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KINEMATIC SIGNIFICANCE OF MYLONITIC FOLIATION (METAMORPHIC).

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8712901

Na ruk, Stephen John

KINEMATIC SIGNIFICANCE OF MYLONITIC FOLIATION

The University of Arizona PH.D. 1987

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University Microfilms International

KINEMATIC SIGNIFICANCE OF MYLONITIC FOLIATION

Stephen John Naruk

A Dissertation Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES

In partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY

In the Graduate College THE UNIVERSITY OF ARIZONA

198 7

As members of the Final Examination Committee, we certify that we have read

the dissertation prepared by stephen J. Naruk entitled ------Kinematic Significance of Mylonitic Foliation

and recommend that it be accepted as fulfilling tte dissertation requirement

Doctor of Philosophy

Date

Date /J I es.... ~~:e \'\'tI ? Date IS-It!?!','! /182 Date I

Final approval and acceptance of this dissertation is contingent upon the candidate's sUbr.lission of the final copy of the dissertation to the Graduate College.

I hereb~ertify that I have read this dissertation prepared under my uirectlon and recommend that it be accepted as fulfilling the dissertation requi ementa~ Date STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allow able without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manu script in whole or in part may be granted by the copyright holder.

SIGNED:~

ii ACKNOWLEDGMENTS

I am greatly indebted to my friends and mentors,

George Davis, Clem Chase, Steve Reynolds, Bill Dickinson, Bob Butler, Arild Andresen, and Chuc]c Thorman for advice,

encouragement, and thoughtful critcisms and reviews.

Research support was provided by SOHIO Petroleum Company,

the University of Tromso, the Geological Society of

America, Sigma Xi, the U.S. Geological Survey, the

University of Arizona Graduate College, Judge and Mrs.

Henry J. Naruk, and Regina M. Capuano.

iii TABLE OF CONTENTS

Page LIST OF ILLUSTRATIONS ...... vi

LIST OF TABLES ...... •...... viii

ABSTRACT ....•••.••••••..•••••....•..••••.•••...•..• ix

1. INTRODUCT.. ION ...... 4. 0 ~ •••••••••••••••••••••••••••••• 1

Interpretations of Foliation Surfaces as Displacement Surfaces ...... 3 Interpretations of Foliation Surfaces as Finite Flattening Surfaces ...... 3 Interpretations of Foliation Surfaces as Minimum Strain Surfaces ...... •..•. 5 Tectonic Implications ...... •...... 9 Summary of Problem and Methods ...... lO 2. BJORNFJELL ZONES ...... 12

Introduction ...... ~ ...... 12

Geometric Analysis ...... 0 ••••••• 14

Kinematic Analysis ...... t •••••• 20 3. PINALENO MOUNTAINS METAMORPHIC-CORE-COMPLEX ZONE ... 23

Int roduct i on ...... fool •••••••••••••••••••••••••••• 23 Geornet ric Analys is ...... 25 Kinematic Analysis ...... 30 4. SANTA CATALINA METAMORPHIC-CORE-COMPLEX ZONE ...... 35

Introduction ...... 35 Geometric Analysis ...... 37 Kinematic Analysis ...... 41 Northwestern Domain ...... 41 Southern Domain ...... 45 Central Domain ...... 49 Summary ...... 51

iv v

TABLE OF CONTENTS--Continued

Page

5. STRAIN SIGNIFICANCE OF S-SURFACES ...... 53

6. DYNAMIC SIGNIFICANCE OF C-SURFACES ...... 56

7. SUMMARY ...... •.....•...... •...... •.. 63

8. LIST OF REFERENCES ...... 66 LIST OF ILLUSTRATIONS

Figure Page

1. Shear zone block diagrams ...... 4

2. Geologic map of Bjornfjell mylonite zones ...... 13

3. Cross sections of Bjornfjell mylonite zones ...... 15

4. Photographs of mylonitic foliation and crenulation cleavage in Bjornfjell zones ...... 17

5. Lower hemisphere, equal area projections of Bjornfjell Zone data ...... •...... •.•.• 19

6. Geologic map of Pinaleno Mountains metamorphic-core-complex mylonite zone ...... 24

7. Lower he~isphere, equal area projections of Pinaleno Zone dnta ...... 26

8. Cross sections of Pinaleno Mountains mylonite zone ...... 27

9. Photographs of shear bands in Pinaleno Mountains mylonitic granite and mylonitic gneiss ...... •...... 29

10. Derivation of strain equation 3 ...... ,...•. 32

11. Geologic map of the Santa Catalina mylonite zone ...... 36

12. Lower hemisphere, equal area projections of Santa Catalina zone data ...... 39

13. Lower hemisphere, equal area projections of poles to di]{es ...... 42

14. Cross section of the Santa Catalina mylonite zone ...... 46

vi vii

LIST OF ILLUSTRATIONS--Continued

Figure Page

15. Stress-strain curve and failure envelope for ideal elasticoviscous material ...... 59 16. Orientations of shear bands, X, and Z in simple shear and pure shear ...... 60 LIST OF TABLES

Table Page I. Calculated and observed values of 8' in Pinaleno zone ...... " ...... 31

f II. Calculated and observed values of 8 in Santa Catalina zone ...... •...... 44

viii ABSTRACT

Geom~tric analyses of three mylonite zones, including two metamorphic-core-complex SC-mylonite zones, show that the mylonitic foliation surfaces (S-surfaces) are consistently discordant to the margins of the shear zones.

Finite-strain analyses show that the foliation surfaces in each zone are consistently oriented parallel to the XY plane of the finite strain ellipsoid. The shear bands within the mylonites (C-surfaces, C'-surfaces, extensional crenulations, and shear-band cleavages) are uniformly oriented subparallel to the margins of the shear zones.

The finite lengths and discontinuous natures of the shear bands require that the displacement along them be accomo dated by the S-surfaces at the tips of the shear bands.

Thus the S-surface elongations and orientations represent the total bulk rock strain, rather than some minimum measure of inter-C-surface strain.

General stress and strain considerations indicate that the shear bands are planes of ma,cimum shear stress, and that they are not only simple-shear slip planes. This interpretation implies that in simple-shear deformation, a

ix x single, irrotational set of shear bands will develop parallel to the shear-zone boundaries. In deformations involving significant components of coaxial strain, however, shear bands may develop in other orientations or in conjugate sets and rotate with progressive deformation. CHAPTER 1

INTRODUCTION

Slaty cleavage, crenulation cleavage, and schis

tosity are generally accepted as the finite principal

flattening surface in deformed rocks (Sharpe, 1847; Sorby, 1853, 1856; Ramsay, 1967; Gray and Durney, 1979; Davis,

1984). In contrast, the strain significance of mylonitic foliation surfaces is not well understood. Lapworth

(1885), in the original definition of mylonite as

"milled-down," brittlely comminuted fault rock, interpreted

the schistosity, or mylonitic foliation surfaces, as "grand

dislocation planes" - in effect, infinitesimally spaced

fault planes. Subsequently, mylonite has generally been

understood to be a fine-grained, cohesive, foliated fauI·t

rock, the presumed ductile analogue of fault breccia

(Tullis et al., 1982). The microstructures, grain-size

reduction processes and crystal deformation mechanisms of

mylonites have been extensively studied (e.g., Nicolas and

Poirer, 1976; Lister and Hobbs, 1979; Lister and Williams,

1979; Tullis and Schmid, 1982). Nevertheless, the basic geometric and strain significance of mylonitic foliations

has remained elusive and controversial. Although many workers have shown that the foliation in centimeter- to

I 2 meter-scale shear zones represents the finite flattening surface within the zones (e.g., Ramsay and Graham, 1970;

Mitra, 1978, 1979; Ramsay and Allison, 1979; Ramsay, 1980;

Simpson, 1981), most geologists assume that the foliation in kilometer-scale zones does not represent the finite flattening surface. Many assume that the foliation surfaces represent actual displacement surfaces (e.g.,

Lapworth, 1885; Higgins, 1971). Others assume that the foliation surfaces (S-surfaces) parallel the finite flat-

~ .'~ tening surface only between millimeter-scale shear bands (C-surfaces, cI -surfaces, extensional crenulations, shear band cleavages), and that these shear bands accomodate major translations which increase the bulk rock strain far beyond that recorded by the S-surface elongations and orientations (e.g., Berth~ et al., 1979; Simpson and

Schmid, 1983, Simpson, 1985 f 1985; Bartley, 1985). Each of these interpretations of the foliation has significantly different geometric, Itinematic, and tectonic implications.

Many of these implications warrant discussion because they contradict two common assumptions regarding shear zones: that the widths of the zones remain constant, and 'that the margins of the zones do not rotate, with progressive simple-shear deformation. 3

Interpretation of Foliation Surfaces as Displacement Surfaces If the foliation surfaces are actual displacement surfaces, i.e., the kinematic slip plane UV (Fig. 1), they

are geometrically required to be oriented parallel to the margins of the shear zone, if the rocks outside the zone

are to remain undeformed and the width and orientation of the zone remain constant (Fig. la). The sense and magni-

tude of strain and displacement across a zone, in this interpretation, can only be determined by restoring the

offset counterparts of unique markers now on opposite sides of the zone. The true strike and dip of the zone is simply equal to be the strike and dip of the foliation within the zone.

