In order to address the heterogeneous character of the Decatur terrane, the rocks that occur within the easternmost San Juan Islands, Washington, are described and compared with respect to lithology, metamorphism, and structure. Based on these observations, a local structural and tectonic history is proposed.
The exposed bedrock in the south part of the study area (Guemes, Huckleberry, Hat, Dot, and Saddlebag Islands) consists primarily of an arc-volcanic intrusive complex (diorite with crosscutting mafic to felsic dikes) as well as ultramafic cumulates of dunite, Iherzolite, and wehrlite. Most of the igneous rocks have no deformational fabric and are relatively unmetamorphosed, containing only very low-grade veins of quartz, calcite, and prehnite. These intrusive and ultramafic rocks are probably correlative with the Fidalgo Igneous Complex of Brown (1977).
Highly deformed oceanic turbidites occur in the central and north part of the study area (Jack, Eliza, and Samish Islands). These rocks consist of chert-rich lithic-quartzofeldspathic metagraywacke with interbedded metasiltstone and phyllite, along with minor intercalated metatuff. The metasedimentary rocks are all distinguished by the presence of at least one well-developed deformational fabric, with textural zones ranging from TZ 2A to TZ 3A-. Metamorphic minerals include abundant white mica, pumpellyite, prehnite, and lesser actinolite. Ar/Ar ages of two white mica samples indicate a Late Jurassic to Early Cretaceous age for the metamorphic fabric (~137 to ~154 Ma). The Ar/Ar spectrum of white mica from a third sample is interpreted to indicate a Carboniferous detrital age. Based on their overall similarities in lithology, geochemistry, metamorphic minerals, metamorphic age, and structure, the metasedimentary rocks are interpreted to represent part of a single terrane, and the apparent age of the metamorphic fabric suggests that they may be equivalent to the westernmost Shuksan Suite, which lies to the immediate east of the study area (Gallagher,1986; Gallagher and others, 1988).
Along the northeast shoreline of Guemes Island, there is a zone of intensely disrupted and intermixed lithologies that appears to separate the metasedimentary (Shuksan-type?) terrane to the north from the Fidalgo Igneous Complex to the south. This zone is interpreted to be a possible terrane-bounding fault and at this location, the Fidalgo Igneous Complex appears to lie structurally above the metasedimentary (Shuksan) package.
In spite of many similarities within the metasedimentary (Shuksan-type) rocks, there are significant differences in fabric development and there appears to be an eastward and southward increase in textural reconstitution, number of fabrics, and possibly metamorphic age within the study area. Since this direction is also down the plunge of an inferred SE-plunging regional antiform, this suggests that the earliest and most recrystallized rocks are now at the structurally highest levels. Internal imbrication and structural inversion of a once coherent stack is proposed to explain this pattern.
An inferred structural history for the study area is as follows: (1) early metamorphism, ductile deformation, and development of metamorphic fabric(s) within the metasedimentary rocks during the Late Jurassic to Early Cretaceous (~137 to ~154 Ma); (2) juxtaposition of the metasedimentary package (Shuksan Suite) with the plutonic rocks and ophiolite (Fidalgo Igneous Complex), probably during the Late Cretaceous thrust event of Brandon and others (1988) and producing an early stage of (north- and east-vergent?) thrust faults; (3) normal/extensional faulting (generally top-to west and south), which is possibly related to uplift; (4) relatively late (Tertiary?) strike-slip faults and regional folds about northwest-trending axes; (5) very recent (post-Tertiary?) folding about east-west trending axes
