A new continental assembly for Pangaea

1-12 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands Tectonophysics, 63 (1980)

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A NEW CONTINENTAL ASSEMBLY FOR PANGAEA M.J. RICKARD and L. BELBIN Department of Geology, The Australian National University, P.O. Box 4, Canberra, A.C.T. 2600 (Australia)

ABSTRACT Rickard, M.J. and Belbin, L., 1980. A new continental assembly for Pangaea. In: M.R. Banks and D.H. Green (Editors), Orthodoxy and Creativity at the Frontiers of Earth Sciences (Carey Symposium), Tectonophysics, 63: 1 -12. A revised Tethyan shear hypothesis leads to a new Pangaea assembly for the Devonian that features no large Tethyan ocean and consequently a more coherent pattern of Early Palaeozoic orogens. The assembly conforms reasonably well with palaeomagnetic data, and produces sep arate apparent pole-wandering paths for Gondwana and Laurasia that cross at the Devon ian position. The short pole-wandering path for Laurasia from Cambrian to Permian times places severe constraints on tectonic explanations of the phenomenon of polar wandering and especially constrains the degree of expansion allowable in an expanding Earth model. INTRODUCTION

The computer-fitted assembly of Gondwana and Laurasia arranged as a single continent (Pangaea) features a wide Tethyan ocean (Bullard et al., 1965; Smith and Hallam, 1970). This BSH-assembly has become a standard basis for tectonic and palaeomagnetic analyses. Within this assembly, however, numerous Gondwanan geometries have been proposed featuring Antarctica, India, Madagascar, and Australia in different positions relative to Africa. These depend on the fact that the continents will fit well geometrically in different positions and the correlation of geological features is often equivocal. McElhinny (1973) claimed that separate continental blocks came together in the Silurian and shared a common apparent pole-wandering path from then until the Mesozoic. More recently it has been suggested that the BSHPangaea and the Tethyan ocean were transient features developed in the latest Palaeozoic just prior to the Mesozoic episodes of sea-floor spreading (Argyriadis, 1975; Irving, 1977). The relationship between Laurasia and Gondwana is uncertain especially for the Palaeozoic. The Early Mesozoic fit (Bullard et al., 1965) depends

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largely on the fit of North America with northwestern Africa along a nearly circular arc (Dietz, 1973; Hayes and Rabinowitz, 1975). Irving (1977) postula te s an He rcy n ian oc ean be twe en the s upe rc on tinen ts o f L aur as ia and Gondwana in the Devonian across which Laurasia is placed arbitrarily northe as t o f A fr ica by a leas t m ov eme nt p roc edu re . A lthough h ypo the se s of dextral and sinistral shear between Laurasia and Gondwana have been dis counted by palaeomagnetic work in the Mediterranean region (Zijderveld and Van der Voo, 1973; Van der Voo and French, 1974; Irving, 1977) large r otations o f L aur as ia re la tive to Gond wana p r io r to the open in g o f the Tethyan ocean have been accepted. During the preparation of a base map for a tectonic map of the Palaeozoic it was found that a new arrangement of Gondwana and a repositioning of Laurasia yielded a coherent supercontinent with no large Tethyan ocean that accords well with available tectonic and geological data (Fig. 1). This R ic k a rd - Be lb in (RB ) -a s s em b ly was m a de u s in g th e c om pu te r p r o gr a m CON TPL OT (B e lb in and C ra in , 19 76 ). Va r iou s a sse mb lies we re tes te d against palaeomagnetic pole dispersions using the well-established data published by McElhinny (1973). New Devonian poles for Australia (McElhinny and Embleton, 1974) and Africa (Hailwood, 1974), and Permian poles for Corsica (Westphal et al., 1976) were added. The Euler rotation poles used are set out in Table I.

