-78 773 EAST93—Geophysical traverse from the Transantarctic Mountains to Wilkes Basin, East Antarctica URI TEN BRINK, U.S. Geological Survey, Woods Hole, Massachusetts 02543 STEPHEN BANNISTER, Institute of Geological and Nuclear Sciences, New Zealand YIZHAQ MAKOVSKY, Department of Geophysics, Stanford University, Stanford, California 94305 RON HACKNEY, Research School of Earth Sciences, Victoria University, New Zealand RAFAEL KATZMAN, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program, Woods Hole, Massachusetts 02543 T he joint U.S.-New Zealand East Antarctic Seismic Traverse (EAST93), carried out during December 1993 and January 1994, was designed to obtain images of the shallow sedimen- tary layers under the east antarctic ice cap. The traverse (fig- ure 1) started 10 kilometers (km) west of the McMurdo Dome drill site and 25 km west of the exposed bedrock of the Transantarctic Mountains at Lashly Mountains and ended 323 km west of the drill site over the Wilkes subglacial basin. The traverse was subparallel to latitude 78°S and 30-50 km north of the 1958-1959 Victoria Land traverse (Crary 1963). It included multichannel seismic-reflection data with 2.5-meter (m) trace intervals, gravity data collected at intervals of 2.1 km, magnetic data collected at average intervals of 0.5 km, and surface radar at intervals of 77 m. The scientific objectives of the study were to test and constrain models of the uplift of the Transantarctic Mountains and the subsidence of the Wilkes Basin, models that involve a flexed lithospheric plate with a free edge at the boundary with West Antarctica (Stern and ten Brink 1989); • to understand whether the Ferrar dolerite sills and basalts originated from an active mantle plume or whether they were the by-product of rifting and passive upwelling (Elliot 1991, pp. 541-548); and • to help resolve the debate about the climatic conditions and the size of the ice sheet in the Cenozoic, in particular whether parts of East Antarctica were deglaciated for much of the Late Cenozoic (Webb 1991, pp. 599-607). To our knowledge, this was the first large-scale geophysical traverse on the polar plateau since the early 1960s. As such, the experiment presented several logistical challenges: • how to collect regional seismic profiles during the short antarctic summer; • how to keep the scientific instruments running with mini- mal protection in the harsh conditions; and • how to move camp daily and still have a full day of work. The fieldwork lasted 54-60 days, of which only 20 days were spent collecting data. The rest of the time was lost to bad weather, mechanical problems, and rearrangement of sup- plies and equipment at the beginning and the end of the experiment and upon arrival to caches. The work was conducted by 10 people: an advance party of three that surveyed and planted flags every 150 m and the main party of seven that carried out all the geophysical measurements. The main party consisted of a group that plowed the snow, planted the primacord, and con- ducted geophysical and weather measurements and a group, trail- ing 2-3 km behind, that shot and recorded the seismic data. A third group, the base camp, moved once a day, measured the radar profile, and set up magnetic and barometric base stations (to be compared later with the roving station). Elevation along the profile latu varied between 2,190 m to 2,470 T" and temperatures varied between -15°C to -30°C. Snow conditions varied from soft to 155 15O Figure 1. Bottom-ice elevation map contoured from aerogeophysical profiles (dotted lines) in the British Antarctic Survey/Scott Polar Research Institute database and showing the location of the EAST93 line. Heavy line—location of figure 2. Top—Map of Antarctica with the location of bottom map. ANTARCTIC JOURNAL - REVIEW 1994 39
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EAST93—Geophysical traverse from the TransantarcticMountains to Wilkes Basin, East Antarctica
URI TEN BRINK, U.S. Geological Survey, Woods Hole, Massachusetts 02543STEPHEN BANNISTER, Institute of Geological and Nuclear Sciences, New Zealand
YIZHAQ MAKOVSKY, Department of Geophysics, Stanford University, Stanford, California 94305RON HACKNEY, Research School of Earth Sciences, Victoria University, New Zealand
RAFAEL KATZMAN, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program, Woods Hole,Massachusetts 02543
The joint U.S.-New Zealand East Antarctic Seismic Traverse(EAST93), carried out during December 1993 and January
1994, was designed to obtain images of the shallow sedimen-tary layers under the east antarctic ice cap. The traverse (fig-ure 1) started 10 kilometers (km) west of the McMurdo Domedrill site and 25 km west of the exposed bedrock of theTransantarctic Mountains at Lashly Mountains and ended323 km west of the drill site over the Wilkes subglacial basin.The traverse was subparallel to latitude 78°S and 30-50 kmnorth of the 1958-1959 Victoria Land traverse (Crary 1963). Itincluded multichannel seismic-reflection data with 2.5-meter(m) trace intervals, gravity data collected at intervals of 2.1km, magnetic data collected at average intervals of 0.5 km,and surface radar at intervals of 77 m.
