From Small-Scale Faults to Plate Kinematics: Palaeostress Determinations in a Fragmented Arc Complex (SE Livingston Island, S Shetlands, Antarctica)

Journal offhe Geological Society, London, Vol. 153, 1996, pp. 1011-1020, 5 figs, 1 table. Printed in Northern Ireland

From small-scale faults to plate kinematics: palaeostress determinations in a fragmented arc complex (SE Livingston Island, S Shetlands, Antarctica)

P. SANTANACH, R. PALLAS, F. SABAT & J. A. MUNOZ Grup de Geodinamica i Andlisi de Conques, Departament de GeologiaDinamica, Geojiiica i Paleontologia, Universitat de Barcelona, Zona Universitiria de Pedralbes, 08028 Barcelona, Spain

Abstract: Analysis of thepolyphase fault population of southeasternLivingston Island led us to establishthree brittle deformation phases characterized byhomoaxial stress tensors. One of the horizontalaxes trends NW-SE, parallel to the transform faults governing the relative movement between the Phoenix and Antarctic plates. On the basis of the principles of symmetry these tensors are interpreted as corresponding to the regional stress field, and the transition between the phases is seen as reflecting changes in the relative values of the principal axes of their corresponding stress tensors.Phases 1 and 2 correspondto strike slip regimes,the first having NW-SE-oriented (rl (maximum principal compressive stress), whereas uI of phase 2 has a NE-SW trend. Phases 2 and 3 show a NW-SE-oriented U, (minimum principal compressive stress). The decreasing magnitudeof the NW-SE stress axis duringthe recorded history is interpretedas being related to thedecreasing velocities of the interacting plates caused by the cessation of the accretion at the Antarctic-Phoenix Ridge. The kinematic evolution of the analysed fault population can be understood assuming that faults form according to the Anderson model, that extensional dykes and veins form perpendicular to u3,and that fault slip on pre-existing fractures occurs parallel to the maximum shear stress direction on those planes.

Keywords: Antarctica, faults, plates, kinematics.

TheAntarctic Peninsula is a Mesozoic to Cenozoic trench separates the South Shetland crustal block from the magmatic arcdeveloped 'along thecontinental margin of former Phoenix plate. Gondwana,in a basement of igneous, metamorphicand The kinematic relationships between the South Shetland deformedsedimentary rocks (Smellie et al. 1984). The trench,the Bransfield Straitand the southern arm of the magmatic arc is related to the subduction of a large piece of Scotia Arcare still poorlyunderstood. It seems that the oceaniclithosphere called the Phoenix plate (Fig. 1). The opening of the Bransfield basin began immediately after the subductionwas driven by accretionand extension at the slowing down or cessation of theaccretion theat segmented ridge (Antarctic-Phoenix Ridge) which bounded Antarctic-PhoenixRidge (Barker & Dalziel 1983). These the Phoenix plate to the north and west (Barker 1982). The authors suggest that extension on the Bransfield rift is due to different segments of the ridge, offset by NW-SE-trending roll-back of the subducting plate determined by the load of transformfaults analogous tothe Shackletonand Hero the subductedslab. It has also been suggested that extension faults, reached the subduction margin diachronously leading of the basin might be controlled by a strike-slip stress regime to a succession of ridge-crest trench collisions that migrated due to anENE-WSW-oriented pushing effect of the oceanic northeastwardsalong themargin. This led theto ridgeslocated close to the Scotia Arc (Tokarski19876, termination of subductionalong most of theAntarctic 1991). Peninsula margin. Infront of thenorthern tip of the Since thepioneer work of Hobbs (1968), different Antarctic Peninsula the accretion at the Antarctic-Phoenix aspects of the structure of the South Shetland Islands have Ridgestopped or sloweddown approximately 4 Maago beenreported by severalauthors. Recently, in a detailed before reaching the subducting margin (Barker 1982). Thus, contribution onthe hydrothermal veins and breccias on part of theformer Phoenix plate has not been entirely Hurd Peninsula, Willan (1994) also described fracture trend subductedand survives aspart of theAntarctic plate at distributions in the area the present paper is dealing with. present (British Antarctic Survey 1985). Tokarski (1991) related plate kinematics to brittle structures The NE-SW-striking SouthShetland archipelago is a of the South Shetland Islands using determinations of stress fragment of the Antarctic Peninsula crustalblock. To the SE field orientations on the basis of joint analysis. However, so it is separated from the Antarctic Peninsula by the 150 km farno attempt has been made to carry outquantitative wide Bransfield Strait(Fig. 2), which seismicity(Forsyth; palaeostress analysis from local slip vectors determined on 1975; Pelayo & Wiens 1989; Vila er al. 1992) and volcanism small faults. (e.g. Smellie 1987) showto be an active rift. Themaster The aim of ourpaper is toanalyse the brittle faults of the Bransfield Basin constitutethe southeastern deformation recorded on the Hurd Peninsula and False Bay boundary of theSouth Shetland block. Part of thisfault area of Livingston Island (South Shetland) in order (1) to systemdetermines the straight southeastern margin of quantitativelycharacterize thestress tensors of the Livingston island and is referred to here as the Bransfield successive brittledeformation phases, (2) to discuss the master-fault zone (Fig. 2). To the NW the South Shetland stressregime history inferred from minor faults in the 1011

