Crustal structure of the Canadian polar margin: results ...

Crustal structure of the Canadian polar margin: results of the 1985 seismic refraction survey1 I. ASUDEHAND D.

A.

FORSYTH

Lithosphere and Canadian Shield Division, Geological Survey of Canada, I Observatory Cr., Ottawa, Ont., Canada KIA OY3

R. STEPHENSON AND A . EMBRY Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 3303 -33rd St. NW, Calgary, Alta., Canada T2L 2A7

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Atlantic Geoscience Centre, Geological Survey of Canada, P. 0. Box 1006, Dartmouth, N.S., Canada B2Y 4A2 AND

D. WHITE^ Pac$c Geoscience Centre, Geological Survey of Canada, 9860 W. Saanich Rd., P.O. Box 6000, Sidney, B. C . , Canada V8L 4B2 Received February 26, 1988 Revision accepted September 12, 1988 The 1985 refraction survey based on Ice Island covered a northern transition zone along the Canadian polar margin north of Axel Heiberg Island. The refraction survey included a 60 km line along the inner shelf, a 180 km parallel line along the outer shelf, and a 60 km connecting line. Shotpoints offset from the line ends recorded upper mantle observations to a distance of 240 km. Along the inner shelf, the upper 700 m, with an interval velocity of 3.7 kmls, is interpreted as Tertiary -Cretaceous strata. The underlying 4 km thick layer has a starting velocity of 5 kmls and a gradient of 0 . 2 s-l. It is thought to consist of mainly deformed lower Paleozoic strata capped by upper Paleozoic - Triassic clastics and carbonates and (or) Cretaceous volcanics. Sequentially, the lower unit, with a starting velocity of 5.8 km/s, most likely consists of Proterozoic - lower Paleozoic rocks. Beneath the offshore line, up to 5 km of strata with a starting velocity of 2.2 kmls and a gradient of 0.5 s-' probably represents Tertiary-Cretaceous clastics. The underlying material with a starting velocity of 4.5 km/s and a gradient of 0.1 s-I is interpreted as a sedimentary succession of either Cretaceous-Tertiary clastics or upper Paleozoic to Cretaceous strata. Beneath this section, a probable Proterozoic - lower Paleozoic lower crustal layer with a starting velocity of 6.2 kmls extends to about 25 km. Apparent upper mantle velocities in the 8.0-8.2 km/s range are observed. Beneath the transitional onshore-offshore line, a Neogene sedimentary basin is interpreted as being floored by faulted blocks of probably deformed Proterozoic to lower Paleozoic rocks on the landward side and possibly Cretaceous to lower Tertiary rocks on the seaward side. La campagne de sismique rkfraction de 1985, avec station de base 9 I'ile Ice, a couvert une zone de transition borkale qui longe la marge polaire candieme au nord de l'ile Axel Heiberg. Le lev6 de sismique rkfraction incluait une ligne de 60 km le long de la plate-forme interne, une ligne parallkle de 180 km suivant la plate-forme externe et une autre de 60 km reliant ces deux premikres lignes. La fengtre d'kmission entre les extrkmitks des lignes a permis d'enregistrer des signaux provenant du manteau supkrieur h une distance de 240 km. Le long de la plate-forme interne, les 700 premiers mktres sous la surface sont carctCrisks par une vitesse de 3,7 kmls, laquelle est interprktke comme correspondant aux strates du Tertiaire-Crktack. La couche sous-jacente de 4 km d'kpaisseur fournit une vitesse de dkpart de 5 krnls et un gradient de 0,2 s-l. Cette couche est assign& 2 des strates du Pla6ozoique infkrieur coiffks par des clastites et des carbonates du Palkozo'ique sup6rieur - Trias et (ou) des volcanites du Crktack. Dans la meme shuence, I'unitk infkrieure qui est caractkrisk par une vitesse de dkpart de 5,8 kmls, semble vraisemblablement correspondre aux roches du Protkrozoique - Palkozoique infkrieur. Sous la ligne au large du rivage, les strates d'une kpaisseur de 5 km, avec une vitesse de dkpart de 2,2 kmls et un gradient de 0,5 s-l, sont formkes probablement de clastites du Tertiaire - Crktack. Le matkriel sousjacent, avec une vitesse de dkpart de 4,5 kmls et un gradient de 0 , l s-l, et interprktk comme une succession skdimentaire formke soit de clastites du Crktack - Tertiaire ou de strates du Palhzoique au Crktack. En dessous de cette section existe probablement une couche de la croiite infkrieure de roches du Protkrozoique - Palkozo'ique infkrieur, fournissant une vitesse de dkpart de 6,2 km/s et s'ktendant jusqu'h 25 km. Les mesures des vitesses apparentes dans le manteau supkrieur varient de 8,O h 8,2 kmls. En dessous de la ligne skparant les plates-formes interne et externe, un bassin saimentaire nkogkne probablement form6 d'un plancher de blocs failles, qui du c6tk interne de I'ile est possiblement composk de roches pilsskes du Protkrozo'ique h Palkozo'ique infkrieur et du c6tk ockanique de roches du CrktacC 2 Tertiaire. [Traduit par la revue] Can. J . Earth Sci. 26, 853-866 (1989)

'Geological Survey of Canada Contribution 44687; Ice Island Contribution 13. 'Present address: Lithosphere and Canadian Shield Division, Geological Survey of Canada, 1 Observatory Cr., Ottawa, Ont., Canada KIA OY3. Printed in Canada 1 Imprim6 au Canada

