Kinematic analysis of deformation at the margin of a regional shear zone, Buehman Canyon area, Santa Catalina Mountains, Arizona

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Authors Bykerk-Kauffman, Ann

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Link to Item http://hdl.handle.net/10150/557985 KINEMATIC ANALYSIS OF DEFORMATION

AT THE MARGIN OF A REGIONAL SHEAR ZONE,

BUEHMAN CANYON AREA,

SANTA CATALINA MOUNTAINS, ARIZONA

by

Ann Bykerk-Kauf fman

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 8 3 Call No. BINDING INSTRUCTIONS INTERLIBRARY INSTRUCTIONS f- O) 0\ a * 60 e 1 of "c 1 to w « m to CM CM «rx 2 STATEMENT BY AUTHOR

This thesis 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 thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate college when in his or her judgment the proposed use of the material is in the interests of scho­ larship. In all other instances, however, permission must be obtained from the author.

APPROVAL BY THESIS DIRECTOR

This l/hesis has been approved on the date shown below:

/ GfJORGE H. DAVIS Date Professor of Geosciences ACKNOWLEDGMENTS

Without the help of numerous others, this research could not have been accomplished. I would like to thank my advisor, George H.

Davis, for his support and guidance throughout the project. His enthu­ siasm coupled with constructive criticism continuously spurred me on.

I would also like to thank Joseph F. Schreiber and William R.

Dickinson for generously giving of their time to visit the study area and to discuss ideas with me. I am grateful to Peter J. Coney and

Joseph F. Schreiber for reading the manuscript and contributing valuable comments.

I am indebted to Gordon Lister from the University of Utrecht and to Carol Simpson from Oklahoma State University for opening my eyes to shear-sense indicators in rocks.

I wish to thank Steve Lingrey and Paula Trever for suggesting the study area. I also thank my fellow graduate students, especially

Jean Crespi, Bob Krantz, Lee DiTullio, Debbie Currier, Steve Naruk and

Tekla Harms for their stimulating discussions, helpful suggestions and moral support.

Funding for this study was provided by the Lab of Geotectonics, the Graduate Student Program Development Fund, and Standard Oil of

California.

I am deeply grateful to Jack and Mary Smallhouse for all they've done for me. Their tremendous hospitality, congenial company, and enor­ mous amounts of help made the field work very pleasant.

ill iv

I wish to thank my parents, Roelof and Inie Bijkerk, for instil­ ling in me a love for learning, for encouraging me to think for myself and for doing everything they could to help me along.

Finally, I wish to express my deepest thanks to my husband,

Mark, whose unfailing help, encouragement, humor, patience and love gave me the strength I needed to complete this thesis. TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS...... vii

ABSTRACT......

1. INTRODUCTION ...... 1

2. THE SANTA CATALINA-RINCON METAMORPHIC CORE COMPLEX...... 8

3. GEOLOGY OF THE BUEHMAN CANYON A R E A ...... 10

General Geologic Framework...... 10 Stratigraphy and Lithology...... 17 Precambrlan Basement ...... 17 Late Precambrlan Rocks ...... 17 Paleozoic Section...... 18 Cretaceous R o c k s ...... 20 Tertiary R o c k s ...... 21 Structural/Metamorphic Transformation of Stratigraphic Units ...... 22 Argillites...... 23 Limestones...... 24 Deformation of Conglomerates ...... 30 Metamorphism ...... 38 Stratigraphic Thickness...... 38 Changes in Orientations of Fabric Elements .... 38 Geometry of the Structural/Metamorphic Transition. 42 Relationship Between Lithology and Intensity of Ductile Deformation ...... 45 Structures...... 46 Large-scale Structures ...... 46 Mesoscopic Structures...... 47 Microstructures...... 52

4. INTERPRETATION...... 59

Kinematics of the Shear Zone...... 59 Direction and Sense of S h e a r ...... 59 Orientation of Shear Zone Walls...... 61 Regional Implications of the Shear Zone Margin . . 63 Thrust-Slip or Normal-Slip Shear?...... 63 Fault-Rock Terminology...... 68 Strain Significance of Pebble Deformation ...... 68

v vi

TABLE OF CONTENTS— Continued

Page

Post-Shear Deformation...... 70 Summary...... 70

5. REGIONAL SIGNIFICANCE...... 71

REFERENCES...... 73 LIST OF ILLUSTRATIONS

Figure Page

1. Schematic cross-section showing structural components of metamorphic core complexes...... 2

2. Schematic diagram showing the structural elements of metamorphic core complexes as products of progressive movement on a low-dipping normal-slip fault zone...... 4

3. Generalized geologic map of the Santa Catalina-Rincon metamorphic core complex...... 7

4. Geologic Map of the Buehman Canyon Area, Pima County in Arizona ...... pocket

5. Geologic Map of the Buehman Canyon A r e a ...... 11

6. Geologic cross-sections A-A*, B-B*, and C-C* of the Buehman Canyon area ...... 12

7. Geologic cross-section D-D* of the Buehman Canyon area. . . 13

8. Composite contoured stereoplots of (a) 769 poles to bedding and (b) 952 poles to foliation...... 14

9. Bedding (SQ) and Foliation (S^) in very strongly deformed

Pennsylvanian-Permian Naco Group limestone...... 15

10. (a) Stereoplot of lineation orientations in strongly deformed Pennsylvanian-Permian Naco Group. (b) Contoured stereoplot of all lineation orientations measured in domain of very strongly deformed rocks...... 16

11. Photomicrograph of crack-seal vein in very strongly deformed Lower Cretaceous Bisbee Group phyllite ...... 25

vii viii

LIST OF ILLUSTRATIONS— Continued

12. Series of photographs showing progressive changes In the Pennsylvanlan-Permlan Naco Group limestones along the structural/metamorphlc gradient. (a) Weakly deformed...... 26

(b) Moderately deformed (c) photomicrograph of one of the dissolution surfaces of (b) truncating a fusulinid...... 27

