Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

TECTONICS OF ANTARCTICA

WARREN HAMILTON

U.S. Geological Survey, Denver. &lo. (U.S.A.)

(Rcceivcd October 21. 1966)

SUMMARY

Antarctica consists of large and wholly continental east Antarctica and smaller west Antarctica which would form large and small islands, even after isostatic rebound, if its ice cap were melted. Most of east Antarctica is a Precambrian Shield, in much of which charnockites are characteristic. The high Transantarctic Mountains, along the Ross and Weddell Seas, largely follow a geosyncline of Upper Precambrian sediment- ary rocks that were deformed, metamorphosed and intruded by granitic rocks during Late Cambrian or Early Ordovician time. The rocks of the orogen were peneplained, then covered by thin and mostly continental Devoniar*Jurassic sediments, which were intruded by Jurassic diabase sheets and overlain by plateau-forming tholeiites. Late Cenozoic doming and block-faulting have raised the present high mountains.

Northeastern Victoria Land, the end of the Transantarctic Mountains south of New Zealand, preserves part of a Middle Paleozoic orogen. Clastic strata laid unconformably upon the Lower Paleozoic plutonic complex were metamorphosed at’low grade, highly deformed and intruded by Late Devonian or Early Carboniferous granodiorites. The overlying Triassic continental sedimentary rocks have been broadly folded and normal-faulted.

Interior west Antarctica is composed of miogeosynclinal elastic and subordinate carbonate rocks which span the Paleozoic Era and which were deformed, metamorphosed at generally low grade, and intruded by granitic rocks during Early Mesozoic time and possibly during other times also. Patterns of erogenic belts, if systematic, cannot yet be defined: but frag- mentation and rotation of crustal blocks by oroclinal folding and strike-slip faulting can be suggested. The Ellsworth Mountains, for example, consist of CambriairPermian metasedimentary rocks that strike northward toward the noncorrelative and latitudinally striking Mesozoic terrane of the Ant- arctic Peninsula in one direction and southward toward that of the Lower Paleozoic, terrane of theTransantarctic Mountains in the other; the three regions may be separated by great strike-slip faults.

The Antarctic Peninsula in west Antarctica, south of South America, consists of metavolcanic and metasedimentary rocks intruded by Late Cretaceous quartz diorite. The pre-granitic rocks are of Jurassic and Early Cretaceous ages wherever they have been dated by fossils, although some crystalline complexes may be older. The S-shape of the peninsula may represent oroclinal bending within Cenozoic time as part of a motion system in which a narrow continental bridge between South America and Antarctica

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was deformed and ruptured. Perhaps this bridge lagged behind as the Iarger continental plates drifted into the Pacific Ocean Basin.

INTRODUCTION

Antarctica is largely covered by a continental ice sheet whose thickness reaches 4.5 km. Around the margins of the continent, however, and in moun- tains projecting through the ice, rock exposures are widespread. Expeditions during the first half of this century reached some of the bedrock regions and sketched their geologic features. The last decade has seen a great accelera- tion in the rate of exploration, with scientists of many nations participating, and most regions of exposed bedrock have now been visited. Most of the geologic work has been of a reconnaissance nature.

A number of summary papers on Antarctic geology have been published recently. Among these are papers by Adie (1962), Gunn (1963), Ford (1964), Voronov (1964), Klimov et al. (1964), Hamilton (1964, in press), Anderson (1965), Harrington (1965), Ravich {I965), Warren (1965), and Ravich et al. (1965). The papers vary widely in view point and emphasis. The reader is referred to these papers for descriptions of regional and local geology. The 76 papers on Antarctic geology in the compendium edited by Adie (1964) provide a wealth of information on recent work and a number of those papers are cited in this paper. I will outline here only briefly the descriptive geology of the continent, but will call attention to important findings of the last few years and then will suggest interpretations of some of the tectonic features.

There is nothing unique about Antarctic bedrock geology - the rocks and structures are all of types that can be matched in other continents - but the rock exposures are among the best in the world. Chemical weathering is negligible and plant cover none~sten~and the exposed-bedrock terranes have been cleaned by recently-higher glaciers. Detailed work yet to come in this huge natural laboratory will certainly much increase our under- standing of geologic processes.

The uniquely polar position of the Antarctic continent moreover makes it particularly important for an evaluation of theories of continental stabil- ity and mobility. Aspects of Antarctic geology bearing on these theories are emphasized here.

This work is supported by a grant to the Geological Survey from the Office of Antarctic Programs of the National Science Foundation.

