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Traces of impact craters in the geoid
41
Tecfonophysics, 216 (1992) 41-44
Elsevier Science Publishers B.V., Amsterdam
Extended Abstract
Traces of impact craters in the geoid Juhani Kakkuri Finnish Geodetic Institute, Ihnalankatu I A, SF-00240 Helsinki, Finland
(Received November 11, 1990; revised version accepted December 30, 1991)
A well-known method for studying the impact craters on the Earth’s surface is gravity measurements. For practical studies, the gravity measured on the surface must first be reduced to a particular standard. In a simpIe procedure, after subtracting the latitude-dependent normal gravity, the topographic masses outside the geoid are either completely removed to infinity or shifted to the mean sea level. The gravity station is then lowered from the Earth’s surface to the geoid. Resulting values are Bouguer gravity anomalies, if the topographic masses are completely removed to infinity, or free air gravity anomalies, if these masses are shifted to the geoid. Since the Bouguer gravity anomalies indicate the effect of disturbing masses, they are often used by geophysicists in their search for shaIlow features in the upper part of the Earth’s crust. In Finland, these anomalies have been used to study impact structures, for example, those associated with Lake Lappaj~~i. According to Elo (1976) and Elo et al. (this issue) there is a negative Bouguer anomaly field with a diameter of 17 km and a relative minimum of - 10 mGa1 at Lake Lappajarvi. Free air gravity anomalies are traditionally applied to geoid computation. The number of details in the geoid calculated depends on the density of the gravimetric net. The highest resolution
Correspondence to: J. Kakkuri, Finnish Geodetic Ilmalankatu 1 A, SF-00240 Helsinki, Finland.
Institute,
which can be obtained is a few centimetres. A gravimetric geoid calculated for Northern Europe by Forsberg (1990) is shown in Figure 1. The spacing of gravity stations behind this geoid varies from 5 to 10 km and the inner accuracy is better than 10 cm. Ahhough numerous geological structures, such as, for example, the boundaries between the Archaean and Proterozoic rocks, the Sveconorwegian rocks, the Caledonian orogeny belt, and the Rapakivi granite areas, are visible in this geoid, the impact structures cannot be seen, due to insuffient resolution. We have computed a new local geoid with a resolution high enough to make the impact structures visible. Computation was made separately for Lake Lappajarvi and the Siljan Ring. The resulting geoids have been compared with the Bouguer anomaly maps of these craters. In order to compute a high resolution local geoid accurate to a few centimetres, a dense gravity net is needed. The density of the Finnish national gravity net, one point per 5 x 5 km’, is not adequate for revealing sufficient details of the structure in an impact crater. Fortunately, there is a local gravity net at Lake LappajHrvi with a density five times that of the national net. The Swedish national gravity net is about as sparse as the Finnish one. Additional data were provided by a special Bouguer anomaly map of the Siljan Ring, published by the Geological Survey of Sweden (1990). This map, which is based on a dense local network, was divided into squares of 1 X 1 km’ and then the mean Bouguer anomaly was estimated for each square. This added about
0040-1951/92/%05.00 0 1992 - Elsevier Science Publishers B.V. AII rights reserved
.I. KAKKURI
42
8,400 new mean values to the data file. These anomalies were further converted into free air anomalies with the aid of a topographic map. Computation of geoid anomalies was performed with software from the National Survey and Cadastre, Copenhagen, Denmark (Forsberg, 1990; Tscherning, 1990). The “remove/restore” principle was followed: first, the globaf model IFE88E2 was removed from the gravity anomalies given: then, from the residual gravity anomalies thus obtained, residual geoid anomalies were computed, and the global model IFE88E2 again added to these. For the Siljan Ring, the OSU8Y~ model was used instead. In this study, however, only residual geoid anomalies (without the re-
Fig. I. The NOI rdic Geoid
computed
by For&erg
(1990).
structure;
The datum
store phase) were used as these show better the local details. The following results were obtained: &‘@r Ring: The Siljan Ring is situated in central Sweden; its diameter is 45 km and its age is 360 Ma. The Ring has a very complex crustal structure, with Phanerozoic sedimentary rocks which form a belt around the granitoids and diabases in the centre of the crater (Gorbunov and Papunen, 1985). There has probably been considerable erosion over the long time span and, therefore, the emerging geoid details are related to the remaining deep crustal structures caused by the impact. Some clear details have emerged, such as the bulge in the centre of the crater. This
is GRSSO and the contour
L = lake Lappajlrvi
impact crater,
interval
25 cm. S = Siljan Ring
TRACESOFIMPACTCRATERSINTHE
43
GEOID
61’ 30’61’30
N
N
I
I
’ I
1
6l”od-
60”30’-
I
15‘J 30’
140130'
(4
----_
rim of the crater Pi
SC&
Scale
0-o
0.lun
km
Fig. 2. (a) Residual geoid of the Siljan Ring. Contours are given in centimetrcs. (b) Bouguer anomalies for the Siljan Ring area according to a map published by the Geological Survey of Sweden (Sveriges Geologiska Undersiikning, 1990).
is known to be a general characteristic of big impact craters. Comparison of the residual geoid (Fig. 2a)
I
\ 23%'
I
with Bouguer anomaly map (Fig. 2b) shows that local density variations, which are due to complex crustal structure and which affect the anomaly
\ 2&50'
23%0 SC&
(b)
6-
10
‘( 20 km
Fig. 3. (a) Residual geoid for Lake Lappajlrvi. Contours are given in centimetres. (b) Bouguer anomalies for Lake Lappajlrvi. The gravity data used are from the Finnish Geodetic Institute. 1990.
44
field, are smoothed away from the residual geoid, showing the general structure of the crater more clearly. The geoid high situated to the east of the crater is probably related to another crustal structure and not to the impact. Lake Lappajiirvi: Lake Lappajarvi is situated in western Finland. The diameter of the crater is 17 km, the latitude and the longitude of its centre are 63.1”N and 23.8”E, respectively, and its age is 77 Ma. The residual geoid, in particular, shows the geometrical beauty of the impact crater (Fig. 3aI. The centre of the crater is not raised, as is often the case in big craters: on the contrary, there is a geoid depression of 12 cm. This is due to fractured bedrock with impact melt and breccias, which fiI1 the centre and which have lower densities than the surroundings. The residual geoid shows less detail than the Bouguer anomaly map (Fig. 3b). In conclusion, we have shown in the present study that local gravimetric residual geoids can be computed with sufficiently high resolution for a useful study of impact craters. Although as smoothed surfaces they lose the smallest details of Bouguer anomaly maps, they reveal the general structure of the impact. This could be beneficial, especially when the gravimetric structure of the impact is hidden due to local crustal complexity.
J. KAKKURl
Acknowledgements
The author is grateful to Dr. Martin Vermeer and Mr. Norbert Haala who have helped in making practical computations. The National Survey and Cadastre of Denmark has offered computer time free of charge which is gratefully acknowledged.
References Elo, S., 1976. A study of the gravity anomaly associated with Lake Lappajiirvi, Finland. Rep. Q20/21/1976/2, Geophysics Dep., Geol. Surv. Finl., Helsinki. Forsberg, R., 1990. NKG Nordic Standard Geoid 1989. In: 0. Bedsted Andersen’ and K. Engsager (Editors), Proc. 11th General Meet. Nordic Geodetic Commission (Copenhagen. 7-l I May), Kort og Matrikkelstyrelsen, Copenhagen, pp. 75-89. Gorbunov, G.I. and Papunen, H. (Editors), 1985. General Geological Map of the Baltic Shield. Maanmittaushai~ituksen karttapaino, Helsinki. Sveriges Geologiska UndersGking, 1990. Tyngdkraftskarta, Siljan Ring Area. 1: 250000, Uppsala, Sweden. Tscherning, CC., 1990. In: 0. Bedsted Andersen and K. Engsager (Editors), Report of the section for the computation of the Nordic Geoid 1986-1990. Proc. 11th General Meet. Nordic Geodetic Commission (Copenhagen, 7- 11 May). Kort-og Matrikkelstyrelsen, Copenhagen, pp. 69-74.
