Austral Summer 1989-1990 at the Executive Committee Range, Marie

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Austral Summer 1989-1990 at the Executive Committee Range, Marie Corey domain. Mount Corey and OConnor Nunataks consist References of alkali granite, associated dikes, and minor intrusive rocks. The granite is pinkish, medium grained, and equigranular. Adams, C.J. 1987. Geochronology of granite terranes in the Ford Ranges, Mafic minerals are hornblende and biotite at OConnor Nun- Marie Byrd Land, West Antarctica. New Zealand Journal Geology Geo- physics, 30, 51-72. ataks, and biotite at Mount Corey. A few pegmatite dikes are Adams, C.J., P. Broady, S.D. Weaver, and P.J. Cleary. 1988. Geological present, but mafic dikes are absent, suggesting that this granite and biological studies on Edward Vii Peninsula, Marie Byrd Land, West is younger than the mafic dikes that intrude the Fosdick and Antarctica. (Immediate Science Report to the Ross Dependency Re- Chester domains. The only deformation features observed in search Committee. New Zealand Antarctic Research Program Field this domain are planar crush zones on Mount Corey and chior- Season 1987/88: Event 151.) itic fractures at OConnor Nunatak. Bradshaw, J.D., P.B. Andrews, and B.D. Field. 1983. Swanson Formation The outcrop pattern in the Fosdick Mountain area is con- and related rocks of Marie Byrd Land and a comparison with the sistent with the migmatite-gneiss complex forming the core Robertson Bay Group of Northern Victoria Land. In R.L. Oliver et al. of a large-scale dome. The deepest crustal level is exposed (Eds.), Antarctic earth science. Canberra: Australian Academy of Science. in the Fosdick Mountains, with the Chester and Mount Corey Cooper, R. A., C. A. Landis, W. E. Le Mauserier, and 1G. Speden. 1982. Geologic history and regional patterns in New Zealand and West domains representing successively higher levels. The anti- Antarctica—Their paleotectonic and paleogeographic significance. formal structure of the range is related to the uplift history In C. Craddock (Ed.), Antarctic geoscience. Madison: University of of the migmatite complex. Halpern (1972) reports seven ru- Wisconsin Press. bidium-strontium biotite ages from the Fosdick Mountains Halpern, M. 1972. Rb-Sr total-rock and mineral ages from the Mar- that cluster from 92 to 102 million years and these ages are guerite Bay area Kohler Range and Fosdick Mountains. In R.J. Adie regarded as a reliable estimate for the uplift and cooling of (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. the migmatite-gneiss complex. These cooling ages are similar Tulloch, A.J., and D.L. Kimbrough. 1989. The Paparoa metamorphic to those determined from metamorphic rocks in New Zea- core complex, Westland-Nelson, New Zealand: Cretaceous exten- land that were uplifted in response to continental extension sion associated with fragmentation of the Pacific margin of Gond- preceding the breakup to the southern Gondwanaland mar- wana. Tectonics, 8, 1,217-1,234. Wade, F. A., C. A. Cathey, and J. B. Oldham. 1977. gin (Tulloch and Kimbrough 1989). Our continuing research Reconnaissance geo- logic map of the Guest Peninsula quadrangle, Marie Byrd Land, Antarctica. is directed at determining the timing and causes of uplift of USARP, Antarctic Geological Map, A-7. the Fosdick migmatite-gneiss complex. Wade, F.A., C.A. Cathey, and J.B. Oldham. 1978. Reconnaissance geo- This research was supported by National Science Founda- logic map of the Gutanko Nunataks quadrangle, Marie Byrd Land, Ant- tion grant DPP 88-17615. arctica. USARP, Antarctic Geological, Map A-il. Austral summer 1989-1990 Kingdom, and New Zealand) to study in detail the geology of volcanoes in Marie Byrd Land, West Antarctica. Marie Byrd at the Executive Committee Range, Land is Antarcticas largest but least studied volcanic province Marie Byrd Land, Antarctica (LeMasurier 1990). It includes 18 large (greater than 2,000- meter elevation) stratovolcanoes and more than 30 smaller eruptive centers. All of these volcanoes show alkaline com- W.C. MCINTOSH and K.S. PANTER positions and are related to crustal extension associated with continental rifting. Most Marie Byrd Land volcanoes have been Department of Geoscience studied only on a reconnaissance level, primarily by helicopter- New Mexico Institute of Mining and Technology supported geological parties during the 1967-1968 and 1977- Socorro, New Mexico 87801 1978 austral summers (LeMasurier and Rex 1989; LeMasurier 1990). During the 1989-1990 austral summer, a snowmobile- J.L. SMELLIE equipped, six-person WAVE team performed 23 days of field work at the southern end of the Executive Committee Range British Antarctic Survey in central Marie Byrd Land. Detailed geologic mapping and Cambridge CB3 OET, United Kingdom sampling was completed on three volcanoes: Mount Waesche, Mount Sidley, and Mount Cummings. This article reports some of our initial field observations. J.A. GAMBLE Mount Waesche. Mount Waesche (3,292 meters) is the south- ernmost and youngest of the Executive Committee Range vol- Department of Geology canoes (figure). It is a coalesced doublet consisting of the Victoria University topographically subdued 1.5 million year old, 10-kilometer- Wellington, New Zealand wide Chang Peak caldera with the younger (0.1 to 1 million year old), higher symmetrical southern peak of Mount Waesche The West Antarctic Volcanological Exploration (WAVE) pro- proper developed on its southern flank (LeMasurier 1990). We gram is a multinational effort (by the United States, the United sampled the only three exposures on Chang Peak caldera and 1990 REVIEW are present in small scoria cones along a radial rift zone in the 0 126° southwest quadrant of the volcano. 