Deep seismic reflection data from the Bay of Fundy and Gulf of Maine: tectonic implications for the northern Appalachians

Deep seismic reflection data from the Bay of Fundy and Gulf of Maine: tectonic implications for the northern Appalachians1

C. E. KEEN AND W. A. KAY Atlantic Geoscience Centre, Bedford Institute of Oceanography, Dartmouth, N.S., Canada B2Y 4A2

D. KEPPIE Nova Scotia Department of Mines and Energy, P.O. Box 1087, Halifax, N.S., Canada B3J 2x1

F. MARILLIER Atlantic Geoscience Centre, Bedford Institute of Oceanography, Dartmouth, N.S., Canada B2Y 4A2

AND

G. PE-PIPER AND J. W. F. WALDRON Department of Geology, Saint Mary's University, Halifax, N.S., Canada B3H 3C3

Received October 12, 1990 Revision accepted March 6, 1991

Three deep-penetration seismic reflection profiles were collected off southwest Nova Scotia to determine the crustal structure and geometry beneath the Avalon and Meguma zones of the Appalachian Orogen in Canada. Onshore geological features have been traced seawards using new gravity and magnetic anomaly maps. The seismic data can also be correlated with the previous United States Geological Survey profile in the central Gulf of Maine.

Two seismically distinct lower crustal blocks are identified: the Avalon and Sable lower crustal blocks, separated by a major north-dipping reflection zone that cuts the entire crust. The recognition of the Sable block adds a fourth block to the three already identified in the Canadian Appalachians. The Sable block is overlain by the Meguma Zone. The Avalon Zone overlies at least the northern part of the Avalon lower crustal block. Although offshore extension of geological features is not unequivocal, it appears that a north-dipping reflection zone southwest of Nova Scotia marks the site of Devonian thrusting of Avalon Zone over Meguma Zone. In the Bay of Fundy to the north, two south-dipping reflection zones are interpreted as major thrusts, possibly placing Avalon lower crust over a unit with different tectonic affinities. The Fundy Fault is a Carboniferous thrust within the Avalon block along the coast of New Brunswick; this was reactivated during Mesozoic extension as a transtensional fault. Extensional displacement farther southwest was probably accommodated along east-west- trending faults and small rift basins associated with them.

Trois profils de sismique reflexion, de penetration profonde, ont kt6 enregistrks au sud-ouest de la ~ouve l le -~cosse , dans le but de determiner la structure et la gComCtrie de la croilte entre les zones d'Avalon et de Meguma de 1'Orogkne des Appalaches du Canada. Les particularites gCologiques obsewkes sur le continent sont tracks au large sur les nouvelles cartes d'anomalies gravim&iques et magnttiques. Les donnCes sismiques ont kt6 mises en correlation avec le profil dCji fait par le United States GeologicaI Survey pour la partie centrale du golfe du Maine.

La sismique permet de reconnaitre I'existence de deux blocs de croilte infkrieure : d'Avalon et de Sable, lesquels sont sCparCs par une importante zone de reflexion, de pendage nord, qui traverse la totalit6 de la croilte. Le bloc de Sable, nouvellement reconnu, forme un quatrikme block qui s'ajoute aux trois autres d6ji CtC identifks dans les Appalaches du Canada. Le bloc de Sable est sous-jacent a la Zone de Maguma. La Zone d'Avalon recouvre au moins la partie nord du bloc de la croilte inferieure d'Avalon. MCme si I'extension au large des particularites gCologiques n'est pas s,ans Cquivoque, il apparait cependant une zone de reflexion de pendage nord qui marque au sud-ouest de la Nouvelle-Ecosse le lieu du chevauchement dCvonien de la Zone d'Avalon sur la Zone de Meguma. Dans la baie de Fundy au nord, les deux zones de rCflexion de pendage sud sont interprCtCes comrne Ctant des chevauchements majeurs, ayant probablement transport6 la croiite infkrieure d'Avalon au-dessus d'une unite de style tectonique diffkrente. La Faille de Fundy est une faille de chevauchement carbonifere, IocalisCe I'intCrieur du bloc d'Avalon et longeant les c6tes du Nouveau-Brunswick; elle fut rCactivCe par la phase d'extension au MCsozofque pour devenir une faille de transtension. I1 est possible que des ajustments le long des failles de direction est-ouest et des petits bassins de distension associCs ont dil se produire lors du diplacement d'extension plus loin en direction du sud-ouest.

[Traduit par la rkdaction] Can. J. Earth Sci. 28. 109&1111 (1991)

Introduction Deep seismic reflection data collected in recent years have

been used to define the deep structure of the Canadian Appalachians. Three lower crustal blocks have been defined on the basis of seismic character (Keen et al. 1986; Marillier et al. 1989a), underlying the geologically defined tectono-stratigraphic zones in the Canadian Appalachians (Williams 1979; Williams and Hatcher 1983; Keppie 1989): (i) the Grenville

'Geological Survey of Canada Contribution 15991.

block, associated with the Grenville craton, is overthrust by the deformed rocks of the late Precambrian - early Paleozoic passive margin (Humber Zone) and western Dunnage Zone; (ii) the Central block is overlain by the eastern Dunnage and Gander zones; and (iii) the Avalon block lies below the Avalon Zone. The geologically defined zones may be allochthonous with respect to the underlying lower crustal blocks, so the boundaries between these blocks do not necessarily correspond vertically to zone boundaries.

The deep seismic studies have provided the basis for a new

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KEEN ET AL.

FIG. 1. Location map showing the positions of the seismic reflection lines 88-2,88-3, and 88-4 with respect to major geological features. Other seismic lines relevant to this paper are lines 88-1, 88-IA, United States Geological Survey (USGS) line IA, and the SoquipUSGS Maine profiles. The tectono-stratigraphic zones (terranes) of the Appalachian Orogen are shown. The Gulf of Maine is divided into three regions, separated by the Fundy Fault and the Nauset magnetic anomaly (NA). The continuation of the Nauset anomaly to the north and into the Bay of Fundy (broken line) is uncertain. Boundaries from Hutchinson et al. (1988) and Williams (1979). Mesozoic basins are shown with stipple. CCFZ, Cobequid-Chedabucto fault zone; ECMA, East Coast magnetic anomaly; GM, Grand Manan Island; NMB, North Mountain Basalt; SMB, South Mountain batholith: FF, Fredricton Fault; NF, Norumbega Fault; TH, Turtle Head Fault.

look at the tectonic framework of the Canadian Appalachians (Stockmal et al. 1987, 1990). However, these studies have not previously imaged significant areas of crust below the Meguma Zone, the most outboard mappable zone in eastern Canada. The Meguma Zone remains the most problematic terrane in terms of the mode of its emplacement and the nature and location of its boundary with respect to the Avalon Zone (Williams 1979; Keppie 1989).

