METAMORPHISM AND DEFORMATION IN THE EASTERN SAN JUAN ISLANDS

Structural analysis and Ar/Ar dating of the metamorphic fabric in Mesozoic rocks in the eastern San Juan Islands (SJI) suggest revisions to the history of the SJI thrust system. Foliation (D1-D3) in metagreywackes on Samish, Eliza, Jack, and Lummi islands is related to shallowly plunging reclined isoclinal folds, pressure solution, and HP/LT metamorphism. There is sparse evidence of ductile non-coaxial shear, suggested by predominance of NE-vergent folds. Ar/Ar ages of white mica samples constrain the age of metamorphism and D1-D3 deformation. The main fabric is dated at 153.7 ± 1.1 Ma on Samish Island and 136.9 ± 2.2 Ma on Eliza Island. The Ar/Ar age of a third sample is ~340 Ma, interpreted as the age of detrital mica. Ar spectra are complex, and also suggest Ar loss in Late Cretaceous time. These ages are broadly similar to 125 Ma dates reported by Brown and Lapen (this meeting) from Lopez Island. Similarities in lithology, geochemistry metamorphism, and structure suggest the metasedimentary rocks on Samish, Jack, and Eliza Islands represent part of a single terrane, and the age of the metamorphic fabric further suggests that they are equivalent to the Shuksan Suite of the northwest Cascades. Textural reconstitution, recrystallization, and the number of mappable foliations increase down the plunge of an inferred SE-plunging regional antiform. The presence of lawsonite at lower structural levels and actinolite at higher levels also suggests metamorphic T increases structurally upward. These patterns are interpreted to be due to post-metamorphic imbrication. Our results suggest that Late Jurassic to Early Cretaceous metamorphism in the SJI predates Late Cretaceous (84-100 Ma, Brandon et al., 1988) juxtaposition of terranes. Terrane juxtaposition in the study area occurred during D4 faulting, which postdates foliation development. The largest D4 faults strike WNW, juxtaposing Fidalgo ophiolite (Decatur terrane) above metagraywacke on Guemes, Cypress, and Lummi Islands. Later brittle events include: ~N-S extension probably related to uplift; kink folds reflecting generally NW-SE shortening, and post-Eocene strike-slip faulting and regional folding about NW-SE axes. Open folding about E-W axes and reactivation of faults may reflect the current stress regime.