The margins of the South Orkney microcontinent

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The margins of the South Orkney microcontinent E. C. KING & P . F . B A R K E R Department of Geological Sciences, Birmingham University, P. 0. Box 363, Birmingham B15 2TT, UK and British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK Abstract. Marinegeophysicalstudies

of theSouthOrkneymicrocontinentshow two intersecting tectonic fabrics of different age. The older fabric is oriented E-W, and represents the magmatic arc and fore-arc created by subduction-related processes at the Pacific margin during the Mesozoic. The younger, oriented NNW to SSE, comprises half-graben related to E-W rifting at both the eastern and western margins of the microcontinent. Both fabrics provide constraints on a 40Ma reconstruction, which showsa highlysystematicdistribution of arcandfore-arcprovincesandpointstothe importance of E-W strike-slip in regional tectonic evolution. The rifting, apparently of Oligocene age, is associatedwithback-arcextension which createdtheoceanicbasinstoeitherside of the microcontinent. This strongly supports the existence of subduction with the same orientation as the present South Sandwich arc as far back as 35 or 40 Ma.

The South Orkney Islands, which lie at 60°35’S, 45”00’W, arethe only exposed parts of a microcontinent. The dimensions of the continental area are approximately 250 km north to south and 350 km east to west. It is thus the largest of the components of the South Scotia Ridge (Fig. 1). The islands are largely composed of deformed and metamorphosed sedimentary and igneous rocks thought to have been emplaced at the Pacific margin of Gondwanaland by subduction-relatedaccretionduring the early Mesozoic (Dalziel 1985). In places these rocks are overlain by gently-folded Upper Jurassic or Lower Cretaceousconglomerates(Thomson 1981) and intruded by rare basic dykes (Thomson 1971). South of the islands lies an E-W sedimentary basin and south of that again a zone of high amplitude magnetic anomalies (Harrington et al. 1972). At present the northern margin of the microcontinent forms part of the Scotia-Antarcticplateboundary, along which approximately E-W sinistral strike-slip motion is taking place (Forsyth 1975; Tectonic Map 1985 and Fig. 1). To east and west, Powell Basin and Jane Basin are thought to have formed by extension, the latter behind Jane Bank, an island arc ancestral to the present South Sandwich arc. Subduction beneathJane Bankceased about20Ma ago when a spreading centre collided with the trench (Barker et al. 1984) but continued farthernorth, giving rise tothe central and eastern Scotia Sea by back-arc extension. There are several reasons why the structure and development of the South Orkney microcontinent(SOM) are of interest. Firstly, its place in the Gondwanaland reconstruction, along with other components of the Scotia Ridge, has long been debated (e.g. Barker & Gnffiths 1972. 1977; Dalziel & Elliott 1973; DeWit 1977). F o r the South Georgiamicrocontinent, a comparison of the variety of geological provinces, onshoreand offshore, with those of southernmost South America (Bruhn & Dalziel 1977; Simpson & Griffiths 1982; Tanner 1982) provides firm constraints. The known onshore geology of the SOM has proved more difficult to match satisfactorily against neighbouring pieces of the ‘jigsaw’, especially the Antarctic Peninsula (e.g. Dalziel 1985; Meneilly & Storey 1986), and the age and origin of the offshore features are uncertain. 317

Secondly, most Gondwanaland reconstructions which consider only a unified Antarctic plate involve overlap of the northernAntarctic Peninsula with SouthAmerica (e.g. Norton 1982). Movement between East and West Antarcticaneeds tobe invokedin orderto avoid this overlap, but the nature and timing of such movement is not yet firmly established (see, for example, Longshaw & Griffiths 1983; Grunow et al. 1987). If the SOM was firmly attached to the Antarctic Peninsula before the opening of Powell Basin, then the problems of overlap are increased. Movementbetween the different components of the Scotia Ridge during the Cenozoic is important because the last barrier the to establishment of the Antarctic Circumpolar Current lay atDrake Passage (Barker & Burrell 1977, 1982). Consideration of the motions of all the major fragments of Gondwanaland appears to require that the Antarctic Peninsula began to move away fromSouth America after anomaly 28 time (64Ma) but before anomaly 13 (35 Ma) (Lawver et al. 1985). This leaves a time gap in our knowledge of the development of the region, because ordered spreading in Drake Passage is only recognised from aboutanomaly 8 time (28Ma, Barker & Burrell 1977). Lawver et al. (1985) speculated that the SOM may have actedas the final link between the Peninsula and South America aftertheformer hadbegun to move southeast towardsitspresentposition,during this period; the SOM then moved independently,opening Drake Passage and ‘catching up’ the Peninsula before assuming its present position by late stage opening of Powell Basin. Thusthe determination of the tectonic history of the SOM, particularly over the last 60 Ma, has importance well beyond the bounds of the block. The marine geophysical investigation presented here is a first step in addressing that history.

Geophysical survey and SOM structure The investigation used data gathered over the period 1970 to 1981 aboard R.R.S. Shackleton, R.R.S. Bransfield, R.R.S. John Biscoe and H.M.S. Endurance, by or on behalf of the Antarctic Marine Geophysics Group at Birmingham University, UK. All shiptracks (Fig. 2) were fixed by

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Fig.1. Regional bathymetry. Contours are in thousandsof metres. M01,M02, margin outliers. Dashed lines show the locations of likely strike-slip lineaments, now inactive. Inset shows present plate boundaries: SAM, South American plate: AFR, African plate: ANT, Antarctic plate:SOM, South Orkney microcontinent: SG, South Georgia: SS, South Sandwich Islands. satellite. Almost all tracks recorded bathymetry and magnetics; those which also recorded gravity or undertook seismic profiling are plotted on Figs 3 and 7, respectively. The seismic reflection source was a single airgun of 0.66 or 2.62 litre capacity. Normal practice was to use the smaller

Fig.2.

gun over shelf areas and the larger in oceanic depths. Acoustic energy detected by the single-channel hydrophone streamer was recorded on analoguemagnetic tapeand displayed on dry paper recorders. Gravity data were recorded by Lacoste and Rombergmeters(Numbers S-40

Detailed bathymetry, in metres of the South Orkney microcontinent. Ship tracks are shown as dotted lines. CI, Coronation Island: LI, Laurie Island; PI, Powell Island;11, Inaccessible Islands; LS, Lethwaite Strait.

