Megathrust faults at subduction zones slip at a broad spectrum of rates from slow creep (centimeters per year) to dynamic rupture (meters per second), with large excess fluid pressures implicated as a control on nucleation style. We report friction measurements on oceanic basalt gouges (IODP Expedition 368X) at elevated temperatures (150-450 °C), stresses (150 MPa), and large fluid overpressures (30-120 MPa) to represent conditions along the descending slab and to link observed rheology to microtextural evolution. With reducing effective stress, slip instabilities are first manifested as slow-slip and evolve through dynamic stick-slip as a result of reduced shear zone width. This transition in rupture style is driven by an increase in effective fault stiffness k'c and a decrease in nucleation length L. Increased intergranular pressure dissolution at elevated effective stress mediates the shear localization width, controls the dynamics of strain localization, and leaves a structurally discernible fingerprint. Our results imply that effective stress-controlled mass transfer, together with strain localization, dictates the styles of instability nucleation manifest as slow earthquakes rationally evolving into dynamic megathrust ruptures.