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Victoria University Antarctic Research Expedition Science and Logistics Reports 1981-82: VUWAE 26

SCIENTIFIC ACHIEVEMENTS

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SCIENTIFIC ACHIEVEMENTS

Erebus Studies and IMESS (K4) - R. Dibble.

Additional data on seismic velocities within and below Erebus were recorded on the IMESS seismic telemetry net (Fig. 1). During epidentre determinations from recordings made early in 1981, Kienle et al. (1981) confirmed the near surface velocity of 1.5km/s determined by the writer in 1975. From recordings of 3 distant earthquakes in November/December 1981 (Table 1), the writer has calculated velocities along the ray paths between sea level and the telemetry station elevations on the assumption that the velocity structure below the Scott Base seismograph is identical to that below sea level under Erebus. Of 8 determinations shown on Fig. 1, 6 lie between 3 and 5.7km/s, and have a mean of 4.5 ± 1.0km/s. The other two values of 1.5 and 15km/s were discarded as anomalous.

Velocities below sea level were determined with the aid of a large seismic shot fired at 04h34m54.48s UT on 23 November 1981 by L.D. McGinniss party. The shot point was at 165.720°E, 77.835°S, and it was recorded at Scott Base, Hoopers Shoulder (166.90°E, 77.538°S, 1900m), Abbott Peak (166.90°E, 77.460°S, 1793m), Bomb (c. 167.43°E, 77.512°S, 1800m), and Terror (c. 168.54°E, 77.518°S, 3230m). Summit (167.15°E, 77.532°S, 3794m) was not operating at the time. The time versus slant-distance graph (Fig. 2) shows an apparent velocity across the net of 7.2km/s with a time intercept of 1.60s. This is more likely to be a downdip than an updip velocity due to isostatic subsidence of the volcanic pile, and indicates oceanic rather than continental type crust under Ross Island.

Table 1. Data for 3 distant earthquakes recorded on the IMESS telemetry net in December 1981. dt (P) is the observed P-time difference t(station) - t(Scott Base); Va is apparent P-velocity across the net from J-B Tables; Ve is the computed velocity within Mount Erebus.

Table 1. Data for 3 distant earthquakes recorded on the IMESS telemetry net in December 1981. dt (P) is the observed P-time difference t(station) - t(Scott Base); Va is apparent P-velocity across the net from J-B Tables; Ve is the computed velocity within Mount Erebus.

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Fifty five eruptive events observed by eye and ear between 27 November and 12 December 1981 are listed in Table 2. Most of these were also recorded on the tape seismograph and infrasonic recorder operated in the summit hut at the levels listed. The larger eruptions again had the characteristic infrasonic signature reported in 1978/79 and 1980/81, and this allowed a further eleven eruptions to be identified instrumentally. The relation between recorded seismic and infrasonic levels are shown in Fig. 3. For a given seismic level, the infrasonic levels (which are related to eruption intensity) varied over a range of 30dB, indicating variation in the partition of energy between seismic and eruptive phenomena.

To check the party's impression that eruptions tended to occur in the early morning, a diagram was constructed of the frequency of occurrence of recorded earthquakes with the time of day (Fig. 4). This was done separately for large earthquakes (which are usually accompanied by eruptions), medium and small earthquakes, and compared with the average diurnal gravity tide (predominantly a diurnal tide of solar origin). There was indeed a peak in large earthquakes between 2 and 6 hours NZST which is just after the maximum tidal gravity (i.e. low earth tide). No obvious correlation with tide exists for medium earthquakes, but small earthquakes were most frequent between 12 and 14 hours NZST which corresponds with minimum tidal gravity (high earth tide). The correlations are of doubtful significance, however, because with the exception of an early morning peak in large earthquakes in December 1974, no correlations were obvious during the other four observation periods.

Further progress was made in the study of earthquake size versus frequency of occurrence. Figure 5 shows that the b-value for small to medium earthquakes in November/December 1981 was 1.6, and not significantly different from that in the 1980/81 season. For the large earthquakes the b-value for combined 80/81 and 81/82 data is only 0.6, indicating that frequency decreases less rapidly with increasing size for eruptive events than for the smaller volcanic earthquakes. Probably there is a mixture of two populations rather than a transition from one slope to the other, and the previous interpretation of a preferred size and upper limit to explosive eruptions of Erebus can no longer be sustained. The concept of different slopes for b-type and eruption earthquakes is important for risk assessment at other volcanoes.

