I 

I 

A Newly Recognized Ductile Shear Zone in 

the Northern Klamath Mountains, OregonImplications for Nevadan Accretion 

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A Newly Recognized Ductile Shear Zone in 

the Northern Klamath Mountains, OregonImplications for Nevadan Accretion 

By MARY M. DONATO 

U.S. GEOLOGICAL SURVEY BULLETIN 2028 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1992 

For sale by 

Book and Open-File Report Sales 

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Library of Congress Cataloging in Publication Data 

Donato, Mary M. 

A newly recognized ductile shear zone in the northern Klamath Mountains, 

Oregon : implications for Nevadan accretion I by Mary M. Donato. 

p. em.- (U.S. Geological Survey bulletin ; 2028) 

Includes bibliographical references. 

1. Rock deformation-Klamath Mountains Region (Calif. and Or.) 

2. Schists-Oregon. 3. Amphibolite-Oregon. 4. Geology, StructuralOregon. I. Title. II. Series. 

QE75.B9 no. 2028 

fQE604] 92-20834 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Purpose of study 1 

Acknowledgments 2 

Geologic setting 2 

Amphibolite 3 

May Creek Schist 4 

Ductile shear zone 4 

Shear sense criteria 6 

Petrographic criteria 6 

Quartz fabrics 7 

Discussion and conclusions 7 

References cited 9 

FIGURES 

1. Generalized geologic map of the northern Klamath Mountains 2 

2. Simplified geologic map of the main part of the contact zone between the May Creek 

Schist and structurally underlying amphibolite 3 

3. Lower-hemisphere, equal-area stereonets comparing structural features measured in 

the amphibolite and in the May Creek Schist 5 

4. Schematic cross sections of the ductile shear zone 6 

5. Photomicrographs showing asymmetric tourmaline porphyroblast with sillimanite 

tails indicating dextral shear and quartz-mica schist S-C surfaces indicating dextral 

shear 7 

6. Equal-area, lower-hemisphere projections of quartz [c]-axes in 12 oriented thin sections of ductilely deformed May Creek Schist 8 

7. Stretching lineations and transport directions for Nevadan and older deformation in 

the northern Klamath Mountains 9 

Contents Ill 

A Newly Recognized Ductile Shear Zone in the 

Northern Klamath Mountains, Oregon-Implications for 

Nevadan Accretion 

By Mary M. Donato 

Abstract 

An 800-to 1,500-m-thick ductile shear zone in the 

northernmost Klamath Mountains marks the contact between 

metasedimentary rocks of the May Creek Schist and structurally underlying amphibolite. The shear zone trends approximately east-west, dips southward, and has been traced about 

13 km along strike. Petrographic criteria and quartz petrofabric analyses of semipelitic schists and quartzofeldspathic 

gneisses of the May Creek Schist in the hanging wall consistently demonstrate a top-to-the-northwest sense of shear, indicating northwestward thrusting (present-day geographic 

framework) of schist over amphibolite in the footwall. Syntectonic sillimanite in deformed May Creek Schist indicates 

that amphibolite-facies conditions prevailed during deformation. The tectonic transport direction is perpendicular to the 

northeast-trending structural grain of the northernmost Klamaths, and in particular to southeast-dipping thrusts related 

to accretion of jurassic volcanic rocks which lie to the west. 

The age of the ductile deformation is inferred to be approximately 145 Ma, based on 40Ar;39Ar cooling age determinations of metamorphic hornblende from amphibolite in the 

footwall. I suggest that the ductile shear zone is a manifestation of Nevadan (144-157 Ma) convergence, during which 

an incipient back-arc spreading center (the protolith of the 

amphibolite) and its sedimentary cover (now the May Creek 

Schist) were shortened by contractive faulting during accretion to North America. Available Nevadan stretching lineations and transport directions from various Klamath 

Mountain localities are disparate and underscore the apparent structural complexity of the Nevadan orogen. 

INTRODUCTION 

Purpose of study 

One of the most frequently encountered problems in 

structural analysis of complexly deformed metamorphic 

terranes is the determination of the sense of movement in 

sheared rocks. In regions composed of accreted terranes, 

Manuscript approved for publication, March 13, 1992. 

such as the Klamath Mountains, such information can 

shed light on the relative motions of large blocks of crust 

and thus may enhance our understanding of the processes 

by which accretion takes place. 

Only a few previous attempts have been made to relate structures in metamorphic tectonites to the regional accretionary tectonic framework of the Klamath Mountains. 

