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

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

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 Triassic to terranes or continental blocks. This paleomagne Early Jurassic continental red beds and plateau tic study of Paleozoic rocks from the Meguma Zone basalts exposed along the Bay of Fundy. [Crosby, was conceived with the hope of finding con 1962]. The age of these rocks has been inferred straints on plate tectonic models involving the from Upper Triassic fossils in the lowermost and Meguma terrane. uppermost sediments and from a K-Ar whole rock age of 198ma for the North Mountain plateau Sampling basalt [Keppie and Smith, 1978]. The syn-rift deposits of this Triassic - Jurassic graben are Siluro-Devonian sections (White Rock and similar in many respects to those in other basins Torbrook Formations) were examined in two locali of the Newark Series in eastern North America. ties along the southern slope of the Annapolis Several plate tectonic models have been deve Valley prior to sampling. Deformation of the loped to explain the relationships between the rocks in the Bear River area was observed to be Meguma terrane, the adjacent parts of the Appala much more penetrative in character than that to chian orogen and cratonic North America. Schenk the east in the Nictaux Falls - Torbrook area. [1978] envisions the Meguma terrane as an Early Here, the White Rock and Torbrook Formations are Paleozoic eugeoclinal sequence marginal to the exposed in the core of the Torbrook Sync line, a Morroccan Meseta, emplaced in its present posi northeasterly trending structure intruded by tion following the Late Paleozoic collision of Devonian granites to the South and onlapped by Gondwana - North America and separated from La~e Triassic sediments on its northern edge Morrocco by subsequent Mesozoic rifting. Recon (Fig. 2). The syncline is a large amplitude structions by McKerrow and Ziegler [1972] and isoclinal fold with subvertical to slightly over Smith et al. [1973] place Meguma against turned limbs, of large enough extent to be appa Colombia, South America, off "northwestern" rent on the scale of the geologic map of Nova Gondwana. Subsequent collision between this part Scotia [1:500,000; Keppie, 1979]. Small scale of Gondwana and the northern Appalachians during folding associated with the major structure has the Devonian, followed by opening of a been observed only in the core of the syncline Carboniferous ocean, left the Meguma terrane in and not on the limbs in the Torbrook - Nictaux its present location relative to North America. Falls area. While a "pencil" cleavage is present Similarly, Keppie [1977] places Early Paleozoic in shaly layers of the Torbrook Formation, coar Meguma off Gondwana adjacent to cratonic basement ser grained sandstones and the volcanics of the of Colombia - Central America previous to colli White Rock Formation show relatively little pene sion with Avalon and North American terranes in trative deformation. 84 SPARIOSU ET AL.

45 5"

N i

Ill "INMI 8EIFRAC1l.IE ~ DEYOtO-CARIONIFERDUS BRANITE ~ LATE TRIA88JC-EMILV.._...... JI.RWSIC /VV ACADIAN CRJBENY

EAALV DEYIINIM TmtMDBIC 11ft """""" tERCVNIAN~ '

LATE CARIDUFERIIUB PICT1JU 8RP •I•SKI lMDIVIDED LATE SILURIAN I LCcl

VISEAN WINDSCit 8RP joswl SILI.ID-ORDOYJCIAN 111-tJTE ~ ROCK Flt

TCUtNAIBIAN I·DU...... ~ ~ Fig. 2. Simplified geologic map of the northwestern portion of the Meguma Zone, Nova Scotia [modified from Keppie, 1979]. Inset shows detail of the Torbrook Syncline area. "X"s mark sam pling localities for the Torbrook and White Rock Fms. (inset) and the Cheverie Fm. (large map).

