Geophysical Studies Bearing on the Origin of the Arctic Basin - DocsLib

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List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Langinen A.E., Gee D.G., Lebedeva-Ivanova N.N. and Zamansky Yu.Ya. (2006). Velocity Structure and Correlation of the Sedimentary Cover on the Lomonosov Ridge and in the Amerasian Basin, Arctic Ocean. in R.A. Scott and D.K. Thurston (eds.) Proceedings of the Fourth International conference on Arctic margins, OCS study MMS 2006-003, U.S. De- partment of the Interior, 179–188. II Langinen A.E., Lebedeva-Ivanova N.N., Gee D.G., Zamansky Yu.Ya. (2009). Correlations between the Lomonosov Ridge, Marvin Spur and adjacent basins of the Arctic Ocean based on seismic data. Tectonophysics, doi: 10.1016/j.tecto.2008.05.029. III Lebedeva-Ivanova N.N., Zamansky Yu.Ya., Langinen A.E., Sorokin M.Yu. (2006). Seismic Profiling across the Mendeleev Ridge at 82°N: Evidence of Continental Crust. Geophys. J. Int., 165, 527–544. IV Lebedeva-Ivanova N.N., Gee, D.G., Sergeyev M. Crustal structure of the East Siberian Continental Margin, Podvodnikov and Makarov basins based on wide-angle seismic data (TransArctic 1989–1991). Memoir series of the Geological Society of London "Arctic Petroleum Geology". (accepted)

Reprints have been made with permission from the respective publishers.

Contents

Introduction...... 11 Outline of the thesis ...... 12 2. Background information about the Arctic Ocean...... 13 2.1 Morphology...... 14 2.2 Gravity field data ...... 14 2.3 Magnetic field data ...... 14 2.4 Previous research of the Amerasia Basin and the Lomonosov Ridge...... 16 Seismic research...... 16 Another geophysical experiment in the central Arctic...... 19 Sampling of the bedrock and sedimentary cover ...... 19 2.5 The tectonic evolution of the Arctic Basin - different scenarios ...20 3. Methods...... 25 3.1 Seismic acquisition, data processing and modelling techniques...... 25 North Pole – 28 (NP-28) reflection seismic data (Paper II)...... 25 TransArctic 1989–1992 projects (Paper I and IV)...... 27 Arctic–2000 project (Paper III)...... 32 HOTRAX seismic data over the central part of the Lomonosov Ridge...... 32 3.2 Resolution and accuracy of seismic data and modelling ...... 38 Resolution and accuracy of reflected arrivals ...... 38 Estimation of the resolution and accuracy in refraction seismic.....40 4. Summary of Papers ...... 49 Paper I: Velocity Structure and Correlation of the Sedimentary Cover on the Lomonosov Ridge and in the Amerasian Basin, Arctic Ocean...... 49 Paper II: Correlations between the Lomonosov Ridge, Marvin Spur and adjacent basins of the Arctic Ocean based on seismic data...... 51 Paper III: Seismic Profiling across the Mendeleev Ridge at 82°N: Evidence of Continental Crust...... 54 Paper IV: Crustal structure of the East Siberian Continental Margin, Podvodnikov and Makarov basins based on refraction seismic data (TransArctic 1989–1991)...... 56

5. The central part of the Lomonosov Ridge (HOTRAX) – first view ...... 59 The surrounding ridges ...... 59 The Intra Basin...... 63 6. Sampling the sedimentary cover and bedrock of the Arctic Basin...... 65 Summary in Swedish ...... 72 Acknowledgments...... 74 References...... 76

Abbreviations

ACEX the Arctic Coring Expedition (IODP, Leg 302) AWI Alfred Wegener Institute bsl below sea level CMP common mid-point CDP common depth-point HOTRAX Healy-Oden Trans Arctic Expedition Hz Hertz, frequency units IBCAO International Bathymetric Chart of the Arctic Ocean IODP Integrated Ocean Drilling Program MCS multi channel seismic Moho Mohorovicic discontinuity ms milliseconds NMO normal moveout NP the ‘North Pole’ drifting ice-stations P-waves primary (compressional) waves RMS root-mean square s seconds SCICEX SCience ICe EXercises research program by U.S. Navy submarines S/N signal to noise S-waves secondary (shear) waves TRA shortening of the TransArctic project TWT two way travel time

Figure 1

Introduction

The Arctic Basin is one of the least explored regions on Earth, mainly due to thick multi-year ice cover. At the same time, it is enigmatic, complicated, largely unexplored and therefore debatable area, with considerable economic potential. That's why it is so attractive for research. The Arctic Basin is divided into the Eurasia and Amerasia basins, separated by the Lomonosov Ridge (Figure 1). The origin of the Eurasia Basin by sea-floor spreading is supported by a broad range of evidence and is not a subject for big debates. The origin of the Amerasia Basin still provides more questions than answers. The reflection and refraction seismic investigations, presented in this thesis, are mainly located in the Amerasia Basin and the central part of the Lomonosov Ridge (Figure 1). The Soviet NP–28, TRA(b), TransArctic 1989–1991 and Russian Arctic– 2000 reflection seismic research from drifting ice define the general character of the sedimentary cover over a large area of the Arctic Basin (Papers I and II). In addition, long-offset reflection seismic observations along the TRA(b) profiles provide information about the P-velocity characteristics of the sediments (Paper I). The main sedimentary units were identified over the research area based on these reflection seismic data. Available geological evidence from the Arctic margins and the results of the ACEX drilling of sediments on the central part of the Lomonosov Ridge allowed an estimation of the ages and composition of the identified sedimentary units. The interpretation of the Arctic–2000 (Paper III) and re-interpretation of the TransArctic 1989–1991 (Paper IV) refraction seismic data have been used for constraining models of the crust down to the mantle. These refraction seismic data do not provide information about the low-velocity sedimentary units, but the interpretation of the refracted waves is strongly dependent on the latter. Therefore, incorporation of the sedimentary structure, defined by the reflection seismic research, was essential for the presented modelling. For interpretation of the derived crustal models, potential field data and geological evidence have also been incorporated. The HOTRAX expedition acquired reflection seismic data over the central part of the Lomonosov Ridge in 2005. The obtained seismic sections, included in this summary, illustrate the sedimentary cover and intra-crustal

Figure 1. Bathymetric map of the Arctic Ocean and location of the seismic research presented in this thesis. TRA 89-92 (TransArctic project) and Arctic-2000 are crustal scale refraction seismic profiles. The TRA 90&91 and Arctic-2000 are also reflection seismic profiles as are NP-28, TRA(b), and HOTRAX.

11 reflectivity over an internal basin and surrounding ridges. Together with the ACEX results and sonobuoy velocity data, an interpretation of the tectonic history of this area has been suggested.

Outline of the thesis Before summarizing the main aspects of the thesis research, general descriptions of the morphology and potential field data over the Arctic Basin are presented in Chapter 2, along with a short historical overview of geophysical and geological research and those discoveries that are most relevant to the work presented here. Also in Chapter 2 are included brief descriptions of different interpretations and controversies concerning the tectonic evolution of the Arctic Basin. The seismic acquisition methods and processing techniques of the presented data which are particularly relevant to the high Arc- tic environment, are described in Chapter 3, along with the sources and approximate values of the uncertainties in the data. Chapter 4 provides summa- ries of the four papers included in the thesis. Chapter 5 contains the HOTRAX results. The geophysical data alone cannot provide unique explanations for the origin of the complex structures, but they give a range of possible interpretations. Based on examination of the seismic data discussed in this thesis, it has been possible to suggest key sites for geological sampling by drilling and coring. These are presented, with short descriptions of site data coverage, in Chapter 6. A short summary of the thesis in Swedish is in Chapter 7.

12 2. Background information about the Arctic Ocean

The high Arctic is dominated by a roughly circular, relatively small (9 534 000 km2), shallow ocean. About half of this ocean is underlain by continental shelves with depths mainly less than 250 m (Jakobsson et al, 2003). All the Arctic margins are passive. The deeper parts of the Arctic Ocean are subdivided into the Eurasia and the Amerasia basins, separated by the Lomonosov Ridge (Figure 2.1). These basins are characterised by very different bathymetry, gravity (Figure 2.2) and magnetic fields (Figure 2.3).

Figure 2.1. International Bathymetric Chart of the Arctic Ocean (Jakobsson et al., 2008).

13 2.1 Morphology The Eurasia Basin is composed of the Amundsen and Nansen basins separated by the Gakkel Ridge (Figure 2.1). The Amundsen and Nansen basins have depths of about 3.5–4.5 km bsl; the Gakkel Ridge is a spreading ridge consisting of rises up to 1 km below sea level. A central rift valley reveals depths of 5.5 km bsl. The Lomonosov Ridge is oriented parallel to the Gakkel Ridge and extends for about 1700 km across the Arctic Ocean from the Canadian to the Siberian margins. The Ridge is 50–150 km wide, with the top at depths of 0.5–1.5 km bsl. The Amerasia Basin is more shallow (maximum depths of about 3.9 km bsl), and has a more complex physiography than the Eurasia Basin. The deeper parts are dominated by the mainly flat Podvodnikov, Makarov and Canada basins with depths of c. 2.8–3.3, c. 3.8 and c.3.5 km bsl respectively. The Alpha and Mendeleev ridges, reaching up to c. 1.5 km bsl, separate the Canada Basin from the Makarov and Podvodnikov Basins1.

2.2 Gravity field data Conspicuous positive gravity anomalies along the Arctic margins (Figure 2.2) are most likely related to the isostatically uncompensated thickening of the sediments beneath the continental slope (Fairhead D., personal commu- nication 2009). The major intra-basinal highs are also characterized by positive anomalies, the relatively short wave-length gravity anomalies are mainly related to the bathymetry variations of the Arctic Ocean.

2.3 Magnetic field data The magnetic field anomalies over the shelves are variable and related to the geology of these continental areas (Figure 2.3). The magnetic field anomalies of the deep Arctic Ocean are readily distinguished into the Eurasia and Amerasia basins.

1 There is some disagreement about the names and boundaries of submarine features in the Arctic Ocean between different authors. In this thesis, the names and boundaries of the bathymetric features are used from Naryshkin & Gramberg (1995); also Grantz et al. (2009). The Makarov Basin is limited to the deep basin between c. 86oN and 89oN; more southerly parts are referred to as the Podvodnikov Basin. The Makarov Basin, on some maps, for example Jakobsson et al. (2003) and Sorokin et al. (1999), extends from the East Siberian Margin to the Lomonosov Ridge near the North Pole. However, Kutschale (1966) had previously referred to this basin as the Siberian Basin and distinguished within it northern (Siberian) and southern (Wrangel) abyssal plains. Often the Alpha and Mendeleev ridges are united and referred to as the Alpha- Mendeleev Ridge, but they are discussed here as separate features.

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Figure 2.2. Free-air gravity anomalies map of the Arctic Ocean (after Glebovsky et al., 2002). The linear magnetic anomalies over the Eurasia Basin are mainly related to ultra-slow asymmetric sea-floor spreading, centred on the Gakkel Ridge. At the termination of the Ridge towards to the Laptev Sea, the spreading rate is only ~6 mm/year; the rate increases towards Greenland termination to ~13 mm/year (Michael et al., 2001). The oldest dated linear magnetic anomalies of C24 (Vogt et al., 1979; Glebovsky et al., 2006) or C25 (Brozena et al., 2003) show that the Eurasia Basin started to form in the late Paleocene either at c. 54 Ma or 58 Ma and sea-floor spreading continues today. The magnetic field anomalies of the Amerasia Basin are mainly characterized by irregular high amplitude variations. The Alpha and Mendeleev ridges are notable for a remarkable mosaic of mostly irregular magnetic anomalies (Leonov, 2000), thought to be related to the Alpha-Mendeleev Large Igne- ous Province (Grantz et al., 2009). The Lomonosov Ridge is defined by generally low amplitude magnetic anomalies. Within the Podvodnikov Basin, sublinear WNW-trending anomalies (Glebovsky et al., 2000) characterize both southern and northern parts of the Basin and the Arlis Gap. These anomalies generally have an amplitude range of –320 nT to +350 nT and a wavelength of 30–70 km along the Geotransect (Figure 2.3). The Canada Basin has lower amplitude magnetic anomalies (in limits of c. ±100 nT) by comparison with other parts of the Amerasia Basin.

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Figure 2.3. Magnetic anomalies map of the Arctic Ocean (after Glebovsky et al., 2002)

2.4 Previous research of the Amerasia Basin and the Lomonosov Ridge This brief summary is focused on the previous research of the central part of the Lomonosov Ridge, the Mendeleev and Alpha ridges, and the Podvod- nikov and Makarov basins i.e. the areas discussed in this thesis (Figure 1).

