Prince Charles Mountains

A broad axpanse of ice ringed by mountains with a person on a quad bike in the middle of the lake.
Lake Radok (Photo: Andrew Cianchi)
un shines on the left-hand side of these mountains as two quad bikes drive on the lake below them.Aerial shot of mounatins ranges covered in snow with glacier passing through

Location and geology

Antarctica, a continent which covers an area of some 14 million km2, is largely (~98%) covered with ice. Scattered bedrock exposures occur only as permanently ice-free dry valley areas, islands and/or coastal outcrops, and mountain ranges, nunataks and massifs extruding through the ice cap. The Prince Charles Mountains represent one such region of bedrock exposure located in the southern part of MacRobertson Land, East Antarctica. They are exposed in a huge (600 by 300 km) depression in the East Antarctic continental ice cap formed as a result of ice drainage along the the Lambert graben through the the Lambert glacier – Amery Ice Shelf region.

The Transantarctic mountains, stretching for ~3500 km across the Antarctic continent from the Ross Sea to the Weddell Sea, mark the geological and geographical boundary between West and East Antarctica. Geologically, West Antarctica represents an accretion of individual continental blocks whereas East Antarctica is dominated by a single ancient Precambrian craton. This craton or continental shield, the East Antarctic Shield, once formed a central part of the Gondwanan supercontinent.

The Prince Charles Mountains comprise a series of nunataks, mountain peaks and flat-topped massifs, up to ~3230 m high, which constitute the largest and most well exposed cross-section of the East Antarctic Shield. Outcrops, ranging in age from Archaean to Eocene or younger, include basement Proterozoic and minor Archaean rocks of sedimentary and igneous origin which were metamorphosed and deformed at ~1000 ma. Subsequent intrusion by Cambrian granites, pegmatites, aplites and alkaline dykes at ~500 to 600 ma was accompanied by a second thermal event and followed by the deposition of Permo-Triassic sediments. Further intrusion, and minor extrusion, of basic alkaline igneous rock types occurred during the Mesozoic and Cenozoic.

Subdivision of the Prince Charles Mountains into northern and southern sections occurs in the region of Mount Willing and is based on the geology of the area as much as the geography. Whereas the southern Prince Charles Mountains are considered to represent an Archaean terrane, comprising both Archaean and Proterozoic basement metamorphic rocks, the northern Prince Charles Mountains are considered to represent part of the Proterozoic mobile belt of the East Antarctic Shield. In addition, metamorphic grade associated with the 1000 ma event, increases northwards from greenschist and lower amphibolite facies in the southern Prince Charles Mountains through upper amphibolite to lower granulite facies in the northern Prince Charles Mountains.

Exploration and earth science research

The Prince Charles Mountains (PCMs) were first sighted and photographed in 1947 during aerial operations carried out by the United States navy as part of ‘Operation Highjump’. They were subsequently sighted and named by ANARE (Australian National Antarctic Research Expeditions) expeditioners during an overland traverse in the winter of 1954, the same year in which Australia’s first mainland Antarctic base (Mawson) was commissioned ~400 km to the northwest.

Early exploration and geological reconnaissance surveys were undertaken in both the northern and southern Prince Charles Mountains by wintering field parties from Mawson between 1954 and 1961, using fixed wing aircraft, motorized vehicles and/or dog sleds for transportation.

With the establishment of advance base camps at Moore Pyramid (northern Prince Charles Mountains) and Mount Creswell (southern Prince Charles Mountains), geological work in this region was intensified in the austral summers of 1968 to 1974 and the use of helicopter transportation was also introduced. From 1965 onwards, members of the SAE (Soviet Antarctic Expeditions) also began undertaking geological fieldwork in the Prince Charles Mountains, eventually establishing a base, Soyuz, on the eastern shore of Beaver Lake in the northern Prince Charles Mountains.

Australian geological work in the Prince Charles Mountains entered its third phase in the summer of 1987/88 when the SAE transported ANARE expeditioners from the Larsemann Hills to the Amery Oasis area of the northern Prince Charles Mountains and a field camp was set up at Radok Lake.

The following summer an advance base camp (Dovers) was established by the Australians near Farley Massif in the Athos Range, making the northern Prince Charles Mountains the centre of focus for geological research in this region. Since the late 1980s, Australian summer field parties, based either at Dovers or a second base camp at Beaver Lake, established in the summer of 1994/95, have been deployed in the northern Prince Charles Mountains on an almost annual basis.

