Terra Antartica2000, 8(2), 71-78

Geological Structure of the South Orkney Microcontinent

M. BUSETTI*, C. ZANOLLA & A. MARCHETTI

Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Borgo Grotta Gigante 42/c, 34010 Sgonico (Trieste) Italy

Received 26 January 2000; accepted in revised form 12 March 2001

Abstract - The South Orkney Microcontinent (SOM) is the biggest (250 x 350 km) continental element of the SouthScotia Ridge between Antarctic and Scotia Plate. The northern margin of the SOM is the Antarctic-Scotia Plateboundary with a sinistral strike-slip movement accomplished by an oblique plate convergence in the western part,and a divergence component eastward toward the Bruce Bank. Seismic and gravity data show two main domains withdifferent structural trends related to the complexity of the geodynamic activity in the surroundings: a) in the southernpart of the SOM the north-south trend of Airy, Bouguer and Eötvös Basins formed during the Oligocene extensionalphase of the Scotia Ridge fragmentation and formation of the Powell and Jane Basins to the south-east and south-westmargins of the SOM respectively, b) in the northern part of the SOM, an east-west trend following the transform plateboundary, affected the structure of the basement high and the Newton Basin.

segments of Gondwanaland during the Early Mesozoicand prior to the break-up. The entire prism reflects acomplex history of accretion. In the South Orkney Islandsthe metamorphic ages which may constrain the time ofaccretion are approximately 200-180 Ma (Trouw et al.,1998).

During the Cenozoic the continuous active marginalong South America and the Antarctic Peninsula, as aconsequence of the Gondwana break up, split away formingthe South Scotia Ridge and several spreading episodesproduced the microplate of the Scotia Sea (Barker et al.,1991). The South Orkney Microcontinent (SOM) is thebiggest (250 x 350 km) lithospheric fragment of the SouthScotia Ridge between Antarctica and the Scotia Plate(Fig. 1), representing a remnant of the original linkconnecting South America with the Antarctic Peninsula.

(Dalziel & Elliot, 1973; de Wit, 1977;Dalziel, 1984).

The new plate tectonic arrangementincludes an active sinistral transformfault at the Antarctic-Scotia Plateboundary, located at the northern marginof the South Scotia Ridge (Forsyth 1975,BAS 1985), (Fig. 1).

In the last decade geophysicalstudies westward of the SOM

INTRODUCTION

The South Orkney Islands are constituted by the samerock assemblage of some locations in the islands of theSouth Scotia Ridge (Smith and Elephant islands)collectively named the Scotia metamorphic complex(Tanner et al., 1982; Dalziel, 1984). Highly deformed andvariably metamorphosed, (greenschist to amphibolitefacies, greywacke-shale sequences with conglomerate)Upper Paleozoic - Lower Mesozoic metasedimentary andmetavolcanic rocks are unconformably overlain by UpperJurassic or Lower Cretaceous and Tertiary strata (Caminos,1962; & Massabie, 1980; Dalziel, 1984; BAS, 1985;Barker et al., 1991; Grunow et al., 1992). They areinterpreted to represent a subduction complex that wasaccreted to the South American and Antarctic Peninsula

Fig. 1 - The South Orkney Microcontinent is thebiggest continental fragment of the South ScotiaRidge. The northern continental margin is asinistral strike-slip plate boundary separating theAntarctic from the Scotia Plate.

*Corresponding author (mbusetti@ogs.trieste.it)

© Terra Antartica Publication

72 M. Busetti et al.

recognised a complex geodynamic setting of this boundary.Besides the strike slip kinemtic also a convergentcomponent producing the subduction of the Scotia Platebelow the Antarctic Plate has been proposed by Kavoun &Vinnikovskaia, (1994), Lodolo et al., (1998), Maldonadoet al. (1998). From seismological studies Pelayo & Wiens,(1989) suggested a transtension tectonic for the plateboundary and Acosta & Uchupi, (1996) recognisedextensional structure at the Antarctic Plate edge. Theeastern part of the plate boundary separates the SOM fromthe Pirie and Bruce Banks with a morphological expressioncharacterised by a 6000 meter deep basin.

The south-east and south-west South OrkneyMicrocontinent passive margins are connected to theproto-oceanic Powell and the oceanic Jane basinsrespectively, formed after the rifting between themicrocontinent and the Antarctic Peninsula in the ?Eocene/Early Oligocene (Coren et al., 1997) and in the Oligocenerespectively (Lawver et al., 1991).

