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Acta Geophysica vol. 61, no. 5, Oct. 2013, pp. 1088-1100 DOI: 10.2478/s11600-013-0126-0 Seismic View on the Svalbard Passive Continental Margin Wojciech CZUBA Institute of Geophysics, Polish Academy of Sciences, Warszawa, Poland e-mail: [email protected] Abstract Deep seismic sounding measurements were performed in the continent-ocean transition zone of the western Svalbard and Barents Sea margin, during the expeditions in 1985-2008. Seismic energy (airgun and TNT shots) was recorded along several profiles by onshore seismic stations and ocean bottom seismometers, and hydrophone systems. Good quality reflected and refracted P waves provided an excellent data base for a seismic modelling along the profiles. TNT sources were recorded even up to 300 km distances. A minimal depth of about 6 km of the Moho interface was found east of the Molloy Deep. The Moho discontinuity dips down to 28 km beneath the continental part of the northernmost profile and down to maximum 32 km beneath other profiles. The evolution of the region is considered to be within a shearrift tectonic setting. The continent-ocean transition zone along the northernmost profile is mostly dominated by extension; therefore, the last stage of the development of the margin can be classified as rifting. The uplifted Moho interface close to the Molloy Deep can be interpreted as a south-western end of the Molloy Ridge. The margin of the southern Spitsbergen is rather of sheared character while the western Barents Sea margin is of slow to ultraslow spreading type. Key words: continent-ocean transition, seismic crustal structure, Svalbard, North Atlantic, Knipovich Ridge. ________________________________________________ © 2013 Institute of Geophysics, Polish Academy of Sciences Unauthenticated Download Date | 6/17/17 10:03 PM SEISMIC VIEW ON THE SVALBARD PASSIVE CONTINENTAL MARGIN 1. 1089 INTRODUCTION Svalbard Archipelago is located at the north-western corner of the Barents Sea continental platform bordered to the west by passive continental margin (Fig. 1, insert map). Spitsbergen is the main island of the archipelago. This region is an interesting and important area for understanding the evolution of the North Atlantic and Arctic Oceans. This is the youngest region of the Fig. 1. Location map of the seismic profiles in the ocean-continent transition zone in the western Svalbard and Barents Sea margin on the background of topography/ bathymetry map (Jakobsson et al. 2000) and simplified tectonic elements (Gabrielsen et al. 1990, Faleide et al. 2008). Stars (yellow for profiles K1 and C1) and triangles are receivers, thin lines and red dots are airgun and chemical (TNT) shots, respectively. HFZ – Hovgård Fracture Zone, MFZ – Molloy Fault Zone, MR – Molloy Ridge, SFZ – Spitsbergen Fault Zone. Colour version of this figure is available in electronic edition only. Unauthenticated Download Date | 6/17/17 10:03 PM 1090 W. CZUBA Atlantic and Arctic Oceans giving a good source to study processes leading to their opening. Rifting and subsequent sea-floor spreading processes in the North Atlantic Ocean and the development of the passive sheared continental margin of the Barents Sea continental platform, these are the processes which form the today’s face of the Earth. The development of this margin is strongly connected with the history of opening of the North Atlantic Ocean (Jackson et al. 1990, Lyberis and Manby 1993a, b, Ohta 1994). The Svalbard passive continental margin has been investigated by geophysical research over the last 30 years This paper concerns deep seismic sounding studies (DSS) performed in the continent-ocean transition zone of the western Svalbard and Barents Sea margin during the international expeditions in 1985-2008. Seismic energy (TNT explosions and airgun) was recorded along several transects by onshore (land) seismic stations and ocean bottom seismographs (OBS), and hydrophone systems (OBH). Good seismic records from airgun shots were obtained up to 200 km distances at onshore stations and 50 km at OBSs. TNT shooting was recorded even up to 300 km distances. 2. TECTONIC SETTING The geological history of Svalbard Archipelago is ranging in age from Precambrian to Cenozoic (e.g., Birkenmajer 1993, Ohta 1994, Harland 1997, Dallmann 1999). Its structure reflects the relative activity of the Eurasian and the North American plates (Eldholm et al. 1987) and shows a multiorogenic development with several prominent tectonic stages (e.g., Dallmann 1999, Harland 1997). The tectonic history of the region can be simplified into terms of three main geological stages (Sellevoll et al. 1991). The first important tectonic event is related to the Caledonian Orogeny (Birkenmajer 1981). Its effects are particularly well visible in the Eastern Svalbard (Sellevoll et al. 1991). The next main tectonic stage is called the Late Devonian Svalbardian event. During this event, the eastern Spitsbergen and Nordaustlandet have moved northward from