Hirokazu Ohmae

Volcanic ash layers in bare ice areas near the Yamato Mountains, Dronning Maud Land and the Allan Hills, Victoria Land, Antarctica

Dirt layers of tephra were found on the bare ice surface in the Meteorite Ice Field near the Yamato Mountains, Dronning Maud Land, and near the Allan Hills, Victoria Land, Antarctica. The grain-size analyses of volcanic ash fragments show that the mean grain size in the Allan Hills region is larger than that in the Yamato Mountains region. This fact indicates that the volcanic sources of the dirt layers in the Yamato Mountains region are farther away than those in the Allan Hills. Their constituent fragments are well-sorted and composed mainly of volcanic glass shards with minor amounts of crystal fragments . Glass shards of the tephra from the Yamato Mountains region have a composition of tholeiitic andesite which is low in alkali and high in iron but not so enriched in titanium, and the associated crystal fragments consist of calcic plagioclase, subcalcic clinopyroxene, orthopyroxene and magnetite. The nature of island arc tholeiite of the tephra indicates that its source is some volcano in the South Sandwich Islands. On the other hand, the tephra from the Allan Hills region is composed of glass shards of trachybasaltic composition and crystal fragments of titanaugite calcic plagioclase, kaersutite, olivine, rMnite and titanomagnetite. A young volcano of the McMurdo volcanic group is suggested as a possible source of this tephra.

Closure rate of a 700 m deep bore hole at Mizuho Station, East Antarctica

Measurements of the bore-hole diameter were made with three-contact-point calipers from the shallow layer to a depth of 700 m at Mizuho Station, East Antarctica, in 1984-86. The minimum strain-rate (i.e. secondary creep rate) of the bore-hole closure was estimated from the observed creep curve at each depth . A relation between stress and strain-rate was deduced for a temperature of -35°C and a stress range from 0.8 to 1.65 MN m-, as E = 6.49 x 10-9 r 2.87 where E is the effective strain-rate (S -1) and r is the effective shear stress (MN m-2). This result shows that, for comparable overburden stresses and temperature, the bore-hole closure rates at Mizuho Station are higher than those measured in bore holes at Byrd Station and in the Northern Hemisphere ice caps (Paterson 1977).

DISTRIBUTION OF REFLECTED POWER FROM THE BED BY RADIO ECHO-SOUNDING IN THE SHIRASE GLACIER DRAINAGE AREA. EAST DRONNING MAUD LAND. ANTARCTICA

A large part of the area of the Shirase Glacier drainage basin has been surveyed by airborne (operating frequency: 179 MHz) and ground-based (60 MHz) radio echo-sounding to define the bedrock topography and to investigate the condition of bed/ ice interface since 1982. It is shown that the reflection intensity from the bed , which is corrected for attenuation in the ice sheet, has a higher value for reflection intensity in the down-stream area of Shirase Glacier than in the up-stream area. The area of strongest intensity of reflection from the bed coincides with the area for which the calculated temperature at the bed is above -1°C. The boundary area between the highest and lowest values of corrected reflected intensity corresponds to the area of decreasing basal shear stress. It is found that the distribution of high corrected reflection intensity corresponds to the area of thinning of the ice sheet, which has been measured by ice-flow observation in the Shirase Glacier drainage basin.

Dynamical Behaviour of the Ice Sheet on Mizuho Plateau, East Antarctica

The Japanese Antarctic Research Expedition (JARE) has continued glaciological work in the Mizuho Plateau, East Antarctica. We have already reported that the ice sheet in Mizuho Plateau, which flows into Shirase Glacier and is classified as a fast-moving outlet glacier was thinning at a rate of about 70 cm/year, and the profile of the basal shear stress along the central flow line was similar to that of surging glaciers. A new 5 year glaciological programme in Mizuho Plateau and East Dronning Maud Land which started in 1981 is now being carried out, and we have obtained new results as follows: