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Palaeomagnetic comparison of a new fit of East and West Gondwanaland with the Smith and Hallam fit
~ecto~ophysics, 61 (1980) 331-390 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
381
PALAEOMAGNETIC COMPARISON OF A NEW FIT OF EAST AND WEST GONDWANALAND WITH THE SMITH AND HALLAM FIT
B.J.J. EMBLETON i, J.J. VEEVERS 2, B.D. JOHNSON 2 and C.Mc.A. POWELL *
1 CSIRO Division of ~i~erai Physics, North Ryde, N.S. W. 2113 (Australiaf * S&o01 of Earth Sciences, ~ac~ua~e Unive~ity, North Ryde, N.S. W. 2113 ~A~tm~ia~ (Received November 25,19’77;
revised version accepted December 5,1978)
ABSTRACT Embleton, B.J.J., Veevers, J.J., Johnson, B.D. and Powell, C.McA., 1980. Palaeomagnetic comparison of a new fit of East and West Gondwanaland with the Smith and Hallam fit. Tectonophysics, 61: 381-390. Palaeomagnetic data for five time intervals in the Phanerozoic are compared for a new fit of East (Antarctica, Australia, India) and West (Africa, Madagascar, South America) Gondwanaland and the hitherto durable fit of Smith and Hallam. The dispersion of the palaeoma~etic poles is marginally less on the Smith and Hallam fit for four time intervals and marginally greater for the remaining one (Permo-Triassic). The Permo-Triassic poles of East Gondwanaland are evenly distributed between India and Australia and the decreased dispersion of the poles for this period on the new fit of East and West Gondwanaland is paralleled by the decreased dispersion of the poles for both India and Australia. Within the limitations of the analysis imposed by the data, the palaeomagnetic comparison shows that there is little to choose between the two fits.
INTRODUCTION
A new fit of East (Antarctica, Australia, India) and West (South America, Africa, Madagascar) Gondw~~~d (Powell et al., 1979) is constrained by recently available information on the extent of continental crust beneath the outer Falkland Plateau (Barker et al., 1974) and off Southeast Africa (Darracott, .1974), and by the pattern of sea-floor spreading between Madagascar and India (Powell et al., 1979). In this fit (Fig. l), Madagascar, whose westem margin has been established as lying against equatorial east Africa (McElhinny et al., 1976), is juxtaposed with the northwestern margin of India, and the Antarctic Peninsula wraps round, without deformation, the Pacific margin of southern South America. East Gondwanaland has been modified from the Smith and Hallam (1970) fit by bringing Australia to Antarctica by a rotation of 31.0” clockwise about a pole at 10.8”N 32.3”E (Griffiths, 1974), and West Gondwanaland by a rotation of Madagascar from
3x2 // ,/’
330’
30’ j
60’
90’ ‘\
\ ‘-\ ‘.
Fig. 1. Averaged palaeomagnetic poles for East (Antarctica, Australia, India) and West (Africa, Madagascar, South America) Gondwanaland, according to (a) the new fit of Powell et al. (1979): full line; (b) Smith and Hallam (1970): fufl line (West Gondwanaland) and dotted line (East Gondwanaland); (c) Smith-Hallam, modified to include the Griffiths’ (1974) Australia-Antarctica fit: same as for the Smith-Hallam fit except Australia’s position is shown with the dashed fine, Drawn relative to Africa’s presentday coordinates.
its position in the Smith and Hallam (1970) fit 4.3” anti-clockwise about a pole 13.2” S 64.1” E. With these changes, the new fit may be constructed from that of Smith and Hallam by a rotation of East Gondwanaland with respect to West Gondwanaland of 16.2” clockwise about a pole at 15.8”N 57.8”W. This fit places East Gondw~al~d in a position similar to that of Tarling’s
383
(1972) fit but has Madagascar in its palaeomagnetically determined position, and obviates the need, as in Tarling (1972), to deform Antarctica. The fit of Powell et al. (1978) is constrained by the pattern of sea-floor spreading in the oceans that now separate the Gondwanaland fragments, and is consistent with the bathymetry of adjacent margins and the trends of geological features before the Late Jurassic breakup. A fourth constraint, continental palaeomagnetism, is discussed here and compared with the hitherto durable fit of Smith and Hallam (1970). DATA SELECTION AND GROUPING
Procedures for selecting palaeomagnetic data are rarely uniformly applied from worker to worker. The nature of the problem and the overall quality of the data available for selection needs to be considered when es~blishing the selection criteria (McElhinny and Embleton, 1976a). Minimum reliability criteria for general application have been used by Hicken et al. (1972) and McElhinny (1973). Although the two procedures differ in detail, they do provide useful guidelines which are adopted here. Apart from the particular requirements regarding the time of acquisition of magnetisation, the age of the rock, and the application of field and laboratory tests of magnetic stability, results are here considered adequate only if based on consistent observations from eight or more samples. However, results from a number of small studies from the same geographical region may be usefully combined to provide an acceptable result, e.g., the Permo-Carboniferous volcanics of southeastern Australia and, ~dividu~ly, the mid-Triassic sediments and basalts from Morocco (Daly and Pozzi, 1976). Some guide to the consistency of observations is acknowledged by excluding all of those results with circles of confidence ((Yap,Fisher, 1953) greater than 25” (cf., Hicken et al., 1972). Data from two tectonic provinces have also been rejected on the grounds that they have possibly undergone some rotation with respect to the main shield and platform areas. One province is the Salt Range, no~hwes~~ India; the rotation of this area has been discussed by Crawford (1974), Wensink (1975a) and Klootwijk (1975), and the other province is the Lachlan fold belt, southeastern Australia, from which pre-Devonian results have been interpreted as representing exotic blocks rather than the main Australian platform (Embleton et al., 1974). The question of partial rema~et~ation has been raised (Wensink, 1975a, b; Klootwijk, 1974, 1975) regarding the time of acquisition of remanent magnetisation in some Gondwana redbeds from India and Pakistan, It was claimed that probable remagnetisation during Late Cretaceous to Early Tertiary times by the Deccan Trap flood basalts is indicated. However, where results are consistent from one formation to another, they are here accepted at face value unless some independent evidence is available that adequately demonstrates some degree of remagnetisation. The palaeomagnetic data used in this analysis are listed in Table I.