Samples were collected using a portable was present to allow application of a fold test diamond-bit coring drill and oriented with a [McElhinny, 1964] although it is still south of magnetic compass according to standard paleomag the zone of penetrative deformation of this for netic practice. Nine sites consisting of six or mation [Boyle, 1963]. more oriented cores each were collected in the White Rock Formation on the south limb of the Rock Magnetic Studies Torbrook Syncline along Fales River, 4 in rhyo lites (rhyodacitic ignimbrites) and 5 in basalt Procedures flows. On the north limb, 11 sites were sampled along the Nictaux Canal, 6 in the rhyolite and 5 Oriented drill core samples were sliced into in the overlying basalts. Sandstones of the 2.4 em specimens, yielding one to three specimens Torbrook Formation were sampled along Spinney per core. Natural remanent magnetizations (NRM) Brook at 4 sites on the north limb of the syn were measured on either a fluxgate spinner magne cline and at 6 sites upstream on the south limb. tometer [Molyneaux, 1972] or a cryogenic magneto Sampling sites were well north of the contact meter [Goree and Fuller, 1974]. Representative metamorphic zone adjacent to the Meguma specimens were then selected for pilot alterna Batholith, except those along the Nictaux Canal ting field (AF) and thermal demagnetization expe which were near, although outside of the aureole. riments. During stepwise thermal demagnetization Red sandstones of the Early Carboniferous procedures, bulk susceptibilities (K) were Cheverie Formation were collected at 11 sampling measured following each heating/cooling cycle for sites along the shore of the Minas Basin in the the purpose of detecting magnetochemical changes Walton-Cheverie area (Fig. 2). This locality was due to the heating/cooling process [Dunlop, chosen because it appeared that enough structure 1972]. MEGUMI TERRANE, NOVA SCOTIA 85

Additionally, thin (0.6 to 1.2 em) disks of sample magnetization whose direction however is samples from the Torbrook and Cheverie sandstones removed from both the present field and the were cut and radially slotted for stepwise chemi dominant southeasterly direction but which may cal demagnetization. These were soaked in 8N HCl correspond to a Triassic overprint; the distribu while kept in field-attenuated space for periods tion of sample directions at this (SWH) and totalling up to greater than 500 hours. Proce another (SWG) basalt site are random. dures followed were generally similar to those Magnetization intensities of rhyolites and described by Henry [1979] and Roy and Park basalts at the Fales River locality are more [1974]. comparable, Sxlo-2 Am-1 and lo-1, respectively, Following analysis of pilot demagnetization and in general a much higher proportion of the experiments, specimens from all remaining samples samples provided usable data. The dominant direc were subjected to thermal, AF or the combination tion is southeasterly with moderate positive thereof deemed likely to isolate and establish inclination, as at Nictaux Canal, and typically the stability of all significant magnetization is revealed over a moderate coercivity range components. All specimens used in the final (Fig. 3e). In one rhyolite site (SWN), all but a analyses have been subjected to a minimum of single sample showed unstable demagnetization three demagnetization steps. Statistical proce behavior; in that sample the southeasterly compo dures used in data reduction include linear nent is observed over intermediate stages of AF regression component analysis [Kent, 1981], demagnetization but is followed by what may be a spherical dispersion analysis [Fisher, 1953], as present-day field direction at higher demagneti well as three level averaging techniques [Irving, zation levels (Fig. 3f). Finally, one basalt 1964]. site (SWQ; Table 1) was characterized by an ano malously high Koenigsberger ratio (>30) which may have resulted from lightening effects. On this Results basis, and also because the directions although well grouped are aberrant, results from site SWQ White ~ Formation. The rhyolites at the are rejected from further consideration. Nictaux Canal locality are typically weakly Site mean directions for the White Rock Forma magnetized, on the order of lo-4 Am-1 compared to tion are listed in Table 1 and plotted in Figure remanent intensities of about lo-1 for the 6a. In situ directions group in the southeast basalts. More pertinent is that linear demagne quadrant, primarily about a moderate (250) incli tization trajectories are either not apparent in nation although some streaking of directions many of the rhyolite samples (11 of 35) or else toward the present field direction is noted (Fig. the directions isolated are often not consistent 6a). The in situ mean directions for Nictaux from sample to sample within a site (3 sites of 6 Canal and Fales River are nevertheless almost have sample direction distributions that cannot identical, less than 5° apart. Following correc be excluded as random at the 95% confidence tion for bedding tilt (finite rotation about level; Table 1). These magnetic characteristics bedding strike) the Nictaux Canal and Fales River may be attributable to the proximity of the site groupings diverge and the overall dispersion rhyolite to the contact metamorphic zone of the increases significantly. Although there is some Meguma Batholith at the Nictaux Canal locality, suggestion that the resulting pattern (Fig. 6a) which may have resulted in complex thermochemical constitutes a dual polarity set, observe that all alteration of the original carriers of magnetiza of the northwesterly directions come from sites tion. Vector end-point demagnetization diagrams on the ~outh limb of the syncline while the of well-behaved rhyolite samples from the Nictaux southeasterly site means are from sites on the Canal locality are shown in Figure 3. Note that north limb. Consideration of the fold geometry the magnetization directions revealed by both AF reveals that the structural correction involves and thermal demagnetization in companion speci near 90° rotations in opposite senses for the two mens from a sample (Fig. 3a) are virtually iden limbs of the syncline since the in situ site mean tical. Magnetite appears to be the dominant directions are nearly perpendicular to bedding carrier of magnetization. Consequently, AF strike. Also note that the two post-correction treatment was used for progresive demagnetization groupings do not form an antiparallel pair, but in the majority of White Rock samples. Figure 3b in fact are only 148° apart. It is thus appro shows an example from a site where individual priate to conclude that the pattern observed in samples display a linear demagnetization trajec the corrected directions is an artifact of the tory but the isolated directions appear to be structural geometry and that the age of magneti randomly distributed (Table 1). zation for the Silurian White Rock Formation is The basalts at Nictaux Canal gave marginally of post-folding (post Early Devonian) origin. better results. The dominant direction (18 sam Other evidence relevant to the possible age of ples from 3 sites) is toward the southeast with this secondary magnetization will be discussed in moderate inclination (Fig. 3c) and is carried by a later section. a magnetic phase of moderate to high coercivity Torbrook Formation. One pilot specimen from and blocking temperatures less than 550°C. each site in the Torbrook Formation was selected Figure 3d shows an example of a univectorial for stepwise AF demagnetization and at least one 86 SPARIOSU ET AL.

