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Research direction: Computational Geodynamics
Key areas:
(1) Numerical methods: numerical modeling, rheology, finite element method, particle-in-cell FEM
(2) Subduction dynamics: dynamics of complex subduction modes, fluid-melt activity, Indo-Eurasian subduction zones.
(3) Lithosphere dynamics: lithospheric evolution, ductile shear zones, strain localization, shear band formation.
(4) Earth system dynamics: mantle plume formation, LLSVPs, deep Earth process, mantle convection, LIPs and Hotspots.
(5) Continental collision: Indo-Eurasian collisional history, ultra-high pressure exhumation, crustal flow, Tibetan plateau, Tethys system.
Double subduction in the Neo Tethys
Double subduction in the Neo Tethys
The Neo-Tethys Ocean saw some complex tectonic evolution, including episodes of same-dip double subduction (SDDS). In this process, both subduction zones dip in the same direction, which makes this setup a bit different from the usual opposite-dip, trench-to-trench configuration. Picture it as two parallel conveyor belts heading downward in the same direction beneath a single overriding plate—essentially, there are two oceanic plates subducting alongside each other. In this study, we build dynamic thermo-mechanical models to explore the initiation and evolution of SDDS systems, guided by Neo-Tethyan paleo-reconstructions. We test three initial plate setups—oceanic, oceanic-continental, and multiple continental configurations—to better understand the complex tectonic history of key Neo-Tethyan subduction zones, specifically the Indo-Eurasian and Andaman systems. In simulations of oceanic SDDS models, we observe that a substantial oceanic plate can drive the simultaneous development of two competing subduction zones. This competition causes unequal evolution between the zones, with one outpacing the other, resulting in extremely high convergence rates of about 16–17 cm per year over a sustained period of 7–8 million years. This finding explains the coeval activity of coupled subduction zones in the Indo-Eurasia convergence during the Cretaceous evolution of the Neo-Tethys. Meanwhile, our ocean-continent SDDS model reveals that subduction tends to concentrate along passive margins where oceanic plates meet continental blocks, forming two subduction zones that grow almost equally and generate a spreading center between the trenches. This pattern aligns with reconstructions of the eastern Neo-Tethyan region in the Cenozoic and the subsequent development of the Andaman subduction zone. Finally, we simulate SDDS dynamics in a setting with multiple continental blocks to track the Indo-Eurasian collision zone's post-Cretaceous evolution. Here, SDDS dynamics eventually trigger slab break-off, converting the double subduction setup into a single subduction zone in just around 3 million years.
Roy, A., Mandal, N., & Van Hunen, J. (2024). Dynamic evolution of competing same-dip double subduction: New perspectives of the Neo-Tethyan plate tectonics. Earth and Planetary Science Letters, 647. https://doi.org/10.1016/j.epsl.2024.119032
Why are deep-Mantle Plumes rare in occurance?
Why are deep-Mantle Plumes rare in occurance?
Mantle plumes arising from deep sources in the Earth are thought to have played a critical role in determining planetary geodynamics. The plumes originate mostly from gravitational or thermochemical instabilities at the core-mantle boundary, triggered by density fluctuations due to thermal or chemical variations. Understanding the initiation mechanics of such instabilities is key to comprehending the formation of these deep-mantle plumes, reflected from hotspots scattered around the globe. Previous studies have explained their growth within a theoretical framework of Rayleigh-Taylor (RT) instabilities. However, a critical aspect that has been largely overlooked is the potential influence of layer-parallel global flows on the dynamics and initiation processes of instabilities. This study combines 2D-finite element particle-in-cell numerical simulations with a linear stability analysis to show the impact of global flows on the growth kinematics of RT instabilities in a thermal boundary layer at the core-mantle boundary. The simulation results indicate that the global flow acts as a counterfactor to dampen the plume growth rates. At a threshold global flow velocity, the dampening effects completely suppress the instabilities, allowing the entire system to advect in the horizontal direction. The stability analysis also predicts a non-linear increase in the instability wavelength with increasing global flow velocity. These new findings imply that the spatial frequency of plumes can drop remarkably in kinematically active regions of Earth's mantle. This study offers a possible explanation for the unusually large spacing between major hotspots in light of instability mechanics under global flows.
Roy A., Ghosh D., Mandal N. (2023) Dampening effect of global flows on Rayleigh–Taylor instabilities: implications for deep-mantle plumes vis-à-vis hotspot distributions, Geophysical Journal International, Volume 236, Issue 1, January 2024, Pages 119–138, https://doi.org/10.1093/gji/ggad414
Ductile materials undergoing shear deformation are characterized by the presence of variedly oriented shear bands, which are narrow zones of significant shear strain localization. Shear band development is a universal geodynamic phenomenon that influences a wide range of natural processes like earthquakes, landslides, and subduction zone dynamics. Studies on shear band formation provide the requisite clues on the failure mechanisms of ductile materials, a topic of great importance for predicting the behaviour of earth materials under extreme conditions. This study is concerned with the factors determining the orientation of shear bands in natural ductile shear zones. The bands occur in two significant orientations, one at a low-angle and the other parallel to the direction of the applied shear. This article aims to address the long-standing question: Under what conditions do shear bands localize parallel to the bulk shear direction in shear deformations? This study combines field observations, analogue experiments, and numerical simulations to constrain the geometrical and rheological conditions of two principal modes of shear band growth. The study findings suggest that ductile shear zones will preferentially enhance the development of shear-parallel bands over low-angle shear bands. A mathematical model is also developed to predict the shear band orientation with respect to the direction of applied shear.
What factors control the growth and development of C-shear bands in ductile shear zones?
What factors control the growth and development of C-shear bands in ductile shear zones?
Roy, A., Roy, N., Saha, P., & Mandal, N. (2021). Factors determining shear-parallel versus low-angle shear band localization in shear deformations: Laboratory experiments and numerical simulations. Journal of Geophysical Research: Solid Earth, 126, e2021JB022578. https://doi.org/10.1029/2021JB022578
Classification of ductile shear zones based on their band patterns.
Classification of ductile shear zones based on their band patterns.
Ductile yielding of rocks and similar solids localize shear zones, which often show complex internal structures due to the networking of their secondary shear bands. Combining observations from naturally deformed rocks and numerical modelling, this study addresses the following crucial question: What dictates the internal shear bands to network during the evolution of an initially homogeneous ductile shear zone? Natural shear zones, observed in the Chotonagpur Granite Gneiss Complex of the Precambrian craton of Eastern India, show characteristic patterns of their internal shear band structures, classified broadly into three categories: Type I (network of antithetic low-angle Riedel (R) and synthetic P-bands), Type II (network of shear-parallel C and P/R bands) and Type III (distributed shear domains containing isolated undeformed masses). Considering strain-softening rheology, two-dimensional viscoplastic models reproduce these three types, allowing the prediction of the condition of shear zone growth with a specific network pattern. This study suggests that complex anastomosing shear-band structures can evolve under simple shear kinematics in the absence of any pure shear component.
Roy N., Roy A., Saha P., Mandal N. (2022) On the origin of shear-band network patterns in ductile shear zonesProc. R. Soc. A.47820220146 http://doi.org/10.1098/rspa.2022.0146
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