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Fig.3.

Free-air gravity, contour interval 10 mgal where data coverage and clarity permit, 50 mgal elsewhere. Regions between 10 mgal and 40 mgal on the SOM are shaded to emphasize lows over sedimentary basins: NB, Newton Basin: BB, Bouauer Basin; EB. Eotvos Basin; AB, Airy Basin. Line G-G' locates profile of Fig..4. Dashed contour is 200ij m water depth.

and S-86), and bathymetric data usually by Kelvin Hughes MS38 depth recorders. Thetotalamount of gravity and seismic profile is about 3200 km.

Morphology The shelf area of the SOM is approximately 88 000 km'. The shelf break lies between 400 and 700 m depth on the northern, western and southwestern margins and between 500 and 1300 m on the southeastern and eastern margins (Fig. 2). The greater part of the western halfof the shelf lies at between 200 and500mdepth,theeastern half between 400 and 1000 m. Two troughs, possibly of glacial origin, extend southward from the islands. Several steep-sideddepressions, either glacial troughs or upperslopecanyons,cut thenorthern margin nearthe islands. Elsewhere, seabed topography on the shelf is subdued, except for a region of steep-sided highs neartheeastern margin. These highs have a relief of between 100 and 200 m. The form of the continental slope will be described below, margin by margin. Firstly however, we consider the geophysical character of the SOM as a whole.

Gravity The gravity data used were collected during the period 1971 to 1981 on various cruises of R.R.S. Shackleton and R.R.S. Bransfield. Base station ties were madeatBarry, South Wales; Montevideo, Uruguay; and Rio de Janeiro, Brazil. All weretied to the International GravityStandardisation Net 1971 (IGSN 71: Morelli et al. 1974). Intermediate checks were carried out at various British Antarctic Survey bases using the values obtained by Sturgeon & Renner (1983). The base values quoted by Sturgeon & Renner were

reduced by 14 mgal to convert from the old Postdam value, to which theywerereferred originally, tothe IGSN 71 standard (Woollard 1979). Navigation for latitudeand Eotvoscorrections was provided by satellite fixes with interpolation by dead reckoning. The mean error for 69 cross-overs in the region of theSOM is 3.5 mgal (RMS = 4.5 mgal). Given the navigational accuracy these errorsare reasonable.They have been distributed only in so far as the free-air anomaly map (Fig. 3) is smoothly contoured. The lowest anomaly values in the region (down to -170mgal) are found to the north of the microcontinent, over the South OrkneyTrough.The field from this low point tothe high over the islands has a gradient of 7.5 mgal km-'. A similar gradient was measured off the northeast corner of the block. The highest anomaly values are found aroundthe islands, reaching 170 mgal nearthe Inaccessible Islands. Another high, exceeding 100 mgal, occurs at the eastern end of the northern margin. This is separated from the islands high by a saddle with values of 70-80 mgal at about 61"05'S, 42"45'W. To the south of the saddle is a spur in the 50 and 60 mgal contours which extends towards the southwest. The gravity field (Fig. 3) includes four lows which coincide with the sedimentary basins seen on the reflection profiles in Fig. 6. The lower limits of the basins cannot be seenonthe profiles but theirhorizontal extent can be mapped (Fig. 9) with reference to the prominent 'basement' highs separating them. Provisionally, we have named these basins in honour of early geophysicists (Fig. 3). The Newton Basin was identified originally by seismic refraction work (Harrington et al. 1972), which showed up to 5 km of sediments lying in a major E-W basin to the south of the

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islands. The associated gravity low has values of less than 30 mgal. Atitseasternend this low connects with a broader, N-S oriented low,extending tothesouthern margin, overtheBouguer Basin. At itswestern endthe NewtonBasin low divides around a small high before closing against a high near the western margin. In the centre of the block a number of isolated gravity highs, oriented N-S, aregroupedaroundthe Airy Basin low. The gradient to the south of the southernmost high, overthe margin, is as steep as thatoverthenorthern margin. This contrasts sharply with the adjacent area to the east where the Bouguer Basin low reaches the margin. The boundary between the Bouguer Basin low and the central region of high anomalies is almost straight and is oriented N-S . A ridge, defined by the 40mgal contour, separates the Bouguer Basin low from a small L-shaped low overthe Eotvos Basin to the east. The other bounds of this eastern low are the northeastern high and an irregularly shaped area of higher anomaly values on the eastern margin. Simple isostatic gravity models (Grow et al. 1979) have been computed across the SOM, using densities of 2.84 and 3.40 X 103kg m-3 for crust and mantle, a fixed mass balance and compensation depth (85.8 X 103kg m-* and 30 km), and with Moho depth as the single variable. By this criterion most of the SOM is in isostatic equilibrium, the only major departure beingalong thenorthern margin. The likely explanation for this departure may be seen in Fig. 4, which shows a more detailed two-dimensional gravity model of a N-S transect of the SOM, extendednorthward to cross Pirie Bank in the central Scotia Sea. The observed gravity and bathymetry used lie close to the line of section (located in Fig. 3) except where it crosses Coronation Island: for this stretch, gravity and bathymetry on a line through Lethwaite Strait wereused. Inthe south the track coincides with seismic reflection lineA.Control on the gravity model came from this and from seismic refraction data on theSOM (Hamngton et al. 1972) andon Pirie Bank(Hamngton 1968). Thiscontrolextends to Moho only on Pirie Bank. G' - 150

