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A discussion of the devonian pole for Europe
Tectonophysics - Elsevier Printed in The Netherlands
Publishing
Company,
Amsterdam
A DISCUSSION OF THE DEVONIAN POLE FOR EUROPE
K.M. STORETVEDT Department (Received
of Geophysics, November
University
of Bergen
(Norway)
4, 1966)
SUMMARY
The palaeomagnetic reliability of European Devonian results, especially those from Great Britain, is discussed. It is concluded that the revised Devonian pole for Europe, estimated by Chamalaun and Creer (1964), appears to have been based on uncleaned rocks. It is suggested that a pole position around 160°E 20°N, based on the stable high temperature directions of Norwegian Old Red Sandstone Bed, is at present the best estimate of the Devonian pole relative to Europe. Also, this result shows a fairly good agreement with many pole positions derived from Old Red Sandstone Beds from elsewhere in Europe and the U.S.S.R.
INTRODUCTION
The palaeomagnetic results, based on the generally accepted working hypotheses in this field, from Devonian rocks of Europe have not given a definite determination of the attitude of the geomagnetic dipole axis during this period. A possible solution of this problem was put forward by Chamalaun and Creer (1964) after having carried out thermal demagnetization experiments on Old Red Sandstone (O.R.S.) samples from south Wales. They showed that the magnetization of this formation was composite and that the direction of magnetization changed considerably during demagnetization. It was stated that the fold test indicates that the high temperature component, which was entirely due to hematite, was acquired before the Permo-Carboniferous folding in that area. The high temperature direction was found to be in good agreement with certain selected n.r.m. directions (directions of natural remanent magnetization) obtained by Stubbs (1958) on the Old Red Sandstone Lavas in the Caledonian Basin (the Midland Valley area) of southern Scotland. The pole position (at 141°E 10°S) calculated from these lavas is quite different from other Devonian poles based on European rocks, some of which are consistent with the well defined Permian palaeomagnetic pole, As Stubbs’ work was considered reliable by Chamalaun and Creer (1964) they suggested that the Scottish lavas give the best estimate of the Devonian field for Great Britain. Furthermore, they concluded that the O.R.S. of southern England was nearly completely remagnetized during Permo-Carboniferous times, but that the high temperature direction was entirely impressed by the Devonian field. The remagnetization process (Chamalaun, 1964) was suggested to be a viscous magnetization acquired at moderate Tectonophysics,
4 (2) (1967) 155-162
155
heating during deep burial. It may be argued that the Devonian strata of central Europe and the U.S.S.R. may likewise have suffered a regional remagnetization at a later time as a thick sedimentary sequence was laid down in this area in Late Palaeozoic as well as in Mesozoic times. However, no adequate experiments have yet been performed to test this hypothesis. In a recent paper on the palaeomagnetism of the Rdragen Devonian of southern Norway (Storetvedt and Gjellestad, 1966) it was suggested that rocks of this age from the Scandinavian landmass may serve as a check on the remagnetization theory, because this area is known to have been geologically extremely quiet in post-Caledonian times and the remains of postDevonian sediments are negligible. It appeared that the Rdragen data were not in favour of the remagnetization theory as the palaeomagnetic pole, within the limits of uncertainty, is not significantly different from those obtained from many of the European red sandstones. As Stubbs’ (1958) work on the Scottish lavas was considered reliable it was concluded that Creer’s (1965) alternative hypothesis of more than one axis of rotation during the Devonian might be relevant. Recently, however, the present author had the opportunity of reading Stubbs’ (1958) thesis and as more data from Norway are now at hand it seems worth while to present a renewed discussion of the Devonian field problem. DISCUSSION OF STUBBS’ RESULTS FROM SCOTLAND