TABLE Rotation poles for Pangaea assembly

Continent

Colatitude

Antarctica Madagascar Africa Australia New Zealand South America North America Greenland Eurasia Southeast Asia Arabia Spain India Italy Sicily Sardinia Corsica Kolyma * anticlockwise rotation +

78.81 81.15 63.33 59.13 45.93 170.81 8.75 16.93 15.42 62.92 90.14 62.98 61.90 29.25 98.76 71.50 67.08

Longitude 159.82 125.54 unrotated 128.04 138.30 329.88 301.99 104.65 156.00 128.23 29.21 3.83 38,41 23.01 35.24 13.86 18.20 91.27

Rotation angle * -56.88 -14.62 -46.44 -55.40 56.68 -58.04 60.13 25.69 18.55 - 9.93 -31.93 -56.16 -80.37 32.66 -28.20 -54.39 20.03

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Fig. 1. Proposed Pangaea reconstruction for the Devonian — showing continental slope ° margins (1000 m) and generalized tectonics; stereographic projection 184 view. Euler rotation poles are listed in Table I. REASSEMBLY OF THE CONTINENTS

The positions of the northern continents have been modified from the Bullard et al. (1965) assembly by moving Greenland northwards (Tarling, 1972) to reduce marginal superposition, and by moving the Kolyma block

4 partly to close the Arctic Ocean following Herron et al. (1974). The Mediterranean segments (Iberia, Sardinia, Corsica, and Italy) are rotated against Europe for reasons discussed later. Uncertainty in making a Gondwana assembly stems mainly from the poor con tro l on the pos ition o f Anta rctica and Ind ia in r ela tion to Afr ica. The sea-floor patterns are useful and support the positioning of India against Antarctica (Sclater and Fisher, 1974; Crawford, 1974). The position of Madagascar has also long been controversial, but palaeomagnetic determinations now support the northern position against Tanzania (Embleton and McElhinny, 1975). The South America—Africa—Madagascar and India—Antarctica— Australia assemblies are essentially those of Smith and Hallam (1970) with modification of the fit be twe en th e two un its . In the RB -as se mb ly An ta rc tic a is rotated anticlockwise and fitted to allow a linear connection between the Antarctic Peninsula and the Patagonian Cordillera as advocated by Dalziel and Elliot (1971), and at the same time maintaining as close a link as possible between India and Madagascar (Crawford, 1974) allowing for the Seychelles, Madagascar and Mozambique rises which are probably continental (Laughton et al., 1970). Except for the positioning of the Antarctic Pen insula, this reconstruction is similar to that of De Wit (1977). Since the junction between Laurasia and Gondwana is everywhere parallel to orogenic belts, the Pangaea reassembly must depend largely on an interp retation o f the comp lex mov ements in the Alp ine—Hima layan b elt and pa laeomagne tism . Ou r recons truc tion is based on an a ttemp t to c los e the Tethyan ocean of the BSH-assembly on the grounds that such a proce du re is a geo me tr ic re qu ir em en t o f p la te te c ton ic r ec ons truc tion s . Th e m an ipu la tio n inv o lve d the re in trodu c tion o f s in is tra l s hea r a long Te thy s as advocated by Carey (1963). The follo wing evidence supports sinistral shear: the general asymmetry of the northern continents with respect to the southern (Carey, 1963) and the tectonic evidence for late ral movements (Wellman, 1966; Reed et al., 1970; Sibuet, 1973; Molnar and Tapponier, 1975; Neev, 1977); the over riding of the northeastern Pacific spreading ridge by North America {Palmer, 1968); the abutment of the Cathaysian orogen against the Indo -China Precambrian block across the major Tonkin wrench fault (Yanshin, 1966; see Fig. 1); the positioning of North America to the northeast of Africa in the Late Precambrian on the basis of palaeomagnetic data (Piper, 1975); studies of plate reconstructions in the Mediterranean indicate that the earliest move ments from Triassic to Cretaceous times involved a southeasterly movement of Africa with respect to Europe (Barberi et al., 1974); and the oroclinal rotations of the Mediterranean segments — Iberia, Corsica, Sardinia, Italy -(Carey, 1955; Zijderveld et al., 1970; Williams, 1975; Westphal et al., 1976). The movements of tectonic elements in the Mediterranean region have been tested extensively by palae omagnetism; a remarkable paradox results. The pa laeomagn etic dec linations f r o m P erm ian to Cre taceous for Ib er ia ,