127 W Debris from the xenolith-rich summit pyroclastic deposit has been transported down the southern slope of Mount Waesche, km probably by a combination of glacial and solifluction/gelifluc- Mt Hampton tion transport processes. This transported debris forms a dis- 0 10 20 tinctive marker horizon for distinguishing older, eroded, debris- 2600m mantled lavas from younger, little-eroded units. We interpret this debris mantle as evidence for former advances of glacial ice centered on Mount Waesche, rather than overriding by the continental ice sheet. 76.5°S Two interesting features were found in the ice sheet adjacent I to Mount Waesche. First, a vertically oriented sequence of 31 englacial tephra layers is present south of the mountain. Layers range from faint dust bands to 4-meter thick beds containing 2200m bombs as large as 2 meters. These tephra layers were probably formed by Holocene activity, and may have potential for cor- relating and dating ashes in deep ice cores elsewhere in Ant- arctica (e.g., Palais et al. 1988). Second, a moraine ridge southeast Mt. Cumming of Mount Waesche is composed of exotic rhyolite, basanite, and mantle xenoliths; analysis of these lithologies may help evaluate pre-Waesche activity. Peralkaline rhyolites were collected at each of three outcrops around the rim of Chang Peak caldera. Mount Sidley. Mount Sidley (4,181 meters) is Antarcticas highest volcano. It is a symmetrical stratovolcano topped by a 5-kilometer-wide, 1,200-meter-deep caldera that is breached Mt. Hartigan on its southern side (figure). The reconnaissance work of LeMasurier (1990) has shown that Mount Sidley consists pri- marily of 4.75 million year old lavas of predominantly phon- olitic composition. We mapped approximately 90 percent of the available outcrops on Mount Sidley, including the east and west faces of the caldera wall. Anorthoclase phonolite (kenyte) lavas dominate the basal 0 portion of the caldera wall sections and are also exposed on 0 S 2200m the eastern, southeastern, and western flanks of Mount Sidley. 77 Most of these lavas show reddened, brecciated bases and tops Mt. characteristic of subaerial eruption and emplacement, but some 2600m ,Z24 M Waesche show basal pillows and/or hyaloclastite indicative of limited interaction with ice and show. The anorthoclase phonolite la- vas are capped by an unconformity along which are discon- 4__JYI,-'— tinuous lenses of hyaloclastite and epiclastic sediment, probably deposited in valleys or summit depressions of the developing volcano. Overlying the unconformity are a variety of relatively t. Sidley crystal-poor, intermediate to silicic lavas, some of which are also exposed on the southern flank of the volcano. ,,__,-.en glacial tephras The caldera rim is capped by a densely welded pyroclastic fall deposit rich in hypabyssal plutonic xenoliths, probably related to the caldera collapse event. Also exposed in the cald- Simplified topographic maps of volcanoes in the Executive Com- era wall are numerous dikes and one stock with an associated mittee Range. (km denotes kilometer.) hydrothermal alteration halo. Several post-caldera-collapse units are present within the caldera, including an extensive unwelded ignimbrite and more mapped approximately 80 percent of the exposures in the 2- local lavas, hydroclastic deposits, and tillites. Dating of these kilometer summit crater and southern flank of Mount Waesches units will help constrain the age of caldera collapse at Mount southern peak. Sidley. The stratigraphically youngest units on Mount Sidley The summit of Mount Waesche is formed by a scoria cone are mafic scoria cones containing abundant crustal and mantle topped by prominent (20-meter) rime ice towers. We found xenoliths. no sign of recent thermal activity (such as fumarolic ice towers, Mount Cumming. A short visit was made to Mount Cum- ice caves, or warm ground). Deposits around the summit crater ming, a small, poorly exposed volcano (figure) from which we are dominantly pyroclastic and contain abundant hypabyssal collected a representative suite of lava, welded pyroclastic fall, plutonic xenoliths. The western and southern slopes are dom- and lherzolitic mantle xenoliths. inated by intermediate-composition lava flows erupted from Laboratory work. Field data for Mount Waesche and Mount summit and flank vents. Vent-proximal pyroclastic deposits Sidley have been compiled on enlargements of U.S. Geological ANTARCTIC JOURNAL Survey 1:250,000 scale maps. Laboratory work in progress in- References cludes argon-40/argon-39 dating and petrographic, geochem- ical, and isotopic analysis of lavas and pyroclastic rocks from LeMasurier, W.E.
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