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obtained by considering thatbrittle structures at different scale are caused by acommon stress field (Mattauer & Mercier 1980; Mattauer 1992). Concerningthe contemporary first-order stress field in thelithosphere ithas been concluded that there is a correlation between the maximum horizontal stress and the directions of plate motions for broad regions of many plates 'I (Zoback & Magee 1991). However, the geometry of plate boundariesalso strongly influences the local stress ansfield Strait orientations, in such a way that the principal stress axes may ..a-.-,. ..a-.-,. deviate from first order stress directions and consequently bedifferent from theabsolute and relative main plate velocity directions (Rebaiet al. 1992). The Mediterranean is a good example of a region with complex plate boundary Fig. 1. Tectonic scheme of the South Scotia Arc. Bold lines geometriesand, as a consequence, a complex stress field represent fracture zones, triangles are subduction zones, double lines are oceanic ridges. Grey rectangle corresponds to location of pattern. Similarstress trajectory deviations are caused by Fig. 2 (modified from British Antarctic Survey 1985). faults at all scales, and therefore the coincidence of stress tensorsinferred from small structures with the regional stress tensor is not obvious. Further difficulties arisewhen considering long periods of time.Although plate motion directions remain framework of the regionalplate kinematics, and (3) to constantduring long time intervals, it is usual to find establish the history of the brittle structures of Livingston reversals faultmovements recording changes in stress Island. As wewill show, quantitative analysisin terms of of regimesduring one of suchperiods. Furthermore, palaeostresses of a scattered and complex fault population thestress tensors deduced from the movements of large will allow us to obtain a fairly simple stress tensor history faults do notalways coincide, in numberand orientation, whichcan be easily correlatedto the movement between with the stress tensors inferred from minor fault kinematics plates. (Guimeri 1988).Frequently the number of stresstensors deducedfrom small fault kinematics is greaterthan that obtained by analysing map-size faults. Method Assuming we areable to determine several stress tensorsand their relative chronology from small-fault Because of the discontinuous character of brittle structures, analysis, we still have to address several problems in order it is generally difficult to establish the genetic relationships to achievecoherent a interpretation includingthe between (1) faults at different scales and (2) faults and plate structuresscalesat from minor structures to plate kinematics. Inspite of this, reasonableresults have been kinematics.Among them, wewill first focus on two questions. (1) Might thededuced main stress directions represent the directions of the stress field that control the plate kinematics, that is to say, the regional stress field? (2) How might the transition from a given stress tensor to the I I I 6ZQW following one have taken place? The answer to both questions will be based on the principles of symmetry (Curie 1894; Schmidt 1926; Sander 1930; Paterson & Weiss 1961). Thesedeal with thecommon elements of symmetry, that is, the same elements in the same orientation, between thecauses and effects of physicalphenomena. These principlespermit certain minimum deductions about phenomena from which information is too poor to allow a complete analysis. Oncethe regionalstress history has been established, further questions have to be answered in order to find out the brittle deformation history of the area considered. When did the present fracture surfaces form, and what was their behaviour? How did pre-existing fracture planes move when affected by successive stress tensors? To discuss the first of these questions we will consider that faults form according to the Anderson (1951) model, and that extensional joints and dykes form perpendicular to the minimum main stress axis, a,. The ideathat fault slip on pre-existing fracture planes is parallel to the maximum shear stress direction will bethe basis of theanswer tothe second question. To Fig. 2. Map of the Bransfield Strait district, to show that the calculate this direction we will apply the equationdeveloped south-eastern margin of the South Shetland Islands is a complex by Bott (1959),which relatesthe maximum shearstress fault zone, schematically represented by a bold dashed line. direction on a plane to the orientation and relative values of

