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Home , Coats Land, Geology of Antarctica, Whichaway Nunataks

Polarforschung 67 (3): 91 - 99, 1997 (erschienen 2000)

Geologie Signifieanee ofRegional Magnetie Anomalies in Coats Land and Western Dronning Maud Land

By Alexander V Golynsky', Garrik E. Grikurov' and Eugene N. Kamenev'

Summary: Some aspects of the Late--Early tectonic INTRODUCTION development of western Dronning Maud Land and Coats Land are examined in this study by using aeromagnetic data. Results of our study neither contra­ dict nor support a postulated connection ofthe Kaapvaal-Zimbabwe Province Coats Land (CL) and western Dronning Maud Land (WDML) into western Droning Maud Land but certainly indicate that the Coats Land are situated in the pivotal region which lay between East crustal block has never been part of the African craton; it appears rather to be and West Gondwana during the transition from typical of the East . Our interpretation contradicts the sug­ gestion that the Coast Land area is a continuation of the Mazatal-Yavapai­ amalgamation to Gondwana fragmentation. In the geodynamic Grenville provinces of North America. Structural relationship of magnetic context, this area has significant interest due to the recent anomalies in the neighboring (Maudheim Province, Shackleton proposal of the SWEAT hypothesis (Southwest United States­ Range and Coats Land) strongly indicate the existence of an ancient cratonic fragment in Coats Land. Magnetic anomalies related to the Ross orogenie East ) which suggests that and event are very limited in western Dronning Maud Land and not discernible in Laurentia may have been connected in an Early Neopro­ the area. This observation together with the geologie data terozoic (MOORES 1991, DALZIEL 1991). allow us to suggest that a Ress-age suture resulting from Gondwana coales­ cence is not presented in western Dronning Maud and Coats Lands, apparently it must lie south of the current exposures of the Shackleton Range. Magnetic Many authors attempted to prove, improve or refuse a new anomaliesassociated with fragmentation of Gondwana during the Mesozoic hypothesis using geological and geophysical information are depicted within the ancient Grunehogna cratonic fragment as isolated (MOYES et al. 1993, 1994, STOREY et al. 1994). For example, intrusions and either a linear chain of intrusions or a dcep-seated ; apparently inherited some ancient inhomogenity in the crust and geochronological, isotopic results and interpretation of are recognized within the Proterozoic mobile belt (the Maudheim Province) as Russian aeromagnetic data allowed STOREY et al. (1994) to intrusions of different composition which are predominantly alkalic in nature. confirm the existence of Grenvillian age rocks within both Three well-known intrusions are located in the eastern shoulder ofthe Jurassie West and East Antarctica and support the juxtaposition of Jutulstraumen-Pencksökket failed rift whose axis is associated with negative magnetic anomalies, suggesting that axial intrusions not developed along the Antarctica and western North America in a Neoproterozoic axis of the rift as aprecursor to crustal splitting. supercontinent reconstruction. The location of the Grenville Front in Antarctica given by DALZIEL (1991; the area between Zusammenfassung: Einige Aspekte der tektonischen Entwicklung vom west­ lichen Dronning Maud Land und Coats Land im späten Proterozoikum und Pensacola and was used as a piercing point) frühen Mesozoikum werden mit Hilfe von flugmagnetischen Daten in dieser has been modified by them. According to MOYES et al. (1993), Studie untersucht. Die Ergebnisse unserer Studie stehen weder im Wider­ the Mid-Proterozoic to Late tectonic history of spruch noch unterstützend zu einer postulierten Verbindung zwischen der Kaapvaal-Zimbabwe-Provinz und dem westlichen Dronning Maud Land, aber WDML does not appear to reflect aperiod of major conti­ zeigen deutlich, dass der Krustenblock von Coats Land niemals ein Bestand­ nental re-organization, but rather a less intense reactivation teil des afrikanischen Kratons gewesen ist; er erscheint eher typisch für den along the major features established during the preceding ostantarktischen Schild. Unsere Interpretation widerspricht dem Vorschlag, Kibaran (Grenvillian) at approx. 1100 Ma. They also dass das Gebiet um Coats Land eine Fortsetzung der Mazatal-Yavapai-Gren­ ville-Provinz von Nordamerika sei. Die strukturelle Beziehung zwischen den concluded that the Pencksökket-Jutulstraumen divide does not magnetischen Anomalien der benachbarten regionalen Einheiten (Maudheim­ represent a continuation of the Mazatal-Yavapai-Grenville Provinz, Shackleton Range und Coats Land) deutet auf die Existenz eines boundary of North America, nor does it represent a Ross age alten kratonischen Fragments im Coats Land hin. Magnetfeldanomalien, die zur Ross-Orogenese in Verbindung stehen, sind für das westliche Dronning suture resulting from Gondwana coalescence. Furthermore, Maud Land sehr limitiert und für das Gebiet der Shackleton Range nicht iden­ GOSE et al. (1997) concluded that the East Antarctica and tifizierbar. Diese Beobachtung in Kombination mit geologischen Daten Laurentia poles of 1100 Ma do not coincide after restoration erlaubt den Vorschlag, dass eine Sutur aus der Zeit der Ross-Orogenese, die ofthe continents to a position suggested by the SWEAT hypo­ aus der Formation von Gondwana resultiert, im westlichen Dronning Maud und Coats Land nicht existiert. Anscheinend liegt sie südlich des jetzigen thesis, based on new paleomagnetic detenninations for grano­ aufgeschlossenen Shackleton Range. Die mit der Fragmentierung von Gond­ phyre and rocks from the Littlewood and Bertrab wana während des Mesozoikums assoziierten Magnetfeldanomalien sind Nunataks. This indicates either that the hypothesis is incorrect innerhalb des alten kratonischen Fragments Grunehogna als isolierte Intru­ sionen, als lineare Kette mafischer Intrusionen oder als eine tiefliegende or that CL and parts of WDML were not part of the East Verwerfung dargestellt. Scheinbar mit Übertragen sind einige Inhomogenitäten Antarctic craton at 1100 Ma. In a new reconstruction given by in der Präkambrischen Kruste, die innerhalb des protero zoischen mobilen GOSE et al. (1997), Coats Land and the Kalahari craton are GÜrtels (Maudheim- Provinz) als Intrusionen von unterschiedlicher Zusam­ mensetzung, meistens alkalischer Natur, erkennbar sind. Drei gut bekannte placed within West Gondwana and offthe present southeastern Intrusionen liegen in der östlichen Schulter des jurassischen Jutulstraumen­ margin of Laurentia. At that time, TESSENSOHN (1997) noted Pencksökket "failed" Rift, dessen Achse mit der negativen magnetischen that flat-Iying, unmetamorphosed, felsic volcanics of the Litt­ Anomalie assoziiert werden kann. Dies lässt vermuten, dass sich axiale Intru­ lewood Nunataks are not particularly good evidence for the sionen nicht als Vorboten vom Krustenabbruch entlang der Rift-Achse entwickelten. presence of the Grenvillian belt in this area. In addition, recent geochemical results allowed DIVAKARA RAo (1996) to suggest that and of the bimodal , VNIIOkeangeologia, I, Angliysky Avenue, 190121, SI. Petersburg, Russia represented in the Bertrab and Littlewood Nunataks have a common source of origin. The source magma, a melt of the Manuscript received 13 January 1999, accepted 25 January 2000