References

Kyle, P.R., Dibble, R.R., Giggenbach, W.F., Keys, J.H., 1982: Volcanic activity associated with the anorthoclase phonolite lava lake, Mt. Erebus, Antarctica, In Craddock (Ed.), Antarctic Geoscience, University of Wisconsin Press, IUGS Series B - 4.

Kienle, J., Kyle, P.R., Estes, S., Takanami, T., Dibble, R.R., Submitted 1981: Seismicity of Mt. Erebus 1980/81, Antarctic Journal of the United States National Science Foundation.

Takanami, T., Terai, K., Osada, N., Kienle, J., Estes, S., Kyle, P.R., Dibble, R.R., 1981: Earthquake Observations at Mt. Erebus, Antarctica. Part 1 (in Japanese), Seismological Society of Japan, Proceedings of the Annual General Meeting, October 1981.

Takanami, T., 1981: Earthquake Observations at the summit of Mt. Erebus, Antarctica (in Japanese), Kyokucki (Polar News, Japan Polar Research Association), 33: 52-7.

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Figure 2: Relation between recorded levels of seismic signals and infrasonic signals of eruptions at the summit of Erebus. For a given seismic level, the range of infrasonic level is 25 dB.

Figure 2: Relation between recorded levels of seismic signals and infrasonic signals of eruptions at the summit of Erebus. For a given seismic level, the range of infrasonic level is 25 dB.

Figure 3: Average diurnal variation of earthquake occurrence rate at Erebus summit between 29 November and 10 December 1981. Large earthquakes (>600 W) were not frequent between 2 and 6 hours NZST, while small earthquakes (0.6-6 W) were most frequent between 10 and 18 hours NZST and correlated with the mean diurnal gravity tide.

Figure 3: Average diurnal variation of earthquake occurrence rate at Erebus summit between 29 November and 10 December 1981. Large earthquakes (>600 W) were not frequent between 2 and 6 hours NZST, while small earthquakes (0.6-6 W) were most frequent between 10 and 18 hours NZST and correlated with the mean diurnal gravity tide.

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Figure 4: Earthquake occurrence rate versus size at the summit of Erebus. The levels and slopes of the graph are similar in the 80/81 and 81/82 seasons, and show different b-values for small earthquakes and for the larger earthquakes which are usually accompanied by eruptions.

Figure 4: Earthquake occurrence rate versus size at the summit of Erebus. The levels and slopes of the graph are similar in the 80/81 and 81/82 seasons, and show different b-values for small earthquakes and for the larger earthquakes which are usually accompanied by eruptions.

Figure 5: Travel time graph of seismic waves from a large explosion fired by L.D. McGinnis in McMurdo Sound which was well-recorded on the IMESS net.

Figure 5: Travel time graph of seismic waves from a large explosion fired by L.D. McGinnis in McMurdo Sound which was well-recorded on the IMESS net.

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Table 2. Eruptions of Erebus between 27 November and 12 December 1981.

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PLATE II: Sphincter corer ready to drop (left), and resulting core (below). (See next page).

PLATE II: Sphincter corer ready to drop (left), and resulting core (below). (See next page).

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McMurdo Sound Sediment Sampling (K5) - A. Pyne/B. Ward.

Sphincter cores 20cm in diameter were recovered from 18 sites in depths ranging between 850 and 109m in McMurdo Sound and Granite Harbour (Fig. 6,7). Most cores were only a few centimetres long on account of the hard sandy bottom, and three were just mounds of disturbed sediment, but the remainder appeared quite undisturbed, with worms, sea spiders and sea-anemonies still alive on the sediment surface. Other common biogenic material in the sediment included sponge spicules and bryozoans. Four cores, three in Granite Harbour (14A, 15, 16) and one off Ferrar Glacier (18) were much longer (41 to 56cm), presumably because of the mud bottom.