The summer of 1997/98 saw a shift in the focus of Australian National Antarctic Research Expeditions (ANARE) operations in the Prince Charles Mountains when the southern Prince Charles Mountains was visited for the first time in more than 20 years. The program reassessed the geology of the most inland and extensive exposure of basement metamorphic rocks of the East Antarctic Shield using modern research and analytical tools. In addition, the Cenozoic glacial deposits located in this region, and the glacial erosion surfaces which characterize many of the southern Prince Charles Mountains massifs, provided vital information regarding the movement of the Lambert Glacier and its tributaries.

Northern Prince Charles Mountains

The northern Prince Charles Mountains, covering an area of approximately 40,000 km2, are located roughly between longitude 64 to 69°E and latitude 70 to 72°S within MacRobertson Land, East Antarctica. Altogether, this mountainous region comprises 140massifs and nunataks which range in area up to ~300 km2 (Fisher Massif) and extend from just above sea level (Else Platform) to 2438 m (Mount Kirkby) in height. The northern Prince Charles Mountains are bounded to the east by the Lambert Glacier – Amery Ice Shelf, to the south by the southern Prince Charles Mountains, and to the north and west by the Antarctic ice cap.

The northern Prince Charles Mountains are dominated by three east-west trending mountain ranges, the Athos, Porthos and Aramis Ranges, which are separated by the Scylla and Charybdis Glaciers. The eastern end of the southernmost Aramis Range is dominated by a number of relatively flat, ice-free areas, the Loewe, Manning and McLeod Massifs, which form the western boundary of the Amery Oasis region. The latter comprises Beaver and Radok Lakes, connected via Pagodroma Gorge and separated from the Lambert Glacier and its tributaries by Jetty Peninsula to the east and Flagstone Bench to the south. Further south, a number of relatively isolated, ice-free areas include Mount Lanyon, Mount Meredith, Fisher Massif and Mount Willing, the latter representing the southernmost exposure of the northern Prince Charles Mountains.

The basement geology of the northern Prince Charles Mountains is dominated by layered and massive felsic gneisses and metasediments, as well as minor mafic granulites. These basement ortho- and para-gneisses were formed from felsic and mafic igneous, together with minor carbonate and sedimentary, protoliths during granulite to upper amphibolite facies metamorphism at ~1000ma. They were subsequently intruded by late syn-tectonic orthopyroxene granitoid (charnockite) plutons, granitic stocks and pegmatite veins as well as younger mafic alkaline dykes. In the Amery Oasis region, a Permo-Triassic sedimentary sequence, the Amery Group, has been faulted against the basement metamorphic rocks. Although unmetamorphosed Mesozoic to Cenozoic mafic dykes of predominantly alkaline affinities are best exposed on the shores of Beaver and Radok Lakes, tholeiitic and alkaline mafic dyke swarms are also present in the southern Fisher Massif, Mount Collins and Mount Willing area of the northern Prince Charles Mountains. Minor olivine leucitite lava flows of Eocene age are exposed on Manning Massif. Cenozoic or younger glacigene deposits have also been identified within the Amery Oasis area, at Fisher Massif and at various locations within the Aramis Range.

The basement gneisses of the Prince Charles Mountains possess metamorphic mineral assemblages which record two main thermal events, at ~1000 ma and ~500 ma. Within the northern Prince Charles Mountains, the 1000 ma thermal event (M1) is considered to have resulted in the general decrease in observed metamorphic grade from lower granulite facies in the north to upper amphibolite facies in the south, continuous with the lower amphibolite to greenschist facies metamorphism of the southern Prince Charles Mountains. In addition, moderate metamorphic pressures have been inferred from the stability of garnet-sillimanite-cordierite mineral assemblages within the metapelites and the absence of garnet within the metabasites. Within the northern Prince Charles Mountains, high-grade metamorphism at ~1000 ma is considered to have been accompanied by at least three deformation events (D1 to D3), as well as charnockite and granite intrusion, and peak metamorphic conditions ( ~800°C, 600–700 mPa) are thought to have been followed by near-isobaric cooling. Later, possibly Palaeozoic, tectonism involved lower-grade, more brittle conditions and resulted in at least two more deformational overprints (D4 and D5).

The early Palaeozoic ~500 ma thermal event (M2) is considered to be of minor importance in the northern Prince Charles Mountains, resulting in overprinting of some M1 mineral assemblages by greenschist facies assemblages, the resetting of biotite Rb-Sr ages and the development of retrograde shear zones.

Three generations of mylonites, associated with crustal thickening and subsequent thinning events, have also been identified within the northern Prince Charles Mountains. Whereas the earliest generation (MY1) predate folding and are considered to have annealed during 1000ma peak metamorphic conditions, the second generation (MY2) formed under conditions which post-date this peak and the third generation (MY3) are considered to have formed much later in the tectonic history of the area, possibly at ~500 ma.