Microstructural and mineral-preferred orientation studies 

were used by Cannat and Boudier (1985) to deduce convergence directions in ophiolitic complexes in the western 

Paleozoic and Triassic belt and the western Jurassic belt 

(names of Irwin, 1966) in northern California. Their results 

indicated northeast-southwest convergence during Mesozoic time, but their interpretations of age relationships, especially for the Josephine ophiolite, are controversial (Harper 

and Harding, 1986; Cannat and Boudier, 1986). Recent 

analysis of structural and shear criteria in amphibolite and 

serpentinite attest to thrusting of the Josephine ophiolite 

toward the north-northeast during Nevadan time (Grady 

and others, 1989; Harper and others, 1990). Cashman's 

(1988) strain analysis of the Galice Formation beneath the 

Orleans thrust fault at the contact between the western Jurassic belt and the western Paleozoic and Triassic belt revealed northwest-southeast-trending stretching lineations 

due to Nevadan convergence; shear-sense indicators are 

compatible with dextral oblique convergence on the fault. 

Many useful criteria for determining the sense of 

shear in mylonitic rocks have been recognized, including 

mesoscopic and microscopic features such as rotated porphyroblasts, pressure shadows, mica fish, and composite 

planar (S-C) fabrics (Berthe and others, 1979; Simpson 

and Schmid, 1983; Lister and Snoke, 1984; Passchier and 

Simpson, 1986). Quartz crystallographic fabric asymmetry 

has also been used successfully as a primary tool for investigating kinematics of large-scale folding and thrusting 

(Behrmann and Platt, 1 982; Patrick, 1988; Klaper, 1988). 

This study uses mesoscopic structural data, petrographic 

criteria, and quartz petrofabric analysis to deduce the 

sense of shear in ductilely deformed quartzites and semipelitic schists from a newly discovered mylonitic shear 

Introduction 
zone separating the May Creek Schist and structurally underlying amphibolite in the northernmost Klamath Mountains. The data consistently indicate Nevadan thrusting of 

the May Creek Schist (usage of Donato, 199lb) over the 

amphibolite in a present-day northwestward direction. 

These results provide additional constraints on the geometry of accretion of the May Creek Schist and adjacent metamorphic terranes and thus contribute to the growing body 

of knowledge about the accretionary history of the region. 

Acknowledgments 

I thank David M. Miller, Dave Brew, Greg Harper 

(State University of New York at Albany), and Sue Cashman (Humbolt State University) for helpful reviews of an 

earlier version of the manuscript, and Miranda Fram and 

Pamela Gemery for their capable assistance in the field. 

GEOLOGIC SETTING 

This report focuses on two lithologic units in the northernmost Klamath Mountains (fig. 1): metasedimentary 

rocks called the May Creek Schist (MCS) and structurally 

underlying hornblende schist and gneiss derived from basaltic igneous rocks (amphibolite). Although the original nature of the contact between the MCS and the amphibolite, 

EXPLANATION 

WESTERN JURASSIC BELT 123° 45' u 

now the locus of intense ductile deformation, is not certain, 

the contact may have originally been depositional. Metamorphism and deformation of the MCS and amphibolite 

accompanied the imbrication and collapse of a small backarc basin during which the basaltic basement was overthrows by its sedimentary cover (Donato, 1987, 1991 a). 

These units are part of the western Paleozoic and Triassic belt (Irwin, 1966), a collage of Jurassic and older 

accreted oceanic and island arc terranes. Tertiary volcanic 

and volcaniclastic rocks of the Western Cascades are in 

possible depositional contact with the Klamath Mountains 

in this region, but this contact has been extensively modified by high-angle faults (Donato, 1991 b). To the north, 

amphibolite and metasedimentary rocks of uncertain age 

are t11JJ1cated by the Tertiary dextral strike-slip Canyonville fault, which also marks the northern boundary of the 

Klamath Mountains Province. To the south, the MCS is in 

probable fault contact with the greenschist-facies oceanic 

metavolcanic and metasedimentary rocks of the Applegate 

group (Smith and others, 1982). Rocks belonging to Irwin's (1966) western Jurassic belt lie immediately to the 

west and include low-grade ophiolitic rocks of the Sexton 

Mountain area and arc-derived hornblende- and pyroxenebearing metavolcanic and metavolcaniclastic rocks, including the middle Jurassic Rogue volcanics of Garcia (1979). 

The western Jurassic belt is cut by numerous northeasttrending, east-dipping thrust faults related to late Jurassic 

(Nevadan) accretion (fig. I). 

123° 00' 

·~ [ D· . Volcanic, volcaniclastic, and 

~ · . . ophiolitic rocks, undivided KILOMETERS -, 

WESTERN PALEOZOIC AND TRIASSIC 

u ~ BELT 

-~ ] ~ [ [,. ~,. ~j Intrusive rocks--Quartz 

2, C~J "E ,. ,. ,. diorite to trondhjemite 42o 45' 

U May Creek terrane 

-~ ~ D May Creek Schist ~:g 

e 0 fault 

2, ~ Amphibolite 

~ Mafic schists and 

~ serpentinite, undivided 

"-... Fault 

../ Thrust fault, barbs on upper plate 

-- Contact 

0 15 30 

Figure 1. Generalized geologic map of the northern Klamath Mountains, modified from Smith and others (1982), Silberling and 

others (1987), and Irwin (in press). 