Interpretations of Foliation Surfaces as Finite Flattening Surfaces

If the foliation represents the finite flattening

plane (i.e., the XY plane of the finite strain ellipsoid having principal semiaxes X>Y>Z), the orientation of the

foliation with respect to the shear zone margins will vary systematically as a function of strain and displacement

(Fig. lb). For zones of simple shear in particular, the angle (0/ ) between the XV surface and the shear zone margin, the shear strain (7) within the zone, and the Figure 1. Shear-zone block diagrams, illustrating possible interpretations of S- and C-surfaces relative to kinematic axes U, V and W. V axis is perpendicular to the plane of the Figure. Slip in all cases occurs parallel to U in UV plane. Light solid lines represent mylonitic foliation. Heavy dotted lines prepresent mylonite-zone margins. (a.) Lapworth-type mylonite zone. Mylonitic foliation surfaces represent kinematic slip plane, UV. (b.) Ramsay and Graham-type simple-shear mylonite zone. Foliation surfaces represent XY surface of finite strain ellivsoid. (c.) SC-mylonite zone. Short heavy lines represent shear bands (C-surfaces). Arrows indicate sense of displacement. Shear surfaces parallel UV plane. Zone thickness and shear-surface orientation may remain constant with progressive shear. (d.) Mylonite zone with shear bands (C/-surfaces or extensional crenulations) at 20 0 -300 angle to UV plane. With prograssive etrain, the shear bands must rotate if zone thickness remains constant, or the thickness must decrease if the shear bands maintain a constant orienta tion. In the latter case, the foliation surfaces may rotate into parallelism with the shear bands, rather than the zone margins, at high strains. (e.) Mylonite zone with conjugate shear bands. With progressive simple shear, shear bands will rotate clockwise. With progressivs pure shear (X parallel to U and Z parallel to W), shear bands will rotate towards the foliation surfaces. In both cases, the zone thickness must decrease. 4

w >

Figure 1. Shear-zone block diagrams. 5 displacement (8) across the zone, are related by the following equations (Ramsay and Graham, 1970):

2 tan 20' = (~quation 1) 'Y

T S = f 'Y(t)dt (equation 2) t=O where: t = height above the shear-zone margin, and T = total thickness of the shear zone. According to this interpretation, the strike and dip of a simple-shear mylonite zone does not equal the strike and dip of the foliation within the zone, unless 'Y is so large that 0'= O. However, if the difference in orientation between the foliation and the zone boundary can be determined, the strain and displacement across the zone can be calculated from equations land 2.

Interpretation of Foliation Surfaces as Minimum Strain Surfaces

Many mylonites contain discretely spaced bands (C- surfaces, CI -surfaces, extensional crenulation cleavages, shear-band cleavages) in addition to the pervasive foliation surfaces (S-surfaces) recognized by Lapworth (1885),

Ramsay and Graham (1970), and Higgins (1971)(Fig. Ie-e).

The term Se-mylonite has generally been adopted for such 6

mylonites (Lister and Snoke, 1984). These spaced surfaces

are generally interpreted as millimeter-scale, planar

domains of high shear strain, or simply slip surfaces~

that serve to increase the bulk-rock finite strain far

beyond that recorded by the angle 0' (Berth~ et ale (1979); Ponce de Leon and Choukroune, 1980; Simpson and Schmid,

1983; Simpson, 1984, 1985; Lister and Snoke, 1984; Bartley,

1985).

In this interpretation, the relative orientations

of foliation surfaces (S-surfaces) and shear bands (C-sur faces, C'-surfaces, extensional crenulation cleavages, shear band cleavages) reflect the senses of shear strain and displacement, but not the magnitudes. The foliation surfaces are assumed to rotate with increasing shear strain, but are also assumed to be oblique to the shear zone margins, except at very high strains. The foliation surface orientations are variously assumed to record some minimum measure of finite strain (Berthe et al., 1979;

Simpson and Schmid, 1983; Simpson, 1984, 1985), or some measure of infinitesimal or incremental strain (Lister and

Snoke, 1984).

Lister and Snoke (1984) make a significant kine matic distinction between granitoid mylonites (type I SC mylonites) and quartzite mylonites (type II SC-mylonites).

They explicitly leave open the possibility that the S surfaces in type I SC-mylonites may represent the finite 7

XY strain surface. However, they do explicitly interpret

the foliation surfaces in type II SC-mylonites as dis

placement surfaces.

The geometric and kinematic significance of the spaced shear bands (C-surfaces, C'-surfaces, extensional

crenulations, shear-band cleavages) is unknown. Physically

identical shear bands have been interpreted to be parallel

to shear-zone margins (Fig. lc) (C-surfaces; Berthe et

al., 1979); to occur parallel to shear-zone margins at low strains, but at 200 -300 to the margins at high strains (C

and C/-surfaces, respectively; Ponce de Leon and

Choukroune, 1980); and to occur in single and conjugate

sets oriented at 200 -300 to the shear-zone margins (Fig.

ld, Ie) (extensional crenulation cleavage, Platt and

Vissers, 1980; shear-band cleavage, Weijermars and Rondeel,

1984) . Apparent conjugate sets of shear bands have been

interpreted as resulting from simple-shear deformation

(Ponce de Leon and Choukroune, 1980; Harris and Cobbold,

1985), and from coaxial (pure-shear) deformation (Platt and

Vissers, 1980).

Each of these geometric interpretations has sig

nificantly different kinematic implications. If the shear bands parallel the shear-zone margins (Fig. Ie); (I) the strike and dip of a zone may be easily determined from the strike and dip of the shear bands; (2) the true thickness 8 of a zone may remain constant with progressive deforma tion; (3) the shear bands will not rotate with progressive deformation; and (4) the foliation surfaces may be rotated a maximum of only 450, into parallelism with the shear bands and the zone margins. However, if the shear bands occur in a single, synthetic set oriented at 20°-30° to the zone margins (Fig. 1d); (1) the strike and dip of the zone must be 20°-30° from the strike and dip of the bands;

(2) the zone thickness must change, or the zone margins must rotate, with progressive deformation; (3) the shear bands must rotate with progressive deformation; and (4) the foliation surfaces may rotate 65°-75°, into parallelism with the shear bands and into 20°-30° discordance with the zone margins. Alternatively, if the shear bands occur in conjugate; synthetic and antithetic sets oriented at 20°-

30° to the zone margins (Fig. Ie); (1) the shear bands must rotate with progressive deformation; and (2) the true thickness of the zone must decrease with progressive deformation.

Thus mylonitic foliation surfaces have been vari ously interpreted as displacement surfaces (Lapworth,

1885; Higgins, 1971; Lister and Snoke, 1984 (mylonitic quartzites», as finite flattening surfaces (Ramsay and

Graham, 1970), and as having no particular finite-strain significance (Berthe et al., 1979; Simpson and Schmid,

1983; Simpson, 1984, 1985). The geometric and kinematic 9 significance of the spaced shear bands is equally unclea~, even though they are uniformly interpreted as displacement surfaces or planar domains of very high shear strain.

Tectonic Implications

This general lack of understanding of mylonites, particularly SC-mylonites, is significant for its impact on tectonic interpretations. In the western North American

Cordillera in particular, SC-mylonites are the major, characteristic structure of the metamorphic core complexes that comprise the Cordilleran hinterland (Davis and Coney,

1979; Crittenden et al., 1980; Lister and Snoke, 1984). In general, these core complex mylonites have been interpreted as zones of major thrusting (e.g., Misch, 1960), zones of major ductile normal faulting (e.g., Davis, 1983, 1987;

Wernicke, 1985; Davis et al., 1986), zones of thrusting reactivated as ductile normal faults (e.g., Thorman, 1977;

Drewes, 1976, 1977, 1978), and zones of coaxial stretching

(e.g., Rehrig and Reynolds, 1980; Miller et al., 1983).

The different geometric and kinematic interpre tations of mylonitic foliation surfaces and shear bands have only fueled these controversies. For example, the foliation ~urfaces in the core complex mylonites generally define broad, doubly plunging antiforms with trends parallel and perpendicular to the mylonitic lineations.

The assumption that the foliation surfaces represent 10 displacement surfaces oriented parallel to the mylonite zone boundaries has lead to the conclusion that the zones are arched or folded (e.g., Keith et a1., 1980; Spencer,

1984; Wernicke, 1985; Reynolds, 1985; Davis et al., 1986).