Seismic research Shallow reflection seismic research The reflection seismic research over the deeper parts of Arctic Ocean began from drifting ice-stations in the 1960s. There were many Soviet (eg. ‘North- Pole’ (NP) and ‘Sever’) and Canadian and US (eg. ‘T-3’, ‘Arlis-II’ and ‘LOREX’) research projects. The researchers on the ice-camps collected various kinds of data (hydrographic, meteorological, seismic, potential field, etc.) continuously over periods of months, even years. The seismic recording equipment was placed on the ice; shots were detonated in the water through ice-holes. It was nearly impossible to plan the research areas and location of the seismic profiles because the tracks of the lines were mostly unpredictable

16 and dependent on ice-drift, with a velocity of generally in the order of 5–10 km per day. These seismic data are characterized by: a) use of one or usually a few seismic receivers on a short array; b) a sparse distribution of shot points (about 0.5 km in between) c) recording usually on analogue tapes; d) acquisition on crooked lines; and f) low coordinate (location) precision. Today, the problems with these data are compounded by: a) the data were recorded on magnetic tapes or even on paper by oscil- lograph, and they are problematic to re-process; b) a lot of the ice-station data are not published. Nevertheless, these seismic investigations gave information about the sedimentary structure, usually down to acoustic basement, over large areas of the Arctic Ocean (ref. Kristoffersen and Mikkelsen, 2004). Ice-camp research has made a huge contribution to the understanding of the Arctic Basin. The Arlis-II ice-station (Kutshale, 1966) crossed the Podvodnikov Basin in 1962 from south to the north. Integration of collected seismic reflection, gravity and magnetic field data led to the following conclusions: below flat sediments, a basement ridge is located in the vicinity of the Arlis Gap; crustal thickness below the Gap is up to 20 km; channels in sediments over the Gap show evidence of northwards transport of the sediments. Probably the longest-track ice-station was the Soviet NP–28 (1987–1989), which collected about 4000 km of reflection data across the Arctic Ocean. Some results of this research are presented in the thesis.

Since the 1990's, multi-channel reflection seismic (MCS) surveys have been carried out from icebreakers (ref. Kristoffersen and Mikkelsen, 2004). Most of these seismic lines over the area of concern here crossed the Lomonosov Ridge (Jokat et al., 1992; Jokat, 2005) and a few crossed the Alpha Ridge (Jokat et al., 2003). The HOTRAX expedition (Coakley et al., 2005) collected MCS data over the Mendeleev, Alpha and Lomonosov ridges and the Makarov Basin in 2005; some of these seismic lines over the Lomonosov Ridge are presented in this thesis summary. The MCS lines were acquired using a short streamer (usually c. 200– 300 m long receiving array) and airguns as a seismic source with shot spacing of 15–60 m. All seismic equipment was towed behind the icebreaker. The MCS lines usually are less crooked and the seismic observations more dense than the acquisition from the drifting ice- stations; they provide information about the sedimentary structure down to acoustic basement and illustrate some reflectivity below that. The seismic data often are contaminated by strong noise due to crushing ice and the icebreaker's engines. Sonobuoys deployed along the MCS lines were used to collect long-offset reflection and refraction seismic data and provide information about seismic velocities along the lines.

17 In 1991, the icebreaker Polarstern acquired two reflection seismic profiles (AWI-91091 and AWI-91090) across the Lomonosov Ridge at 88oN (Jokat et al., 1992). These data showed that the top of the Ridge is covered by a nearly flat-lying sequence of low-velocity sediments (velocities of <2.2 km/s) c. 500 m thick, resting on a prominent unconformity. The rocks below this Lomonosov Unconformity have significantly higher velocities (more than 4.0 km/s). Those seismic lines were the basis for locating the IODP-ACEX drilling sites (see below).

The old drifting ice-camps collected in average c. 500 km of new seismic data per year in contrast to c. 300 km per year collected from the icebreakers since the seismic data acquisition started (Kristoffersen and Mikkelsen, 2004). This is probably due to the short operation season of icebreakers (about 3–4 summer months) by comparison with the year-long operations from drifting ice; also probably due to the great expense of icebreaker operations. Alternative seismic acquisition methods (eg. using hovercraft) from ice- stations may be valuable in the future (Hall and Kristoffersen, 2009).

Crustal-scale refraction seismic experiments Most of the crustal-scale refraction seismic experiments in the Arctic Ocean were carried out from drifting ice. All seismic equipment was placed on the ice and trotyl was used for the seismic source, detonated in the water. One of the first experiments in the central part of the Arctic Ocean was carried out during the LOREX ice-island expedition (1979) over the central part of the Lomonosov Ridge and the Makarov Basin, near the North Pole. Long-offset seismic observations demonstrated the presence of c. 25 km thick continental crust below the Ridge and c. 10 km thick oceanic crust below the Basin. The Ridge crust was shown to be composed of two layers with P-velocities of 4.7 km/s and 6.6 km/s (Mair and Forsyth, 1982). Subsequently, a Soviet expedition, TransArctic 1992, also collected a wide-angle seismic profile across the Siberian side of the Lomonosov Ridge (re-processed by Ivanova et al., 2002) confirming that the Ridge was made up of c. 28 km thick continental crust comprising three main layers, with velocities of 5.0–5.6 km/s, 6.1–6.4 km/s and 7.0–7.5 km/s of approximately similar average thickness, below the sedimentary cover. In the 1980s, the CESAR refraction seismic experiment crossed the Alpha Ridge and showed the presence of anomalously thick crust (up to c. 35 km thick), which was interpreted to be similar to Icelandic oceanic crust (Jack- son et al., 1986). The Soviet TransArctic 1989–1991 profile from the East- Siberian continental margin, across the Podvodnikov and the Makarov basins to the flank of the Lomonosov Ridge and the Russian Arctic–2000 across the Mendeleev Ridge experiment are described in Papers III and IV.

18 Another geophysical experiment in the central Arctic The U.S. Navy SCICEX program used nuclear submarines to collect 'chirp', high resolution shallow seismic profiles (down to c. 100 m below the sea-floor), swath bathymetry and high resolution gravity field data (for ref. Edwards and Coakley, 2003; Cochran et al., 2006). The gravity field data provided detailed information about depth variations of the basement below the sediments. Cochran et al., (2006) described the structure of the Lomonosov Ridge and Podvodnikov and Makarov basins using the SCICEX data. In particular, these authors concluded that the basins contained many nearly linear pieces of continental crust, rifted off of the Lo- monosov Ridge, often buried by little consolidated sediments. The results of the SCICEX research are discussed and compared with seismic data presented in this thesis.

Sampling of the bedrock and sedimentary cover Most of the geological samples collected in the deep Arctic Ocean have been taken by piston coring (eg. Darby et al., 2005b) and are comprised of up to c. 20 m of unconsolidated Quaternary sediments. Unfortunately, they are not suitable for interpretation of the reflection and refraction seismic data, because they are below the resolution of the data. A few samples of consolidated sedimentary rocks of Jurassic age have been collected by piston coring on the Amerasia slope of the Lomonosov Ridge (Grantz et al., 2001) from beneath the Lomonosov Unconformity (Paper II). Subsequently, an IODP (Integrated Ocean Drilling Program) expedition sampled the entire sedimentary cover succession on the top of the Lomonosov Ridge (Backman et al., 2006). This ACEX expedition located three drillholes on the AWI-91090 seismic line and reached a total depth of c. 430m, nearly all in little consolidated Cenozoic sediments. The ACEX expedition showed the importance of drilling for identifying the character and age of the sedimentary cover, and how these can be correlated with the reflection seismic (Figure 2.4), both along the Ridge and into the basins.

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Figure 2.4. Correlation between a synthetic seismogram representing the stratigraphy of the ACEX sites and seismic reflection profile AWI-91090 with seismic units LR6–LR3 as inferred in Jokat et al. (1995) are on left panel. The ACEX density and velocity records presented on the central panel. The ACEX age model and composition of sediments are on right panel. After Jakobsson et al. (2007).

2.5 The tectonic evolution of the Arctic Basin - different scenarios The creation of the Arctic Basin apparently began in the Mesozoic, nearly two hundred million years ago, with the fragmentation of the megaconti- nent Pangea and the first stages of opening of the Amerasia Basin. Subse- quently, in the early Cenozoic, a thin slice of continental crust (now the Lomonosov Ridge) was rifted off the edge of the Barents-Kara Shelf, and during the following fifty million years, the Eurasia Basin was formed by sea-floor spreading. Whereas the Cenozoic development of the Eurasian Basin can be defined in some detail by analysis of the sea-floor spreading anomalies, the older history of the Amerasia Basin is controversial. It has been suggested (eg. Grantz, 1979; Jackson and Gunnarsson, 1990) that the evidence of Mesozoic sea-floor spreading may be obscured by thick younger sediments and that the Amerasia Basin originated in a similar way to the Eurasia Basin, but earlier (Figure 2.5).

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Figure 2.5. Tectonic setting of the Arctic showing the inferred opening of the Amerasia Basin in the Mesozoic, and the Eurasia Basin and North Atlantic in the Cenozoic. Based on Jackson and Gunnarson, (1990); re-drowned by Lorenz (2004). The Amerasia Basin is surrounded by continents without clear evidence of a mid-oceanic ridge and connection to any major spreading system (Tes- sensohn and Roland, 2000). In contrast with the Eurasia Basin, it is nearly impossible to trace magnetic 'spreading' anomalies in the Canada Basin

Figure 2.6. Summary of possible locations of the Mesozoic spreading axis in the Amerasia Basin by Scott et al. (2006).

21 (Figure 2.3), though a linear gravity low at the centre of the Basin marks the possible location of a central rift valley related to sea-floor spreading (Figure 2.2). A summary of possible locations of Mesozoic spreading axes that have been proposed by different authors is presented on Figure 2.6; it illustrates the broad range of interpretations. More recently, Grantz et al (2009) have suggested that the Amerasia Ba- sin was created by two stages of extension during the Mesozoic. The first resulted in the creation of the ocean-continent transitional crust by the stretching of continental crust during the Jurassic and possibly the early Early Cretaceous, and the second created mid-ocean ridge basalts in the centre of the Amerasia Basin by seafloor spreading during the Early Cretaceous. In particular, based on the shape of the margins near Alaska and the Cana- dian Arctic Islands, the rotational model for the opening the Canada Basin (Figure 2.7) was initially proposed by Carey (1958) and developed by Grantz et al., (1979), and is discussed in this thesis.

Figure 2.7. The rotational model for the opening the Canada Basin after Grantz et al. (1979). The origin and age of the other parts of the Amerasia Basin is even more debatable. The creation of the Podvodnikov and the Makarov basins by rota- tion, as proposed by (Grantz et al., 1979), has been supported by many researchers based on geological (eg. Drachev and Saunders, 2006) and geophysical data (eg. Cochran et al., 2006). Sublinear WNW-trending magnetic anomalies within the Podvodnikov Basin, have been interpreted as evidence of Cretaceous sea-floor spreading (Kovacs et al., 1999; Glebovsky et al., 2000), but also as the signature of mafic intrusions related to rifting (Scott et al., 2006), By contrast, based on land-based geological, structural and other studies, supplemented by analysis of the bathymetry of the basins and Alpha and Mendeleev ridges, Miller and Verzhbitsky (2009) suggested latitude-

22 parallel opening of the basins by rifting during Cretaceous-Tertiary time. Linear magnetic anomalies in the Makarov Basin were described by Vogt et al. (1979) and interpreted as evidence of Cretaceous sea-floor spreading, but they can be related to linear fragments of continental crust (eg. Marvin Spur) rifted off the Lomonosov Ridge (Papers II and IV). Speculations about the composition, structure and origin of the Men- deleev Ridge have included a wide variety of possibilities. Most authors have regarded the Alpha and Mendeleev ridges to be part of a related system crossing the high Arctic from the East Siberian Shelf to the Canadian Shelf, north of Ellesmere Island (Jakobsson et al., 2003). Whereas some authors (King et al., 1966; Johnson et al., 1978; Verba & Petrova, 1986) have proposed that these ridges may be sunken continental shelf, like the Lomonosov Ridge, others have favoured an oceanic character (Hall, 1973; Jackson et al., 1986; Forsyth et al., 1986; Jokat, 2003), maybe related to the Mesozoic track of a hotspot that started beneath the Siberia Plate in the early Triassic and is now located below Iceland (Lawver & Muller, 1994). Weber (1990) suggested that the Alpha Ridge formed by funnelling of volcanic material from a nearby hotspot into a spreading centre like the Iceland-Faroe Ridge; however, he regarded the Mendeleev Ridge to be of continental origin. A number of scenarios for the origin of the Arctic Basin (Figure 2.8) have been tested by a new method of gravity inversion with embedded lithosphere thermal gravity anomaly corrections incorporated into plate reconstructions (Alvey et al., 2008). These authors concluded that models containing older “trapped” and younger oceanic crust within the Makarov Basin (Figure 2.8c) were most consistent with the seismic models (Papers III and IV). However, a good knowledge of the sediment successions and their thickness is required for predictions of the age of formation of the older crust; unfortunately this is lacking in the Arctic Ocean.

