Geophysical observations bearing upon the origin of the newfoundland ridge

Tectonophysics, 59 (1979) 71-81 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

GEOPHYSICAL OBSERVATIONS THE NEWFOUNDLAND RIDGE

BEARING

UPON THE ORIGIN

71

OF

A.C. GRANT Geological Survey of Canada, Atlantic Geoscience Oceanography, Dartmouth, N.S. (Canada)

Centre, Bedford Institute of

(Received March 9,1979)

ABSTRACT Grant, A.C., 1979. Geophysical observations bearing upon the origin of the Newfoundland Ridge. In: C.E. Keen (Editor), Crustal Properties across Passive Margins. Tectonophysics, 59: 71-81. Multichannel reflection seismic profiles extending southward from the Grand Banks show gently dipping reflectors within “basement” features underlying the Newfoundland Ridge. These reflections appear to be from sedimentary strata, indicating that the Newfoundland Ridge is a remnant of a former sedimentary basin, rather than a ridge of oceanic crust as prescribed by plate tectonic models. Probably this feature is underlain, and to some extent surrounded by, continental crust. INTRODUCTION

The Newfoundland Ridge is a physiographic feature that extends about 900 km southeast from the southern tip of the Grand Banks of Newfoundland, separating the Sohm Abyssal Plain to the south from the shallower Newfoundland Basin to the north (Fig. 1). Watson and Johnson (1970) interpreted seismic profiles from the Newfoundland Ridge as showing buried basement blocks, which had been uplifted and distorted by faulting. From the magnetic anomalies associated with these blocks they inferred a volcanic origin. Plate tectonic-type predrift reconstructions of the North Atlantic have defined the southwestern edge of the Grand Banks as a transform margin, which accommodated displacement of the African plate from that area beginning probably sometime in the Mesozoic (Williams, 1975). It has been concluded that the Newfoundland Ridge is a fracture zone in oceanic crust marking the seaward extension of this transform margin (Auzende et al., 1970; LePichon and Fox, 1971). Gradstein et al. (1977) and Grant (1977) have discussed geological and geophysical data supporting the interpretation that the Newfoundland Ridge is underlain by continental crust, and that the J-anomaly Ridge adjacent may also be continental in origin. This is contrary to the conventional view

0

100 Km

SOHM ABYSSAL PLAIN GS

Fig. 1. Bathymetric map of the Grand ~k~NewfoundK~d Ridge area (contours in metres). Lettered lines show locations of profEes drawn in Figs. 2 and 3. Heavy parts of lines A-D and surrounding crosshatch denote area where seismic reflectors interpreted as denoting sedimentary strata are apparent beneath event U. Dotted line indicates the minimum extent of these sedimentary strata inferred on the basis of physiography. Crosshatched areas enclosed by dashed lines indicate the extent of sub-unconformity basins on the Grand Banks, The heavy dashed Ime is the axis of the Avalon Uplift. Crosses mark refraction profiles discussed in text. Black dots mark w&s (Pu = Puffin; Te = Tern; Ma = Mallard; Tw = Twiliiek; Bi = Battern; Ju = Jaeger). (From Nature, 270,1977, p. 23.)

that the Newfoundland Ridge marks the location of an oceanic fracture zone, and that the “J” magnetic anomaly is a sea-floor spreading isochron expressing normal oceanic basement, This paper persists in the argument that the conventional plate tectonic model is too simple to explain the complexities observed in this area, and reviews the data that illustrate these complexities. Although the Newfoundland Ridge and the J-anomaly Ridge are not very large or imposing features on a global scale, geophysical and geological observations from this area pose questions regarding definition of continental vs. oceanic crust, and processes of crustal subsidence at continental margins. In the context of this symposium these observations provide a casehistory illustrating the nature of the questions that must be confronted to reconcile global-scale models with real data. Fart of this paper reviews multichannel reflection seismic data described by Grant (1977). These data are from a nonp~~~ie~ survey by S&can Delta Ltd. that the Canadian Government has purchased to provide regional

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KM 0

100

GSC Fig. 2. Seismic and magnetic profiles from locations indicated in Fig. 1. Vertical scale for the seismic profiles is in seconds reflection time. The U event has been emphasized. The vertical scale exaggeration (at water velocity) is about 6 : 1. (From Nature, 270, 1977, p. 23.)

control. Reduced diagrams of record sections from this survey are presented in Fig. 2. REGIONAL

GEOLOGY

The subsurface geology of the Grand Banks area has been outlined through geophysical surveys and exploratory drilling for petroleum (Amoco and Imperial, 1973; Upshaw et al., 1974; Gradstein et al., 1975; Swift et al., 1975; Jansa and Wade, 1975). An outstanding feature of the geology revealed by these studies is a late Early Cretaceous unconformity, which separates a blanket of Cretaceous and Tertiary sediments from lower Cretaceous, Jurassic, and older formations in several fault-bounded basins

