In the frame of the SCAR GPS Campaigns a joint German research group carried out GPS observations in the Atlantic sector of Antarctica and at the Antarctic Peninsula. Larger expeditions took place in 1995 and 1998, where approximately 20 points were reoccupied.
The GPS data were analysed with different software packages (BERNESE, GAMIT/GLOBK, GIPSY, GEONAP). The horizontal position accuracy is in the order of 1 cm for the whole Antarctic continent and in the order of a few millimeters for the regional network of the Antarctic Peninsula.
We will discuss the obtained network deformations and relate them to tectonic structures. This includes deformation rates in relation to neighbouring tectonic plates, the stability of the Antarctic plate and regional deformation patterns in the Antarctic Peninsula area.
We would like to express our thanks for the scientific cooperation and the logistic support by institutions and colleagues from Argentina, Australia, Bulgaria, Chile, China, France, India, Japan, Russia, United Kingdom, United States, Ukraine and Uruguay.
A geodetic GPS network was established in 1990-91 italian expedition to Antarctica for the study of deformation control of Mount Melbourne volcano (Al Bayari et al, 1996). The network has been surveyed four time: 1990-91, 1993-94, 1995 and 1997-98. The results showed a significant coordinates variations after the fourth repetition, in comparison with method precision. A deformation analysis has been made integrating geodetic measurements with geophysical observations. The italian geodetic network, covering a wider area around Terra Nova Bay, italian base in Antarctica, was monumented in 1990-91 as a reference frame for scientific activites (photogrammetry, cartography, geology) and with geodynamics purposes (Capra et al., 1996). The network was completely surveyed two times in 1990-91, 1993-94 and it will surveyed in the next December 1998. The data processing and analyssis of the three campaigns will be presented. An analysis of data processing results obtained with different software and different approach to GPS data solutions (Lc,L1,L3) will presented. The different algorythms and the effects of the ionospheric noise (scintillation effects) were studied, particularly critical at those latitude (Capra et al.,1998, Al Bayari et al. 1998). As a contribute to the study of Antarctic geodynamics, the TNB1 station, coordinates emanation point(0100) of geodetic network, has been included in SCAR GPS Epoch campaigns 1996, 1998 and in the next 1999. Moreover it became a permanent GPS station in January 1998 (Capra and Gandolfi, 1998). The data acquired are not discharged continously in international net at the moment. The data stored in 1998 year will be available at the end of summer expedition, end of January 1999, and a preliminary data analysis will be presented here. Moreover GPS measurements will be obtained at very far away stations (about 1000 km each other): TNB1 (Italian base), Dome C station (during Dome C Strain Net surveying in January 1999), Durmont D'Urville (French base)and Mc Murdo (USA base). The data will be acquired from GPS permanent stations and for very long occupation and a preliminary network processing will be made.
Al Bayari O, Capra A, Radicioni F, Vittuari L GPS network for the deformation control in the Mount Melbourne area (Antarctica): preliminary results of third measuremnets campaign, Reports on Surveying and Geodesy. Pubblicazione del DISTART- Edizioni Nautilus, Bologna, Italy, 64-72, (1996).
Al Bayari O, Capra A, Mancini F, Vittuari L Comparison between TEC values derived from antrctic GPS measuremnets and atmospheric soundings, Proceedings of Workshop of 6W1 of ISPRS- Archives ISPRS (in press), XXXIII, (1998).
Capra A, Gubellini A, Radicioni F, Vittuari L Italian geodetic activities in Antarctica, Italian Geophysical Observatories in Antarctica - Editrice Compositori - Bologna- Italy, (1996).
Capra A, Gandolfi S Italian GPS permanent station in Antarctica (Terra Nova Bay- Victoria Land), Proceedings of Workshop Standards and services of GPS network- ASI - Matera - Italy, (1998).
Capra A, Radicioni F, Vittuari L Italian geodetic network as reference frame for geodynamics purposes (Terra Nova Bay- Victoria Land- Antarctica), IAG symposia 1997- Geodesy on the Move. Gravity, Geoid, Geodynamics and Antarctica. Springer, 119, 498-503, (1998).
