ギャジョ氷河の構造

Structural studies on Gyajo Glacier in Khumbu region, east Nepal

 本報告では、東ネパールのクンブ地域にあるギャジョ氷河の構造に関する下記論文から、現地調査の結果をまとめた前半部分を紹介します（実験的研究の後半部分は省略）。

 氷河上流部分に降った雪が氷河氷になり、氷河流動の過程で、氷河の構造がどのように変化（変成）するのかを調べました。世界の山岳地帯で見られる片麻岩などの変成岩の構造と氷河構造との類似点は、岩石や氷河研究者が従来から注目しています。

記

ON PREFERRED ORIENTATION OF GLACIER AND EXPERIMENTALLY DEFORMED ICE.. 1972, JOURNAL OF　GEOLOGICAL SOCIETY OF JAPAN, 78, 12, 659-675.

 　　　　　　Plate１　東ネパールのクンブ地域にあるギャジョ氷河の構造

Structural studies on Gyajo Glacier in Khumbu region, east Nepal

 1. Introduction

The present author engaged in the investigations of the structural glaciology at Gyajo glacier in the Solu-Khumbu district of East Nepal from June to December in 1970 (Fig. 1).

The summer monsoon season, which lasts from June to September in East Nepal, brings rain falls to the Himalayan highland below 5400 m above sea level. The rain turns into snow fall above this level. During the season of 1970, the amount of snow accumulation reaches up to 4 m which is ten times that of the other whole seasons approximately. It should be noted that the snow accumulates mainly in the summer season and develops to glacier ice in the Himalayan highland.

The mean temperature during the summer monsoon is about O°C at near the glacier tongue. In day time, the temperature goes up above the　freezing point and then the snow thaws. On the contrary, the temperature comes down below　the freezing point in night time so that it freezes　up the melted water. It is, therefore, clear that　there is an apparent difference in the firnification　between the Gyajo glacier and the glacier in the　polar regions where the snow metamorphoses to　ice without melting. The Gyajo glacier should　be classified into the temperate glacier.

This glacier has an area of 1 km by 1.5 km and　its terminus is situated at 5230 m above sea level,　while the highest point of the accumulation area is at 5650 m (Fig. 2). In the present report, the　accumulation area above 5420 m is defined to be　the upper-stream, and the ablation area to be the　down-stream. The midstream between the upper and　the down-stream is steeper than other areas.　A line which connects every center of traverse　sections is named the center line. Through the firnification, the snow turns into　ice of which the grain size ranges 2 to 4 mm. The fine ice in the upper-stream flows down to the downstream　and it grows into large ice crystals of more　than 70 mm in grain size (Fig. 7). The present　author has kept particulary interest in the relation　between crystal growth and preferred orientation　of the crystal axis of ice as the glacier flows down.

In order to measure the surface flow, forty three　stakes were set up and a base line was settled at X and Y points on　the left bank of the glacier  (Fig. 2).

2. Structure of Gyajo Glacier

Bubble foliation is defined by the alternations of　bubbly ice and bubble-free transparent ice layer　which is strongly developed in the down-stream　near the left bank of the glacier (Fig. 3, 6, 9). Stratification is represented by thin layers marked　by fine sands, deposited by strong wind which　occurs twice a year; once in the pre-monsoon　season and the other in the post-monsoon season.　There are two kinds of bubble foliation, one of　which developed at the upper part of the glacier,　has the strike parallel to the flow direction with　steep inclination towards the center line. The　other occurs at the lower part of the glacier which　is able to be observed only at the terminus, and　it has not much bubbles but is marked by the thin　layers of fine clay and sand which are resulted　from the intense shear movement, which brings　fine clay and sand out of the bed rock. (Figs. 3, 6, 11).

The structure of the stratification at the upper　part in the down-stream shows either gentle or　complicated fold system, while it reveals to be　intense shear movement parallel to the bed rock　to produce the foliation above mentioned at the　lower part. Then, joints which signify closed fractures　and open to be crevasses, which develop within　the upper part of the glacier and hardly cut　down to the lower part (Fig. 5, 10). A thrust　fault divides the upper part and the lower in the　glacier. Consequently, the structure of the glacier　is vertically divided into two structural units which　should represent the structural levels.

