Along the subduction megathrust, the up-dip limit of the seismogenic zone is thought to occur at 100 to 150 °C and the shallower region hosts diverse modes of fault slip. The transition to the seismogenic zone is thought to occur due to evolving material properties that promote seismicity; however, which properties are responsible and how they evolve is poorly understood. The Sestola-Vidiciatico Unit (SVU) has been interpreted as an analog of the sedimentary component of active subduction megathrusts, near the up-dip transition to seismicity. The SVU accommodated Miocene convergence between the subducting Adriatic plate and the overriding accretionary prisms of the European plate. Deformation was dominantly accommodated within a 500 m thick shear zone along a well-exposed basal decollement and inaccessible roof decollement. Here we evaluate the role that diagenesis of sedimentary rocks plays in controlling fault zone architecture and strain partitioning. We use HAWK pyrolysis to determine new temperature constraints for deformation at multiple outcrops along the basal decollement and show that deformation occurred at temperatures between ~180 to 190 °C at our four field sites. We characterize the macroscopic fault architecture through field measurements and rock composition and microfabrics of the basal decollement using scanning electron microscopy and electron probe microanalysis. Diagenetic reactions such as silicification, chloritization, and pyritization occur in samples from each outcrop. We show that fault architecture is variable even at similar deformation conditions. We quantify the abundance and occurrence of diagenetic reactions and discuss their correlations with variations in fault architecture and deformation textures.