91 74

Fig. 1: Selected aeromagnetic profiles flown by the Polar Marine Research Expedition over western Dronning Maud 76 Land (l cm = 1000 nT).

Abb. 1: Ausgewählte flugmagnetische Profile, geflogen von der Polar Marine -25 -20 -15 -10 -5 5 10 15 20 Research Expedition über das westliche Dronning Maud Land (l cm = 1000 nT).

upper mantle appears to have evolved in a compressional The continuation ofthe Maudheim Province mobile belt to the tectonic regime and was intruded and extruded in an exten­ west is only evidenced in the occurrence of coeval metamor­ sional rift tectonic setting. phics in the Haag Nunataks area westward of the Embayment. In this respect the rocks of the Touchdown Hills In this paper we intend to further examine the relevant aero­ nunataks are of crucial importance. It is obvious, that com­ magnetic data pertaining to the Late Proterozoic - Early Meso­ parison of the structural features of two neighboring terranes zoic tectonic development of WDML and Coats Land. (Maudheim Province and Coats Land) is meaningless due to Magnetic data have potential for resolving existing questions the great distance between them and due to existence on the regarding the structural relations in this area. CL side ofonly three tiny outcrops with total area less than 0.1 km'. The predominant rock type at the Bertrab and Littlewood Nunataks is of granitic composition (MARSH & THOMSON 1984). They describe the Bertrab Nunatak granitoid as a GEOLOGIC PROVINCES granophyre, which intruded by a dyke-like body of flow­ banded rhyolite, and by a fine-grained dyke of altered micro­ Several distinct geologie provinces were recognized in WDML diorite. and CL. The oldest ofthese is an to Mid-Proterozoic cratonic fragment known as the Grunehogna Province The Shackleton Range lies close to the proposed position of (GROENEWALD et al. 1991). The Archean crust is characterized the Grenville front within Antarctica in reconstructions of by the limited exposures at Annandagstoppane and immedia­ Rodinia (DALZIEL 1991, STOREY et al. 1994). As was shown by tely surrounding nunataks, where granite dominates (e.g. TESSENSOHN et al. (1991), geological observations do not RAVICH & SOLOVIEV 1966, KNOPER et al. 1995). This Archean support this hypothesis. There are several tectonic models that crustal fragment is apparently overlain by the relatively unde­ can explain our knowledge about the present structure of the formed and unmetamorphosed intracratonic-basin sediments Shackleton Range and its relation to the Ross Orogen as the of the Ritscherflya Supergroup (WOLMARANS & KENT 1982). whole. Some of these models were considered by KLEIN­ The sediments are intruded by tholeiitic sills and dykes of the SCHMIDT & BUGGISCH (1994) and they concluded that all ofthe Borgmassivet Intrusions. A recently determined V-Pb zircon models imply that the parts of Antarctica north of the Shack­ age of 1135 Ma from felsic volcanic rock (KNOPER et al. 1995) leton Range (WDML and CL, perhaps inc1uding the north­ indicates deposition of the Ritscherflya Supergroup at the western parts of the Shackleton Range itself) are more likely same time as the deformation at 1200-1000 Ma in the adjacent related to (Paleo-) Africa than to the East Antarctic Craton. orogenie belt,the Maudheim Province (GROENEWALD et al. 1991). The highly-deformed orogenie belt extends longitudi­ New isotopic characteristics of two infracrustal basement nally from the Weddell Sea depression northeastward through groups (Read and Stratton Groups) suggest a common proto­ the Heimefrontfjella, Kirwanveggen, and H.U. Sverdrupfjella lith (MILLAR & HENJES-KuNST 1997). This argues against the towards Gjelsvikfjella and Muhlig-Hofmannfjella (GROENE­ existence of apre-Ross oceanic basin between the northern WALD 1993, OHTA et al. 1990). From the west, north and east and southern parts of the range. The supracrustal gneis ses and the orogenie belt is juxtaposed against the Archean crustal metasedimentary rocks of the Pioneers Group that are tecton­ fragment, at that time its southern boundary did not extend ically interleaved with high-grade metamorphie basement beyond the Shackleton Range as was suggested by STOREY et rocks belonging to the Stratton Group have mid-Proterozoic al. (1994) and presumably juxtaposed against the unexposed (Grenvillian) Nd-model ages indicating a different type of East Antarctic craton. crust more related to the Heimefrontfjella (ARNDT et al. 1991,

