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Crustal structure of the Makarov Basin, Arctic Ocean determined by seismic refraction
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Earth and Planetary Science Letters 168 (1999) 187–199
Crustal structure of the Makarov Basin, Arctic Ocean determined by seismic refraction M.Yu. Sorokin a , Yu.Ya. Zamansky a , A.Ye. Langinen a , H.R. Jackson b,Ł , Ron Macnab b b
a Polar Marine Geosurvey Expedition, Pobeda Street 24, St. Petersburg-Lomonosov, 189510, Russia Geological Survey of Canada (Atlantic), 1 Challenger Drive, Box 1006, Dartmouth, N.S., B2Y 4A2, Canada
Received 27 April 1998; revised version received 8 January 1999; accepted 8 February 1999
Abstract Wide-angle seismic reflection=refraction data, recorded on ten seismographs from five large explosive charges, clearly define the crustal and upper mantle velocities of the Makarov Basin. These data are combined with a tangential seismic reflection profile and with Russian navy refraction velocities to produce a seafloor to upper mantle velocity–depth function. The gently dipping sedimentary horizons contrast with the hummocky basement topography on the reflection profile. The seismic basement is interpreted as oceanic layer 2 consistent with the Russian navy velocities. Velocities of 6.7 km s 1 determined for the lower crust and 8.0 km s 1 for the upper mantle are constrained by the seismic record sections. The 6.7 km s 1 velocity layer is interpreted as oceanic crust layer 3. The well-resolved thickness of this layer of 15 km is substantially greater than the average thickness for layer 3 of 4 km. The velocities and thickness from the Makarov Basin, oceanic plateaus and hotspots, and continental crust are summarized for comparison. The character of seismic reflection basement and the velocity–depth function are consistent with thick oceanic crust in the Makarov Basin. 1999 Elsevier Science B.V. All rights reserved. Keywords: Arctic Ocean; seismic profiles; velocity structure; oceanic crust
1. Introduction A seismic refraction experiment was carried out in the Makarov Basin by the Polar Marine Geosurvey Expedition (PMGE). This experiment was run to study the outer limit of the Russian continental margin in the Arctic Ocean [1] and the adjacent basin. The seismic data displayed here are part of the 1400 km Delong Islands–Makarov Basin Geotransect. The transect includes seismic reflection profiling, closely spaced gravity and bathymetry stations, and airborne Ł Corresponding
author. Fax: C1-902-426-6152; E-mail: [email protected]
magnetics within a 100 km wide zone along the transect. The initial results of the experiment were published in Russian [2]; however, the seismograms were not shown and the interpretation techniques were different from those employed in this paper. The results of the analysis — the velocities and the thickness of the crustal layers — are used to infer the type of crust underlying the basin. A summary of the bathymetric and subsurface features of the Makarov Basin reveals its complexity and provides a framework for interpreting the refraction data. The basin is located between the Lomonosov Ridge and the subparallel Alpha–Mendeleev Ridge complex (Fig. 1). It is also bounded by the East
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Fig. 1. Major bathymetric features of the Arctic Ocean. The long white dashed line indicates the Makarov Geotransect, one section is shown in this paper. The white circles are the locations of the shots for the refraction experiment described here. The irregular white line is the position of the seismic reflection profile conducted from ice station NP-28. The straight white lines display the locations of refraction experiments in the region: CESAR, LOREX and two Polarstern profiles. The box encloses the area displayed in Fig. 2.
Siberian continental margin (Fig. 2), the widest shelf in the world. The basin is wedge-shaped, being 500 km across near the East Siberian Sea [3], and narrowing northwards. The abyssal plain adjacent to the East Siberian margin is called Podvodnikov in Rus-
sian literature and Wrangel in English (Fig. 2). This plain, with water depths of about 2800 m, is separated from the deeper Siberia Abyssal Plain at 3900 m close to the Arlis Gap [4]. Based on shallow reflection, refraction, gravity and magnetic data, a promi-
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Fig. 2. Bathymetry and configuration of the shots (circles) and receivers (triangles) for the refraction experiment. The letters A and B indicate the segment of the reflection profile NP-28 shown here.
nent basement ridge subparallel to the East Siberian continental margin was identified beneath the Arlis Gap. The basement high beneath the Gap acts as a dam that ponds sediment behind it. Magnetic anomalies of 800 nT and gravity anomalies of C10= 15 mGal were observed over the basement high [4]. This basement ridge was interpreted as a spur of the Alpha Ridge due to the continuous nature of the aeromagnetic anomalies. Ostenso [5], based on the available bathymetry, seismic reflection, magnetics and gravity data estimated the depth to the M discontinuity to be 22 km under the buried ridge. The experiment described here is located near the Arlis Gap.
