EVIDENCE FOR CRACKING AND SILICA REDISTRIBUTION AT THE LOWER END OF THE SEISMOGENIC ZONE, KODIAK, ALASKA AND OTAGO, NEW ZEALAND

Microstructural analysis of veins from two ancient megathrusts indicates crack-seal deformation and silica redistribution that may accompany microseismicity and shear associated with slow slip events near the lower end of the seismogenic zone. The Otago schist and the central belt of the Kodiak Formation in Alaska both originated as oceanic lithologies that were underthrust along ancient subduction zones and underplated at depths of pumpellyite-greenschist grade metamorphism (280-300˚C). Both examples show evidence for progressive simple shear. In the Otago case, this simple shear deformation is distributed on anastomosing arrays of slip surfaces with veins in dilational jogs that show evidence for episodic cracking and sealing. In the Kodiak Formation , simple shear is concentrated in en echelon arrays of veins that are regularly spaced throughout the shear zone. As in the Otago schist, veins from Kodiak exhibit crack-seal microstructures. There is also microstructural evidence in both cases for anisotropic growth kinetics in veins, which is consistent with competition for growth within open fractures. Open fractures at the depths experienced by these shear zones requires high fluid pressures and hydrofracturing, with fluid pressure equal to σ₃ plus the tensile strength— a likely consequence of dehydration reactions in a regime of low permeability. In both these examples, the ratio of average slip (10-100 microns, in Otago, 5-20 microns in Kodiak) to length of fractures (m’s) is on the order of 10^-5-10^-6, consistent with earthquakes that have stress drops of 3-30 kPa. The opening of hydrofractures leads to chemical potential gradients that drive local diffusion of silica from the wall rock or from compressional jogs along slip surfaces into cracks, where precipitation seals the fractures and relocks slip surfaces. A 1-D model for transport and silica redistribution for the Kodiak example suggests that fractures can be sealed in the time scales relevant for slow earthquakes (days to months). Crack seal deformation could be important at the lower end of the seismogenic zone for healing slip events and providing the dilatancy needed to stabilize slow slip instabilities. At these depths and at low effective stresses, the megathrust behaves as a brittle-ductile shear zone.