Palinspastic restoration of the Olympic orocline and its implications

Abstract

The topographic grain and bedrock structure of the Olympic Mountains define a convex-landward horseshoe shape in map view (the Olympic orocline) in the upper plate of the Cascadia subduction zone. This map curvature has been explained as a product of orogen/trench-normal deformation due to accretion and underplating. The basaltic Crescent Formation of the Siletzia terrane outcrops around the periphery of the Olympic Mountains and continues south, underlying much of the “Oregon forearc block.” Regional GPS and paleomagnetic studies demonstrate that the limbs of the Olympic orocline record opposite rotations during overall northward motion relative to cratonic North America. Here, we compile paleomagnetic and structural data to show that the Olympic orocline is a vertical-axis fold. We palinspastically undo the fold, showing that oroclinal folding accommodated 110 km of shortening of the Cascadia forearc. We suggest that subduction underthrusting initially produced a margin-parallel, oceanward-verging fold belt. Oroclinal folding resulted from pinning of the deformed, northward-translating forearc against the Coast Range buttress to the north. Oroclinal bending of the upper plate resulted in the landward retreat of the hinge zone away from the trench, giving rise to passive upward warping of the downgoing Juan de Fuca oceanic lithosphere beneath the growing orocline. Our model provides a deformation sequence that explains the Olympic Mountains as a result of margin-parallel translation and shortening of the Cascadia forearc.