Did northern Victoria Land collide with East Antarctica in the ...

K.D. Lapham. 1983. Bowers graben and associated tectonic features across northern Victoria Land, Antarctica. Nature, 304, 334-336. Tessensohn, F., K. Duphorn, H. Jordan, G. Kleinschmidt, D.N.B. Skinner, U. Vetter, T.O. Wright, and D. Wyborn. 1981. Geological comparison of basement units in north Victoria Land, Antarctica. Geologisches Jahrbuch, B41, 31-88. Weaver, S.D., J.D. Bradshaw, and M.G. Laird. 1984. Geochemistry of

Cambrian volcanics of the Bowers Supergroup and implications for the Early Paleozoic tectonic evolution of northern Victoria Land, Antarctica. Earth and Planetary Science Letters, 58, 128-140. Wodzicki, A., J.D. Bradshaw, and M.G. Laird. 1982. Petrology of the Wilson and Robertson Bay Groups and Bowers Supergroup, northern Victoria Land, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press.

Did northern Victoria Land collide with East Antarctica in the Cretaceous?

field and thermal demagnetization were employed for magnetic cleaning. Reliability of individual samples was judged using Briden and Arthur's (1982) method on single measurements, consistency of directions through demagnetization, and similarity of directions from multiple specimens. Coherence of directions between samples and the stability of the directions to demagnetization reflect the reliability of the data from a site. Only two sites in the Salamander Range proved to have stable magnetization judging by these criteria. The results of these two acceptable sites, each divided into two lithologic groups, are listed in the table and shown as virtual geomagnetic poles (vGP's) in figure 2. Comparing these data, figure 2 shows an apparent polar wander path (APwr') for Gondwana (Thompson and Clark 1982) rotated to Antarctica using Smith and Hallam's (1970) pole. Also plotted as MR (Mesa

R. F. BURMESTER and J. K. ANDERSON

Department of Geology Western Washington University Bellingham, Washington 98225

The disparate lithologies and tectonic histories across northern-Victoria. Land have prompted speculation of major displacement on faults between what are now recognized to be separate terranes (Stump et al., 1983; Weaver, Bradshaw, and Laird 1984). We report on results primarily from granite rocks of northern Victoria Land (figure 1) that suggests a preposterous tectonic model. Six sites at three localities shown on figure 1 were sampled using conventional methods adapted to subfreezing conditions (Schmierer, Burmester, and Wodzicki 1982). Both alternating

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Figure 1. Generalized geologic map showing paleomagnetic localities at Lillie Marline (LM), Salamander Range (AP), Monte Cassino (Mc) and Mesa Range (MA) with respect to the Bowers Supergroup. Modified after Stump et al. (1983). 1984 REVIEW

Figure 2. Stereographic projection centered on the south pole showing virtual geomagnetic poles from northern Victoria Land (3, 4, 5, MA, table) as well as a paleomagnetic pole from other Ferrar Supergroup localities (J)tabulated in McIntosh et al. (1982) and Thompson and Clark's (1982) APWP for Gondwana rotated to Antarctica. Crosses are at 10 million years, labeled at 50-million-year intervals. C is approximate equatorial Ealer pole required to explain divergence of northern Victoria Land VGP'S from East Antarctica poles due to relative motion. 31



Range) is the mean VGP for 174 million-year-old Kirkpatrick Basalt flows in the Mesa Range (McIntosh et al. 1982) and a mean pole (J) calculated from other quality studies of Ferrar rocks tabulated by McIntosh et al. (1982). This mean pole compares favorably with the APWP lending credence to its validity. Both sites are in rocks mapped as Devonian Admiralty Intrusives. Potassium-argon ages on biotite and hornblende from other Admiralty Intrusives are 360 to 367 million years (Kreuzer et al. 1981) but the rocks in the Salamander Range may be as young as 320 million years old. (Stump, Borg, and Holloway personal communication). The granite host (5 on figure 2) and mafic dike (4 on figure 2) of site 23 have directions similar to the Jurassic paleopole. The simplest explanation is that the granite was remagnetized during emplacement of the dike which is probably Ferrar, but the dike provides a more reliable record of the Jurassic field because of greater stability to laboratory demagnetization and lower dispersion. The other site (3 on figure 2) preserves two essentially antiparallel magnetizations strikingly different from the present or Jurassic fields. The upward directions reside in samples of coarse-grained granite (1) above a horizontal contact. The downward directions are recorded by the younger, mediumgrained porphyry (2 on figure 2) beneath. Magnetic mineralogy of both units is similar. Homogenous magnetite is the dominant magnetic mineral in both units, existing as embayed large crystals and smaller (10 micrometers) grains in altered mafic minerals. The low blocking temperature of the remanence and probable reconstitution of the magnetic cleaning deuteric alteration indicate the remanence is not a primary thermal magnetization. Nevertheless, both directions are discordant with both the Jurassic field and present field and probably reflect two directions of an earlier paleomagnetic field. Therefore, the data were combined by inverting the directions of (1 in figure 2) to provide the best possible estimate of the paleomagnetic field (3). Comparison of (3) with the APWP in figure 2 shows that site 3 may well preserve an original Devonian magnetization. All the best results from northern Victoria Land (3, 4, MR) depart from their reference poles in roughly the same direction. The consistency of the discrepancy between the northern Victoria Land VGP'S and the APWP suggest that the discrepancies