Tecfonophysics, 216 (1992) 41-44
Elsevier Science Publishers B.V., Amsterdam
Extended Abstract
Traces of impact craters in the geoid Juhani Kakkuri Finnish Geodetic Institute, Ihnalankatu I A, SF-00240 Helsinki, Finland
(Received November 11, 1990; revised version accepted December 30, 1991)
A well-known method for studying the impact craters on the Earth’s surface is gravity measurements. For practical studies, the gravity measured on the surface must first be reduced to a particular standard. In a simpIe procedure, after subtracting the latitude-dependent normal gravity, the topographic masses outside the geoid are either completely removed to infinity or shifted to the mean sea level. The gravity station is then lowered from the Earth’s surface to the geoid. Resulting values are Bouguer gravity anomalies, if the topographic masses are completely removed to infinity, or free air gravity anomalies, if these masses are shifted to the geoid. Since the Bouguer gravity anomalies indicate the effect of disturbing masses, they are often used by geophysicists in their search for shaIlow features in the upper part of the Earth’s crust. In Finland, these anomalies have been used to study impact structures, for example, those associated with Lake Lappaj~~i. According to Elo (1976) and Elo et al. (this issue) there is a negative Bouguer anomaly field with a diameter of 17 km and a relative minimum of - 10 mGa1 at Lake Lappajarvi. Free air gravity anomalies are traditionally applied to geoid computation. The number of details in the geoid calculated depends on the density of the gravimetric net. The highest resolution
Correspondence to: J. Kakkuri, Finnish Geodetic Ilmalankatu 1 A, SF-00240 Helsinki, Finland.
Institute,
which can be obtained is a few centimetres. A gravimetric geoid calculated for Northern Europe by Forsberg (1990) is shown in Figure 1. The spacing of gravity stations behind this geoid varies from 5 to 10 km and the inner accuracy is better than 10 cm. Ahhough numerous geological structures, such as, for example, the boundaries between the Archaean and Proterozoic rocks, the Sveconorwegian rocks, the Caledonian orogeny belt, and the Rapakivi granite areas, are visible in this geoid, the impact structures cannot be seen, due to insuffient resolution. We have computed a new local geoid with a resolution high enough to make the impact structures visible. Computation was made separately for Lake Lappajarvi and the Siljan Ring. The resulting geoids have been compared with the Bouguer anomaly maps of these craters. In order to compute a high resolution local geoid accurate to a few centimetres, a dense gravity net is needed. The density of the Finnish national gravity net, one point per 5 x 5 km’, is not adequate for revealing sufficient details of the structure in an impact crater. Fortunately, there is a local gravity net at Lake LappajHrvi with a density five times that of the national net. The Swedish national gravity net is about as sparse as the Finnish one. Additional data were provided by a special Bouguer anomaly map of the Siljan Ring, published by the Geological Survey of Sweden (1990). This map, which is based on a dense local network, was divided into squares of 1 X 1 km’ and then the mean Bouguer anomaly was estimated for each square. This added about
0040-1951/92/%05.00 0 1992 - Elsevier Science Publishers B.V. AII rights reserved
.I. KAKKURI
42
8,400 new mean values to the data file. These anomalies were further converted into free air anomalies with the aid of a topographic map. Computation of geoid anomalies was performed with software from the National Survey and Cadastre, Copenhagen, Denmark (Forsberg, 1990; Tscherning, 1990). The “remove/restore” principle was followed: first, the globaf model IFE88E2 was removed from the gravity anomalies given: then, from the residual gravity anomalies thus obtained, residual geoid anomalies were computed, and the global model IFE88E2 again added to these. For the Siljan Ring, the OSU8Y~ model was used instead. In this study, however, only residual geoid anomalies (without the re-
Fig. I. The NOI rdic Geoid
computed
by For&erg
(1990).