Figure 2.8. (After (Alvey et al., 2008) Present day oceanic age grids and coastlines for the Arctic region. Plate reconstruction models: (a) 1: Mendeleev Ridge is rifted from the Canadian continental margin at 150Ma with a conventional rotational opening of the Amerasia Basin. (b) 2: the Mendeleev Ridge is rifted from the Lomonosov Ridge/Barents Sea Margin at 71 Ma with the Amerasia Basin opening in a rotational fashion beginning at 140 Ma. (c) 3: a hybrid model containing older trapped and younger oceanic floor within the Makarov Basin.

23 All scenarios of the Amerasia Basin tectonic evolution depend upon interpretation of the structure and composition of the crust and identification of the character and age of the sedimentary cover. The geophysical research reduces the number of alternative interpretations. Due to extremely poor coverage by geological sampling (drilling and piston coring), the interpretation of the physical parameters of the rocks cannot provide reliable models of the tectonic evolution of the Amerasia Basin; they leave too broad a range of alternative models. Systematic sampling of the sea-floor from key areas of the Basin and correlation with collected geophysical data could give a much improved understanding of the Amerasia Basin tectonic evolution.

24 3. Methods

3.1 Seismic acquisition, data processing and modelling techniques The seismic data presented in the four papers in this thesis were acquired from drifting ice. These experiments were designed with a number of pre- cautions in order to take into account the unpredictable movement of the ice, possible destruction of receiver arrays by cracking of the ice, and technical and environmental limitations. The average coordinate location errors of the old seismic data (i.e. collected before year 2000) were in the range of 200–300 m. The old data have been recorded on magnetic tapes; during the 1990’s the data were digitised with a 4 ms sampling rate and processed with modern techniques. All presented data contain compressional seismic P-waves only, because shear seismic S-waves do not propagate through the water. The presented seismic data cannot be considered as ‘conventional’ and therefore they require challenging ‘unconventional’ acquisition and processing methods. This chapter focuses not on the seismic methods as such, but rather on a clarification of the ‘unconventionality’ and the methods of deal- ing with it. The basics of the conventional seismic methods, processing and interpretation can be founded in e.g. Sheriff and Geldart (1995), Yilmaz (2001).

North Pole – 28 (NP-28) reflection seismic data (Paper II) In 1987–1989, the Soviet ice-station NP-28 crossed the Arctic Ocean from the southern part of the Podvodnikov Basin to the Yermak Plateau and collected about 4000 km of reflection seismic data (Figure 1). Shot spacing was about 0.5 km; shots were exploded by 3-5 electric caps at water depths of c. 8 m. The receiver array consisted of two arms, arranged orthogonally (Figure 3.1), each 550 m in length, with a receiver spacing of 50 m. This design of the receiver array provided attenuation of out-of-plane waves. Syn- thetic shot gathers and ray-paths for the array geometry are presented in Figure 3.1. These examples were calculated for within-plane waves reflected from flat sea-bottom at 3 km bsl and a velocity of 1.5 km/s (Figure 3.1 a), and for the out-of-plane reflections (Figure 3.1) that were calculated from the same distance of 3 km from a typical slope with a dip of 10o (like the Lomonosov Ridge slope into the Podvodnikov or the Makarov basins). The out-of-plane reflections for three possible orientations of the arms relative to the slope are presented on Figure 3.1. Figure 3.1a illustrates the case, when

25

Figure 3.1. Receiver array geometry of the NP-28; ray-paths and synthetic seismo- grams for the near-slope environment are shown: a) Reflection points on the horizontal within-plane surface and corresponding reflection signal are marked by '1'; out-of-plane reflections at the same distance from the slope and corresponding reflections are marked by '2'. b) Two other possible orientations ('3' and '4') of the receiver array relative to the slope and corresponding out-of-plane reflections are presented. Ray-paths and synthetics were generated by a code in Kashubin and Juhlin (submitted). one arm is parallel to the slope and the other is orthogonal to it. The ‘parallel’ arm will receive the out-of-plane reflected signal at the same time as from the within-plane structure; the 'orthogonal' arm will receive the seismic signal with larger delays than from the within-plane structure. For the two cases where the arrays are oriented at 45o to the slope (Figure 3.1 b) the out- of-plane signal will have a larger delay or an earlier arrival time in comparison to within-plane signal. NMO corrections and further stacking will increase the amplitude of the within-plane signal and attenuate out-of-plane reflections. When the NP-28 track passed over a slope area, only data from

26 that arm, which was most parallel to the slope (or traces of the nearest offsets) were used for stacking. In theory, the acquisition geometry allows processing of the data by the CMP-method and this would increase the resolution of the final stack. But this way of processing was rejected, because the navigation error of 200–300 m is comparable to the length of the receiver array and the shot spacing; the CMP processing would create significant artefacts. Therefore, the data have been analysed and stacked in the shot- gather domain. The data processing steps are presented in Table 3.1.

Table 3.1. Processing steps of the NP-28 reflection seismic data. 1 Creating straight line geometry from the crooked line 2 Sort to common source gathers 3 Define zero time 4 Trace editing and muting 5 AGC window 1 s 6 Velocity analysis in source gathers and NMO correction 7 Stack of source gathers 8 Minimum phase predictive deconvolution – prediction distance – 8 ms – operator length – 200 ms – white noise – 0.1% – deconvolution – wide gate 9 Bandpass filter 15–18–45–55 Hz.

TransArctic 1989–1992 projects (Paper I and IV) The integrated TransArctic 1989–1992 research projects (Gramberg et al., 1993; Sorokin et al., 2003) were carried out over the East Siberian continental margin near Zhokhov Island, across the Podvodnikov and Makarov basins to the slope of the Lomonosov Ridge near the North Pole in 1989–1991 (Figure 1 and Table 3.2) and across the Lomonosov Ridge in 1992 (Ivanova et al., 2002). The main goal of the research was to define and compare the crustal structure beneath the continental shelf and the Arctic Basin. The TransArctic 1989–1992 projects included the following parts (Table 3.2): refraction profiling, shallow reflection measurements, and gravity measurements at receiver points along the refraction profiles; TRA(b) reflection seismic experiments with long offsets (up to 7 km) for determination of the interval velocities in the sedimentary cover (Paper II); and airborne magnetic surveys over a 100 km wide zone along the refraction seismic lines (Glebovsky et al., 2000).

27 Table 3.2. TransArctic and Arctic projects acquisition parameters Name of transect TransArctic TransArctic TransArctic Arctic– Parameters 1989 1990 1991 2000 of transect Main morpho- Podvodnikov Makarov East Siberian Mendeleev logical structure Basin Basin Margin Ridge Location of ends 164.5ºE / 80.8º'N 171ºE / 85.1ºN 153.1°E / 76°23′N 150º E / 82ºN of profiles 170.2º'E / 85º'N 154.1ºW / 88.9ºN 166°E / 81.5°N 150º'W / 82ºN Length of 470 km 450 km 630 km individual 485 km transects Total length of the Geotransect is 1487 km 3 3 Number of seg- 3 (+1 supplemen- 3 (+1 supplemen- ments (+1 supplementary) tary) tary) 40 km, Shot spacing 30–70 km 30–70 km 30–60 km 10-40km on suppl.section 15 23 24 Number of shots (+3 supplemen- 16 (+8 supplemen- (+5 supplementary) tary) tary) 205 km; Maximum offset 170 km 170 km (270 km 200 km for suppl.section) 5 km Receiver spacing 10 – 14 km c. 10 km c. 7 km 2 km on suppl.section Number of re- 25 ceivers per seg- 10 13 16 16 on ment suppl.section Recording sys- analog analog analog digital tem Shallow reflection seismic no yes yes yes along transect Seismic velocity research in the TRA(b)-89 TRA(b)-90 TRA(b)-91 no sedimentary cover Gravity field measurements at yes yes yes yes seismic acquisition points Aeromagnetic survey along the yes yes yes no transect

28 TRA(b) 89–92 reflection experiments for determination of velocity characteristics in the sedimentary units (Paper I) During the acquisition, the research-team camped on drifting ice-islands near the transect lines. The drift paths of the camps are shown on Figure 1 where they are marked by "TRA(b)". The seismic reflection data were collected along the path of the drifting camp. Each profile was about 100 km long. Seismic acquisition along the lines and data processing were done in a similar way to the NP-28 project (described above). The TRA(b) ‘cross’ geometry of the receiver array (Figure 3.2A) was designed for attenuation of out- of-plane waves in the same way as the ‘orthogonal’ geometry in the NP-28 profile, but with as much more data provided for the stack. In addition, long-offset (2.0–6.5 km) reflection seismic data were collected every 3–6 km along the profiles. These data provided systematic observations of the velocities in the sediments. The offset length was chosen according to the estimated sea-floor depth for each area. During the first long-offset experiment, the TRA(b)-89, over the Arlis Rise the receiver array was designed with a length of 3.5 km and three shot points (Figure 3.2B, TRA(b)-89). It provided a forward and reverse observation setup for better definition of velocities for dipping layers. However, this acquisition geometry was difficult to operate and partly destroyed due to ice-cracking (Figure 3.2C, TRA(b)-89). The next year, the TRA(b)-90 experiment collected data over the deepest area in the Makarov Basin; therefore, offset length was increased to 7.0 km, but receiver arrays were shortened (Figure 3.2B, TRA(b)-90). During the following years, the TRA(b) 91&92 acquisition over the East-Siberian continental slope and the Lomonosov Ridge, long- offset data were collected on a shorter receiver array; shot points were located within the array and outside of it (Figure 3.2 B, TRA(b)-91&92). This setup reduced the amount of data for velocity analysis, since a single seismogram became a combination of two or three shot points, relatively flat- lying reflection horizons were suitable for velocity estimations (Figure 3.2D, TRA(b)-91&92). However, this acquisition geometry was easier to operate and proved to be more stable against ice-cracking.

TransArctic 1990 and 1991 seismic reflection experiment along the seismic refraction line (Paper IV) Shallow reflection seismic observations were carried out along the TransArctic 1990 and 1991 refraction seismic lines. The experiment involved a single shot point with a short 6-channel receiver array at each receiver point of the refraction seismic experiment. The data were processed in a similar way to the NP-28 data. The shot-gather based stacked sections illustrate the structure of the main sedimentary units.

29

Figure 3.2. Acquisition configurations of the reflection (A) and the long-offset (B, C) reflection TRA(b) survey; examples of the acquired reflection data with the main identified reflections on D. V – location of the shot points; 'ShP' – shot points and corresponding reflection seismic data on D; thick solid lines – receiver arrays.

TransArctic refraction seismic data (Paper IV) The TransArctic 1989–1991 was composed of three separate profiles, referred to as TransArctic 1989, 1990 and 1991 (Figure 1). These three profiles partly overlap and together comprise a 1487 km long Geotransect. The experiments were carried out using similar methods throughout (see Table 3.2 and fig. 12 in Paper IV). Each of the three transects was subdivided into three segments. Receiver points consisted of short arrays of 6 channels on the ice. The receiver points were distributed along the profile line over one

30 segment first and then, after acquisition, they were moved to the next segment. The shot points were located along the line both within and outside of the receiver segments, thus providing longer offsets so that the rays sampled the lower crust and the upper mantle. In addition, data along the two supplementary segments were acquired to fill in the data gaps due to faulty re- corders and in order to extend maximum offsets in the case of very thick crust in the southern part of the TransArctic 1991 profile. Presented in this thesis is a re-interpretation of the refraction seismic data of the entire TransArctic 1989–1991 profile (referred as the Geotransect), combined with reflection seismic and other geophysical data. A brief summary of the Geotransect investigations was published by Gramberg et al. (1993). The first published interpretation of the data was done by Zamansky et al. (1999); a part of the profile TransArctic–1989 over the Podvodnikov Basin was presented by Sorokin et al. (1999).

Processing and interpretation of the TransArctic 1989–1991 (Paper IV) At the end of the 1990's, the magnetic tapes with seismic data from the Geo- transect were digitized using a sampling rate of 4 ms and were re-processed and re-interpreted. Re-processing of the refraction seismic data included stacking of the six-channel array for each receiver point and sorting to common shot gathers thereafter. Due to the drift of the ice, offsets were calculated for each of the receiver points at the times of each detonation. A bandpass filter of 1–3–8–10 Hz provided the best signal quality of the seismic data. The modelling presented here was done for the entire Geotransect in a consistent manner. The procedure involved two steps. The refraction seismic data generally do not provide information on the low velocity sedimentary cover, but the interpretation of the deeper layers is strongly influenced by these sediments. Therefore, firstly, the depth section of the sedimentary cover was constructed, based on the shallow reflection seismic data collected along the Geotransect and the velocities obtained by the TRA(b) observations. The gap in reflection data on TransArctic 1989 over the southern part of the Podvodnikov Basin was partly filled by projecting reflection seismic data from the nearby and parallel part of the profile NP–28 onto the Geotran- sect line. The rest of the gap was covered by information from the bathymetry map (Naryshkin & Gramberg, 1995) and by estimation of the sedimentary structure using the NP-28, TRA(b)-89 and other reflection seismic data from Soviet drifting ice stations. Thereafter, forward modelling of the layers with velocities greater than 5 km/s for the entire Geotransect’s refraction and wide angle reflection seismic dataset was performed, using the velocities of the sedimentary units and their boundaries that were obtained in the previous step. The interactive package SeisWide, provided by Dr. Deping Chian (Geological Survey of Canada), based on a software package by Zelt and Smith (1992), was used for the modelling of the refraction seismic data (fig. 12, Paper IV). The layers were modelled sequentially from the bottom of the sedimentary cover to the Moho. First arrivals were assumed as the most reliable data; secondary arrivals (including the wide angle reflections) were

31 used as less reliable data and were re-tested by first arrivals from other shot- gathers when it was possible.

