Fluid-mediated transition from dynamic rupturing to aseismic slip at the base of the seismogenic continental crust

• Dynamic rupturing replaced by viscous aseismic creep through fluid-induced weakening. • Fluid pressure transients promote dynamic rupturing followed by frictional shear. • Reaction weakening leads to aseismic creep on short timescales (10 −1 -10 3 years). • Physio-chemical evolution of H 2 O-CO 2 fluid promotes cyclical creep and embrittlement. • Aseismic phyllonites host brittle veins due to daily/monthly fluid pressure cycles. Chemo-mechanical fluid-rock interactions are critical in controlling the frictional-viscous transition in the continental crust and the competition between seismic and aseismic deformation in fault zones. In this study, we provide quantitative constraints on the timing and magnitude of weakening, and associated changes in slip rates, due to fluid-rock interactions at the base of the seismogenic continental crust. Integrating field, microstructural analyses, and micromechanical modelling we constrain the microstructural and mechanical evolution of phyllosilicate-rich, carbonate-bearing brittle-ductile faults/shear zones developed in the Rieserferner granitoid pluton (Eastern Alps). Here, transient overpressure of (H 2 O + CO 2 )-rich fluids triggered dynamic rupturing in the strong host rock (>100 MPa), and promoted the development of weak phyllonites through long-term fluid-mediated feldspar-to-mica reactions. These phyllosilicate-rich fault rocks accommodated frictional-viscous aseismic creep at very low differential stresses (<10 MPa) and near-lithostatic fluid pressure conditions. Microscale vein networks overprinting the phyllonite indicate cyclical embrittlement related to increased creep rates (up to 10 −7 s −1 ) that occurred over a timeframe of days to months and potentially related to slow earthquakes (slip rates of 10 −8 m/s). These findings offer new constraints on the development and seismogenic potential of phyllosilicate-rich fault zones and on the effect of fluid chemistry on fault zone rheology. Fluid-mediated fault weakening can occur in rather short time (months-to-years) comparable to the interseismic period, progressively promoting long-term, viscous aseismic creep on a previously strong fault zone developed by dynamic rupturing. The combined effect of strain localization, the low permeability of the phyllonitic cores, as well as of fluid chemistry evolution and CO 2 -enrichment, may lead to the development of brittle-frictional instabilities during transient accelerated-creep events. Therefore, the fluid-mediated microstructural evolution of phyllosilicate-rich fault rocks controls their seismogenic behaviour, potentially leading to accelerated creep, slow earthquakes and slow slip on otherwise aseismically creeping faults.