(d) Very strongly deformed...... 28

13. Series of photomicrographs showing grain-size reduction in the PennsyIvanian-Permian Naco Group Limestones. (a) Weakly deformed, (b) Moderately deformed (c) Very strongly deformed...... 29

14. Series of photographs showing progressive changes in the Lower Cretaceous Glance Conglomerate. (a) Weakly deformed ...... 31

(b) Moderately deformed, outcrop surface oriented perpendicular to foliation (c) Moderately deformed, outcrop surface oriented parallel to foliation ...... 32

(d) Strongly deformed, outcrop surface oriented perpendicular to foliation (e) Strongly deformed, outcrop surface oriented parallel to foliation ...... 33

(f) Very strongly deformed, outcrop surface oriented perpendicular to both foliation and lineation (g) Very strongly deformed, outcrop surface oriented perpendicular to foliation but parallel to lineation...... 34

(h) Very strongly deformed, outcrop surface oriented parallel to foliation, showing lineation...... 35

15. Variations in thicknesses of (a) Mississippian Escabrosa Limestone and (b) PennsyIvanian-Permian Naco Group...... 39

16. Contoured stereoplots of poles to bedding in the Mississippian Escabrosa Limestone, the Pennsylvanian-Permian Naco Group and the Cretaceous Bisbee Group. (a) Weakly deformed, (b) Moderately deformed, (c) Strongly deformed, (d) Very strongly deformed ...... 40 lx

LIST OF ILLUSTRATIONS— Continued

17. Contoured stereoplots of poles to foliation in the Mississippian Escabrosa Limestone, the Pennsylvanian-Permian Naco Group and the Cretaceous Bisbee Group. (a) Weakly deformed, (b) Moderately deformed, (c) Strongly deformed, (d) Very strongly deformed ...... 41

18. Bar graphs showing the frequency distribution of the stereographically determined angle between bedding and foliation in 5° increments. (a) Weakly deformed, (b) Moderately deformed, (c) Strongly deformed, (c) Very strongly deformed and (e) composite plot...... 43

19. Graph showing progressive swing of lineation trend...... 44

20. Stereoplot of poles to normal faults which cut bedding at a high a n g l e ...... 48

21. Contoured stereoplot of 100 poles to foliation around major syncline in the southeast part of the area...... 49

22. (a) Photograph of mesoscopic shear zones in Precambrian diabase sill (b) Photograph of lineations on foliation surface in shear z o n e ...... 50

23. Contoured stereoplots of (a) 31 poles to shear zone orientations and (b) 27 lineations in shear zones...... 51

24. Crenulation cleavage in very strongly deformed Cretaceous Glance Conglomerate...... 53

25. Contoured stereoplot of 51 poles to crenulation cleavage surfaces ...... 54

26. Photomicrograph of crack-seal vein truncated by crenulation cleavage surfaces ...... 55

27. (a) Photomicrograph of part of a deformed limestone pebble in the very strongly deformed Cretaceous Glance Conglomerate (b) Kinematic interpretation of the photomicrograph...... 57

28. Photomicrograph of shear bands in the Late Precambrian Pioneer Shale...... 58 X

LIST OF ILLUSTRATIONS— Continued

Figure Page

29. Hansen (1971) plot of S- and Z-folds by Schloderer (1974) from an area immediately south of the Buehman Canyon a r e a ...... 62

30. Four possible interpretations for the position of the Buehman Canyon area relative to the regional shear zone . . . 64

31. Schematic cross-sections of five possible types of top-to-the-east slip on the shear zone...... 66 ABSTRACT

The Buehman Canyon area occupies the gradational northern margin of a regional shear zone in the Santa Catalina-Rincon metamorphic core complex of southeastern Arizona• An unmetamorphosed, predominantly sedimentary sequence of Precambrian through Cretaceous rocks becomes progressively more metamorphosed and deformed towards the south. The increase in intensity of deformation is marked by progressive 1) tecto­ nic thinning of stratigraphic units, 2) development of foliation and lineation, 3) grain-size reduction, and 4) flattening and stretching of pebbles in conglomerate units. Limestone pebbles with initial aspect ratios of 2:1 are deformed to pebbles with aspect ratios as high as

60:1.

Foliation is subhorizontal, consistently more shallowly-dipping than the moderately east-dipping bedding. Lineation generally trends east. East-verging intrafolial folds, east-directed subhorizontal shear zones, and asymmetric microstructures in the tectonites indicate an eastward sense of shear. The upright thinned section suggests normal- slip shear.

xi CHAPTER 1

INTRODUCTION

The importance of Cordilleran metamorphic core complexes and associated Tertiary low-angle normal faults has become increasingly apparent in recent years (Anderson, 1971; Armstrong, 1972; Wright and

Troxel, 1973; Proffett, 1977; Davis and Coney, 1979; Crittenden and others, 1980). These features have been identified within a broad belt along the North American Cordillera from southwestern Canada into north­ western Mexico (Coney, 1980). This belt is characterized by terranes of moderately- to shallowly-dipping rotational normal faults. These normal faults commonly sole into subhorizontal "detachment faults" which are generally underlain by a zone of cataclasites derived from foot-wall lithologies (Davis and Coney, 1979; Davis and others, 1980; Davis,

1980). In the metamorphic core complexes the detachment faults place brittlely-deformed unmetamorphosed hanging-wall rocks upon ductilely- deformed, commonly mylonitic, foot-wall tectonites (Figure 1).

Foliations in the foot-wall tectonites are subhorizontal and lineations

have remarkably consistent regional trends (Davis and Coney, 1979; Crit­

tenden and others, 1980). Within the footwall terrane the metamorphic

tectonites commonly grade structurally upward (Davis and others, 1980;

Rehrig and Reynolds, 1980) or downward (Howard and others, 1982; Rehrig

and Reynolds, 1980; Davis, 1980) into nontectonite equivalents (Figure

1).