In 1988, therefore, deep seismic reflection data were collected along three lines in the Bay of Fundy and the Gulf of Maine southwest of Nova Scotia (Fig. I), across part of the Meguma Zone and the adjacent Avalon Zone. The profiles also provide information about along-strike variations in the deep structure of the Appalachians, because they lie between the deep seismic transects across the Gulf of St. Lawrence (Marillier et al. 1989b) and northeast of Newfoundland (Keen et al. 1986) on the one hand and across southern Quebec, Maine, and the Gulf of Maine (Steward et al. 1986; Hutchin- son et al. 1988; Doll et al. 1989) (Fig. 1) on the other.

Geological and geophysical setting Major geological features

On land, the Meguma Zone of southern Nova Scotia lies south of the Cobequid-Chedabucto fault zone (CCFZ, Fig. 1). The metasedimentary Cambro-Ordovician Meguma Group is overlain by White Rock Formation metasedimentary and metavolcanic rocks and Siluro-Devonian metasedimentary rocks. These were first deformed during the Early Devonian Acadian orogeny (Keppie and Dallmeyer 1987; Muecke et al.

1988) and intruded by Late Devonian and Carboniferous granitoid plutons (Clarke et al. 1988). Gneisses and schists in intrusive rocks of the Liscomb Complex and xenoliths in dykes that cut the Meguma Group may represent sub-Meguma basement (Giles and Chatterjee 1986; Clarke and Chatterjee 1988; Eberz et al. 1991). The xenoliths isotopically resemble Avalon basement, and Nd model ages for the Meguma Group metasediments are older than for the granitoids and the Liscomb Complex, reflecting either a source from an older continental terrain or a tectonic break beneath an allochthonous Meguma Group (Ebertz et al.). Because of these uncertainties we restrict the term "Meguma Zone" to the metasediments and granites observed at or near the surface.

The Avalon Zone crops out in northern Nova Scotia, southern New Brunswick, and neighbouring Maine (Fig. 1). It consists of variably deformed late Precambrian arc-related igneous and sedimentary rocks onlapped by early Paleozoic sedimentary rocks (Dostal et al. 1990; Keppie et al., in press). The Fredericton Fault may mark the Gander-Avalon zone boundary in southern New Brunswick (Williams 1979; Williams and Hatcher 1983). This boundary has been extended northeast into the Gulf of St. Lawrence by Durling and Marillier (1990).

The Avalon-Meguma zone boundary in Nova Scotia is marked by the Cobequid-Chedabucto fault zone (CCFZ, Fig. 1) (Keppie 1982): this fault is interpreted to continue west- wards beneath the Bay of Fundy. Early Devonian folds and associated slaty cleavage in the Meguma Zone south of the fault suggest sinistral motion on the fault and are interpreted

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1098 CAN. .I. EARTH SCI. VOL. 28, 1991

as marking the docking of the Meguma and Avalon zones (Keppie and Dalmeyer 1987; Mawer and White 1987); Middle to Late Devonian structures in the same area indicate dextral motion (Keppie and Dalmeyer 1987). Immediately north of the fault zone in Nova Scotia and southern New Brunswick, north- vergent thrusts of Carboniferous age are widespread (Nance and Warner 1986; Waldron et al. 1989), and dextral motion on the fault continued through the Late Carboniferous with the formation of pull-apart basins (Yeo and Gao 1987).

Mesozoic rifting of the Bay of Fundy was predominantly listric and may have been localized along the Carboniferous structures. Triassic - Early Jurassic rifting was accompanied by approximately 75 km of east-west sinistral movement on the CCFZ (Keppie 1982; Keppie and Dallmeyer 1987). The Bay of Fundy is now a half-graben, bounded to the northwest by a normal fault and filled with about 6 km of Triassic - Early Jurassic syn-rift sediments and the plateau basalts of the North Mountain Formation. The geochemically similar Shelburne dike cuts the southwestern Meguma Zone (Papezik and Barr 1981) within the study area (Fig. 2). In mid-Jurassic time rifting, which was concentrated along the present continental margins, apparently stopped in the Bay of Fundy region. Extension also appears to have changed direction at that time from east-west to north-northwest - south-southeast.

Offshore correlation of upper crustal geology The ease with which deep seismic data may be shot at sea,

compared with transects on land, is offset by the difficulties of extrapolating terrane boundaries, mapped on land, into marine areas. In this study, we rely principally on aeromagnetic (Fig. 2) and gravity Fig. 3) data to extend offshore the geological features mapped on land. We also were guided by previous correlations based primarily on seafloor samples of bedrock by BalIard and Uchupi (1975), Hermes et al. (1978), and Pe-Piper and Loncarevic ( 1989).

Magnetic anomalies near the southwestern coast of Nova Scotia clearly show the offshore extension of the Meguma Group and White Rock Formation, with structural trends parallel to those on land. Locally, these anomalies are disrupted by Permian plutons and north-northwest-trending faults (Hutchinson et al. 1988; Pe-Piper and Loncarevic 1989). A region of relatively smooth magnetic field occurs along the northern part of line 88-2 (Fig. 2), south of the offshore continuation of the North Mountain basalts on land. The geological structure of this area of smoother magnetic field is uncertain.

Within the Fundy Basin, shallow structure is known from industrial seismic lines (Brown 1986) that are tied to the Chinampas N-37 and Cape Spencer P-79 wells (Pe-Piper et al., in press). The North Mountain basalt forms a prominent reflector and separates Jurassic and Triassic syn-rift sedimentary units (Olsen 1988). Metamorphic basement underlies the Triassic Blomidon Formation in the Cape Spencer P-79 well and outcrops beneath North Mountain Basalt on Grand Manan Island (Stringer and Pajari 1981): the onshore correlation of these rocks is uncertain, although there are similar rocks within the Avalon Zone (Stringer and Pajari 1981).

In the central Gulf of Maine deep seismic transect, Hutchin- son et al. (1988) used potential-field data and seabed samples, together with the seismic character of the upper crust, to divide the Gulf of Maine into four zones. These are, from north to south (i) the "Fault Zone," bounded on its south side by the Fundy Fault (FF, Fig. 2); (ii) the Central Plutonic Zone (P, Fig.

2), bounded on its south side by the Nauset magnetic anomaly (NA, Fig. 2); (iii) the Southern Plutonic Zone (M, Fig. 2), bounded-on its south side by the border fault of the Mesozoic rift basins; and (iv) the "Rifted Zone" along the outer part of the continental shelf, most of which is occupied by the Mesozoic Georges Bank Basin (see Fig. 1) (Schlee and Klitgord 1988). The aeromagnetic data (Fig. 2) can be used to extend these zones eastwards towards the Canadian land area.

The Fault Zone shows a series of southeasterly dipping reflections in the deep seismic profile of Hutchinson et al. (1988). It corresponds to a northeast-trending belt of strong, linear magnetic anomalies that may be traced into the Avalon Zone onshore in southeastern Maine. Some lineations are truncated near Grand Manan Island by the north-northwest Oak Bay Fault (0 , Fig. 2) (McCutcheon and Robinson 1987), which is coincident with a strong gravity gradient (Fig. 3). Similar magnetic characteristics to those of the Fault Zone continue northeastward along the north side of the Bay of Fundy in the Avalon Zone of southern New Brunswick. The Fundy Fault, marking the southern margin of the Fault Zone, thus appears to correlate with the northern margin of the Fundy Basin.

