~ I-· l1c~ S 11 ~lt t'~lttt ~<- ~ < l f. tJ 1 t' 

JUL 31 1978 

PLEAS;: RETU1N 

JQ j..i2.~ .. \~~r: 

< )lyrn pic :\fountains, \Yashington--

~- \ 11 ~ 1l \-s i s c l f · ~ t S 11l J r ltt (, 1 i ( J t 1 Zr > r 1 t ~ ., 

:_ J,-..~i~-,;.: 

... ~~. ·~ 
The Structure of the 

Olympic Mountains, Washington-. 

Analysis of a Subduction Zone 

By R. W. TABOR and W. M. CADY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1033 

UN I TED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1978 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Tabor, Rowland W. 

The structure of the Olympic Mountains, Washington. 

Geological Survey Professional Paper 1033 

Bibliography: p. 24-25. 

1. Geology-Washington (State)-Olympic Mountains. I. Cady, Wallace Martin, 1912- joint author. 

II. Title. III. Series: United States Geological Survey Professional Paper 1033 

QE176.038T3 557.97'94 76-606187 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock No. 024-001-03079-9 
CONTENTS 

Page 

~bstract____________________________________________________ 1 

Introduction ------------------------------------------------ 1 

Summary of regional geology ---------------------------- 1 

Procedure and acknowledgments ------------------------ 3 

Structural geology of the eastern core ------------------------ 3 

Overview of the structure________________________________ 3 

Major rock units------------------------------------ 3 

Faults bounding the core ---------------------------- 5 

Significance of top directions ------------------------ 5 

Tectonic versus soft-sediment slump structure ________ 7 

Tectonic fabric ___ :_______________________________________ 7 

Cleavage ------------------------------------------ 7 

Folds ---------------------------------------------- 10 

Structural geology of the eastern core-Continued 

Tectonic fabric-Continued 

Page 

Pencil structures ____________________________________ 10 

Stretched clast lineations ____________________________ 12 

Major structural terranes ________________________________ 12 

Multiple folding ________________________________________ 14 

Early folding-formation of the core units ------------ 14 

Late folding and doming-formation of the pencils ____ 19 

Summary of the deformation ---------------------------- 22 

Plate margin tectonics ---------------------------------- 23 References cited ____________________________________________ 24 

Supplemental information----------------------------------- 28 

Frequency diagrams for subdomains ______________________ 28 

Computerized structural diagram program---------------- 36 

ILLUSTRATIONS 

Page 

FIGURE 1. Sketch map showing major geologic terranes of the Olympic Peninsula ------------------------------------------ 2 

2. Geologic map of the eastern core ------------------------------------------------------------------------------ 4 

3. Sketch map showing major folds and faults on the Olympic Peninsula-------------------------------------------- 6 

4-8. Photographs showing: 

4. Beds sheared off by cleavage in zone of disruption, Hurricane Ridge fault zone south of Mount ~ngeles ____ 7 

5. Block of undisrupted thin-bedded sandstone and slate in broken formation, western Olympic lithic assemblage 8 

6. Shear fold in thin-bedded sandstone with slate core, southwest of Mount Olympus ------------------------ 8 

7. Sheared-off sandstone bed in weakly developed slate, south side Mount ~ppleton -------------------------- 8 

8. Sketches showing beds in sandstone and slate disrupted by cleavage______________________________________ 9 

9-15. Photographs showing: 

9. Blocks and lenses of sandstone in contorted slate matrix, southeast of Muncaster Mountain ---------------- 9 

10. Tectonic lenses of sandstone in phyllite, southwest side Mount Barnes------------------------------------ 9 

11. Small blocks of sandstone in slate, east of Mount Christie ________________________________________________ 11 

12. Disrupted beds and tectonic lenses in slate, west side Mount Olympus ------------------------------------ 12 

13. Recumbent fold juxtaposed by faulting with inclined fold, northwest of Grand Pass ------------------------ 12 

14. Sharply hinged fold in sandstone with slate interbeds, northeast shoulder McCartney Peak ---------------- 12 

15. Fold with rounded hinge in sandstone with siltstone laminations, north side Mount Cameron --------· _____ 12 

16. Sketch showing folded isoclinal fold northwest of Mount Olympus---------------------------------------- 12 

17. Large overturned drag fold on Mount ~nderson -------------------------------------------------------- 13 

18-23. Photographs showing: 

18. Folds in cleavage, phyllite, south ridge Mount Norton -------------------------------------------------- 14 

19. Pencil structures in limestone northeast of Grand Pass -------------------------------------------------- 14 

20. Pencil structures in slate, northwest McCartney Peak -------------------------------------------------- 14 

21. Large pencils in slate and siltstone at high angle to bedding and fold axis, north of Mount Cameron ________ 15 

22. Crude pencils parallel to axial plane of fold northeast of Grand Pass---------.----------------------------- 15 

23. Lineated granule conglomerate, east of Ludden Peak :... ___________________________________________________ 15 

24-32. Sketches showing: . 

24. Pencil lineations in the eastern core-------------------------------------------------------------------- 16 

25. Domains and subdomains eastern core __________________________________________________________________ 17 

26. Principal structural elements for each subdomain ·------------------------------------------------------ 18 

27. Development of eastern core by folding, faulting, and shear folding _____________ :.. ________________________ 19 

28. Ninety-five poles to bedding in volcanic rocks in the Crescent Formation __________________________________ 19 

29. Summary diagrams, structural elements in Domain West_ _______________________________________________ 20 

30. Summary diagrams, structural elements in Domain East ------------------------------------------------ 21 

31. Intersection of principal shear cleavage, late stage deformation with girdle of early fold axes and constructed 

kinematic axis------------------------------------------------------------------------------------- 22 

32. Generalized section through Olympic Mountains at plate margin ---------------------------------------- 23 

33. Computer form for structural data ______________________________________________________________________ 37 

34. Computer output for structural program---------------------------------------------------------------- 38 

III 
IV 

TABLE 

CONTENTS 

TABLES 

Page 

1. Probable ages for units in the eastern core and adjacent peripheral rocks on the Olympic Peninsula ---------------- 5 
THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTONANALYSIS OF A SUBDUCTION ZONE 

By R. W. TABOR and W. M. CADY 

ABSTRACT 

Long thin packets of structurally disrupted rocks in the eastern 

core of the Olympic Mountains mostly top eastward, but the overall 

age of the rocks decreases westward, suggesting early folding around 

subhorizontal axes with imbricate thrusting or imbricate thrusting 

alone. 

