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Crustal variations in the Baltic Sea; a geologic evaluation of satellite altimetry data
Precambrian Research, 64 ( 1993 ) 311-317
311
Elsevier Science B.V., Amsterdam
Crustal variations in the Baltic Sea; a geologic evaluation of satellite altimetry data K.O. Wann~is*'a and K.L. Haylingb'** aDepartment of Geology and Geochemistry, Universityof Stockholm, S-10691 Stockholm, Sweden bPetroScanAB, Lilla Bom men 1, S-41104 Gothenburg, Sweden Received April 30, 1991; revised version accepted October 10, 1991
ABSTRACT The sea surface closely agrees with the marine geoid, a surface of constant gravitational potential. If the observed undulations of the sea surface are adjusted for time-dependent effects, such as tide, currents and wind, the remaining undulations reflect density variations within the Earth. By studying the relative undulations of the mean sea surface instead of the mean sea surface heights relative to the reference ellipsoid, the precision and the resolution of satellite radar altimeter observations can be greatly improved. The noise level can be reduced to a few centimetres and wavelengths as short as a few tens of kilometres can be detected. Satellite radar altimeter data, from the three satellites GEOSAT, SEASAT and GEOS-3 have been combined, to compute a map of the marine geoid in the southern Baltic Sea. Harmonic filtering of the computed sea surface has been applied to separate the long wavelength undulations (220-450 km) of the marine geoid. This wavelength band of the marine geoid in the southern Baltic Sea is characterized by a pronounced gravity low southwest of the Tornquist-Teisseyre ( T - T ) Zone and a high northeast of this border between the Baltic Shield and central Europe. This gravity low is caused by the lower mass of the crust on the European side of the T - T Zone relative to the crust on the Baltic side of the T - T Zone. Thick accumulations of low-density sediments on the European side, density variations in the crust, or a combination of these, are possible explanations for this negative gravity anomaly. In the eastern part of the Baltic Sea several gravity lows, having similar or higher gravity gradients than those over the T - T Zone, are following a more or less north-southerly trend. The explanation of these very strong mass deficiencies is not clear, but suggests the possibility of an extension of the Central Swedish Gravity Low which continues southward into the Central Baltic Sea Gravity Low. A strong positive gradient in the marine geoid can be observed between the island of Gotland and the Swedish mainland, probably related to the relatively higher density of rocks of the Bergslagen Province.
1. Introduction The determination of geodetic and geodynamic parameters by means of space geodetic methods has brought an enormous progress in earth science during the last decades. Since the first measurements of the sea surface from Spacelab, several satellites equipped with radar altimeters have measured the undulations of the sea surface. Undulations of this surface, *Corresponding author. **Present address: G e o R e m AB, Agatan 32, S-43135 M~51ndal, Sweden.
0301-9268/94/$07.00
or the marine geoid, are caused by mass variations within the Earth. The very long wavelengths (thousands of kilometres ) of these undulations are generally caused by mass variations deep within the Earth, while the short wavelengths (a few tens to a few hundreds of kilometres) are caused by mass variations in the upper lithosphere. Geoid highs are caused by excess mass, such as seamounts or oceanic ridges, while areas with a geoid low are associated with mass deficiencies, for example oceanic trenches. Various bathymetric features of the ocean floor have been identified from this correlation (e.g.
© 1994 Elsevier Science B.V. All rights reserved.
SSD10301-9268 ( 9 3 ) E O 0 6 5 - K
312
K.O. WANN,~,S AND K.L. HAYLING
Sandwell, 1984; Haxby, 1987 ). Gahagan et al. (1988) used discrete data points, rather than gridded averages, which increased the resolution of satellite altimetry data, and made it possible to detect previously unknown features of the ocean floor and to resolve better the shape of known tectonic features. Gahagan et al. (1988) also found an overprint in the signal over the slope of continental plateaus and passive margins, which may reveal basement structures. If adjustments are made for the variations in geoidal height that are caused by the topography of the sea-floor and isostatic effects, the remaining undulations reflect density variations in the underlying crust. Application of harmonic filtering to the adjusted sea surface into different wavelength bands offers the possibility for detailed studies of proposed crustal segments as well as small-scale geological features. The purpose of this paper is to examine and compare major structural geological features onshore with possible continuations in the offshore area based on undulations in the marine geoid from the Baltic Sea. 2. S a t e l l i t e
radar altimetry
The data have been selected from satellite passes over the southern Baltic Sea (Fig. 1 ) from GEOS-3, SEASAT and GEOSAT. Sufficient data density was obtained in the entire area investigated, except for some parts offthe Swedish coast. Each individual satellite track was inspected and repeated passes were checked for coherency• About 7% of all tracks in the area were rejected during the first inspection. The investigation of the remaining satellite passes resulted in rejection and adjustment of another 2% of the tracks. The cut-off level for high-frequency noise was estimated at about 25 km. The choice of this filter level was based on the signal-to-noise ratio determined as the standard error and standard deviation of the altimeter data points to the regression surface of the estimated sea surface. Similar
