VEINS IN THE OLYMPIC SUBDUCTION COMPLEX: IMPLICATIONS FOR THE STRESS FIELD IN THE PALEO-CASCADIA ACCRETIONARY WEDGE

The Olympic Subduction Complex (OSC) is an ancient part of the modern Cascadia accretionary wedge, uplifted and now exposed in the central Olympic Mountains. The Lower OSC comprises clastic sediments that were underthrust and underplated beneath the wedge to ~275°C and ~20 km depth. This implies that the Lower OSC should preserve geological records reflecting characteristics of shear zones associated with subduction thrust faults. However, unlike other ancient accretionary complexes, mineral veins in this area have not been extensively investigated to validate this statement. Here, we report our field and microstructural observations of veins in the Lower OSC. Quartz slickenfibres are commonly found along the NE-dipping foliation planes, indicating a top-SW shear direction, consistent with the subduction direction of the Juan de Fuca Plate at the latitude of the Olympic Mountains. Crack-sealing textures, such as wall rock inclusion bands parallel to vein margins, are observed microscopically in veins that develop parallel to foliations. This suggests that fractures opened perpendicular to the foliations, indicative of a stress state with a sub-vertical σ3 and near-lithostatic pore fluid pressure. Vein studies in other exhumed underthrust sediments recording similar maximum burial temperatures to those of the Lower OSC, such as the Arosa Zone in the Central Alps and the Makmine mélange in the Shimanto belt of Japan, report similar findings. Conversely, sub-vertical extension veins perpendicular to the foliations of the matrix are found in sandstone boudins of the Lower OSC. Their presence reflects an opposite stress field characterized by a sub-vertical σ1. An interpretation of low differential stress and transiently rotating principal stresses during seismic cycles has been suggested for sediments subducted to the inner wedge. The same explanation may apply to the Lower OSC, providing a framework to reconcile the coexisting but conflicting geological records. In summary, our preliminary analysis of veins in the Lower OSC shares similarities with those observed in other exhumed underthrust sediments. Further structural and geochemical studies on them can reveal the hydrogeology and deformation kinetics in the paleo-Cascadia accretionary wedge, providing valuable insights into the subduction processes of the modern Cascadia subduction zone.