<p>Magmatic overpressure in shallow- and mid-crustal magma chambers (MC) can deform the crustal host rocks. Stress field produced by such deformation often control the nucleation and subsequent crack formation for magma emplacement. A direction of physical volcanology is concerned with determination of the volcanotectonic ground surface displacements that can aid in monitoring and sometimes forecasting magmatic eruptions. The existing Mogi Model can analytically calculate surface displacements due to overpressure in a single MC by considering elastic deformation of a finite crustal section. Many geological and geophysical studies report that magma plumbing systems represent an array of randomly placed interconnected MCs, and there is a need of theoretical estimation of their ground surface displacement. In this study we present a new analytical formulation to estimate surface displacement in terms of both vertical as well as horizontal directions above a dual MC setting. Our analytical solution finds support from finite element (FE) models performed with the same set of geometrical and physical parameters. The off-axis chambers considered in our model are separated along both vertical and horizontal directions. The present study suggests that with increasing horizontal chamber separation (<em>S</em><em><sub>h</sub></em>) the vertical ground displacement above the two chambers gradually changes from a single peak into an indistinct double-peak, and finally two prominent independent, high-amplitude peaks. On the other hand, on increasing the vertical separation (<em>S</em><em><sub>v</sub></em>) between two off-axis chambers we observed that the initial double peaks merged to produce a single peak situated roughly above the middle of the two chambers. Stress map obtained from the FE models shows that the deformation of two MCs can only interact when located within a critical distance, else their deformation remains independent. Interestingly, our study suggests that the magnitude of stress field strongly depends on the strength of the mechanical interaction between two neighboring chambers.</p>
To understand the physico-chemical processes associated with migmatisation is an interesting petrological problem. New developments in microfluidics and chaotic mixing experiments have helped us to better perceive these processes from the migmatic rocks of the Proterozoic Chotanagpur Granite Gneiss Complex (CGGC), eastern India. The migmatic rocks of CGGC have preserved folded leucocratic veins in amphibolites representing viscous folding. The viscous folding phenomenon occurred due to the interaction between leucosome and melanosome. Based on textural features and mineral chemical data interpretations, we infer that when granitic and pegmatitic magmas intruded the gneissic rocks and amphibolites of our study area, diffusion of heat and volatiles from the hotter felsic magmas to the colder country rocks initiated partial melting in the amphibolites, forming melanosomes. After their formation, the highly viscous felsic magmas veined into the melanosomes, by progressively melting them and then interacting, leading to chaotic mixing dynamics. The development of chaotic mixing allowed the leucosome to venture into the melanosome as veins by stretching and folding dynamics. As the leucocratic veins or leucosome traversed through the partially molten rock or melanosome due to advection, the veins underwent viscous folding owing to the exertion of compressional stress brought about by the viscosity difference between the two mediums. The occurrence of viscous folding exponentially increased the contact area between the leucosome and the melanosome, eventually leading to enhanced diffusion and augmented mixing between the two mediums. Evidence of mixing through elemental diffusion is well documented by the compositions of amphibole and biotite occurring in the leucosome and melanosome. These minerals show substitution of magnesium and ferrous ion that show linear variation between the endmember compositions.
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