TABLE 1. Site mean directions for the White Rock Formation

In situ Site Lithology N/na Dec!. Incl. k a95 (0) (0)

1. Nictaux Canal (Bedding strike/dip .. 32°/71°SE)

SWA Felsic 4/6 144.5 60.5 193. 6.6 SWB Felsic 4/6 ( 87.4 60.7 2.2 83. )b swc Felsic 4/6 ( 350.5 26.6 1.8 104. )b SWD Felsic 5/6 153.0 12.5 72. 9.1 SWE Felsic 3/6 150.7 -35.3 2.4 111. )b SWF Felsic 4/5 158.5 -12.5 17. 23. SWG Mafic 6/6 ( 180.6 65.6 1.9 68. )b SWH Mafic 6/6 ( 285.1 74.0 1.5 89. )b SWI Mafic 6/6 147.2 28.7 27. 13. SWJ Mafic 6/6 148.7 32.9 33. 12. SWK Mafic 6/6 141.4 9.6 107. 6.5 Kean (6/11 sites}: 149.3 21.9 10 22 (after tilt correction 156.4 -41.1

2. Fales River (Bedding strike/dip = 229°/88°NW} SWL Felsic 6/6 158.9 17.1 21. 15. SWM Felsic 4/6 167.6 23.3 24. 19. SWN Felsic 1/6 )C swo Felsic 5/6 145.9 45.8 65. 9.6 SWP Mafic 4/6 146.9 20.7 58. 12. SWQ Mafic 5/5 289.0 71.8 103. 7.6 )d SWR Mafic 6/6 146.7 18.5 48. 9.8 sws Mafic 4/4 132.2 27.5 19. 22. SWT Mafic 6/6 142.4 28.4 138. 5.7 Kean (7/9 sites): 148.9 26.2 33 11 (after tilt correction 298.4 64.0

Formation mean (13/20 sites, in situ): D-= 149.1° I-= 24.3° k-= 18 a. 5"' 10° corrected for post-Triassic tilt (strike, dip "' 250° /7°N~: 148.4° 31.2°

Pole position: 21.9° N Lat., 147.7° E Long., dp ,dm 6.2, 11.2

aNumber of samples used in mean calculation/total number of samples. bsite excluded because random distribution hypothesis could not be rejected at the 95% confidence level. conly one sample gives an interpretable demagnetization trajectory. dsite SWQ excluded because of anomalously high Koenigsberger ratio (>30), possibly a result of lightning strike.

for detailed thermal demagnetization. Stepwise coercivity/blocking temperature magnetization thermal demagnetizations of the samples already components, but they appear to be of minor sig AF demagnetized were also carried out. Vector nificance in terms of the magnitude of their end-point diagrams of typical demagnetization contribution to the NRM and possess no signifi curves are shown in Figure 4. Initial demagneti cant directional grouping. Subsequent thermal zation steps of lOmT or 2000C remove low demagnetization to 660° reveals three main clas- MEGUMI TERRANE, NOVA SCOTIA 87

