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Weddell and break-up: an introduction

E. C. KING, R. A. LIVERMORE & B. C. STOREY British Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK

The , part of the circumpolar Aim of the volume Southern , is probably the most remote, least known and least accessible sea in the world. Many of the major controversies concerned with Difficult ice conditions have limited the acqui- 're-uniting Gondwana', such as the original sition of ship data, although this has been partly position of Madagascar relative to East , offset in recent years by access to satellite radar and that of India relative to have now altimetry data. The Weddell Sea was originally been resolved to the satisfaction of most defined by the Admiralty Hydrographic Depart- scientists. The major exception is the question of ment in 1932 and redefined by the Antarctic how the continental fragments surrounding the Place Names Committee in 1976 (Hattersley- Weddell Sea should be reconstructed so as to Smith 1991). It is bounded on the western side by avoid overlaps and provide a plausible break-up the east coast of the , on the history. The major overlap is that of the southern side by the Ronne and Filchner ice Antarctic Peninsula with the Falkland Plateau, fronts, and on the southeastern side by the which was highlighted by the earliest attempts to Dronning Land and coasts of quantify the fit of the southern (Smith (Fig. 1). The South Ridge & Hallam 1970). separates the Weddell Sea from the to Far from providing a simple explanation for the north and a line joining Southern Thule in this overlap, new data have identified a series of the South Sandwich Islands and Kapp Norvegia enigmatic physical features and tended towards in Dronning Maud Land, separates it from the more complicated models, involving the move- South to the NE. ment of several microplates. Its remoteness and Within this volume, papers relate to the harsh climatic conditions have protected the Weddell Sea as defined above, together with Weddell Sea from the type of comprehensive part of the adjoining South Atlantic Ocean up to investigations carried out in other key oceanic 50°E, and to the of the once neighbour- , so that progress towards a generally ing continents of Gondwana. The term Weddell acceptible solution is slow and characterised by Sea embayment is also used informally through- controversy. Such a solution depends upon out this volume to include the embayment area knowledge of the structure and age of the to the south of the Weddell Sea now covered by oceanic crust to the north, the stretched con- the Ronne and Filchner ice shelves, including tinental or oceanic crust to the south, and the , and the continental shelf north nature of the -ocean boundaries sur- of the Ronne and Filchner ice fronts (Figs 1 & 2). rounding the Weddell Sea. The Weddell Sea was first penetrated by The identification of marine magnetic anoma- in the sealing brig Jane to c. lies and fracture zones provides the basis for 74°15'S, 34°17'W in 1823 and labelled Sea of most plate kinematic models, at least for the George the Fourth or King George the Fourth's major plates. Surprisingly, motions between the Sea on his charts but he later settled on the name South American, African and Antarctic plates Weddell Meer (Hattersley-Smith 1991) and that are still poorly known for times earlier than name, in its Anglicized form, now has common chron C34 (83 Ma). Improved models of plate currency. It is perhaps most well known as the separation, based upon correlation of pre- place where Sir 's ship Endur- break-up features, rifting history and marine ance was crushed in the pack ice and sank in studies, should permit the application of a test 1915. for closure of the three-plate system, to deter-

From Storey, B. C., King, E. C. & Livermore, R. A. (eds), 1996, WeddellSea Tectonics and Gondwana Break-up, Geological Society Special Publication No. 108, pp. 1-10. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

2 E.C. KING ET AL.

+

North Scotia Ridge

PL, 50~N 3o~ ~RICE Anta/c~ Ridge EV,nNG BANK i

'-.... '''' ""...... /ii ...... /

Fig. 1. Place names around the Weddell Sea. Abbreviations are BI, Berkner Island; ST, Southern Thule; KN, Kapp Norvegia. Inset shows plates and plate boundaries in the .

