Geología de la Antártida Argentina

• Evolución del la Antártida Oriental         Geología virtual de la Antártida

• Geología de la Antártida

Geological Evolution Of The Transantarctic Mountains, Southern Victoria Land, Antarctica.

 History Of Exploration

In late 1839-early 1840 Capt James Clark Ross in charge of two vessels, HMS Terror and HMS Erebus, sailed south from Hobart with the aims of recording magnetic observations at high southern latitudes and, if feasible, of reaching the South Magnetic Pole. Ross sighted land near Cape Adare in Northern Victoria Land (NVL), and observed the high peaks of the Transantarctic Mountains (TAM). He collected geological specimens from offshore islands, and sailed south into the Ross Sea, to the foot of Mt Erebus and the barrier of the Ross Ice Shelf. Whaling expeditions were to follow in subsequent years.
In 1899, a British-sponsored expedition, led by an Australian Carstens Borchgrevink, wintered over in NVL, and next year, on the Southern Cross sailed south to Ross Island, collecting samples of Tertiary volcanic rocks en route.
The major geological advances in SVL, however, were made in later expeditions led by Scott and Shackleton, first in 1901-4 on Discovery, then in 1907-9 on Nimrod, and finally the ill-fated Terra Nova expedition of 1910-13. As you all doubtless remember, it was Scott's insistence on retaining rock specimens collected on his return from the South Pole that may have contributed to his demise. I sympathise with his dilemma, all of the Otago field seasons have involved either the back-packing or man-hauling by sledge of rock specimens, with a constant temptation to offload surplus specimens into the nearest crevasse!

Geological Summary

So what of the geology of the Transantarctic Mountains? The mountain range itself extends across the Antarctic continent for a distance in excess of 3500 km, and comprises peaks over 4000 m high. One of the lasting impressions of SVL is the view from near Ross Island, where Scott Base and McMurdo Base are situated, of the majesty of the Transantarctic Mountains rising steeply from the Ross Ice Shelf. This segment of the TAM was named by Scott's party as the Royal Society Range, in recognition of the financial support for the expedition; many of the peaks carry the names of past presidents of that society.

The Transantarctic Mountains divide Antarctica into two geological provinces. East Antarctica, the larger of the two, is geologically a very stable area, composed of very old rocks, which we refer to as a craton. In SVL, most of the craton is covered by ice of the Polar Plateau, but elsewhere the rocks have been dated as Archean, extending back in time to 2.5 Ga (2,500,000,000 years). In contrast, West Antarctica is composed of a collage of crustal segments, unrelated to the craton, and much younger. Effectively the TAM is a suture, dividing yet uniting the continent.
To answer the question of how the TAM was formed, we need to go back approximately 750 million years into the Precambrian period and then, according to ideas espoused by Moores and Dalziel, the East Antarctic craton used to form the nucleus of a land mass called Rodinia. Their idea, referred to as the SWEAT hypothesis, derives from SouthWestern U.S. East Antarctica, since the proponents believe that the best correlation of rocks from the margin of East Antarctica lie along the margin of the Laurentian craton of North America.

Fig. 2.A Reconstruction of the supercontinent Rodinia at ca 725 Ma (E-W- Ellsworth Whitmore, W.AFR-West African craton, KALAH-Kalahari craton).	Fig. 2B. Reconstruction of Rodinia at the Cambrian-Precambrian boundary, showing subduction of the Paleo-Pacific ocean crust at the Delamerian-Ross convergent margin (S.FR-Sao Francisco craton, SIB-Siberia craton)

Acceptance of mobility of crustal fragments around the surface of the globe, undreamt of in Scott's time, or even in the early years when I trained as a geologist (!), was made possible through the revolutionary Plate Tectonic concept. Rodinia is thought to have fragmented approximately 700 million years ago, North America drifting off to its place on the surface of the Earth, opening up an expanding ocean, the fore-runner of the Pacific Ocean perhaps, in its wake. Evidence for an old oceanic component on the edge of East Antarctica lies in suite of basalts of mid-ocean ridge character, dated at 668 Ma, now incorporated into the central sector of the Transantarctic Mountains.
Subsequently, with successive plate readjustments, the paleo-Pacific Ocean closed, and the rifted margin of East Antarctica became a war zone. Here oceanic crust of the newly-opened Pacific Ocean was destroyed by subduction and any crustal fragments adrift in the ocean were rafted towards the subduction zone to collide with, and be accreted onto, the leading edge of the craton. Plate convergence, with associated subduction and collision, was responsible for uniting East and West Antarctica. The association of geological processes is referred to as the Ross Orogeny. An orogeny is a mountain building process, a time when there is upheaval of the earth's crust, with associated magmatic activity and metamorphism of any sediments that had formed in the ocean basin, or along its margin, and which were deformed and thermally altered on incorporation into the convergent plate margin. This convergence, occurring in latest Precambrian or Cambrian times, approximately 500 million years ago, is not unique to Antarctica. The same aged orogenic rocks occur in New Zealand (as part of Fiordland), and in Australia. The inescapable conclusion from these correlations is that Antarctica, New Zealand and Australia used to be part of the same margin, they were part of a single supercontinent, which is called Gondwanaland. Africa, South America, and India were also our neighbours.

It is these Ross Orogeny rocks that were later to become the so-called basement to the TAM. They are the oldest rocks exposed over much of the mountain range, and they have been uplifted, eroded, subjected to later orogenies, and overlain by later rocks.