THE CONTINENT OF ANTARCTICA

Surface exploration, coupled with seismic sounding to determine the ice thickness along oversnow traverses, has outlined the bedrock topo~ra~y of most of the continent. Bentley (1964) has recently summarized the know- ledge of topography and crustal structure. Were the ice sheet to melt and the continent to rise isostatically in consequence, the landmass of east Antarctica, about the size of Australia, would stand above sea level, mostly in east longitudes (Fig.1). West Antarctica, in west longitudes, would con- sist of mountainous islands of various sizes up to that of Japan. Some of these islands are separated by deep straits. The continental shelf forms a

556 Tectonophysics, 4(4-6) (1967) 555-568

Fig.1. Tectonic provinces of Antarctica.

narrow rim about the outer coast of east Antarctica, but broadens to en- close the lands of west Antarctica, although broken by troughs. Crustal structure of east Antarctica is that of a typical continent, whereas that of west Antarctica is on average thinner; presumably the west Antarctic is- lands have a thick crust but the intervening basins a thin one.

One of the earth’s great mountain chains, the ‘hansantarctic Mom- tains, continuously bounds east Antarctica along the west Antarctic side. Summit altitudes reach 4 km above sea level in most sectors of the range. High mountains are present also in Queen Maud Land and lesser mountains elsewhere around the outer coast of east Antarctica and beneath the ice of the interior, although the bedrock surface of much of the interior is rel- atively low.

The height of Antarctic mountains indicates continuing tectonism and yet the continent is virtually aseismic. Voronov and Klushin (1963) sug- gested that the lack of earthquakes may be due to the plastic deformation of the crust and upper mantle by the ice cap and to illhibitio~ of shallow crustal fracture by the weight of the ice, rather than to a lack of tectonic activitv; ice-capped Greenland also is now aseismic.

Tectonophysics. 4(4-6) (1967) 555-568 557

TECTONIC ELEMENTS OF ANTARCTICA

East Antarctica consists of a large Precambrian Shield, bordered on the west Antarctic side by an Early Paleozoic erogenic belt, which is in turn bounded, in northeastern Victoria Land, by a Middle Paleozoic mobile belt (Fig.1). In west Antarctica, Marie Byrd Land and the Ellsworth Moun- tains display complexes whose major deformation probably occurred during Early Mesozoic time, whereas the Antarctic Peninsula displays the effects primarily of Mesozoic orogeny. The relationship of the west Antarctic terranes to each other and to the other suites cannot yet be certainly defined and is one of the subjects for interpretation here.

Antarctica was long assumed to consist simply of a large Precambrian Shield, forming all of east Antarctica,and a Mesozoic orogen, forming all of west Antarctica. By 1960, radiometric age determinations (Klimov and Solov’ev, 1964) and geologic relationships (Hamilton, 1960) indicated that between Shield and Mesozoic mobile belt must lie a broad region deformed primarily in Paleozoic time. The voluminous geologic information that has since come from the Antarctic has proved this general argument correct, while adding much detail.

Precambrian Shield

The outer coast of east Antarctica, between the meridians of Z’W and 145’ E, displays the crystalline complexes of the Precambrian Shield. Soviet geologists (as Ravich et al., 1965) have published the most information on shield geology. Basic schists (as, pyroxene-labradorite rocks) and silicic, aluminous gneisses rich in intermediate plagioclase, are the dominant basement types, but talc-silicate rocks, granulites, charnockites, and granitic rocks are abundant. Rocks bearing cordierite, sillimanite, or hypersthene are widespread. Polymetamorphism has variably affected the rocks. Soviet age determinations, mostly by whole-rock potassium-argon methods, on specimens from these complexes show a wide scatter, from 400 to 1,500 m.y. (million years); even small areas show considerable scatter. Potassium-argon age determinations on biotite separated from similar suites of rocks in one region in Wilkes Land, however, show a tight grouping at about 1,100 m.y. (Webb et al., 1964). Low-grade metasedimentary rocks are present in some regions (Trail, 1964) and like the crystalline complexes are partly polymetamorphic. At least some of the low-grade rocks are younger than the high-grade rocks. Nonmetamorphosed and flat- lying or gently-dipping strata, including sedimentary rocks bearing a Permian GZossoPteris flora, overlie the basement complex in a few areas (McLeod, 1964).

Late Precambrian and Early Paleozoic mobile belt of the Transantarctic Mountains

The Transantarctic Mountains are approximately coextensive with a mobile belt which consists of geosynclinal elastic sedimentary rocks and subordinate interbedded limestones, metamorphosed at low to high grade

558 Tectonophysics, 4(4-6) (1967) 555-568

and intruded by batholiths during Early Paleozoic time. Volcanic rocks are locally abundant in the geosynclinal section. Lower and Middle Cambrian fossils have been found in the limestones in a number of localities. No fossils have yet been found in the elastic or volcanic sections, the complex structure, monotonous lithologies and discontinuous exposures of which have so far discouraged stratigraphic studies; presumably the Upper Precambrian is represented as well as parts of the Cambrian. The numerous age deter- minations on granitic rocks, which were intruded largely after the deforma- tion and metamorphism of the geosynclinal materials, and on gneisses, are mostly within the range 450-520 m.y., or Early Ordovician and Late Cambrian. Recent reports of such ages include Deutsch et al. (1964) and Minshew (1965). A few age determinations are markedly older, as the 630 m.y. K-Ar biotite age reported for a gneiss by McDougall and Grindley (1965) and the 1,000 m.y. Sr/Rb ages reported for two porphyry dikes in gneiss by Deutsch et al. (1964); these determinations apparently represent minimum ages for inliers of basement rocks upon which the geosynclinal materials were deposited. Schmidt et al. (1965), working at the Weddell Sea end of the Transantarctic Mountains, found an unconformity separating fossiliferous Cambrian rocks from highly deformed older materials. Other geologists have inferred that high grade complexes elsewhere in the mountain system also represent pre-geosynclinal basement rocks.

