Fault-bounded slices of allochthonous Paleozoic to Mesozoic bedrock of the San Juan Islands in northwest Washington provide a locality in which to study terrane translation and ductile and brittle deformation in an accretionary wedge setting, as well as the factors involved in preservation of blueschist-facies terranes. This study contributes to understanding of the tectonic evolution of the Lopez Structural Complex, a major Late Cretaceous terrane-bounding fault zone in the San Juan Thrust System. Structural study is combined with X-ray diffraction and fluid inclusion analysis to constrain the relative timing, kinematics, and P-T conditions of fabric formation and post-fabric brittle deformation in the Lopez Structural Complex.
Deformation is characterized by multiple generations of ductile and brittle structures. After formation of a regional flattening fabric by two processes, pressure solution and localized bi-directional shearing, the area was crosscut by brittle structures including: 1) early strike-slip structures related to bi-directional northwest/southeast shear, 2) southwest-vergent thrusts, 3) extension veins and normal faults related to northwest/southeast extension, and 4) conjugate strike-slip structures also related to northwest/southeast extension. The presence of comparable structures in the eastern San Juan Islands (Lamb and Schermer, 2003) and adjacent to the Lopez Structural Complex indicates this sequence of brittle deformation is widespread in the San Juan nappes.
X-ray diffraction and petrographic observations of vein material identify aragonite- and prehnite-bearing veins associated with thrust, normal, and strike-slip structures, although most of the latest strike-slip faults contain only prehnite and calcite. High pressure minerals constrain brittle deformation to have occurred at greater than ~ 20 km depth and at most ~ 200° C. Fluid inclusion analyses of aqueous and methane-bearing inclusions in vein quartz are in agreement with low temperature conditions after vein formation and tentatively suggest rapid exhumation to shallow depths under near isothermal conditions.
Sustained high-pressure - low-temperature conditions and the preservation of widespread aragonite are possible only if structures formed in an accretionary prism during active subduction. Therefore, meter-scale brittle structures in rocks of the Lopez Structural Complex record a pattern of internal wedge deformation at depth or early during uplift of the San Juan nappes. The sequence observed is generally consistent with internal orogen-normal contraction and vertical thickening followed by vertical thinning and lateral along-strike extension. Possible mechanisms for brittle deformation include a change in the angle or magnitude of plate convergence vectors, a large underplating or accretion event, or the collision of the Cretaceous prism with the margin of Wrangellia
