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Magnetic modelling as a tool in the evaluation of impact structures, with special reference to the Tvären Bay impact crater, SE Sweden
TECTONOPHYSICS ELSEVIER
Tectonophysics 262 (1996) 291-300
Magnetic modelling as a tool in the evaluation of impact structures, with special reference to the Tv iren Bay impact crater, SE Sweden J. Ormi5 a,*, G. Blomqvist b.~ " Department o['Geology and Geochemistry, Stockhohn Lh*iversiO', S-10691 Stockhohn. Swede. i~ Di~'i.~ion ~?['Land and Water Resources, Royal hr~titute ~!/+Technolo,W, S-10044 Stockholm, Swede.
Received 24 July 1995: accepted 5 January 1996
Abstract Until recently computer modelling of radial-symmetric structures, like impact craters, has presented difficulties. As a result of this, such models are scarce in the literature. In this study, the magnetic structure of the marine Tvfiren Bay impact crater (58°46'N, 17°25'E) south of Stockholm is investigated with a sophisticated commercially available computer program developed for gravimetric and magnetic modelling of geological structures. Aeromagnetic data along three profiles across the crater have been used. It is concluded that the method is an important supplement to other geological and geophysical investigations in estimating the shape, depth and extension of the crater and the surrounding region of basement that was fractured by the impact.
1. Introduction
Fredriksson and Wickman (1963) were the first to suggest that the unusually deep Tvfiren Bay (Fig. 1) is an impact crater. Seismic investigations (Flod~n et al., 1986) and the recovery of two drill cores from within the structure where planar deformation features in quartz were found (LindstriSm et al., 1994), have proven this hypothesis to be correct. The impact event is now dated by means of chitinozoans of late Kukrusean age (c. 455 Ma) (Grahn and N61vak, 1993). At that time the area was covered by a shallow shelf sea (Flod~n et al., 1986; Lindstrtim et
Corresponding author, e-mail: [email protected] I Fax +46 8 4110775. E-mail: [email protected].
al., 1994), too deep for the crater rim to prevent the water from resurging into the crater. The almost circular bay is approximately 3.5 km across and has in its central part a circular depression 2 km in diameter with a depth to the fractured Precambrian crystalline bedrock somewhat more than 200 m. The crystalline bedrock consists in the northern, eastern and southern surroundings of the bay of gneissose granitoids of intermediate composition+ with relatively high magnetic susceptibilities. To the west of the bay the rock types are the same but with lower susceptibility values resulting in lower magnetic anomalies on the aeromagnetic map (Lundst65m. 1976). Four major northwest-southeast-striking fracture zones and several smaller ones cut through the outer parts of the structure and the surrounding area (Fig. 1). The structure itself is partly filled with
0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights rcserved. Pll S0040- 195 I (96)000 15-7
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Palaeozoic and Quaternary sediments. In the most complete of the two drillcores mentioned above, Tv~iren 2, 137 m of the Paleozoic sediments were recovered above 5 m of brecciated crystalline bedrock. The other drilling, Tv~iren 1, almost directly hit the crystalline breccia and was stopped after only 4 m. The lower part of the Palaeozoic sequence was
formed by the resurge of water into the crater immediately after the impact. While flowing back into the crater the water carried masses of fragments of the crystalline bedrock and the sedimentary rocks that already existed in the area, mostly Orthoceratite Limestone (OrmiS, 1994). The sequence starts with a calcareous breccia grading upwards into finer tur-
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Fig. 1. Location of the Tv~iren Bay impact structure, the two areas sampled for magnetic data, the Tv~iren 1 and 2 drillings, major fracture zones and the three investigated profiles (A, B, C).
J. Ormi~, G. Blomqvist / Tectonophysics 262 (1996) 291-300
biditic sandstones and finally mudstones. Above this succession follow post impact secular sediments, mostly bituminous carbonatic mudstones and nearly 40 m of nonlithified Quaternary sediments (LindstriSm et al., 1994). The maximum water depth in the structure today is 80 m. Below the sedimentary infill there is a region of brecciated crystalline bedrock to an uncertain depth. The work described herein produces a model of the crater shape and tries to constrain the extension and shape of this fractured region, based on magnetic data.