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the mainstress axes of anyconsidered stress tensor the different sites will allow us to determine which of the (GuimerB 1994). tensors represent general stress configurations and which of them correspond to local situations. If this were not taken intoaccount, possible local stressconfigurations could be Determination of stress tensors misinterpretedrepresentingas different phases of The determination of the stress tensor history from brittle deformation. structures is the starting point in our reasoning. Better than any other brittle structure (joints, for example), the slicken linedsmall faults make it possible to determine the stress Geology of the Hurd Peninsula and False Bay area tensorwhich caused its kinematics. Inaddition, valuable Most of the Hurd Peninsula is constituted by the (?)Early information on the relative chronologyof stress tensors may Triassic Miers Bluff Formation (Willan et al. 1994), which be obtained from cross-cutting relationships between faults, consists of turbiditic siliciclastic sandstones, mudstones and dykes and veins.For these reasons we havecentred our conglomerates (Hobbs 1968; Arche et al. 1992; Pallhs et al. effort on the analysis of small-fault populations. 1992: Smellie et al. 1995). The formation is predominantly Several quantitative computer aided methods have been overturned, youngs from W to E, and dips about 45" to the proposed to infer thestress tensor from the study of NW (Dalziel 1972; Muiioz et al. 1992). small-scalefaults (Carey & Brunier 1974: Angklier & TheMoores Peak BrecciaBeds cropout close tothe Goguel 1979; Armijo & Cisternas1978; Angklier & False Bay coast of Hurd Peninsula, stratigraphically above Manousis1980; Etchecopar et al. 1981).These methods the Miers Bluff Formation,and are in contact with the assume that the maximum shear stress on a plane is parallel massive volcaniclastic breccias of the (?)Cretaceous Mount tothe fault slip direction,and are based on theequation Bowles Formation. Moores Peak Beds could be either part developed by Bott (1959). They solve the inverse problem of the Miers Bluff Formation or a separate unit (Fig. 3). The determiningthe deviatoric component of thestress field contact between the Moores Peak Beds and Mount Bowles relative to agiven fault population by usingfault-plane Formationmay beeither unconformable or tectonic orientation,striae pitch anddirection of slip. Mathemati- (Smellie et al. 1995). cally, four striated planes are necessary to solve the inverse Tonaliticrocks are present along the eastern coast of problem,but real casesusually show thatmore striated False Bay and probably constitute most of the mountainous planes areneeded to give reliable results. The resultant area in thesoutheastern part of theisland (Smellie et al. stress tensor is described by the plunge and azimuth of its 1995). Tonalitesat Barnard Point yielded a K-Ar age of main stress axes and the parameter R = (U, - u3)/(u,- u3) 46 f 1 Ma (Smellie et al. 1984). Onthe Hurd Peninsula that describes the relative magnitude of the main stress axes there are small outcrops of tonalitic rocks. The one north of and ranges between 0 and 1. When R = 1 the magnitudes of Johnsons Dock has recently been dated as 73 Ma (Kamenov U, and U, are close to each other and when R = 0, U, and U, 1995) and the one at northern False Bay may correspond to are very similar in magnitude. apophyses of the large pluton located to the southeast. A useful method to analyse polyphase fault populations At one locality on the eastern side of False Bay there is a is described by Etchecopar et al. (1981). The algorithm of sequence of schists that formxenoliths within the foliated the Etchecopar method is based on the following steps: (1) a marginalzone of themajor tonalitic pluton. These rocks large number of stress tensors are randomly generated and were interpreted by Hobbs (1968) asfragments of a the theoretical orientations of the striae are calculated for all (?)Precambrianbasement incorporated into the pluton thefault planes measured in the field, (2) theangular during its emplacement. They were reinterpreted by Smellie deviationsbetween theoretical and real striae (the ones (1983) asbasic dykes which hadbeen deformed and measured in the field) are calculated, and (3) the quadratic metamorphoseddue to the emplacement of theEocene sum (S) of the deviations between the theoretical and the tonalitic pluton. This author refers to the False Bay schists real striae is calculated for every stress tensor. Among all andthe locally associated layered diorites as metagabbro, stresstensors the one yielding aminimum S value is and considers them to be synplutonic and, hence, Tertiary in consideredcorrespondto thetostress regime that age. determinedthe formation of the fault population.The Dykes of uncertainage, intermediate in composition number of stress regimes that determine a fault population (Caminos et al. 1973),usually porphyritic toaphanitic in as well as the group of faults corresponding to each of them texture (less frequentlymicrogranular) and a few cen- have to be determined by carefulsorting of the solutions timetres to some decimetres thick, are present. Calcite and given by thealgorithm. The minimumnumber of fault quartz veins a few centimetres thick are also common along planesneeded to yieldreliable results in theanalysis of the western coast of the Hurd Peninsula. An extensive and polyphase fault populations is about twenty. detailedstudy determining vein distribution,compositions Whenconsidering polyphasea fault population, the andtextures, as well astheir relation to brittlestructures Etchecoparmethod makes it possible to infer the (different kind of faults, dykes), volcanic and intrusive rocks configuration of several stress tensors, but cannot give any was carriedout by Willan (1994). AlthoughRb-Sr informationabout their relative age. In order to establish determinationsindirectly suggest that hydrothermal altera- the relative chronology, cross-cutting relationships between tion (and henceveining) in theHurd Peninsula maybe structures consistent with the different stress tensors must be Cretaceous in age (Willan 1994; Willan et al. 1994) younger used. ages cannot be ruled out. Unless clear homogeneity between sites is ensured, the Pervasivefaults ranging from map size (PallBs et al. analysis of a fault population has to be performed by taking 1995) tometre size (Santanach et al. 1992) affectall the single sites separately and cannot be applied to all the sites lithostratigraphicunits of thearea studied. The constant as a whole. Only a careful comparison of the results from attitude of the stress axes obtained from small fault analysis

