Microstructural analysis of Greater Himalayan rocks in northern Bhutan
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Authors
Penfold, Melissa L.
Issue Date
2014
Type
Thesis
Language
Keywords
Bhutan , Greater Himalayan rocks , kinematic vorticity , orogenic flattening , quartz CPO , quartz microstructure
Alternative Title
Abstract
Across the Himalayan fold-thrust belt, high-grade metamorphic rocks of
the Greater Himalayan (GH) zone are juxtaposed between low-grade
metasedimentary rocks structurally above and below. In Bhutan, the highergrade GH rocks lie structurally over lower-grade Lesser Himalayan rocks and are
separated by the Main Central Thrust. However, many aspects of the
deformation path, deformation conditions, and the emplacement mechanism that
led to the exhumation of GH rocks are poorly understood.
In this study geologic mapping and quantitative microstructural analysis
are utilized to gain insight into the deformation history of GH rocks in Bhutan, and
to test the applicability of end-member emplacement models. Microstructural
datasets include characterization of kinematic indicators, determination of
deformation temperatures through analysis of quartz deformation microstructures
and quartz crystal-preferred orientation (CPO) data, and classification of strain
and shear type using CPO and kinematic vorticity data.
Semi-quantitative deformation–temperature estimates obtained from
cataloguing quartz-recrystallization mechanisms, combined with quantitative
temperature estimates from CPO plot opening angles, suggest that GH rocks
were deformed at temperatures of ca. 500 to 750°C at both structurally-lower and
higher levels, and were later overprinted by a lower-temperature recrystallization
event around that occurred at conditions of ca. 400–500°C. The highertemperature recrystallization event is interpreted to be associated with earlier slip
(~22–15 Ma) along the Main Central Thrust, at or near peak metamorphic
temperature conditions. The lower-temperature overprint is interpreted to have
occurred at a higher point along the pressure–temperature–deformation path as
GH rocks were passively translated and structurally elevated southward,
concurrent with duplexing of Lesser Himalayan rocks (~18–10 Ma). Internal
deformation within structurally-lower and higher GH rocks consisted of
components of both coaxial (pure shear) and non-coaxial (simple shear) strain,
as indicated by localities with opposite shear-sense kinematics, type-I crossgirdle CPO patterns and kinematic vorticity, which suggests that exhumation of
GH rocks was accompanied by significant flattening in north-central and
northeast Bhutan.
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