2. Data and method The constraints required in order to make a good magnetic model of the structure are first of all the magnetic susceptibilities for all major rock types within and around the structure. Susceptibility values from two characteristic localities (Fig. 1) were kindly put at our disposal by Herbert Henkel, Royal Institute of Technology, Stockholm. The values from the two localities were treated separately and therefore, in some cases, two slightly different values for the same rocktypes were achieved. These were used respectively for the closest profile. From the susceptibility distributions (Fig. 2) type values were calculated and associated with the bodies in the models representing the different rocktypes of the area. Measurements of the susceptibility of the crystalline breccia in the lowermost parts of the two drill cores gave values used for this lithology in the model. Susceptibility measurements were also made on the sediments in the Tv~iren 2 drill core. The resulting values were used for the different types of sedimentary rock infill in the crater. In profile A, the sediments in the crater are divided into post-impact sedimentary rocks and Quaternary sediments. As the magnetic properties of the Quaternary sediments are similar to water both the B and C profiles have been simplified in that aspect. The Quaternary sediments and the water were given a value close to zero. The fracture zones intersecting the crater were given susceptibility values approximately 100 times lower than the surrounding rocks, according to Henkel and Guzmfin (1977). Values for the local declination and inclination of the magnetic field were found in Sveriges Nation-
293
alatlas (Eriksson and Henkel, 1994). The magnetic anomalies in the area were taken from the aeromagnetic map published in Lundstrt~m (1976). Measurements for the construction of the map have been made at a flight altitude of 30 m, a line spacing of 200 m, and with measurement spacing along the lines of 40 m. The flight direction was east-west. The original map has 40 nT contours of the total magnetic field. A simplified version of the map for the Tv~iren structure (Fig. 3) shows most of the magnetic features discussed in the text. The profiles are based on the original map and are therefore more detailed than Fig. 3. Since felsic plutonic rocks prevailing in this area usually lack significant remanence (Henkel, 1994) the modelling has been made without remanence in the unaltered bedrock. Three profiles (for location see Fig. 1) were interpolated from the aeromagnetic map intersecting the Tv~iren 2 drillcore which is located somewhat to the north-west of the centre of the crater. They are transverse to the crater rim and continue for a distance of about 2 km outside the crater. Due to an area of almost no differences in the magnetic anomalies to the north-west of the crater (see Fig. 3), the profile A was chosen to extend only from Tv~iren 2 and over the south-eastern crater rim. The part of the profile modelled to the left of the point 0 is therefore to be considered as a conjectural complement. Magnetic anomaly values interpolated from the map at 35 m intervals along these profiles were used in the modelling. Having the profiles in the chosen directions also made it possible to use information about the crater form and the thickness of the sedimentary infill from seismic profiles of approximately the same directions published in Flod~n et al. (1986) and Lindstr~Sm et al. (1994). The observations along these profiles were used to fix the inner crater wall which should not change during the modelling. The shape of the crater and the surrounding fractured regions in Fig. 6 were drawn by using the information received from the modelled profiles and connecting these by following the shape of the isomagnetic lines of the anomalies on the aeromagnetic map. The inter-active program GMM (Gravity and Magnetic Modelling) provided by GeoVista AB (1994), was used in the computer modelling. The features of this program include the possibilities to work in both horizontal and vertical view, to change
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that however has very little effect on the modelling because of the distance between the corners of the bodies and the profile. After feeding the computer with the magnetic field parameters of the area and the susceptibility values for all known bodies in the
the strike length and strike direction of the bodies in the model and also to offset bodies in relation to the profile. The shape of the bodies in the horizontal dimension have to be quadrangular. This explains why the crater shape in Fig. 4 is not circular, a fact
25
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Fig. 2. Histograms of the susceptibilities from the two sampled areas and the drillcores.
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J. Orm6, G. Blomqt'ist / Tectonophysics 262 ( 19961 291-300
295
model, the program can produce a magnetic response curve. By changing parameters such as shape, direction, depth below surface etc. for nonfixed bodies introduced into the model one can make this response curve fit with the one obtained from the map. The model created in this way is an approximation of the shape of the crater at depth and along the profile.
3. Model interpretations and discussion
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The three model profiles (Fig. 5) show a magnetic low over the Tv~iren Bay with almost no internal variations of the magnetic anomalies. Similar magnetic lows can be seen in other impact structures, even in much larger ones like the Saint Martin crater (Grieve and Pesonen, 1992), the complex Lake Lapp@irvi crater (Elo et al., 1992a), the Lake S~i~ik@irvi crater (Elo et al., 1992b), the K~irdla crater, which is very similar to Tv~iren in other aspects as well (Puura and Suuroja, 1992), Lockne (Ormi5 et al., 1995), Mien (Henkel, 1982) and Dellen (Henkel, 1992). As this magnetic-low area appears in craters of various sizes and with different lithologies, it can be considered a typical feature in crater magnetometry. It is, however, most obvious where the structure is developed in a high magnetic crystalline bedrock. The magnetic low is explained as due to the oxidation of magnetite in the brecciated zone (Henkel and Pesonen, 1992) and the depth to the buried unaffected basement under the low-magnetic sedimentary crater infill (Henkel, 1992). Another relevant factor may also be the disruption and disordering of magnetization vectors caused by brecciation (Beals et al., 1963). This might be of interest in structures with significant remanence which is not the case in the mentioned crater structures. The effects of oxidation of fractured rocks are known from investigations of fracture zones (Henkel and Guzmfin, 1977). A significant difference between Tv~iren and the larger craters among those mentioned above, is that the central area of the magnetic low of larger craters often contains high-amplitude, short-wavelength anomalies. There are several processes that can lead to such anomalies; based on data from the Mien crater, Henkel (1982) suggests them to be caused by near-surface impact-melt bodies with strong remahence+ and by bodies of unaltered magnetic target rocks within the brecciated region. Grieve and Peso-
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J. OrmS, G. Blomqvist / Tectonophysics 262 (1996) 291-300
(a) lProfile A
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J. OrmiJ, G. Blomqeist / Tectonophysics 262 (1996)291-300
