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 Arc are 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/1011/4889280/gsjgs.153.6.1011.pdf by guest on 23 September 2021 1012 P. SANTANACH ET AL. obtained by considering that brittle 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 analysis in 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