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Deformational Temperatures Across the Lesser Himalayan Sequence in Eastern Bhutan and Their Implications for the Deformation History of the Main Central Thrust
Author(s) -
Grujic Djordje,
Ashley Kyle T.,
Coble Matthew A.,
Coutand Isabelle,
Kellett Dawn A.,
Larson Kyle P.,
Whipp David M.,
Gao Min,
Whynot Nicholas
Publication year - 2020
Publication title -
tectonics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.465
H-Index - 134
eISSN - 1944-9194
pISSN - 0278-7407
DOI - 10.1029/2019tc005914
Subject(s) - geology , closure temperature , thermochronology , metamorphic rock , shear zone , fission track dating , muscovite , shear (geology) , nappe , quartz , main central thrust , sequence (biology) , zircon , mineralogy , geochemistry , paleontology , tectonics , chemistry , biochemistry
We postulate that the inverted metamorphic sequence in the Lesser Himalayan Sequence of the Himalayan orogen is a finite product of its deformation and temperature history. To explain the formation of this inverted metamorphic sequence across the Lesser Himalayan Sequence with a focus on the Main Central Thrust (MCT) in eastern Bhutan, we determined the metamorphic peak temperatures by Raman spectroscopy of carbonaceous material and established the deformation temperatures by Ti‐in‐quartz thermobarometry and quartz c axis textures. These data were combined with thermochronology, including new and published 40 Ar/ 39 Ar ages of muscovite and published apatite fission track, and apatite and zircon (U‐Th)/He ages. To obtain accurate metamorphic, deformation, and closure temperatures of thermochronological systems, pressures and cooling rates for the period of interest were derived by inverse modeling of multiple thermochronological data sets, and temperatures were determined by iterative calculations. The Raman spectroscopy of carbonaceous material results indicate two temperature sequences separated by a thrust. In the external sequence, peak temperatures are constant across the structural strike, consistent with the observed hinterland‐dipping duplex system. In the internal temperature sequence associated with the MCT shear zone, each geothermometer yields an apparent inverted temperature gradient although with different temperature ranges, and all temperatures appear to be retrograde. These observations are consistent with the quartz microfabrics. Further, all thermochronometers indicate upward younging across the MCT. We interpret our data as a composite peak and deformation temperature sequence that formed successively and reflects the broadening and narrowing of the MCT shear zone in which the ductile deformation lasted until ~11 Ma.