92 JACOBS et al. 1997). The zircon data also suggest that Grenvil­ Provinces, as weIl as within the Coats Land crustal block and lian affected Early Proterozoic Stratton Group adjacent areas (GOLYNSKY & ALESHKOVA 2000). The over­ of the northern Shackleton Range (MILLAR 1995). printing character of magnetic anomalies is rather different The situation is complicated by radiometric ages of unknown within the study area, but we have not recognized any significance between 2300 and 2700 Ma obtained for evidence in the onshore areas that fragmentation ofGondwana emplaced granitoids from the Lagrange Nunataks and from during the Mesozoic disregarded to full extent of older suture the southern Haskard Highlands (PANKHURST et al. 1983), lines or boundaries between major provinces. One of the best possible parts of the African Kaapvaal Craton or surrounding examples of the Mesozoic influence on the basement anomaly mobile belts as suggested BROMMER et al. (1997) or a more pattern is observed in WDML. Here, the central part of the northern continent, microcontinent or as considered by Grunehogna Province and northern continuation of the Maud­ KLEINSCHMIDT & BUGGISCH (1994). The brief overview of the heim Province is transected by the linear Princess Martha tectonic setting of the Shackleton Range shows that many Coast Magnetic Anomaly (PMCMA) which can be interpreted problems have not really been solved at the moment, the kind as a linear chain of mafic intrusions or a deep-seated fault of connections between the basement of the zone. Apparently, this anomaly may delineate a zone of and the northern Shackleton Range, as well as with the CL structural weakness that initially developed during or prior the crustal block remains unc1ear. Gondwana separation. Several circular highs with amplitude up to 1700 nT occur within the Grunehogna Province supposedly caused by intrusions similar to those spatially attri­ MAGNETIC ANOMALY IMPRINTS OF THE MAJOR buted to the Maudheim Province and exposed in Utpostane TERRANES and Muren (olivine gabbro) or near Straumsvola and Sistenup (syenite). The structural relationship between the WDML and CL terranes has remained uncertain due to a lack of information It is known that the Jurassie dolerite dykes from Vestfjella are concerning sub-ice crustal structure. Russian regional aero­ strongly magnetized (2xlO· 3 emu.) relative to the basaltic magnetic surveys of this area, completed as part of an inte­ (5xlO'5 emu.) and predominantly of normal polarity (KRI­ grated geophysical and geological investigation provide the STOFFERSEN & AALERUD 1988, RUOTOISTENMÄKI & LENTIMÄKI opportunity to study the ice-covered territory where several 1997). Comparison of the regional magnetic anomaly map Precambrian terranes are recognized. with the outcrop geology of Vestfjella shows that only the flows near Plogen and Salryggen occur over a magnetic The recently compiled aeromagnetic anomaly map of the high; the others are associated with negative magnetic anoma­ Weddell Sea region allowed GOLYNSKY & ALESHKOVA (2000) lies or lie along the flanks of highs. For this reason, it is diffi­ within the study area to distinguish six distinct magnetic units cult to infer the real extent ofbasalts or to correlate them with attributed to different crustal terranes. It was shown that a specific magnetic anomalies of regional extent. The causative magnetic low area occupied the Princess Martha Coast and source of the positive magnetic anomaly near Plogen and linear short-wavelength magnetic anomalies over the Salryggen is not c1ear, and it may be caused by infrastructural Ahlmannryggen-Borgmassivet area correspond to the Archean variations in the crust of the Maudheim Province mobile belt. to Mid-Proterozoic Grunehogna Province. A linear belt of However, coincidence of this anomaly with the gravity magnetic highs and lows observed over RU. Sverdrupfjella high (ALESHKOVA et al. 2000) indicates that it may be also and Kirwanveggen is associated with the Middle to Late associated with gabbro intrusion. Proterozoic orogenic belt (Maudheim Province). The pattern of magnetic anomalies over the CL defines the extent of the It should be noted that NE-SW-trending low-amplitude CL crustal block and delineates structural elements and magnetic minima close to the major outcrops of the Vestfjella characteristic trends ofthe Shackleton and Argentine Ranges. coincides with ice-surface lineaments and observed fracture trends such as the trends of the horsts, grabens and escarp­ Correlative analysis of the magnetic field with the outcrop ments, implying major bedrock fault control. The older dole­ geology allowed the conc1usion that the dominant source of rite dykes known in Vestfjella (200-250 Ma) were intruded anomalies is the Precambrian basement. Additional com­ predominantly along the same trending fractures and may plexity of the magnetic fabric is caused due to the Mesozoic represent the initial period of rifting (JACOBS et al. 1996). magmatism associated with the break-up of Gondwana and Moreover, PETERS et al. (1991) pointed out that the con­ manifested in WDML as mafic vo1canic rocks in Kirwan­ spicuous parallelism in strike between the Middle Jurassie veggen (RAVICH & SOLOVIEV 1966, HARRIS et al. 1990), dykes of Vestfjella and the Explora Escarpment, and between Heimefrontfjella (JUCKES 1972) and Vestfjella (HJELLE & the Early Jurassie dykes in Ahlmannryggen and the WINSNES 1972), alkaline intrusions in Sverdrupfjella (RAVICH Jutulstraumen-Pencksökket (J-P) graben, is a conc1usive & SOLOVIEV 1966) and Kirwanveggen, and dolerite dykes evidence for their mutual geodynamic formation. (HARRIS et al. 1990) which intrude throughout the area. At Vestfjella basaltic flows are intruded by a 130 km 2 olivine Also of special note is the hinge zone magnetic lineament gabbro, which is in turn intruded by dolerite dykes (HJELLE & (HZL) c1early distinguished on magnetic images (see Figs. 1 WINSNES 1972, LUTTINEN et al. 1994). Sub-horizontal dolerite through 5 in GOLYNSKY & ALESHKOVA 2000). Admittedly this sills range in thickness from <1 m to >200 m and are exposed lineament may represent a faulted boundary delineating major in the , as well as at . crustal extension along the continental margin ofthe southern Weddell Sea as a result of stretching and faulting during The magnetic anomalies associated with the Mesozoic mag­ Jurassic/Cretaceous time and related to the initial rifting matism are present in both the Grunehogna and Maudheim events of Gondwana. It is parallel to other outstanding