mine whether the Weddell Sea has formed mainly Tectonic setting as a consequence of seafloor spreading between and Antarctica, or whether Weddell Sea motions of other fragments are recorded. The Weddell Sea forms part of the present-day Wider questions related to (Fig. 1, inset), most or all of the break-up are also addressed. The Weddell Sea oceanic crust within it having formed on the region was clearly affected by major magmatic southern flank of a spreading system which is events associated with, and following, rifting of now represented by the South American - southwest Gondwana (White & McKenzie 1989; Antarctic Ridge (Lawver& Dick 1983; Barker Cox 1992). Correlation of igneous events in the &Lawver 1988). Most of the lithosphere created Weddell Sea, and the regions which surrounded on the northern, South American, flank of this it prior to and during break-up, with the system has been subducted beneath the Scotia kinematic history, potentially provides an in- Sea, a process which terminated when the ridge sight into the causes of continental separation. crest interacted with the former South Scotia Of these topics this volume addresses the trench, as at Jane Bank (Fig. 2a); (Barker et al. following: 1984). The South Scotia Ridge has since evolved (1) history of seafloor spreading in the Weddell into a predominantly sinistral strike-slip bound- Sea; ary, separating the Weddell Sea from the Scotia (2) origin and nature of the Weddell Sea Sea, and forming the boundary between the embayment area: continental or oceanic? Antarctic and Scotia plates. (3) origin and nature of continent--ocean boundaries and the prominent escarpments bounding the Weddell Sea and embayment area; Geology of surrounding regions (4) kinematic history of microplates based on palaeomagnetic data; The Weddell Sea and embayment regions are (5) kinematic history of crustal shear zones bounded by three of the five main crustal blocks related to Weddell Sea evolution; or microplates within (6) Gondwana break-up processes and the (Fig. 2b); (Dalziel & Elliot 1982). These include: initial rifting history in the Weddell Sea area. on the western side, the Antarctic Peninsula, on Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

INTRODUCTION 3 _

I --4--

f..... L

'i .... ,\ ...~.-. H J ! ! i ~ Anomaly-T

Andsnes

Fig. 2. (a) Structural features in the Weddell Sea related to Gondwana break-up and early seafloor spreading. Fracture zone trends derived from satellite altimetry form the herring-bone pattern in the northern Weddell Sea. Polar stereographic projection. (b) Microplates of the Weddell Sea region. West Antarctic blocks, shown in white lettering, are: ANP, Antarctic Peninsula; THI, block; MBL, ; EWM, Ellsworth-Whitmore Mountains (block includes Haag Nunataks, HN). South American blocks, shown in black lettering, are: FIB, block; FPB, Falkland Plateau Basin; MEB, Maurice Ewing Bank. The regions in black show bedrock above sea level (expect for East Antarctica where the interior basins are not differentiated). Medium grey fill extends to the 2000 m bathymetric contour, light grey fill extends to the 3000 m bathymetric contour off South America only.

the southwestern side, Haag Nunataks and the ary complex on the fore-arc side (Storey & Ellsworth-Whitmore mountains, and on the Garrett 1985), and thick back- arc basin se- southeastern side, part of the East Antarctic quences on its eastern Weddell Sea side. The craton (Tingey 1991). latter include the Middle and Upper infill of the Latady Basin to the SE (Rowley etal. Antarctic Peninsula. The Antarctic Peninsula is 1983) and Cretaceous and Tertiary sedimentary predominantly a Mesozoic magmatic arc (Pank- rocks of the Larsen Basin to the NE (Macdonald hurst 1982; Leat et al. 1995) that formed along et al. 1988; Crame et al. 1991). The peninsula was the active margin of Gondwana and West deformed by wide ductile shear zones which may Antarctica by of Pacific and proto- relate to the history of the Weddell Sea region floor. It contains a wide accretion- (Storey et al.). Subduction ceased by a series of Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

