Cyclic fluid flow through a regionally extensive fracture network within the Kodiak accretionary prism

Two types of periodic textures observed in veins from the Kodiak accretionary prism attest to cyclic fluid flow through a regionally extensive fracture network buried at 8-12 km depth: 1) crack-seal microstructures with bands of mica inclusions and 2) collapse microstructures with jagged bands of residue embedded within euhedral crystals of quartz. Analysis of chemical composition on either side of four large euhedral growth veins and whole rock analysis of slates across the Kodiak Formation reveal local depletion of silica adjacent to veins but no evidence for long-distance silica transport within the system. Both crack-seal and collapse textures are observed in a regionally extensive vein system. The mesoscopic distribution and textural variability of the vein network suggests that the development of the vein system involved early nucleation and growth of vertical hydrofractures. As the fracture density increased, arrays of fractures locally provided zones of weakness and southeast dipping brittle-ductile shear zones nucleated. These en echelon cracks remained open and provided small reservoirs of fluid. Periodic inflation and deflation of en echelon arrays may reflect periodic slip on crosscutting faults and rupture of the seals that separate reservoirs. -from Authors