title: Structural Geology of the Central San Juan Islands, Northwest Washington
Creators: name: Belanger, Todd I.
Publisher: name: Western Washington University
Year: 2008

description: <p>Within the Late Cretaceous San Juan thrust system, northwest Washington, tectonically juxtaposed melange separates the relatively unmetamorphosed, arc-affinity Fidalgo Complex and the high pressure-low temperature, MORB-affmity Ocean Floor terrane. Structural analysis. X-ray diffraction, and fault-slip analysis are used to constrain the relative timing, kinematics, and pressure-temperature conditions of postfabric ductile and brittle deformation in the Fidalgo Complex, Ocean Floor, and melange. Structural analysis of the melange, Fidalgo Complex, and Ocean Floor terranes help to constrain the tectonic development of the structurally highest terranes in the San Juan thrust system and provide insight into the mechanisms required for uplift of high-pressure terranes in accretionary wedge settings.</p>
<p>The Fidalgo Complex and Ocean Floor terranes have experienced multiple episodes of both ductile and brittle deformation. The intensity of ductile deformation is different in each terrane with ductile deformation characteristic to the Ocean Floor terrane and not the Fidalgo Complex. Folds of bedding and the regional fabric in the Ocean Floor are crosscut by a sequence of brittle faults that include reverse faults followed by extension veins, normal faults, and lastly, strike-slip faults. Kinematic axes from reverse faults show consistent north-south shortening and subvertical extension. Normal faults are consistent with subvertical shortening and average north-south extension. Kinematics of strike-slip faults are scattered and could not be resolved at the scale of the study area. The timing and kinematics of brittle deformation are found to be consistent across the Ocean Floor and Fidalgo Complex and with recent studies in the Lopez Structural Complex and eastern San Juan Islands (Gillaspy, 2005; Lamb, 2000). However, scattered kinematic directions and inconsistent structures are common to each stage of faulting in the central San Juan Islands. Stress inversion of strike-slip data is in agreement with calculated strain axes, but suggests intermixing of structural stages due to poor controls on crosscutting relations.</p>
<p>Scattered kinematics observed in all stages of brittle faulting and inconsistent structures in the strike-slip stage are not simply restored or explained by later block rotations or folding. Examination of the assumptions inherent to the calculation of kinematic axes and stress inversions implies that homogeneous stress and independent slip may not be valid assumptions for fault-slip data in the San Juan Islands.</p>
<p>The preservation of the HP-LT mineral aragonite, identified by X-ray diffraction, in veins associated with reverse faults and extensional structures constrains deformation within the Ocean Floor terrane to >20km and ~200°C. Strike-slip faults do not contain aragonite. Blocks within the melange contain aragonite-bearing structures but are derived from the Ocean Floor terrane. The Fidalgo Complex lacks veins with HP-LT mineralogy. The structures and mineralogy of the Fidalgo Complex and Ocean Floor terranes and melange indicate that brittle deformation was active at various depths throughout an accretionary prism. The character of meter-scale brittle deformation, pressure-temperature conditions, and presence of melange at the Ocean Floor-Fidalgo Complex terrane boundary are compatible with observations in the Coast Ranges, California and in western Baja California, Mexico. Therefore, it is possible that similar mechanisms of uplift, related to oblique subduction and orogen-parallel extension, affected the Ocean Floor terrane and Fidalgo Complex. The Buck Bay fault and faults bounding the melange zones are possibly normal faults related to uplift of the Ocean Floor terrane and its juxtaposition with the Fidalgo Complex.</p>