1 2

brittlely deformed hanging wall

detachment fault cataclasite nontectonite protolith

metamorphic tectonite + + + + + + + + + + + + + + + ■ + + + + + + + + + + + + + + + + 4" + 4" + 4" + + + + + + + + 4" nontectonite • + + + + + + + + + + + + + + + protolith . * .+ -.A. + *.A + + +

Figure 1. Schematic cross-section showing structural components of metamorphic core complexes, (after Davis, 1983). 3

Abundant evidence confirms the Tertiary age and extensional nature of the brittle faulting (Coney, 1974; Compton and others, 1977;

Cheney, 1980; Snoke, 1980; Reynolds and Rehrig, 1980; Rehrig and Rey­ nolds, 1980; Shackelford, 1980; Davis, 1980; Davis and others, 1980,

1982; Davis and Hardy, 1981; Price, 1981; Brown and Murphy, 1982;

Harms, 1982; Garner and others, 1982; Martin and others, 1982; Logan and

Hirsch, 1982; Lingrey, 1982; Reynolds, 1982; among others). However, the kinematic significance and timing of the ductile deformation below the detachment fault remains unclear. Historically, both the detachment faults and the ductilely-deformed tectonites were interpreted as Meso­ zoic thrust-related features (for example, Misch, 1960). Many workers still relate the ductile deformation to Mesozoic tectonics (Davis and others, 1980, among others). This hypothesis is supported by the almost universal truncation of foot-wall fabrics by detachment faults (Davis and others, 1980) and by geochronological studies in southeastern Cali­ fornia which suggest that the ductile deformation is late Cretaceous in age (Davis and others, 1982; Hamilton, 1982; Martin and others, 1982).

However, transport directions of the detachment faults and lineatlon orientations in the foot-wall tectonites are commonly remarkably paral­ lel; in addition, the sense of movement on the detachment faults and the shear sense derived from the mylonitlc tectonites are commonly the same

(Davis and others, 1981; Davis and Hardy, 1981; Reynolds, 1983; Davis and others, 1983; Harms and Price, 1983). These relationships have led several workers to conclude that the tectonites represent a shear zone which is essentially the deeper-level extension of the detachment fault

(Figure 2) (Davis, 1981; Davis and Hardy, 1981; Lingrey, 1982, Lingrey 4

nontectonite hanging wall detachment fault

normal-slip shear zone nontectonite with metamorphic tectonites protolith

Figure 2. Schematic diagram showing the structural elements of metamorphic core complexes as products of progressive movement on a low- dipping normal-slip fault zone — The zone is a ductile shear zone at depth and a brittle fault at shallow levels. As displacement progresses and the footwall moves to higher crustal levels, plastically deformed metamorphic tectonites formed in the shear zone move out of the ductile shear regime and into the brittle faulting regime. The detachment fault thus truncates earlier tectonite fabrics. Modified from Ramsay (1980) and Davis (1983). 5 and Davis, 1982; Davis, 1983). This hypothesis is supported by geochro- nological studies which indicate the approximate synchroneity of detach­ ment faulting and ductile deformation in the South Mountains (Reynolds,

1982, 1983), and along the Newport Fault (Harms, 1982, Harms and Price,

1983). Clearly, the problem is a highly complex one in a terrane which has undergone multiple episodes of deformation (Burchfiel and Davis,

1975).

This paper addresses the issue of the kinematic significance of

ductile deformation in the Buehman Canyon area, located in the northeas­

tern part of the Santa Catalina-Rincon metamorphic core complex of

southeastern Arizona (Figure 3). Previous mapping in the area has

revealed the general geology: a moderately east-dipping Precambrian

through Mesozoic sedimentary and metasedimentary section which is cut by

normal faults (Moore and others, 1941; McKenna, 1966; Schloderer, 1974;

Creasy and Theodore, 1975). McKenna (1966) studied the Paleozoic stra­

tigraphy in the northern part of the area and reported an unmetamor­

phosed section. Schloderer (1974) worked in the southern part of the

area and described a Paleozoic section that had been converted to meta­

morphic tectonites. Neither discussed the nature of the contact between

the metamorphic tectonites and the unmetamorphosed rocks. Schloderer

(1974) studied asymmetric folds in the metamorphic tectonites and con­

cluded that the tectonites had been sheared with upper layers moving

toward the east.

The major purposes of this study were 1) to determine the nature

of the contact between the metamorphosed and unmetamorphosed sedimentary

rocks, 2) to expand on Schloderer*s work in determining the kinematic 6 significance of the ductile deformation in the metasedimentary rocks, and 3) to explore the implications of this work for the kinematics of ductile deformation in the Santa Catalina-Rincon metamorphic core com­ plex as a whole. Figure 3. Generalized geologic map of the Santa Catalina-Rincon metamorphic core complex — showing distribution of unmetamorphosed sedimentary rocks and metasedimentary tectonites. Compiled from Banks (1976), Benson (1981), Creasy (1967), Creasy and Theodore (1975), Drewes (1980), Keith and others (1980), Lingrey (1982), Thorman and Drewes (1981), and Warner (1982). Early to Mld-Tertlary Sedimentary Rocks p|||] Unmetamorphosed Pre- l i i l Tertiary Sedimentary Rocks

Metasedimentary Tectonites Precambrian Pinal Schist, Younger intrusive Rocks and Mylonite Gneiss

Detachment Fault Normal 10 km >>>z Thrust Fault Fault CHAPTER 2

THE SANTA CATALINA-RINCON METAMORPHIC CORE COMPLEX

The Santa Catalina-Rincon metamorphic core complex, located in southeastern Arizona (Figure 3), exemplifies the controversies surroun­ ding the ductile deformation below the detachment fault. Three very different models have been proposed to explain the ductile deformation structures in this complex: 1) northeast-directed Laramide thrusting

(Drewes, 1981), 2) southwest-directed Eocene thrusting (Keith, 1983), and 3) southwest-directed Mid-Tertiary low-angle normal shearing (Davis,

1981, Davis and Hardy, 1981; Lingrey, 1982, Lingrey and Davis, 1982).