Continued east-west compression overturned beds eastward, bending the packets into an arc within the horseshoe bend of the Crescent 

Formation, a foldlike structure that formed as the core rocks were 

imbricated or was extant from the original arcuate distribution of 

basaltic seamounts. In the western part of the eastern core, secondary structures (B elements) (mostly small-scale folds) developed parallel to the steep axis of the fold of the basaltic horseshoe; in the 

southern part of the core, the rock packets were sheared off beneath 

the basalts on the south limb of the horseshoe bend. Later shear 

folding on a cleavage fan oriented parallel to a north-northwesttrending subhorizontal kinematic axis rotated B elements (folds) 

formed earlier, producing widespread late pencil structures, in part 

centripetal to the basaltic horseshoe, where the late cleavage intersected earlier deformed bedding and cleavage. This late folding produced a domelike structure extended asymmetrically eastward. 

The complex structure of the core rocks is consistent with current 

models of accretionary prisms in subduction zones and developed its 

form as the thick mass of volcanic rocks ofthe Crescent Formation to 

the east resisted the eastward movement of the accreted sedimentary 

prism. 

INTRODUCTION 

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 ear]y 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). 

The slaty rocks of the eastern core are Eocene to 

1 
::::::::::::::::::::::::::: 

··························· 

:::::::::;::::::::::::::::: 

....... II. ! • 

OF 

·········· .......... .... . . .......... .... . .......... .... . ::::::::::::::: 

·········· ..... ::::::::::::::: 

............... 

•••••••••• •••• 0 

............... 

•••••••••• •••• 0 

............... 

·········· .... . .......... .... . ::::::::::::::: 

............... 

::::::::::::::: 

·········· ..... ::::::::::::::: 

............... ·········· ..... ·········· .... . .... . ::::::::::::::: 

0 

I 

0 10 

10 

I 

20 MILES 

I 

20 KILOMETERS 

N 

FIGURE I.-Major geologic terranes of the Olympic Peninsula. 

Area of m 

WASHINGTON 

EXPLANATION 

CORE ROCKS PERIPHERAL ROCKS 

Upper Oligocene to middle 

Miocene rocks with small 

areas of Eocene rocks 

along coast 

Eocene to upper 

Miocene rocks 

Upper Eocene and 

Oligocene rocks 

Eocene rocks Early and middle Eocene 

basaltic rocks of the 

Crescent Formation 

Contact 

--?-

Fault 

Queried where uncertain 

\\ \ \ \ \ 

\ \\ \ \ l \ ,' 

Area of well-developed 

slaty cleavage 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 3 

early Oligocene in age. They are pervasively sheared 

and best characterized as broken formation in the terminology of Hsii (1968, p. 1065--1066). At many places, 

the rocks resemble melanges described in the Franciscan rocks of California (Page, 1966, p. 260; Hsii, 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 

ltSupplemental 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 structual 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. 

STRUCTURAL GEOLOGY 

OF THE EASTERN CORE 

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. 

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 
0 

0 

10 MILES 

10 Kl LOMETERS 

EXPLANATION 

Core rocks 

Western Olympic lithic assemblage 

Two, sandstone, foliated sandstone, and 

semischist with less than 40 percent siltstone and slate 

Twos, slate and phyllite with less than 30 

percent sandstone and semischist 

phacoids 

Twot, thick-bedded sandstone with less 

than 20 percent sla.ie . 

Needles -Gray Wolf lithic assemblage 

Tnm, micaceous sandstone with less than 

60 percent slate 

Tnms, thick-bedded micaceous sandstone 

Tn b, basalt, minor diabase, and gabbro 

Tnt, thin-bedded slate and siltstone with 

less than 30 percent sandstone 

Elwha lithic assemblage Grand 

~ 

~ 

Valley lithic 

assemblage Tes, slate and phyllite 

with less than 20 

percent foliated 

sandstone and 

semischist 

Tess, foliated sandstone 

and semischist with 

Tgs, 

10- 50 percent slate Tgss, 

and phyllite 

Tet, thick-bedded foliated 

sandstone, conglomerate, and semischist 

Teg, greenstone and 

greenschist 

Contact 

sandstone and 

foliated sandstone with 40 -

70 percent siltstone, slate, and 

phyllite 

thick-bedded sandstone and folia ted 

sandstone 

FIGURE 2.-Geologic map of the eastern core. 

Rocks of the Southern 

fault zone 

Slate, phyllite, foliated sandstone, semischist, basalt, and 

diabase 

Peripheral rocks 

Sedimentary and 

igneous rocks 

Ts, shale, sandstone, 

and conglomerate 

Tcb, Crescent Formation; 

basalt, diabase, 

and gabbro 

Fault 

Q) 

c:: 

Q) 

~ g Q)~ 

(.) ... 0 Q) 

bOO. 

:.:o. 

O::s "0 

c:: 

~ 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 5 

TABLE !.-Probable ages of units in the eastern core and 

adjacent peripheral rocks on the Olympic Peninsula 

Unit 

Crescent Formation and associated sedimentary rocks. 

Needles-Gray Wolf lithic assemblage. 

Grund Valley lithic assemblage. Elwha lithic assemblage. 

Western Olympic lithic assemblage. 

Age Fossil evidence and reference 

Peripheral units 

Early and middle Eocene 

la~~ o~~~~ 

Many microfossils, some megafossils, coccoliths (Rau, 1964, p.G3--G4; Tabor and others, 1972; Cady, 

1972; P.D. Snavely, Jr., N.S. MacLeod, 

and J.E. Pearl, written commun., 1974). 

Core units 

Late Eocene------

Tertiary----------

Early and middle Eocene. 

Late Eocene to early Oligocene. 

Some microfossils, a few megafossils 

(Cady and MacLeod, 1963; Cady, Tabor, MacLeod and Sorenson, 1972). 

Very sparse microfossils (Tabor and others, 1972). 

Very sparse microfossils (Tabor and others, 1972). 

Lithologic continuity with fossilif- erous rocks to west and northwest (Gower, 1960; R.J. Stewart, written commun., 1970; Harvey, 1959, p. 45-46; P.D. Snavely, Jr., N.S. MacLeod, and J.E. Pearl, written commun., 1974). 

moved far, for the general lithology of the surrounding 

terrane is the same as that of local rocks containing the 

fossils. 

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 NeedlesGray Wolf lithic assemblage in the core from volcanicrich 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). 

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. Snav.ely, 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 ro~ks (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 
6 

Cape 0 Flattery 

THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

124° 

~ZEITELAK£ 

0 10 20 MILES 

I I 

0 10 20 KILOMETERS 

EXPLANATION 

......,.......?....,...... 