Arctic–2000 project (Paper III) The Arctic–2000 integrated research project has been carried our across the central part of the Mendeleev Ridge in 2000 (Figure 1) in order to define the crustal structure of the ridge. In general, the project was carried out in a similar way as the TransArctic (see Table 3.2 for acquisition parameters). The differences were that: the research team was based on an ice-class vessel ‘Akademik Fedorov’, geological samples were collected with dredges and piston cores, and magnetic observations were not included. Concerning the seismic research, the differences were that: shot and receiver point spacing was decreased, digital seismic equipment was used and there was no long- offset reflection seismic survey. Instead, a supplementary 32 km long section with 2 km receiver spacing was used for estimation of velocities in the sediments in the western part of the profile (see acquisition geometry on fig. 4 in Paper III).

Processing and interpretation of the seismic data Processing of the shallow reflection data along the refraction line and modelling of the crust have been done in the same way as for the TransArctic transect (described above). The velocity values in the sediments were interpo- lated from the TRA(b) studies and were used to create the depth section for further crustal modelling. The results of geological sampling were incorporated for interpretation of the crustal model later on.

HOTRAX seismic data over the central part of the Lomonosov Ridge HOTRAX expedition During the period 5 August - 30 September 2005, the HOTRAX (Healy- Oden Trans Arctic Expedition) expedition on the US Coast Guard icebreaker Healy crossed the Arctic Ocean from the Aleutian Islands (USA) to the Tromsø (Norway) via the North Pole. This transect runs over the Northwind Ridge, Chukchi Plateau, Mendeleev Ridge, Alpha Ridge, Makarov Basin, Lomonosov Ridge (near the North Pole), Amundsen Ba- sin, Gakkel Ridge, Nansen Basin and, finally, the Yermak Plateau (Figure 3.3). This international multi-disciplinary expedition included geological, geophysical, ice, water, and marine-mammal research on the way (Darby et al., 2005a). Healy and the Swedish icebreaker Oden operated together from the Alpha Ridge to the Nansen Basin. This joint effort was supposed to optimize seismic acquisition and safety through the heaviest multi-year ice in that area. Scientists onboard Oden conducted independent oceanographic and ecological studies as part of the "Beringia 2005" project (http://www.polar.se/expeditioner/beringia2005/).

32

Figure 3.3. HOTRAX expedition track-line (red line) across the Arctic Ocean (from www. of Bergen University). The further discussed area is marked by the rectangle. During the research program, Healy acquired integrated underway geophysical data sets, consisting of multi-beam swath (up to 10 km) bathymetry and 'chirp' sub-bottom profiler data (first 100 meters of sediments), gravity field measurements, multi-channel seismic reflection and sonobuoy seismic refraction data. The geological program collected core samples from all the main morphological structures. The geological studies provided information about the sediments (depth down to 17 m below sea floor) and paleohistory of the ocean from c. Pliocene till recent (Darby et al. 2005b). The multi-channel seismic (MCS) and sonobuoy data were acquired mainly in the Amerasian Basin (Coakley et al., 2005). Extremely difficult ice conditions did not allow seismic surveying in the Eurasian Basin. The total length of the HOTRAX MCS reflection profiles was about 2200 km. Two airguns were used as the seismic energy source, with a discharge volume of 250 in3 each, for a total volume of 500 in3. Depending on ice conditions, the length of the hydrophone streamer was up to 300 m; the distance between the hydrophones was 12.5 m. The airguns and the streamer were

33 towed by the icebreaker along the survey lines. Seismic pulses were emit- ted at intervals of 20 s and recorded with 2 ms sampling rate. The 20 s spacing corresponds to a shot interval of 30-40 m at the normal cruise speed. The time of recording was 8 or 16 s. The seismic data imaged the sedimentary units and the uppermost crust. Along with the MCS acquisition, about 100 sonobuoys were deployed on the MCS profiles (Coakley et al., 2005). They collected long-offset reflection and refraction data for estimation of velocities in the sediments and deeper. Sonobuoys recorded the signal from the same airgun source that was used for the MCS. The maximum distance between source and receiver was about 25 km.

Seismic data over the central part of the Lomonosov Ridge: Acquisition The central part of the Lomonosov Ridge near the North Pole between 88- 89o N includes an interior basin (the Intra Basin, Bjork et al., 2007). Seven MCS lines for about 280 km of total length were collected over the area. Six of them (Lines 39–44) are presented in the summary (Figure 5.2). These lines were acquired nearly at the end of the HOTRAX seismic survey. Due to very difficult ice-conditions the lines were very crooked; about 50 % of the geophones were malfunctioning; airguns were partly damaged and did not work properly. The data were contaminated by very strong noise from the crushing of the surrounding ice by Healy and its propellers. The processing flow was designed to deal with the listed problems and to extract useful signal from the noise.

Processing of the MCS data The geometry of the MCS crooked lines was assigned by a MatLab script developed in the University of Bergen by Gjengedal (2004). The script calculates distances and directions between shot points and positions of the receivers and location of the corresponding mid-points (Figure 3.4). In- stead of CDP binning, the seismic line was divided onto chosen summation intervals. The mid-points were projected onto a straight line between the two nearest shot-points. CDP-like gathers contained the projected mid- points within the summation intervals. The MCS data with the geometry assigned as described above have been further processed as CDP gathers.

34

Figure 3.4. Principle schema of the binning along the crooked line developed by Gjengedal (Gjengedal, 2004), after Årthun (2008).

Main processing steps for the six MCS lines were: 1. Binning to summation intervals of 50 m (50 m was chosen to increase the fold (c. 20) and to improve the S/N ratio at depth). 2. Trace editing (c. 60 % of traces were removed due to low S/N ratio or faulty hydrophones). 3. Time variant bandpass filter 10-20-80-120 Hz (for about first second below sea-floor reflection), 7-15-55-75 Hz, (for lower part). 4. Min. phase predictive deconvolution: decon operator 200ms, operator prediction distance 20ms, decon gate parameters 0-800 ms. 5. Trace equalization at water bottom gate reference. 6. Trace weighting. 7. Time-variant scaling. 8. NMO. Results of the above processing steps are presented on Figure 3.5

35 0

0.4 0.4 Time, s Time, Time, s Time,

0.8 0.8

Row dataRaw – CDP data gather processed CDP gather (before NMO) 3.6

4.0 4.0

Figure 3.5. Example of the raw MCS data from Line 43 (left panel) and result of its processing (before step 8, right panel). Main processing steps for the six MCS lines (continuation) were: 9. CDP stack. 10. Projection of stacked time section onto straight line/lines. 11. Time variant bandpass filter as in step 3. 12. Trace equalization at water bottom gate reference (in wide window). 13a. F-X decon 8-75 Hz, 700ms 13b. Lines 42&43 only: Kirchhoff Time Migration (max. frequency 75 Hz) 14. AGC 700ms.

HOTRAX sonobuoy data - estimation of the velocities Eleven sonobuoys were deployed on the Lines 39–44 over the Lomonosov Ridge (Figure 5.2). Four sonobuoys (SB) were used for determination of the velocities along the MCS lines: SB 85 on Line 39, SB 86 on Line 40, SB 92&93 on Line 43. Unfortunately, the rest of the sonobuoys did not provide reliable information due to weak source signal, strong noise level and complicated sea floor relief. The sonobuoy data were processed and interpreted in two main steps: velocity analysis of the reflection data at offsets of up to 5 km for estimation of RMS velocities, which were further converted to appar- ent interval velocities (Figure 3.6); and modelling of the true velocity using the reflection and refraction sonobuoy data in combination with MCS data (Figure 3.7).

36

Figure 3.6. Example of the velocity analysis for offsets shorter than 5 km. Sonobuoy 86 on the MCS Line 40.

Figure 3.7. Modelling of the true velocities and reconstruction of depth section using MCS time section (Line 40) and sonobuoy 86 seismic data. Black discontinuities are constrained by MCS and reflection data; red discontinuity and velocities below are modelled using reflected and refracted waves.

37 Processing of the sonobuoy data included: 1. Offset calculations using coordinates of the sonobuoy deployment and source location. 2. Offset corrections using direct water wave for unknown drift of the sonobuoy. 3. Filtering with the parameters similar to the MCS data. 4. Trace editing. 5. Identification of the main reflection horizons, those observed in the MCS section and in the sonobuoy seismic data. Thereafter, sonobuoy data with offsets less than 5 km were processed for velocity analysis. 6. Top mute of the data till sea-floor reflection. 7. Velocity analysis of the main horizons for estimation of the RMS velocities (Figure 3.6). The velocity-depth model was reconstructed iteratively by forward modelling. In the initial model, the depth of each velocity discontinuity was obtained from the MCS time section by time to depth conversion with the initial interval velocities derived from the reflection data velocity analysis. Layer velocity was manually adjusted with each forward run in order to achieve a better fit of both reflection and refraction arrivals for a few itera- tions. A constant lateral velocity was assumed within a layer. Requirements for a successful model were that the assigned velocities match the calculated and observed reflection and refraction sonobuoy phases and that the converted depth to time section reproduces the main identified reflection events on the MCS time section. This trial and error technique was repeated sequentially from the top to the bottom of the section and provided an estimate of the true velocities in the layers (Figure 3.7).

3.2 Resolution and accuracy of seismic data and modelling The resolution and accuracy of the seismic methods are dependent on the physics of seismic wave propagation, the physical properties of the investigated medium, the acquisition geometry, and the techniques and methods of processing and interpretation. Some of these are discussed below in the ap- plication to the presented seismic data.

Resolution and accuracy of reflected arrivals

The vertical resolution (Rvert) is a measure of the ability to recognize individual, closely spaced reflectors and is determined by the pulse length of the recorded seismic signal. The vertical resolution for vertical incidence reflection data (the quarter wavelength criteria) is defined as

Rvert= ¼ λ = V/4f (1)

38 where λ is the dominant wavelength of the pulse, V the velocity in the layer, and f the dominant frequency. The vertical resolution of the presented reflection seismic data for the range of observed velocities and dominant frequen- cies2 are given in Table 3.3.

Table 3.3. The vertical resolution in km for the range of observed velocities and dominant frequencies Velocity, 20 Hz 40 Hz 60 Hz km/s 1.5 0.02 0.01 0.01 2.5 0.03 0.02 0.01 3.5 0.04 0.02 0.01 5.0 0.06 0.03 0.02 However, resolution is not the same thing as detectability. A the thin layer with a thickness in the order of λ/20 – λ/30 can be detected in the reflection seismic data (Kallweit and Wood, 1982), but its thickness cannot be determined.

As a limit on the horizontal resolution the first Fresnel zone is commonly used. Features in the reflectors separated by a distance smaller than this cannot be individually distinguished. The radius of the first Fresnel zone for near-vertical incidence reflections can be defined as (Thore and Juliard, 1999; Yilmaz, 2001) λ z VRMS t0 VRMS z Rfr = = = (2) 2 2 f 2 f where z is reflector depth, VRMS is the root-mean-square velocity of the layer overlying the reflector, and t0 the two way travel time of a vertically reflected ray. Computed Fresnel zones for the observed ranges of VRMS and depths, mainly in the sediments, are given in Table 3.4.

Table 3.4. The size of the first Fresnel zone in km for a range of observed RMS velocities for typical depths and dominant frequencies RMS Depth velocity 20 Hz 40 Hz 60 Hz range, km km/s 1.5 1.5–4.0 0.2–0.4 0.2–0.3 0.1–0.2 1.9 1.8–4.5 0.3–0.5 0.2–0.3 0.2–0.3 2.1 2.0–5.0 0.3–0.5 0.2–0.4 0.2–0.3 3.0 2.5–10.0 0.4–0.9 0.3–0.6 0.3–0.5

2 The NP-28, TransArctic and Arctic-2000 reflection data have dominant frequencies in the range of about 20-40 Hz. The HOTRAX reflection data have a dominant frequency range from c. 60 Hz for the upper reflections in sediments to c. 20 Hz or lower for crustal reflections.

39 However, the trace spacing on the NP-28, TransArctic and Arctic-2000 reflection seismic sections is 0.5 km or larger, thus the horizontal resolution for those data is limited by this, instead of the Fresnel zone.

The accuracy of the identified interval velocities in the sediments by TRA(b) observations can be estimated by their deviation from the average value within the layer. These deviations are in the order of 4% (i.e. about ±0.1– 0.15 km/s) within a single unit except the Unit 1 on the Lomonosov Ridge (TRA(b)-92) where the deviation is up to 7% (i.e. about ±0.15 km/s).