Throughout SVL the Ross Orogeny basement rocks have been eroded down over a long period of geological time (perhaps up to 100 million years), to form an almost planar upper surface, on which later rocks have been deposited. This surface, an unconformity, has been termed the Kukri erosion surface or peneplain. It is overlain by a vast (c 2.5 km thick) sequence of horizontally bedded sediments, dominated by quartz-rich alluvial sandstones and conglomerates, but containing Carboniferous glacial deposits and Permian coal measures. Collectively the sequence is referred to as the Beacon Supergroup (colloquially “Beacon Sandstone”), and is interpreted as having been deposited in elongate non-marine or at times shallow marine basins parallel to the present axis of the TAM. In SVL its age ranges from Devonian to Triassic, a period of nearly 200 Ma. Fossil fish and land vertebrates have been found, indicating establishment of fresh-water lakes at various stages of the sedimentary history.
The next event recorded in the geological history of SVL occurs over the length of the TAM and represents the intrusion of copious volumes (estimated between 1.0 and 1.7 x 107 km3) of basaltic magma. This was injected into the basement and into the overlying Beacon Sandstone. The dominant form is of subhorizontal sheets, intruded into the basement, along the peneplain, and into the overlying sediments, where they define bodies called sills. Occasional discordant, steeply dipping intrusions called dykes connect the sill horizons. Individual sills often reach thicknesses of 420 m and can be traced, or extrapolated, over very large areas (>20000 km2). Because of the thermally insulating effect of large magma volumes, the sills cool slowly and form rocks called dolerites, rather than basalts. However, magmas are also injected to the Earth's surface and are erupted as lavas that cool more slowly and form the Kirkpatrick Basalt. Recent work within the Department has shown that some of these high level magmas react explosively with groundwater in the Beacon Sandstone and form extensive phreatocauldera. Basalts and dolerites are collectively referred to as the Ferrar Supergroup, named after H.T.Ferrar, geologist on Scott's Discovery expedition. Recent work has suggested that Ferrar magmas have such similar compositions that they must have been generated in a single, point-source, partial melting event in the mantle of the Weddell Sea area, and subsequently injected and magmatically transported up to 3000 kilometres along the length of the TAM.
Irrespective of whether the magmas are indeed far-travelled, it is clear that Ferrar magmatism marks a very important event in terms of its distribution. Igneous rocks have been isotopically dated, and as analytical accuracy and precision have improved, it is evident that the copious outpourings have occurred over a very short time frame in the Jurassic (177-183 Ma). What is more, similar magmatism occurs elsewhere in Gondwanaland, in Africa (the Karoo basalts), in Tasmania and New Zealand, and in the Falklands. Such Large Igneous Provinces (LIPs) are a characteristic consequence of, or precursor to, continental fragmentation.
In many theories, the impact of a deep mantle-generated plume or superplume onto the lithosphere is instrumental in the break-up of a supercontinent such as Gondwanaland. Plumes are considered to be a diapiric uprise, perhaps from the core-mantle boundary, of a vertical column of plastic rock. When the plume intersects the base of the lithosphere its head flattens out, dragging the lithosphere with it, and initiating plate spreading. If a superplume was responsible for the fragmentation of Gondwanaland, it is argued that the likely impact site would be the area of the present Weddell Sea.
New plate boundaries were established during the dispersal of Gondwanan fragments, but until recently there has been considerable debate about whether a boundary still existed through the Antarctic continent. Through Deep Sea Drilling projects, through marine geophysical investigations, and through paleomagnetic observations we now know that the Ross Sea Embayment contains extensive sedimentary basins that have accumulated considerable thicknesses (up to 7 km) of Tertiary sediments. East and West Antarctica have also undergone at least 300 km of displacement and perhaps 40° to 90° of rotation in the last 100 Ma. Recently it was established that much of this separation occurred in the Eocene and Oligocene, related to spreading in the Adare Trough at the mouth of the Ross Sea Embayment.

Fig.4. The Ross Sea area showing the Transantarctic Mountains, the offshore sedimentary basins of the Ross Embayment, and location of Eocene-Oligocene spreading in the Adare trough.

Fission track dating had established many years earlier that uplift of the TAM started approximately 55 Ma ago. The documented extension and subsidence of the crust in the Ross Embayment is compatible with complementary uplift of the Transantarctic Mountains on the western shoulder of this rift system. As such, uplift of the TAM occurs by normal faulting, in marked contrast to the collisional events at convergent margins typical of other mountain belts around the world (e.g. Himalayas, European and New Zealand Alps etc).
Extensional Tertiary sedimentary basins received the erosional debris from the rising TAM, and the extension is also manifest in one other concurrent geological phenomenon, namely magmatism. Volcanic rocks were erupted in NVL from 48 Ma (Eocene) and in SVL from 24 Ma (Oligocene) to the present day. Mt Erebus, formed as a consequence of this magmatism, is the most southerly active volcano on Earth.
Climatic deterioration occurred in the late Tertiary, but fine details of the glacial chronology and the extent of the glaciations are still uncertain. Resolution of these issues is the subject of multi-national drilling incentives (e.g. ANDRILL) at the present day. The geological history of SVL is summarised in the attached stratigraphic column.

Geology Of Southern Victoria Land

The Otago programme investigating the basement terrain formed during the Ross Orogeny has had to deal with a variety of mainly plutonic igneous rocks intruding into and inducing contact metamorphism of a metasedimentary sequence of uncertain age.

Further south, sediments include fossiliferous, archeocyathid-bearing limestones, enabling a biostratigraphic age of Cambrian to be assigned. In SVL the grade of metamorphism of calcareous and calc-silicate sediments is reasonably high, and any fossils have been destroyed by metamorphic recrystallisation. Ages of the parent rock type (protolith) are therefore unknown.
Intrusive igneous rocks had been mapped and classified previously on the basis of either the presence or absence of a foliation (a planar mineral fabric such that a rock splits into parallel-sided slabs) or their colour (pink, grey, etc.). Those granites where the individual grains have a parallel alignment to form a foliation, were inferred to have been recrystallised by a metamorphic event subsequent to intrusion and were designated as pre-tectonic. Those that were non-foliated were assumed to have been unaffected by metamorphism, and hence were post-tectonic. Unfortunately this scheme doesn't work, as our first batch of post-graduate researchers quickly found out. The policy was adopted instead, of mapping the basement igneous rocks as separate intrusions (or plutons). The practice was labour intensive in the field, but using field relationships it gave rise to an unambiguous relative age for each pluton. The presence of fragments of one type of granite enclosed within another clearly indicated that the fragments belonged to an earlier pluton that had been broken off and entrained into a later intrusion. Similarly, an igneous intrusion such as a dyke, emplaced across another igneous body is clearly later.
Geochemical analysis back in the laboratory resulted in an additional chemical classification of the granite. These first studies, conducted on granites in the Dry Valleys area, were instrumental in defining different igneous suites, whose chemical characteristics were broadly compatible with formation in a convergent, plate-margin environment, above the previously inferred subduction zone where the paleo-Pacific Ocean crust was consumed beneath the East Antarctic craton.

As the programme evolved, it became clear that in order to make regional correlations and to constrain the duration of the Ross Orogeny we needed absolute ages on specific plutons in order to define the timing of intrusion of the various suites. Given that K-Ar dating gives uplift rather than intrusive ages, and is susceptible to resetting by subsequent thermal events, we opted to try U-Pb dating instead, which is potentially a more robust chemical system.

