Physical properties of surface outcrop cataclastic fault rocks, Alpine Fault, New Zealand. Geochemistry Geophysics Geosystems 13

[1] 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 experi-ments on fault gouge and intact samples of cataclasite were conducted at 30–33 MPa effective normal stress (sn′) 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 1020 m2 to k = 1.15 1016 m2), (2) a transition from frictionally weak (m = 0.44) fault gouge to frictionally strong (m = 0.50–0.55) cataclasite, (3) a change in friction rate depen-dence (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 (m = 0.31 to m = 0.44) and fault-perpendicular permeability (k = 3.10 1020 m2 to k = 7.45 1020 m2). Comminution of frictionally strong (m = 0.51–0.57) cataclasites forms weaker (m = 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 (Mw 8) surface ruptures on the Alpine Fault, petrological observations indicate tha