The boundary between the Transantarctic geosyncline and the older Precambrian Shield is buried beneath the inland ice except at the outer coast of east Antarctica. South of Australia, the geosynclinal elastic rocks are low-grade (greenschist facies) metamorphic rocks, whereas the older basement rocks to the west,and in a large inlier, are polymetamorphic plutonic and sedimentary rocks whose latest metamorphism dates from about 500 m.y., according to Ravich et al. (1965). Such Early Paleozoic re-metamorphism of the ancient rocks apparently was widespread also at the other end of the Transantarctic geosyncline south of the Atlantic Ocean (Picciotto et al., 1964; Ravich et al., 1965), where again the Transantarctic geosynclinal rocks were metamorphosed at low grade only.

The crystalline rocks are overlain throughout the length of the Trans- antarctic Mountains by the unmetamorphosed nonmarine elastic strata of the Beacon Sandstone, which contains Devonian ( ?&Early Jurassic “Gondwana” floras. Marine Lower Devonian rocks lie locally at the base of the section and Carboniferous or Permian tillite occurs at many places (Frakes et al., 1966). Middle Jurassic plateau basalt overlies. and diabase intrudes, the Beacon.

The present mountain system owes its height to Cenozoic arching and block faulting. The crust thins abruptly between the mountains and the flanking Ross and Weddell depressions (Behrendt et al., 1966).

A younger Paleozoic mobile belt trends obliquely southeastward across the Pacific end of the Transantarctic Mountains and forms the northeastern corner of Victoria Land. The basement complex beneath the geosynclinal filling of this younger orogen consists of phyllite, schist. gneiss and minor marble, intruded by granitic rocks (Gair, 1964:

Tectonophysics, 4(4-G) (1967) 555-568 559

Ravich et al., 1965, p.356363; D.F. Crowder and W. Hamilton, unpublished data). Potassium-argon whole-rock (Ravich et al., 1965, p.440-441) and biotite (Webb et al., 1964) age determinations are mostly clustered between 450 and 500 m.y., hence date the metamorphism and intrusion as probably Early Ordovician. This basement complex presumably is part of the Late Precambrian-Early Paleozoic Transantarctic Mountains mobile belt, al- though no fossils have yet been found in this region to prove the Paleozoic age of any pre-metamorphic rocks.

Unconformably overlying these crystalline rocks are low-grade metamorphosed elastic sedimentary rocks (Hamilton, 1965; D.F. Crowder and W. Hamilton, unpublished data). The basal and southwesternmost formation consists of polymict conglomerate and black slate, which form a northwest-striking outcrop belt 10 km wide of steeply dipping rocks which presumably are isoclinally folded. The next higher formation consists of quartzite and quartz conglomerate, which also forms a single northwest- trending outcrop belt, 8-15 km wide, through the Leitch Massif and the Bowers Mountains. The remaining met~e~mentary rocks belong to the Robertson Bay Formation of metasiltstone and metagraywacke whose outcrop belt, 150 km wide, extends from the Millen Range to Robertson Bay and which are tightly folded throughout about northwest-trending axes. Carbonate and volcanic rocks are absent from the geosynclinal filling. Bedding-plane thrusting preceded the folding and minor axial-plane thrusting accompanied and followed it. Metamorphism was to greenschist facies except in the contact aureoles about the numerous stocks and small batholiths of horn- blende-biotite granodiorite and quartz monzonite. Potassium-argon deter- minations indicate these granitic rocks to be about 350 m.y. old (Webb et al., 1964; Ravich et al., 1965, p,441), or probably Late Devonian or Early Carboniferous. No fossils have been found in the sedimentary rocks, which are, however, bracketed by the Early Ordovician and Late Devonian age determinations.

Both Early Paleozoic and Middle Paleozoic suites in northeastern Victoria Land are overlain by the unmetamorphosed Beacon Sandstone, here Upper Triassic and Lower Jurassic (Norris, 1965). The Beacon Sandstone has only very gentle dips where it lies upon the older high-grade rocks, but it has moderate dips and is broken by many normal faults where it lies upon the younger low-grade rocks.