Copyright American Geophysical Union Geodynamics Series Plate Reconstruction From Paleozoic Paleomagnetism Vol. 12 a) s b) JOO SWA SWE 4A c) w UP s 5mT I s

lA """'iD-4 ,/...... IB /.--- E ON E,DN I NRM 0 "!J_ __ N NRN SWKGA ~ E D~

d) SWH 3A

-~-

NRN

e) SWT 4A f) SWN lA 10 s ------tl~

ON NRM N Fig. 3. Orthogonal demagnetization diagrams [Zijderveld, 1967] of specimens from the White Rock Formation. Open circles represent projections on the vertical, North-South plane, closed circles projections on the horizontal plane. Demagnetization fields in millitesla (mT), temperatures in degrees C~lsius. Magnetization intensity units (scale bars or axes units) all in amperes per meter (Am- ). (a) AF and thermal demagnetization of two specimens from the same core sample of a basaltic flow. (b) Example of a linear demagnetization trajectory in a specimen from a site with a random distribution of directions. (c) AF demagnetization of a basalt specimen from the Fales River section. (d) A stable direction which is thought to represent a Triassic overprint. (e) Single component magnetization in a basalt from Fales River. (f) Multicomponent magnetization in a rhyolite from Fales River. 88 SPARIOSU ET AL.

a) DTBJ-IA UP

b) DTBC-5A W UP 660° DTBB-18

1.0

zof

NRM E,DN E DN Fig. 4. Demagnetization plots for specimens from the Torbrook Formation. Symbols and units are the same as in Figure 3, all are plotted in in situ coordinates. (a) A specimen illustrating typical behavior of "normal" polarity magnetized samples. (b) Example of a reversely magnetized specimen. (c) Specimen showing evidence of components of both polarities, normal from 400°C to 600°C, reversed from 600° to 660°. (d) An example of predominant behavior observed during chemical demagnetization. Note the difference in direction compared to thermal demagnetization.

ses of directional behavior: 1) generally linear most effective cleaning procedure for the bulk of decay to the origin of a south to southwesterly the remaining samples, which were demagnetized component with shallow negative inclinations with a minimum of four temperature steps to 640- (Fig. 4a), 2) similar behavior of a component 6600. directed north to northeasterly with 20-30° posi Site mean results for the thermally demagne tive inclinations (Fig. 4b), and 3) removal of tized Torbrook sandstones are listed in Table 2 either the northerly or southerly component, al and plotted in Figure 6b. In situ site mean though missing the origin, followed either by directions form an apparently dual polarity set removal of the other or unresolvable directional and in fact, mean reversed (southerly) and mean behavior. Figure 4c illustrates the removal of normal (northerly) group means do not differ the northerly, down component between 400 and significantly from bipolarity. After correction 6000, followed by removal of the southerly compo for bedding tilt, however, dispersion increases nent from 600°. AF demagnetization to lOOmT most dramatically in both normal and reversed polarity often results in stable directional behavior in groups. Dispersion also increases when the entire either the northerly or southerly directions group is treated as a dual polarity set of the noted above but is ineffective in substantially same paleomagnetic field (reversed directions reducing magnetization intensities, hence step inverted). We infer from this that the magneti wise thermal demagnetization was selected as the zation is post-folding (post-Early Devonian).

MEGUMI TERRANE, NOVA SCOTIA 89

TABLE 2. Site Mean Directions for the Torbrook Formation

In situ Tilt Corr. 8 Site Bedding N/n Pol. Decl. Incl. a95 Decl. Incl. (0) (0) (0) (0)

DTBA 36/858 6/7 N 6.5 26.9 4.7 63.8 27.8 DTBB 36/85S 5/6 M 22.9 60.1 21.5 96.4 10.8 DTBC 36/858 5/5 M 19.2 28.0 13.7 63.8 17.2 DTBD 36/85S 3/4 R 199.1 -18.0 9.0 233.2 -17.6 DTBE 215/94N 5/6 N 42.1 22.7 22.7 11.8 5.0 DTBF 215/94N 6/6 N 26.5 30.3 12.4 4.9 -9.4 DTBG 215/94N 6/6 N 15.2 31.8 11.3 2.7 -18.9 DTBB 233/93N 3/4 Mb 13.6 30.4 35.3 17.4 -35.0 DTBI 233/93N 5/5 M 350.3 2.5 27.0 53.4 -62.3 DTBJ 233/938 4/6 M 6.9 38.6 22.9 5.4 -36.5

Formation mean:c in situ D= 15.8°, I .. 29.6°, a95= 11.7o • k= 18.1, B= 10 sites tilt corrected 36.2°, -9.1°, 29.1°, 3.7

Pole (in situ): 58.0° R Lat., 85.3° E Long., dp= 7.1°, dm= 12.9° Corrected for post-Triassic tilt: (strike= 70°, dip= 7°N) 55.5° N Lat., 90.7° E Long.