The bulk of the Santa Catalina-Rincon metamorphic core complex consists of granitic intrusions of four distinct ages (Keith and others,

1980): 1440-1450 my old, 75-64 my old, 50-44 my old, and 26 my old.

Parts of the three older suites of plutons and possibly part of the youngest suite are tectonized (Keith and others, 1980). The complex is flanked on all sides by Precambrian through Cretaceous sedimentary rocks, their metamorphic equivalents, and Mid-Tertiary sedimentary rocks

(Figure 3).

Foliation dips gently and defines several north-northeast trending doubly-plunging folds (Davis, 1980). In the Rincon and south­ western Santa Catalina Mountains, lineation trends are consistently

N50°E to N70°E (Davis, 1980). Two low angle detachment faults serve to separate foliated and lineated tectonites from nontectonites:

8 9 the Catalina Fault and the San Pedro Fault (Figure 3). Unmetamorphosed rocks generally make up the hanging-wall while metamorphic tectonites make up the foot-wall. The detachment faults are gently folded along

the same trends as the folds in the foliation (Lingrey, 1982; DiTullio,

1983). Interpretations of hanging-wall displacement direction are

southwestward for both the Catalina and San Pedro faults (Lingrey, 1982,

Davis and Hardy, 1981). Intrafolial folds in metasedimentary tectonites

in the eastern Rincon Mountains (Lingrey, 1982) and microstructures in mylonitic tectonites in the western Rincon and Santa Catalina Mountains

indicate southwest-directed shear, parallel to the inferred transport

direction on the detachment faults (G. S. Lister, 1982, personal commu­

nication; C. Simpson, 1982, personal communication).

The northeastern part of the Santa Catalina-Rincon metamorphic

core complex, where the Buehman Canyon area is located, does not follow

the aforementioned pattern. Lineation trends vary from north-northeast

to east to even west-northwest (Davis, 1980; Schloderer, 1974; Keith and

others, 1980; this study). In addition, there is no detachment fault

separating unmetamorphosed rocks from metamorphic tectonites. Rather,

metamorphic tectonites grade toward the north-northeast into their plu-

tonic and sedimentary equivalents (this study) (Figure 3). CHAPTER 3

GEOLOGY OF THE BUEHMAN CANYON AREA

General Geologic Framework

In the Buehman Canyon area, an unmetamorphosed, predominantly sedimentary sequence of Precambrian through Cretaceous rocks grades over several kilometers toward the south into equivalent lower greenschist

facies metamorphic tectonites (Figures 4 and 5). Lithologic units are

progressively attenuated southward. They are consistently upright and

in normal stratigraphic order (Figures 4, 5, 6 and 7). Throughout the

area, bedding generally dips moderately to the east (Figure 8a). Folia­

tion dips shallowly to the east, consistently at a shallower angle than

bedding (Figures 8b and 9). Lineation, where well developed, trends

east (Figure 10b). Both bedding and foliation are locally folded into

NNW-trending upright folds and cut by steeply east-dipping crenulation

cleavage surfaces (Figures 4, 5 and 6). An east-dipping reverse fault

also cuts the tectonite foliation (Figures 4 and 5). These folds and

the reverse fault are in turn cut by predominantly west-dipping normal

faults.

10 Figure 5. Geologic map of the Buehman Canyon Area Scale 1:20,000 EXPLANATION /- T T O \ | T c o err Late Tertiary r 0 94 0 \ 110°31,30" \o o i Qulburis Formation \ o o 7o\ ,5/0 o 01 jo Qxx n,r7 ? r\ - 1 tX n o o o o / v . Late Cretaceous Leather- ; ! \ , ) o o o x . wood Quartz Diorite o o y se o \ °-^2a ° V»7 0*^!’ ° ° ^ ai V -o o„ ox o o i Early Cretaceous Bisbee Group \; v ^ \ # ~-u' o ( X l 1 r i\\ r p n Weakly deformed D C o Moderately deformed .N(/ r 4 A , ipQ) o - ^ o o oi4 o o o O *0 nc o I -VifSSSp o O O O O JO % t 20 y Strongly deformed \ liTX i rl yg-O o o — > o o * o -*e _xtj-js

Very strongly deformed ^ 0 o ° r ? h ‘V 'o , i34' xu-RTnv o o o o o o c n X 0 t Pennsylvanian-Per mi an Naco Group cpTTBrT~Ao o o o o o o /\ O \ TJ_TL'Ay I y I 71y\ ,000000 to oX,/V-^b O o 340 -•— t'l n n In \ ■TTTI Weakly deformed •|y1,11V:4:1^rJ4^4t) c o o vq q o I t \ i I i^1-'|1-['1\ i v i4o r \ Q 0 0 0 cVb \ _ O O O/ Moderately deformed r, o o o o o o \_ririn o o- i,y,l, \ rI_3z o o o o o o o \iyl yl yf o \ 'x’b'-vA v -- Strongly deformed 0 o O 0 O 60 O o «XX 1 1 / ooooocfoo \4b O' rtrYr Very strongly deformed \_ y :. I ,L' OOOOOOOO D O O o o oxo o o o o o o O ^ O or T'^'f o t) o o o Mississippian Escabrosa Limestone ,1- - I- -I -I - .TV b o o o o o o ( J Weakly deformed o f y o ,y

Moderately deformed o o o S m m \ o 0a0p a o Strongly deformed \*e> o o o o # _ _ _ o o o I lylyl IT Very strongly deformed < /UaoXooooo XA Fnl o o q o o o c x ^ *, A o p o o x # ' K n Devonian Martin Formation —32°26' 5p oxooo o / r . 32°26‘- C V31Q oq^o / ^2.;ox"' V-P o — ^ o Y ( Vt 4-r: Cambrian Abrigo Formation <• 76-1