The amplitude of reflected phases is governed by the reflection coefficient (r). For the normal-incidence case for compressional waves without any conversion to shear waves the reflection coefficient is: ρ V − ρ V r = 2 2 1 1 (3) ρ + ρ 2V2 1V1 where 1 is the density and V1 is velocity in the first (upper) media, and 2 and V2 are the density and velocity, respectively, in the second (lower) media. Frenje (2000) showed by numerical modelling that the reflection coefficient needs to be larger then 0.1 for producing detectable reflections under certain assumptions about noose. At low noise conditions, the reflection coefficient can be considerably less than 0.1. A typical reflection coefficient for a strong reflection is about 0.2, and the reflection coefficient for the reflection from the hard sea-floor is about 0.3 (Yilmaz, 2001).

Estimation of the resolution and accuracy in refraction seismic The resolution analysis of refraction seismic modelling is not as straightfor- ward as for near-vertical incidence reflection data. Some estimates of the resolution and modelling errors, relevant to the acquisition geometries and modelling techniques in this thesis are discussed below.

Error analysis of the refracted wave velocities

Figure 3.8. Travel-time curves for a refractor with the modelled velocity Vmod and V’ and V’’ velocities representing the limits of the time uncertainty dt.

40 The velocity of the refracted wave (Vmod ) can be derived from the ratio of a travel-time segment length (L) and time difference (t) (Figure 3.8). Using this ratio, the velocity uncertainty can be analytically derived as

V ' −V ' ' dtL dV = = (4) 2 (Δt 2 − dt 2 ) were dt is the time uncertainty. The dependence of the velocity uncertainty versus the travel-time segment length are given on Figure 3.9 for the range of the modelled velocities over the TransArctic 1989–1991 and the Arctic– 2000 profiles and for dt=0.1 s, about half the period of the dominant wave length. The velocity (Vmod) is assumed constant along the segment L along the refracting boundary. However, the RMS misfit of the modelling was less than dt=0.1 s for most of the interpreted layers (see table 2 in the Paper IV), i.e. for an accu- rate velocity interpretation a shorter length of the travel-time segment was required. Examples of sensitivity analysis for two shot points on the TransArctic 1989–1991 model are shown on Figure 3.10 and Figure 3.11. The analysis has been done as described in Grad et al. (2003). Travel-time curves are computed for V=6.1 km/s (Figure 3.10) and V=8.0 km/s (Figure 3.11) and velocity variations of +0.2 km/s and –0.2 km/s for both. It is clear from those examples that modelled velocities have uncertainties less than ±0.2 km/s and in good agreement with analytical estimations by equation (4) as shown in Figure 3.9.

0,7

0,6

0,5 v= 3. 7

s v= 5. 0 0,4 v= 6. 1 v= 6. 7 0,3 v= 7. 0 +/- dV, km/ dV, +/- 0,2 v= 8. 0

0,1

0,0 0 102030405060708090 L, km

Figure 3.9. The velocity uncertainty versus the length of the travel-time segment are calculated using equation (3) and dt=0.1 s for the modelled velocities on the TransArctic 1989–1991 and the Arctic–2000.

41

Figure 3.10. Velocity sensitivity analysis for the Trans-Arctic 1989–1991 final model, shot point 4 TRA-1991 (fig. 6, Paper IV). The calculated travel-time curves from the modelled layer with V=6.1 km/s and ±0.2 km/s velocity perturbation are given with the observed seismic wave field. The horizontal scales show the location in the full model (Figure 4.3).

Figure 3.11. Velocity sensitivity analysis for the Trans-Arctic 1989–1991 final model, shot point 22 TRA-1991 (fig. 7, Paper IV). The calculated travel-time curves from the modelled layer with V=8.0 km/s and ±0.2 km/s velocity perturbation are given with the observed seismic wave field. The horizontal scales show the location of the example in the full model (Figure 4.3).

42 Summarizing the velocity error analysis: 1) Velocity uncertainties for the TransArctic 1989–1991 and Arctic–2000 models derived by forward modelling are on the order of about ±0.1 km/s within a layer for the relatively simple crustal structures and it is in good agreement with analytical estimates. However, small lateral velocity variations can not be resolved due to the large spacing between the observation points. 2) The presented analysis can be used for estimating the velocity uncertainties for a similar group of models.

Spatial resolution of refracted arrivals The interface Fresnel zone of head waves The resolution limits of refracted waves can be analytically estimated using the Fresnel zone concept of the head wave as described in Kvasnika and erven (1996).

Figure 3.12. Boundary of the Fresnel volume of the head waves (after Kvasnika and erven, 1996). S is the location of a seismic source, R the receiver location and X the distance between those. L is the length along the refracting boundary W. It is assumed that the velocities in the two layers are V1

(6) and for the high-frequency approximation equation (6) may be simplified to

λ l 1 1 (7) 1 − n 2

43 Kvasnika and erven, (1996) provide calculations for equations (6) and (7) for various frequencies. They found, that using equation (7) instead of (6) generates an error for finite frequencies; calculated by (7) is about 30% larger for f = 10Hz in comparison with calculations using exact equation (6). The transverse semi-axis r+ of the interface Fresnel zone is 1 r+ λ X . (8) 2 2

The penetration distances D and D* below and above interface W (Figure 3.12) are 1 D λ L (9) 2 2 and 1 λ D* 1 . (10) 4 (1 − n2 )

In equations (5) to (10) the subscripts ‘1’ and ‘2’ represent the upper and lower layers respectively; λ is the wavelength of the pulse, V is the velocity in the layer, f is the frequency, l is the length of the one-way ray-path through the layer, X is the distance between source and receiver (Figure 3.12), and n = V1/V2 is the refraction index.

Using these equations it is possible to estimate the resolution of the refracted waves with frequency of 6 Hz for the TransArctic and Arctic–2000 models: • Horizontal resolution can be approximated as two times that of . It ranges from 12 km for the Moho boundary at a depth of 20 km to about 20 km for a 35 km deep Moho. The horizontal resolution for refracting boundary with velocity of 6.1 km/s is about 4 km. • Vertical resolution can be characterized by two parameters: 1) r+ near the edges of the refracting boundaries (points A and B in Figure 3.12) and 2) D+D* in general along the surface. r+ is about 4 km and 6 km for the Moho at 20 km and 35 km depth, respectively, and about 1.5– 2.0 km for mid-crustal boundaries. D+D* is about 1 km for all refraction boundaries.

Accuracy of the modelling of the refraction seismic boundaries Preliminary estimation of the modelling error of the depths to boundaries can be done analytically using e.g. the equation

44 t V V z = i 1 2 (11) 2 − 2 1/ 2 2(V2 V1 ) where z is the refractor depth, ti is the intercept time, and V1 and V2 the velocity in the layers above and below the refractor, respectively. Using equation (11), the estimation of errors for the depths of the boundaries can be done in two ways: 1) Perturbations in the depth of the boundary for various velocities (Figure 3.13) can be computed using (11) for various depths and velocities. The deviations used for the parameters were dV = 0.1 km/s and dt = 0.1 s (reasons for these values were discussed above).

1,8 1,6 1,4 v=5,1 m 1,2 v=6,1 v=6.7 1,0 v=7,0 0,8 v=8,0 0,6 v=5, basins + / - depth error, k error, - depth / + 0,4 v=3.5 0,2 0,0 0 5 10 15 20 25 30 35 40 depth, km

Figure 3.13. Error of the boundary depth versus depth for layers with different velocities in km/s (v) on the TransArctic and Arctic–2000 models using equation (11). 2) Maximum errors can be obtained by a logarithmic differentiation of equation (11) as

dt dV dV dz = z + 1 + 2 . (12) 2 − 2 2 − 2 ti V1 (V2 V1 ) V2 (V2 V1 )

Using dV1 = dV2 = 0.1 km/s and dt = 0.1 s in equation (12) the error of the boundary depth versus depth for layers with different velocities (V) are as shown on Figure 3.14.

45 5,5

5,0

4,5

4,0

3,5 m v= 5 3,0 v= 6 v= 6. 7 2,5 v= 7 Moho + / - depth error, k error, /+ - depth

2,0

1,5

1,0

0,5

0,0 0 5 10 15 20 25 30 35 40 depth, km

Figure 3.14. Error of the boundary depth versus depth for layers with different velocities in km/s (v) on the TransArctic and Arctic–2000 models using equation (12). As expected, the second method gives larger values for the errors. The errors related to the depth to a layer with a velocity V=6.7 km/s is markedly worse than the others due to a lower velocity contrast (the velocity is about 6.3–6.5 km/s above it). This layer has been identified in the East-Siberian margin only. This sensitivity analysis gives some explanation for the large difference in depths to this layer (c. 10km) between models by Zamansky et al. (1999) and the ones presented in this thesis (see fig. 9, Paper IV). Note, that the model by Zamansky et al. (1999) was constrained using first arrivals only, whereas the boundary between the layers in the model presented here have been constrained using refracted and reflected waves together, and should therefore be a more reliable depth estimate. Examples of sensitivity analysis like in Grad et al. (2003) are shown for the two shot points on the TransArctic 1989–1991 on Figure 3.15 and Figure 3.16 for the Moho boundary at the depth of c. 20 km and c 35 km, respectively. The entire boundary has been systematically shifted up and down for ±1 km and ±3 km. Similarly, an example of a sensitivity analysis for a small segment of the Moho boundary at the depth of c. 20 km is shown on Figure 3.17, the same shot point as in Figure 3.16. The segment length chosen was c. 10 km, similar to the spacing between receivers. The depth of the segment is varied for ±1.5 km.

46

Figure 3.15. Depth sensitivity analysis for the Trans-Arctic 1989–1991 final model, shot point 1 TRA-1991 (fig. 6, Paper IV). The calculated travel-time curves for the modelled Moho depth at about 35 km, ±1 km and ±3 km depth perturbation for the entire boundary are given with observed seismic wave field and picked arrivals; horizontal scales show the location of the example in the full model (Figure 4.3).

Figure 3.16. Depth sensitivity analysis for the Trans-Arctic 1989–1991 final model, shot point 22 TRA-1991 (fig. 7, Paper IV). The calculated travel-time curves for the modelled Moho depth at about 20 km, ±1 km and ±3 km depth perturbation for the entire boundary are given with observed seismic wave field and picked arrivals; horizontal scales show the location of the example in the full model (Figure 4.3).

47

Figure 3.17. Depth sensitivity analysis for the small segment of the Moho boundary (the Trans-Arctic 1989–1991 final model), shot point 22 TRA-1991 (fig. 7, Paper IV). The calculated travel-time curves for the modelled Moho depth at about 20 km, ±1.5 km depth perturbation are given with observed seismic wave field and picked arrivals; horizontal scales show the location of the example on the model (Figure 4.3).

It is possible to conclude that the mean depth of a large segment of the modelled boundary for a quite simple structure can be estimated with some confidence. These estimates are similar to those on Figure 3.13. However, depth deviations of a small segment of the boundary may be large, as given by equation (12). The sensitivity analysis does not take into account the possible errors in the velocities above the refractor; it is somewhat optimistic, thus the maximum errors analysis provided by the logarithmic differentiation is not unreasonable.

Finally, it is worth noting, that the presented models do not contain structures smaller than the Fresnel zones or the spatial density of the observations.

48 4. Summary of Papers

Paper I: Velocity Structure and Correlation of the Sedimentary Cover on the Lomonosov Ridge and in the Amerasian Basin, Arctic Ocean. Motivation The velocity parameters of the sedimentary cover in the Podvodnikov and Makarov basins and East Siberian continental slope are known only locally. This information provides the basis for interpretation of the main sedimentary units in these areas. Controlling the velocity structure of the sedimentary cover along the main seismic profiles has been an essential prerequisite for interpretation of the deep structure of the Basin, which is based on wide- angle refraction profiles (Papers III and IV).

Methods The long-offset (2.0–6.5 km) reflection seismic data, together with the reflection seismic profiles, were acquired over the Podvodnikov and Makarov basins, East Siberian continental slope and the Lomonosov Ridge, from drifting ice. Each profile was about 100 km long. The long-offset reflection seismic data provided information about velocity variations in the sediments along the profiles, with a spacing of 3–6 km. It allowed estimation of the mean interval velocities for the main sedimentary units that were identified over the four morphological provinces. The drifting ice-station NP-28 crossed all these provinces and continuously collected reflection seismic data on the way. This gave an opportunity to correlate the main reflectors in the sediments over a wide area and identify the main sedimentary units. The results of the IODP ACEX 2004 drilling on the Lomonosov Ridge provided the basis for estimating the ages of the units.

