Kumarapeli & Saull, 1966, CJES, St. Lawrence rift system

THE ST. LAWRENCE VALLEY SYSTEM: A NORTH AMERICAN EQUIVALENT OF THE EAST AFRICAN RIFT VALLEY SYSTEM

P. S. KUMARAPELI AND V. A. SAULL Department of Geological Sciences, McGill University, Montreal, Quebec

Received June 9, 1966

ABSTRACT

The St. Lawrence valley system (including the St. Lawrence, Ottawa, and Champlain valleys, and the St. Lawrence or Cabot trough) is coextensive with a well-defined pattern of seismic activity. The valley system is in a region of general updoming, normal faulting, and alkaline igneous activity of a distinctive type. The main phase of tectonic activity probably dates back to Mesozoic time. The above and other evidence presented in this paper indicate the existence of a major rift valley system that may be called the St. Lawrence rift system.

The Rough Creek - Kentucky River fault zone, and the normal fault zones in Texas and Oklahoma, and the Lake Superior fault zone probably represent extensions of the St. Lawrence rift system. However, current seismicity indicates that the present tectonic activity is along a straight zone running through lakes Ontario and Erie into the Mississippi embayment. The St. Lawrence rift system may also be connected with the mid-Atlantic rift, in the region of the Azores plateau.

The rift hypothesis presented may be useful as a regional guide in the search for niobium-bearing alkaline complexes and diamond-bearing kimberlites.

Crustal tension in the St. Lawrence region may be genetically related to the opening of the Atlantic basin as postulated in the hypothesis of continental drift.

INTRODUCTION

The St. Lawrence valley system (including the St. Lawrence, Ottawa, and Champlain valleys, and the St. Lawrence or the Cabot trough) (Fig. 1) has, time and again, attracted the attention of geologists. Since 1900 alone, literally hundreds of publications have referred to the distinctive physiography, stratigraphy, and structural setting of this region, to the uncommon petro- chemistry and mode of occurrence of the intrusive bodies it contains, and to the nature and origin of its well-defined seismicity.

In this paper it is argued that a large body of evidence strongly supports a rift origin for the St. Lawrence valley system. In view of the significance presently attached to rift systems (Beloussov 1965) this suggestion is of particular interest. The present views are based on a study of published geological and geophysical data on the St. Lawrence region made in connection with Ph.D. thesis research by Kumarapeli (1966).

GENERAL DESCRIPTION OF THE REGION

Except for the interruptions caused by a group of eight intrusive stocks, the floor of the St. Lawrence valley forms plain-like lowlands underlain by almost flat-lying rocks of Lower Palaeozaic age. The stocks make up the Monteregian Hills, which lie approximately along an east-west trending arc through Montreal (Fig. 1). They rise abruptly 190 to 380 m above the surrounding plain, the average elevation of which is about 75 m. The valley is widest (being about 80 km) in the vicinity of Montreal, but narrows to about 25 km below Quebec

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KUMARAPELI AND SAULL: ST. LAWRENCE VALLEY SYSTEM 641

City. Beyond this point the valley floor is more or less fully occupied by the St. Lawrence estuary.

Near the mouth of the Saguenay, the St. Lawrence estuary deepens rather abruptly to form a steep-sided submarine trough with a nearly flat base. This feature extends to the edge of the continental shelf and has a depth of over 200 m for a distance of nearly 1 200 km (Fig. 1). At two places it attains a depth of about 550 m and in this region has a closed basin about 250 km long, lying below the 400-m bathymetric contour. One large tributary of the trough runs towards the Strait of Belle Isle and a minor branch extends along the north side of Anticosti Island.

PREVIOUS WORK

The earliest attempt to account for the seismicity of the region was by Laflamme (1908), who interpreted the St. Lawrence valley as a strip of land sunk between parallel faults. Studies of the Grand Banks earthquake of 1929 led Gregory (1929), Keith (1930), and Hodgson and Doxee (1930) to suggest a graben origin for the St. Lawrence trough. Kay (1942) recognized a graben structure which runs along the Ottawa valley, and called i t the Ottawa-Bonne- chere graben. Dufresne (1948) recognized widespread normal faulting in the St. Lawrence valley and concluded that the region as a whole had been de- pressed. By means of a seismic survey, Press and Beckmann (1954) showed that the St. Lawrence trough is of structural origin. Carey (1958) drew attention to the radial disposition of the St. Lawrence trough, and of the Rhine graben, with respect to the Alaskan orocline.