Cambrian Bolsa Quartzite 1 f e y Precambrian Y Dripping M /& f e q f e a =v ; j ’ & Spring Quartzite and Dia­ ' A a t'' ^ \ 25 base Sills, undifferentiated Tfe'jiiSS'r-' Til S R 18 E Precambrian Y Pioneer j “A Shale and Diabase Sills, C X O ^ O o o l o | T12S.R18E undifferentiated O O CZ> O O Ol - V / C> CD < h f- T - ' n c l O o O r X-2B ~T~ Precambrian X Pinal Schist \ 31 ‘ o O o O ; ; t ( « ,^26, ^ O c? , I .-W \ / o o3Jfe w/v 42 SCALE

Geology by ' A . Ann Bykerk-Kauffman, 1982 111 >” r Base map: U S G S S k " Buehman Canyon 7’// Quadrangle, Arizona SYMBOLS k ,«y^ Contact * 80 • Normal Fault,* r , * showing dip; ball and bar on down- */ thrown side L;t j # W Bedding-aubparallel ", =»|) vS. Normal Fault.* / teeth on hanging wall TZ ^T=T=I % tt 7 r ,' - 1 7 Reverse Fault,* J l 2* A W : 1/ ** teeth on hanging wall ‘ 47 \ V 2 2S A A i-r a 4 8 / Strike and dip : ie ^ t1 1 / of bedding ■V, Strike and dip of ZT V/ _3J 23 y foliation, trend and plunge of N llneatlon \ f e : 1 24 Strike and dip of 2J y shear zone, trend ^ 3 '-a * aA /A l“!° ^ and plunge of y • 7 7 errs errs cr* llneatlon In zone TE7 = rs Strike and dip of (A crs3»r-> /T\AT ( 78V crenulatlon cleav­ <=. errs <=> cX*. 23\ V 12 x age surface ,44 5 5 errs errs errs errs A i r-» 35 f k . _ i \ B0 - 7 Trend and plunge s «rr> < errs errs c= > A ^A ers v s \ of pencil structure ' ' rrs cm errs er T - Nx errs errs errs errs errs errs c s C ^ CZS e s \ 37 < Axial trace of s errs ,3 0 _ errs errs errs errs V m errs cm, X syncline errs , r * 27 • ’ \ ' > d > errs errs errs.A ' cm errs ers errs errs errs errs m s errs errs ^ ■ \ 6 3 X rr. Pebble-measuring errs errs errs ers i#rs errs errs errs errs errs e r A r s \ •—* location s errs ers errs N p-16 e , errs ^^rs errs err/ y« e m / errs ers errs errs » r> v . « = ' •All contacts and 3 k!!7CT xto*31'3

. 2000 ft

1000 ft

r 4 ooo ft 1200 m -

1000 m - 3000 ft

800 m

Figure 6. Geologic cross-sections A-A1, B-B' , and C-C’ of the Buehman Canyon area — See Figure 5 for explanation of units. Scale 1:20,000. D

4000 ft

1000

• 3000 ft

2000 ft 500

Figure 7. Geologic cross-section D-D’ of the Buehman Canyon area — See Figure 5 for explanation of units. Scale 1:20,000. Figure 8. Composite contoured stereoplots of (a) 769 poles to bedding and (b) 952 poles to foliation — Contours 0.2-l-5-9% per 1% area. 15

Figure 9. Bedding (SQ ) and Foliation (S^) in very strongly deformed Pennsylvanian-Permian Naco Group limestone — Photograph taken looking north. 16

N N

Figure 10. (a) Stereoplot of lineation orientations in strongly deformed Pennsylvanlan-Permlan Naco Group. (b) Contoured stereoplot of all lineation orientations measured in domain of very strongly deformed rocks — Contours 1-6-13% per 1% area. 17

Stratigraphy and Lithology

Almost the entire geologic column of southeastern Arizona is represented in the Buehman Canyon Area. This geologic column closely resembles those described in detail by Creasy (1967) in the Mammoth

Quadrangle and by Cooper and Silver (1964) in the Dragoon Quadrangle.

In general, the stratigraphy consists of Late Precambrian through late

Tertiary sedimentary and metasedimentary rocks, resting on Precambrian metamorphic basement. A number of generations of igneous rocks invade parts of the sequence. These include Late Precambrian diabase sills,

Late Cretaceous(?) quartz diorite plugs, and intermediate to mafic Mid-

Tertiary dikes.

Precambrian Basement

The Pinal Schist makes up the Precambrian metamorphic basement.

It crops out in the southwestern corner of the Buehman Canyon area

(Figures 4 and 5) and consists of a chlorite-muscovite schist. It was derived from sedimentary and volcanic protoliths during the Mazatzal

Revolution about 1,500-1,700 my ago (Cooper and Silver, 1964; Drewes,

1981).

Late Precambrian Rocks

The Pioneer Shale and Dripping Spring Quartzite of the Upper

Precambrian Apache Group crop out in the southwestern corner of the area

(Figures 4 and 5), unconformably overlying the Pinal Schist. The

Pioneer Shale is generally a reddish sequence of argillite and quartzite

with a basal quartzite pebble conglomerate, the Scanlan Conglomerate

(Creasy, 1967; Cooper and Silver, 1964). The argillites are 18 metamorphosed to a muscovite schist and display a strong foliation and lineation. Quartzite pebbles in the conglomerate are flattened parallel to foliation and elongated parallel to lineation. The overlying Drip­ ping Spring Quartzite is an arkosic quartzite with a basal quartzite cobble conglomerate, the Barnes Conglomerate (Creasy, 1967). The cob­ bles in the conglomerate are not deformed and the quartzite is weakly foliated or unfoliated* The Apache Group is intruded by diabase sills which have been dated as 1,100-1,200 m.y. old elsewhere by Silver and

Damon (as reviewed by Shride, 1967). These sills dilate the section, approximately doubling the thickness to around 300 m (Creasy, 1967).

The sills are locally foliated and lineated and contain numerous shear zones which will be discussed in a later section.