8 Magnetization polarity; N=normal, R=reversed, M=mixed polarity, some sam ple directions inverted to dominant site polarity. bAll samples from Site DTBB had reversed directions at intermediate temperatures, normal at high temperature (>600°C); only the high temperature component is included in the site mean calculation. csite DTBD direction inverted.

Although the presence of both polarities was once spurious components in the first (4 hour) step thought to be an indication of a primary magneti (Fig. 4d). The directions of these three lie zation [McElhinny, 1973], dual-polarity seconda along the present day field direction for the ry magnetizations have been documented elsewhere sampling locality (in situ coordinates). After [eg., Kent, et al., 1982]. Rote that in the case bedding correction, however, directions from the of the Torbrook sandstones, both polarities are two specimens sampled on the southern limb of the observed within single sites and sometimes within syncline move away from the direction of the single specimens. Also, there is no apparent specimen from the northern limb. We believe that relationship between structural position in the the component removed by chemical demagnetization syncline and polarity nor any obvious stratigra (and not recognized during thermal demagnetiza phic control over polarity zonation. The combi tion) represents a Tertiary to Recent magnetiza nation of dual polarity and secondary charac tion that is unstable under AF and high tempera teristics of this magnetization points, we think, ture but resistant to dissolution in BCl, to a long process of magnetization, most likely a although the small number of samples chemically slow chemical process spanning at least the time demagnetized makes any such statement equivocal. required for a field reversal. A possible explanation for the behavior of the In order to learn more about the magnetization sandstones during these different demagnetization mechanism of these rocks, 6 specimens were procedures is: 1) the recent magnetization is subjected to the chemical demagnetization proce carried by large multidomain (detrital?) magne dures described above. One of the specimens tite or hematite grains with very low coercivi disintegrated during early stages of the process ties and blocking temperatures but whose grain and results are not used further. Of the re size renders them resistant to attack by the maining five, two of the specimens showed scatter acid; and 2) the stable secondary magnetization and/or unresolvable directional behavior during revealed during thermal demagnetization is the demagnetization process, three demagnetized carried by fine-grained hematite which was either univectorially to the origin following removal of deposited interstitially long after deposition or

90 SPARIOSU ET AL.

-3 MCB lA W,UP

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-s~[o- .00.• ~·zOo- 300.'~· 400• 500-·~I sao- too•I E,DN T b) c) d) MCA-28

1.0 o.s MCJ-2A

o.s

NRM lxi0-1 A/m

E,DN

100° E,DN E,DN Fig. 5. Typical demagnetization behavior of samples from the Cheverie Formation; symbols and units as in Figure 3. (a) Zijderveld plot alongside temperature vs. bulk susceptibility plot for a typical specimen; note the sudden increase in intensity and the unstable directional behavior associated with the sharp rise in bulk susceptibility beginning around 600°C. (b) Typical behavior during thermal and AF demagnetization of samples from 9 of the 11 sites. (c) Example of behavior noted in samples from two sites. The trajectory initially does not trend toward the origin but at higher temperatures begins to reverse and approach the origin from a direction perhaps antiparallel to that exhibited in (a) and (b). Unstable behavior above 600° prohibits isolation of this component. (d) Chemical demagnetization in 8 Normal BCl. Demagnetization levels are in cumulative hours in the acid solution (B).