Stubbs (1958) investigated in his survey of the British Devonian the lavas of the Caledonian Basin of southern Scotland as well as the O.R.S. Beds of the Orcadian Basin in the northeast of Scotland. In both basins the Lower and Middle O.R.S. were folded prior to the deposition of the Upper O.R.S. A factor of potential significance in connection with the present problem is that a considerable amount of effusive material accumulated in the Caledonian Basin during the time interval Lower Devonian-Permian. Also, the downward faulting of this basin is most likely connected with the great displacement of magma which must have occurred. In addition, the Caledonian Basin is also affected by Tertiary igneous activity. The Orcadian Basin, on the other hand, does not show signs of significant igneous activity in the post-Devonian history of this area. The conclusion we can draw from these geological observations is that the Midland Lavas very likely have suffered a quite complex thermal history and therefore may have been remagnetized in geological epochs subsequent to their formation. Rocks remagnetized at a somewhat elevated temperature may be magnetically extremely stable, and it may be difficult to reveal the primary component of magnetization. Therefore, the geological evidence seems to suggest that the sediments of the Orcadian Basin probably are a much more reliable palaeoma~eti~ medium than the O.R.S. Lavas. In Fig.1 Stubbs’ (1958) results from O.R.S. Lavas are sbown. In selecting these data he applied a rather strict criterion; the dispersion of directions within any flow should give a circle of confidence of less than 30’. On this basis 14 of a total collection of 44 flows were rejected. It is interesting to note that all the rejected flows were of Middle Devonian age and were lying above a certain stratigraphic level (the Arbuthnott Group). It may be 156
Te~tonop~y8i~6,
4 (2) (1967) 155-162
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.
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.
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Fig.1. Directions of natural remanent magnetization of Old Red Sandstone Lavas of the Midland Valley. All samples are from below the top of the Arbuthnott Group and from flows with a circle of confidence of less than 30’. Full circles indicate points on the lower hemisphere and crosses are points on the upper hemisphere. (After Stubbs, 1958.) argued that the lower part of the lava succession was probably more extensively reheated at some later date than the upper part and therefore became more easily remagnetized, resulting in a better internal consistency of the n.r.m. directions. The stability and age of Stubbs’ (1958) magnetization direction were derived from the following considerations: fold test, contact studies and reversals. The fold test indicates that the magnetization was acquired before folding. A fold test on uncleaned rocks, however, does not necessarily indicate palaeomagnetic reliability of the data, but merely that a primary component may dominate over secondary magnetizations. Furthermore, it is an important fact that the contact test does not seem to be valid in this case, as there is no straightforward explanation of the Tectonophysics, 4 (2) (1967) 155-162
157
thermoremanent magnetization in the baked rock at increasing distances from the contact. This fact may suggest that both lava and baked rock (in this case the Wormit Tuff) have been subjected to a remagnetization which post-dates the lava eruption. The occurrence of reversals in untested rocks does not seem convincing with respect to the problem of deriving a reliable field direction, because the presence of normal and reversed zones might well have been impressed long after the eruption of the lavas. It must be pointed out that if the thermal history of the Midland area is as complex as its geological history seems to indicate, nearly everything might have happened to the original thermoremanent magnetization. One should, therefore, be very careful to draw conclusions from the palaeomagnetism of this formation before extensive laboratory studies have been performed. In Fig.2 Stubbs’ (1958) results from the Caithness O.R.S. (of the Orcadian
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x I
.
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.
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Fig.2. Directions of natural remanent magnetization of the Caithness Old Red Sandstone sediments. Key to plots as in Fig.1. (After Stubbs, 1958.) 158
Tectonophysics,
4(2) (1967) 155-162
Basin) are shown. As the age of this sandstone is almost certainly younger than the Midland Lavas and their magnetic directions are different, Stubbs concluded that the two sets of results may represent progressive stages in a shift of the earth’s field relative to Britain. Before leaving the discussion of British results it is necessary to refer again to Chamalaun and Creer’ s (1964) high temperature direction of the O.R.S. of south Wales. It is evident from their paperthat a stable high temperature component could not be deduced and the validity of their high temperature direction is therefore highly disputable.
