Large-displacement, hydrothermal frictional properties of DFDP-1 fault rocks, Alpine Fault, New Zealand: Implications for deep rupture propagation

The Alpine Fault, New Zealand, is a major plate-bounding fault that accommodates 65-75% of the total relative motion between the Australian and Pacific plates. Here we present data on the hydrothermal frictional properties of Alpine Fault rocks that surround the principal slip zones (PSZ) of the Alpine Fault and those comprising the PSZ itself. The samples were retrieved from relatively shallow depths during phase 1 of the Deep Fault Drilling Project (DFDP-1) at Gaunt Creek. Simulated fault gouges were sheared at temperatures of 25, 150, 300, 450, and 600°C in order to determine the friction coefficient as well as the velocity dependence of friction. Friction remains more or less constant with changes in temperature, but a transition from velocity-strengthening behavior to velocity-weakening behavior occurs at a temperature of T = 150°C. The transition depends on the absolute value of sliding velocity as well as temperature, with the velocity-weakening region restricted to higher velocity for higher temperatures. Friction was substantially lower for low-velocity shearing (V < 0.3 µm/s) at 600°C, but no transition to normal stress independence was observed. In the framework of rate-and-state friction, earthquake nucleation is most likely at an intermediate temperature of T = 300°C. The velocity-strengthening nature of the Alpine Fault rocks at higher temperatures may pose a barrier for rupture propagation to deeper levels, limiting the possible depth extent of large earthquakes. Our results highlight the importance of strain rate in controlling frictional behavior under conditions spanning the classical brittle-plastic transition for quartzofeldspathic compositions.

Clay mineral transformations and weakening mechanisms along the Alpine Fault, New Zealand

The formation of clay minerals within active fault zones, which results from the infiltration of aqueous fluids, often leads to important changes in mechanical behaviour. These hydrous phyllosilicates can (1) enhance anisotropy and reduce shear strength, (2) modify porosity and permeability, (3) store or release significant volumes of water, and (4) increase fluid pressures during shearing. The varying interplay between faulting, fluid migration, and hydrous clay mineral transformations along the central Alpine Fault of New Zealand is suggested to constitute an important weakening mechanism within the upper section of this crustal discontinuity. Well-developed zones of cataclasite and compacted clay gouge show successive stages of hydrothermal alteration, driven by the cyclic, coseismic influx of meteoric fluids into exhumed amphibolite-facies rocks that are relatively Mg rich. Three modes of deformation and alteration are recognized within the mylonite-derived clay gouge, which occurred during various stages of the fault’s exhumation history. Following initial strain-hardening and frictional melting during anhydrous cataclastic breakdown of the mylonite fabric, reaction weakening began with formation of Mg-chlorite at sub-greenschist conditions (<320 °C) and continued at lower temperatures (<120 °C) by growth of swelling clays in the matrix. The low permeability and low strength of clay-rich shears are suitable for generating high pore-fluid pressures during faulting. Despite the apparent weakening of the c. 6 km upper segment of the Alpine Fault, the upper crust beneath the Southern Alps is known to be actively releasing elastic strain, with small (<M 5) earthquakes occurring to 12 km depth. We predict that larger events will nucleate at c. 6–12 km along an anhydrous, strain-hardened portion of the fault.