~

Seismic refraction velocities (2.1, 2.65, 4.7, 6.3 and 8.2 km S-') were converted to densities using the data of Ludwig et al. (1970). Use of the Pirie Bank Moho dataas control on the entire section means that the crust in the extreme north and south has to be about 2 km thicker than the 'standard oceanic' crust of Worzel (1974). However, it lies within the range of other oceanic crustal thickness measurements in the region (Allen 1966; Harrington 1968; Ewing et a l . 1971). The water depth in both areas is also abnormally shallow, and we suspect these characteristics reflect the 'back-arc' nature of south-eastern Powell Basin and the central Scotia Sea. To fit the gravity anomaly across the northern margin of theSOM,steep densitycontrasts and a thick crust are required beneaththe South Orkney trough. The isostatic anomaly is, therefore, the result of the difference between this andthestandard isostatic model, which would have neither.It is presumably the active (Scotia-ANT, Fig. 1) strike-slip motion along the margin which has created such sharpandsteep densitycontrastsandpreventedtheir dissipation. On the SOM, seismic refractioncontrol is confined to the shallower layers. One prominent feature is the great thickness of the 2.55 X 103kgm-3 layer beneaththe non-magnetic Newton Basin (Harrington et al. 1972), compared with the magnetically disturbed area to the south (around Airy Basin). It cannot be concluded, however, that this layer has the same characteristics right across the SOM: in some places in the southern area, Harrington et al. found magneticmaterial at all levels up toand including the overlying 2.3 X 103kg m-3 .layer. Unfortunately, we cannot add to this conclusion here, for lack of additional seismic refraction control. Although the model is not an exact fit to the short wavelength gravity anomaliesover the southern SOM, there are clearly many ways in which it can be made

so. The crust of theSOM is unusually thin, a conclusion which is largely independent of the assumptions of the model, despite the lack of a direct determination of depth to G -

NORTH

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50 r-

IOOkm

L

.cn__-,

500

400

Pirie Bank

-

l

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1

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South Orkney Microcontinent

-L

-

I

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Fig.4. North-south crustal section from the central Scotia Sea to south-eastern Powell Basin, crossing Pirie Bank and the SOM. Located in Fig. 3. Observed free-air gravity is continuous line and values calculated from the model are dots. Model density units are kg mP3X i d . Seismic refraction control (Harrington 1968; Harrington et al. 1972) shown by short, thick lines.

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Fig. 5. Magnetic anomaly profiles

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over the SOM. Shading onpositive parts of anomalies indicates direction

orprojectich from ships ;rack. Moho other than beneath Pine Bank. Use of a lower mantle density,forexample, would tendto thin the crust (by c. 2 km for 100 kgm-3) and our introduction of an undetected but typical (3 X 103kg m-3; Carlson et al. 1980) lower crustallayer will havethickened it.The crustal thinning beneath Newton Basin, similarly, is virtually unavoidable.

Magnetics The magnetic data over the SOM contain both long and short wavelength components. The wide spacing of ship tracks precludes contouring of the short wavelength anomalies, so individual profiles are shown in Fig. 5. Low anomaly values are found in the Powell and Jane Basins and in the area around the South Orkney Islands, whereas high amplitudeanomalies dominatethesouthernpart of the SOM. Four magnetic provinces can be distinguished. Main anomaly. The main anomaly hasaprominentpeak of between 500 and 1000nT which runs across the block from the centre of the west margin to the northeast corner. There are numerous short wavelength anomalies superimposed on the main anomaly (including its southern, negative part), on some but not all of the lines. Harrington et al. (1972) modelled the causative bodyasa granitegabbro intrusive complex. The variableamplitude of the short wavelength anomalies could be due to changes in the depth to the upper surface of the intrusive body, variations in its susceptibility or the presence of lavas in associated sediments. Hamngton et al. concluded that some magnetic materialhad to occur within alayer having a seismic refraction velocity of only 3.5 km S-’. It is notable that the short wavelength anomalies are subdued across the Bouguer Basin, but the main long wavelength anomaly persists.

Islands quiet zone. The magnetic anomaly field overthe north-central and north-western parts of the block is of low amplitude (Fig. 5). There are two areas wherethis is not the case, one to the south of Laurie Island, the other south of the Inaccessible Islands. In these regions thereare numerous short wavelength anomalies of upto500nT positive amplitude. Harrington et al. (1972) believed these anomalies to be associated with the dykes observed on the islands. The low anomalyamplitudeelsewhere in this region probably indicates that the metamorphic and sedimentary rocks seen on theislands extend under much of the northern part of the shelf.

East marginhigh. There is a broad region of positive anomalies of between 300 and 500 nT amplitude called the east margin high (Fig. 5) between the main anomaly and the eastern margin of the block. At the north-east corner of the block the two magnetic provinces merge. The south-easternedge of the high is oriented parallel to the margin and at this edge there are some short wavelength anomalies. These occur in the region of rough topography noted above. A dredge haul in this region included in situ altered basalts with a whole-rock K-Ar age of 78 f 2 Ma (Barber et al. 1987). West margin high. Inthe western margin areathe main anomaly extendsnorthwardtowards the islands. There is also an increase in short wavelength anomalies in this region (noted by Harrington et al. 1972), and the magnetic high, here termedthe west margin high,extends beyond the 2000 m contour. The increase in the high frequency component of the main anomaly on approaching the western margin, and the short wavelength anomaliessouth of the Inaccessible Islands, may be related to thepresence of basic dykes on the eastern end of Coronation Island.

