STRUCTURAL ARCHITECTURE OF THE FIDALGO OPHIOLITE ON FIDALGO ISLAND, NORTHWESTERN WASHINGTON

Fidalgo Island in northwestern Washington is underlain by a succession of oceanic and arc-affinity igneous and sedimentary rocks that constitute a melange and overlying sedimentary succession dismembered and imbricated during the mid-Cretaceous. The lowest unit is an argillite-basalt matrix melange, containing olistolithic to olistostromal blocks of tonalite, keratophyre, gabbro, peridotite, chert, and basalt. The melange is overlain in angular unconformity by a Jurassic to Cretaceous clastic sedimentary succession. Melange blocks and parts of the argillite-dominated matrix were silicified, fractured (locally to cataclasite), and veined prior to the onset of early penetrative deformation. Within the melange, a penetrative foliation (S_1a) formed and varies substantially in orientation. First structures formed as localized shear zones developed primarily within the melange matrix and at block boundaries. Shear zones contain carbonate and quartz dominated veins and are locally transposed into shallow variable orientations by a subsequent penetrative foliation (S_1b) that is best developed within argillite units and which crosscuts both sets of veins. The overlying sedimentary succession is at least 500 m thick and passes upward from a basal breccia containing clasts of keratophyre, gabbro, tonalite, chert, and basalt into argillite containing Late Jurassic radiolarians. The argillite is overlain by poorly-sorted greywacke and conglomerate with clast populations similar to those of the basal breccia. The conglomerate fines upward into a massive to bedded, feldspathic-lithic arenite and greywacke that yields mid-Cretaceous detrital zircons. The sedimentary succession and underlying melange are deformed by NNE- to NW-trending close folds with a well-developed axial planar cleavage (S₂). The melange and sedimentary rocks are structurally overlain by a thrust sheet composed of porphyritic tonalite that was emplaced prior to and/or during D₂ deformation during the mid-Cretaceous. All units are further deformed by three generations of penetrative structures that are pervasive throughout the San Juan Island region of northwestern Washington. The successively younger WNW to NW, NE, and E-W to WNW striking folds have foliations that cross-cut the earlier structural fabrics and faults.