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(Fig. 1). The gently dipping Cretaceous and Tertiary deposits above the unconformity thicken gradually outward from the northwesterly trending axis of the Avalon Uplift. The strata in the structural basins beneath the unconformity have been deformed by salt diapirism. The tectonism that occurred in the Early Cretaceous was accompanied by extrusion of volcanic rocks, which have been drilled at the level of the unconformity at three locations on the Grand Banks (Fig. l), Bittern M-62, Mallard M-45, and Twillick G-49 (Amoco 1972a, 1973b, and 1974b), and have also been encountered at this stratigraphic level in wells on the Labrador Shelf to the north (McWhae and Michel, 1975). The basement underlying the Grand Banks, insofar as it has been sampled by drilling, is composed of Paleozoic metasedimentary and igneous rocks. To the south of the Grand Banks, DSDP site 384 was drilled in a water depth of 3910 m on the J-anomaly Ridge, which projects southwest as a spur from the Newfoundland Ridge (Fig. 1). This hole bottomed in altered diabasic basalt 330 m below the sea floor (Tucholke et al., 1975). The basalt, which was regarded as oceanic basement, was overlain by Aptian reefal carbonates. The occurrence of these carbonates has been interpreted as evidence that the J-anomaly Ridge has subsided more than 4100 m since Early Cretaceous time. A high-amplitude (500-1000 nT) magnetic anomaly is associated with the J-anomaly Ridge (Ballard et al., 1976). Although radiometric age determinations have not been reported for the basalts from site 384, the indicated age of the immediately overlying carbonates can be interpreted as agreeing with the young Keathley age (approximately 110 m.y.) proposed for the “J” magnetic anomaly (Pitman and Talwani, 1972). GEOPHYSICAL

DATA

The early Cretaceous unconformity on the Grand Banks is an excellent seismic boundary. On the profiles in Fig. 2 this event (labelled U) has been traced southward from the Grand Banks to deep water in the vicinity of the Newfoundland Ridge. On individual seismic records it is usually difficult to follow this event through the zone of the shelf break, where the change in bottom slope and probable changes in sediment lithology combine to degrade record quality. Fortunately, however, the correlation from the shelf to deep water can be checked by tracing the U event around the several loops of survey coverage (Fig. 1). In the vicinity of the Newfoundland Ridge the U event corresponds to the acoustic basement recorded with singlechannel seismic profiling systems (Auzende et al., 1970; Watson and Johnson, 1970; Jackson et al., 1975; Sullivan and Keen, 1978). However, the multichannel system used to record the data presented in Fig. 2 provides penetration below the level of the U event, and this necessitates a revised geological intkpretation. Section G and the southern ends of sections B, C, and D (Fig. 2), iocated on the Newfoundland Ridge, show seismic reflectors to depths approaching

four seconds (reflection time) below the U event, with no indication of any underlying basement. These sub-U reflectors are smooth and gently sloping, and are interpreted as denoting gently dipping sedimentary strata. Velocity analyses performed in the course of data processing yield velocities in these strata of 3-4 km/s, indicating that their thickness reaches at least 6-8 km. On the southern end of section D the reflectors define a gentle synclinal structure which appears to strike no~hwest-southe~t, approximately parallel to the physiographic trend of the Newfoundland Ridge. These strata form apparent cuestas at the U event interface (e.g., section GG’), although it is not clear from the seismic data whether the scarps associated with these features are entirely erosional or whether displacement by faulting has occurred. The heavy parts of the lines in Fig. 1 indicate the areas where reflectors beneath the unconformity are interpreted as representing sedimentary strata. The crosshatch defines the interpolated, minimum extent of these inferred strata. Seismic records from the southeastern slope of the Grand Banks show sub-U penetration of a much different appearance. Reflections beneath the U event on the eastern end of line E, and along the northern three-quarters of line D (Fig. 2) are highly variable in attitude, amplitude, and continuity. This seismic character contrasts in turn with that beneath the southwestern slope of the Grand Banks, where large anomalies traced by the U event on lines A and E are interpreted as buried seamounts. The seismic record sections show reflectors on the flanks of these features, but no apparent penetration beneath their central, highest parts. The character and disposition of these flanking reflectors give the impression of talus deposits. Jackson et al. (1975) recorded a mantle refraction at a depth of 17 km in the area of the seamounts southwest of the Grand Banks (Fig. 2), and described the irregular surface traced by the U event as oceanic basement. A sonobuoy refraction measurement near the north end of line D indicated a refractor at the level of the U event with a velocity of 5.8 km/s, likewise interpreted as oceanic basement. These are standard marine seismic refraction measurements of the type that have long been used to define crustal structure, including diagnosis of continental vs. oceanic crust. It is evident, however, that they do not detect the obvious difference in geologic structure at the level of the U event that is shown by reflection seismic data. Thus there is a question as to which - if either, of these refraction measurements defines “true” oceanic basement, apart from the problem in this area of defining continental vs. oceanic basement. The single-channel reflection seismic profile drawn in Fig. 3 (from Sullivan and Keen, 1978) shows about 2 km relief on the U event beneath the Newfoundl~d Ridge. The profile does not show the sub-U reflectors recorded on the southern end of multichannel profile D (Fig. 2). The gravity profile in this diagram is of particular interest because of the low amplitude of the anomaly associated with the Newfoundland Ridge. Two-dimensional modelling using a basalt density (2.9 g/cm3) for the acoustic basement ridge

