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Geology of Mt. Everest

02.02.2008  |  129× přečteno      vytisknout článek 

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Geologically, the Himalayas is divisible into four longitudinal morph tectonic zones: 

1.Tethys Himalaya

2. Higher Himalaya

3. Lower Himalaya

4. Sub Himalaya.

Those morph tectonic zones are characterized by well discernible geological and morphological features. Tethys Himalaya is comprised of sedimentary sequences. It is separated from sedimentary and volcanic rocks of the Lhasha region by Indus Tsangpo Suture Zone. This delimiting zone is considered to be the conventional boundary between Himalaya and Trans Himalaya. Its southern boundary with the Higher Himalaya is broadly transitional. In some sectors, however, faults and thrust separate the two belts.

The ketazonal  metamorphic of the Higher Himalaya is delimited to the south by the Main Central Thrust (MCT). Along this plane Higher Himalayan rocks have been transposed over the Lower Himalayan belt. The later belt is comprised of unfossiliferous  meta-sedimentary sequences with remanent klippes and napes of the Higher Himalaya. Its southern boundary is define by the Main Boundary fault/thrust (MBT). The sub Himalayan belt extends to the south of MBT and is made upon Tertiary mollase deposits. This belt is the southern belt of the Himalayas.

Study area is a part of the Tethys and Higher Himalayan morph tectonic zones. It lies to the east central part of the Alpine Himalayan Belt.

STRATIGRAPHY of the Mt. Everest Region:

The rock succession of the area dip due No, N, and Nw. It comprises a basal sequence of Ketaxonal metamorphic and migmatites followed transitionally by meta-sedimentary and sedimentary rock. A thick granite sill occur near the boundary between metamorphic and sedimentary sequences. By comparison of the lithclogy, metamorphic character and structure with the other parts of the Himalayas, the rocks of the study area have been divided into two distinct belts. One ketazonal motamorphics, migmatites and associated granite from the rock sequences of the Higher Himalayan belt whereas the transitionally overlying sedimentary sequences represent the Tethys Himalayan belt.

(A) Higher Himalayan Belt :

Statigraphic reconstruction requires a detail litho logical structural information from the whole of the area. In a mountain system of a magnitude of the Mt. Everest, collection of the required information was not an easy task. Terrian difficulties did not allow mapping in the whole of the area naturally field work was limited to accessible routes only. Consequently, geological interpretations based on the limited information differed diagonally among the geoscientists working in the region. 

The Higher Himalaya comprises a rather monotonous metamorphic rocks devoid of any fossil traces. Further, the rocks are highly mobilized into migmatites. Migmatisation is so intense that the original character of the rocks are completely or nearly completely masked.

Petrographic characters of the rocks across the structural succession do not show drastic differences except in the intensity of migmatisstion. Structurally as well, similarity within sequence is beyond doubt. There are some distinct shear zones, however, do not indicate any major dislocation and transportation. It is  therefore visualized that the Highest Himalayan succession of the region is normal and undisturbed. This view is in contrast to the observations of Lombard and Hagen and fits well with the observations of Bordet (op cit). Stratigraphic division of the Higher Himalayan rocks in the Khumbu valley is difficult. Structural litho logical, metamorphic, petrographic etc characters of the rocks are so similar that any classification is rather theoretical only. For the purpose of providing litho logical variation, however, the Higher Himalayan rocks have been divided into following formations.

(1) Chautikharka Gneisses.

(2) Namche Migmatites and Schists. 

(3) Black gneisses and schists .

 

(1) Chaurikharka Gneisses

The gneiss derived its name after the village of Chaurikharka on the left bank of the Dhudh Kosi river some 25km south of Namche Bazar. The rocks of this formation are best exposed it the cliff sections of the Chaurikharka village. This formatic rocks are not developed in the study area, however, it is described here to provide complete account of the geology of the region.

 The formation was named differently by various workers viz. Kathmandu series (in the sense of Kathmandu Nappe) Hagen (1969); Lower Khumbu gneisses (in the sense of Khumbu Nappes) and Ectinitic gneisses of the Barun gneisses, Bordet, (1961). 

