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Southern Coats Land nunataks
Shackleton Range
References |
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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
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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.;
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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;
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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:
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both normal and reversed polarities in samples from the quartz arenite
unit and
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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.
References
Aughenbaugh, N.B., R.W. Lounsbury, and J.C. Behrendt. 1965. The Littlewood
Nunataks, Antarctica. Journal of Geology, 73(6), 889-894.
Dalziel, I.W.D. 1991. Pacific margins of Laurentia and East Antarctica/Australia
as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent.
Geology, 19(6), 598-601.
Dalziel, I.W.D. 1992. Antarctica: A tale of two supercontinents? Annual
Review of Earth and Planetary Sciences, 20, 501-526.
Dalziel, I.W.D., M.A. Helper, F.E. Hutson, and S.W. Grimes. 1994. Geologic
investigations in the Shackleton Range and Coats Land nunataks, Antarctica.
Antarctic Journal of the U.S., 29(5), 4-6.
DiVenere, V., D.V. Kent, and I.W.D. Dalziel. 1995. Early Cretaceous
paleomagnetic results from Marie Byrd Land, West Antarctica: Implications
for the Weddellia collage of crustal blocks. Journal of Geophysical
Research, 100(B5), 8133-8151.
Golovanov, N.P., V.E. Mil'shteyn, V.M. Mikhaylov, and O.G. Shulyatin.
1979. Stromatoliths and microphytoliths of the Shackleton Range (western
Antarctica). Doklady Akademii Nauk, SSSR. 249(4), 977-979. [In Russian]
Gose, W.A., I.W.D. Dalziel, M.A. Helper, F.E. Hutson, and J.N. Connelly.
1997. Paleomagnetic data and U-Pb isotopic ages from Coats Land, Antarctica:
A test of the Laurentian-East Antarctic ("SWEAT") connection. Journal
of Geophysical Research, 102(B4), 7887-7902.
Grunow, A.M. 1995. Implications for Gondwana of new Ordovician paleomagnetic
data from igneous rocks in southern Victoria Land, East Antarctica. Journal
of Geophysical Research, 100(B7), 12589-12603.
Helper, M.A., S.W. Grimes, and I.W.D. Dalziel. 1995. Basement-cover
relations and fabrics of the central Read Mountains, Shackleton Range,
Antarctica. Seventh International Symposium on Antarctic Earth Sciences,
Siena, Italy. [Abstract]
Hodgkinson, G.R. 1989. Palaeomagnetic studies in western Dronning Maud
Land, Antarctica. (Unpublished Masters of Science thesis, Department of
Geophysics, University of Witwatersrand, Republic of South Africa.)
Hutson, F.E., W.A. Gose, and I.W.D. Dalziel. 1995. Paleomagnetic results
from the Neoproterozoic Watts Needle Formation, Shackleton Range, Antarctica.
Seventh International Symposium on Antarctic Earth Sciences, Siena, Italy.
[Abstract]
Kirschvink, J.L., M. Magaritz, R.L. Ripperdan, A.Yu. Zhuravlev, and
A.Yu. Rozanov. 1991. The Precambrian/Cambrian boundary: magnetostratigraphy
and carbon isotopes resolve correlation problems between Siberia, Morocco,
and South China. GSA Today, 1(4), 69-71, 87, 91.
Marsh, P.D. 1983. The Late Precambrian and Early Paleozoic history of
the Shackleton Range, Coats Land. In R.L. Oliver, P.R. James, and J.B.
Jago (Eds.), Antarctic earth science. Canberra: Australian Academy
of Science.
Marsh, P.D., and J.W. Thomson. 1984. Location and geology of nunataks
in north-western Coats Land. British Antarctic Survey Bulletin,
65, 33-39.
Moores, E.M. 1991. The Southwest-U.S.-East Antarctica (SWEAT) connection:
A hypothesis. Geology, 19(5), 425-428.
Moyes, A.B., J.M. Barton, Jr., and P.B. Groenewald. 1993. Late Proterozoic
to Early Paleozoic tectonism in Dronning Maud Land, Antarctica: Supercontinental
fragmentation and amalgamation. Journal of the Geological Society London,
150, 833-842.
Olesch, M., H.M. Braun, E.N. Kamenev, G.I. Kamenev, and W. Schubert.
In press. Read Group. In J.W. Thomson (Ed.), British Antarctic Survey
Geomap 4.
Pankhurst, R.J., P.D. Marsh, and P.D. Clarkson. 1983. A geochronological
investigation of the Shackleton Range. In R.L. Oliver, P.R. James, and
J.B. Jago (Eds.), Antarctic earth science. Canberra: Australian
Academy of Science.
Peters, M. 1989. Igneous rocks in western and central Neuschwabenland,
Vestfjella and Ahlmannryggen, Antarctica: Petrography, geochemistry, geochronology,
paleomagnetism, geotectonic implications. Berichte zur Polarforschung
(Vol. 61). Bremerhaven, Germany: Alfred-Wegener-Institute for Polar and
Marine Research.
Storey, B.C., R.J. Pankhurst, and A.C. Johnson. 1994. The Grenville
Province within Antarctica: A test of the SWEAT hypothesis. Journal
of the Geological Society London, 151, 1-4.
Toubes Spinelli, R.O. 1983. Geology of the Bertrab Nunatak, Argentinian
sector of Antarctica. Contribucion Instituto Antarctico Argentino,
296, 1-9. [In Spanish]
Weber, K. 1991. Microfossils in Proterozoic sediments from the Southern
Shackleton Range, Antarctica: A preliminary report. Zeitschrift für
Geologie Wissenschaft, 19(2), 185-197. |