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Margins The four margins of the microcontinent each have distinctive morphologies,sedimentdistributions and structural styles. They will now be described in turn.

Northern margin The northern margin, trending 110-290", is almost straight at the 1000m isobath over most of its length. The trend at the western and eastern ends is 095" (Fig. 2). Outlying highs M 0 1 and M 0 2 (Fig. l ) , one in the west and one in the east andbothelongated E-W, areseparatedfromthe main block by saddles. The change in the trend of the margin in theeast is accompanied by an offset tothesouth.The margin is very steep everywhere, with slopes of up to 1 in 3. The base of the continental slope lies below 5000 m along much of the margin. On the single channel seismic reflection profiles it can be seen that theshelf is free of superficial sediments adjacent to the margin and the seabed is highly reflective. The slope is also free of sediments of significant thickness. Atthe bottom of the South Orkney Trough(Fig. 1) lies a sediment pond about 1 km deep and 25 km wide. The seabed rises steeply to the north, reaching about 1400 m depth on Bruce Bank. Interpretation. Asstatedearlier, a major active sinistral transform fault lies along the north side of the SOM. The twooutliers (M01 and M02, Fig. 1) areinterpreted as slivers of continental crust separated from the SOM by branches of the transformfault. If so, they are migrating along the margin. The nature and origin of the deep, narrow basin off the northeast corner of the microcontinent (Fig. 1)is of interest. The orientation of the long axis of the basin lies across the gross orientation of thetransform, suggesting thatit is a transtensional or pull-apart basin. However, the history of this section of the South ScotiaRidge involves a complex series of ridge-crest/trench collisions with subsequent readjustments of plate boundaries (Barker & Hill 1981; Barker et al. 1984). Hence the inference of a straightforward pull-apart origin for the basin may be too simplistic.

Western margin Line drawings of four single-channel seismic reflection profiles across the western margin form part of Fig. 6. The original of one of these(LineL) is shown in Fig. 7 to illustrate data quality andthenature of the reflectors. Bathymetrically the western margin is diverse, with both steep and gentle slopes and detached elevations in places. However,there is underlyingdepositional and structural unity, which the profiles bring out. Three seismic sequences can be distinguished on the single channel reflection profiles available. The uppermostsequence(Sequence 1) has wide distribution over the microcontinent but the one underlying it (Sequence 2) is restricted to fault-bounded basins. The third sequence is exposed at the seabed on the continental slope and in the north of the SOM, but elsewhere underlies Sequences 1 and 2. Sequence 3. The upper surface of the sequence is rough on a small scale and dissected on a large scale into half-graben which have corners planed off by an erosional unconformity. The acoustic impedencecontrastat this upper surface is

usually high. The rotational faults delineating the half-graben throw down to the west. Reflections from the top of the sequence are frequently difficult to see on the records available, due to interference by seabed multiples. Internal reflectors are evident only on one horst,where they are truncated at the top of the sequence. The spacing of the faults bounding the half-graben is between 5 and 30 km, and the zone affected extends 150 km from the ocean basin (Fig. 6). The exposure of Sequence 3 on the slope below 1700m on line E (Fig. 6) was dredged on Shackleton cruise 1/80, yielding very young alkali basalts and late-Cretaceous calc-alkaline basalts (Barber et al. 1987). The former indicate the possibility of somerecent volcano-tectonic modification of the margin in this area, but nothing of this nature can be seen on the reflection profiles. Sequence 2. Thissequence is confined tothe half-graben and is characterzed by low amplitude reflectors which diverge toward the bounding fault. For example, in Fig. 7, dips within Sequence 2 are eastward in the lower part (i.e. parallel tothetop of Sequence 3) and westward in the uppermost part. The top of the sequence is usually marked by a strong reflector. Sequence 1. This sequence produces parallel-layered reflections of good continuity and medium to high amplitude. It is unconformable on bothSequence 3 and Sequence 2. On the continental slope, chaotic and mounded reflector configurations indicate slumping. Truncation of Sequence 1 reflectors at the seabed is observed in places on the slope. Internal unconformities are evident near the shelf break (Fig. 7). Some minor faults are observed, mainly in the region of slumping on the continental slope. Interpretation. Lines E and L(Fig. 6) cross the southern part of the western margin from west to east and are spaced about 18 km apart. These profiles show the classic features of ariftedcontinentalmargin, which we assume to have been created during the earlystage of opening of Powell Basin. Sequence 3 represents pre-rift material cut by rotational faults down-throwing tothe west and formedduring the rifting phase of crustalextension.Sequence 2 is syn-rift sediment, possibly locally derived from the eroded corners of the basement blocks. The unconformity at the base of Sequence 1 is thebreak-up or drift-onsetunconformity, which marksthe transition from crustal extension (with associated uplift and erosion) to thermalsubsidence. The major half-graben can be correlated between tracks and are oriented slightly west of north. Although thebreak-up unconformity is equally clear on Lines C and G which cross the margin further north, the underlying prisms of Sequence 2 are much less extensive.This could be explained if the uplift associated with rifting here hadbeengreaterthan further south, resulting in deeper erosion. Sequence 1, comprising thermal-subsidence-phase sediments, is nowhere more than 1 km thick. Thus, either sediment sources have been limited or the margin is young or both. At present, only the islands and their immediate surroundings are providing terrigenous sediments. From the seismic evidence it is likely that the only additional sources of temgenous sediment in the post break-up period would

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Fig. 613 Fig. 6. Line drawings of seismic reflections profiles. Sections have been converted to depth assuming a velocity of 2 km S-' in the sediments. Heaviest lines are inferred faults, medium weight lines are sequence boundaries. Sequence 3 is shaded. Vertical exaggeration approximately X 10.