Samples from the cores were treated with alcohol soon after recovery so that the living and dead foraminifera could be distinguished later in the laboratory (Ward, 1982). Splits were treated with rose bengal stain, and tests containing stained protoplasm extracted for further study. Non-stained tests were also separated for comparative purposes. Three areas of varying foraminiferal distribution have been identified in the McMurdo Sound area as follows: (1) below 560m there exists an assemblage of agglutinated foraminifera with Reophax spp. as the dominant taxa; (2) between 560 and about 210m there is a mixed assemblage, again with Reophax spp. as the dominant agglutinated taxa, and Trifarina earlandi, Globocassidulina cf. subglobosa and Cassidulinoides porrectus as the dominant calcareous taxa; this includes the Granite Harbour area; and (3) the New Harbour area supports an agglutinated population similar to that found below 560m in the open Sound.

Comparison of living (stained) and dead assemblages from the top 20m of five 22cm-diameter cores indicates that post-mortem alteration of assemblages, specifically, disappearance of calcareous tests, increases progressively with greater water depth until the CCD is reached, somewhere between 560 and 850 metres. The difference in proportions of calcareous and agglutinated foraminifera in live and dead assemblages increases the difficulty in the ecological interpretation of ancient (dead) assemblages.

Grain size analyses were carried out in all cores, and from these and samples from previous seasons several significant conclusions were drawn concerning marine sedimentation in the area (Barrett et al., 1982): The most important processes operating in the Sound today are ice-rafting of wind-blown sand by sea ice and sedimentation of fine terrigenous and biogenic material from suspension. Glacier and shelf ice locally make small contributions. Wind-blown sand is a major component, as much as 70 percent in places, of sea floor sediment on the western shelf and slope. However, the deep basins around Ross Island are floored with mud mostly containing less than 5 percent sand. Superglacial debris appears limited to the southern and western parts of the Sound. Coarse basal glacial debris was not found even close to the floating terminus of Mackay Glacier, where it appears to have already melted out.

References

Barrett, P.J., Pyne, A.R., ward, B.L. 1982. Modern sedimentation in McMurdo sound, Antarctica (abs.), Fourth International Symposium on Antarctic Earth science, Adelaide. August 16-22

Ward, B.L. 1982. Benthic foraminifera of McMurdo Sound (abs.), Fourth International Symposium on Antarctic Earth Science, Adelaide, August 16-22.

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McMurdo Sound Bathymetry and Oceanography (K5)- P. Barrett.

Over the last four seasons about 200 soundings have been made from surveyed positions on the sea ice in the course of gravity survey (Sissons, 1980) and sediment sampling (Barrett, 1979; Pyne, 1981; and this season). Most measurements were made with a weighted terylene line, and compare well with measurements from a meter wheel on 5mm steel cable taken during coring, the terylene gave measurements from 0 to 3m greater than the steel cable along the western side of the Sound and in Granite Harbour in depths as gerat as 550m, though in the central part of the Sound, where there may have been significant current activity measurements ranged from 0 to 16m greater.

Bathymetry beyond the coastal fringe was obtained from continuous depth recorder records off cruises by VUW and DSIR personnel since 1972 and plotted by B. Ward. The ships track was obtained from satellite navigation data, radar fixes and dead reckoning, and depths were read off at km intervals. Most depths at track crossings were within 10m, the greatest different being 24m. Comparison of depths from ships tracks and line soundings are hard to make because of errors in the ships position of several hundred metres, and local relief of the sea floor, but appear to be of the order of a few metres. 500 data points were plotted from the shipboard data, and contoured along with the nearshore data (Fig. 7). Soundings were also made in Granite Harbour, allowing for the first time a coherent view of its bathymetry (Fig. 6).

Measurements of current velocity and tide were made over a 2 day period in 167m of water 0.6km off the edge of the Strand Moraines (77°45S; 164°30E). Velocities ranged from less than 0.02 (sensitivity of the meter) to 0.12m/s, but appeared unrelated to the phase of the tide, which had a range of 0.6m in that period. No current was recorded near the sea floor but became detectable about 5m above. Flow was invariably to the north, judging from the deflection of the wire.

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Figure 6 (above) & 7 (below): Bathymetry of McMurdo Sound and New Harbour respectively. Locations of cores 1 to 18 are also shown.