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faults with shear criteria are approximately distributed on a nearly horizontal great circle but, in spite of their scatter, they show a well developed maximum which corresponds to faultstrending around N115 (Fig. 4a). Thefaults yielding clear shear criteria can be split into 56 dextral (Fig. 4b) and 57 sinistral faults with striae pitch angles of less than 35" (Fig. 4c), 11 normal-oblique faults with striae pitch angles between 35" and 55", 30 normal faults with striae pitch of morethan 55" (Fig. 4d), five reverse-obliqueand four reverse faults. Strike-slipfaults clearly predominate, suggesting the presence of strike-slip regimes, and they are followed in number by normal faults. Reversefaults are extremely scarce. Considering both single sites and the total faultpopulation, trends of dextraland sinistral strike-slip faults show a high degree of coincidence (Figs 4b and c). Furthermore,some fault planes showboth dextral and sinistral shear criteria. This coincidence in direction of the sinistral and dextral strike-slip faults observed in the Hurd Peninsula and False Bay area indicates that deformation is polyphase since at least two stress regimes are required to explain the whole fault population. Dykesalso generally have vertical attitudes but their trends are scattered in all directions. The density diagram of theirpoles shows the existence of three well-developed maximacorresponding to dykestrending around N020, NlOO and N155 (Fig. 4e). Veins are mainly concentrated on outcrops flanking the western coast of the Hurd Peninsula. They are subvertical and, although they have a slight dispersion in direction, the stereoplot of their poles shows a well-developed maximum LEGEND corresponding to planes trending around NO20 (Fig. 4f). 0ice-covered areas Fault analysis plutonic rocks In theHurd Peninsula,only sites 1, 2and 3 have the Mount Bowles Fm. minimum of 20 small-faults needed for the application of the Etchecopar method. We first worked out these three major Moores P. Breccias sites and obtained a succession of stress tensors. To test the validity of theresults and determine the total amount of g% Miers Bluff Fm. Barnard consistent faults, we later applied the stress tensors to the Point remaining sites. In a first step, the application of the Etchecopar method Fig. 3. Geological sketch-map of the Hurd Peninsula and False Bay to sites 1, 2 and 3 led us to the determination of a stress area. Arrows indicate location of the sites where data were tensor(tensor 1: U, = 031155; u2= 831270; U, = 061065; collected. R = 0.80) which is well defined by a significant amount of faults in the three sites. After suppression of those faults from the populations of the corresponding sites, successive regardless of thebedding attitude on the different sites application of themethod to remainingfaults and the allows us to considerthat the analysed brittle structures carefulselection of thesolutions given by the algorithm formed later than folding. allowed us to establish two additional stress tensors (tensor 2: U, = 131248; u2=681124; u3=181342; R = 0.09 and The brittle structures tensor 3: U, = 881123; U, = 05/241; U, = 01/331; R = 0.81), Wehave analysed faults, veins anddykes on 13 sites tensors 2 and 3 being fairly well-defined in site 1, tensor 3 in scatteredover the Hurd Peninsula and FalseBay area site 2, and tensor 2 in site 3. Finally, stress tensors 1,2 and 3 (Fig. 3). wereapplied to thefaults of theremaining sites. The Metre to decametre size fault planes showing slickenside number of faults being explained by more than one of the striations are common, and a great number of them show accepted stress tensors was minimized as much as possible. shearcriteria, mainly growth fibres. Thetotal amount of Table 1 shows the amount of faults consistent with the stress striatedplanes observed is288, of which163 yield shear tensors deduced. The accepted maximum deviation of the criteria. Three sites with more than twenty well determined realstriation from the theoretical one according to Bott's faults, four sites where the number of suitable faults ranges formula is 23". Upto 66% of the fault population is between12 and 17, and six siteswith less than ten faults explained by the deduced stress tensors while 34% of the showing shear criteria have been analysed (Fig. 3 and Table wholefault population is not. The faults which are 1). In stereoplot the fault plane polesof the total amount of incompatible with the stress tensors may correspond either

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Table 1. Number of studied faults per site and their relation with stress tensors 1, 2 and 3

Sites 1 2 3 4 5 6 7 1-13

Geological location Mien Bluff LateMiers Bluff Eocene Eocene Miers Bluff Miers Bluff See Fig. 3 Fm. Fm. plutonic rocks plutonicrocks Cretaceous Fm. Fm. plutonic rocks Number of faults 32 24 21 17 15 14 12 163 Faults consistent with 10 11 4 10 3 1 1 48 stress tensor l 31 yo 45 % 47 yo 17% 26 Yo 7% 8 YO 29 ?'o Faults consistent with 12 2 4 3 1 6 3 40 stress tensor 2 37 % 8 ?'o 19% 17?'o 6 '!40 42% 25 ?'o 24 % Faults 8 consistent1 with 4 4 1 4 8 33 stress tensor 3 12% 34 yo 5 yo 24 To 6 Yo 28% 66% 20 % Faults consistent20 with 24 14 8 6 8 9 108 stress tensors 1+2+3 YO of faults consistent 75 70 83Yo 66 ?'o 47 % 40% 57 % 75% 66% with stress tensors 1+2+3

