Physical properties of surface outcrop cataclastic fault rocks, Alpine Fault, New Zealand

TLDR

The study presents a unified analysis of the physical properties of cataclastic fault rocks from surface exposures of the central Alpine Fault at Gaunt Creek and Waikukupa River, New Zealand. Friction experiments on fault gouge and intact cataclasite were conducted at 30–33 MPa effective normal stress using a double‑direct shear configuration with controlled pore fluid pressure in a true triaxial pressure vessel. Samples reveal that fault normal permeability increases by orders of magnitude, friction shifts from weak to strong with a change from velocity‑strengthening to both strengthening and weakening behavior, mineral alteration and comminution produce weaker, foliated cataclasites associated with aseismic creep, and these petrological observations support that large Alpine Fault ruptures involved sliding within velocity‑strengthening gouge.

Abstract

We present a unified analysis of physical properties of cataclastic fault rocks collected from surface exposures of the central Alpine Fault at Gaunt Creek and Waikukupa River, New Zealand. Friction experiments on fault gouge and intact samples of cataclasite were conducted at 30–33 MPa effective normal stress ( σ n ′) using a double‐direct shear configuration and controlled pore fluid pressure in a true triaxial pressure vessel. Samples from a scarp outcrop on the southwest bank of Gaunt Creek display (1) an increase in fault normal permeability ( k = 7.45 × 10 −20 m 2 to k = 1.15 × 10 −16 m 2 ), (2) a transition from frictionally weak ( μ = 0.44) fault gouge to frictionally strong ( μ = 0.50–0.55) cataclasite, (3) a change in friction rate dependence ( a‐b ) from solely velocity strengthening, to velocity strengthening and weakening, and (4) an increase in the rate of frictional healing with increasing distance from the footwall fluvioglacial gravels contact. At Gaunt Creek, alteration of the primary clay minerals chlorite and illite/muscovite to smectite, kaolinite, and goethite accompanies an increase in friction coefficient ( μ = 0.31 to μ = 0.44) and fault‐perpendicular permeability ( k = 3.10 × 10 −20 m 2 to k = 7.45 × 10 −20 m 2 ). Comminution of frictionally strong ( μ = 0.51–0.57) cataclasites forms weaker ( μ = 0.31–0.50) foliated cataclasites and fault gouges with behaviors associated with aseismic creep at low strain rates. Combined with published evidence of large magnitude ( M w ∼ 8) surface ruptures on the Alpine Fault, petrological observations indicate that shear failure involved frictional sliding within previously formed, velocity‐strengthening fault gouge.