a/73; 14/303; Cassange Series (Valencio and Vilas, 1976)
no data 7133; 7134; a/so
8/93;
Grds rouge and Tracbyandesites (Daly and Pozzi, 1976)
no data 7139; 81103; 81104; 81105; mean of 7140, 8179 and 9197
91117; 141361
no data 141352; 141368; 141369; Yetholme adametlite (Facer , 19’76; Embleton, 1977a); Mulga Downs; (Embleton, 1977b); Victorian
4132; 81144; Cambrian of Morocco (Daly and Pozzi, 1977)
10/140; 14/408
141376; 141393; 141395; 14/405; 14/409; 14/413; 141416; Dundas Group (Embleton and Giddings, 1974)
Africa
Antartica Australia
PermoTriassic “.___.~~~
__ PermoCarboniferous __._
Silurian-early Carboniferous
Quasi-static interval
mid-cambrianOrdovician
Land mass
Data sources
TABLE I
-..._-._- --.~
__-
Tasmanian dolerite; Kangaroo Island basalt; Jurassic intrusives; West Victorian basalt; Garrawilla volcanics (Schmidt 1976b, c)
2127; 6136; 10170; 9163
6140-43; 8159; 8163; S/67; 8172; 10177; 13135; 13136; 13140; 12193; 141248; 141 249; 141250; 141288; 141 290; 141573; 141289; Morocco Lavas; Jerada Sediments (Daly and Pozzi. 1976)
.-.._ . .--------.-
Triassic-Jurassic
no data
12/140; 121146
Madagascar
South America
12/X41; no data
no data
no data
12/116; 141309; 141333; Tubarao encio et 12/124
12/117; 141332; 14/345 Group (Vaiai., 1975)
Sakoa Group (McElhinny and Embleton, 1976b); Sakoa Group (Razafindrazaka et ai., 1976)
x4/329
14/302;
11/46;
14/269
1x/43;
14/241;
11/45
14/274
and McElhinny (1968-1977).
11/49, 11/56; 11/60; 11/61; 11/62; 13142; 141286; 14/291; 14/306; Passa Dois Group (Vaiencio et al., 1975); Irati Formation (Pascholati et al., 1976)
Sakamena Group (McElhinny and Embleton, 1976b); Sakamena Group (Razafindrazaka et al., 1976)
14/302; 11/57
Palaeomagnetic pole numbers refer to the Geophys. J.R. Astron. Sot. pole list of Irving (1960-1965)
no data
India
Lavas; Housetop Granite; Canning Reef (Schmidt, 1976a)
386
The data have been grouped into five time intervals covering the midCambrian to mid Jurassic part of the Phanerozoic. The grouping follows the concept of quasi-static intervals (Briden, 1967). The intervals used are: midCambrian-Ordovician which covers the period following the mid-Cambrian episode of apparent polar wander (McElhinny and Luck, 1970; Embleton, 1972) until the mid-Palaeozoic polar transition from northern to southern Africa, e.g. (McElhinny and Briden, 1971); Silurian-Early Carboniferous for the period following the mid-Palaeozoic polar transition until the midCarboniferous transition; Permo-Carboniferous for the late Palaeozoic reversed magnetic interval (Kiaman); Permo-Triassic; and Triassic-Jurassic. The last three quasi-static intervals were recognised by Daly and Pozzi (1976), and Embleton and Schmidt (1977) demonstrated that the palaeomagnetic data grouped discretely according to those time intervals. After midJurassic times, Gondw~~~d began to fragment (Embleton and Schmidt, 1977). DATA
ANALYSIS
Following data selection (Table I) the first and foremost observation is that the numbers of palaeom~etic pole positions are unevenly dist~buted for a given time interval from one land mass to another. Six major land masses comprise Gondwanaland and only for the most recent time interval considered, the Triassic-Jurassic, are there data available from all six regions. The worst case is the Silurian-Early Carboniferous interval in which data from only Africa (two poles) and Australia (eight poles) are available. The analysis has been carried out by assigning unit weight to palaeomagnetic pole positions irrespective of the number of observations (poles) from each landmass. In order to compare the results for each reconstruction model, palaeomagnetic poles have been averaged for (1) the six land masses reconstructed according to the constraints of each model (Table II) and (2) the East Gondw~~~d and West Gondwanal~d components (Table III). To reduce any bias due to the modified position of Australia in East Gondwanaland (Griffiths, 1974), a similarly modified Smith and Hallam fit has been included (Fig. 1). All latitude and longitude values are quoted with respect to Africa’s present geographic coordinates. COMPARISON
OF RESULTS
When the palaeomagnetic poles are averaged for the six land masses (Table II) they show a marginal increase in dispersion for the new Gondwanaland model compared to the Smith-Hallam model for four time intervals and a marginal decrease in dispersion for the Permo-Triassic interval. Table III lists the averaged p~aeom~etic poles for the East and West components of Gondwanaland and Table IV lists the angular differences between the respective pole pairs for the two blocks. The angles calculated
16 10 18 22 34
mid-Cam./Ord. Sil-early Carb. Permo-Carb. Permo-Triassic Triassic-Jurassic
k
36.8 20.6 35.3 53.1 65.8
ON OS OS OS ‘8
011.3 025.1 066.1 080.3 071.4
11.1 13.1 5.7 4.2 3.3
11.8 14.5 37.7 54.5 54.9
2
Sil.-early
12
16
23
Permo-Carb.