The southern sector of the Priestley Fault, in the Southern Deep Freeze Range, north Victoria Land, Antarctica, has been studied during the XII Italian Antarctic Expedition (1996-1997). The Priestley Fault consists of an anastomosed array of fault zones which constitute a highly deformed belt running NW-SE from the head of the Priestley Glacier to the Terra Nova Bay. From the Black Ridge to the coast, good exposures allowed us to collect detailed data, mostly on faults and fractures, together with attitudes of the main Paleozoic foliation and related folds. Data also includes the orientation of the Cenozoic mafic dikes in the area. Statistical analysis of fault azimuthal trends highlighted the dominance of steeply dipping to near vertical, NW-SE trending faults, with subordered NNW-SSE, E-W, and NE-SW ones. Slickenline analysis pointed out the dominance of right-lateral strike-slip to oblique-slip motions. Dip-slip motion characterises steeply dipping, reverse faults in positive flower structures at Black Ridge and along the Boomerang Glacier shoulders. Pseudotachylites have been found in the core of a master fault at Gondwana Station, and along reverse faults at Black Ridge. This may suggest a seismogenetic activity of the Priestley Fault during the Cenozoic. Mafic dikes belonging to the Cenozoic McMurdo Volcanic Group injected in a releasing bend zone in the coastal region, testifying for the role of the Priestley Fault to provide preferred pathways for the ascent of sub-crustal magma.The deformational pattern in the Southern Deep Freeze Range, along the Priestley Fault, can be explained by an overall right-lateral strike-slip kinematics. It well fits the geodynamic framework proposed by Salvini et al. (1997) for the whole Ross Sea Region during the Cenozoic. In this framework, the geodynamic evolution of this region of Antarctica is dominated by right-lateral shear along NW-SE trending intraplate faults, which represent the south-eastward prosecution of transform faults in the Southern Ocean (Balleny, Tasman, Gambier, and Carey fracture zones).
Salvini F., Brancolini G., Busetti M., Storti F., Mazzarini F., Coren F, J. Geophys. Res., 102, 24,669-24,696, (1997).
Cenozoic mafic dikes belonging to the McMurdo Volcanic Group are exposed along the eastern coastal region of Victoria Land, Antarctica. During the XII Italian Antarctic Expedition (1996-1997), detailed field work was carried out in the area from the Priestley Glacier to the Mawson Glacier, southward of the Drigalski Ice Tongue. Fault and fracture data, dike orientation, and details of their intrusive forms were recorded and organised in a georeferenced database. Fault slip data indicate right-lateral strike-slip faulting as the main Cenozoic tectonic regime in the study area. Strike-slip faulting occurred along regionally-sized, NW-SE trending deformation belts, characterized by local transpressional components. In the sectors inbetween NW-SE regional faults, the strike-slip motion was partially transferred to pre-existing and/or newly formed, N-S to NNE-SSW trending extensional and transtensional faults. Dike injection frequently occurred along either NW-SE trending or N-S to NNE-SSW trending faults. In places, mafic dikes are crosscutted by strike-slip faults, while in other exposures strike-slip faults are crosscutted by the dikes. Moreover, at Starr Nunatak, fault segment linkage occurs through magma filled tension gashes whose geometries are compatible with the NNW-SSE right-lateral strike-slip faulting in this sector. The Cenozoic tectonics is thus responsible of both faulting and dike swarm emplacement. Frequency histograms of fault and dike strikes were prepared and automately best fitted with a function sum of gaussian curves. The result was the identification of their preferred orientations and statistical parameters, including mean values and standard deviations. A mechanical model relates the mean azimuthal trend of the dikes, the average regional fault azimuths, and the regional stress field orientation. A clockwise rotation of the principal compressional stress from NW-SE to NNE-SSW occurs moving from the northwest, where strike-slip dominates (Rossetti & Storti, 1998)to the southeast, where E-W transtension acts (Wilson, 1995). Rotation of the regional stress field fits into the Cenozoic geodynamics of the whole Ross Sea Region, where dextral intraplate shear and extensional processes concurrently occur (Salvini et al., 1997).
Rossetti F & Storti F, Terra Antartica Reports, 2, 39-41, (1998).
Salvini F, Brancolini G, Busetti M, Storti F, Mazzarini F, & Coren F, J. Geophys. Res, 102, 24,669-24,696, (1997).
Wilson TJ, Tectonics, 14, 531-545, (1995).
