The Mesozoic Rimrock Lake inlier, southern Washington Cascades: Implications for the basement to the Columbia Embayment

Abstract

The Mesozoic Rimrock Lake inlier lies between the North Cascades, Blue, and Klamath Mountains, and thus has significant implications for the basement to the Columbia Embayment. The inlier consists of the Late Jurassic Indian Creek complex and the voluminous Late Jurassic-Early Cretaceous Russell Ranch complex. The Indian Creek complex consists of trondhjemitic to gabbroic rocks, some of which underwent dynamo-thermal medium-grade metamorphism that was probably broadly synchronous with plutonism. Lithologies and geochemical data indicate that the complex represents the roots of an arc. The Russell Ranch complex is a tectonic melange, consisting mainly of arkose and mudstone, with subordinate chert, conglomerate, and pillowed greenstone, and minor green tuff. Deposition may have occurred on an inner fan or the inner part of a mid-fan. Sandstones were derived from a plutonic terrane, presumably a dissected are. The geochemistry of greenstones apparently is most compatible with eruption as within-plate tholeiites or MORB. The complex displays a block-in-matrix mesoscopic fabric; greenstone and the foliated Twin Sisters Lakes silicic metavolcanic unit occur as kilometer-scale blocks. Predominantly steep faults riddle the complex. Overlapping time relations between normal and thrust faults, several generations of folds, and cleavages record protracted deformation. Steep faults separate the units and are bracketed between Early Cretaceous and earliest Eocene, as is deformation of the Russell Ranch complex. These are mostly brittle structures, but one mylonite zone has a gentle stretching lineation suggestive of strike slip. Large displacements probably occurred between the Indian Creek and Russell Ranch complexes, but an original basement and cover relationship cannot be precluded. The complexes are broadly correlative with rocks to the north. The Late Jurassic are, represented by the Indian Creek complex, may include rocks in other inliers in central Washington, the melange belts of north-western Washington, and the San Juan Islands. The Russell Ranch complex most closely resembles parts of the western melange belt and the Constitution Formation. Late Jurassic arc-type rocks also occur in the Klamath and Blue Mountains, and the Russell Ranch complex may correlate with the Yolla Bolly terrane of the Franciscan Complex. These correlations suggest that lithological belts are more continuous between Washington and California than implied by models which propose major tectonic dispersal. The Mesozoic arc-type crust and continentally derived clastic rocks in the inlier demonstrate that the basement to the Columbia Embayment includes a variety of tectonic slices or terranes and that the embayment is not simply underlain by Tertiary or Mesozoic oceanic crust as is commonly proposed.