The age and rate of displacement along the Main Central Thrust in the western Bhutan Himalaya

Tobgay, Tobgay; McQuarrie, Nadine; Long, Sean P.; Kohn, Matthew J.; Corrie, Stacey L.

doi:10.1016/j.epsl.2011.12.005

Cited by 101 publications

(113 citation statements)

References 60 publications

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“…Large shear strains accu-mulated over ∼ 150-250 km of displacement along the MCT (e.g., Srivastava and Mitra, 1994;Hodges, 2000;Mitra et al, 2010;Tobgay et al, 2012;Law et al, 2013) and yet quartz OH contents (50-150 ppm) in our MCT samples are far lower than required for water weakening, thereby challenging our understanding of dislocation creep and the role of water in deformation deep in the continental crust. With only a few exceptions, IR spectra of our MCT samples have OH bands of the same character and size as dry natural quartz crystals, which are strong and have not been deformed by dislocation processes in laboratory experiments (e.g., Heard and Carter, 1968;Blacic, 1975;Blacic and Christie, 1984).…”

Section: Water Weakening In Nature?mentioning

confidence: 90%

Synchrotron FTIR imaging of OH in quartz mylonites

et al. 2017

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Abstract. Previous measurements of water in deformed quartzites using conventional Fourier transform infrared spectroscopy (FTIR) instruments have shown that water contents of larger grains vary from one grain to another. However, the non-equilibrium variations in water content between neighboring grains and within quartz grains cannot be interrogated further without greater measurement resolution, nor can water contents be measured in finely recrystallized grains without including absorption bands due to fluid inclusions, films, and secondary minerals at grain boundaries.Synchrotron infrared (IR) radiation coupled to a FTIR spectrometer has allowed us to distinguish and measure OH bands due to fluid inclusions, hydrogen point defects, and secondary hydrous mineral inclusions through an aperture of 10 µm for specimens > 40 µm thick. Doubly polished infrared (IR) plates can be prepared with thicknesses down to 4-8 µm, but measurement of small OH bands is currently limited by strong interference fringes for samples < 25 µm thick, precluding measurements of water within individual, finely recrystallized grains. By translating specimens under the 10 µm IR beam by steps of 10 to 50 µm, using a software-controlled x − y stage, spectra have been collected over specimen areas of nearly 4.5 mm 2 . This technique allowed us to separate and quantify broad OH bands due to fluid inclusions in quartz and OH bands due to micas and map their distributions in quartzites from the Moine Thrust (Scotland) and Main Central Thrust (Himalayas).Mylonitic quartzites deformed under greenschist facies conditions in the footwall to the Moine Thrust (MT) exhibit a large and variable 3400 cm −1 OH absorption band due to molecular water, and maps of water content corresponding to fluid inclusions show that inclusion densities correlate with deformation and recrystallization microstructures. Quartz grains of mylonitic orthogneisses and paragneisses deformed under amphibolite conditions in the hanging wall to the Main Central Thrust (MCT) exhibit smaller broad OH bands, and spectra are dominated by sharp bands at 3595 to 3379 cm −1 due to hydrogen point defects that appear to have uniform, equilibrium concentrations in the driest samples. The broad OH band at 3400 cm −1 in these rocks is much less common. The variable water concentrations of MT quartzites and lack of detectable water in highly sheared MCT mylonites challenge our understanding of quartz rheology. However, where water absorption bands can be detected and compared with deformation microstructures, OH concentration maps provide information on the histories of deformation and recovery, evidence for the introduction and loss of fluid inclusions, and water weakening processes.

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Section: Water Weakening In Nature?mentioning

confidence: 90%

Synchrotron FTIR imaging of OH in quartz mylonites

et al. 2017

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“…Monazite, a light rare earth element phosphate (LREEPO 4 ), is ubiquitous in the Himalayan metapelitic rocks (Tobgay et al 2012;Rubatto et al 2012). This accessory mineral is a powerful geochronometer (Parrish 1990;Rubatto et al 2012) and, due to its ability to undergo dissolution/ re-precipitation in response to deformation events Gibson et al 2004).…”

Section: Methodsmentioning

confidence: 99%

Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal

Carosi

Montomoli

Langone

et al. 2014

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Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet\ud of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali\ud Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In\ud situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt\ud inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial\ud melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan\ud event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW\ud sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings\ud highlight the occurrence of an older melting event in the GHS during prograde metamorphism in\ud the kyanite stability field before the more diffuse Miocene melting event.\ud The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of\ud the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS\ud should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew\ud during the same time span and they grew in different tectonic settings

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“…The Greater Himalaya is divided into two structural levels: the lower unit is above the MCT but below the out-of-sequence Kakhtang Thrust (KT), while the higher unit is in the hanging wall of the KT (Grujic et al, 2002). Estimates for the initiation of motion on the MCT range from ∼ 25 to 20 Ma (e.g., Hodges et al, 1996;Daniel et al, 2003;Tobgay et al, 2012), with continued shearing in the Bhutan Himalaya through 18-16 Ma (Grujic et al, 2002;Daniel et al, 2003;Kellett et al, 2009). The age of motion on the KT is notably younger (14-8 Ma; Daniel et al, 2003;Grujic et al, 2002Grujic et al, , 2011Coutand et al, 2014).…”

Section: Tectonostratigraphymentioning

confidence: 99%

Testing the effects of topography, geometry, and kinematics on modeled thermochronometer cooling ages in the eastern Bhutan Himalaya

et al. 2018

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Abstract. In this study, reconstructions of a balanced geologic cross section in the Himalayan fold-thrust belt of eastern Bhutan are used in flexural-kinematic and thermokinematic models to understand the sensitivity of predicted cooling ages to changes in fault kinematics, geometry, topography, and radiogenic heat production. The kinematics for each scenario are created by sequentially deforming the cross section with ∼ 10 km deformation steps while applying flexural loading and erosional unloading at each step to develop a high-resolution evolution of deformation, erosion, and burial over time. By assigning ages to each increment of displacement, we create a suite of modeled scenarios that are input into a 2-D thermokinematic model to predict cooling ages. Comparison of model-predicted cooling ages to published thermochronometer data reveals that cooling ages are most sensitive to (1) the location and size of fault ramps, (2) the variable shortening rates between 68 and 6.4 mm yr −1 , and (3) the timing and magnitude of out-of-sequence faulting. The predicted ages are less sensitive to (4) radiogenic heat production and (5) estimates of topographic evolution. We used the observed misfit of predicted to measured cooling ages to revise the cross section geometry and separate one large ramp previously proposed for the modern décollement into two smaller ramps. The revised geometry results in an improved fit to observed ages, particularly young AFT ages (2-6 Ma) located north of the Main Central Thrust. This study presents a successful approach for using thermochronometer data to test the viability of a proposed cross section geometry and kinematics and describes a viable approach to estimating the first-order topographic evolution of a compressional orogen.

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The age and rate of displacement along the Main Central Thrust in the western Bhutan Himalaya

Cited by 101 publications

References 60 publications

Synchrotron FTIR imaging of OH in quartz mylonites

Synchrotron FTIR imaging of OH in quartz mylonites

Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal

Testing the effects of topography, geometry, and kinematics on modeled thermochronometer cooling ages in the eastern Bhutan Himalaya

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