During lithospheric deformation, the transition from brittle to ductile deformation—commonly referred to as the brittle-to-ductile transition (BDT)—marks a critical depth zone that influences earthquake nucleation and propagation. Understanding the physical controls on deformation in this transitional regime is essential for improving our understanding of fault mechanics and seismic hazard.

At the BDT, deformation occurs through a combination of localized slip and distributed flow. Recent studies highlight the role of strain partitioning—the division of deformation between fault slip and off-fault bulk strain—in governing fault stability, yet its dependency on effective stress and temperature remains poorly constrained. Using the HighSTEPS biaxial apparatus (LEMR, EPFL, Switzerland) we performed shear experiments on Carrara marble under varying stress conditions to explore these dependencies.

We show that under increased confining pressure and lower effective stress, strain becomes increasingly distributed in the bulk, coinciding with a reduction in frictional strength and the suppression of stick-slip behavior. Velocity-step experiments further reveal that strain partitioning decreases with increasing slip rate, consistent with a transition from semi-brittle to more brittle behavior.

These findings provide direct experimental evidence that effective stress and slip rate modulate strain localization at the BDT. The results bridge laboratory and natural observations of fault behavior in the middle crust and offer new constraints on the mechanical processes that may govern the initiation and propagation of earthquakes in carbonate-rich lithologies.

We anticipate that this work will inform numerical models of dynamic rupture in transitional crustal zones and support ongoing efforts to map the mechanical limits of the seismogenic zone.