There are limited seismic reflection profiles from the Makarov Basin and the adjacent ridges. Ice station LOREX [6] drifted across the North Pole from the Lomonosov Ridge into the adjacent basins. The high-resolution airgun profile showed a well-stratified sedimentary sequence at least 1 km thick overlying basement with considerable relief in the Makarov Basin. Data collected from the Polarstern show up to 3 s of sedimentary strata on the flanks of the Makarov Basin [7]. On the Lomonosov Ridge, a thin sequence of stratified sediments overlies several slightly tilted en-echelon fault blocks. The track of part of the 1800 km long reflection
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Fig. 3. Seismic reflection profile NP-28, A and B indicating the ends of the drift track shown in Fig. 2. The north (N) and south (S) ends of the profiles are also indicated.
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profile recorded at ice station NP-28 that crosses the refraction line is shown in Fig. 2. The positions were determined by satellite navigation. The sound source for each shot consisted of 3–5 detonators (electric caps) and the receiver was an on-ice cross-shaped array with 12 channels along each branch. The seafloor (Fig. 3) is nearly horizontal in the survey area. Near the north end of the profile the seabed rises and falls, and the water depths increase by 1 s. A prominent sloping reflector buried about 1 s below the seabed at the south end of the line shoals northwards. The profile is transparent above and below this reflector except in the middle of the section where the region below the reflector is stratified. At the south end of the profile, a hummocky reflector buried by 2 s of presumed sedimentary section is displayed that shallows to the north and then deepens. No later events are recorded and this reflector is interpreted as basement. A seismic reflection profile was recorded on the U.S. ice station Arlis II near the southern portion of the refraction line. A sedimentary thickness of at least 3.5 km was observed by Kutschale [4] that thinned above the basement high observed at the Arlis Gap. The topographically rough basement and the velocity of 5.0 km s 1 determined from Russian navy refraction experiments [8] is compatible with oceanic layer 2. Crustal refraction data were shot on the Alpha Ridge and in the nearby Makarov Basin (Fig. 1) during the CESAR experiment [9,10]. The Makarov Basin and the Alpha Ridge were interpreted by Forsyth et al. [9] to be composed of thick oceanic crust. The velocity–depth profile of the Makarov Basin is similar to that of the Alpha Ridge but thinner. This suggested that the origins of these two features were closely tied [11]. The portion of the LOREX refraction profile (Fig. 1) beneath the Makarov Basin was also interpreted as sampling oceanic crust [12]. Lineated magnetic anomalies are usually associated with oceanic crust and provide another line of evidence for interpreting the crust as oceanic. Taylor et al. [13] distinguished a set of anomalies with a peak to trough maximum of 700 nT that he correlated with anomalies 34 to 21. This correlation was tenuous and the effect of topography on the anomalies was unknown. Due to these uncertainties in interpreting the magnetic anomalies, the plate
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reconstructions of the Amerasia Basin are poorly constrained. Since only short sedimentary cores [14] and no bedrock samples have been obtained in the basin, no dates are available for the age of the basin. Heat flow values of 60–70 W m 2 for the Makarov Basin near the Lomonosov Ridge, however, are consistent with a Cretaceous age for the basin [3]. In the Makarov Basin Soviet scientists at North Pole 15 in the 1960s made 13 measurements with values of 43–105 W m 2 [15,16]. These values are anomalously high compared with other regions in the Amerasia Basin. This could suggest younger lithosphere underlying this region. The lack of samples, the irregular magnetic anomalies and the meagre amount of seismic profiles have made it difficult to determine the age and origin of the Makarov Basin [3].
2. Experimental method The Makarov Basin is a frontier for geological information because of the logistic difficulties of carrying out experiments on the ice-covered Arctic Ocean. The wide-angle reflection=refraction experiment presented here was carried out in April of 1989 [17]. The seismic arrivals were recorded by ten TAYGA-2 analog tape recorders placed on the drift ice with assistance of an MI-8 helicopter (Fig. 4). The intervals between the recording instruments varied from 10 to 15 km. Every TAYGA-2 station had a 6-channel array of low-frequency geophones (resonance frequency 5 Hz). The array’s length was between 200 and 500 m. The analog seismic recorders were operated by a radio unit on the helicopter from an altitude of approximately 2.5–3 km. The signal from the radio unit commanded the on-ice recorders to turn on and put precise 4 ms time marks on every field recorder for up to 3 min while the seismic signals were received. A seismic recorder was installed near all five shots to register the time marks and the shot instant. The shots were fired in succession with intervals of no less than 3–5 min. The locations of the sources and the receivers were determined by GPS units (MX 4400) with an accuracy of about 100 m. The sources were trinitrotoluene (TNT) charges detonated underwater at a depth of about 50 m. Charge weights
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Fig. 4. Configuration of the seismic refraction experiment.
varied from 100 to 500 kg. A split spread of 220 km was obtained with a total of 45 seismic traces with a high signal to noise ratio. The sedimentary and upper crustal arrivals were not constrained due to the paucity of shots near the origin; however, the lower crust and M discontinuity are resolved.