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854

CAN. J . EARTH

Introduction The Canadian polar margin stretches for more than 2000 km from the Alaskan border to the Lincoln Sea. The width of the continental shelf varies from nearly zero off northern Ellesmere Island to about 200 km off Mackenzie Delta. Remoteness and the continuously shifting ice pack in combination with temperatures in the -40" to -25°C range have limited exploration of this margin. Regional aeromagnetic and gravity data are adequate for the margin (Forsyth et al. 1988); however, pre-1985 seismic data are limited to industry reflection lines in the Beaufort Sea and reconnaissance refraction lines offshore of Brock and Ellef Ringnes islands (Hobson and Overton 1967; Overton 1970; Berry and Barr 1971). As part of the Frontier Geoscience Program of the Department of Energy, Mines and Resources Canada, seismic refraction surveys were undertaken in 1985 and 1986 to determine the thickness and distribution of potential sedimentary basins on the continental shelf as an aid to resource assessment. In addition to focusing on the general crustal structure of the middle to upper crust, the surveys were designed to gather data pertinent to the upper mantle to improve our understanding of the tectonic evolution of the polar margin. Both the 1985 and 1986 refraction surveys were operated from the Canadian Ice Island, named Hobson's Choice, located offshore from Nansen Sound between Ellesmere and Axel Heiberg islands (see Fig. 1). Logistic support was provided by the Polar Continental Shelf Project (Asudeh et al. 1985, 1986). The 1985 refraction data are discussed in detailed in this report, with brief reference to the 1986 data where required. A comprehensive analysis of the 1986 data will appear in a future publication. Geological setting The regional geology of the Arctic Islands has been established during the last 30 years by the Geological Survey of Canada (Thorsteinsson and Tozer 1970; Trettin and Balkwill 1979; Trettin, in press). Three tectono-stratigraphic sequences, separated by major unconformities, are mapped on the land areas adjacent to the northern portion of the polar continental shelf (Fig. 1). Schematic stratigraphic sections for northwestern Ellesmere Island, northern Axel Heiberg, and Meighen Island are shown in Fig. 2. The oldest sequence consists of highly indurated Upper Proterozoic(?) - lower Paleozoic clastics, carbonates, and basalts that are intruded by gabbro and granite. These strata, part of the Franklinian Basin, were severely deformed during the Late Silurian - Early Carboniferous Ellesmerian Orogeny (Trettin and Parish 1987). Based on the continuity of the pattern of potential-field anomalies from immediately onshore (Forsyth et al. 1988), these deformed lower Paleozoic strata probably extend under the continental shelf into the area of the 1985 refraction survey. The next sequence consists of Upper Carboniferous to lowest Cretaceous strata that belong to the (pre-Amerasian Basin rift) Sverdrup Basin succession (Balkwill 1978). The Sverdrup Basin developed in response to substantial rifting and thinning of continental crust, mainly during the Carboniferous and Permian periods, and to subsequent passive thermal subsidence (Stephenson et al. 1987). Upper Paleozoic strata consist mainly of carbonates with lesser amounts of evaporites, terrigenous clastics, and minor volcanics. The overlying Triassic - lowest Cretaceous strata consist almost entirely of

SCI. VOL. 26,

1989

terrigenous clastics. These strata were subsequently intruded by Cretaceous diabase dykes and sills, locally overlain by similarly aged volcanic flows and subsequently deformed during the early Tertiary Eurekan Orogeny. Trends of major Eurekan structures are mainly northwest - southwest on Axel Heiberg Island and southwest-northeast on northern Ellesmere Island. The Sverdrup Basin succession thins markedly northward away from the rift axis of the Sverdrup Basin depocentre as a result of depositional thinning and pre-Late Cretaceous truncation (Meneley et al. 1975). There is sedimentological evidence that the terrigenous -volcanic succession had, in part, a northerly source (Thorsteinsson and Tozer 1970; Embry, in press), and its occurrence over much of the 1985 refraction survey area may be minor. The youngest sequence is Cretaceous -Tertiary in age and is part of the continental shelf succession deposited during and subsequent to Amerasian Basin opening. The CretaceousTertiary sequence on Meighen Island (Figs. 1, 2) consists of 2500 m of sandstone and conglomerate overlying 600 m of shale and siltstone. Seismic reflection data on Meighen Island (Van Altena 1974) indicate that this succession thickens northwestward toward the newly developed continental margin. It is suggested that these strata underlie much of the continental shelf in the refraction survey area, as they are coeval with the shelf development. A lower Tertiary sandstone has been cored from the shelf about 20 krn north of Axel Heiberg Island (P. Mudie, personal communication, 1986). The present continental margin in this area is thought to have been formed as a result of rifting of the continental crust north of the Sverdrup Basin, mainly in the Early Cretaceous, followed by a period of sea-floor spreading that ended in the Late Cretaceous (Embry and Osadetz 1988). The present shelf structure in the vicinity of the 1985 refraction survey will reflect the effects of the opening of the adjacent ocean basin, the development of the nearby Alpha Ridge (Asudeh et al. 1988), and the Eurekan Orogeny, all superimposed on Franklinian basement rocks.

