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| Titel |
Structural Geology of the Higher Himalayan Shear Zone, Sutlej Section |
| VerfasserIn |
S. Mukherjee |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250031522
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| Zusammenfassung |
| Throughout the Higher Himalayan Shear Zone (HHSZ), the northeasterly dipping main
foliation planes that acted as the primary shear planes (the C-planes) are rarely
sub-horizontal. S-C fabrics, sigmoid quartz veins and leucosomes, and asymmetric intrafolial
folds denote a top-to-SW sense of ductile shearing. The angles between the S-planes of this
shear sense and the C-planes in the shear zone are within 25-43,0which is close to the higher
angular limit of 450. The axial planes of the intrafolial folds are at 12-220 to the NE direction
of the C-planes. The long axes of sigmoid leucosomes and quartz veins are at very low
angle 3-80 to the C-planes. The aspect ratios of these sigmoid elements show a
wide variation between 3.6-9.5. Some of the S-fabrics are occasionally affected by
secondary C--shearing synthetic to the main shearing event. The angle between
the C- and the C--planes range between 15-250. This is well within the limit of
15-350 previously compiled from different shear zones. The C--planes are straight
but are shorter than the C-planes. The C--planes are much less ubiquitous in the
HHSZ.
Interestingly, an additional phase of a top-to-NE sense of extensional ductile shearing,
deciphered from intrafolial folds of quartz rich layers, S-C fabrics, and sigmoid-shaped
quartz veins and leucosomes are found to be confined within two zones in macro-scale in the
HHSZ. The top-to-NE sense of ductile shearing is restricted in two zones within the
northeasterly dipping C-planes parallel to hence the same as that for the top-to-SW sense.
One of these zones, designated as the STDSL, is delineated at the locality Karcham. The
other zone of top-to-NE shearing occurs within the topmost level of the HHSZ and is
equated with the STDS or the STDSU. Its lower boundary is demarcated between
the locations Pangi- and Kashang. The upper boundary is the same as the contact
between the HHSZ and the overlying Tethyan Sedimentary Zone. In these ductile
extensional shear zones, fabrics of a top-to-SW sense of shearing also occur less
frequently.
In the STDSU and the STDSL in macro-scale, the angles between the S- and the C-
fabrics showing a top-to-NE sense of shearing and those between the C- and the C--planes
show a wide variation between 18-450 and 10-480, respectively. The former range touches the
higher angular limit of 450 betweenthe respective fabrics as previously mentioned. However,
the later range crosses the upper limit of variation of angles between the C- and
the C--planes as 15-350 as compiled from different shear zones. The long axes
of the sigmoid leucosomes and veins are at 8-200 with the C-plane. The aspect
ratios of the sigmoid leucosomes and quartz veins show a wide range between
2.8-8.21. The axial planes dip towards SW and make 10-170 to the NE direction of the
C-planes.
Oriented thin-sections of rocks of the STDSL and the STDSU reveal dominantly a
top-to-NE sense of ductile shearing most abundantly by mineral fishes of sigmoid
geometries, S-C fabrics and rarely by the sigmoidally oriented inclusion pattern
inside porphyroblasts. Some of the mica fishes possess a mouth at one of their
corners possibly due to their dynamic recrystallisation. The top-to-NE sheared higher
grade rigid porphyroblastic index minerals indicate the extensional shearing to be
quite intense. In the STDSL, pronounced extension parallel to the main foliation is
deciphered from crystal-plastic extension of rigid and high-grade minerals such as garnet
with aspect ratios as high as 9.5. The sheared geometries of these index minerals,
however, are partially destroyed by migration of quartz grains from the matrix into
them. The S-fabrics are sometimes defined by a number of mutually separated
mica grains. The C-shear planes are short, remarkably straight and are defined by
(a) foliation minerals such as biotite; and (b) trails of minerals at the corners of
mineral fishes. The long axes of the mineral fishes are at 7-150 to the C-planes.
Their aspect ratios vary within 1.7-4.8, which is a narrower range than previously
reported 2-16 from different shear zones. As in field-scales, the ductile synthetic
secondary C--shear planes of both the compressional and extensional shearing are
straight, less frequently developed, affect individual mineral grains but are shorter than
the C-planes. For both the compressional and extensional and senses of ductile
shearing in macro- and micro-scales, the S-planes are in most cases sigmoid and attain
progressively lower angles as they come closer to the C-planes. The S-fabrics vary in
thickness, length and curvature. The shearing secondary to top-to-SW and top-to-NE
senses took place either simultaneous to or later than the respective primary shearing
event.
Brittle-ductile secondary shear C1--planes are rarely developed in the STDSL around 750
to the primary shear C- planes. In the HHSZ, including the STDSU and the STDSL, a
different manifestation of brittle-ductile extension parallel to the main foliation is deciphered
from different varieties and sizes of boudins. Throughout the HHSZ, including the
STDSU and the STDSL, a late stage brittle shearing consistently in a top-to-SW
sense is deciphered from duplexes of dimensions ranging from few cm to several
meters. The roof- and the sole thrusts bounding these duplexes, i.e. the Y-planes, dip
northeasterly usually within 30-400. Individual thrust slices are most commonly
sigmoid-shaped. Both isolated and a number of sigmoid-shaped stacked thrust slices are
noted. The P-planes of brittle shear are defined as the contact planes between individual
thrust slices. The angles between the Y- and the P-planes widely vary between
30-680. Sometimes, the angles between the Y- and the P-planes in adjacent thrust
slices vary widely. Some of these thrust slices are affected by secondary brittle
shearing along discontinuous and locally developed synthetic secondary ‘R’ shear
planes. The angles between the Y- and the R-planes vary within a wide range of
25-860. The range is much higher than the previously compiled average value of 150
from different shear zones. The Y-planes are found to be parallel to or same as the
primary C-shear planes. In other words, the anisotropy created by the pre-existing
ductile primary shear planes acted later as the preferential sites of brittle primary
shearing.
The brittle shearing prevalent in macro-scale is also found to affect individual grains in
micro-scale in terms of duplexes of stacked-up minerals in XZ oriented thin-sections that
show the sense of shearing same as those in field-scale. The micro-duplexes are identified by
their morphological resemblance with those well established from field-scales. The thrust-up
grains are typically hat- or trapezium-shaped with their straight boundaries unaffected by
migration of the adjacent grains. The longest boundaries of these trapeziums are identified as
the ‘P’ shear planes. The ‘P’ planes dip northeasterly from moderate angle of 400to
sub-parallel to the Y-plane. Few trapezium-shaped mica grains that are surrounded by
quartzo-feldspathic minerals have their longest and straight boundaries sub-parallel to the
Y-planes. These grains probably represent thrust slices that were transported to relatively
longer distances to the extent that they got completely detached from the underthrust
counterpart grains. Such grains were exempted from determination of brittle shear sense. |
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