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Fig. 6b

havebeensomesmall,isolatedhighs.Furthermore, shelf depthsonthe SOM are such that very largesea level changes are required to expose significant additional areas toerosion.Henceit is probablethatthemargins of the SOM have received only limited sediment supply since their formation. The effect of thisdown thecontinentalslope, where typically most Sequence of 1 outcrops, is compounded by slumping and by bottom-current scour. One wayof assessing theage of the margin in the

absence of direct geological evidence is to examine the age of the adjacent ocean basin using the empirical age-depth relationships of TrChu (1975) or Parsons & Sclater (1977). Oceanic basement can be seen on theprofiles to rise toward the centre of Powell Basin (Fig. Age-depth 8). determinations were made using the sediment loading correction of LeDouaran & Parsons (1982) andamean age-depth relationship of depth (km) = 2.53 + 0.336fi Ma (TrChu 1975). The ages so computedforan E-W Line

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Fig. 6c

across thecentre of the basin are plottedin Fig. 8. A linear fit to these data gives a maximum age forthe basin of 29 Ma . If the rifting episode which preceded the onset of seafloor spreading took 5-10 Ma(Royden & Keen 1980; Sclater et al. 1980), thenthe initiation of extension is dated

atabout 35-40 Ma (Late Eocene-EarlyOligocene). The oceanic spreading appearsto haveceasedatabout 23 Ma (Fig. 8). The age-depth relationship is not entirely reliable, especially when using a restrictedsamplein a small ocean basin. For example, the West Philippine and Parece Vela

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Basins in the western Pacific have ocean crust deeper than would be expected from the known age (Sclater et al. 1976), as does the eastern Scotia Sea (Barker & Hill 1981). If this were the case in the Powell Basin then the ages determined would be too great. The quality of the seismic data precludes definitive measurement of superficial extension by comparing the bed length of the top basement surface to the extended length. The value so obtained would not necessarily be a good measure of whole-crustalextension, as hasbeendemonstrated for the Goban Spur margin (Masson et al. 1985). To assess which of the numerous models for crustal extension applies tothe westernmargin of the SOM, it would be necessary tomeasurethe detail of crustal thickness and upper crustal extensional faulting on both this margin and its conjugate on the Antarctic Peninsula side of Powell Basin. Other parameters needed include the direction of spreading in Powell Basin, the orientation of rotational faulting, and of any transfer offsets in the fault pattern, and the asymmetry or otherwise of faulting styles on opposite sides

, 1

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of Powell Basin. Onefeature worthnoting is the clear suggestion, fromthe steadyoceanwarddeepening of the truncatedfault blocks (on Line E particularly), of an oceanward increase in stretching factor.

Eastern margin Three profiles (G2, E2, F) in Fig. 6 cross the eastern margin of the SOM. The eastern continental slope is irregular, but less steep overall than the slope of the western margin. The shelf break (though always difficult to discern), deepens from 600 m in thenorthtoabout 1300 m in the south. Seismic sequences with the character of those onthe western margin are recognized on the eastern margin also. However, there are some differences of detail. Sequence 3. Rotated fault blocks are only observed directly about 70 km continent-ward of the shelf break (Line E, Fig. 6), and show no obvious signs of truncation at wave-base. Closer to the margin the sequence lacks a mappable fault pattern.

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Fig. 8(a) Age-depth relationship in Powell Basin. Ages calculated from the present depth of oceanic layer 2, corrected for sediment loading, are plotted against distancefrom the spreading axis. Wide scatter, reflected in the low correlation coefficient, is due to irregularityof top of layer 2. Best fit indicates a maximum age for the Basin of about 29 Ma. (b) Part of a seismic reflection profiler record over the centre of the Basin. Figures indicate two-way timein seconds.

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Sequence 2. The distinctive configuration of divergent reflectors occupying half-graben is not observed on this margin. Some tilted blocks are seen on line E2 between 225 and 265 km (Fig. 6) but the first seabed multiple obscures details in Sequence 2 hereabouts. The top of the sequence is marked by a high amplitude reflector at between 500 and 600ms (TWT) below seabed over most of the region and at about 700 ms in Eotvos Basin. A number of faults downthrown to the east cut the top of the sequence. The majorityhavethrows up to 200ms;the largest, atthe western edge of Eotvos Basin, has a throw of about 500 ms. Sequence 1. The character of this sequence is more variable on the eastern margin than in the west. It is possible to subdivide the sequence on the basis of an apparent unconformity observed at the edge of Eotvos Basin. Reflectors in the upper part of the sequence appear to onlap a reflecting surface which is traceable through much of the region. This disconformity also marks a change from high amplitude reflectors above to low to mediumamplitude reflectors below. The faults which cut the base of the sequence die out well before the seabed, with the probable exception of the western boundary fault of Eotvos Basin. Mounded reflector configurations are frequentlyobserved, especially adjacent to highs and in the upper slope basins. Thisindicates thatcurrent action may have influenced deposition of the sequence. Slumping and possible current erosion are observed on the continental slope, as on the western margin. Adjacent ocean basin. The floor of Jane Basin (Fig. 1) lies at about 3300mdepth.It is flanked onone side by the SOMandontheother by JaneBank, which has been interpreted as an inactive island arc(Barker et al. 1984). Subduction stopped at about 20 Ma due to the collision of the ridge crest with the trench. The depth of the Basin and the thickness of sediments are comparable with Powell Basin. It is therefore possible thatthe two basins are of similar age. Preliminary heat flow datafromJane Basin (Lawver et al. 1987) suggest an age of about 25 Ma. Interpretation. We interpret the formation of Jane Basin as due to back-arc extension behind the Jane Arc. The growth of Jane Basin would very probablyhaveceased when ridge-crest/trench collision (Barker et al. 1984) took place at the arc (c. 20Ma ago) if it had notceasedalready. The observation of normal faulting on the eastern margin of the SOM also suggests that extension has taken place. However, activity on some of thefaultsappears to have continued through to the middle of Sequence 1. Thus if the base of thissequence is timeequivalent oneasternand western margins then the eastern margin has been tectonically active more recently than the west. The identification of Jane Bank as an inactive island arc (Barker et al. 1984) and the assumption that Jane Basin is a back-arc extensional feature provide a possible explanation of some of the complexities of the eastern margin tectonics. There is no sign (in the area traversed by profiles E , F and G at least) of Sequence 3 having been elevated to wavebase during rifting. Hence, the sharp distinction between faulted pre- and syn-rift sequences and unfaulted post-rift sediments, usually created by the period of uplift and subaerialerosionassociated with rifting, is missing here. The unconformity betweenSequences 1 and 2observed