The foregoing summary represents the views of those who based their conclusions on structural and seismic evidence, and the common theme of these opinions is that the region as a whole has dropped down. However, those who attempted to account for the rather unusual shape of the valleys and troughs in the region from a physiographic point of view, came to very different conclusions.

For example, Spencer (1903) and later Johnson (1925) believed that the St. Lawrence trough was formed by river erosion and subsequent drowning of a lowland. Shepard (1931), after a careful study of the morphology of the trough, concluded that it was primarily a result of glacial erosion. McNeil (1956) suggested that turbidity currents running along the submerged channel may have modified it to its present shape.

DISCUSSION O F EVIDENCE FOR SUGGESTED ORIGINS FOR T H E ST. LAWRENCE TROUGH

The morphological features generally regarded as characteristic of drowned river valleys (sinuous trends, seaward-sloping valley floor) are not those displayed by the St. Lawrence trough. The trough also does not have any of the features typical of a submarine valley shaped by turbidity currents (a winding, roughly V-shaped gorge, sloping continuously downwards and running down the continental slope to great depths). Therefore, neither river erosion nor the activity of turbidity currents can be accepted as responsible for the present morphology of the trough.

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Shepard (1931) found good evidence for ice movement along the trough. This, however, is not surprising because the area adjacent to the present shelf in this region came under Pleistocene glaciation. The box-like shape of the trough is compatible with a tectonic as well as a glacial origin, and Shepard concedes that the great width of the trough is a little unusual for glacial erosion. Shepard also marshalled several arguments against a tectonic origin. Part of his argument is based on the fact that the detailed pattern of trans- atlantic cable breaks caused by the Grand Banks earthquake of 1929 does not bear out Gregory's (1929) contention that the breaks formed two lines which were continuations of the sides of the trough. However, in the light of work done by Heezen and Ewing (1952) on the role turbidity currents, initiated by the earthquake, played in causing cable breaks, Shepard's argument has lost its relevance.

Shepard (1931) also saw no reason to assume a connection between the earthquakes in the St. Lawrence valley and those off the Grand Banks. Since then, however, the shocks off Newfoundland have been recognized as eastern members of the St. Lawrence group of shocks (Gutenberg and Richter 1949, p. 81).

Yet another objection raised by Shepard was that the bend of the trough below Anticosti Island is unusual for a graben. An examination of the rift valley patterns of East Africa and elsewhere shows that this objection is not valid: sharp deflections are characteristic features of rift valleys (Fig. 6).

EVIDENCE THAT T H E ST. LAWRENCE VALLEY SYSTEM IS A R I F T VALLEY SYSTEM

( A ) Evidence from Dimensions and Form (i) Dimensions Holmes (1964, p. 1061) commented on the remarkable similarity in width

of most of the rift valleys. Generally, these widths lie in the range of 50 & 15 km, which is of the same order of magnitude as the thickness of the continental crust. Approximate values of mean widths quoted below are from Girdler (1964, p. 123) with the approximate mean widths of members of the St. Lawrence system added for comparison.

Gulf of Aquaba 50 km Rhine Valley 40 km Lake Baikal rift 50 Gulf of Suez 35 East African rifts 50 St. Lawrence valley 60 Dead Sea rift valley 35 St. Lawrence trough 75 Midland valley of Ottawa-Bonnechere

Scotland 75 graben 55

( i i ) Form The remarkable similarity of the patterns produced on the continents by

rift valleys demonstrates the similarity of the mechanical processes producing them. This similarity seems to be virtually independent of the scale of the patterns (Beloussov 1962, p. 587).


Comparison of Fig. 1 with Figs. 6B and 6C shows the similarity in form between the St. Lawrence valley system and the rift valleys of East Africa and the Rhine. The bifurcating patterns and sharp angular deflections of the former have their counterparts in the latter.