The Central Plutonic Zone corresponds to a zone of more chaotic magnetic anomalies, whose southern boundary is defined by the Nauset Anomaly (this anomaly, indicated on the magnetic derivative map in Fig. 2 by NA, is seen only as a change in the character of the magnetic derivative). On the seismic data of Hutchinson et al. (1988) the upper crust is almost reflection free, interpreted to indicate the presence of extensive plutons. Bedrock samples of these plutons have been correlated with Ordovician to Silurian peralkalic granites in the Avalon Zone of Massachusetts (Hermes et al. 1978; Hermes and Zartman 1985).

The Southern Plutonic Zone and Rifted Zone correspond to a second belt of northeast-trending anomalies, which to the northeast are traced into the Meguma Zone in Nova Scotia. Hutchinson et al. (1988) preferred a correlation of the Southern Plutonic zone with the Avalon Zone, since the boundary between the Central and Southern plutonic zones does not have a strong magnetic anomaly similar to that associated with the CCFZ. We reject this reasoning, since the CCFZ anomaly is associated with Carboniferous mafic plutonism and is therefore a younger feature distinct from (but coincident with) the Meguma-Avalon zone boundary.

Acquisition and processing of the seismic data A total of 220 km of seismic reflection data was collected

in the study region by Western Geophysical Limited under contract to the Geological Survey of Canada. An additional 315 km of data was collected across the continental margin off Nova Scotia (lines 88-1, 88-1A; Fig. 1); these data are the subject of a separate paper (Keen et al. 1991). The portion of line 88-1 closest to mainland Nova Scotia does give us an additional sample of the crustal block underlying the Meguma Zone and so will be discussed briefly here. The seismic lines were collected in a similar manner to other deep seismic reflection lines off eastern Canada. A large air-gun array (8 100 in.3 or 132.7 L capacity, fired at a pressure of 1800 psi or 12.4 MPa) was the sound source, and a digital seismic streamer was the receiver.Two basic system configurations were used: (i) a 120 channel, 3200 m long streamer, with a 53.3 m shot interval; or (ii) a 180 channel, 4500 m long streamer, with an 80 m shot interval. The 180 channel data were mixed during processing to give 90 channels. This provides a 30-fold

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1 1m CAN. J. EARTH SCI. VOL. 28. 1991

FIG. 3. Free-air gravity anomaly at sea, Bouguer anomaly on land. Data after Shih et al. 1991b. Shot points are given along the seismic lines. Note that lines 88-2 and 88-3 were each shot in two segments, with the break indicated by 2-2A or 3-3A in this figure. The two deep exploratory wells in the Bay of Fundy are shown: C, Chinampas N-37; S, Cape Spencer P-79.

common midpoint stack section for either configuration. The record length varied from 20 to 25 s (two-way traveltime).

Standard methods of data processing were applied to these data, including f - k velocity filtering, designature, predictive deconvolution, velocity analysis, f- k demultiple, stack, and migration. All data shown in this paper (Figs. 4-6) are migrated, and care was taken in the interpretation to compare the migrated and unmigrated sections to minimize interpretation of artifacts as real reflectors. Line drawings (Figs. 7-9) were also produced from the migrated sections, omitting multiples and other obvious artifacts.

Seismic results Line 88-2 setting

Line 88-2 (Figs. 1-3) is located about 60 km west of the southwestern tip of Nova Scotia. The line, 139 km long, was located to sample the Meguma Zone and the Avalon-Meguma boundary. Its location is a compromise between science and logistics: a line closer to land that tied with known land geology would have been preferable but was prevented by strong currents and the presence of rough, highly reflective sea floor in the inshore region.

Offshore magnetic anomaly trends (Pe-Piper and Loncarevic 1989) (Fig. 2) show that at least the southern part of the line lies within the Meguma Zone. The line crosses the Shelbume dike near shot point (SP) 1750. The northern part of the line enters a region of relatively smooth magnetic field, which Pe- Piper and Loncarevic (1989) suggest is underlain by a granitoid pluton. This region is along strike from the Central Plutonic Zone of Hutchinson et al. (1988). Near its north end, the line crosses magnetic lineations that mark the seaward extension of the basalts of North Mountain and terminates within the Fundy rift basin.

The free-air gravity anomalies along the line show that it lies outside the major regional gravity low associated with the South Mountain batholith (SMB, Fig. 1) and its offshore extensions (Fig. 3) (Pe-Piper and Loncarevic 1989). It does cross a gravity maximum of about 20 mGal, with the maximum falling roughly in the centre of the line.

Line 88-2 results The seismic line drawing, shown in Fig. 7, displays the

prominent characteristics of these data. Starting with the upper crust, the Fundy rift basin sedimentary rocks are observed

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FIG. 5. Seismic data for line 88-3. Note that the line was shot in two sections, with a break indicated by 3-3A above the data.

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1102 CAN. J . EARTH SCI. VOL. 28. 1991

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krn FIG. 6. Seismic data for line 88-4.

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KEEN ET AL.

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FIG. 7. Results, line 88-2. (a ) Potential-field data along the profile. (b) Line drawing for line 88-2. (c) Interpreted line drawing for line 88-2. Letters are referred to in the text. The geologically defined terranes and the lower crustal blocks are shown above and below the interpreted section, respectively.

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down to about 3.5 s in the northern part of the line, terminat- Paleozoic or Mesozoic in age. Below about 1 s the uppermost ing laterally at about SP 500. Within the basin, the North crust through most of the remainder of this line is essentially Mountain basalts give bright reflections at depths of about transparent. In the middle crust, at about 3-4 s, there is a 0.5 s (A, Fig. 7). South of this region there is no evidence for group of weak, diffuse reflectors (C, Fig. 7), and below this any major Mesozoic rift basins, although in the upper 1 s there group the crust is again transparent down to 6-7 s. may be remnants of small basins, which could be either Below this second transparent level, the lower crust is highly

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CAN. J. EARTH SCI. VOL. 28, 1991

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FIG. 8. Same as Fig. 7 but for lines 88-3 and 88-4.

reflective and contains both dipping and horizontal groups of reflectors. The reflection Moho is interpreted to lie at the base of this reflective lower crust. The lower crust can be divided into two zones of differing characteristics, separated by a prominent zone of north-dipping reflections (B, Fig. 7). In the southern part of the line, the upper surface of the reflective lower crust is not flat; rather, it appears to form two packages of reflections (Dl, and D2, Fig. 7) whose upper surfaces dip shallowly to the south. The character of the reflective layer changes to the north; at the extreme northern end of the line the reflectivity of the lower crust appears to be much more diffuse (E, Fig. 7) than that to the south. Within zone E there are some very bright reflections (I, Fig. 7) that dip very shallowly to the south and may extend as deep as the Moho but do not appear to continue into the upper mantle.