93 magnetic features such as a wide linear belt of positive types. The same is true also for the Shaekleton Range where magnetic anomalies along the coast of WDML and CL in­ the Ross event was much greater than in the WDML (MOYES eluding the Explora Anomaly and the Weddell Sea failed rift et al. 1993, TESSENSOHN 1997). These observations support the anomaly. Near the Prineess Martha Coast the HZL transeets earlier eonelusion of MOYES et al. (1993) that Gondwana the PMCMA and separates moderate-amplitude, long-wave­ amalgamation occurred along suture lines not eurrently length anomalies to the north-west from moderate-to short­ exposed in outcrop in Dronning Maud Land. We aeknowledge wavelength anomalies to the southeast. The inerease in their suggestion that boundaries which would be consistent anomaly wavelength is due to a corresponding substantial with the SWEAT hypothesis must either lie polewards, or the increase in depth to Preeambrian basement under the Weddell hypothesis is wrong. Sea shelf. The observed oval magnetie high of amplitude 350 nT immediately south of the HZL (74° 05' GOLYNSKY & ALESHKOVA (2000) have suggested that the 19° 55' W) is coincided with the gravity anomaly (see Fig. 3 at Maudheim Provinee magnetic unit eorresponds to the highly ALESHKOVA et al. 2000) and supposedly may be interpreted deformed Mid- to Late Proterozoic orogenie belt which erops due to igneous roeks such as olivine gabbro in Vestfjella. out in Heimefrontfjella, along the Kirwanveggen escarpment Farther southwest, over the CL shelf, several similar magnetie and in H.U. Sverdrupfjella. The major gneissie unit of the highs are loeated in vieinity of the mega-lineament. This belt, the Sverdrupfjella Group, eomprises various tonalitie, implies that eausative sourees of the magnetie anomalies may quartzofeldspathic and metapelitic gneisses with minor but be associated with an extensional episode along the Coats and extensive marbles and ealc-silicate gneisses (GRANTHAM et al. western Dronning Maud Lands margin, whereas the HZL may 1988). Various intrusive units into these gneisses are present, be interpreted as deep-seated structure of regional seale that and these suites were complexly deformed during a main event provided channel ways for the ascending magma during the that has been correlated with the Kibaran or Grenvillian Mesozoie time. orogeny at approx. 1100-1000 Ma (BARTON & MOYES 1990).

Large areas of Gondwana were affeeted by a major period of The NS-trending magnetic anomalies of the northern part of tectonism during the Late Proterozoic-Early , which the RU. Sverdrupfjella segment of the SKA extend as far culminated in aperiod of Late Neoproterozoic-Early Paleo­ north as the border ofthe survey area. The eausative sourees of zoie deformation andlor magmatism tenned the Ross 01' the these anomalies are not obvious, and they may have been Pan-African event (e.g. GROENEWALD 1993). The geologieal caused by infrastructural variations in the crust. At that time its terranes of the study areas have had signifieantly different central part eorresponds to the Roerkulten granite of the teetonie histories during the . In western orogenie event approx. 1100 Ma and located westward from Dronning Maud Land, the cratonic Grunehogna Provinee the mountain range. Furthermore, GRANTHAM & HUNTER appears to have remained stable sinee Grenvillian time, (1991) postulated that the spatial distribution of lithologies in whereas the approx. 1100 Ma mobile belt was deformed the western and central H.U. Sverdrupfjella shows a litho­ during the Ross event although relatively gently when logieal zonation with the various formations paralle1ing the eompared to the Shaekleton Range 01' to the Transantaretie boundary between the Ahlmannryggen eratonie terrane and Mountains (MOYES et al. 1993). Aecording to MOYES et al. the Sverdrupfjella terrane to the east. The lithologies elosest to (1993) the Ross event in WDML is charaeterized by pervasive Ahlmannryggen inelude intermediate orthogneisses (the Gray mineralogical isotopie resetting, relative to open folding and Complex of the Jutulrora Formation), which have the thrusting towards the eraton and limited magmatism, with only ehemieal eharaeteristics of calc-alkaline volcanics, inter­ one large intrusion at Brattskarvet. layered with banded paragneisses (the Banded Gneiss Complex). The sequence is interpreted as interealated meta­ Examination of the aeromagnetic map eovered the mountain volcanics and arenitic to pelitie sediments (GRANTHAM & areas of the WDML eompiled by using 5 km flight-line HUNTER 1991, GROENEWALD & GRANTHAM 1991), thus a spacing (see Fig. 7 in GOLYNSKY & ALESHKOVA 2000, this vol.) possible interpretation is that the north-south trends may together with outcrop geology lead us to eonelude that it yields refleet the western extent of the Maudheim Provinee and important elues to the tectonic framework and the regional magnetic anomalies are possibly eaused by orthogneisses. distribution of basement-rock types. For example, in the northern parts ofthe Heimefrontfjella, week narrow anomalies From these observations (structural grain of magnetic anoma­ are not eollinear with dominant trends of the Maudheim lies and known geology), the Sverdrupfjella segment of the Province magnetie unit and apparently tend to reflect the SKA, that consists of three major anomaly segments along Grenville-aged thrust/shear zones reactivated during the Ross which different geologie processes have occurred, may orogeny (JACOBS 1991, JACOBS et al. 1997). Similar magnetic delineate a zone of crustal weakness that initially developed anomalies are diseernible in the Kirwanveggen area and the during 01' prior to Kibaran/Grenville time. Subsequently, this nature of their sourees is as yet unknown. At that time, the part of mobile belt was deformed during the Ross event, albeit discrete high-frequeney peak of around 500 nT amplitude relatively gently when eompared to the effects of the Kibaran oeeurred in vieinity ofthe thrust-related Brattskarvet intrusion orogeny (MOYES et al. 1993). of approx. 519 Ma and appear to be shifted northward, albeit after redueing to the pole the correspondenee ofa maximum to During the Jurassie time, this zone ofweakness beeame aetive granitoids is improved. This high is located elose to the when the Jutulstraumen-Pencksökket thrust was reactivated in Sverdrupfjella segment (GOLYNSKY & ALESHKOVA 2000) ofthe an extensional regime that led to intrusion of swarms of mafic Sverdrupfjella-Kirwanveggen Anomaly as diseussed below. and alkaline dykes and the intrusion of two plutonie alkali ne We have not diseovered any additional signifieant anomalies, and syenite complexes whieh are refleeted by eireular high­ whieh would be assoeiated with the Ross-aged basement-roek intensity magnetie anomalies (GOLYNSKY & ALESHKOVA 2000).