Interior west Anlarctica

The mountainous tops of islands project through the ice cap of interior west Antarctica, between the Ross and Weddel Seas and the Antarctic Penin- sula. The pre-Tertiary rocks of these islands consist of low-grade meta- sedimentary rocks intruded in some regions by granitic batholiths. Avail- able data do not permit assigning these complexes to through-going mobile belts, but suggest on the contrary that the complexes are disconnected from each other and from the terranes of east Antarctica.

The Ellsworth Mountains form a range 350 km.long in interior west Antarctica, elongate north-northwestward parallel to the str&ike of the sed- imentary rocks which comprise it (Craddock et al., 1964a; Craddock et al., 1965). The miogeosynclina1 stratigraphic section is thought to be about

560 Teotonophysics, 4(4-6)(1967)555-568

12 km thick. The lower half of the exposed section is dominated by lime- stone and marble but contains intercalated elastic sediments and has yielded Upper Cambrian fossils from its upper part. Presumably the lower part of the section is in part Precambrian. Next higher is very thick quart- site, which is overlain by thick, unsorted conglomerate believed to be sub- aqueous tillite. At the top of the section is sandstone and argillite which contains a Permian Glossopteris flora. All rocks are folded, the general direction of overturning being southwestward,and in most of the region they have been slightly metamorphosed. There are no granitic intrusions.

The geology of the mountains of western Marie Byrd Land, near the northeast part of the Ross Sea, is still known primarily from the work of Wade (1945) and Warner (1945). Low-grade metasiltstone and metasand- stone are tightly folded about northwest-trending axis and are invaded by granodiorite, quartz monzonite and granite. None of the rocks have been dated by either fossils or radiometric methods.

Farther east, near the coast of Bellingshausen Sea, quartz diorite gneiss of Thurston IsIand yielded a strontiu~rubidium biotite age of 280 m.y. (Craddock et al., 1964~) and granite of the Jones Mountains a potassium- argon muscovite age of 199 m.y. (Craddock et al., 1964b).

Antuvctic Perzinsula mobile belt

The Antarctic Peninsula is formed mostly of metavolcanic and meta- sedimentary rocks and quartz diorite. The pre-granitic rocks are of Jurassic and Cretaceous ages wherever they have been dated by fossils, although Adie (1962) infers that crystalline complexes of several older ages are present also. Ra~ometrically dated granitic rocks are of Cretaceous and Early Tertiary age. Among the recent papers describing peninsular geology are those by Laudon et al. (1964), Halpern (1965), and Scott (1965).

GONDWANALAND

Antarctic geology was almost u~nown when Du Toit (1937) assigned Antarctica a central position within the Late Paleozoic supercontinent of Gondwanaland. The geologic information that has since come from east Antarctica is so consistent with that required by such a Gondwanaland con- cept that strong support is given to explanations in terms of continental drift.

Some of the evidence is paleontological and paleoclimatical. Thus the Antarctic Cambrian contains thick limestones, probably indicators of warm water, and bioherms of archeocyathids like those of Australia (Hill, 1965), whereas cooler-climate indicators abound in much of the Cambrian of the Northern Hemisphere. The rich Lower Devonian cool-water marine fauna of the Transantarctic Mountains is strikingly similar to that of South Africa (Doumani et al., 1965). Tillite is widespread at the base of Antarctic sections bearing Permian Glossopteris floras, as is the case throughout the other Gondwanaland masses, including now-tropical parts of South America, Africa, India and Australia; and yet correlative rocks in the Atlantic sector of the present Arctic display abundant evidence of tropical and subtropical

Tectonophysics. 4(4-F) (1967) 555-568 561

climates. Plumstead (1962) described the then-known 27 species of leaves, mostly of the characteristic Gondwana genera Glossopteris and GangamopteJ-is, from the temperate Permian flora of Antarctica between 76O S and 87” S, and 20 of the same species are known in the Permian of peninsular India, 16 in central and southern Africa, 15 in South America and 14 in Australia. (The Permian floras of the rest of the world are very different.) Triassic and Lower Jurassic floras of Antarctica are also of typical Gondwana aspect (Plumstead, 1962; Norris, 1965).

The tectonic pattern of east Antarctica provides further support for the concept of Gondwanaland. The major structural units of Australia project southward to their truncation by the continental slope at the margin of the continent and similar units appear in east Antarctica to the south. The Precambrian Shields of both continents contain abundant charnockites. (It appears premature to argue for or against correlations of separate Pre- cambrian terranes between the two continents on the basis of age deter- minations, structural trends, or specific lithologies.) This Shield on each continent is overlain on the east by a miogeosynclinal assemblage (the Adelaide geosyncline in South Australia and the Transantarctic geosyncline in Antarctica) of Upper Precambrian and Cambrian rocks containing sim- ilar Cambrian faunas and intruded by strikingly similar Upper Cambrian or Lower Ordovician granites. These Early Paleozoic mobile belts are bounded in turn on the east by Middle Paleozoic ones. The completely nonvolcanic Middle Paleozoic section of northeastern Victoria Land cannot, however, be matched with the correlative sections of the exposed Paleozoic terranes of Victoria and Tasmania, and it appears as though the Victoria Land analogue in Australia is buried beneath the Tertiary cover of southeastern South Australia, where indeed the belt should be if the two continents are fitted together to aline their contacts between Precambrian Shields and Early Paleozoic mobile belts.

