The brittle–ductile transition, typically located near the base of the seismogenic zone, controls strain localization and the evolution of physical properties, with direct implications for fluid transport and fault mechanics. However, quantitative constraints on strain partitioning and porosity evolution across this transition remain limited, particularly in low-porosity carbonate rocks. We conducted triaxial compression experiments on dry Carrara marble at confining pressures of 5 and 100 MPa using two complementary techniques: (1) acoustic emission (AE) monitoring coupled with P-wave velocity measurement, and (2) in-situ X-ray microtomography combined with digital volume correlation (DVC). AE activity remains low at all pressures, with source mechanisms becoming increasingly compaction-dominated at the highest confinement. P-wave velocities fall markedly during deformation by up to 60% at 5-10 MPa, ∼50% at 40 MPa, and ∼40% at 100 MPa, reflecting pervasive microcrack development. DVC reveals that the proportion of dilative volumetric strain decreases with confinement from nearly 90% at 5 MPa to 50% at 70-90 MPa, yet dilation remains a major component and is comparable to compaction even at the highest pressures. X-ray-derived bulk porosity increases from <1% to ∼1.5% at low confinement prior to failure, and to ≥10% at higher confinements, indicating substantial porosity generation during distributed dilatant cataclastic flow. Together, these results provide the first quantitative link between P-wave velocity reduction, volumetric strain partitioning, and transient porosity generation across the brittle to semi-ductile transition, revealing how the interplay of dilation and compaction governs both elastic properties and strain localization in low-porosity crystalline carbonate.