94 These are excellent examples illustrating that magmatic as a marginal mobile belt of the ancient cratonic fragment activity associated with the earlier stages of crustal thinning located in CL. It should be noted that three aeromagnetic and magma underplating during the pre-rift stage appears to profiles flown in the south-eastern part of CL (Fig. I) show have a tendency to be alkalic in nature (BOTT 1979). remarkable similarities of the observed anomalies with those Accordingly to ALLEN (1991), the Jutulstraumen-Pencksökket attributed to the Maudheim Province. It seems that they may structure is a failed Jurassie rift, formed by reactivation of a swing towards the Shackleton Range and in this way outlining pre-existing strike-slip fault forming the suture between the the extent of the CL crustal block as weil as they may turn in polydeformed high-grade terranes of RU. Sverdrupfjella and the ice-covered interior of Antarctica. In any case, AWI aero­ Kirwanveggen, and the undeformed Grunehogna Province to magnetic data coverage of the south-eastern part of CL (Jokat the west. Several aeromagnetic intersections of the rift struc­ pers. comm.) could test these ideas. ture (Fig. I) completed in the northern part of the rift show that its axis is mostly associated with negative anomalies. This We also suggest timt magnetic anomalies of northern Berkner is eventually showing that axial intrusions not developed along Island are also associated with this zone, but they are largely the axis ofthe rift as aprecursor to crustal splitting. superimposed by N-S trends probably caused by Jurassie intrusions along the western border of the Filchner rift under­ We believe that the magnetic anomaly pattern observed in the lain by subsided and highly modified continental crust. CL area represents a mixture (jumble) of sources that vary in Undoubtedly, it is difficult to be sure from the presented age as weil as in size, depth and strike direction, possibly images (see Figs. 1 through 5, GOLYNSKY & ALESHKOVA 2000) reflecting petrologie variations in more ancient basement that several W-E and NW-SE trending, subtle features (Archean?) that probably underlies the rocks of the granite­ recognized as a composite part ofthe Druzhnaya Anomaly, but rhyolite terrane. Such inferences should be made with extreme not so obvious for northern part of the Berkner Anomaly caution, however, geological data strongly suggest the (northward from 80° 20'S) constitute the northwestern conti­ existence of another cratonic fragment to the south of the nuation ofthe Shackleton magnetic unit. However, iftrue, this Maudheim Province mobile belt (MOYES et al. 1993). MOYES suggests that the structural trends of two belts of the et al. (1993) mentioned that crystalline basement rocks of the Shackleton Range extend northwestwards beneath the Shackleton Range are suitable candidates for such a southern sedimentary sequences of the Weddell Sea Embayment. The craton. Indirect evidence is also found in western Dronning observed pattern of NW-SE and near W-E features invite Maud Land, since ARNDT et al. (1991) postulate such a speculation. Are they of different origin than the former cratonic presence in order to explain decreasing Nd values original structures within the Precambrian and Early Paleozoic southwards in Heimefrontfjella. Other indirect evidence basement, the later Paleozoic intrusions during the Ross comes from East and . It is known that the orogeny or fault zones subsequently reactivated in extension greenschist facies metamorphic rocks of Archean age (Ruker during pre-breakup phase? Or do they reflect volcanic rocks Terrain) occupy the southern Prince Charles Mountains and forming the Explora Wedge (HINZ & KRAUSE 1982, HINZ & southern parts ofthe Mawson Escarpment produce a magnetic KRISTOFFERSEN 1987) as suggested by HUNTER et al. (1996) or pattern similar to that observed over CL (GOLYNSKY et al. by integrated effect of the thick volcanic pile of Explora 1995). Wedge and the underlying intrusive dyke suite as considered by LEITCHENKOV et al. (1996)? In our opinion it is hard to agree with the interpretation of the Coats Land magnetic anomaly pattern given by STOREY et al. It was mentioned above that some magnetic anomalies over (1994). It seems unlikely that the Grenvillian Front stretching the northern part ofGrunehogna Province are caused by mafic from North America through the West Antarctica and towards intrusions. The similar appearance of the magnetic anomalies WDML represents a single indivisible orogenie belt due to ab­ of the Explora Anomaly in the eastern Weddell Sea area may solutely different magnetic signature over the Haag Nunataks be formed by similar intrusions. This is supported by observa­ and CL areas even if the last one admittedly has similarity tion ofmagnetic trends for the area between 30 °W and 20 °W with that observed across the Grenvillian Front in North which are in general discordance with the volcanic continental America (HINZE & ZIETZ 1995). The observed structural margin. It is difficult to agree with the suggestion that the pattern ofthe magnetic anomalies over CL and adjoining areas Explora anomaly produced by integrated effect of volcanic (WDML and Shackleton Range) do not allow us to support the layers and underlying dyke suite. It is known that magnetic suggestion that the Coast Land area is a continuation of the properties of the Jurassie basalts from the Vestfjella are Mazatal-Yavapai-Grenville provinces ofNorth America as was relatively low, whereas dolerite dykes are characterized by initially inferred by MOORES (1991) and DALZIEL (1991). Our high levels of magnetic susceptibility (KRISTOFFERSEN & arguments are based on the distinction of the magnetic AALERUD 1988, RUOTOISTENMÄKI & LENTIMÄKI 1997). Other anomaly pattern of the neighboring terranes which is re­ probable sources of NW-SE and W-E-trending magnetic markably visible on the magnetic anomaly map and other anomalies which might be considered as component parts images (Figs. 1 through 5 at GOLYNSKY & ALESHKOVA 2000). the Druzhnaya and Berkner anomalies, such as Paleozoic The boundary between the Maudheim Province and CL intrusions not known in vicinity of discussed area and must be terranes may be drawn rather distinctly along gradient zone of excluded from consideration. Therefore it is reasonable to the westernmost positive magnetic anomaly of the Maudheim assume that these anomalies may be associated with the NW Province magnetic unit. Furthermore, the arcuate shape oftwo continuation of the Shackleton Range Proterozoic belts. At magnetic belts of the Shackleton Range bounded the CL that time N-S-running residual magnetic anomalies along the crustal block from south and by the Druzhnaya Anomaly eastern margin of the and the eastern com­ (GOLYNSKY & ALESHKOVA 2000) from the west suggests that ponent of the Druzhnaya Anomaly are apparently genetically these anomalies constitute a single tectonic zone that evolved associated with Jurassie intrusions similar to those exposed in