4 E.C. KING ETAL. collisions of the Pacific-Phoenix spreading ridge Microplates within Gondwana and break- with the trench during Tertiary times, except for up models a small currently active segment off the (Barker 1982). Gondwana reconstructions Although the fit of the major continents within Haag Nunataks. This small crustal block, situ- Gondwana is well established, there is consider- ated between the base of the Antarctic Peninsula able debate over the number and palaeoposition and the (Fig. 2b), is of microplates within the Weddell Sea region. A formed of possessing precise Gondwana reconstruction is an import- Grenvillian Rb-Sr whole-rock ages between ant starting point in trying to understand 1176 and 1003Ma (Millar & Pankhurst 1987). break-up mechanisms and processes, and the Aeromagnetic surveys have revealed the extent history of the Weddell Sea region. The main of this crustal block and Maslanyj & Storey controversies surround the original position of a (1990) have suggested that similar Proterozoic combined Ellsworth-Whitmore mountains- basement may underlie part of the Weddell Sea Haag Nunataks block (EWH), and the Falkland embayment region. Islands crustal block, both displaced parts of the Gondwanian belt. Most reconstructions Ellsworth-Whitmore mountains. The Ellsworth place these microplates within the Natal em- Mountains are formed of a 13 km thick Cam- bayment (e.g. Dalziel & Grunow 1992 for the brian to sedimentary succession folded EWH, and Mitchell et al. 1986 for the Falkland during the Permian- Gondwanian Islands) between southern Africa and Antarc- (Webers et al. 1992). Their anomalous tica. However, Curtis & Storey point out that structural trend relative to the Transantarctic there is unlikely to be sufficient space in the Mountains, together with palaeomagnetic data Natal embayment for both of these blocks (Watts & Bramall 1981) have led geologists to together with the Maurice Ewing Bank as consider the mountains as a displaced part of the suggested by Marshall (1994), and have pro- Gondwanian Cape fold belt of southern Africa posed, based on a review of existing data, a (Schopf 1969; Dalziel & Grunow 1992; Curtis & position for the EWH outboard of the Cape fold Storey). Comparable sedimentary rocks belt. The palaeoposition of the Falkland Islands throughout the remainder of the Elisworth- is equally uncertain. Adie (1952), based on Whitmore mountains block are intruded by geological comparisons, and Mitchell et al. granitic rocks related petro- (1986) and Taylor & Shaw (1989) based on genetically to the widespread magmatism palaeomagnetic data, have suggested that the emplaced during the initial stages of Gondwana Falkland Islands rotated 180° from a position off break-up (Storey et al. 1988). SE Africa during break-up, whereas Riehards et al., from an interpretation of newly acquired East Antarctic craton. The East Antarctic cra- seismic data, conclude that there is no evidence ton bordering the Weddell Sea region (Tingey to support rotation models. They interpret the 1991) can be divided into three basement sedimentary basins adjacent to the Falkland provinces separated by distinct physiographic Islands to have formed initially as Triassic divides (Storey et al. 1994). These, from north to through earliest Cretaceous E-W extensional south are (1) the Grunehogna province which rifts during break-up, and they see no evidence consists of Archaean granitic and for complicated structures that one would sedimentary rocks of similar associate with large rotations. age to the Kalahari craton in southern Africa to which it was once joined (Groenewald et al. Break-up magmatism 1991), (2) the Maudheim province which con- sists of Proterozoic high-grade metamorphic The initial stage of Gondwana break-up was rocks of Grenvillian age (1163-850 Ma), and (3) associated with a major outpouring of continen- the which consists of Palaeo- tal flood in southern Africa (Karoo to Mesoproterozoic magmatic and metamorphic province, Cox 1992), Antarctica (Dronning rocks. The basement rocks are unconformably Maud Land and Ferrar provinces, Elliot 1992) overlain by and younger sedimentary and (Tasman province, Hergt et al. rocks of the Beacon Supergoup, and intruded 1989) during Mid-Jurassic times. The Karoo and and overlain by parts of the Middle Jurassic Dronning Maud Land provinces, together with flood province related to Gondwana postulated volcanic flows, imaged as seaward break-up. dipping reflectors off the Dronning Maud Land Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