Refraction survey parameters The survey consisted of five 60 km refraction spreads, each with 12 seismic recorders deployed on the sea ice at nominal 5 km intervals. The recorders were programmed to digitally record the output of a 16-element circular array (Mair and Lyons 1981) of 4.5 Hz geophones in predetermined 2 -3 min. time windows. Instrument failure resulted in inadequate coverage for the DE spread (Fig. 1). Shots were detonated at the ends of the 60 km spreads as well as at offset distances of 2.5 (half the recorder spacing), 60, 120, 180, and, when possible, 240 km from each recording spread. The use of the shots at 2.5 km from the end points was to double the volume of crustal data obtained beneath a given spread of 12 recording instruments. With this shooting technique, a composite record from two shots 2.5 km apart fired into 12 recorders deployed at spacings of 5 km provided an apparent double coverage with 24 traces for a 60 km long line. The data collected in this manner may be composited and interpreted as a pseudosection if structure beneath the adjacent shot points is simple and one dimensional. For more complex structures, this offset shooting does provide additional information, but the data for each shot should be analyzed separately before being composited. Larger shots were fired at greater offsets from the spreads

ASUDEH ET AL.

855

Upper Cretaceous

r]

-

Upper Paleozoic Lower Cretaceous Clartics,


Carbonates, Minor Volcanics

..........

Lower Paleozoic Clastics, Carbona+er,Gobbro,Glonits

1 7 0 0

FIG.1. Onshore geological setting and geometry of the 1985 refraction survey showing Borden (B.I.), Ellef Ringnes (E.R.I.), Axel Heiberg (A.H.I.), and Ellesmere (E.I.) islands. The three major stratigraphic sequences mapped onshore are separated by regional unconformities and are discussed in the text. Seismic shots were fired at A, B, C, D, E, F, I, and G and recorded at sites denoted by circles. Ice Island was located near shotpoint I during the survey. Syledis navigation beacons were located at Lokk, Hubbard, Bjarenson, and Meighen.

to maintain good signal-to-noise ratios for the entire range of 0-240 km covered during the survey. For distance ranges of 0-60, 60-120, 120-180, and 180-240 krn, shots of 136, 217.60, 435.20, and 652.80 kg, respectively, were detonated while suspended at a depth of 100 m below the ice. The signal-to-noise ratio was generally good to about 120 km; however, some sections exhibit an increase in noise beyond 60 km. This effect is probably due to variations in energy scattering and attenuation caused by crustal structure as well as shot size. Three "fan" shots were also fired broadside from offshore shotpoints into nearshore spreads to provide additional control on the Moho depth beneath the continental margin. In total, 31 shots were fired into five recording spreads, producing more than 300 seismograms. The naming convention for shotpoints throughout this paper uses a one character shotpoint name and a sequential shot number; e.g., the 10th shot of the survey at shotpoint C is named C10 (see Table 1). The clocks in the shooting and recording instruments were routinely calibrated against a master clock, which was corrected daily to the standard time signals. The corrections were normally on the order of a few milliseconds, but the recorder clocks showed erratic jumps of about 100 ms in two cases. In addition to the clock correction, a 16 ms delay had to be

applied to all shot times to account for the time of travel of shock waves through 100 m of primacord used to detonate the charges. All shots were detonated by a voltage pulse triggered by a chronometer, except for shots B15, C10, C11, C19, C21, D07, and DO8, which were fired manually because of instrument malfunction. The correct shot times (to within k0.1 s) for these seven shots were obtained from analysis of the waterwave arrivals (Asudeh et al. 1985). Navigation The positioning for the experiment was provided by four Sercel ultrahigh-frequency (UHF) pulse-positioning (Syledis) beacons established on elevated coastal sites by a team from Marine Electronics, Bedford Institute of Oceanography, along the coast of Ellesmere, Axel Heiberg, and Meighen islands (Fig. 1). The original positions of the four Syledis beacons were obtained by the Geophysics Division of the Geological Survey of Canada using the Global Positioning System (GPS) in stationary mode. Mobile Syledis units aboard the helicopters were used to lock into signals transmitted by the Syledis beacons and to navigate to planned survey sites. Four Marconi SMA 76 1 satellite doppler receivers provided by the Geodetic Survey of Canada were used to position shot-

CAN. J.

856

MEIGHEN ISLAND CROCKER 1-53

Q,

0,

a

a

a

a,

u Q,

u

.

V)

....... .......... ....... )........... S&:.-:.,...... ......... I:........... ........ :& ......... ........ I7.r....... .......... -...... .......... ::... :Y?.; '-.-:. ... .......... ..... .UU. ..... ......... .......... :. .: .......... .......... .::.......... 3+ .......... .......... .......... ........ ..... .&.' .......... .......... .......... .......... .......... .......... .......... .......... ----------------- --m


0

t

V)

Tertiary

-

uppermost sandstone, conglomerate, siltstone

NORTHERN AXEL HEIBERG ISLAND Upper Triassic sandstone, siltstone Cretaceous d i a b a s e sills 500 m Lower - Upper Triassjc shale and s ~ l t s t o n e Cretaceous d~abase sills and d y k e s 1000 m

2

---------

Upper P a l e o z o i c clastics and

2500 m

.111....._---------

--

Upper Cretaceous shale, siltstone 600 m

- - --------Upper Triassic

.....

--------

2 -,L.L-~

------

------------

---

shale, siltstone Cretaceous diabase sills

Q,

Q,

carbonates , ,

u Q,

V)

Upper Cretaceous basalt and trachyte

3 minor sandstone and shale

I n t r u d e d by D e v o n i a n granite

1000 m

--

-Upper - - -Paleozoic -----

2

carbonates 500 m

- - - -1 0-0-0 -m- - - - -Highly deformed Lower Paleozoic sandstone, shale, carbonates, chert, volcanics

NORTHWEST ELLESMERE ISLAND

a

----------

Cretaceous

. ' I ? . . . . . .

