Terrestrial geology and geophysics Uplift of the shoulder escarpment rift system and the rift shoulder. If a maximum uplift of 5-6 kilometers in the southern Victoria Land or 10 kilometers in of the west antarctic northern Victoria Land (Fitzgerald 1989) area is correct, then rift system and its relation 1-2 kilometers and as much as 6 kilometers, respectively, of erosion has occurred from the highest topography in these two to Late Cenozoic climate change areas to result in the maximum altitude of about 4 kilometers observed now. The variation of the high topography seen in figure 2 along the rift shoulder is probably caused by erosion JOHN C. BEHRENDT and differential uplift (Behrendt and Cooper in press). We interpret that the main cause of uplift, along the west antarctic U.S. Geological Survei rift shoulder, is late Cenozoic tectonism associated with rifting, Denver Federal Center probably as modeled by Stern and ten Brink (1989), for a con- Denver, Colorado 80225 tinental lithospheric plate heated at the free edge. The maximum elevations (figure 2) vary from about 4 kilo- meters in Victoria Land and the Queen Maud Mountains (fig- ALAN COOPER ure 1), dropping to about 3 kilometers in the Horlick and Whitmore mountains (perhaps resulting from a greater rate of U.S. Geological Survey erosion) and rising again to 5 kilometers in the Ellsworth Menlo Park, California 94025 Mountains. In contrast, elevations along the Transantarctic Mountains are much lower toward the Weddell Sea (see the The Cenozoic West Antarctic rift system (Behrendt and Cooper solid-line profile of figure 2). The lower topography at the in press) extends over a 3,000-by-750-kilometer, largely ice- Weddell Sea end of the Transantarctic Mountains is not a part covered area from the Ross Sea to the Bellingshausen Sea, of the late west antarctic rift, but appears to be significantly comparable in area (Tessensohn and Woerner in press) to the older than that of the Cenozoic west antarctic rift shoulder. Basin and Range Province and the East African rift systems Because the Jurassic Dufek intrusion is faulted by this older (figure 1). A spectacular Cenozoic rift-shoulder scarp extends uplift, however, it must post-date its emplacement. from northern Victoria Land to the Queen Maud Mountains Fitzgerald (1989), using fission track dates, identified 5-6 to the Horlick-Whitmore-Ellsworth mountains along which kilometers of uplift in the Transantarctic Mountains of south- peaks reach 4-5 kilometers maximum elevation. The rift shoul- ern Victoria Land beginning about 60 million years ago and der has maximum present relief of 5 kilometers in the Ross calculated an average uplift rate of about 100 meters per million embayment and 7 kilometers in the Ellsworth Mountains/Byrd years since that time. Seismic reflection data from the Ross Sea Subglacial basin area. The west antarctic rift system is char- adjacent to the Transantarctic Mountains show several angular acterized by bimodal alkaline Cenozoic volcanic rocks (Le- unconformities that have been interpreted by Cooper et al. Masurier 1990) in Marie Byrd Land and the Transantarctic (1987) as evidence for episodic uplift. Therefore, we infer that Mountains bordering the Ross embayment ranging from about the uplift of the Transantarctic Mountains and entire rift shoul- Oligocene to the present. In contrast, Jurassic tholeiites (Ferrar der was also episodic, probably being as much as an order of dolerites, Kirkpatrick basalts) marking the Jurassic Transant- magnitude faster at times (probably including the present) than arctic rift (Schmidt and Rowley 1986) crop out coincidently with the mean rate of 100 meters per million years calculated by the late Cenozoic volcanic rocks only along the section of the Fitzgerald (1989) for the last 60 million years. Various lines of Transantarctic Mountains from northern Victoria Land to the geomorphic, glacial geological, sedimentalogical, paleontolog- Horlick Mountains (Craddock et al. 1969). The Jurassic thol- ical, and geophysical evidence, led Behrendt and Cooper (1990) eiites continue to be exposed (including the Dufek intrusion) and Behrendt and Cooper (in press) to conclude that the Trans- along the lower-elevation (1-2-kilometer) section of Trans- antarctic Mountains part of the rift shoulder (and possibly the antarctic Mountains to the Weddell Sea, whereas the late Cen- entire shoulder) has been rising at a rate of the order of 1 ozoic alkalic volcanic rocks are exposed throughout Marie Byrd kilometer per million years probably since early of mid-Pli- Land (figure 1) to the southern Antarctic Peninsula but not in ocene time rather than the 100 meters per million years average the Ellsworth Mountains area. since 60 million years ago. We examined the amount of uplift of the rift shoulder in the It is interesting to note that the estimated start (Behrendt west antarctic rift system in light of extreme elevations (figure and Cooper in press) of the latest episode of uplift (2-4 million 2). The steep scarp suggests a youthful topography; it is in- years ago) is approximately coincident with the most recent terpreted to be the expression of a major normal or extensional "warm conditions" cited by Webb (1990) for Antarctica. While fault that defines the boundary of the Cenozoic west antarctic he notes that "overall the present Antarctic ice sheet appears 1990 REVIEW oC: çS orw 90E 180 Bathymetric contours Ice-sheet surfacecontours 0 500 1000km .,,,,... .,...., Ice shelves Areas of exposed rocks Cenozoic West Antarctic rift shoulder Jurassic tholeiitic rift Figure 1. Index map of Antarctica showing some of the features discussed in the text. Shaded area is approximate location of west antarctic Cenozoic rift system. Heavy line is approximate rift shoulder. 6— 6 Ellsworth Mts. SHOULDER Queen Maud Mts Victoria Land 4-i Whitmore Mts CI ol ltck Weddell Sea > — Thiel Mts. W I go 2 LLJ I\%TMts. TRANSANTARCTIC MOUNTAINS 7ShackletonMts Theron Mts. n_I-a 0 3000 2000 1000 0 distance (km) Figure 2. Generalized topographic profile for crest of highest peaks along Interpreted rift shoulder (figure 1) (dashed line) from the north coast of Victoria Land at right to the Ellsworth Mountains at left compared with lower topography (solid line) of highest peaks In Transantarctic Mountains from Thiel Mountains at right to Weddell Sea coast at left. The interpreted Cenozoic west antarctic rift shoulder has highest topography of any rift in the world. Low areas within the mountains probably are the results of glacial erosion and differential uplift and are not shown in this profile. (km denotes kilometer.) 2 ANTARCTIC JOURNAL 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.