Lithologic Controls on Frictional Behavior Along the Shallow Subduction Interface: Constraints From an Exhumed Accretionary Wedge (McHugh Complex, Alaska)

Subduction zone megathrusts accommodate a wide range of slip modes, from earthquakes to slow slip events (SSEs) and aseismic creep. Understanding why different slip modes localize in specific regions of the shallow subduction interface remains a significant challenge. Exhumed accretionary complexes are an important natural laboratory for addressing this problem. In this study, we examine the mechanical behavior of representative lithologies and faults within the McHugh Complex in the Kenai Mountains of southern Alaska. The McHugh Complex is a Mesozoic accretionary wedge that exposes lithologies and fault zones representative of the shallow subduction interface. We integrate field observations with compositional and microstructural analyses and laboratory friction experiments to evaluate both fault failure conditions and potential slip modes. Direct shear experiments were conducted on powdered fault gouges and host rocks under dry and water-saturated conditions at normal stresses of 10–40 MPa, representative of shallow subduction zone conditions. 
Our results demonstrate that mineralogical composition exerts a first-order control on fault strength and frictional stability. Increasing proportions of phyllosilicates reduce friction coefficients (μ_f) and promote velocity-strengthening behavior. Argillitic fault gouges rich in organic matter exhibit the lowest frictional strength (μ_f = 0.33), consistent with strain localization observed in these rocks in the field. Conversely, stronger lithologies, such as pillow basalts and cherts, display higher frictional strengths (μ_f = 0.53) and frictional stabilities that promote seismic slip initiation. However, fault zones within basaltic units that have undergone significant alteration and chlorite enrichment evolve toward velocity-neutral behavior, suggesting the potential to nucleate SSEs rather than earthquakes. 
A cross-section through the exhumed accretionary wedge reveals that contrasts in mechanical strength often coincide with contrasts in permeability, suggesting that stress concentrations and transient fluid overpressure likely act together to trigger fault failure. Overall, our findings emphasize the role of lithologic heterogeneity in controlling both fault failure and slip mode along the shallow subduction interface. This provides a framework for linking rock composition to the spatial distribution of seismic and aseismic behavior. Future work will apply this integrated approach to additional cross-sections across the McHugh Complex.