K.D. Lapham. 1983. Bowers graben and associated tectonic features Cambrian volcanics of the Bowers Supergroup and implications for across northern Victoria Land, . Nature, 304, 334-336. the Early Paleozoic tectonic evolution of northern Victoria Land, Tessensohn, F., K. Duphorn, H. Jordan, G. Kleinschmidt, D.N.B. Skin- Antarctica. Earth and Planetary Science Letters, 58, 128-140. ner, U. Vetter, T.O. Wright, and D. Wyborn. 1981. Geological com- Wodzicki, A., J.D. Bradshaw, and M.G. Laird. 1982. Petrology of the parison of basement units in north Victoria Land, Antarctica. Wilson and Robertson Bay Groups and Bowers Supergroup, northern Geologisches Jahrbuch, B41, 31-88. Victoria Land, Antarctica. In C. Craddock (Ed.), geoscience. Weaver, S.D., J.D. Bradshaw, and M.G. Laird. 1984. Geochemistry of Madison: University of Wisconsin Press.

Did northern Victoria Land collide with field and thermal demagnetization were employed for magnetic cleaning. Reliability of individual samples was judged using in the Cretaceous? Briden and Arthurs (1982) method on single measurements, consistency of directions through demagnetization, and sim- R. F. BURMESTER and ilarity of directions from multiple specimens. Coherence of J. K. ANDERSON directions between samples and the stability of the directions to demagnetization reflect the reliability of the data from a site. Department of Geology Only two sites in the Salamander Range proved to have stable Western Washington University magnetization judging by these criteria. Bellingham, Washington 98225 The results of these two acceptable sites, each divided into two lithologic groups, are listed in the table and shown as The disparate lithologies and tectonic histories across north- virtual geomagnetic poles (vGPs) in figure 2. Comparing these ern-Victoria. Land have prompted speculation of major displace- data, figure 2 shows an apparent polar wander path (APwr) for ment on faults between what are now recognized to be separate Gondwana (Thompson and Clark 1982) rotated to Antarctica terranes (Stump et al., 1983; Weaver, Bradshaw, and Laird 1984). using Smith and Hallams (1970) pole. Also plotted as MR (Mesa 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

90E 9M W An

ris

\ g 11 S \

(f1 ° Robertson 0. ii 6 Terrane 72 RangoWe cjc1 CO LOS 4 Neve 3S . 0) Zoo w 0 71 eIIE PAC Ut Figure 2. Stereographic projection centered on the show- ing virtual geomagnetic poles from northern Victoria Land (3, 4, 5, MA, table) as well as a paleomagnetic pole from other Ferrar Super- group localities (J)tabulated in McIntosh et al. (1982) and Thompson and Clarks (1982) APWP for Gondwana rotated to Antarctica. Figure 1. Generalized geologic map showing paleomagnetic lo- Crosses are at 10 million years, labeled at 50-million-year intervals. calities at Lillie Marline (LM), Salamander Range (AP), Monte Cassino C is approximate equatorial Ealer pole required to explain diver- (Mc) and Mesa Range (MA) with respect to the Bowers Supergroup. gence of northern Victoria Land VGPS from East Antarctica poles Modified after Stump et al. (1983). due to relative motion.

