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Geology of Metamorphic Rocks and Granitoids: Ford Ranges of Western

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Geology of Metamorphic Rocks and Granitoids: Ford Ranges of Western to have had only a small effect on uplift of the Transantarctic Pacific Council for Energy and Natural Resources Earth Science Se- Mountains" (about 100 meters), we suggest that the converse ries. is not necessarily true. If uplift of about 1 kilometers per million Craddock, C., R.J. Campbell, S.J. Adie, S.J. Carryer, A.B. Ford, H.S. years since early- or mid-Pliocene did occur, it may in fact have Gair, G.W. Grindley, K. Kizaki, L.L. Lackey, M.G. Laird, T.S. Lau- triggered the most recent advance of the east antarctic ice sheet don, V.R. McGregor, I.R. McLeod, A. Mirsky, D.C. Neethling, R.L. by a mechanism similar to that proposed for the norther hemi- Nichols, P.M. Otway, P.G. Quilty, E.F. Roots, D.L. Schmidt, A. Sturm, T. Tatsumi, D.S. Trail, T. VanAutenboer, F.A. Wade, and sphere (winter cooling and increased precipitation caused by G. Warren. 1969. Geologic maps of Antarctica: American Geographical mountain uplift culminating in the Plio-Pleistocene ice ages) Society, Folio 12. New York: American Geographic Society. by Ruddiman and Kutzbach (1989) although their model sug- Fitzgerald, P.G. 1989. Uplift and formation of Transantarctic Moun- gested plateau uplift. We suggest a probable synergistic rela- tains—Applications of apatite fission track analysis to tectonic prob- tion between episodic tectonism in the Cenozoic west antarctic lems. 28th Geological Congress, Abstracts. Washington, D.C.: Geological rift system and the waxing and waning of the antarctic ice Congress. sheet (Webb 1990) approximately coincident in time with rift- LeMasurier, W.E. 1990. Late Cenozoic volcanism on the Antarctic ing, mountain uplift, and volcanism since Oligocene or earlier plate—An overview. In W.E. LeMasurier and J.W. Thomson (Eds.), time. Volcanoes of the Antarctic plate and southern oceans. (Antarctic Research Series, Vol. 48.) Washington, D.C.: American Geophysical Union. Ruddiman, W.F., and J.E. Kutzbach. 1989. Forcing of the Late Cen- ozoic northern hemisphere climate by plateau uplift in Southern References Asia and the American West. Journal of Geophysical Research, 94(D15), 18,409-18,427. Behrendt, IC., and A.K. Cooper. 1990. Speculations on the uplift of Schmidt, D.L., and P.D. Rowley. 1986. Continental rifting and trans- the shoulder escarpment of the Cenozoic West Antarctic rift system form faulting along the Jurassic Transantarctic rift, Antarctica. Tec- and its relation to Late Cenozoic climate change. In A.K. Cooper tonics, 5, 279-291. and P.N. Webb (Eds.), ANTOSTRAT. (U.S. Geological Survey Open Stern, T.A., and U. ten Brink. 1989. Flexural uplift of the Transantarctic File Report 90-309.) U.S. Geological Survey. Mountains. Journal of Geophysical Research, 94, 10,315-10,330. Behrendt, J.C., and A.K. Cooper. In press. Evidence of rapid Cenozoic Tessensohn, F., and G. Woerner. In press. The Ross Sea Rift system uplift of the shoulder escarpment of the west antarctic rift system (Antarctica)—Structure, evolution, and analogues. In M.R.A. and a speculation of possible climate forcing. Geology. Thomson and J.W. Thomson (Eds.), Proceedings of the Fifth Interna- Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratig- tional Symposium on Antarctic Earth Science. Cambridge: Cambridge raphy and structure of the Victoria Land Basin, western Ross Sea, University Press. Antarctica. In A.K. Cooper and F.J. Davey (Eds.), The Antarctic Con- Webb, P.N. 1990. The Cenozoic history of Antarctica and its global tinental Margin geology and geophysics of the western Ross Sea. Circum- impact. Antarctic Science, 2(1), 3-21. Geology of metamorphic rocks western Marie Byrd Land was adjacent to portions of New Zealand and the Campbell Plateau prior to the late Cretaceous and granitoids: Ford Ranges and early Tertiary breakup of the southern margin of Gond- of western Marine Byrd Land wanaland (e.g., Cooper et al. 1982). The goal of our project is to gain insight into the Mesozoic and Cenozoic tectonics of West Antarctica, to explore geologic ties with New Zealand, and to understand the geologic history of separation of New D.L. KIMBROUGH Zealand and the Campbell Plateau from this margin of the Gondwanaland supercontinent. Department of Geological Sciences A multidisciplinary investigation of crystalline basement rock San Diego State University in the Ford Ranges of Marie Byrd Land was initiated during San Diego, California 92182 austral summer 1989-1990. Work was concentrated on the high- grade metamorphic rocks of the Fosdick Mountains and plu- tonic rocks in the adjacent Chester Mountains. The scientific B.P. LUYENDYK, S.M. RICHARD, and C. SMITH party consisted of Bruce P. Luyendyk, Stephen M. Richard, and Christine Smith of the University of California at Santa Institute for Crustal Studies Barbara, and David L. Kimbrough of San Diego State Univer- University of California sity. Thirty-nine days were spent in the field area during De- Santa Barbara, California 93106 cember and January with logistic