3. Processing and analysis In 1995–96 the analog data (seismic tape records and time marks) were digitized by a unit created at PMGE and reformatted into SEG-Y files. The
data have been bandpass-filtered between 6 and 10 Hz and a linear scaling of amplitude with distance was applied. The seismic sections were interpreted by iterative forward modelling. A two-dimensional model was developed from the seabed to the M discontinuity. The model (Fig. 5) was perturbed until the calculated and observed travel times matched as shown in the record sections. The model was refined by amplitude modelling with synthetic seismograms. Asymptotic ray theory was used to calculate the synthetic seismograms. The synthetic sections were processed and plotted with the same parameters as the data. Gaussian noise was added to the synthetics
Fig. 5. Two-dimensional velocity–depth model. The shots are indicated by numbered circles and the receivers by squares. The arrow identifies the point that the seismic reflection profile crosses the model. The shaded area indicates the region where the section is reversed. The progressively darker tones of grey indicate sedimentary rocks, the crust and the upper mantle. The first number in each block is the velocity in km s 1 along the top boundary and the number in parentheses is the linear velocity gradient in s 1 .
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to facilitate comparison with the data. All record sections are displayed reduced to 7 km s 1 . Bathymetry data were collected at the shots and receivers. The velocity–depth profile for the sedimentary and upper crust was based on several sources. They included seismic refraction experiments carried out on drift stations sponsored by the Russian navy in 1970–71 [8,18] and refraction experiments organized by PMGE at the drift stations near this transect [19]. The sedimentary and upper crustal velocities of 1.6 to 5.0 km s 1 were determined from the Russian navy refraction experiments. The sedimentary thick-
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ness of up to 3.5 km is consistent with the estimates of Kutschale [4]. A 5.5 km s 1 velocity was assumed to make the arrival times of the well-determined refraction velocity of 6.7 km s 1 match the intercepts observed on the seismic sections. The upper mantle velocity of 8.0 km s 1 is observed as a Pn arrival indicating a velocity gradient in the upper mantle. A probable PmP reflection is consistent with a velocity contrast at the M discontinuity. On record section 6 (Fig. 6) five of the nine instruments have an excellent signal to noise ratio. The first arrivals (Pg) at a range of 70 to 100 km define a travel time curve that has an apparent velocity
Fig. 6. The observed seismic section (top) for shot 6 with the travel time curves superimposed. The synthetic section (middle) and the ray paths (bottom) are plotted. Pg D the refracted event through the lower crust with a velocity of 6.7 km s 1 ; Pn D the refracted arrival from the upper mantle with a velocity of 8.0 km s 1 ; PmP D is the reflection from the M discontinuity.
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Fig. 7. The seismic sections (top) for shot 7 with the travel time curves superimposed. The synthetic section (middle) and ray paths (bottom) are displayed. Pg D the refracted event through the lower crust with a velocity of 6.7 km s 1 ; PmP D the reflection from the M discontinuity.
of 6.7 km s 1 . A higher-velocity refracted arrival (Pn) is plotted based on information from other shots; unfortunately, the noise on this section makes it difficult to distinguish. Possible PmP reflections, particularly at offsets of 100 to 140 km, are matched by the synthetic sections. On seismic section 7, the signal to noise ratio is high (Fig. 7). At offsets of 20 to 100 km, Pg is the prominent first arrival and
its velocity is constrained. The split spread shown on section 8 (Fig. 8) with a maximum range of 60 km has insufficient traces to define the sedimentary and upper crustal structure. There are only four seismic traces at offsets of less than 30 km. The Pg branch of the travel time curve at ranges of 30 to 60 km is consistent with the observed first arrivals and the distribution of amplitudes on the synthetic section.
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Fig. 8. The seismic section (top) for shot 8 with the travel time curves superimposed. The synthetic section (bottom) is plotted. Pg D the refracted event from the lower crust with a velocity of 6.7 km s 1 .
On profile 9 at offsets of 20 to 120 km, the Pg event is the first arrival constraining the apparent velocity (Fig. 9). On record section 10 (Fig. 10) with offsets from 60 to 160 km good signal to noise is observed particularly on the most distant instruments. The Pg event is a first arrival to about 100 km, after that Pn arrives first. Five traces are available to resolve the mantle velocity of 8.0 km s 1 . The horizontal resolution of the experiment is poor because of the large spacing between the shots and receivers. Error analysis of the seismic sections has not been done because the paucity of arrivals on each line would produce large statistical uncertainties. Fortunately, the large signal to noise ratio on the seismic traces for the lower crustal and upper mantle arrivals does provide velocity control for these layers.