SUMMARYOF NORWEGIANRESULTS At present palaeomagnetic results from two Norwegian Red Bed Formations, the Rdragen (62ON 12OE) Formation (Storetvedt and Gjellestad, 1966) of Lower Devonian age and the Ringerike (60°N ll”E) Sandstone (Halvorsen, 1966), which is of Upper Silurian (Late Ludlow-Early Downton) age, are now available. Both formations have been affected by Caledonian erogenic movements. The Rdragen complex appears to have been folded and tilted while the Ringerike Sandstone, which lies on the border zone of Caledonian activity, has been only slightly folded. On the other hand, the Ringerike Formation lies within the Oslo Graben, where extensive igneous activity took place in Permian times. The sandstone was therefore very likely heated during this time. Samples from both formations have been thermally demagnetized and cooled in zero magnetic field to room temperature. The stable directions of magnetization derived in this way are given with respect to the bedding plane, which is thought to represent the palaeohorizontal. Specimens from the RBragen Formation showed stability in their directions of magnetization throughout the whole range of temperatures. Normal and reversed zones occur. The directions of magnetization are shown in Fig.3. The mean direction irrespective of sign, treating the mean direction of each sample as a single observation, is N 210°E, + 9’. The radius of the circle of confidence at the 95% probability level is 15O. The pole position derived is, in the present coordinates, 160°E 19ON. The Ringerike Sandstone, which will be more fully treatedina separate paper, showed n.r.m. directions in very good agreement with the Permian direction (Van Everdingen, 1960) of this region. The thermal analysis, however, revealed that the magnetization was composite resulting in a directional change like that observed in the O.R.S. Beds of southern England. However, contrary to the British rocks, a stable high temperature direction was obtained from three sites, in which no directional change was observed above 6OOOC.All directions (six samples) were normal and very well grouped, giving a pole position at 159OE 21°N. The results from these two Norwegian Red Bed Formations (of slightly different age) are therefore in has not been directly complete agreement with each other. This direction confirmed by igneous control. In the district just north of Trondheim, however, a minor ultrabasic dike cuts through Ordovician Limestone. The dike seems not to have been affected by the Caledonian orogeny and Carstens (1961) tentatively suggested a Permian age. The present author has investigated the remanent direction of this dike and found that it is stable against thermal Tectonophysics, 4 (2) (1967) 155-162
159
demagnetization. The results in agreement with the direction deduced above. As the dike occurs in the centre of the Caledonian zone and the distance to the nearest Permian intrusive is about 300 km, a Late Caledonian Age appears very likely. Therefore, the probability of a Permian age of the dike is statistically very small. Fig.4 shows the pole positions of the Norwegian O.R.S. Formations in relation to other results from Europe and the U.S.S.R.
CONCLUSION It appears that the thermal history of the Caledonian Basin of Scotland has been too complex to enable one to draw a palaeomagnetic conclusion from Stubbs’ (1958) n.r.m. directions, In addition, the author is of the opinion that a stable high temperature direction of the O.R.S. of south Wales has
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I
I
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I
I
1
*E
S
Fig.3. Directions of natural remanent magnetization red sandstone samples. Key to plots as in Fig.1. 160
Tectonophysics,
of the Rdragen
4 (2) (196’7, 155-162
NP
SP
Fig.4. Devonian palaeomagnetic poles from Europe and northern Asia. Numbers are according to Irving (1964). R and RS denote the pole position from the Rdragen and Ringerike Formations, respectively. The triangle shows the mean Permian pole, as given in Irving (1964, pp.125), for Europe and the U.S.S.R. not been satisfactorily established. In fact, the two Norwegian rock formations of O.R.S. age, mentioned in this paper, are the only formations of this age from Europe, so far investigated, where a stable high temperature direction has been obtained. Also, the n.r.m. directions of the Caithness Sandstone seem to agree much better with this result than do the Midland Lavas. It is concluded, therefore, that the revised Devonian pole, as proposed by Chamalaun and Creer (1964), appears to have been based on uncleaned rocks, and that a pole position around 160°E 20°N is at present the best estimate of the Devonian pole relative to Europe. As the Norwegian pole positions of Early O.R.S. age are not significantly different from those obtained from many of the west European and U.S.S.R. rock formations, and taking a certain amount of polar movement within the Devonian into account,an extensive remagnetization in Late Palaeozoic times does not seem to be the general explanation of the majority of results from this area.
