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

OVERVIEW OF THE STRUCTURE

MAJOR ROCK UNITS

The rocks of the eastern core are characterized by lithologic monotony and pervasive penetrative deformation. The rocks are mostly shale, siltstone, and sandstone, but have in minor amounts conglomerate, basalt, basaltic volcaniclastic rock, diabase, and gabbro. The sedimentary rocks have been variously metamorphosed to slate, semischist, and phyllite, mostly in the pumpellyite, prehnite-pumpellyite, and low-rank greenschist facies of regional metamorphism. Basaltic rocks are now greenstones or greenschists. Sandstones are mostly feldspathic to volcanic subquartzose sandstones (in the terminology of Crook, 1960). Graded beds, bottom structures, and rhythmic bedding suggest that the sandstones are turbidites.

Core units form long, irregular curved packets roughly convex eastward (fig. 2). They vary from relatively intact interbedded sandstone and slate to completely disrupted broken formations of foliate sandstone or semischist in a matrix of slate or phyllite. Although we mapped most unit boundaries on the basis of lithology, units are separated locally by wide zones of intensely sheared rock. In places, faulting is clearly indicated by truncation of folds in the sandstone of one unit by slate of an adjoining unit (see Tabor and others, 1972). Because of these relations and the ages based on fossils (see below), we mapped major faults at the boundaries of the major units. Probably there are many more faults than shown.

FIGURE 2.—Geologic map of the eastern core. (click on image for an enlargement in a new window)

The four major structural units of the eastern core are the Needles-Gray Wolf lithic assemblage, the Grand Valley lithic assemblage, the Elwha lithic assemblage, and the western Olympic lithic assemblage (fig. 2; Tabor and Cady, 1978). Fossils are very sparse in the eastern core; probable ages based on fossils are summarized in table 1. Because the units are broken formations and some fossils are in probable tectonic blocks, we do not know whether the rocks containing the fossils have moved long distances tectonically into the units in which they are now found. A few have not moved far, for the general lithology of the surrounding terrane is the same as that of local rocks containing the fossils.

TABLE 1.—Probable ages of units in the eastern core and adjacent peripheral rocks on the Olympic Peninsula

On the basis of whole-rock potassium-argon ages, regional metamorphism culminated in the growth of new minerals about 29 million years ago. A later episode of fault brecciation and quartz veining (along faults during uplift?) took place about 17 million years ago (Tabor, 1972).

FAULTS BOUNDING THE CORE

The highly disrupted core rocks appear to be separated from the peripheral rocks by anastomosing steeply dipping faults that we interpret to be thrust faults (figs. 2, 3). Some of these faults are between units of contrasting rock types; one such fault, the Hurricane Ridge fault, separates micaceous lithic to feldspathic subquartzose sandstone of the Needles—Gray Wolf lithic assemblage in the core from volcanic-rich lithic subquartzose sandstone associated with the Crescent Formation. Shearing associated with the Hurricane Ridge fault is well exposed on the Hurricane Ridge Road (fig. 4), on Mueller Creek northeast of Gray Wolf Ridge, and along a logging road up Boulder Creek east of Mount Stone. An unnamed westward extension of the Hurricane Ridge fault separates the Crescent Formation from relatively undeformed sandstone and shale north of the western core. This extension is a moderately dipping thrust fault that has moved the Crescent Formation over younger sedimentary rocks similar to peripheral rocks that lie stratigraphically above the Crescent Formation to the north (P. D. Snavely, Jr., N. S. MacLeod and J. E. Pearl, written commun., 1972).

The continuation of the Hurricane Ridge fault in the southern Olympic Mountains separates rocks of less contrasting lithology. Mica is common in both peripheral sedimentary rocks and in core units (Tabor and Cady, 1978), but the fault can be traced along slaty tectonic-breccia zones such as in Slate Creek near Staircase and northwest of Capitol Peak, near the head of the Wynoochee River. Most of the rocks in the southeastern core area are intensely sheared; neither core units nor bounding faults can be traced far in them. To the south near Lake Quinault, the faults that bound the core units probably merge through this zone of intense deformations to form a fault zone at the base of the peripheral rocks (fig. 3).

FIGURE 3.—Major folds and faults on the Olympic Peninsula. Geology modified from Tabor and Cady (1978). (click on image for an enlargement in a new window)

FIGURE 4.— Beds sheared off by weakly developed cleavage in zone of disruption marking the Hurricane Ridge fault zone, Hurricane Ridge Road, south of Mount Angeles.

The southern fault zone is critical to the interpretation of structure here. The faults shown on the map tend to truncate southeast- to south-trending structures and lithologic units. This tendency is especially evident west of Mount Stone (fig. 2), where a prominent shear zone northwest of the high basalt ridge forming Mount Stone and other peaks truncates major core units. On the northern side of the core, where units and structures tend to curve around parallel to the peripheral basaltic horseshoe, the recognition of tectonic truncation is more difficult, except farther west where fossils are more abundant.

The extension of faulting southwest of Lake Quinault is speculative because the area is completely covered by glacial deposits. By analogy to the faulted contact between the highly deformed core rocks and the less deformed peripheral rocks elsewhere and interpretation of drill-core data and aeromagnetic surveys (P. D. Snavely, written commun., 1976), a fault or complex of faults probably extends southwestward to the coast.

The Calawah fault zone (Gower, 1960) along the north margin of the western core cuts only core rocks where it is best exposed east of Sappho as a wide zone of sheared argillite set with many blocks of sandstone, conglomerate, and basaltic volcanic rocks. Westward from near Sappho, the probable extension of the Calawah fault separates highly deformed core rocks from peripheral rocks (P. D. Snavely, Jr., N. S. MacLeod, and J. E. Pearl, written commun., 1974). Eastward, the Calawah fault appears to splay to the southeast into several faults separating the slaty units of the eastern core.

SIGNIFICANCE OF TOP DIRECTIONS

Of great significance to the structural interpretation of the Olympic Mountains is the distribution of bedding tops. In the peripheral rocks, sedimentary beds and the pillow basalts of the Crescent Formation generally top away from the core, although locally there are a few folds (fig. 3). Within the rocks of the eastern core, about 23 percent of the beds measured yielded top data such as graded beds, ripple marks, load casts, and pillows in basalt. Of the known tops, about 70 percent face north, east, and southeastward, away from the core. If each lithic assemblage or structural unit is considered separately, about the same proportion of tops faces away from the core. This suggests that the core rocks are older coreward or westward, but the best paleontologic evidence indicates that in general the rocks are as young or younger westward and axially to the horseshoe, with one possible age reversal in the Elwha lithic assemblage (table 1). This enigmatic relation supports the interpretation that the major rock units are separated by faults (Tabor and others, 1970), as indicated by the penetrative deformation within the units and local severe disruption at their margins.

TECTONIC VERSUS SOFT-SEDIMENT SLUMP STRUCTURES

Many of the structures of highly disrupted beds found in rocks of the Olympic core could have originated through soft-sediment slumping. We did not find specific evidence to identify olistrostromes, although it is likely that they were common in the original depositional environment. The slaty cleavage and recrystallization, closely associated with the folding, indicate that most of the structures in the eastern core are closely related to the metamorphism of the core rocks and are therefore tectonic.