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across theeastern margin is probably the result of the suddenchangein thepattern of deposition,from Jane Arc-derived volcaniclastic sedimentbeforedrift to sparse terrigenous and biogenic sedimentation, possibly influenced by enhanced bottom current activity, after drift had started andthe nascent Jane Basin hadbegun to collect the volcaniclastics. Thiscontrast would have beengreatest directly at the eastern margin, since Bouguer Basin appears tohave beenaconduit forthe southwardtransport of temgenous sediment from the more elevated north, leaving only biogenic sedimentation in the east. The pattern of faulting on the easternmargin is no more complicated in a gross sense than that on thewestern margin of theSOM,oncethe effects of an initially irregular basement topography are accounted for. As on the western margin,faultingextendscontinentward of the shelf break for more than 100 km. Hence most, if not all, of the SOM has been affected by extension. Southern margin The coverage and quality of data over this margin are poor compared to the othermargins of the SOM. However, from the available seismic reflection profiles (Fig. 6) it is evident thatthe western andeastern portions of this southern margin are influenced strongly by theextension at the western and eastern margins, respectively. The movement of theSOM away fromthe Antarctic Peninsula would require some strike-slip motion along the southern edge of the microcontinent if the N-S orientation of normalfaults at the western margin indicates an E-W direction of initial opening of Powell Basin. However, both the continent-ocean boundary and the expected strike-slip zone are difficult to locate. The very steep section of the margin near 45"W could have had a strike-slip origin, but only if the south-western margin has been built out extensively by the much later alkali volcanism represented in dredge hauls and noted previously (Barber et al. 1987).

Tectonic fabric The principal tectonic elements of the SOM aresummarized in Fig. 9. Features can be divided into two sets, a younger NNW-SSE set superimposed on an older E-W set. In the southern portion of the block, the faults and their attendant half-graben have predominant a NNW-SSE trend. Those structures related to extensional rifting of the western margin areseparated from their eastern margin equivalents by a narrow central horst. If the faults on the west side of Bouguer Basin and the east side of Airy Basin are the main bounding faults (Bosworth 1985) for the two extensional systems, east andwest,then the centralhorst may bethe only portion of theSOM unaffected by stretching. It is notable thatthe South Orkney Islands themselves show no obvious signs of N-S faulting except for the straits separating the three main islands. Faulting there, however, is considered be to of the same Early Cretaceous-LateJurassicageas the subaerially exposed conglomerates (Elliott & Wells 1982). Further west, a suite of dykes of early Tertiary age (King et al. 1987), intruded intothe metamorphic complex, may be related tothe separation of the SOM from part of the Antarctic Peninsula. The peak of the mainmagnetic anomaly trends E-W across thecentre of the SOM, swinging to SW-NE near Eotvos Basin. The anomaly appears to be truncated at the

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Fig. 9. Principal tectonic elements of the SOM. Note E-W trends of Newton Basin and the main magnetic anomaly and the NNW-SSE trends of the faults defining extensional half-graben.

margins which indicates thatthe emplacement of the causativebody took place beforebreak-up of thearc (Hamngton et al. 1972). Thisdeduction is reinforced by study of the high amplitude and short wavelength anomalies superimposed on the mainmagnetic anomaly. Wherethe peak of the anomaly passes under Bouguer Basin the short wavelength componentsbecomemoresubduedand field gradients are lower, implying deeper burial, but the anomaly is neither destroyed nor laterally offset. This supports ourthesis that the N-S features areassociated with the separation of the SOM from the Antarctic Peninsula and Jane Bank, and thus are younger than the E-W features. NewtonBasinalso has an E-W trend. The basin lies between accretionary wedge material,represented by the Scotia Metamorphic Complex rocks on the islands (Dalziel 1985), andthearc,represented by the intrusive complex which gives rise tothe magneticanomaly.This position suggests that Newton Basin may have commenced its history as a fore-arc basin.Such a setting is envisaged forthe fluvial, deltaic andsubmarinefan deposits of theupper Mesozoic Fossil Bluff FormationonAlexander Island, Antarctic Peninsula and for some marineconglomerates and turbiditesonAdelaide Island(Storey & Garrett 1985). Magnetic results over other parts of the Antarctic Peninsula (Renner et al. 1982) and over Tierra del Fuegoshow no firm indications of deep fore-arc basins. Hence the development of such basins along the Pacific margin appears to have been irregular. On the basis of both gravity and seismic reflection data it is evident that Newton Basin closes atbothends before reaching the margins of the block. The large positive isostaticanomaly andthe peneplanation of Sequence 3 along the northern margin both indicate recent uplift of the area, probably related to the active strike-slip motion along the margin. This uplift has probably affected the shape and form of the northern edge of Eotvos Basin and altered the limits of Newton Basin.

