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• Magnetotelluric images were obtained across the Xiaojiang fault system (XJFS). • Fluid-based fault zone conductors divide the resistive upper crust into fault-parallel blocks. • Lower crustal conductors may be associated with shear-enhanced partial melting. • A dynamic model is proposed for understanding the deformation mechanism of the XJFS. It is widely accepted that large-scale strike-slip faults play an important role in accommodating the India‐Eurasia convergence, but the relationships between surface displacements and deep deformation along the faults remain largely uncertain. To address this issue, new magnetotelluric (MT) images were obtained along two profiles across the Xiaojiang strike-slip fault system (XJFS), one of the most tectonically active areas in southwest China. Although different in details, these two profiles show similar overall crustal structures. The upper crust is generally resistive but separated by several narrow, subvertical conductors that broadly coincide with the surface traces of the XJFS. These fault zone conductors (FZCs) are interpreted to represent fault damage zones that formed by strike-slip faulting/shearing along the faults and filled with deeply-sourced magmatic and/or metamorphic fluids. The lower crust is broadly characterized by enhanced conductivity and likely associated with partial melting at high temperatures. These observations are consistent with other geophysical measurements in this area and in favor of the development of ductile flow along the XJFS, although the pattern of flow is more complex than previously suggested. Based on these observations, we propose that active deformation along the XJFS can be better described, in the brittle upper crust, by strike-slip motion along the fluid-weakened faults, and by shear-enhanced ductile flow in the partially molten lower crust. These two mechanisms are not mutually exclusive but rather can be complementary to each other. [ABSTRACT FROM AUTHOR]