4. Discussion The seismic reflection profile (Fig. 3) shows the characteristics of the seafloor to the basement. Beneath the sedimentary section a hummocky reflector, identified as seismic basement and interpreted as oceanic layer 2, are mimicked by the two-dimensional model used to calculate the ray paths (Fig. 5). The unpublished Russian refraction velocities make for a degree of uncertainty in the sedimentary and upper crustal layers; however, the velocities are sup-
ported by the regional published velocities. For example, the velocity–depth profile of the Makarov Basin near the Alpha Ridge (CESAR experiment) is consistent with the velocities developed by the Russian navy. In particular, the 5.0–5.2 km s 1 highgradient layer underlying the sedimentary section was measured [9,10] and supports our upper crustal velocity. The lower crust and upper mantle velocities of 6.7 and 8.0 km s 1 , and the thickness of the lower crust is constrained by the refraction record sections. Globally, the most consistent velocities measured in oceanic crust are for layer 3 (6.7–7.2 km s 1 with a low gradient) and the upper mantle (near 8.0 km s 1 ) [20,21]. The average thickness for layer 3 is about 4 km; in contrast, we measured a thickness of 15 km. The thickness of the igneous sections of oceanic crust can increase near mantle plumes and plateaus [22,23]. Mantle plumes generate anomalously hot asthenosphere up to 1000 km away from the central plume [24,25]. The average thickness of igneous crust affected by the Icelandic plume is 10:3 š 1:7 km and the mean thickness directly above a plume is 20:5 š 1:3 km. Limited evidence for large crustal thicknesses above aseismic ridges indicates a mean crustal thickness of 20 š 1 km [23]. The Alpha Ridge based on geological and geophysical data has been interpreted as an oceanic plateau similar to the Pacific plateaus such as the Ontong-Java or the Manihiki [26]. The crustal layers of the Ontong-Java plateau are similar to oceanic crust but five times
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Fig. 9. The seismic section (top) for shot 9 with the travel time curves superimposed. The synthetic section (middle) and the ray paths (bottom) are shown. Ps D the reflected and refracted waves from the sedimentary section; Pg is the refracted event through the lower crust with a velocity of 6.7 km s 1 ; PmP D the reflection from the M discontinuity.
thicker [27]. The velocity and thicknesses measured here for the lower crust and mantle are compatible with thick oceanic crust. The regional tectonic setting, although not well known, is also consistent with crust of oceanic origin. Velocities for the lower crustal of 6.5 km s 1 and upper mantle of 8.2 km s 1 were recorded for the Makarov Basin near the Alpha Ridge [9,10]. The depth to the M discontinuity was 21–25 km similar to that derived from this data set. However, a 7.2 km s 1 velocity measured on the CESAR experiment is
not seen in our data, either because it does not exist or because the spacing between the seismic traces is too large to resolve it. The high-velocity lower crustal layer may be related to the proximity of the Alpha Ridge that exhibits this velocity as well. The Alpha Ridge and adjacent basin were interpreted as oceanic [9,10]. Two refraction profiles intersect the Lomonosov Ridge (Fig. 1), generally considered a continental fragment [6,7]. On the refraction line parallel to the ridge crustal velocities of 4.7 and 6.6 km s 1
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Fig. 10. The seismic section (top) for shot 10 with the travel time curves superimposed. The synthetic section (middle) and the ray paths (bottom) are shown. Pg D the refracted event through the lower crust with a velocity of 6.7 km s 1 ; Pn D the refracted arrival from the upper mantle with a velocity of 8.0 km s 1 .
were measured with the M discontinuity at a depth of 27 km. A summary of velocities for continental crust [28] assumes that a measurement near 6.0 km s 1 is typical. This velocity is conspicuously absent from both our observations and from the Lomonosov Ridge. Not surprisingly, the 1700 km long, 65–200 km wide Lomonosov Ridge does not exhibit standard continental crustal velocities [12]. However, the seis-
mic reflection profiles from the Lomonosov Ridge show basement as a series of rotated fault blocks [7] consistent with a continental origin. In contrast, the basement topography as imaged by profile NP-28 is an irregular reflector typically produced by oceanic crust. On the perpendicular profile, a crustal velocity of 6.6 km s 1 and a thickness of 9 km [12] was determined for the edge of the Makarov Basin. The
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M discontinuity was observed as a post-critical reflection and its depth was predicted to be 23 km. The 6.6 km s 1 velocity was interpreted as oceanic layer 3. Our preferred interpretation of the seismic reflection and refraction sections is oceanic crust. Nevertheless, the lack of basement samples, the poorly lineated magnetic anomalies and the restricted amount of seismic data make it impossible to rule out the possibility of continental crust.
[3]
[4]
[5]
5. Conclusions The seismic reflection and refraction profile collected by PMGE in the Makarov Basin provides new insights into the velocity structure and origin of the basin. The seismic reflection profile shows several kilometres of sedimentary strata overlying an irregular reflector with an associated velocity of 5.0 km s 1 (based on unpublished Russian Navy data) inferred to be oceanic basement. The refraction profile clearly delimits a 6.7 km s 1 velocity layer overlying a 8.0 km s 1 upper mantle velocity at 23 km depth. The data constrains the thickness of the lower crust to 15 km. Based on a comparison with other refraction experiments in the Arctic and worldwide averages, the crust is interpreted as thick oceanic.