EARTH SCI. VOL. 26, 1989

1

- - - - - - ---Highly deformed L o w e r Paleozoic strata Intruded by g r a n i t e and gabbro 1 0 + km

1 0 + km

I

FIG.2. Schematic stratigraphic sections for land areas adjacent to refraction survey area, showing the three major stratigraphic sequences discussed in the text and the bounding unconformities.

points and fuel caches outside the range of the Syledis network. Post-survey processing of the SMA 761 data yielded 6 h interval solutions with standard deviations in latitude and longitude of the order of 6 m. Comparison of nearly simultaneous Syledis and SMA 761 observations showed a maximum discrepancy of 50 m. Water waves Recording windows were designed to record many seconds of water-wave data from each shot (Asudeh et al. 1985). This proved to be most useful both in cross-checking the quality of the navigation fixes and in correcting detonation times for shots that were fired manually because of instrument malfunctions. For manually fired shots (e.g., shot B15) the shot times were corrected such that all water-wave first arrivals were positioned on a line with a zero intercept time.

Seismic sections and velocity modelling The seismic refraction data were modelled with the program of Spence et al. (1984). In this method, the velocity model is specified by the coordinates of the upper boundary of a number of blocks and by the velocity and velocity gradient below

the boundary. A choice must be made between dividing the model into a number of blocks with fixed velocities (near-zero gradients) or dividing it into blocks with varying velocities (with nonzero gradients). This choice is made by examining the fit of modelled traveltimes and amplitudes to the data. A fixed-velocity model with velocity discontinuities across block boundaries will generate large-amplitude reflected arrivals after the first breaks. Alternatively, a gradient zone with a gradual increase in velocity distributes the seismic energy more evenly and diminishes strong-amplitude reflected arrivals. Whenever clear, large-amplitude, reflected arrivals are not observed, a gradient model is favoured for the interpretation of our data. Interpretation of the data was carried out in three stages. Initially, all the first-arrival picks for the recording spreads were analyzed in order to determine the main features of the data. he-waveform data for each spread were then considered and velocity structures were modified as needed to model the upper to middle crustal structure beneath the survey area. Finally, composite sections were constructed from different spreads to investigate the lower crust to upper mantle structure of the polar margin.

ASUDEH I!T

TABLE1. Survey shots


Shot name

Spread name

Date (April)

Time (GMT)

Weight (kg)

AL.

857

velocity of 4.4 kmls. First arrival picks for line IG (Fig. 3c) do not fit this velocity and require a higher velocity of' 5.0 kmls. First arrivals for line IE fit a 4.4 kmls velocity on the offshore end and 5.0 kmls onshore.

Offsets of 30- 60 km This is generally a more complex range pertaining mainly to structures farther offshore, and most of the data show evidence of lateral and vertical variations in the velocity structure. The complexity of the structure is best illustrated in Fig. 4 , where picks from five shots fired into different spreads are plotted together. The general trend of the data, however, point strongly to a structure with a basement velocity begining at 6 kmls. Materials with smaller or larger velocities intruded into this structure (Forsyth et al. 1988) could account for the observed complexities. Offsets of 60- 120 km The coverage in this range is limited but critical, since the crossover to mantle velocities lies within this range. Shot Dl7 fired into line BC offers good data for this range, resulting in a crossover around 80 km, beyond which upper mantle arrivals with a velocity of 8.0 kmls are observed (Fig. 5 ) . The fan-shot data (D23, D31, and E28, Fig. 6 ) cover a small range from 55 to 85 km and provide useful information on the crossover distance. The first-arrival picks also show a difference in intercept time between D and E shots, which is most probably due to thinner sedimentary cover beneath E.

First arrivals The first-arrival picks for four of the recording spreads are illustrated in Fig. 3 (the sparse data for spread DE are not shown here). When the picks for shots fired in forward and reverse directions into the same spread lie on straight lines, the velocity structure beneath the spread is considered one dimensional and can be determined by simple least-squares fit of straight-line segments (e.g., distance range of 10-30 km for spread BC in Fig. 3a). First-arrival picks do not in general line up in a reversed sense and point to more complex and laterally varying structures beneath the survey lines (e.g., distances less than 45 km in Fig. 3d). The first-arrival picks are grouped in four distance ranges and are discussed below. Offsets of 0 - 1 0 km The number of first arrivals from 0 to 10 km is limited for each shot, and details of the near-surface structure may not be resolved. However, a velocity of 2.1 km/s is obtained from a least-squares fit to a composited section from lines BC and C D . Picks for the nearshore line IG show an early crossover to velocities near 5.0 kmls, and accordingly the 2.1 kmls sedimentary cover material appears to be too thin to be detected here. A similar sedimentary cover is measured beneath line IE but may not be verified for line DE because of a lack of data. Offsets of 10-30 km Both BC and CD lines have good coverage in the 10 - 30 km range, and first-arrival picks plotted in Figs. 3a and 3b indicate a simple one dimensional structure with a least squares