76 H

H’

-400 I?\ / ' ,'.\\---MAGNETIC ANOMALY too:: ' '\ ,I.‘\-0 ,,' '\ I\ y\ a i,', I ; \ / ( _.,' ,y 1 ? '. ' L ?i 2 o-..____-J1 / \_I ^u+ ,--* r x!__/-I Ii*P / \ '.T 1--400 ! : '1 ' , \,: -ioo’ I

0

-

FREE-AIR GRAVITY

200

100

300

KILOMETRES

Fig. 3. (From Sullivan and Keen, 1978). Magnetic anomaly, free-air gravity, and reflection seismic profile of the Newfoundland Ridge. Location shown in Fig. 1.

in Fig. 3 and a density of 2.1 g/cm3 for the overlying sediment indicates that the free-air gravity anomaly should be several times as large as recorded. The absence of an anomaly on this scale is supportive evidence that the Newfoundland Ridge is composed of lighter material - possibly sedimentary strata, than the oceanic crust adjacent. Available magnetic coverage for the area of the Newfoundland Ridge (Keen et al., 1977) shows a complex assemblage of large magnetic anomalies, but does not contribute to refining the areal limits of the different types of basement structure indicated by seismic data. The large anomalies in Fig. 3 generally reflect sources several km below the sea floor. Occassional short wave length anomalies appear to be from sources near the sea floor. DISCUSSION

The multichannel reflection seismic data illustrated in Fig. 2 are interpreted to indicate that the so-called basement blocks underlying the Newfoundland Ridge are in fact composed of sedimentary strata. Assuming that the U seismic event has been traced correctly from the Grand Banks to the Newfoundland Ridge, and that it is roughly coeval in both areas, then the age of these inferred sedimentary strata must be Early Cretaceous or older. The seismic character of these strata can be compared with that of the Lower Cretaceous-Jurassic sedimentary fill of the sub-unconformity basins on the Grand Banks. Comparisons may also be drawn with the Grand Banks basins on the basis of the apparent thickness of these strata, and their areal extent as depicted by crosshatching in Fig. 1. These similarities suggest that the inferred sedimentary strata beneath the U event on the Newfoundland Ridge may represent a basinal accumulation analogous to the sub-unconformity basins on the Grand Banks. The Newfoundland Ridge, therefore, is

77

perhaps best defined as a vestigial sedimentary basin, which apparently subsided about 3 km relative to the Grand Banks by displacement across a hinge line or zone of faulting beneath the present continental slope. If these strata extend to the physiographic limit of the Newfoundland Ridge, as traced by the dotted line in Fig. 1, this feature would represent a subsided, remnant basin roughly equivalent in area to the total of the several sub-unconformity basins on the Grand Banks. The features of the Newfoundland Ridge described above point to a continental origin for this structure, with the implication that the inferred basinal deposit of sedimentary strata is underlain by a continental basement. By analogy with the Grand Banks, this basement would consist of Palaeozoic metasedimentary and igneous rocks. A further implication of the inferred basinal setting of these strata is that basement rocks adjacent have subsided more deeply than the sediments, and may extend some distance laterally from the Newfoundland Ridge. The relevance of this question is highlighted by the apparent protrusion of the Newfoundland Ridge into the supposed realm of oceanic crust (Fig. 1). Multichannel seismic coverage (Fig. 1) is not sufficient to pursue the above hypothesis in detail. Possibly, however, the contrast in seismic character at the level of the U event between the southeastern and southwestern margins of the Grand Banks, as displayed by multichannel lines A, E, and D (Fig. Z), may relate to differences in the composition of continental basement rocks, now subsided. Magnetic data from the area of the Newfoundland Ridge may also indicate a difference in parent crustal properties on either side of this feature. The well-lineated sea-floor magnetic anomalies to the southwest, described by Kovacs et al. (1978) contrast with the much less regular pattern, not clearly lineated, to the northeast (Keen et al., 1977). Identification of sea-floor spreading anomalies in the latter area is problematic. The magnetic anomalies associated with the Newfoundland Ridge (Figs. 2 and 3) suggest a significant component of igneous rock. By analogy with the Grand Banks this magnetic component could consist in part of extrusive volcanic rocks at the level of the Early Cretaceous unconformity. Volcanic rocks at the level of the unconformity in the Twillick G-49 well (Fig. 1) presumably are responsible for the large magnetic anomaly (about 2000 nT) in that area, first reported by Hood and Godby (1965). This pronounced magnetic disturbance contrasts with relatively featureless magnetic profiles in the vicinity of the Bittern M-62 and Mallard M-45 wells, where early Cretaceous extrusive volcanics have also been drilled. This apparently rather drastic variation in the magnetic properties of volcanic rocks associated with the early Cretaceous unconformity should be heeded as a cautionary note in pursuing interpretations of magnetic data in this region. The possible extent of subsided continental crust peripheral to the Newfoundland Ridge can be examined in the larger context of pre-drift reconstructions of the North Atlantic. The reconstruction to initial time of