Interior west Antarctica should by analogy be correlative tectonically with eastern Australia; but the geology of Marie Byrd Land is too poorly known and that of Ellsworth Mountains too distant from the possible join, to permit any detailed suggestions regarding specific correlations.

South Africa and the Weddell SertAtlantic sector of east Antarctica also share features consistent with their having been adjacent in Paleozoic time. The Precambrian Shields and overlying Late Paleozoic and Early Mesozoic platform sedimentary rocks and diabases are similar. The Cape Mountains geosyncline and granitic intrusions of South Africa are analogous to the Transantarctic Mountains suite.

Paleomagnetism of Middle Jurassic diabase sheets provides evidence for marked drift of East Antarctica since the break-up of Gondwanaland. Samples from widely separated regions along the length of the Transant- arctic Mountains have been studied independently in four laboratories and consistently indicate a paleomagnetic pole near 55’S 140°W in the South Pacific Ocean (Blundell, 1965). If the earth’s magnetic field was then (as it has certainly been throughout Cenozoic time) a dipole with its axis near the geographic axis of the earth, then these results indicate that Antarctica in Jurassic time lay at middle rather than polar latitudes.

562 Tectonophysics, 4(4-6)(1967)555-568

SCOTIA ARC AND ANTARCTIC PENINSULA

The island-sprinkled submarine ridge of the Scotia Arc connects South America and the Antarctic Peninsula by a hairpin curve that ex- tends 2,300 km east of the direct line, only 800 km long, between the two continents. Southernmost South America and the Antarctic Peninsula both consist chiefly of Jurassic and Lower Cretaceous eugeosynclinal materials metamorphosed and intruded by granitic rocks during Cretaceous time. South Georgia Island, on the north limb of the Scotia Arc, consists of sim- ilarly metamorphosed and intruded eugeosynclinal materials, Lower Creta- ceous in their upper part (Trendall, 1959). The South Orkney Islands on the south limb of the arc consist of shallow-water sedimentary rocks meta- morphosed during Late Triassic or Early Jurassic time (Matthews, 1959: Miller, 1960). The South Sandwich Islands, which form the outer eastern bend of the Scotia Arc, are young volcanoes of basalt, andesite and dacite, such as are typical of volcanic island arcs elsewhere in the world; a sub- marine trench lies along the outer side of the bend.

The remanent magnetic field directions in the Cretaceous granitic rocks of the northern half of the Antarctic Peninsula were determined by Blundell (1962). His data can be interpreted to show a progressive clock- wise swing totalling about 25O, along the peninsula from latitude 68OS northeastward to the northeast tip (Hamilton, 1966). The peninsula itself bends about 40° clockwise in the same interval - so its shape may be due largely to bending since Early Cretaceous time.

The shallow-water Cenozoic brachiopods of South America and the Antarctic Peninsula are more similar than are those of any other pair of the southern lands now separated by deep water and the similarity suggests that South America and Antarctica were joined by a continuous coastline in Early Tertiary time (Allan, 1963). Lower Tertiary beech-podocarp land floras of southern South America also are quite similar to those of Ant- arctica.

These and other aspects of the geology and paleobiogeography can be explained in terms of lateral disruption of the continental crust (Hawkes, 1962; Hamilton, 1966). The geometry and geology of the arc permit the interpretation that the arc was produced by tensional thinning and frag- mentation of a narrow belt of continental rock which in Cretaceous and (?) earliest Tertiary time more directly connected South America and the Antarctic Peninsula. The young South Sandwich volcanoes apparently have grown across an oceanic gap between the disrupted ends of the initially continental belt. Long, straight channels along the south side of Tierra de1 Fuego and along the north end of the Antarctic Peninsula perhaps mark strike-slip faults whereby the Scotia Sea block has moved relatively east- ward. Scott (1965) noted that such channels in one Antarctic Peninsula region apparently mark shear zones? across which the rocks do not correlate. The Caribbean Arc is so similar to the Scotia one that they must both be ex- plained by the same mechanism,and the lateral-disruption interpretation can be supported by much evidence in the Caribbean region (Hamilton, 1966).