Tectonophysics, 4(2) (1967) 155-162
161
REFERENCES Carstens, H., 1961. A post-Caledonlan ultrabasic biotite lamprophyre dyke of the Island Ytterdy in the Trondheimsfjord, Norway. Norg. Geol. Under&k., 215: 19-21. Chamalaun, F.H., 1964. Origin of the secondary magnetization of the Old Red Sandstones of the A~l~Welsh cuvette. J. Geophys. Res., 69: 4327-4337. Chamalaun, F.H. and Creer, KM., 1964. Thermal demagnetization studies of the Old Red Sandstones of the Anglo-Welsh cuvette. J. Geophys. Res., 69: 1607-1616. Creer, K.M., 1965. Psleolatitudes of the continents in the Devonian as revealed by the natural remanent magnetization of rocks. In: A.E.M. Nairn (Editor), Problems in Paieoclimatology. Interscience, London, pp.269-284. Hsivorsen, E., 1966. Paleomagnetisk UndersBkelse av Ringeriksandsteinen. Thesis, Univ. Bergen, 80 pp. Irving, E., 1964. Paleomagnetism and its Application to Geological and Geophysical Problems. Wiley, New York, N.Y., 399 pp. Storetvedt, K.M. and Gjellestad, G., 1966. Paiaeomsgnetic investigation of an Old Red Sandstone Formation of southern Norway. Nature, 212: 59-61. Stubbs, P.M., 1958. Continental Drift and Polar Wandering, a Paleomagnetic Study of the British and European Trias and of the British Old Red Sandstone. Thesis, Univ. London, 158 pp. Van Everdingen, R-O., 1960. Paleomagnetic analysis of Permian extrusives in the Oslo region, Norway. Skrifter Norske Videnskaps-Akad. Oslo, I: Mat. Waturv. ICI., 1960: 80 pp.
162
Tectonophysics, 4 (2) (1967) 155-162
Publishing
Company,
Amsterdam
A DISCUSSION OF THE DEVONIAN POLE FOR EUROPE
K.M. STORETVEDT Department (Received
of Geophysics, November
University
of Bergen
(Norway)
4, 1966)
SUMMARY
The palaeomagnetic reliability of European Devonian results, especially those from Great Britain, is discussed. It is concluded that the revised Devonian pole for Europe, estimated by Chamalaun and Creer (1964), appears to have been based on uncleaned rocks. It is suggested that a pole position around 160°E 20°N, based on the stable high temperature directions of Norwegian Old Red Sandstone Bed, is at present the best estimate of the Devonian pole relative to Europe. Also, this result shows a fairly good agreement with many pole positions derived from Old Red Sandstone Beds from elsewhere in Europe and the U.S.S.R.
INTRODUCTION
The palaeomagnetic results, based on the generally accepted working hypotheses in this field, from Devonian rocks of Europe have not given a definite determination of the attitude of the geomagnetic dipole axis during this period. A possible solution of this problem was put forward by Chamalaun and Creer (1964) after having carried out thermal demagnetization experiments on Old Red Sandstone (O.R.S.) samples from south Wales. They showed that the magnetization of this formation was composite and that the direction of magnetization changed considerably during demagnetization. It was stated that the fold test indicates that the high temperature component, which was entirely due to hematite, was acquired before the Permo-Carboniferous folding in that area. The high temperature direction was found to be in good agreement with certain selected n.r.m. directions (directions of natural remanent magnetization) obtained by Stubbs (1958) on the Old Red Sandstone Lavas in the Caledonian Basin (the Midland Valley area) of southern Scotland. The pole position (at 141°E 10°S) calculated from these lavas is quite different from other Devonian poles based on European rocks, some of which are consistent with the well defined Permian palaeomagnetic pole, As Stubbs’ work was considered reliable by Chamalaun and Creer (1964) they suggested that the Scottish lavas give the best estimate of the Devonian field for Great Britain. Furthermore, they concluded that the O.R.S. of southern England was nearly completely remagnetized during Permo-Carboniferous times, but that the high temperature direction was entirely impressed by the Devonian field. The remagnetization process (Chamalaun, 1964) was suggested to be a viscous magnetization acquired at moderate Tectonophysics,