U is a radioactive element, with ²³⁸U decaying with time to ²⁰⁶Pb, and ²³⁵U decaying to ²⁰⁸Pb. U is an element that is accommodated into the crystal structure of the mineral zircon, ZrSiO₄. Zr, in turn, is an element that occurs in some abundance in the continental crust, so is concentrated in granitic magmas formed by melting of crustal materials. Zircon is a highly refractory, physically resistant mineral, which, once formed, is difficult to destroy and it requires exceedingly high temperatures to recrystallise or modify the stored age signature within a chemically zoned grain. Zircon is separated from crushed granites using density and magnetic properties, and suitable grains are then chemically treated to isolate the appropriate elemental concentrates. Isotope ratios are determined by Thermal Ionisation Mass Spectrometry, and an age calculated. All of this U-Pb geochronology has been undertaken in collaboration with scientists Drs Dave Parkinson and Nick Walker who have access to overseas facilities at UCSB and Brown University respectively.
We have also experimented with microbeam techniques for age determination, whereby individual grains, or zones in grains are bombarded by either a laser beam, or a beam of oxygen ions which vaporises a small area of the mineral. Products of the vaporisation are fed directly into a mass spectrometer, where isotope ratios are determined. The two techniques, available at the Australian National University are referred to as ELA-ICP-MS (excimer laser ablation induced coupled plasma mass spectrometry) and SHRIMP (sensitive high resolution ion microprobe). Both techniques are capable of analysing separately a relict core of a zircon grain, which might be inherited from a previous cycle of geological events, and the latest growth around the crystal rim, which has formed by igneous crystallisation during residence in a granitic magma chamber.
Results of the dating give intrusive igneous ages ranging from 551 Ma to 488 Ma, with some zircons being recycled during successive melting and intrusive events. The Ross Orogeny, therefore, occurred over an interval of time of at least 65 million years, equivalent to the time since the dinosaurs disappeared on Earth at the end of the Cretaceous. Since many of the granites intrude previously deformed and metamorphosed country rock materials, this figure has to be considered as a minimum estimate for the duration of the orogeny.
The timing of metamorphism is difficult to determine directly, but it has recently been constrained by microbeam (LAP-ICP-MS and SHRIMP) study of the ages of detrital zircons in the metasediments. Zircon retains its early pre-metamorphic history despite the enclosing rock recrystallising totally to a new fabric. Sedimentary rocks accumulate detrital zircon that is washed into the basin of sedimentation from the surrounding river catchment. These zircons will be of varying ages, but clearly none can be younger than the age of sedimentation. The minimum age of detrital zircon will give a minimum age for the age of sedimentation and therefore a maximum age for the subsequent metamorphism. So far we have only determined zircon age spectra from two metasediments from SVL, and have minimum ages of detrital zircons of c. 630 Ma. These sediments are likely, therefore, to be late Precambrian in age, with subsequent metamorphism during the early Ross Orogeny, prior to the magmatic climax marked by granitoid intrusion. We are hoping to expand the detrital zircon technique to look at the age and regional provenance of other sediments metamorphosed during the Ross Orogeny. This analysis will also enable us to put a maximum estimate on the inception of the Ross Orogeny.

Abstract.  We present new paleomagnetic and isotopic data from the Transantarctic Mountains in East Antarctica that constrain the paleogeographic position of this region during the late Early and Middle Cambrian. Two new poles have been determined from volcanic and granitic rocks in the Scott Glacier area. The first pole is from the Wyatt and Ackerman formations (~525 Ma) and the Mt. Paine tonalite (40°E, 1°N, A95=6°, N=11 VGP’s).  The second pole is from the Zanuck granite (36°E, 7°S, A95= 9°, N=9 VGP’s).  These poles differ from the Gondwana Early Paleozoic reference poles and could indicate that the Scott Glacier area (and by geologic correlation, North Victoria Land, eastern Australia and West Antarctica?) was part of a terrane accreted to East Gondwana in the Cambrian.  Another possibility is that these new poles support rapid apparent polar wander in the late Early Cambrian and Middle Cambrian.  This apparent polar wander event could be related to rapid plate motions or to a true polar wander event.  Lastly, there may have been a local vertical axis rotation of the Scott Glacier area if the Wyatt pole is compared with the African Ntonya pole.  (Tectonics, 2000, v. 19, pp.168-181)

New geologic constraints on basement rocks from the Shackleton Glacier region

Anne Grunow, John Encarnacion, and Tim Paulsen, Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210   A.J. Rowell, Museum of Invertebrate Paleontology and Department of Geology, University of Kansas, Lawrence, Kansas 66045

During austral summer 1995-1996, Grunow, Encarnacion, and Paulsen, plus Mike Roberts, were put-in by LC-130 on 20 November to a field camp just north of Cape Surprise. The objective of our field programs was to collect paleomagnetic, geochronologic, paleontologic, and structural samples from basement granitoids, sedimentary, and volcanic rocks to improve understanding of the Early Paleozoic tectonic evolution of the Transantarctic Mountains. We established a Ski-doo route between Cape Surprise and the Bravo Hills for our second base camp in early December (figure 1). We encountered many large sastrugi and crevasses in the Gabbro and Bravo Hills areas making Ski-doo travel quite slow. From 9 December, our fieldwork was done by helicopter from the MacGregor camp where Bert Rowell joined us for the remainder of our season. The weather was excellent until 10 December whereafter, on most days, cloud cover obscured many of the basement exposures between Lubbock Ridge and the Ross Ice Shelf. The localities visited by Ski-doo, Twin Otter, or helicopter are shown on figure 1.

Prior knowledge of the age of basement rocks in the Shackleton Glacier area did not allow good geologic correlation with events elsewhere in Antarctica. In the field area, a thick succession of silicic volcaniclastic rocks, lava flows, and some limestones forms the Taylor Formation. It is widely correlated with the Fairweather Formation of Liv Glacier region to the east. The Henson marble forms the Fairweather Formation's upper member and was commonly regarded as the equivalent of the limestones in the Taylor Formation (Wade 1974). On lithological grounds, Wade (1974) correlated the Henson Marble with the Lower Cambrian Shackleton Limestone. One of our samples from the Henson Marble at Mount Fairweather contains what appears to be recrystallized solitary, cone-shaped, double-walled archaeocyath .

Well-preserved trilobites from the upper part of the succession of the Taylor Formation at Taylor Nunatak reveal that the limestones containing them are late Middle Cambrian and much younger than the Henson Marble. Seemingly, the Taylor Formation is not correlative with the Fairweather Formation. The trilobites include Amphoton sp. cf. A. oatesi Palmer and Gatehouse (1972)  and Nelsonia sp., cf. N. schesis Palmer and Gatehouse, which can be tied to Middle Cambrian successions. Nelsonia is endemic to Antarctica, but N. schesis has been reported from northern Victoria Land (Cooper and Shergold 1991, pp. 20-62), where it occurs with cosmopolitan late Middle Cambrian trilobites. This age is compatible with a 515±6-million-year uranium-lead zircon date from Taylor Formation metarhyolites on Lubbock Ridge (Van Schmus et al. in press) and suggests that the enigmatic Cloudina? tubes from Taylor Nunatak (see Stump 1995) may have limited stratigraphic value.