( B ) Geomorphological Evidence ( i ) Fault Scarps and Fault-Line Scarps The escarpments on the north side of the Ottawa-Bonnechere graben have

a maximum relief of about 300 m (Kay 1942, p. 610). Fault scarps or fault-line scarps are prominent along the western margin of the Champlain valley, and behind them the land rises rapidly to about 1000 m to form the Adirondack Mountains. A large part of the Champlain valley is occupied by Lake Cham- plain, the surface of which is 30 m above sea level and the bottom a few meters below sea level. On the northwest side of the St. Lawrence valley an escarpment (Laurentide escarpment) generally separates the plain from the more rugged topography of the Laurentian uplands. In some stretches i t is remarkably straight and regular, and presents an abrupt rise of about 75 to 100 m. In these parts i t is undoubtedly a fault-line scarp. About 40 km below Quebec City, the Laurentide escarpment rises to about 600 m above the water level of the St. Lawrence estuary.

A rather ill-defined, low escarpment is found to separate the monotonously flat landscape of the St. Lawrence valley from the rolling topography of the Appalachians. This feature, however, takes on imposing dimensions between St. Vallier and St. H616ne (Fig. I) , a distance of about 110 km. Here it rises about 300 m above the floor of the St. Lawrence valley, and has been described as a fault scarp (Dresser and Denis 1944, p. 366). The steep to vertical sides of the St. Lawrence trough have already been mentioned.

( i i ) Tilted Blocks and Horsts Grabens are known to occur typically along the crests of upwarps. Hence

the edges of many grabens are more elevated than the adjacent highlands traversed by the troughs. Taber (1927) attributed the tilting to isostatic effects controlled by the shape of the crustal blocks when they are disjointed by faulting.

The Madawaska Highlands (Fig. 1) south of the Ottawa-Bonnechere graben has been described as a fault-block tilted to the south (Kay 1942, p. 638). The Adirondack massif is probably a horst of Precambrian rock tilted to the northwest. A spur of this nearly circular uplift extends across the St. Lawrence valley as the Frontenac Axis, forming a corridor into the Canadian Shield. Tilted fault-blocks can also be recognized in the Laurentian uplands adjacent t o the St. Lawrence valley. For example, north tilting of a triangular block lying between the St. Maurice, Saguenay, and St. Lawrence river valleys has tended to back-pond the south-flowing streams, which in turn have entrenched themselves into hard crystalline rocks to form gorges up to 500 m deep. The Gasp6 Peninsula is believed to be block-tilted to the south (Dresser and Denis 1944, p. 293). The indented, irregular line of the southern and southeastern

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coasts (as compared with the relatively smooth and sweeping curve of the north coast, which for most part gives way directly to high land) is believed to be due to drowning. Corroborative evidence for the southward tilting of Gasp6 Peninsula was found by Mattinson (1958), who recorded antecedent and reversed drainage patterns in this region.

( C ) Structural Evidence Studies of the East African rift valleys have shown that the basic graben-type

structure is too simple to represent the structural complexity of large graben systems (UMC 1965). Some of the individual faults can be traced for long distances and show parallelism with the general direction of the troughs, but many others are shorter and commonly arranged en echelon a t considerable angles with the valley margins. Faulting may be present only on one side of a rift valley, the other side showing flexuring. A glance a t Fig. 2 shows that all these characteristics are displayed by the'structure of the St. Lawrence region.

Kay (1942) described the normal faults associated with the Ottawa-Bonne- chere graben and his estimates of vertical movements along some of the faults

FIG. 2. Structure map of the St. Lawrence, Ottawa, and Champlain valley region.


are as much as 450 m. Dufresne (1948), after an extensive study of faulting in the St. Lawrence valley, concluded that the region has undergone structural deformation chiefly by normal faulting. The faults on the northwest side of the valley are arranged en echelon and make an angle of about 30" with the general direction of the valley, their cumulative effect being to depress the valley floor with respect t o the Laurentian uplands. Widespread normal faulting has also been recognized on the western side of the Champlain valley and the downthrow of these faults is almost'always to the east (Quinn 1933). Vertical displacements of 900 to 1 200 m have been observed on some of these faults (Swinnerton 1932 ; Quinn 1933).

