SCALY FABRIC DEPLETION WITHIN THE GHOST ROCKS MÉLANGE IN RELATION TO FAULT HEALING AT SUBDUCTION ZONE INTERFACES

A geochemical investigation of a shear zone within the Ghost Rocks Formation of the Kodiak, Accretionary Complex illustrates the importance of dissolution, cracking, and mineral redistribution during deformation along an ancient subduction interface. The shear zone is 10’s of centimeters wide and marks the footwall of an imbricate thrust at the base of a sliver of pillow basalt and within the mudstone-sandstone mélange. Thus, the shear zone represents the stepping down of the décollement into the underthrusting oceanic crust, or underplating by duplex accretion. In the Ghost Rocks Formation, evidence of repeated fracturing and healing can be observed within the mélange terranes. The mélange displays anastomosing microfault networks—called scaly fabrics—within the mudstone matrix that have undergone localized mineral depletion and volume loss. This study characterized the local redistribution within the shear zone using a combination of SEM backscatter imagery, x-ray diffraction data, and LA-ICPMS data to compare depleted scaly fabrics with adjacent parent rock. Findings suggest a significant loss of mobile elements and reduction in volume in the scaly fabric zones of the mudstones compared with the adjacent wall rock. The sample, taken from adjacent to the contact with the pillow basalt, also displays high chlorite and albitization that suggests metasomatism. The redistribution of material out of scaly fabrics and into cracked sandstone blocks, and the potential introduction of material from nearby greenstone to the mélange, document the importance of pressure solution and mineral reactions during interseismic deformation along the subduction plate interface at temperatures associated with the seismogenic zone (200-250˚ C).