Alpha Ridge and iceland-products of the same plume?

JOURNAL OF GEODYNAMICS6, 197 214 (1986)

ALPHA RIDGE PLUME?

AND

ICELAND-PRODUCTS

197

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SAME

D. A. FORSYTH, P. MOREL-A-L'HUISSIER, I. ASUDEH and A. G. GREEN

Lithosphere & Canadian Shield Division, Geological Survey of Canada, Energ),, Mines and Resources, Ottawa, Ontario, Canada KIA OY3 (Accepted June 6, 1986)

ABSTRACT Forsyth, D. A., Morel-a-l'Huissier, P., Asudeh, I. and Green, A. G., 1986. Alpha Ridge and Iceland Products of the Same Plume? In: G. L. Johnson and K. Kaminuma (editors), Polar Geophysics. Journal o['Geodynamics, 6: 197-214. Refraction results from the 1983 Canadian Expedition to Study the Alpha Ridge suggest a crustal model with many similarities to the region of Iceland. The overall character of the seismic sections, the derived velocity structure, the lateral dimensions of crustal transition zones and the general crustal thicknesses are comparable from the Arctic and Atlantic regions. Additional similarities are noted in the alkalic basalts, the subaerial to subaqueous, within-plate volcanic environment and the positive regional magnetic responses at satellite altitudes. Based on the geophysical semblances and the apparent evolutionary sequence of within-plate volcanism and tectonic events extending from the northern Ellesmere-Greenland area to Iceland, it is proposed that the Alpha Ridge and Iceland may have been affected by the same mantle plume.

INTRODUCTION

The locations of four reversed refraction lines, surveyed as part of the 1983 CESAR (Canadian Expedition to Study the Alpha Ridge) project, are shown in Figure 1. Profiles were recorded along the CESAR South Ridge (Strike Line), along the CESAR North Ridge (Camp Line), in the Basin area 175 km to the north (Basin Line) and perpendicular to the previous three lines (Dip Line). Preliminary results from the Strike and Basin lines showing similarities with the Iceland crustal structure were reported by Forsyth et al. (1986). Figure 2, from Forsyth et aL (1986), shows a comparison of the crustal Geological Survey of Canada Contribution 16086 CESAR contribution No. 30 0264-3707/86/$3.00

© 1986 Geophysical Press Ltd.

FORSYTH ET AL.

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model derived for the region from southern Iceland to the east flank of the Reykjanes Ridge with the model for the CESAR Strike Line. The Iceland model extends from the region of anomalous crust affected by recent plume and spreading activity to the offshore ridge flank underlain by thickened but more normal 10 my oceanic crust (Goldflam et al., 1980). Forsyth et al. (1986) suggested that the similarities of crustal structure between Iceland and the Alpha Ridge, considering the relative ages and the current states of tectonism, indicate the Icelandic model may be a current tectonic analog to the Alpha Ridge. Detailed models for all the CESAR refraction data will be published by

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Asudeh et al. This report examines several general features of the record sections from the Alpha Ridge, their similarity with features from the IcelandReykjanes Ridge data and explores the hypothesis that Iceland and the Alpha Ridge are, at least in part, products of the same mantle plume. RECORD SECTION FEATURES-ALPHA RIDGE/ICELAND SIMILARITIES

Figure 3 shows the true amplitude sections (seismograms adjusted for shot weight, geometrical spreading with respect to distance and normalized

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with respect to the largest amplitude on the section) recorded along the Strike and Camp Lines. On the Strike Line section shot from the west, the first arrivals are remarkably coherent and uniform out to a distance of about 120 km. Similar responses are evident in the data recorded to the west from the Strike Line central shotpoint (Figure 3c) and on the Camp Line section (Figure 3a). In contrast, as illustrated in Figures 3c and 4, first arrival energy transmitted from the central and eastern shotpoints into the eastern half of the Strike line shows a different mid to upper crust response. Further similarities between the Alpha Ridge and Iceland crustal sections (Figure 4) may be noted by comparing the section recorded from the eastern end of the Strike line with the Iceland data along line BIO1SW recorded from the Iceland shelf to the east flank of the Reykjanes Ridge