Fig. 4. “Pre-drift” reconstruction

of the North Atlantic (after Kristoffersen, 1977).

opening shown in Fig. 4 places the Iberian Peninsula opposite the southeastern edge of the Grand Banks, with Galicia Bank situated as an obstacle to tight juxtaposition of the two margins. The gap to the south of Galicia Bank in this particular fit obviously could accomodate a considerable expanse of foundered continental crust northeast of the Newfoundland Ridge. In addition to variations in the geophysical properties of the crust in this region it is evident that there are variations in degree and style of crustal subsidence. Gradstein et al. (1977) have pointed out that the 4 km subsidence of DSDP site 384 (Fig. 1) since early Cretaceous time is similar to that documented for the Puffin B-90 (Amoco, 1972b) and Tern A-63 (Amoco, 1973a) wellsites on the Grand Banks. Other parts of the Grand Banks, however, have undergone much less subsidence (Gradstein et al., 1975). At the Jaeger A-49 (Amoco, 1974a) well, for example, to the east of the Puffin and Tern sites, post-early Cretaceous subsidence as recorded by the level of the early Cretaceous unconformity is less than 1 km. Subsidence increases laterally from the northwesterly trending axis of the AvaIon Uplift, and the 3 km differential in subsidence between the Jaeger well and the Puffin and Tern wells takes place over a distance of about 200 km normal to this axis. Whereas this differential in subsidence occurs smoothly over a distance of 200 km, profiles in Fig. 2 (A, B, C, E) show abrupt displacement of the unconformity at the shelf edge. The indicated differentials in rates and degree of crustal subsidence within the area of the southern Grand Banks and the Newfoundland Ridge are difficult to explain in terms of any simple model of cooling lithosphere. If post-early Cretaceous subsidence of this segment of continental margin is modelled as relating primarily to mantle processes on a “plate tectonic”

79

scale, probably these local differentials in subsidence as expressing inhomogenieties at a “crustal” scale. SUMMARY

must be accounted

for

AND CONCLUSJONS

The Newfoundland Ridge appears to be underlain by blocks of sedimentary strata, which may be the subsided remnant of a sedimentary basin analogous to those on the Grand Banks, and may likewise be intruded and locally flooded by early Cretaceous volcanics. An implication of this inferred basinal setting is that continental basement rocks may extend beneath the sea floor adjacent to the Newfoundland Ridge. This interpretation derives from analysis of multichannel reflection seismic data, which in this area bridge the gap between the coarse resolution of sea-surface refraction seismic measurements and the limited penetration of single channel reflection seismic systems. The importance of integrated interpretation of geophysical data is further emphasized by drilling results from the Grand Banks, which provide evidence of large variation in the magnetic properties of extrusive volcanic rocks. The variations in degree and style of crustal subsidence across this segment of the continental margin requires further study. Two key problems amenable to investigation by multichannel reflection seismic profiling are: (1) the nature of the zone of junction of the Newfoundland Ridge and the J-anomaly Ridge; and (2) the possible extent of subsided continental crust laterally from the Newfoundland Ridge. If the Newfoundland Ridge is indeed continental in origin, it constitutes an additional crustal fragment that must be accomodated in establishing the pre-drift fit of North America and Iberia. If oceanic in origin, then the multichannel reflection seismic data indicate some process of formation of oceanic ridges other than the intrusive mechanisms generally postulated. The zone of the Newfoundland Ridge is generally regarded as the northern limit of the old, Jurassic-Cretaceous sea-floor off eastern North America. It is proposed that an IPOD drill site on the Newfoundland Ridge would resolve the question of the origin of this feature, and if the suspected sedimentary strata are encountered, their age and depositional environment would illuminate the nature of the early seaway between North America and Iberia. ACKNOWLEDGEMENTS

I thank A. Jackson for modelling geophysical data, G. Grant and G. Cooke for preparing illustrations, and F.M. Gradstein and R.T. Haworth for critical review of the manuscript.

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