Figure 6 (above) & 7 (below): Bathymetry of McMurdo Sound and New Harbour respectively. Locations of cores 1 to 18 are also shown.

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Triassic strata at Mt. Bastion (K7A) - B. Walker.

The Triassic Beacon Supergroup of south Victoria Land is divided into 5 stratigraphic units, the Fleming Member of the Feather Conglomerate and Members A-D of the Lashly Formation, and all are exposed at Mount Bastion. Preliminary investigations show that at least 4 distinct paleohydraulic regimes operated during the deposition of the Triassic strata.

The Fleming member consists of medium to large trough-x-bedded, medium to coarse grained quartzose sandstones with intervals of planar-x-beds. Paleocurrent variability is low as was also found for the other members. The Fleming member is considered to have been deposited in a braided stream environment where dune migration along the floors of wide but shallow river channels and transverse bar formation were the dominant processes.

Member A of the Lashly Formation has a greater diversity of sedimentary structures and variation in grain size. A change in grain composition from quartzose to volcaniclastic is evident from the weathering behaviour of the sandstones. The 80 metres of member A can be broadly divided into two facies, a trough-x-bedded medium grained sandstone facies and a parallel and ripple laminated very fine sandstone to siltstone facies with mudstones containing white rootlets. River channel cross-sections can be clearly seen at some exposures and show channel widths in the order of 100 metres with depth of 3 to 5 metres. Both facies are considered to be "in channel" facies with the mudstones representing still water deposition and alter plant colonisation in abandoned channels. The trough-x-bedded sandstone facies may represent deposition of sand within channels during floor events. A 0.5 metre thick sandstone with very low angle cross-stratification interbedded with horizontal laminae is at present unique to the section at Mount Bastion and requires further investigation as does a cross-stratified sedimentary structure whose geometry has not been reported in the scientific literature.

Member B is 96 metres thick and consists almost entirely of medium grained well-sorted sandstone. The bedforms are of a larger scale than found elsewhere with individual trough and planar-x-beds several metres thick. Large scours are common and often associated with thick (up to 14 metres) massive sandstones containing large silicified logs. Channel cross-sections exposed within a bluff at the northern end of Mount Bastion indicate channel depths of over 10 metres. Member B is interpreted as representing the deposits of wide and at times deep rivers that transported large amounts of sand, large mudstone blocks (up to 1 metre across) and plant debris during the peak flow of big floods. Channel avulsion was common accounting for the occasionally recorded 90° paleocurrent different from the general flow direction. In at least one place channel avulsion has cut through 20 metres of ripple laminated carbonaceous and non-carbonaceous fine sandstone. As yet unexplained is a 180° paleocurrent reversal that occurs within an interval of 5 metres and can be traced for the length of the outcrop (400m).

A complete change in paleohydraulic regime occurs in Member C which consists of thinly bedded carbonaceous and non-carbonaceous ripple laminated very fine sandstone and siltstone The 78 metres of Member C includes 2 thick sandstone units which suggest inundation of coarser flood debris onto what may have been a shallow lake environment. Conditions were at times stable long enough to allow for the accumulation of peat which eventually formed thin coal seams. Plant material is abundant suggesting that the Triassic landscape was well covered in vegetation.

Member D is 205 metres thick and consists largely of trough and planar-x-bedded medium to coarse grained sandstones. It is similar to parts of Member B and the Fleming member and similar conditions for its deposition are envisaged. Towards the top of Member D several sandstone units show a return to a quartzose composition last seen in the Fleming Member.

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Lower Feather Conglomerate at Mount Bastion (K7A) - P. Fitzgerald.

The lower part of the Feather Conglomerate at Mt. Bastion was deposited in late Permian times on a moderately steep west-to-northwest dipping paleoslope (Fitzgerald, 1982). Deposition was characterised by sheetflow during flood events and low sinuosity braided rivers during normal flow. The dominant facies present, trough cross-bedded sandstone, is a result of dune migration in the upper lower flow regime. The planar cross-beds are a result of linguoid bar formation, whereas other minor facies are a consequence of low water modification processes and overbank sedimentation. Sediments are from a dominantly granitic provenance and consist of submature quartzarenites and subarkoses. Roundness of quartz pebbles (0.55) and mineralogical immaturity indicate deposition within a few hundred kilometres of source.