Implications for reconstruction The original aim of the work reported herewas to establish the position of theSOM(a) immediatelybefore Powell Basin opened (c. 35 Ma ago) and (b) at the Pacific margin when Gondwanaland broke up (c. 150 Ma ago?). It seemed necessary to discover the age and direction of spreading in Powell Basin andthenature of its margins in orderto constrain the reconstruction. Then using those components of the structural fabric of the SOM which certainly existed before Powell Basin opened, we would perhaps be able to constrain its position in earlier reconstructions going back to 150 Ma should this prove to bein any way different from the 35 Ma position. We do not yet have all of this information. Nevertheless, it seems worthwhile toattempt (Fig. 10) a tentative reconstruction to c. 35 Ma ago, using the criteria now available. Characteristics of this reconstruction are: (1) The western SOM margin was extensional and the southern margin (partly obscured by later alkali volcansim) strike-slip. The southwest margin of Powell Basin (Fig. 1) also has a (steep-to, smoothly-curving) strike-slip appearance. The eastern margin of the SOM was also extensional, related to the opening of Jane Basin. Both the orientationandtheshape of the margins of Jane Basin suggest that opening was E-W. BothJane Basin and Powell Basin started to open after 35 Ma. (2) The E-W orientation of the basement high around the South Orkney Islands, the basin to the southand the main magnetic anomaly (Fig. 9), are older and therefore not related directly to the opening of Powell and Jane Basins. Thesefeatures can therefore be used to examine the difference between the 35 Ma andearlier reconstructions. Thestructure of otheradjacent elevated areas is known from the geophysical work of Renner et al. (1982-Antarctic Peninsula) andWatters (1972-South Scotia Ridge),from the land geology (TectonicMap 1985) and fromsparse unpublished Birmingham geophysical data.

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Fig.10. Reconstruction of Scotia Arc region to c . 35 Ma. Component blocks are Antarctic Peninsula (APen), South Shetland Islands (SS), northern non-magnetic (N) and southern magnetic (M) parts of south Scotia Ridge, the SOM, South Georgia (SG), Jane (J) and Discovery (D) Arcs, Burdwood Bank (BB), Bruce (B) and Pine (P) Banks, Shag Rocks block (SR), South America (SAm), including the Falkland Plateau (FP). Fore-arc (non-magnetic) areas, dotted, and magnetic (arc batholith and break-up-related) areas, lined, shown a flattened cusp or ‘fore-arc sandwich’ pattern, suggesting earlier N-S convergence. (3) Scotia Seareconstructions to 10 and20Ma by Barker et al. (1984) illustrate part of the history, including the opening of Bransfield Strait (Fig. 1) and the effects of present-day sinistral strike-slip motionalong the South Scotia Ridge. (4) Other likely strike-slip lineaments, active at some stageduring the past 35 Ma,arethe Gibbs Island fault (Ashcroft 1972; de Wit et al. 1977), and the northwestern margin of Powell Basin (also steep-to and smoothlycurving), with its likely westward extensionthrough the South Scotia Ridge (Fig. 1). Closure of Bransfield Strait aligns these two. (5) In reconstructing the area to the north of the SOM in Fig. 10, it is assumed that almost all of the undated areas of the oceanic Scotia Sea floor are younger than 35 Ma, and that Pine and Bruce Banks are thinned, subsided continental fragments. The reconstruction is therefore rather speculative inthisrespect.However, it is useful to attempt such a reconstruction in order to assess the validity of the South Scotia Ridge subset we are mostly concerned with. As a way of assessing the validity and permanence of the 35 Ma reconstruction, we have shaded two provinces in Fig. 10: (a) the magnetic province (the magnetic arc, of mid-Jurassic age and younger, plus any undetected magmatism related to post-35 Ma break-up). (b) the fore-arc(i.e. Pacific margin accretionary prism and fore-arc basin, possibly post-Permian, essentially non-magnetic). Back-arc regions are not shaded. The great range of ages of rocks within these provinces makes theirdelineation of value only if there has been no significant change of role (e.g. migration of younger arcinto olderfore-arc). The

coherence of the patterns in Fig. 10 suggest that this has not happened to any great extent since the mid-Jurassic. The closure of Powell Basin on the basis of (1) and (3) above cannot be accomplished without moving the southern, magnetic block of the South Scotia Ridge. We did this along the lineament of (4), shortening the block meanwhile to allow for stretching and addingsome of the minor fragments. The thin northern non-magneticpart of the South Scotia Ridge is also displaced by such motion. Thus the eastward separation of the South Orkneyblock from the Peninsula may have caused thin slivers of the northern edge of the Peninsula, originally arc and fore-arc of the Pacific margin, to follow in its wake. This contrast in the history of the western margin of theSOM, between Powell Basin opening in the south and a more difise, less wholehearted extensional regime in the north, is reflected in the nature of the margin (Fig. 9): thesouthernpart shows stretching, block-faulting and later subsidence, while the northern part is not block-faulted and has not obviously subsided, but did apparently suffer an episode of dyke injection when extension started. As already noted, there are several more general features of the reconstructionin Fig. 10 which we cannot discuss indetail here. Some areimportant, however, and stem directly from the work on the SOM, atleast in part. (1) The reconstruction includes subduction of the South American Plate beneath the recombined SOM-Jane Bank. Given the existence of subductionbeneath Jane Bank before the 20 Ma ridge-crest/trench collision (Barker et al. 1984), it seems most likely that Powell Basin and Jane Basin were both essentially back-arc extensional. Indeed,an alternative environment is difficult to imagine. Thus, Atlantic ocean floor was being subducted as long agoas 35-40 Ma. Some of the magnetic parts of the SOM, as well as Jane Bank, may well be magmatic bodies related to this