5 Corsica, Sardinia, and Italy are displaced some 30 ° to 50 ° from stable Europe but are parallel to those for stable Africa; if on the other hand, the islands and peninsulas are rotated clockwise hack to the European continent, the palaeomagnetic declinations parallel those for stable Europe. Early work summarized by Zijderveld et al. (1970) supported oroclinal bending and showed that the magnetic inclinations for rotated segments agreed with the palaeolatitudes of stable Europe. From this it was concluded that no shearing had occurred between the rotated segments and stable Europe. The possibility of Tethyan shear to the south of these segments was incorrectly dismissed. More recent data (Zijderveld and Van der Voo, 1973, Van der Voo and French, 1974; Channell and Tarling, 1975) emphasized the parallelism with African declinations and led to the interpretation of the Mediterranean segments as prongs of the African plate advancing into the Alpine orogen. The geological consequences of this interpretation have not been explained. Zijderveld and Van der Voo (1973) admit the possibility of a sinistral shear interpretation but favour the African association. These authors invoke an anticlockwise rotation of the segments with Africa to account for the present divergence of the magnetic declination with respect to Europe. Thus the junction between the rotational African and European plates must lie to the north of the Mediterranean segments in a position that had previously been "proved" to be a line of no shear translation! The key to this paradox lies in more accurate determinations of the palaeo-isoclines because those for stable Europe and Africa are extrapolated from unreliable data (Channell and Tarling, 1975). The latest results from peninsula Italy (Channel and Tarling, 1975) mismatch the African palaeoisoclines of Zijderveld and Van der Voo, 1973 (fig. 9) by as much as 45°. Irving (1977) claims that the match of Jurassic palaeolatitudes between adjacent North America and Africa precludes Tethyan shear later than the Jurassic. However, his sequential reconstructions do not preclude earlier sinistral movements and the possibility of post-Jurassic shear depends entirely on the nature of the initial opening pattern of the Atlantic. In favouring the orocline and sinistral-shear hypothesis we wish to point out first, that the sea-floor magnetic pattern in the Bay of Biscay (Williams, 1975) strongly supports a European attachment for Iberia, second, that Sardinia and Corsica have geological affinities with France (Westphal et al., 1976), and finally, that palaeomagnetics alone are not capable of detecting latitudinal mega-shearing. Thus we suggest that Laurasia moved westwards with respect to Africa during the Late Palaeozoic and Early Mesozoic. In making our reconstruction Laurasia has been repositioned somewhat arbitrarily by rotation around the arcuate northwestern shelf of Africa far enough to unwind the Mediterranean oroclines (Fig. 1). TESTING THE REASSEMBLY The most significant feature of this RB -assembly is that it closes the Tethyan ocean of the familiar BSH-assembly. The validity of any assembly

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must be tested by detailed matching of tectonic and geological features, and by palaeomagnetism. This is proceeding, and preliminary examination reveals no major violations (Fig. 1). The closer proximity of Australia and eastern Asia is supported by faunal similarities in the Early Cambrian (Jell, 1974). Moreover, Tarling (1972), Crawford (1974), and Veevers et al. (1975) have argued for juxtaposition of p arts of southern Asia with India and Australia. Silurian and Devonian coral localities accord well with the palaeo -equator

°

F i g . 2. P a n g a e a — 1 2 1 3- a ss e m b l y : s t er e o g r a ph i c p ro j e c t io n 1 9 0 v i e w. P a l a e o m a gn e t i c p o l es aft er M cElh inn y (1973), McE lhinn y an d E mbl eton (1974), H ail wo o d (1974), an d West phal et al. ( 197 6). Po les for each su bcontin ent are plott ed with dif fer ent s ymbols (so li d Lau r as i an, o p en = G o n d w an an ) an d c lus t er s f or d i f f er ent p er i o ds ar e o ut l in ed . Th e g en eral tren ds of the apparent po le- wanderin g path s for Laur asia an d Gondw ana ar e indi c at ed w ith bro ad arr ows.