Structurally, the Lower Cambrian? Fairweather Formation is tightly folded, foliated, and metamorphosed to greenschist/lower amphibolite facies, whereas the late Middle Cambrian Taylor Formation is relatively unmetamorphosed and largely only tilted with no penetrative deformation. Several north-south trending subvertical shear zones, including mylonites, cut probable correlatives of the Taylor Formation along the Shackleton Glacier. These shear zones have downdip stretching lineations and may be associated with tilting of the Taylor Formation. We believe that the structural differences between the Taylor and Fairweather formations reflect structural level such that both formations were deformed during a single event. It is possible, however, that a late Early to early Middle Cambrian deformation event may have caused tight folding of the Fairweather Formation, and a second deformation event in post-late Middle Cambrian time resulted in ductile shearing and tilting of the Taylor Formation. At O'Brien Peak, a granite that intrudes deformed marbles and clastics has an S-C fabric indicating sinistral shear parallel to the mountain front. High-grade metamorphic rocks were observed at the small Sage Nunatak, Bravo Hills, Mount Woodall, and Fallone Nunataks. Most of the granitoids between the Shackleton and Liv Glaciers are undeformed except at their margins. Approximately 500 paleomagnetic drill cores and approximately 40 samples for isotopic dating were collected at the locations shown on figure 1.

We thank Mike Roberts for his excellent mountaineering assistance and A.R. Palmer for confirming the trilobite identifications. This work was supported by National Science Foundation grant OPP 93-17673 to Grunow; paleontological analyses were supported from grant OPP 91-17444 to Rowell.

References

• Cooper, R.A., and J.H. Shergold. 1991. Palaeozoic invertebrates of Antarctica. In R.J. Tingey (Ed.), The geology of Antarctica. Oxford: Blackwell.
• Palmer, A.R., and C.G. Gatehouse. 1972. Early and Middle Cambrian trilobites from Antarctica (U.S. Geological Survey, Professional Paper 456-D). Washington, D.C.: U.S. Government Printing Office.
• Stump, E. 1995. The Ross Orogen of the Transantarctic Mountains. Cambridge: Cambridge University Press.
• Van Schmus, W.R., L.W. McKenna, D.A. Gonzales, A.H. Fetter, and A.J. Rowell. In press. U-Pb geochronology of parts of the Pensacola, Thiel, and Queen Maud Mountains, Antarctica. Proceedings volume VII ISARS, Siena, Italy.
• Wade, F.A. 1974. Geological surveys of Marie Byrd Land and the central Queen Maud Range. Antarctic Journal of the U.S., 9(5), 241-242.

The Geology of Antarctica

The geology of Antarctica is similar in many respects to that of the other southern continents that once formed the larger continent of Gondwana. Because of this former union with other land masses, mineral resources in Antarctica are probable, but none of any significance have been found, perhaps because of the widespread cover (97%) of ice over the continent. Offshore oil and gas are presently unknown, but would seem to hold the best prospect for any development in the near future, but perhaps not until questions of sovereignty and ownership of potential resources are resolved.

   The geology of Antarctica has become known in detail only within about the last 25 years, largely as a result of the multinational research efforts that started with the International Geophysical Year in 1957-1958. Although the continent was discovered in 1820, very little was known of its geology until about the turn of the century. James Eights, an American on a United States expedition in 1829-1831 to the South Shetland Islands (Fig. 1), described the first fossil (carbonised wood) recorded from the Antarctic and made remarkably accurate observations on the geology of the islands, even though he was trained as a physician. Some early expeditions collected continental rocks from icebergs, but the first geologic specimens obtained directly from the continent were collected at Cape Adare in the 1898-1900 expedition of C.E. Borchgrevink. The first report of mineral resources from Antarctica came from the Shackleton expedition of 1907-1909, when Frank Wild discovered coal measures at Mount Buckley near the head of the Beardmore Glacier. Perhaps the hardest-won geologic specimens were the 16kg of rocks that were collected by R.F. Scott and his companions during their fatal return from the South Pole in 1911-1912. All five of Scott's field party died, but the rocks, which they had manhauled with them until their final camp, were recovered intact when the bodies were discovered the next season.
    The general geologic framework of Antarctica thus became known slowly over a period of about half a century. Even though the geology of the interior of Antarctica was essentially unknown at the time, in 1937 the South African geologist Alexander Du Toit published a book, Our Wandering Continents, which sets forth in detail the geologic evidence then available for continental drift and for the existence of Gondwana, a protocontinent of the southern hemisphere. Du Toit's Gondwana reassembly, which included Antarctica, predicted the geologic patterns to be expected in the continent's interior.
    A contemporary view of this reassembly is shown in Fig. 2. The implications of this major southern supercontinent of Gondwana with regard to potential mineral resources are discussed later.