[6]
[7]
[8]
[9]
[10]
Acknowledgements [11]
We wish to thank Matt Salisbury and Gordon Oakey who reviewed the manuscript internally. Thanks are also due to Art Grantz and two anonymous reviewers whose careful reviews have improved this paper. Geological Survey of Canada contribution # 1998197. [CL]
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Earth and Planetary Science Letters 168 (1999) 187–199
Crustal structure of the Makarov Basin, Arctic Ocean determined by seismic refraction M.Yu. Sorokin a , Yu.Ya. Zamansky a , A.Ye. Langinen a , H.R. Jackson b,Ł , Ron Macnab b b
a Polar Marine Geosurvey Expedition, Pobeda Street 24, St. Petersburg-Lomonosov, 189510, Russia Geological Survey of Canada (Atlantic), 1 Challenger Drive, Box 1006, Dartmouth, N.S., B2Y 4A2, Canada
Received 27 April 1998; revised version received 8 January 1999; accepted 8 February 1999
Abstract Wide-angle seismic reflection=refraction data, recorded on ten seismographs from five large explosive charges, clearly define the crustal and upper mantle velocities of the Makarov Basin. These data are combined with a tangential seismic reflection profile and with Russian navy refraction velocities to produce a seafloor to upper mantle velocity–depth function. The gently dipping sedimentary horizons contrast with the hummocky basement topography on the reflection profile. The seismic basement is interpreted as oceanic layer 2 consistent with the Russian navy velocities. Velocities of 6.7 km s 1 determined for the lower crust and 8.0 km s 1 for the upper mantle are constrained by the seismic record sections. The 6.7 km s 1 velocity layer is interpreted as oceanic crust layer 3. The well-resolved thickness of this layer of 15 km is substantially greater than the average thickness for layer 3 of 4 km. The velocities and thickness from the Makarov Basin, oceanic plateaus and hotspots, and continental crust are summarized for comparison. The character of seismic reflection basement and the velocity–depth function are consistent with thick oceanic crust in the Makarov Basin. 1999 Elsevier Science B.V. All rights reserved. Keywords: Arctic Ocean; seismic profiles; velocity structure; oceanic crust
1. Introduction A seismic refraction experiment was carried out in the Makarov Basin by the Polar Marine Geosurvey Expedition (PMGE). This experiment was run to study the outer limit of the Russian continental margin in the Arctic Ocean [1] and the adjacent basin. The seismic data displayed here are part of the 1400 km Delong Islands–Makarov Basin Geotransect. The transect includes seismic reflection profiling, closely spaced gravity and bathymetry stations, and airborne Ł Corresponding
author. Fax: C1-902-426-6152; E-mail: [email protected]
magnetics within a 100 km wide zone along the transect. The initial results of the experiment were published in Russian [2]; however, the seismograms were not shown and the interpretation techniques were different from those employed in this paper. The results of the analysis — the velocities and the thickness of the crustal layers — are used to infer the type of crust underlying the basin. A summary of the bathymetric and subsurface features of the Makarov Basin reveals its complexity and provides a framework for interpreting the refraction data. The basin is located between the Lomonosov Ridge and the subparallel Alpha–Mendeleev Ridge complex (Fig. 1). It is also bounded by the East
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Fig. 1. Major bathymetric features of the Arctic Ocean. The long white dashed line indicates the Makarov Geotransect, one section is shown in this paper. The white circles are the locations of the shots for the refraction experiment described here. The irregular white line is the position of the seismic reflection profile conducted from ice station NP-28. The straight white lines display the locations of refraction experiments in the region: CESAR, LOREX and two Polarstern profiles. The box encloses the area displayed in Fig. 2.
Siberian continental margin (Fig. 2), the widest shelf in the world. The basin is wedge-shaped, being 500 km across near the East Siberian Sea [3], and narrowing northwards. The abyssal plain adjacent to the East Siberian margin is called Podvodnikov in Rus-
sian literature and Wrangel in English (Fig. 2). This plain, with water depths of about 2800 m, is separated from the deeper Siberia Abyssal Plain at 3900 m close to the Arlis Gap [4]. Based on shallow reflection, refraction, gravity and magnetic data, a promi-
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Fig. 2. Bathymetry and configuration of the shots (circles) and receivers (triangles) for the refraction experiment. The letters A and B indicate the segment of the reflection profile NP-28 shown here.
nent basement ridge subparallel to the East Siberian continental margin was identified beneath the Arlis Gap. The basement high beneath the Gap acts as a dam that ponds sediment behind it. Magnetic anomalies of 800 nT and gravity anomalies of C10= 15 mGal were observed over the basement high [4]. This basement ridge was interpreted as a spur of the Alpha Ridge due to the continuous nature of the aeromagnetic anomalies. Ostenso [5], based on the available bathymetry, seismic reflection, magnetics and gravity data estimated the depth to the M discontinuity to be 22 km under the buried ridge. The experiment described here is located near the Arlis Gap.