INTRODUCTION 5 margin (Hinz 1981; Hinz & Krause 1982), have motion occurred between the EWH and East been related to a mantle plume beneath Gond- Antarctica, 130-100Ma. Divenere et al., in a wana (White & McKenzie 1989) which may have review of new palaeomagnetic data from the had a fundamental control on Gondwana break- Marie Byrd land crustal block, divide Marie up. Initial estimates of the age of this continental Byrd Land into an eastern and a western block. flood basalt province were for a prolonged They place the constraint that the EWH, period of magmatism during Early and Mid- Thurston Island, East Marie Byrd Land, and the Jurassic times (e.g. Fitch & Miller 1984). eastern province of New Zealand did not attain However, new Ar-Ar age dating for the Karoo their mid-Cretaceous positions until after and Ferrar provinces indicates short-lived mag- 117 Ma, due to a plate reorganization during the matic events of 182 + 2 Ma (Hooper et al. 1993) Cretaceous long normal polarity interval that and 176.6 + 1.8 Ma (Heimann et al. 1994) re- was unrelated to the initial opening of the spectively. Furthermore, Brewer et al., also Weddell Sea. using Ar-Ar geochronology, report new ages Another approach to understanding the initial within the Dronning Maud Land province of rifting period is to investigate palaeostress at the 172.4 + 2.1 Ma, and 182.1 + 1.9Ma for basaltic time of break-up as recorded by structural and a dolerite respectively. They also features. Grantham, in a review of new and give new ages within the range 172.6-181.9 Ma existing data on Mesozoic and dyke trends (mean 176.8_ 1.7Ma) from the transition re- in parts of Dronning Maud Land shows that the gion between the Dronning Maud Land and trend of the oldest dyke suites are consistent Ferrar provinces in the , and a with initial E-W separation (in an African new age of 182 + 2.4Ma for the large gabbro framework) of East and West Gondwana, and intrusion of the Dufek Massif within the Ferrar that faults and joints which postdate the Middle province in the . Jurassic magmatic rocks are consistent with a second N-S spreading phase. Reimer etal. apply a similar technique in southern South America Break-up models by analysing ERS-1-SAR images and Landsat- Uncertainties in the palaeoposition and number Thematic Mapper images combined with field of microplates within Gondwana in the South mapping. Storey et al. review the structural and Atlantic region have led to many different tectonic evolution of the Antarctic Peninsula Gondwana break-up models, and to different crustal block which was deformed by wide views of the history of the Weddell Sea region. crustal-scale shear zones during formation of the Models that involve rotation and translation of Weddell Sea. They relate their structural evol- the EWH and Falkland Islands from positions ution to a Gondwana break-up model. off SE Africa will inevitably be more complex for the Weddell Sea region than those that do not (e.g. Storey 1991; Grunow 1993; Marshall Rifts, failed rifts and continent-ocean 1994). The most detailed models are primarily boundaries driven by palaeomagnetic data. They take into account geological and geophysical data, and The determination of an accurate reconstruction involve rotations and translations of the West of SW Gondwana also depends on knowledge of Antarctic microplates either singly or grouped the location and nature of the continent-ocean together to form the larger plate of Weddellia boundary of each component microplate and on (e.g. Grunow etal. 1987,1991). The main, initial an assessment of the degree of stretching within rotation of the Ellsworth Mountains occurred the continental blocks. Even more fundamental prior to c. 175 Ma based on a palaeomagnetic is the determination of what should be regarded pole from the Middle Jurassic granites within the as continental crust and what oceanic. The EWH (Dalziel & Grunow 1992). A series of identification of significant intra-plate defor- these models based on acquisition and incorpor- mation or rotations affects the shapes to be fitted ation of more palaeomagnetic data have led together and the matching of features across Grunow (1993) to suggest that a southern plate boundaries. Weddell Sea, up to 1000km wide, opened between the EWH and the base of the Antarctic Location of continent-ocean boundary Peninsula, 175-155 Ma. The southern Weddell Sea was subsequently subducted beneath the A number of features have been described in the eastern margin of the Antarctic Peninsula and Weddell Sea which may mark the transition southern margin of the Thurston Island block, from continent to ocean. In the SE, off Dronn- 155-130Ma. In addition c. 750km of sinistral ing Maud Land, the lower continental slope has Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