Results and conclusions In the four areas selected for the study, three main stratigraphic units were recognized – I, II and III. In general, the interval Vp velocity of Unit I is 1.7 – 1.9 km/s, that of Unit II, 2.0 to 3.0 km/s and Unit III, 3.0 – 4.0 km/s (mainly). Interval velocities show a variation of up to 4%, except for over the Lomonosov Ridge, where unit I shows a variation of 7%. Units I and II compose the Cenozoic section. Unit I has a fairly uniform thickness of c. 300–400 m over most of the described area and is lying con- formably on Unit II. Unit II has various thicknesses up to c. 1.5 km towards the East Siberian slope and in the Makarov Basin and wedging out over the

49 Arlis Rise. Units II and III are separated by a well pronounced composite reflection packet 'A'. The reflection package is widespread in the Amerasia Basin and on the Lomonosov Ridge, it marks the base of the Cenozoic succession, lying unconformably, on the irregular surface of the acoustic basement. Unit III has various velocities and composition of Mesozoic and perhaps older strata. A prominent structural high beneath the Cenozoic sediments of the Makarov Basin appears to be a southerly continuation of the Marvin Spur, i.e. a fragment of continental crust rifted off Lomonosova (subsequently, today's Lomonosov Ridge) in the Mesozoic. Similar fragments of continental crust may exist beneath the Podvodnikov Basin and Alpha and Mendeleev ridges.

Aldona Langinen processed the reflection data which were essential for interpretation of the refraction data of the deeper structure (see papers about those below). My contributions to the presented research were participation in interpretation of seismic data and correlation of the sedimentary units with the ACEX results; I wrote some parts of the chapters about results and con- cluding remarks. I was the corresponding author for the paper.

50 Paper II: Correlations between the Lomonosov Ridge, Marvin Spur and adjacent basins of the Arctic Ocean based on seismic data. Motivation Reflection seismic data were continuously collected over the Arctic Ocean over a distance of about 4000 km by the Soviet drifting ice-station NP–28 in 1987–1989. The NP-28 crossed the central part of the Lomonosov Ridge three times. In combination with other seismic data in the area, new gravity and magnetic data and the ACEX drilhole results, it was possible to suggest some stages in the tectonic evolution of the central part of the Arctic Ocean.

Methods The reflection seismic data were acquired continuously along a crooked line from the drifting ice-station on the two receiving-arrays, each c. 550 m long, and arranged orthogonally. A shot point was located at the vertex of arrays and fired every 2-4 hours; in order to obtain seismic data at c. 0.5 km intervals. The collected data were processed by shot-gather; interpretation of the seismic time-sections are presented in the paper.

Results and conclusions The three segments of the NP-28 profile interpreted in the paper provide images not only of the Lomonosov Ridge and Marvin Spur, but also of rela- tionships to the adjacent Makarov and Amundsen basins. The similar character of the main reflectors the ACEX drill cores allowed correlation of the main four sedimentary units over the whole area. The uppermost Unit I (about 200 m thick) is composed of deep marine mud and distal turbidites of mid Miocene (c. 14 Ma) and younger ages; Unit II spans the late Eocene to the early Miocene and is composed of estuarine to freshwater sediments; Unit III (characterized by the highly reflective package 'A', included in Unit III in Paper I), is of early Eocene and late Palaeocene (and probably also late Cretaceous) ages and apparently dominated by shallow marine siliciclastic strata; and Unit IV and IV' (they are unconformably underlain Unit III) are of variable character, probably mainly of Mesozoic, but also perhaps Palaeo- zoic and older age (different in different regions), underlain by higher velocity acoustic basement. The presented interpretation suggests that the Marvin Spur is a sliver of thinned continental crust rifted off the Lomonosov Ridge. The similar features can be traced continuously into the Makarov Basin; this would imply that the Basin had a Mesozoic (perhaps earlier) rifting and graben history, followed by uplift and erosion in the late Mesozoic and then deposition of Palaeocene–early Eocene shallow marine successions, prior to rapid subsi- dence to present depths (Figure 4.1). Spurs in the Makarov and Podvodnikov basins of similar morphology to the Marvin Spur suggests that this part of the Amerasia Basin was influenced, but not controlled by major transcurrent

51

Figure 4.1

52 Figure 4.1. Reflection seismic data, potential fields and seismic model of the crust beneath the Makarov Basin, showing the basement highs. (a) Bathymetry map enlarged from the Figure 1 with location of profiles. Lines 1–2 and A–A' show parts of the NP-28 profiles presented on the figure; white dotted lines show the probable continuation of the Marvin Spur and similar features. (b) Reflection time section of TRA(b)–90 (Paper I) and free air gravity anomalies along the profile (Cochran et al., 2006). (c) Part of NP–28 seismic time section showing basement high 'LMS' (Paper II). (d) Part of NP–28 seismic time section showing basement high 'MS' (Paper II). (e) Gravity and magnetic anomalies (Figure 4.3). (f) Seismic model (Figure 4.3). Dashed pink line (III?) marks the possible Layer III on 'MS' high (below modelling resolution thickness). (g) Reflection depth section (Figure 4.3). On the seismic sections, letters refer to the inferred surfaces of the main reflection boundaries within the sedimentary cover.

(transform) faulting. Differences in character of acoustic basement in the Amundsen Basin, in combination with magnetic data, suggest that the oldest spreading magnetic anomaly is C24. The previously dated anomaly C25, on the Canadian side of the Basin is thought to be related to a linear rift-related mafic intrusive complex along the flank of the Lomonosov Ridge.

Aldona Langinen processed the reflection data which were essential for interpretation of the refraction data of the deeper structure (see papers about those below). My contributions to this paper were participation in interpretation of seismic data and correlation of the sedimentary units with the ACEX results. Visualisation of the results and writing the second part of the article (from the results to conclusions) were done by me with the assistance of my co-authors. I was the corresponding author for the paper.

53 Paper III: Seismic Profiling across the Mendeleev Ridge at 82°N: Evidence of Continental Crust. Motivation The Mendeleev Ridge is a substantial part of the so-called Alpha-Mendeleev Large Igneous Province. The origin of the Province and the Ridge, in particular, are highly debatable. Determination of the crustal structure and origin of the Mendeleev Ridge was the main goal of the research presented in this paper.

Methods The Arctic–2000 crustal-scale refraction seismic profile, with maximum offsets of 200 km, crossed a central part of the Mendeleev Ridge; it was combined with shallow reflection seismic and gravity measurements, and bottom samples were collected by piston coring and dredging. The refraction seismic data were interpreted by forward modelling with incorporation of reflection seismic data for the upper part of the model. Interpretation of the crustal structure of the derived model was carried out together with potential field data and the results of the geological sampling.

Figure 4.2. Potential fields and seismic model of the crust along the “Arctic-2000” geotraverse. Concerning the potential field profiles: GF – Free Air anomalies; GB2.3 – 3 Bouguer anomalies ( = 2.3 g/cm ); GB2.3-reg - regional Bouguer anomalies; Ta - magnetic anomalies. Concerning the model of the crust: thick lines show seismic boundaries; thin lines are velocity isolines through 0.2 km/sec; the dotted line is the inferred upper boundary of the crust-mantle mixed layer. Roman numerals mark the seismic sequences of the sedimentary cover and layers of the consolidated crust; letters mark the seismic boundaries; triangles show the locations of explosions.

54 Results The crustal thickness below the axis of the Mendeleev Ridge is up to c. 32 km, and it thins to 20 km below the Podvodnikov Basin and to 13 km under the Mendeleev Basins (Figure 4.2). The Mendeleev Ridge is composed of sediments in the uppermost part of 1.5–2.0 km thickness and up to 3.5 km thick towards the Podvodnikov Basin. The underlying basement is divisible into three main layers, an upper one (ca. 4 km) with a Vp velocity of 5.0–5.4 km/s, a middle one (c. 4 km) with 5.9–6.4 km/s Vp velocity and a lower one (c. 19–20 km) ranging in Vp velocity from 6.7 to 7.3 km/s. Separating these from the mantle (Vp velocity 7.9–8.0 km/s), is a layer (up to 7 km thick) of ‘mixed’ crust–mantle composition with Vp from 7.4 to 7.8 km/s.

Conclusions The velocity structure and thickness of the Mendeleev Ridge crust is consistent with highly attenuated and underplated continental or thickened oceanic crust. The geological results of the Arctic–2000 expedition suggested that the upper units of the consolidated crust (Vp of 5.0–5.4 km/s) are composed of platform sedimentary rocks. The higher velocity of the lower crust may be related to the formation of the Large Igneous Province. This favours the conclusion that the Mendeleev Ridge is composed, at least in part, of attenuated underplated continental crust.

My contributions to this paper: I participated in the seismic data acquisition, processed and modelled wide angle seismic data. Interpretation and visualisation of results and writing the article were mainly done by me, with the assistance of my co-authors. I was the corresponding author for the paper.

55 Paper IV: Crustal structure of the East Siberian Continental Margin, Podvodnikov and Makarov basins based on refraction seismic data (TransArctic 1989–1991). Motivation The main goal of the crustal-scale seismic research, integrated with other methods, was to define the crustal structure beneath the East Siberian continental shelf and the Podvodnikov and the Makarov basins. The presented research is a re-interpretation of the refraction seismic data of the entire TransArctic 1989–1991 profile, combined with reflection seismic results and other geophysical data from the area.

Methods The c. 1500 km long TransArctic 1989–1991 wide-angle seismic profile was acquired over a vast part of the Arctic Ocean. Long-offset (up to 270 km) refraction seismic data provided images of the crust down to the Moho discontinuity. Refraction data were interpreted by forward modelling in combination with reflection seismic results for the upper part of the model. Geological evidence and potential field data were incorporated into the interpretation.

Results A four-layer crust has been identified throughout the TransArctic 1989–1991 Geotransect. Layer I, mainly constructed using reflection seismic data, is composed of three main sedimentary units, with velocities increasing downwards: V=1.7–1.9 km/s (Unit 1), V=2.0–3.0 km/s (Unit 2), V=3.5–4.0 km/s (Unit 3);

Figure 4.3. Potential fields and seismic model of the crust along the Geotransect (TransArctic 1989–1991). Cross-points with Arctic–2000 (Paper III) and NP–28 (Paper II) profiles are indicated. a) Free Air gravity anomalies (GF) are shown by red line; magnetic anomalies (T)a by blue line. b) In the model with a vertical exaggeration (V.E.) of 8:1, thick lines show seismic boundaries; thin lines are velocity isolines with intervals of 0.1 km/s. Roman numerals mark the sedimentary cover and layers of the consolidated crust; letters mark the seismic boundaries; triangles show the locations of shot points. Arabic numbers in ovals are velocities in sediments, derived from TRA(b) reflection seismic data (Paper I); other Arabic numbers are velocities at the top of seismic layers, derived from refraction seismic data. Grey arrows show location of 1-D models on the fig. 9 in Paper IV. c) The model (b, above), with vertical and horizontal scale equal. d) Depth section of sedimentary cover (Units 1, 2 and 3) which constitute Layer I along TransArctic 1989–1991 seismic profile. The data are combined from reflection seismic data with TransArctic–1991 (TRA-1991), TransArctic–1990 (TRA-1990) and drifting ice-station NP–28 (Paper I and II). Red lines and related letters mark the main reflection boundaries within the sedimentary cover; interval velocities are in ovals (in km/s); blue dashed line marks the boundary to the multiple reflections.

56

Figure 4.3

57 Layers II-IV, modelled on refraction seismic data, have P-wave velocities of 5.0–5.4 km/s, 5.9–6.5 km/s and 6.7–7.3 km/s, respectively. The East Siberian margin has a c. 40 km thick continental crust, mainly composed of layers III and IV, both c.15 km thick. Beneath the Podvodnikov Basin, the Moho depth varies from c. 20 km bsl at the southern and northern ends to c. 30 km bsl at the centre beneath the Arlis Gap. The edge of the Alpha-Mendeleev Ridge, separating the Podvodnikov and Makarov basins, has a crustal thickness of c. 25 km, mainly composed of layers III and IV. In the Makarov Basin, the crust has a thickness of 8–15 km.

Conclusions The uppermost part of the sedimentary succession (Units 1 and 2), of probable Cenozoic age, can be recognised on reflection data from the Makarov Basin across the Podvodnikov Basin to the continental slope, north of the East Siberian shelf. Correlation of the TransArctic reflection profile with published MCS data both over the Siberian shelf and the Lomonosov Ridge with the ACEX drillhole stratigraphy indicates that the deepest part of the sedimentary succession (Unit 3, below reflection 'A') is probably dominated by Mesozoic formations. Layer II can be formed by older sedimentary rocks similar to those on the shelf and possibly also mafic volcanics in the basins; Layer III and Layer IV compose the crystalline crust. Below the De Long Plateau on the East Siberian Margin, Layer II is composed of Late Mesozoic and/or Paleozoic successions (Sekretov 2001). The thinning of the continental crust beneath the continental slope is characterized by major faults, pronounced thickening of Unit 3 (Layer I) and local absence of Layer III. The geophysical data alone allow different interpretations of the Podvod- nikov Basin structure. Taken in combination with the geological evidence, an origin is favoured by N–S extension (stretching factor of c. 2.5) of continental crust during the Mesozoic, perhaps starting in the Jurassic during the closing of the Angayucham (South Anyui) Ocean. Transcurrent faulting along the Lomonosov margin of the Amerasia Basin (the Amerasia Basin Transform Fault) may have accompanied this Podvodnikov longitudinal (present coordinates) extension. If so, it was partly obscured by late Cretaceous and Cenozoic rifting and latitudinal extension during opening of the Eurasia Basin. Similarity of velocities and the ratio (1 : 2.5–3.0) of the thicknesses of Layers III and IV below the Arlis Gap and the slope of the Alpha-Mendeleev Ridge suggest that these features may be of similar origin. The thin crust beneath the 4-km-deep Makarov Basin varies greatly in composition. Some parts may well be of oceanic in origin (8–12 km thick), but the Basin includes c. 15 km thick fragments of thin continental crust, related to the Marvin Spur, rifted off the Lomonosov Ridge. My contributions in the presented research: I modelled the whole set of wide angle seismic data. Interpretation and visualisation of the results, writing of the article were mainly done by me with assistance and geological in-put of my co-authors. I was the corresponding author.

