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Late Paleozoic Motions of the Meguma Terrane,Nova Scotia: New Paleomagnetic Evidence

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Late Paleozoic Motions of the Meguma Terrane,Nova Scotia: New Paleomagnetic Evidence Geodynamics Series Plate Reconstruction From Paleozoic Paleomagnetism Vol. 12 LATE PALEOZOIC MOTIONS OF THE MEGUMA TERRANE,NOVA SCOTIA: NEW PALEOMAGNETIC EVIDENCE Dann J. Spariosu and Dennis V. Kent Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964 J. Duncan Keppie Nova Scotia Department of Mines and Energy, 1690 Hollis Street, Halifax, Nova Scotia, Canada B3J 2Xl Abstract. Three rock units from southern Nova Introduction Scotia were sampled for a paleomagnetic study of the relationship of the Meguma terrane to the Differing paleomagnetic signatures of adjacent Atlantic-bordering continents during the Paleo­ geologic provinces have provided evidence for the zoic. These include the Si lure-ordovician White existence of terranes in the northern Appala­ Rock Formation volcanics, sandstones of the Lower chians that were exotic to cratonic North America Devonian Torbrook Format ion and red beds of the for portions of Paleozoic time, including Acadia Lower Carboniferous Cheverie Formation. Pro­ [Kent and Opdyke, 1978, 1979, 1980], Armorica gressive thermal and alternating field demagneti­ [Vander Voo, 1982], and the Traveler terrane zation of the White Rock basalts and rhyolites [Spariosu and Kent, 1981, in press, 1983]. Recen­ reveal a single component magnetization with a tly, plate tectonic models of the Appalachians mean direction of D • 149.10, I "' 24.3°, a 95 = have been constructed with the incorporation of 10°, for N = 13 sites. Rotation of the site mean the concept of "exotic" or "suspect" terranes in directions about the axis of the Torbrook the Paleozoic evolution of the orogen [Williams Syncline suggest a post-folding (post Middle and Hatcher, 1982; Keppie, in press, 1983]. Such Devonian) age for this magnetization, which cor­ terranes have been defined by as "areas characte­ responds to a pole at 21.90 N Lat., 147.7° E rized by an internal continuity of geologic Long. after correction for post-Triassic regional record, with features that contrast sharply with tilting. The magnetization of the Torbrook For­ those of nearby provinces." To date, however, no mation (D = 15.80, I .. 29.60, a 95 = 11.7°) is paleomagnetic data have been published for the clearly a secondary magnetization whose pole Paleozoic of the Meguma terrane of southern Nova (55.5° N Lat., 90.7° E. Long.) lies near Triassic Scotia, a terrane long viewed as exotic to pre­ poles from both North America and southern Nova Mesozoic North America on the basis of its litho­ Scotia. The age of the Cheverie Formation magne­ stratigraphic character [Schenk, 1978; Keppie, tization (D • 146°, I = 250, a 95 = 6°, tilt 1977b, 1982a,b, in press, 1983; Williams and corrected) appears to pre-date folding in the Hatcher, 1982]. This study presents paleomagne­ area (pre-Westphalian) and the corresponding pole tic evidence that the Meguma terrane did indeed (24° N Lat., 152° E Long.) lies near to the White move independently of North America for at least Rock pole, suggesting a similar age of magnetiza­ part of Carboniferous time. tion. The White Rock and Cheverie poles, which are constrained to have Early Carboniferous ages, Geologic Setting are 30° or more away from the North American APW path over the same age range, a discrepancy which The Meguma is the outermost terrane of the can be explained by a 15 - 19° northward motion northern Appalachians, lying farthest from the of Meguma with respect to the North American early Paleozoic cratonic margin of North America craton along with a 20 - 25° counter - clockwise [Williams and Hatcher, 1982; Keppie, in press, rotation. There is no paleolatitude discrepancy 1983], (Fig. 1). The Meguma Zone is juxtaposed between these results and paleomagnetic results against the adjacent Avalon composite terrane of from the adjacent Avalon Zone although a similar northern Nova Scotia and New Brunswick along the rotational discrepancy is evident. These infer­ Minas Geofracture (Chedabucto-Cobequid fault red motions of the Meguma terrane most likely system) [Keppie, 1982a] and presumably forms the took place during the Carboniferous, prior to the basement beneath the present day continental formation of Pangea. margin to the South and East. The name is de- 82 Copyright American Geophysical Union Geodynamics Series Plate Reconstruction From Paleozoic Paleomagnetism Vol. 12 70° Fig. 1. Map of the northern Appalachians adapted from Williams and Hatcher [1982], showing major terranes. rived from the Meguma Group, a thick (up to 13km) Devonian), [Boucot, 1960]. The Torbrook Formation succession of Cambrian - Ordovician graywackes marks the end of a long period of seemingly and shales