In the middle of the line, the zone of north-dipping reflec-

tions, noted above, cuts through most of the crust (B, Fig. 7). The upward projection of the top of this zone intersects the surface near the offshore projection of the Shelburne dike (near SP 1700). The dipping reflections extend downward into the reflective lower crust and interpenetrate there with the fabric of reflections (D3, Fig. 7). This apparent overlap of B reflections with more gently dipping reflections in D3 probably results from the oblique orientation of the boundary repre- sented by the B reflections, relative to the line of the profile. There appears to be an offset in the Moho at this feature (F, Fig. 7; see also Fig. 4), with a deeper Moho to the north.

The reflection Moho, interpreted to lie at the base of this reflective lower crust, rises below the centre of the line (SP 750 - 1500) from about 10 s to about 9 s. This may be responsible in part for the free-air gravity maximum in the centre of the line (Fig. 3). The upper mantle is fairly transpar-

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1106 CAN. J. EARTH SCI. VOL, 28, 1991

ent below Moho. However, there are some quite strong of zone E, which forms a zone separating highly reflective and reflections (G and H, Fig. 7) that cannot be discounted as nonreflective crust, dipping to the south. artifacts. Their origin is not clear, but a well-defined north- Unit E is characterized by moderately strong reflections and dipping reflector within the upper mantle on line 88-1 (Fig. 1) most resembles the "diffusely" reflective lower crustal section (Keen et al. 1991) projects to the locations of reflections H on at the north end of line 88-2 and on line 88-3. Unit E is this line. underlain by unit K, which is highly reflective down to the

Line 88-3 setting and results This short (20.8 km) line, which lies entirely within the

Fundy rift basin, was intended to tie lines 88-2 and 88-4. However, strong tides and currents (velocity in excess of 2.6 m/s) in the region created high noise levels and did not permit extension of this line to cross line 88-4.

The data show the same basic features as the northern end of line 88-2 (Figs. 5 , 8): a transparent upper crust below the basin sedimentary rocks, a reflective lower crust, and a transparent upper mantle below Moho at 10 s. The two-way traveltimes to these units agree with those observed on the north end of line 88-2. Sedimentam seauences. 3-3.5 s thick.

d .

infill the Fundy basin and appear to be truncated, perhaps by steeply dipping faults (J, Fig. 8). These might be northwest- trending transfer faults, associated with formation of the rift basin. One of these "faults" coincides with a merge of data from two separate line segments, and the truncation of reflectors may be simply a mismatch of the two parts of the line. However, this does not explain other, similar features. The basement to the basin is not observed but is inferred to lie below the stratified reflections indicative of sediments. The North Mountain basalts form a bright reflector within the sedimentary section (A, Fig. 8).

Line 88-4 setting This line, 46.6 km long, crosses much of the Fundy rift

basin just east of Grand Manan Island. At its north end, the line was terminated early due to strong tidal currents that made high-quality data collection impossible across the Fundy Fault zone near the coast of New Brunswick. The Chinarnpas N-37 well (Fig. 3) (Pe-Piper et al., in press), 13 km along regional strike from the north end of the line, provides stratigraphic control for the basin sediments Metamorphic rocks, similar to those in southern New Brunswick, occur in the Cape Spencer P-79 well 60 km to the northeast (Pe-Piper et al.) and on Grand Manan Island (Stringer and Pajari 1981). The south coast of New Brunswick is part of the Avalon Zone, where late Precambrian through Carboniferous rocks were affected by Late Carboniferous dextral transpression (Nance 1987) within a fault zone bordering the Fundy coast. This region is associated with strong, northeast-trending magnetic anomalies (Fig. 2). At least one of these faults may have been reactivated during a later phase of Mesozoic extension and sinistral transtensional motion and now forms the bounding fault of the Fundy Basin. The seismic line is located within the basin, in an area of smooth magnetic field (Fig. 2), and does not appear to extend far enough north to sample the surface expression of these faults.

Line 88-4 results The seismic data across the Fundy Basin show the presence

of a deep sedimentary basin, filled mainly with synrift sedimentary rocks, with the flood basalts (A, Fig. 8) providing a prominent marker. The sediments are displaced down to the south on normal faults (J, Fig. 8). The base of the sediments is not clearly defined, being obscured by multiple reflections from the overlying basalts. They may terminate against the top

Moho; unit K is not observed on the other lines. The upper surface of this unit dips to the southeast and is subparallel to the top of unit E. Locally, unit K may have a more westerly strike than the perceived regional strike; otherwise it would be imaged on line 88-3. Alternatively this unit may terminate immediately west of line 88-4, as suggested by the abrupt increase of the gravity field (Fig. 3) across the Oak Bay Fault near Grand Manan Island. The lower surface of unit K is the reflection Moho, at a depth of 12 s. This is 2 s deeper than Moho on the other lines. This apparent change in Moho depth between lines 88-3 and 88-4 is approximately coincident with a change in regional free-air gravity from positive to negative.

Lower crustal block identification

Lower crustal blocks have been previously defined through parts of the Canadian Appalachians crossed by deep seismic reflection lines (Marillier et al. 1989b). This definition is based on the seismic character of the blocks. Here we try to use the same criteria to identify lower crustal blocks from the new seismic data.

There is a striking difference in the seismic character of different parts of the lower crust observed on line 88-2: units Dl-D3 and E (Fig. 7). These suggest the existence of two lower crustal blocks, separated by the north-dipping event B. The southern block, which we call the Sable block, partly underlies the Meguma Zone and is terminated to the north by dipping event B. It has not been defined on earlier surveys, although a similar, very reflective lower crust is observed below the inner part of line 88-1, where Meguma rocks can be traced offshore (Keen et al. 1991), and below the southern Grand Banks, in a region thought to be occupied by the Meguma Zone (Keen et al. 1987, 1990). However, on line 88-1 the reflection Moho is 2 s deeper than observed here, and the lower crustal reflections are somewhat different in character (see Keen et al. 1991).

We interpret unit E on lines 88-2, 88-3, and 88-4 to be the Avalon lower crustal block because it shows the same diffuse reflectivity that the Avalon block exhibits elsewhere in the Canadian Appalachians (Marillier et al. 1989a, 1989b). The top of unit E on line 88-4 (Fundy Fault, Fig. 8) projects to the surface in the coastal region of southern New Brunswick, occupied by the Avalon Zone. Zone E on lines 88-2 and 88-3 1 correlates along strike with the Fault Zone of Hutchinson et al. (1988), which they suggest is part of the Avalon Zone. Such an identification would imply that the dipping feature B

i represents the boundary between the Avalon and Sable lower crustal blocks.

In depth and geometry, as defined by industrial seismic lines in the region, the top of unit E can be correlated with the "Fundy Decollement" of Brown (1986), which he interprets to be a Paleozoic thrust fault, reactivated as an extensional (or transtensional) fault during the Mesozoic. Hutchinson et al. (1988) observed similar thrust features within their Fault Zone. We will refer to this surface as the Fundy Fault and correlate it with the most southerly fault of the Fault Zone mapped to the southeast by Hutchinson et al. (1988).