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values for the shortest wavelength resolvable have been found by other investigators, e.g. Sandwell and McAdoo (1990). Besides the mass variations within the Earth, which are the most important ones, gravitational sources in space (e.g. the moon) make their imprints on the sea surface. These sources normally produce very long wavelength undulations of the geoid that are time-dependent. These effects can be adjusted for either by removing the tide effect based on tidal models, or by observing the undulations of the sea surface over a long time period and using the average. In this study the averaging technique has been used together with low-cut filtering. When the effects of space sources have been removed, the seafloor topography makes the most pronounced imprint on the sea surface. This effect has to be removed when studying the signal from basement structures. Bathymetric data were digitized from nautical charts. Each data point was then adjusted by using an average crustal density of 2.67 g/cm 3. Adjustment was also made for the isostatic effect associated with increased water depths. A multilayer model of the lithosphere, similar to that of Litinsky ( 1987 ), was applied. However, the
C R U S T A L V A R I A T I O N S I N T H E BALTIC SEA
313
water depth in the southern Baltic Sea is quite small and the effect on the sea surface from isostatic compensation was negligible. Harmonic filtering was then applied by computing local "spherical harmonic (SH) models" of a spherical cap covering the study area. Since the undulations of the sea surface over the cap do not have to be determined relative to a reference level, such as the reference ellipsoid, different symmetrical filters (described by e.g. Dobrin, 1976; Vani~ek and Krakiwsky, 1986 ) were applied to remove specific wavelengths within the cap. SH-models including sea surface undulations having wavelengths between 450 and 220 km were computed, resulting in a long wavelength map of the marine geoid (Fig. 2 ). A detailed theoretical description of satellite radar altimetry in geodesy and the different computational techniques used to determine the undulations of the sea surface, the gravitational potential and gravity fields are beyond the scope of this paper. For further details on the subject the reader is referred to e.g. Heiskanen and Moritz ( 1967 ), Vani~ek and Krakiwsky (1986). I
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The Baltic Sea forms a complex depression within the Baltic Shield developed during several tectonic phases and filled with variable thicknesses of sedimentary rock. The Precambrian crystalline basement of the Baltic Shield is exposed along most of the Swedish and Finnish coasts. A generalized description of the rock units in Sweden, related to the investigated area, is shown in Fig. 3. The northern part (the Bergslagen Province (Fig. 3) and adjacent areas in southern Finland) consists of an east-west trending belt of metamorphosed felsic volcanics, sedimentary units and granitoid intrusions. Scattered areas of supracrustal rock units of somewhat different character exist in the Vetlanda, V&istervik and Valdermarsvik areas (Lundquist, 1979). These units occur either as mega-xenoliths
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Fig. 3. Surface geology surrounding the southern Baltic Sea. Further description of each area is given in the text. T-T Zone denotes the Tornquist-Teisseyre Zone.
314
within the Transscandinavian Granite-Porphyry Belt or at the southernmost margin of the Bergslagen Province. They are dominated by metamorphosed mafic volcanics and quartzrich sediments. The Transcandinavian Granite-Porphyry Belt extends from southern Sweden to northern Norway (Gorbatschev, 1980 ). In the south, this belt passes into the Blekinge Province which is dominated by gneisses and granitoids (Johansson and Larsen, 1989). Anorogenic rapakivi granite intrusions are known in the Proterozoic basement, but have also been reported from the Baltic States (Pozaryski and Kotanski, 1978; Urban and Tsybulya, 1988; Ryka, 1990). The largest massif in this area is the Riga Massif which occupies the main part of the Gulf of Riga, including the southern part of Saaremaa Island (Fig. 3 ). The West Lithuanian Granulite Massif and the Pomorze Massif are crustal units identified in the tectonic map of the southwestern border of the East European Platform (IGCP-86, 1986). The Pomorze Massif in Poland and the offshore area have been described by Ryka (1990), who indicates a proposed Precambrian domain onshore, generally divided into the "Karelides" (gneisses, crystalline schists and granitoid massifs) and the "Pre-Karelides" (greenstone belts and grey gneisses). However, the major part of the offshore area is described as Proterozoic granitoids and migmatites comparable to the Transscandinavian Granite-Porphyry Belt and rapakivi-like granitoids. One of the major tectonic zones in the Baltic Sea is the Tornquist-Teisseyre (T-T) Zone, which has been active since the Precambrian and was reactivated during several orgeneses. This zone extends from the Black Sea in the southeast to the North Sea in the northwest. Southwest of this zone, the crystalline basement consists of possible Precambrian units covered by extensive Phanerozoic sediments. The Protogine Zone (Gorbatchev, 1980) is one of the most important tectonic structures
ICO. WANNesSAND K.L. HAYLING