6 E.C. KING ET AL. a steep, linear morphology, which was first Weddell Sea embayment: continent or identified as an escarpment by Hinz & Krause ocean? (1982). It was named Explora Escarpment by Kristoffersen & Haugland (1986) who also On the basis of it's magnetic signature, the reported a buried basement ridge further to the Explora Wedge is interpreted as extending west, the Andenes Escarpment. They proposed southwestwards, into the Weddell Sea em- that the two escarpments formed a linear trend bayment, possibly to Berkner Island and inland marking the plate boundary between the An- to the Dufek Massif (Hunter etal.). This matches tarctic continent and the Weddell Sea. A high the general trend of a deep linear graben amplitude magnetic anomaly (the Orion Anom- proposed by Kadmina et al. (1983) based on aly) lying between the Andenes Escarpment and interpretation of Soviet Antarctic Expedition the Antarctic Peninsula was discovered by deep seismic sounding data. Additional seismic long-range aeromagnetic surveys (LaBrecque et evidence for a graben in this location was al. 1986). The apparent continuity of the Orion presented by Hinz & Kristoffersen (1987), who Anomaly with the Explora and Andenes used the term 'failed rift'. Kristoffersen & Hinz escarpments led a number of authors to the view (1991) proposed that the feature was formed that this combination of bathymetric, basement during the initial fragmentation of Gondwana, and magnetic features marked a continuous, but a change in the regional stress field resulted in E-W-trending continent-ocean boundary, lo- the cessation of stretching within the embayment cated at the foot of the present continental slope and the establishment of a new plate boundary to (Kristoffersen & Haugland 1986; Kristoffersen the north. & Hinz 1991). This interpretation was first The Weddell Sea embayment also contains a challenged by Miller et al. (1991) who discovered large basin, with a sedimentary thickness of an isolated basement high, Polarstern Bank, 12-15 km (Kudryavtzev et al. 1987). This was when searching for the continuation of the thought by some to be underlain by highly Explora Escarpment. The case for a complete stretched continental crust (Grikurov etal. 1991), revision of the relationships between these whereas the presence of oceanic crust generated features is made by Jokat et al. during the migration of the Ellsworth-Whitmore The linear nature of the Orion magnetic Mountains microplate is implied by the palaeo- anomaly suggests that it marks an important magnetic models of Grunow (1993). The present crustal boundary. If this boundary is actually the dimensions of the embayment are approximately continent--ocean boundary, then it would be 1000 km by 700 km. If it is of continental origin, expected to continue to the east, and to be then an additional sizeable microplate (Filchner accompanied by a characteristic gravity anomaly microplate of de Wit et al. 1988) must be included (e.g. Rabinowitz & LaBrecque 1979). Conti- in Gondwana reconstructions. This question of a nuity to the west is established by the combined continental or oceanic origin for the Weddell Sea aeromagnetic data of Hunter et al., while free-air embayment is addressed using seismic data by gravity anomalies, derived from retracked Geo- Hiibscher et al. and using aeromagnetic and sat and ERS-1 radar altimeter data over sea ice gravity data by Hunter et ai., with both papers by MeAdoo & Laxon appear to show a related favouring a continental origin. anomaly, as well as giving the first image of such It has been clear for some time that the data south of 72°S. Antarctic Peninsula must have moved relative to A further significant feature of the Donning East Antarctica during the process of Gondwana Maud Land margin is the wedge of seaward- break-up because, without such movement, an dipping reflectors named the Explora Wedge by overlap with the Falkland Plateau results. A Hinz (1981), which lies to the landward side of number of authors have speculated on the the Explora Escarpment. The Explora Wedge is presence of a maj or strike-slip zone on the eastern interpreted as an example of 'subaerial seafloor side of the Antarctic Peninsula, extending spreading' similar to the Outer V~ring Plateau through the Weddell Sea embayment, to ac- off NW Norway (Mutter et al. 1982). If this comodate this movement (Barker & Griffiths interpretation is correct, then the wedge must lie 1977; Kellog & Rowley 1989, 1991; Storey 1991; at or close to the continent-ocean boundary. Grunow 1993). The validity of such speculations The crustal structure and the location of the is examined by King & Bell using seismic data continent-ocean boundary off Dronning Maud recorded over the Ronne . Land is discussed by l-liibscher et al. and Initial rifting Leitchenkov et al. The previous assumptions that the boundary lay at the foot of the Explora The presence of the failed rift in the Weddell Sea Escarpment are challenged. embayment and it's apparent truncation by the Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