structure;
The datum
store phase) were used as these show better the local details. The following results were obtained: &‘@r Ring: The Siljan Ring is situated in central Sweden; its diameter is 45 km and its age is 360 Ma. The Ring has a very complex crustal structure, with Phanerozoic sedimentary rocks which form a belt around the granitoids and diabases in the centre of the crater (Gorbunov and Papunen, 1985). There has probably been considerable erosion over the long time span and, therefore, the emerging geoid details are related to the remaining deep crustal structures caused by the impact. Some clear details have emerged, such as the bulge in the centre of the crater. This
is GRSSO and the contour
L = lake Lappajlrvi
impact crater,
interval
25 cm. S = Siljan Ring
TRACESOFIMPACTCRATERSINTHE
43
GEOID
61’ 30’61’30
N
N
I
I
’ I
1
6l”od-
60”30’-
I
15‘J 30’
140130'
(4
----_
rim of the crater Pi
SC&
Scale
0-o
0.lun
km
Fig. 2. (a) Residual geoid of the Siljan Ring. Contours are given in centimetrcs. (b) Bouguer anomalies for the Siljan Ring area according to a map published by the Geological Survey of Sweden (Sveriges Geologiska Undersiikning, 1990).
is known to be a general characteristic of big impact craters. Comparison of the residual geoid (Fig. 2a)
I
\ 23%'
I
with Bouguer anomaly map (Fig. 2b) shows that local density variations, which are due to complex crustal structure and which affect the anomaly
\ 2&50'
23%0 SC&
(b)
6-
10
‘( 20 km
Fig. 3. (a) Residual geoid for Lake Lappajlrvi. Contours are given in centimetres. (b) Bouguer anomalies for Lake Lappajlrvi. The gravity data used are from the Finnish Geodetic Institute. 1990.
44
field, are smoothed away from the residual geoid, showing the general structure of the crater more clearly. The geoid high situated to the east of the crater is probably related to another crustal structure and not to the impact. Lake Lappajiirvi: Lake Lappajarvi is situated in western Finland. The diameter of the crater is 17 km, the latitude and the longitude of its centre are 63.1”N and 23.8”E, respectively, and its age is 77 Ma. The residual geoid, in particular, shows the geometrical beauty of the impact crater (Fig. 3aI. The centre of the crater is not raised, as is often the case in big craters: on the contrary, there is a geoid depression of 12 cm. This is due to fractured bedrock with impact melt and breccias, which fiI1 the centre and which have lower densities than the surroundings. The residual geoid shows less detail than the Bouguer anomaly map (Fig. 3b). In conclusion, we have shown in the present study that local gravimetric residual geoids can be computed with sufficiently high resolution for a useful study of impact craters. Although as smoothed surfaces they lose the smallest details of Bouguer anomaly maps, they reveal the general structure of the impact. This could be beneficial, especially when the gravimetric structure of the impact is hidden due to local crustal complexity.
J. KAKKURl
Acknowledgements
The author is grateful to Dr. Martin Vermeer and Mr. Norbert Haala who have helped in making practical computations. The National Survey and Cadastre of Denmark has offered computer time free of charge which is gratefully acknowledged.
References Elo, S., 1976. A study of the gravity anomaly associated with Lake Lappajiirvi, Finland. Rep. Q20/21/1976/2, Geophysics Dep., Geol. Surv. Finl., Helsinki. Forsberg, R., 1990. NKG Nordic Standard Geoid 1989. In: 0. Bedsted Andersen’ and K. Engsager (Editors), Proc. 11th General Meet. Nordic Geodetic Commission (Copenhagen. 7-l I May), Kort og Matrikkelstyrelsen, Copenhagen, pp. 75-89. Gorbunov, G.I. and Papunen, H. (Editors), 1985. General Geological Map of the Baltic Shield. Maanmittaushai~ituksen karttapaino, Helsinki. Sveriges Geologiska UndersGking, 1990. Tyngdkraftskarta, Siljan Ring Area. 1: 250000, Uppsala, Sweden. Tscherning, CC., 1990. In: 0. Bedsted Andersen and K. Engsager (Editors), Report of the section for the computation of the Nordic Geoid 1986-1990. Proc. 11th General Meet. Nordic Geodetic Commission (Copenhagen, 7- 11 May). Kort-og Matrikkelstyrelsen, Copenhagen, pp. 69-74.