Geochronology

Early reconnaissance dating work in the Northern Prince Charles Mountains resulted in Rb-Sr isochron ages of 550 to 1090ma for whole-rock samples and 400–600 ma for biotite and muscovite separates. These ages were interpreted to represent the ages of major metamorphic events. Subsequent Rb-Sr isochron and conventional U-Pb zircon dating studies on rocks from Jetty Peninsula obtained ages of 1100+14/-11 ma (U-Pb) for a granulite facies gneiss, 940+24/-17 Ma (U-Pb) and 718+/-32 Ma (Rb-Sr) for a gneissic leucogranite, and ~480 ma (Rb-Sr mineral) for a biotite granite. Monazite U-Pb dating of the same granites resulted in ages of 530 to 540 ma, and an age of 495 to 505 ma (U-Pb) was obtained for a pegmatite from the same area. A separate study reported a Sm-Nd isochron age of 1233+/-160ma for a layered gabbronorite complex on Mount Willing as well as a zircon U-Pb age of 1400 +/-80 ma for a granite on Mount Collins. Further studies have reported a Rb-Sr whole-rock age of 882+/-140 ma for intrusive charnockite rocks of the Porthos Range, and Sm-Nd ages of 635 to 555ma for monazite inclusions within garnets in a leucogneiss sheet from Mount McCarthy. An age of 630ma was also obtained, via the same method, for a two pyroxene granulite from the same general area.

The most recent SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb zircon dating work undertaken on northern Prince Charles Mountains samples has revealed that two important magmatic episodes occurred at 1300 to 1280ma and at 1020 to 980ma. The first episode involved multiple intrusions of mafic to felsic magmas in the southern Fisher Massif to Mount Willing area of the northern Prince Charles Mountains, as well as volcanic extrusions on Fisher Massif. The second episode, coeval with the major ~1000 ma tectonothermal event, resulted in extensive granitic (charnockite, granite and syenite granite) intrusion and the formation of minor leucogneiss bodies and granitoid intrusion within the northern Prince Charles Mountains. In addition, detrital zircon populations within a paragneiss on Mount Meredith have been dated at ~2800 ma and 2100 to 1800 ma.

Ion microprobe (232Th/208Pb) dating of monazite grains within MY2 mylonite zones from the northern Prince Charles Mountains resulted in an average age of 800+/-16 Ma.

Alpine-type valley and cirque glaciers in the northern Prince Charles Mountains

In addition to the larger ice sheet outlet glaciers found within the northern Prince Charles Mountains, such as the Scylla and Charybdis Glaciers, small-scale drainage systems of alpine-type valley glaciers or cirque glaciers also occur on many of the massifs. Although such small-scale drainage systems have already been extensively studied in the Transantarctic Mountains, where they flow into dry valleys or snow-free oases, they remain the subject of on-going research within the Prince Charles Mountains.

The alpine-type glaciers of the Prince Charles Mountains reveal information regarding the behaviour of the East Antarctic Ice Sheet and the world's largest drainage system, the Lambert Glacier. For example, the present day snowline altitudes are influenced by snow-bearing winds, particularly the summer winds that carry moisture inland from the open waters of Prydz Bay. The retreat of these glaciers during the Last Glacial Maximum was in contrast to the expansion of the larger ice-sheet outlet glaciers and is thought to have occurred in when sea-ice cover around the continent expanded, resulting in the reduction sea-water as a moisture source.

These alpine-type glaciers have formed through a leeward or snowfence effect on Loewe Massif, located within the northern Prince Charles Mountains approximately 240 km inland from the coast. These glaciers have redeposited drift (carried by the katabatic winds) which was deposited on the leeward or sheltered side of the massif at 600m above sea level. The snouts are located at approximately 220 to 250m above sea level and show evidence of minor indentation on the moraine sweep left behind from the Charybdis Glacier during the Last Glacial Maximum (LGM) approximately 20,000 years ago.

Just like seeing the tip of the iceberg, the northern Prince Charles Mountains are just showing their mountain tops above the glaciers (>1000m in thickness) that flow through the ranges. In the northern Prince Charles Mountains, the Nemesis, Charybdis and Scylla Glaciers, as well as the Lambert Glacier, flow past the mountains that support the smaller scale alpine-type glaciers that are typically 1 km wide and 10 km long.