However, interpretation of the foliation surfaces as finite flattening surfaces contributes to the conclusion that the antiforms trending perpendicular to lineation, represent variations in shear strain, and that the zones are planar (e.g., Davis, 1983). The mere occurrence of shear bands (C-surfaces) has been cited as evidence of simple shear (Reynolds, 1985; Davis et al., 1986), and both constant shear band (C-surface) orientations (Naruk,

1986) and variable shear band (C-surface) orientations

(Reynolds, 1985; Davis et a1., 1986; Reynolds and Lister,

1987), have been cited as evidence of uniform-sense simple shear.

Summary of Problem and Methods

Different interpretations of mylonitic foliation surfaces and mylonitic shear bands result in radically different structural and tectonic interpretations of kilometer-scale mylonite zones. To resolve these different interpretations, geometric and kinematic analyses of foliation surfaces and shear bands were carried-out in three different mylonite zones. The first zone is a

Caledonian mylonite zone with components of reverse-slip 11 simple shear and volume loss. The second zone is a Cor- dilleran metamorphic core complex zone of normal-slip simple shear, in which the foliation does not define an antiform. The third zone is a Cordilleran core complex zone in which the foliation does define an antiform, and which involves components of normal-slip simple shear, and normal-slip simple shear plus coaxial flattening or volume loss.

Each zone is one-half kilometer or more in struc tural thickne~s, and tens of kilometers in strike length.

The boundaries of each zone were determined by mapping the contacts between the mylonites and their non-mylonitic equivalents. The orientations of the boundaries were subsequently calculated from structure contour maps, and compared with the orientations of the various mylonitic surfaces measured in the field. The finite strain magni tudes and principal orientations within each zone were calculated from displacements of contacts and dikes which cross the zones, and the calculated orientations were compared with the observed orientations of foliation surfaces. CHAPTER 2

BJORNFJELL ZONES

Introduction

The Bjornfjell mylonite zones are located along the northern edge of the Rombak structural window in the northern Scandinavian Caledonides (Fig. 2). The general geology and tectonics of the region have been most recently reviewed by Stephens et a1. (1985), Tull et a1. (1985), and Gee et ale (1985). Most of the Rombak Window exposes

1.7 b.y. old, undeformed Rombak Granite (Gustavson, 1974).

Along the perimeter of the Window, a thin sequence of late

Proterozoic- to Cambrian-age quartzite and schist are locally preserved in original depositional contact on the granites. These autocthonous, miogeoclinal rocks are tectonically overlain by four major, southeast-vergent thrust nappes composed of miogeoclinal and eugeoclinal roclcs.

The Bjornfjell zones represent an approximately 10 lcilometer-long system of east-dipping, reverse-slip shear zones that are analogous to late-kinematic back-thrusts in the southern Norwegian Caledonides (Thon, 1980), the U.S.

Sierra Nevadas (Day et al., 1985), and the Canadian

Cordillera (Murphy, 1987). At the northern end of the

12 13

1km

Schist

Quartzite

Mylonitic Granite

Figure 2. Geologic map of Bjornfjell mylonite zones. Inset map of Scandinavia shows location (star). 14

Bjornfjell zones, the individual zones are interconnected

by a broad zone of foliated but non-mylonitic granite

(Fig. 2). Here the zones fold both the autocthonous rocks

and the overlying nappe complexes about shallowly north

plunging axes (Fig. 3). To the south, towards the center

of the Window, the zones cut progressively down-section,

truncating the autocthonous rocks, and branching into smaller, diverging and anastomosing zones at the southern most end of the system. Outside of these zones, the granite and quartzite exposed within the Window generally

retain primary igneous and sedimentary fabrics. The granite typically has a coarse-grained, hypidiomorphic

granular texture, and the quartzite locally contains ripple marks and beds of undeformed pebble conglomerate.

Within the mylonite zones, both granite and quartzite are pervasively mylonitic.

Geometric Analysis

The main Bjornfjell zone has well-defined boundar ies for most its length. In the area of cross sections BB' through EEl, there is sufficient structural relief to construct structure-contour maps of the boundaries and compare the foliation orientations directly with the orientations of the shear zone boundaries (Fig. 3). The lower boundary is the contact between the undeformed, autocthonous quartzite and the overlying, strongly cleaved Figure 3. Cross sections of Bjornfjell mylonite zone. Light solid lines represent foliation trajectories. Boundary of mylonite zone shown by dashed line where constructed from structure contour data, and by dotted line where projected. Patterns as in Figure 2, except in cross sections BB' - FF', where patterns for schist and mylonitic granite have been omitted. 15

+500m

Sea level

I B B 800m

, c c' E E 800 m 800m

600 m 600 m

·~,,~,,~:~:,,\),~~·:~)~.!3;~) 400m I.;.;"':"':';;;:';;~;":~"'~,,",---: _.. .:.~'";;~,,,\' ..~.\ .. ',.,' \,,";;.'\ .. '~':.;.\',;;':.;',;"..:' '_' .. "~ 400 m

D F 800 m 800 m

Figure 3. Cross sections of Bjornfjel1 mylonite zone. 16

schist. This boundary strikes N25°-30 0 E and dips approximately 40 0 east, parallel to the autocthonous quartzite

bedding. The upper boundary of the zone is defined by a

few-meter-wide gradational contact between coarse-grained,

hypidomorphic-granular granite, and strongly foliated and

lineated mylonite derived from the same granite. This

upper boundary strikes N30 0 -35°E, and dips an average of

40 0 east. In detail, the dip decreases structurally

downward, from 900 at the highest elevations (630 m eleva

tion), to 300 -40c at the lowest exposed elevations (470 m

elevation) (see Fig. 3).

Two types of S-surfaces, a mylonitic foliation and

a crenulation cleavage occur, within the zone. These are oblique to the upper boundary of the zone, and subparallel

to the lower boundary. The mylonitic foliation occurs in the granite, where it is defined by ribbons of quartz and

feldspar, and planar alignments of feldspar augen (Fig.

4a). The crenulation cleavage occurs in the schist, and

is defined by planar concentrations and alignments of phyllosilicates (Fig. 4b). Both the mylonitic foliation

and the crenulation cleavage strike NIoo-400 E and dip 50°-

800 E (Fig. 5). The dip angle generally decreases from

approximately 75 0 at the upper boundary of the zone, to

approximately 45 0 at the lower boundary (see Fig. 3). The

foliation and cleavage trajectories thus are discordant to the upper boundary of the zone, and subparallel to the 17

Figure 4. Photographs of (a.) mylonitic foliation and (b.) crenulation cleavage in Bjornfjell mylonite zone. 18 lower boundary, The trajectories generally resemble half .. of an ideal reverse-slip; simple-shear zone.

Both extension and intersection lineations occur within the zone. The mylonitic foliation surfaces contain a well-developed, steeply plunging lineation defined by pronounced down-dip elongation of quartz ribbons, feldspar porphyroclasts, and mica porphyroclasts. The crenulation cleavage surfaces locally contain two lineations. One is a moderately developed, steeply plunging mineral lineation that parallels the mylonitic lineation (Fig. 5). The second is an intersection lineation that plunges variably northeast, parallel to crenulation axes and minor fold axes (Fig. 5). The physical character of the mylonitic lineation indicates that it is an extension lineation.

The fact that the mineral lineation in the schist is parallel to the mylonitic lineation indicates that the mineral lination is also an extension lineation. The intersection lineation represents only the line of intersection between the surface being crenulated (Sn), and the imposed cleavage surface (Sn+1), and has no particular kinematic significance.

Shear bands are rare in the Bjornfjell zone. The observed shear bands (C-surfaces) uniformly dip east less steeply than the foliation surfaces, indicating a reverse sense of shear. Complete strikes and dips were obtained for only two shear bands. These strike parallel to the 19

~OO/ • • ~30'"

lineation

N N

~.,.

lineation

Figure 5. Lower hemisphere, equal area projections of data from Bjornfjell zone. a.) Mylonitic lineation (n=14) and poles to mylonitic foliation (S-surfaces)(n=41) in granite. b.) Mineral lineations (n=8) and poles to crenulation cleavage (n=24) in schist. c.) Intersection lineations, crenulation axes and minor fold axes in schist (n=32). 20

zone boundaries, but dip 600 and 680 southeast in an area where the zone boundaries dip 40 0 southeast. Thus the

shear bands are oblique to the boundaries by angles of

20 0 -280 , but in a sense opposite to that expected for C -

surfaces, extensional crenulation cleavages or shear band cleavages. For the observed sense of shear in the

Bjornfjell zone, these latter types of surfaces would be expected to dip 100 -200 southeast.

Kinematic Analysis

The angular discordance between the foliation

surfaces and the mylonite-zone upper boundary indicates

that the foliation surfaces cannot be displacement sur

faces. Material cannot be translated parallel to the

foliation trajectories without deforming the granite

outside the upper boundary of the zone, and such deformation is not observed.