Paleozoic Section

Bolsa Quartzite. The Middle Cambrian Bolsa Quartzite crops out in the southwestern part of the area (Figures 4 and 5). It is 76 m thick in the Buehman Canyon area (McKenna, 1966). It unconformably overlies the Apache Group and consists of a thick-bedded, coarse­ grained, gray to purplish gray cliff-forming quartzite (Drewes, 1981;

Cooper and Silver, 1964). It is unfoliated except for within rare

several-mm-thick shear zones located only in the extreme southern part of the area.

Abrigo Formation. The overlying Middle to Upper Cambrian Abrigo

Formation crops out in the southwestern and east-central parts of the

area (Figures 4 and 5). It is 136 m thick where most complete (McKenna,

1966) and is made up of three members: a lower member of thinly 19

Interbedded sandstone and shale, a middle pure quartzite member and an upper sandy dolomite member. These members correspond to the Three C,

Southern Belle and Peppersauce members described by Creasy (1967) in the

Mammoth Quadrangle. The Abrigo Formation is weakly foliated to unfolia­ ted.

Martin Formation. The unconformably overlying Devonian Martin

Formation crops out in the southwestern and east-central parts of the area. Where most complete, it is 80 m thick (McKenna, 1966) but the thickness varies dramatically (Figures 4, 5 and 6). It consists of thin-bedded dolomite and sandstone (Creasy, 1967). It is generally weakly foliated or unfoliated.

Escabrosa Limestone. The overlying Mississippian Escabrosa

Limestone crops out throughout the Buehman Canyon area (Figures 4 and

5). Where most complete, it is 170 m thick. It is a thick-bedded, coarse-grained, cliff-forming limestone. The lower 30 m of the forma­ tion is a very dark gray coarse-grained dolomite while the rest is a pure light grey limestone with only subordinate dolomite beds (Creasy,

1967). Limestone beds are unfoliated in the northern part of the area but are well foliated and commonly lineated in the southern part of the area as discussed in the next section. Dolomites are commonly fractured but not foliated, even when interbedded with intensely foliated lime­ stones.

Naco Group. 'The overlying Pennsylvanian-Permian Naco Group crops out throughout the Buehman Canyon area (Figures 4 and 5). The two lower formations of this group, the Pennsylvanian Horquilla Limestone and the Pennsylvanian-Permian Earp Formation are represented. Where 20 most complete these formations together are 400 m thick. The Horquilla

Limestone consists of thin—bedded blue—gray limestones interbedded with red shale and shaley limestone (Cooper and Silver, 1964). The Earp

Formation contains more shale and also contains sandstone, dolomite and a distinctive 1/2 - 1 m thick reddish conglomerate bed composed of red chert, jasper and limestone pebbles (Cooper and Silver, 1964). Since the contact between the two formations is gradational, it was not mapped. However, the upper member of the Earp Formation above the con­ glomerate unit is quite distinctive and was mapped as a separate unit

(Figure 4). The Naco Group rocks are unfoliated in the northern part of the area but are strongly foliated and lineated in the southern part of the area as discussed below.

Cretaceous Rocks

Bisbee Group. The sedimentary and metasedimentary rocks which unconformably overlie the Pennsylvanian-Permian Naco Group were mapped by Creasy and Theodore (1975) as Tertiary(?) in the northern part of the area and as Mesozoic in the southern part of the area. The northern

sequence consists of a 10-15 m thick basal limestone-cobble conglomerate overlain by at least 300 m of interbedded maroon and light green sand­

stones, siltstones and shales; white quartzarenites; conglomerates and

rare silty limestones. The southern sequence is composed of a basal

stretched-pebble tectonite overlain by purple and green phyllites and white quartzites. The southern and northern sequences grade into each

other in the central part of the area. This has led me to conclude that

the two sequences are not of different ages but, rather, that the 21 southern sequence is the metamorphic equivalent of the northern sequence. The basal conglomerate strongly resembles the Glance Conglo­ merate , the basal conglomerate of the Early Cretaceous Bisbee Group

(J.F. Schreiber, 1983, personal communication, and W.R. Dickinson, 1982, personal communication). The rest of the sequence also resembles the

Bisbee Group as described by Cooper and Silver (1964) in the Dragoon

Quadrangle. Therefore, I have interpreted both northern and southern sequences as Lower Cretaceous Bisbee Group (Figures 4 and 5).

Leatherwood Quartz Diorite. Several quartz diorite plugs intrude the Precambrian lithologies in the southwestern part of the area

(Figures 4 and 5). Creasy and Theodore (1975) correlated them with the

Late Cretaceous Leatherwood Quartz Diorite located 5 km to the west.

The quartz diorite is weakly foliated and llneated.

Tertiary Rocks

Intrusives. Several minor intermediate-to-mafic dikes and plugs intrude Pre-Tertiary lithologies throughout the area, commonly intruding

faults (Figures 4 and 5). Most are only a few meters thick and there­

fore too small to map. These dikes are not foliated.

Quiburis Formation. The flat-lying Late Tertiary Qulburis

Formation surrounds the Buehman Canyon area on three sides (Figure 4 and

5). It lies in angular unconformity on all lithologies listed above. 22

Structural/Metamorphlc Transformation of Stratigraphic Units

The most prominent characteristic of the Buehman Canyon area is the gradational southward increase in ductile deformation of certain lithologies. These lithologies are the limestones of the Hississippian

Escabrosa Limestone, the limestones and shales of the Pennsylvanian-Per- mian Naco Group, and the conglomerates and shales of the Cretaceous Bis- bee Group (Figures 4 and 5). They are transformed from unmetamorphosed sedimentary rocks into foliated and lineated metamorphic tectonites.

The gradational transition from sedimentary rocks to tectonites was mapped by dividing the aforementioned units into domains based on their degree of ductile deformation (Figures 4 and 5). The degree of ductile deformation was defined as the intensity of tectonite fabric development; i.e. the presence and intensity of foliation and lineation and the relative degree of pebble deformation. The four domains are designated as weakly deformed, moderately deformed, strongly deformed and very strongly deformed*. There is no strain significance attached to these particular divisions; they were simply a convenient way to map the gradual transition. The domains are defined as follows:

1) Weakly Deformed: foliation in shale units is a weak

slaty cleavage; limestones and limes tone** pebble conglo­

merates are unfoliated.