~ ~ ~~ ~~ ~~ ~~ ~~ ~ ~ ~ Sites 1 to 7 are those where the most significant amount of faults was measured. Notice that the total amount of faults consistent with stress tensors 1 + 2 + 3 is less than the addition of values corresponding to tensors 1, 2 and 3 separately. This is because some faults can be explained by more than one of the deduced stress tensors.

to inheritedMesozoic faults recording older brittle local slip vectors may be partly determined by local block deformation phases, in the case of faults affecting the Miers geometries, Mercier & Carey-Gailhardis 1989). Bluff Formation, to periods of transitionbetween the Thus,the brittle deformation history of theHurd definedstress tensors, or to local stress disturbances (e.g., Peninsula and False Bay area, as deduced from the analysis

N N N

all faults faults

N N N

Fig. 4. Lower-hemisphere stereoplots of the poles of the analysed brittle structures. n = number of plotted planes. Density contours: 1, 2, 5% (diagram a) and 2. 5, 10% (diagrams b to f).

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of small-faults,can be divided into three tectonic phases the SE (Fig. 2) and are interpreted as the master extensional characterizedby stress tensors 1, 2 and 3. Tensor 1 faults of the BransfieldStrait. We consider them to have corresponds to a strike-slipregime but is quite close to beenformed during the last regional-scaletectonic stage uniaxialextensional, tensor 2 alsocorresponds to astrike which may be still active at present. Thus, regional criteria slip regimebut in thiscase itis veryclose to uniaxial seem to indicate that phase 3 postdates phase 2. compressional, and tensor 3 corresponds to an extensional Thefaults compatible with theoldest stress tensor regime but quite close to uniaxial extensional. Stress tensors (tensor 1) are well represented in all sites, including site 3 1, 2 and 3 are homoaxial, with the horizontal axes oriented (Table 1). This site consists of diorites corresponding to the NW-SE and NE-SW (Fig. 5). False Bay metagabbro assemblage of Smellie (1983). According to this author these rocks are Tertiary in age and Chronology coeval with the emplacement of the Eocene Barnard Point tonalitic pluton(Smellie et al. 1984). Thus,the history of In order to establish therelative chronology of thestress brittle deformation caused by stress tensors 1, 2 and 3 may tensors 1, 2 and 3 the followinggeometrical relationships correspond to the period from the Eocene to the Present. were taken into account: veins are cut by dykes and faults Veins may be related to hydrothermal processes associated consistent with stress tensors 2 and 3. Faults consistent with with the smallJohnsons Dock tonalite intrusion (Willan stress tensor 1 are cut by faults consistent with stress tensors 1994), which recently has been attributed an age of 73 Ma 2 and 3. Theserelationships clearly indicate that vein (Late Cretaceous; Kamenov 1995). generation and the tectonic phase corresponding to tensor 1 In spite of the arguments given above, some of the faults are prior to tectonic phases corresponding to tensors 2 and considered here could be older than the Eocene. The Miers 3. Whatever their orientation, dykes are found either cutting Bluff Fm., Triassic in age,could have recorded Mesozoic or indistinctly beingcut by faultsconsistent with stress brittledeformation phases (e.g. faultsrelated to the arc tensors1, 2 and 3. No cross-cuttingrelationships were extensionimplied by the Byers Group). Nevertheless, if observed between structures compatible with stress tensors 2 duringMesozoic times the regional stress field hadbeen and 3. Stress tensor 3 is consistent with a preferential normal similarly oriented and with similar principal axes values to faulting along a NO61 direction. This direction is very close theones we havedetermined, the corresponding faults tothe trend of thefaults (suchas the NO58 oriented wouldbe indistinguishable from the Tertiary faults. If the Bransfield master-fault zone) that bound the archipelago to

phase 1 phase 2 phase 3 r N N N

Fig. 5. Stress tensors of the three phases and kinematics of related faults. Lower left corner: Attitude of the principal stress axes of the tensors corresponding N to phases 1, 2 and 3. Upper row: fractures formed in response to tensors @20; ! 1, 2 and 3 (continuous line great circles represent theoretical conjugate fault sets according to Anderson’s model, dashed line great circles correspond to theoretical conjugated faults assuming a relief of the main stresses of similar value, and dotted lines great circles represent tensional fractures. Second and third E \

rows: calculated kinematics of the N fracture planes caused by tensors 1 and 2 when subject to the successive tensors after their formation. All diagrams are lower hemisphere plots. Arrows indicate the movements of hangingwall. Notation of plane poles: numbers indicate the phase responsible for the formation of the plane, letters indicate the kind of m phase 1 (R = 0.8, i.e. ol m a) fracture (in capitals the kinematics A phase 2 (R = 0.1, i.e. a m 03) during the phase considered, in small 0 phase 3 (R = 0.8, i.e. oI m a) capitals during previous phases).