95 the Dufek Massif and the Forrestal Range of the Pensacola Plateau) originated possibly during Pan-African orogenic Mountains (FORD & HIMMELBERG 1991). event as was assumed by CORNER et al. (1991) 01' even in the Mesozoic time were correlated with another one in Antarctica There is no doubt that the magnetic anomalies of two units related with Grenvillian age rocks. (Shackleton Range and Maudheim Province), reflected by high-grade metamorphic rocks have bounded character to the From our point of view it is incorrect to compare magnetic CL crustal block. Some questions in this respect immediately signature of two remote Gondwanian terranes without taking come to the mine!, i.e. do they form one and the same magnetic into account the data from the Mozambique plains and the unit representing an invisible mobile belt that is truncated on continental margin ofAfrica. The same is true for the Mozam­ the Weddell Sea shelfby NE-SW-trending magnetic anomalies bique Ridge which comprises a continental crust (MOUGENOT (Failed Rift Magnetic Anomaly)? 01' do they turn in the oppo­ et al. 1991). Integration of all magnetic observations from the site direction and may be outlined within the Gondwana southern Africa continental margin with those over continent assemblages? Such inference was made by CORNER et al. parts could shed light upon probable sources of the magnetic (1991) for the Dronning Maud Land anomalies (the anomalies and could possibly be useful to constrain the recon­ Sverdrupfjella-Kirwanveggen Anomaly, in our interpretation) struction of Gondwana. At the moment it is unc1ear, what the by correlation them with the Falklands Plateau anomalies and relationship between magnetic anomalies in onshore and off­ the Beattie magnetic anomaly which has a strike length of shore areas is. Are they continued on the continent as in case about 900 km and a maximum amplitude of 1050 nT in the ofWDML 01' not? east and occurred in the poly-metamorphic Namaqua-Natal belt, that rims the southern margin of the Kaapvaal craton of Other aspect on which we are going to draw the attention is the South Africa. The causative body of the Beattie Anomaly was obvious similarity between the Beattie Anomaly and the found to have a depth ofabout 6 km. It was also suggested that Princess Martha Coast Magnetic Anomaly (GOLYNSKY & it possibly is related to the Pan-African orogeny at circa 500 ALESHKOVA 2000). Both of them are parallel to respective Ma (CORNER et al. 1991). The origin of the anomaly is rather corresponding continental margins. At that time, the magnetic speculative. CORNER (1989) proposed that the major anomaly anomaly map published by CORNER & GROENEWALD (1991) in the Natal Metamorphie Province is genetically related to the (Fig. 2) exhibits a northwest-southeast-trending grain ofshort­ Beattie Anomaly. It was also suggested that since the Natal wavelength magnetic anomalies consistent with the boundary Anomaly arises from craton-directed thrusts in a granite­ between the Kaapvaal Province and the Namaqua Belt. This gneiss terrane, a similar geological setting is possible for the grain is c1early overprinted in the eastern part of Karoo Basin Beattie Anomaly. This is in conflict with the interpretation of by the Beattie Anomaly and therefore it is reasonable to DE BEER & MEYER (1984) and JOHNSTON (1998) who favored assurne that it might be apparently associated with mafic mafic lithologies. For example, in accordance with the inter­ intrusions injected into upper crust presumably during early pretation given by JOHNSTON (1998), the Beattie Anomaly Mesozoic rifting and magmatism affected both continental extends through the length of the Karoo basin composed of a margins. This suggestion is also arguable, however at any rate thick package of to Triassie detrital sedimen­ magnetic anomalies associated with Jurassie volcanic tary rocks and may indicate the presence of a mafic intrusion sequences broadly developed along the eastern boundary of coincident with the basin axis. It was suggested also that this the Kaapvaal Province and within the Limpopo Province dyke-like body probably intruded during the early stages of (CORNER & GROENEWALD 1991, HUNTER et al. 1991). The extension, and may have localized subsequent basin develop­ thick section of flood basalts (more than 10 km) within the ment. Although we find the hypothesis of CORNER et al. (1991) Karoo Basin provides additional evidence that this region intriguing, the following observations are probably worth where the Beattie Anomaly is observed has a much langer and nothing. more complex history in comparison with WDML comprising remnants of Jurassie basaltic lavas of variable thickness but The tectonic history ofWDML during the Middle Proterozoic not exceeded 1000 m in Vestfjella. to Late Cambrian is very similar to southern Africa and does not appear to reflect aperiod of major re-organization in that The magnetic anomaly pattern observed over the north-eastern area, but rather a less intense reactivation along the major part of Kaapvaal craton in position suggested by CORNER & features established during the preceding Kibaran orogeny at GROENEWALD (1991) is not compatible with the Grunehogna 1100 Ma (MoYES et al. 1993). Province. More 01' less similar character of the magnetic anomalies within the Kaapvaal craton with those in Antarctica Magnetic anomaly pattern of two counterparts in Gondwana is recognized over the south-eastern part of the craton, i.e. in reconstruction is quite similar (Fig. 2), however there are some position proposed by HUNTER et al. (1991). In this recon­ differences which do not allow us to accept Corner's interpre­ struction the most pronounced magnetic anomalies of both tation. Correlation of the Beattie Anomaly of unknown origin continents, the Beattie and DML Anomalies, also might form with the Falklands Plateau anomalies related to unknown one indivisible magnetic belt whereas the Natal Anomalies sources over the Falkland microplate margin and the western and other linear magnetic anomalies within the Maudheim Dronning Maud Land magnetic anomalies associated with the Province would have not any continuation into counterpart Grenvillian age rocks of the Maudheim Province is to some regions. degree subjective and incompatible idea, just for the sake of idea, to constrain the reconstruction ofGondwana by using the We strongly believe that an attempt to match two cratonic frag­ aeromagnetic and marine data. 11 is incomprehensible at all ments by using similarity oftheir patterns when all data (aero­ why two fragments of proposed Gondwanian magnetic belt magnetic and marine) from the African part are not (Beattie Anomaly and magnetic anomalies of the Falkland incorporated into the analysis, and when the full extent of the

96 KAAPVAALCRATONBO~pARY

Fig. 2: Colour composite image of magnetic data covering southern Africa, the Falkland Plateau and western Dronning Maud Land based on CORNER & GROENEWALD (1991). The arrows point to intcrpreted extensions of the Beattie Anomaly into the Falkland Plateau and western Dronning Maud Land.

