Control of mechanical stratification on stress localization in a shallow magma plumbing system containing a pre-existing fracture: Findings from thermo-mechanical, visco-plastic models

Abstract

Shallow magma chamber hosted in mechanically heterogeneous crustal segments often contain pre-existing fracture (PEF), which can influence the nucleation and growth of dikes due to magma excess pressure (P_e). Employing thermo-mechanical finite element models using visco-plastic rheology, this study investigates how P_e and competency contrast (C_r) between the crustal layers control stress localization, yield strength distribution, and ground displacement for different chamber temperatures (T_c). We show that condition with higher P_e and C_r, enhance tensile stress localization around the chamber flanks and near PEF tips, amplifying mechanical interaction between such zones. FE model results obtained with high C_r (=1:20) condition produce uneven stress distribution between the layers with contrasting stiffness and develops potential zone for dike deflection. This agrees with the analysis of elastic mismatch parameter (α_D), which increases by up to ∼36 % when C_r is increased from 1:5 to 1:20. PEF lowers the yield strength near the chamber tip and along the chamber-facing side of the PEF, and such low yield zones widen progressively with increasing P_e and C_r. We show that vertical ground displacements amplify with increasing T_c and P_e, which hold an inverse relation with C_r. Calculated stored strain energy (U₀) from the model results are utilized to evaluate the strike dimensions of feeder dikes, showing an increasing trend with an increase in P_e. Finally, we present a comparison between the modelled ground displacements and observations from the Campi Flegrei caldera (Italy), which shows a good agreement between the modelled and natural surface uplift magnitudes.