The scientific objectives of the study wereto test and constrain models of the uplift of theTransantarctic Mountains and the subsidence of theWilkes Basin, models that involve a flexed lithosphericplate with a free edge at the boundary with West Antarctica(Stern and ten Brink 1989);
• to understand whether the Ferrar dolerite sills and basaltsoriginated from an active mantle plume or whether theywere the by-product of rifting and passive upwelling (Elliot1991, pp. 541-548); and
• to help resolve the debate about the climatic conditionsand the size of the ice sheet in the Cenozoic, in particularwhether parts of East Antarctica were deglaciated for muchof the Late Cenozoic (Webb 1991, pp. 599-607).
To our knowledge, this was the first large-scale geophysicaltraverse on the polar plateau since the early 1960s. As such,the experiment presented several logistical challenges:• how to collect regional seismic profiles during the short
antarctic summer;• how to keep the scientific instruments running with mini-
mal protection in the harsh conditions; and• how to move camp daily and still have a full day of work.
The fieldwork lasted 54-60 days, of which only 20 dayswere spent collecting data. The rest of the time was lost to badweather, mechanical problems, and rearrangement of sup-plies and equipment at the beginning and the end of the
experiment and upon arrival tocaches. The work was conductedby 10 people: an advance party ofthree that surveyed and plantedflags every 150 m and the mainparty of seven that carried out allthe geophysical measurements.The main party consisted of agroup that plowed the snow,planted the primacord, and con-ducted geophysical and weathermeasurements and a group, trail-ing 2-3 km behind, that shot andrecorded the seismic data. A thirdgroup, the base camp, movedonce a day, measured the radarprofile, and set up magnetic andbarometric base stations (to becompared later with the rovingstation).
Elevation along the profilelatu varied between 2,190 m to 2,470
T" and temperatures variedbetween -15°C to -30°C. Snowconditions varied from soft to
155 15OFigure 1. Bottom-ice elevation map contoured from aerogeophysical profiles (dotted lines) in the BritishAntarctic Survey/Scott Polar Research Institute database and showing the location of the EAST93 line.Heavy line—location of figure 2. Top—Map of Antarctica with the location of bottom map.
ANTARCTIC JOURNAL - REVIEW 199439
Distance along profile (km)58 5960 61 6256 57
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5657585960616263Distance along profile (km)
Figure 2. Preliminary seismic data, total magnetic intensity anomaly, free-air gravity anomaly, and elevation along a section of the traversebetween km 55.65 and km 61 (nT c1nnt nintki
steel-narci. sastrugi were encountered between km 122 and265 and had dominant southwest-northeast direction andvariable amplitude and wavelength, reaching as much as 1.5meters. Katabatic wind persistently blew from the west andthe southwest at 10-50 kilometers per hour (km/h) (6-30knots), but the sky was clear. The wind picked up snow atspeeds above 30 km/h (17 knots). Daily maximum windoccurred around 6:00 p.m. local time. Weather fronts from thesouth occasionally brought warmer temperatures, low winds,snow fall, and whiteout conditions.
Our seismic source was a 40- to 60-rn-long detonatingcord (primacord) (1.6 to 2.4 kilogram explosives) per shot laidat 150-rn intervals along the traverse. The primacord wasburied by snow plow at a depth of 16.5 centimeters below the
surface. A 200-rn-long Norsk Hydro's snow streamer with 60active channels was towed and used as a seismic receiver. Theseismic signals were recorded and demultiplexed by Geomet-rics Strataview R-60 seismic system, which was also used forquality-control processing. The gravity field was measured bya LaCoste-Romberg gravimeter. Two Geornetrics G-856 mag-netometers measured the magnetic field, one used as a rovingstation and the other as a temporary base station. The radarprofile was collected by towing low-frequency (1.25 mega-hertz), 80-rn-long receiving and transmitting antennas behinda moving snowmobile.
The experimental design and the scientific equipmentchosen for this experiment were successful in achieving thefollowing.