The finite strain analyses of Gray and Durney

(1979) show that crenulation cleavage surfaces, in general,

are always within 40 or less of the XY surface of the finite strain ellipsoid. In the Bjornfjell zone, the

parallelism of the two types of surfaces thus indicates that the mylonitic foliation surfaces are also within 4 0

of the finite XY surface.

The foliation in the lowest 20 m of the zone is subparallel to the lower boundary of the zone. In this 21

area, 0' can only be constrained as ~ 8°. At such low

values of 0 1 (corresponding to 7 L 7; 1+e1/1+e3 > 50/1) the foliation surfaces may become active slip surfaces,

although no evidence of such slip was observed in the field.

1 Comparison of cross sections BB' through FF

indicates that the thickness of the Bjornfjell zone de-

creases from 240 m in Bt to a constant 145-160 m in DD' , EE 1 and FF I , corresponding to an area decrease of 33%-42%.

Interpretation of this decrease as representing actual

volume loss is consistent with the conclusion that crenu-

lation cleavage develops primarily through pressure solu-

tion (Gray, 1979; Gray and Durney, 1979), and presumably

entails significant mass transfer and volume decrease. In

the Bjornfjell zone, this volume decrease implies that the

values of 0' , at least in the schist, are a function of both simple-shear strain and volume change, and are not

only a function of simple shear.

In summary, the mylonitic foliation surfaces of the Bjornfjell zone parallel the crenulation cleavage

within the zone, and thus are oriented within 4 0 of the bulk, finite XY strain surface. At the high-strain lower

margin of the zone, the foliation surfaces are subparallel

to the margin, and may act as slip surfaces. The orienta-

tion of the foliation surfaces, and the resultant values

I of 0, are not exclusively a function of simple-shear 22 strain, but rather are a function of both shear strain and volume change. The shear bands within the zone are dis-

cordant to the margins by 200 -280 , but in a sense opposite to that expected for CI -surfaces, extensional crenulations, or shear-band cleavages. CHAPTER 3

PINALENO MOUNTAINS METAMORPHIC-CORE-COMPLEX ZONE

Introduction

The Pinaleno Mountains metamorphic-core-complex

mylonite zone strikes west-northwest and dips moderately

northeast along the northern flank of the Pinaleno Moun

tains of southeastern Arizona (Fig. 6). The zone cuts

across a variety of non-foliated igneous rocks, and am

phibolite-grade metamorphic rocks with northeast-trending

contacts and a northeast-trending, steeply dipping crys

talloblastic foliation. Within the mylonite zone, mylon

itic foliation surfaces, lineations, and shear bands are

well-developed in all roc}c types. The shear bands resemble both C-surfaces and extensional crenulation cleavages.

The geology of the non-mylonitic rocks is described in detail by Thorman (198la, 1981b), and the geology and

geometry of the mylonite zone are described in detail by

Davis (1980) and Naruk (1986). Detailed map relations are

shown by Houser et al., (in press, a, b).

23 "tINI:j .-. 0 .-. ::s ::s 0;), III m f! ~ .. .., } m m 20 , ::so ....~ en r+. =~/~~'1:::'''-'''-,,,;-, 20 /'~ 3:::r' "','I' ", 'i/~;:'~I '/---:/ ~ 2 o 7-'''-'''_,''",- ,'/, 0 / /'",~~ /, f! .-. G') wlY.'I/,I/,I. 2 ;"'''_~''" ,'-',, ~ ::s ::s m '" '''~'''-~'''20./ "\,..... ""/,'/,', " ("t-CilO , l» m ~ 1/,1,"'.....,,'I/"/,~/ "'-'"-'~''''' '-'''>-'/-~''(-''(~20''' ,'/,'/ 1/ '" .-. r+ 0 ,-:""-~",-, / ,,,,//"-", - , '-' "~-'/ ",I /,1, / '-''''-,-2~/_. ~ ".... ::s 0;), : : : : : . . 0 : : : : ,I,,: til S to'. ,'.'(.'.'~,~~"js-iS.f'~i~~':':,":,. ,'/ __ / ...... '" . I'll n "'*~,,':.,,,--'"'~.6@'''J ~ "//'" ...... 'd " ' '"' "' '" " , -' 0 , ...... \ S 'j/';-"",o~ ""/"'~~ '-"-",",'20... " ...... 1. I'll ..... o '/"""-'~'''-'/." \'/\I'-"'/'~'V-', 2" ~"-. ...•.••.•.~...... ::'~ ,. 27 to!,"C ~'-,',''/'/ /,1/,1 "/~J?"'",-"~~" ~"'""\,~-, '/\, 1)/"/' ...... )'0 '" ( "":':::":;.,;R:. • ••.• : : . 3p/ >0 ~''-~''-~'';:' ';;'~"~";!'~-'!~'''''o /,1 0 •.•. .••.. .., to!, "//'((i'/" ";:-'''-' .... -, \- ~/ J .. :~ ~. 0 ••• 0 : : : • '13 . • •••••• \ .... ''-''-''-'i.... '" ',/' ,/~"" ''.., -"" "-='/ ...." '"' -II • " •••• 16 • N "tI .. ,"~/.....,, . '/-,-""(>-'/'-'/\-~"\-~/\I-:\I/~i/.'..... " // //", ' ...... ~~: ~.... ~."'~":.~.~ ~ Q o .-. -,'c "i"'" '" "-"-" ,' ...... '. Oil;'• . . . .•...•• .: ..... ::s t:S " "', , ' ">'-"-"" ';;" ...... 4S . -",CIJ o '-V-J,., \;;", ""'" .:...... : ...... " .... ., I'll I» \7:. \V\~/~I/' \/' '/-'/- , J ...... '';'''-'' ...... -~ ~ 0", c,' '_Y_, ":: .. :: :: ...... ~ ::::: ...... "/-\"• " , - '"- ,,,,,-...... -. • •••••••••••••••••••••••••• 70 • ••• 0 ••••••••••••••••••• ! Cil m o~. ::r'::s -'!{,. 0' ,-' ':::::::::::::::::::::::"':::::::::::::::::::::::.;c::-: IS o 0 v 0...... ::;,..- ..... '" ::: : ":':" .-. :s: .....'" '. A:..-:::::::::::::::60 ...... 55.::::::. ::s 0 Ir.!.f! ::s ~("t- o I'll ~ Mylonitic. foliation & lineation n .-. I'll t:S Crystal I oblast ic foliation o ("t-tIl .... , Mylonite-zone boundary o a ::s« N ~ D Alluvium o 0 Q to!,::s " ..... f;!-'J Granite & granodiorite rt" m ~ Porphyritic gneiss ~ L:...:...:J Quartzofeldspath ic gneiss N o 0.5 1 2 km ~ § Biotite schist 25

Geometric Analysis

The lower boundary of the mylonite zone is a 10-

20 m thick gradational contact between non-mylonitic rocks

to the southwest, and their mylonitic equivalents to the

northeast (Naruk, 1986). Along its trace length, the

boundary ranges in strike from N600W to N90 0 W, and in dip

from 200 northeast to 360 northeast (Fig. 7) (Naruk, 1986).

The mylonitic foliation surfaces (S-surfaces)

throughout the zone are a pervasive fabric defined by the

coplanar alignment of quartz ribbons, feldspar ribbons, mica ribbons, and the long dimensions of feldspar por

phyroclasts. These foliation surfaces strike N45°-75°W

and dip 10°-250 northeast (Fig. 7). The surfaces are

everywhere oblique to the lower boundary of the zone (Fig.

8). Unlike the foliation surfaces in many metamorphic

core complexes, the foliation surfaces in the Pinaleno

zone do not define an antiform trending parallel to the

strike of the zone.

The lineation occurs on both foliation and shear

surfaces, but is normally observable only on foliation surfaces. On both types of surfaces, it is defined by the

linear alignment of extremely elongate quartz ribbons and feldspar and mica porphyroclasts. It trends uniformly

N40 0 E (Fig. 7). Figure 7. Lower hemisphere, equal area projections of data from Pinaleno zone. In a-e, contours equal 1, 10, 20 and 40% of data points per 1% of area. (a.) Poles to lower boundary of zone (n=2l). (b.) Lineations (n=310) and poles to foliation (S surfaces)(n=439). (c.) Poles to shear bands (C-surfaces) in mylonite derived from granite and granodiorite (n=77). (d.) Poles to shear bands (extensional crenulation cleavages) in mylonite derived from quartzofeldspathic gneiss (n=7). (e.) Striation lineations (n=30) and poles to minor faults and shear zones (n=41) structurally below mylonite zone. (f.) Synoptic diagram comparing mean orientations of foliation (N55°W, 20+7°NE), lower boundary of shear zone (N63°W, 28+6°NE), C-surfaces (N52°W, 32+8°NE), extensional crenulation cleavages (N46°W, 28+6°NE), and minor faults and shear zones (N73°W, 34+100 NE). Foliation surfaces consistently dip approximately 20 0 northeast, while the boundary of the shear zone, C-surfaces, extensional crenu lation cleavages, and minor faults all dip approximately 300 northeast. 26