nen (1992) give some suggestions on how shock and thermal processes may generate remanent magnetization within the breccias. Because no such anomalies can be seen inside Tv~iren, but only at the edge of the crater, we interpret that those observed result from relatively large bodies of less fractured target rocks a n d / o r original high-magnetic parts of the bedrock. Another alternative is impact melt. Though most high-magnetic anomalies are cut at the edge of the fractured region, in some places high magnetic structures in the surroundings can be traced as weaker anomalies through the fractured region surrounding the crater. On the eastern side of the bay this can be seen as a north-south-striking anomaly. In the case of the small high-magnetic slab on the eastern side of the crater in profile B, an alternative model could be a larger body of original high-magnetic signature, with connection down into the unaltered bedrock below. The model chosen here gives the best fit of the curves and is therefore a more likely alternative. The same arguments are valid for the slab in the northern part of profile C. The small slab to the south in profile C creates a sharp anomaly with steep gradients. It is most likely a small body with a position close to the surface. As its position is somewhat outside the fractured region and in the lowmagnetic part of the bedrock surrounding Tv~iren, it is most likely an original high-magnetic structure. The three slabs can not be interpreted as impact melt bodies as the small size of the crater and the position of the anomalies close to, and even outside, the crater rim speak against that alternative. In the drillcores, only small amounts of melt appear between the fragments in the crystalline authochtonous breccia and most likely, in accordance with the small size of the crater, melt is rare in the whole structure. Similar experiences from drillings and magnetic investigations in the Lockne impact crater (Orm/5 et al., 1995), almost four times larger than Tv~iren, reveal that not only the small size of the crater may be responsible for the absence of substantial melt bodies; unlike Mien, Dellen and many other craters with large amounts of melt, both Tv~iren and Lockne were formed at sea with the crystalline bedrock covered with relatively thick sedimentary deposits, mostly carbonates, which preferentially dissociate. Potential impact related remanence in the brecciated basement would also be strongly reduced in connec-
297
tion with the oxidation process that changed the magnetic mineralogy after the impact. In Tv~iren the low-magnetic area is surrounded on the northern, eastern and southern sides by areas of high-magnetic anomalies corresponding to unaffected gneissose granitoids of high susceptibilities. On the aeromagnetic map it can be seen how highmagnetic structures within the gneissose granitoids, striking towards the crater, are cut at the edge of the low-magnetic area (Fig. 3). The difference between the highest anomalies of the surroundings and the lowest values of the low area, along the investigated profiles, is about 1800 nT. The model computations, together with the information from earlier seismic (Flodrn et al., 1986) and drillcore investigations (Lindstrtim et al., 1994), resulted in a shallow bowl-like shape of the crater structure. This fiat bowl-shaped appearance is described by Melosh (1989) as common for simple craters, that is craters with a diameter less than 4 km when formed in crystalline rocks (Dence et al., 1977). The depth from present sea-level to the top of the brecciated crystalline bedrock is about 230 m in the centre of the crater. The fractured region underneath the crater has been divided into a highly fractured part and a less fractured part representing the transition into unaltered bedrock. As the modelling is dependent on differences in susceptibilities between the rock types, the transition into unaltered bedrock can bee seen as different degrees of oxidation to depth. Fracturing and oxidation are, as already mentioned, closely connected but the effect of healing of fractures could somewhat decrease the depth to which oxidation is able to reach. This may result in the depth to unaltered bedrock obtained in the modelling being a minimum depth. The depth to the unaltered rocks is in our model found to be about 650 m. A change of this depth results in a distinct deviation from the measured anomaly, which could not be adjusted without changing other fixed parameters. The deviation is more of a change in the shape of the calculated curve at the edges of the structure than a change of amplitude in the central part. This is more obvious in the north-south striking profiles as the magnetic inclination amplifies these edge effects. The depth to the unaltered rocks has been set to the same value in the intersection point between the three profiles. The depth is therefore a compromise
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between the results of the modellings of the three profiles. This explains the remaining discrepancy between observed and measured anomaly in the central part of the structure. The area occupied by the low-magnetic part of the anomaly is interpreted as representing the extent of the fractured and oxidized bedrock. When the extent of the fractured bedrock from the three profiles is synthesized on a map (Fig. 6), the diameter of the resulting area is approximately 4 kin, thus reaching 1 km outside the actual 2-kin-wide sediment filled crater. This is consistent with the observation mentioned by Flod~n et al. (1986), that an area to a distance of 2r from the center of a crater with the radius r is fractured with outwards decreasing intensity. However, Gurov and Gurova (1982) come to
another conclusion after studying the Siberian Elgygytgyn structure. There they found that increased fracturing can extend to approximately one crater diameter outside the crater rim. Elgygytgyn though, is more than 10 times larger than Tv~iren. The limit of the fractured region in Tv~iren does not clearly show topographically. In the field, a more intense tYacturing can be observed on some of the capes along the northern and western sides of the bay; on one of them, meteoric water drained down into fractures in the ground. In the same way as with the fractured region, the contact between the high-magnetic rocks in the east and the low-magnetic rocks in the west can be drawn from the information from the profiles and the aeromagnetic map. In the simplified magnetic map (Fig. 3) the low-magnetic dotted areas
Fig. 6. Extension of the apparent crater and the surrounding fractured region. Major fracture zones and the border between low-magnetic gneissose granitoids in the west and higher magnetic rocks of the same type in the east. are indicated. Interpreted from the modelled profiles and the aeromagnetic map (LundstriSm, 1976).
J. Ormg, G. Blomqcist / Tectonophysics 262 (1996)291-300
are due to edge effects caused by the dip of the contact. The area around the Tv~iren crater is dissected by several distinct N W - S E striking fracture zones seen both in the topography and in the magnetic data. The fracture zone in profile B (see Fig. 1), which was deeply excavated by later Quaternary ice movements, causes a magnetic low. The north-southmoving ice is also most likely responsible for the deep trough along the steep eastern side of the crater best seen in profile B. The fracture zones are likely to have existed in the crystalline basement prior to the impact due to the high age of the basement, although this assumption does not affect our modelling. What can be seen in the magnetic model is that the fracture zone that intersects the impact generated fractured region in profile B, has lower susceptibility than the region fractured by the impact. This may indicate reactivation of an already existing fracture zone long after the impact or that the fracture zone is younger than the impact. Because of movements in the fracture zone the oxidation process could continue in the otherwise healed fractured region that would have prevented further water circulation. Even the second fracture zone that crosses the the southwestern part of the crater structure is reconstructed as cutting the fractured regions. This could not be proven in the modelling. The fracture zone cuts through a low-magnetic part of the profile but because of the small contrast to the surrounding rocks it does not affect the anomalies. However, where it cuts the profile a minimum occurs that is not reproduced by the model. It may be a contact minimum of a north-facing edge of the high-magnetic area to the south. If this high-magnetic rock is somewhat upthrusted at the fracture zone, another shape of the southwestern part of the crater is achieved. It would be a steeper wall with high-magnetic rocks closer to the surface on the southern side which, in this connection, would be able to generate the magnetic minimum. However, in our modellings we could not produce this effect and therefore we consider the present model as the most likely alternative. Although the fractures are very distinct in several data sets, the still very regular shape of the crater indicates that any displacements that have occurred after the impact event are within the spatial resolution of the magnetic data i.e. 200 m.