Abb, 2: Zusammengestellte Farbabbildung der magnetischen Daten, die das südliche Afrika und das westliche Dronning Maud Land abdecken, basierend auf CORNER & GROENEWALD (1991). Die Pfeile deuten auf die interpre­ tierte Ausdehnung der Beattie-Anomalie in das Falkland Plateau und in das westliche Dronning Maud Land.

Sverdrupfjella-Kirwanveggen Anoma1y is not known, will be Shack1eton Range bounded the CL crusta1 block from south subjective 01' specu1ative. It is also necessary to carefully and by Druzhnaya Anomaly from the west provides the ana1yze existent geophysica1 information derived by gravity opportunity supposedly to admit that these anomalies and seismic investigations in both regions. The available constitute single tectonic zone evo1ved as marginal mobile belt seismic data show that the crust of the African Kaapvaa1 ofthe ancient cratonic fragment Iocated in Coats Land. There­ craton exhibits lower velocities than the crust in DML fore it is reasonab1e to assurne that two magnetic units (HÜBSCHER et a1. 1996). Two possibi1ities follow from these (Maudheim Province and Shack1eton Range) might outline observations: either the Antarctic seetion of the Kaapvaal one and the same unit representing the Proterozoic mobile belt craton experienced a different 01' additional tectonic 01' continuous1y stretching from WDML around the CL cratonic magmatic influence relative to the African part, 01' the fragment towards the Shack1eton Range. To a certain degree suggestion that the Kaapvaa1 craton continues into DML is this is in agreement with the new geochrono1ogical data incorrect. Despite there are some indicators that the Kaapvaa1 obtained for the Pioneers Group characterized by Grenvillian craton not continued into DML as was assumed by BARTON & Nd-model ages (MILLAR & HENJES-KUNST 1997) indicating a MOYES (1990) who consider the Grunehogna Province to be a different type of crust more re1ated to the Heimefrontfjella microp1ate unre1ated to the Kaapvaa1-Zimbabwe Province, (BROMMER et a1. 1997). HÜBSCHER et a1. (1996) prefer the interpretation oflower crust affected by magmatism. CONCLUSIONS Undoubted1y, at this stage of investigations, when the AWI dataset from the south-eastern part of CL is not avai1ab1e for Results of our study neither contradict nor support a interpretation and comprehensive analysis and there is an postulated connection of the Kaapvaal-Zimbabwe Province essential gap in the aeromagnetic data coverage between H.U. into western Droning Maud Land but certain1y indicate that Sverdrupfjella and the Sor-Rondane Mountains, any the CL crusta1 block has never been part ofthe African craton. suggestions about continuity and corre1ation ofthe Maudheim It appears instead to be typica1 of the . Province magnetic anomalies with others either within Ant­ Bounding character of magnetic anomalies in the neighboring arctica 01' counterparts of Gondwana assemb1ages will be terranes (Maudheim Province and Shack1eton Range) relative specu1ative. This problem remains to be e1ucidatecl, but as to the above mentioned crusta1 block strong1y indicate the stated above that arcuate fashion of two magnetic belts of the existence ofan ancient cratonic fragment in Coats Land.

