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

SUMMARY OF THE DEFORMATION

By our analysis, deformation of the rocks of the Olympic Mountains can be summarized as four episodes:

(1) The thick sequence of lower Tertiary sandstone and shales with thin interbeds of pillow basalt (fig. 27A) was isoclinally folded along roughly northwest-southeast trending axes, then faulted, imbricated, and overturned westward (fig. 27B). Imbricate faulting without major folding would also produce the pattern seen today. Rocks may not have been highly lithified during early stages of deformation, but cleavage could have developed very early (Moore and Geigle, 1974, p. 509). The folding and imbrication may well have begun before deposition of Oligocene rocks.

(2) Continued deformation under east-west compression pressed the core rocks into an arc of basaltic rocks that was forming in response to deformation or was already extant from the initial distribution of sea-mounts. The easternmost core rocks were bent into the horseshoe fold on the north but were mostly sheared off below the basalt on the south and southeast along the southern fault zone.

(3) Deformation continued, and as the pile of sedimentary rocks, now thoroughly lithified, became even more constrained by the basaltic horseshoe, which may have ceased yielding, the core rocks yielded upward and outward by shear folding (fig. 27D). An overall mushroomlike dome, highly asymmetric to the east and northeast, developed with a fan of cleavage. The conspicuous pencil structures formed where the new cleavage intersected older, deformed bedding and cleavage. In much of Domain East, the sense of shear in this last stage of deformation was opposite that of the earlier stages. Any one of the east-dipping shear planes in the eastern core that had undergone eastward underthrusting in the early stages of deformation was now bent over eastward, and movement continued as eastward overthrusting.

There is no way to tell from data in the eastern core if the deformation process was continuous or sporadic, although Tertiary unconformities on the west (Snavely and Pearl, 1975) suggest distinct episodes of deformation.

(4) The final uplift of the Olympic Mountains could well have been isostatic because of the thickened pile of sedimentary rocks. This uplift would increase the overturning of structures on the east, especially if there was considerable differential uplift in the center of the range as suggested by uplift to higher elevations of rocks of the highest metamorphic grade (see Tabor, 1972, p. 1811).