Permo-Triassic
Triassic-Jurassic
35.8 OS (6.1) 56.3 OS (4.6) 67.8 OS (4.1)
060.9 (51.1) 080.4 (85.6) 070.5 (53.7)
004.6 (8.2) 025.9 35.8 OS (6.2) 56.1 OS (4.6) 67.8 OS (4.1)
@95)
(k)
(~9s)
34.5 ON (24.7) 13.7 OS -
lat.
long. ‘E
lat.
061.4 (49.2) 081.1 (82.6) 070.9 (53.4)
(k)
long. ‘E
11
6
6
8
10
37.9 ON (13.0) 22.4 OS (16.1) 33.7 OS (11.3) 44.4 OS (10.3) 61.7 OS (5.6)
(ass)
lat.
12.2 13.8 36.3 55.3 53.5
k
015.3 (14.7) 024.9 (12.7) 076.3 (36.0) 080.1 (42.7) 072.7 (66.8)
(k)
long. ‘E
Smith-Hallam
N
Smith-Hallam
N
6
Csrb.
10.9 13.4 5.8 4.2 3.3
East Gondwanaland Powell et al.
008.1 024.6 066.6 081.1 073.2
West Gondwanaland
mid-Cam./Ord.
Interval
ON OS OS OS OS
095
+ Griffiths
24.0 37.0 38.7 55.4 69.0
lat.
ON OS OS OS OS
007.2 019.8 067.8 084.4 080.5
long. OE
New Gondwanaland
43.8 OS (9.6) 61.1 OS (5.5)
(11.0)
32.5 ON (12.9) 27.6 OS (16.1) 32.9 OS
(a951
(14.9) 024.3 (12.7) 077.8 (3 7.6) 082.4 (49.2) 077.5 (68.1)
010.0
Smith-Hallam + Griffiths ~ lat. ;;;g. “B
of Gondwanaland (estimates of precision are italicised)
33.2 24.8 35.1 53.0 65.6
long. OE
lat.
a95
lat. long. OE
Smith-Hallam
Smith-Hallam
Average paiaeomagnetic poles for the east and west components
TABLE III
N
Interval
Average palaeomagnetic poles for the different reconstructions of Gondwanaland
TABLE II
11.0 11.0 31.1 65.9 46.7
k
18.0 ON (12.8) 42.9 OS (16.1) 43.4 “s (11.0) 52.8 OS (9.6) 69.7 OS (5.5)
008.5 (I 5.0) 017.6 (12.7) 082.5 (3 7.6) 092.5 (49.4) 102.5 (67.6)
long. ‘E (k)
lat. (~95)
Powell et al.