The separation of the Americas and Antarctica from the mid-Atlantic and Atlantic-Indian Ridges probably indicates drift of the continents since Triassic time, as most advocates of drift assume. The breadth of the conti- nental plates, as they existed in Late Mesozoic time, apparently controlled

Tectonophysics, 4(4-6) (1967) 555-568 563

Thin port of conlinantal connwtion much bent

Cenozoic volcbnic bridge across oceanic qop

\ SOUTH \

AMERICA

1. ond disfuohd by lo9

wk?st Antarctica Q Islands

Fig.2. Relation of width of the continental plates to the fragmentation which produced the Scotia and Caribbean Arcs.

the pattern of Cenozoic deformation (Fig.2). The broad continental plates moved independently of one another, but narrow parts of the plates bent and the narrowest parts ruptured. The strength of a continental plate must vary with its width: and it is an obvious inference from the geometry of Scotia and Caribbean deformation that the moving continents bent where they were weak

564 Tectonophysics, 4(4-G) ~1967) X%--568

and broke where they were weakest. The narrowest parts of the continents may have lagged behind and been disrupted. The presence of Cretaceous sediments in the Atlantic Ocean Basin (Ewing et al., 1966), however, places severe limits on the amount of westward drift of the Americas and Antarctica that can be postulated to have occurred since Early Cretaceous time. If the Scotia and Caribbean Arcs have indeed lagged behind continents drifting toward the Pacific, then much of the lag indicated by the U-shapes of the arcs antedates the middle of Cretaceous time. Alternatively, the bending and fragmentation of the narrow parts of the Mesozoic belt might have been caused by drift of those parts independent of the broad adjacent continents, or by eastward ejection of Pacific mantle that carried with it the narrow continental masses. The young volcanic island arcs and flanking trenches may have developed where Pacific and Atlantic crust and mantle came in contact.

DEFORMATION WITHIN WEST ANTARCTICA

If the rationale just set forth is valid and the bend of the northern part of the Antarctic Peninsula is due to oroclinal folding, then it is likely also that the entire S-shape of the Peninsula represents such deformation. Drag along a right-lateral fault system south of the Peninsula could explain the 90° clockwise swing made by the southern half of the Peninsula. The existence of such a fault can be inferred from the geology of the Ellsworth Mountains, the Paleozoic miogeosynclinal rocks of which trend northward toward the latitudinally-striking Mesozoic eugeosynclinal rocks of the base of the Peninsula. The T junction, buried beneath the ice cap, may mark a great strike-slip fault.

Another strike-slip fault may be responsible for the similar T junction, again a subglacial trough, between the southern projection of the Ellsworth Mountains and the Early Paleozoic crystalline terrane of the Transantarctic Mountains. The Ellsworths may be a crustal fragment rotated between strike- slip faults bounding it on both north and south.

The Middle Paleozoic terrane of metasiltstone, metagraywacke and granodiorite of northeastern Victoria Land strikes southeastward, and the undated terrane of argillite, metagraywacke and granitic rocks of Marie Byrd Land appears on strike on the east side of the Ross Sea. The tempting correlation between these two regions has been suggested by many workers, but the granitic rocks are so different that it seems likely to me that they formed in different erogenic belts. Strike-slip faults may intervene here also.

The crustal geometry of west Antarctica - large thick-crust islands, separated by thin-crust depressions - is itself perhaps suggestive of rifting into large blocks by a combination of strike-slip faulting and tensional thinning. The abrupt rise of the Transantarctic Mountains for a length of 4,000 km from the Ross and Weddell Seas and the buried lowlands that con- nect them reflects a comparably abrupt change in crustal thickness; perhaps this crustal break also is related to rifting or strike-slip faulting. The degrees of freedom are many.

Tectonophysics, 4(4-6) (1967) 555-568 565

REFERENCFS 1

Adie, R.J., 1962. The geology of Antarctica. Geophys. Monograph, 7: 26-39. Adie, R.J. (Editor), 1964. Antarctic Geology. North-Holland, Amsterdam, 758 pp. Allan, R.S., 1963. On the evidence of Austral Tertiary and Recent brachiopods on

Antarctic biogeography. In: J.L. Gressitt (Editor), Pacific Basin Biogeography. Bishop Museum Press, Honolulu, pp.451-454.

Anderson, J.J., 1965. Bedrock geology of Antarctica: a summary of exploration. 1831-1962. Am. Geophys. Union, Antarctic Res. Ser., 6: l-70.

Behrendt, J.C., Meister, L. and Henderson, J.R., 1966. Airborne geophysical study of the Pensacola Mountains of Antarctica. Science, 153: 137>1376.

Bentley, C.R., 1964. The structure of Antarctica and its ice cover. In: H. Odishaw (Editor), Research in Geophysics, 2. Mass. Inst. Technol. Press, Cambridge, Mass., pp. 335389.

Blundell, D.J., 1962. Palaeomagnetic investigations in the Falkland Islands Dependencies. Brit. Antarctic Surv., Sci. Rept., 39: l-24.

Blundell, D.J., 1965. Palaeomagnetism of the dolerite intrusions. Trams-Antarctic Expedition 1955-58, Sci. Rept., 8 (4): 61-67.

