The lesser Himalayan Duplex in Sikkim: Implications for variations in Himalayan shortening

Mitra, Gautam; Bhattacharyya, Kathakali; Mukul, Malay

doi:10.1007/s12594-010-0016-x

Cited by 117 publications

(108 citation statements)

References 26 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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“…Overall shortening estimates are 530 km across north Pakistan (Coward and Butler 1991) and 349 km across the Sulaiman fold belt along the western limits of the Himalayas (Jadoon et al 1994). This is comparable with 502 km of total shortening in the main Himalayas (Mitra et al 2010).…”

Section: Tectonics Of Pakistanmentioning

confidence: 55%

“…The collision was accompanied by the development of an active fold-and-thrust system. This is divided into three major tectono-stratigraphic units in north Pakistan along major longitudinal faults (Yin 2006;Rehman et al 2007;Mitra et al 2010; Fig. 1).…”

Section: Tectonics Of Pakistanmentioning

confidence: 99%

Structural interpretation and geo-hazard assessment of a locking line: 2005 Kashmir earthquake, western Himalayas

Jadoon

Hinderer²,

Kausar

et al. 2014

Environ Earth Sci

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The 08 October 2005, magnitude (Mw) 06 Kashmir earthquake occurred along the Balakot-Bagh fault (BBF) with about 30°dip toward NE in the internal part of the western Himalayas in north Pakistan. It was accompanied by a ground rupture of about 75 km with an average slip of about 5 m along the causative fault. The epicenter of the thrust was located at about 19 km to the NE of its surface trace in Muzaffarabad with about 11 km depth of the hypocenter. The geometry of the fault based on a structural cross section has allowed us to interpret it as a thrust restricted to a roof sequence along a triangle zone across the Hazara-Kashmir syntaxis (HKS). The triangle zone is occupied at depth by a wedge of the Higher Himalayan Sequence (HHS) in the core zone of the HKS. The core-wedge is bounded between the NE-dipping BBF and SE-to SW-dipping thrust stack of the Lesser Himalayan Sequence (LHS) along the northeastern and southwestern limb of the HKS, respectively. Based on surface geology, the overlapping BBF and MBT are interpreted to merge at depth in a roof thrust of Pre-Cambrian (Late Proterozoic) rocks above a duplex which is inferred to have a floor thrust in Early Proterozoic/Archean rocks. The core-wedge is located over a ramp which is connected to the floor thrust in the basement. The BBF is inferred to be active, at least since 1-0.5 Ma, with recurrence interval of about 625 ± 125 years. This out-of-sequence deformation is represented by linear seismicity, both along emergent and blind thrusts in the system, with likelihood of major events as a result of strain buildup due to slow convergence rates (*7 mm/year) in the region. Many towns located along the active fault trace were destroyed or largely damaged due to the earthquake. Major destruction of human dwellings and infrastructure occurred as a consequence of earthquake-triggered landslides, mostly along fault, high river terraces, and road cuts. To minimize future damages in earthquake-prone areas, several mitigation measures are suggested including: (1) avoiding new settlements near the fault trace and landslide susceptible areas, (2) establishment of new township schemes in relatively safer areas with earthquake-sustainable structural designs, and (3) extensive forestation for slope stability, erosion control, and provision of wood for flexible earthquake-resistant structures. The measures are needed for the sustainable development of the region.

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“…Therefore, we construe that the recumbent folds are perhaps the result of near-vertical compression with shortening direction nearly parallel to the foliation rather than horizontal shearing after careful consideration of all possibilities. We interpret that the formation of the recumbent fold structure in selective parts of the Lesser Himalaya in the DSH is due to the effect of the weight of the vertical column of rocks (~6-8 km) 29 . Before erosion, the weight of the rock column induced a substantial vertical compressional force on the underlying mass of the rock to initiate folding on the foliation in phyllite of the RT sheet.…”

Section: Genesis Of the Small-scale Recumbent Foldsmentioning

confidence: 84%

“…The area of study encompasses a small part of the RT sheet in the Lesser Himalaya in the DSH (Figure 1). The frontal part of the Darjiling-Sikkim-Tibet (DaSiT) wedge has been studied by earlier workers 8,15,[24][25][26][27][28][29] . In this area, the Tista half-window was formed as a result of erosion of folded thrust sheets by the Tista River (Figure 1).…”

Section: Geological Settingmentioning

confidence: 99%

Overburden-Induced Flattening Structure in the Himalaya: Mechanism and Implication

Banerjee¹,

Matin²,

Mukul³

2015

Current Science

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Small-scale structures in fold-thrust belt are mainly formed in response to the emplacement of thrust sheets. However, some small-scale structures may not be developed directly in response to the emplacement of thrust sheets, but might be genetically tied with the orogenic process. Metre-to centimetre-scale late-stage folds on foliation in phyllite with near-recumbent fold geometry are selectively developed with a specific spatial distribution, particularly in places where the foliation is steeply dipping, in the Ramgarh thrust sheet in the Darjiling-Sikkim Himalaya. The recumbent-fold structures appear to have been formed in response to overburden-induced vertical compressive deformation on steep dipping foliation, especially in the subvertical southern limb of the antiformal structure of the Lesser Himalayan Duplex in the Darjiling-Sikkim Himalaya. The role of gravity and overburden in the formation of these structures from worldwide orogenic belts may be considered to validate their genesis.

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The lesser Himalayan Duplex in Sikkim: Implications for variations in Himalayan shortening

Cited by 117 publications

References 26 publications

Synchrotron FTIR imaging of OH in quartz mylonites

Synchrotron FTIR imaging of OH in quartz mylonites

Structural interpretation and geo-hazard assessment of a locking line: 2005 Kashmir earthquake, western Himalayas

Overburden-Induced Flattening Structure in the Himalaya: Mechanism and Implication

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