299
4. Conclusions The computer modelling of the magnetic anomalies in the Tv~iren area permits the following conclusions to be drawn about the method and the structure: (1) Knowledge of the variations in the magnetic field of the area and the magnetic properties of the rocks made it possible to create a computer model of the crater structure. (2) The method is an important supplement to other geological and geophysical investigations in estimating the shape, depth and extension of the crater and the surrounding region of basement that was fractured by the impact. (3) Autigenic brecciation affected by oxidation extends 2 km from the center of the crater and reaches a depth of at least 650 m. (4) The structure contains no major amounts of magnetic impact melt or other remanent material. (5) Magnetic anomalies related to structures within the bedrock of the area are cut at the edge of the crater, i.e. 1 km from the center. (6) A flat bowl-like shape of the crater was obtained, similar to other impact structures of comparable size. (7) The rather intact shape of the crater indicates that no severe post impact deformation has occurred along the fracture zones intersecting the structure.
Acknowledgements We are indebted to Herbert Henkel (Royal Institute of Technology, Stockholm) and professor Maurits Lindstr~m (Stockholm University) for great help during the work and valuable comments on the manuscript. The work of Jens Orm~5 was supported by The Bank of Sweden Tercentenary Foundation and the Royal Swedish Academy of Sciences.
References Beals, C.S., lnnes, M.J.S. and Rottenberg, J.A., 1963. Fossil meteorite craters. In: B.M. Middlehurst and G.P. Kuipe (Editors), The Moon, Meteorites and Comets. Univ. Chicago Press, Chicago, IL, pp. 235-284. Dence, M.R., Grieve. R.A.F. and Robertson, P.B., 1977. Terres-
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trial impact structures: Principal characteristics and energy considerations. In: D.J. Roddy, R.O. Pepin and R.B. Merrill (Editors), Impact and Explosion Cratering. Pergamon, New York, NY, p. 247. Elo, S., Jokinen, T. and Soininen, H., 1992a. Geophysical investigations of the Lake Lapp@irvi impact structure, western Finland. Tectonophysics, 216: 99-109. Elo, S., Kivek~is, L., Kujala, H., Lahti, S.I. and Pihlaja, P., 1992b. Recent studies of the Lake S~i~iksjRrvimeteorite impact crater, southwestern Finland. Tectonophysics, 216:163-167. Eriksson, L. and Henkel, H. 1994. Geofysik. In: C. Fred~n (Editor), Sveriges Nationalatlas. Berg och Jord. pp. 76-101. Flod~n, T., Tunander, P. and Wickman, F.E., 1986. The Tv~iren Bay structure, an astrobleme in southeastern Sweden. Geol. F~Sren. Stockholm FiSrh., 108: 225-234. Fredriksson, K. and Wickman, F.E., 1963. Meteoriter. Sven. Naturvetensk., 16: 121-157. GeoVista AB, 1994. GMM, Interactive Gravity and Magnetic Modelling Program, Version 1.4. User's Manual, pp. 1-31. Grahn, Y. and N~lvak, J., 1993. Chitinozoan dating of Ordovician impact events in Sweden and Estonia. A preliminary note. Geol. F/3ren. Stockholm F/Srh., 115: 263-264. Grieve, R.A.F. and Pesonen, L.J., 1992. The terrestrial impact cratering record. Tectonophysics, 216: 1-30. Gurov, E.P. and Gurova, E.P., 1982. Some regularities of the areal spreading of fractures around Elgygytgyn crater. Lunar Planet Sci., 13: 291-292. Henkel, H. 1982. The Lake Mien structure. Geological Survey of Sweden. Geophysical department report. 8221.
Henkel, H., 1992. Geophysical aspects of meteorite impact craters in eroded shield environment with special emphasis on electric resistivity. Tectonophysics, 216: 63-89. Henkel, H., 1994. Standard diagrams of magnetic properties and density - - a tool for understanding magnetic petrology. J. Appl. Geophys., 32: 43-53. Henkel, H. and Guzm~n, M., 1977. Magnetic features of fracture zones. Geoexploration, 15: [73-181. Henkel, H. and Pesonen, L.J., 1992. Impact craters and craterform structures in Fennoscandia. Tectonophysics, 216: 31-40. Lindstrtim, M., Flod~n, T., Grahn, Y. and Kathol, B., 1994. Post-impact deposits in Tv~iren, a marine Middle Ordovician crater south of Stockholm, Sweden. Geol. Mag., 131 : 91 - 103. Lundstr~Sm, I., 1976. Berggrundskartan, NykiSping SO. Sver. Geol. Unders., Ser. Af 114. Melosh, H.J., 1989. hnpact cratering, a geologic process. Oxford Monographs on Geology and Geophysics, I 1: 1-245. Orm~5, J., 1994. The pre-impact stratigraphy of the Tv~iren Bay impact structure, SE Sweden. Geol. F~iren. Stockholm FiSrh., 116: 139-144. Ormti, J., Blomqvist, G., Sturkell, E.F.F. and T~Srnberg, R., 1995. Modelling of impact craters based on magnetometry. In: A. Montanari and R. Coccioni (Editors), Abstracts and Field Trips. Impact Cratering and Evolution of Planet Earth - - 4th International Workshop, Ancona, Italy, 130 pp. Puura, V. and Suuroja, K., 1992. Ordovician impact crater at K~.rdla, Island of Hiiumaa, Estonia. Tectonophysics, 216: 143-156.
