 33°S. In contrast, pillow lavas of the upper part of the Late Cambrian to Lower Ordovician Moreton's Harbour Group, which are currently juxtaposed to the Chanceport Group along the Lobster Cove-Chanceport Fault, acquired their remanence at 11 °S in a marginal island arc setting. Subaerial deposits of the mid-Silurian Botwood Group that unconformably overlie marine sequences in northeastern Newfoundland yield a primary magnetization with a paleolatitude of 24°S, which is indistinguishable from the Early Silurian position of the southeast-facing Laurentian margin. Silurian closure of Iapetus is supported by the timing of thrusting and folding and by the age of angular unconformities in the Central Mobile Belt.The combined paleomagnetic and tectonostratigraphic data present a working hypothesis for the geometry and tectonic evolution of the northern Appalachians. In Early Ordovician times, a volcanic arc and a back-arc basin (Iapetus I) were located near the Laurentian margin. Following Middle Ordovician obduction of ophiolites onto the Laurentian margin when Iapetus I closed, convergence of Avalon and Laurentia by northward subduction continued until closure of Iapetus II was complete by the Late Silurian.In view of these data, the traditional subdivision of Ordovician "Taconic" and Devonian "Acadian" orogenies needs to be revised. Maintaining the terminology of orogenic phases, one either has to expand the time interval of the Acadian orogeny to include the Silurian or add an orogenic phase (Caledonian?) in between Taconic and Acadian. In either case, Early to Middle Paleozoic closure of Iapetus should be viewed in terms of a progressive deformation history with peak deformation pulses rather than temporally discrete orogenies.van der Pluijm, B. A., Johnson, R.
Volcanogenic sediments of the Fourchu Group and a gabbmic intrusion, which are found in the Avalonian terraile of southeastern Cape Breton Island, have been sampled for paleomagnetic analysis. Upon detailed thermal and alternating-field demagnetization, three often-superimposed components of magnetization are obtained. One of these is aligned with the present-day geomagnetic field direction in Nova Scotia and is assumed to be of recent origin. The second group of directions is south-southeasterly and shallow, is postfolding in age, and is inferred to represent a Carboniferous overprint. The third direction, carried almost always by hematite, is also postfolding and yields a dual-polarity mean direction to the northwest or southeast, with a Fdirly steep inclination ( D = 132", 1 = -63 "). This last direction is not seen in Avalonian or other North American rocks of Devonian or younger age; it is, therefore, bracketed in age between the earliest folding of the rocks and the latest Silurian. Given that Taconic folding has not been reported for this area, we assume that this magnetization was introduced in the rocks during uplift and oxidation after an Avalonian folding phase. For the Avalon terrane of Nova Scotia. the available paleomagnetic data reveal a set of moderately high paleolatitudes for the Late Precambrian and early Paleozoic, in contrast to the near-equatorial values predicted for the area under the assumption that it remained fixed with respect to the craton. On the other hand, strong similarities exist between Avalonian paleolatitudes and those for Annorica and Gondwana; a tentative reconstruction is proposed in which Avalon is adjacent to Armorica and Gondwana in the Late Precambrian and early Paleozoic.Les skdiments volcanogkniques du groupe de Fourchu et une intrusion gabbro'ique du terranc D7Avalon, dans le sud-est de l'ile du Cap Breton, ont Ct C CchantillonnCs pour l'analyse palCon~agnCtique. L'examen detail16 de la dksaimantation thermique et par champ alternatif rtivkle trois composantes d'aimantation, souvent surimposCes. Une de ces composantes est align& avec la direction actuelle du champ gComagnCtique en Nouvelle-Ecosse, et dont l'origine semble rkcente. Le deuxikme groupe de directions est sud-sud-est et de faible profondeur, d'iige postkrieur h la dCformation et considCr6 comme une surimpression datant du Carbonifere. La troisikme direction, portCe presque toujours par 17hCmatite, est Cgalement posterieure 5 la deformation, et elle fournit pour la polaritC-double une direction moyenne nord-ouest ou sud-est et une inclinaison passablement abrupte ( D = 132", 1 = -63 "). Cette dernikre direction n'est pas observCe dans les roches devoniennes ou plus jeunes d7Avalon ou ailleurs en Amerique du Nord; par consequent leur Age est confink entre entre le dCbut de la dCformation des roches et la fin du Silurien. Etant donnC que la dkfonnation taconique se~nble absente dans cette rCgion, nous supposons que cette aimantation a Ct C introduite dans les roches durant le soulkvement et l'oxydation aprks...
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