formed as an oxidation product from ferrosili Cheverie Formation. NRM directions of Cheverie cates and/or oxide grains, thus leaving it vulne Formation samples group predominantly in the rable to dissolution in acid. More rock magnetic southeast quadrant with inclinations varying from and petrographic work is needed to better under steeply positive (down) to -25 to -30°. AF stand the magnetization history of the Torbrook demagnetization to fields up to lOOmT generally sandstones. fails to remove more than 10 to 20% of the MEGUMI TERRANE, NOVA SCOTIA 91

remanence, although demagnetization trends are A) WHITE ROCK toward the origin. Decay during thermal demagne N N tization follows the same trajectories observed during AF in most cases, up to about 550°-600°C, although heating is much more effective in a a reducing total magnetization intensities; above a these temperatures spurious directional behavior 270" 90° is observed, often along with sudden increases in ...... + intensity (Fig. Sa). This behavior occurs in 0 conjunction with increases in bulk susceptibility from one to four orders of magnitude above ini tial values. Such behavior has been associated s s with the formation of magnetic mineral phases m situ fih corrected during heating and cooling in air [Dunlop, 1972; Bl TORBROOK Kent and Opdyke, 1978]. This characteristic of N N these rocks prevents us from isolating any dis tinct components residing at blocking tempera tures above 6oooc, a blocking temperature range usually important in redbed magnetizations. Nonetheless, the linear demagnetization trajecto 270" 90° ries observed at lower temperatures and their direct trends toward the origin lead us to believe that we have effectively isolated the dominant magnetization of the Cheverie Formation. Exceptions to the behavior described above were s s some samples from sites MCI and MCJ. The initial Fig. 6. Equal area projections of site mean component removed by AF demagnetization and directions from the studied rock units. Filled thermal demagnetization up to 3oooc in these (open) symbols are projected on the lower (upper) appears to be the same southeast and down magne hemisphere. (a) White Rock Formation (Table 1); tization observed in the others, however, the also shown are the direction of the present day linear trend clearly bypasses the origin (Fig. field at the locality (*) and the bedding plane 5c). Subsequent thermal demagnetization begins orientations at the two sections sampled. (b) to remove a component directed north to northeas Torbrook Formation; point marked ''H" represents terly with intermediate positive inclination. the intermediate temperature reversed component Another example (thermal demagnetization only) in site DTBB samples and was not used in the shows the northerly component alone. Unfortu formation mean calculation. nately, the spurious magnetizations due to heating precluded isolation of this component in any of the other samples. chemical demagnetization. AF demagnetization Chemical demagnetization experiments reveal no revealed the southeast, down component in every magnetization components not noted above from sample, while acid leaching removes the south thermal demagnetization. In fact trajectories of east, up component observed during thermal demag the demagnetization plots look remarkably similar netization of the "a" specimens from this site in for either thermal or chemical procedures (Fig. 4 out of 5 instances, the other revealing the 5d). One specimen from site MCJ shows the north downward component. These two components are - northeasterly, down component observed during significantly different (Table 3). The component heating in site MCI. This component is not removed by AF demagnetization is directionally isolated during thermal demagnetization of MCJ nearer the single component magnetization ob samples although trajectories of the southeaster served during both AF and thermal studies of ly component "miss" the origin, thus suggesting samples from the rest of the sites. While the its presence. The principal magnetization compo unique behavior of samples from site MCD remains nents of the Cheverie sandstones, in contrast to unexplained, we note that the southeasterly, up those of the Torbrook Formation, show no directions from the thermal studies more closely differences between blocking temperature and matches the secondary magnetization direction grain solubility spectra. obtained by Scotese et al. [this volume] from Two magnetization components are observed in similar age rocks of the nearby Schubenacadie samples from site MCD, a southeasterly, down basin. direction removed ty AF demagnetization, or, in Site mean directions of the southeasterly one instance, below 200°C thermal demagnetiza component (interpreted as the characteristic tion, and a southeasterly, up direction removed magnetization) of the Cheverie are lisLad in by routine thermal demagnetization. Because this Table 3 and plotted in Figure 7b. Although a was the only site which appeared to possess two simple correction for bedding tilt (one-stage distinct, isolatable magnetizations, ''b" speci finite rotation about bedding strike) produces no mens were halved and one specimen from each better grouping of site mean directions, the samrle subjected to stepwise AF and stepwise variations in site mean inclinations seem gene- 92 SPARIOSU ET AL.