There are limited seismic reflection profiles from the Makarov Basin and the adjacent ridges. Ice station LOREX [6] drifted across the North Pole from the Lomonosov Ridge into the adjacent basins. The high-resolution airgun profile showed a well-stratified sedimentary sequence at least 1 km thick overlying basement with considerable relief in the Makarov Basin. Data collected from the Polarstern show up to 3 s of sedimentary strata on the flanks of the Makarov Basin [7]. On the Lomonosov Ridge, a thin sequence of stratified sediments overlies several slightly tilted en-echelon fault blocks. The track of part of the 1800 km long reflection
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Fig. 3. Seismic reflection profile NP-28, A and B indicating the ends of the drift track shown in Fig. 2. The north (N) and south (S) ends of the profiles are also indicated.
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profile recorded at ice station NP-28 that crosses the refraction line is shown in Fig. 2. The positions were determined by satellite navigation. The sound source for each shot consisted of 3–5 detonators (electric caps) and the receiver was an on-ice cross-shaped array with 12 channels along each branch. The seafloor (Fig. 3) is nearly horizontal in the survey area. Near the north end of the profile the seabed rises and falls, and the water depths increase by 1 s. A prominent sloping reflector buried about 1 s below the seabed at the south end of the line shoals northwards. The profile is transparent above and below this reflector except in the middle of the section where the region below the reflector is stratified. At the south end of the profile, a hummocky reflector buried by 2 s of presumed sedimentary section is displayed that shallows to the north and then deepens. No later events are recorded and this reflector is interpreted as basement. A seismic reflection profile was recorded on the U.S. ice station Arlis II near the southern portion of the refraction line. A sedimentary thickness of at least 3.5 km was observed by Kutschale [4] that thinned above the basement high observed at the Arlis Gap. The topographically rough basement and the velocity of 5.0 km s 1 determined from Russian navy refraction experiments [8] is compatible with oceanic layer 2. Crustal refraction data were shot on the Alpha Ridge and in the nearby Makarov Basin (Fig. 1) during the CESAR experiment [9,10]. The Makarov Basin and the Alpha Ridge were interpreted by Forsyth et al. [9] to be composed of thick oceanic crust. The velocity–depth profile of the Makarov Basin is similar to that of the Alpha Ridge but thinner. This suggested that the origins of these two features were closely tied [11]. The portion of the LOREX refraction profile (Fig. 1) beneath the Makarov Basin was also interpreted as sampling oceanic crust [12]. Lineated magnetic anomalies are usually associated with oceanic crust and provide another line of evidence for interpreting the crust as oceanic. Taylor et al. [13] distinguished a set of anomalies with a peak to trough maximum of 700 nT that he correlated with anomalies 34 to 21. This correlation was tenuous and the effect of topography on the anomalies was unknown. Due to these uncertainties in interpreting the magnetic anomalies, the plate
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reconstructions of the Amerasia Basin are poorly constrained. Since only short sedimentary cores [14] and no bedrock samples have been obtained in the basin, no dates are available for the age of the basin. Heat flow values of 60–70 W m 2 for the Makarov Basin near the Lomonosov Ridge, however, are consistent with a Cretaceous age for the basin [3]. In the Makarov Basin Soviet scientists at North Pole 15 in the 1960s made 13 measurements with values of 43–105 W m 2 [15,16]. These values are anomalously high compared with other regions in the Amerasia Basin. This could suggest younger lithosphere underlying this region. The lack of samples, the irregular magnetic anomalies and the meagre amount of seismic profiles have made it difficult to determine the age and origin of the Makarov Basin [3].
2. Experimental method The Makarov Basin is a frontier for geological information because of the logistic difficulties of carrying out experiments on the ice-covered Arctic Ocean. The wide-angle reflection=refraction experiment presented here was carried out in April of 1989 [17]. The seismic arrivals were recorded by ten TAYGA-2 analog tape recorders placed on the drift ice with assistance of an MI-8 helicopter (Fig. 4). The intervals between the recording instruments varied from 10 to 15 km. Every TAYGA-2 station had a 6-channel array of low-frequency geophones (resonance frequency 5 Hz). The array’s length was between 200 and 500 m. The analog seismic recorders were operated by a radio unit on the helicopter from an altitude of approximately 2.5–3 km. The signal from the radio unit commanded the on-ice recorders to turn on and put precise 4 ms time marks on every field recorder for up to 3 min while the seismic signals were received. A seismic recorder was installed near all five shots to register the time marks and the shot instant. The shots were fired in succession with intervals of no less than 3–5 min. The locations of the sources and the receivers were determined by GPS units (MX 4400) with an accuracy of about 100 m. The sources were trinitrotoluene (TNT) charges detonated underwater at a depth of about 50 m. Charge weights
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Fig. 4. Configuration of the seismic refraction experiment.
varied from 100 to 500 kg. A split spread of 220 km was obtained with a total of 45 seismic traces with a high signal to noise ratio. The sedimentary and upper crustal arrivals were not constrained due to the paucity of shots near the origin; however, the lower crust and M discontinuity are resolved.