Geology

    Antarctica can be divided into two major geologic provinces. The larger part of the continent (East Antarctica) lies south of mainly the Atlantic and Indian Oceans and is nearly all in east longitude. This geologic province consists of a typical continental Precambrian shield, similar to shield areas of the other segments of Gondwana. West Antarctica appears to be small, detached continental plates which would appear as island archipelagoes if the ice sheet were removed. Ice sheet thicknesses in East Antarctica are as much as 4500m, with rock exposures found mainly around the coastline. Most of East Antarctica's bedrock would be above sea level, after allowing for isostatic rise, or crustal rebound, following removal of the weight of the ice sheet. Less than about 3% of the continent is icefree (Fig. 1), which is one of the reasons that detailed geologic studies are made difficult. Antarctica's current glacial history began perhaps in Miocene time, or about 25 million years ago or more.
    An interesting feature of the ice sheet is that a storehouse for meteorites. As Fig. 3 shows, meteorites have rained at different places on Antarctica's vast expanse of ice and at different times for many thousands of years. They are frozen in by subsequent snow accumulation and carried seaward by the ice at a rate of 1-10m a year. Many of the meteorites thus reach the edge of the continent and vanish out to sea in icebergs, melting out later and falling to the sea bottom. In some cases, however, the horizontal flow of ice is stopped by a mountain barrier, and the stagnating ice will push upward against it, depositing its cargo of meteorites on the surface at the end of an eternal conveyor belt of ice. Knowledge of this mechanism of entrapment, transport and resurfacing of meteorites dates back to about 1973 when Japanese scientists discovered an unusual concentration of meteorites on the surface of the ice sheet. Since that time, more meteorites have been found in Antarctica than have been found in previous occurrences in all other parts of the world.
    Rocks of East Antarctica are as old as 3.8-3.5 billion years, as found in an Archaean crators block in Enderby Land, and possibly as old as 4.0 billion years. The East Antarctic shield rocks include older igneous and metamorphic rocks, overlain by younger, flat-lying stratified sedimentary rocks. By contrast, West Antarctica is composed of generally younger rocks that are widely deformed and metamorphosed. Current volcanic activity can be seen in the Antarctic Peninsula and islands of the Scotia Arc, and also along the boundary of East Antarctica in the Ross Sea area.
    A striking feature of East Antarctica is the 3000 km long Transantarctic Mountains, a linear chain of outcrops of mostly Proterozoic and Palaeozoic rocks overlain unconformably by rocks of the Beacon Supergroup, a sequence of mostly flat-lying, middle Palaeozoic and lower Mesozoic rocks. Beacon rocks contain evidence of Gondwana strata similar to that of the other southern continents. Some of this common evidence includes (1) a much older glacial history than at present, in late Palaeozoic time (Permian), and of continental dimensions; (2) overlying coal beds of younger Permian age; (3) fossil material, both in Palaeozoic and Mesozoic strata, of plants (Permian-Carboniferous Glossopteris and related plants associated with the coal beds) and animals (Triassic reptiles and amphibia that moved along land routes in Gondwana); and (4) Jurassic igneous rocks (Ferrar Dolerite) which intruded Beacon rocks as sills and dikes over much of the Transantarctic Mountains Much more recently in geologic time and in the breakup history of Gondwana, the first discovery of a fossil land mammal (a marsupial) was made in 1982 in late Eocene rocks (about 40 million years old) near the tip of the Antarctic Peninsula, thus adding more evidence for a  land connection between this part of Antarctica and South America.
    Breakup of Gondwana into the present continents of the southern hemisphere began in about Late Triassic or Early Jurassic time by separation of crustal plates, and was characterized by rifting and emplacement of Ferrar Dolerite in the Transantarctic Mountains. Breakup of the various components of Gondwana continued, however, for many millions of years, culminating in the separation of Australia and Antarctica during Eocene time, and South America from West Antarctica in late Oligocene time. The crustal plates responsible for movement and relocation of the Gondwana segments are presumably still active, although motion is exceedingly slow and possibly cannot be measured within the short time span of humans on the Earth.

Initial results of geologic investigations in the Shackleton Range and southern Coats Land nunataks, Antarctica

FREDERICK E. HUTSON, MARK A. HELPER, IAN W.D. DALZIEL, and STEPHEN W. GRIMES, Department of Geological Sciences and Institute for Geophysics, University of Texas, Austin, Texas 78712

We present here initial results of geologic investigations conducted during the 1993-1994 field season in the Shackleton Range and the southern Coats Land nunataks (Dalziel et al. 1994). The major goal of this study is to test the "SWEAT" (Southwest U.S.-East Antarctica) hypothesis, which proposes that Laurentia and East Antarctica-Australia were juxtaposed in the Proterozoic and formed part of the supercontinent, Rodinia (Dalziel 1991; Moores 1991). The SWEAT hypothesis suggests that the approximately 1.0-billion-year-old rocks of the southern Coats Land nunataks are a continuation of the 1.0- to 1.3-billion-year-old Grenville Province of North America and that approximately 1.6- to 1.8-billion-year-old rocks of the Yavapi/Mazatzal Province in the southwestern U.S. are correlative with broadly similar-age rocks in the Shackleton Range. We are examining the hypothesis by

• comparing the igneous rocks of the southern Coats Land nunataks and basement rocks of the Shackleton Range with their proposed equivalents in the southwestern U.S.;
• attempting to correlate the late Neoproterozoic Watts Needle Formation, which is exposed in the southern Shackleton Range, with similar-age sequences in Australia and western North America;
• determining paleomagnetically the position of the east antarctic craton relative to Laurentia between approximately 1.0 and 0.7 billion years ago.

Southern Coats Land nunataks

The Bertrab, Littlewood, and Moltke nunataks are exposed along the southeastern Weddell Sea coast and are herein collectively referred to as the southern Coats Land nunataks. We mapped and sampled the Bertrab and Littlewood nunataks but were unable to visit Moltke Nunatak, which is exposed in an ice-fall. Marsh and Thomson (1984) discuss the confusion over the exact location of the Bertrab Nunataks. Using air photographs and satellite data, these authors determined the position of the largest nunatak of the group as 7753'S 3438'W. We confirmed this position using a hand-held global positioning system device, which was also used to locate and map the other nunataks of the Bertrab and Littlewood Groups.

The Bertrab Nunataks are composed of red-to-gray weathering, fine- to medium-grained, oligoclase-phyric, isotropic granophyre, which is cut by flow-banded rhyolite dikes and altered, mafic dikes (figure 2 C ) (Toubes Spinelli 1983; Marsh and Thomson 1984; Gose et al. 1997). The five small outcrops of the Littlewood Nunataks (figure 2 D ) are composed of red-weathering, densely silicified rhyolite (Aughenbaugh, Lounsbury, and Behrendt 1965). Storey, Pankhurst, and Johnson (1994) report a whole-rock rubidium-strontium (Rb-Sr) age of 1,076±7 million years for the Bertrab granophyre and a recalculated whole-rock Rb-Sr age of 976±35 million years for a mixture of samples from Bertrab and Littlewood nunataks. Aughenbaugh et al. (1965) report a whole-rock potassium-argon (K-Ar) age of 840±30 million years for rhyolite at the largest outcrop of the Littlewood Nunataks.

Uranium-lead (U-Pb) isotopic analyses of two fractions of zircon from the Littlewood rhyolite and two fractions of titanite from the Bertrab granophyre yield concordant U-Pb ages of 1,112±4 million years and 1,106±3 million years, respectively (Gose et al. 1997). The ages represent a crystallization age for the rhyolite and a cooling age for the granophyre. These ages support earlier suggestions of a cogenetic origin for the granophyre and rhyolite and indicate cooling of the granophyre below the magnetite Curie Point (580C) by approximately 1.1 billion years ago.

Eighty-four oriented samples were collected from six sites (four in the granophyre and two in rhyolite dikes) at the Bertrab Nunataks and three sites in the rhyolite at the Littlewood Nunataks (figures 2 C and D ). Rock magnetic and petrologic studies indicate that magnetite is the dominant carrier of magnetic remanence in the Bertrab granophyre and hematite is the carrier for the Littlewood rhyolite. Site means of the Bertrab and Littlewood samples are indistinguishable and yield a mean pole position of 23.9S 258.5E with an error of a95=4.00 (Gose et al. 1997). The remanent magnetization is interpreted as a primary thermal remanent magnetization. This interpretation is supported by a lack of evidence for later thermal resetting (Aughenbaugh et al. 1965; Marsh and Thomson 1984; Gose et al. 1997), as well as a broad similarity of the Coats Land pole position with paleopoles obtained from approximately 1.0-billion-year-old rocks in Queen Maud Land (Hodgkinson 1989; Peters 1989)  and dissimilarity to poles obtained from younger rocks in Antarctica (cf. DiVenere, Kent, and Dalziel 1995; Grunow 1995).