N N

fOI.@

N N

ext. c~ens'-:8e.Loliation fltS'~ c-surfaces ---lc:J-faults \. lower ff boundary

Figure 7. Lower hemisphere, equal area projections of data from Pinaleno Mountains mylonite zone. Figure 8. Cross sections of the Pinaleno Mountains mylon ite zone. Random dash pattern represents· non-mylonitic rocks. Gravel pattern represents Quaternary-Tertiary gravels. Heavy solid and dotted lines represent the mapped and projected portions of the lower boundary of the mylonite zone. In AA' and BB', light solid lines represent foliation (S-surface) trajectories, and short heavy lines I J represent shear bands. In CC and DD , heavy sol id 1 ines represent 1 i thologic contacts. Angles a and a: represent angles used to calculate shear strains. 27

B s' 2.5 km

2·0

1·5

1·0

c c' 2.0 km

1·5

1·0

I D D 2·0 km

1·5

1·0

Figure 8. Cross sections of Pinaleno Mountains mylonite zone. 28

The shear bands throughout the zone strike N20o-

90 0 W and dip 20 0 -500 northeast. They are subparallel to the lower boundary of the zone, and also subparallel to minor normal-slip faults and shear zones structurally below the boundary (Fig. 7). In mylonites derived from granite and granodiorite, the shear bands are a spaced planar fabric defined by syst~~~tic, coplanar alignment of the ends of sigmoidally shaped po~phyroclasts and ribbons.

The shear bands are approximately 1 mm thick and 10 cm in length, and are spaced approximately 1 cm apart.

Mineralogically, the bands are comprised of major propor tions of quartz and feldspars" and do not contain any high concentrations of phyllosilicates, opaque, or mafic miner als, such as ~ight be construed as insoluble residue related to pressure solution. These bands are excellent examples of those typically described as C-surfaces (Fig.

9a).

In mylonites derived from the gneisses, the shear bands resemble those described as C -surfaces, extensional crenulations and shear band cleavages (Fig. 9b). The Pinaleno bands are subparallel to the zone margin, rather than at a systematic 200 -30 0 angle to it, however (Fig.

7e). These surfaces are also only a few centimeters in length, and are not continuous, through-going shears (cf.

Weijermars and Hondeel, 1984). 29

b. Figure 9. Photographs of (a.) mylonitic granite with shear bands that resemble C-surfaces, and (b.) mylonitic gneiss with shear bands that resemble extensional crenulations. 30

Kinematic Analysis

The obliquity of the foliation surfaces to the lower boundary of the mylonite zone indicates that the foliation trajectories are not slip trajectories. The

orientations of the shear bands, in contrast, are consistent with their interpretation as slip planes.

The finite strain within the zone, and the orientation of the finite XV surface, can be calculated from contact displacements. In the area of cross section eel, a lithologic contact is rotated from N25°E, 18°SE outside the mylonite zone, to N34°W, l8°NE within the zone (see

Fig. 8)(Naruk, 1986). In the area of cross section DD I , a

lithologic contact is similarly rotated from N400E, 16°SE outside the mylonite zone, to N40 o W, 13°NE within the

zone. Assuming exclusively simple-shear deformation, the changes in contact orientations are related to the shear strain by the following equation (Fig. 10)(Ramsay, 1967):

I 'Y = cotan a cotan a (equation 3) where; 'Y = shear strain; a = the angle between the U kinematic axis and the line of intersection of the unsheared 31

contact with the UW plane); and I a = the angle between the U axis and the line of intersection of the sheared contact with the

UW plane.

The results of equation 3 can then be used in equation 1 to calculate the orientation of the finite KY surface. The

1 results are listed in Table I, where f}calculated represents the calculated angle between the KY surface and the _I mylonite zone boundary, and 60bserved represents the observed angle between the foliation surfaces (S-surfaces) and the boundary. The calculated and observed angles agree within the combined standard deviations of + go.

Table 1. Calculated and Observed Values of 8'

in Pinaleno Zone I I Domain 'Y 8Calculated OObserved

I CC 2.5 + 0.8 200 + 40 140 + 8 0

DD' 1.6 + 0.5 250 + 40 190 + 80

One implication of the preceding analysis is that integration of the shear strain across the Pinaleno zone should yield the minimum displacement across the zone

(equation 2). The integral represents the minimum displacement in this case because the upper boundary of Figure 10. Derivation of equation 3. a.) Undeformed state. The position of any point in plane P can be specified in terms of coordinates Ul, Vl, Wl. The angle a ,between U and the line of intersection between planes P and UW is given by:

Wl tana = ( i ) Ul

b.) After simple shear 7 parallel to U. The new coordi nates of (Ul, Vl, Wl) are given by:

o Ul + I Vl (ii) = = ( o Wl

W2 Wl I and tana = = (iii) U2 Ul + \\11

Solving i and iii for Wl, equating the two expressions ~nd eliminating Ul and Wl yields equation 3:

1 I 'Y = tanaI tana 32

W A:\

Figure 10. Derivation of strain equation 3. 33

the zone is not exposed, and the full width of the zone is

unknown.

Integration of the Pinaleno shear strains, calcu-

lated on the basis of foliation orientations and equation

1, yields a minimum displacement of 7 km (Naruk, 1987).

In comparison, the offset along the apparent shallow-level

continuation of the Pinaleno mylonite zone, the Eagle Pass

detachment fault (Davis, 1980; Davis and Hardy, 1981;

Spencer, 1984), is between 6 km and 9 km, as estimated

from an offset contact (Naruk, 1987). The agreement between the two displacement estimates verifies the pre-

ceding interpretation of the Pinaleno foliation surfaces

as finite XY surfaces.

Naruk (1986) used a different, and unfortunately

incorrect, method to calculate the strains in the Pinaleno

zone. This method equated the results of an irrotational

strain calculation (Ramsay, 1967; eqns. 4.2la-4.2lc) directly with the results of a simple-shear, and hence

rotational, strain calculation (equation 1). This is equivalent to equating the two transformation matrices;

0 0 Sy and 1 0 (~ 0 ~) without multiplying the left-hand matrix by an appropriate

rotation matrix. In general, the use of irrotational 34 strain equations to calculate simple-shear strains yields significant overestimates of 7. In the case of the

Pinaleno zone, the irrotational strain calculations of

Naruk (1986) overestimate 7 by 70%. In summary, the discordance between the foliation surfaces (S-surfaces) and the lower boundary of the mylonite zone indicates that the foliation trajectories do not represent displacement trajectories, and that the foliation surfaces do not represent displacement surfaces. Further- more, the foliation surfaces are within at least +9 0 of the XY surface of the bulk rock finite strain ellipsoid.

Thus the foliation surfaces appear to represent the bulk rock finite flattening surface. The shear bands, including those that resemble CI -surfaces and extensional crenulation cleavages, are consistently oriented subparallel to the boundary of the mylonite zone. The observation that the shear bands parallel minor faults and shear zones with demonstrable displacements, supports interpretation of the shear bands as millimeter-scale domains of displacement and very high shear strain. CHAPTER 4

SANTA CATALINA METAMORPHIC-CORE-COMPLEX ZONE

Introduction

The Santa Catalina mylonite zone occurs along the southwest flank of the Santa Catalina Mountains in south eastern Arizona (Fig. 11). These and the adjacent Rincon

Mountains contain a variety of Proterozoic, Mesozoic and

Cenozoic intrusives, as well as Paleozoic and Mesozoic sedimentary rocks, that have been affected by at least one pre-Eocene deformational event and one post-Eocene deform ational event (Keith et al., 1980; Bykerk-Kauffman and

Janecke, 1987). The mylonites of the Santa Catalina zone are derived primarily from the Eocene granite, and thus are late Eocene or younger in age. The northeastern boundary of the mylonite zone is a gradational contact with non-mylonitic granite. The southwestern boundary of the zone is the southwest-dipping, normal-slip, Santa Catalina detachment fault (Fig. 11).

35 36

0 5 10 km

15~ 20 •• ~ Mylonitic foliation & lineation ~'··' Mylon1te-zone. boundary ~ Detachment fault ~ Northwestern domain Central domain Southern domain Undifferentiated mylonites Metasedimentary rocks Granites & quartz diorite

Figure 11. Geologic map of the Santa Catalina mylonite zone, with inset map of Arizona, showing location of Santa Catalina Mountains. 37

Geometric Analysis

The northeastern boundary of the mylonite zone is a gradational contact between non-mylonitic Eocene granite, and pervasively foliated and lineated mylonite derived from the same granite. Along the northeastern side of this contact, the granite commonly contains a weak, north east-striking, steeply-dipping foliation defined by biotite schlieren and by alignment of quartz and biotite grains.

Local flattening of quartz grains indicates that this foliation is not a primary igneous fabric. This fabric decreases in intensity to the northeast, grading into granite with a primary-igneous, hypidiomorphic-granular texture.