Tectonophysics 262 (1996) 291-300
Magnetic modelling as a tool in the evaluation of impact structures, with special reference to the Tv iren Bay impact crater, SE Sweden J. Ormi5 a,*, G. Blomqvist b.~ " Department o['Geology and Geochemistry, Stockhohn Lh*iversiO', S-10691 Stockhohn. Swede. i~ Di~'i.~ion ~?['Land and Water Resources, Royal hr~titute ~!/+Technolo,W, S-10044 Stockholm, Swede.
Received 24 July 1995: accepted 5 January 1996
Abstract Until recently computer modelling of radial-symmetric structures, like impact craters, has presented difficulties. As a result of this, such models are scarce in the literature. In this study, the magnetic structure of the marine Tvfiren Bay impact crater (58°46'N, 17°25'E) south of Stockholm is investigated with a sophisticated commercially available computer program developed for gravimetric and magnetic modelling of geological structures. Aeromagnetic data along three profiles across the crater have been used. It is concluded that the method is an important supplement to other geological and geophysical investigations in estimating the shape, depth and extension of the crater and the surrounding region of basement that was fractured by the impact.
1. Introduction
Fredriksson and Wickman (1963) were the first to suggest that the unusually deep Tvfiren Bay (Fig. 1) is an impact crater. Seismic investigations (Flod~n et al., 1986) and the recovery of two drill cores from within the structure where planar deformation features in quartz were found (LindstriSm et al., 1994), have proven this hypothesis to be correct. The impact event is now dated by means of chitinozoans of late Kukrusean age (c. 455 Ma) (Grahn and N61vak, 1993). At that time the area was covered by a shallow shelf sea (Flod~n et al., 1986; Lindstrtim et
Corresponding author, e-mail: [email protected] I Fax +46 8 4110775. E-mail: [email protected].
al., 1994), too deep for the crater rim to prevent the water from resurging into the crater. The almost circular bay is approximately 3.5 km across and has in its central part a circular depression 2 km in diameter with a depth to the fractured Precambrian crystalline bedrock somewhat more than 200 m. The crystalline bedrock consists in the northern, eastern and southern surroundings of the bay of gneissose granitoids of intermediate composition+ with relatively high magnetic susceptibilities. To the west of the bay the rock types are the same but with lower susceptibility values resulting in lower magnetic anomalies on the aeromagnetic map (Lundst65m. 1976). Four major northwest-southeast-striking fracture zones and several smaller ones cut through the outer parts of the structure and the surrounding area (Fig. 1). The structure itself is partly filled with
0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights rcserved. Pll S0040- 195 I (96)000 15-7
J. OrmiL G. Blomqrist / TectonolYhysics 262(1996)291-300
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Palaeozoic and Quaternary sediments. In the most complete of the two drillcores mentioned above, Tv~iren 2, 137 m of the Paleozoic sediments were recovered above 5 m of brecciated crystalline bedrock. The other drilling, Tv~iren 1, almost directly hit the crystalline breccia and was stopped after only 4 m. The lower part of the Palaeozoic sequence was
formed by the resurge of water into the crater immediately after the impact. While flowing back into the crater the water carried masses of fragments of the crystalline bedrock and the sedimentary rocks that already existed in the area, mostly Orthoceratite Limestone (OrmiS, 1994). The sequence starts with a calcareous breccia grading upwards into finer tur-
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Fig. 1. Location of the Tv~iren Bay impact structure, the two areas sampled for magnetic data, the Tv~iren 1 and 2 drillings, major fracture zones and the three investigated profiles (A, B, C).
J. Ormi~, G. Blomqvist / Tectonophysics 262 (1996) 291-300
biditic sandstones and finally mudstones. Above this succession follow post impact secular sediments, mostly bituminous carbonatic mudstones and nearly 40 m of nonlithified Quaternary sediments (LindstriSm et al., 1994). The maximum water depth in the structure today is 80 m. Below the sedimentary infill there is a region of brecciated crystalline bedrock to an uncertain depth. The work described herein produces a model of the crater shape and tries to constrain the extension and shape of this fractured region, based on magnetic data.
2. Data and method The constraints required in order to make a good magnetic model of the structure are first of all the magnetic susceptibilities for all major rock types within and around the structure. Susceptibility values from two characteristic localities (Fig. 1) were kindly put at our disposal by Herbert Henkel, Royal Institute of Technology, Stockholm. The values from the two localities were treated separately and therefore, in some cases, two slightly different values for the same rocktypes were achieved. These were used respectively for the closest profile. From the susceptibility distributions (Fig. 2) type values were calculated and associated with the bodies in the models representing the different rocktypes of the area. Measurements of the susceptibility of the crystalline breccia in the lowermost parts of the two drill cores gave values used for this lithology in the model. Susceptibility measurements were also made on the sediments in the Tv~iren 2 drill core. The resulting values were used for the different types of sedimentary rock infill in the crater. In profile A, the sediments in the crater are divided into post-impact sedimentary rocks and Quaternary sediments. As the magnetic properties of the Quaternary sediments are similar to water both the B and C profiles have been simplified in that aspect. The Quaternary sediments and the water were given a value close to zero. The fracture zones intersecting the crater were given susceptibility values approximately 100 times lower than the surrounding rocks, according to Henkel and Guzmfin (1977). Values for the local declination and inclination of the magnetic field were found in Sveriges Nation-
293