4 (2) (1967) 155-162
155
heating during deep burial. It may be argued that the Devonian strata of central Europe and the U.S.S.R. may likewise have suffered a regional remagnetization at a later time as a thick sedimentary sequence was laid down in this area in Late Palaeozoic as well as in Mesozoic times. However, no adequate experiments have yet been performed to test this hypothesis. In a recent paper on the palaeomagnetism of the Rdragen Devonian of southern Norway (Storetvedt and Gjellestad, 1966) it was suggested that rocks of this age from the Scandinavian landmass may serve as a check on the remagnetization theory, because this area is known to have been geologically extremely quiet in post-Caledonian times and the remains of postDevonian sediments are negligible. It appeared that the Rdragen data were not in favour of the remagnetization theory as the palaeomagnetic pole, within the limits of uncertainty, is not significantly different from those obtained from many of the European red sandstones. As Stubbs’ (1958) work on the Scottish lavas was considered reliable it was concluded that Creer’s (1965) alternative hypothesis of more than one axis of rotation during the Devonian might be relevant. Recently, however, the present author had the opportunity of reading Stubbs’ (1958) thesis and as more data from Norway are now at hand it seems worth while to present a renewed discussion of the Devonian field problem. DISCUSSION OF STUBBS’ RESULTS FROM SCOTLAND
Stubbs (1958) investigated in his survey of the British Devonian the lavas of the Caledonian Basin of southern Scotland as well as the O.R.S. Beds of the Orcadian Basin in the northeast of Scotland. In both basins the Lower and Middle O.R.S. were folded prior to the deposition of the Upper O.R.S. A factor of potential significance in connection with the present problem is that a considerable amount of effusive material accumulated in the Caledonian Basin during the time interval Lower Devonian-Permian. Also, the downward faulting of this basin is most likely connected with the great displacement of magma which must have occurred. In addition, the Caledonian Basin is also affected by Tertiary igneous activity. The Orcadian Basin, on the other hand, does not show signs of significant igneous activity in the post-Devonian history of this area. The conclusion we can draw from these geological observations is that the Midland Lavas very likely have suffered a quite complex thermal history and therefore may have been remagnetized in geological epochs subsequent to their formation. Rocks remagnetized at a somewhat elevated temperature may be magnetically extremely stable, and it may be difficult to reveal the primary component of magnetization. Therefore, the geological evidence seems to suggest that the sediments of the Orcadian Basin probably are a much more reliable palaeoma~eti~ medium than the O.R.S. Lavas. In Fig.1 Stubbs’ (1958) results from O.R.S. Lavas are sbown. In selecting these data he applied a rather strict criterion; the dispersion of directions within any flow should give a circle of confidence of less than 30’. On this basis 14 of a total collection of 44 flows were rejected. It is interesting to note that all the rejected flows were of Middle Devonian age and were lying above a certain stratigraphic level (the Arbuthnott Group). It may be 156
Te~tonop~y8i~6,
4 (2) (1967) 155-162
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I
I
l
2:*
.
.*
x
.*
.
. . .*.
. .-•* . ,* . .
.
l
N
_y.’.
.
. .*
, l .
* . ..
a.
. . l2
.. l
4
.