After rotation of the east antarctic craton about an Euler pole consistent with the SWEAT reconstruction, our new Coats Land pole falls directly on the Laurentian apparent polar wander path (APWP), lending support to the Rodinian reconstruction of Dalziel (1991) (figure 3). Our approximately 1,100-million-year-old Coats Land pole, however, overlaps poles that define the 1,000-million-year-old segment of the Laurentian APWP. Uncertainties in the age of magnetization acquisition for both the poles of the Laurentian APWP and the Coats Land pole may account for this discrepancy.

Shackleton Range

The Shackleton Range is composed of Paleo- to Mesoproterozoic basement gneisses and granitoids overlain by upper Neoproterozoic and lower Paleozoic supracrustal rocks (Marsh 1983; Pankhurst, Marsh, and Clarkson 1983). Concurrent studies of the basement and supracrustal rocks are underway with the aim of comparing the tectonic history of the range with equivalent age rocks in the southwestern United States. Our initial efforts have focused on isotopic and structural studies of basement rocks and a paleomagnetic study of the overlying Neoproterozoic clastic and carbonate rocks of the Watts Needle Formation of the Read Mountains in the southern Shackleton Range.

In the central Read Mountains, the basement comprises middle amphibolite to granulite-grade gneisses, amphibolites, and migmatites intruded by variably foliated to unfoliated granitoids (Read Group; Olesch et al. in press). Foliated but nonmylonitic migmatites and relict granulites occur north of an east-west striking, south-dipping zone of intense mylonitization, the Read Mountain Mylonite Zone (RMMZ) (Helper, Grimes, and Dalziel 1995), that transects the central part of the range. Grain size reduction textures in quartz and feldspar within mylonites of a variety of lithologies are consistent with shearing at amphibolite facies conditions. Subparallel zones of phyllonite and lower temperature mylonite within the southern portion of the RMMZ indicate renewed or continued motion at greenschist facies conditions. Both fabrics are cut by subhorizontal to moderately north-dipping, brittle shears and faults. Maximum ages of mylonitization and dynamic metamorphism are constrained by new U-Pb zircon ages of approximately 1,790 million years and approximately 1,785 million years (Helper unpublished data) for a slightly discordant, dioritic layer of mylonitic orthogneiss and a concordant deformed tonalite dike, respectively. These ages are interpreted as crystallization ages of the igneous precursors. The tonalite dike is subparallel to the mylonitic foliation and is boudinaged but not internally foliated, possibly indicating late-kinematic emplacement. Further U-Pb dating of cross-cutting dikes and granitoids, as well as high-grade orthogneisses, is presently underway to constrain the minimum age of ductile deformation and to directly date the metamorphism.

The Watts Needle Formation is composed of a lower clastic and upper carbonate unit that rests nonconformably on Mesoproterozoic granitoids (Marsh 1983). A Vendian age has been assigned on the basis of acritarchs, stromatolites, and a whole-rock Rb-Sr model age of 720 million years (Golovanov et al. 1979; Pankhurst et al. 1983; Weber 1991). A detailed study of this unit may enable us to correlate it with other well-studied Vendian units worldwide (cf. Kirschvink et al. 1991).

We collected oriented samples from both the granitic basement (31 samples) and overlying Watts Needle Formation (157 samples) at Mount Wegener and Nicol Crags. Samples were drilled at approximately 1.0-meter intervals and 10 or more cores were collected at selected stratigraphic horizons.

Paleomagnetic results from basal red siltstones and sandstones of the Watts Needle Formation at Mount Wegener yield a preliminary mean pole position at 18.5S 44.3E with an a95=7.50 (Hutson, Gose, and Dalziel 1995). A quartz arenite layer that underlies the upper carbonate section at Mount Wegener yields a preliminary mean pole position at 4.3S 56.4E with an a95=11.10 (Hutson et al. 1995). A well-defined component of primary remanent magnetization for these units was not reset during later tectonic events (e.g., Ross Orogeny). Evidence for this interpretation includes the following:

• both normal and reversed polarities in samples from the quartz arenite unit and
• our pole positions, which are clearly different from published Early Paleozoic pole positions for the antarctic craton (cf. Grunow 1995).

Paleopoles from the Watts Needle Formation fall close to North American paleopoles of similar age after rotation of East Antarctica into a position adjacent to western North America, as suggested by the SWEAT hypothesis. The paleomagnetic data from the Watts Needle Formation support the juxtaposition of the Laurentian and east antarctic cratons at approximately 750 million years ago.

Paleomagnetic studies of basement rocks of the Read Mountains and the lower Paleozoic Blaiklock Glacier Group are underway. Initial results from a conglomerate test in the Blaiklock Glacier Group suggest that a primary magnetization component may be recovered from these clastic rocks.

This research is supported by National Science Foundation grant OPP 91-17996. We thank J. Connelly and Kathy Manser for assistance and technical support with U-Pb isotopic work.

References

• Aughenbaugh, N.B., R.W. Lounsbury, and J.C. Behrendt. 1965. The Littlewood Nunataks, Antarctica. Journal of Geology , 73(6), 889-894.

• Dalziel, I.W.D. 1991. Pacific margins of Laurentia and East Antarctica/Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent. Geology , 19(6), 598-601.

• Dalziel, I.W.D. 1992. Antarctica: A tale of two supercontinents? Annual Review of Earth and Planetary Sciences , 20, 501-526.

• Dalziel, I.W.D., M.A. Helper, F.E. Hutson, and S.W. Grimes. 1994. Geologic investigations in the Shackleton Range and Coats Land nunataks, Antarctica. Antarctic Journal of the U.S. , 29(5), 4-6.

• DiVenere, V., D.V. Kent, and I.W.D. Dalziel. 1995. Early Cretaceous paleomagnetic results from Marie Byrd Land, West Antarctica: Implications for the Weddellia collage of crustal blocks. Journal of Geophysical Research , 100(B5), 8133-8151.

• Golovanov, N.P., V.E. Mil'shteyn, V.M. Mikhaylov, and O.G. Shulyatin. 1979. Stromatoliths and microphytoliths of the Shackleton Range (western Antarctica). Doklady Akademii Nauk , SSSR. 249(4), 977-979. [In Russian]

• Gose, W.A., I.W.D. Dalziel, M.A. Helper, F.E. Hutson, and J.N. Connelly. 1997. Paleomagnetic data and U-Pb isotopic ages from Coats Land, Antarctica: A test of the Laurentian-East Antarctic ("SWEAT") connection. Journal of Geophysical Research , 102(B4), 7887-7902.