11.6 15.1 6.2 3.8 3.6
(~95
E
388 TABLE
IV
Angular differences between averaged palaeomagnetic poles for East and West Gondwanaland calculated for the Smith and Hallam (1970) fit (S-H); the Smith-Hallam fit modified following Griffiths (1974); and the new fit of Powell et al. (1979) Interval
S-H
S-H
mid-Cam./Ord. Sil.-early Carb. Permo-Carb. Permo-Triassic Triassic-Jurassic
9.3 8.8 12.9 12.0 6.2 -____-
5.0 14.0 14.3 12.6 7.4
+ Griffiths
Powell et al. ~_______ 16.9 30.1 18.3 7.9 11.7
for the new fit show an increase for four time intervals compared to the angles calculated on the basis of the Smith-Hallam fit, and a decrease for the Permo-Triassic time interval. The Permo-Triassic poles for East Gondwanaland are evenly distributed between India and Australia; none is available for Antarctica. The decrease in the angle between East and West Gondwanaland in going from the Smith-Hallam fit to the new fit, cannot be attributed solely to the Indian data. The angular separation between the average palaeomagnetic poles for West Gondwanaland and Australia is also much lower for the new fit. These results are summarised in Table V. CONCLUSIONS
A significant variable in any analysis of this type remains the initial setting up of the reliability criteria on which to base selection of the palaeomagnetic data. As pointed out earlier, the data are unevenly distributed between land masses and in these circumstances arbitrary weighting is unavoidable. During the process of averaging the palaeomagnetic poles, no account was taken of the number of poles available from individual land masses and the size of the land mass represented. With these limitations, the results indicate that for four time intervals (mid-Cambrian-Ordovician, TABLE
V
Average palaeomagnetic poles compared for the Permo-Triassic time interval Region
W. Gondwanaland Australia India
N
16 3 3
for Australia
and India with West Gondwanaland
New Gondwanaland
Smith-Hallam lat. “S
long. ‘E
angle
lat. OS
long. ‘E
angle
56.3 48.5 40.0
080.4 073.0 086.1
9.1 16.8
56.1 57.4 48.3
081.1 089.8 094.6
5.0 11.4
389
Silurian-Early Carboniferous, Permo-Carboniferous and Triassic-Jurassic) the palaeomagnetic poles are more closely grouped for the Smith-Hallam reconstruction than they are for the new fit, and for the remaining interval, the Permo-Triassic, the poles are less closely grouped. Similar conclusions are reached from an analysis comparing continent average poles. Reducing the dispersion of the palaeomagnetic poles by invoking minor movements among the blocks and continents is unwarranted by the data. On palaeomagnetic grounds there is little to choose between the two models should we wish to apply any single reconstruction to that part of Phanerozoic time for which Gondwanaland is understood to have existed. ACKNOWLEDGEMENTS
We thank Dr. R.V. Dingle and Dr. M.W. McElhinny for reviewing an earlier draft of this manuscript. Supported in part by the Australian Research Grants Committee. REFERENCES Barker, P.F. et al., 1974. Preliminary results of Deep Sea Drilling Project Leg 36; the southernmost Atlantic Ocean Basin. Geol. Sot. Am., Abstr., 6: 644-645. Briden, J.C., 1967. Recurrent continental drift of Gondwanaland. Nature, 215: 13341339. Crawford, A.R., 1974. The Salt Range, the Kashmir Syntaxis and the Parmir Arc. Earth Planet Sci. Lett., 22: 371-379. Daly, L. and Pozzi, J.-P., 1976. Resultats palaeomagnetiques du Permien inferieur et du Trias marocain; comparaison avec les donnees africaines et sud americaines. Earth Planet. Sci. Lett., 29: 71-80. Daly, L. and Pozzi, J.-P., 1977. Determination d’un nouveau pole palaeomagnetique africaine sur des formations cambriennes du Maroc. Earth Planet. Sci. Lett., 34: 264-272. Darracott, B.W., 1974. On the crustal structure and evolution of southeastern Africa and the adjacent Indian Ocean. Earth Planet Sci. Lett., 24: 282-290. Embleton, B.J.J., 1972. The palaeomagnetism of some Proterozoic-Cambrian sediments from the Amadeus Basin, central Australia. Earth Planet. Sci. Lett., 17: 217-226. Embleton, B.J.J., 1977a. Discussion. Palaeomagnetism, radiometric age and geochemistry of an adamellite at Yetholme, N.S.W. J. Geol. Sot. Aust., 24: 121-123. Embleton, B.J.J., 1977b. A Late Devonian palaeomagnetic pole for the Mulga Downs Group, Western New South Wales. J. Proc. R. Sot. N.S.W., 110: 25-27. Embleton, B.J.J. and Giddings, J.W., 1974. Late Precambrian and Lower Palaeozoic palaeomagnetic results from South Australia and Western Australia. Earth Planet. Sci. Lett., 22: 355-365. Embleton, B.J.J. and Schmidt, P.W., 1977. Revised palaeomagnetic data for the Australian Mesozoic and a synthesis of Late Palaeozoic-Mesozoic results for Gondwanaland. Tectonophysics, 38: 355-364. Embleton, B.J.J., McElhinny, M.W., Crawford, A.R. and Luck, G.R., 1974. Palaeomagnetism and the tectonic evolution of the Tasman erogenic zone. J. Geol. SOC. Aust., 21: 187-194. Facer, R.A., 1976. Palaeomagnetism, radiometric age and geochemistry of an adamellite at Yetholme, N.S.W. J. Geol. Sot. Aust., 23: 243-248.