Basaltic rocks of the 

Crescent Formation 

Folded thrust fault 

Queried where uncertain. 

Sawteeth on upper plate 

High-angle fault 

Dashed where inferred 

--tSyncline 

Showing direction of plunge 

Anticline 

Showing direction of plunge 

Overturned anticline 

Showing direction of dip of limbs 

45'?'GLarge-scale drag fold 

Showing plunge of axis 

FIGURE 3.-Major folds and faults on the Olympic Peninsula. Geology modified from Tabor and Cady (1978). 

N 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 7 

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. 

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. 

TECTONIC FABRIC 

CLEAVAGE 

Cleavage is the dominant structural element of the 
8 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

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). 

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. 

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). 

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 recogFIGURE 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. 

nizable 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). 

Whereas pelitic rocks through the area of this study 

display fairly well developed slaty cleavage, the cleavFIGURE 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. 

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. 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 9 

age 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 ~he south-central part of the area in subdoFIGURE B.-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. 

mains 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. 

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. 
10 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

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 augen 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). 

Pelitic material appears to have been highly mobile. 

FIGURE 11.-Small tectonic blocks of sandstone in slate. The clea vage 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 Bucking horse Creek. 

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 . 

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 offolding 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. 

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 offolds 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. 

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. 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 11 

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

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. 

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. 

FIGURE 13.-Recumbent fold (left) juxtaposed by faulting with inclined fold (right). Subdomain 5, northwest of Grand Pass. 
12 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

STRETCHED CLAST LINEATIONS 

In subdomains 3, 8, 9, 10, and 14, foliated sandstones, especially those ofthe 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. 

MAJOR STRUCTURAL TERRANES 

We selected two major structural domains and 19 

subdomains (fig. 25) on the basis of pencil orientation 

because it is the most consistent structural element 

(fig. 24). Although some of the subdomains roughly 

coincide with the major core units, most are unrecognizable in the field. The boundary between the principal domains, Domain West and Domain East, is recognizable in the field by opposing ·dips and plunges. 

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. 

1 meter 

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 FIGURE 16.--Sketch of folded isoclinal fold in slate bed. Subdornain 

shoulder of McCartney Peak. 16, northwest of Mount Olympus. 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 13 

Planar structures east of the boundary dip west and 

southwest, and most pencils plunge westward. In contrast, planar structures west of the boundary dip to the 

east and northeast, and pencils plunge eastward (fig. 

26). Many structures along the boundary have steep to 

vertical dips. Pencils in subdomains 1 and 2 (east ofthe 

boundary) actually plunge south to southeast, but the 

direction of plunge rotates smoothly and becomes dominantly westward approaching the center of Domain 

East. Subdomains 16, 8, and 10 best illustrate the 

progressive change from east to west dip of bedding 

and cleavage and from east to west plunge of pencils 

and other linear structures. 

Subdomains 1 and 2 extend beyond the core into the 

peripheral rocks. In subdomain 1, only 18 percent of 

the data fall outside the core, and these are mostly 

bedding. In subdomain 2, about 50 percent of the data 

come from the peripheral rocks; for that reason, we 

broke subdomain 2 into two small domains, 2A and 2B, 

separated by the Hurricane Ridge fault. Except for 

cleavage, not present in most ofthe peripheral rocks in 

subdomain 2A, structural elements do not suggest that 

the peripheral rocks of this area have undergone a 

structural history highly different from the adjoining 

core rocks. As will be shown, the difference is mostly 

one of intensity of deformation. 

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 45o S. 35°W. 
14 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

Elements of the structure are summarized for each 

subdomain in figure 26. The near-coplanar orientation 

of all the structures (note the near parallelism of principal beds, cleavage, and axial planes in most subFIGURE 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. 

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. 

domains) is to be expected m rocks where cleavage 

dominates. 

MULTIPLE FOLDING 

EARLY FOLDING-FORi\IATIO:'\ OF THE CORE C"!ITS 

The arcuate structural packets of broken formations 

that make up the eastern core of the Olympic 

Mountains probably formed when the depositional sequence (fig. 27 A) was folded and faulted (fig. 27B, C). 

A scheme of this sort, though highly simplified, can be 

used to explain both the distribution of rocks of various 

ages in the eastern core units and the overturning of 

beds away from the core. In figure 27C, the west-facing 

limbs of the major folds are shown sheared off. The 

style offolding and sense of yielding in the early stages 

of the deformation (fig. 27B) are suggested by structures in the western core, where folds are overturned 

westward (Stewart, 1970, pl. 1; Rau, 1975), and on the 

northwestern peninsula, where the Crescent FormaFIGURE 20.-Pencil structures in slate. Subdomain 5, northwest of 

McCartney Peak. 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 15 

tion is thrust over younger rocks (P. D. Snavely, Jr., 

written commun., 1976; fig. 3). Eastward underthrusting without large-scale folding could also account for 

FIGURE 21.-Large pencils in slate and siltstone at high angle to 

steep bedding and fold axis. Subdomain 5, ridge north of Mount 

Cameron. 

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 structures in the eastern core. Considerable disruption of bedding probably took place in this early stage 

of large-scale folding and or thrusting. The formations 

were broken. 

Horizontal fold axes or other B lineations formed at 

this stage are not evident in the eastern core. As we 

will show, early fold axes have been rotated by later 

shear folding. The arcuate bend of the core units did 

form in this early stage, for the bend controls the orientation of later-formed structures, and the bend indicates that the basaltic horseshoe was forming or already extant. This horseshoe could have formed during 

this early stage of deformation, or, as an initial reentrant formed by the arcuate distribution of several volcanic seamounts (see Cady, 1975, p. 575), it could have 

been enhanced as the rocks of the core, predominantly 

sedimentary, were pressed into and against the more 

rigid basalt. 

The similarity of structural fabric in the peripheral 

rocks (subdomains 2A and 2B) adjacent to the main 

fault separating core rocks from peripheral rocks is to 

be expected in this scheme (see discussion of "Major 

Structural Terranes"). Although the rigid mass of 

basalts protected the peripheral rocks from being 

highly deformed, they were acted upon by the same 

forces. 

The entire eastern core has the aspect of a large 

steeply plunging fold sheared off on the south side. Although there is no independent way of showing that 

FIGURE 23.-Lineated granule conglomerate. Subdomain 9, east of 

Ludden Peak. 
16 

I 

I 

....... 

THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

0-45° 46-79° 80-90° 

Bearing and plunge of pencils 

Contact 

EXPLANATION 

Fault 

N 

0 4 MILES 

I I 

0 4 Kl LOMETERS 

Crescent Fonnation 

Domain boundary 

FIGURE 24.-Pencil lineations in the eastern core . 
16 

STRUCTURAL GEOLOGY OF THE EASTERN CORE 

L- ".l.., v 

"'V > v ""-" < .lv>v~L-" ~ v v 

\" L- '\ 

~ ~ 1.- 0 

L- r- I 

0 ., £. 4 

4 

I 

,..< 

8 Ml LES 

I 

8 Kl LOMETE RS 

FIGURE 25.-Domains and subdomains in the eastern core. 

17 

N 
18 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

the peripheral rocks have been folded into the horseshoe configuration (see Cady, 1975, p. 578), a pole diagram of bedding attitudes in the basaltic horseshoe 

crudely defines a f3 fold axis that plunges steeply eastward (fig. 28). In Domain East, the B elements (fig. 30) 

do not in any way parallel this ccfold." In Domain West, 

the B lineation and f3 axes of beds and cleavage (fig. 29) 

generally correspond to this axis, suggesting that at 

Area of most 

fold axes 

or pencils 

EXPLANATION 

FIGURE 26.-Summary of principal structural elements for each subdomain. Data from section at end of text "Frequency Diagrams 

for Subdomains." 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 19 

WEST EAST some stage in the deformation the basaltic horseshoe 

Lower and middle Eocene 

-- -- -- -- -- A. Depositional sequence prior to folding. 

B. Early folding and position of faults. WO, E, and NG denote 

the western Olympic, Elwha and Needles- Gray Wolf lithic 

assemblages, respectively. 

Oligocene 

/"-..... 

'·-·-':\·-· 

Upper~ 

Eocene\ 

\ 

WO\ 

\ 

\ \ 

\ 

\ 

\ 

\ 

C. After early folding and faulting, major core units established. 

P- pi Is form line to show later deformation In D. 

D. After shear folding, doming, and erosion occurred. Deformation of P- pi exaggerated for Illustration. 

FIGURE 27 .-Development of the eastern core by folding, faulting, 

and shear folding. Diagram is simplified because deformation and 

deposition may have gone on at the same time and may have been 

episodic. 

either influenced structures in Domain West or developed with them. 

The southern fault zone must have developed in this 

stage, for structures parallel to it are rearranged by 

later cleavage (see below) and it seems to be a part of 

the horseshoe fold. Although some of the major rock 

units are truncated by the fault zone, others curve 

around sharply to the southwest to parallel the southern arm of the horseshoe. On an outcrop scale, north- to 

northeast-trending bedding is broken and rotated by 

shear movement along later northwest-trending cleavage; bedding maxima (fig. 26) reflect the rearranged 

bedding even though the gross lithologic units curve 

around to the southwest. Subdomain 18 contains elements of both the fault zone and later northwesttrending cleavage. 

LATE FOLDING AND DOMING-FORMATION 

OF THE PENCILS 

Summary diagrams for Domain East (fig. 30) indicate considerable complexity, but the data reveal major 

features of the structure not as well shown in the subdomains. In Domain East, there is considerable scatter 

of orientations, especially of bedding, cleavage, and 

axial planes, a pattern to be expected in multiple folding. From the field evidence, we have assumed that 

rocks were deformed predominantly by shear folding, 

FIGURE 28.-Ninety-five poles to bedding in volcanic rocks in the 

Crescent Formation within lower 1-3 km of main part of basaltic horseshoe. Overturned beds, mostly in northeast part of horseshoe, were left out. Contours at 1, 2.1, 4.2, 5.3, and 7.4 percent 

per 1 percent area plotted by hand. 
20 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

and we have not considered flexural slip in the following analysis. In figure 31A a selected maximum for the 

summary cleavages-pole diagram (fig. 30B) is plotted 

as a principal cleavage orientation that approximately 

coincides with the partial girdle in the summary diagram for pencils (fig. 30C). This principal cleavage 

orientation is well represented by cleavages-pole maxima in subdomains 1, 2B, 4, 5, 6, 7, 9, and 11. In southern subdomains 13 and 15, few cleavages correspond to 

this orientation. 

Cleavage of the principal orientation is probably one 

of the principal planes bounding the late-formed pencils, hence is itself late formed. It intersects earlier 

planar structures of diverse orientation (mostly earlier 

formed cleavage and subparallel bedding which are 

isoclinally folded as well as partially bent around into 

the arc of the basaltic horseshoe; see strikes of bedding 

and cleavage in fig. 26). The summary fold-axis girdle 

Pole=.B axis0 

-:- D 

A 

(fig. 30E) intersects the principle cleavage in projection 

at a (fig. 31A). Most fold axes represent bedding-plane 

folds that probably formed in the earlier stage in the 

deformation. They have been rotated in a plane that 

defines the girdle (great circle) of figure 30E. Fold-axis 

maxima near a may indicate that the late shear folds· 

are in part isoclinal. Ideally, the late shear folding, if 

isoclinal, rotates earlier formed fold axes into the a 

direction. Intersection a represents a west-plunging 

line (kinematic a) that parallels the direction of movement during the latest stage in shear folding (Weiss, 

1959, p. 100-103). The latest movement in Domain 

East, then, was in general east-west direction on shear 

plan·es (cleavage) dipping southwest. The b kinematic 

axis for this late shear folding should lie in the cleavage, 90° from a (fig. 31A). 

Axes of crinkle folds and folds in cleavage (fig. 30/) 

tend to lie in the synoptic fold-axis girdle, indicating 

c 

209 poles to bedding with contours 

at 1, 2, 3, and 5 percent per 1 percent area 

199 poles to cleavage with contours 

at 1, 2, 3, and 5 percent per 1 percent area 

156 pencils with contours at 1, 2, 6, 10, 

and 13 percent per 1 percent area. Includes 26 pencils from area north of 

subdomain 16 

D 

25 poles to axial planes 

of folds in bedding 

E 

. .. _I_ 

I 

.. 

. . 

. . . 

29 fold axes of folds 

in bedding 

FIGURE 29.-Summary diagrams for structural elements in Domain West. 
2,337 poles to bedding with contours 

at 0.1, 0.5, 1.0, 1.5, 2.0, and 2.5 percent per 1 percent area 

331 poles to axial planes of folds in 

bedding with contours at 0.3, 1, 2, 

3, and 4 percent per 1 percent area 

66 fold axes of folds in cleavage with 

contours at 2, 3, and 5 percent per 

1 percent area 

STRUCTURAL GEOLOGY OF THE EASTERN CORE 

1, 716 poles to cleavage with contours 

at 0.1, 0.5, 1.5, 2.5, and 3.5'percent 

per 1 percent area 

401 fold axes of folds in bedding 

with contours at 0.2, 1, 2, and 

2.5 percent per 1 percent area 

16 long axes of stretched slate 

chips in sandstone and pebbles 

in conglomerate 

1,104 pencils with contours at 1, 2, 3, 

4, and 5 percent per 1 percent area 

F 

I 

40 poles to axial planes of folds in 

cleavage with contours at 3, 5, 10, 

and 13 percent per 1 percent area 

81 crinkle folds in cleavage of 

phyllite and slate 

FIGURE 30.-Summary diagrams for structural elements in Domain East. 