INTRODUCTION 7

Explora-Andenes trend has been put forward as Riiser-Larsen Basin off Dronning Maud Land evidence of at least two stages of rifting by Roeser et al. shows good agreement with a (Kristoffersen & Hinz 1991). Leitchenkov et al. model involving moderately slow spreading expand on this theme and relate the features between M24 and M2. Fracture zone trends and associated with rifting to distance from the magnetic polarity reversal boundaries derived centre of the mantle plume which generated from vector magnetic anomalies near Gunnerus extensive flood basalts in the Karoo province of Ridge, are also used (Nogi et al.) to constrain South Africa and in western Dronning Maud early separation of Gondwana. Combining Land (White & McKenzie 1989). The sub- AFR-ANT and SAM-AFR plate rotations, sequent history of uplift and erosion of the Livermore & Hunter show that the trends of basalts in Donning Maud Land is described by flowlines observed on satellite gravity maps can Jaeobs et al. who conclude that the initial rifting be approximated by assuming that Weddell Sea affected only a narrow zone while the advanced floor north of 72°S was created entirely by South rift stage affected a broader area. America-East Antarctica separation, although the possibility of additional plates cannot be discounted at this stage. Seafloor spreading history Concluding remarks Age of earliest spreading and plates involved The volume contains much new data, new interpretations of previously known features, High-resolution Geosat altimetry, extended and new ideas on the tectonic evolution of the south of 72°S by the addition of more recent Weddell Sea region during Gondwana break- ERS-1 data, has illuminated the spreading up. Much remains to be done, problems and history of the Weddell Sea. It shows major controversies still exist and some of our ques- changes in spreading direction expressed in the tions remain unanswered. The most unusual 'herring-bone' pattern of gravity ridges and aspect of the tectonic development of this troughs, which reflect underlying fracture zone remote region is the number of small micro- trends (Fig. 2a). Abrupt changes at about Chron plates that formed during the break-up of C29 and C21 were documented (Livermore & Gondwana. Such extensive disintegration may Woollett 1993) and correlated with similar in turn be related to the effect of a mantle plume changes observed on the southern Mid-Atlantic on the process. A concensus view on the Ridge and . Apart from movement history of these microplates has not this, WNW-ESE spreading appears to have been reached and there would still appear to be a predominated since a gradual re-orientation of dichotomy between the reasonably simple spreading which occurred some time within the spreading history proposed by the marine Cretaceous magnetic quiet zone. Prior to this geophysicists for the Weddell Sea after c. re-orientation, a period of NNE-SSW spreading 165Ma, compared to the more complicated created a comparable set of closely-spaced break-up models based primarily on palaeomag- fracture zones. These disappear abruptly near netic data from the crustal blocks. The latter 69°S, where a narrow, linear gravity high, known models imply different histories within the as Anomaly-T (Livermore & Hunter), divides Weddell Sea region, than those models based this crust from presumably older seafloor to the primarily on interpretations of marine geophysi- south. Within this area magnetic anomalies M20 cal data. These differences provide a potential to M29 and even older lineations have been focus for new research in the Weddell Sea region recognized (LaBrecque & Barker 1981), but this which may eventually unravel the mysteries of interpretation has been challenged by later this challenging environment. workers. If seafloor spreading in the Weddell Sea We are very grateful to the many people who assisted commenced at about the same time as that during the preparation of this volume, and during the between Africa and Antarctica, as recorded in international conference on the subject that was held conjugate sets of M-series marine magnetic at Madingley Hall in Cambridge, 6-7 June 1994 anomalies off Dronning Maud Land (Bergh organized by the and Tec- 1987) and in the Mozambique Basin (Segoufin tonic Studies Group of the Geological Society Lon- 1978; Simpson et al. 1979), then the earliest don. These include our colleagues in Geoscience Division at BAS, in particular Gill McDonnell for her flowline trends in the Weddell Sea will be secretarial support, and the referees (listed below) for copolar with those further east. A revised their careful reviews, and for responding to our interpretation of magnetic anomalies in the deadlines. Financial support received from The Royal Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