58 5. The central part of the Lomonosov Ridge (HOTRAX) – first view

The central part of the Lomonosov Ridge near the North Pole between 88– 89o N includes an interior flat-bottomed basin (the Intra Basin of Bjork et al., 2007) at c. 2.6 km bsl. It is about 100x30 km in area and elongated along the Ridge axis, surrounded mainly by c. 1.5 km bsl ridges (Figure 5.2). The geometry of the basin suggests it was created by dextral movement along faults parallel to the Ridge axis. The six seismic lines (Lines 39–44) of the HOTRAX MCS data have been processed, as described above, and a preliminary interpretation has been made by using ACEX results. Arrows on the time sections mark the correlated reflection horizons and their colour shows how they are correlated along the ridges into the Intra Basin. The nomenclature of the main reflections and units are used as in Paper II. For correct comparison of the HOTRAX data with the reflections from the NP–28 seismic sections, similar filtering parameters need to be applied (Figure 5.1).

The surrounding ridges The surrounding ridges of the Intra Basin on the Amerasian side have steep slopes (up to 10o–20o dip). Correlation of reflection horizons were started from Line 40, this being the closest line to the ACEX drillholes; it has also better quality data and clearer reflectivity. The interval velocities have been estimated on this line (Figure 5.3). Unit I has a thickness of 0–0.15 km and velocity of c. 1.6 km/s. A pronounced flat reflection on the top of the ridge is correlated with the ACEX hiatus at 16–44 Ma (Figure 2.4) and ‘d1’ on the NP–28 reflection seismic sections. Unit II has low- frequency and low amplitude reflections and velocities of c. 1.8 km/s; the reflections lie unconformably at low-angles to the overlying ‘d1’. This character of the reflectivity (or transparency of the unit) is typical for a formation with ooze that was observed in the ACEX drill cores. The composite reflection ‘A’ defines the boundary between the Cenozoic and Mesozoic sediments.

59 Figure 5.1

60

Figure 5.1. Fragments of the NP-28 (c) and Line 44 (b and c) time sections. On b) higher frequency filters {f1} as described in the processing steps for the HOTRAX data are applied to the time section; on a) and c) lower frequency filters {f2} (Table 3.1) are applied to the NP-28 and Line 44 time sections. The major fault separating the surrounding ridges from the Intra Basin, as seen on line 40, appears to have a roll-over anticline in the hanging wall, suggesting that the basin was created by a growth fault; also, there exists the possibility of a hydrocarbon trap in the anticline. The same reflections as described for Line 40 are marked on the other lines across the ridges (Figure 5.4). However, correlation of reflections on Line 41 (Figure 5.5) is tricky, since some of the pronounced reflections at 2– 3 s on the time sections might be related to out-of-plane reflections. 3-D modelling of the reflections from the sea-floor may aid in the interpretation of Line 41.

Figure 5.2. Bathymetry map (Jackobson et al. 2008) of the central part of the Lo- monosov Ridge and research tracks over the area (see Figure 3.3 for the area location). Lines 38–44 are HOTRAX MCS lines, dotted lines on those are lines for projection of stacked time sections; stars on those are deployment locations of the sonobuoys; SB 86, 92&93 are sonobuoys used for estimation of velocities. Approximate locations of AWI 90090 and NP-28 reflection seismic profiles are marked by dashed lines. ACEX (Backman et al. 2006) and 94-PC27&29 (Grantz et al. 2001) geological sampling sites marked by V.

61

Figure 5.3. Time section along Line 40. The main reflections in accordance with the NP-28 nomenclature are marked by arrows; numbers are related to estimated interval velocities. Inserted diagrams (Hyne, 2001) illustrate a possible mechanism of the creation the Intra Basin by a growth fault.

Figure 5.4. Time section along Line 39; numbers are related to estimated interval velocities using SB 85. Enlarged parts of the section on Figure 5.6 are marked by rectangular.

62

Figure 5.5. Fragment of time section along Line 41. Arrows mark possible out-of- plane reflections.

The Intra Basin The Intra Basin succession is characterized by a c. 0.7 km thick section of Cenozoic sediments with velocities less then 2.3 km/s. An erosional surface in the middle of the Cenozoic succession is observed in the south-eastern part of the basin. Evidence of sediment slides can be observed near the ridge slopes (Line 43, Figure 5.7). The end of Line 39 (Figure 5.6) in the basin probably illustrates the most complete section of the Cenozoic sediments. Downlap patterns of seismic reflections (Line 39) in the middle and late Cenozoic succession suggests that the western (Siberian) part of the basin was filled mainly from a north-west direction; by contrast, the main source of sediments in the eastern part (Canadian) was from the south-east. Older sediments have various thicknesses (from 0.5 to about 1.5 km), velocities (2.6-3.9 km/s) and show gentle deformation. Seismic lines over the Intra Basin, parallel to the axis of the Lomonosov Ridge, illustrate gently folded basement at about 4.5 km bsl; on the surrounding ridges this surface is at c. 3 km bsl. Intra basement reflections are usually parallel to the upper surface and in combination with higher velocities (c. 4–5 km/s), suggest old well-consolidated sediments.

63 a)

b) Figure 5.6. Fragment of the time section along Line 39 (Figure 5.4) without (a) and with (b) interpretation of reflections.

Figure 5.7. Time section along Line 43 with interval velocities (numbers) and interpretation. Erosive surface are marked by dark blue on lower panel.

64 6. Sampling the sedimentary cover and bedrock of the Arctic Basin.

The Amerasia Basin has a complex origin; alone, the geophysical data can support very different hypotheses (Figure 6.1). For understanding the tectonic evolution of the Basin and origin of the ridges and troughs we need to collect geological data. Bedrock samples from key locations are especially needed, with full video or photo documentation of the sampling for avoiding later debates about whether bedrock or ice-drift was collected. Full stratigraphic sections though the Cenozoic and older sedimentary successions are needed at key locations for understanding the tectonic evolution of the Ba- sin. The depositional environment of the composite reflection package ‘A’ related to Cenozoic shallow water environments, as recorded in the ACEX drillholes, needs to be investigated in other locations. We will then be able to better define the nature of particular morphological features and construct more reliable models of the Amerasia Basin, in general. Based on the research presented in this thesis, some key sites are suggested for geological sampling (drilling and/or gravity piston coring), where there is good coverage of geophysical data and access to particular geological formations (Figure 6.2 and Table 6.1). The Mendeleev Ridge sites have already been presented at a workshop for the Arctic drilling proposals (Lebedeva-Ivanova, 2008).

65

Figure 6.1. Seismic P-velocity versus density at a pressure of 200 MPa for common crystalline rocks (after Salisbury et al. 2003) with velocity variations over the main morphological features on TransArctic 1989–1991 and velocity-density (Paper III and Astafurova et al., 2006) variations over Arctic–2000. Ellipses have areas corresponding to standard deviations of velocity and density. Black dots are values, taken from Christensen and Mooney (1995); BPP - prehnite–pumpelliyite facies basalt, BGR - greenschist facies basalt.

66

Figure 6.2. Map of the presented seismic research and proposed sites for drilling.

67 Table 6.1. Suggested sampling sites (see Figure 6.2 and some comments about sites below) Site; Some references; Goal What data were collected on the site? What to do? location; depth bsl. Figures in the thesis L1; Consolidated bedrock 1. HOTRAX* Line 40 reflection seismic. Gravity piston- Cochran et al., 2006; central part of the of Mesozoic age and 2. SCICEX** cruises. cores, lateral Brozena et al., 2003; Lomonosov Ridge; maybe older from the 3. Detailed aeromagnetic surveys (Brozena et al., 2003) 'profiling' Grantz et al., 2001; c. 2.0-2.5 km scarp Figure 5.3 L2; Full stratigraphic 1. HOTRAX*: Drilling Cochran et al., 2006; central part of the section of Cenozoic Line 39 reflection seismic; piston core. up to c.1 km Brozena et al., 2003; Lomonosov Ridge sediments and maybe 2. SCICEX** (hb9801 profile) below sea-floor Figure 5.6 (‘Intra’ Basin); older 3. Detailed aeromagnetic surveys (Brozena et al., 2003) c. 2.7 km MS1&2; 1. Stratigraphic section 1. NP-28, profile BC and DC. Drilling Cochran et al., 2006; the Marvin Spur; of probably Cenozoic 2. AWI-98511&98510 profiles are near by up to c.0.6 km Brozena et al., 2003; c. 1.5 km and sediments. 3. SCICEX** (Pargo5 profile) below sea-floor. Jokat, 2005; 4. Detailed aeromagnetic surveys (Brozena et al., 2003) Gravity piston- Paper II (fig. 3&4b); c.2.1–2.3 km 2. Consolidated bed- cores, lateral rock from the scarp 'profiling' or shallow drilling. M1; Full stratigraphic 1. Refraction /reflection seismic TrancArctic-1990 Drilling Cochran et al., 2006; central part of the section of Cenozoic profile. up to 2 km below Brozena et al., 2003; Makarov Basin; sediments and maybe 2. Reflection seismic NP-28 profile. sea-floor Paper II (fig. 9); c. 3.8 km older 3. SCICEX** (Cavalla3 profile) Figure 4.1 4. Detailed aeromagnetic surveys (Brozena et al., 2003) M2; Consolidated bedrock; 1. Refraction /reflection seismic TrancArctic-1990 Shallow drilling Cochran et al., 2006; central part of the defining nature of profile. or gravity piston- Brozena et al., 2003; Makarov Basin layer with P-velocity 2. Reflection seismic NP-28 profile. cores Paper IV (fig. 10); (continuation of the 5.2 km/s 3. SCICEX** (Cavalla7 and hb9801 profiles). Figure 5.6 Marvin Spur); 4. Detiled aeromagnetic surveys (Brozena et al., 2003) c. 3.8 km

A1; Consolidated bedrock, 1. Refraction/reflection seismic TransArctic-1990 pro- Drilling Cochran et al., 2006; slope of the Alpha defining nature of file. Brozena et al., 2003; Ridge; layers with P-velocity 2. Reflection seismic NP-28 profile is near by Paper IV; c. 3.8 km 5.2 and maybe 6.2 3. SCICEX** (Pargo2, Cavalla5 (?) profiles) is near by. Figure 4.1 km/s 4. Detailed aeromagnetic surveys (Brozena et al., 2003) MR1; Consolidated bedrock; 1. Refraction /reflection seismic Arctic-2000 profile and Shallow drilling; Kaban'kov et al., 2004; slope of local high defining nature of gravity measurements at points of seismic observations. or maybe gravity Lebedeva-Ivanova, on the Mendeleev layers with P-velocity 2. Arctic-2000: bottom samples were collected by pis- piston-cores 2008; Ridge; 5.0 km/s ton coring and dredging. Paper III c. 1.5-2.0 km 3. HOTRAX*. (figs. 5a, 9 & 10); Figure 4.2 MR2; Consolidated bedrock; 1. Reflection seismic Shallow drilling Paper III (fig. 5b); slope of the Men- defining nature of NP-26 profile. or gravity piston- deleev Ridge; acoustic basement 2. AWI-2008 reflection seismic profile (?) cores c. 3.0 km HOTRAX* expedition collected 'chirp' subbottom profiles, swath bathymetry, gravity field data on the way and reflection seismic data (where it was possible). SCICEX** expeditions collected 'chirp' subbottom profile, swath bathymetry and high resolution gravity field data on the way.

Some additional remarks on the Table 6.1. All sites presented here fit well with most of the Site Survey Data Require- ments (IODP) for a drilling site (www.ssdb.iodp.org/documents/site-survey- data.php).

L1 – Central part of the Lomonosov Ridge. – Older history. The MCS Line 40 (HOTRAX’05) illustrates near horizontal sedimentary strata of probable early Cenozoic and older age. These outcrops are covered by very thin soft sediments on a scarp of the Ridge, dipping 18-20o into the ‘Intra’ Basin. It should be possible to sample the different formations and determine their age by systematically piston coring down the slope of the scarp (like in Grantz et al., 2001).