Unit K (Fig. 8) could also be part of the Avalon block.

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KEEN ET AL. 1107

However, given its distinctive character, with a strongly reflective lower crust near Moho depths, we interpret this unit as equivalent to the Central lower crustal block observed elsewhere underlying the Canadian Appalachians (Marillier et al. 1989a, 1989b). Its upper boundary, when projected to the surface, corresponds with the Fredericton Fault in New Brunswick which probably marks the Gander-Avalon zone boundary. On the basis of previous seismic results, we would also expect this zone boundary to correspond to a change from Avalon to Central crustal blocks. One problem with this interpretation is that the Avalon-Central boundary has been interpreted elsewhere to be a steeply dipping feature that is unlikely to correlate with the fault shown in Fig. 8 (Stewart et al. 1986; Keen et al. 1986). Conversely, Durling and Marillier (1990) show that the northeast extension of the Fredericton Fault into the Gulf of St. Lawrence correlates with a shallow- dipping feature in the upper crust. Because of the uncertainties in interpreting the land geology and in the extrapolation of our results into an unknown region, this interpretation is considered to be highly speculative.

Comparison with other deep seismic data in the region In Fig. 9a, a geological interpretation of the seismic data is

proposed, along a composite profile in which line 88-4 is projected onto the end of line 88-2. There have been no adjustments made for the differences in trend (35") of these two lines because the assumption of two-dimensionality of structure is, in any case, an approximation. Line 88-3 was used to provide the link between the other two profiles. Figure 9b shows the line-drawing interpretation of the deep seismic reflection line collected by the United States Geological Survey (USGS) in the Gulf of Maine (Hutchinson et al. 1988) and located about 150 km southwest of line 88-2 (Fig. I).

The interpretation of USGS line 1 and our composite profile are similar in the following ways:

(1) The lower crust beneath the northern end of both is interpreted to be a stack of south-dipping thrust faults (TI to T3 on USGS line 1). T1, the Fundy Fault, was not significantly displaced during the Mesozoic below USGS line 1. We note that TI extends to the Moho. We do not observe this on our data, perhaps because of the gaps in seismic coverage (Fig. 9a).

(2) The Avalon lower crustal block is reflective, and reflections within it are subhorizontal in the middle of both transects (E, Fig. 9).

(3) There is a north-dipping feature on both lines (our feature B). This feature is less clear in the lower crust on

I USGS line 1. We have interpreted this to be the Avalon-Sable crustal block boundary. Hutchinson et al. (1988) place the equivalent boundary farther south, at the beginning of the rifted block. However, their data equally support an interpretation in which the boundary coincides with the north-dipping reflections on USGS line 1, as shown in Fig. 9b.

(4) South-dipping features (Nl-N3, Fig. 9b) in the lower crust on line USGS 1 are reminiscent of the zones Dl-D3 on line 88-2. These occur in both cases just south of the suggested Avalon-Sable block boundary.

There are also some differences in the two data sets, for example the depth to the top of the reflective lower crust. Moho is also deeper below line 88-4 than below the north end of USGS line 1. However, the good correlation of major geological features greatly strengthens our interpretation of the crustal blocks beneath the Gulf of Maine. In particular, it is

less likely that Mesozoic overprinting has greatly affected USGS line 1, as it is farther from a major rift basin. Thus we can place more confidence in our identification of features, such as the north-dipping boundary, as being Paleozoic in age.

The major boundaries identified in Fig. 9 are shown in map form in Figs. 1 and 2. The Avalon-Sable boundary trends northeast in the north-central Gulf of Maine. It lies just north of the Nauset anomaly of Hutchinson et al. (1988), which separates northeast-trending (Meguma Zone) magnetic anomalies from less well-organized anomalies to the north (Fig. 2). The trace of the Fundy Fault, representing the southeastern boundary of a stack of thrusts along the coast of Maine and New Brunswick, is based on magnetic data (Fig. 2).

Paleozoic tectonics in the Gulf of Maine region One of the major issues in delineating the Paleozoic tectonic

history of the study area is the nature of emplacement of the Meguma Zone. The deep seismic data show the distribution and geometry of the boundary of the Avalon and Sable lower crustal blocks, which underlie the Avalon and Meguma zones. The present data do not unequivocally resolve the relationship of the near-surface zones to these lower crustal blocks.

First, we must consider whether the Sable block is a distinct crustal block or whether it is part of the Avalon block but modified by partial melting or Mesozoic extension. A corollary is whether the north-dipping event B represents a major Paleozoic tectonic suture. The Sable block is characterised by high reflectivity. The reflective nature of the lower crust is a feature common to Phanerozoic crust in many areas, and a variety of explanations have been proposed (Warner 1990). If the Sable block is not compositionally distinct from the Avalon block, it may have acquired its distinct reflectivity by one of two processes: (i) Mesozoic extension, causing either igneous underplating-intrusion or the development of shear fabrics within the lower crust; and (ii) lower crustal partial melting during Paleozoic tectonic thickening of the crust. It is difficult to ascribe the reflectivity to Mesozoic extension because of the absence of Mesozoic rift basins along line 88-2. Furthermore, the seismic data along line 88-1 that extend into the rifted zone show no evidence for an increase in reflectivity towards the edge of the continent (Keen et al. 1991). The abrupt change in lower crustal reflectivity just where the zone B cuts the lower crust is also difficult to account for by either hypothesis. Thus, although lower crustal melting or Mesozoic extension cannot be ruled out, the simplest explanation is that the observed seismic signature of the Sable block reflects a compositionally distinct unit.

A second issue is the nature of the surface expression of the boundary between the Avalon and Sable lower crustal blocks. This is related to questions as to the location of the boundary of the Meguma and Avalon zones in the northeastern Gulf of Maine and whether the Meguma Zone is autochthonous or allochthonous with respect to the Sable lower crustal block. To elucidate these issues we present here two possible models of Paleozoic plate kinematics in the region (Fig. 10).

