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HIGHLIGHTS
HIGHLIGHTS
Combine field-based and experimental rock deformation (high P-T) to reveal how microstructures and creep processes observed in exhumed rocks relate to stress, temperature, and strain-rate conditions deep within the Earth.
Rheology and flow-laws of different geomaterials.
Role of secondary mineral and fluid fractions on the processes of strain localization. Additionally, I aim to understand and isolate the relative contribution of different weakening processes in the subsequent evolution of fault-zone material property.
Using laboratory-based and numerical modelling techniques as well as tectonic reconstructions (geological mapping, cross-sections, GIS etc.) to quantify the deformation and forces associated with large-scale geodynamic processes.
Wet polycrystalline coarse-grained quartz rheology under high confining pressure.
Wet polycrystalline coarse-grained quartz rheology under high confining pressure.
I am conducting triaxial compression experiments on coarse/fine-grained quartzite aggregates at the ISTO, using the new generation GRIGGS type apparatus. Quartz is one of the most important rock-forming minerals of the planet Earth. Our creep results for both the as-is and 0.1 wt.% H2O-added samples yield Q = 110 kJ/mol and n = 2 Our microstructural analysis suggests that the bulk sample strain is accommodated by grain-scale crystal-plasticity, i.e., dislocation glide (dominantly in prism <a>) with minor recovery by subgrain formation/rotation, accompanied by grain boundary migration and micro-cracking. It is inferred that strain incompatibilities induced by dislocation glide are accommodated by grain boundary processes, including dissolution-precipitation and grain boundary sliding. These intra-grain and grain-boundary processes together resulted in a lower n-value of 2 for the quartzite.
Our new flow law predicts strain rates that are in much better agreement with the inferred natural values than the earlier flow laws. It further suggests that the strength of the continental crust considering quartz rheology is significantly lower than previously predicted.
Diffusion creep of diopside.
Diffusion creep of diopside.
I have conducted uniaxial compression experiments on fine-grained diopside with either 4-10 vol% forsterite, and 10 vol% anorthite at the Hiraga Lab. I learned to develop highly dense nanocrystalline samples with controlled microstructures for starting materials, using SPS. Mechanical data were obtained at a stepped load for a temperature range of 1050 to 1170 °C. My experimental results show that the plastic deformation in clinopyroxene is dominantly accommodated by lattice diffusion, which led to significantly high activation energy (~720 kJ) than olivine or plagioclase. I have compared previously reported diffusion creep rates of diopside in nondimensional stress and strain-rate space, constructed based on the newly developed flow law, which highlights the effect of aluminum (Al) and iron (Fe) on the creep rates (Ghosh et al., 2021).
Control of the mechanically weak layers in the upper crust on thrust progression.
Control of the mechanically weak layers in the upper crust on thrust progression.
The analytical solution used in traditional critically tapered Coulomb wedge theory does not explicitly include any spatio-temporal analysis of shear failure. Initially, I have integrated detailed field observations with analogue (sandbox) experiments to show the mechanical effects of the upper crustal lithologies on the development of the frontal Himalayan thrust system (Ghosh et al., 2018). I have shown how the coal-shale bearing mechanically weak Gondwana stratigraphy can localize strain and instigate faulting. In line with this study, I have modelled the characteristic flat-ramp-flat geometry of the Main Himalayan Thrust (MHT), using self-consistent numerical simulations (Underworld2) that successfully predicts the locations of high strain zones, long-term uplift, and erosion patterns across the Himalayas. This work (Ghosh et al., 2020) is crucial for future earthquake predictive studies from the Himalayas, as the mechanical framework for the development of the most important seismically active structural asperity on the basal Main Himalayan mega-Thrust i.e. the mid-crustal ramp is given. Moreover, I have analyzed why the mid-crustal ramp on the MHT has developed in a segmented manner and how that controls the along-strike rupture propagation. Although, it can't be observed directly, yet we can predict the rheology of the MHT fault zone material from field observations.
Strain localization along the Daling Thrust (DT) zone in Darjeeling-Sikkim Himalaya.
Strain localization along the Daling Thrust (DT) zone in Darjeeling-Sikkim Himalaya.
The Daling Thrust (DT) (also known as the Shumar Thrust in Bhutan) delineates a zone of intense shear localization in the Lesser Himalayan Sequence (LHS) of the Darjeeling-Sikkim Himalaya. From microstructural studies of deformed quartzite samples, we show a transition in the dynamic recrystallization mechanism with increasing distance from the DT, dominated by grain boundary bulging (BLG) recrystallization closest to the DT, and progressively replaced by sub-grain rotation (SGR) recrystallization away from the thrust. The transition is marked by a characteristic variation in the fractal dimension (D) of grain boundaries, estimated from the area–perimeter method. For the first time, a increasing deformation temperature away from the DT in the hanging wall is documented, which is later verified from Bhutan by independent geothermometry. Deformation conditions like differential stresses, strain rate and ambient temperature from quartz dislocation creep microstructures were estimated, under which a continental scale ductile shear zone (i.e. DT) in the Eastern Himalaya has evolved. Then, using numerical modelling, we have correlated such data obtained from microstructural analysis with the large-scale Himalayan tectonics (Ghosh et al., 2016).
Orogen-transverse tectonic windows in the Eastern Himalayan fold belt.
Orogen-transverse tectonic windows in the Eastern Himalayan fold belt.
The Eastern Lesser Himalayan fold-thrust belt is punctuated by a row of orogen-transverse domal tectonic windows. Based on new structural maps, coupled with outcrop-scale field observations, we recognize at least four major episodes of folding in the litho-tectonic units of Darjeeling-Sikkim Himalaya (DSH). We propose a new genetic model for the RW, invoking the mechanics of superposed buckling in the mechanically stratified litho-tectonic systems. We substantiate this superposed buckling model with results obtained from analogue experiments. The model explains contrasting F3-F4 interferences in the Lesser Himalayan Sequence. The lower-order (terrain-scale) folds have undergone superposed buckling in Mode 1, producing large-scale domes and basins, whereas the RW occurs as a relatively higher-order dome nested in the first-order Tista Dome. The Gondwana and the Proterozoic rocks within the RW underwent superposed buckling in Modes 3 and 4, leading to Type 2 fold interferences, as evident from their structural patterns (Bose et al., 2014).
Tectonic overpressure.
Tectonic overpressure.
In this paper (Marques et al., 2018), we worked on a very complex geodynamic problem of dynamic overpressure. Overpressure is the tectonic pressure (stress) in excess to lithostatic pressure (ρgh). The concept is still hotly debated among the geoscientists. However, using numerical models approximated to the Himalayan setting, we explored the origin of high pressure (HP) and ultra HP rocks in the Himalayan orogen as a consequence of tectonic overpressure.
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