The eastern margins of the Champlain and St. Lawrence valleys were first recognized as a fault zone by Logan (1863). "Logan's Line" (Fig. 2), as this zone of dislocation is sometimes called, extends from Lake Champlain to about 100 km below Quebec City. From Logan's time i t has been customary to regard this feature as due to the frontal thrust of the Appalachians. In one or two isolated and rather restricted instances, i t has, in fact, been seen as a low angle thrust fault (Clark 1951, p. 20). Along most of its length, however, Logan's Line has merely been placed to explain stratigraphic peculiarities and widespread brecciated zones, and its precise nature is in doubt (Clark 19643, p. 75). Nor is there any general agreement regarding its age, although a Taconic (Upper Ordovician) age has been favored, solely on the basis that Ordovician rocks have been affected by it.

Apart from the dislocations represented by the problematic Logan's Line, there are other faults on the southeastern side of the St. Lawrence valley. These, unlike the former, are well documented and betray the graben structure of the valley. For instance, the St. BarnabC fault, which has been traced for a distance of about 55 km, is a normal fault and has a downthrow of about 600 m to the west (Clark 1964a, p. 53). The faults that give rise to the escarpments between St. Vallier and St. HCl&ne have already been mentioned. In some places along this fault zone, parallel escarpments give rise to a step fault relief, and the downthrow is always towards the St. Lawrence River (Dresser and Denis 1944, p. 337).

As a result of a refraction seismic survey across the Cabot Strait, Press and Beckmann (1954) discovered a downfaulted prism of sediments along the St. Lawrence trough. The downthrow on the northeast side is about 4 300 m. They, however, did not give any estimate of the extent of downfaulting along the southwest margin, although according to their cross section (Fig. 3) the amount is likely to be small.

The structural evidence outlined above indicates that the system of valleys, lakes, and submarine troughs, extending from the heads of the Ottawa- Bonnechere graben and the Champlain valley to the edge of the continental shelf, has been formed by down-faulting. The rift valley nature of the Ottawa valley was recognized in 1942, but recognition of the graben structure of the St. Lawrence and Champlain valleys has been lacking until now, perhaps because the interpretation of the large body of structural evidence for i t has

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C o b o t S t r o i t T r o u q h St. Pier re

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Semi-consolidated sediment Basement

LINE OF SECTION A - d MARKED I N FIG. I

FIG. 3. Section across Cabot Strait based on seismic velocities. Modified slightly after Press and Beckmann (1954).

been influenced unduly by some of the controversial and still unsolved problems of the adjacent Appalachian region. This, in fact, is reflected in some of the hypotheses that have been advanced to account for the downfaulted nature of the St. Lawrence valley. For instance, Dufresne (1948) suggested that loading of the region by nappes (from the Appalachian region) may have tended to depress it. Numerous geologists (Bucher 1957; Craddock 1957) in recent years have denied the existence of thrust sheets on the scale originally postulated to explain some of the structural and stratigraphical peculiarities of the Appalachian front adjacent to the Hudson, Champlain, and St. Lawrence valleys.

(D) Earthquake Evidence Shallow shocks are usually associated with rift zones in continents (Gutenberg

and Richter 1949). The Dominion Observatory in Ottawa recently compiled earthquake

records of eastern Canada going back about 400 years (Smith 1962, 1964). Records from long-period seismographs are available from about 1900, but short-period instruments have been installed in the region only since 1928. The records clearly show that the epicenters are closely concentrated along the St. Lawrence and Ottawa valleys and the St. Lawrence trough. The seismicity is low, but forms a well-defined pattern, which appears to be related a t least in some places to the complex fault pattern in the region. Six large earthquakes (Richter scale magnitude 5.5 to 7.2) have occurred in the region since the turn of this century (Hodgson 1965) and on the average about 20 minor shocks


(magnitude 2 to 5) per year were recorded during the period 1954 to 1959 (Smith 1964). The earthquakes of eastern Canada can be felt for larger distances compared with the shocks of the Pacific coast region. Therefore, the activity of the former area used to be regarded as deep-seated. However, recent work has shown that this is not the case: the depths of earthquake foci of eastern Canada and the Pacific coast are comparable, being on the average about 15 km (Hodgson 1965).

( E ) Evidence from the Association of Alkaline Intrusbes Many writers (for example, Holmes 1964, p. 1053) have commented on the

worldwide association of alkaline rocks, carbonatites, and kimberlites with continental rift valleys.