_
Fig.1. Directions of natural remanent magnetization of Old Red Sandstone Lavas of the Midland Valley. All samples are from below the top of the Arbuthnott Group and from flows with a circle of confidence of less than 30’. Full circles indicate points on the lower hemisphere and crosses are points on the upper hemisphere. (After Stubbs, 1958.) argued that the lower part of the lava succession was probably more extensively reheated at some later date than the upper part and therefore became more easily remagnetized, resulting in a better internal consistency of the n.r.m. directions. The stability and age of Stubbs’ (1958) magnetization direction were derived from the following considerations: fold test, contact studies and reversals. The fold test indicates that the magnetization was acquired before folding. A fold test on uncleaned rocks, however, does not necessarily indicate palaeomagnetic reliability of the data, but merely that a primary component may dominate over secondary magnetizations. Furthermore, it is an important fact that the contact test does not seem to be valid in this case, as there is no straightforward explanation of the Tectonophysics, 4 (2) (1967) 155-162
157
thermoremanent magnetization in the baked rock at increasing distances from the contact. This fact may suggest that both lava and baked rock (in this case the Wormit Tuff) have been subjected to a remagnetization which post-dates the lava eruption. The occurrence of reversals in untested rocks does not seem convincing with respect to the problem of deriving a reliable field direction, because the presence of normal and reversed zones might well have been impressed long after the eruption of the lavas. It must be pointed out that if the thermal history of the Midland area is as complex as its geological history seems to indicate, nearly everything might have happened to the original thermoremanent magnetization. One should, therefore, be very careful to draw conclusions from the palaeomagnetism of this formation before extensive laboratory studies have been performed. In Fig.2 Stubbs’ (1958) results from the Caithness O.R.S. (of the Orcadian
TRUE
N
x
I
I
I
I
l
,T
, .
r .
x I
.
l*
.
S
Fig.2. Directions of natural remanent magnetization of the Caithness Old Red Sandstone sediments. Key to plots as in Fig.1. (After Stubbs, 1958.) 158
Tectonophysics,
4(2) (1967) 155-162
Basin) are shown. As the age of this sandstone is almost certainly younger than the Midland Lavas and their magnetic directions are different, Stubbs concluded that the two sets of results may represent progressive stages in a shift of the earth’s field relative to Britain. Before leaving the discussion of British results it is necessary to refer again to Chamalaun and Creer’ s (1964) high temperature direction of the O.R.S. of south Wales. It is evident from their paperthat a stable high temperature component could not be deduced and the validity of their high temperature direction is therefore highly disputable.
SUMMARYOF NORWEGIANRESULTS At present palaeomagnetic results from two Norwegian Red Bed Formations, the Rdragen (62ON 12OE) Formation (Storetvedt and Gjellestad, 1966) of Lower Devonian age and the Ringerike (60°N ll”E) Sandstone (Halvorsen, 1966), which is of Upper Silurian (Late Ludlow-Early Downton) age, are now available. Both formations have been affected by Caledonian erogenic movements. The Rdragen complex appears to have been folded and tilted while the Ringerike Sandstone, which lies on the border zone of Caledonian activity, has been only slightly folded. On the other hand, the Ringerike Formation lies within the Oslo Graben, where extensive igneous activity took place in Permian times. The sandstone was therefore very likely heated during this time. Samples from both formations have been thermally demagnetized and cooled in zero magnetic field to room temperature. The stable directions of magnetization derived in this way are given with respect to the bedding plane, which is thought to represent the palaeohorizontal. Specimens from the RBragen Formation showed stability in their directions of magnetization throughout the whole range of temperatures. Normal and reversed zones occur. The directions of magnetization are shown in Fig.3. The mean direction irrespective of sign, treating the mean direction of each sample as a single observation, is N 210°E, + 9’. The radius of the circle of confidence at the 95% probability level is 15O. The pole position derived is, in the present coordinates, 160°E 19ON. The Ringerike Sandstone, which will be more fully treatedina separate paper, showed n.r.m. directions in very good agreement with the Permian direction (Van Everdingen, 1960) of this region. The thermal analysis, however, revealed that the magnetization was composite resulting in a directional change like that observed in the O.R.S. Beds of southern England. However, contrary to the British rocks, a stable high temperature direction was obtained from three sites, in which no directional change was observed above 6OOOC.All directions (six samples) were normal and very well grouped, giving a pole position at 159OE 21°N. The results from these two Norwegian Red Bed Formations (of slightly different age) are therefore in has not been directly complete agreement with each other. This direction confirmed by igneous control. In the district just north of Trondheim, however, a minor ultrabasic dike cuts through Ordovician Limestone. The dike seems not to have been affected by the Caledonian orogeny and Carstens (1961) tentatively suggested a Permian age. The present author has investigated the remanent direction of this dike and found that it is stable against thermal Tectonophysics, 4 (2) (1967) 155-162
159
demagnetization. The results in agreement with the direction deduced above. As the dike occurs in the centre of the Caledonian zone and the distance to the nearest Permian intrusive is about 300 km, a Late Caledonian Age appears very likely. Therefore, the probability of a Permian age of the dike is statistically very small. Fig.4 shows the pole positions of the Norwegian O.R.S. Formations in relation to other results from Europe and the U.S.S.R.
