CRUSTAL-SCALE DETACHMENT WITHIN AN ACCRETIONARY SETTING, SEIAD COMPLEX, RATTLESNAKE CREEK TERRANE, KLAMATH MOUNTAINS

The Seiad metaophiolite complex (Rattlesnake Creek terrane, Klamath Mountains; N. California, S. Oregon) records a history of tectonism in an accretionary setting. The complex consists of ultramafic rocks, amphibolites, marbles, and quartzites that are deformed at amphibolite- to granulite-facies conditions. We propose that a crustal-scale detachment exists within the Seiad complex, which is localized along a ~60 m thick marble unit. The marble unit records high strain, including well-developed kinematic sense-of-shear indicators and sheath folds. The marble and interlayered quartzites separate ultramafic complexes with different petrological and microstructural characteristics. Structurally above the detachment is the Kangaroo harzburgite and dunite, which contains relict fabrics that are similar to those in unmetamorphosed ophiolite sections. Fabrics in the marble and the overlying Kangaroo unit are not parallel at their contact, suggesting that they are separated by a fault. Structurally at the base of the detachment lies a discrete fault, below which are the West Fork ultramafic complex (lherzolite, dunite, chromitite, wehrlite, and pyroxenite), Lily Pad harzburgite and dunite, and amphibolites. The West Fork and Lily Pad units are highly deformed and have experienced significant deformation at granulite-facies conditions (~7 kbar; 700-750 °C). Further, the foliations of the West Fork complex strike nearly orthogonal to the structurally overlying metamorphosed sedimentary rocks. The West Fork complex shows a stronger CPO, smaller grain size, and more strongly developed fabric than does the Kangaroo unit. Together, these structural relations indicate that detachments may occur at deeper levels in accretionary settings within rheologically weak lithological horizons such as marble. Further, the microstructural fabrics in the West Fork complex illustrate how mantle rocks deform at granulite-facies conditions in a crustal setting.