in southern Sweden and constitutes the border between the Southwest Swedish Gneiss Region and the Transscandinavian Granite-Porphyry Belt. Gravity measurements in central Sweden over the Protogine Zone clearly show a strong gravity gradient between the Southwest Swedish Gneiss Region and the Transscandinavian Granite-Porphyry Belt (Zuber and 0hlander, 1990). Another important tectonic zone is the Filipstad-V~istervik Shear Zone (Henkel and Eriksson, 1987), which is the border between the Transscandinavian Granite-Porphyry Belt and the Bergslagen Province. The sedimentary bedrock in the Baltic Sea ranges from Neoproterozoic to Late Cretaceous in age. The southeastern part of the Baltic Sea forms an area of subsidence in the Baltic Shield, the Baltic Synclise, which is filled with thick deposits of Paleozoic to Cenozoic rocks. The Neoproterozoic strata underwent intense erosion, leading to the formation of a sub-Cambrian peneplain, which explains the rather patchy distribution of Proterozoic sedimentary rocks in the Baltic Sea. These rocks have mainly been preserved in several basins, where they often attain considerable thicknesses, e.g. the Landsort Basin (Flod6n, 1980). The sub-Cambrian peneplain and the crystalline basement have an overall southeasterly dip, which means that the average thickness of the Paleozoic to Cenozoic sequences increases from about 500 m on Gotland, to about 3500 m in northern Poland (Winterhalter et al., 1981 ). An exception to this general distribution is the area south of the Tornquist-Teisseyre Zone. Southwest of this zone where, it traverses the Baltic Sea, the sedimentary bedrock reaches a thickness of almost 7 km (Liboriussen et al., 1987). 4. The marine geoid in the Baltic Sea
The regional crustal segments of the southern Baltic Sea were tentatively interpreted within a quite narrow spectrum, D&O 90 to
CRUSTAL VARIATIONS IN THE BALTIC SEA
180, of the sea surface height variations. These anomalies have wavelengths between ~ 200 and 500 km. The anomalies seen in this filter level (Fig. 2) are well above the noise floor. The amplitude of the undulations is almost 1 m. The maximum gradient, found in the northwestern part of the map, is about 1 m over 120 kin. This deflection of the vertical is 8/trad, or approximately 8 mgal (Heiskanen and Moritz, 1967 ). McAdoo and Sandwell ( 1988 ) found from coherency analysis of adjacent GEOSAT tracks in the Southern Ocean and Antarctic Margin that the precision of alongtrack deflections of the vertical was ~ 1/trad. They set their resolution limit to 20 km. The minimum variation in sea surface height over 20 km is then ~ 2.0 cm, which is in close agreement with the noise floor determined for relative height variations in the Baltic Sea. Four distinct areas can be distinguished from the strong gradients in the marine geoid in Fig. 3: ( 1 ) a low southwest of Bornholm that has its centre towards the German coast; (2) a high extending from Blekinge to northern Poland; (3) a north-south trending low in the eastern part of the Baltic Sea interrupted by two highs along the coasts of Estonia and Lithuania; and (4) a high extending from the Swedish coast in a southeasterly direction to the island of Gotland. 5. Discussion
The geologic interpretation of these four units of the southern Baltic Sea must be considered as a tentative one. The mass variations in the crust related to interpreted units must therefore not necessarily refer to just one unit or rock complex. The strong gradient in the altimeter map between Units 1 and 2 corresponds perfectly with the Tornquist-Teisseyre Zone separating the Baltic Shield and the European Platform. It is therefore evident that this well established fault-controlled zone can be clearly recognized in the marine geoid.
315
Unit 2 appears to consist of two major geologic complexes: gneisses and granitoids of the Blekinge region, and the Transscandinavian Granite-Porphyry Belt. This unit may also include the Precambrian crust of the Pomorze Massif. The boundary between Units 2 and 3 is apparently of a magnitude similar to the gradient of the T - T Zone, which may indicate a major tectonic zone at this site. However, the gradient in the marine geoid between Units 2 and 3 is not related to major vertical sedimentary offsets, which is a probable explanation for the T-T Zone. The hypothesis that the strong gradient between Units 2 and 3 reflects a major tectonic zone is supported by the presence of the Filipstad-V~istervik Shear Zone on the Swedish mainland (Henkel and Eriksson, 1987). The extension of this zone under the Baltic Sea may very well follow the border between Units 2 and 3, but may also indicate a lithologic boundary between the two units. Unit 3 cannot easily be compared with Precambrian onshore crustal units because of the lack of exposure in the Baltic States. However, it is possible that the density low of Unit 3 is linked to the Central Swedish Gravity Low (Werner et al., 1977; Zuber, 1985). This gravity low in the central part of the Bergslagen Province consists mainly of Early Svecofennian granitoids and felsic volcanics. The Central Swedish Gravity Low strikes into Aland Sea and continues as a density low southwards to Unit 3 in the central Baltic Sea. The possibility that thick accumulations of sediments are the sources of the strong negative anomalies in Unit 3 was examined by forward gravity modelling of the Landsort Basin (located east of Unit 4). This sedimentary basin was selected because it is bordered by crystalline bedrock, and the extension and depth of the basin are fairly well known from seismic studies (T. Flodrn, pers. commun., 1990). The gravity modelling of the sea surface over the Landsort Basin shows that the major mass deficit of the basin cannot be explained solely by the density
316