Interpretation of data by refraction line The interpretation of the data by offset distance in the previous section illustrated the general features of the structure beneath the survey area, but significant variations from the simple one-dimensional structures are evident and discussed here. In this section each of the five refraction lines is analyzed separately to further detail the structure. Seismogram sections are plotted in true-amplitude sense and are compensated by a factor proportional to distance squared to compensate for simple geometrical spreading. Line BC Line BC data offer the most complete coverage in terms of number of shots and successfully recorded traces. The structure beneath this line appears relatively one dimensional. A structure with a surface cover of apparent velocity of 2.1 km/s overlying material with an apparent velocity of 4.4 kmls fits the data well for the four near offset shots of B15, B16, C19, and C21. As a result of this simple structure, data for all shots fired into line BC (i.e., shots B02, B09, B15, B16, C19, and C21) are combined to produce pseudosections (Figs. 7 a , 8a) and interpreted as a reversed profile. Notable features of these composite sections are the lower amplitude of the first-arrival coda near 25 km; high-amplitude secondary arrivals at 25-40 km (Fig. 7a); complex firstarrival times near 40 km (Fig. 8a); a crossover from crustal velocity of about 6 kmls to upper mantle velocity of about 8 kmls near 80 km distance; and large-amplitude, wide-angle reflections and weak Pn arrivals (Fig. 7 a ) . The low-amplitude arrivals near 25 km are modelled by a surface cover with velocity of 2.1 kmls and gradient of 0.5 s-I overlying a thin layer of 4.4 kmls material at depth of 4.3 km. At a depth of 5 km the velocity increases to 4.5 kmls, with a slight change in gradient from 0.15 to 0.1 s-l. The

CAN. J . EARTH SCI. VOL. 26, 1989

858

z ';EN"'

m-

id

LWE CD

gl

x +

I

t+


LINE IG X

I30 C27

+

X

Cll DO7

4

DO8

#

4 - r n

X

I28

C10

3

xnx 0

xq

"0,

xu

X

x

x+%x""$ x+

#+

xgX+

x+#+

LINE IE E22 E26

X

FIG.3. First-arrival picks for all shots fired into spreads BC, CD, IG, and IE. Note that in this and the next three figures all data are plotted from a common origin to emphasize the similarities of the structure. Shots in this and all following figures are numbered using one shotpoint letter and a time-sequential number (see text for more details).

4.4 -4.5 kmls material extends to a depth of 12 km below the surface, where it is underlain by a block of lower gradient (0.04 s-l) 6.0 kmls material. This block is required to satisfy secondary reflected arrivals observed at 25 -40 km (Fig. 7a). Beneath this block at a depth of 15 km, "basement" material with a starting velocity of 6.12 kmls and a low gradient of 0.02 s-I extends to a Moho depth of about 25 km. A Pn velocity of 8.0 kmls with a small gradient of 0.01 s-I explains both the weak Pnarrivals and a wide-angle reflection amplitudes of Fig. 7a. In summary, the structure beneath line BC can be explained by a model (Figs. 7c, 8c) that begins with a velocity of 2.1 kmls and a relatively high gradient of 0.5 s-I and extends to a depth of 5 km. The velocity gradient then diminishes over a mid-crustal section with a thickness of 7 km and a velocity ranging from 4.5 to 5.2 kmls. The lower crust is modelled with a velocity ranging from 6.0 to 6.3 kmls. Minor perturbations to this model, as explained above, generate synthetic seismograms (Figs. 7b, 8b) that match the first few seconds of the observed data.

less than the optimum 2.5 km separation. The combined data from these shots shown in Figs. 8a and 8d correlate but do not offer the increase in recording density of data that shotpoint B does. The two small offset shots at the D end of the line (i.e., DO7 and DO8) were separated by too great a distance to be composited, and accordingly only data for shot DO8 (Fig. 8d) are considered here. The uppermost layer and most of the upper crust beneath this line appear similar to the velocity model derived for line BC. At depths of 9- 10 km, a higher velocity of 6.2 km/s is required beneath this line to explain the observed data at 30 -50 km from shots C10, C11, and DO8 (Fig. 8c). This indicates a change in mid-crustal structure beneath the overall BD spread. The rest of the structure beneath this line is identical to that of the BC model, and the synthetic seismograms generated for this model match the observed P waveform coda well. This model generates some higher amplitude reflected arrivals that cannot be confirmed because of a 15 km gap in the recorded data at 20-35 km (Fig. 8 4 .

Line CD Because of environmental and navigational restrictions, shots C10 and C11 fired into line CD were at an offset much

Line DE The data available for this line are sparse and may not be modelled independently. The shallow structure beneath line

ASUDEH ET AL.

All l i n e s a] 1 x


rn

Fan s h o t s

B15

FIG.4. First-arrival picks compared for typical shots from each of the four short spreads in the survey. Note major differences in intercept times for I24 and E22 shots, which were fired at the two ends of the IE spread. An apparent velocity of 6 kmls is observed at distances beyond 40 km.

Ex~xnx xxx? @Y

Distant s h o t s

X%

F04

X

X

F20 F13

0

FIG. 5. First-arrival picks for shots fired at offsets of 50 km or more from the refraction spreads. All picks indicate an average upper mantle velocity of 8 kmls from a crossover near 80 km. Small differences in intercept times indicate variations in average crustal time term.