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Range) is the mean VGP for 174 million-year-old Kirkpatrick have a common cause, such as post-Jurassic motion between Basalt flows in the Mesa Range (McIntosh et al. 1982) and a northern Victoria Land and East Antarctica. Left oblique con- mean pole (J) calculated from other quality studies of Ferrar vergence about pole C, figure 2, would fit all data adequately. rocks tabulated by McIntosh et al. (1982). This mean pole com- This is consistent with a prediction of Stock and Molnar (1982) pares favorably with the APWP lending credence to its validity. that East and West Antarctica converged in the Late Cretaceous Both sites are in rocks mapped as Devonian Admiralty Intru- and Early Tertiary. Other explanations such as failure of the sives. Potassium-argon ages on biotite and hornblende from dipole hypothesis are possible if less interesting, but demon- other Admiralty Intrusives are 360 to 367 million years (Kreuzer stration of the validity of any requires more extensive collection, et al. 1981) but the rocks in the Salamander Range may be as measurement, and analysis. young as 320 million years old. (Stump, Borg, and Holloway We would like to thank E. Stump, J.R. Holloway, S.C. Borg, personal communication). The granite host (5 on figure 2) and K. E. Lapham, A. Wodzicki, K. S. Schmierer, and C. Dumont for mafic dike (4 on figure 2) of site 23 have directions similar to the their kind assistance and cooperation in collecting the samples. Jurassic paleopole. The simplest explanation is that the granite This work was supported in part by National Science Founda- was remagnetized during emplacement of the dike which is tion grant DPP 80-20726. probably Ferrar, but the dike provides a more reliable record of the Jurassic field because of greater stability to laboratory de- magnetization and lower dispersion. References The other site (3 on figure 2) preserves two essentially anti- parallel magnetizations strikingly different from the present or Briden, J. C., and G. R. Arthur. 1982. Precision of measurement of rema- Jurassic fields. The upward directions reside in samples of nent magnetization. Canadian Journal of Earth Science, 18, 527-538. coarse-grained granite (1) above a horizontal contact. The Kreuzer, H., A. Hohndorf, H. Lenz, U. Vetter, F. Tessensohn, P. Muller, downward directions are recorded by the younger, medium- H. Jordan, W. Harre, and C. Besang. 1981. K/Ar and Rb/Sr dating of grained porphyry (2 on figure 2) beneath. Magnetic mineralogy igneous rocks from northern Victoria Land, Antarctica. Geologisches of both units is similar. Homogenous magnetite is the dominant Jahrbuch, 841, 267-273. magnetic mineral in both units, existing as embayed large crys- McIntosh, W.C., P.R. Kyle, E.M. Cherry, and H.C. Noltimer. 1982. Paleomagnetic results from the Kirkpatrick Basalt Group, Victoria tals and smaller (10 micrometers) grains in altered mafic miner- Land. Antarctic Journal of the U.S., 17, 20-22. als. The low blocking temperature of the remanence and proba- Schmierer, K. E., R. F. Burmester, and A. Wodzicki. 1982. Paleomagnetic ble reconstitution of the magnetic cleaning deuteric alteration investigations of the Sledgers Group, Bowers Mountains, northern indicate the remanence is not a primary thermal magnetization. Victoria Land. Antarctic Journal of the U.S., 17, 8-9. Nevertheless, both directions are discordant with both the Smith, A. G., and A. Hallam. 1970. The fit of the southern continents. Jurassic field and present field and probably reflect two direc- Nature, 225, 139-144. tions of an earlier paleomagnetic field. Therefore, the data were Stock, J. , and P. Molnar. 1982. Uncertainties in the relative positions of combined by inverting the directions of (1 in figure 2) to provide the Australia, Antarctica, Lord Howe and Pacific plates since the Late the best possible estimate of the paleomagnetic field (3). Com- Cretaceous, Journal of Geophysical Research, 87, 4697-4714. Stump, E., S.G. Borg, and J.R. Holloway. 1984. Personal parison of (3) with the APWP in figure 2 shows that site 3 may communication. well preserve an original Devonian magnetization. Stump, E., M.G. Laird, J.D. Bradshaw, J.R. Holloway, S.G. Borg, and All the best results from northern Victoria Land (3, 4, MR) K.E. Lapham. 1983. Bowers graben and associated tectonic features depart from their reference poles in roughly the same direction. cross nothern Victoria Land, Antarctica. Nature, 304, 334-336. The consistency of the discrepancy between the northern Vic- Thompson, R., and R.M. Clark. 1982. A robust, least-squares Gond- toria Land VGPS and the APWP suggest that the discrepancies wana apparent polar wander path and the question of paleomagnetic

Paieomagnetic directions and virtual pole positions, Admiralty Intrusive and Mafic Dike, Salamander Range

Remanence Virtual geomagnetic pole Semi-angle Approximate peak Mean Number of samples of cone of Site alternating field declination Mean used in calculation 95 percent South East Spa Sma (in millitessla) inclination divided by total confidence latitude longitude number of samples

81ap03Ab 24.2 -50.7 4/5 12.0 20 -47.20 15.60 10.9 16.2 81apO313c 195.2 +57.5 17/20 5.9 30 -55.00 5.70 6.3 8.6 81ap03d 197.1 +56.2 21/25 5.2 _9 _5350 7.90 5.4 7.5 81ap23De 249.6 -82.4 6/6 7.6 20 -63.20 197.00 14.4 14.8 81 ap23G1 279.3 -71.0 6/10 13.0 10 -54.40 238.40 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 Cambrian volcanics of the Bowers Supergroup and implications for Letters, 57, 152-158. the Early Paleozoic tectonic evolution of northern Victoria Land, Weaver, S.D., J.D. Bradshaw, and M.G. Laird. 1984. Geochemistry of Antarctica. Earth and Planetary Science Letters, 68, 128-140.

Mount Siple volcano, Marie Byrd Land 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 OConner. The scientific party included Pamela Ellerman (on her birthday), David Johnson, William W. E. LEMASURIER McIntosh, and me. We spent a full day visiting Lovill Bluff and two other western flank localities before steaming north to the Geology Department vicinity of Maher Island (figure 1). Because of poor weather University of Colorado at Denver conditions, we were forced to examine the northern flank ex- Denver, Colorado 80202 posures with binoculars over the next 4 days and finally left the area on the morning of 27 February to maintain the ships Mount Siple, one of the largest volcanoes in Antarctica, was schedule. A very useful series of close-up photographs along visited for the first time on 22 February 1984 during cruise II of the north flank of the volcano was taken by Seaman A.R. Sul-

Limit of fast ice - Maher (February 1965) Island - 125000W 126° 1280 (<\c:) 73°00S -1-- Lauff I Island Burtis C_; C^l \ 0 Island I ri?- 0 Cape Dart Recely Bluff MOUNT SIPLE 3110 I KEY Possible source Trachyte Lovill of felsic beach U° C— — .- cobbles ¼ Bluff ._® c. Felsic bea Basalt ch .j I — .. fflJ cobbles P cha n k r a t z I I F r1 Hyaloclastite L_J Bay (hydroclastic tuff) ) +d 73°30S Inferred limit of / q summit caldera I •i._ —ID I Scale 1:500,000 , . 10 0 10 20 30 i—a i—a i—i 1 km

Figure 1. Preliminary geologic map of Mount Siple. Base map is from U.S. Geological Survey (1965). Revised geographic coordinates based on Polar Sea cruise ii data were not available at the time of writing. However, shipboard estimates place the true position of Mount Siple approximately 40 kilometers west of the position shown In this figure. Localities A, C, D, and J are field designations used for unnamed rock exposures. ("km" denotes kilometer.)

1984 REVIEW 33