21 
22 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

that they too are earlier formed B structures. As they 

themselves formed after cleavage had formed, they indicate multiple shear folding prior to the last stage of 

shear folding under consideration here. Although 

stretched-clast lineation (fig. 30H) usually coincides 

with the a direction, probably indicating extension in 

the east-west direction, the small amount of data, especially in subdomains to the northeast, precludes recognition of a possible girdle. If the long axis of the clasts 

lay in the plane containing the fold axes, some clasts at 

least would have to parallel earlier formed B elements. 

Summary diagrams for Domain West display a fair 

monoclinic symmetry with a steeply dipping northeast-striking symmetry plane (fig. 29). They are not so 

amenable to the same analysis as Domain East because they contain much less data and the pencil girdle 

is less discrete (fig. 29C), making identification of a late 

cleavage less certain. The summary pencil diagram 

(fig. 29C) contains data from the area north of subdomain 16 (figs. 24, and 25), where pencil structure is 

poorly defined. This area was not otherwise included in 

the analysis, partly because of sparsity of welldeveloped cleavage, but the pencils help define a partial girdle. 

There is considerable continuity in pencil structures 

across the eastern core, suggesting they all formed by 

the same process. The fan of cleavage from east to west 

suggests that the pencils formed together in the latest 

stage of deformation and that, on the west, steeply 

eastward-dipping late cleavage and earlier planar 

A 

a

Selected 

cleavage 

maxima 

structures controlled pencil formation. By analogy to 

the features discussed for Domain East, then, a weak 

girdle of early-formed fold axes (fig. 29E) intersects a 

principal cleavage plane near a possible pencil girdle 

at a (fig. 31B). At 90° from a in the cleavage is the b 

axis of the late shear folding. It is subhorizontal and 

trends north northwest; its proximity to the theoretical 

b axis for Domain East shows the results of the 

analysis for the two domains to be consistent. In Domain West, the a movement direction is roughly parallel to the monoclinic symmetry plane of the total structural fabric, as is required by the symmetry. 

The late shear folding is present in both major domains. The direction of yielding, based on the orientation of a, was steeply up and westward in Domain West 

and more gently up and eastward in Domain East. The 

analysis of only two major domains has generalized 

kinematic direction a into two directions, whereas actually it probably is distributed in a fan from west to 

east, radiating upward from a sub-horizontal northnorthwest-trending b axis. We picture a mushrooming 

dome, with fan of cleavage, extended asymmetrically 

to the east (fig. 27D). 

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. 

B 

FIGURE 31.-Intersection (a) of the principal shear cleavage of the late stage of deformation with girdle of early fold axes and 

constructed b kinematic axis. Ai, Domain east. B, Domain west. 
STRUCTURAL GEOLOGY OF THE EASTERN CORE 23 

27A) was isoclinally folded along roughly northwestsoutheast 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 seamounts. 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 e~stward 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). 

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. 

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 (197 4, 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 

WEST EAST 

FIGURE 32.-Generalized section through Olympic Mountains at 

plate margin. Modified from Karig and Sharman (1975). 
24 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

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. 

REFERENCES CITED 

Brown, R. D., Jr., Gower, H. D., and Snavely, P. D., Jr., 1960, Geology of the Port Angeles-Lake Crescent area, Clallam County, 

Washington: U.S. Geol. Survey Oil an<l Gas Inv. Map OM-203, 

scale 1:62,500. 

Cady, W. M., and MacLeod, N. S., 1963, Geological Survey research 

1963-Summary of investigations: U.S. Geol. Survey Prof. 

Paper 4 75-A, p. A96. 

Cady, W. M., Tabor, R. W., MacLeod, N. S., and Sorensen, M. L., 

1972, Geology of the Tyler Peak quadrangle, Washington: U.S. 

Geol. Survey Geol. Quad. Map GQ-970, scale 1:62,500. 

Cady, W. M., Sorensen, M. L., and MacLeod, N. S., 1972, Geology of 

the Brothers quadrangle, Washington: U.S. Geol. Survey Geol. 

Quad. Map GQ-969, scale 1:62,500. 

Cady, W. M., 1975, Tectonic setting of the Tertiary volcanic rocks of 

the Olympic Peninsula, Washington: U.S. Geol. Survey Jour. 

Research, v. 3, no. 5, p. 573--582. 

Cheney, E. S., and Stewart, R. J., 1975, Subducted graywacke in the 

Olympic Mountains, USA-implications for the origin of Archaean sodic gneisses: Nature, v. 258, p. 60-61. 

Cowan, D. S., 1974, Deformation and metamorphism of the Franciscan subduction zone complex northwest of Pacheco Pass, 

California: Geol. Soc. America Bull., v. 85, p. 1623--1634. 

Crook, K. A., 1960, Classifica,tion of arenites: Am. Jour. Sci., v. 258, 

p. 419-428. 

Danner, W. R., 1948, A contribution to the geology of the Olympic 

Mountains, Washington: Washington Univ., Seattle, M.S. 

thesis, 67 p. 

--1955, Geology of Olympic National Park: Seattle, Olympic 

National History Assoc., Univ. Washington Press, 68 p. 

Glassley, W. E., 1974, Geochemistry and tectonics of the Crescent 

volcanic rocks, Olympic Peninsula, Washington: Geol. Soc. 

America Bull., v. 85, p. 785-794. 

Glassley, William, Lyttle, N. A., and Clarke, D. B., 1976, New 

analyses of Eocene basalt from Olympic Peninsula, Washington-Discussion and reply: Geol. Soc. America Bull., v. 87, p. 

1200-1204. 

Gower, H. D., 1960, Geologic map of the Pysht quadrangle, Washington: U.S. Geol. Survey Geol. Quad. Map GQ-129, scale 1:62,500. 

Grow, J. A., 1973, Crustal and upper mantle structure of the central 

Aleutian arc: Geol. Soc. America Bull., v. 84, p. 2169-2191. 

Harvey, J. L., 1959, Geologic reconnaissance, S. W. Olympic Peninsula: Washington Univ., Seattle, M.S. thesis, 53 p. 

