UC Santa Cruz

The Duck Creek mélange is one of a series of diapiric mud intrusions into the slope cover sequence of the Cascadia Accretionary Wedge. The diapir originated deep within the accreted Hoh sequences and has intruded approximately 1 km up into the 2–3 km thick Pliocene Quinault slope cover sequences at its current level of exposure in coastal seacliffs. Analyses of the geometry and kinematic indicators at the current exposure level strongly suggest that this part of the diapir intruded at a low angle to form an inclined sill or dyke-like body at least 0.5 km in diameter. Microstructural analysis indicates a zonation of deformational features between the core and margins of the diapir resulting from an evolving series of fabrics and the increasing concentration of strain towards the margins. Specifically, we interpret early independent particulate flow (IPF) fabrics as indicating deformation in a low effective stress environment with the possibility of turbulence during initial mobilization and intrusion. The subsequent transition to cataclastic flow indicates deformation occurring in the presence of increasing effective stress, and the creation of broad marginal shear zones suggests strain hardening. The final transition to IPF fabrics limited to narrow strain softened marginal shear zone suggests decreasing effective stresses during the latest intrusion phase. In addition, the abundance of veining phases associated with the latter part of the intrusion history indicates that the diapir became a major fluid conduit for mineralizing fluids toward the end of its emplacement history.