Observations and Implications of Cyclical Slip in DFDP-1 Principal Slip Zone Gouges, Alpine Fault, New Zealand

Abstract:

The Alpine Fault accommodates up to 75% of the total relative Australia-Pacific plate boundary motion and ruptures episodically in large magnitude (Mw~8) earthquakes. An aim of the Deep Fault Drilling Project (DFDP) is to understand the processes that govern earthquake rupture nucleation and propagation on the Alpine Fault. In January 2011, DFDP-1A drilling recovered a c. 25 cm-thick principal slip zone (PSZ) at Gaunt Creek. Assuming serial partitioning occurred at c. 2 km depth, this narrow PSZ has accommodated up to 5 km of displacement. We document the frictional, hydrological, structural, mineralogical, and chemical attributes of an 8 cm x 8.5 cm cylindrical section of PSZ core containing the oblique thrust contact (055/29SE, lin. 23/109) between hanging wall gouges and footwall gravels. Two main gouge types comprise the DFDP-1A core: smectitic brown gouges and chloritic, micaceous blue gouges. Smectitic gouges occur primarily at the plate boundary contact. However, smectitic gouges are also present as injection veins, lenses, recumbent folds, and clasts within overlying blue gouges, providing evidence for cyclical slip and material mixing within the PSZ. The room-temperature frictional properties of PSZ gouges were measured at 30 MPa effective normal stress (σ_n’) and varying sliding velocities (v=0.1-10 μm/s) using the direct-shear configuration. The friction coefficient (μ=0.60) of a blue gouge agrees with that published in Boulton et al. (2014). Two brown gouges are slightly stronger (μ=0.49 and 0.54) than similar gouges recovered in a nearby borehole (DFDP-1B). All gouges exhibit velocity-strengthening behavior in 0.3-10 μm/s velocity steps. These results indicate that although they are not frictionally weak at low sliding velocities, fault slip repeatedly localizes in the PSZ gouges. Additional measurements of fault gouge physical and chemical properties will elucidate the pre-seismic, interseismic, and post-seismic behavior this major plate boundary structure.