2 A Newly Recognized Ductile Shear Zone in the Northern Klamath Mountains, Oregon-Implications for Nevadan Accretion 
Two post-metamorphic plutons intrude the western 

part of the MCS and underlying amphibolite (fig. 2), but 

extensive weathering limits exposures in all but a few localities. Contact-metamorphic effects of the White Rock 

pluton (141 Ma, biotite K/Ar, Hotz, 1971) extend probably 

no more than 100 meters into surrounding amphibolite. 

The Wimer pluton intrudes both the May Creek Schist and 

amphibolite, but isotopic age determinations for this pluton are not available. The Wimer, White Rock, and the 

. ()• 

·~ 

. ·~ 

8· . 

·~ 

.. 30 . 

·~··· . 40 ... 

nearby Grants Pass pluton (139 Ma, U-PB, Saleeby, 1984) 

belong to Irwin's (1985) Grants Pass plutonic belt, which 

he considered Early Cretaceous in age. 

Amphibolite 

The amphibolite consists chiefly of variably layered 

hornblende schist and gneiss. In spite of thorough meta145± 7 Ma 

(hbl 

40 

Ar/ 

39 

Ar) 

~ *~ 27 

145±2 Ma 

0 

31 

~ 

1 2 

EXPLANATION 

D . 

D 

• II 

May Creek Schist ~ 

Amphibolite ~ 

18 

Strike and dip of 

foliation 

Trend and plunge of 

lineation 

Metamorphosed 

serpentinite 

Pre- or syn-tectonic 

intrusive rocks 

Post-tectonic intrusive 

rocks 

:::~::::::::::::::::::::. Approximate limit of 

··::t: ductile deformation 

j Thrust fault, barbs on 

upper plate 

40 !9 * Ar Ar age sample "Fault 

Figure 2. Simplified geologic map of the main part of the contact zone between the May Creek Schist and 

structurally underlying amphibolite. The shear zone trends approximately east-west and dips moderately 

southward. Oblique lineations suggest either oblique motion within the zone or post-deformation tilting or 

rotation of the shear zone. 

Geologic Setting 3 
morphic recrystallization, relict igneous textures such as 

plagioclase phenocryst pseudomorphs and chilled intrusive 

contacts are commonly visible in less deformed rocks. 

These features suggest that the protolith was an igneous 

complex which included basaltic dikes or sills, shallow diabase intrusions, and gabbro. The chemistry of the amphibolite is homogeneous and strongly resembles some 

back-arc basin basalts. Based on this geochemical similarity and on regional field relations, the amphibolite is believed to have formed in a back-arc basin, probably in 

middle Jurassic or earlier time (Donato, 1991a). 

The most common metamorphic mineral assemblage 

in the amphibolite is green hornblende+plagioclase+sphene+opaque oxide, indicating middle amphibolite-facies 

conditions. New 40 Ar/39 Ar analyses of metamorphic 

hornblende from two samples of amphibolite (see fig. 2 

for sample locations) give isotopic ages of 145 ± 2 and 

145 ± 7 Ma (Donato, 1991a). The protolith (igneous) age 

of the amphibolite is unknown. 

The amphibolites are LS-tectonites with a pervasive 

schistosity or foliation defined by alternating hornblendeand plagioclase-rich layers. The foliation generally strikes 

northeast and dips southeast, although local and regional 

broad warping of the foliation has occurred. A strong, consistent southeast-plunging hornblende lineation is also 

present throughout (fig. 3). In some cases, elongate, recrystallized plagioclase grains or aggregates define a 

measurable flattening plane, or rarely, a stretching lineation parallel to the hornblende lineation. Foliation and lineation become more pronounced in the amphibolite with 

proximity to the contact with the overlying MCS, but 

shear criteria and textural evidence of intense ductile deformation and dynamic recrystallization are best-developed in the basal, quartzose part of the MCS. 

Rare outcrop-scale isoclinal reclined folds deform the 

compositional layering. Available data indicate that the 

folds generally plunge southeast, parallel to the hornblende lineation and parallel to stretching lineations measured in metasedimentary rocks from the ductile shear zone 

(see fig. 3 and below); the axial planes of reclined folds 

are parallel to the measured foliation. 