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(Pavlenkova, 1985). Despite differences in station density and apparent frequency content, there is a distinct similarity in the character (ampitude and times) of the first arrivals. The Iceland section out to near 70 km is similar to the Strike Line section out to near 100 kin. In addition, the reflected energy in the 120-190 km range of Strike Line data strongly resembles the response in the 90-130 km range of Iceland data. The greater offset distances for the Strike Line reflections are largely due to the thicker oceanic crustal section in the CESAR area. Note that in both cases the lateral dimension of the transition zone from more oceanic type crust to anomalously thick crust is about 50 km (Figure 2). The differences in the general nature of the first arrivals observed over the eastern and western areas of the Strike Line suggests differences in crustal properties beneath the eastern and western regions of the CESAR study area. As suggested in Figure 2, the eastern Strike Line would be underlain by crust similar to that beneath Iceland while the crust beneath the western

202

FORSYTH ET AL.

Strike Line and Camp Line areas would have crustal affinities with the east flank of the Reykjanes Ridge as described by the RRISP Working Group (Angenheister et al., 1980). The crustal structure in the transition zone (Asudeh et aL) from the eastern Strike Line to the northern basin is compared in Figure 5 to a ALPHA RIDGE

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ALPHA RIDGE AND ICELAND

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similar length of model for the region from Iceland eastward to the IcelandFaeroe Ridge (Bott and Gunnarsson, 1980). Although, in both the Iceland and Alpha Ridge studies, data along these "dip" sections is more sparse than on lines along ridge strike, there are obvious similarities in crustal thickness and velocity structure. Comparable observations to those made by Bott and Gunnarsson (1980) for the Iceland-Faeroe region on the nature of the sedimentary cover and the lateral variations in short-wavelength gravity and magnetic anomalies have been made by Jackson (1985) and Weber (in preparation) for the CESAR study area.

SATELLITE MAGNETIC DATA

Satellite magnetic data over the Alpha Ridge have been extensively described by Langel and Thorning (1982), Taylor (1983), Coles (1985). Figure 6 is derived from Haines (1985). In noting the relationship between major magnetic anomalies and Arctic tectonic features, both Langel and Thorning (1982) and Coles (1985) emphasize the prominent magnetic responses over the Alpha Ridge, northern and southern Greenland and Iceland. They note that the anomaly which peaks over the Alpha Ridge, but also covers the Mendeleyev Ridge and Chukchi Plateau, is among the most significant satellite magnetic anomalies on earth. In considering the Iceland data, Coles (1985) suggests that a thick volcanic pile, with an average magnetization of 3 A/m estimated from core sampling, persisting to a Curie isotherm depth near 10 km may explain the amplitude of the Iceland Magsat anomaly. From Figure 2, note the 10 km depth corresponds to the general crustal thickness beneath Iceland. The much larger anomaly over the Alpha Ridge may result from a deeper Curie isotherm within the older tectonic environment or an increased thickness of volcanic material that is reflected in the greater crustal thickness shown in Figure 2. From POGO data, Langel and Thorning (1982) draw attention to a possible Iceland-Greenland relationship and an extension of the North Greenland magnetic high, southwards, to separate the relative lows on either coast. It is tempting to speculate that the observed anomaly pattern from the Alpha Ridge to Iceland is a consequence of the emplacement of plume material in the lithosphere. The larger amplitude anomaly over the Alpha Ridge would result from material emplaced relatively near-surface in oceanic crust, perhaps along a failed rift arm from the opening of the Canada Basin, whereas the north Greenland anomaly would result from emplacement of material into the already rifted region of northeastern

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Greenland. The Greenland anomaly is broken from the Alpha anomaly as a result of the continued adjustment of Greenland relative to North America throughout Cretaceous-Tertiary time. The relative lows observed over the central Greenland volcanic provinces (Clarke and Pedersen, 1976, Deer, 1976) may reflect a lower net magnetization produced by the reaction of plume material with a more felsic continental crust, the effect of a greater number of reversed polarity periods from about 70 to 50 my or, as Vogt (1983) suggested, a variation in plume discharge. While the GreenlandIceland relationship and the southward extension of the north Greenland anomaly remain unresolved on the maps of Coles (1985) and Haines