contrast between Neoproterozoic sedimentary rocks and the crystalline bedrock. In the eastern part of Unit 3, the southernmost of the two pronounced highs in Fig. 3 may indicate an extension of the West Lithuanian Granulite Massif bordering the Riga Rapakivi Massif farther north. This assumption is based on the fact that Rapakivi granites generally have lower densities than the surrounding crystalline basement (Eriksson and Henkel, 1983). The Riga Rapakivi Massif seems also to be connected to the gravity low of Unit 3. Unfortunately, the geologic information about the crystalline basement in this area is not sufficient to verify this possibility. The density low of the ]~land Sea is another example suggesting the derivation of the mass deficit from the rapakivi granite on Aland Island. The gradient between Units 3 and 4 north of Gotland Island extends in a northwesterly direction and appears to correspond to the southeastern boundary of the Bergslagen Province and the Central Baltic Sea Gravity Low. 6. Conclusions Four areas, two gravity lows and two gravity highs, can be distinguished in the long wavelength undulations of the marine geoid in the southern Baltic Sea. The strong gradient in the marine geoid between a gravity low over the European Platform and a gravity high over the Baltic Shield corresponds perfectly with the Tornquist-Teisseyre Zone. The gravity high extends across the Baltic Sea from the Blekinge Province to the Pomorze Massif. In the central and eastern parts of the Baltic Sea, this high is bordered by a gravity low, revealing the presence of a low-density crustal unit(s) in the central Baltic Sea, which possibly is an extension of the Central Swedish Gravity Low. A positive anomaly, extending from the Swedish coast to the northern part of Gotland Island, forms a pronounced gradient towards the gravity low in the central part of the Baltic Sea. This strong gravity gradient probably denotes
K.O. WANNA.SAND K.L.HAYLING
the border between the Bergslagen Province and the Central Baltic Sea Gravity Low. Acknowledgements The authors wish to thank PetroScan AB for permission to publish these results. Thanks are also due to Dr. T. Flod6n and Dr. K. Sundblad for valuable comments. Reviews by Dr. Lars Sj6berg and an anonymous reviewer led to improvements in the manuscript. References Dobrin, M.B., 1976. Introduction to Geophysical Prospecting. McGraw-Hill, New York, 630 pp. Eriksson, L. and Henkel, H., 1983. Deep structures in the Precambrian interpreted from magnetic and gravity maps of Scandinavia. Int. Basement Tectonics Assoc. Publ., 4: 351-358. Flod6n, T., 1980. Seismic stratigraphy and bedrock geology of the Central Baltic. Stockholm Contrib. Geol., 35: 1-240. Gahagan, L.M., Scotese, C.R., Royer, J.Y., Sandwell, D.T., Winn, J.K., Tomlins, R.L., Ross, M.I., Newman, J.S., Muller, R.D., Mayes, C.L., Lawyer, L.A. and Heubeck, C.E., 1988. Tectonic fabric map of the ocean basins from satellite altimetry data. Tectonophysics, 155: 1-26. Gorbatschev, R., 1980. The Precambrian development of southern Sweden. Geol. F6ren. Stockholm F6rh., 102: 129-136. Haxby, W.F., 1987. Gravity field of the world's oceans, map. Natl. Geophys. Data Center, Natl. Oceanic and Atmos. Admin., Boulder, Colo. Heiskanen, W.A. and Moritz, H., 1967. Physical Geodesy. Freeman, San Francisco, Calif., 364 pp. Henkel, H. and L. Eriksson, L., 1987. Regional aeromagnetic and gravity studies in Scandinavia. Precambrian Res., 35: 169-180. IGCP-86, 1986. Southwest Border of the East European Platform, Tectonic Map. Zentrales Geologisches Institut, Berlin. Johansson, A. and Larsen, O., 1989. Radiometric age determination and Precambrian geochronolgy of Blekinge, southern Sweden. Geol. F/Sren. Stockholm F6rh., 111 : 35-50. Liboriussen, J., Ashton, P. and Tygesen, T., 1987. The tectonic evolution of the Fennoscandian Border Zone in Denmark. Tectonophysics, 137:21-29. Litinsky, V.A., 1987. Isostatic reduction at sea: a new version. Paper presented at the 57th Int. Meeting, Society of Exploration Geophysicists, New Orleans.
CRUSTAL VARIATIONS IN THE BALTIC SEA
Lundquist, T., 1979. The Precambrian of Sweden. Sver. Geol. Unders., Ser. C, 768: 1-87. McAdoo, D.C. and D.T. Sandwell, D.T., 1988. Marine gravity, GEOSAT's exact repeat mission. EOS, Am. Geophys. Union, 69, 1569. Pozaryski, W. and Kotanski, Z., 1978. Baikalian, Caledonian and Variscan events in the forefield of the EastEuropean Platform. Z. Dtsch. Geol. Ges., 129: 391402. Ryka, W., 1990. Podloze krystaliczne Pomorza i Polsiega Baltyku. Przegl. Geol., 5-6 (445-446), ROK XXXVIII: 227-229. Sandwell, D.T., 1984. A detailed view of the South Pacific geoid from satellite altimetry. J. Geophys. Res., 89: 1089-1104. Sandwell, D.T. and D.C. McAdoo, 1990. High-accuracy, high-resolution gravity profiles from 2 years of the Geosat Exact repeat Mission. J. Geophys. Res., 95: 3049-3060. Urban, G. and Tsybulya, L., 1988. Thermal field of the Riga Pluton (with English summary). Eesti NSV
317 Teaduste Akadeemia Toimetised, ISSN, 0201-8136, pp. 49-54. Vani6ek, P. and Krakiwsky, E., 1986. Geodesy, the concept. Amsterdam, 697 pp. Werner, S., Aaro, S. and Lagmansson, M., 1977. Gravimeterunders/Skningar inom Bergslagstraversen. Styrelsen for teknisk utveckling, STU, 75-5084: 1-61. Winterhalter, B., Flod6n, T., Ignatius, H., Axberg, S. and Niemist6, L., 1981. Geology of the Baltic Sea. In: A. Vipio (Editor), The Baltic Sea. Elsevier Oceanographic Series, Amsterdam, 30:1-21. Zuber, J.A., 1985. Geological interpretation of gravity and aeromagnetic surveys over the Fellingsbro-Blixterboda granite. Geol. F/3ren. Stockh. F6rh., 107: 203213. Zuber, J.A. and Ohlander, B., 1990. Geophysical and geochemical evidence of Proterozoic collision in the western marginal zone of the Baltic Shield. Geol. Rundsch., 79: 1-11.
