Geology of Mount Everest

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The geology of Mount Everest is dominated by sedimentary and metamorphic rocks , which from the Eocene onwards slid over the granulitic , archaic continental crust of India in the course of the collision with Eurasia that began in the Upper Cretaceous. The enormous height of the mountain is explained on the one hand by the expansion of leucogranitic magma , which intruded the metasediments of the substructure from the beginning of the Miocene , and on the other hand by the presence of two upper crustal ceilings .

introduction

The north side of Mount Everest as seen from Rongpu Monastery . The North-Col formation with the yellow band in the hanging wall and above it the summit pyramid with the Everest formation can be easily recognized.

It is believed that the upper two formations of Mount Everest originally consisted of marine shelf sediments that had accumulated on the northern passive continental margin of India before the collision. The collision itself occurred during the Cenozoic , around 54 to 50 million years ago in the Eocene. The consequence was the closure of the Tethys , whose last marine sediments along the Indus-Yarlung-Tsangpo suture are exposed and date from the early Eocene. They are 50.5 to 49 million years old and belong to the Ypresium . The collision deformed and metamorphosed the previously deposited sediments, which were pushed up in a southerly direction.

The lowest formation of Mount Everest is made up of highly metamorphic rocks of sedimentary origin. During the collision process, it was sunk to a depth of 15 to 20 kilometers in a northerly direction, heated, metamorphosed, partially melted and in the early to mid Miocene between 24 and 12 million years it was interspersed with veins of leucogranitic material . Surrounded by two large Abscherhorizonten - one each in both the hanging wall and the footwall - it was then to the south under kanalartigem flow ( English channel flow ) produces pressed again. The amount for this is assumed to be 100 to 200 kilometers south.

The constant advancement of India northwards into the Eurasian continent resulted in a doubling of the crust thickness to 70 kilometers both below the Himalayas and below the Karakoram-Lhasa block. The highlands of Tibet arose , the largest high plateau on earth with heights of over 5000 meters.

Currently, the Himalayas are rising at a rate of 5 millimeters per year and the tectonic narrowing occurs at a rate of 17 to 18 millimeters per year.

Structure of the Himalayas

Geological overview map of the Himalayas, in black the leuco granites. Mount Everest is located in the northeast of Kathmandu .

The Himalayan Orogen forms a clearly defined arch structure, the wavy front of which has smaller bulges and indentations in the hundreds of kilometers. It is made up of five lithotectonic belts that run more or less parallel to each other:

  • the Transhimalayan batholith in the north (red)
  • the Indus-Yarlung-Tsangpo-Sutur zone (green)
  • the Tethyalen High Himalayan sediment sequence (light blue)
  • the metamorphic Greater Himalaya Sequence (orange)
  • the front Himalayas in the south (yellow)

The upper crust of the Tethys Himalaya consists of 10 to 12 kilometers thick, folded and overturned sediments of the Phanerozoic ( Ediacarium to Eocene ) - referred to in English as the Higher Himalayan Sedimentary Series ( HHSS - High Himalayan Sediment Series ). They are cut off in the north by the Indus-Yarlung-Tsangpo-Sutur and to the south find their end in the flat shear horizon of the South Tibetan Detachment System (abbreviated STDS - South Tibetan Shear System ) with top to the north as a sense of movement. To the south of this is the 15 to a maximum of 40 kilometers thick Greater Himalayan Sequence ( GHS ) - metamorphic rocks of the Barrow type, migmatites and leukogranites. Their stratigraphically deepest metapelites ( Kuncha-Pelit ) are somewhat older than 1830 million years and thus come from the Proterozoic . The GHS ends to the south in a 2 to 4 kilometer thick zone with reversed metamorphic isogrades that reach back from the sillimanite-thistle zone to the biotite-chlorite zone. At the base there is a ductile thrust zone with the top to the south as a sense of movement, the Main Central Thrust ( MCT - central main thrust ). The Front Himalaya ( Lesser Himalaya ) lying south in front of it contains rocks from the Indian Plate, including Proterozoic basement and Paleozoic deck sediments of relatively small thickness. The Himalayan Orogen ends with the two thrust systems, the Main Boundary Thrust on the northern edge of the Front Himalayas and the Main Frontal Thrust in the northern Siwaliks foreland of Pakistan and India . The lower crust of the Himalayas, which is nowhere exposed, is probably composed of granulite facial shield rocks from India.