The cirque is oriented to the southwest and assumed to be receiving accumulation from the drift laden katabatic winds. Distinct lateral and terminal moraines suggest that this glacier has retreated significantly from its maximum extent. The snout terminates ~500m above the surface of the outlet glacier, over-riding a Last Glacial Maximum/pre-Last Glacial Maximum surface that appears as polygon patterns (due to their ice core nature).

The location of the moraine, relative to the glacier, indicates both the maximum dynamics/behaviour of the glacier and the current dynamics. This glacier is retreating from its maximum extent/volume. The debris constituting the moraine till can be traced to both the cirque/headwall and the terrain that the alpine-type glacier flowed over.

Their cirques are oriented to the northeast to receive maritime precipitation. The elevation of the cirque base is 960m above sea level (right hand side) and 1160m above sea level (left hand side) and the snouts are at 80 and 60m above sea level, respectively. Lateral moraines are clearly visible with both alpine-type glaciers at, or near, their maximum extent. These glaciers are approximately 320 km from the open water and they are flowing towards the Lambert Glacier (in the foreground).

This has occurred on the northern face of Mount Jacklyn near the Australian National Antarctic Research Expeditions (ANARE) summer base of Dovers, Athos Range, northern Prince Charles Mountains. These moraines, like most others in the Prince Charles Mountains, are ice-covered, meaning that they are solid ice with a coating of rocks and dirt which prevents melting due to a lack of exposure.

A frozen melt lake is visible immediately in front of a lateral moraine from the Charybdis Glacier.

The ratio of the ice cliff to the apron (debris at the ice cliff base) indicates that this glacier is near equilibrium (not advancing or retreating, northern Prince Charles Mountains.

The rock and dirt acts as an insulation cover over the ice that hampers sublimation. The small channels in the ice surface occur as a result of the brief period in summer when ablation can occur due to melt, not sublimation (from solid to vapour), northern Prince Charles Mountains.

The Pagodroma Tillite

Geographic and stratigraphic location

The Pagodroma Tillite has two main areas of exposure within the northern Prince Charles Mountains, the Amery Oasis and Fisher Massif. Within the Amery Oasis area this semi-lithified glacigene deposit is widely distributed west of Beaver Lake where it unconformably overlies the Amery Group, forms a prominent exposure in the walls of Pagodroma Gorge, and comprises subhorizontal sequences which are mostly <30m in thickness. West of Radok Lake, the Pagodroma Tillite forms a more than 100m thick deposit which directly overlies Precambrian metamorphic basement rocks. South of the Amery Oasis, the Pagodroma Tillite is present on both the flanks and crest of Fisher Massif. In addition, it has also been sighted at Mt. Bunt and near Gorman Crags, both of which are located within the Aramis Range at some distance (>90 km) west of Pagodroma Gorge, as well as further south at Mount Collins. Within the southern Prince Charles Mountains, the glacial deposits observed at Mount Menzies may also comprise Pagodroma Tillite.

Where Pagodroma Tillite overlies the Permian fluvial sediments of the Amery Oasis region, between Beaver and Radok Lakes, its basal contact is characterised by fragmentation and disruption of the underlying Amery Group sedimentary sequence. However, where the Pagodroma Tillite directly overlies basement metamorphic rocks, it rests upon a glacially smoothed erosion surface. The exposed and scree-blanketed top of the Pagodroma Tillite represents an undulating ablation surface, the partial erosion of which is considered to reflect a subsequent, but little known, glacial event.

Lithologies

The Pagodroma Tillite is a marine lodgement tillite comprising massive, matrix-rich diamictite. Individual sequences measure up to several metres in thickness and are interbedded with thinner beds (<0.5m thick) of matrix-free pebble- and cobble-conglomerate as well as rare beds (<3m thick) of laminated silty sandstone. The Pagodroma Tillite comprises subangular to rounded clasts of both Amery Group and Precambrian basement lithology's, which are generally less than 1m in diameter and widely dispersed throughout a non-sorted matrix. The preferred orientation of the clasts is often indicative of a pronounced lodgement till depositional fabric. Occasional thin (<1m thick) matrix-rich, fine-grained diamictites contain shelly fragments and may represent gravity-flow deposits. Although no distinct stratification has been recognised within the Pagodroma Tillite, the basal few metres of diamictite overlying the sedimentary sequences of the Amery Oasis region are dominated by Amery Group clasts as well as a dark matrix which contains abundant carbonaceous material derived from the underlying Bainmedart Coal Measures.