TABLE 3. Site Mean Directions for the Cheverie Formation

In Situ Unfolded8 Site Bedding N/n Decl. Incl. Decl. Incl. (0) (0) (0) (0)

MCA 56/14S 3/3 143.1 36.0 18.3 142 26 MCB 73/14S 5/5 142.6 40.7 24.0 149 32 MCC b 43/20S 4/5 157.9 26.5 16.7 153 15 MCDTh 52/US 6/6 151.7 -18.4 13.2 152 -27 MCDAF 52/US 5/6 146.9 37.6 16.7 146 27 MCE 272/12N 5/5 144.8 21.3 11.4 142 30 MCF 286/14& 5/5 151.4 29.5 10.0 147 35 MCG 286/14N 5/5 152.1 15.6 13.8 149 25 MCB 23/75E 5/5 166.4 47.3 10.2 133 7 MCI 18/41E 3/5 160.4 42.4 30.5 141 23 MCJ 28/77E 4/4 180.6 56.9 10.9 139 22 MCK 18/07E 6/6 165.1 29.5 U.l 162 27

Formation mean (11 sites): in situ D• 155°, Ia 35°, k• 30 unfolded 146°, 25°, 59

Pole position (unfolded directions): 24° N Lat, 152° E Long, dp • 3°, dm = 6°

:corrected for flexural slip distortion and bedding tilt (see text) Th • thermal demagnetization result, AF = alternating field de magnetization result; Th not included in format ion mean calculation (see text)

rally related to the dip of the beds. Examina Devonian) and its mean in situ direction yields a tion of the minor folds in the area suggests that paleomagnetic pole at 24.70 H. Lat., 147 .2o E. flexural-slip or flexural-flow were the main fold Long. While there are no constraints on the mechanisms, which would modify the site mean minimum age of this magnetization, the pole does directions as shown in Figure 7a. Ramsay [1967] not fall near any post Devonian segment of the describes the geometry of this type of deforma North American apparent polar wander path and tion and a method for removing its effects. thus may be considered anomalous in that respect. Unfolding of the Cheverie site mean directions The mean direction of the Torbrook Formation was accomplished by first rotating them to bring (D .. 15.80, 1=29.60, Cl 95-11.70) corresponds to a the fold axes to the horizontal, and then post-folding (post Early Devonian) magnetization rotating them along small circles about the with a pole at 58.00 H. Lat., 85.30 E. Long., far appropriate fold axis (031/12NE or 077/5NE) removed from the White Rock pole. Unlike the through the angle [e-

[Beck, 1972; de Boer 1 1968i Opdyke, 1961]. Paleopoles and Implications If the Torbrook and White Rock magnetizations are Late Triassic - Early Jurassic or older, then The White Rock Formation magnetization is the mean directions should be corrected for the clearly post folding in age (post-Middle sloping of the pre-Late Triassic peneplane and MEGUMI TERRANE, NOVA SCOTIA 93

N N c) 330

300

w E

\~ ~ 210 s s Fig. 7. (a) Cross section of a flexural-slip fold showing effect of folding on magnetization vectors or other linear features. The correction for this effect is explained in the text. (b) Equal area plot showing in situ site mean directions, poles to bedding planes with fold axes and unfolded site mean directions. The only direction not included in the formation mean calculations is the one shown on the upper hemisphere and represents thermal demagnetization ~esults from site D.

overlying strata, which in the Annapolis Valley reported in abstract by Seguin et al. [1981], who strike 700 and dips approximately 70 to the found two magnetization components in Torbrook North. This rotation yields a direction of sandstones with directions of D=320o, I=-100 (in D=l3.8°, 1=23.8° for the Torbrook, corresponding situ), and D=2300, 1=30° (tilt corrected, in situ to a pole at 55.5° N. Lat., 90.7° E. Long. in unknown). Details of this work are not yet close proximity to poles from the Middle Triassic known, although most of their samples came from a Hanicouagan impact site, Quebec [Larochelle and different locality than in this study [Seguin, Currie, 1966; Robertson, 1967]. The same rotation personal communication, 1982]. It is interesting, on the White Rock gives a direction of D=l48.4°, however, that their secondary direction from the 1=31.2°, with a pole at 21.9N, 147.7E. The Torbrook is roughly antiparalle~ to the secondary Torbrook direction is not similar to results direction in the White Rock Formation. 94 SPARIOSU ET AL.