DE must be similar to that of line BC, since the available data compare well with data from line BC. Line ZG Data from two of the three shots fired at the I and G shotpoints (shots I29 and G27, Fig. 9) are used to model the structure beneath line IG. The structure beneath this line is different from all of others because the crossover to high upper crustal velocities (5.0 kmls) is close to the shotpoint. This indicates little or no low-velocity cover beneath this line. The 5.0 km/s material with a 0.12 s s l gradient extends to a depth of 5 km and overlies "basement" material with a velocity of 5.8 km/s. and a gradient of 0.1 s-l. This simple model adequately

D23

X

EZ8

D

D31

0

X' (km)

FIG. 6. First-arrival picks for fan shots. Shots D23 and D31 were fired into spread IE; E28 was fired into spread IG. Changes in intercept times are due to varying upper sedimentary thickness beneath E and D.

explains the observed first-arrival data and reflections from the top of the basement structure observed at ranges near 25 km (see Fig. 9). No crossover to Moho velocities is observed because of the short length of the profile. Line ZE The four shots fired into this line (I24, 125, E22, and E26) offer the most complex data set for the 1985 survey. The structure beneath this line is clearly two or three dimensional, and only its general features may be resolved by the present data. Constraints are provided by data from the intersecting IG line on the onshore end and DE line offshore. An important feature of the data for this line is a large difference in intercept times between data recorded from the I and E shotpoints. This can be attributed to a seaward-thickening low-velocity surface layer with a complicated topography at its base. The low-velocity surface layer must be about 3 km thicker beneath the E shotpoint to account for the large difference in the intercept times (see Figs. 10c, 10f). Another important feature of the data is the sharp change in apparent velocity near 30 km (Fig. 10a) and the higher amplitude arrivals beyond 40 km (Figs. 10a, 10d). Data for small offset shots beneath this line may not be composited because of rapid changes of the structure, and only two of the four shots (I24 and E26) are discussed here. Models obtained by observations from these two shots fit the data from the adjacent 2.5 km offset shots. The structure beneath the I end of this line must connect to the structure found for the adjoining IG line, and the offshore E end point must connect to the structure of line DE. These constraints were considered in arriving at a simple model to satisfy the observed data. A surficial cover with a velocity of 2.1 kmls and a gradient of 0.2 s-' extending to a depth of 2.9 km at E and reaching a maximum depth of 4.5 km about 30 km away from E explains the near distance arrivals well. The thickness of the sediments decreases to 700 m about 50 km away from E and is fixed to this value beneath the I shotpoint.

-


CAN. J. EARTH SCI. VOL. 26. 1989

FIG. 7. (a) Observed "true-amplitude" data, (b) synthetic seismograms, and (c) ray-tracing diagram for shots fired into spread BCD from shots near B (SW end of the line).

Beneath the top sedimentary cover, the typical 4.4 4.5 kmts upper crustal velocity is observed at the offshore end of the profile, whereas the higher velocity (5.0 kmls) structure, similar to the onshore line IG, is detected near the I end of this line. This upper crustal structure is disrupted by a body

with a velocity more typical of the deeper structure (5.8 krnts). The limits of this body are rather well determined by the change in the observed traveltime data (Figs. IOU, 10d). The deeper structure beneath this line is generally similar to those of other lines, but here the top of the 6.0 kmls material


ASUDEH ET AL.

FIG.8. (a, d) Observed data, (b, e) synthetic seismograms, and (c,f) ray-tracing diagrams for shots fired into spreads BC (C end in C) and CD (D end = NE end of the line).

is marked by a structure dipping seaward, with the E end at a depth of 11.5 km and the I end at depth of 5 km. Note that the lower crustal velocity of 6.0 km/s appears laterally continuous. No Moho velocity near 8.0 kmls is observed beneath this line because of the short length of the line. In summary, the synthetic seismograms generated for the model for line IE show good resemblance to the first few seconds of the observed data (see Fig. lo), indicating that the

= midpoint

upper crustal portion of this velocity model is well constrained by the data.

Summary of crustal models The fence diagram in Fig. 11 summarizes the crustal models presented in this work. Simple, constant-velocity blocks were not used in the modelling; rather, gradients were used, result-

CAN. J. EARTH SCI. VOL. 26, 1989


862

FIG.9. (a, d) Observed data, (b, e) synthetic seismograms, and (c,nray-tracing diagrams for shots fired from shotpoints I and G into spread IG from SW and NE, respectively.

ing in a P-wave velocity structure that shows five main divisions, each one covering a range of velocities. The velocity gradients are also given on this figure to indicate that the quoted values are in fact "starting" (i.e., upper boundary) velocities for each unit. This representation is used in comparing the seismic refraction results with the regional onshore geology.

It should be noted that the thin (700 m) sequence of unit 5 beneath line IG is not well resolved by the refraction data. The starting velocity of 2.1 km/s is used because it was the velocity of the shallowest strata on all other lines. A. Overton (personal communication, 1987) reported an interval velocity of 3.7 kmls for uppermost material beneath a reflection line about 35 km to the southwest of shotpoint G. The 700 m thick-


ASUDEH ET AL.

FIG.10. (a, d) Observed data, (b, e) synthetic seismograms, and (c, j)ray-tracing diagrams for shots fired from shotpoints I and E into spread IE from NW and SE, respectively.

ness is required to arrive at the intercept time for the traveltime branch with a velocity of 5 kmls. The changes in depth to various velocity horizons, such as beneath l i n i 1 and ~ the mid-crustal sectioh beneath BD, suggest considerable structural com~lexitv.The limitations inherent in the 5 km receiver spacini and h e frequency content of the seismograms mean that although the regional structural

features are well modelled the view is a smoothed picture of a more complex structure.

Geological significance of refraction results In terms of the three major tectono-stratigraphic sequences mapped in the adjacent Arctic Islands-Precambrian - lower


CAN. J. EARTH SCI. VOL. 26, 1989

5 Cret. - Tert. 4 Cret. - Tert.

2.1 krn /s

4.4

- 4.5

0.2 - 0.5 s-' 0.08-0.15

3 L.Paleo. -U.Mes.?

FIG. 11. Summary fence diagram for the 1985 survey. See text for details.