Figure 10a presents an interpretation of upper crustal geology that is simply related to the geometry of lower crustal blocks. The Avalon block is interpreted as thrust northwest- ward over the Central block. The Fundy Fault (FF, Fig. 10) and parallel deeper faults within the Avalon block reflect Carboniferous transpression in southern New Brunswick, although these may be reactivated older features. The Fundy Fault may also have been reactivated in the early Mesozoic as

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1108 CAN. J . EARTH SCI. VOL. 28, 1991

N AVALON-MEGUMA s BOUNDARY - 8 8 - 4 -4 -88 -2 Y

\ AVALON YMEGUMA -

EARLIER THRUST

N SHEAR ZONE (?)

s - 8 8 - 4 -1- 88-2-

CENTRAL(?) . -- FIG. 10. Two interpretative models of the seismic data. (a) Sable block is distinct lower crustal block overlain by Meguma terrane;

Avalon-Meguma terrane boundary lies southwest of Bay of Fundy. (b) Sable block is modified Avalon block; Meguma terrane is allochthonous and extends northwards into the Bay of Fundy. See text for further explanation.

the primary Mesozoic bounding fault of the Fundy basin but test of this model. In this model, the boundary between the shows no significant Mesozoic extension farther west. To the Avalon and the Maguma zones lies near the Fundy Fault, in east, the Fundy Fault merges with the Avalon-Meguma the northern Bay of Fundy and Gulf of Maine. The Meguma boundary in the Cobequid-Chedabucto fault zone (CCFZ). Zone is allochthonous over the Avalon-Sable lower crustal

The Avalon Zone continues south below line 88-2 as far as block, an interpretation consistent with isotopic data (Eberz et the dipping feature B marking the thrust contact of the Avalon al. 1991). The northwest-dipping feature B on line 88-2 in this and Sable lower crustal blocks: it is therefore interpreted as model lies entirely within the Meguma Zone and would be a thrust over the Meguma Zone. At some point east of the seismic lines, the northwest-dipping Meguma-Avalon boundary seen on line 88-2 must be cut by the south-dipping Fundy Fault seen on line 88-4 if the above interpretation is correct, but the exact geometry of this intersection is unknown.

This model implies that the Avalon Zone is thrust over Meguma, in contrast with the situation in mainland Nova Scotia, where Meguma is inferred to be thrust over Avalon in the Carboniferous (Nance 1987; Keppie and Dallmeyer 1987; Waldron et al. 1989). The thrust contact in Nova Scotia is marked by the northern limit of the distinctive magnetic trends within the Meguma Zone (Fig. 2). In the vicinity of line 88-2 the thrust may be correlated with the change in direction of

shear zone, postdating thrusting of the Meguma Zone over the Avalon lower crust along the line of the Fundy Fault, and would be similar perhaps to the Tobeatic lineament in southwest Nova Scotia (Giles and Chatterjee 1988). This interpretation implies that the Central Plutonic Zone of Hutchinson et al. (1988) is not part of the Avalon Zone, and the correlations of Hermes et al. (1978) and Hermes and Zartman (1985) would require reevaluation. The difference in seismic character between the Avalon and Sable lower crustal blocks observed on line 88-2 would be due to processes such as partial melting or Mesozoic extension.

We have not attempted to show directions of subduction or other structural effects within the mantle that would result from

Meguma magnetic trends, from northeast-southwest to north- either of these models, as little observational evidence is south. We suggest that this change is a result of collision of available. We note, however, the occurrence of deep mantle the Meguma sediments with an irregular Avalon boundary. The reflections on both line 88-2 (H, Fig. 7) and on line 88- 1 trace of the Avalon boundary would then lie near the Nauset (Keen et al. 1991). On line 88-2 these reflections dip gently to anomaly and its northeastward continuation into the Bay of the north, and on line 88-1 they dip at an apparent angle of Fundy region. However, any distinctive magnetic trend is about 20". If the latter are proiected west-southwest onto the masked below the Fundv basin sediments, which limits south end of line 88-2. thev tie with reflections H. The

, , tracking of the zone boundary north of the immediate vicinity assumed strike is about the same as that of the proposed of line 88-2. Avalon-Meguma boundary. We speculate that these reflections

The alternative model presented in Fig. lob illustrates an may be remnants of a partially subducted mantle slab. interpretation in which the upper crustal Meguma Zone is allochthonous and in which the Meguma-Avalon zone bound- Mesozoic tectonics ary is located north of the boundary between the Avalon and Later Mesozoic motion reactivated at least part of the Fundy Sable lower crustal blocks. The Liscomb Complex is inter- Fault as a normal fault bounding the Fundy basin. The preted as representing a basement unit to the Meguma Zone geometry of the Fundy Fault is unclear in our study area (see that extends northwards into the area of complex magnetic Fig. 10). One possibility is that it flattens or terminates near anomalies in the southern Bay of Fundy and outcrops in Grand the top of the reflective lower crust of the Avalon-Sable Manan Island. Reexamination of the basement geology of blocks. If this were the case the top of the Avalon and Sable Grand Manan (Stringer and Pajari 1981) would thus provide a lower crustal blocks on line 88-2 would be a decollement and

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the Avalon-Sable boundary would be disrupted. The observa- tions suggest that some horizontal displacement of that boundary at the top of the lower crust (tens of kilometres) is possible, given the diffuse nature of that boundary.

A second possibility is that the Fundy Fault may extend through the lower crust to the Moho or into the upper mantle. We do not observe this, although the extension of the fault to Moho depths may occur in a region of no seismic data, so there is no way of testing this interpretation. We prefer this alternative, as the Fundy Fault observed on USGS line 1 has this kind of geometry.

If there was eastward movement of a large crustal block along the Cobequid-Chedabucto Fault (CCFZ) system during Triassic - earliest Jurassic rifting, then this displacement must be accommodated to the southwest, in the Gulf of Maine region. Hutchinson et al. (1988) suggested that accommodation could occur along north-northwest-trending transfer faults in the vicinity of our line 88-2, connecting the Bay of Fundy with the Georges Bank basin. However, the orientation of the CCFZ constrains Traissic displacement to be east-west, not northwest-southeast, so the proposed transfer faults would be expected to show a major component of normal dip-slip motion. The absence of rift basins near line 88-2, as also indicated on industrial seismic lines through the region, therefore argues against linkage along the northwest-striking transfer faults.

The aeromagnetic map (Fig. 2) shows many east-west trends, which are interpreted to be faults, thoughout the Gulf of Maine region. We suggest that these accommodated displacement in the Fundy region, with the strain distributed throughout the Gulf of Maine, as exemplified in the presence of small rift basins observed there, west of our study area (Ballard and Uchupi 1975). During the Jurassic, when continental breakup was proceeding to the southeast, there was little further development of these Traissic basins or of the Fundy basin, removing the necessity for such linkages.

Summary of results Deep marine seismic reflection data have allowed the

geometry of crustal blocks, related to Appalachian compression and perhaps modified by Mesozoic extension, to be delineated in the vicinity of southwestern Nova Scotia. Most of the structures have been interpreted as related to Paleozoic compression, with reactivation of a Paleozoic thrust as the basin-boundary normal fault of the Mesozoic Fundy basin.

On the basis of seismic character and magnetic trends, the major results are as follows:

(1) Variations in the reflection time to Moho of up to 2 s are observed. The reasons for these variations are not wholly clear, although they may be related to Appalachian compressional tectonics.

(2) The crust is cut by a series of north-verging thrusts along the north coast of the Bay of Fundy. These thrusts have placed the Avalon over the Central(?) lower crustal block below the Bay of Fundy.

(3) Two seismically distinct lower crustal regions are observed in the northeast Gulf of Maine: the Avalon lower crustal block to the north and the Sable crustal block to the south. The latter, which has not been previously defined, lies below the Meguma Zone. The boundary between these two blocks is a north-dipping zone of reflections, which cut the whole crust and possibly offset the Moho.