390
Fisher, R.A., 1953. Dispersion of a sphere. Philos. Trans. R. Sec. London, Ser. A, 217: 295-305. Griffiths, J.R., 1974. Revised continental fit of Australia and Antarctica. Nature, 249: 336-338. Hicken, A., Irving, E., Law, L.K. and Hastie, J., 1972. Catalogue of palaeomagnetic directions and poles. Publ. Earth Phys. Branch, E.M.R., Canada, 45: 135 pp. Irving, E., 1960-1965. Palaeomagnetic directions and pole positions. Parts II, IV, VI, VII. Geophys. J.R. Astron. Sot., 3: 444-449; 6: 263-267; 8 (with P.M. Stott): 249-257; 9: 185-194. Klootwijk, C.T., 1974. Palaeomagnetic results from some Panchet Clay Beds, Karanpura Coalfield, northeastern India. Tectonophysics, 21: 79-92. Klootwijk, C.T., 1975. Palaeomagnetism of Upper Permian red beds in the Wardha Valley, central India. Tectonophysi~s, 25: 115-137. ~~cElhinny, M.W., 1968-1977. Palaeomagneti~ directions and pole positions. Parts VIII, IX, X, XI, XII, XIII, XIV. Geophys. J.R. Astron. Sot., 15: 409-430; 16: 207-224; 19: 305-327; 20: 417-429; 27: 247-257; 30: 281-293; 49 (with J.A. Cowley): 313-356. McElhinny, M.W., 1973. Palaeomagnetism and Plate Tectonics. Cambridge Univ. Press, Cambridge, 358 pp. McElhinny, M.W. and Briden, J.C., 1971. Continental drift during the Palaeozoic. Earth Planet. Sci. Lett., 10: 407-416. McElhinny, M.W. and Embleton, B.J.J., 1976a. Precambrian and Early Palaeozoic palaeomagnetism in Australia. Philos. Trans. R. Sot. London, Ser. A, 280: 417-431. McElhinny, M.W. and Embleton, B.J.J., 1976b. The palaeoposition of Madagascar: remanenee and magnetic properties of late Palaeozoic sediments. Earth Planet. Sci. Lett., 31: 101-112. ~IcElhinny, M.W. and Luck, G.R., 1970. The palaeomagnetism of the Antrim Plateau Volcanics of northern Australia. Geophys. J.R. Astron, Sot., 20: 191-205. McElhinny, M.W., Embleton, B.J.J., Daly, L. and Pozzi, J.-P., 1976. Paleomagnetic evidence for the location of Madagascar in Gondwanaland. Geology, 4: 355-358. Pascholati, EM., Oacca, I.G. and Vilas, J.F., 1976. Paleomagnetism of sedimentary rocks from the Permian Irati Formation, Southern Brazil. Rev. Bras. Geocienc., 6: 156163. Powell, CMcA., Johnson, B.D. and Veevers, J.J., 1980. The fit for East and West Gondwanaland. The Carey Symposium, Hobart, Feb. 1977. Tectonophysics, in press. Razafindrazaka, G,, Daly, L., Pozzi, J.-P. and Black, R., 1976. Position de Madagascar dans le Gondwana B partir de l’etude magndtique des formations du Karroo. C.R. Acad. Sci. Paris, 282: 17-20. Schmidt, P.W., 1976a. The late Palaeozoic and Mesozoic Palaeomagnetism of Australia. Thesis, Aust. National Univ., 260 pp. Schmidt, P.W., 1976b. A new palaeomagnetjc investigation of Mesozoic igneous rocks in Australia. Tectonophysics, 33: l-14. Schmidt, P.W., 1976c. The non-uniqueness of the Australian Mesozoic palaeomagnetic pole position. Geophys. J.R. Astron. Sot., 47: 285-300. Smith, A.G. and Hallam, A., 1970. The fit of the southern continents. Nature, 225: 139-144. Tarling, D.H., 1972. Another Gondwanaland. Nature, 238: 92-93. Valencio, D.A. and Vilas, J.F., 1976. Sequence of the continental movements occurred prior to and after the formation of the South Atlantic. An. Acad. Bras. Cienc., 48: 377-386. Valencio, D.A., Rocha-Campos, A.C. and Pacca, I.G., 1975. Paleomagnetism of some sedimentary rocks of the Late Paleozoic Tubarao and Passa Dois Groups, from the parana Basin, Brazil. Rev. Bras. Geoscienc., 5: 186-197. Wensink, H., 1975a. The paieom~netism of the Speckled Sandstones of early Permian age from the Salt Range, Pakistan. Tectonophysics, 26: 281-292. Wensink, H., 1975b. The structural history of the India-Pakistan sub-continent during the Phanerozoic. In: Progress in Geodynamics. Royal Netherlands Academy of Arts and Science, Amsterdam, pp. 190-207.
381
PALAEOMAGNETIC COMPARISON OF A NEW FIT OF EAST AND WEST GONDWANALAND WITH THE SMITH AND HALLAM FIT
B.J.J. EMBLETON i, J.J. VEEVERS 2, B.D. JOHNSON 2 and C.Mc.A. POWELL *
1 CSIRO Division of ~i~erai Physics, North Ryde, N.S. W. 2113 (Australiaf * S&o01 of Earth Sciences, ~ac~ua~e Unive~ity, North Ryde, N.S. W. 2113 ~A~tm~ia~ (Received November 25,19’77;
revised version accepted December 5,1978)
ABSTRACT Embleton, B.J.J., Veevers, J.J., Johnson, B.D. and Powell, C.McA., 1980. Palaeomagnetic comparison of a new fit of East and West Gondwanaland with the Smith and Hallam fit. Tectonophysics, 61: 381-390. Palaeomagnetic data for five time intervals in the Phanerozoic are compared for a new fit of East (Antarctica, Australia, India) and West (Africa, Madagascar, South America) Gondwanaland and the hitherto durable fit of Smith and Hallam. The dispersion of the palaeoma~etic poles is marginally less on the Smith and Hallam fit for four time intervals and marginally greater for the remaining one (Permo-Triassic). The Permo-Triassic poles of East Gondwanaland are evenly distributed between India and Australia and the decreased dispersion of the poles for this period on the new fit of East and West Gondwanaland is paralleled by the decreased dispersion of the poles for both India and Australia. Within the limitations of the analysis imposed by the data, the palaeomagnetic comparison shows that there is little to choose between the two fits.