Chaurikharka gneisses consist of intensely banded, finely crystalline, rather monotonous, dark colored quartzo-feldspathic layer. The lower limit of the Chaurikharka gneisses is unknown. It rests tectonically on the marble, black schist, quartzites and migmatites of the Lower Himalaya along MCT. 

In the studied section of the Dudh Kosi valley, the passage is noticed with the profound shearing and injection of secondary quartz segregation. The distinction is also visible in the type of gneisses across the boundary. The Lower Himlalayn migmatites comprises two mica, coarsely crystalline gneisses with large augends of feldspar. In contrast, the Chaurikharka gneisses are finely crystalline, banded and muscovite free. Feldspar augends are occasional and smaller in size.

The succession commence with dark colored banded gneisses with idiomorphic and xenomorphic garnets weaped but lepidoblastic predominantly biotitic mica. Mineral silliminite occur occasionally in the dark biotitic layer. Bordet (1961), also mentioned occurrence of silliminite forming local glide planes. The feldspathic light layer is finely crystalline with rare small feldspar augends.

Associated with the gneisses in the basal section are layers of 10's of meter thick amphibolites and cal silicates. The amphibolites are dark green, coarsely crystalline and consists of hornblende, actionolite, tremolite, epidote, plasioclase etc.

In the middle part of the section feldspar layers are comparatively coarser, feldspar augends are frequent and big. The minerals like garnet and kyanite are rare. In the upper half of the sequence the gneisses are rather mobilized and consists of thin layers of amphibolites and calc silicate similar to lower section. The mineral garnet, kyanite, and silliminite are very rare and small in size. Significantly aplitic apophyses are seen crossing the gneissic country rocks parallel and across the regional foliation.

Mobilisation and migmatisation of the banded Chaurikharka gneisses marks the beginning of the Namche migmatites and schist.

(2) Namche Migmatites and Schists :

It is named after the small township Namche on the southern part of the Khumbu valley. The migmatites are best exposed in the gorge sections of Bhote Kosi and Dudh Kosi around Namche settlement. It is this migmatites and schists that cover the most part of the study area. It has been described under different names by the carlier workers viz. Superieur formation of Khumbu Nappe (Lokbard, 1958); Barun Migmatites of Barun Gneisses (Bordet, 1961) and Khumjun gseries, Pangboche series, Samso series etcof Khumbu nappes (Hagen, 1969).

Namehce migmtites and schists show wide range of variation in textures and structures due to variations in the intensity of migmatisation. In some section, effect of migmatization is least and original rock fabric is least and original rock fabric is still preserve. Where as in other sections original textures and structures are completely destroyed. Migmatitic structures ranging from injection, lit par lit, to complete mobilizaion are observed.

The fabric in the basal section of the formation indicate that the migmatisation startedin the granulose feldspathic bands of the gneisses at very high temperature conditions. The micaceous part or bands were not effected. In the middle and upper section, however, injection, lit par lit to complete mobilistin is observed even affecting the micaceous bands. Further, the different migmatitic structures an textures grade into each other laterally and vertically that the rocks are very difficult to be described in ascending order. IN conclusion, the Namche migmatites and schists consists of banded gneisses, lit part lit migmatites, injection migmatites to migmatites at different stages of homogenization. Complete anatectic rock types can also be met. The parent rocks preserved in the basal and middle sections are thinly bedded gneisses, bedded quartzites, schists an dcalc silicate rocks with occasional layers of marble beds.

Lower limit of the Namche migmatites and schists is not well defined. It starts with the progressive migmatisation of the granulose layers of the thinly banded Chaurikharka gneisses. The chage is estimated from the bandin gintervals, the Namche migmatites and schists show thick banding in contrast to the thinly banded Chaurikharka geisses.