(4) The seismic character of the Avalon deep crustal block is similar to that observed elsewhere in the Canadian Ap- palachians.

(5) The geometry of major reflections within the crust is remarkably similar to that observed on a seismic reflection profile collected in the Gulf of Maine about 150 km to the southwest of the data described in this paper (Hutchinson et al. 1988). This suggests that the deep structure may exhibit a respectable degree of two-dimensionality.

(6) The simplest interpretation of the relationship of the Appalachian tectono-stratigraphic zones to the lower crustal blocks is that the Sable block is a distinct crustal block, associated with the overlying Meguma Zone. The Avalon- Meguma boundary is identified with the north-dipping zone, southwest of Nova Scotia, representing Early Devonian docking of the two zones. Carboniferous transpression on the Cobequid-Chedabucto fault zone resulted in later thrust faulting within the Avalon Zone.

(7) An alternative mode, in which the allochthonous Meguma Zone extends well north of the Sable block, is consistent with the seismic data but is difficult to reconcile with the correlation of the plutons of the central Gulf of Maine with the Avalon Zone of Massachusetts.

(8) Mesozoic extension is interpreted to have reactivated the Fundy Fault, the southernmost Carboniferous thrust near the top of the Avalon block along the coast of New Brunswick, as a transtensional fault. Extensional displacement farther southwest was probably accommodated along east-west-trending faults and small rift basins associated with them.

Acknowledgments The seismic data form a contribution to the Geological

Survey of Canada's Frontier Geoscience Project and to Lithoprobe. B. C. MacLean contributed to data acquisition and was particularly helpful in processing the seismic data. G. S. Stockmal contributed to the early interpretation of the data. We thank K. G. Shih and R. Macnab for helping us with the displays of potential-field data used in this paper. M. J. Keen and D. J. W. Piper provided helpful reviews of an earlier version. D. Hutchinson very kindly provided copies of USGS line 1 to help our interpretation. D. Hutchinson and L. Fyffe provided helpful and thoughtful reviews.

BALLARD, R. D., and UCHUPI, E. 1975. Triassic rift structure in Gulf of Maine. American Association of Petroleum Geologists Bulletin, 59: 1041-1072.

BROWN, D. W. 1986. The Bay of Fundy: Thin-skinned tectonics and resultant Early Mesozoic Sedimentation. In Basins of eastern Canada and worldwide analogues. Atlantic Geoscience Society, Programme with abstracts, p. 28.

CLARKE, D. B., and CHA'ITERJEE, A. K. 1988. Meguma Zone basement I: Sr-Nd isotopic study of the Liscomb Complex and the origin of peraluminous granites in the Meguma zone. Geological Association of Canada - Mineralogical Association of Canada, Program with Abstracts, 13: A21.

CLARKE, D. B., HALLIDAY, A. N., and HAMILTON, P. J. 1988. Neodymium and Strontium isotopic constraints on the origin of the peraluminous granitoids of the South Mountain Batholith, Nova Scotia. Chemical Geology, 73: 15-24.

DOLL, W. E., COSTAIN, J. K. CORUH, C., DOMORACKI, W. D., PO'ITS, S. S., LUDMAN, A., and HOPECK, J. 1989. Results of a seismic reflection and gravity study of the Bottle Lake Complex, Maine. EOS, 70: 401.

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DOSTAL, J., KEPPIE, J. D., and MURPHY, J. B. 1990. Geochemistry of Late Proterozoic basaltic rocks from southern Cape Breton Island, Nova Scotia. Canadian Journal of Earth Sciences, 27: 619-631.

DURLING, P. W., and MARILLIER, F. J. Y. 1990. Structural trends and basement rock subdivision in the western Gulf of St. Lawrence, northern Appalachians. Atlantic Geology, 26: 79-95.

EBERZ, G. W., CLARKE, D. B., CHAITERJEE, A. K., and GILES, P. S. 1991. Chemical and isotopic composition of the lower crust beneath the Meguma lithotectonic zone, Nova Scotia: evidence from granulite facies xenoliths. Contributions to Mineralogy and Petrology. (In press.)

GILES, P. S., and CHATTERJEE, A. K. 1986. Peraluminous granites of the Liscomb Complex. Nova Scotia Department of Mines and Energy, 10th Annual Open House and Review of Activities, Program and Summaries, pp. 83-89.

1988. The Tobeatic Fault Zone - post-Visean sinistral fault movement in southern Nova Scotia [abstract]. Atlantic Geoscience Society Colloquium '88, Antigonish, N.S. Maritime Sediments and Atlantic Geology, 24: 196.

HERMES, 0. D., and ZARTMAN, R. E. 1985. Late Proterozoic and Devonian plutonic terrane within the Avalon Zone of Rhode Island. Geological Society of America Bulletin, 96: 272-282.

HERMES, 0. D., BALLARD, R. D., and BANKS, P. 0 . 1978. Upper Ordovician peralkaline granites from the Gulf of Maine. Geological Society of America Bulletin, 89: 1761-1774.

HUTCHINSON, D. R., KLITGORD, K. D., LEE, M. W., and TREHU, A. M. 1988. U.S. Geological Survey deep seismic reflection profile across the Gulf of Maine. Geological Society of America Bulletin, 100: 172-104.

KEEN, C. E., KEEN, M. J., NICHOLS, B., REID, I., STOCKMAL, G. S., COLMAN-SADD, S. P., O'BRIEN, S. J., MILLER, H., QUINLAN, G., WILLIAMS, H., and WRIGHT, J. 1986. Deep seismic reflection profile across the northern Appalachians. Geology, 14: 141-145.

KEEN, C. E., BOUTILIER, R., DE VOOGD, B., MUDFORD, B., and ENACHESCU, M. E. 1987. Crustal geometry and extensional models for the Grand Banks, eastern Canada: Constraints from deep seismic reflection data. In Sedimentary basins and basin-forming mechanisms. Edited by C. Beaumont and A. J. Tankard. Canadian Society of Petroleum Geologists, Memoir 12, pp. 101-1 15.

KEEN, C. E., KAY, W. A,, and ROEST, W. R. 1990. Crustal anatomy of a transform continental margin. Tectonophysics, 173: 527-544.

KEEN, C. E., MACLEAN, B. C., and KAY, W. A. 1991. A deep seismic reflection profile across the Nova Scotia continental margin, offshore eastern Canada. Canadian Journal of Earth Sci- ences, 28: 11 12-1 120.

KEPPIE, J. D. 1982. The Minas Geofracture. In Major structural zones and faults of the northern Appalachians. Edited by P. St-Julien and J. BCland. Geological Association of Canada, Special Paper 24, pp. 263-280.

UPPIE, J. D., and DALLMEYER, R. D. 1987. Dating transcurrent zone accretion: An example from the Meguma and Avalon composite terranes in the northern Appalachians. Tectonics, 6: 831-847.