ALPHA RIDGE AND ICELAND

205

(1985), it is notable that the proposed drift track of the Iceland hotspot (Vink, 1984) (Figure 6) does suggest a link between northern Greenland and Iceland. Further work on the magnetic data is required to help resolve the thesis of hotspot passage beneath Greenland. AEROMAGNETIC DATA

For the CESAR study area of Figure 7 there is a change in the magnetic anomaly pattern (Perry et al., 1985) from west to east along the Strike Line, with shorter wavelength, east-west trending anomalies overlying the eastern region of proposed thickened oceanic-type crust. The magnetic high lying

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FORSYTH ET AL.

immediately east of centre on the Strike Line overlies the mid-crustal region of lower average velocity in the model shown in Figure 2. It may be noted that the magnetic map for the principal islands of Hawaii (Godson, 1984) shows anomalies of similar wavelength and character to that shown in Figure 7. Figure 5 shows a comparison between the seismic model and the estimated magnetic source depth (EMSD) values, from Figure 20 of Kovacs and Vogt (1982), for the Dip Line across the north flank of the Alpha Ridge. Agreement between magnetic and seismic models is reasonable, except over the central region where the magnetic data predict a basement source 2-3 km deeper than the seismic model. We know the profile runs across strike of the bathymetry and magnetic contours and there is a regional change suggested in upper crustal (refraction) structure from the Ridge to the Basin (Figure 5). It is thus probable that the profile crosses susceptibility variations as well, which could produce erroneously deep EMSD estimates (Klitgord and Behrendt, 1979) over the central region.

OTHER GEOLOGICAL~3EOPHYSICAL CONSIDERATIONS

New information on the age and composition of the Alpha Ridge was obtained from the shallow seismic profiling and sampling program (Initial Geological Report on Cesar, 1985) carried out from the CESAR camp as it drifted over the CNR and CSR (Figure 1). Analyses of the dredged basalt samples suggests the probable widespread eruption of alkali basalt in a shallow water, within-plate tectonic environment (Van Wagoner, 1985). Paleomagnetic stratigraphy of the CESAR cores (Aksu, 1985) and lithostratigraphic analysis (Mudie and Blasco, 1985) indicate no significant tectonic disturbance of the CESAR area since about 73 my. Magnetic anomaly traits (Vogt et al., 1979) suggest a time of formation for the CESAR area during the Cretaceous positive polarity period between about 80 and 120 my. The development of the Alpha Ridge was therefore contemporaneous with the generation of oceanic crust in the Canada Basin between about 125 and 79 my (Sweeney, 1985). Heat flow data (Taylor et al., 1986) support the formation age interpreted from magnetic data. The geothermal study suggests about 100 my has passed since the last major thermal event in the CESAR area and considers a mantle plume model to most simply explain the known heat flow, bathymetry and gravity data. The above geophysical and geological considerations strongly favour an oceanic origin for the Alpha Ridge.

ALPHA RIDGE AND ICELAND

207

ALPHA AND ICELAND EFFECTS OF THE SAME PLUME?