Paleozoic (Franklinian), upper Paleozoic - lowest Cretaceous (Sverdrup), and Cretaceous -Tertiary (polar continental rnargin)-the five major units of Fig. 11 are interpreted as follows, beginning with the one closest to the surface. Unit 5, with a starting velocity of 2.1 kmls, reaches a maximum thickness of about 4.3 km beneath spread BC. Its velocity at this depth is 4.4 kmls. This material is interpreted as being part of the essentially undeformed Cretaceous -Tertiary continental margin succession that becomes more indurated with depth and contains volcanic material. By comparison, a sonic log for the upper 2.5 km of Tertiary section from a well at the northeast comer of Meighen Island shows a starting velocity of 2.1 kmls and a gradient of 0.7 s-'. The top of unit 4 underlying unit 5 along line BD is represented by a narrow zone of changing velocity gradient at a depth of 5 km beneath spread BC. The change to a lower velocity gradient explains the diminished first-arrival arnplitudes observed at about 20 km from shotpoint B. This layer

-

extends to a depth of 9 - 12 km along line BD, and the geological interpretation of this unit is more difficult than for unit 5. One hypothesis is that it consists mainly of CretaceousTertiary clastic strata with volcanic flows and associated dykes and sills and represents the lower portion of the continental shelf succession. Sonic logs for wells penetrating the Mackenzie-Beaufort Basin, for example Niglintgak B-19 in the north-central basin, frequently show velocities of 4.24.5 kmls where the logs extend to lower Tertiary strata at depths of 3 -4 km. Thus, units 4 and 5 together would constitute the polar margin sequence, and the "boundary" between them may not indicate a fundamental tectonic event. In this respect, however, it is worth noting that there were two geological events that may have punctuated the polar margin sequence in this area and which may, therefore, be related to the velocity-gradient boundary between units 4 and 5. One is the transition from synrift and postrift (' 'syndrift") sedimentation along the polar margin such has been postulated off Nova


ASUDEH ET AL.

Scotia by Reid (1987), an the other is the extent to which the nearby (generally less than 100 km) Tertiary Eurekan Orogenic tectonism may have affected the polar margin sequence. The refraction data along spread IE suggest that unit 4, but not unit 5, is bounded by moderately complex structures. Unit 4 therefore may predate or be synchronous with Eurekan tectonism, with the top of unit 4 marking the end of the Eocene (e.g., Miall 1981; Balkwill 1981). The emerging Eurekan Orogen may have been the source area for much of the postulated thick Cretaceous -Tertiary sequence. A different hypothesis for the geological interpretation of unit 4 could assign the entire succession to the upper Paleozoic - Jurassic tectono-stratigraphic sequence, which, in the Sverdrup Basin, displays similar seismic velocities. For unit 4 to be dominated by strata of this sequence implies the existence of a distinct and major late Paleozoic - Mesozoic depocentre in the area of the present continental shelf. Such a hypothesis is problematic because upper Paleozoic - Jurassic Sverdrup Basin strata thin markedly toward the north from the basin depocentre and suggest a basin edge in the area of the present continental shelf. However, it is possible that upper Paleozoic - Jurassic strata constitute the basal portion of Unit 4 in some areas. Unit 3 directly underlies unit 5 along the nearshore line IG. It has a starting velocity of 5.0 krnls and a gradient of 0.12 s-I and is modelled as being 4.5 km thick. This material is interpreted as predominantly deformed and metamorphosed lower Paleozoic Franklinian strata capped by minor upper Paleozoic - Triassic strata of the Sverdrup Basin sequence including dense Carboniferous carbonates and (or) Cretaceous volcanic rocks. The incoherent ringing nature of the secondary coda on the refraction section from shotpoint I suggest considerable scattering by nonplanar, probably highly deformed, upper to mid-crustal structure. This succession is exposed on nearby Axel Heiberg and Ellesmere islands. By comparison, Sobczak and Overton (1984) assigned densities of 2.62.7 g/cm3 and velocities in excess of 5.0 krnls to the lower Paleozoic assemblage beneath the Sverdrup Basin to the south. Unit 2, modelled as material with a velocity of 5.86.0 km/s and a gradient of 0.1 s-l, underlies unit 3 along line IG and also unit 4 beneath the seaward end of line IE and lines BC-CD. This unit is interpreted as undivided crystalline basement composed, at least in part, of the deformed and metamorphosed Proterozoic - lower Paleozoic section. The depth of the velocity change to greater than 6.0 kmls varies from 9 to 12 km beneath spread CD, indicating more complex structure. This variation in depth to the 6.0 krnls horizon is in accord with the wavelength of potential-field anomalies along the line (Forsyth et al. 1988). Unit 1 (identified only on section BD), with a velocity greater than 8.0 km/s, is upper mantle. The crustal thickness of 25 km suggests a region of thinned and stretched continental -transitional crust consistent with a rifted passive margin.