7 (Pickett, 1975). The Palaeozoic orogens circumscribe Pangaea (Fig. 1) in a simpler more coherent distribution than on the BSH -assembly (cf. Hurley, 1974), and in particular the Cathaysian and Tasman orogens are brought into closer proximity. There is also a remarkable alignment of the Rhine-Rhone rift system with that of the Red Sea African rifts (Fig. 1) — Tertiary structures for which basement controls have long been argued (lilies, 1974). The new Gondwana assembly was tested palaeomagnetically by comparing the positions of the Australian pole -wandering paths with respect to those for Africa and South America against the BSH assembly. The RB-assembly gives a somewhat closer fit for the Early Palaeozoic and a tighter cluster of Cambro-Ordovician poles. The latter poles cannot give a conclusive tes t because the Cambrian was a time of rapid polar wandering, and the position of the Ordovician poles in relation to Africa is being revised (Thompson, 1973; Hailwood, 1974; McElhinny and Embleton, 1974). The test was also carried out with Hailwood's (1974) revised data with comparable results. The palaeomagnetic data for each subcontinent of the RB assembly is plotted in Fig. 2. Apparent pole -wandering paths for Gondwana and Laurasia are indicated by joining the clusters for each period; these paths cross a t app rox imately the Devon ian pos ition . Unfo rtuna tely the palaeo magnetic data for the Gondwanan Devonian is poorly clustered and there are only sin g le r e s u lts fo r A fr ic a an d S ou th A m eric a ; th e y p lo t in two g r ou ps h e r e (Fig. 2) one of which coincides with the Laurasian cluster. Rapid pole movement during the Devonian has been invoked to explain such double clustering (Creer, 1973; Schmidt and Morris, 1977) but this is unlikely since the Laurasian results are well clustered (Fig. 2). It is noteworthy that a ll the Devonian poles for Laurasia are derived from sites separated from their pole positions by the Hercynian or Alpine orogenic belts. Allowance for shorten ing across these belts could be expected to move the whole Laurasian polewandering path by an unknown amount with respect to the Gondwana path, and Creer (1973) advocated a similar shift to improve the cluster ing of Devonian poles on the BSH-assembly. Thus, we maintain that, although the RB -assembly may be too tight, it satisfies the available palaeomagnetic data reasonably well and that it may represent the positions of the major continents in Devonian times (possibly Late Silurian to Middle Carboniferous) after the major Early Palaeozoic "Caledonian" orogeny. CONCLUDING COMMENTS The symmetrical Y -shaped distribution of the Gondwana pole wandering path about the shorter Laurasian one (Fig. 2) suggests that Laurasia under went a slow general northward movement throughout the Palaeozoic, whereas Gondwana made larger excursions, closing with Laurasia in the Devonian and separa ting aga in . Te thys opened dur ing th e La te Pa laeoz oic, and by Early Mesozoic the supercontinents had probably moved to a position repre -

8 sented by the BSH-assembly on which the Mesozoic poles cluster well (Creer, 1973; McElhinny, 1973), or a modified version of it (Tarling, 1972. Van der Voo and French, 1974; Embleton and Valencio, 1977; Irving, 1977). The RB-assembly has an appealing geometric simplicity (Fig. 1); the four symmetrically placed re-entrants giving a clover-leaf pattern remarkably similar to the tetrahedral convection forms suggested by Hughes (1973). These re-entrants became the sites of important smallocean basins during later tectonic developments. Finally we note that the RB-assembly places constraints on the expanding Earth hypothesis (Carey, 1958, 1976). Interpretation of the four re-entrants as expansion gapes requires less expansion than postulated by Carey (1958) for a wide Tethyan gape. Moreover, although the northward drift of Laurasian continents indicated by their pole-wandering path may be explained by expansion (Carey, 1976), model studies (A.R. Coleman, personal communication, 1978) indicate that such a short pole-wandering path (Fig. 2) can be produced by an expansion of less than 20% of radius. Thus if expansion is a tectonic reality it must be regarded as a background process that nevertheless may provide the triggering mechanism for more extensive plate adjustments by creating rotational instabilities. ACKNOWLEDGEMENTS The computer work was carried out by Belbin and the tectonic interpretations by Rickard. The Australian National University Computer Centre gave much patient assistance. M.W. McElhinny kindly provided palaeomagnetic data and many critical comments; Professors D.A. Brown and R.W. Rutland, and B.A. Duff and K.A.W. Crook kindly read various drafts of the manuscript. The authors are pleased to acknowledge their indebtedness to Professor S.W. Carey for the inspiration that motivated this work. ADDITIONAL NOTE Recent publications since this paper was written have not clarified the tectonic and palaeomagnetic situation in the Mediterranean region and the position of Italy with respect to Africa and Europe remains in doubt. It has been argued that Italy and Sardinia formed part of an Adriatic block associated with the African continent during post Hercynian times until the Early Tertiary (Horvath and Channell, 1977; VandenBerg, 1979). Since the Mesozoic palaeomagnetic data from Africa is inadequate, the association with Africa is dependent on comparison with a pole-wandering path extrapolated from North American data. VandenBerg et al. (1978) demonstrate that the Italian palaeomagnetic data is in good agreement with the extrapolated African poles from the Early Jurassic, after allowance has been made for a post-Eocene 25 ° anticlockwise rotation of Italy. This stud y places great reliance on the shape of the short section of the Jurassic to