Description of Mount Everest

Geologically , Mount Everest - at 8,848 meters above sea ​​level the highest mountain on earth - can be divided into three lithotectonic units, which are separated from one another by flat-lying faults with a sense of movement Top to the north, belonging to the STDS. The following units can be distinguished from hanging to lying :

- Qomolangma Detachment -

- Lhotse Detachment -

The North-Col-Formation, occasionally also North-Col-Formation and Everest-Formation together, are often referred to as Everest-Serie (English Everest Series ).

Everest formation

The summit structure of Mount Everest with the gray Everest formation in the hanging wall and the dark North Col formation in the recumbent position, separated by the yellow ribbon .

The Everest formation is above the yellow band (engl. Yellow tape ) to approximately 8600 meters through the flat with 5 incident to 20 ° to the north-east Qomolangma Detachment of the North-Col formation separated. It also forms the summit of Mount Everest, so 225 to 250 meters are exposed from it. The formation, which plunges at around 15 ° to the north-northeast, is made up of gray to dark gray, sometimes white, thick-layered, micritic limestone of the Lower to Middle Ordovician , in which subordinate recrystallized dolomites and clayey-silty layers are interposed. Gansser (1964) was originally of the opinion that lime contained crinoids . Subsequent petrographic investigations on samples near the peaks showed that the formation contained carbonate pellets ( peloids ) and finely chopped remains of trilobites , crinoids and also ostracods . However, many samples were so sheared and recrystallized that the original components could not be determined. About 70 meters below the summit there is a 60-meter-thick, weathered white thrombolite layer that includes the third step and extends to the base of the summit pyramid. These are stromatolites-like, shallow marine sediments that have been captured, bound and cemented by the secreted biofilm of microorganisms, especially cyanobacteria . The lowest 5 meters of the formation above the Qomolangma Detachment are very badly deformed.

The Everest Formation is traversed by numerous steep faults , all of which end in the flat, brittle Qomolangma Detachment. This fault separates the formation from the underlying Yellow Band of the North Col Formation.

North Col Formation

The summit area of ​​Mount Everest between 7000 and 8600 meters consists of the 1600-meter-thick, Central Cambrian, upper greenshist to lower amphibolite faced North Col Formation . Your hanging wall 8200-8600 meters above sea level is the most northeast ridge up to the First Step approach reaching Yellow Ribbon . The 172-meter-thick yellow ribbon is mainly composed of a coarse-grained, calcite-rich diopside-epidote marble that weathered to a striking yellow-brown, but also contains layers of muscovite-biotite-phyllite and slate . Samples from a height of 8,300 meters showed a content of around 5 percent of the remains of recrystallized arm and stalk members of crinoids. The top 5 meters of the Yellow Ribbon in the immediate vicinity of the Qomolangma Detachment are extremely deformed; a 5 to 40 centimeter thick fault breccia separates it from the overlying Everest Formation.

Below the Yellow Band between 8,200 and 7,000 meters above sea level, the North Col Formation leads alternately deformed slates, phyllites and, to a lesser extent, marbles. The upper 600 meters between 8200 and 7600 meters are mainly biotite-quartz and chlorite-biotite-phyllites, into which less significant biotite-sericite-quartz-schists intervene. This is followed by biotite-quartz-slate between 7,600 and 7,000 meters in height with intercalations of epidote-quartz-slate, biotite-calcite-quartz-slate and thin layers of quartz-containing marble.

