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.