alatlas (Eriksson and Henkel, 1994). The magnetic anomalies in the area were taken from the aeromagnetic map published in Lundstrt~m (1976). Measurements for the construction of the map have been made at a flight altitude of 30 m, a line spacing of 200 m, and with measurement spacing along the lines of 40 m. The flight direction was east-west. The original map has 40 nT contours of the total magnetic field. A simplified version of the map for the Tv~iren structure (Fig. 3) shows most of the magnetic features discussed in the text. The profiles are based on the original map and are therefore more detailed than Fig. 3. Since felsic plutonic rocks prevailing in this area usually lack significant remanence (Henkel, 1994) the modelling has been made without remanence in the unaltered bedrock. Three profiles (for location see Fig. 1) were interpolated from the aeromagnetic map intersecting the Tv~iren 2 drillcore which is located somewhat to the north-west of the centre of the crater. They are transverse to the crater rim and continue for a distance of about 2 km outside the crater. Due to an area of almost no differences in the magnetic anomalies to the north-west of the crater (see Fig. 3), the profile A was chosen to extend only from Tv~iren 2 and over the south-eastern crater rim. The part of the profile modelled to the left of the point 0 is therefore to be considered as a conjectural complement. Magnetic anomaly values interpolated from the map at 35 m intervals along these profiles were used in the modelling. Having the profiles in the chosen directions also made it possible to use information about the crater form and the thickness of the sedimentary infill from seismic profiles of approximately the same directions published in Flod~n et al. (1986) and Lindstr~Sm et al. (1994). The observations along these profiles were used to fix the inner crater wall which should not change during the modelling. The shape of the crater and the surrounding fractured regions in Fig. 6 were drawn by using the information received from the modelled profiles and connecting these by following the shape of the isomagnetic lines of the anomalies on the aeromagnetic map. The inter-active program GMM (Gravity and Magnetic Modelling) provided by GeoVista AB (1994), was used in the computer modelling. The features of this program include the possibilities to work in both horizontal and vertical view, to change
294
J. OrmS. G. BlomqHst / Tectonophysics 262 (1996) 291-300
that however has very little effect on the modelling because of the distance between the corners of the bodies and the profile. After feeding the computer with the magnetic field parameters of the area and the susceptibility values for all known bodies in the
the strike length and strike direction of the bodies in the model and also to offset bodies in relation to the profile. The shape of the bodies in the horizontal dimension have to be quadrangular. This explains why the crater shape in Fig. 4 is not circular, a fact
25
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Fig. 2. Histograms of the susceptibilities from the two sampled areas and the drillcores.
SusceDtibility (SI)
J. Orm6, G. Blomqt'ist / Tectonophysics 262 ( 19961 291-300
295
model, the program can produce a magnetic response curve. By changing parameters such as shape, direction, depth below surface etc. for nonfixed bodies introduced into the model one can make this response curve fit with the one obtained from the map. The model created in this way is an approximation of the shape of the crater at depth and along the profile.
3. Model interpretations and discussion
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The three model profiles (Fig. 5) show a magnetic low over the Tv~iren Bay with almost no internal variations of the magnetic anomalies. Similar magnetic lows can be seen in other impact structures, even in much larger ones like the Saint Martin crater (Grieve and Pesonen, 1992), the complex Lake Lapp@irvi crater (Elo et al., 1992a), the Lake S~i~ik@irvi crater (Elo et al., 1992b), the K~irdla crater, which is very similar to Tv~iren in other aspects as well (Puura and Suuroja, 1992), Lockne (Ormi5 et al., 1995), Mien (Henkel, 1982) and Dellen (Henkel, 1992). As this magnetic-low area appears in craters of various sizes and with different lithologies, it can be considered a typical feature in crater magnetometry. It is, however, most obvious where the structure is developed in a high magnetic crystalline bedrock. The magnetic low is explained as due to the oxidation of magnetite in the brecciated zone (Henkel and Pesonen, 1992) and the depth to the buried unaffected basement under the low-magnetic sedimentary crater infill (Henkel, 1992). Another relevant factor may also be the disruption and disordering of magnetization vectors caused by brecciation (Beals et al., 1963). This might be of interest in structures with significant remanence which is not the case in the mentioned crater structures. The effects of oxidation of fractured rocks are known from investigations of fracture zones (Henkel and Guzmfin, 1977). A significant difference between Tv~iren and the larger craters among those mentioned above, is that the central area of the magnetic low of larger craters often contains high-amplitude, short-wavelength anomalies. There are several processes that can lead to such anomalies; based on data from the Mien crater, Henkel (1982) suggests them to be caused by near-surface impact-melt bodies with strong remahence+ and by bodies of unaltered magnetic target rocks within the brecciated region. Grieve and Peso-
296
J. OrmS, G. Blomqvist / Tectonophysics 262 (1996) 291-300
(a) lProfile A
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J. OrmiJ, G. Blomqeist / Tectonophysics 262 (1996)291-300
nen (1992) give some suggestions on how shock and thermal processes may generate remanent magnetization within the breccias. Because no such anomalies can be seen inside Tv~iren, but only at the edge of the crater, we interpret that those observed result from relatively large bodies of less fractured target rocks a n d / o r original high-magnetic parts of the bedrock. Another alternative is impact melt. Though most high-magnetic anomalies are cut at the edge of the fractured region, in some places high magnetic structures in the surroundings can be traced as weaker anomalies through the fractured region surrounding the crater. On the eastern side of the bay this can be seen as a north-south-striking anomaly. In the case of the small high-magnetic slab on the eastern side of the crater in profile B, an alternative model could be a larger body of original high-magnetic signature, with connection down into the unaltered bedrock below. The model chosen here gives the best fit of the curves and is therefore a more likely alternative. The same arguments are valid for the slab in the northern part of profile C. The small slab to the south in profile C creates a sharp anomaly with steep gradients. It is most likely a small body with a position close to the surface. As its position is somewhat outside the fractured region and in the lowmagnetic part of the bedrock surrounding Tv~iren, it is most likely an original high-magnetic structure. The three slabs can not be interpreted as impact melt bodies as the small size of the crater and the position of the anomalies close to, and even outside, the crater rim speak against that alternative. In the drillcores, only small amounts of melt appear between the fragments in the crystalline authochtonous breccia and most likely, in accordance with the small size of the crater, melt is rare in the whole structure. Similar experiences from drillings and magnetic investigations in the Lockne impact crater (Orm/5 et al., 1995), almost four times larger than Tv~iren, reveal that not only the small size of the crater may be responsible for the absence of substantial melt bodies; unlike Mien, Dellen and many other craters with large amounts of melt, both Tv~iren and Lockne were formed at sea with the crystalline bedrock covered with relatively thick sedimentary deposits, mostly carbonates, which preferentially dissociate. Potential impact related remanence in the brecciated basement would also be strongly reduced in connec-
