COMPARISON OF DEFORMATION IN TWO ACCRETIONARY WEDGE TERRANES, CENTRAL SAN JUAN ISLANDS, WASHINGTON

Within the San Juan Thrust System, Washington, two contrasting terranes, the relatively unmetamorphosed, arc-affinity Fidalgo Complex and the HP/LT, MORB affinity Ocean Floor terranes, have been juxtaposed during Late Cretaceous orogeny. Structural analysis within both terranes is used to address the deformational history of the FC and OF terranes, investigate a mélange zone that separates FC/OF rocks as a possible terrane boundary, and examine the tectonic evolution of an accretionary prism setting.

The FC and OF terranes have experienced ductile, and multiple episodes of brittle, deformation. A marked difference exists between terranes regarding the timing and intensity of ductile deformation. Early ductile features characteristic to the OF terrane are not found within the FC. Although rotation of structures has occurred, kinematics and timing of brittle deformation are found to be consistent across both terranes and with recent studies in other SJI areas. A progression of dominantly north-south thrusting is followed by scattered subhorizontal extension related to normal faulting and extension veins both of which are followed by strike-slip faulting. Domainal grouping of extension axes is found across both terranes, but regional scatter is attributed to folding and dominant, east-west oriented, left-lateral strike-slip motion. Brittle structures within mélange zones have similar kinematics but also include several highly sheared, anastomosing zones of serpentinite with evidence for dip-slip and strike-slip motion. These dismembered zones are interpreted as a possible terrane boundary with a localized reactivated strike-slip component. Within the OF terrane, XRD results indicate the presence of aragonite and prehnite (constraining deformation to >20km and 200°C) within extension veins and veins associated with normal faults. The FC lacks veins with HP mineralogy suggesting brittle deformation was also active at shallower depths in the accretionary wedge. Additionally, significantly less veining is associated with the FC suggesting lower fluid volume and/or fluid pressure higher in the wedge. Future results from paleostress analysis may help to identify stress orientations for each brittle deformation phase and relate these to paleo-plate boundary and accretionary wedge conditions.