The tectonic activity of the area affected also the SOM.King and Barker, (1988) from geophysical data suggestedthe presence of two major structural trends: an east-westtrend comprising a magnatic arc, fore-arc basin andaccretionary complex created when the SOM was at thePacific margin, and a younger, approximately north-southtrend comprising the Airy, Bouguer and Eötvös Basins,related to the rifting of eastern and western margins.

DATA USED

About 5000 km of multichannel seismic reflectionprofiles were collected in the past decade by theOsservatorio Geofisico Sperimentale (Italy), the MarineArctic Geological Expedition (Russia), the British AntarcticSurvey (United Kingdom) and the Japanese National Oil

Company (Japan) (Fig. 2). The seismic interpretationswere tied to the ODP wells 695, 696 and 697 (Barker &Kenneth, et al., 1988) (Fig. 2). The well stratigraphy on thecontinental shelf can be simplified by dividing thestratigraphic column into three units, that can also berecognised in the seismic data interpretation: 1) a ?LateEocene/Early Oligocene condensed terrigenoussedimentary succession; 2) Upper Middle Miocene/?LowerPliocene mainly biosiliceous deposits; 3) Upper Pliocene/Quaternary diatom bearing silty and clayey muds (Fig. 3).

Gravity data from ERS-1 and Geosat GeodeticAltimetry (Andersen & Knudsen, 1998) were used tocompile a free air anomaly map (Fig. 4), for correlation ofthe geological structure inferred from seismicinterpretations. The crustal structure of the South OrkneyMicrocontinent has been investigated through modellingof gravity data collected during a geophysical survey bythe Osservatorio Geofisico Sperimentale along seismicprofiles. The modelling is constrained by seismicinterpretation.

RESULTS

From a comparison of seismic data interpretations andthe gravity anomaly map, two main structural domainswith their own trend have been identified. The east-westtrend in the northern domain is expressed by the structuralhigh of a mainly undeformed acoustic basement, by theisland chain, and by the epicontinental Newton Basin(Fig. 5). In the southern area an approximately north–south trend is followed by the Airy, Bouguer and EötvösBasins (Fig. 5), and by mainly undeformed or low deformedstructural highs or blocks, bordering the eastern and westernmargin of the SOM. Orientation of the basins range fromnorth-north west/south-south-east of Airy Basin, on the

Fig. 2 - Estimated Sea FloorTopography from satellite data(Smith & Sandwell, 1995) andlocation of multichannel seismiclines. The continental shelf of theSouth Orkney Microcontinent isdelimited by the 1000 meterscontour, with an average depth ofaround 300 meters; severaldepressions are present in it. TheSouth Orkney Trough, more than6000 meters deep, separates theSouth Orkney Microcontinentfrom the Bruce Bank on the easternpart. The Jane and Powell Basinsare present on the eastern andwestern sides respectively.

73Geological Structure of the South Orkney Microcontinent

Fig. 3 - The stratigraphy of the ODP Leg 113wells (modified after Barker, Kenneth, et al.1988). Three main units can be identified: I) acondensed terrigenous sediment dating fromprobably Late Eocene to Early Oligocenedeposited before separation of the South OrkneyMicrocontinent from the Antarctica Peninsula;II) a middle unit constituted by an expandedUpper Middle Miocene and ?Lower Pliocene,dominated by biosiliceous sediment, about the90% of diatoms, deposits after separation fromthe Antarctic Peninsula; III) condensed unit,ranging from Upper Pliocene to Quaternary,constituted by diatom bearing silty and clayeymuds, related to the development during theLatest Miocene, of a major and probablypermanent West Antarctica Ice-Sheet, the mainsupplier of fine terrigenous sediments to theWest Antarctic continental margin (Barker,Kenneth, et al. 1988). Hz-1 and Hz-2 indicatethe boundary between Miocene - Pliocene and?Eocene/Oligocene - Miocene respectively thathave been used as seismic marker (See Fig. 6aand 6b).

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western side for the SOM, to north-south for the BouguerBasin in the central part, to north-north-east/south-south-east of the Eötvös Basin on the eastern side.