Hawkins, J. W., Jr., 1967, Prehnite-pumpellyite facies metamorphism of a graywacke-shale series, Mount Olympus, Washington: 

Am. Jour. Sci., v. 265, p. 79~818. 

Hsii, K. J., 1968, Principles of melanges and their bearing on the 

Franciscan-Knoxville paradox: Geol. Soc. America Bull., v. 79, 

p. 1063--1074. 

--1969, Preliminary report and geologic guide to Franciscan 

melanges of the Morro Bay-San Simeon area, Californi~: 

California Div. Mines and Geology Spec. Pub. 35, 46 p. 

Karig, D. E., and Sharman, G. F., 3d, 1979, Subduction and accretion 

in trenches: Geol. Soc. America Bull., v. 86, p. 377-389. 

Koch, A. J., 1968, Petrology of the "Hoh Formation" ofTertiary age 

in the vicinity of the Raft River, western Washington: Washington Univ., Seattle, M.S. thesis, 41 p. 

Lindquist, J. W., 1961, Geology and paleontology of the fork area, 

Dungeness and Greywolf River, Clallam County, Washington: 

Washington Univ., Seattle, M.S. thesis, 185 p. 

Lyttle, N. A., and Clarke, D. B., 1975, New analyses of Eocene basalt 

from the Olympic Peninsula, Washington: Geol. Soc. America 

Bull., v. 86, p. 421-427. 

McKee, Bates, 1972, Cascadia-The geologic evolution of the Pacific 

Northwest: New York, McGraw-Hill Book Co., 394 p. 

McMichael, L. B., 1946, Geology of the northeastern Olympic Peninsula, Washington: Washington Univ., Seattle, M.S. thesis, 33 p. 

Maxwell, J. C., 1974, Anatomy of an orogen: Geol.Soc. America Bull. 

v. 85, p. 1195-1204. 

Miller, M.S., 1967, The bedrock geology of the southeast quarter of 

Mount Steel quadrangle, Washington: Washington Univ., 

Seattle, M.S. thesis, 78 p. 

Moore, J. C., and Geigle, J. E., 1974, Slaty cleavage-incipient occurrences in the deep sea: Science, v. 183, p. 509-510. 

Moore, J. C., and Karig, D. E., 1976, Sedimentology, structural geology, and tectonics of the Shikoku subduction zone, southwestern 

Japan: Geol. Sqc. America Bull., v. 87, p.-1259-1268. 

Muecke, G. K., and Charlesworth, H. A. K., 1966;· Jointing in folded 

cardium sandstones along the Bow River, Alberta: Canadian 

Jour. Earth Sci., v. 3, p. 579-596. 

Page, B. M., 1966, Geology of the Coast Ranges of California, in 

Bailey, E. H., ed., Geology of northern California: California 

Div. Mines and Geol. Bull. 190, 508 p. 

Park, C. F., Jr., 1950, Structure in the volcanic-rocks of the Olympic 

Peninsula, Washington: Geol. Soc. America Bull., v. 61, no. 12, 

pt. 2, p. 1529. 

Rau, W. W., 1964, Foraminifera from the northern Olympic Peninsula Washington: U.S. Geol. Survey :prof. ·Paper 374-G, 33 p. 

--1973, Geology of the Washington coast between Point Grenville and the Hoh River: Washington Dept. Natural Resources 

Bull. 66, 58 p. 

--1975, Geologic map of the Destruction Island and Taholah 

quadrangles, Washington: Washington Geologic and Earth Resources Div. Map GM 13, scale 1:62,500. 

Snavely, P. D., Jr., and Wagner, H. C., 1963, Tertiary geologic history of western Oregon and Washington: Washington Div. Mines 

and Geology Rept. Inv. 22, 25 p. 

Snavely, P. D., Jr., and Pearl, J. E., 1975, G~ological Survey research 

1975-Summary of investigations: U.S. Geol. Survey Prof. 

Paper 975, p. 128. 

Stewart, R. J., 1970, Petrology, metamorphism,· and structural relations of graywackes in the western Olympic Peninsula, Washington: Stanford Univ., Stanford, Ph.D. Thesis, 129 p. 

--1971, Structural framework of the western Olympic Peninsula, Washington: Geol. Soc. America Abs. with Programs, v. 3, 

no. 2, p. 229. 

Suppe, John, 1973, Geology of the Leech Lake Mountain-Ball 

Mountain region, California: California Univ. Pubs. Geol. Sci., 

v. 107, 82 p. 

Tabor, R. W., 1971, Origin of ridge-top depressions by large-scale 

creep in the Olympic Mountains, Washington: Geol. Soc. 
REFERENCES CITED 25 

America Bull., v. 82, p. 1811-1822. 

--1972, Age of the Olympic metamorphism, Washington-K-Ar 

dating of low-grade metamorphic rocks: Geol. Soc. America 

Bull., v. 83, p. 1805--1816. 

--1975, Geologic guide to Olympic :National Park: Seattle, Univ. 

Washington Press, 144 p. 

Tabor, R. W., Cady, W. M., and Yeats, R. S., 1970, Broken formations 

and thrust faulting in the northeastern Olympic Mountains, 

Washington: Geol. Soc. America Abs. with Programs, v. ~'no. 2, 

p. 152. 

Tabor, R. W., Yeats, R. S., and Sorensen, M. L., 1972, Geologic map of 

the Mount Angeles quadrangle, Washington: U.S. Geol. Survey 

Geol. Quad. Map GQ-958, scale 1:62,500. 

Tabor, R. W., and Cady, W. M., 1978, Geologic map of the Olympic 

Peninsula: U.S. Geol. Survey Map I-994, scale 1:125,000. · 

Weaver, C. E., 1937, Tertiary stratigraphy of western Washington 

and northeastern Oregon: Washington Univ. Pub. in Geology, v. 

4, 266 p. 

Weiss, L. E., 1959, Geometry of superposed folding: Geol. Soc. 

America Bull., v. 70, p. 91-106. 

Weiss~nborn, A. E., and Snavely, P. D., Jr., 1968, Summary report on 

the geology and mineral resources of the. Flattery Rocks, Quillayute Needles, and Copalis National Wildlife Refuges: U.S. 

Geol. Survey Bt~ll. 1260--F, 16 p. 

SUPPLEMENTAL INFORMATION 
28 THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON 

SUPPLEMENTAL INFORMATION 

FREQUENCY DIAGRAMS FOR SUBDOMAINS 

Frequency diagrams for subdomains shown on figures 25 and 26. 