311
Elsevier Science B.V., Amsterdam
Crustal variations in the Baltic Sea; a geologic evaluation of satellite altimetry data K.O. Wann~is*'a and K.L. Haylingb'** aDepartment of Geology and Geochemistry, Universityof Stockholm, S-10691 Stockholm, Sweden bPetroScanAB, Lilla Bom men 1, S-41104 Gothenburg, Sweden Received April 30, 1991; revised version accepted October 10, 1991
ABSTRACT The sea surface closely agrees with the marine geoid, a surface of constant gravitational potential. If the observed undulations of the sea surface are adjusted for time-dependent effects, such as tide, currents and wind, the remaining undulations reflect density variations within the Earth. By studying the relative undulations of the mean sea surface instead of the mean sea surface heights relative to the reference ellipsoid, the precision and the resolution of satellite radar altimeter observations can be greatly improved. The noise level can be reduced to a few centimetres and wavelengths as short as a few tens of kilometres can be detected. Satellite radar altimeter data, from the three satellites GEOSAT, SEASAT and GEOS-3 have been combined, to compute a map of the marine geoid in the southern Baltic Sea. Harmonic filtering of the computed sea surface has been applied to separate the long wavelength undulations (220-450 km) of the marine geoid. This wavelength band of the marine geoid in the southern Baltic Sea is characterized by a pronounced gravity low southwest of the Tornquist-Teisseyre ( T - T ) Zone and a high northeast of this border between the Baltic Shield and central Europe. This gravity low is caused by the lower mass of the crust on the European side of the T - T Zone relative to the crust on the Baltic side of the T - T Zone. Thick accumulations of low-density sediments on the European side, density variations in the crust, or a combination of these, are possible explanations for this negative gravity anomaly. In the eastern part of the Baltic Sea several gravity lows, having similar or higher gravity gradients than those over the T - T Zone, are following a more or less north-southerly trend. The explanation of these very strong mass deficiencies is not clear, but suggests the possibility of an extension of the Central Swedish Gravity Low which continues southward into the Central Baltic Sea Gravity Low. A strong positive gradient in the marine geoid can be observed between the island of Gotland and the Swedish mainland, probably related to the relatively higher density of rocks of the Bergslagen Province.
1. Introduction The determination of geodetic and geodynamic parameters by means of space geodetic methods has brought an enormous progress in earth science during the last decades. Since the first measurements of the sea surface from Spacelab, several satellites equipped with radar altimeters have measured the undulations of the sea surface. Undulations of this surface, *Corresponding author. **Present address: G e o R e m AB, Agatan 32, S-43135 M~51ndal, Sweden.
0301-9268/94/$07.00
or the marine geoid, are caused by mass variations within the Earth. The very long wavelengths (thousands of kilometres ) of these undulations are generally caused by mass variations deep within the Earth, while the short wavelengths (a few tens to a few hundreds of kilometres) are caused by mass variations in the upper lithosphere. Geoid highs are caused by excess mass, such as seamounts or oceanic ridges, while areas with a geoid low are associated with mass deficiencies, for example oceanic trenches. Various bathymetric features of the ocean floor have been identified from this correlation (e.g.
© 1994 Elsevier Science B.V. All rights reserved.
SSD10301-9268 ( 9 3 ) E O 0 6 5 - K
312
K.O. WANN,~,S AND K.L. HAYLING
Sandwell, 1984; Haxby, 1987 ). Gahagan et al. (1988) used discrete data points, rather than gridded averages, which increased the resolution of satellite altimetry data, and made it possible to detect previously unknown features of the ocean floor and to resolve better the shape of known tectonic features. Gahagan et al. (1988) also found an overprint in the signal over the slope of continental plateaus and passive margins, which may reveal basement structures. If adjustments are made for the variations in geoidal height that are caused by the topography of the sea-floor and isostatic effects, the remaining undulations reflect density variations in the underlying crust. Application of harmonic filtering to the adjusted sea surface into different wavelength bands offers the possibility for detailed studies of proposed crustal segments as well as small-scale geological features. The purpose of this paper is to examine and compare major structural geological features onshore with possible continuations in the offshore area based on undulations in the marine geoid from the Baltic Sea. 2. S a t e l l i t e
radar altimetry
The data have been selected from satellite passes over the southern Baltic Sea (Fig. 1 ) from GEOS-3, SEASAT and GEOSAT. Sufficient data density was obtained in the entire area investigated, except for some parts offthe Swedish coast. Each individual satellite track was inspected and repeated passes were checked for coherency• About 7% of all tracks in the area were rejected during the first inspection. The investigation of the remaining satellite passes resulted in rejection and adjustment of another 2% of the tracks. The cut-off level for high-frequency noise was estimated at about 25 km. The choice of this filter level was based on the signal-to-noise ratio determined as the standard error and standard deviation of the altimeter data points to the regression surface of the estimated sea surface. Similar