Structure contours and three-point geometric solutions of the northeastern boundary show that west of point a (see Fig. 11), the boundary is a planar, grada tional contact with a strike and· dip of N550 W, 210 +

100SW. East of point a, the mapped trace permits two possible interpretations; the contact may be either trough shaped, plunging approximately 40°, S65°W, or it may be planar with a strike and dip of N35°W, 27°NE.

Within the mylonite zone, mylonitic foliation surfaces (S-surfaces), lineations, and shear bands (C- and 38 , C -surfaces) are all well developed. As in the Bjornfjell and Pinaleno zones, the foliation surfaces are a pervasive

planar fabric defined by the parallel alignment of quartz ribbons, phyllosilicate ribbons, and feldspar ribbons and

porphyroclasts. The shear bands (C- and C'-surfaces) are a spaced planar fabric defined by parallel alignment of

asymmetric, retort-shaped porphyroclast tails and ribbon tails. The lineations are defined by pronounced, parallel elongation of pophyroclasts and ribbons within the foliation surfaces.

The orientations of foliation surfaces and shear bands define three different subdomains that trend approx imately parallel to the margins of the zone (Fig. 11).

The northwestern domain represents approximately the northeastern third of the zone west of point a. The orientations of the foliation surfaces and shear bands in this domain define a normal, southwest-vergent sellse of shear. Foliation surfaces generally dip 5° northeast, lineations trend N300E, and shear bands strike N35°W and dip 200 + 6°SW (Fig. 12). The shear bands thus parallel the mylonite-zone boundary in this domain, while the foliation surfaces are significantly oblique to it.

The central domain represents approximately the middle third of the zone west of point a (Fig. 11). The southern boundary of the domain approximately coincides Figure 12. Lower hemisphere, equal area projections of mylonitic fabrics in the Santa Catalina zone. (a.) Poles to foliation along the lower boundary of the mylonite zone. (b.) Lineation and poles to foliation surfaces (S surfaces) in northwestern domain. (c.) Poles to shear bands in northwestern domain. (d.) Lineation and poles to foliation surfaces (S surfaces) in central domain. (e.) Poles to shear bands in central domain. (f.) Lineation and poles to mylonitic foliation (S surfaces) in southern domain. (g.) Poles to shear surfaces in southern domain. 39

Fo~ D.

Figure 12. Lower hemisphere, equal area projections of mylonitic fabrics in Santa Catalina zone. 40 with the hinge of the Forerange arch. In this domain, the foliation surfaces and shear bands define a normal, but northeast-vergent, sense of shear. The foliation surfaces generally dip 120 northeast, and the lineations trend

N45°E. The shear bands generally strike N35°W and dip 300

+ 70 northeast (Fig. 12). The shear bands are thus parallel to one of the possible orientations of the mylon ite-zone boundary east of point s.

The southern domain extends from approximately the hinge of the Forerange arch to the detachment fault. The foliation surfaces and shear bands in this domain define a normal, southwest-vergent sense of shear, as in the north western domain. The foliation surfaces generally dip 120 southwest, the lineations trend S65°W, and the shear surfaces strike N35°W and dip 210 + 7 0 southwest (Fig.

12). The shear bands in this domain thus are parallel to the shear bands and the mylonite-zone boundary in the northwestern domain.

Within each domain, the sense of shear is invari ant. Except along domain boundaries, no northeast-vergent shear indicators occur in the southwest-vergent domains, and no southwest-vergent indicators occur in the northeast- vergent domain. The boundaries between domains are defined by distinct reversals in the dip directions of the shear bands, although the foliation and lineation orientations 41

change continuously between domains. Along the boundary

between the northwestern and central domains, the dips of

the shear bands change from 200 southwest in the

northwestern domain, to 300 northeast in the central domain. Along the boundary between the southern and

central domains, southwest-vergent shear bands appear to

be rotated approximately 100 , from a dip of 210 southwest

to a dip of 100 southwest. These gently southwest-dipping

shear bands are cut by shear bands which dip approximately

30 0 northeast.

Kinematic Analysis

Northwestern Domain

In the northwestern domain, as in the Bjornfjell

and Pinaleno zones, the discordance between the foliation and the boundary of the zone indicates that the foliation

surfaces are not slip su~faces. Finite strain analyses described below further indicate that the foliation sur

faces parallel the XV surface of the finite strain ellip soid.

Shear strains within the northwestern domain can

be calculated from the changes in orientation of aplite and pegmatite dikes (Fig. 13). In the undeformed granite

north of the lower boundary of the zone, peraluminous aplite and pegmatite dikes are undeformed, and have a 42

Figure 13. Lower hemisphere, equal area projections of poles to; (a. ) undeformed dikes, (b. ) deformed dikes 100 m above the mylonite zone boundary, and (c. ) deformed dikes 600 m above the mylonite zone boundary. 43 well-constrained mean orientation of N400W, 540 + l6°NE.

Within the mylonite zone, the same dikes are pervasively deformed and reoriented. Approximately 100 m above the lower boundary of the mylonite zone, these dikes have an orientation of N55°W, 150 + 5°NE. Structurally higher in the same domain, 600 m above the boundary, the dikes have an orientation of NooE, 60 + 3°E.

The lines of intersection between the mylonite zone boundary and the undeformed dikes, and between the mylonite-zone boundary and the deformed dikes, do not change orientation significantly. One-hundred meters above the boundary, the two lines differ by only 100.

Six-hundred meters above the boundary, the two lines of intersection have exactly the same orientation. This constant orientation indicates that ey = 0, or that the deformation is a plane strain.

Assuming that the lower boundary of the mylonite zone equals the shear plane of simple shear, the bulk-rock finite shear strains can be calculated from the dike orientations and equation 3 (see Fig. 10). The orientation of the finite XY strain plane can then be calculated from equation 1. Table II lists the results of these strain calculations, and compares the observed 'orientations of the mylonitic foliation surfaces (O~bS), with the calculated orientations of the finite XY surface (O~alc). 44

The results show very close argeement between the observed foliation orientations and the calculated orientations of the finite XY strain plane.

Table II. I Calculated and Observed Values of {) in Santa Catalina Zone

Height above mylonite-zone I I boundary 'Y 8Calculated 80bserved

100 m 1.2 + 0.6 290 + 6 0 320 + 110

600 m 1.9 + 0.6 230 + 50 230 + 110

The shear bands in the northwestern domain parallel the mylonite-zone boundary, and thus parallel the simple- shear slip plane (see Fig. lc). Where shear bands inter- sect dikes, offsets of the dike margins are apparent in some cases, and the sense of offset is always sympathetic to that of the overall zone. The magnitudes of slip are small, however. Displacements were measured along 100 shear bands in mylonite with a shear strain of 1.9. The aver.age displacement on 22% of the shear bands is 8 + 5 mm. On 78% of the bands, any displacement is less than 5 mm, the average grain size of the rock. These small displacements nevertheless produce very high shear strains within the individual bands. Because thp. bands are gener- 45

ally less than 0.5 mm thick, displacements of only 5 mm

yield shear strains greater than 10 within the bands.

Southern Domain

The southern domain appears to be the southwestern projection of the northwestern domain (Fig. 14). The

mylonites in both domains have a southwest vergence. The

shear bands, in particular, are parallel in the two do mains. In the northwestern domain the mean strike and dip

of the shear bands is N35°W, 20 ± 6°SW, and in the southern,

domain it is N35°W, 21 + 7°SW. This similarity suggests

the domain may be related.

The lower boundary of the northwestern domain

projects to an elevation of -3500 feet in the southern

domain. However, at +3500 feet elevation, the structurally

lowest mylonites of the southern domain are underlain by non-mylonitic rocks with a northeast-striking, sub-vertical

foliation. The foliation and the non-mylonitic character of these rocks strongly resemble the foliation and non mylonitic character of the rocks immediately below the boundary of the northwestern domain. These observations,

combined with the observations of normal vergence in the central domain, suggest that the southwest-dipping lower

boundary of the northwestern domain has been offset, from the projected elevation of -3500 feet, to a present eleva

tion vf +3800 feet, by northeast-vergent normal shear in 46

Figure 14. Cross section of the Santa Catalina mylonite zone. Location shown in Figure 11. Dashed pattern represents non-mylonitic rocks. Gravel pattern represents Quaternary-Tertiary gravels. Heavy solid line represents detachment fault. Light solid lines represent mylonitic foliation trajectories. Short heavy lines represent shear bands. Heavy solid line and dotted line represent the mapped and projected portions of the mylonite-zone boundaries. Lower boundary of southwest-vergent zone is interpreted as being offset from A to ~ by normal slip on northeast-dipping zone (central domain). 47 the central domain. Thus the southern domain is interpreted as the displaced, southwest projection of the

northwestern domain.