Contours drawn by hand on percentages calculated and plotted by 

computer to nearest whole number; contours shown to lower right of 

each diagram. All percentages calculated per one percent area. Data 

not contoured if less than 40 points. 
Subdomain 1 -

all data 

Subdomain 2A -

all data 

Subdomain 28 -

all data 

235 poles to beds; 

1, 2, 4, 5, 6 percent 

196 poles to beds; 

1, 2, 4, 6 percent 

109 poles to beds; 

1, 2, 4, 6 percent 

77 poles to cleavage; 

1, 3, 5, 7 percent 

17 poles to cleavage 

· .... 

19 poles to cleavage 

86 pencils; 1, 3, 5, 7, 12 percent 

·. . .... .. . . .. 

. , . 

27 pencils 

. -:-

.... . -.. 

28 pencils 

20 fold axes (•); 19 poles 

to axial planes(+) 

15 fold axes (•); 12 poles 

to axial planes (+) 

+ + 

+ + 

. . 

11 fold axes (•); 8 poles 

to axial planes (+) 

+ 
Subdomain 3 -

all data 

Subdomain 4 -

all data 

I -~-

24 poles to beds 

74 poles to beds; 1, 3, 4, 5 percent 

... 

29 poles to cleavage 

143 poles to cleavages; 

1, 2, 4, 8 percent 

+ ..... 

. . . .... : .. :. 

33 pencils 

91 pencils; 1, 4, 8, 12 percent 

+ 

_I_ 

I 

3 fold axes (•); 2 poles 

to axial planes ( +) 

+ 

+ 

3 fold axes (•); 5 poles 

to axial planes (+) 

+ 

1-3 

::r: 

tr:l 

rn 

1-3 

~ 

e 

(') 

1-3 

e 

~ 

tr:l 

0 

1-rj 

1-3 

::r: 

tr:l 

0 

t'"' 

>-<: 

s:: 

'"0 

0 

s:: 

0 

e 

z 

1-3 

> -z 

sn 

~ 

> rn 

::r: -z 

0 

1-3 

0 

z 
Subdomain 5 -

all data 

235 poles to beds; 

1, 2, 4, 5, 6 percent 

Subdomain 6 -

Subdomain 7 -

all data 

164 poles to beds 

1, 2, 3, 4 percent 

153 poles to beds; 

1, 2, 3, 4, 5 percent 

211 poles to cleavages; 

1, 2, 3, 4 percent 

66 poles to cleavage; 

2, 3, 6, 9 percent 

108 poles to cleavages; 

1, 2, 4, 6, 8 percent 

146 pencils; 65 fold axes; 2, 5 percent 

1, 3, 6, 9, 12 percent 

~ 0 

~ '0 ~ 

~0 ® 

D =<:0 . 

68 pencils; 44 fold axes; 

1, 4, 10, 18 percent 2, 5, 7, 9 percent 

43 pencils; 

2, 5, 9, 14 Percent 

~ 

66 poles to axial planes; 

2, 5, 8 percent 

+ + 

+ + 

+ 

-'-I 

+ 

+ + 

38 poles to axial planes 

17 fold axes (•); 14 poles 

to axial planes (+) 

en 

c 

""0 

'i:l 

t"" 

t%j 

~ t%j 

z 

t-3 

> t"" 

z 

"'%j 

0 

:::0 

~ 

> t-3 

~ 

0 

z 
Subdomain 8-

all data 

Subdomain 9-

best data 

selected 

Subdomain 10 -

best data 

selected 

45 poles to beds; 

2, 4, 7, 11 percent 

·. 

+ 

25 poles to beds 

102 poles to beds; 

1, 3, 5, 7, 8 percent 

.. 

• I 

144 poles to cleavage; 

1, 3, 6, 9 percent 

71 poles to cleavages; 

1, 3, 5, 11 percent 

294 poles to cleavages; 

1, 2, 4, 6, 8 percent 

88 pencils; 1, 3, 9, 20 percent 

48 pencils; 2, 8, 16, 29 percent 

+ 

208 pencils; 1, 4, 8, 12,16 percent 

,. 

.. 

29 fold axes (•); 36 poles 

to axial planes(+) 

1 fold axis (•) 

+ .... + ++ 

+ 

++ * I + -~-

+ ++ 

+ .t 

45 fold axes; 2, 4, 8, 13 percent; 

26 poles to axial planes (+) 
Subdomain 11 -

all data 

Subdomain 12-

all data 

Subdomain 13-

all data 

73 poles to beds; 

1, 3, 7, 10 percent 

134 poles to beds; 

1, 2, 4, 5, 6 percent 

87 poles to beds; 

1, 3, 5, 8 percent 

69 poles to cleavages; 

1, 3, 6, 10 percent 

50 poles to cleavages; 

2, 4, 8, 10 percent 

86 poles to cleavages; 

1, 2, 5, 6, 8 percent 

43 pencils; 2, 6, 9, 12 percent 

. . .. -:-

9 pencils 

74 pencils; 1, 5, 7, 12 percent 

29 fold axes (•); 18 poles 

to axial planes (+) 

FA • 

East hinge~· • 

I 

o I 

AP pole® 

East hinge 

+ 

11 fold axes (•) 7 poles to axial 

planes(+); Constance Pass fold® 

.. . 

·. 

. -:-

. . 

• + 

+ 

+ 

++ 

+ 

+ 

++ 

+ 

+ 

24 fold axes (•J; 16 poles 

to axial planes(+) 
Subdomain 14-

163 poles to beds; 1, 2, 4, 5 percent 

Subdomai n 15 -

114 poles to beds; 1, 2, 4, 6, 10 percent 

Subdomain 16-

all data 

56 poles to beds; 2, 5, 9 percent 

189 poles to cleavage; 

1, 2, 4, 6, 8 percent 

74 poles to cleavage; 

1, 3, 5, 7, 11,14 percent 

46 poles to cleavages; 

2, 4, 8, 11 percent 

73 pencils; 

1, 4, 8, 12, 16 percent 

5 pencils 

. . . 

29 pencils 

63 fold axes; 

2, 5, 10 percent 

41 poles to axial planes; 

2, 7, 12 percent 

+ 

+ 

+ 

,I 

-,. 

+ 

+ 

+ + 

8 fold axes (•); 9 poles 

to axial planes (+) 

.. : 

+ 

10 folds axes (•); 8 poles 

to axial planes (+) 
Subdomain 17-

all data 

Subdomain 18-

all data 

81 poles to b d 1 2 e s; • • 4, 7 percent 

50 poles to bed . 

