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 accre tionary prism attest to cyclic fluid flow through a regionally extensi ve fracture network buried at 8-12 km depth: (1) crack-seal microstruc tues with bands of mica inclusions and (2) collapse microstructures wi th jagged bands of residue embedded within euhedral crystals df quartz . The difference in texture reflects the closure of cracks: crack-seal microstructures record the complete chemical sealing of the crack aft er each fracture event, whereas the collapse features record longer fl uid-filled periods followed by more rapid draining of fractures. Colla pse features consist of pressure solution selvages trapped within vein s and in the wall rock adjacent to euhedral growth terminations; the h igh concentrations of immobile elements in these selvages indicate tha t these fractures closed by collapse and penetration of quartz crystal s into wall rock. Analysis of chemical composition on either side of f our large euhedral growth veins and whole rock analysis of slates acro ss the Kodiak Formation reveal local depletion of silica adjacent to v eins but no evidence for long-distance silica transport within the sys tem. Both crack-seal and collapse textures are observed in a regionall y extensive vein system that displays a regular geometry, with thin, c losely spaced (0.5-3 cm), near-vertical crack-seal veins that connect vertically and laterally with thicker euhedral growth veins arranged i n widely spaced (similar to 500 mm) southeast dipping en echelon sets. The mesoscopic distribution and textural variability of the vein netw ork suggests that the development of the vein system involved early nu cleation and growth of vertical hydrofractures. As the fracture densit y increased, arrays of fractures locally provided zones of weakness an d southeast dipping brittle-ductile shear zones nucleated. These en ec helon cracks remained open and provided small reservoirs of fluid. Tex tures show that en echelon fractures remain open but periodically grow by upward and downward propagation. Crack tips are then sealed with l ocally derived silica, and fluid drains back into en echelon fracture arrays. This local fluid movement is punctuated by less frequent event s where the system links up over a greater distance, fractured reservo irs become interconnected, and the fluid within reservoirs is drained upward or laterally. Periodic inflation and deflation of en echelon ar rays may reflect periodic slip on crosscutting faults and rupture of t he seals that separate reservoirs.