According to the ODP well stratigraphy (Barker &Kenneth, et al., 1988) the basins are filled by ?LateEocene/Oligocene terrigenous sediments (Fig. 3). In thedepocentre, below or in the ?Late Eocene/Oligocenedeposits there is, occasionally, a seismo-stratigraphicsequence characterised by high amplitude and continuousreflectors, faulted and deformed by rifting activity. Theterrigenous sequence is present in the basins of the southernpart of the SOM, and is sealed by a strong reflector (Hz-2) merging with the surface of the structural highs. Themarine Miocene deposits seal the basins onlapping on thehighs like the older sequence (Fig. 6). In most cases the top

of this unit (Hz-1) merges with the top of ?Eocene/Oligocene (Hz-2) in correspondence with the structuralhighs. Occasionally this unit is present also on the basementhighs. The mainly terrigenous Plio-Quaternary sequencedrapes over the whole SOM (Fig. 6).

The northern margin of the SOM is a transform plateboundary with a complex morphological expression. Thecontinental shelf close to the border is a flat surface atabout 300 meters deep, draped with stratified sediment. Atthe shelf edge a steep wall with a thin sedimentary coverand a complex series of blocks settings marks the transitionto the oceanic domain (Fig. 7). Also the block morphologyis characterised by steep flanks and small tectonicallycontrolled basins between the blocks. The base of the slopeends at a sea floor depression, more than 5000 meters in

Fig. 4 - Free air gravity anomaly mapfrom ERS-1 and Geosat GeodeticAltimetry (Andersen & Knudsen,1998). Strong negative and positivegravity anomalies are located incorrespondence with the SouthOrkney Trough (up to -200 mGal),and in the South Orkney Islands (upto 200 mGal) respectively. The mainstructural elements like basins andbasement highs have a goodcorrelation in the anomalies (seeFig. 5).

Fig. 5 - Structural map of the SouthOrkney Microcontinent, obtainedfrom interpretation of seismic data.Two main domains with differentstructural trends are recognizable:a) the east-west trend following thetransform plate boundary, affectedthe structure of the basement highand the Newton Basin; b) the north-south trend of Airy, Bouguer andEötvös Basins formed during theOligocene extensional phase of theScotia Ridge fragmentation andformation of the Powell and JaneBasins to the south-east and south-west margins of the SOM,respectively.

75Geological Structure of the South Orkney Microcontinent

depth, constituted by a basin with about 2 km of sediment.In the western part of the northern border of the SOM anaccretionary prism is probably present at the base of theslope (Kavoun & Vinnikovskaia, 1994; Maldonado et al.,1998). Evidence of the accretionary prism is given by thesmooth step morphology, no evidence of stratified layers(as they are deformed in the melange complex), an abruptchange in the seismic characteristics occurring at the outerdeformation front and continentward dipping reflectors(Fig. 7). Moving towards the east the convergence evidencebecome less clear, producing mainly compressionalstructures in the oceanic sediment with folds and reversefaults, rather than forming an accretionary wedge. Theconvergent structures end at the north – east corner of theSOM. A big trough up to 6000 meters deep separates theSOM from the Bruce Bank (Fig. 8), suggesting that it ispossibly a shifted continuation of the South Orkney Trough,from which it is separated by a shoulder of about 3000meters in depth. The Bruce Bank in the north and a 2000meter deep bank in the south delimit the eastern part of thetrough. Steep morphology characterises the slope of theSOM and the banks, including, besides the transformfeature, also some extensional faults, which together withthe stratified sedimentary wedge on the Bruce Bank andno evidence of downward reflectors, suggest a

transtensional tectonic regime in the area (Fig. 8).Gravity data (Fig. 4) show a steep gradient delimiting

the SOM in the northern part, plus articulated features allalong the plate boundary, with strong negative anomalies,up to -180 mGal in the South Orkney Trough. Positiveanomalies with values of about or less than 40 mGalcharacterise the basins, while the structural highs havehigher values of positive anomalies.

To investigate the crustal structure of the SOM agravity modelling was calculated along the gravimetricand seismic profiles IT91AW93 and IT91AW94 usinggeological constrains from seismic interpretations for theupper part of the model and the estimated densities ofrocks according to depth (Fig. 9). Modelling results providea crustal thickness of around 25 km for the SOM and theBruce Bank, and thinning in between both, arguing for anextensional component in the transcurrent regime, andalso towards the proto-oceanic Powell Basin.

DISCUSSION

Due to the complexity of the structural setting of thearea and the different tectonic regime (transform,convergence and divergence) in the surroundings, the

Fig. 6 - Seismic profiles across the Bouguer Basin (Fig. 6a) and the Eötvös Basin (Fig. 6b). The horizons Hz-1 and Hz-2 are seismic markers tied toODP well 696 stratigraphy (Barker, P.F., Kenneth, J.P., et al., 1988). The basins are filled by ?Eocene/Oligocene terrigenous sediments. In thedepocentre, there is, occasionally, a seismo-stratigraphic sequence characterised by high amplitude and continuous reflectors, faulted and deformedby rifting activity (Fig. 6a). The marine Miocene deposits seal the basins onlapping on the top of the basement highs. The mainly terrigenous Plio-Quaternary sequence drapes over the whole SOM.