2 • 4 • 6, 8 perce~~ 

7 1 1 poles 3 7 o t cleavages· 

• • • 10 percent • 

7 7 1 poles 3 4 o t cleavages· 

• • • 6 percent • 

67 pencils 

0~ ~ 

~ 

D-~-1 

0 

3 5 pencils 

0 .. 

+ + 

I 

-~-

17 fold axes(·)· 16 to axial 1 • poles Panes(+) 

-:-

+ 

2 fold axes (•). • 1 . ax1al plane(+) 
36 THE STRUCTURE OF THE OL YMPJC MOUNTAINS, WASHINGTON 

COMPUTERIZED STRUCTURAL DIAGRAM PROGRAM 

In order to handle the large amounts of complex structural data 

obtained while mapping the Olympic Mountains, we established a 

two-part computer program in Fortran IV. The first part allows storage of the structural data on punchcards with provisions to select the 

data by type and geographic location (subdomain). The second part of 

the program prepares a pole-density orientation diagram from the 

selected data. The computer program for pole-density orientation 

diagrams was made at the University of Alberta by Muecke and 

Charlesworth (1966). Our orientation program deck is a duplicate of 

the Canadian deck, lent to us by Dr. Charlesworth. Ming Ko, of the 

U.S.G.S. Computer Center, modified the Canadian program for our 

needs. 

The storage of data was facilitated by the use of forms (fig. 33) that 

we filled out in the field. Any type of line or plane can be entered on 

the form and treated by the program. We measured dips to the 

nearest 5°, strikes to the nearest 2°. Before the data were entered on 

punch cards, station coordinates were punched by use of a coordinatograph. 

The program allows subdomains to be selected by coordinates and 

(or) indicated stations. If coordinates are used, each subdomain is 

outlined as nearly as possible by two rectangles, specified by their 

southwest and northeast corners. Stations can be added or subtracted 

individually to fill in irregular subdomains. 

Because the Olympic rocks are particularly prone to sliding and 

slumping (see Tabor, 1971), we included an option of select data on 

the basis of field observation of reliability, a somewhat subjective 

judgment. We found that the results for all data and for only the best 

data were about the same. Most subdomains were run with all the 

data. 

The following summary of the pole-density orientation program is 

modified from a description of the program by H. A. K. Charlesworth 

(written commun., 1967). 

The program counts out points on the reference sphere, not on 

their projection in the equatorial plane. The pole densities obtained 

are then projected onto the equatorial plane and faithfully reflect the 

true pole densities. 

The computer converts the strike, dip, and direction of a plane to 

the bearing and plunge of the normal to the plane. All the bearings 

and plunges are then changed into direction cosines of unit length. 

The counting out of the poles in the reference sphere is accomplished by the use of a circular cone with semivertical angle, the 

value of which is determined by the percentage of the total area of 

the projection that the counter is required to cover. The percentage of 

the total area counted and the number of counting locations are 

chosen such that the counting cones overlap so as to assure that all 

points on the sphere are counted. At the same time, the counting 

area is kept small in order to obtain the best possible definition of 

any differences in the densities of the points. 

The number of data readings in each data set falling within the 

counting cone at each counting locality is converted into the percentage of the total number of points processed. 

In order to facilitate a simple output format for a line printer, the 

counting locations were chosen to form a regular grid and projected 

from the lower hemisphere of the reference sphere onto the equatorial plane by equal-area projection (fig. 348 ). For the convenience of 

accurate contouring, these locations were augmented by 14 additional points near the periphery of the projection. We contoured the 

printed densities by hand; a subprogram that plots the projections of 

the pole intersections (fig. 34A) was especially useful in subdomains 

with small amounts of data. 

In keeping with the spirit established by Muecke and Charlesworth, we maintain a program card deck with our modifications 

(data retrieval and plotting subroutine), available for loan and duplication. Address inquiries to R. W. Tabor, U.S. Geological Survey, 

345 Middlefield Road, Menlo Park, Calif. 94025. 
1 !2 314 s e I' 16 91101'']•z •31t4ltsl•~ 171•sl•9 a::[zt 2~13 2417~[ 2!' vi~ ;s :s~ 311:•1~3 )~IE )c J71)/j\J9 4(1,41 42 ~ 31441

4 s rG 147 _.__~ l4_c.{so s1Jszj_s ~~sjs6 s71sa s~L•c.lol 62~3 ~4 -, ~ £516 7 6816~ 7~711., 7~7~75 7< nj1~ 1~e'l 

Field number Coordinates Beds Beds Cleavage Cleavage Pencil Axial Plane Fold Axis Axial Plane Fold Axis Joint 

~1. Yr. Sta. X y strike dip d T strike dip d T strike dip d strike dip d bear pl. strike dip d bear pl. strile dip d bear pl. strike dip d R 

I I l 

I _!_ I 

_!_ I I I 

L I 

I j _!__l _L 

1 J I I _L_l I I l__l _L l 

_L _1 I I l I I I I I I l__l_ 

_1 I _i 

_l _L I I I ~_L 

_L _L I I I I I 

I I .L I I 

I .L L l l _ _l J •- I I 

__._ _l I _l 1 _l__l_ I I L 

I I _.L L 1 J 1 .L I I 

I I _l_ -'- L I I . I I I L_l_ 

__._ I I L_i_ _l___l_ -' _._ • _j j 

_!_ _l_ _L _l_ I I I l I I I 

I _l_ _ _I J I 1 I • t I 

I .l___L __._ J 

I I I L _l _l •- I l 

I I I I I I 1 I 

,__._ -'- I I I I 

_l_ I l_ I . I L l 

_1_.1 I _.__ t ;.__. 

I _L I I I l I 

_I I I I I I I I I I 1 I 

1 jz 314 5 &1718 9l1oj11j12 131t4jtslt6 11ltej19 3)121 22 23 24125126 27128 293) 31132133 >~I:J5 3637,38,39 '*)1•11 424~4~45 46,47 4otl49! 50 51152 sJ 54jssj56 57158 58 _\6oj6t 6Z,6l 64 6~66167 68169 JDI7117z 7~14 7~ 76 7717el79jao 

FIGURE 33.-Computer form for structural data. 
..... 

! 

0 

..... 

\0 

A 

. . 

·. .. 

.. 

.. , . .,..· .. -:-· . . . ... I I •• 

....... .. .. . ... 

, . 

... .. .. 

. . 

150 LINES PENCILS 

FIGURE 34.-Computer output for structural program. Circles reduced from 10 inches. A, Plotter output for 150 pencils. Circular 

alinement is an artifact of recording dips to the nearest 5°. B, Frequency plot of same data in percentages per 1 percent area. 

Contoured by hand. ·