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Mesozoicstress fields hadbeen different from those through an intermediate stage (phase 11) where u1was in a determined,the corresponding fault movements would broad N-S position. The qualitativemethod used by probablycorrespond to those of some of thefaults not Tokarski does not make it possible to calculate R values but explained by the stress history that has been deduced. only to estimate stress orientations. It should also be noted that the estimation of stress orientations made by Tokarski is mainlybased on twoassumptions: (i) all thejoints and Discussion dykesobserved are due to strike-slip tectonicregimes, an assumptionfounded on the predominance of subvertical The three calculated tensors correspond to three phases or joints and dykes and subhorizontal slickenlines on the scarce stages of theevolution (which may be continuous) of the smallfaults observed, and (ii) both joints and dykes are stress field responsible for the brittle fracturingof Livingston considered to be formed perpendicular to and parallel to Islandfrom Eoceneto Present. Due to some unknown u3 the and (T~plane. Therefore, every set of joints would reason,fault kinematic indicators corresponding to these (T, represent a different orientation of (T~.Recently, in spite of stages have been better preserved than those from possible interpreting some of the joints on Livingston Island as shear intermediate stages. It has to be borne in mind that there joints, Tokarski still advocated rotations of thestress field are notenough data to evaluate if thesestages represent (Doktor et al. 1994). events of similarimportance with similartectonic results According tothe interpretation of Barker & Dalziel (i.e. similar offset alongmap-size faults). Althoughthese (1983), and refined by Jeffers et al. (1991), the subduction of limitationsare significant wheninterpreting the results in former-Phoenix beneath Antarctica has been driven by the terms of deformationphases on a large scale, important spreading of theAntarctic-Phoenix Ridge. The slowing inferences can be made from our results. downand finally the cessation of the accretion atthe Antarctic-Phoenix Ridge led to a slow passive subduction at Relation with regional tectonics the South Shetland Trench by a trench suction mechanism (Forsyth & Uyeda1975) that causedthe opening of the Stress tensors 1, 2 and 3 are homoaxial, and their horizontal BransfieldStrait. Thethree dynamicstages reflected by NW-SEaxes are parallel with the trend of the Hero and stresstensors 1, 2 and3 are easily understandable in the Shackletonfracture zones, which governtherelative framework of this model, the variations in magnitude of the movementbetween the former Phoenix and Antarctica NW-SEstress axis, parallel tothe direction of plate plates. Theseobservations imply acommon element of movements,being controlled by velocitychanges of the symmetry (a symmetry plane) shared by the stress tensors interacting plates. andthe movement between the plates. Through the Stresstensors with horizontal u1 oriented NW-SE symmetryargument we considerthat the stress tensors (compressional and strike-slip regimes) would correspond to obtainedfrom the analysis of smallfaults may represent stagesprior tothe cessation of the accretionatthe stages in the evolution of the regional stress tensor, which Antarctic-PhoenixRidge, and had to berelated tothe wouldbe related tothe relative plate motions between activesubduction at the South Shetland Trench. Attenua- former-Phoenix and Antarctica. tion of stressespassing from compressional to strike-slip If the relative plate motions of the former Phoenix and regimes have been described (i) in space, within plates when Antarcticaplates have been controlled since (at least) moving away from convergence boundaries (Tapponnier & Eoceneto Present by theHero and Shackleton fracture Molnar 1976; Bousquet & Philip1981), and (ii) in time, zones,as well as othertransform faults parallel tothe during transitions from compressional to tensional regimes former, because of the same symmetry argument considered (Guimerh 1984, 1988).Stress tensor 1 corresponds to a earlier,the orientation of the mainstress axes of the strike-slip regime with (T,oriented NW-SE (Fig. 4). Because regional stress tensors must have remained constant during thearea studied is small, it is notpossible to distinguish theperiod mentioned. Therefore the stress tensors of any between the followingtwo possibilities: (a)the strike-slip intermediate stage should have the same orientation as that regime corresponds to a stress state in an area away from of stress tensors 1, 2 and 3. This conclusion implies that the the trench while a compressional regime is being developed transition between the calculated stress tensors would have closer tothe convergence boundary, (b) the strike-slip taken place by means of a progressive change of R values, regimecorresponds to anintermediate stage between that is to say, by the progressive change in the magnitudes of compressionaland tensional regimes while there is a theirmain axes. Such a kind of progressivetransition has transitionfrom active to passive subduction. Nevertheless, been well documented in studies carried out on populations the absence of fracture planes dipping less than 45" suggests of smallfaults in theIberian Chain, Spain, and has been that in the studied area thrust faults never developed, and proposed on a regional scale for NE Iberia (Guimerh 1984, therefore it hasnot been subjected to anycompressional 1988, 1994). stress regime. In any case, stress tensor 1 would correspond Thisinterpretation differs fromthe one proposed by to a stage prior to the inception of the Bransfield Strait. Tokarski(1981, 1984,1987a, 6, 1991),who systematically In the framework of the plate model considered, stress studied the joints and dykes of King George Island, an area tensors with horizontal (T~oriented NW-SE, like tensors 2 located in thesame tectonic setting as LivingstonIsland. and 3, could be related to the cessation of the accretion at Instead of homoaxiality of thestress tensors with the Antarctic-Phoenix Ridge, and thus to the replacement progressivechanging of R values, and,as a consequence, of active by passivesubduction at the South Shetland changing of stress regimes, he suggested the permanence of Trench leading to the extension which caused the Bransfield a strike-slip regime. He inferred a clockwise rotation of its Strait.However the compressionalcharacter of the orientation, from a position with u1trending NW-SE (phase NE-SW-orientedaxis of tensors 2 and 3 canhardly be I)to an end positionhaving a NE-SW trend(phase 111) explained by this processalone. It could be dueto the