The Monteregian stocks together with their associated sills, dyke swarms, and diatreme breccias make up a well-defined petrographic province extending across the southern part of the St. Lawrence valley in a broad east-west trending arcuate belt (Figs. 5 and 6A). The main rock types are nepheline syenites, essexite, alnoite, other alkaline kindreds, and carbonatite (Gold 1963). The roughly circular plugs appear to have punched through the crust (very likely along a deep-seated fracture zone), probably by a process akin to jet piercing and fluidization (Holmes 1964, p. 271). Evidence is present in some of the associated breccias that gases a t high pressure (mainly COz and HzO) have streamed through. The breccias contain angular as well as rounded fragments, derived from widely separated stratigraphic horizons (Osborne and Grimes- Graeme 1936), indicating agitation and considerable movement.

The volcanic activity associated with a rift may not be concentrated in the valley itself, because the enormous weight of the rift block is likely to keep the boundary faults tightly closed most of the time in most of the places (Holmes 1964, p. 1051). For example, in the Rhine graben the bulk of the volcanic activity has broken through across the northern part of the rift zone and along the southeast margin of the upwarped area.

Figures 6A and 6B illustrate the similarity in spatial distribution of igneous activity in the St. Lawrence valley and in the Rhine graben. In the former, intrusive activity is concentrated in the south. Alkaline complexes a t Meach Lake near Ottawa (Bkland 1951), and on Manitou Islands (Rowe 1958) and Iron Islands in Lake Nipissing are probably related to the Ottawa-Bonnechere graben. However, these alkaline complexes are believed to be older (pre- Ordovician) than the Monteregian intrusives, which are Mesozoic in age. The alnoite, mica peridotite, and kimberlite dykes in New York State (Martens 1924) may be related to the uplift of the Adirondack massif.

( F ) Evidence from Gravity Anomalies Bullard's (1936) gravity measurements in East Africa demonstrated for the

first time the presence of negative Bouguer anomalies over the rift valleys of that region. Since then such anomalies have been found to be present over most rift valleys.


Gravity measurements in the St. Lawrence valley region were carried out by Thompson and Garland (1957), and their Bouguer anomaly map shows a prominent change in the general gravity pattern of the region a t the St. Lawrence valley. Here the trend shows a marked parallelism with the valley. A gravity low can be traced from north of Lake Champlain northward to the vicinity of the St. Lawrence river and thence northeastwards along the valley. The amplitude of the anomaly in the vicinity of Lake Champlain is about 50 mgal. The overall gravity picture appears to be very similar to that of the East African rift valleys and the Rhine graben. Thompson and Garland's map does not cover the Ottawa-Bonnechere graben area fully.

THE ST. LAWRENCE RIFT SYSTEM

( A ) General Statement The various lines of evidence that have been presented in this paper indicate

the presence of a major rift system that may appropriately be named the St. Lawrence rift system (Fig. 4). The St. Lawrence rift system forms a branching zigzag pattern across the eastern part of the North American continent. Between Newfoundland and Nova Scotia it cuts through the Appalachian trend almost a t right angles. Its length is about 2 200 km and possibly more (see below). The branch along the Ottawa valley appears to extend as far as Lake Nipissing and Lake Timiskaming. The Saguenay valley (including the Lake St. John depression) has a structure (Dresser 1916, p. 46) compatible with that of a graben and very likely belongs to the St. Lawrence rift system. A similar relationship is inferred for the submarine trough that extends towards the Strait of Belle Isle. The Lake Melville depression was interpreted (Kranck 1947) (see Fig. 6A) as a graben that originated during Tertiary time. This feature is on line with the Montreal - Sept lies section of the St. Lawrence valley. Surface indications of a direct connection between the two rift valleys have not been traced up to now, but detailed mapping of the intervening area may eventually reveal a connection.

( B ) Age Relations Dixey (1965) summarized the main points arising from the Upper Mantle

Committee's symposium on the East African rift system. One important point is that almost no generalization can be made about its age. The Luangwa-mid- Zambesi rifts (see Fig. 6C), which are now largely occupied by Karroo sediments (ranging in age from Permo-Carboniferous to Upper Triassic and possibly to Jurassic), are older features than, for instance, the Kenya rifts, which are essentially Miocene or later in age. Another significant fact is that the directions of many later faults are dictated by pre-existing lines of weakness in the basement. This is very conspicuous in the mid-Zambesi region where Karroo and post-Karroo faults largely conform to Precambrian tectonic trends.

