Geological Survey Professional Paper 1033 The Structure of the Olympic Mountains, Washington—Analysis of a Subduction Zone

PLATE MARGIN TECTONICS

The sedimentary history and style of deformation in the western core led Stewart (1970, p. 66) to propose plate margin deformation in the Olympic Mountains. The severe disruption of the eastern core rocks and the overall progression of ages from oldest to youngest westward, even though tops indicate eastward younging, is strong evidence for subduction (Maxwell, 1974, p. 1195). Recent studies of trench deposits have developed a scenario of deformation that in part at least fits the history of deformation in the Olympic core rocks (see Grow, 1973, fig. 6; Karig and Sharman, 1975, figs. 2, 7; Moore and Karig, 1976, p. 1266-1267). A generalized section (fig. 32) based on a figure from Karig and Sharman (1975, p. 3791) illustrates how Olympic core rocks may have been emplaced in the accretionary prism. The absence of blueschist-facies rocks and ultramafic rocks is evidence that Olympic rocks were accreted high in the subduction zone, not dragged down the descending slab or mixed with mantle material.

FIGURE 32.—Generalized section through Olympic Mountains at plate margin. Modified from Karig and Sharman (1975).

There is no exact analogy between Olympic rocks and typical island-arc systems because massive accumulations of submarine basalt similar to the probable seamounts of the Crescent Formation (Snavely and Wagner, 1963, p. 3-5; Lyttle and Clarke, 1975) close to the continental margin (see Cady, 1975, p. 579-580) have not been reported elsewhere. In the alternative model proposed by Glassley (1974, p. 792) and Glassley, Lyttle, and Clarke (1976), oceanic basalt of a ridge or intraplate seamount occurring in the lower part of the Crescent Formation is juxtaposed by faulting with Hawaiian Island-type basalt of the upper part of the Crescent Formation. This interpretation implies that at least the lower part of the Crescent Formation was partially subducted in the early stages of deformation. Whatever the origin of the basalt, its great mass provided a resistant bulwark to the subducted sediments. The deformation history of the eastern core, beginning with imbricate underthrusting and ending with mushroom-like shear folding, is appropriate for an accreted mass of sediments pushed against a rigid mass. The less intensely deformed peripheral sedimentary rocks accumulated back (east) of the subducted part of the accretionary prism, protected from severe deformation by the rigid horseshoe of basalt.

This summary of deformation simplifies a very complex process. There may have been considerable overlap of each stage. Deformation of lower and middle Eocene rocks of the eastern core may well have begun while upper Eocene and Oligocene rocks were still being deposited to the west. Underthrusting and imbrication in the western core may have continued while late-stage shear folding and doming progressed in the eastern core.

We believe that this study has successfully analyzed the very complex deformation of an exposed subduction zone, mainly owing to the large amount of data and the availability of computers and programs to handle it. Broken formations and melanges elsewhere that appear inordinately complex structurally may yield a coherent deformational scheme through statistical structural analysis.