Signatures of localization control transition between rupture styles on basaltic megathrusts

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°C−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 manifest 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 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 discernable 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.