Craddock, C., Anderson, J.J. and Webers, G.F., 1964a. Geologic outline of the Ellsworth Mountains. In: R.J. Adie (Editor), Antarctic Geology. North-HolIand, Amsterdam, pp.155-170.

Craddock, C., Bastien, T.W. and Rutford, R.H., 1964b. Geology of the Jones Mountains area. In: R.J. Adie (Editor), Antarctic Geology. North-Holland, Amsterdam, pp.171-187.

Craddock, C., Gast, P.W., Hanson, G.N. and Linder, H ., 1964c. Rubidium-strontium ages from Antarctica. Bull. Geol. Sac. Am., 75 (3): 237-240.

Craddock, C., Bastien, T.W., Rutford, R.H. and Anderson, J.J., 1965. Glossopteris discovered in west Antarctica. Science, 148: 634-637.

Deutsch, S. and Webb, P.N., 1964. Sr/Rb dating on basement rocks from Victoria Land; evidence for a 1,000 million year old event. In: R.J. Adie (Editor), Antarctic Geology. North-Holland, Amsterdam, pp.557-562.

Doumani, G.A., Boardman, R.S., Rowell, A.J., Boucot, A.J., Johnson, J.G., McAlester, A.L., Saul, J., Fischer, D.W. and Miles, R.S., 1965. Lower Devonian fauna of the Horlick Formation, Ohio Range, Antarctica. Am. Geophys. Union, Antarctic Res. Ser., 6: 241-281.

Du Toit, A.L., 1937. Our Wandering Continents. Oliver and Boyd, Edinburgh, 366 pp. Ewing, M., Le Pichon, X. and Ewing, J., 1966. Crustal structure of the mid-ocean

ridges. 4. Sediment distribution in the South Atlantic Ocean and the Cenozoic history of the Mid-Atlantic Ridge. J. Geophys. Res., 71, (6): 1611-1636.

Ford, A.B., 1964. Review of Antarctic geology. Trans. Am. Geophys. Union, 45 (2): 363-381.

Frakes, L.C., Matthews, J.L., Neder, I.R. and Crowell, J.C., 1966. Movement direc- tions in Late Paleozoic glacial rocks of the Horlick and Pensacola Mountains, Antarctica. Science, 153: 746-748.

Gair, H.S., 1964. Geology of upper Rennick, Campbell and Aviator glaciers, northern Victoria Land. In: R.J. Adie (Editor), Antarctic Geology. North-Holland, Amsterdam, pp.18&-198.

Gunn, B.M., 1963. Geological structure and stratigraphic correlation in Antarctica. New Zealand J. Geol. Geophys., 6 (3): 423-443.

Halpern, M., 1965. The geology of the General Bernard0 O’Higgins area, northwest Antarctic Peninsula. Am. Geophys. Union, Antarctic Res. Ser., 6: 177-209.

Hamilton, W., 1960. New interpretation of Antarctic tectonics. U.S., Geol. Surv., Profess. Papers, 400-B: 379-380.

Hamilton, W., 1964. Tectonic map of Antarctica - a progress report. In: R.J. Adie (Editor), Antarctic Geology, North-Holland, Amsterdam, pp.676679.

Hamilton, W., 1965. Geology of northeastern Victoria Land. Bull. Antarctic Proj. Officer U.S. Navy, 6 (7): 68-69.

Hamilton, W., 1966. Formation of the Scotia and Csribbean Arcs. Geol. Surv. Can. Paper, 6615: 178-187.

566 Tectonophysics, 4(4-6) (1967) 555-568

Hamilton, W., in press. Antarctica and Pacific tectonics. Intern. Geol. Congr., 22nd, New Delhi, 1966, Sect. 17.

Barrington, H.J., 1965. Geology and morphology of Antarctica. In: P. van Oye and J. van Mieghem (Editors), Biogeography and Ecology in Antarctica - Mono- graphiae Biologicae, 15: l-71.

Hawkes, D.D., 1962. The structure of the Scotia Arc. Geol. Msg., 99: 85-91. Hill, D., 1965. Archaeocyatha from Antarctica and a review of the phylum. Trans-

Antarctic Expedition 195558, Sci. Rept. 10: l-151. Klimov, L.V. and Soloviev, D.S., 1964. Correlation of geological formations of the

Ross Sea region and Oates Coast. In: Soviet Antarctic Expedition, 2. Elsevicr, Amsterdam, pp.171-174.

Klimov, L.V., Ravich, M.G. and Soloviev, D.S.: 1964. Geology of the Antarctic Platform. In: R.J. Adie (Editor), Antarctic Geology, North-Holland, Amsterdam, pp.681-691.

Laudon, T.S., Behrendt, J.C. and Christensen, N.J., 1964. Petrology of rocks collected on the Antarctic Peninsula traverse. J. Sediment. Petrol., 34 (2): 36e-364.