INTRODUCTION
A new fit of East (Antarctica, Australia, India) and West (South America, Africa, Madagascar) Gondw~~~d (Powell et al., 1979) is constrained by recently available information on the extent of continental crust beneath the outer Falkland Plateau (Barker et al., 1974) and off Southeast Africa (Darracott, .1974), and by the pattern of sea-floor spreading between Madagascar and India (Powell et al., 1979). In this fit (Fig. l), Madagascar, whose westem margin has been established as lying against equatorial east Africa (McElhinny et al., 1976), is juxtaposed with the northwestern margin of India, and the Antarctic Peninsula wraps round, without deformation, the Pacific margin of southern South America. East Gondwanaland has been modified from the Smith and Hallam (1970) fit by bringing Australia to Antarctica by a rotation of 31.0” clockwise about a pole at 10.8”N 32.3”E (Griffiths, 1974), and West Gondwanaland by a rotation of Madagascar from
3x2 // ,/’
330’
30’ j
60’
90’ ‘\
\ ‘-\ ‘.
Fig. 1. Averaged palaeomagnetic poles for East (Antarctica, Australia, India) and West (Africa, Madagascar, South America) Gondwanaland, according to (a) the new fit of Powell et al. (1979): full line; (b) Smith and Hallam (1970): fufl line (West Gondwanaland) and dotted line (East Gondwanaland); (c) Smith-Hallam, modified to include the Griffiths’ (1974) Australia-Antarctica fit: same as for the Smith-Hallam fit except Australia’s position is shown with the dashed fine, Drawn relative to Africa’s presentday coordinates.
its position in the Smith and Hallam (1970) fit 4.3” anti-clockwise about a pole 13.2” S 64.1” E. With these changes, the new fit may be constructed from that of Smith and Hallam by a rotation of East Gondwanaland with respect to West Gondwanaland of 16.2” clockwise about a pole at 15.8”N 57.8”W. This fit places East Gondw~al~d in a position similar to that of Tarling’s
383
(1972) fit but has Madagascar in its palaeomagnetically determined position, and obviates the need, as in Tarling (1972), to deform Antarctica. The fit of Powell et al. (1978) is constrained by the pattern of sea-floor spreading in the oceans that now separate the Gondwanaland fragments, and is consistent with the bathymetry of adjacent margins and the trends of geological features before the Late Jurassic breakup. A fourth constraint, continental palaeomagnetism, is discussed here and compared with the hitherto durable fit of Smith and Hallam (1970). DATA SELECTION AND GROUPING
Procedures for selecting palaeomagnetic data are rarely uniformly applied from worker to worker. The nature of the problem and the overall quality of the data available for selection needs to be considered when es~blishing the selection criteria (McElhinny and Embleton, 1976a). Minimum reliability criteria for general application have been used by Hicken et al. (1972) and McElhinny (1973). Although the two procedures differ in detail, they do provide useful guidelines which are adopted here. Apart from the particular requirements regarding the time of acquisition of magnetisation, the age of the rock, and the application of field and laboratory tests of magnetic stability, results are here considered adequate only if based on consistent observations from eight or more samples. However, results from a number of small studies from the same geographical region may be usefully combined to provide an acceptable result, e.g., the Permo-Carboniferous volcanics of southeastern Australia and, ~dividu~ly, the mid-Triassic sediments and basalts from Morocco (Daly and Pozzi, 1976). Some guide to the consistency of observations is acknowledged by excluding all of those results with circles of confidence ((Yap,Fisher, 1953) greater than 25” (cf., Hicken et al., 1972). Data from two tectonic provinces have also been rejected on the grounds that they have possibly undergone some rotation with respect to the main shield and platform areas. One province is the Salt Range, no~hwes~~ India; the rotation of this area has been discussed by Crawford (1974), Wensink (1975a) and Klootwijk (1975), and the other province is the Lachlan fold belt, southeastern Australia, from which pre-Devonian results have been interpreted as representing exotic blocks rather than the main Australian platform (Embleton et al., 1974). The question of partial rema~et~ation has been raised (Wensink, 1975a, b; Klootwijk, 1974, 1975) regarding the time of acquisition of remanent magnetisation in some Gondwana redbeds from India and Pakistan, It was claimed that probable remagnetisation during Late Cretaceous to Early Tertiary times by the Deccan Trap flood basalts is indicated. However, where results are consistent from one formation to another, they are here accepted at face value unless some independent evidence is available that adequately demonstrates some degree of remagnetisation. The palaeomagnetic data used in this analysis are listed in Table I.
