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Fault zones exhibit 3D variable thickness, a feature that remains inadequately explored, particularly with regard to the impact on fluid flow. Upon analyzing an analytic solution, we examine 3D thermal‐hydraulic (TH) dynamical models through a benchmark experiment, which incorporates a fault zone with thickness variations corresponding to realistic orders of magnitude. The findings emphasize an area of interest where vigorous convection drives fluid flow, resulting in a temperature increase to 150°C at a shallow depth of 2.7 km in the thickest sections of the fault zone. Moreover, by considering various tectonic regimes (compressional, extensional, and strike‐slip) within 3D thermal‐hydraulic‐mechanical (THM) models and comparing them to the benchmark experiment, we observe variations in fluid pressure induced by poroelastic forces acting on fluid flow within the area of interest. These tectonic‐induced pressure changes influence the thermal distribution of the region and the intensity of temperature anomalies. Outcomes of this study emphasize the impact of poroelasticity‐driven forces on transfer processes and highlight the importance of addressing fault geometry as a crucial parameter in future investigations of fluid flow in fractured systems. Such research has relevant applications in geothermal energy, CO2 storage, and mineral deposits. Plain Language Summary: Exploring critical parameters affecting the fluid flow within fault zones in the Earth's crust is of fundamental scientific and economic interest. Among them, fault zone thickness and tectonic regimes are two parameters whose role remains unexplored. The results of this study show that fluid flow by convection is vigorous in the zone where fault thickness is largest. In this area of interest, the tectonic regimes impact convective dynamics and modify the thermal distribution of the system. These generic outcomes are discussed in relation to real‐cases scenarios and emphasize the fundamental role of considering a Geometric parameter (G), the fault zone thickness variation, as well as tectonic regimes, during the exploratory phase for geothermal energy, CO2 storage, and mineral exploration. Key Points: Based on a critical Rayleigh number analysis and 3D numerical modeling, we explore the effects of fault zone thickness variations on fluid flowIn line with the critical Rayleigh number analysis, our numerical results reveal that thermal convection is more efficient in areas where the fault zone thickness is largestThe poroelasticity‐driven force alters the convective dynamics within the zone of interest in the fault zone (i.e., the thickest part) [ABSTRACT FROM AUTHOR]