Aseismic fracturing and cataclasis involving reaction softening within core material from the Cajon Pass Drill Hole

Abstract

Cataclastic deformation features in crystalline rocks from the Cajon Pass drill hole, located some 4 km NE of the San Andreas fault trace in southern California, appear to have developed at slow strain rates. There is no clear evidence of seismic deformation. Most of the observed structures and microstructures are inferred to have formed during pre‐Pliocene distributed deformation, before the San Andreas fault became active in this area. Extension fractures are filled by laumontite which increases in grain size from a fine amorphous habit to coarser prismatic crystals unidirectionally across fractures and does not generally display crack‐seal textures. Fragments within fractures were derived from adjacent grains without rotation or shear. Fracture opening and cementation were therefore synchronous and slow. Fluids were present at all times during fracture opening, but there was no repeated hydrofracturing after fracture formation. However pore fluid pressures probably stayed close to the least principal stress (σ3) during subsequent fracture growth. Plagioclase compositions in all fractured areas change from oligoclase to albite or anorthite as a consequence of albitization and laumontization. Particle size distributions in dilational areas, and in extension and in shear fracture fillings, show that alteration of the major phase, plagioclase, is the fundamental process of grain size reduction in a variety of rock types. Cataclastic stresses and strain rates were controlled by alteration reaction rates in a coupled process that is a form of transformation‐modified deformation. Fracturing leading to weakening was assisted by stresses due to the 60% volume increase accompanying in situ laumontization of plagioclase and by stress corrosion and subcritical crack growth facilitated by alteration. Deformation was also enhanced by replacement of plagioclase by weaker laumontite. Such dilatant cataclasis is consistent with fluid pressure levels having remained high during deformation with effective least principal compressive stress close to zero. This transformation‐modified deformation at low temperatures (90° < T < 250°C) may be a common process in feldspar‐rich rocks. The structures and microstructures described here could be used to distinguish the products of slow and fast Cataclastic deformation.