Some of the normal faults forming the en echelon pattern on the northwest side of the St. Lawrence valley can be traced from the Palaeozoic rocks of the valley into the Precambrian rocks of the Laurentian uplands (Bkland 1961,



p. 36). Glassy pseudotachylite from one of these faults has given a radiometric age of 975 f 45 m.y. (Philpotts and Miller 1963), indicating movements going well back into the Precambrian. The above finding substantiates Dufresne's conclusion that the faults of the St. Lawrence valley followed lines of weakness determined by Precambrian faults.

There is a t present no way of dating precisely the beginning of the episode of updoming and normal faulting in the St. Lawrence region. The youngest rocks cut by the normal faults in the region are Silurian in age. Kay (1942), using long distance correlations and assuming that the Monteregian igneous activity and the normal faulting were synchronous, suggested a late Cretaceous or early Tertiary age. Later, Dufresne (1948) came to similar conclusions. Several radiometric ages are now available for the Monteregian intrusives and these indicate a lower Cretaceous age for the igneous activity (Fairbairn et al. 1963). The widespread normal faulting in the region also is generally regarded as dating back to the same period. The marked physiographic expression of some of the fault-line scarps also indicates movements of a relatively recent age. Miller (1913) found evidence for post-Pleistocene faulting in the Timiskam- ing region. The current seismicity indicates that the rift zones are active a t present.

( C ) Possible Extensions within the Continent Hodgson (1964, p. 9) pointed out that a straight zone running from the New

Madrid area in Missouri along lakes Erie and Ontario to the St. Lawrence valley includes the positions of many earthquakes in the eastern part of North America (Fig. 5). This observation gives rise to the interesting possibility that the tectonic trend of the St. Lawrence rift system may continue towards the Mississippi embayment, and the geological evidence for this and other possible extensions of the St. Lawrence rift system is discussed below.

The southwesterly continuation of the earthquake zone follows, more or less closely, the upwarp of the Findlay Arch (Fig. 5). Although conspicuous fault zones are lacking, three normal faults occur in the axial region of this upwarp (Tectonic Map of United States, 1962). Farther south, the earthquake belt is related to the fault zones just north of the Mississippi embayment in southwest Kentucky. Two fault zones, one trending east-west and the other northeast-southwest, intersect in this region. These are late Palaeozoic or, probably, younger. The east-west zone consists of the Rough Creek and Kentucky River fault zones and can be traced more or less continuously from Llano uplift in Texas northward to Oklahoma, northeastwards to Ozark uplift, and thence eastward to West Virginia (Fig. 5). They are zones of dominantly normal faulting and contain graben and horst structures, particularly in the southern part. As evidenced by the southwestward extension of the earthquake belt, the northeast-southwest trending fault zone probably extends beneath the recent sediments of the Mississippi embayment. This synclinal downwarp may well have a rift valley structure. I t formed largely during the Eocene period (Eardley 1962, p. 653). Does Hodgson's seismic trend define a zone of crustal weakness between the rifts of the St. Lawrence region and the crustal

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sag of the Mississippi embayment? The structural evidence is fragmentary and weak in itself, but there is supporting evidence in the spatial distribution of alkaline intrusives, kimberlites, and carbonatites.

For example, the Rough Creek fault zone in southern Illinois, west Kentucky, and southeast Missouri has been intruded by dykes of alnoite, kersantite, mica peridotite, and kimberlite (Koenig 1956). The Arkansas alkaline province (Tolman and Landes 1939; Fryklund et al. 1954) containing various alkaline kindreds, diamond-bearing peridotite, and one carbonatite, is on the western margin of the Mississippi embayment. These alkaline rocks, like the Montere- gian intrusives, are Cretaceous in age (Miser and Ross 1922). Moreover, the line defined by these two areas of alkaline igneous rocks and the Monteregian petrographic province approximately coincides with the line of seismicity suggested by Hodgson (Fig. 5).

