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

TECTONIC FABRIC

CLEAVAGE

Cleavage is the dominant structural element of the eastern core. It is less pronounced in rocks of the northeastern part, where bedding is relatively consistent and continuous; southwestward, beds are increasingly disrupted by cleavage. In the most disrupted areas, bedding and other sedimentary features are preserved in isolated blocks in the sheared matrix of slate and siltstone (fig. 5).

FIGURE 5.—Block of undisrupted thin-bedded sandstone and slate (foreground) in broken formation in the western Olympic lithic assemblage. Tightly appressed isoclinal folds are found in some thin-bedded blocks like this one. Subdomain 17, southwest side of Mount Olympus.

We have not made a distinction between shear cleavage and fracture cleavage in this analysis, but most cleavage is parallel to axial planes of folds (fig. 6) and many folds are highly attenuated.

FIGURE 6.—Shear fold in thin-bedded sandstone with slate core. Axial-plane cleavage is alined with hammer handle. Note divergence of bedding and cleavage in sandstone and irregular shape of fold. Subdomain 17, southwest of Mount Olympus, upper South Fork of the Hoh River.

The hinges of folds are commonly sheared off by movement along cleavage (fig. 7), and, in some areas, thin beds of sandstone are alined in a crisscross pattern (fig. 8A), suggesting that all hinges have been sheared off; the beds are realined roughly parallel to the cleavage. Sandstone lenses and large blocks of thinly bedded sandstone and slate may display divergent beds or folded beds, truncated by the cleavage in slate or phyllite enveloping the sandstone (figs. 6, 8B) or folds and beds may be in total chaos (fig. 8C).

FIGURE 7.—Sheared-off sandstone bed in weakly developed slate, The structure probably began as a clockwise drag fold. North of subdomain 16, south side of Mount Appleton.

FIGURE 8.—Notebook sketches of disrupted beds in sandstone and slate. A, Crisscross beds in thin-bedded slate; folded beds sheared off and juxtaposed. Subdomain 6, north side of Hurricane Ridge. B, Sandstone blocks and lenses in slate with bedding partially rearranged by movement along cleavage. Subdomain 10, northwest of Mount Anderson. C, Parts of folds and beds juxtaposed by movement along slaty cleavage. Subdomain 14, east side of Chimney Peak.

The scale of disruption of bedding by movement along cleavage ranges from outcrop dimensions (figs. 9, 10, 11) to entire mountainsides (figs. 5, 12), although its prominence may well be dependent on the bedding and lithologic features of the original rocks. In outcrops of slate, where bedding is visible in siltstone laminations, isoclinal folding is commonly evident, and the beds are not disrupted on an outcrop scale. In very thick beds of sandstone and slate, disruption is recognizable only at map scale (fig. 2). The most severe disruption is found in thinly bedded sandstone and slate where each outcrop bears many small tectonic lenses (compare fig. 11). Where the rock is strongly sheared, it is commonly crisscrossed by many curving cleavages and studded with blocks and lenses (fig. 9).

FIGURE 9.—Highly disrupted zone with blocks and lenses of sandstone in contorted slate matrix. Subdomain 13, southeast of Muncaster Mountain.

FIGURE 10.—Tectonic lenses of sandstone in phyllite. Subdomain 8, southwest side of Mount Barnes.

FIGURE 11.—Small tectonic blocks of sandstone in slate. The cleavage runs left to right and the rock has fractured parallel to a quartz vein (parallel to plane of photo) which has weathered differentially, leaving quartz caps on the sandstone clasts. Subdomain 14, east of Mount Christie, head of Buckinghorse Creek.

FIGURE 12.—Disrupted beds and tectonic lenses of sandstone in slate, Subdomain 17, west side of Mount Olympus.

Whereas pelitic rocks through the area of this study display fairly well developed slaty cleavage, the cleavage in sandstone is more subtle. Development of cleavage or low-rank schistosity in sandstones, as in other rocks, progresses from northeast to southwest, mostly independent of the units mapped (fig. 2). The most highly schistose and more recrystallized rocks occur in the south-central part of the area in subdomains 8 and 10 and the northern parts of subdomains 13 and 14 (see fig. 25).

The development of cleavage (schistosity) and new minerals in sandstones can be best seen in thin section. In the initial stages of metamorphism (or diagenesis), clastic irregular haloes and wisps of white mica and chlorite grow around grains of quartz and feldspar. Where penetrative deformation increases, the interstitial micas are smeared out into thin layers anastomosing between lithic fragments and microaugen of quartz and feldspar. Lithic fragments and plagioclase are replaced more and more by white mica (and calcite and chlorite). Clastic grains are crushed and lose their identity. As deformation and recrystallization progressed, the micas apparently became more abundant and both quartz and mica more segregated. In the most metamorphosed sandstones, the semischists, a schistose mica fabric prevails with rare carugen of quartz and plagioclase-bearing relict clastic textures (see Tabor, 1972, p. 1812).

FOLDS

Folds of outcrop scale are common throughout the area, although hinges are hard to find and many folds are revealed only by opposing tops of beds. Folds in thinly bedded rocks most commonly have sharp hinges and are open to tightly appressed (figs. 13, 14). Fold hinges in thick-bedded sandstones are more rounded (fig. 15).

FIGURE 13.—Recumbent fold (left) juxtaposed by faulting with inclined fold (right). Subdomain 5, northwest of Grand Pass.