9 Cretaceous pole-wandering path and dismisses the possibility of a greater rotation back to the European pole position and hence dismisses the possibility of sinistral movement between Africa and Europe. Horvath and Channel] (1977) explain the meridional structures in the Adriatic block as due to collision with the eastward moving Africa; thus a sinistral transform is invoked to separate these units from Europe. Fromm and Pitcher (1978) invoke independent movement patterns since the late Carboniferous, with consistent anticlockwise rotation of Africa and clockwise rotation of Europe superimposed on differential meridional movements between the continents. On the other hand, others still support the argument that Italy and Sardinia have rotated anticlockwise from Europe (Soffel, 1978; Storetvedt and Markhus, 1978). Moreover, Channel! et al. (1978) have shown that the Late Cretaceous magnetic inclination for northern Umbria is 44 ° compared with a predicted inclination of 44 ° from. Europe and 36 ° from Africa. For southern Umbria the magnetic inclination is 39 ° ; but the closeness to the African result is considered fortuitous. An additional complication is the controversy over the autochthony of many of the Italian sampling sites (Kligfield and Channel!, 1979; VandenBerg and Wonders, 1979). The Hercynian wrench fault pattern in Europe has been interpret ed as a largely. dextral system (Arthaud and Matte, 1975) although Le Pichon et al. (1977) have pointed out that the existence and sense of many strike-slip movements are still in doubt. In his reconstruction of Mediterranean palaeogeography Hsu (1977) indicated that Spain and Europe moved sinistrally with respect to Africa from Triassic to Cretaceous times. Thus, we conclude that extensive sinistral shear between Gondwana and Laurasia is still a possibility and that our proposed rec onstruction is a possible Pangaea configuration. REFERENCES Argyriadis, I., 1975. Mósogée permienne, chaine hercynienne et casgure tethysienne. Bull. Soc. Geol., Fr., 17: 56 -67. Arthaud, F. and Matte, P., 1975. Les decrochements tardi -hercyniens du sudouest de L'Europe. Geometric , et essai du reconstitution des conditions de la deformation. Tectonophysics, 25: 139 -171. Barberi, F., Civetta, L., Gasparini, P., Innocenti, F. and Scandone, R., 1974. Evolution of a section of the African—Europe plate boundary; palaeomagnetic and volcanological evidence from Sicily. Earth Planet. Sci. Lett., 22: 123-132. Belbin, L. and Crain, T., 1976. CONTPLOT: a program to draft continental reconstructions. Comput. Geosci., 1: 279-308, Bullard, E.C., Everett, J.E. and Smith, A.G., 1965. The fit of the continents around the Atlantic. Philos. Trans. R. Soc., London, 258: 41 -51. Carey, S.W., 1955. The orocline concept in geotectonics. Proc. R. Soc. Tasmania, 89: 255-288. Carey, S.W., 1958. A tectonic approach to continental drift. Symposium, University of Tasmania.

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