3. Processing and analysis In 1995–96 the analog data (seismic tape records and time marks) were digitized by a unit created at PMGE and reformatted into SEG-Y files. The
data have been bandpass-filtered between 6 and 10 Hz and a linear scaling of amplitude with distance was applied. The seismic sections were interpreted by iterative forward modelling. A two-dimensional model was developed from the seabed to the M discontinuity. The model (Fig. 5) was perturbed until the calculated and observed travel times matched as shown in the record sections. The model was refined by amplitude modelling with synthetic seismograms. Asymptotic ray theory was used to calculate the synthetic seismograms. The synthetic sections were processed and plotted with the same parameters as the data. Gaussian noise was added to the synthetics
Fig. 5. Two-dimensional velocity–depth model. The shots are indicated by numbered circles and the receivers by squares. The arrow identifies the point that the seismic reflection profile crosses the model. The shaded area indicates the region where the section is reversed. The progressively darker tones of grey indicate sedimentary rocks, the crust and the upper mantle. The first number in each block is the velocity in km s 1 along the top boundary and the number in parentheses is the linear velocity gradient in s 1 .
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to facilitate comparison with the data. All record sections are displayed reduced to 7 km s 1 . Bathymetry data were collected at the shots and receivers. The velocity–depth profile for the sedimentary and upper crust was based on several sources. They included seismic refraction experiments carried out on drift stations sponsored by the Russian navy in 1970–71 [8,18] and refraction experiments organized by PMGE at the drift stations near this transect [19]. The sedimentary and upper crustal velocities of 1.6 to 5.0 km s 1 were determined from the Russian navy refraction experiments. The sedimentary thick-
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ness of up to 3.5 km is consistent with the estimates of Kutschale [4]. A 5.5 km s 1 velocity was assumed to make the arrival times of the well-determined refraction velocity of 6.7 km s 1 match the intercepts observed on the seismic sections. The upper mantle velocity of 8.0 km s 1 is observed as a Pn arrival indicating a velocity gradient in the upper mantle. A probable PmP reflection is consistent with a velocity contrast at the M discontinuity. On record section 6 (Fig. 6) five of the nine instruments have an excellent signal to noise ratio. The first arrivals (Pg) at a range of 70 to 100 km define a travel time curve that has an apparent velocity
Fig. 6. The observed seismic section (top) for shot 6 with the travel time curves superimposed. The synthetic section (middle) and the ray paths (bottom) are plotted. Pg D the refracted event through the lower crust with a velocity of 6.7 km s 1 ; Pn D the refracted arrival from the upper mantle with a velocity of 8.0 km s 1 ; PmP D is the reflection from the M discontinuity.
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Fig. 7. The seismic sections (top) for shot 7 with the travel time curves superimposed. The synthetic section (middle) and ray paths (bottom) are displayed. Pg D the refracted event through the lower crust with a velocity of 6.7 km s 1 ; PmP D the reflection from the M discontinuity.
of 6.7 km s 1 . A higher-velocity refracted arrival (Pn) is plotted based on information from other shots; unfortunately, the noise on this section makes it difficult to distinguish. Possible PmP reflections, particularly at offsets of 100 to 140 km, are matched by the synthetic sections. On seismic section 7, the signal to noise ratio is high (Fig. 7). At offsets of 20 to 100 km, Pg is the prominent first arrival and
its velocity is constrained. The split spread shown on section 8 (Fig. 8) with a maximum range of 60 km has insufficient traces to define the sedimentary and upper crustal structure. There are only four seismic traces at offsets of less than 30 km. The Pg branch of the travel time curve at ranges of 30 to 60 km is consistent with the observed first arrivals and the distribution of amplitudes on the synthetic section.
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Fig. 8. The seismic section (top) for shot 8 with the travel time curves superimposed. The synthetic section (bottom) is plotted. Pg D the refracted event from the lower crust with a velocity of 6.7 km s 1 .
On profile 9 at offsets of 20 to 120 km, the Pg event is the first arrival constraining the apparent velocity (Fig. 9). On record section 10 (Fig. 10) with offsets from 60 to 160 km good signal to noise is observed particularly on the most distant instruments. The Pg event is a first arrival to about 100 km, after that Pn arrives first. Five traces are available to resolve the mantle velocity of 8.0 km s 1 . The horizontal resolution of the experiment is poor because of the large spacing between the shots and receivers. Error analysis of the seismic sections has not been done because the paucity of arrivals on each line would produce large statistical uncertainties. Fortunately, the large signal to noise ratio on the seismic traces for the lower crustal and upper mantle arrivals does provide velocity control for these layers.