Discussion

The paleomagnetic poles from the White Rock and Cheverie formations can provide useful cons traints on the relationships between the Meguma Zone and adjacent terranes in the northern Appalachians. Figure 8 shows these poles plotted relative to the APW paths for North America and the Acadia (Avalon) terrane for Late Paleozoic to Early Mesozoic time. Comparing them first with the APW for cratonic North America, we note that the Meguma poles are about 30° away from North American mean poles for either early or late Carboniferous time (Fig. 8). This implies a 20 to 25° counter-clockwise rotation of Meguma with respect to North America. Also implied is a northward translation of Meguma with respect to North America, considering the 13-17° South paleolatitude inferred from the Meguma poles and the near equatorial paleolatitude of North America at this time. These motions must both occur at some time between the Tournaisian and late Westphalian, sometime before the Westphalian assembly of Pangea [Vander Voo et al., this volume]. There is no room for Meguma to translate Fig. 8. Paleomagnetic poles from this study northwards at any other time prior to the opening (diamonds) plotted against the apparent polar of the present Atlantic Ocean. wander path for North America (solid circles). The White Rock - Cheverie poles are also about North America APW from Van der Voo and French 300 away from Acadia mean poles for the early to [1974] with Early and Late Triassic mean poles late Carboniferous. This can be accounted for a recalculated to shift Manicouagan poles from similar 20 to 250 counter-clockwise rot at ion of early to late Triassic based on K-Ar ages of Meguma with respect to Acadia. There does not Wolfe [1971]. The open circle represents the appear to be a significant paleolatitude diffe Early Carboniferous pole for Acadia (Avalon rence (paleolatitude=l7° s. for the White Rock, Terrane) from Roy and Park [1974]. Asterisks (*) 13° s. for the Cheverie, and 9° S. for Acadia; denote Late Triassic to Early Jurassic poles from Fig 9) within the range of statistical error and southern Nova Scotia (see text). possible age differences of these magnetizations. The degree of rotation inferred from comparison to older results from the Avalon terrane [eg., Roy and Park, 1974] is substantiated by new As noted above, the fold test for the Cheverie results from the Nova Scotia Avalon terrane Formation suggests an Early Carboniferous age of (North of the Chedabucto- Cobequid fault) pre magnetization. This further implies that the sented in this volume by Scotese et al. This unfolded formation mean direction (D=l46°, 1•25°) rotation is consistent with Keppie's [ 1982a] and the corresponding pole (24° N. Lat., 152° E. hypothesis of a collision of the Meguma terrane Long.) are representative of the Early Carbonife with southern New Brunswick following transcur rous paleomagnetic field with respect to the rent displacement along the Minas geofracture Meguma terrane. A noteworthy aspect of this during the Hercynian Orogeny. Although the paleopole is its proximity to the White Rock Triassic opening of the Fundy Basin could be Formation secondary pole (Fig. 8), suggesting associated with a counter-clockwise rotation, its either similar magnetization ages for the two width is far too narrow ("'lOOkm) to account for rock units or slow polar wander rates with the implied twenty degree rotation. We infer respect to Meguma between the times of their from these results that independent motion of the magnetization. Indeed, if the White Rock remag Meguma terrane relative to both North America and netization is due to a thermal resetting caused the Avalon terrane during the Carboniferous, by the intrusion of nearby plutons, its age (361- although Meguma and Avalon may have already been 371ma) may not be much older than the depositio in close association [Keppie, 1983]. nal age of the Cheverie Formation (Tournaisian). Unfortunately, these data do not permit a test If this is true, then these poles, from rock of the hypothesis that the Meguma Zone was once units affect,ed by different deformation events closely associated with Gondwanaland. As noted and sampled in localities separated by 50 km, above, the oldest that the White Rock and must represent the position of the Meguma Zone Cheverie magnetizations can be is Early Carboni for some time near the Early Carboniferous. ferous. During the Early Carboniferous, paleola-

MEGUMI TERRANE, NOVA SCOTIA 95

counter-clockwise with respect to Avalonia and North America. The Carboniferous basins of the Maritime provinces may have formed and deformed in response to these tectonic motions. It seems unlikely that the Meguma terrane was internal to _19~ _N_----- a larger Armorica plate as proposed by Van der Voo and Scotese [1981] considering this evidence for its independent motion, although it may yet prove to have an origin in close association with some part of Gondwanaland.

Acknowledgments. This study was funded by National Science Foundation grant EAR 80-07748. The manuscript was reviewed by G. Bond, C. Scotese, and L. Tauxe. Field assistance provided by S. Coughlin and C. Kent, laboratory assistance by D. Lafferty, Lamont-Doherty Geological Obser vatory contribution #3509.

References

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96 SPARIOSU ET AL.

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