Matthews, D.H., 1959. Aspects of the geology of the Scotia Arc. Geol. Mag., 96: 42rr441 McDougall, I. and Grindley, G.W., 1965. Potassium-argon dates on micas from the

NimrodBeardmoreAxel Heiberg region, Ross Dependency, Antarctica. New Zealand J. Geol. Geophys., 8 (2): 304-313.

McLeod, I.R., 1964. An outline of the geology of the sector from longitudq45“E-80°E., Antarctica, In: R.J. Adie (Editor), Antarctic Geology. North-Holland. Amsterdam, pp.237-247.

Miller, J.A., 1960. Potassiumargon ages of some rocks from the South Atlantic. Nature, 187: 1019-1020.

Minshew, V.H., 1965. Potassiumargon age from a granite at Mount Wilbur, Queen Maud Range, Antarctica. Science, 150: 741-743.

Norris, G., 1965. Triassic and Jurassic miospores and acritarchs from the Beacon and Ferrar Groups, Victoria Land, Antarctica. New Zealand J. Geol. Geophys., 8 (2): 236-27’7.

Picciotto, E., Deutsch, S. and Pasteels, P., 1964. Isotopic ages from S$r-Rondane Mountains, Dronning Maud Land. In: Adie, R.J. (Editor), Antarctic Geology. North-Holland, Amsterdam, pp.57&578.

Plumstead, E.P., 1962. Fossil floras of Antarctica. Trans-Antarctic Expedition 195558, Sci. Rept. 9: l-154.

Ravich, M.G., 1965. Das kristalline Fundament der antarktischen Plattform. Frei- berger Forschungsh., C-190: 99128.

Ravich, M.G., Klimov, L.V. and Solov’ev, D.S., 1965. Dokembriy vostochnoy Antarktidy. Tr. Inst. Geol. Arktiki, 138: l-470.

Schmidt, D.L., Wiliiams, P.L., Nelson, W.H. and Ege, J.R., 1965. Upper Prccambri~ and Paleozoic strat~graphy and structure of the Neptune Range, Antarctica. U.S. Geol. Surv. Profess. Papers, 525-D: 112-119.

Scott, K.M., 1965. Geology of the southern Gerlache Strait region, Antarctica. J. Geol., 73 (3): 51c-527.

Trail, D.S., 1964. Schist and granite in the southern Prince Charles Mountains. In: R.J. Adie (Editor), Antarctic Geology. North-Holland, Amsterdam, pp.49*497.

Trendall, A.F., 1959. The geology of South Georgia, 2. Falkland 1~1s. Depend. Surv., Sci. Rept., 19: l-48.

Voronov, P.S., 1964. Tectonics and neotectonics of Antarctica. In: R.J. Adie (Editor), Antarctic Geology. North-Holland, Amsterdam, pp.629-700.

Voronov, P.S. and Klushin, LG., 1963. 0 probleme aseysmichnosti Antarktidy i Grenlandii. Problemi Arktiki i Antarktiki, Sb Statei. Arktichi Antarkt. Nauchn. Issled. Inst., 12: 513.

Wade, F.A., 1945. The geology of the Rockefeller Mountains, King Edward VII Land, Antarctica. Proc. Am. Phil. Sot., 89: 67-77.

Warner, L.A., 1945. Structure and petrography of the southern Edsel Ford Ranges, Antarctica. Proc. Am. Phil. Sot., 89: 78-122.

Tectonophysics, 4(4--6) (1967) 555-568 567

Warren, G., 1965. Geology of Antarctica. In: T. Hatherton (Editor), Antarctica. Reed Wellington.

Webb, A.W., McDougall, I. and Cooper, J.A., 1964. Potassium-argon dates from the Vincennes Bay region and Oates Land. In: R.J. Adie (Editor), Antarctic Geology. North-Holland, Amsterdam, pp.597~-600.

NOTE ADDED IN PROOF

The contrast between interior west Antarctica and the Transantarcti~ Mountains is increased by several new isotopic age determinations. A quartz monzonite batholith in western Marie Byrd Land has concordant potassium- argon and rubidium-strontium ages of about 140 m.y. (Boudette et al., 1966), or very Late Jurassic; no correlation with the Paleozoic granitic rocks of the terrane now on strike across the Ross Sea in northeastern Victoria Land appears possible. The rubidium-strontium age of a pink granite from a nunatak between the Ellsworth and Transantarctic Mountains is 17’7 m.y. (Balpern, 1966), Early Jurassic.

REFERENCES

Boudette, E.L., Marvine, R.F. and Hedge, C.E., 1966. Biotite, potassium-feldspar, and whole-rock ages of adamellite, Clark Mountains, West Antarctica, U.S., Geol. Surv., Profess. Papers, 550-D: 190-194.

HaIpern, Martin, 1966. Rubidium-strontium date from Mt. Byerly, West Antarctica, Earth Planetary Sci. Letters, l(6): 455-457.

568 Tectonophysics, 4(4-Q (1967) 55fi-568