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Fig. 7 - Seismic profiles across the northern plate boundary margin of the South Orkney microcontinent. The transform fault is located at the steepmargin of the SOM. At the base of the slope an accretionary prism is probably present (Kavoun and Vinnikovskaia, 1994; Maldonado et al., 1998).Evidence of the accretionary prism are the smooth step morphology, no stratified layers (as they are deformed in the melange complex), an abruptchange in seismic characteristics occurring at the outer deformation front and continent deeping reflectors (modified after Kavoun and Vinnikovskaia,1994).

Fig. 8 - Seismic profiles across the Antarctic - Scotia Plate between the SOM and the Bruce Bank. Faults geometries suggest the location of the mainstrike slip motion on the very steep flank of the SOM couple with some extensional faults on the Bruce Bank margin side.

77Geological Structure of the South Orkney Microcontinent

small microcontinent has a complicated geological history.The Eocene-Oligocene age of the terrigenous sediment

filling of the basins in the SOM is coeval with the openingof the Powell (Coren et al., 1997) and Jane Basin (Lawveret al., 1991) suggesting that the tectonic phase of basinformation is related to the rifting of the Powell and JaneBasins. Drifting of the Powell Basin occurred in theMiocene (Coren et al., 1997), causing the isolation of theSOM from the Antarctic Peninsula and hence the end ofterrigenous supplies and the starting of the deposition ofmarine sediment. During this period no sedimentationoccurred on the structural highs separating the basins.

From Pliocene to Recent all the area was affected bythe sedimentation of a draping of diatom-bearing silty andclayey mud deposit influenced by the glacial/interglacialconditions. This unit produced sea floor topographyconformable to the basin structure (Fig. 6).

The Antarctica and Scotia Plate boundary at the northernmargin of the SOM is probably a large area of deformationbetween the sinistral strike slip fault at the shelf edge of themicrocontinent and the deepest part of the South OrkneyTrough, about 30-40 km north, where the compressivestructures are present (Fig. 5 & 7). This geometry has beenmodelled, considering the distribution of crustaldeformation, by Braun & Beaumont (1995), and theirresults indicate that the strain partitioning with a thrustzone that accommodates convergent motion in the oceanarea and the vertical strike-slip zones that accommodatethe transcurrent motion at the border of the steep escarpmentcorrespond to an obliquely convergent plate boundarydominated by a transcurrent condition.

Going eastward the convergent component seems todecrease until the vertical component of deformation atthe plate boundary appears to change in the area betweenthe SOM and the Bruce Bank. Block faulting and steepescarpment also on the eastern flank of the SOM suggesta transtensional regime of the margin (Fig. 8). The keypoint of change seems to be the north eastern corner of theSOM where an abrupt variation in the direction of theSouth Orkney Trough is present (Fig. 4).

Fig. 9 - Gravity modelling along IT91AW93 and IT91AW-94. The upperpart of the model is constrained by seismic data. Rock densities areestimated according to depth. Modelling results provide a crustalthickness of around 25 km for the SOM and the Bruce Bank, and thinningin between both, arguing for an extensional component in the transformregime.

CONCLUSIONS

The main tectonic activity in the north-south elongatedAiry, Bouguer and Eötvös basins started locally in the preEocene/Oligocene time, related to the rifting of the SouthScotia Ridge fragmentation, and consequently the PowellBasin development. The separation was completed beforethe beginning of the Miocene, testified by a change fromterrigenous, hence continental, supply, to marinesedimentation. During the Miocene the basins were filledby deposits, and from Pliocene to Recent draping ofsediment all over the SOM indicate the end of the mainextensional phase.

The northern plate boundary is an obliquely convergentplate boundary dominated by a transcurrent conditionexhibiting strain partitioning of convergent motionaccommodated by a thrust zone in the ocean area andvertical strike-slip zones at the border of the steepescarpment of the SOM. Between the SOM and the BruceBank, the plate boundary changes, exhibiting a transcurrentand extensional regime.

ACKNOWLEDGEMENTS - This work has been carried out aspart of the Italian Programma Nazionale di Ricerche in Antartide.The paper benefitted from the useful reviews by E. Cristalliniand A.E. Rapalini.

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