4. Discussion The seismic reflection profile (Fig. 3) shows the characteristics of the seafloor to the basement. Beneath the sedimentary section a hummocky reflector, identified as seismic basement and interpreted as oceanic layer 2, are mimicked by the two-dimensional model used to calculate the ray paths (Fig. 5). The unpublished Russian refraction velocities make for a degree of uncertainty in the sedimentary and upper crustal layers; however, the velocities are sup-
ported by the regional published velocities. For example, the velocity–depth profile of the Makarov Basin near the Alpha Ridge (CESAR experiment) is consistent with the velocities developed by the Russian navy. In particular, the 5.0–5.2 km s 1 highgradient layer underlying the sedimentary section was measured [9,10] and supports our upper crustal velocity. The lower crust and upper mantle velocities of 6.7 and 8.0 km s 1 , and the thickness of the lower crust is constrained by the refraction record sections. Globally, the most consistent velocities measured in oceanic crust are for layer 3 (6.7–7.2 km s 1 with a low gradient) and the upper mantle (near 8.0 km s 1 ) [20,21]. The average thickness for layer 3 is about 4 km; in contrast, we measured a thickness of 15 km. The thickness of the igneous sections of oceanic crust can increase near mantle plumes and plateaus [22,23]. Mantle plumes generate anomalously hot asthenosphere up to 1000 km away from the central plume [24,25]. The average thickness of igneous crust affected by the Icelandic plume is 10:3 š 1:7 km and the mean thickness directly above a plume is 20:5 š 1:3 km. Limited evidence for large crustal thicknesses above aseismic ridges indicates a mean crustal thickness of 20 š 1 km [23]. The Alpha Ridge based on geological and geophysical data has been interpreted as an oceanic plateau similar to the Pacific plateaus such as the Ontong-Java or the Manihiki [26]. The crustal layers of the Ontong-Java plateau are similar to oceanic crust but five times
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Fig. 9. The seismic section (top) for shot 9 with the travel time curves superimposed. The synthetic section (middle) and the ray paths (bottom) are shown. Ps D the reflected and refracted waves from the sedimentary section; Pg is the refracted event through the lower crust with a velocity of 6.7 km s 1 ; PmP D the reflection from the M discontinuity.
thicker [27]. The velocity and thicknesses measured here for the lower crust and mantle are compatible with thick oceanic crust. The regional tectonic setting, although not well known, is also consistent with crust of oceanic origin. Velocities for the lower crustal of 6.5 km s 1 and upper mantle of 8.2 km s 1 were recorded for the Makarov Basin near the Alpha Ridge [9,10]. The depth to the M discontinuity was 21–25 km similar to that derived from this data set. However, a 7.2 km s 1 velocity measured on the CESAR experiment is
not seen in our data, either because it does not exist or because the spacing between the seismic traces is too large to resolve it. The high-velocity lower crustal layer may be related to the proximity of the Alpha Ridge that exhibits this velocity as well. The Alpha Ridge and adjacent basin were interpreted as oceanic [9,10]. Two refraction profiles intersect the Lomonosov Ridge (Fig. 1), generally considered a continental fragment [6,7]. On the refraction line parallel to the ridge crustal velocities of 4.7 and 6.6 km s 1
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Fig. 10. The seismic section (top) for shot 10 with the travel time curves superimposed. The synthetic section (middle) and the ray paths (bottom) are shown. Pg D the refracted event through the lower crust with a velocity of 6.7 km s 1 ; Pn D the refracted arrival from the upper mantle with a velocity of 8.0 km s 1 .
were measured with the M discontinuity at a depth of 27 km. A summary of velocities for continental crust [28] assumes that a measurement near 6.0 km s 1 is typical. This velocity is conspicuously absent from both our observations and from the Lomonosov Ridge. Not surprisingly, the 1700 km long, 65–200 km wide Lomonosov Ridge does not exhibit standard continental crustal velocities [12]. However, the seis-
mic reflection profiles from the Lomonosov Ridge show basement as a series of rotated fault blocks [7] consistent with a continental origin. In contrast, the basement topography as imaged by profile NP-28 is an irregular reflector typically produced by oceanic crust. On the perpendicular profile, a crustal velocity of 6.6 km s 1 and a thickness of 9 km [12] was determined for the edge of the Makarov Basin. The
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M discontinuity was observed as a post-critical reflection and its depth was predicted to be 23 km. The 6.6 km s 1 velocity was interpreted as oceanic layer 3. Our preferred interpretation of the seismic reflection and refraction sections is oceanic crust. Nevertheless, the lack of basement samples, the poorly lineated magnetic anomalies and the restricted amount of seismic data make it impossible to rule out the possibility of continental crust.
[3]
[4]
[5]
5. Conclusions The seismic reflection and refraction profile collected by PMGE in the Makarov Basin provides new insights into the velocity structure and origin of the basin. The seismic reflection profile shows several kilometres of sedimentary strata overlying an irregular reflector with an associated velocity of 5.0 km s 1 (based on unpublished Russian Navy data) inferred to be oceanic basement. The refraction profile clearly delimits a 6.7 km s 1 velocity layer overlying a 8.0 km s 1 upper mantle velocity at 23 km depth. The data constrains the thickness of the lower crust to 15 km. Based on a comparison with other refraction experiments in the Arctic and worldwide averages, the crust is interpreted as thick oceanic.
[6]
[7]
[8]
[9]
[10]
Acknowledgements [11]
We wish to thank Matt Salisbury and Gordon Oakey who reviewed the manuscript internally. Thanks are also due to Art Grantz and two anonymous reviewers whose careful reviews have improved this paper. Geological Survey of Canada contribution # 1998197. [CL]
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