• Grunow, A.M. 1995. Implications for Gondwana of new Ordovician paleomagnetic data from igneous rocks in southern Victoria Land, East Antarctica. Journal of Geophysical Research , 100(B7), 12589-12603.

• Helper, M.A., S.W. Grimes, and I.W.D. Dalziel. 1995. Basement-cover relations and fabrics of the central Read Mountains, Shackleton Range, Antarctica. Seventh International Symposium on Antarctic Earth Sciences, Siena, Italy. [Abstract]

• Hodgkinson, G.R. 1989. Palaeomagnetic studies in western Dronning Maud Land, Antarctica. (Unpublished Masters of Science thesis, Department of Geophysics, University of Witwatersrand, Republic of South Africa.)

• Hutson, F.E., W.A. Gose, and I.W.D. Dalziel. 1995. Paleomagnetic results from the Neoproterozoic Watts Needle Formation, Shackleton Range, Antarctica. Seventh International Symposium on Antarctic Earth Sciences, Siena, Italy. [Abstract]

• Kirschvink, J.L., M. Magaritz, R.L. Ripperdan, A.Yu. Zhuravlev, and A.Yu. Rozanov. 1991. The Precambrian/Cambrian boundary: Magnetostratigraphy and carbon isotopes resolve correlation problems between Siberia, Morocco, and South China. GSA Today , 1(4), 69-71, 87, 91.

• Marsh, P.D. 1983. The Late Precambrian and Early Paleozoic history of the Shackleton Range, Coats Land. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science . Canberra: Australian Academy of Science.

• Marsh, P.D., and J.W. Thomson. 1984. Location and geology of nunataks in north-western Coats Land. British Antarctic Survey Bulletin , 65, 33-39.

• Moores, E.M. 1991. The Southwest U.S.-East Antarctica (SWEAT) connection: A hypothesis. Geology , 19(5), 425-428.

• Moyes, A.B., J.M. Barton, Jr., and P.B. Groenewald. 1993. Late Proterozoic to Early Paleozoic tectonism in Dronning Maud Land, Antarctica: Supercontinental fragmentation and amalgamation. Journal of the Geological Society London , 150, 833-842.

• Olesch, M., H.M. Braun, E.N. Kamenev, G.I. Kamenev, and W. Schubert. In press. Read Group. In J.W. Thomson (Ed.), British Antarctic Survey Geomap 4 .

• Pankhurst, R.J., P.D. Marsh, and P.D. Clarkson. 1983. A geochronological investigation of the Shackleton Range. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science . Canberra: Australian Academy of Science.

• Peters, M. 1989. Igneous rocks in western and central Neuschwabenland, Vestfjella and Ahlmannryggen, Antarctica: Petrography, geochemistry, geochronology, paleomagnetism, geotectonic implications. Berichte zur Polarforschung (Vol. 61). Bremerhaven, Germany: Alfred-Wegener-Institute for Polar and Marine Research.

• Storey, B.C., R.J. Pankhurst, and A.C. Johnson. 1994. The Grenville Province within Antarctica: A test of the SWEAT hypothesis. Journal of the Geological Society London , 151, 1-4.

• Toubes Spinelli, R.O. 1983. Geology of the Bertrab Nunatak, Argentinian sector of Antarctica. Contribucion Instituto Antarctico Argentino , 296, 1-9. [In Spanish]

• Weber, K. 1991. Microfossils in Proterozoic sediments from the Southern Shackleton Range, Antarctica: A preliminary report. Zeitschrift für Geologie Wissenschaft , 19(2), 185-197.

Reprinted from the December 1997 online issue of Antarctic Journal of the United States (volume 32, number 4).

Cambrian magmatic rocks of the Ellsworth Mountains, West Antarctica

MARGARET N. REES, EUGENE I. SMITH, and DEBORAH L. KEENAN, Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154  ERNEST M. DUEBENDORFER, Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011

A critical element in reconstructing the paleo-Pacific margin of Gondwanaland is the Ellsworth-Whitmore mountains terrane that lies between the Transantarctic Mountains and Antarctic Peninsula (Storey et al. 1988). Paleomagnetic data suggest that during the Cambrian, the terrane lay near the juncture of Africa and Antarctica (Grunow 1995). Nevertheless, much of the tectonic history of this terrane is equivocal and numerous conflicting models have been proposed regarding its tectonic setting and timing of magmatism (Vennum et al. 1992; Grunow 1995; Curtis and Storey 1996; Dalziel 1997). Thus, one aspect of our larger Ellsworth Mountains project focused on the geochemistry and geochronology of magmatic rocks in the northern Heritage Range of the Ellsworth Mountains. We conducted fieldwork during the 1996-1997 austral summer and subsequently completed laboratory analyses.                   The Cambrian Heritage Group is composed of volcanic and sedimentary rocks (figure 2) (Webers et al. 1992) that are unconformably overlain by the Ordovician(?)-Devonian siliciclastic Crashsite Group (Duebendorfer and Rees in press). The dominant structures in the range, which are attributed to the Triassic Ellsworth/Gondwanide Orogeny, are north-northwest-trending folds and a series of east-vergent stacked thrust sheets that have disrupted the stratigraphic succession. In addition, the Heritage Group preserves evidence of an earlier pre-Crashsite Group deformation that is attributed to deformation within the Ross orogen (Duebendorfer and Rees in press).

Volcanic rocks within the dominantly sedimentary succession of the Heritage Group are present in the Union Glacier and Springer Peak formations (figure 2) (Webers et al. 1992). In the Union Glacier Formation, basalt to andesite hyaloclastite deposits, and flows and interbedded sedimentary rocks locally are cut by dikes of basalt. The hyaloclastite deposits have yielded uranium/lead (U/Pb) zircon ages of 512±14 million years (Van Schmus personal communication). This date, together with other stratigraphic data (Duebendorfer and Rees in press) and the timescale of Shergold (1995), suggests deposition during the late Early Cambrian or early Middle Cambrian.