1989. Northern Appalachian terranes and their accretionary history. In Terranes in the circum-Atlantic Paleozoic orogens. Edited by R. D. Dallmeyer. Geological Society of America, Special Paper 230, pp. 159-192.

KEPPIE, J. D., NANCE, R. D., MURPHY, J. B., and DOSTAL, J. In press. Northern Appalachians Avalon and Meguma terranes. In The West African orogens and circum-Atlantic correlatives. Edited by R. D. Dallmeyer and J. P. LCcorchC. Springer-Verlag, Berlin.

MARILLIER, F., KEEN, C. E., and STOCKMAL, G. S. 1989a. Laterally persistent seismic characteristics in the lower crust: examples from the Northern Appalachians. In Properties and processes of Earth's lower crust. Edited by R. F. Mereu, S. Mueller, and D. M. Fountain. American Geophysical Union, Geophysical Monograph Series, No. 51; pp. 45-52.

MARILLIER, F., KEEN, C. E., STOCKMAL, G. S., QUINLAN, G., WILLIAMS, H., COLMAN-SADD, S. P., and O'BRIEN, S. J. 1989b.

Crustal structure and surface zonation of the Canadian Appa- lachians: implications of deep seismic reflection data. Canadian Journal of Earth Sciences, 26: 305-321.

MAWER, C. K., and WHITE, J. C. 1987. Sense of displacement on the Cobequid-Chedabucto fault system, Nova Scotia, Canada. Canadian Journal of Earth Sciences, 24: 217-223.

MCCUTCHEON, S. R., and ROBINSON, P. T. 1987. Geological constraints on the genesis of the Maritimes basin, Atlantic Canada. In Sedimentary basins and basin-forming mechanisms. Edited by C. Beaumont and A. J. Tankard. Canadian Society of Petroleum Geologists, Memoir 12, pp. 287-297.

MUECKE, G. K., ELIAS, P., and REYNOLDS, P. H. 1988. HercynianIAl- leghanian overprinting of an Acadian terrane: 40Ar/39Ar studies in the Meguma Zone, Nova Scotia, Canada. Chemical Geology, 73: 153-167.

NANCE, R. D. 1987. Dextral transpression and late Carboniferous sedimentation in the Fundy coastal zone of southern New Bruns- wick. In Sedimentary basins and basin-forming mechanisms. Edited by C. Beaumont and A. J. Tankard. Canadian Society of Petroleum Geologists, Memoir 12, pp. 363-377.

NANCE, R. D., and WARNER, J. B. 1986. Variscan tectonostratigraphy of the Mispec Group, southern New Brunswick: structural geometry and deformational history. In Current research, part A. Geological Survey of Canada, Paper 86-lA, pp. 351-358.

OLSEN, P. E. 1988. Paleontology and paleoecology of the Newark supergroup (early Mesozoic, eastern North America). In Triassic- Jurassic rifting. Edited by W. Manspeizer. Developments in Geo- tectonics, No. 22; Elsevier, pp. 185-230.

PAPEZIK, V. S., and BARR, S. B. 1981. The Shelburne dike, an Early Mesozoic diabase dike in Nova Scotia: mineralogy, chemistry, and regional significance. Canadian Journal of Earth Sciences, 18: 1346-1355.

PE-PIPER, G., and LONCAREVIC, B. D. 1989. Offshore continuation of Meguma Terrane, southwestern Nova Scotia. Canadian Journal of Earth Sciences, 26: 176-191.

PE-PIPER, G., JANSA, L. F., and LAMBERT, R. St. J. In press. Early Mesozoic magmatism on the eastern Canadian Margin: petrogenetic and tectonic significance. Geological Society of America, Special Paper.

SCHLEE, J. S., and KLITGORD, K. D. 1988. Georges Bank Basin: A regional synthesis. In The Atlantic continental margin, U.S. Edited by R. E. Sheridan and J. A. Grow. Geological Society of America, The Geology of North America, Vol. 1-2, pp. 243-267.

SHIH, K. G., WILLIAMS, H., and MACNAB, R. 199la. Magnetic anomalies and major structural features of southeastern Canada and the Atlantic margin. Geological Survey of Canada, Map 1778A, scale 1 : 3 000 000. (In press.)

1991b. Gravity anomalies and major structural features of southeastern Canada and the Atlantic margin. Geological Survey of Canada, Map 1777A, scale 1 : 3 000 000. (In press.)

STEWART, D. B., UNGER, J. D., PHILLIPS, J. D., and GOLDSMITH, R. 1986. The Quebec - western Maine seismic reflection profile: Setting and first year results. In Seismology: The continental crust. Edited by M. Barazangi and L. Brown. American Geophysical Union, Geodynamics Series, No. 14, pp. 189-199.

STOCKMAL, G. S., COLMAN-SADD, S. P., KEEN, C. E., O'BRIEN, S. J., and QUINLAN, G. 1987. Collision along an irregular margin: a regional plate tectonic interpretation of the Canadian Appalachians. Canadian Journal of Earth Sciences, 24: 1098-1 107.

STOCKMAL, G. S., COLMAN-SADD, S. P., KEEN, C. E., MARILLIER, F., O'BRIEN, S., and QUINLAN, G. 1990. Deep seismic structure and plate tectonic evolution of the Canadian Appalachians. Tectonics, 9: 45-62.

STRINGER, P., and PAJARI, G. E. 1981. Deformation of pre-Triassic rocks of Grand Manan, New Brunswick. In Current research, part C. Geological Survey of Canada, Paper 81-lC, pp. 9-15.

WALDRON, J. W. F., PIPER, D. J. W., and PE-PIPER, G. 1989. Deformation of the Cape Chignecto pluton, Cobequid Highlands,

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

CO

NC

OR

DIA

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IV o

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/08/

14Fo

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nly.

KEEN ET AL,. 1111

Nova Scotia: Thrusting at the Meguma-Avalon boundary. Atlantic mountain chains. Edited by R. D. Hatcher, Jr., H. Williams, and I. Geology, 25: 51-62. Zietz. Geological Society of America, Memoir 159, pp. 33-53.

WARNER, M. 1990. Basalts, water, or shear zones in the lower YEO, G., and GAO, R.-X. 1987. Stellarton graben: an upper Carbonif- continental crust? Tectonophysics, 173: 163-174. erous pull-apart basin in northern Nova Scotia. In Sedimentary

WILLIAMS, H. 1979. The Appalachian Orogen in Canada. Canadian basins and basin-forming mechanisms. Edited by C. Beaumont and Journal of Earth Sciences, 16: 792-807. A. J. Tankard. Canadian Society of Petroleum Geologists, Memoir

WILLIAMS, H., and HATCHER, R. D., JR. 1983. Appalachian suspect 12, pp. 299-309. terranes. In Contributions to the tectonics and geophysics of

Can

. J. E

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Documents

Deep seismic reflection data from the Bay of Fundy and Gulf of Maine: tectonic implications for the northern Appalachians