The paleosols have been recognised from several features including vein networks, which result from desiccation produced by clay contraction. Mottling and gleying in the underlying sandstone along with gammate structure in the paleosols themselves indicate periodic, possibly seasonal, waterlogging of the soil profile. Concretions associated with the paleosol have formed both syngenetically and epigenetically. These along with absence of any carbonaceous material suggest an oxidising environment and a low level of contemporaneous vegetation. Climate was temperate to moderately cool and humid.

Trace fossils included Skolithos and hypichnial ridges. Skolithos occurs throughout the section, in some instances destroying almost all sedimentary structures. It was not found in fine grained sediments or in paleosols. The presence of Skolithos as well as clean, moderately well sorted sands indicate a high energy environment and abrupt sediment aggradation or degradation. Its presence here in alluvial plain sediments indicates that it can occur in non-marine as well as marine sediments.

A metamorphic overprint assemblage has resulted from the intrusion of the Ferrar Dolerite in Jurassic times. Both 7 Å and 14 Å chlorite are present in the sandstones but only 7 Å in the paleosols. 14 Å oxy-chlorite is a result of alteration of detrital biotite and 7 Å a result of recyrstallisation of clays due to thermal metamorphism. The presence of pyrophyllite also indicates metamorphism and its co-existence with kaolinite and quartz suggests a temperature of 310-315°C.

References

Fitzgerald, P.G., 1982: Environment of deposition of the Feather Conglomerate at Mount Bastion, south Victoria Land, Antarctica. Unpubl. BSc Hons Thesis, Victoria Univ. of Wellington Library, 89pp.

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Permian strata in north Victoria Land (K18) - B. Walker.

Alluvial sediments:

The sediments of the Beacon Supergroup in north Victoria Land occur as scattered outcrops exposed along cliff faces and valley walls, that have been cut by glaciers. The Permian sediments (recognised from the presence of the Glossopteris flora) rest on the flat-lying sub-Beacon surface that cuts a basement of metasedimentary and plutonic rocks.

The Takrouna Formation proposed by Dow and Neall (1974) for the sediments of north Victoria Land is from this seasons field work divided into two facies (Walker, 1982).

Facies Association 1 which outcrops in the Alamein Range, Morozumi Range and Helliwell Hills is characterised by variation in type, scale and grain size of sedimentary structures. Medium scale trough-x-bedding and ripple lamination are the most common bedforms. Mudstone lenses, planar-x-bedding, scour fill and coal measures (including coal seams, green siltstones and fining upwards sandstones) are also found. Coarse to fine grained ratios vary within sections and between outcrops and it is evident that a variable paleohydraulic regime operated during the deposition of this facies. The trough and planar-x-bedding and ripple laminated units are considered to be deposits reflecting the varied flow conditions associated with the different water level stages of flood episodes. The different scales of the same sedimentary structures reflects the varying intensity of those floods. The fine grained units including the coal measures are most probably in channel facies that were deposited when a channel became separated by the processes of channel avulsion and migration from the main flow of water. Channel cross-sections, low paleocurrent variability and sequence of sedimentary structures suggest that the rivers that deposited facies 1 had high width to depth ratios and were of low sinuosity and due to an apparent variable climatic regime could expect a variable discharge.

Facies Association 2 is exposed in the Moawhanga Never Gair Mesa, Neall Massif, Jupiter Amphitheatre (Morozumi Range) and is in marked contrast with Facies Association 1. It is dominated by multi-storey "sheet-like" sandstone bodies comprised almost entirely of medium to large scale trough and planar-x-bedding with individual sandstone bodies separated either by erosion surfaces or mudstone drapes. Facies Association 2 is considered to have been deposited by braided rivers with high width to depth ratio channels. Individual sandstone bodies probably represent discrete flood events which transported large quantities of sand by the processes of dune and transverse bar migration. Flood events were relatively frequent and allowed no time for overbank deposits to develop.

Paleocurrents for Facies Association 1 show that it was deposited on a paleoslope dipping towards the northeast and that Facies Association 2 was deposited on a paleoslope dipping west (Fig. 8).

