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. Evolution of slow slip to dynamic rupture on seabed megathrusts in basalt can be explained by strain localization and a transition to strain weakening behaviour encoded as a microtextural signature in triaxial shear experiments on oceanic basalt gouge.