GEOLOGICAL NOTES ON THE K2 (CHOGO-RI) MASSIF IN THE KARAKORAM

ARDITO DESIO

THE K2 PEAK (8610 m) is a gigantic isolated pyramid of gneissic rock rising about 3600 m above its base, which lies 5000 m above sea level.

The six sides of the mountain face to the north-northeast, east, south-southeast, south-southwest, west, and north-northwest. The ridges are well defined from the crest down to 7500 m, while below this height, some of them branch out into a number of spurs. The number of faces of mountain thus increases towards its base.

On the north-northeast side of K2, at an altitude of 7000-7500 m, is a shoulder covered by an ice-blanket with precipitous walls descending on either side. Above the shoulder the peak rises as another smaller pyramid. The north slope is a tremendous wall, so that, viewed from this side, the top of the mountain appears very slender.

The divide between the basins of the Indus and Yarkand rivers runs across the top of K2, but only the two northern slopes belong to the Yarkand basin.

The pyramid of K2 is connected with the main Karakoram range by two ridges, one oriented in a northwest direction and ending at the Savoia saddle, and the other in an east-northeast direction, towards the Skyang Kangri (7544 m), and, further eastwards, the Skyang-la. The Godwin Austen glacier flows from the Skyang-la, grazing the foot of the southeast slope of K2. The Savoia glacier flows from the Savoia saddle grazing the foot of the eastern slope and joining the Godwin Austen glacier at the end of the south- southwest spur (Angelus spur). The two slopes of the north side of K2 are grazed by the K2 glacier towards the north-west and by the Sughet glacier towards the north-east.

The K2 peak is largely covered by ice, which flows for the most part into the Godwin Austen glacier which is divided into two main branches, the Savoia glacier and the De Filippi glacier. Other branches are very steep, and smaller than the preceding ones, and have independent glaciers. Except on the west side of the mountain, the country rock is very poorly exposed and usually occurs where the slope is too steep to support the ice-cap. However, the walls are also often coated with snow, so that, even during the summer, clean rock surfaces are reduced. Snow falls are frequent also during the summer and the mountain is rarely free from clouds. Under such condition, favourable occasions for geological inspection of the rock slopes also from a distance are very few and of short duration.

For the purposes of the geological description of K2 we shall divide the mountain into four slopes, instead of six : the south slope, the east slope, the north slope, and the west slope. The ridge dividing the four slopes are, descending from the top: for the south slope, the Angelus ridge; for the east slope, the Abruzzi spur, and the one ending at the gap at 6526 m, opening at the head of the Marpophong glacier; the north slope of the mountain stands to the north of the watershed ridge; the west slope is between the ridge leading to the Savoia saddle (6666 m) and the Angelus peak.

South slope. The most common rock type is a porphyroblastic gneiss, more or less rich in biotite and generally strongly laminated, among which augen gneiss prevails. The granite-like gneiss occurs for the most part on the walls west of the De Filippi glacier.

The carbonate rocks are rather frequent on the Gilkey-Fuchoz spur and on the walls east of the end of the De Filippi glacier.

We called 'G. P. sequence' (Gilkey-Puchoz spur calcareous sequence) this complex. It is composed of arenaceous limestone, arenaceous gneiss, biotitic paragneiss and graphitic phyllite.

White marble was observed in situ along the Bill couloir and SE of the summit, but also as glacial debris, near the left corner of the confluence of the De Filippi glacier and upstream in the last glacier below the shoulder of the mountain. Lamprophyric dykes cross-cut here and there the rocks.

From the stratigraphic point of view the complex of layers containing carbonate rocks on the south slope of K2 is well differentiated from the other rock associations of the whole south slope.

East slope. The upper eastern slope of K2 above 6500 m is mostly made of porphyroblastic gneiss, locally very laminated, while the lower slopes are composed of Falchan Kangrigneiss. This meta- morphic formation—which largely outcrops on the western side of Falchan Kangri — is mostly composed of an association of quartz- feldspar arenaceous gneiss with fine-grained biotite gneiss and biotite paraschist, sometime garnetiferous.

Near the south foot of the eastern slope calcareous rocks appear and, as at the north-east end, a passage from Falchan gneiss to 'black slates' is to see. Numerous bands of granite gneiss are enclosed in these rocks.

The layers dip prevailingly north-east, and the inclination increases from the shoulder to the foot of K2, while it decreases from the shoulder to the top. From the shoulder downwards the layers describe a knee- shaped anticline.

North slope. The northern slope of K2 is one of the highest mountain walls in the world and probably the highest. It amounts to 3600 m from the foot to the top of the mountain. The slope is divided into two faces by a long ridge which separate the two upper branches of the K2 glacier. The moraines of this glacier supplied informations on the geological composition of the wall. The prevailing lithotypes are porphyroblastic gneiss of the type of the K2 gneiss, associated with 'black slates'. Marble outcrops more downstream from the wall on the left side of the valley of K2 glacier. Possibly some blocks may come down also from the calcareous sequence near the top of K2.

West slope. The lithological composition of this slope is not different from that of the neighbouring southern and northern slopes. Viewed from a distance the rock is identical both in colour and in layering, and is continuous from one slope to another. The rock is prevailingly composed of grey and greenish, rather massive gneiss, and often augen gneiss (K2 gneiss) associated with granite gneiss.

The western spur of the satellite peak Angelus (6855 m) is made of K2 gneiss and Falchan gneiss and is crossed by an irregular agmatitic lens of white matrix including blocks and fragments probably belonging to the Falchan gneiss.

Tectonic and orogeny

The massif of K2 is a mosaic of faulted blocks elongated prevailingly in north-south or north-northwest direction, and gently folded. The main fault crosses the mountain near the top.

Three anticlines are to identify. Two of them, with their axes striking NNE, cross the eastern portion of the mountain. Between them there is a syncline. The other anticline runs near the western foot of the K2 pyramid.

The information gathered on the age of the rocks of the K2 massif suggests that some deformations occurred during the Hercynian orogeny. The sedimentary formations may have undergone a primary folding and a longitudinal faulting during the last events of the Hercynian tectogenic cycle.

During the Alpine orogeny a few faults were reactivated. The block, already made rigid during the previous folding, was repeatedly fractured in prevailing direction north-south. It was later submitted to a granitisation process which, after consolidation, made the block more and more rigid, until it eventually fractured into large blocks as previously recorded.

 

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