have a common cause, such as post-Jurassic motion between northern Victoria Land and East Antarctica. Left oblique convergence about pole C, figure 2, would fit all data adequately. This is consistent with a prediction of Stock and Molnar (1982) that East and West Antarctica converged in the Late Cretaceous and Early Tertiary. Other explanations such as failure of the dipole hypothesis are possible if less interesting, but demonstration of the validity of any requires more extensive collection, measurement, and analysis. We would like to thank E. Stump, J.R. Holloway, S.C. Borg, K. E. Lapham, A. Wodzicki, K. S. Schmierer, and C. Dumont for their kind assistance and cooperation in collecting the samples. This work was supported in part by National Science Foundation grant DPP 80-20726. References Briden, J. C., and G. R. Arthur. 1982. Precision of measurement of remanent magnetization. Canadian Journal of Earth Science, 18, 527-538. Kreuzer, H., A. Hohndorf, H. Lenz, U. Vetter, F. Tessensohn, P. Muller, H. Jordan, W. Harre, and C. Besang. 1981. K/Ar and Rb/Sr dating of igneous rocks from northern Victoria Land, Antarctica. Geologisches Jahrbuch, 841, 267-273. McIntosh, W.C., P.R. Kyle, E.M. Cherry, and H.C. Noltimer. 1982. Paleomagnetic results from the Kirkpatrick Basalt Group, Victoria Land. Antarctic Journal of the U.S., 17, 20-22. Schmierer, K. E., R. F. Burmester, and A. Wodzicki. 1982. Paleomagnetic investigations of the Sledgers Group, Bowers Mountains, northern Victoria Land. Antarctic Journal of the U.S., 17, 8-9. Smith, A. G., and A. Hallam. 1970. The fit of the southern continents. Nature, 225, 139-144. Stock, J. , and P. Molnar. 1982. Uncertainties in the relative positions of the Australia, Antarctica, Lord Howe and Pacific plates since the Late Cretaceous, Journal of Geophysical Research, 87, 4697-4714. Stump, E., S.G. Borg, and J.R. Holloway. 1984. Personal communication. Stump, E., M.G. Laird, J.D. Bradshaw, J.R. Holloway, S.G. Borg, and K.E. Lapham. 1983. Bowers graben and associated tectonic features cross nothern Victoria Land, Antarctica. Nature, 304, 334-336. Thompson, R., and R.M. Clark. 1982. A robust, least-squares Gondwana apparent polar wander path and the question of paleomagnetic

Paieomagnetic directions and virtual pole positions, Admiralty Intrusive and Mafic Dike, Salamander Range Remanence Mean Site Mean declination inclination

Semi-angle Number of samples of cone of used in calculation 95 percent divided by total confidence number of samples

81ap03Ab 24.2 -50.7 81apO313c 195.2 +57.5 81ap03d 197.1 +56.2 81ap23De 249.6 -82.4 81 ap23G1 279.3 -71.0

4/5 17/20 21/25 6/6 6/10

Virtual geomagnetic pole Approximate peak alternating field South East Spa Sma (in millitessla) latitude longitude

12.0 5.9 5.2 7.6 13.0

20 -47.20 30 -55.00 _5350 _9 20 -63.20 10 -54.40

15.60 10.9 16.2 5.70 6.3 8.6 7.90 5.4 7.5 197.00 14.4 14.8 238.40 19.9 22.8

a Sp and Sm = 95 probability ellipse around pole along and at right angles to the paleomeridian, respectively. b One sample excluded for inconsistent directions on demag. ° Three samples with divergent direction and poor stability excluded because near oplite dike. d At B combined by inverting A directions. Mafic dike. Direction based on six most stable samples. U-,, denotes not applicable. 32

ANTARCTIC JOURNAL



assessment of Gondwana reconstructions. Earth and Planetary Science Letters, 57, 152-158. Weaver, S.D., J.D. Bradshaw, and M.G. Laird. 1984. Geochemistry of

Mount Siple volcano, Marie Byrd Land W. E. LEMASURIER Geology Department University of Colorado at Denver Denver, Colorado 80202

Mount Siple, one of the largest volcanoes in Antarctica, was visited for the first time on 22 February 1984 during cruise II of

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Cambrian volcanics of the Bowers Supergroup and implications for the Early Paleozoic tectonic evolution of northern Victoria Land, Antarctica. Earth and Planetary Science Letters, 68, 128-140.

USCGC Polar Sea. The initial landing was made at Lovill Bluff (figure 1) by helicopter, piloted by LCDR Rick McLean and crewed by AM2 Robert O'Conner. The scientific party included Pamela Ellerman (on her birthday), David Johnson, William McIntosh, and me. We spent a full day visiting Lovill Bluff and two other western flank localities before steaming north to the vicinity of Maher Island (figure 1). Because of poor weather conditions, we were forced to examine the northern flank exposures with binoculars over the next 4 days and finally left the area on the morning of 27 February to maintain the ship's schedule. A very useful series of close-up photographs along the north flank of the volcano was taken by Seaman A.R. Sul-

Limit of fast ice Maher ' (February 1965) Island - 125000'W 126° 73°00'S

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