297
tion with the oxidation process that changed the magnetic mineralogy after the impact. In Tv~iren the low-magnetic area is surrounded on the northern, eastern and southern sides by areas of high-magnetic anomalies corresponding to unaffected gneissose granitoids of high susceptibilities. On the aeromagnetic map it can be seen how highmagnetic structures within the gneissose granitoids, striking towards the crater, are cut at the edge of the low-magnetic area (Fig. 3). The difference between the highest anomalies of the surroundings and the lowest values of the low area, along the investigated profiles, is about 1800 nT. The model computations, together with the information from earlier seismic (Flodrn et al., 1986) and drillcore investigations (Lindstrtim et al., 1994), resulted in a shallow bowl-like shape of the crater structure. This fiat bowl-shaped appearance is described by Melosh (1989) as common for simple craters, that is craters with a diameter less than 4 km when formed in crystalline rocks (Dence et al., 1977). The depth from present sea-level to the top of the brecciated crystalline bedrock is about 230 m in the centre of the crater. The fractured region underneath the crater has been divided into a highly fractured part and a less fractured part representing the transition into unaltered bedrock. As the modelling is dependent on differences in susceptibilities between the rock types, the transition into unaltered bedrock can bee seen as different degrees of oxidation to depth. Fracturing and oxidation are, as already mentioned, closely connected but the effect of healing of fractures could somewhat decrease the depth to which oxidation is able to reach. This may result in the depth to unaltered bedrock obtained in the modelling being a minimum depth. The depth to the unaltered rocks is in our model found to be about 650 m. A change of this depth results in a distinct deviation from the measured anomaly, which could not be adjusted without changing other fixed parameters. The deviation is more of a change in the shape of the calculated curve at the edges of the structure than a change of amplitude in the central part. This is more obvious in the north-south striking profiles as the magnetic inclination amplifies these edge effects. The depth to the unaltered rocks has been set to the same value in the intersection point between the three profiles. The depth is therefore a compromise
298
J. OrmiJ, G. Blomqt'ist / Tectonophysics 262(19961 291 300
between the results of the modellings of the three profiles. This explains the remaining discrepancy between observed and measured anomaly in the central part of the structure. The area occupied by the low-magnetic part of the anomaly is interpreted as representing the extent of the fractured and oxidized bedrock. When the extent of the fractured bedrock from the three profiles is synthesized on a map (Fig. 6), the diameter of the resulting area is approximately 4 kin, thus reaching 1 km outside the actual 2-kin-wide sediment filled crater. This is consistent with the observation mentioned by Flod~n et al. (1986), that an area to a distance of 2r from the center of a crater with the radius r is fractured with outwards decreasing intensity. However, Gurov and Gurova (1982) come to
another conclusion after studying the Siberian Elgygytgyn structure. There they found that increased fracturing can extend to approximately one crater diameter outside the crater rim. Elgygytgyn though, is more than 10 times larger than Tv~iren. The limit of the fractured region in Tv~iren does not clearly show topographically. In the field, a more intense tYacturing can be observed on some of the capes along the northern and western sides of the bay; on one of them, meteoric water drained down into fractures in the ground. In the same way as with the fractured region, the contact between the high-magnetic rocks in the east and the low-magnetic rocks in the west can be drawn from the information from the profiles and the aeromagnetic map. In the simplified magnetic map (Fig. 3) the low-magnetic dotted areas
Fig. 6. Extension of the apparent crater and the surrounding fractured region. Major fracture zones and the border between low-magnetic gneissose granitoids in the west and higher magnetic rocks of the same type in the east. are indicated. Interpreted from the modelled profiles and the aeromagnetic map (LundstriSm, 1976).
J. Ormg, G. Blomqcist / Tectonophysics 262 (1996)291-300
are due to edge effects caused by the dip of the contact. The area around the Tv~iren crater is dissected by several distinct N W - S E striking fracture zones seen both in the topography and in the magnetic data. The fracture zone in profile B (see Fig. 1), which was deeply excavated by later Quaternary ice movements, causes a magnetic low. The north-southmoving ice is also most likely responsible for the deep trough along the steep eastern side of the crater best seen in profile B. The fracture zones are likely to have existed in the crystalline basement prior to the impact due to the high age of the basement, although this assumption does not affect our modelling. What can be seen in the magnetic model is that the fracture zone that intersects the impact generated fractured region in profile B, has lower susceptibility than the region fractured by the impact. This may indicate reactivation of an already existing fracture zone long after the impact or that the fracture zone is younger than the impact. Because of movements in the fracture zone the oxidation process could continue in the otherwise healed fractured region that would have prevented further water circulation. Even the second fracture zone that crosses the the southwestern part of the crater structure is reconstructed as cutting the fractured regions. This could not be proven in the modelling. The fracture zone cuts through a low-magnetic part of the profile but because of the small contrast to the surrounding rocks it does not affect the anomalies. However, where it cuts the profile a minimum occurs that is not reproduced by the model. It may be a contact minimum of a north-facing edge of the high-magnetic area to the south. If this high-magnetic rock is somewhat upthrusted at the fracture zone, another shape of the southwestern part of the crater is achieved. It would be a steeper wall with high-magnetic rocks closer to the surface on the southern side which, in this connection, would be able to generate the magnetic minimum. However, in our modellings we could not produce this effect and therefore we consider the present model as the most likely alternative. Although the fractures are very distinct in several data sets, the still very regular shape of the crater indicates that any displacements that have occurred after the impact event are within the spatial resolution of the magnetic data i.e. 200 m.
