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Southern Coats Land nunataks

Shackleton Range


Initial results of geologic investigations in the Shackleton Range and southern Coats Land nunataks, Antarctica

Frederick E. Hutson, Mark A. Helper, Ian W.D. Dalziel, and Stephen W. Grimes, Department of Geological Sciences and Institute for Geophysics, University of Texas, Austin, Texas 78712

We present here initial results of geologic investigations conducted during the 1993-1994 field season in the Shackleton Range and the southern Coats Land nunataks (Dalziel et al. 1994) (figure 1). The major goal of this study is to test the "SWEAT" (Southwest U.S.-East Antarctica) hypothesis, which proposes that Laurentia and East Antarctica-Australia were juxtaposed in the Proterozoic and formed part of the supercontinent, Rodinia (Dalziel 1991; Moores 1991). The SWEAT hypothesis suggests that the approximately 1.0-billion-year-old rocks of the southern Coats Land nunataks are a continuation of the 1.0- to 1.3-billion-year-old Grenville Province of North America and that approximately 1.6- to 1.8-billion-year-old rocks of the Yavapi/Mazatzal Province in the southwestern U.S. are correlative with broadly similar-age rocks in the Shackleton Range. We are examining the hypothesis by

  • comparing the igneous rocks of the southern Coats Land nunataks and basement rocks of the Shackleton Range with their proposed equivalents in the southwestern U.S.; 
  • attempting to correlate the late Neoproterozoic Watts Needle Formation, which is exposed in the southern Shackleton Range, with similar-age sequences in Australia and western North America; 
  • determining paleomagnetically the position of the east antarctic craton relative to Laurentia between approximately 1.0 and 0.7 billion years ago. 

Figure 1. Map of the Weddell Sea margin of the east Antarctic Precambrian craton (see inset), showing location of the "Grenville Front," as suggested by Dalziel (1992), the Maudheim and Grunehogna Provinces (from Moyes, Barton, and Groenewald 1993), the Shackleton Range, and the location of the southern Coats Land nunataks. Generalized areas of rock exposure are shown in black. Abbreviations: Ahl, Ahlmannryggen; B, Borgmassivet; D.L.M., Queen (Dronning) Maud Land; T.A.M., Transantarctic Mountains (after Gose et al. in press). (Ga denotes billion years.)

Southern Coats Land nunataks

The Bertrab, Littlewood, and Moltke nunataks are exposed along the southeastern Weddell Sea coast and are herein collectively referred to as the southern Coats Land nunataks (figure 2). We mapped and sampled the Bertrab and Littlewood nunataks but were unable to visit Moltke Nunatak, which is exposed in an ice-fall. Marsh and Thomson (1984) discuss the confusion over the exact location of the Bertrab Nunataks. Using air photographs and satellite data, these authors determined the position of the largest nunatak of the group as 7753'S 3438'W. We confirmed this position using a hand-held global positioning system device, which was also used to locate and map the other nunataks of the Bertrab and Littlewood Groups.

Figure 2. Maps of the Bertrab and Littlewood nunataks showing the location, geology, and sampling sites. The location of inset B is shown by the black box in inset A. Inset B shows the location of the Bertrab and Littlewood Nunataks. Medium shading in main figure (C) and inset D denotes granophyre at the Bertrab nunataks and rhyolite at the Littlewood Nunataks. Within the Bertrab Nunataks, solid, black lines are mafic dikes and northeast-trending, shaded dikes are rhyolite (after Gose et al. in press).

