100 kmEAST-.'-JH-WEST

0

100

UJ 20C

30C.Figure 2. A cross section through the north end of the SouthSandwich Islands arc, displaying the hypocenters located by datafrom 10 or more observatories. The majority of intermediatedepth focus earthquakes are concentrated in the northern 100-kilometer segment of the arc. A recomputation of the hypocentersjointly should clarify the subduction zone that appears to bedipping at an angle of approximately 45 0 to a depth of slightly

less than 200 kilometers.

occurred during 1972. The December 22, 1972, eventin the South Sandwich Islands region had 9 aftershocksof magnitude 5.0 or above. It is unusual for shocks inthis area to have extensive aftershock sequences. TheSouth Sandwich Islands region was the site of 3 of 5major shocks, and 51 of 109 earthquakes located duringthe year.

The profile of deep focus shocks in 1972 also wasnearly the same as in 1971. Eight shocks occurred withintermediate depths of focus greater than 60 kilometers,and the deepest was computed at 157 kilometers. Thecorresponding values for 1971 are 7 and 183 kilometers.

A relatively unusual shock occurred at 55.7 0 S. 47.90W., on July 14, 1972, with a magnitude of 5.0. It wasalong the northern boundary of the Scotia Sea plate neara similar hypocenter that occurred on October 7, 1970.

Antarctic seismological studies

LEON KNOPOFF and GREGG VANE

Institute of Geophysics and Planetary PhysicsUniversity of California, Los Angeles

The surface of the earth is covered by a small numberof rather large plates, each in motion relative to theothers, according to the model of plate tectonics. Theseplates glide over the low-velocity channel in the earth'smantle at the rate of centimeters per year (see Knopoff,1969, among others). This channel is probably due topartial melting and therefore is likely to be a region ofreduced viscosity, probably acting as a lubricant and

allowing the lithospheric matter to move relatively freely.Under some of the ancient pre-Cambrian shields of

the earth, however, the low-velocity zone of the mantleis either absent or poorly developed (Biswas, 1971;Biswas and Knopoff, 1973) It has been shown satisfac-torily that the low-velocity channel is absent or nearly sobeneath the Canadian and Baltic shields (Fouda, 1973),and there are strong indications that this is also true forthe South African (Block et al., 1969; Fouda, 1973),Australian (Bolt and Niazi, 1964), Brazilian (Sherburneet al., 1971), and Indian (Gabriel and Kuo, 1966;Fouda, 1973) shields. For all of these regions, the ageof the basement rocks is greater than I billion years.The purpose of this study is to determine whether theantarctic shield has an upper mantle structure similar tothe old, Canadian and Baltic shields, and hence, to makean inference regarding the age of the antarctic basement.

In the investigation we will use the two-station surfacewave method in which the dispersion of long-period sur-face waves is measured by using data from two similarlyinstrumented seismographic stations separated by a dis-tance of some hundreds of kilometers. Briefly, the pro-cedure involves harmonic analysis of the surface waves ofa selected earthquake after selective frequency filteringand velocity windowing to minimize the effects of echoesand multipath transmissions, computation of the phasevelocity dispersion of surface waves, and inversion ofthese results to obtain the upper mantle structure beneaththe path between the two stations.

Instruments of the World Wide Standardized Seismo-graphic Network (wwssN) have been operated for sev-eral years at Amundsen-Scott South Pole Station and atScott Base. We expect to obtain adequate records for oneside of the antarctic shield from the seismograms re-corded at these stations. To complete the two-stationpath across Antarctica, long-period seismographic instru-ments, identical to those at the WWSSN sites, were set upand operated at Soviet Antarctic Station Novolazarevska-ya during the 17th Soviet Antarctic Expedition, therebymaking possible the investigation of the structure beneaththe paths Novolazarevskaya-South Pole and Novolaza-revskaya-Scott Base, which are shown in the figure. TheSouth Pole-Scott Base line is complicated by the presenceof the Transantarctic Mountains and the fact that the linelies along the very edge of the shield area. Thereforeit is improbable that analysis of data obtained for thisline will at this time yield fruitful results.

