Abstract

Sampling of granitoid basement rocks to determine the late Mesozoic and Cenozoic uplift history of the Transantarctic Mountains in the McMurdo Sound region was completed this season. The study will utilise fission-track dating techniques on apatites at the University of Melbourne to provide uplift rates across the mountains.

Granitoid samples were collected from the New Harbour area including "basement" from the CIROS 2 drill hole. Topographic features attributed to faulting are an eastwards decrease of peak heights along the Kukri Hills and a saddle on the east flank of Mt Coleman.

The Transantarctic Mountain Front from Miers Valley across the Blue Glacier to the Royal Society Range was also sampled. The upper Blue Glacier lies in a graben bounded on the west by a number of normal faults and to the east by the block of Miers to Salmon Valleys area.

This was the second field season of a PhD study to determine the uplift history of the Transantarctic Mountains using fission-track dating. This technique depends on the determination of the gradient of apatite fission-track age with elevation, which requires the taking of samples at regular intervals over significant elevation ranges in order to gain information representing the greatest possible time period. The main area visited this season was the Blue Glacier which contains exposures with relief of up to 2500 m over short horizontal distances. Miers Valley, the New Harbour area and Robertson Ridge were also visited.

Field work last season as well as previous reconnaissance fission-track studies (Gleadow, 1982; Gleadow et al., 1984; Gleadow and Fitzgerald, 1984; Fitzgerald and Gleadow, 1984) show that step-faulting is important in controlling the eastern front of the Transantarctic Mountains. They also reported that uplift during the Mesozoic was a steady 15 m/Myr but this increased dramatically close to 50 Myr ago to about 90 m/Myr.

The geology of the field area consists of the multiply-deformed Pre-Cambrian Koettlitz Group which is composed of marbles, conglomerate, amphibolitic, quartzofeldspathic and pelitic schists intercalated with quartzofeldspathic gneisses. Intruding this are the Cambro-Ordovician Granite Harbour Intrusives composed of a number of pre- to post-tectonic granitoids. These basement rocks are unconformably overlain by the Devonian-Jurassic Beacon Supergroup which is made up of glacial, alluvial and shallow marine strata. Both the Beacon and the basement rocks were intruded in the Jurassic by the Ferrar Dolerite in the form of a number of essentially horizontal sills. These sills can be traced with little offset over distances of up to tens of kilometres, especially in the basement. They can therefore be used as reference surfaces to determine displacements that occurred in post-Jurassic times and which may be related to the tectonic development of the Transantarctic Mountains.

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In areas where the Ferrar Dolerite does not crop out, fission-track dating can be used to generate reference surfaces. Because it has been demonstrated that the apatite ages vary with elevation in a number of profiles, each age can be taken to represent a certain tectonic level. Sampling for fission-track studies is limited to those rocks which contain suitable uranium-enriched minerals. This study is looking mainly at apatite which is common in granitic rocks and in the majority of areas visited this season, dolerite did not crop out, hence the need for artificial reference planes.

New Harbour

CIROS 2, drilled in October-November 1984 at the snout of the Ferrar Glacier, struck basement at 167 m sub-bottom, approximately 378 m below sea-level. This gave us our lowest sample yet and the chance to quantitatively measure offset across the mountain front. A vertical sampling profile was taken off the eastern end of the Kukri Hills, starting from the top of Mt Barnes. Another vertical sample line was also taken on the south side of New Harbour, off the peak lying just to the west of spot height 1204 m.

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Individual samples were taken from Hjorth Hill, Mt Coleman, Mt Falconer and the south ridge of Mt McLennan as well as Mt Herb (spot height 1097) and spot height 1204 m. This was to try and locate the position of any faults, such as the Bowers Fault (Gunn and Warren, 1962), postulated to run across the front of the Transantarctic Mountains. The topographic expression of this fault can be seen on the east flank of Mt Coleman. The abrupt drop in peak height as one moves east along the Kukri Hills, is possibly further evidence of faulting, as suggested by Wrerm and Webb (1982). Fission-track data from this area should do much towards solving this problem.

Miers Valley

The Miers Valley was briefly visited in the 1983/84 season and a vertical sampling profile of 800 m was taken off surveyors Peak. This yielded apatite fission-track ages of 56 ± 3.7 to 43 ± 1.6 Myrs and again shows the change of uplift rate from 15 m/Myr prior to 50 Myr, to approximately 95 m/Myr (Fig. 14). It became clear from this that ages younger than 50 Myr are useless as tectonic markers as the errors in age compared to change in elevation overlap considerably. The errors in ages older than 50 Myr are small compared to change in elevation and so are good tectonic markers, a change of a few 100 m or so producing a significant age difference. It was therefore necessary to take samples from the tops of ridges or summits of peaks when they were to be used as tectonic markers. A horizontal sampling line across the Transantarctic Mountain Front was started at the coast and continued along the ridge between the Marshall and Miers Valleys almost as far west as spot height 1430 m.

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Blue Glacier area

The sample line across the Transantarctic Mountain Front started in the Miers Valley area, was picked up at spot height 1430 and extended across the Blue Glacier and up the ridge south or Salient Glacier. Samples were also taken from other ridges to try and tie down structural trends. A vertical sampling profile of 800 m was taken from spot height 1890. Samples were also taken across the north end of the Blue Glacier/Briggs Hills area in an extension of work done in the New Harbour area.

A prominent scarp west of Bettle Peak suggesting a fault downthrown to the west is also seen further south to the west of Williams Peak. This is the Foothills Fault of Gunn and Warren (1962) and defines the eastern margin of the Blue Glacier graben; the western margin being defined by the Lister Fault running along the end of ridges which come down off the Royal Society Range lying to the west. However, there exists another fault between the Lister Fault and the Royal Society Range. This is seen in ridges underneath Chaplins Tableland just north of Mt Lister where the basement dolerite sill is downfaulted approximately 200 m to the east. It continues south to Armitage Col and north to the Ferrar Glacier, however no evidence of it is seen north of the Ferrar. Numerous notches in these ridges suggest the presence of more faults in the Southern Foothills running parallel to this fault but no slickensides were seen in what in some cases were possibly poorly indurated crush zones.

In summary: The upper Blue Glacier appears to lie in a graben, its western boundary defined by a number of normal faults. Its eastern boundary is delineated by what appears to be a single block, consisting of the Miers-Marshall-Salmon Valleys and probably also including a small block to the north of the lower Blue Glacier. The block is bounded on its western side by a large normal fault and most probably on its eastern side by a number of step faults similar to those seen further north along the Wilson Piedmont. This graben is terminated to the north by the Ferrar Glacier.

Samples taken for fission-track dating will be processed at the University of Melbourne and representative samples of granitoid rocks from the area covered will be studied in detail at Victoria University to determine mineralogy and geochemistry.