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values for the shortest wavelength resolvable have been found by other investigators, e.g. Sandwell and McAdoo (1990). Besides the mass variations within the Earth, which are the most important ones, gravitational sources in space (e.g. the moon) make their imprints on the sea surface. These sources normally produce very long wavelength undulations of the geoid that are time-dependent. These effects can be adjusted for either by removing the tide effect based on tidal models, or by observing the undulations of the sea surface over a long time period and using the average. In this study the averaging technique has been used together with low-cut filtering. When the effects of space sources have been removed, the seafloor topography makes the most pronounced imprint on the sea surface. This effect has to be removed when studying the signal from basement structures. Bathymetric data were digitized from nautical charts. Each data point was then adjusted by using an average crustal density of 2.67 g/cm 3. Adjustment was also made for the isostatic effect associated with increased water depths. A multilayer model of the lithosphere, similar to that of Litinsky ( 1987 ), was applied. However, the
C R U S T A L V A R I A T I O N S I N T H E BALTIC SEA
313
water depth in the southern Baltic Sea is quite small and the effect on the sea surface from isostatic compensation was negligible. Harmonic filtering was then applied by computing local "spherical harmonic (SH) models" of a spherical cap covering the study area. Since the undulations of the sea surface over the cap do not have to be determined relative to a reference level, such as the reference ellipsoid, different symmetrical filters (described by e.g. Dobrin, 1976; Vani~ek and Krakiwsky, 1986 ) were applied to remove specific wavelengths within the cap. SH-models including sea surface undulations having wavelengths between 450 and 220 km were computed, resulting in a long wavelength map of the marine geoid (Fig. 2 ). A detailed theoretical description of satellite radar altimetry in geodesy and the different computational techniques used to determine the undulations of the sea surface, the gravitational potential and gravity fields are beyond the scope of this paper. For further details on the subject the reader is referred to e.g. Heiskanen and Moritz ( 1967 ), Vani~ek and Krakiwsky (1986). I
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The Baltic Sea forms a complex depression within the Baltic Shield developed during several tectonic phases and filled with variable thicknesses of sedimentary rock. The Precambrian crystalline basement of the Baltic Shield is exposed along most of the Swedish and Finnish coasts. A generalized description of the rock units in Sweden, related to the investigated area, is shown in Fig. 3. The northern part (the Bergslagen Province (Fig. 3) and adjacent areas in southern Finland) consists of an east-west trending belt of metamorphosed felsic volcanics, sedimentary units and granitoid intrusions. Scattered areas of supracrustal rock units of somewhat different character exist in the Vetlanda, V&istervik and Valdermarsvik areas (Lundquist, 1979). These units occur either as mega-xenoliths
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314
within the Transscandinavian Granite-Porphyry Belt or at the southernmost margin of the Bergslagen Province. They are dominated by metamorphosed mafic volcanics and quartzrich sediments. The Transcandinavian Granite-Porphyry Belt extends from southern Sweden to northern Norway (Gorbatschev, 1980 ). In the south, this belt passes into the Blekinge Province which is dominated by gneisses and granitoids (Johansson and Larsen, 1989). Anorogenic rapakivi granite intrusions are known in the Proterozoic basement, but have also been reported from the Baltic States (Pozaryski and Kotanski, 1978; Urban and Tsybulya, 1988; Ryka, 1990). The largest massif in this area is the Riga Massif which occupies the main part of the Gulf of Riga, including the southern part of Saaremaa Island (Fig. 3 ). The West Lithuanian Granulite Massif and the Pomorze Massif are crustal units identified in the tectonic map of the southwestern border of the East European Platform (IGCP-86, 1986). The Pomorze Massif in Poland and the offshore area have been described by Ryka (1990), who indicates a proposed Precambrian domain onshore, generally divided into the "Karelides" (gneisses, crystalline schists and granitoid massifs) and the "Pre-Karelides" (greenstone belts and grey gneisses). However, the major part of the offshore area is described as Proterozoic granitoids and migmatites comparable to the Transscandinavian Granite-Porphyry Belt and rapakivi-like granitoids. One of the major tectonic zones in the Baltic Sea is the Tornquist-Teisseyre (T-T) Zone, which has been active since the Precambrian and was reactivated during several orgeneses. This zone extends from the Black Sea in the southeast to the North Sea in the northwest. Southwest of this zone, the crystalline basement consists of possible Precambrian units covered by extensive Phanerozoic sediments. The Protogine Zone (Gorbatchev, 1980) is one of the most important tectonic structures
ICO. WANNesSAND K.L. HAYLING