Fossil record and age of deposition

The fossil assemblage of the Pagodroma Tillite includes reworked microfossils of both marine and non-marine (?lacustrine) origin. Since the age of this unit must be younger than that of the youngest identified reworked fossils, the presence of marine diatom flora of Upper Miocene (Denticulopsisdimorpha and Denticulopsishustedtii zones) to Middle Pliocene (Cosmiodiscusinsignis and Nitzschiakerguelensis zones) age has been interpreted to suggest a maximum Late Pliocene age of deposition. In addition to planktonic and benthic marine diatoms, other fossils identified within the Pagodroma Tillite include non-marine diatoms, silicoflagellate and radiolarian fragments, sponge spicules and reworked, but in situ, shelly macrofossil fragments.

Environment of deposition

Like the Sirius Group of the Transantarctic Mountains, deposition of the Pagodroma Tillite is considered to reflect a Late Pliocene or Early Pleistocene expansion of the East Antarctic Ice Sheet within a major ice drainage basin. In the case of the Pagodroma Tillite, this drainage basin is the Lambert Graben system. The reworked microfossil assemblages within the diamictite sequences are thought to derive from higher latitude sites located well below sea level within the Lambert Graben. The presence of marine fossils within the sediments at these sites are accounted for by proposed marine incursions into the Lambert Graben in response to previous glacial retreats. Subsequent expansions or advances of the grounded Lambert Glacier would then have resulted in the erosion of these marine sediments and their eventual incorporation into the Pagodroma Tillite. The fact that the base of this unit has been recognised at a range of altitudes, including sea level in the Amery Oasis area, 300m above sea level on the flanks of Fisher Massif and ~1450m above sea level on the crest of Fisher Massif, indicates that phases of deposition were interspersed with periods of considerable uplift and erosion.

Glacial erratics, which comprise basement metamorphic lithology's and measure up to 5m in diameter, as well as terminal and lateral moraines rest upon the upper ablation surfaces of the Pagodroma Tillite in the Amery Oasis area. Although such features support the idea of a younger glacial event, the limited extent of these younger glacial moraines indicates that the ice sheet expansion recorded by the Pagodroma Tillite was of a much greater magnitude than any subsequent glacial event in this region.

Fisher Massif

Fisher Massif, located towards the southern end of the northern Prince Charles Mountains, is a large (300 km2) isolated, uplifted highland block with a northeast-southwest orientation. It is one of only a limited number of outcrops located within the transitional zone between the Archaean cratonic block exposed in the southern Prince Charles Mountains and the Proterozoic mobile belt exposures of the northern Prince Charles Mountains.

General geology

Although Fisher Massif represents part of the Proterozoic mobile belt exposed in the northern Prince Charles Mountains, it is geologically from the more northern exposures in that it comprises a much higher proportion of basic metavolcanic rock and underwent lower grade metamorphism and less intense deformation. Fisher Massif is considered to be Antarctica's closest approach to a greenstone belt, although it is not Archaean in age.

The layered greenstone sequence of Fisher Massif, known as the Fisher Terrane, comprises mafic and felsic to intermediate volcanics and volcaniclastics, with minor pelite, psammopelite, meta-conglomerate, banded iron formation, chert and carbonate intercalations. The metavolcanic rocks are dominated by amygdaloidal basalt flows, although basaltic andesite, andesite, dacite, rhyodacite and rhyolite are also present. Intrusive lithology's include gabbro, diorite, plagiogranite, granodiorite, S- and I-type granites and a series of minor mafic and felsic dykes. The metavolcanic rocks, which constitute at least half of the outcrop on Fisher Massif, comprise a series of NNW-NW dipping flows with an overall thickness of more than 3300m. They are best exposed on the southeastern cliffs of Fisher Massif. Deposition by lava and mass flow is considered to have occurred in a subaqueous and locally subaerial environment within an intercontinental rift.

As opposed to the higher pressure metamorphic mineral assemblages identified elsewhere in the northern Prince Charles Mountains, the metamorphic grade at Fisher Massif has only reached greenschist to lower amphibolite facies, and rock exposures show evidence for only relatively weak deformational events. Peak P-T estimates for the ~1000ma (M1) and ~500ma (M2) metamorphic events are 625 to 675°C at 4.5 to 5.3 kbar and 640 to 675°C at 3.8 to 4.5 kbar, respectively. Although M1 was accompanied by pervasive deformation (D1), followed by granite intrusion and a second deformation event (D2), M2 was purely a thermal event. Regional scale open folding, minor faulting and mylonite development occurred after M2.

Geochronology

Preliminary U-Pb zircon data which indicated a ~1300ma age of emplacement for the felsic and intermediate metavolcanic rocks of Fisher Massif, are consistent with the recent SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb zircon dating of a metadacite (1283+/-21ma) and a granodiorite (1293+/-28 Ma).