The stratification in the down-stream shows　the fold-axes parallel to the flow direction with　gentle undulation at the surface, while the acute　and complicated fold system such as recumbent　and slip folds, develops down to the lower structure　(Figs. 4, 6, 10, l l). Furthermore, another　fold system, which is partly observed in the downstream,　has the fold axis normal to the direction　of the glacier flow. Such a superimposed fold　system is never seen in the upper- and the midstream　(Plate 1).

The average grain size in the upper-stream is　2 to 4 mm and grows to be 70 to 80 mm in the　down-stream and 110 mm in the lower structure　(Plate 2). The ice crystal  becomes especially　large near the left bank in the down-stream (Fig.　7). The shape of the ice crystal is not polygonal　in the mid- to the down-stream and generally has　two elongated directions on the horizontal plane.　The strongly elongated direction is parallel to that　of glacier flow, but the weak one is normal to the　flow direction (Fig. 8 and Plate 2).

In the ice of the upper-stream, fine and circular　bubbles are homogeneously scattered but it becomes　large and is more elongated toward the　down-stream. The elongated direction of bubbles　is parallel to the direction of glacier flow　(Fig. 9).

The glacier surface in the upper-stream flows　six times faster than that in the down-stream does　during the summer monsoon season. The maximum　flow velocity is 1 18 m y^-1  in the upper- stream and 18 m y^-1  in the down-stream (Fig. 12).

3. Fabric Pattern of Glacier Ice

The locations for measuring the orientation of　the crystal axis of ice are F7, J5, H0, F3, F0, D0, B0, B3, F4, and B4 (Fig. 2). F7, J5, H0 and F3 belong　to the upper-stream, F0 and D0 to the mid-stream.　B0 and B3 belong to the down-stream with weak　bubble foliation. F4 and B4 are obtained from the down-stream with strong bubble foliation.　Each sample was cut out parallel to the stratification　plane. And also the ice samples were cut out　horizontally at an ice cliff of the glacier terminus.　The ice crystals were divided by the grain size　into two groups; smaller and coarser grains, and　the crystal axes of 100 ice grains are measured out　of each ice sample. In this report, the orientation　diagrams of the crystal axis plotted on the lower　hemisphere of SCHMIDT net is called "fabric　pattern". The great circle girdle drawn on the fabric pattern indicates the bubble foliation plane.

3-1. Fabric Pattern of the upper-stream

In this part, the fabric pattern shows no particular preferred orientation but a great circle girdle　is seen perpendicular to the bubble foliation plane　at J5 and F3L. The mean grain size is 2 to 5 mm and the texture of ice is generally polygonal (Fig. 13 and Plate 2). The bubble foliation is not well developed.

3-2. Fabric Pattern of the mid-stream

In this part, the fabric pattern of the coarser crystal grain gives a small circle with several maxima centered at the pole of the bubble foliation (FOM and DOL), but that of the smaller crystals shows the similar pattern to those of the upper-stream which shows the great circle girdle perpendicular to the bubble foliation plane (FOS) . The grain size reaches up to 40 mm (Fig. 14 and Plate 2).

3-3. Fabric Pattern of the down-stream

The fabric pattern with weak bubble foliation shows no clear preferred orientation (Fig. 15),  but that with strong bubble foliation gives clear preferred orientation with three maxima (Fig. 16). There is a difference between the fabric pattern of coarser grains and smaller ones; the former has strong concentration in maxima (F4L, B4L), but the latter shows the great circle girdle normal to the bubble foliation plane (F4S). The maxima of the pattern from the coarser grain distribute on a smaller circle of which the center coincides to the pole of the bubble foliation plane at location B4. The crystal grain then grows up to 75 mm (Fig. 16).

Both of the fabric patterns of the upper structure with complicated fold system and of the lower structure, give very strong preferred orientation with maxima. The fabric pattern of the lower structure (TL) has a tendency to concentrate into single maximum on a small girdle of which the center situates at the pole of the bubble foliation plane (Fig. 17). The ice crystals of the lower structure are  immensely elongated to be 110 mm along its spindle axis and the crystal texture of fine grains are cataclastic (Plate 2).

　　　　　　　　　Plate 2　氷河氷の結晶形を示す薄片写真（ギャジョ氷河で撮影）