Rifting within West Antarctica, uplift of the Transantarctic Mountains rift flank, and associated magmatism, are the results of intraplate processes associated with progressive rifting within the Antarctic plate. The degree of neotectonic activity within this linked geodynamic system is debated, because of the apparent conflict between the modern aseismicity of the region as a whole and the evidence for Quaternary faulting and volcanism. Morphotectonic analysis of satellite imagery (Landsat TM, SPOT, SAR) is interpreted to reveal the late Cenozoic structural architecture of the Transantarctic Mountains in southern Victoria Land, where the rift-flank is offset across the Discovery accommodation zone. The normal-sense border fault system between the mountain uplift and the adjacent rift basins, known as the Transantarctic Mountains Front, steps progressively eastward from south to north. Sinistral shear along west-northwest transfer faults accommodated changes in extension magnitude along the offset rift-flank. The transverse accommodation zone continues into the offshore rift system, where Neogene and active volcanism has been focused. The oblique orientation of structures with respect to the rift flank trend indicates transtensional deformation along the Transantarctic Mountains Front. These kinematic models, and the magnitude of contemporary crustal motions, are being tested by deployment of the TAMDEF GPS array throughout southern Victoria Land. The design of the array is intended to distinguish between crustal motions due to tectonic activity and due to rebound from glacial unloading. The contemporary stress regime associated with the Transantarctic Mountain Front is being determined from analysis of fractures in core and borehole walls as part of the Cape Roberts Project drilling program in southern Victoria Land. This integrated program of neotectonic studies will document both crustal motions and stresses associated with the current neotectonic regime along one of the key tectonic boundaries within the Antarctic continent.
After having separated from Tierra del Fuego in the Early Tertiary the Antarctic Peninsula shifted southward to its present location along the Shackleton Fracture Zone with an average velocity of 1-2 cm/yr. Recent geodetic GPS experiments demonstrate however a slight shift of the Antarctic Peninsula towards South America, while the Bransfield Strait is opening at a velocity of 1 cm/yr. Subduction roll-back and steepening of the subduction zone as well as low stress deformation below the South Shetland Islands probably cause the scarcity and weakness of the earthquake activity along the subduction zone. A migration of the magmatic arc due to the development of the Bransfield Strait basin is well known from age determinations and sample correlation to arc and backarc settings (Saunders and Tarney, 1991). The aim of this study is to highlight more the role of changes in the source regions in correlation to plate tectonics. We will consider here particularly the case of the Seal Nunataks as an example of the Weddell Sea coast and the basalts of central Livingston Island as an example of the South Shetland Islands. From Pliocene to present the volcanoes of the Seal Nunataks developed from polar marine to subglacial eruptions. Here the low SiO2 and TiO2 contents in combination with high MgO and CaO/Al2O3 ratios are comparable with picritic or komatiitic basalts. Fluid inclusions in olivine from ultramafic nodules show high contents of water, CO2 and CH4 under very high pressure. The tectonic setting can be considered as pull-apart basin like with influence from the down-going slab. The level of the source region changed from spinel-peridotite for the back-arc to garnet-peridotite for the arc. In central Livingston Island the basalts show close relationships to boninitic volcanism, which indicates an early stage in arc volcanism or supra-subduction volcanism after developing of an ensialic back-arc basin in the Bransfield Strait. All central Livingston volcanoes erupted under subglacial conditions.
We thank Dr. Rodolfo del Valle, Instituto Antártico Argentino, and Prof. Dr. Christo Pimpirev, Bulgarian Antarctic Institute, for logistic support, kind field assistance and valuable discussions. This work is funded by the BMBF 03 PL 022 D project.
Saunders AD & Tarney J, In: Floyd P A. (ed.), Oceanic Basalts, 219-263, (1991).
The Seismic Experiment in Patagonia and Antarctica (SEPA) is a deployment of eleven broadband seismographs in the Antarctic Peninsula, South Shetland Islands, and Chilean Patagonia. The goals of the deployment are to investigate earthquakes and upper mantle structure associated with possible subduction along the S. Shetland Trench, extension in the Bransfield Strait, and slow convergence in Drake Passage and Southernmost South America (Barker and Dalziel, 1983; Pelayo and Wiens, 1989). Some of the stations are located at field sites which can only be reached during the Antarctic summer and which must receive all their power from solar panels and banks of batteries. We were pleased to find that most of the stations ran successfully throughout the Antarctic winter. An additional deployment of 14 ocean bottom seismographs is currently underway in cooperation with Leroy Dorman (Scripps). Although the South Shetland Island region displays a low level of seismicity in global catalogs, the data obtained from the first year of the experiment indicates a high level of local seismicity (mb 2-4). The earthquake locations and level of seismicity demonstrate that slow subduction is occurring along the South Shetland Trench. Further studies of this data should allow us to determine the depth and extent of the South Shetland slab subducting beneath the South Shetland Islands. Earthquake activity has also been detected at some of the large underwater volcanoes along the central rift of the Bransfield Strait, suggesting ongoing volcanic activity. Seismic waveform inversion indicates that the upper mantle seismic velocity beneath the Scotia plate is about 2-5% slower than global averages throughout the upper 300 km, consistent with other backarc regions. Preliminary results from shear wave splitting analysis of SKS and SKKS arrivals indicate that the fast polarization direction of the Antarctic Peninsula region is oriented NE-SW, perpendicular to the fast splitting direction in Patagonia (Helffrich et al., 1997). This data therefore does not support a pattern of mantle flow through Drake Passage from the Pacific to the Atlantic, as was proposed by Alvarez (1982).
Barker PF, Dalziel IWD, Geodynamics of the Eastern Pacific Region, Caribbean and Scotia Arcs, 137-170, (1983).
Pelayo AM, Wiens DA, J. Geophys. Res, 94, 7293-7320, (1989).
Helffrich G, Wiens D, Barrientos S, Vera E, EOS Trans. Am. Geophys. Un, 78, 708, (1997).
Alvarez W, J. Geophys. Res, 87, 6697-6710, (1982).
The tectonic structures of the southern part of the Shackleton Fracture Zone have been investigated using seismic reflection measurements. The multichannel seismic profile IT89AW-42 was acquired by the SV OGS Explora during the austral summer 1989/90 and processed by the Exploration Geophysics Group, University of Trieste. It is a 130 km-long line and crosses perpendicularly the fracture zone near the 60th parallel south. The NW-SE trending Shackleton Fracture Zone is one of the most impressive oceanic fractures: it forms the western boundary of the Scotia Plate and connects the Chile trench with the South Shetland trench and South Scotia Ridge. The SFZ is characterised by a topographic high, more than 2500 m above the surrounding sea floor, and by a 40 km wide half-graben located to the east of the relief and bounded by a big synsedimentary listric fault. Sediments more than 1000 m thick of Cenozoic age present at the base of the eastern flank of the relief are folded and deformed. The relative motion of the two adjacent plates, the Antarctic and the Scotia Plates, is mainly transpressive: a set of small positive flower structures clearly identifiable within the well defined sedimentary package indicates a strike-slip regime with a compressional component. Significant deformations affect not only the shallow sedimentary cover but also the deepest structures: in the migrated section it is possible to follow the plane of the listric fault starting from the base of the crust up to the youngsters sediments. A conventional processing permitted to outline the main structures, but only the use of particular techniques, such as the prestack partial migration, provided a good definition both of the acoustic basement and the sedimentary package. In particular a strong horizontal reflection is evident at about 8 s (twt) and it may represent the Moho discontinuity. Interval velocities obtained from DMO corrected CMP gathers were used to depth convert the prestack time-migrated section. The results indicate that the sediment thickness varies between 0.5 km and 1.5 km from the western to the eastern flank of the SFZ respectively. More than 3.5 km of sediments are deposited in the trough as a consequence of the synsedimentary deformation of the half graben.
Geodesy was recognised as an important Antarctic scientific discipline by the Scientific Committee on Antarctic Research (SCAR) when the Working Group on Geodesy and Cartography was established in 1958. However the main task of Geodesy at that time was to support the mapping of the unknown continent. The precise measurement of the motion of the Gowanaland fragments was not possible between continents until Space Geodesy using artificial satellite was applied to Antarctica in the early 1970s.
Twentyfive years later the geodetic measurement technology of contemporary motion between, and within, tectonic plates is now considered to be almost routine. In 1992 The SCAR Geodetic Infrastrucuture for Antarctica (GIANT) program was developed to provide a linkage between rock based isolated geodetic networks across Antarctica as the basis for uniform reporting the results from scientific research. Thistechnology initiated the gradual establishment of continuous GPS base stations in Antarctica and the repeated epoch occupations of scientific sites for measuemnt of surface geodynamics. This has resulted in a basic uniform framework of geodetic positions on rock within the global reference frame being established, and the computation of of precise velocities for these sites for comparision with long term geoghysical models and geological records.
This work will form the backbone of a special study of Antarctic neotectonics approved by the SCAR executive at the XXV meeting in Concepion in July 1998.