All these now metamorphic rocks of the middle to upper green slate facies are likely to have emerged from a deep-sea frog of the Middle or Upper Cambrian , which was originally composed of alternating layers of claystone, slate, clayey sandstone, calcareous sandstone and sandy lime. In the lying position, the North Col Formation is cut flat by the ductile Lhotse Detachment .

Rongbuk formation

The underlying upper amphibolite facial Rongbuk Formation (or Rongpu Formation ) forms the substructure of Mount Everest below 7000 to 5400 meters above sea level. It already belongs to the central crystalline zone of the Himalayas ( Greater Himalayan Sequence or GHS for short ) and consists of slate and gneiss (dark, biotite-rich sillimanite-garnet-cordierite-gneiss), which is supported by numerous storage corridors and corridors made of leucogranite - Everest Nuptse Granite - to be intruded. Simpson and colleagues (2000) dated the accompanying contact metamorphosis to be 17.9 ± 0.5 million years. The foliation of the Rongbuk Formation generally strikes east-west and falls flat to N 005 to N 020, the associated linear stretching strike east-northeast. Kinematic criteria such as SC structures, mica fish and asymmetrical porphyroclast endings prove a high-temperature shear sense top to the north with simultaneous pressing out of the GHS to the south. Microstructures in quartz and the c-axes of quartz indicate temperatures of over 500 ° C in the shear zone.

Above this, there are folds stretching in northerly directions in the kilometer to ten kilometer range.

Everest-Nuptse granite

The Everest-Nuptse-Granite, also Pumori-Everest-Granite , is a decidedly peraluminous two- mica tourmaline-leucogranite, which contains the minerals quartz , plagioclase , alkali feldspar ( microcline or orthoclase ) and the mica muscovite and biotite as well as tourmaline . Andalusite , cordierite and garnet can also be added , as well as zircon , monazite , xenotime and apatite as accessories . The thickness of the leucogranite dikes is very variable and can range from centimeters to swellings in the thousands of meters. The highest thickness attained the 3000-meter-reach inflation of Kangshung east wall , which almost up to 7800 meters at the South Col zoom ranges. Ultimately, this should be responsible for the enormous height of Mount Everest and Lhotse .

The leukogranites form part of a belt of Oligocene to Miocene intrusives - the HHL ( High Himalaya Leucogranites - Leukogranites of the High Himalayas). They were created in two phases by partial melting of the high-grade palaeoproterozoic to Ordovician metasediments of the GHS ( Greater Himalayan Sequence ) synkinematically 24 to 17 million years ago in Aquitanium and Burdigalium and post-kinematically 16.4 million years ago in Langhian . The ultimate trigger for this was the subduction of the Indian under the Eurasian plate .

The leuco granites were created from very viscous minimal melts. Two processes are considered here - a low-temperature, wet melting of a pelitic parent rock in the presence of hot liquids or a higher-temperature, dry melting process. The latter is realized, for example, in the incongruent melting of muscovite, which takes place without a vapor phase, but generates a higher proportion of melt.

Metasediments are to be assumed as parent rocks, as the isotopes of the elements strontium, neodymium and lead suggest. A shell participation is excluded. In particular, the isotope ratios 87 Sr / 86 Sr are extremely high at 0.74 to 0.79 and at the same time very heterogeneous, which implies a one hundred percent crustal protolith. Muscovite-bearing pelites and quartz-feldspar gneisses of the Neoproterozoic Haimanta formation are now considered to be the most likely parent rock . The heat source required for the melting process can only come about through a high concentration of radioactive elements in the parent rock. It is known that the leukogranites of the Himalayas have very high contents of radiogenic lead isotopes and that their protoliths must therefore be enriched in uranium and thorium. The uranium concentrations in Himalayan granites are among the highest in the world.