May Creek Schist 

The structurally lowest units of the MCS, including 

the newly discovered ductilely deformed rocks, were described by Diller and Kay (1924) as "highly metamorphosed mica schist and mica slate." They consist of finegrained and strongly layered metapelite, metapsammite, 

and quartzofeldspathic gneiss consisting mainly of quartz, 

plagioclase, biotite, and white mica, with accessory spessartine, tourmaline, and opaque oxides. These rocks grade 

upward into grayish-green to purplish fine-grained metagraywacke and volcaniclastic metasedimentary rock and 

impure calcareous units containing rare marble lenses. 

Diagnostic metamorphic mineral assemblages are 

rare, owing to the lack of appropriate bulk compositions, 

but amphibolite-facies conditions are clearly indicated locally. Sillimanite occurs with quartz, plagioclase, biotite, 

garnet, and white mica at two known localities. In calcareous tuffaceous rocks higher in the section, the assemblage 

is diopside+plagioclase+quartz+actinolitic amphibole+calcite±biotite. No direct determination of the metamorphic 

age of the MCS has been made, but the age is inferred to 

be 145 Ma, on the basis of apparent synchroneity of metamorphism of the MCS and the amphibolite. 

The MCS is everywhere well foliated but strongly 

lineated only where it has undergone intense ductile deformation near its base. In pelitic, semipelitic, and psammitic 

varieties, foliation is defined by alternating micaceous and 

quartzofeldspathic layers, and by flattened lensoidal aggregates of plagioclase or mica. Rocks within the ductile 

shear zone display a conspicuous lineation caused by elongated quartz, feldspar, and mica grains within the foliation 

plane. Structural elements in the MCS are generally parallel to those in the amphibolite (fig. 3). 

DUCTILE SHEAR ZONE 

A zone of ductilely deformed quartz-mica schist and 

quartzofeldspathic gneiss marks the basal part of the M CS 

along its contact with the amphibolite (fig. 2). The ductile 

shear zone trends approximately east-west, is estimated at 

two localities to be approximately 800 to 1,500 m thick, 

and is traceable along strike for about 13 km. These rocks 

are "mylonitic," according to the criteria accepted by 

Penrose Conference attendees (Tullis and others, 1982): 

( 1) they have undergone grain size reduction, (2) they 

occur in a relatively narrow planar zone, and (3) they display enhanced foliation and lineation due to strain concentration. Field evidence for mylonitization includes a strong 

south- to southeast-dipping laminar foliation and a pronounced southeast-plunging lineation defined by aligned 

mica flakes and stretched quartz and plagioclase feldspar 

grains. In quartzofeldspathic rocks, comminuted, broken 

feldspar porphyroclasts are visible on surfaces exposing 

the X-Z plane of the strain ellipsoid. Thin sections of such 

rocks cut in this plane commonly display beautiful fluxion 

structure. Plagioclase grains are blocky or subrounded and 

occasionally broken, whereas quartz has deformed ductilely, flowing around resistant plagioclase grains and forming 

elongate ribbons up to 5 mm long. 

Mylonitic metasedimentary rocks are only locally in 

direct contact with the underlying amphibolites. Various 

highly deformed rocks, including metaserpentinite, metasomatic rocks, and deformed dioritic intrusive rocks, com4 A Newly Recognized Ductile Shear Zone in the Northern Klamath Mountains, Oregon-Implications for Nevadan Accretion 
monly occur between the mylonites and the amphibolite. 

For example, linked traverses in Evans Creek, traversing 

down section through the contact zone (fig. 4), lead from 

mylonitic quartzofeldspathic schist through lenses of 

sheared metaserpentinite, talc-tremolite schist, thinly layered quartzite, hornblende-biotite schist, hornblende-epidote schist, mylonitic quartzite, gneissic hornblende 

diorite, and mylonitic amphibolite. The structural thickness 

of the tectonically disrupted zone at this locality is about 

1,500 m. A similar zone of interlayered ultramafic and 

mafic intrusive rocks separating amphibolite from mylonitic quartzofeldspathic gneisses is exposed in Sprignett 

Creek (fig. 4). Here the zone is probably only 600 to 800 

m thick. 

Dioritic intrusive rocks which display textural evidence of ductile deformation occur at several localities 

within the contact zone between amphibolite and the May 

Creek Schist. A striking example occurs in Evans Creek, 

where foliated hornblende quartz diorite is exposed (sec. 

23, T. 34 S., R. 3 W.). The foliation of the quartz diorite, 

the attitude of which is variable, is defined by 1- to 5-mmthick bands or layers of alternating mafic and felsic material, which impart a gneissic appearance to the outcrop. 