This work is based on the processing and interpretation of selected MCS reflection lines obtained by the Italian geophysical expeditions to the Ross Sea from 1988 to 1990. The focus is on the quantitative assessment of the tectonic deformation in the area of the Discovery Graben, Lee Arch and Coulman High with particular reference to the recent (Cenozoic to Present) evolution. The western sector of the Ross Sea shows evidences of the tectonic cycles which deformed the area located between the Transantarctic Mountains to the west and Marie Byrd Land to the east, from Mesozoic to the Present. The seismostratigraphic analysis and the correlation with seismic profiles obtained in different parts of the Ross Sea indicate that the deepest unconformities interpreted in the Discovery Graben most probably mark the top of Late Paleozoic-Early Mesozoic sequences. Such sequences are interpreted at the western margin of the Discovery Graben, where they are locally disrupted by faults which show no evidence of synsedimentary activity in this stratigraphic interval. The listric faults at the eastern margin of the Discovery Graben show that the most prominent tectonic activity is concentrated in three successive cycles which allowed the deposition of a sedimentary wedge with average thickness exceeding 9,000 m at the hanging wall of the fault. The first cycle correlates with evidences of extensional activity observed through the Ross Sea area and probably associated with the Late Jurassic-Lower Cretaceous continental breakup phase. An estimated 10% of the total stretching experienced by the crust of the western sector of the Ross Sea is due to this event. A second extensional phase, associated with a prominent unconformity of regional extension, stretched the western sector of the Ross Sea during Paleogene and reactivated most of the faults generated by the first tectonic cycle. The average stretching effect of this cycle is around 35% of the total deformation of the area. A Neogene to Present extensional phase is responsible for more than 50% of the stretching of the western Ross Sea sector bounded to the north by Cape Washington. The late cycle is associated with evidences of strike-slip deformation. The strike-slip component produces local transpressive features and locally activates or reactivates normal faults. The distribution of transpressive and transtensive features provides indications about local variations in azimuth and bends of the strike-slip lineaments which are correlated with the main tectonic alignments at regional scale. The crustal thickness in the area of study is estimated from direct observation of reflections from the Moho discontinuity at depths ranging between 19.0 and 22.0 km.
In 1997, during the First Ukrainian Antarctic expedition on r/v "E.Krenkel", to the station "Faraday", the geological survey has been made on the Antarctic Peninsula and offshore islands. It is taking 42 samples of the rock from 33 denudations. The present data are based also on the petrological study of make slices. In the region there are only magmatic rocks, which can be divided in there complexes: piroclastic-effusive (Jurassic time, seria Oskar), intrusive (Cretaceous time) and vien-dyke (Neogene?) according to Greakurov (1973). The rocks composition varies from basic to acid. Due to the petrological investigations, more abundant representatives of the effusive complex are quartz andesites, andesitic and dacitic porphirites and andesite-dacites. There are paleotypal rocks, which characterized by full or partial changes of darc-coloured minerals into epidot, calcitite, and chlorite. The ore material is present by pyrite or ilmenite and magnetite. The acid effusive are less abundant that middle composition effusive rocks. Dacites and quartz dacites are predominated varieties, and they are by metasomatic changed light-coloured acid rocks at the slices. The pyroclastic rocks of S. Oskar presented by tuffs and tufflaves. They are abundant on the Argentine Island and on the nearest coast of the Antarctic Peninsula. The intrusive complex is abundant on the Barchans Islands and on the coast of Antarctic Peninsula. The xenolites of the effusive in intrusives coast of Antarctic Peninsula. The xenolittes of the effusives in intrusives proves the active contact, but it can't be observed, owing to the petroligical characteristics, intrusives of middle composition (diorite set) and acid composition (granite set) have been defined. The pegmatitic bodies are often found among intrusives. The most developed are straight-linear pegmatitic viens, their thickness is from the first centimeters up to meters. The pegmatites are characterized by chloritization of primary darc-coloured minerals. The dyke complex is characteristic of the Galindez Island and the Little Barchans Islands. Here there are of basic composition (diabase and diabase-andesites) and the middle compositions (andesite-dacites, andesites, dacites). Their structure is fine-grained and their texture is massive. The most part of dykes has dropped its thickness from a part of meter to 10 meters. The vien complex is abundant on all area. It's present by quartz viens of different size. The quartz viens of the Little Barchan Island are of particular interest, where they have nest intrusion of coarsely crystalline of magnetite, pyrite and other sulfides. The zones of secondary changes are closely connected with tectonic processes. Along the zones of elects the processes of serpentinization, pyritazations, quartzitization and other are developed. The rocks of vien complex, pegmatite zones and the zones of secondary changes are of special interest for the further investigation.
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