The Union Glacier volcanic rocks are subalkaline, tholeittic basalt and picritic basalt with 50 to 62 weight percent silica (SiO₂). Alumina (Al₂O₃), titania (TiO₂), ferric iron (FeO), lime (CaO), magnesia (MgO), and soda (Na₂O) decrease with increasing SiO₂. Their magnesium number (Mg#; magnesium divided by the sum of magnesium plus iron) varies from 42 to 65. These rocks are enriched in light rare earth elements (LREE) when compared to chondritic abundances (60-200x) and display negative niobium (Nb), tantalum (Ta), and titanium (Ti) anomalies. Epsilon neodymium (Nd) varies from +2 to -1, and initial strontium-87/strontium-86 (⁸⁷Sr/⁸⁶Sr) between 0.7043 and 0.7095. Their geochemistry is very similar to that of mid-oceanic ridge basalt from the Gulf of California (Saunders et al. 1982), and they have trace element abundances reflecting asthenospheric and lithospheric mantle and crustal components. Nd model ages of 0.9 to 1.0 billion years from the Union Glacier volcanic rocks (Walker personal communication) may suggest the age of the underlying lithosphere of the newly formed narrow ocean basin

Pillow basalt and flows cut by diabase and gabbro dikes and sills are present in the Springer Peak Formation (figure 2). Locally, isolated basalt flow lobes are interbedded with latest Middle Cambrian fossiliferous shale and limestone that indicate mafic magmatism continued in the region until approximately 500 million years ago, using the timescale of Shergold (1995).

Springer Peak Formation volcanic rocks are subalkaline, calc-alkalic basalt, andesite, and trachyandesite with 37 to 50 percent SiO₂. Al₂O₃, TiO₂, and CaO decrease with increasing SiO₂. Their Mg# varies from 42 to 65. These rocks have low rubidium (Rb), potassium (K), and Sr due to alteration and lack high field strength element (Nb, Ta) anomalies when normalized to primitive mantle. They are moderately enriched in LREE (40-80x chondrite), have an epsilon Nd of +5, and initial ⁸⁷Sr/⁸⁶Sr of 0.705. The basalt is similar to enriched mid-oceanic ridge basalt (MORB) although their higher barium (Ba) and Sr may suggest either source heterogeneity, alteration, or minor sediment input. We suggest that they erupted in an ocean basin wider than that represented by Union Glacier volcanics and in which the lithospheric mantle had been delaminated (figure 3 B ).

Dacite and rhyolite sills and dikes were observed in the Springer Peak Formation on Yochelson Ridge in the Heritage Range. These rocks, however, have yielded zircon U/Pb dates of 4986 million years (Van Schmus personal communication). Again using the timescale of Shergold (1995), these rocks could represent a Late Cambrian magmatic episode that postdated the Springer Peak Formation and predated deposition of the Crashsite Group.

These intrusions are calc-alkaline dacite and rhyolite with SiO₂ content between 75 to 87 percent SiO₂, and Mg# between 22 and 40. They are enriched in large-ion lithophile elements (LIL) and LREE (500x chondrite) but depleted in Ba and Sr. They have negative anomalies at Nb and zirconium (Zr), a very strong negative anomaly at Ti when compared to primitive mantle, and distinct negative europium (Eu) anomaly compared to chondrite. Epsilon Nd is +0.5 and intitial ⁸⁷Sr/⁸⁶Sr is 0.713. Tectonic discrimination diagrams suggest that the dacite and rhyolite intrusives formed in a continental arc setting. Furthermore, the geochemical and isotopic differences preclude these more felsic rocks of Yochelson Ridge from being produced by fractional crystallization of magmas that produced the mafic succession in the Springer Peak. Thus, the later felsic rocks represent closing of the narrow ocean basin and onset of subduction related magmatism (figure 3 C ).

Our geochemical and geochronological study of the Union Glacier and Springer Peak formations of the Heritage Group in the Ellsworth Mountains indicates opening of a narrow ocean basin during late Early through Middle Cambrian time. The subsequent Late Cambrian arc magmatism together with deformation and low-grade metamorphism of the Heritage Group and the angular unconformity at the base of the overlying Crashsite Group are compelling evidence that the Ellsworth-Whitmore mountains terrane lay within the Cambrian mobile belts of the paleo-Pacific-facing margin of Gondwanaland.

This research was supported by National Science Foundation grants OPP 92-20395 and OPP 93-12040.

References

• Curtis, M.L., and B.C. Storey. 1996. A review of geological constraints on the pre-break-up position of the Ellsworth Mountains within Gondwana: Implications for Weddell Sea evolution. In B.C. Storey, E.C. King, and R.A. Livermore (Eds.), Weddell Sea tectonics and Gondwana break-up. London: The Geological Society.
• Dalziel, I.W.D. 1997. Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation. Geological Society of America Bulletin , 109(1), 16-42.
• Duebendorfer, E.M., and M.N. Rees. In press. Evidence for Cambrian deformation in the Ellsworth-Whitmore mountains terrane, Antarctica: Stratigraphic and tectonic implications. Geology .
• Grunow, A.M. 1995. Implications for Gondwana of new Ordovician paleomagnetic data from igneous rocks in southern Victoria Land, East Antarctica. Journal of Geophysical Research , 100(B7), 12589-12603.
• Saunders, A.D., D.J. Fornari, J.L. Joron, J. Tarney, and M. Trevil. 1982. Geochemistry of basic igneous rocks, Gulf of California. In Deep Sea Drilling Project Leg 64. Washington D.C.: U.S. Government Printing Office.
• Shergold, J.H. 1995. Timescales 1. Cambrian. In Australian Phanerozoic timescales biostratigraphic charts and explanatory notes (second series). Canberra, Australia: Australian Geological Survey Organisation.
• Storey, B.C., I.W.D. Dalziel, S.W. Garrett, A.M. Grunow, R.J. Pankhurst, and W.R. Vennum. 1988. West Antarctica in Gondwanaland: Crustal blocks, reconstruction and break-up processes. Tectonophysics , 155, 381-390.
• Van Schmus, W.R. 1997. Personal communication.
• Vennum, W.R., P. Gizycki, V.V. Samsonov, A.G. Markovich, and R.J. Pankhurst. 1992. Igneous petrology and geochemistry of the southern Heritage Range, Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Craddock, and J.F. Splettsoesser (Eds.), Geology and paleontology of the Ellsworth Mountains, West Antarctica. Boulder: Geological Society of America.
• Walker, J.D. 1997. Personal communication.
• Webers, G.F., R.L. Bauer, J.M. Anderson, W. Buggisch, R.W. Ojakangas, and K.B. Sporli. 1992. The Heritage Group of the Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Craddock, and J.F. Splettsoesser (Eds.), Geology and paleontology of the Ellsworth Mountains, West Antarctica. Boulder: Geological Society of America.

Tomado de Antarctic Journal of the United States Review 1997. Marine and Terrestrial Geology and Geophysics

CICLO TRANSAMAZONIANO DEL CRATON DEL RIO DE LA PLATA

CICLO URUGUAYANO DEL CRATON DE LA ANTARTIDA ORIENTAL

Cátedra de Geología Argentina                INICIO                                CURSO 2006