Records for 11 events of possible use were obtained atNovolazarevskaya during the period of instrument opera-tion. Data processing will begin for the lines Novola-z arevskaya -South Pole and Novolazarevskaya-Scott Baseas soon as the records of the corresponding events arereceived from South Pole and Scott Base. Especially im-portant will be the record quality at South Pole, whereinstrument magnification is lower than at either Novola-zarevskaya or Scott Base.

256 ANTARCTIC JOURNAL

Paths across Antarctica, overwhich the two-station surfacewave method is to be ap-

plied.

• Bellingauzen (USSR)

Capitán Arturo Prat (Ch.)4

DECEPTION ISLANDARGENTINE ISLANDS

I . IVEDDELL SEA

Palmer (USA) General

•\.,RONNEflICE

SHELF/

C z Siple (USA)

.r.1

50005001000I' I_i

KILOMETERS

Showa (lap.)

ISER'Y—..._.iolodezhnaYa (USSR)C RONDANEC

Q U c NMOUNTAINS

Halley Bay (UK)

\)2.0Mawson (Ausl.)5

*Plateau (USA; closed)

Amun en-ScottSouth Ic (USA)

NROSS ICE SHELF

ROSS ISLAND

ROSS SEA

Hallett (USA and N.E.)

11(0'

90E

Mirnyy )USSR(C

*Vostok (USSR)

lCasey (Aunt.)

colt Base Ni.)tcMurdo (USA)

DRY VALLEYS

VICTORIA LAND

L , ..'4Dumont d'Urville (Fr.)Leningradskaya (USSR)

References

Geologic survey of Marie Byrd Landand western Ellsworth Land

Biswas, N. N. 1971. The upper mantle structure of theUnited States from the dispersion of surface waves. Ph.D.dissertation, University of California, Los Angeles.

Biswas, N. N., and L. Knopoff. In press. The structure of theupper mantle under the United States from the dispersionof Rayleigh waves. Geophysical Journal of the Royal Astro-nomical Society.

Block, S., A. L. Hales, and M. Landisman. 1969. Velocitiesin the crust and upper mantle of Southern Africa from multi-mode surface wave dispersion. Seismological Society of Amer.lea. Bulletin, 59: 1599-1629.

Bolt, B. A., and M. Niazi. 1964. Dispersion of Rayleigh wavesacross Australia. Geophysical Journal of the Royal Astro.nolnical Society, 9: 21-35.

Fouda, A. A. 1973. The upper mantle structure under thestable regions. Ph.D. dissertation. University of California,Los Angeles.

Gabriel, V. G., and J . T. Kuo. 1966. High Rayleigh wavephase velocities for the New Delhi, India-Lahore, Pakistanprofile. Seismological Society of America. Bulletin, 56:1137-1146.

Knopoff, L. 1969. The upper mantle of the earth. Science,163: 1277-1287.

Sherburne, R. W., and S. S. Alexander. 1971. Crust and uppermantle structure for continental South America from surfacewave measurements (abstract). American Geophysical Union.Transactions, 52: 281,

F. ALTON WADE

The MuseumTexas Tech University

Progress continues on the reduction of data and analy-ses of specimens collected during the 1934, 1940, 1966,1967, and 1968 field seasons in Marie Byrd Land andEllsworth Land. The task of unravelling the geologichistory of this large sector of West Antarctica is de-pendent upon detailed microtextural studies of late Pre-cambrian-early Paleozoic metasedimentary rocks andinterpretations of orogenic and metamorphic eventsrecorded in these rocks. Four orogenic events in thissector have been radiometrically dated and reported.These are: (1) 445-475 million years, Ross orogenyequivalent (Krylov ci al., in press) ; ( 2) a mid-Paleozoicevent, 325-360 million years (Klimov, 1967; Halpern,1968; Wade, 1972); (3) a late-Paleozoic event, 265-285million years (Halpern, 1972; Wade, 1972); and (4)Cretaceous, a circumpacific event about 100 million

September-October 1973 257