Acknowledgments We would like to thank the director, G. Hobson, and the officers of the Polar Continental Shelf Project for the base camp and logistic support making this survey possible; J. Davidson and M. Wright for providing the essential Syledis navigation support; R. Duval and M. Schmidt for operation of the SMA 761 positioning system and postsurvey processing of the 761 data; R. Schieman, M. Gorveatt, and J. Ardai for

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technical support; and the air crews from Bradley and Quasar for operational flying without incident. The logistic support of the Instrumentation Laboratory of the Geophysics Division of the Geological Survey of Canada is also gratefully acknowledged. Computation of velocities from sonic logs in the Mackenzie-Beaufort Basin by R. Peach and suggestions from C. Spencer, J. Sweeney, I. Reid, C. Harrison, A. Jones, and K. Louden during manuscript review are greatly appreciated. A~UDEH, I., FORSYTH, D. A., JACKSON, H. R., STEPHENSON, R., and WHITE,D. 1985. 1985 Ice Island refraction survey phase I report. Geological Survey of Canada, Open File 1196. 1986. 1986 Ice Island refraction survey phase I report. Geological Survey of Canada, Open File 15 11. ASUDEH,I., GREEN,A. G., and FORSYTH, D. A. 1988. Canadian expedition to study the Alpha Ridge complex: results of the seismic refraction survey. Geophysical Journal of the Royal Astronomical Society, 92: 283 -301. BALKWILL, H. R. 1978. Evolution of the Sverdrup Basin, Arctic Canada. American Association of Petroleum Geologists Bulletin, 62: 1004-1029. BERRY, M. J., and BARR,K. G. 1971. A seismic refraction profile across the polar continental shelf of the Queen Elizabeth Islands. Canadian Journal of Earth Sciences, 8: 347-360. EMBRY, A. F. In press. Mesozoic history of the Arctic Islands. In Innuitian Orogen and Arctic Platform: Canada and Greenland. Edited by H. P. Trettin. Geological Survey of Canada, Geology of Canada, No. 3. (Geological Society of America, Decade of North American Geology, The Geology of North America, Vol. E.) K. G. 1988. Stratigraphy and tectonic EMBRY, A. F., and OSADETZ, significance of Cretaceous volcanism in the Queen Elizabeth Islands, Canadian Arctic Archipelago. Canadian Journal of Earth Sciences, 25: 1209-1219. FORSYTH, D. A., BROOME, J., EMBRY, A. F., and HALPENNY, J. 1988. Features of the Canadian polar margin. Tectonophysics. HOBSON, G. D., and OVERTON, A. 1967. A seismic section of the Sverdrup Basin, Canadian Arctic Islands. In Seismic refraction prospecting. Edited by A. W. Musgrave. Society of Exploration Geophysicists, Section 7, pp. 550 -562. MAIR,J. A., and LYONS, J. A. 1981. Crustal structure and velocity anisotropy beneath the Beaufort Sea. Canadian Journal of Earth Sciences, 18: 724-741. MENELEY, R. A,, HENAO, D., and MERRITT, R. K. 1975. The northwest margin of the Sverdrup Basin. In Canada's continental margins and offshore petroleum exploration. Edited by C. J. Yorath, E. R. Parker and D. J. Glass. Canadian Society of Petroleum Geologists, Memoir 4, pp. 557-587. MIALL,A. D. 1981. Late Cretaceous and Paleogene sedimentation and tectonics in the Canadian Arctic Islands. In Sedimentation and tectonics in alluvial basins. Edited by A. D. Miall. Geological Association of Canada, Special Paper 23, pp. 221 -272. OVERTON, A. 1970. Seismic refraction surveys, western Queen Elizabeth Islands and polar continental margin. Canadian Journal of Earth Sciences, 7: 346-365. REID,I. 1987. Crustal structure of the Nova Scotian margin in the Laurentian Channel region. Canadian Journal of Earth Sciences, 24: 1859-1868. SOBCZAK, L. W., and OVERTON, A. 1984. Shallow and deep crustal structure of the western Sverdrup Basin, Arctic Canada. Canadian Journal of Earth Sciences, 21: 902-919. SPENCE, G. D., WHITTALL, K. P., and CLOWES, R. M. 1984. Practical synthetic seismograms for laterally varying media calculated by asymptotic ray theory. Bulletin of the Seismological Society of America, 74: 1209- 1223. STEPHENSON, R. A., EMBRY,A. F., NAKIBOGLU, S. M., and HASTAOGLU, M. A. 1987. Rift-initiated Permian to Early Cretaceous subsidence of the Sverdrup Basin. In Sedimentary basins and

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basin-forming mechanisms. Edited by C. Beaumont and A. J. Tankard. Canadian Society of Petroleum Geologists, Memoir 12, pp. 213-231. THORSTEINSSON, R., and TOZER, E. T. 1970. Geology of the Arctic Archipelago. In Geology and economic minerals of Canada. Edited by R. J. W. Douglas. Geological Survey of Canada, Economic Geology Report 1, pp. 547 -590. TRETTIN,H. P. In press. Summary and remaining problems. In Innuitian Orogen and Arctic Platform: Canada and Greenland. Edited by H. P. Trettin. Geological Survey of Canada, Geology of Canada. No. 3. (Geological Society of America, Decade of North American Gwlogy, The Gwlogy of North America, Vol. E.)

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TRETTIN,H. P., and BALKWILL, H. R. 1979. Contributions to the tectonic history of the Imuitian Province, Arctic Canada. Canadian Journal of Earth Sciences, 16: 748 -769. TRETTIN,H. P., and PARRISH,R. 1987. Late Cretaceous bimodal magmatism, northern Ellesmere Island: isotopic age and origin. Canadian Journal of Earth Sciences, 24: 257-265. VANALTENA, P. J. 1974. Report of a reflection seismic survey conducted on Meighen Island, 1972 and 1973. Canada Oil and Gas Lands Administration, Report 005-06-10-037.