One thesis that follows from consideration of the evidence described, is that the Alpha Ridge, by analogy with Iceland, represents the product of plume and spreading activity. The thesis is not new, but rather an extension and confirmation of the views already suggested by Vogt and Ostenso (1970) and Vogt et al. (1979). We further suggest that both Iceland and Alpha may be products of the same plume. The possible role of hotspots or mantle plumes in the development of the North Atlantic has been described at length by Morgan (1983), Vink (1984) and Vink et al. (1985). The following discussion attempts to rationalize the track of one plume, the Icelandic plume, in terms of features lying between the CESAR area and Iceland. We propose here (Figure 8), that the hotspot was beneath the CESAR survey area (86N, l l 0 W ) at about lOOmy. Morgan (1983) suggested the Iceland hotspot would leave a track from northwest Greenland at about 90 my. For Figure 8, the plume track positions for 50 my and 60 my are from the fit by Vink (1984), shown in Figure 6, while the positions at 68 my and 80 my were derived using the pole positions and plate reconstruction outlined on the map by Bernero et al. (1985) at Mid-Maestrictian and Santonian times. As the Canada Basin continued to open during early Late Cretaceous time, (see, for example, Sweeney et al., 1978, Jackson et al., 1986) the hotspot effects might then be evident beneath the northern Ellesmere to Greenland area and beneath areas further south during the Tertiary. Vink (1984) has suggested that, in addition to affecting the immediately overlying area, the plume's effects may also be seen some distance laterally as the plume material rises to the nearest regions of thinnest lithosphere. From stratigraphic and geochronologic data, Ricketts et al. (1985) conclude that the Strand Fiord volcanics of northern Axel Heiberg and Ellesmere Islands range in age from about 95-88 my. They conclude that the Strand Fiord rocks are the product of an early Late Cretaceous Hawaiian style volcanism from an extensive source to the north and propose that the "volcanics represent the cratonward extension of the Alpha Ridge". Trettin and Parrish (in preparation) would support this view. They report U/Pb zircon ages of 92 my + - 1 for the Wootton intrusion and 88 + 2 0 / - 2 1 my for the Hansen Point volcanics of northwest Ellesmere Island and suggest the rocks are correlative with the lower part of the neighbouring Strand Fiord Formation. They point out a possible relationship between the Hansen Point volcanics and the somewhat younger Kap Washington Group of nothern Greenland. Batten et al. (1981) and Soper et al. (1982) JOG 6,1-4-14

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ALPHA RIDGE AND ICELAND

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describe a tectonic setting of crustal extension and regional doming that accompanied the extrusion of alkali dykes and sills in a within-plate environment prior to emplacement of the Kap Washington Group about 75-65 my. Batten et al. (1981) suggest the ages 82 to 66my (Figure 8) are the most reliable dates away from areas of Tertiary tectonism. Osadetz and Moore (1986) infer that subalkaline basic rocks from the vicinity of Lake Hazen, northeast Ellesmere Island, represent magmatic activity during the late stages of Amerasian Basin opening. For the West Greenland Volcanic Province, Clarke and Pedersen (1976) suggest a period of intense basaltic volcanism between 63 an 56 my. Rolle (1985) reports the presence of subaqueous and subaerial basalt flows from wells offshore of the West Greenland Volcanic Province with ages from about 68 to 53 my (K/Ar). The location of the plume beneath northern Greenland at about 70-80 my is in accord with the CESAR core data that require no significant adjustment to sediments deposited in the CESAR study area after about 73 my.

POSSIBLE IMPLICATIONS OF A PLUME PASSAGE BENEATH GREENLAND

The Late Cretaceous Early Tertiary passage of northern Ellesmere Island and Greenland over the hotspot may have produced rifting between Greenland and the eastern Canadian Arctic and contributed to basin and arch development in the eastern Sverdrup Basin during phase 1 of the Eurekan Orogeny (Balkwill, 1978). The presence of a plume beneath Greenland may also have initiated or provided impetus to the Late Cretaceous opening of Baffin Bay (Srivastava, 1978), as well as providing the forces required to complicate an otherwise uniform spreading in the Labrador sea that had already begun about 84my (Srivastava and Tapscott, 1985). The result of these tectonic adjustments to plume activity beneath Greenland is perhaps evident in the "diffuse plate boundary" along Nares Strait (Miall, 1984). The plume position beneath central Greenland may have fed lithospheric material to the nearest regions of thinnest lithosphere beneath the Thule volcanic provinces of west Greenland - Baffin Island, the Voring Plateau (Vink, 1984) and the Scoresby Sund region (Figure 8). While present beneath central Greenland in Early Tertiary time, the plume may have supported spreading activity both in Baffin Bay and in the NorwegianGreenland Sea. With continued drift of Greenland to the northwest, the plume became less effective in the Baffin Bay - Labrador Sea opening, ceasing at about 35 my (Srivastava and Tapscott, 1985), and more effective in the spreading to the east. The passage of the northern Greenland-Ellesmere