The majority of the leukogranite ducts appear synkinematic, but they can also be post-kinematic. Synkinematic corridors are based on the foliation in the Rombuk formation and are themselves weakly foiled, recognizable by the relatively indistinct muscovites and elongated feldspar phenocrystals. Their microtectonic structure proves their deformation, for example through the undissolved erosion of quartz and feldspar, through deformation twins in plagioclase and through late, brittle breaking of quartz and feldspar. Post-kinematic dikes are massive, have no internal deformation and can also penetrate parallel to the foliation of the cladding rock. Often, however, they cross the gneiss foliation, knock out large blocks of gneiss and synkinematic leukogranite and adjust them by rotation.

The synkinematic leukogranites could be injected sideways into hydraulic fracture systems due to simple shear. It can also be observed how migmatite leukosomes combine to form huge storage corridor systems , which in turn can merge into larger pluton-like collections according to the Christmas tree principle. The storage corridors are always more or less parallel to the foliation in gneisses of the sillimanite facies. The melt migration was predominantly horizontal and not vertical. The accumulations of enamel did not actively intrude into higher elevations, but rather behaved like billowing storage corridors. The clearest example of this is the warehouse corridor in the Nuptse south wall.

As a source region of the magma further north lying, deeper storage tunnels of large dimensions are considered. The physical conditions for melting were generally between 0.4 and 0.6 gigapascals , which corresponds to mid-crustal depths of 15 to 20 kilometers.

Deformation, metamorphosis and anatexis

In the Himalayan orogen, two significant deformation phases can be distinguished, which in turn are linked to metamorphic events:

  • an Eohimalaya phase in the Middle Eocene to the Upper Oligocene, which led to thickening of the crust and reached its regional climax between 33 and 28 million years ago
  • a Neohimalaya phase from the Lower Miocene 23 million years ago that continues to this day . It attained the sillimanite grade with more than 620 ° C and intrusive leukogranites were formed with anatectic melting. It brought a clear change in the tectonic deformation style, which has not changed so far and therefore suggests an orogen in a state of equilibrium.

The Eohimalaya phase was preceded by the continental collision, which hit the northern edge of the Indian plate in the Lutetium around 46.4 million years ago . Ultra-high pressure conditions (Coesit-Eklogit-Facies) with pressures of up to 2.75 GPa (which corresponds to depths of over 100 kilometers) and temperatures of 720 to 770 ° C were achieved. This initial UHP metamorphosis then gave way to the regional metamorphosis of the Eohimalaya phase in the disthene facies and then the Neohimalaya phase in the sillimanite facies.

The metamorphic conditions of the sillimanite facies persisted in the hanging wall of the GHS for 16.9 million years. This points to a greatly increased topography during this period already during the early Miocene.

A final fourth metamorphic event is characterized by very low pressures but high temperatures and was accompanied by metasomatosis and cordierite-bearing leukogranites. However, it could only be detected in the syntaxes (indentations in a ceiling front) of the Nanga Parbat and Namjagbarwa .

On Mount Everest, the course of the metamorphosis can be divided into two events. The first high pressure event M 1 was of the Barrow type and progressed progressively up to the disthene facies. The PT conditions were 550 to 560 ° C and 0.8 to 1.0 GPa. The subsequent event M 2 in the sillimanite facies was heated to a higher temperature at 650 to 740 ° C. under a pressure drop (0.7 to 0.4 GPa). M 1 began 39 million years ago in the Upper Eocene and was dated by Simpson and colleagues (2000) to be 32.2 ± 0.4 million years ago. M 2 then established itself between 28 and 18 million years, dated by Simpson and colleagues (2000) to be 22.7 ± 0.2 million years. The high temperatures thus lasted for a good 20 million years. The pressure drop is generally associated with anatexis and the production of leuco granites, such as the Everest-Nuptse granite. The last post-kinematic leuco granites were secreted in the Kangshung valley 16.7 million years ago, otherwise 16.4 million years ago. The ductile foliation in the Rongbuk Formation is clearly older than this date of the late Burdigali .

Geodynamics

Shearings at the STDS

Everest panorama, taken from the north from the Gyawu La pass in Tibet. Makalu on the left, Gyachung Kang and Cho Oyu on the right.