It is possible that Facies Association 1 represents the deposits of rivers that flowed over a flat low gradient flood plain from the area of the retreating Permian ice cap that was centred in north Victoria Land at this time (Barrett et al., 1972). This depositional episode ended when uplift of mountains to the east reversed and increased the gradient of the paleoslope resulting in a new paleohydraulic regime that determined the depositional character of Facies Association 2.

Glacial beds:

An early Permian (?) tillite sequence is exposed on the western side of the Lanterman Range, north of the Orr Glacier (McKelvey, 1982). The tillite is faulted against the metamorphic rocks of the Wilson Group and is tightly folded. The following description is from McKelvey and Walker (1982).

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The glacial beds are of acid plutonic and metasedimentary provenance and are exposed on two spurs about 2 kilometres apart. On the northern spur the oldest exposed strata consist of at least 70 metres of poorly sorted lensoidal or channel-fill outwash conglomerates, pebbly sandstones and diamictites associated with laminated or cross-bedded coarse sandstones. The coarser rock types are most common at the top and bottom of this interval. A dark mudstone and shale unit containing interbeds of thin sandstone, lensoidal conglomerate and a fine diamictite overlies. This unit thickens eastwards to at least 150 metres and changes rapidly in fades to a monotonous mudstone sequence containing in its uppermost 80 metres thin laterally persistent graded and laminated turbidites sometimes with erosional bases and containing infraformational sandstone clasts. No dropstones are present. More than 100 metres of coarse outwash strata similar to those at the base of the section overlie.

On the southern spur a similar coarse sandstone sequence with lensoidal conglomerates and pebbly sandstones at the base overlies more than 150 metres of fissile or massive mudstones alternating with very coarse tillite sheets and lenses. Most of the mudstone and shale horizons contain sparsely scattered dropstones.

The contact between the folded glacigene sequence in the Lanterman Range and the widespread Permian fluvial Takrouna Formation of northern Victoria Land has not been observed. Coarse metasediment breccias up to 50 metres thick and associated sometimes with minor petromict conglomerate lenses frequently mantle the pre-Permian basement complex beneath the Takrouna Formation. No definite glacial features have been observed in these breccias.

References

Barrett, P.J., Grindley, G.W., Webb, P.N. 1972. The Beacon Supergroup of East Antarctica. In: Adie, R.J. (Ed.), Antarctic Geology and Geophysics, Universitetsforslaget, Oslo: 319-332.

McKelvey, B.C., Walker, B.C. 1982. Late Paleozoic glacigene strata in northern Victoria Land (abs.), Fourth International Symposium on Antarctic Earth Science, Adelaide, August 16-22.

Walker, B.C. 1982. The Beacon Supergroup of northern Victoria Land, Antarctica (abs.), Fourth International Symposium on Antarctic Earth Science, Adelaide, August 16-22.

Figure 8 Localities visited in north Victoria Land by K18. Arrows are paleocurrent directions from the Beacon strata.

Figure 8 Localities visited in north Victoria Land by K18. Arrows are paleocurrent directions from the Beacon strata.

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Fission-Track dating (K7B) A. Gleadow.

Twenty-one granite samples (several Kg each) were collected for fission-track dating at various elevations in the lower Wright Valley and along the McMurdo Sound coast between Gneiss Point and Spike Cape.

Field observations suggest that step faulting on a number of different faults may be important in controlling the eastern front of the Transantarctic Mountains in this area (Fig. 9). Fission-track ages of apatites from these samples were reported at the Fourth International Symposium on Antarctic Earth Sciences, and confirm the step-like nature of the faulting. They also indicate a steady uplift rate of 15m/m.y. throughout the Mesozoic (Gleadow, 1982).

References

Gleadow, A.J.W., 1982: Fission-track geochronology of granitoids and uplift history of the Transantarctic Mountains, Victoria Land, Antarctica (abs.). Fourth International Symposium on Antarctic Earth Science, Adelaide, August 16-20.

Figure 9: Latitudinal section through Mt. Doorly, showing the step-like nature of the faulting in the basement near the Victoria Land coast.

Figure 9: Latitudinal section through Mt. Doorly, showing the step-like nature of the faulting in the basement near the Victoria Land coast.