By use of the tectonic maps of Canada (1950) and of United States (1962) the faults of the Lake Nipissing region can be traced westward to the Lake Superior region and thence southwestwards into Wisconsin and Minnesota (Fig. 5). The Lake Superior trough is a downfolded synclinal basin bounded on the south by the Keweenaw fault and on the north probably by the northeast continuation of the Douglas fault. Despite its great depth (215 m below sea level) the Lake Superior depression has generally been regarded as a result of glacial erosion, probably for want of a better explanation. Recently, Smith and others (Smith et al. 1966) found the Mohovoricic discontinuity to vary from about 20 km in the area just west of Lake Superior to about 55 km in the eastern half of the lake. They have suggested the possibility of a "fossil rift" structure in this region. The carbonatite-alkaline complexes in the eastern part of northern Ontario (Parsons 1961) and north of Lake Superior (Fig. 5) may be related to such a rift structure. Some of these complexes have been dated radiometrically as Precambrian (Hurley et al. 1958), indicating that the Lake Superior trough may be a much older feature than the rift valleys of the St. Lawrence system.

Possible extensions of the St. Lawrence rift system are sketched in Fig. 5. The tectonic trend suggested by the southwestward continuation of the Lake Superior fault zones and the north-south trending fault zones of the Llano and Arbuckle Mountain uplifts may be continuous through the Nemaha uplift. This trend also coincides with a zone of strong gravity anomalies. The align- ment of the Rough Creek - Kentucky River fault zones with the Kelvin Seamount group is another possible tectonic trend.

The distribution of cryptoexplosion structures in east-central United States is also included in Fig. 5 because of the apparent relation of their distribution to the postulated extensions of the St. Lawrence rift system. Bucher (1933) recognized the fact that these intensely disturbed centers of localized uplift were more or less restricted to domes and arches in the basement. He believed them to be the result of volcanic gas explosions, similar to the circular structures in the Ries basin, southern Germany. Dietz (1960) advanced a meteorite impact theory to account for some of them. However, as Bucher (1963) pointed out, the systematic relation of these features to structures of terrestrial

652 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 3, 1966

origin is a strong argument in favor of the volcanic explosion hypothesis. The cryptoexplosion structures in the United States mostly affect almost flat-lying Palaeozoic rocks, but in one case Lower Cretaceous rocks are involved (Hoppin and Dryden 1958).

The St. Lawrence rift system and its possible extensions into the continent are compared with the East African rift system in Figs. 6A, 6C, and 6D. The

FIG. 6. Comparison of the St. Lawrence, East African, and Rhine rift zones. Note that the scale of B is two and a half times that of the other maps.

-40 Scale inKilometen

Possible Extensions of the


patterns formed on the two continents are very similar. The lengths of the two rift systems are also comparable. The position of the Great Lakes (excluding Lake Superior) with respect to the westerly and southerly extensions of the St. Lawrence system are very similar to the positions occupied by lakes Victoria and Kyoga between the eastern and western rifts of East Africa. The St. Lawrence trough and its branch running towards the Strait of Belle Isle are comparable in position with the Gulf of Aden and the Red Sea, respectively. In this context Newfoundland occupies a position equivalent to that of Arabia. The similar geographical relation of the St. Lawrence rift system and the East African rift system to the mid-Atlantic and mid-Indian-Carlsberg ridges, respectively, is also noteworthy, and is being studied further by the authors.

( D ) The Possible Connection with the Global System of Rifts The St. Lawrence trough can be traced eastwards to the edge of the con-

tinental shelf, where it loses its identity as a prominent physiographic feature. However, what is probably a complementary ridge structure extends south- eastward from the southern tip of the Grand Banks towards the mid-Atlantic ridge. This is the southeast Newfoundland ridge, separated from the mid- Atlantic ridge by a narrow abyssal gap (Heezen et al. 1959, p. 70). The profiles of Press and Beckmann (Fig. 3) show that the St. Lawrence trough may be a half graben, with the faulting largely restricted to the northeast side. This probably explains why a ridge is present only on the north side. At this point in the argument it is tempting to go one step further and suggest a possible connection between the St. Lawrence rift - southeast Newfoundland ridge system and the mid-Atlantic ridge (Fig. 7). This connection would be in the region of the Azores plateau. If this be true, the St. Lawrence rift system is connected with the world girdling ridge-rift system of which the mid-Atlantic ridge is a part.