thought to be derived from a low-lying continuous deposition in a marine environment. metamorphic terrane to its southeast; a succes­ A Middle Devonian orogeny affected all of the sion with a distinctly different lithology than above described rock units. The deformation pro­ the Cambro-Ordovician sections of any of the duced northeasterly trending folds associated other northern Appalachian terranes. The Meguma with an axial plane cleavage and was followed Group is disconformably and unconformably over­ closely by regional metamorphism grading from lain by the White Rock Formation, an undated lower greenschist facies in the north through succession of sandstones and shales locally con­ amphibolite facies in the southwestern corner of taining thick piles of mafic and felsic volcanic Nova Scotia. Subsequently the deformed pile was rocks. Locally, the sediments of the Kentville intruded by the granitoid rocks of the Meguma and New Canaan formations conformably overlie the Batholith, a composite intrusion with radiometric White Rock; these contain fossils of probable ages ranging from 371±2 ma to 361±1 ma [Keppie Late Silurian (Ludlovian) age [Keppie, 1977a]. and Smith, 1978]. Regional structures and Conformably above these Silurian units more than metamorphic isograds are truncated by the grani­ !300m of the Torbrook Formation sandstones, silt­ toid plutons. The age relationships between the stones and shales are exposed along the southern uppermost Torbrook Formation and the intrusions edge of the Annapolis Valley. These paralic thus constrain the age of deformation to be post­ marine sediments contain an abundant shelly fauna early Emsian, pre-360ma, close in time to the of Rhenish-Bohemian affinity, ranging in age from Acadian orogeny in the rest of the northern early Gedinnian to possibly early Emsian (Early Appalachians. MEGUMI TERRANE, NOVA SCOTIA 83 Copyright American Geophysical Union Geodynamics Series Plate Reconstruction From Paleozoic Paleomagnetism Vol. 12 The Meguma terrane contains no strata of Middle the Devonian. In his model, however, Late Paleo­ and Late Devonian age. Early Carboniferous zoic dextral transcurrent motion follows between (Tournaisian) red beds of the Horton Group rest Euramerica and Gondwana. In contrast, Williams unconformably on the pre-Carboniferous rocks. [1979] and Bird and Dewey [1970] cite the simi­ The Horton Group contains 1500m of conglomerate, larity between Meguma Group graywacke& and arkose, sandstone, siltstone, shale and coal Cambrian deposits of the Welsh Basin [Rast et [Hacquebard, 1972] deposited in subaerial, flu­ al., 1976], speculating that the Meguma terrane viatile and lacustrine environments. Conformably could represent a "faulted trough that developed overlying the Horton sediments are Visean supra­ within the Avalon Zone~ This interpretation tidal to shallow marine clastics and carbonates implies that the current geographic relationship of the Windsor Group. The Horton and Windsor between the zones of the Appalachian orogen was Groups are thought to have been deposited in established by the end of the Ordovician and has pull-apart basins formed in a broad region of not changed significantly since. Van der Voo transcurrent faulting in the Maritimes [Bradley, [1982] incorporates the Meguma Zone as part of an 1982; Fralick and Schenk, 1981; LeFort and Van Armorica plate, which includes the Avalon Zone, der Voo, 1981; Keppie, 1982a,b]. The fluviatile Great Britain south of the Great Glen fault, and clastics of the Scotch Village Formation of late the Armorican Massif of France. In his model, Westphalian age [Hacquebard, 1972] disconformably Armorica first rifts away from Gondwana in the overlie the Horton-Windsor succession. A paleo­ Late Precambrian - Early Paleozoic, collides with magnetic study of some of these Windsor to North America adjacent to the southern Appala­ Westphalian age rocks is reported by Scotese et chians in the Ordovician, and moves northward to al. [this volume]. its present position during Carboniferous sinis­ The Carboniferous rocks of southern Nova Scotia tral transcurrent motion between North America were deformed during the Hercynian orogeny which and an assemblage of Armorica and the Baltic is generally correlative with the Alleghanian shield. orogeny in the United States Appalachians. De­ Paleomagnetism offers the opportunity to con­ formation ocurred mainly during Westphalian and strain models of the motions of suspect terranes Permian times [Keppie, 1982b] with the most by either comparison of segments of apparent intense deformation along the northern margin of polar wander (APW) paths of geologic terranes the terrane [Keppie, 1982a]. over the same time period or by comparison of A regional unconformity separates the under­ paleolatitudes with those inferred for other lying Paleozoic section from Late
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