210

FORSYTH ET AL.

area over the plume in early Late Cretaceous time and subsequently, southeastern Greenland in Late Eocene time may have supported the rotational motion of Greenland prior to anomaly 25 time at about 59 my (Srivastava and Tapscott, 1985). The present position at 64 N, 16 W implies a net apparent drift of the plates over the plume of about 2.6 cm/yr. SPECULATIVE IMPLICATIONS FOR THE CANADA BASIN OPENING

The Late Cretaceous opening of the Canada Basin is not well constrained. One of problems with plate reconstructions is the overlap of the Chukchi Plateau-Northwind Rise (Chukchi Borderland) area on the Sverdrup Basin region prior to the opening of the Canada Basin. Suggested tectonic evolutionary models may be found in Sweeney et al., 1978, Vogt et al., 1982 and Jackson et al., 1986. Here, we wish to emphasize the possibility of the presence of a plume beneath the northern Canada Basin that may have initiated a triple junction opening of the Arctic Ocean during the Late

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ALPHA RIDGE AND ICELAND

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Jurassic-Early Cretaceous. The propagating arms of the rift would have been directed towards the southern Canada Basin, the Ellesmere-Greenland region and Eurasia. The considerable volume of basalt erupted in phases throughout the central and northeast Sverdrup Basin during the Cretaceous may be evidence that the plume was beneath the North American side of the rift rather than the Alaska Arctic plate. During the initial opening of the Canada Basin, plume material was emplaced on both the Arctic Alaska plate, to create or add to the Chukchi Borderland and on the North American plate, to begin formation of the Alpha Ridge. Later, with the passage of the northern Greenland-Ellesmere area over the plume between about 85-75 my, continued spreading in the Canada Basin effectively separated the Chukchi borderland from the eastern end of the Alpha Ridge across the Mendeleyev Abyssal Plain (Figure 9). The approximate fit of the southeast Alpha Ridge to the Chukchi Borderlands at a depth of 2500~3000 m suggests a separation of about 400 km, or a period of about 10 my at a spreading rate of 2 cm/yr. The product of the rift arm which developed toward the Eurasian plate is represented by the Mendeleyev Ridge. The drift of Greenland and North America over the plume along a track approximately coincident with the rift arm toward Greenland-Ellesmere produced the Alpha Ridge. The tapered shape of the the Makarov Basin toward Greenland may indicate the rift was largely a failed arm fed from the nearby plume. SUMMARY

Crustal seismic refraction data from the Alpha Ridge and the IcelandReykjanes Ridge show many similar features. The structural models derived both along stike and normal to the Arctic and Atlantic Ridges also exhibit strong similarities. If, in fact, the Alpha and Iceland crustal models reflect the effects of similar tectonic processes with a difference in age of about 100 my, the present relative seismic responses are notable. MAGSAT anomalies over the region from the Alpha Ridge, through northern Greenland to Iceland, are among the most intense satellite magnetic anomalies observed on Earth. Aeromagnetic anomalies over the Alpha Ridge are similar in wavelength to those observed over Hawaii and also reflect features in the crustal seismic model. The results of the geophysical and geological studies strongly favour an oceanic origin for the Alpha Ridge. It is proposed that the Alpha Ridge and Iceland may have developed in an analogous fashion as products of possibly both plume and spreading activity. The plume may also have had a major influence in the opening of

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FORSYTH ET AL.

the Canada Basin and the later adjustments of Greenland with respect to North America. Volcanic lithologies, ages and interpreted tectonic environment from the Alpha Ridge to Iceland provide evidence in support of the proposal.

AC KNOWLEDG E M ENTS

Although the authorship must be held accountable for the views expressed herein, we sincerely appreciate the input contributed both through unpublished data and discussions with N. Van Wagoner, R. Jackson, H. Trettin, K. Osadetz, R. Coles, G. Haines, J. Sweeney, L. Johnson and A. Embry. The work has arrived in timely, final form largely through the efforts of the drafting and photography sections of the Earth Physics Branch.

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