The sequence commences with thickly banded gneisses. Each bands are one to five centimeter thick. In the dark band biotite forms the chief mica minerals. Muscovite is missing. Mineral biotite occurs in the form of concentrated heaps of strongly preferred oriented mineral. Silliminite is occasionally associated with the biotite. Garnet and kyanite are also seenoccasionly. The light coloured band consists of quartz and feldspar only. Granular in texture, they do not show any orientation in contrast to the oriented nature of the Chaurikharka gneisses which indicate their neosome character.

The basal banded gneisses comprises ten to 15 meter thick bands of green, coarsely crystalline calc gneisses at Gomilla. These gneisses consist of actionolite, tremolite, plagioclase and epidote as chief minerals. Diopside occur very rarely. Apatite, sphene and iron ores are the main accessory minerals.

At around Jorsalle and Monja the basal banded migmatites are succeeded by coarsely  crystalline, light coloured  porphyritie and occasionally augean structured migmatites (photo no 13,14). It has a character of injection migmatite. The leucosome constituents are granular an dare randomly oriented. The melanosome constituents consisting of biotite occur in the form of stacks of tabular flakes which show ill defined foliation trend in the migmatites. Up in the succession these types of migmatites occur regularly alternating with the banded migmatites. Occasionally lit par lit migmatites are also seen. Northeast of Namche, the rock successions highly sheared. At Khumjung, Phorche and Thulunga and rocks are green colored. Due to excessive chloritizaition. The rocks are brecciated and mylonitise indicating the shearing internsity. Similar shear zone is noted in the middle section of Namche migmatites and schist at Lungden and north of Goma in the Bhote Kosi valley.

In all these areas, thinly bedded quartzites, calc silicates and schists occur as unmiugmatised country rocks in various stages of retrogression and crushing. Thin layers of amphibokite are also associated with these rocks. Migmatites in this zone occur as retrograded horizons regularly alternating with the unmigmatised rocks. In some section the unmigmatised rocks occur in the form of lensoidal bodies within the migmatites.

Up in the sequence the migmatites of different fabric and structure succeed with thin and thick bands of dominantly  biotitic gneisses. At the level of Pangbouche layers of marble ,calc silicates and amphibolites occur within the migmatites. Those rocks are sheared and show retrogressive minerals. North and cast of Pangbouche biotite chlorite schist occur regularly in the different types of migmatites. It is this rock types that form the youngest rock types of the Namche migmatites and schists in the study area.

Upper limit of the Namche migmatites and schists is marked but the gradual disappearence of migmatites with corresponding appearance of thinly bedded and banded dark coloured biotites and chlorite gneisses and schist.

(3) Black Gneisses and Schists:  

The name to the formation is adopted from the stratigraphy of Bordet 1961. Typical section of the formation is exposed in the steep south face of Nuptse-Lhotse range. As the name implies, the Black gneisses and schist are dark in colour. Rocks are regularly thinly bedded and banded which contrast them from the underlying Namche migmatities and schist. Texturally the rocks are fine grained. The banding structure is between the dark biotite, chlorite minerals and light quartzofeldspathic minerals. The dark colour is due to the abundance of biotite and chlorite minerals.

In the field disappearance of the migmatites and start of the thinly bedded and banded black coloured gneisses sequence mark the beginning of the Black gneisses and schist.

In our field traverses we could not examine the sequence. Since, it marks distinct and recognizable unit even from distances due to its dark color and bedded and banded nature we have incorporated from the stratigraphy  of Bordet, 1961. The rock formation is seen exposed in the southern spur of Nuptse and in the western upper mountain slopes of Amadablan – Kangtega. According to Bordet 1961, the formation passes upward transitionally from the underlying migmatites. The gneisses are regularly finely bedded and dark in colour. The dark colour is due to the abundance of biotiete.It also contains violet cordiorite in grains of few mm across of even idiomorphic crystals of several cm. Locally, layers of garnet-amphibolite are intercalated. Near the top, it is intruded into but the Nuptse granite. In the top, it is intruded into but the Nuptse granite. In the west face of Nuptse it appears again above the Nuptse granite. The formational boundary with the granite is distinct and sharp and parallels the foliation plane of the gneisses.