eastern Greenland to a position north of Greenland (Sellevoll et al. 1991). This is a disputable hypothesis (Harland 1997). Thus, the eastern part of Svalbard has attached to western Spitsbergen. Western Spitsbergen was supposed to be located there before these movements or it has slightly moved northward from a shorter distance (Harland et al. 1974, Sellevoll et al. 1991). Cenozoic tectonic processes in the Svalbard area were related to the structural history of the western Barents Sea margin. The relative motion between Svalbard and Greenland took place along the Hornsund Fault Zone, traced from just south of Bjørnøya at ca. 75ºN to about 79ºN (Sundvor and Eldholm 1979, Eldholm et al. 1980). This regional fault zone has acted as an Unauthenticated Download Date | 6/17/17 10:03 PM SEISMIC VIEW ON THE SVALBARD PASSIVE CONTINENTAL MARGIN 1091 incipient plate boundary between the opening Arctic Ocean and the Barents Sea shelf. The Barents Sea region has been affected by several stages of tectonism from the time of the Caledonian Orogeny (Birkenmajer 1981, Talwani and Eldholm 1977). The western sector of the Barents Sea has been tectonically most active during the Mesozoic and Cenozoic times (Gabrielsen et al. 1990). There has occurred magmatic activity during the Palaeocene and Eocene in the western part of the Barents Sea, probably related to the breakup of the North Atlantic Ocean. It has started with regional dextral shear in the Early Palaeocene and continued with rifting from about 36 Ma ago (Talwani and Eldholm 1977, Myhre et al. 1982, Eldholm et al. 1987). The evolution of the North Atlantic Ocean can be divided into two main phases. The continental break-up occurred and sea floor spreading started along the Reykjanes, Aegir, and Mohns Ridges (Talwani and Eldholm 1977) during the first phase in the Early Eocene. The shearing along faults between northeast Greenland and Svalbard has created the Spitsbergen fold and thrust belt to the north. The result of this process was the Western Spitsbergen Orogeny (Harland et al. 1974, Steel et al. 1985). Rapid erosion has increased sedimentary load along the Svalbard western margin and turned the epicontinental, littoral basin of central Spitsbergen into a rapidly subsiding foreland basin (Eiken and Austegard 1987). Conversely, dense mantle material was intruded most significantly at the Vestbakken Volcanic Province after thinning and weakening of the crust caused by transtension (Eiken and Austegard 1987, Eldholm et al. 1987, Sundvor and Eldholm 1979). The second phase of North Atlantic evolution was marked by a change in the spreading direction from NNW-SSE to NW-SE. It has resulted the termination of the Western Spitsbergen Orogeny. This has begun in Early Oligocene when spreading in the Labrador sea stopped (Mosar et al. 2002, Talwani and Eldholm 1977). The beginning of the phase has unlocked the northward development of the Mid Atlantic Ridge. The spreading axis has developed into the Spitsbergen Shear Zone creating the asymmetric, ultraslow and obliquely-spreading Knipovich Ridge. Around 23 Ma ago spreading started further, along the Molloy Ridge, and around 10 Ma ago continental break-up occurred along the Fram Strait. This has established connection between the Arctic and the Northern Atlantic ridges (Lundin and Doré 2002, Crane et al. 1991). 3. CONTINENT-OCEAN TRANSITION Good quality records of refracted and reflected P waves were obtained along the entire profiles lengths (Fig. 1). They were an excellent data base for following seismic modelling (Fig. 2) along the profiles. Details about experiUnauthenticated Download Date | 6/17/17 10:03 PM 1092 W. CZUBA Fig. 2. 2D seismic P-wave velocity models developed by ray-tracing technique along profiles shown on Fig. 1, from north (top) to south (bottom), respectively. Black lines represent seismic discontinuities (boundaries), colours represent the distribution of the P-wave velocity and numbers in the model are P-wave velocities in km/s. Gray-shaded belt marks ocean-continent transition zones in every model. All the models are in the same geometric and colour scale. C1 – marks the crossing place of the K1 profile with the C1 profile. Colour version of this figure is available in electronic edition only. Unauthenticated Download Date | 6/17/17 10:03 PM SEISMIC VIEW ON THE SVALBARD PASSIVE CONTINENTAL MARGIN 1093 ments, data processing and quality, as well as modelling procedures and interpretations are described in Czuba et al. (1999, 2005, 2008, 2011), Libak et al. (2012), Ljones et al. (2004), and Ritzmann et al. (2004). Although all the profiles are WARR profiles, their setup was different because of polar region logistics, instruments availability, and geometry. Seismic sources were used in the sea only for all the profiles. Profiles K1 and C1 were performed using TNT sources and seismic land stations (5 channels, vertical component) only. Profiles 99200, 99400, Horsted ’05, and BIS-2008 were performed using seismic land stations, OBSs and OBHs (99200 and 99400 only) as receivers, and dense system of airgun shots as well as TNT shots to extend distance of recording. The profile 99200 geometry setup was partly off-line but seismic modelling was performed like for full in-line setup using real distances between sources and receivers. The Profile 8 was full marine profile with airgun shots and OBSs only. The profile BIN-2008 was performed using seismic land stations, OBSs and airgun shots. All data were processed in the same way, transformed at the end to SEG-Y files with 100 Hz sampling. Different filters were used dynamically during picking of arrivals of seismic energy. Seismic models were calculated using P waves by ray-tracing trial-and-error method using wide available software. A minimal depth of about 6 km of the Moho discontinuity was determined east of the Molloy Deep. Here, the upper mantle exhibits P-wave velocity of about 7.9 km/s, and the crustal thickness does not exceed 4 km It is associated with the Molloy Transform Fault Zone or with the Molloy Ridge connecting the Molloy Transform Fault Zone with the Spitsbergen Transform Fault Zone. The Moho discontinuity dips down to 28 km beneath the continental part of the 99200 (northernmost profile) and K1 profiles, and down to maximum 32 km beneath the other profiles. The high P-wave velocity below the Moho interface increases generally up to 8.2 km/s, reaching maximal 8.6 km/s beneath the continental part of the profile located in the central part of the west coast of Spitsbergen (K1). The high P-wave velocity 8.6 km/s could seem to be anomalous but it is modelled with accuracy in the range of 0.10.2 km/s. It could be connected somehow with the close location of young active rifting processes (Knipovich Ridge). This is the closest location to the continental crust along profiles described here. The continental crust consists of two or three crystalline layers. The lowermost crustal continental layer with the P-wave velocity in the order of 7 km/s does not exist in the continental crust along three of the profiles described in this paper. It is completely missing (BIN-2008) or it exists in the transition zone only (99200, Horsted ’05). These layers or even high velocity bodies are connected with serpentinizing or partial melting in the magma-reach region. A layer characterised by P-wave velocities signifiUnauthenticated Download Date | 6/17/17 10:03 PM 1094 W. CZUBA cantly above 7 km/s is found along the continental part of the BIS-2008 profile but it is very thin and it is hard to define as a normal continental lower crust. Profiles 99400 and K1 show clear continental lower crustal layer characterised by P-wave velocities of 6.7-7.0 km/s and 7.1-7.2 km/s, respectively. The oceanic crust is generally similar in terms of thickness along all the profiles studied here but it is composed of more layers in those three southern profiles, where the opening of Northern Atlantic happened earlier. Differences in crustal thickness are the result of different spreading rate in the tectonic history. The thickness is minimal in the vicinity of mid-oceanic ridge. There, the P-wave upper mantle velocities are lowest along these profiles, being even lower than 7.9 km/s (Profile 8, BIN-2008). The sedimentary part of the continent-ocean transition zone is characterised by a complex basin structure, known in the Spitsbergen region as west Spitsbergen foreland basin (Eiken and Austegard 1987). The P-wave velocity at the topmost layers is very low, being even 1.8-1.9 km/s (BIS-2008, 99200), which indicates high water saturation of the rock body. Margins crossing by several of the profiles studied here have a character of sheared margin with rather short continent-ocean transition and abrupt change of the crustal thickness there (99400, K1, Horsted ’05). The shearing took place mostly along the Hornsund Fault Zone (Faleide et al. 1991, 2008). Margins crossing by the other profiles have more complex history and their transition zone structure indicates the transform character in the past, but in the present they can be classified rather as rifted margins with ultra-slow spreading. 4. MARGIN ANISOTROPY Seismic modelling described in Czuba et al. (1999) for profiles K1 and C1 was conducted to coincide parameters at the crossing area. An isotropic medium was assumed. Remodelling of the C1 profile was done for the purpose of this study to fit better the recorded seismic data. In this case, assuming anisotropic medium, seismic velocities at the crossing area can be different along both profiles. The same ray-tracing trial-and-error technique was used during both modelling cases. The remodelling has resulted in changing of the seismic model. The new model is shown in Fig. 3. The difference is obtained in the lower crustal layer, where P-wave velocities in the corrected model are 0.15 km/s in an average higher, than those along K1 profile in the same place. Moreover, the Moho depth agrees better in both models. According to accuracy discussion in several papers with the same seismic modelling method used (Janik et al. 2002, Grad et al. 2003, 2006, 2008, Środa et al. 2006), this velocity difference is significant. Such a difUnauthenticated Download Date | 6/17/17 10:03 PM SEISMIC VIEW ON THE SVALBARD PASSIVE CONTINENTAL MARGIN 1095 SYNTHETIC SECTION 10 RED. TIME T-X/8 (s) 8 6 4 2 0 150 200 250 300 350 400 SECTION & TRAVELTIMES 10 RED. TIME T-X/8 (s) 8 6 PmP Pn 4 Pg 2 0 150 200 250 N 300 350 400 S K1 0 0 4.80 6.25 Depth [km] 10 10 6.40 7.30 20 Moh 20 o 7.40 Moho 30 40 8.63 30 Profile C1 150 200 Vertical exaggeration x3 250 300 350 Vp [km/s] 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 40 400 Distance [km] Fig. 3. Corrected 2D seismic modelling along C1 profile: P-wave velocity model developed by ray-tracing technique (bottom), observed record section with calculated travel times (middle), and synthetic seismograms (top). K1 marks the crossing place of the C1 profile with the K1 profile. Main P-wave phases: Pg – crustal refraction, Pn – Moho refraction, PmP – Moho reflection. Other descriptions as in Fig. 2. Colour version of this figure is available in electronic edition only. Unauthenticated Download Date | 6/17/17 10:03 PM 1096 W. CZUBA ference is probably caused by seismic anisotropy. It is rather small anisotropy, in the order of 2%. At pressures of 200 to 300 MPa, corresponding to about 10-15 km depth, much of the anisotropy disappears, primarily due to the closure of cracks. Therefore, any crustal anisotropy below these depths is probably caused by other phenomena (Meissner et al. 2006, Środa 2006). Thus, the anisotropy in this case is rather not connected with cracks and faults existing closer to the surface. The C1 profile runs along the Spitsbergen coast and it is approximately parallel to main geological structures, while the K1 profile crosses these structures (e.g., Knipovich Ridge and Hornsund Fault Zone in Fig. 1) and the C1 profile almost perpendicularly. Thus, the anisotropy could be connected with stresses caused by extensional regime acting in the lower crust during spreading processes in the relatively young Knipovich Ridge. 5. CONCLUSIONS Thanks to the dense system of airgun shots it was possible to model very accurately the seismic crustal structure along profiles located in the western Svalbard and Barents Sea continental passive margin. There were found sedimentary basins (probably of Cenozoic sequence) with a low seismic velocity and the thinned continental crust in the continent-ocean transition zone. The evolution of this region appears to be within a shear-rift tectonic setting. The continent-ocean transition zone along the northernmost profile (99200) is mostly dominated by extension; therefore, the last stage of the development of the margin is classified as rifting. It can be interpreted as the result of an extensional regime, acting here from the anomaly 13 time (Eiken and Austegard 1987, Faleide et al. 1991, Harland 1997), which probably has hidden previous shear structure of the margin crossed by the 99200 profile (Czuba et al. 2005). The margins along southern Spitsbergen profiles are rather of sheared character, while the profiles crossing western Barents Sea margin are of slow to ultraslow spreading type. It could be explained by the tectonic history of the region. The most advanced spreading processes occur in the southern part of the study region, where the opening of the North Atlantic has started. Then new spreading processes have appeared in the north (Molloy Deep, Molloy Ridge) and the central part of the region (profiles 99400, K1, Horsted ’05) has been keeping the transform regime for longer time. P-wave anisotropy was determined in the lower crust in the central part of the region. This is the first anisotropy investigation using DSS P-wave modelling in the region known to the author. A c k n o w l e d g m e n t s . The public domain GMT software (Wessel and Smith 1991, 1998) was used to produce the map with profiles (Fig. 1). Unauthenticated Download Date | 6/17/17 10:03 PM SEISMIC VIEW ON THE SVALBARD PASSIVE CONTINENTAL MARGIN 1097 References Birkenmajer, K. (1981), The geology of Svalbard, the western part of the Barents Sea, and the continental margin of Scandinavia. In: A.E.M. Nairn, M. Churkin Jr., and F.G. Stehli (eds.), The Ocean Basins and Margins, The Arctic Ocean, Vol. 5, Plenum Press, New York, 265-329. Birkenmajer, K. (1993), Tertiary and cretaceous faulting in a proterozoic metamorphic terrain, SE Wedel Jarlsberg Land, Spitsbergen, Bull. Pol. Acad. Sci. Earth Sci. 41, 3, 181-189. Crane, K., E. Sundvor, R. Buck, and F. Martinez (1991), Rifting in the northern Norwegian-Greenland Sea: Thermal tests of asymmetric spreading, J. Geophys. Res. 96, B9, 14529-14550, DOI: 10.1029/91JB01231. Czuba, W., M. Grad, and A. Guterch (1999), Crustal structure of north-western Spitsbergen from DSS measurements, Pol. Polar Res. 20, 2, 131-148. Czuba, W., O. Ritzmann, Y. Nishimura, M. Grad, R. Mjelde, A. 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