The Bertrab Nunataks are composed of red-to-gray weathering, fine- to medium-grained, oligoclase-phyric, isotropic granophyre, which is cut by flow-banded rhyolite dikes and altered, mafic dikes (figure 2C) (Toubes Spinelli 1983; Marsh and Thomson 1984; Gose et al. 1997). The five small outcrops of the Littlewood Nunataks (figure 2D) are composed of red-weathering, densely silicified rhyolite (Aughenbaugh, Lounsbury, and Behrendt 1965). Storey, Pankhurst, and Johnson (1994) report a whole-rock rubidium-strontium (Rb-Sr) age of 1,076±7 million years for the Bertrab granophyre and a recalculated whole-rock Rb-Sr age of 976±35 million years for a mixture of samples from Bertrab and Littlewood nunataks. Aughenbaugh et al. (1965) report a whole-rock potassium-argon (K-Ar) age of 840±30 million years for rhyolite at the largest outcrop of the Littlewood Nunataks.

Uranium-lead (U-Pb) isotopic analyses of two fractions of zircon from the Littlewood rhyolite and two fractions of titanite from the Bertrab granophyre yield concordant U-Pb ages of 1,112±4 million years and 1,106±3 million years, respectively (Gose et al. 1997). The ages represent a crystallization age for the rhyolite and a cooling age for the granophyre. These ages support earlier suggestions of a cogenetic origin for the granophyre and rhyolite and indicate cooling of the granophyre below the magnetite Curie Point (580C) by approximately 1.1 billion years ago.

Eighty-four oriented samples were collected from six sites (four in the granophyre and two in rhyolite dikes) at the Bertrab Nunataks and three sites in the rhyolite at the Littlewood Nunataks (figures 2C and D). Rock magnetic and petrologic studies indicate that magnetite is the dominant carrier of magnetic remanence in the Bertrab granophyre and hematite is the carrier for the Littlewood rhyolite. Site means of the Bertrab and Littlewood samples are indistinguishable and yield a mean pole position of 23.9S 258.5E with an error of a95=4.00 (Gose et al. 1997). The remanent magnetization is interpreted as a primary thermal remanent magnetization. This interpretation is supported by a lack of evidence for later thermal resetting (Aughenbaugh et al. 1965; Marsh and Thomson 1984; Gose et al. 1997), as well as a broad similarity of the Coats Land pole position with paleopoles obtained from approximately 1.0-billion-year-old rocks in Queen Maud Land (Hodgkinson 1989; Peters 1989) (figure 3) and dissimilarity to poles obtained from younger rocks in Antarctica (cf. DiVenere, Kent, and Dalziel 1995; Grunow 1995).

After rotation of the east antarctic craton about an Euler pole consistent with the SWEAT reconstruction, our new Coats Land pole falls directly on the Laurentian apparent polar wander path (APWP), lending support to the Rodinian reconstruction of Dalziel (1991) (figure 3). Our approximately 1,100-million-year-old Coats Land pole, however, overlaps poles that define the 1,000-million-year-old segment of the Laurentian APWP. Uncertainties in the age of magnetization acquisition for both the poles of the Laurentian APWP and the Coats Land pole may account for this discrepancy.

Shackleton Range

The Shackleton Range is composed of Paleo- to Mesoproterozoic basement gneisses and granitoids overlain by upper Neoproterozoic and lower Paleozoic supracrustal rocks (Marsh 1983; Pankhurst, Marsh, and Clarkson 1983). Concurrent studies of the basement and supracrustal rocks are underway with the aim of comparing the tectonic history of the range with equivalent age rocks in the southwestern United States. Our initial efforts have focused on isotopic and structural studies of basement rocks and a paleomagnetic study of the overlying Neoproterozoic clastic and carbonate rocks of the Watts Needle Formation of the Read Mountains in the southern Shackleton Range (figure 1).

Figure 3. Precambrian virtual geomagnetic pole positions (VGP's) from Antarctica with their 95 percent circles of confidence and ages shown after rotation of the antarctic poles around an Euler pole consistent with the SWEAT reconstruction. Abbreviations: COATS, southern Coats Land (Gose et al. in press); BORG, Borgmassivet (Hodgkinson 1989); AHL, Ahlmannryggen (Peters 1989). Crosses indicate North American paleopoles which define the Laurentian APWP with generalized ages shown for the path. North America is shown in present-day coordinates with Antarctica and Australia restored to the SWEAT reconstruction at approximately 750 million years. The Grenville Province and its proposed continuation into Antarctica is indicated by dark shading (after Gose et al. in press). (Ma denotes million years.)

In the central Read Mountains, the basement comprises middle amphibolite to granulite grade gneisses, amphibolites, and migmatites intruded by variably foliated to unfoliated granitoids (Read Group; Olesch et al. in press). Foliated but nonmylonitic migmatites and relict granulites occur north of an east-west striking, south-dipping zone of intense mylonitization, the Read Mountain Mylonite Zone (RMMZ) (Helper, Grimes, and Dalziel 1995), that transects the central part of the range. Grain size reduction textures in quartz and feldspar within mylonites of a variety of lithologies are consistent with shearing at amphibolite facies conditions. Subparallel zones of phyllonite and lower temperature mylonite within the southern portion of the RMMZ indicate renewed or continued motion at greenschist facies conditions. Both fabrics are cut by subhorizontal to moderately north-dipping, brittle shears and faults. Maximum ages of mylonitization and dynamic metamorphism are constrained by new U-Pb zircon ages of approximately 1,790 million years and approximately 1,785 million years (Helper unpublished data) for a slightly discordant, dioritic layer of mylonitic orthogneiss and a concordant deformed tonalite dike, respectively. These ages are interpreted as crystallization ages of the igneous precursors. The tonalite dike is subparallel to the mylonitic foliation and is boudinaged but not internally foliated, possibly indicating late-kinematic emplacement. Further U-Pb dating of cross-cutting dikes and granitoids, as well as high-grade orthogneisses, is presently underway to constrain the minimum age of ductile deformation and to directly date the metamorphism.

The Watts Needle Formation is composed of a lower clastic and upper carbonate unit that rests nonconformably on Mesoproterozoic granitoids (Marsh 1983). A Vendian age has been assigned on the basis of acritarchs, stromatolites, and a whole-rock Rb-Sr model age of 720 million years (Golovanov et al. 1979; Pankhurst et al. 1983; Weber 1991). A detailed study of this unit may enable us to correlate it with other well-studied Vendian units worldwide (cf. Kirschvink et al. 1991).

We collected oriented samples from both the granitic basement (31 samples) and overlying Watts Needle Formation (157 samples) at Mount Wegener and Nicol Crags. Samples were drilled at approximately 1.0-meter intervals and 10 or more cores were collected at selected stratigraphic horizons.

Paleomagnetic results from basal red siltstones and sandstones of the Watts Needle Formation at Mount Wegener yield a preliminary mean pole position at 18.5S 44.3E with an a95=7.50 (Hutson, Gose, and Dalziel 1995). A quartz arenite layer that underlies the upper carbonate section at Mount Wegener yields a preliminary mean pole position at 4.3S 56.4E with an a95=11.10 (Hutson et al. 1995). A well-defined component of primary remanent magnetization for these units was not reset during later tectonic events (e.g., Ross Orogeny). Evidence for this interpretation includes the following:

  • both normal and reversed polarities in samples from the quartz arenite unit and 
  • our pole positions, which are clearly different from published Early Paleozoic pole positions for the antarctic craton (cf. Grunow 1995). 
Paleopoles from the Watts Needle Formation fall close to North American paleopoles of similar age after rotation of East Antarctica into a position adjacent to western North America, as suggested by the SWEAT hypothesis. The paleomagnetic data from the Watts Needle Formation support the juxtaposition of the Laurentian and east antarctic cratons at approximately 750 million years ago.

Paleomagnetic studies of basement rocks of the Read Mountains and the lower Paleozoic Blaiklock Glacier Group are underway. Initial results from a conglomerate test in the Blaiklock Glacier Group suggest that a primary magnetization component may be recovered from these clastic rocks.

This research is supported by National Science Foundation grant OPP 91-17996. We thank J. Connelly and Kathy Manser for assistance and technical support with U-Pb isotopic work.


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