The foliation is crosscut at a high angle by discrete 1 to 2 

em wide shear bands spaced 1 to 2 m apart. These shear 

bands strike N. 75°-90° W. and dip moderately to steeply 

southward, subparallel to the local orientation of the metasedimentary rocks-amphibolite contact zone. Within these 

narrow shear bands, the foliation in the quartz diorite is 

drawn into parallelism with the shear band, and grain size 

diminishes. The foliation becomes mylonitic and is composed of finely laminated hornblende, plagioclase porphyroclasts, and quartz ribbons. 

In thin section, hornblende is brown with dark-green 

rims and is ovate or lenticular in shape. Plagioclase is also 

lenticular, crudely aligned, and commonly displays bent 

twin lamellae, recrystallized grain margins, and undulatory 

extinction. A fine-grained mosaic of recrystallized feldspar 

Amphibolite May Creek Schist 

• • + + + 

•• • • 

••• • 

• • 

~.\ . + + .... • ~ • + • 

+ + 

• + + 

+ ~+ + 

++[j ~ + + • + + + + 

EXPLANATION 

AmEhibolite May Creek Schist Shear zone 

Number a[ measurements 

• Pole to foliation 134 123 18 + Lineation 92 31 17 

+ Fold axis 9 

~ Pole to axial plane 9 

Azimuth and l!.lunge 

0 Pole to best-fit great circle 158, 31 155,32 

* Mean lineation vector 142,31 135,26 

Figure 3. Lower-hemisphere, equal-area stereonets comparing structural features measured throughout the amphibolite and the 

May Creek Schist. Lineations measured in amphibolite are hornblende lineations; those in the May Creek Schist are S1 x S2 

intersection lineations or quartz or feldspar stretching lineations measured in mylonitic rocks. 

Ductile Shear Zone 5 
and quartz surrounds lensoidal quartz and feldspar grains 

in the felsic laminae. 

These mesoscopic and microscopic features are characteristic of solid-state ductile deformation in g'ranitic 

rocks (Paterson and others, 1989), but the alignment of 

plagioclase (particularly albite twin lamellae) suggests that 

ductile deformation may have been superimposed on a 

pre-existing magmatic foliation. Brown hornblende cores 

suggest locally higher temperatures compared with the adjacent amphibolite. Although there is no direct petrographic evidence of magmatic-state deformation, it is possible 

that the diorite was intruded during early phases of deformation and cooled below its solidus as deformation 

proceeded. 

A 

Southeast 

B 

South 

0 200 400 

SHEAR SENSE CRITERIA 

Petrographic Criteria 

Oriented thin sections of mylonitic rocks from localities along the length of the shear zone were examined to 

determine the sense of shear. All thin sections were cut 

parallel to the lineation and perpendicular to the foliation 

(that is, in the X-Z plane of the strain ellipsoid) and oriented so that apparent dextral shear in the thin section indicates top-to-the-northwest sense of motion. 

Excellent examples of o-type tourmaline porphyroblasts (Passchier and Simpson, 1986) indicating dextral 

shear were observed. In one sample, which also contains 

Northwest 

North 

600 800 1000 

meters 

I I 

--

no vertical exaggeration 

EXPLANATION 

Amphibolite 

Syntectonic diorite 

and quartz diorite 

Quartzose metasedimentary rocks 

and quartzofeldspathic gneiss 

Serpentinite 

Figure 4. Schematic cross sections of the ductile shear zone based on traverses in Evans Creek (A) and Sprignett 

Creek (8). 

6 A Newly Recognized Ductile Shear Zone in the Northern Klamath Mountains, Oregon-Implications for Nevadan Accretion 
biotite and garnet, curved syntectonic sillimanite fibers in 

the tails of a rotated tourmaline porphyroblast demonstrate 

that amphibolite-facies metamorphic conditions prevailed 

during mylonitization (fig. SA). Well-developed S-C fabrics (fig. 5B) and sigmoidal mica "fish," all indicating dextral shear, are also commonly found in the more 

micaceous lithologies. 

Quartz Fabrics 

Quartz petrofabric studies of 12 oriented thin sections 

of quartz-rich metasedimentary rocks from localities along 

the shear zone were performed in order to confirm the 

sense of shear in the mylonites. Sections were cut parallel 

to the lineation and perpendicular to the foliation. No lineation was measurable in the field for 5 of the 12 samples, 

so the sections were cut parallel to the local lineation, 

Figure 5. A, Asymmetric tourmaline porphyroblast (t) with 

sillimanite (si) tails indicating dextral shear in sample 156. 

Garnet (g), ribbon-like quartz, plagioclase, and opaque oxide 

are also present. Scale bar represents 1 mm. B, Quartzofeldspathic schist with micaceous S-C surfaces indicating dextral 

shear (sample 258-A 1 ). Small, round, high-relief grains are 

garnet. Scale bar represents 1 mm. 

measured either elsewhere at that outcrop, or at a nearby 

outcrop. The orientations of [c)-axes were measured with 

a standard four-axis Universal Stage and plotted on lowerhemisphere, equal-area stereographic projections. The contoured results are shown in figure 6. 