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subduction. The subduction zone is shown extending from the South AmericafAntarctic(SAM-ANT) boundary in the southtothe forelandfold-thrustbelt of the Magallanes Basin and (much shortened)North ScotiaRidgein the north. The simplicity of these relationships and their implications for the earlier history of the region, including the origin of subduction of South American plate,are discussed by Barker et al. (1987). (2) We have interpreted the olderE-W structures of the SOM as an accretionary prism, fore-arc and magmatic arc with apolarity which implies subduction off the northern margin, presumably of Pacific oceanic lithosphere. However, the magnetic and fore-arc provinces on the SOM arenot aligned exactly with their equivalents onthe Antarctic Peninsula. The 35 Ma position for the SOM was, therefore, temporary, and not the original (Gondwanaland) position. Thus,although our reconstruction differs from that of Dalziel (1985), we have not contradicted the conclusions he drew from the onshore geology, and there is no necessary incompatibility. (3) The abundance of thin slivers of continental material indicates the importance of E-W strike-slip motion within the regionaltectonicevolution overthe past 35-40 Ma. This feature provides ample opportunity for decoupling the northern and southern zones of the reconstruction. We see this as a complication superimposed upon the considerable simplictiy of Fig. 10, in which the Pacific margin couplet of magmatic arcandfore-arctakethe general form of a flattened cusp: a‘fore-arc sandwich’. The significance and possible origin of this form are discussed by Barker et al. (1987). Suffice to say herethatthe existence of the ‘sandwich’ in the 35 Ma reconstructionpoints to relative motion(mostsimply, N-S convergence)between about 90 Ma or earlier, when South Georgia and probably other blocks were at a subducting Pacific margin, and 35 Ma when theywere not. Such convergentmotion may also have offset the SOM from the AntarcticPeninsula. (4) The existence of E-W strike-slipmeans thatthe more speculative features of thenorthernpart of the reconstruction are less importantto this study. These include the originalintervention of a recompactedBruce BankandPineBank between Tierradel Fuego andthe geologically similar South Georgia microcontinent, and the apparent connection at the eastern end of the ‘sandwich’ of the South Georgia and South Orkney magmatic arcs. To resolve these and other uncertainties of the reconstruction will require more precise estimates of the relative motion of theSouthAmericanandAntarctic plates,from further marine magneticsurvey intheSouth Atlantic. For the period between 35 and 90 Ma, which is not represented in theonshore geology of the continentalfragments lying between South America and the Antarctic Peninsula, there is no alternative source of information.

Summary By means of the marine geophysical work described above, the most important elements of the structure and tectonic evolution of the South Orkney microcontinenthavebeen described. (1) Two major structural trends have been recognized. Theolder,oriented E-W, comprises magmatic a arc, fore-arc basin and accretionary complex created when the SOM was atthe Pacific margin, beforeandafterthe

break-up of Gondwanaland. These features help constrain a reconstruction. (2) The younger trend, oriented slightly west of north, comprises half-graben related to rifting of both the eastern and the western margins, and pervades the entire block. (3) The rifting was essentially back-arc extensional, and Oligocene in age. Itdemonstratesthe existence of an east-facing arc and trench at Jane Bank, subducting South American oceanic lithosphere, as far back in time as 35 or 40 Ma. (4) The subsequent opening of Jane and Powell Basins was completed at or shortlybefore the collision of a spreading centre with the trench east of Jane Bank about 20 Ma ago, when subduction stopped. ( 5 ) Since their formation, the margins of the SOM have been essentially sediment-starved. The subaerial part of the SOM hasdecreasedin size with time, by subsidence and erosion, and strong bottom currents combined with sediment slumping have caused the entire post-rift sedimentary sequence to be exposed at the seabed on parts of the upper continental slope. ( 6 ) From earlier earthquake studies it is known that the northern margin of the SOM is the site of active sinistral strike-slip motion, at the Scotia-Antarctic plate boundary. The features we have described, although imperfect and incomplete, can be used to guide areconstruction of the situation c. 35 Ma ago, before the opening of Powell or Jane Basins. From this attempt it is clear that: (7) The SOM was not then in its original (Gondwanaland)position, since the older structuralelements are not aligned with those of itsneighbours (in particular the Antarctic Peninsula). (8) E-W strike-slip motion has been prominent a feature of Scotia Arc evolution. (9) The 35 Ma reconstruction contains a simple pattern forthe Pacific margin arc-fore-arc couplet: that of a flattened cusp, suggesting earlier N-S convergence, to exclude from the Pacific margin some regions originally located there. (10) The pattern of Atlantic-directed subduction 35 Ma ago was also simple and suggests how it may have originated. Despite the uncertainties, the reconstruction was a fruitfulexercise, in pointing out how constraints may be made tighter. Further study of SouthAmerican-Antarctic platemotion will further constrain the wider context of Scotia Arc development, while the details require a better understanding of the growth of the numerous small oceanic basins in the region. We aregrateful for the logistic support provided bytheBritish Antarctic Survey and by Research Vessel Services, Barry. In particular we would like to thank the captains and crews of R.R.S. Bransfield and R.R.S. Shackleton for their unstinting efforts during our field seasons. We acknowledge with thanks the facilities provided byBirmingham University and Britoil p.1.c. duringthe production of this paper. Data collection and reduction was greatly assisted by R. Whittington, P. Barber, A . Wilson and B . Hall. The work was supported financially by NERC research grant GR3/3279.

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Received 17 February 1986;revised typescript accepted 23 July 1987