a/73; 14/303; Cassange Series (Valencio and Vilas, 1976)
no data 7133; 7134; a/so
8/93;
Grds rouge and Tracbyandesites (Daly and Pozzi, 1976)
no data 7139; 81103; 81104; 81105; mean of 7140, 8179 and 9197
91117; 141361
no data 141352; 141368; 141369; Yetholme adametlite (Facer , 19’76; Embleton, 1977a); Mulga Downs; (Embleton, 1977b); Victorian
4132; 81144; Cambrian of Morocco (Daly and Pozzi, 1977)
10/140; 14/408
141376; 141393; 141395; 14/405; 14/409; 14/413; 141416; Dundas Group (Embleton and Giddings, 1974)
Africa
Antartica Australia
PermoTriassic “.___.~~~
__ PermoCarboniferous __._
Silurian-early Carboniferous
Quasi-static interval
mid-cambrianOrdovician
Land mass
Data sources
TABLE I
-..._-._- --.~
__-
Tasmanian dolerite; Kangaroo Island basalt; Jurassic intrusives; West Victorian basalt; Garrawilla volcanics (Schmidt 1976b, c)
2127; 6136; 10170; 9163
6140-43; 8159; 8163; S/67; 8172; 10177; 13135; 13136; 13140; 12193; 141248; 141 249; 141250; 141288; 141 290; 141573; 141289; Morocco Lavas; Jerada Sediments (Daly and Pozzi. 1976)
.-.._ . .--------.-
Triassic-Jurassic
no data
12/140; 121146
Madagascar
South America
12/X41; no data
no data
no data
12/116; 141309; 141333; Tubarao encio et 12/124
12/117; 141332; 14/345 Group (Vaiai., 1975)
Sakoa Group (McElhinny and Embleton, 1976b); Sakoa Group (Razafindrazaka et ai., 1976)
x4/329
14/302;
11/46;
14/269
1x/43;
14/241;
11/45
14/274
and McElhinny (1968-1977).
11/49, 11/56; 11/60; 11/61; 11/62; 13142; 141286; 14/291; 14/306; Passa Dois Group (Vaiencio et al., 1975); Irati Formation (Pascholati et al., 1976)
Sakamena Group (McElhinny and Embleton, 1976b); Sakamena Group (Razafindrazaka et al., 1976)
14/302; 11/57
Palaeomagnetic pole numbers refer to the Geophys. J.R. Astron. Sot. pole list of Irving (1960-1965)
no data
India
Lavas; Housetop Granite; Canning Reef (Schmidt, 1976a)
386
The data have been grouped into five time intervals covering the midCambrian to mid Jurassic part of the Phanerozoic. The grouping follows the concept of quasi-static intervals (Briden, 1967). The intervals used are: midCambrian-Ordovician which covers the period following the mid-Cambrian episode of apparent polar wander (McElhinny and Luck, 1970; Embleton, 1972) until the mid-Palaeozoic polar transition from northern to southern Africa, e.g. (McElhinny and Briden, 1971); Silurian-Early Carboniferous for the period following the mid-Palaeozoic polar transition until the midCarboniferous transition; Permo-Carboniferous for the late Palaeozoic reversed magnetic interval (Kiaman); Permo-Triassic; and Triassic-Jurassic. The last three quasi-static intervals were recognised by Daly and Pozzi (1976), and Embleton and Schmidt (1977) demonstrated that the palaeomagnetic data grouped discretely according to those time intervals. After midJurassic times, Gondw~~~d began to fragment (Embleton and Schmidt, 1977). DATA
ANALYSIS
Following data selection (Table I) the first and foremost observation is that the numbers of palaeom~etic pole positions are unevenly dist~buted for a given time interval from one land mass to another. Six major land masses comprise Gondwanaland and only for the most recent time interval considered, the Triassic-Jurassic, are there data available from all six regions. The worst case is the Silurian-Early Carboniferous interval in which data from only Africa (two poles) and Australia (eight poles) are available. The analysis has been carried out by assigning unit weight to palaeomagnetic pole positions irrespective of the number of observations (poles) from each landmass. In order to compare the results for each reconstruction model, palaeomagnetic poles have been averaged for (1) the six land masses reconstructed according to the constraints of each model (Table II) and (2) the East Gondw~~~d and West Gondwanal~d components (Table III). To reduce any bias due to the modified position of Australia in East Gondwanaland (Griffiths, 1974), a similarly modified Smith and Hallam fit has been included (Fig. 1). All latitude and longitude values are quoted with respect to Africa’s present geographic coordinates. COMPARISON
OF RESULTS
When the palaeomagnetic poles are averaged for the six land masses (Table II) they show a marginal increase in dispersion for the new Gondwanaland model compared to the Smith-Hallam model for four time intervals and a marginal decrease in dispersion for the Permo-Triassic interval. Table III lists the averaged p~aeom~etic poles for the East and West components of Gondwanaland and Table IV lists the angular differences between the respective pole pairs for the two blocks. The angles calculated
16 10 18 22 34
mid-Cam./Ord. Sil-early Carb. Permo-Carb. Permo-Triassic Triassic-Jurassic
k
36.8 20.6 35.3 53.1 65.8
ON OS OS OS ‘8
011.3 025.1 066.1 080.3 071.4
11.1 13.1 5.7 4.2 3.3
11.8 14.5 37.7 54.5 54.9
2
Sil.-early
12
16
23
Permo-Carb.