Dextral shear is displayed by the clockwise rotation 

of the girdles with respect to the foliation plane in eight 

samples ("leading edge asymmetry"; Raiser, 1990), indicating a thrust sense of movement between the M CS and 

amphibolite. The remaining four samples displayed strong 

lattice preferred orientation of quartz, but did not yield 

shear sense information. No samples indicated sinistral 

sense of shear. The [c)-axis patterns shown are typical of 

those described in major shear zones where dextral shear 

is independently verified by other criteria (see Bouchez 

and Pecher, 1981; Behrmann and Platt, 1982; Schmid and 

Casey, 1986). Such independent verification is important, 

because experimental results show that for a single sense 

of shear the [c)-axis asymmetry with respect to the shape 

fabric may be either "leading edge" or "trailing edge," depending on pre-existing fabric (Raiser, 1990). 

Samples 258-C2 and 323A show atypical doublemaximum patterns occurring at 45° to 55° to the foliation 

plane, and cannot be used to deduce sense of shear. The 

patterns may reflect previously existing fabrics, lower 

strain, or the effects of feldspar or mica on quartz deformation. Two other samples from east of Sprignett Creek 

(no lineations available) have strong fabrics but do not indicate sense of shear (fig. 6). Rotating the data in attempts 

to compensate for incorrectly estimated lineation directions did not improve the results. 

DISCUSSION AND CONCLUSIONS 

From the combined petrographic and petrofabric data, 

I conclude that the ductile shear zone was the locus of 

northwest-directed overthrusting of the MCS. Although the 

isotopic age of the ductile deformation is not directly 

known, a Nevadan age (approximately 153-144 Ma; Harper and Wright, 1984; Harper and others, 1986) is inferred 

on the basis of metamorphic and structural continuity between the ductilely deformed rocks and the underlying 

amphibolite, which has a cooling age of 145 Ma (Donato, 

1991a). Although there is some uncertainty that the ductile 

deformation was synchronous with metamorphism, this is 

a reasonable assumption given that there is no petrographic evidence for multiple metamorphic events, ~nd given 

the structural and metamorphic continuity across the shear 

zone. 

The N. 38°-45° W. transport direction is approximately perpendicular to the structural grain of the northernmost 

Klamath Mountains, which is defined primarily by the 

trend of major southeastward-dipping thrust faults which 

bound and displace terranes of the western Jurassic belt 

Discussion and Conclusions 7 
(fig. 1; Smith and others, 1982). These faults are probably 

related to the Nevadan accretion of this material to North 

America. The geometric relation between the trend of the 

faults and the transport direction suggests that the ductile 

shear zone is a smaller thrust inboard of the locus of accretion and reflects imbrication of the hanging wall of a larger 

thrust system. Convergence, therefore, was probably northwest-southeast in a present-day geographic framework. 

The significance of this ductile shear zone with regard 

to large-scale tectonic plate motions is not clear. The Nevadan continental margin was undoubtedly complex. The 

angular relationship between stretching lineations and relative plate motion is not simple and may depend on subduction angle and finite strain (Ellis and Watkinson, 1987). 

Early stretching lineations formed in the footwall may develop subparallel to the trend of the orogen and may reflect relative plate motion. However, later lineations 

317-A 302-A2 

formed during or after the peak of metamorphism may not 

reflect relative convergence and are more likely related to 

imbrication and isostatic emergence of the hanging wall 

normal to the orogen (Ellis and Watkinson, 1987). The observed stretching lineations probably correspond to the latter type, because they developed during the metamorphic 

peak, and therefore may have formed perpendicular to the 

trend of the orogen. 

Figure 7 summarizes existing data for Nevadan tectonic transport directions and stretching directions in the 

northern Klamath Mountains. Two pre-Nevadan transport 

directions are also shown. It should be noted that none of 

the data represented have been corrected for post-Nevadan 

tectonic rotation (for example, up to 60° of clockwise rotation is postulated for the Josephine ophiolite; Harper and 

others, 1990). The amount of post-Nevadan rotation of the 

MCS, if any, is unknown. 

258-A1 

13B 

1, 2, 5, 8, 12, 16 (20.8) 1, 2, 4, 8, 12, 16 (20.8) 1, 2, 3 (3.5) 1, 3, 5, 8, 12 (13.6) 

Figure 6. Equal-area, lower-hemisphere stereographic projections of quartz [c)-axes in 12 oriented thin sections of ductilely 

deformed May Creek Schist. To facilitate comparison among 

samples, all data have been rotated so that the foliation is vertical and strikes east-west, and the lineation (where known) is 

horizontal and trends east-west. The number of measurements 

is 200 for samples 293-A, 258-C2, and 317-A; all others are 

100. F = foliation plane and L = lineation (shown only where 

measured on the sample). Contour intervals beneath each plot 

are in percent per 1 percent counting area, with the maximum 

in parentheses. Arrows indicate stretching lineations measured 

at sample localities. Patterns are same as those used in figure 2. 