The South Tibetan Shear System (STDS) is represented on Mount Everest by two shear surfaces - the brittle Qomolangma detachment in the hanging wall with a minimum offset of 34 kilometers and the ductile Lhotse detachment in the horizontal with a minimum offset of 40 kilometers. The Lhotse Detachment was created earlier and is folded in places. Wedged between the two detachments is the North Col formation with the yellow ribbon. This formation is clearly sheared and has reached temperatures of up to 450 ° C, but overall it is significantly less metamorphic than the underlying Rombuk formation and was not infiltrated by leukogranites. The ductile movements at the Lhotse Detachment are believed to have occurred between 18 and 16.9 million years ago in the Burdigalium. The brittle movements at the Qomolangma Detachment, on the other hand, did not occur until after 16 million years and are therefore younger.

The two detachments then merge into a single shear area north of Mount Everest at Rongpu Monastery , so that here a ductile shear zone in Cambrian layers is directly covered by a flat fault area. Further to the north-east of the monastery, the STDS is finally formed as a sole, 1000-meter-thick ductile shear zone, which plunges at 35 ° to the north, whereby Ordovician and younger sediments come to lie over sheared Cambrian silicate rocks and mylonites . In the Kharta Valley, 57 kilometers north of Mount Everest, GHS sillimanite gneisses, interspersed with leukogranite dykes, appear on the surface.

Also further west in Nyalam in southern Tibet the two detachments unite to form a single shear area, here too Cambrian layers lie below the combined shear. Below this, the degree of deformation and metamorphosis increases rapidly. That the STDS not only split up like on Mount Everest, but can also take very variable positions, is shown in Zanskar , where it reaches down to the Neoproterozoic.

Khumbu thrust

Below the Lhotse Detachment, southwest of Mount Everest, at the foot of the Nuptse, appears the Khumbu Thrust , where a 3 to 6 kilometer thick blanket of flat leucogranite storage corridors and bodies was pressed up to 25 kilometers to the south. In addition to the Nuptse Pluton, this cover includes the leuco granite peaks Ama Dablam , Kantega and Thamserku , which were probably all connected as a single layer before the onset of the current erosion.

Channel flow zone

The partially melted zone of the Channel Flow , which includes the Rombuk Formation, is, as already mentioned, bounded at its base by the ductile Main Central Thrust, whereby the metamorphic isogrades experience a reversal. In the top it is cut off flat by the Lhotse detachment of the STDS - but the isogrades are correct here. The interior of this zone documents geodynamically pure shear , whereas the upper and lower edges show a combination of pure and simple shear. The ductile lower rim is between 1 and 2 kilometers thick and ends in a brittle thrust - the MCT, which in the Everest region can be dated between 23 and 20 million years ago.

The south-facing extrusion ultimately took place due to the gravitational potential difference, caused by the different crust thickness and the difference in height between the highlands of Tibet and the foreland of India. The Tibetan hinterland has a crust thickened to 70 to 80 kilometers and is at an average altitude of 5000 meters, whereas the north Indian crust is only 35 to 40 kilometers thick and reaches low heights of up to 1000 meters.

Further development from the Middle Miocene

The ductile shear movements at the STDS and the mid-crustal extrusion of the channel flow came to an end 16 million years ago, as the entire Himalayas had already cooled below 350 ° C. Subsequently, the High Himalayas were only moved and lifted using the piggy-back method on younger thrusts that had developed southward in the pre-Himalayas. The thrusts on the Main Boundary Thrust occurred from 10 million years ago and the Main Frontal Thrust was only activated around 3 million years ago.

At the STDS itself, only brittle movements took place from the Middle Miocene 16 million years ago, as the Rongbuk formation of the GHS had already cooled below the muscovite sealing temperature of 350 ° C. In the period 16 to 2.5 million years, the further cooling proceeded only very slowly with a cooling rate of 20 to 22.5 ° C per million years. With a geothermal gradient of 25 to 35 ° C per kilometer, the exhumation speeds were 0.2 to 2.0 millimeters per year. From the Gelasian 2.5 million years ago, the speed of exhumation increased again, as the rate of erosion had increased sharply due to the onset of the Quaternary Ice Ages and the associated climatic deterioration.