Permo-Triassic
Triassic-Jurassic
35.8 OS (6.1) 56.3 OS (4.6) 67.8 OS (4.1)
060.9 (51.1) 080.4 (85.6) 070.5 (53.7)
004.6 (8.2) 025.9 35.8 OS (6.2) 56.1 OS (4.6) 67.8 OS (4.1)
@95)
(k)
(~9s)
34.5 ON (24.7) 13.7 OS -
lat.
long. ‘E
lat.
061.4 (49.2) 081.1 (82.6) 070.9 (53.4)
(k)
long. ‘E
11
6
6
8
10
37.9 ON (13.0) 22.4 OS (16.1) 33.7 OS (11.3) 44.4 OS (10.3) 61.7 OS (5.6)
(ass)
lat.
12.2 13.8 36.3 55.3 53.5
k
015.3 (14.7) 024.9 (12.7) 076.3 (36.0) 080.1 (42.7) 072.7 (66.8)
(k)
long. ‘E
Smith-Hallam
N
Smith-Hallam
N
6
Csrb.
10.9 13.4 5.8 4.2 3.3
East Gondwanaland Powell et al.
008.1 024.6 066.6 081.1 073.2
West Gondwanaland
mid-Cam./Ord.
Interval
ON OS OS OS OS
095
+ Griffiths
24.0 37.0 38.7 55.4 69.0
lat.
ON OS OS OS OS
007.2 019.8 067.8 084.4 080.5
long. OE
New Gondwanaland
43.8 OS (9.6) 61.1 OS (5.5)
(11.0)
32.5 ON (12.9) 27.6 OS (16.1) 32.9 OS
(a951
(14.9) 024.3 (12.7) 077.8 (3 7.6) 082.4 (49.2) 077.5 (68.1)
010.0
Smith-Hallam + Griffiths ~ lat. ;;;g. “B
of Gondwanaland (estimates of precision are italicised)
33.2 24.8 35.1 53.0 65.6
long. OE
lat.
a95
lat. long. OE
Smith-Hallam
Smith-Hallam
Average paiaeomagnetic poles for the east and west components
TABLE III
N
Interval
Average palaeomagnetic poles for the different reconstructions of Gondwanaland
TABLE II
11.0 11.0 31.1 65.9 46.7
k
18.0 ON (12.8) 42.9 OS (16.1) 43.4 “s (11.0) 52.8 OS (9.6) 69.7 OS (5.5)
008.5 (I 5.0) 017.6 (12.7) 082.5 (3 7.6) 092.5 (49.4) 102.5 (67.6)
long. ‘E (k)
lat. (~95)
Powell et al.
11.6 15.1 6.2 3.8 3.6
(~95
E
388 TABLE
IV
Angular differences between averaged palaeomagnetic poles for East and West Gondwanaland calculated for the Smith and Hallam (1970) fit (S-H); the Smith-Hallam fit modified following Griffiths (1974); and the new fit of Powell et al. (1979) Interval
S-H
S-H
mid-Cam./Ord. Sil.-early Carb. Permo-Carb. Permo-Triassic Triassic-Jurassic
9.3 8.8 12.9 12.0 6.2 -____-
5.0 14.0 14.3 12.6 7.4
+ Griffiths
Powell et al. ~_______ 16.9 30.1 18.3 7.9 11.7
for the new fit show an increase for four time intervals compared to the angles calculated on the basis of the Smith-Hallam fit, and a decrease for the Permo-Triassic time interval. The Permo-Triassic poles for East Gondwanaland are evenly distributed between India and Australia; none is available for Antarctica. The decrease in the angle between East and West Gondwanaland in going from the Smith-Hallam fit to the new fit, cannot be attributed solely to the Indian data. The angular separation between the average palaeomagnetic poles for West Gondwanaland and Australia is also much lower for the new fit. These results are summarised in Table V. CONCLUSIONS
A significant variable in any analysis of this type remains the initial setting up of the reliability criteria on which to base selection of the palaeomagnetic data. As pointed out earlier, the data are unevenly distributed between land masses and in these circumstances arbitrary weighting is unavoidable. During the process of averaging the palaeomagnetic poles, no account was taken of the number of poles available from individual land masses and the size of the land mass represented. With these limitations, the results indicate that for four time intervals (mid-Cambrian-Ordovician, TABLE
V
Average palaeomagnetic poles compared for the Permo-Triassic time interval Region
W. Gondwanaland Australia India
N
16 3 3
for Australia
and India with West Gondwanaland
New Gondwanaland
Smith-Hallam lat. “S
long. ‘E
angle
lat. OS
long. ‘E
angle
56.3 48.5 40.0
080.4 073.0 086.1
9.1 16.8
56.1 57.4 48.3
081.1 089.8 094.6
5.0 11.4
389
Silurian-Early Carboniferous, Permo-Carboniferous and Triassic-Jurassic) the palaeomagnetic poles are more closely grouped for the Smith-Hallam reconstruction than they are for the new fit, and for the remaining interval, the Permo-Triassic, the poles are less closely grouped. Similar conclusions are reached from an analysis comparing continent average poles. Reducing the dispersion of the palaeomagnetic poles by invoking minor movements among the blocks and continents is unwarranted by the data. On palaeomagnetic grounds there is little to choose between the two models should we wish to apply any single reconstruction to that part of Phanerozoic time for which Gondwanaland is understood to have existed. ACKNOWLEDGEMENTS
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