8 A Newly Recognized Ductile Shear Zone in the Northern Klamath Mountains, Oregon-Implications for Nevadan Accretion 
The MCS transport directions concur with those of 

Cashman (1988), who reported northwest-southeast dextral 

oblique convergence on the Orleans fault, about 135 km to 

the southwest, during Nevadan time. Conversely, Harper 

and others (1990) found north-northeast-directed Nevadan 

obduction of the Josephine ophiolite (fig. 7). Two possible 

explanations for this difference are (1) the Josephine Ophiolite and the MCS were in different stress regimes in the 

Nevadan orogen and were transported in approximate! y 

perpendicular directions and (2) one or both terranes were 

affected by post-Nevadan rotation and the indicated transport directions do not represent original transport directions. Clearly, a comprehensive tectonic model for 

Nevadan plate motions and accretion requires a better understanding of chronology, structure, and large-scale block 

rotations, particularly in the western Jurassic belt. The disparity among the available Nevadan lineations alone underscores the apparent complexity of the Nevadan and 

younger continental margin. 

REFERENCES CITED 

Behrmann J.H., and Platt, J.P., 1982, Sense of nappe emplacement from quartz c-axis fabrics; an example from the Betic 

Pacific 

Ocean 

Coast 

Range 

Province 

Cordilleras (Spain): Earth and Planetary Science Letters, v. 

59, p. 208-215. 

Berthe, D., Choukroune, P., and Jegouzo, P., 1979, Orthogneiss, 

mylonite and non-coaxial deformation of granites: the example of the South Armorican Shear zone: Journal of Structural Geology, v. 1, p. 32-42. 

Bouchez J.L. and Pecher, A., 1981, The Himalayan Main Central 

Thrust pile and its quartz-rich tectonites in central Nepal: 

Tectonophysics, v. 78, p. 23-50. 

Cannat M., and Boudier, F., 1985, Structural study of intra-oceanic thrusting in the Klamath Mountains, northern California: implications on accretion geometry: Tectonics, v. 4, p. 

435-452. 

--1986, Reply: Tectonics, v. 5, p. 693-695. 

Cashman, S.M., 1988, Finite-strain patterns of Nevadan deformation, western Klamath Mountains, California: Geology, v. 

16, p. 839-843. 

Diller J.S., and Kay, G.F., 1924, Description of the Riddle quadrangle, Oregon: U.S. Geological Survey Atlas, Riddl~ Folio, 

No. 218. 

Donato, M.M., 1987, The May Creek Schist, southwestern 

Oregon: remnant of an incipient back-arc basin? [abs.]: 

Geological Society of America Abstracts with Programs, 

v. 19, p. 373. 

---1991a, Geochemical recognition of a captured back-arc 

basin metabasaltic complex, southwestern Oregon: Journal 

of Geology, v. 99, no. 5, p. 711-728. 

May Creek Schist 

?Nevadan 

-145 Ma 

(this study) 

Nevadan 

-155 Ma 

(Grady and 

. others 1 1989) 

Pre-middle Jurassic 

(Cannat and Boudier1 1985) 

Pre-Nevadan 

(Cannatand 

Boudier 1 1985) 

[] 

IT] 

Western Jurassic belt 

Western Paleozoic and 

Triassic belt 

Nevadan 

140-153 Ma 

Cashman (1988) 

Figure 7. Stretching lineations and transport directions for Nevadan and older deformation in the 

northern Klamath Mountains. Data are from Cannat and Boudier (1985), Cashman (1988), Grady 

and others (1989), and this study. 

References Cited 9 
---1991b, Geologic map showing part of the May Creek 

Schist and related rocks, Jackson County, Oregon, U.S. 

Geological Survey Miscellaneous Field Investigations Map 

MF-2171, scale 1:62,500. 

Ellis, M., and Watkinson, A. J., 1987, Orogen-parallel extension 

and oblique tectonics: The relation between stretching lineations and relative plate motions: Geology, v. 15, p. 1022-1026. 

Garcia, M.O., 1979, Petrology of the Rogue and Galice Formations, Klamath Mountains, Oregon: identification of a Jurassic island arc sequence: Journal of Geology, v. 86, p. 29-41. 