In contrast to the shear. bands, the foliation

surfaces in the northwestern and southern domains have significantly different orientations. The foliation sur

faces in the southern domain strike N65°W and dip 120 southwest, while the surfaces in the northwestern domain strike N22°W and dip 50 northeast. Thus the foliation surfaces in the southern domain are rotated approximately

150 about a subhorizontal, N50 0 W-S60 0 E axis, relative to the foliation surfaces in the northwestern domain.

Qualitatively, the mylonites in the southern domain appear to be highly strained rocks. The ribbon structures are much more well developed, and porphyroclasts are fewer and smaller than in correlative rocks in the northwestern domain. Petrographically, the southern domain mylonites resemble those described as having shear bands at 200 -30 0 to the shear zone margins (i.e., a' surfaces, extensional crenulation cleavages, and shear band cleavages). Unfortunately, the orientation of the mylon ite-zone boundary in the southern domain cannot be directly determined. However, the parallelism of the southern domain and northern-domain shear bands, suggests that the southern-domain boundary dips 210 southwest, parallel to 48

the northern domain boundary. If the southern domain · shear bands are oriented at 20°-30° to the zone margins,

the entire southern domain must have been coincidentally

rotated by 20°-30°, but in an exactly opposite sense, to

maintain the observed parallelism. It seems more lik~ly

that both the shear bands and the zone margins have main-

tained constant orientations. The southern-domain shear bands are interpreted here to be parallel to the mylonite

zone margins, and the lower margin is consequently inferred to strike N35°W and dip 21° ± 7° southwest.

Assuming that the southern domain boundary dips

21° southwest, the foliation surfaces are oblique to the

boundary by approximat~ly 9° ± 7°. This is inconsistent with interpretation of the foliation surfaces as slip

surfaces, and consistent with their interpretation as finite XY surfaces. The 15° rotation of the foliation

surfaces, from the northern to the southern domain, thus appears to represent an increase in simple-shear strain,

consistent with the higher-strained appearance of the rocks.

The principal strain magnitudes and orientations

in the domain can only be inferred. In the preceding domains, the dikes used as strain markers were intruded

into Eocene granite, and thus have relatively simple deformation histories. In the southern domain, dikes may 49

be of multiple generations, and intrude primarily Proter-

ozoic and Cretaceous host rocks. Thus their initial

orientations are unknown, and they are not useful strain

markers. However, the interpretation of the southern

domain as the offset correlative of the northwestern

domain, does imply that the deformation involves only . , simple shear. In th~s case, the mean value of (J (90 +

7 0 ) implies a mean shear strain of approximately 6.2 in the

southern domain. This estimate is poorly constrained,

however, in that the error of + 7 0 implies that the shear

strain may be as low as 3.6 ( 0'= 160 ) or as high as 22.5

( 0' = 2 0 ).

Central Domain

In the central domain, the maximum concentration

of shear bands (C-surfaces) parallels one of the possible

orientations of the mylonite-zone boundary east of point

a. Thus the shear bands appear to be slip surfaces

oriented parallel to the shear-zone margin, consistent with the preceding examples. The foliation surfaces (S-

surfaces), however, are oblique to the boundary, as in the preceding examples, and thus cannot be slip surfaces.

This interpretation of the mylonites, combined with the proposed correlation of the southwest-vergent domains, implies that the central domain is a separate, northeast-dipping, normal-slip shear zone (Fig. 14). 50

Local cross-cutting relationships support this interpreta tion. Along the southwestern margin of the central domain, northeast-dipping, northeast-vergent shear bands locally offset and rotate apparent southwest-vergent shear bands. In the northwestern domain, northeast-dipping, northeast vergent, minor shear zones locally offset the pervasive southwest-vergent fabrics. In the central domain, however, southwest-vergent minor zones are not observed. Thus northeast-vergent, normal-slip minor structures are super imposed on southwest-vergent, normal-slip structures.

As described above, dikes in the structurally high levels of the northwestern domain have a subhorizontal orientation. In the central domain, these dikes dip 200 +

50 northeast. This change in orientation is consistent with rotation of the dikes by northeast-vergent normal shear, following an initial rotation to the horizontal by southwest-vergent normal shear.

At one specific location within the domain, the shear strain calculated from these dike orientations is

5.8 + 3.6, corresponding to a value of 0' of 100 + go, assuming exclusively simple-shear deformation. The ob served angle between the foliation surfaces at this loca tion and the mylonite-zone boundary is 130 ± 50. Thus the observed foliations surfaces appear to be within 3 0 of the finite XY surface of the bulk rock at this location, 51 although the potential error margins are relatively large.

The mean shear strain within the zone can also be estimated from the displacement of the southwest-dipping boundary (see Fig. 14). Assuming that the offset of the boundary (2.6 km) is due entirely to simple shear, the mean shear strain within the zone is 0.9, corresponding to a mean value of 0' of 330 • In contrast, the observed mean , value of 0 is 150 + 90 • Thus the observed mean orientation of the foliation does not equal the mean orientation of the finite flattening surface predicted for simple- shear deformation alone. If the foliation orientations do correspond to the XV-surface for the total deformation, as in the preceding examples, the deformation must involve signficant components of coaxial strain and/or volume change in addition to simple shear.

Summary

The Santa Catalina mylonite zone is composed of two separate shear zones; a southwest-vergent normal- displacement zone, and a northeast-vergent, normal dis- placment zone that offsets the southwest vergent zone. In the northeast-vergent zone and the northern half of the southwest-vergent zone, the foliation surfaces are oblique to the zone boundaries, and the shear bands are parallel to the boundaries. In the southern half of the southwest- vergent zone, the orientation of the boundary can only be 52 inferred. However, the foliation surfaces are slightly oblique to this inferred boundary, and the shear bands, which resemble C/-surfaces, are parallel to it.

At three specific locations in two domains, the foliation surfaces are within 30 , or less, of the XY finite strain surface. In the northeast-vergent zone, however, the mean orientation of foliation surfaces differs by 180 from the mean orientation of the finite XY surface, unless the deformation involves significant coaxial strain or volume change.

Interpretation of the central domain mylonites as a separate shear zone has significant implications for the origin of metamorphic-core-complex anti forms. Specifical ly, the Santa Catalina antiform, or Forerange arch, appears to be formed by the intersection of two shear zones, rather than by significant folding or arching of a single zone, or by variations in simple-shear strain within a single zone. This origin is consistent with gravity data that suggest that uplift of the Santa Catalina-Rincon mountains is attributable to a low density crustal root, rather than to isostatic arching and folding of the mylon ite zone related to tectonic denudation and unloading

(Wallace et al., 1986; Holt et al., 1986). CHAPTER 5

STRAIN SIGNIFICANCE OF S-SURFACES

All of the studied areas contain SC-mylonites,

even though some of the domains have undergone significant components of volume change or coaxial strain in addition

to simple shear. The Pinaleno zone and the northwestern

Santa Catalina domain have been deformed by simple shear

alone. The Bjornfjell zone haa experienced simple shear plus volume loss, and the central Santa Catalina domain

has experienced simple shear plus volume change or coaxial strain. The deformation of the southern Santa Catalina

domain appears to involve only simple shear r but may also involve volume change or coaxial strain. Thus the occur

rence of SC-mylonites is not, in itself, evidence of strictly simple-shear deformation.

In four of the five studied domains, the mylonitic foliation surfaces (S-surfaces) are demonstrably and significantly oblique to the mylonite zone margins. The foliation surfaces thus cannot, in general, be slip surfaces. They may act as slip surfaces when the angle between the foliation surfaces and the mylonite-zone margins becomes smaller than approximately 8°, although evidence of such slip was not observed in the field. A

53 54 significant practical implication of this observed discord ance, is that cross section projections of mylonite zones should be based directly on the strikes and dips of the boundaries of the zones, and not on the foliation strikes and dips.

In three of the zones and at least part of a fourth, the foliation surfaces are within 50 of the finite

XY surface for the bulk rock. This includes domains of exclusively simple-shear deformation, as well as one domain of simple shear plus volume loss.

All of the observed spaced shear bands are subparallel to the mylonite-zone margins. Observable offsets verify that they are slip surfaces, or millimeter-scale, planar domains of very high shear strain. Although some of the shear bands resemble C -surfaces, extensional crenulation cleavages, and shear-band cleavages, no shear bands were observed at a systematic 20 0 -30 0 angle to the shear-zone margins. These observations are limited, however, to shear strains ~ 6, and to domains of predomin ately, if not exclusively, simple shear.

