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

The marked horseshoe outcrop pattern of the Eocene basalts on the Olympic Peninsula has long fascinated geologists. Weaver (1937, pl. 2) was the first to recognize this pattern; in his interpretation, the peninsula was traversed by a large east-southeast-trending anticline, a concept that has persisted to the present. The rocks in the core of this fold were thought to be mostly pre-Tertiary (Weaver, 1937, p. 17-18), probably Cretaceous (Danner, 1955, p. 24-27). The structural complexity of the mountainous core became apparent as small areas were investigated in detail (compare McMichael, 1946; Danner, 1948; Harvey, 1959; Lindquist, 1961; Hawkins, 1967; Miller, 1967). Park (1950) and Walter Warren (written commun., 1955) cited evidence that core rocks included younger strata and anticipated some of the structural complexities described here.

We began systematic mapping of the mountainous core in 1961 (Cady, Tabor, MacLeod, and Sorenson, 1972; Cady, Sorenson, and McLeod, 1972; Tabor and others, 1972; Tabor, 1975; Tabor and Cady, 1978). Our study and unpublished work of the U.S. Geological Survey done earlier by Walter Warren, Charles Park, and others indicates that: the mountainous core of the range is mostly, if not all, Tertiary in age; much of the core is about the same age or younger than the partly encircling Eocene basalts; the rocks are highly deformed, having been thickened by imbricate faulting, and, in the eastern part, underwent multiple shear folding. Deformation probably took place in an accretionary prism during underthrusting of the continent by an oceanic plate.

In this report we describe and analyze the megascopic and macroscopic structures in homogeneous domains and subdomains of the mountainous core and develop a tectonic model for their formation.

SUMMARY OF REGIONAL GEOLOGY

Two major geologic terranes dominate the Olympic Peninsula. Surrounding the mountainous core on three sides is a horseshoelike belt of early and middle Eocene, mostly oceanic, basalt, the Crescent Formation of Brown, Gower and Snavely (1960, fig. 1). Overlying the basalt on the north, east, and south are upper Eocene to Miocene and minor Pliocene marine sedimentary rocks. The peripheral sedimentary rocks are fossiliferous and, though folded and faulted, are in general stratigraphically continuous. Within the arms of the basaltic horseshoe, the core consists of two major terranes. The western core of Eocene to Miocene rocks is nonslaty and at least locally includes coherent areas of rocks that are for the most part stratigraphically continuous. Complex folds and faults are common, and some areas are so totally disrupted that the rocks have the aspect of melange (Weissenborn and Snavely, 1968, p. F8-F9; Koch, 1968; Stewart, 1970; Rau, 1973, 1975; Tabor and Cady, 1978).

FIGURE 1.—Major geologic terranes of the Olympic Peninsula. (click on image for an enlargement in a new window)

The slaty rocks of the eastern core are Eocene to early Oligocene in age. They are pervasively sheared and best characterized as broken formation in the terminology of Hsu (1968, p. 1065-1066). At many places, the rocks resemble melanges described in the Franciscan rocks of California (Page, 1966, p. 260; Hsu, 1969; Suppe, 1973, p. 17-22; Cowan, 1974, p. 1625-1628), but the Olympic core rocks lack blocks of recognizable exotic material. And there are no blueschists or eclogites. Although there are a few small feldspathic peridotite dikes, no mantle material has been recognized. Nevertheless, because of the oceanic basalt and the structural complexity, numerous workers have suggested that the Olympic core rocks were emplaced during subduction of oceanic lithosphere (Stewart, 1970, p. 66; 1971, p. 201; McKee, 1972, p. 166-169; Rau, 1973, p. 10-11; Glassley, 1974, p. 792-793; Snavely and Pearl, 1975; Tabor, 1975, p. 33-36; Cady, 1975, p. 578-581; Cheney and Stewart, 1975; Tabor and Cady, 1978).

This report is focused on the slaty rocks of the eastern core. One might question whether conventional structural analysis techniques can be applied to melange or broken formation where shearing may have destroyed any order or symmetry commonly displayed by folded beds. As we will show, although the imprint of shearing is dominant everywhere in the eastern core, the tectonic fabric has considerable order and symmetry.

PROCEDURE AND ACKNOWLEDGMENTS

We collected most of the structural data used in this analysis between 1961 and 1972. The Mount Angeles, Tyler Peak, and The Brothers quadrangles and local smaller areas such as the region north of Staircase (fig. 2) have more data than the remaining area, which was mapped in reconnaissance fashion.

Data was stored on punch cards and plotted in projection on the lower hemisphere of an equal-area net with the help of an IBM 360-65 computer (see section "Supplemental Information"). We thank G. K. Muecke and H. A. K. Charlesworth, of the University of Alberta, for giving us their structural plotting program (see Muecke and Charlesworth, 1966). We combined their program with a storage and retrieval program designed and operated with the help of computer programmer Ming Ko and other specialists of the U.S.G.S. computer facility.

We thank the following workers for considerable help in collecting the structural data in the field: John Alan Bartow (1965), Wyatt Gilbert (1965), Robert Koeppen (1972), Norman MacLeod (1961-62), Kenneth Pisciotto (1971), Eduardo Rodriguez (1972), Martin Sorensen (1964-68), Richard Stewart (1967-71), Robert Tallyn (1969-70), and Robert Yeats (1968-69).

We have spent many hours discussing Olympic structure with our colleagues, in particular William Glassley, Norman MacLeod, Parke D. Snavely, Jr., Richard J. Stewart, and Robert Yeats. M. Clark Blake, Jr., Richard J. Stewart, and Parke D. Snavely, Jr., made many helpful suggestions. We are particularly indebted to Ronald Kistler and Robert Loney for considerable and patient guidance in the structural analysis.