South Tibetan Detachment System

Rocks from the South Tibetan Detachment shear zone, illustrating the typical fabrics. The first set (E03 and ME samples) are from Mount Everest, where the shear zone is ~1500m thick, and the rocks are dominated by semipelitic or metapelitic clastic sediments.
The second series (KA samples, John Cottle thesis) is from the Dzakaa Chu, 50 km NE of Mt Everest, where the shear zone is ~1000m wide, and developed mostly in calcareous rocks. The images are arranged in order of increasing structural depth: there is a systematic progression downwards from the top of the zone involving increasing metamorphic grade, and for at least the upper half of the zone, increasing intensity and complexity of the fabrics.
E03-09-x4p-256  Sample E03-09, plane polarised light, field of view 2.5mm. Collected for us by David Hamilton at the South Col on Mt Everest. This sample evidently came from a low-strain zone in the upper part of the Everest Series. It preserves a microstructure of equant random porphyroblasts of biotite, which appear to overgrow the matrix fabric (this would, therefore, be S1). E03-11-x4p-270  Sample E03-11, plane polarised light, field of view 2.5mm. Another South Col sample, but with a markedly different fabric. There is a strong, spaced foliation. The elongate biotites, remarkably, do not show a lattice preferred orientation - this is a shape fabric only and we must infer that they did not grow in the fabric (where they would certainly align their (001) faces in the schistosity) but are older biotite crystals in random orientations, like those in E03-09, that have been truncated during fabric development. The fabric is S2, therefore. E03-10-x4p-263  Sample E03-10, plane polarised light, field of view 5mm. A fine muscovite-biotite phyllite from the South Col. Note the larger quartz grains, probably relics of sedimentary clasts. There is a strong schistosity (most likely S2) parallel to compositional layering, which is crossed at about 25° by slightly irregular c' type shear bands that are frequent enough to give the rock a second cleavage - an extensional crenulation cleavage. E03-10-x10p-267  Sample E03-10, plane polarised light, field of view 1mm. A detailed view of the shear-band fabric, here aligned E-W. It is defined in part by biotite. Therefore, all the fabrics so far - S1, S2 and shear-band cleavage - occurred under biotite-grade conditions. ME131ps-x4p-307  Wager's sample ME-131, plane polarised light, field of view 2.5mm. A biotite-hornblende schist from deeper in the Everest Series, below the North Col, also shows a strong S2 fabric with abundant shear bands (dextral in this view).
ME131-x4p-98  Sample ME-131, plane polarised light, field of view 2.5mm. S2 fabric cut by shear bands in a biotite-rich part of the sample. ME131-x10p-99  Sample ME-131, plane polarised light, field of view 1mm. Detail of a shear band, clearly defined by the same biotite-bearing mineral assemblage as the main S2 foliation. ME126-x4p-92  Wager's sample ME-126, plane polarised light, field of view 2.5mm. This is a calcareous phyllite from the lower magin of the "Yellow Band" at about 8300m on Mt Everest's NE Ridge. It encapsulates the whole history of shear and extensional deformation on the South Tibetan detachment system in the upper part of the Everest Series. A strong schistosity (S2?) wraps winged porphyroclasts of calcite indicating dextral simple shear.  A spaced shear-band cleavage (s-c' fabric or extensional crenulation cleavage) cuts across it at about 30°. Cross-cutting extensional veins with a dark wall-rock reaction zone cut the foliations at about right angles. ME126-x10p-87  Sample ME-126, plane polarised light, field of view 1mm. Detail of winged sigma porphyroclast. The porphyroclast is calcite, with an inclusion-rich core, containing, among other phases, chlorite, which is absent from the matrix. We infer that the calcite grew as a poikiloblast during prograde metamorphism. During dextral shear new inclusion-poor calcite was added, filling the asymmetric strain shadows. Note the stair-stepping that indicates the shear sense. A dextral shear band is also visible to the right and above the porphyroclast tail. ME126-x10p-96  Sample ME-126, plane polarised light, field of view 1mm. Another asymmetric winged porphyroclast showing a very clear contrast between old and new growth, with a strong suggestion that material for the wings was derived by dissolution of calcite from the upper and lower faces of the porphyroclast. Shear bands (c') are visible at top and bottom. Biotite is stable both in the main foliation and in the shear bands.