97 Our interpretation eontradiets the suggestion that Coats Land VNIIOkeangeologia and supported by the German Federal area is a eontinuation of the Mazatal-Yavapai-Grenville Ministry for Research and Teehnology (grant BMFT provinees of North Ameriea due to the ineonsistent struetural 03F08GUS9). pattern of magnetie anomalies within the proposed orogenie belt. Magnetie anomalies related to the Ross orogenie event are very limited in western Dronning Maud Land and not References diseernible in the Shaekleton Range area. This, together with the geologie observations, allow to admit that Ross-age suture Aleshkova, ND, Golynsky, A. V, Kurinin, R. G. & Mandrikov, vs. (2000): Gravity mapping in the southern Weddell Sea Region.- Polarforschung resulting from Gondwana coalescenee is not presented in 67: 163-177. western Dronning Maud and Coats Lands. It must lie south­ Allen, A.R. (1991): The tectonic and metamorphic evolution of RU. Sver­ wards of the current exposures of the Shackleton Range 01' at drupfjella, western Dronning Maud Land, Antarctica.- In: M.R.A.THOMSON, IA. CRAME & IW THOMSON (eds), Geological least in the Shackleton Range. The teetonic evolution of evolution of Antarctica, 53-60, Cambridge University Press, Cambridge. WDML from the Late Proterozoic to Cambrian time does not Arndt, NT. Todt. W, Chauvel, C, Tapfer, M. & Webei; K. (1991): U-Pb zircon appear to reflect aperiod of major continental reorganization age and Nd isotopic composition of granitoids, and in that area, but a less reactivation along the structures arisen supracrustal rocks from Heimefrontfjella, Antarctica.- Geol. Rundschau 80: 759-777. during the preceding Grenville orogeny at 1100Ma. Barten, JM. (JI) & Moyes, A.B. (1990): Cooling patterns in western Dronning Maud Land, Antarctica and southern Africa and their implication to Magnetic anomalies associated with the fragmentation of Gondwana.- Zbl. Geol. Paläont. Teil I, (1/2): 33-43. Bott. M.HR (1979): Subsidence mechanism at passive continental margins.­ Gondwana during the Mesozoie are widespread and varied in In: IS. WATKINS (ed.), Geological and Geophysical Investigations of form, size and length. The causative sources are pre­ Continental Margins, AAPG Memoir 29: 3-10. dominantly igneous rocks. StructuraIly, they are depicted Brommer, A., Henjes-Kunst, F, Millar; J.L.. Talarico, F & Kleinschmidt, G. (1997): Implication for Antarctica Pre-Panafrican crustal evolution ­ within the aneient Grunehogna cratonic fragment as isolated indications for Ross-reworked Proterozoic basement-terranes from the intrusions and linear chain of mafic intrusions or deep-seated northern Shackleton Range.- Abstracts EUG 9, Strasbourg, France, 223. fault (the PMCMA); apparently they inherited some ancient Corner; B. (1989): The Beattie anomaly and its significance for crustal inhomogeneity in the Precambrian crust and are recognized evolution within a Gondwana framework. - Abstracts, SA Geophysical Ass., First Technical Meeting, Johannesburg. within the Proterozoie mobile belt (the Maudheim Province) Corner, B., Maccelari, JC & Niccol, S. (1991): Major magnetic anomalies in as intrusions of different eomposition but predominantly western Dronning Maud Land: their possible origin and correlates in alkalic in nature. Three well-known intrusions are located in Southern Africa.- Abstracts Sixth Internat. Sympos. Antarctic Earth Sci., Tokyo,Japan,113. the eastern shoulder of the Jurassie Jutulstraumen­ Corner; B. & Groenewald, RB. (1991): Gondwana reunited. 1991.- South Pencksökket failed rift whose axis is associated with negative African T. Nav. Antarkt., 21 No 2: 172. anomalies eventually suggesting that axial intrusions were not Dalziel, I. WD. (1991): Pacific margins of Laurentia and East Antarctica­ developed along the axis of the rift as aprecursor to crustal as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent.- Geology 19: 598-601. splitting. Overprinting character of magnetic anomalies is De Beet; JH & Meyer; R. (1984): Geophysical characteristics of the rather different within the study area, but we have not Namaqua-Natal bell and its boundaries.- South Africa. I Geodyn. 1: recognized any evidences in the onshore areas that fragmen­ 473-494. Divakara Rao, V (1996): Precambrian bimodal volcanism in Weddell Sea, tation of Gondwana during the Mesozoic disregarded to full West Antarctica.- Journ. Geol. Soc. India 45(2): 163-174. extent older suture lines 01' boundaries between major Ford, A.B. & Himmelberg. G.R. (1991): Geology and crystallization of the provinces. Dufek intrusion.- In: R.J. TINGEY (ed.), The , Oxfard Monographs on Geology and Geophysics, 17: 175-214. Golynsky, A. V & Aleshkova, ND. (2000): Regional magnetic anomalies ofthe The origin ofthe source ofmany anomalies is obvious, where Weddell Sea region.- Polarforschung 67: 101-117. they eorrespond to known outcrop geology. Others relate to Golynsky, A. V, Masolov, VN & Jokat, W (2000): Magnetic Anomaly Map of structures coneealed under ice-cover and their origin is more the Weddell Sea region: a new compilation ofthe Russian data.- Polarfor­ schung 67: 125-132. speculative. However, in these cases the aeromagnetic data Golynsky, A. V, Aleshkova, ND., Grikurov, G.E., Kamenev, E.N. Kurinin, R.G. assist considerably in eorrelating known features and trends Masolov, VN & Volnukhin, Vs. (1995): Main tectonic provinces of the across large regions. We have attempted to suggest how some Lambert Graben region of East Antarctica, based on airborne magneue data.- Abstracts, 7th Internat. Sympos. Antarctic Earth Sci., Siena, 160. magnetic anomaly patterns can be interpreted in terms of the Gose, WA., Helper. M.A., Connelly, JN, Hutson, FE. & Dalzie/, I. WD. teetonie evolution and crustal development ofWDML and CL. (1997): Paleomagnetic data and U-Pb isotopic age determinations from Further aeromagnetie investigations in particular over areas Coats Land, Antarctica: Implications for Late Proterozoic plate recon­ between the Sverdrupfjella Mountains in the west and the S0r­ structions.- I Geophys. Res. 102 (B4): 7887-7902. Grantham, G.H & Hunter, D.R. (1991): The geology of the western and Rondane Mountains in the east as weIl over adjaeent offshore central RU. Sverdrupfjella, Antarctica: a Proterozoic collision zone.­ areas up to Astrid Ridge and Queen Maud Ridge possibly Abstracts 6th Internat. Sympos. Antarctic Earth Sci., Japan, 178-183. underlain by continental crust would greatly increase the Grantham, G.H, Groenewald, RB. & Hunter; D.R. (1988): Geology of the northern H.U. Sverdrupfjella, western Dronning Maud Land and geological knowledge of the area. implication for Gondwana reconstructions.- South African Journ Ant­ arctic Res. 18: 2-10. Groenewald, RB., Grantham, G.H & Watkeys, M.K. (1991): Geological evidence for a Proterozoic to Mesozoic link between south-eastern Africa ACKNOWLEDGMENTS and Dronning Maud Land, Antarctica.- In: R.H. FINDLAY, H.R. BANKS, II VEEVERS & R. UNRUG (eds), Gondwana 8: Assembly, We are pleased to aeknowledge all our colleagues of the Ant­ evolution and dispersal. AA Balkema, Rotterdam. aretic Geology Department, VNIIOkeangeologia for helpful Groenewald, RB. & Grantham, G.H (1991): Geology ofRU. Sverdrupfjella, Dronning Maud Land, Antarctica.- Abstracts 6th Internat. Sympos. Ant­ diseussion regarding this work. We wish to thank Phil Leat arctic Earth Sci., Japan, 195. who provided a useful review of an earlier version of the Harris, C, Marsh, is, Duncan, A.R. & Erlank, AJ (1990): The petrogenesis manuseript and Joaehim Jacobs for constructive review. This of Kirwan basalts of Dronning Maud Land, Antarctica.- I Petrology 31: 341-369. work was done within the joint project of AWI, PMGRE and Hinz, K. & Krause, W (1982): The continental margin of ,

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