CONCLUSION It appears that the thermal history of the Caledonian Basin of Scotland has been too complex to enable one to draw a palaeomagnetic conclusion from Stubbs’ (1958) n.r.m. directions, In addition, the author is of the opinion that a stable high temperature direction of the O.R.S. of south Wales has
TRUE
I
N
I
I
I
I
I
1
*E
S
Fig.3. Directions of natural remanent magnetization red sandstone samples. Key to plots as in Fig.1. 160
Tectonophysics,
of the Rdragen
4 (2) (196’7, 155-162
NP
SP
Fig.4. Devonian palaeomagnetic poles from Europe and northern Asia. Numbers are according to Irving (1964). R and RS denote the pole position from the Rdragen and Ringerike Formations, respectively. The triangle shows the mean Permian pole, as given in Irving (1964, pp.125), for Europe and the U.S.S.R. not been satisfactorily established. In fact, the two Norwegian rock formations of O.R.S. age, mentioned in this paper, are the only formations of this age from Europe, so far investigated, where a stable high temperature direction has been obtained. Also, the n.r.m. directions of the Caithness Sandstone seem to agree much better with this result than do the Midland Lavas. It is concluded, therefore, that the revised Devonian pole, as proposed by Chamalaun and Creer (1964), appears to have been based on uncleaned rocks, and that a pole position around 160°E 20°N is at present the best estimate of the Devonian pole relative to Europe. As the Norwegian pole positions of Early O.R.S. age are not significantly different from those obtained from many of the west European and U.S.S.R. rock formations, and taking a certain amount of polar movement within the Devonian into account,an extensive remagnetization in Late Palaeozoic times does not seem to be the general explanation of the majority of results from this area.
Tectonophysics, 4(2) (1967) 155-162
161
REFERENCES Carstens, H., 1961. A post-Caledonlan ultrabasic biotite lamprophyre dyke of the Island Ytterdy in the Trondheimsfjord, Norway. Norg. Geol. Under&k., 215: 19-21. Chamalaun, F.H., 1964. Origin of the secondary magnetization of the Old Red Sandstones of the A~l~Welsh cuvette. J. Geophys. Res., 69: 4327-4337. Chamalaun, F.H. and Creer, KM., 1964. Thermal demagnetization studies of the Old Red Sandstones of the Anglo-Welsh cuvette. J. Geophys. Res., 69: 1607-1616. Creer, K.M., 1965. Psleolatitudes of the continents in the Devonian as revealed by the natural remanent magnetization of rocks. In: A.E.M. Nairn (Editor), Problems in Paieoclimatology. Interscience, London, pp.269-284. Hsivorsen, E., 1966. Paleomagnetisk UndersBkelse av Ringeriksandsteinen. Thesis, Univ. Bergen, 80 pp. Irving, E., 1964. Paleomagnetism and its Application to Geological and Geophysical Problems. Wiley, New York, N.Y., 399 pp. Storetvedt, K.M. and Gjellestad, G., 1966. Paiaeomsgnetic investigation of an Old Red Sandstone Formation of southern Norway. Nature, 212: 59-61. Stubbs, P.M., 1958. Continental Drift and Polar Wandering, a Paleomagnetic Study of the British and European Trias and of the British Old Red Sandstone. Thesis, Univ. London, 158 pp. Van Everdingen, R-O., 1960. Paleomagnetic analysis of Permian extrusives in the Oslo region, Norway. Skrifter Norske Videnskaps-Akad. Oslo, I: Mat. Waturv. ICI., 1960: 80 pp.
162
Tectonophysics, 4 (2) (1967) 155-162