Nuptse Granite :

It derives its name from the mountain Nuptse. The rocks type is best exposed in the south face of Nuptse where it forms a visible wall of several hundred meters.

It is a sill like intrusive body. Its upper and lower boundaries are distinct and sharp. The lower boundary shows local shearing features. Its boarder with the country rock (Black gneisses and schist) is often characterized by swarms of dykes and pegmatites. The Nuptse granite is a leucocratic granite. Biotite, muscovite and tourmaline are the chief mineralogical constituents besides quartz and plagioclase. Texturally, the granite is fine to medium grained. It is neither banded nor bedded. According to Bordeet 1961, the body of the granite shows minimum crushing features.

In our field traverses we have examined the granite body in the Kalapathar above Gorakhchep. It was massive, fine to medium grained and devoid of any structural orientation. This granite was possible the western continuationof Nuptse south face exposer across the Khumbu glacier. In the Nuptse range the rock type is exposed forming a distinct anticlinal arch. According to Bordect 1961, the granite body extends further east across Nuptse, Lotshe range upto Makulu. The mountain tops of Amadablan and Kangtega is also formed by this granits.

(B) TETHYS HIMALAYAN BELT :

The rock succession of the Tethys Himalaya have a very limited distribution in the study area. The sequence is found to occur in the high mountain peaks in the region of Mt. Everest, Lhotse and Nuptse only. In the study areas, Higher Himalayan rocks broadly transitionally grade into the Tethys Himalayan rocks.

Direct geological observation of the sedimentary succession of the Tethys Himalaya by a geologist is very rare. Our description of the succession is also based upon the observation made from distance. Few samples collected on the Khumbu glacier has provided some information on their mineralogical association texture and structure. The lithological division descriptions presented here is base upon the handspecimen study of the samples collected by mountaineers and presented in the text of the earlier workers. In the region, the stratigraphic division adopted by wager 1934, has been found valid and adopted. According to him, the Tethys Himalayan rocks in the MT Everest region is broadly visible into following formations.

(1) Lower calcareous layers,

(2) The Everest pelites and

(3) Everest Limestone

Lithological and petrographical descriptions of these formations are available in the literatures. The most significant is the carboniferous age of the Everest Limestone. The Limestone forms the summit of the worlds highest peak Mt Everest.  

TECTONTCS AND STRUCTURE :

With the limited geological information's, tectonic interpretation in areas of such altitudinal contrast is extremely difficult. As described earlier the area is characterized by very monotonous lithological formations. Beside the fock formations are migmatised and their nature is unpredictably variable both laterally and vertically. Tectonic division based upon the lithological features only is rather theoretical. Surprisingly, our observations in the study area do not show any differences in the structure which seems to be uniform throughout the area.

Previous workers Lombard 1958, have divided the rocks of the area into numbers of tectonic units whose explanations is very difficult to understand. Our observations in the area suggest that their tectonic division is mostly based upon their observation of the shear zone in the structural succession. For example, the shear structure north of Namche bazaar demarcates the boundary between the Khumbu Nappe 1and 2 of Lombard and Hagen  (op cit) .

Present investigators have also come across many shear zones in the structural succession of the Khumbu valley. But considering the lithological, petrographical, metamorphic and structural characters across the shear zones, we have come to the conclusion that the shear structures of the area are of local significance only and the area is represented by a single tectonic unit with normal stratigraphic set up. In our opinion the worlds highest ground is tectonically simple as proposed by Bordet 1961.

Regional Structure :

The rock succession of the Khumbu region do not show megascopic fold structures except very complicated due to the presence of criss-cross normal and transerverse structures. Further, there are structures which do not fit into either of the tectonic directions and create confusion in the interpretations.