Grady, K., Harper, G., Heitzler, M., 1989, N-NE obduction of 

the Josephine ophiolite along an amphibolite sole, SW Oregon [abs.]: Geological Society of America Abstracts with 

Programs, v. 21, p. 86. 

Harper, G.D., Grady, K., and Wakabaiashi, J., 1990, A structural 

study of a metamorphic sole beneath the Josephine ophiolite, western Klamath terrane, California-Oregon, in 

Harwood, D.S. and Miller, M. M., eds., Paleozoic and early 

Mesozoic paleogeographic relations; Sierra Nevada, Klamath Mountains, and related terranes: Geological Society of 

America Special Paper 255, p. 379-396. 

---and Harding, D., 1986, Comment on "Structural study of 

intra-oceanic thrusting in the Klamath Mountains, northern 

California: implications on accretion geometry" by Mathilde 

Cannat and Fran~oise Boudier: Tectonics, v. 5, p. 687-692. 

--Saleeby, J.B., Cashman, S.M., and Norman, E., 1986, 

Isotopic age of the Nevadan orogeny in the western Klamath Mountains [abs.]: Geological Society of America Abstracts with Programs, v. 18, p. 114. 

---and Wright, J. E., 1984, Middle to Late Jurassic tectonic 

evolution of the Klamath Mountains, California-Oregon: 

Tectonics, v. 3, p. 759-772. 

Hotz, P.E., 1971, Plutonic rocks of the Klamath Mountains, California and Oregon: U.S. Geological Survey Professional 

Paper 684-B, p. B1-B19. 

Irwin, W.P., 1966, Geology of the Klamath Mountains province, 

in Bailey, E.H., ed., Geology of Northern California: California Division of Mines and Geology Bulletin 190, p. 19-38. 

---1985, Age and tectonics of plutonic belts in accreted terranes of the Klamath Mountains, California and Oregon, in 

Howell, D.G.,ed., Tectonostratigraphic terranes of the circumPacific region: Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series Number 1, p. 187-199. 

---in press, Geologic map of the Klamath Mountains: U.S. 

Geological Survey Miscellaneous Field Investigations Map 

MF-2148, scale 1:500,000. 

Klaper, E.M., 1988, Quartz c-axis development and large-scale 

post-nappe folding (Wandfluhhorn Fold, Penninic nappes): 

Journal of Structural Geology, v. 10, p. 795-802. 

Lister, G.S, and Snoke, A.W., 1984, S-C mylonites: Journal of 

Structural Geology, v. 6, p. 617-638. 

Passchier, C.W., and Simpson, C., 1986, Porphyroclast systems 

as kinematic indicators: Journal of Structural Geology, v. 8, 

p. 831-843. 

Paterson, S.R., Vernon, R.H., and Tobisch, O.T., 1989, A review 

of criteria for the identification of magmatic and tectonic 

foliations in granitoids: Journal of Structural Geology, v. 11, 

p. 349-363. 

Patrick, B.E., 1988, Synmetamorphic structural evolution of the 

Seward Peninsula blueschist terrane, Alaska: Journal of 

Structural Geology, v. 10, p. 555-565. 

Raiser, S., 1990, Shear zones developed in an experimentally deformed quartz mylonite: Journal of Structural Geology, v. 

12, p. 1033-1045. 

Saleeby, J.B., 1984, Pb/U zircon ages from the Rogue River 

area, western Jurassic belt, Klamath Mountains, Oregon: 

Geological Society of America Abstracts with Programs, v. 

16, p. 331. 

Schmid, S.M. and Casey, M., 1986, Complete fabric analysis of 

some commonly observed quartz c-axis patterns, in Hobbs, 

B.E., and Heard, H.C., eds., Mineral and Rock Deformation: 

Laboratory studies, The Paterson Volume: American Geophysical Union, Monograph 36, p. 263-286 

Silberling, N.J., Jones, D.L., Blake, M.C. Jr., and Howell, D.G., 

1987, Lithotectonic terrane map of the western conterminous United States: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1874-C, scale 1 : 2,500,000. 

Simpson, Carol, and Schmid, S.M., 1983, An evaluation of criteria to deduce the sense of movement in sheared rocks: Geological Society of America Bulletin, v. 94, p. 1281-1288. 

Smith, J.G., Page, N.J, Johnson, M.G., Moring, B.C., and Gray, 

F., 1982, Preliminary map of the Medford 1 o x 2° sheet, 

Oregon, U.S. Geological Survey Open File Report, 82-955. 

Tullis, J., Snoke, A.W., and Todd, V.R., 1982, Penrose Conference Report-Significance and petrogenesis of mylonitic 

rocks: Geology, v. 10, p. 227-230. 

10 A Newly Recognized Ductile Shear Zone in the Northern Klamath Mountains, Oregon-Implications for Nevadan Accretion 


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