The conclusion that the pervasive foliation sur faces represent the bulk-roc]{, finite XY surface, and the conclusion that the spaced surfaces represent shear bands, are not contradictory. In general, SC-mylonites are composed of the spaced shear bands (C-surfaces), plus the inter-shear-band domains, or lithons, that contain the 55 pervasive foliation surfaces (S-surfaces). The shear bands have both finite lengths and discrete spacings, and are not directly interconnected. Displacement along an, individual shear band consequently must produce strains in the lithons at the tips of the band. The total strain in the lithons thus includes a component of strain that is directly related to the shear-band displacements. The foliation-surface elongations and orientations within the

lithons incorporate the shear-band displacements, and thus record the total strain within the rock. Shear-band displacements do not add an increment of bulk strain beyond that manifested by the foliation surfaces. CHAPTER 6

DYNAMIC SIGNFICANCE OF C-SURFACES

The spaced shear bands in mylonites cannot, in

general, represent only the kinematic slip plane (UV) of

deformation (see Fig. 1). In any single episode of simple-

shear or pure-shear deformation, the slip plane maintains

a single, constant orientation. If mylonitic shear bands represent only slip planes, mylonites deformed by a single

episode of simple shear should contain a single set of shear bands with a constant orientation. Similarly, mylonites deformed by a single episode of coaxial deforma tion should also contain a single set of shear bands with

a constant orientation, in this case parallel to the foliation surfaces, and presumably indistinguishable from

them. If shear bands represent only slip planes, conjugate sets of shear bands should not develop, nor should rotation of shear bands occur, in any single deformation. However,

Ponce de Leon and Choukroune (1980), and Platt and Vissers

(1980) describe conjugate sets of shear bands formed in a single deformation. Platt and Vissers (1980) also describe multiple gener~tions of shear bands in which early sets are rotuted and offset by later sets formed during the same deformation. With regard to metamorphic-core-complex

56 57 mylonites in particular, shear bands with 0?posing dips and vergences, and rotated sets of shear bands, have been reported in South Mountains mylonites (Reynolds, 1985, 1986 personal commun.), Whipple Mountains mylonites (Davis et al., 1986), and the Harquahala, White Tank, Picacho and

Tortolita mountains mylonites (Reynolds and Lister, 1987).

These observations contradict any general interpretation of shear bands as simply kinematic slip planes.

In simple shear, the kinematic slip plane (UV) coincides with a maximum shear-stress plane. Direct stress measurements in simple-shear deformation e}Cperiments show empirically that 01 (the principal compres~ive stress) rotates into a 450 angle with the simple-shear slip plane during deformation (Mandl et al., 1977). Thus simple-shear slip planes, and the shear bands observed in mylonites deformed by simple shear, coincide with planes of maximum shear stress.

In a perfectly plastic material, the potential failure surfaces are oriented at 45 0 to aI, and thus they coincide with the planes of maximum shear stress. Mohr-

Coulomb criteria predict failure at an angle ~ to aI, where;

~ = 2 and ~ is defined as the angle of internal friction, and 58

represents the slope of the Mohr-Coulomb failure envelope.

~ typically has a value of approximately 300 • However,

the definition of plastic behavior implies that ~ equals

zero in perfectly plastic material, and that ~ conse

quently equals 45°.

Figure l5a is a schematic stress-strain curve for

an ideal elasticoviscous material. Differential stress,

60, is defined as 01 - 03. For differential stresses

below the elastic limit, the slope of the stress-strain

curve has a positive value. In the region of perfectly

plastic behavior beyond the elastic limit, the slope of

the curve becomes equal to zero. Because the slope is

zero in the plastic region, a constant value of differen-

tial stress, ~amax, is required for failure in the

plastic region. The Mohr stress circles representing

failure conditions in the plastic region consequently have

a constant diameter ( 0 1 - 03) of AOmax , and the slope

(~) of the failure envelope that is tangent to the circles

becomes zero (Fig. 15b). Thus in a perfectly plastic

material, the potential failure surfaces are oriented at

450 to ala

In simple-shear deformation in particular: because

0 1 is oriented at 45 0 to the slip plane, one set of

potential failure surfaces is oriented parallel to the

kinematic slip plane (Fig. 16). The conjugate set is

oriented at 90 0 to the slip plane, Because material 59

dQa = 0 ~de A Om ax ...... ,.---.;;..----

'1:-0

Figure 15. (a.) Stress-strain curve for ideal elastico viscous material. Differential stress, ~a, equals 01- 03. Material cannot support a differential stress greater than the yield strength, ~amax. (b.) Mohr-Coulomb failure envelope for the same material. At differential stress below ~Omax, the shear stress at failure (r), and the orientation of the failure surfaces with respect to a1 (~), are both functions of~. At 01- 03 = D.O max , the shear stress at failure becomes constant, and ~ becomes a constant 45°. 60

0- ?~l~ ,

o.~ ! .. @P4S;;>\ I .... ~, .... ' f) .' . .... ~

~C~3-? l'

Figure 16. Ca.) Relative orientations of al, X, Z, and the potential failure surfaces in an elasticoviscous material in simple shear. Because a1 becomes oriented at 450 to the simple-shear plane, one potential failure surface maintains a constant orientation, parallel to the simple shear slip plane. Failure parallel to the conjugate surface is unlikely because the sense of shear stress will be opposed to the sense of simple shear as the failure surface rotates. Cb.) Relative orientations of a1, X, Z, and the potential failure surfaces in an elasticoviscous material in pure shear. Failure surfaces are initially oriented at 45 0 to al, but will rotate towards the XY plane with progressive deformation. Failure is equally likely on both conjugate surfaces. 61 planes perpendicular to the slip plane will rotate with progressive simple shear, the sense of shear stress on the

planes is opposed to the overall sense of simple-shear displacement. Thus actual failure on the conjugate planes

is unlikely. In contrast, material planes parallel to the plane of slip do not rotate with progressive simple shear.

These planes remain parallel to a potential failure surface throughout simple-shear deformation, and the sense of shear stress on the plane is sympathetic to the sense of simple-shear displacement. Consequently, failure parallel

to these planes is likely. Any failure surfaces formed in this orientation will not rotate, and should remain active during progressive simple-shear deformation. Thus in the

Pinaleno and Santa Catalina domains where the deformation has occurred by simple shear, the observed shear surfaces

(C-surfaces) parallel the maximum shear stress planes and the potential failure planes, as well as the kinematic slip planes, assuming that the mylonites approximate a perfectly plastic material.

In contrast, in irrotational pure-shear deforma tion, the kinematic slip plane coincides with the finite

XV plane, and not a maximum shear-stress plane. Assuming that Z and a 1 are coincident, there will be two poten tial failure surfaces oriented at 45 0 to the slip and finite XV planes (Fig. 16b). All these planes maintain a 62

constant orientation with respect to a 1 with progressive strain. However, material planes which initially parallel maximum shear-stress planes will rotate toward the slip and finite XY planes with progressive strain. Thus in coaxial deformation, if failure surfaces (C-surfaces) form initially parallel to maximum shear-stress planes rather

than kinematic slip planes, (1) they may form in conjugate sets, and (2) they will rotate towards the foliation with progressive strain, eventually becoming unfavorably oriented for additional displacement, and be cut by new failure surfaces. CHAPTER 7

SUMMARY

Geometric analyses of three mylonite zones, in cluding two metamorphic-core-complex zones, show that the foliation surfaces (S-surfaces) in each are significantly oblique to the mylonite-zone boundaries. Finite-strain analyses show that the foliation surfaces are within at least 9 0 of the bulk-rock finite XY surface. The deforma tion in each zone is primarily, but not exclusively, simple-shear deformation. Volume change or coaxial strain are important in two of the zones.

In the Pinaleno core-complex zone, and in at least one of three domains of the Santa Catalina core-complex zone, the deformation is exclusively simple shear. In these domains, the siillple-shear strain can be calculated from the angle between the foliation and the shear-zone margin, using the equations of Ramsay and Graham (1970).

In these simple-shear domains, this angle is equivalent to the angle between the foliation and the statistical maximum concentration of shear bands. In the Pinaleno and Santa

Catalina zones the angular values thus obtained generally have standard deviations of approximately ± 50. Thus the shear strains calculated on the basis of these angles in

63 64

highly strained rocks, have potentially large errors.

In the Santa Catalina core-complex zone, the zone

boundaries are planar and show no evidence of arching or

folding of the zone. The antiformal geometry of the foliation surfaces is not due to folding or isostatic

rebound of the zone, nor is it due to variations in the

magnitude of simple-shear stl'ain. Rather it is primarily

the product of two intersecting shear zones having normal,

but opposite, senses of shear.

The shear bands (C-surfaces) in each of the zones

are slip surfaces, or millimeter-scale planar domains of

very high shear strain, oriented subparallel to the

boundaries of the zones. The occurrence of such bands

does not contradict the conclusion that the foliation

surfaces (S-surfaces) represent the XY surface of the bulk-rock finite strain. The non-inter-connected nature of

the shear bands requires that their displacements be

incorporated in the foliat~on-surface elongations and

orientations in the inter-shear-surface lithons.

In general, shear bands appear to be failure

surfaces oriented parallel to the planes of maximum shear stress. In simple shear, the slip plane coincides with a

plane of maximum shear stress. Consequently, a single s~t of shear bands with a constant orientation develops in

simple-shear deformation. In coaxial deformation, however, 65 the maximum shear-stress and slip planes do not coincide.

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