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influence of the accretion at the submeridian oceanic ridges Phase 3 is characterized by an extensional regime with U, causing the opening of the Scotia Sea. approximately perpendicular to the Bransfield Strait which causednormal faults trending around NO61 (3Nand 3N’, Fig. 5c). Previously developed fault planes would have been Concerning small and map-size faults reactivated as sinistral (1DsS, Fig. 5f, and 2sS, Fig. 5e) and The sets of fractures to be explained in the framework of the dextral (IsDD, Fig. 5f, and ~DD,Fig. 5e) strike-slip faults. suggestedstress tensor history arethe faults, dykesand Tensionalfractures formed during phases 1 and 2 would veins represented on the stereoplots of Fig. 4. The conjugate behave respectively as dextral (~TsD,Fig. 5f) and sinistral faultsrelated tothe three determined stress tensors (~TS,Fig. 5e) faults. according thetoAnderson model, as well theas The history of dyke emplacement seems to be complex. corresponding tensional fractures, are represented in Fig. 5. Mismatching of the extensionaldirections related tothe Wehave hypothesised that whenfractures are being three phases (lT, 2T and 3T, Fig. 5a, b, c) with some of the developedunder the influence of uniaxialstress regimes, maximashown onthe stereoplot (Fig. 4e),leads us to relief of stresses with similar values is produced in such a assume that dyke intrusion would have outlasted the whole way that two orthogonal sets of conjugated faults may be brittledeformation history described, from pre-phase 1 to formed.The conjugate faults related to these subordinate post-phase 3. Dykes trending around N020, the same trend tensorshave also been represented (Fig. S, discontinuous shown by veins whichmay be LateCretaceous in age great circles). Applying Bott’s equation we have calculated (Willan 1994), couldhave intruded prior to phase 1. the kinematics of the fracture planes caused by stress tensors Cross-cutting relationships suggest that dykes do not follow 1 and 2 when subject to the successive stress tensors after simple patterns relatingtheir directions to time of their formation (Fig. 5). Comparison of the data shown on emplacement.Moreover, thecoincidence in direction Figs 4 and 5 allows us to establish the history of the brittle between dykes and faults suggests that intrusion might have deformation on Livingston Island. occupied former planes of weakness which could have been On the one hand, most of the faults may be explained by subjected to extension during later phases of deformation. the fault systems formed according to Anderson’s model in In thatrespect, if thesuggested age for the veins is not relation to the calculated stress tensors. Only some normal correct,veins and dykes oriented around NO20 might faultscould be interpreted as caused by thesubordinate correspond to former strike-slip faults formed during phases stress tensor developed during phase 1. Faults related to the 1 and 2 (1S, Fig. 5a, and 2D, Fig. 5b). This would make it subordinatestress tensor of phase2 would correspond to possible to explain the virtual absence of these strike-slip reversefaults which are nearlyentirely absent, and those faults, as they would have been filled by later intrusions. In which might eventually form in relation to the subordinated addition, in orderto understand properlydyke patterns, stress tensor of phase 3 are indistinguishable from the main local andtransient stress fields related to intrusionphases strike-slip faults of phases 1 and 2 because of their similar should be taken into account, an aspect which is beyond our trends. On the other hand, not all the theoretically expected purpose in this article. sets of faultswere observed in the field. Faultstrending Havingdemonstrated that the calculated stress tensors between NO00 (around 005/84NW, sinistral, tensor1) and correspond tothe regional stress field, thereasoning and NO45 (around 041/68SE,dextral, tensor 2) are weakly calculationsapplied to small faults canbe applied to represented in our data (Figs. 4 and 5), as well as the NW map-size faults. However, due mainly to outcrop constraints dipping normal faults, one of the conjugate sets of phase 3. (isolatedoutcrops in anextensive ice cap),attitudes of Trends of dykes are scattered in all directions, and they map-sizefaults arequite imprecise. Fault strikes are show no simplerelation with theextensional directions relatively well established, whereas fault dips are unknown related to the stress tensors of phases 1, 2 and 3 (lT, 2T and in most of the cases. These facts make it difficult to deduce 3T, Fig. S). Thedirection of the dykescorresponding to precise fault kinematichistories from the regional stress their mostconspicuous maximum coincides with the field. Nevertheless,these kind of deductions,although preferredorientation of the veins.This direction, around somewhat speculative, may be of help in understanding fault N020, is coincident with that of the weakly represented sets histories. In order to illustrate the possibilities and of faults 1s and 2D (Fig. 4), related, respectively, to tensors limitations of this kind of reasoningone fault will be 1 and 2. considered: the Barnard-Renier Fault. Takinginto account theabove observations, the The Barnard-Renier Fault (trend: N058)is one of the following history of the brittle structures may be established. most conspicuous map-size faults on Livingston Island and During phase 1 dextral faults trending around N125 (lD, constitutes part of the system determining the northwestern Fig. 5a) developed. The sinistral set, around NO05 ( 1S, Fig. margin of the Bransfieldgraben shown asBransfield 5a)developed weakly. Subordinate normal faults trending master-faultzone inFig. 2.If itwas formedduring the aproximately N155 (1Nand lN’, Fig. 5a)formed as a history governed by the calculated stress tensors, it had to consequence of the relief of stresses with similar value (a2 be during phase 3 and as a normal fault (compare Bransfield and a3). master fault direction with the plots of Fig. 5). However, the Phase 2 also corresponds to a strike slip regime, but with possibility that this fault was present prior to phase 1 can not a, nearlyparallel to a, of phase 1. New sinistral faults beneglected. If so, duringphase 1 it wouldhave formed around N102/82SW (2S, Fig. 5b) and the previously experiencedasinistral-normal movement, thedegree of developeddextral faults would invert their movement to obliquitydepending on the value of the SE dipassumed. sinistral faults (1DS, Fig. 5d). Conjugated dextral faults (2D, Duringphase 2 its calculatedkinematics range from a Fig. 5) seem to be less developed.Tensional fractures normal-dextralfault for anearly vertical fault plane to developedduring phase 1 would move with aprominent adextral-reverse fault (dipping 60SE) passingthrough a sinistral component (ITS, Fig. 5d). dextral-normal one (dipping 75SE). Finally, during phase 3