2) Moderately Deformed: both limestones and shales show a

foliation; foliation in limestone is spaced.

1. The Escabrosa Limestone was divided into only two general domains: 1) weakly deformed and 2) deformed. This is because varia­ tions in intensity of fabric are more subtle than in the other units. 23

anastomosing; limestone pebbles in conglomerates are

slightly flattened.

3) Strongly Deformed: limestones, limestone-pebble

conglomerates and shales are penetratively foliated on a

hand sample scale; limestone pebbles are very flattened

but still easily distinguishable; limestones are weakly

lineated.

4) Very Strongly Deformed: limestones, limestone-pebble

conglomerates and shales are penetratively foliated and

lineated, limestone pebbles are so deformed as to be

almost indistinguishable.

Various beds of the same lithologies commonly show different intensities of deformation. Therefore, when classifying the rocks in a particular area, the maximum degree of tectonite fabric development was used.

As mapped in the Buehman Canyon area, the above domains form a

series of bands oriented roughly east-west across the area, with the

gradient of increasing intensity of deformation going from north to

south (Figures 4 and 5).

A detailed description of the changes which occur in the

Mississippian Escabrosa Limestone, the PenneyIvanian-Permian Naco Group

and the Cretaceous Bisbee Group through the four domains follows.

Argillites

Argillites are transformed from maroon shales to purple phyl-

lites. In the weakly deformed Cretaceous Bisbee Group and the

Pennsylvanian-Permian Naco Group the maroon shales display a weak slaty 24

cleavage and/or a pencil structure. The pencil structure is nearly

horizontal and trends north, paralleling the line of intersection

between bedding and slaty cleavage. Moderately deformed argillites

display a very penetrative slaty cleavage. Strongly deformed argillites

are slightly phyllitic. Very strongly deformed argillites are purple

phyllites rich in muscovite.

Calcite-filled crack-seal veins (after Ramsay, 1980b) are common

throughout the Buehman Canyon area but are especially common and well

developed in the phyllites of the very strongly deformed rocks (Figure

11); these veins are generally oriented perpendicular to both foliation

and lineation; the fibrous calcite crystals of the vein fillings are

aligned parallel to lineation.

Limestones

Weakly deformed limestones appear relatively unfoliated in out­

crop (Figure 12a). Moderately deformed limestones in the Pennsylvanian-

*Permian Naco Group display spaced, anastomosing dissolution-cleavage

surfaces (Figure 12b). Evidence for volume loss is particularly

striking in thin section. For example, in figure 12c a fusulinid is

truncated and half dissolved by such a surface. Strongly deformed lime­

stones display a foliation which is penetrative on a handsample scale;

it is defined by both mesoscopic color banding and microscopic mineral

alignment. A weak lineation is present in the shaley limestones of the

Pennsylvanian-Permian Naco Group but not in the purer limestones of the

Mississippian Escabrosa Limestone. The lineation is defined by the

alignment of fossils and uncommonly by mineral trails on foliation 25

Figure 11. Photomicrograph of crack-seal vein in very strongly deformed Lower Cretaceous Bisbee Group phyllite. Figure 12. Series of photographs showing progressive changes in the Pennsylvanian-Permian Naco Group limestones along the structural/metamorphic gradient, (a) Weakly deformed — unfoliated. 26

( a ) Figure 12. Series of photographs showing progressive changes in the Pennsylvanian-Permian Naco Group limestones along the structural/metamorphic gradient (continued). (b) Moderately deformed - foliation in the form of spaced, anastomosing dissolution surfaces. (c) photomicrograph of one of the dissolution surfaces of (b) truncating a fusulinid — Note the clayey and carbonaceous residue along the surface. 27

(b)

(c ) Figure 12. Series of photographs showing progressive changes in the Pennsylvanian-Permian Naco Group limestones along the structural/metamorphic gradient (continued), (d) Very strongly deformed — foliation penetrative, lineation well developed.

Figure 13. Series of photomicrographs showing grain-size reduction in the Pennsylvanian-Permian Naco Group limestones, (a) Weakly deformed, (b) Moderately deformed, (c) very strongly deformed — All three samples were taken from highly fossiliferous, shale-rich, pink-colored beds. 29

(c ) 30 surfaces. Both foliation and lineation are well-developed and penetra­ tive in strongly deformed shaley limestones (Figure 12d). Foliation is well defined in pure limestones and a penetrative lineation is uncom­ monly present. Foliation is defined by microscopic mineral alignment.

Lineation is defined by abundant mineral-trails and fine grooves on foliation surfaces, by microscopic mineral alignment and commonly by alignment of deformed fossils.

Grain size in the limestones generally decreases as the degree of deformation increases. This is demonstrated in figure 13. All three

samples were collected from highly fossiliferous, shale-rich, pink-

colored beds in the Pennsylvanian-Permian Naco Group.

Deformation of Conglomerates

The Lower Cretaceous Glance Conglomerate is composed of Upper

Paleozoic limestone cobbles with subordinate chert and sandstone cobbles

in a coarse sand matrix (Figure 14a). The shapes of limestone cobbles

are very sensitive indicators of the degree of deformation of the rock;

the chert and sandstone cobbles are less useful because they are neither

as abundant nor as strongly deformed. Only a qualitative evaluation of

the degree of limestone cobble deformation was used to define the

domains and map the structural/metamorphlc transition (see previous

section). However, after the mapping was completed, aspect ratios of

limestone pebbles were measured at four sites in order to evaluate the

relative strain across the transition.

The locations of the four sites are shown on figures 4 and 5.