in southern Sweden and constitutes the border between the Southwest Swedish Gneiss Region and the Transscandinavian Granite-Porphyry Belt. Gravity measurements in central Sweden over the Protogine Zone clearly show a strong gravity gradient between the Southwest Swedish Gneiss Region and the Transscandinavian Granite-Porphyry Belt (Zuber and 0hlander, 1990). Another important tectonic zone is the Filipstad-V~istervik Shear Zone (Henkel and Eriksson, 1987), which is the border between the Transscandinavian Granite-Porphyry Belt and the Bergslagen Province. The sedimentary bedrock in the Baltic Sea ranges from Neoproterozoic to Late Cretaceous in age. The southeastern part of the Baltic Sea forms an area of subsidence in the Baltic Shield, the Baltic Synclise, which is filled with thick deposits of Paleozoic to Cenozoic rocks. The Neoproterozoic strata underwent intense erosion, leading to the formation of a sub-Cambrian peneplain, which explains the rather patchy distribution of Proterozoic sedimentary rocks in the Baltic Sea. These rocks have mainly been preserved in several basins, where they often attain considerable thicknesses, e.g. the Landsort Basin (Flod6n, 1980). The sub-Cambrian peneplain and the crystalline basement have an overall southeasterly dip, which means that the average thickness of the Paleozoic to Cenozoic sequences increases from about 500 m on Gotland, to about 3500 m in northern Poland (Winterhalter et al., 1981 ). An exception to this general distribution is the area south of the Tornquist-Teisseyre Zone. Southwest of this zone where, it traverses the Baltic Sea, the sedimentary bedrock reaches a thickness of almost 7 km (Liboriussen et al., 1987). 4. The marine geoid in the Baltic Sea
The regional crustal segments of the southern Baltic Sea were tentatively interpreted within a quite narrow spectrum, D&O 90 to
CRUSTAL VARIATIONS IN THE BALTIC SEA
180, of the sea surface height variations. These anomalies have wavelengths between ~ 200 and 500 km. The anomalies seen in this filter level (Fig. 2) are well above the noise floor. The amplitude of the undulations is almost 1 m. The maximum gradient, found in the northwestern part of the map, is about 1 m over 120 kin. This deflection of the vertical is 8/trad, or approximately 8 mgal (Heiskanen and Moritz, 1967 ). McAdoo and Sandwell ( 1988 ) found from coherency analysis of adjacent GEOSAT tracks in the Southern Ocean and Antarctic Margin that the precision of alongtrack deflections of the vertical was ~ 1/trad. They set their resolution limit to 20 km. The minimum variation in sea surface height over 20 km is then ~ 2.0 cm, which is in close agreement with the noise floor determined for relative height variations in the Baltic Sea. Four distinct areas can be distinguished from the strong gradients in the marine geoid in Fig. 3: ( 1 ) a low southwest of Bornholm that has its centre towards the German coast; (2) a high extending from Blekinge to northern Poland; (3) a north-south trending low in the eastern part of the Baltic Sea interrupted by two highs along the coasts of Estonia and Lithuania; and (4) a high extending from the Swedish coast in a southeasterly direction to the island of Gotland. 5. Discussion
The geologic interpretation of these four units of the southern Baltic Sea must be considered as a tentative one. The mass variations in the crust related to interpreted units must therefore not necessarily refer to just one unit or rock complex. The strong gradient in the altimeter map between Units 1 and 2 corresponds perfectly with the Tornquist-Teisseyre Zone separating the Baltic Shield and the European Platform. It is therefore evident that this well established fault-controlled zone can be clearly recognized in the marine geoid.
315
Unit 2 appears to consist of two major geologic complexes: gneisses and granitoids of the Blekinge region, and the Transscandinavian Granite-Porphyry Belt. This unit may also include the Precambrian crust of the Pomorze Massif. The boundary between Units 2 and 3 is apparently of a magnitude similar to the gradient of the T - T Zone, which may indicate a major tectonic zone at this site. However, the gradient in the marine geoid between Units 2 and 3 is not related to major vertical sedimentary offsets, which is a probable explanation for the T-T Zone. The hypothesis that the strong gradient between Units 2 and 3 reflects a major tectonic zone is supported by the presence of the Filipstad-V~istervik Shear Zone on the Swedish mainland (Henkel and Eriksson, 1987). The extension of this zone under the Baltic Sea may very well follow the border between Units 2 and 3, but may also indicate a lithologic boundary between the two units. Unit 3 cannot easily be compared with Precambrian onshore crustal units because of the lack of exposure in the Baltic States. However, it is possible that the density low of Unit 3 is linked to the Central Swedish Gravity Low (Werner et al., 1977; Zuber, 1985). This gravity low in the central part of the Bergslagen Province consists mainly of Early Svecofennian granitoids and felsic volcanics. The Central Swedish Gravity Low strikes into Aland Sea and continues as a density low southwards to Unit 3 in the central Baltic Sea. The possibility that thick accumulations of sediments are the sources of the strong negative anomalies in Unit 3 was examined by forward gravity modelling of the Landsort Basin (located east of Unit 4). This sedimentary basin was selected because it is bordered by crystalline bedrock, and the extension and depth of the basin are fairly well known from seismic studies (T. Flodrn, pers. commun., 1990). The gravity modelling of the sea surface over the Landsort Basin shows that the major mass deficit of the basin cannot be explained solely by the density
316