The rock sequence to the south  of Namche bazaar show monotonous NW-SE trend with moderate to steep NE dips. Contour diagram (fig 4) of this section show this tendency with a weak and feeble NNE tectonic direction. To the north, west and east of Namche Bazar structural picture chages considerable. Sturecture parallel to the Himlalayn trend control the geological picture of the area but due to the prominent transevers structures, the Himalayan

Tectonic trends are hidden in the background. The Bhote Kosi section, (fig 5) and the geological map clearly manifests the E-W Himalayan tectonics superimposed and masked by the transevers structures. A similar situation is met with in the section or Namche-Gokyo and Namche-Emja Khola (fig 6 and 7). In conclusion, the transevers structures are more prominent in the study area. The structures at Bhote Kosi, Dudh kosi and Imja Khola are such structures. These structures are of smaller wave length compared to the broad Himalayan tectonic directions. The Himalayan trends, though masked by the superposing transverse structures are the principle geological structures of the area.

Minor structures :

The minor structures described here are from the Namche Migmatites and schist which cover most of the ground investigated. The recorded minor structures occur throughout the structural succession of the Namche Migmatites and schist. Only few structures are confined in the non migmatised sections. The migmatised rocks in general are plastically flow folded and mostly show structures that are impressed in periods during and after migmatisation. Altogether four generation of compressional orogenic fold types wee received. These structures and their character will be discussed according to their probable age sequence.

The first and the earliest fold type recorded in the area are represented by isoclinal recumbent folds. The fold orients NE-NNW-SSW and show variable plunge. Distinct cleavage is associated with the fold which parallels the bedding at the fold Limb. At the fold hinge the cleavages fan at angles at bedding. These shortening structures of intra formational character are mostly confined in the calcareous and quartzitic lithologies of the Namche Migmatites and schist. Prominent lineations related to these structures are cleavage bedding intersections. Some of the quartz mullion also define this lineation. The fold type of the F1 generation in the field are represented by folds of 1 to 3m scale. The photographs and field sketches (photo no 15,16,17,18 and fig 8,9,10,11) represents the typical example. The photographs are from the Thare locality in the Dhudh Kosi valley.

The second generation folds (F2) are also isoclinal folds. These folds are upright and orient NW-WNW/SE-ESE with variable plunge. Distinct cleavage is associated with these folds which parallels the bedding plane. Like F1 folds these folds are also confined in the calcareous and quartzitic horizons. Occasionaly, these folds are even preserved in t he migmatites. The beautiful outcrops (photo No 19,20 and fig 12,13) on the road section at Gumilla are the typical representatives of the fold type.

The third genraation folds (F3) have developed throughout the area. Oriented NW-WNW/SE-ESE , the F3 folds are represented by upon an dupright undulations. Chief lineations of this folds are micopuckers, crenultions and axis of the small undulations. In general the fold type has not generated any cleavage and are formed by the warping of the earlier cleavage and bedding.

Associated and oriented similarly are locally developed chevron folds. A distinct fracture slip cleavage is seen at the hinge of these folds.

The fold type (F4) is also represented by open and upright gentler undulations. These folds have effected all the earlier fold episodes and extend across the Himlayan trend I e NE-NNE/SW-SSW. Lineations associated with the fold type is micropuckers and crenulations.

Out of the four generations of orogenic fold types two of the fold episodes orient across the Himalayan axis. Folds at perpendicular disposition to the Himalayan trend were also recorded and explained by the previous workers in the High Himalayan (Heim and Gansser 1939);  Lambard 1958; Bordet 1961; Gansser 1964 and Hagen 1969etc). The F1 compressional folds are formed due to WNW-ESE shortening. These structures predates F2 and F3 structures formed by the N-S shortening which gave rise to the Himalayn trend. The F1 structures have been opined to be the remained of the Aravalli trend in the Himalaya (Auden 1935; Heim and Gansser 1939, Gansser 1964 etc).

The open F4 cross structures, as the field relationships indicate, are the younger structures. These compressional structures in a dominantly N-S shortening stress field of the Himalayas, is very difficult to explain. These structures are believed to be the rejuvenation of the Aravalli trend.

Shear Zones:

The rocks of the khumbu valley show features of shearing and remobilization. Inour traverses, we have been able to identify some widely sheared zones whose lateral continuation, however, is very uncertain.

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