Evolving Early Mesozoic Convergent Margin Deformation, central Klamath Mountains, northern California

Abstract

The Stuart Fork terrane in the central Klamath Mountains of northern California is a coherent Late Triassic blueschist belt which records early plate convergence in western North America. Complex deformation and polyphase metamorphism of the terrane directly reflect evolving convergent margin processes. Two major periods of deformation are recorded in the Stuart Fork terrane: D1, an early (Late Triassic) phase of synmetamorphic deformation which produced complex folds and possible southwest directed reverse faults, and D2, a late stage (mid‐Jurassic) deformation which formed simple, open folds as a result of westward thrusting of the entire terrane over neighboring terranes during regional shortening. D2 thrust imbrication of the Stuart Fork and adjacent terranes also produced a mélange zone characterized by disrupted tectonic slivers of these terranes. Each of these episodes of shortening is related to convergence, and at least one (D1) accompanied high‐pressure blueschist facies metamorphism during subduction. More importantly, however, it is possible to distinguish deformation features which formed during initial subduction from those which formed as a result of final thrusting (or accretion) of the entire assemblage to the continental margin as regional contraction progressed through the arc and forearc region. The Stuart Fork terrane is a lithologically and structurally distinctive lithotectonic assemblage which cannot be directly correlated with other terranes of the western Paleozoic and Triassic subprovince. Nonetheless, the Stuart Fork and other similar terranes of the central Klamath Mountains share a common origin as oceanic materials which were accreted against the continental convergent margin in mid‐Mesozoic time during east directed subduction.