Glaciation

View over the Khumbu Himal

In the Khumbu Himal three glacial advance stages can be distinguished:

  • the Periche stage
  • the Chhukung stage
  • the Lobuche stage.

OSL dating indicated an age of 25,000 to 18,000 years BP for the Periche stage . It thus corresponds to the oxygen isotropic stage MIS 2 and coincides with the last ice age maximum (LGM). The Chhukung stage is around 10,000 years old at the beginning of the Holocene . The Lobuche stage finally located between 2000 and 1000 years before today and represents a spätholozänen thrust that even before the Little Ice Age expired.

The Khumbu Glacier is currently retreating at a rate of 20 meters per year. In addition, Everest Base Camp has lost 40 meters in height over the past 55 years. This loss of thickness is even more pronounced uphill, so that the glacier overall loses speed.

Individual evidence

  1. ↑ a b M. P. Searle, RL Simpson, RD Law, RR Parrish and DJ Waters: The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal - South Tibet. In: Journal of the Geological Society . tape 160 , 2003, p. 345-366 , doi : 10.1144 / 0016-764902-126 .
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  20. ↑ RF Weinberg: Himalayan leucogranites and migmatites: nature, timing and duration of anatexis. In: Journal of Metamorphic Geology . tape 34 , 2016, p. 821-843 , doi : 10.1111 / jmg.12204 .
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  24. ↑ a b Jessup, MJ, Cottle, JM, Searle, MP, Law, RD, Newell, DL, Tracy, RJ and Waters, DJ: PT-tD paths of Everest Series schist, Nepal. In: Journal of Metamorphic Geology . tape 26 , 2008, p. 717-739 .
  25. ↑ Parrish, RR, Gough, SJ, Searle, MP and Waters, DJ: Plate velocity exhumation of ultrahigh-pressure eclogites in the Pakistan Himalaya . In: Geology . tape 34 , 2007, p. 989-992 .
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  28. ↑ Simpson, RL, Parrish, RR, Searle, MP and Waters: Two episodes of monazite crystallization during metamorphism and crustal melting in the Everest region of the Nepalese Himalaya . In: Geology . tape 28 , 2000, pp. 403-406 .
  29. ↑ John M. Cottle, Michael P. Searle, Matthew SA Horstwood and David J. Waters: Timing of Midcrustal Metamorphism, Melting, and Deformation in the Mount Everest Region of Southern Tibet Revealed by U (-Th) -Pb Geochronology . In: The Journal of Geology . volume 117, 2009, p. 643-664 , doi : 10.1086 / 605994 .
  30. ↑ Carosi, R., Lombardo, B., Molli, G., Musumeci, G. and Pertusati, PC: The South Tibetan detachment system in the Rongbuk valley, Everest region. Deformation features and geological implications . In: Journal of Asian Earth Sciences . v. 16, 1998, pp. 299-311 , doi : 10.1016 / S0743-9547 (98) 00014-2 .
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  33. ↑ Nelson, KD et al .: Partially molten middle crust beneath southern Tibet; synthesis of Project INDEPTH results . In: Science . tape 274 , 1996, pp. 1684-1688 .
  34. ^ Ben WM Richards et al .: Timing of late Quaternary glaciations south of Mount Everest in the Khumbu Himal, Nepal . In: Geological Society of America Bulletin . v. 112; no. 10, 2000, p. 1621-1632 , doi : 10.1130 / 0016-7606 (2000) 112 <1621: TOLQGS> 2.0.CO; 2 .
  35. ↑ Bajracharya, SR, Mool, PK and Shrestha, BR: Impact of climate change on Himalayan glaciers and glacial lakes; case studies on GLOF and associated hazards in Nepal and Bhutan . ICIMOD, 2007.