FIGURE 14.—Sharply hinged fold in thick-bedded sandstone with thin slate interbeds. Axial plane cleavage and a second cleavage form pencil structure in the slate beds. Subdomain 5, northeast shoulder of McCartney Peak.

FIGURE 15.—Fold with rounded hinge in sandstone with dark siltstone laminations. Fold is partially sheared off along right limb. Subdomain 5, north side of Mount Cameron.

Pelitic material appears to have been highly mobile. In folds with tightly appressed limbs, the core material is almost totally squeezed out. Folded folds are common (fig. 16), and the juxtaposition of fold hinges along shears (or small faults) make a chaotic terrane (fig. 13). The prevalence of axial-plane cleavage and flowage of material into the crest of similar folds indicates that most folding is shear folding.

FIGURE 16.—Sketch of folded isoclinal fold in slate bed. Subdomain 16, northwest of Mount Olympus.

Very few large folds greater than outcrop scale have been found in core rocks; they appear to be more common in peripheral rocks and in rocks of the western core (fig. 3). A large fold, of drag-fold form and measuring several kilometers across the limbs, crops out west of Mount Constance (fig. 2) in subdomain 12 (see section at end of text "Frequency Diagrams for Subdomains"); a poorly developed draglike fold lies east of Steeple Rock in subdomain 1, and a moderately large fold is exposed on the ridge of Mount Anderson (fig. 17). As very few individual folds or structures can be related to particular phases of folding by style or orientation, we do not know how these large-scale folds fit into the tectonic sequence. The shapes of the fold and steeply plunging axes suggest that they are simply large versions of the small folds.

FIGURE 17.—Large overturned drag fold on Mount Anderson viewed from the south, subdomain 14. Data on the tops of beds show the folding although most of the hinges are sheared off. Sketch shows probable shape of the fold, axis plunges about 45° S. 35° W.

Folds in cleavage are numerous in subdomains 8 and 10 (fig. 25) and southward. The larger cleavage folds are cylindrical and open (fig. 18). A later cleavage parallels axial planes of folds in the early cleavage. Small crinkle folds on cleavage surfaces are especially prominent in subdomain 10. The fold axes of the crinkles tend to parallel larger fold axes in cleavages and pencil structures (see below and fig. 30). Locally crinkle folds are folded.

FIGURE 18.—Folds in cleavage. Traces of (younger) axial-plane cleavage in these folds can be seen to left and below hammer handle. Phyllite in subdomain 10, south ridge of Mount Norton.

Conventional structural analysis depends heavily on field recognition of the relative ages of folds by their style, superposition, or orientation. We were unable to recognize various generations of folds or other structures by style. That the Olympic core rocks have been subjected to several episodes of folding is shown by crinkle folds, folds in cleavage, and a few isolated folded folds and by cleavage girdles, axial-place girdles, and fold-axis girdles in most plots of the structural elements (see section at end of text "Frequency Diagrams for Subdomains").

PENCIL STRUCTURES

The most eye-catching, consistently oriented, and characteristic structures in the eastern core of the Olympic Mountains are thin slivers of rock or pencils, formed by the intersections of either two or more cleavages or cleavage and bedding. The pencils range in length from a few centimeters to 1 or 2 m (figs. 19, 20, 21). They are prominent in slate but also occur in sandstone, where they are blockier and less perfectly formed.

FIGURE 19.—Pencil structures in red limestone. Pencils parallel to wooden pencil; bedding strikes from left to right and dips steeply away from plane of photograph. Subdomain 7, northeast of Grand Pass.

FIGURE 20.—Pencil structures in slate. Subdomain 5, northwest of McCartney Peak.

FIGURE 21.—Large pencils in slate and siltstone at high angle to steep bedding and fold axis. Subdomain 5, ridge north of Mount Cameron.

In many outcrops, especially in the western and northeastern parts of the eastern core, pencil structures lie in the bedding; in the central part, especially where several generations of cleavage occur, pencils do not lie in bedding (figs. 19, 21, 22) but stand almost perpendicular to fold axes or the crests of folds. These pencils athwart fold axes are clearly formed by two cleavages and are probably later formed than their associated folds. Because of their consistent orientation (homogeneity) relative to other structures, pencils appear to be late-formed structures, but we observed a few outcrops where pencils are folded with the cleavage.

FIGURE 22.—Crude pencils parallel to axial plane of a fold but roughly perpendicular to bedding and a nearly horizontal fold axis. Subdomain 5, northwest of Grand Pass.

The orientations of pencils plotted in figure 24 are direct measurements of pencil bearings and plunge. We found that plotting intersecting cleavage and bedding, especially where they intersect in a small angle, gave unreliable pencil orientations. A small error in the measurement of two nearly parallel planes leads to a very large error in orientation of their intersection. The strong maximum of pencil orientations (fig. 30C) is produced partly by the large amount of data collected in the axial region of the eastern core where pencils are particularly well formed.

STRETCHED CLAST LINEATIONS

In subdomains 3, 8, 9, 10, and 14, foliated sandstones, especially those of the Elwha lithic assemblage, commonly show a strong linear fabric of uniformly distributed slate chips, 1-2 mm long. Foliated granule conglomerates display a similar lineation (fig. 23). These lineations tend to parallel pencils (fig. 30C) in the same area.

FIGURE 23.—Lineated granule conglomerate. Subdomain 9, east of Ludden Peak.