contrast between Neoproterozoic sedimentary rocks and the crystalline bedrock. In the eastern part of Unit 3, the southernmost of the two pronounced highs in Fig. 3 may indicate an extension of the West Lithuanian Granulite Massif bordering the Riga Rapakivi Massif farther north. This assumption is based on the fact that Rapakivi granites generally have lower densities than the surrounding crystalline basement (Eriksson and Henkel, 1983). The Riga Rapakivi Massif seems also to be connected to the gravity low of Unit 3. Unfortunately, the geologic information about the crystalline basement in this area is not sufficient to verify this possibility. The density low of the ]~land Sea is another example suggesting the derivation of the mass deficit from the rapakivi granite on Aland Island. The gradient between Units 3 and 4 north of Gotland Island extends in a northwesterly direction and appears to correspond to the southeastern boundary of the Bergslagen Province and the Central Baltic Sea Gravity Low. 6. Conclusions Four areas, two gravity lows and two gravity highs, can be distinguished in the long wavelength undulations of the marine geoid in the southern Baltic Sea. The strong gradient in the marine geoid between a gravity low over the European Platform and a gravity high over the Baltic Shield corresponds perfectly with the Tornquist-Teisseyre Zone. The gravity high extends across the Baltic Sea from the Blekinge Province to the Pomorze Massif. In the central and eastern parts of the Baltic Sea, this high is bordered by a gravity low, revealing the presence of a low-density crustal unit(s) in the central Baltic Sea, which possibly is an extension of the Central Swedish Gravity Low. A positive anomaly, extending from the Swedish coast to the northern part of Gotland Island, forms a pronounced gradient towards the gravity low in the central part of the Baltic Sea. This strong gravity gradient probably denotes
K.O. WANNA.SAND K.L.HAYLING
the border between the Bergslagen Province and the Central Baltic Sea Gravity Low. Acknowledgements The authors wish to thank PetroScan AB for permission to publish these results. Thanks are also due to Dr. T. Flod6n and Dr. K. Sundblad for valuable comments. Reviews by Dr. Lars Sj6berg and an anonymous reviewer led to improvements in the manuscript. References Dobrin, M.B., 1976. Introduction to Geophysical Prospecting. McGraw-Hill, New York, 630 pp. Eriksson, L. and Henkel, H., 1983. Deep structures in the Precambrian interpreted from magnetic and gravity maps of Scandinavia. Int. Basement Tectonics Assoc. Publ., 4: 351-358. Flod6n, T., 1980. Seismic stratigraphy and bedrock geology of the Central Baltic. Stockholm Contrib. Geol., 35: 1-240. Gahagan, L.M., Scotese, C.R., Royer, J.Y., Sandwell, D.T., Winn, J.K., Tomlins, R.L., Ross, M.I., Newman, J.S., Muller, R.D., Mayes, C.L., Lawyer, L.A. and Heubeck, C.E., 1988. Tectonic fabric map of the ocean basins from satellite altimetry data. Tectonophysics, 155: 1-26. Gorbatschev, R., 1980. The Precambrian development of southern Sweden. Geol. F6ren. Stockholm F6rh., 102: 129-136. Haxby, W.F., 1987. Gravity field of the world's oceans, map. Natl. Geophys. Data Center, Natl. Oceanic and Atmos. Admin., Boulder, Colo. Heiskanen, W.A. and Moritz, H., 1967. Physical Geodesy. Freeman, San Francisco, Calif., 364 pp. Henkel, H. and L. Eriksson, L., 1987. Regional aeromagnetic and gravity studies in Scandinavia. Precambrian Res., 35: 169-180. IGCP-86, 1986. Southwest Border of the East European Platform, Tectonic Map. Zentrales Geologisches Institut, Berlin. Johansson, A. and Larsen, O., 1989. Radiometric age determination and Precambrian geochronolgy of Blekinge, southern Sweden. Geol. F/Sren. Stockholm F6rh., 111 : 35-50. Liboriussen, J., Ashton, P. and Tygesen, T., 1987. The tectonic evolution of the Fennoscandian Border Zone in Denmark. Tectonophysics, 137:21-29. Litinsky, V.A., 1987. Isostatic reduction at sea: a new version. Paper presented at the 57th Int. Meeting, Society of Exploration Geophysicists, New Orleans.
CRUSTAL VARIATIONS IN THE BALTIC SEA
Lundquist, T., 1979. The Precambrian of Sweden. Sver. Geol. Unders., Ser. C, 768: 1-87. McAdoo, D.C. and D.T. Sandwell, D.T., 1988. Marine gravity, GEOSAT's exact repeat mission. EOS, Am. Geophys. Union, 69, 1569. Pozaryski, W. and Kotanski, Z., 1978. Baikalian, Caledonian and Variscan events in the forefield of the EastEuropean Platform. Z. Dtsch. Geol. Ges., 129: 391402. Ryka, W., 1990. Podloze krystaliczne Pomorza i Polsiega Baltyku. Przegl. Geol., 5-6 (445-446), ROK XXXVIII: 227-229. Sandwell, D.T., 1984. A detailed view of the South Pacific geoid from satellite altimetry. J. Geophys. Res., 89: 1089-1104. Sandwell, D.T. and D.C. McAdoo, 1990. High-accuracy, high-resolution gravity profiles from 2 years of the Geosat Exact repeat Mission. J. Geophys. Res., 95: 3049-3060. Urban, G. and Tsybulya, L., 1988. Thermal field of the Riga Pluton (with English summary). Eesti NSV
317 Teaduste Akadeemia Toimetised, ISSN, 0201-8136, pp. 49-54. Vani6ek, P. and Krakiwsky, E., 1986. Geodesy, the concept. Amsterdam, 697 pp. Werner, S., Aaro, S. and Lagmansson, M., 1977. Gravimeterunders/Skningar inom Bergslagstraversen. Styrelsen for teknisk utveckling, STU, 75-5084: 1-61. Winterhalter, B., Flod6n, T., Ignatius, H., Axberg, S. and Niemist6, L., 1981. Geology of the Baltic Sea. In: A. Vipio (Editor), The Baltic Sea. Elsevier Oceanographic Series, Amsterdam, 30:1-21. Zuber, J.A., 1985. Geological interpretation of gravity and aeromagnetic surveys over the Fellingsbro-Blixterboda granite. Geol. F/3ren. Stockh. F6rh., 107: 203213. Zuber, J.A. and Ohlander, B., 1990. Geophysical and geochemical evidence of Proterozoic collision in the western marginal zone of the Baltic Shield. Geol. Rundsch., 79: 1-11.