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therange of possiblefault kinematics would be from a BARKER,D. H. N. & AUSTIN JR.,J. A. 1994. Crustal diapirism in Bransfield sinistral-normalfault for nearlya vertical plane to a Strait, West Antarctica:Evidence fordistributed extension in marginal-basin formation. Geology, 22,657-660. normal-sinistral fault for a 60SE dipping fault plane. In that BARKER,P. F. 1982. The Cenozoic subduction history of the Pacific margin of casethe opening of the Bransfield Basin wouldhave a theAntarctic Peninsula: ridge crest-trenchinteractions. Journal oj fhe transtensionalcharacter. The idea of transtensionala Geological Society, London, 139,787-801. opening of the BransfieldStrait has alsobeen reached by - & DALZIEL,1. W. D. 1983. Progress in geodynamics in the Scotia Arc region. American Geophysical Union Geodynamic Series, 9, 137-170. Tokarski (1991) andBarker & Austin (1994) following Borr, M. H. 1959. The mechanism of oblique slip faulting. Geological different and independent approaches. 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Analyse thtoriqueet numirique d’un explained by asimple stress field historylasting from the mod2le michanique iltmentaire appliqut i I’etude d’une population de failles. Compfes rendus de I’AcadCmie des Sciences, Paris, sir. D, 279, Eoceneto Present. Three successivestress tensors have 891-894. been determined, one of the horizontal principal stress axes CURIE,P. 1894. Sur la symttrie dans les phtnom2nes physiques, symttrie d’un of each tensor being always parallel to the transform faults champilectrique et d’un champmagnetique. Journal dephysique that governed the movements between the Antarctica and rhiorique et appliqui, Sir. 3, 3, 393-415. DALZIEL,1. D. 1972. Large-scale folding in the Scotia Arc. In: ADIE,R. J. Phoenix plates. Thisparallelism has been interpreted, (ed.) Anfarcric Geology and Geophysics. Universitetsforlaget, Oslo, according tothe symmetry principles,as supporting a 47-55. genetic relationship between the deduced stress tensors and DOKTOR. M,, SWIERCZEWSKA,A. & TOKARSKI,A. K. 1994. Lithostratigraphy the movement between tectonic plates. Thus, the established andTectonics of the Mien Bluff FormationatHurd Peninsula, Livingston Island (WestAntarctica). Studio GeologicaPolonica, 104, stress field history is considered to be the regionalstress 41 -1 04. field history, and the determined stress tensorsallowed us to ETCHECOPAR,A., VASSEUR,G. & DAIGNIERES,M.1981. An inverse problem discuss the kinematic history of faults of different sizes. The in microtectonicsfor the determination of stresstensors from fault complex pattern of thedykes, however, is noteasily striation analysis. Journal of Structural Geology. 3, 51-65. understandable if only theproposed regional stress field FORS~H,D. W.1975. Fault planesolutions and tectonics of theSouth Atlantic and Scotia Sea. Journal of Geophysical Research, SO, 1429-1443. history is takeninto account. The case reportedhere FORS~H.D. & UYEDA,S. 1975. 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Received 10 May 1996; revised typescript accepted 16 July 1996. Scientific editing by Alex Maltman.

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