ME126-x10p-93  Sample ME-126, plane polarised light, field of view 1mm. Detail of the hydrothermal vein, one of a series of regularly-spaced such veins cutting these rocks. The dark colour is mainly caused by newly-grown biotite. Its composition suggests that its growth temperature was, if anything, higher than that in the foliation or shear bands, despite its occurence in features that post-date all ductile deformation in the shear zone. These veins therefore also post-date most of the exhumation that the detachment achieved in its ductile phase of movement. The explanation probably lies in two factors: rapid exhumation coupled with thinning across the zone keeps hot rocks a short distance below this level; and leucogranite sills generated by partial melting in the gneisses of the footwall (and abundantly visible in the Everest area) will be crystallising at this stage, releasing hot fluids by hydraulic fracturing into the overlying rocks. KA100-1a  Dzakaa Chu STD section, sample KA-100. A siltstone from the Tethyan sedimentary series above the shear zone. The dark seams and mineral alignment is most probably a diagenetic compaction fabric parallel to the bedding. The mica flakes are detrital. The temperature determined from the crystallinity of carbonaceous matter is about 300°C. KA76-i7  Dzakaa Chu STD section, sample KA-76. About 50 metres lower down, this is the first sample with a tectonic fabric - a pressure-solution cleavage seen here wrapping around a clay pellet in fine siltstone. KA78-i4  Dzakaa Chu STD section, sample KA-78. A sandstone showing an extensional vein filled with elongate columnar quartz fibres. Plane polarised light. KA78-i5  Dzakaa Chu STD section, sample KA-78. A sandstone showing an extensional vein filled with elongate columnar quartz fibres. Crossed polars.
KA91-i1  Dzakaa Chu STD section, sample KA-91. Beneath the siliciclastic sediments are carbonate rocks. This is the first marble to show a shape fabric in calcite. The foliation is at a small angle to the bedding, here marked by darker bands and nodules containing dolomite. The nodules may represent detached fold hinges in the layering. We are now in the shear zone proper. KA91-i3  Dzakaa Chu STD section, sample KA-91. Detail of the foliation defined by shape fabric and elongated aggregates of finely recrystallised calcite. Note the angular quartz grains, not deformed or recrystalised because they are stronger than calcite under these conditions. The temperature here is about 450°C. KA69-i1  Dzakaa Chu STD section, sample KA-69. A little deeper in the shear zone, a calc-schist reveals that the strong fabric is actually a spaced fabric developed from a crenulation cleavage - therefore it is the second tectonic fabric in these rocks. This rock also marks the crossing of the biotite isograd. KA69-i3  Dzakaa Chu STD section, sample KA-69. A detail of one of the larger microlithons showing its continuous foliation defined by pale biotite, at a high angle to the strong second foliation of the shear zone. KA70-i4  Dzakaa Chu STD section, sample KA-70. Very strong shape fabric of elongated calite grains in fine-graiined marble. This sample gave a temperature of more than 500°C from the crystallinity of carbonaceous matter.
KA70-i5  Dzakaa Chu STD section, sample KA-70. Here the foliation wraps around more rigid clusters of dark, iron-stained dolomite. KA71-i1  Dzakaa Chu STD section, sample KA-71. A greenschist containing an intense, closely-spaced foliation (almost continuous, but not quite - there are narrow microlithons and lenses with discordant fabric). Also towards the top there are small dextral-sense shear bands. These are the first shear bands seen below the top of the shear zone. KA71-i7  Dzakaa Chu STD section, sample KA-71. Detail of the fabric, showing a small lenticular microlithon. The rock contains chlorite, biotite and two plagioclase feldspars: albite and oligoclase-andesine. This first occurrence of a more calcic feldspar typically marks the very beginning of amphibolite-facies conditions. Note the shear band at lower left. KA93-i1  Dzakaa Chu STD section, sample KA-93. Mylonitic fine-grained calc-silicate gneiss. The first truly amphibolite-facies rock (T about 540°C). There are relics of early diopside but most of the green mineral grains are actinolite. This is also the first sample to show all three styles of shear zone fabric: the continuous mylonitic foliation and mineral banding (ENE-WSW in the photo), abundant spaced dextral shear bands (ESE-WNW), and extensional vein fractures filled with actinolite and quartz at right angles to the mylonitic fabric. Actinolite was evidently stable at all three stages. KA93-i13  Dzakaa Chu STD section, sample KA-93. Detail of extensional actinolite-calcite-quartz vein with S-shaped actinolite fibres.
KA93-i10  Dzakaa Chu STD section, sample KA-93. Detail of the spaced shear-band fabric making an angle of about 25° with the mylonitic banding. Actinolite grains form sigma-style porphyroclasts with recrystallised tails drawn out into the shear bands. KA73-i1  Dzakaa Chu STD section, sample KA-73. A calcareous biotite schist in which some layers show large early poikiloblasts of diopside forming lenticular domains wrapped by the mylonitic foliation. The metamorphic temperature has increased by almost 250°C over a vertical depth of 800m into the shear zone - an apparent gradient, strongly condensed by the shearing, of 300°C/km. KA95-i1  Dzakaa Chu STD section, sample KA-95. Phlogopite-diopside marble. Deeper in the shear zone, higher temperatures of around 600°C have allowed the grain size to coarsen, and the shear zone fabrics are less distinct. Nevertheless we can distinguish here the principal foliation and some widely-spaced shear bands.