( E ) Implications in Continental Drift Almost directly east of the southeast Newfoundland ridge an ill-defined

irregular ridge runs from the east margin of the Azores plateau to the Straits of Gibraltar (Fig. 7). On the basis of its seismicity, Heezen, Tharp, and Ewing (1959, p. 98) inferred that this feature is similar in structure and topography to the mid-Atlantic ridge. The southeast Newfoundland and Azores-Gibraltar ridges have equivalent positions on either side of the mid-Atlantic ridge. If one assumes that the continents on either side of the Atlantic have drifted apart, the question arises whether the shoreward ends of these ridges define two points that were once in contact. These would then be the Cabot Strait and the Strait of Gibraltar (Fig. 8). The shoreward ends of aseismic ridges have been used as guides for locating conjugate points on continents (Wilson 1964). In the present case, however, the Azores-Gibraltar ridge is not altogether aseismic.

I t is possible that the rift faulting in the St. Lawrence region may have had some connection with the actual separation of Africa from North America. The two events appear to date back to approximately the same time. Nairn and-others (1959) compared palaeomagnetic data from Lower Carboniferous

654 CANADIAN JOURNAL O F EARTH SCIENCES. VOL. 3. 1966

FIG. 7. Possible connection of the St. Lawrence rift system with the global rift system. Modified after Girdler (1964).

rocks of Newfoundland with results obtained by other workers in the western United States, and showed that Newfoundland may have rotated 20' counter clockwise in post-Carboniferous time. Du Bois (1959), from palaeomagnetic studies on Carboniferous rocks of New Brunswick and the Gasp6 Peninsula, found no evidence for relative rotation between Newfoundland and the Maritime Provinces since Carboniferous time. He suggested, therefore, that New Brunswick, Gasp6 Peninsula, and Newfoundland must have rotated as a unit. Quinn (1933), from a study of normal faulting in Lake Champlain region, showed that crustal stretching in this area is in an eastward direction. Normal faulting in the St. Lawrence valley also indicates crustal stretching similar t o that of the Lake Champlain region. Crustal movements indicated above are shown in Fig. 4 and from the overall pattern i t would appear that the crustal block east of the St. Lawrence and Champlain valleys, together with New- foundland, has undergone an easterly translation and an anticlockwise rotation.

FIG. 8. Relation of the St. Lawrence rift system to Bullard's (Girdler 1965) assembly of continents.

The above movements are similar to the movements of the African block relative to North America (Carey 1958, p. 275), which, according to the hypothesis of continental drift, led to the development of the Atlantic basin in Mesozoic and later times.

( F ) Economic Implications Kimberlites and carbonatites are genetically related to alkaline rocks, and

recent studies have shown a close relationship in space and time between these rock types and rift valleys, more or less on a worldwide scale.

Kimberlite pipes are the chief primary source of diamonds in the world. So far as is known, the North American continent is poorly endowed with diamond resources. Several thousand small stones have been obtained from a kimberlite intrusion in the alkaline petrographic province of Arkansas. Also, good stones of fair size have been found from time to time in the region of the Great Lakes of North America (Fig. 5). They are presumed to have come from an unknown source in Canada which according to Hobbs (1899) is in the vicinity of James Bay. However, since the Great Lakes region appears to have rift structures, and intrusives genetically related to kimberlites, the interesting possibility that the diamonds discovered in this region have not travelled far from their source cannot be discounted.


Carbonatite is the principal source of niobium and the cerium group rare- earth elements. Thorium also is produced from carbonatites as a by-product. In the search for carbonatite complexes it may be possible to use the rift valley hypothesis outlined in this paper as a regional guide.

ACKNOWLEDGMENTS

The writers are grateful to Dr. J . S. Stevenson and Dr. A. R. Philpotts, and in particular to Dr. J. E. Gill, all of the Department of Geological Sciences, McGill University, for critically reading the manuscript of this paper. Thanks are also due to graduate students in the Department of Geological Sciences, McGill University, for offering critical comments and helpful suggestions. Financial assistance from McGill University is gratefully acknowledged.

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Kumarapeli & Saull, 1966, CJES, St. Lawrence rift system