BEDROCK STUDIES (K043)
|1)||by determining pre-Devonian structural and metamorphic history of the basement in the Victoria and Wright valleys, with emphasis on the depth of burial and amount of uplift prior to Beacon sedimentation.|
|2)||by determining the Late Mesozoic and Cenozoic uplift history of the Royal Society Range.|
Structure and Metamorphism of the Basement Complex (K043A) - R.J. Korsch
The lower Victoria and Wright valleys are relatively ice-free and hence ideal to determine the pre-Devonian structural and metamorphic history of the basement complex. The rocks have suffered polyphase deformation with possibly four fold generations being recognised. F1 produced isoclinal folds which have been refolded during later events. F2 is the dominant fold generation, producing close to isoclinal asymmetric folds and a well-developed, subvertical, NW-SE striking axial surface foliation. Fold axes have variable plunges within this plane. F3 and F4 are localised in extent. Some Theseus Granodiorite dykes contain the S3 foliation and hence are pre-F3 in age, whereas the Vanda porphyries and lamprophyres are unfolded and hence post-F3 in age. The field studies confirm a complicated pre-Devonian structural and metamorphic history of the McMurdo region of the Transantarctic Mountains.
Field work during the 1984-85 summer season concentrated on the Robertson Ridge in the lower Victoria Valley and the ridges between the Meserve and Goodspeed valleys in the lower Wright Valley. Field objectives were to carry out detailed structural mapping, and the collection of structural data and a representative suite of the metamorphic rocks. As well, nine large (>35 kg) samples of granitoids and related rocks were collected for U-Pb zircon dating.
The basement complex in the lower Victoria and Wright valleys consists of multiply-deformed metasedimentary rocks intruded by granitoids and related rocks of the Wright and Victoria Intrusive Suites. The metamorphic rocks have been referred to as the Asgard Formation (McKelvey and Webb, 1962) or the Meserve Member of the Hobbs Formation (Findlay, 1983). They are now biotite schist and gneiss, hornblende gneiss, quartz-feldspar gneiss, augen gneiss and calc-silicate rocks along with rarer pure marble, amphibolite, quartzite and pelitic schist. Prior to metamorphism they represented a sedimentary sequence dominated by sandstone but also containing mudstone, pure limestone and impure limestone suggestive of a passive continental margin depositional environment.
Although most rock types are represented at each locality, the proportions vary; at the Robertson Ridge the dominant lithology is quartz-feldspar-biotite-hornblende gneiss, and augen gneiss is rare; between the Meserve and Hart glaciers the sequence is dominated by augen gneiss and between the Hart and Goodspeed glaciers, hornblende schist is most abundant and augen gneiss less common.
Preliminary structural results determined in the field, suggest that, with minor differences, the lower Victoria and lower Wright valleys have very similar deformational histories.page 19
At Robertson Ridge, evidence was observed for at least three deformational events, here labelled F1, F2 and F3.
|F1:||The earliest deformation produced isoclinal folds with fold axes now having variable orientations. Apart from in the hinge zones, lithological layering has been transposed into parallelism with the axial surface foliation (S1). It is possible that the lithological layering represents original bedding, but the lack of recognisable sedimentary structures hinders confirmation. The axial structures of the F1 folds also have variable orientations due to later refolding by F2 and F3 events (Fig. 11). F1 folds are relatively rare in comparison with F2 folds.|
|F2:||This is the dominant fold generation, producing abundant, close to isoclinal, asymmetric folds, along with a well-developed axial surface foliation (S2) in certain lithologies. The plunges of the fold axes vary from subhorizontal to moderately plunging within a subvertical, NW-SE striking axial surface. Where folding has been intense enough, the lithological layering has been transposed into parallelism with S2. Elsewhere, the S2 foliation is oblique to the lithological layering and/or S1 foliation. Minor structures associated with this fold event include the long axes of boudins, mineral lineation and intersection lineation, all of which parallel the orientations of the fold axes.|
|F3:||This deformation produced gentle to tight folds, crenulations and occasionally an axial surface cleavage (S3). The F3 folds refold both F1 and F2 folds and have axial surfaces which dip 50° - 70° to the NW. Most of the fold axes plunge gently to the north. Although this deformation is only of localised extent it is extremely useful in that it enables some members of the Victoria Intrusives to be dated relative to this deformational event. Dykes of Theseus Granodiorite contain the S3 foliation but no evidence of S2, and hence are post-F2 but pre-F3 in age. The Vanda Porphyry and Lamprophyre are unfoliated and hence post-F3 in age.|
The structural history of the lower Wright Valley is very similar to that of the Robertson Ridge, the main exception being orientations of the F2 fold axes. Structurally, the area between the Meserve and Hart glaciers can be subdivided in two, with the steeper part of the ridge separated from the lower, flatter part by a band of scree. In the upper part of the ridge, the F2 fold axes normally plunge very gently to the SE, whereas on the flatter section the F2 fold axes plunge subhorizontally to steeply both to the NW and SE. In the areas between the Hart and Goodspeed glaciers, the F2 fold axes plunge gently to the NW and SE. The large (λ = 1 km) folds between the Meserve and Goodspeed glaciers visible in aerial photographs (Fig. 12) are here interpreted as F2 folds.
Evidence for a fourth, very localised fold generation was observed between the Hart and Goodspeed glaciers and on the flatter section between the Meserve and Hart glaciers. Gentle to open upright folds plunge steeply to the E and W, and boudins with long axes of the same orientation occur also. This generation post-dates F2 but no time relationship with the more common F3 structures could be established. Late, minor faulting occurred at some time after F3.
The structural history outlined above differs to some extent to that proposed by Murphy (1971). My F2 is mainly equivalent to Murphy's F3 but could possibly include some of his F2, and my F1 is equivalent to Murphy's F1 plus F2. Murphy did not recognise equivalents of my F3 or F4 (Table 9).page 20
Figure 11. Minor F1 fold with curved axial surface being refolded by F2 folds. Ridge between Meserve and Hart glaciers, lower Wright Valley.
Figure 12. Part of oblique aerial photograph TMA360-00166 (F33) looking south, showing the south wall of the Wright Valley between the Goodspeed and Hart Glaciers. Note the obvious large F2 folds visible in the valley wall. Photograph courtesy of the United States Geological Survey.
|1)||metamorphic petrology to assess the pressure-temperature conditions and allow an estimate of the original depth of burial of the rocks.|
|2)||U-Pb isotope dating of zircons separated from the granitoid samples.|
These studies coupled with structural analysis of the field data should provide an integrated tectonic history of the McMurdo portion of the Transantarctic Mountains prior to commencement of deposition of the Beacon Supergroup in the Devonian.
Findlay, R.H. 1983. Basement Geology of the McMurdo Sound Region, Antarctica. Unpublished report to RDRC.
McKelvey, B.C. and Webb, P.N. 1962. Geological investigations in southern Victoria Land, Antarctica. Part 3 - Geology of the Wright Valley. New Zealand Journal of Geology and Geophysics. Vol. 5, pp. 143-162.
Murphy, D.J. 1971. The petrology and deformational history of the basement complex, Wright Valley, Antarctica with special reference to the origin of the augen gneisses, unpublished PhD Thesis, University of Wyoming.