support by VXE-6 LC-130 flights. We completed an east-west traverse of the Fosdick Mountains, with a linking north-south traverse to the Chester The Ford Ranges provide the most widespread exposures Mountains, mapping, making detailed structural observations, of pre-Cenozoic (more than 65 million-year-old) rocks in Marie and collecting rock samples for paleomagnetic, geochronol- Byrd Land (Bradshaw, Andrews, and Field 1983; Adams 1987). ogic, and petrologic studies (figure). Geologic similarities have led previous workers to suggest that Three major geologic domains were recognized in the base- 1990 REVIEW Generalized map of the Fosdick Mountains and Chester Mountains area adapted from Wade, Cathey, and Oldham (1977, 1978) showing camps and traverses made during the 1989-1990 field season. BP denotes Birchall Peaks; Ml denotes Mount Iphigene; OG denotes Ochs Glacier; MA denotes Mount Avers; ML denotes Mount Lockhart; BB denotes Bird Bluff; GN denotes Griffiths Nunataks; ON denotes OConnor Nunataks; MR denotes Mount Richardson; MG denotes Mount Getz; NN denotes Neptune Nunataks; MC denotes Mount Corey. (ml denotes mile; km denotes kilometer.) ment rocks. These are the Fosdick domain, consisting of mig- Migmatization in the Fosdick Range was accompanied by matitic paragneiss, and orthogneiss; the Chester domain, the intrusion of mafic dikes. The youngest dikes are vertical, consisting mostly of granodioritic plutons; and the Mount Corey strike northerly in general, and consist of hornblende and calcic domain, consisting of granite plutons. The contacts between plagioclase. Progressively older dikes are more deformed by these domains are buried under the intervening glaciers, but buckling and boudinage, rotated into parallelism with the en- our observations suggest that a high strain zone separates the closing gneiss, and metamorphosed to fine-grained biotite- Fosdick and Chester domains, and that the Mount Corey gran- feldspar granofels. Many of the blocks in the block gneisses itoids are younger than rocks in either of the other domains. are relict mafic dikes. Fosdick domain. The Fosdick Mountains consist of a mig- The foliation in the migmatite defines a gentle, upright an- matite-gneiss complex of strikingly uniform lithologic character tiform, with limbs dipping approximately 200 on both flanks from Birchall peaks at the west to Bird Bluff on the east. The of the range. Foliation is defined by compositional banding in complex consists of interlayered migmatitic metasedimentary the gneiss; no lineation has been observed. Numerous exten- rocks and granodiorite. These rocks provide the best oppor- sion faults cut the foliation at a low to moderate angle. These tunity for direct observation of the deeper levels of continental faults were observed to form conjugate sets in any given out- crust in this segment of Marie Byrd Land. A small area of crop, suggesting bulk flattening of the gneiss. Intrusion of heterogeneous high-grade paragneiss and lesser orthogneiss neosome along many of these fault planes suggest that they at Scott Nunataks and Mount Swadener in the Edward VII are syn-metamorphic. Sparse intrafolial isoclinal folds were Peninsula are the only comparable rocks in the Ford Ranges also observed, but hinge lines could rarely be determined. The (Adams et al. 1988). foliation is folded into a large-scale close, recumbent fold pair, In rare places, the protolith of the Fosdick Mountains me- with an east-trending hinge, along the northern side of the tasedimentary material can be seen to be dark gray feldspathic range between Mount Lockhart and the Ochs Glacier. sandstone and argillite, similar to pendants of Swanson For- Chester domain. The Chester Mountains consist of medium mation we observed in Chester Mountains granodiorite. The grained biotite granodiorite. Pendants of dark gray sandstone mineral assemblage includes biotite, sillimanite, garnet, cor- and argillite with sparse calcareous nodules are present. The dierite, quartz, and K-feldspar throughout the range. Grano- granodiorite is intruded by medium to fine-grained muscovite- diorite bodies are generally thick concordant layers. The biotite granite, abundant pegmatite dikes, and by mafic dikes metasedimentary migmatite has a layered internal structure, resembling those that intrude the Fosdick migmatite-gneiss with dark and light layers of paleosome and neosome. Partial complex. At Neptune Nunataks along the south side of the melting was apparently quite advanced in the metasedimen- central Fosdick Mountains and at Griffith Nunataks to the east, tary component, leading to the formation of block gneiss, in are outcrops of foliated granodiorite in which the foliation is which lenses of paleosome 1-4 meters in diameter are engulfed concordant to that in nearby Fosdick migmatite-gneiss expo- in leucogranitic neosome. In the western part of the complex, sures. Pegmatite dikes crosscut this foliation. These outcrops bodies of block gneiss have been mobilized and crosscut the may represent a deeper level of the Chester Mountains bath- regional foliation. Distinctive subunits based on variations in olith.
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