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• High data-acquisition rate. In a full working day, 60-80shots were fired (maximum of 108 shots) in addition toother geophysical and weather measurements. Sastrugi,encountered along 60 percent of the traverse, did notcause any particular problems except slowing vehiclemovement.
• Data acquisition in harsh weather. Surveying, measuringpotential field, and primacord planting could be per-formed in almost any weather. Shooting was limited towind speeds no greater than 30-35 km/h because of windnoise on the streamer and because the blowing snowposed a risk of premature detonation by electrical shorten-ing. In addition, low temperatures due to wind chill (-50°Cto -70°C) made plastic cover of shooting wires brittle, elec-tric tape nonadhesive, and exposed fingers unbearablycold. Radar and magnetic equipment that had to operateoutside were boxed and kept warm by hot water bottles.The remaining equipment (seismic recorder and tapedrive, gravimeter, computers) were housed in the cabin ofone of the large tracked vehicles. The cabins were heatedonly during data acquisition.
• Efficient air support. A necessary logistical component tothe geophysical traverse was an efficient air support. Toreduce the load on the traverse vehicles, caches of food,fuel, and explosive were put along the profile in advance ofthe experiment by a Twin-Otter plane. Reference stationsfor absolute location and elevation were measured duringthese flights. Airdrops of explosives by a Royal NewZealand Air Force C-130 plane flying from Christchurch toMcMurdo were highly accurate and did not damage theexplosives. A Twin-Otter plane was available on demand tohelp with mechanical, logistical, and medical problems.
A coincident suite of geophysical measurements for asmall section of the traverse is shown in figure 2. Preliminaryprocessing of the seismic data indicates that the ice thicknessis about 1,900 m, the subglacial topography appears rough ona scale of 100 m, and no indication of sediments is evident.The magnetic anomaly drops by 40 nanotesla toward km 61where the seismic reflection ends, a reading that may implythat the strong seismic reflection represents a magma sill at ornear the rock surface. The gravity anomaly, however, is notconsistent with this interpretation. Preliminary analysis of theseismic data indicates that the primacord generated a
broader band signal (6-500 hertz) and stronger surface wavescompared to a down-hole shot. The burial of the primacordimproved the energy coupling with the snow, minimizedenvironmental pollution and energy loss to the surface, andsaved one person's work. The gravity data, processed alongthe entire profile, show that the Bouguer anomaly drops by100 milligals in the eastern 100 km of the line and by addi-tional 25 milligals in the remaining 212 km. Superimposed onthis long-wavelength trend are 30-km-long anomalies withamplitudes of 20-30 milligals.
In summary, EAST93 was the first major geophysical tra-verse on the polar plateau of East Antarctica in 30 years. Theexperiment combined state-of-the-art seismic methods withother geophysical techniques for the purpose of solvingremote geological problems. The experiment was successfulin its major logistical objectives—the collection of a long seis-mic profile in a short field season, reliable operation of theinstruments, and efficient camp support. Scientific results aretoo preliminary to address but the data are promising.
We thank E. Rygg and Norsk-Hydro Co.; R. Huggins andGeometrics, Inc.; J. Behrendt, S. Hodge, and W. Robinson ofthe U.S. Geological Survey; and E. Waddington and D. Morseof the University of Washington for lending scientific equip-ment and guiding us in their use. We thank D. Benson and L.Hotham, U.S. Geological Survey, for locating reference points,and the U.S. Antarctic Program, Antarctic Support Associates,VXE-6, and the New Zealand Antarctic Programme for theirexcellent field support. We thank S. Colman and D. Hutchin-son for their reviews. This study was supported by NationalScience Foundation grant OPP 92-20462.
References
Crary, A.P. 1963. Results of United States traverses in East Antarctica,1958-1961. New York: American Geographical Society.
Elliot, D.H. 1991. Triassic-Early Cretaceous evolution of Antarctica. InM.R.A. Thomson, A. Crame, and J.W. Thomson, (Eds.), Geologicalevolution ofAntarctica. Cambridge: Cambridge University Press.
Stern, T.A., and U.S. ten Brink, 1989. Flexural uplift of theTransantarctic Mountains. Journal of Geophysical Research,94(12), 10315-10330.
Webb, P.N. 1991. A review of the Cenozoic stratigraphy and paleon-tology of Antarctica. In M.R.A. Thomson, J.A. Crame, and J.W.Thomson (Eds.), Geological evolution of Antarctica. Cambridge:Cambridge University Press.