8 E.C. KING ET AL.

Society and from British Petroleum Plc underpinned associated with Gondwanaland break-up: an the success of the meeting and is gratefully acknow- Antarctic perspective. In: STOREY,B. C., ALABAS- ledged. TER, T. & PANKHURST, R. J. (eds) Magmatism and the Causes of Continental Break-up. Geological Society of , Special Publications, 68, Referees 165-184. FITCH, F. J. & MILLER, J. A. 1984. Dating karoo P. Barker, J. Bradshaw, R. Brown, K. Cox, L. Dalla igneous rocks by the convention K-Ar and Salda, M. Degutsch, D. Elliot, R. Evans, D. Fairhead, 4°Ar/39Ar age spectrum methods. In ERLANK, A. S. Garrett, M. Ghidella, A. Grunow, C. Harris, B. J. (ed.) Petrogenesis of the volcanic rocks of the Jacobs, S. Kelley, R. Larter, L. Lawver, P. Leat, D. Karoo Province. Special Publications of the Macdonald, R. Meisnner, B. O'Reilly, S. Rooney, J. Geological Society of South Africa, 13,247-266. Y. Royer, P. Patriat, D. Pirrie, G. Stuart, E. Stump, GRIKUROV, G. E., IVANOV, V. L., TRAUBE, V. V., U. ten Brink, A. Vaughan, F. Vine, A. Watts, R. LEITCHENKOV, G. L., ALESHKOVA, N. n., GO- White, R. Whitmarsh. LYNSKY, A. V. • KURININ, R. G. 1991. Structure and evolution of sedimentary basins in the Weddell province. Abstracts: 6th International References Symposium on Antarctic Earth Sciences. National ADIE, R. J. 1952. The position of the Falkland Islands Institute of Polar Research, Japan, 185-190. in a reconstruction of Gondwanaland. Geological GROENEWALD, P. B., GRANTHAM, G. H. & WATKEYS, Magazine, 89,401-410. M. K. 1991. Geological evidence for a Protero- BARKER, P. F. 1982. The subduction history zoic to Mesozoic link between southeastern of the Pacific margin of the Antarctic Peninsula: Africa and Dronning Maud land, Antarctica. ridge crest-trench interactions. Journal of the Journal of the Geological Society, London, 148, Geological Society, London, 139,787-801. 1151-1123. --& GRI~VrrHs, D. H. 1977. Towards a more certain GRUNOW, A. 1993. 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