299
4. Conclusions The computer modelling of the magnetic anomalies in the Tv~iren area permits the following conclusions to be drawn about the method and the structure: (1) Knowledge of the variations in the magnetic field of the area and the magnetic properties of the rocks made it possible to create a computer model of the crater structure. (2) The method is an important supplement to other geological and geophysical investigations in estimating the shape, depth and extension of the crater and the surrounding region of basement that was fractured by the impact. (3) Autigenic brecciation affected by oxidation extends 2 km from the center of the crater and reaches a depth of at least 650 m. (4) The structure contains no major amounts of magnetic impact melt or other remanent material. (5) Magnetic anomalies related to structures within the bedrock of the area are cut at the edge of the crater, i.e. 1 km from the center. (6) A flat bowl-like shape of the crater was obtained, similar to other impact structures of comparable size. (7) The rather intact shape of the crater indicates that no severe post impact deformation has occurred along the fracture zones intersecting the structure.
Acknowledgements We are indebted to Herbert Henkel (Royal Institute of Technology, Stockholm) and professor Maurits Lindstr~m (Stockholm University) for great help during the work and valuable comments on the manuscript. The work of Jens Orm~5 was supported by The Bank of Sweden Tercentenary Foundation and the Royal Swedish Academy of Sciences.
References Beals, C.S., lnnes, M.J.S. and Rottenberg, J.A., 1963. Fossil meteorite craters. In: B.M. Middlehurst and G.P. Kuipe (Editors), The Moon, Meteorites and Comets. Univ. Chicago Press, Chicago, IL, pp. 235-284. Dence, M.R., Grieve. R.A.F. and Robertson, P.B., 1977. Terres-
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trial impact structures: Principal characteristics and energy considerations. In: D.J. Roddy, R.O. Pepin and R.B. Merrill (Editors), Impact and Explosion Cratering. Pergamon, New York, NY, p. 247. Elo, S., Jokinen, T. and Soininen, H., 1992a. Geophysical investigations of the Lake Lapp@irvi impact structure, western Finland. Tectonophysics, 216: 99-109. Elo, S., Kivek~is, L., Kujala, H., Lahti, S.I. and Pihlaja, P., 1992b. Recent studies of the Lake S~i~iksjRrvimeteorite impact crater, southwestern Finland. Tectonophysics, 216:163-167. Eriksson, L. and Henkel, H. 1994. Geofysik. In: C. Fred~n (Editor), Sveriges Nationalatlas. Berg och Jord. pp. 76-101. Flod~n, T., Tunander, P. and Wickman, F.E., 1986. The Tv~iren Bay structure, an astrobleme in southeastern Sweden. Geol. F~Sren. Stockholm FiSrh., 108: 225-234. Fredriksson, K. and Wickman, F.E., 1963. Meteoriter. Sven. Naturvetensk., 16: 121-157. GeoVista AB, 1994. GMM, Interactive Gravity and Magnetic Modelling Program, Version 1.4. User's Manual, pp. 1-31. Grahn, Y. and N~lvak, J., 1993. Chitinozoan dating of Ordovician impact events in Sweden and Estonia. A preliminary note. Geol. F/3ren. Stockholm F/Srh., 115: 263-264. Grieve, R.A.F. and Pesonen, L.J., 1992. The terrestrial impact cratering record. Tectonophysics, 216: 1-30. Gurov, E.P. and Gurova, E.P., 1982. Some regularities of the areal spreading of fractures around Elgygytgyn crater. Lunar Planet Sci., 13: 291-292. Henkel, H. 1982. The Lake Mien structure. Geological Survey of Sweden. Geophysical department report. 8221.
Henkel, H., 1992. Geophysical aspects of meteorite impact craters in eroded shield environment with special emphasis on electric resistivity. Tectonophysics, 216: 63-89. Henkel, H., 1994. Standard diagrams of magnetic properties and density - - a tool for understanding magnetic petrology. J. Appl. Geophys., 32: 43-53. Henkel, H. and Guzm~n, M., 1977. Magnetic features of fracture zones. Geoexploration, 15: [73-181. Henkel, H. and Pesonen, L.J., 1992. Impact craters and craterform structures in Fennoscandia. Tectonophysics, 216: 31-40. Lindstrtim, M., Flod~n, T., Grahn, Y. and Kathol, B., 1994. Post-impact deposits in Tv~iren, a marine Middle Ordovician crater south of Stockholm, Sweden. Geol. Mag., 131 : 91 - 103. Lundstr~Sm, I., 1976. Berggrundskartan, NykiSping SO. Sver. Geol. Unders., Ser. Af 114. Melosh, H.J., 1989. hnpact cratering, a geologic process. Oxford Monographs on Geology and Geophysics, I 1: 1-245. Orm~5, J., 1994. The pre-impact stratigraphy of the Tv~iren Bay impact structure, SE Sweden. Geol. F~iren. Stockholm FiSrh., 116: 139-144. Ormti, J., Blomqvist, G., Sturkell, E.F.F. and T~Srnberg, R., 1995. Modelling of impact craters based on magnetometry. In: A. Montanari and R. Coccioni (Editors), Abstracts and Field Trips. Impact Cratering and Evolution of Planet Earth - - 4th International Workshop, Ancona, Italy, 130 pp. Puura, V. and Suuroja, K., 1992. Ordovician impact crater at K~.rdla, Island of Hiiumaa, Estonia. Tectonophysics, 216: 143-156.