L2 – Central part of the Lomonosov Ridge. – The 'Intra' Basin history. The end of the MCS Line 39 (HOTRAX’05) into the 'Intra' Basin illustrates the presence of a thick undisturbed sedimentary section of low- velocity sediments of Cenozoic age and possibly older. Based on SCICEX high resolution gravity data, Line 39 is located in the area of deepest basement, where the most complete sedimentary succession has accumulated.

MS1&2 – Marvin Spur. – Comparison with the Lomonosov Ridge For testing the hypothesis that the Marvin Spur rifted off the Lomonosov Ridge we need to compare the low-velocity sedimentary sections and underlying bedrock from both features. The NP-28 seismic section DC (fig. 3, Paper II), shows an undisturbed sedimentary succession on the top of the Marvin Spur that would be suitable for sampling of Cenozoic sediments by drilling. The slopes of the Spur towards the Lomonosov Ridge dip about 10o and basement rocks are close to the sea-floor on the scarps on the BC and CD seismic profiles of NP-28. These can be sampled by gravity coring or shallow drilling. SCICEX ‘chirp’ data can be helpful for defining the sites more precisely and choosing the best method.

M1 – Central part of the Makarov Basin. – When did it form? To define the history of the Makarov Basin and the relationship of this feature to surrounding ridges and spurs, it is necessary to sample the sedimentary units in the Basin and compare them with the ACEX results from the Lomonosov Ridge and possible results from other sites, proposed above. The NP-28 reflection seismic section shows mostly horizontal thick sedimentary units and also has good geophysical data coverage by the SCICEX and TransArctic–1990 projects. This would be a suitable site for drilling and collecting the stratigraphic column of Cenozoic and probably older age and testing the composition of the ‘A’ reflection package.

70 M2 – Central part of the Makarov Basin. – Testing the continuation of the Marvin Spur. The NP-28 data set near to the M1 site provides information about out- cropping acoustic basement which Cochran et al., (2006) and Paper II suggested was a probable continuation of the Marvin Spur beneath the Makarov Basin. Samples of this basement will provide a basis for understanding the origin of several similar features in the Podvodnikov and Makarov basins, as traced by Cochran et al. (2006).

A1 – Slope of the Alpha Ridge. – Continental or oceanic? Sampling the bedrock. The origin of the Alpha Ridge is one of the most debatable subjects in the tectonic evolution of the Arctic Ocean. The model of the crust below the slope of the Ridge on the TransArctic–1990 profile suggests that not only acoustic basement, but even the layer with P-velocity of 6.2 km/s are close to the sea-floor on that slope. This provides an unique chance to drill the upper crust, sample this layer and define its origin. Unfortunately, the site area has less coverage of geophysical data than the other sites described above.

MR1 – Mendeleev Ridge. – Continental or oceanic? Sampling the bedrock. The origin of the Mendeleev Ridge is one more highly debatable subject. Was it formed together with and in the same way as the Alpha Ridge, or has it another geological history? Sampling of the consolidated rocks can answer these questions. The Arctic–2000 research expedition collected seismic and gravity data over the site and bottom samples were collected by piston coring and dredging. The collected data suggest there may be outcrops on the slope of the local high. HOTRAX collected 'chirp' subbottom profiles, and swath bathymetry over the same slope of the high. More details about that site are described in Lebedeva-Ivanova (2008).

MR2 – Slope of the Mendeleev Ridge towards the Mendeleev Basin. Acoustic basement near the surface of the Mendeleev Ridge is seen on the NP-26 reflection profile (fig. 5b in Paper III; Lebedeva-Ivanova, 2008). Sampling of this scarp will give information about the composition of the basement of the Ridge.

Siberian continental margin – Relationship to the Canadian margin. Geological sampling of the De Long Plateau is also needed. This area has the northernmost outcrops on the East-Siberian Shelf. Only very limited geological research and sampling have been carried out, mainly in the 1960- 1970's (Vol'nov et al. 1961, 1970; Vinogradov, 1975) and in the late 1980’s (Drachev and Saunders, 2006) on the Plateau.

71 Summary in Swedish

Den arktiska bassängen är en av de minst utforskade regionerna i världen, huvudsakligen beroende på att den tjocka fleråriga havsisen gör området svåråtkomligt. Samtidigt innebär den komplicerade och omtvistade geologin i kombination med en avsevärd ekonomisk potential att området är attraktivt för forskning. Den arktiska bassängen är indelad i den Eurasiska och den Amerasiska bassängen, separerade av Lomonosovryggen (Figur 1). Det råder en utbredd uppfattning om att den Eurasiska bassängen har skapats genom oceanbotten- spridning. Däremot är ursprunget för den Amerasiska bassängen fortfarande omtvistat. De reflexions- och refraktionsseismiska studierna som presenteras i den här avhandlingen är huvudsakligen lokaliserade inom den Amerasiska bassängen och den centrala delen av Lomonosovryggen (Figur 1). Reflexionsseismiska data från de sovjetiska projekten NP-28, TRA(b) och ”TransArctic 1989-1991” samt det ryska projektet ”Arctic-2000” visar sedi- mentens utbredning inom en stor del av den arktiska bassängen (artikel I och II). Tilläggas bör att mätningarna utfördes från den drivande havsisen. De seismiska mätningarna inom TRA(b) tillför även information om P- vågshastigheten i sedimenten (artikel I). Huvudenheterna i den sedimentära lagerföljden kan följas över hela undersökningsområdet. Tillgängliga geologiska data från den arktiska bassängens randområden samt resultat från ACEX-borrningarna inom den centrala delen av Lomonosovryggen (utförda inom ramen för Integrated Ocean Drilling Project) har gjort det möjligt att uppskatta ålder och sammansättning för de identifierade sedimentära enheterna. Genom tolkning respektive omtolkning av refraktionsseismiska data från Arctic-2000 (artikel III) och ”TransArctic 1989-1991” (artikel IV) har det varit möjligt att skapa modeller av jordskorpan ned till manteln. De refraktionsseismiska mätningarna tillför ingen information om de ytliga sedimenten med låga seismiska hastigheter och tolkningen av de refraktionsseismiska mätningarna är starkt beroende av tolkningen av de reflexionsseismiska mätningarna som nämns ovan. Vid tolkningen av de seismiska mätningarna har även magnetfälts- och tyngdkraftsdata samt geologiska observationer utnyttjats. Inom expeditionen HOTRAX utfördes under 2005 reflexionsseismiska mätningar över den centrala delen av Lomonosovryggen. Mätningarna visar den sedimentära lagerföljden samt strukturer i jordskorpan inom den interna bassängen och kringliggande tektoniska ryggar. Genom att även utnyttja data från ACEX-borrningarna samt data från speciella seismiska mätningar med flytbojar så har en tolkning av den geologiska utvecklingen inom området föreslagits.

72 Avhandlingens upplägg Innan de huvudsakliga resultaten i avhandlingen beskrivs så presenteras i kapitel 2 en översiktlig genomgång av den arktiska bassängens morfologi samt de huvudsakliga mönstren i magnetfältet och tyngdkraftsfältet, till- sammans med en kort historisk återblick över relevant geofysisk och geolo- gisk forskning inom undersökningsområdet. I kapitlet beskrivs även de olika modellerna som ställts upp för den tektoniska utvecklingen i den arktiska bassängen. Kapitel 3 beskriver olika aspekter av mätförfarande och databe- arbetning för de seismiska mätningarna i den speciella arktiska miljön, även inkluderande en feluppskattning. Kapitel 4 innehåller sammanfattningar av de fyra artiklar som ingår i avhandlingen. I kapitel 5 presenteras resultat från HOTRAX. Avslutningsvis presenteras i kapitel 6 förslag till lämpliga nyckellokaler vid vilka borrning och kärnprovtagning skulle kunna utföras för att testa de geofysiska modeller som presenterats i avhandlingen. Sammanfattning av de huvudsakliga resultaten i avhandlingen Modeller av jordskorpan baserade på seismiska undersökningar i kombination med magnetfälts- och tyngdkraftsmätningar över det Östsibiriska randområdet, Podvodnikov- och Makarovbassängerna samt Mendeleevryggen utgör ett ramverk för att förstå denna tektoniskt svårtolkade region (artikel III och IV). Jordskorpan under det Östsibiriska randområdet är upp till 40 km tjock, för att tunnas av till omkring 20 km inom Podvodnikovbassängen. Modellerna över Arlisgapet, vid mitten av Podvodnikovbassängen, samt Mendeleevryggen visar en ovanligt tjock jordskorpa (upp till 28-32 km tjock) samt en seismisk hastighetsstuktur som tyder på förekomsten av ett lager kraftigt uttunnad på stort djup kontinental jordskorpa. Inom Makarovbassängen är jordskorpan betydligt tunnare (8 till 15 km). De reflexionsseismiska mätningarna visar den sammanlagda tjockleken av samt de interna strukturerna i den sedimentära lagerföljden (huvudsakligen av kenozoisk eller senmesozoisk ålder), både vid ryggarna samt under trågen (artikel I och II). I artikel II presenteras belägg för att vissa av ryggarna (ex. Marvin Spur) består av fragment av kontinental jordskorpa som bru- tits loss från Lomonosovryggen (med ett liknande diskordant kenozoiskt sedimenttäcke), förefaller stupa flackt in under sedimenten, ex. vid Maka- rovbassängen. I synnerhet visar de reflexionsseismiska mätningarna i samband med expeditionen HOTRAX över den centrala delen av Lomonosovryggen den sedimentära lagerföljden både vid ryggens mest upphöjda del samt inom en intern bassäng. De huvudsakliga sedimentära enheterna kan urskiljas genom jämförelse med de tidigare omnämnda ACEX-borrningarna. Huvudförkast- ningen som separerar de intilliggande ryggarna från den interna bassängen förefaller ha speciella antiklinalstrukturer i hängväggen som tyder på att bassängen skapades genom en tillväxtförkastning. De seismiska mätningarna visar belägg för ett mjukt veckat kristallint underlag under Lomonosovryg- gen med interna strukturer som är parallella med överytan, vilket i kombination med relativt låga hastigheter (4-5 km/s) antyder förekomsten av välkon- soliderade gamla sedimentära bergarter.

73 Acknowledgments

I appreciate my dear daughter Irina for her friendship, patience and humour over the years; and for assisting me with this chapter. I'm lucky that I've got you!

First of all I would like to say thank you to my supervisors Professor David Gee and Professor Christopher Juhlin for guiding me with my PhD research and improving my thesis book. They always gave me their geological and geophysical shoulders for support and wisely helped me to avoid mistakes. Special thanks to David for making my English understandable for the international society.

I appreciate Uppsala University and Statoil for support of my PhD research. These gave me not only funds, but friends as well. Extra thanks to Statoil for funding all my research trips and also for the new experience from internal scientific workshops, and for Statoil student conference (2007) which was like an adventure into the future.

Thanks to Geophysical Institute (University of Alaska Fairbanks) for letting me being on board of USCS Healy during HOTRAX expedition; thanks to GSA for funding my participation in 'Workshop to Prepare for Arctic Ocean Scientific Drilling' in 2008, and thanks to Swedish Institute for earlier support.

I acknowledge all reviewers of my articles. They forced me to do a lot of work, but they were very constructive and improved the papers a lot.

I had a great time at the Geophysical Department of Uppsala University, thanks to all of you! Special thanks to Artem Kashubin for his assistance with scripts (Figure 3.1), and thanks to him and Ari Tryggvason for a constructive discussion of chapter 3, Niklas Juhojuntti for translating chapter 7 to Swedish. Thank you, Taher, you are a tenable person not only to me, but for many people in our Geocentrum.

I'm happy to take a chance to say thanks to all my Arctic colleagues/friends for inspiring work and discussions during the expeditions and after. The Arctic–2000 expedition had inpiring spirit, because of people and Michael Sorokin's leadership. (Sorry about his death, we miss him.) I was impressed by enthusiastic and hard work of the young geological team lead by wise Irina Andreeva and the Geon seismic team lead by Andrey Mauhin.

74 I found good wise friends Lena Sokolova and Oleg Ganzha, who stay with me from Arctic–2000. This thesis is because I have been on the Arctic–2000 expedition. Thanks to Yury Zamansky, Aldona Langinen, Mikhail Sergeev, Natasha Kosteva, Alexander Tebenkov and many other former colleagues from PMGRE for heated arguments about the cold Arctic. Thanks to HOTRAX expedition scientists and the Healy crew; and to Bernie Coakley for being as he is. Thanks to Yngve Kristoffersen, Tore Årthun and Vibeke Bruvoll from University of Bergen for helpful discussions of the HOTRAX results.

I appreciate my parents for supporting my education which allowed me to get this far.

I'm happy, that I participated in the Krasnoyarsk Summer School (www.klsh.org) in 1988; it turned my life around and inspired the whole of my way. Without the School, my life would be different and probably bor- ing. Thanks to the Faculty of Physics (St. Petersburg State University) for knowledge and friends, whoes good humour has supported me over the decades. "It is much more than you can imagine!"

Finally, I am very sorry that I am not able to mention everyone who was helpful with discussions about research, who was good team-mates, and who encouraged me; otherwise it will be one more book.

Nina Lebedeva-Ivanova, Uppsala, 30 March 2010

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