Pebbles were measured on surfaces oriented roughly 1) perpendicular to Figure 14. Series of photographs showing progressive changes in the Lower Cretaceous Glance Conglomerate, (a) Weakly deformed — aspect ratios on all outcrop surfaces average around 2:1, regardless of orientation. 31

( a ) Figure 14. Series of photographs showing progressive changes in the Lower Cretaceous Glance Conglomerate (continued). (b) and (c) Moderately deformed — aspect ratios 6:6:1, (b) outcrop surface oriented perpendicular to foliation, (c) outcrop surface oriented parallel to foliation. 32 Figure 14. Series of photographs showing progressive changes in the Lower Cretaceous Glance Conglomerate (continued). (d) and (e) Strongly deformed — aspect ratios average 13:13:1, (d) outcrop surface oriented perpendicular to foliation, (e) outcrop surface oriented parallel to foliation. 33

(d)

(e) Figure 14. Series of photographs showing progressive changes in the Lower Cretaceous Glance Conglomerate (continued). (f) and (g) Very strongly deformed — aspect ratios average 60:16:1, (f) outcrop surface oriented perpendicular to both foliation and lineation, (g) outcrop surface oriented perpendicular to foliation but parallel to lineation. 34

(g) Figure 14. Series of photographs showing progressive changes in the Lower Cretaceous Glance Conglomerate. Very strongly deformed (continued). (h) outcrop surface oriented parallel to foliation, showing lineation. 35 36 foliation and parallel to lineation and 2) perpendicular to both folia­ tion and lineation. Such surfaces should approximate principal planes of the ellipsoidal deformed pebbles. The long and short dimensions of thirty pebbles were measured as exposed on these outcrop surfaces. The aspect ratios were computed for each pebble; and then averaged for each outcrop surface. The average aspect ratios for the two surfaces were then combined to yield approximations of the average three-dimensional aspect ratios. In the absence of lineation, measurements were made on two approximately perpendicular surfaces, both perpendicular to folia­ tion. In the absence of both lineation and foliation, measurements were made on two approximately perpendicular surfaces, both perpendicular to bedding.

The data from the four sites clearly indicate a dramatic

increase in strain accompanying the increase in intensity of tectonite

fabric.

Site #1. This site is in weakly deformed rocks (Figures 4, 5 and

14a). The comglomerate is neither foliated nor lineated. Aspect ratios

of limestone pebbles average 1.7:1, 1.8:1 and 2.1:1 on three outcrop

surfaces of widely varying orientations. The long dimensions of the

pebbles show no preferred orientations.

Site #2. This site is in moderately-to-strongly deformed rocks

(Figures 4, 5, 14b and 14c). There is a well developed foliation but no

lineation. Aspect ratios of limestone pebbles average around 6:1 on two

mutually perpendicular outcrop surfaces, both oriented perpendicular to

foliation. The long dimensions are subparallel to the trace of folia­

tion. Thus the composite aspect ratio is approximately 6:6:1. 37

Site #3. This site is in strongly-1o-very-strongly deformed rocks (Figures 4, 5 and I4d and 14e). Both foliation and lineation are well developed. Aspect ratios of limestone pebbles on surfaces oriented perpendicular to foliation (both perpendicular and parallel to linea­ tion) average around 13:1. The long dimensions are parallel to the trace of foliation. Thus the composite aspect ratio is approximately

13:13:1. Interestingly, these pebbles are not elongated parallel to lineation, even though lineation is commonly well developed.

Site #4. This site is in very strongly deformed rocks (Figures

4, 5, 14f, 14g, and I4h). Aspect ratios on surfaces oriented perpendi­ cular to both foliation and lineation average around 16:1; by contrast, on surfaces oriented perpendicular to foliation and parallel to linea­ tion, the aspect ratios average around 60:1! Thus the composite aspect ratio is approximately 60:16:1.

Chert and Sandstone Pebbles. In the moderately deformed Glance

Conglomerate, chert and sandstone pebbles are cotmonly cut by closely- spaced (several mm apart) fractures; these fractures are oriented at a high angle to foliation and commonly form conjugate sets. Spacing between the fractures becomes closer (less than a mm apart) in strongly deformed rocks; the traces of the fractures on the foliation plane generally parallel lineation. Chert and sandstone pebbles in the very

strongly deformed Glance Conglomerate are slightly flattened and

elongated; trails of these pebbles are common on the foliation plane, making the lineation in these rocks very prominent. 38

Metamorphism

Weakly, moderately and strongly deformed rocks are sub-green-

schist facies. Very strongly deformed rocks commonly contain chlorite,

epidote and muscovite, indicating a lower greenschist facies of metamor­

phism.

Stratigraphic Thickness

Stratigraphic units become progressively thinner as the rocks

become more deformed (Figures 6 and 7). For example, the Mississippian

Escabrosa Limestone is thinned from 168 m to 49 m (Figure 15). The

Pennsylvanian-Permian Naco Group is thinned from 400 m to 3 m! It must

be noted, however, that a small percentage of this thinning could be due

to the erosional unconformity at the upper contact of the Naco Group.

Changes in Orientations of Fabric Elements

For the most part, bedding, foliation and lineation maintain

remarkably constant orientations regardless of the degree of deforma­

tion, although some systematic changes in the orientations of fabric

elements occur.

Bedding. Average bedding orientation remains remarkably con­

stant at approximately N20oW,35°NE, although the scatter increases with

increasing deformation (Figure 16).

Foliation. Figure 17 shows foliation orientations for each of

the four domains defined by degree of fabric development. In the

strongly and very strongly deformed rocks the average foliation

orientation is N35°W, 15°SE. The average foliation orientation in the Figure 15. Variations in thicknesses of (a) Mississippian Escabrosa Limestone and (b) Pennsylvanian-Permian Naco Group — Thicknesses calculated from outcrop width on map. Horizontal axis of each graph is the distance north of the southern boundary of the study area. Vertical axis is the thickness of the unit. Bar on right side of graph indicates range of standard thicknesses (after Bryant, 1968; Drewes, 1981). 39

Very Strongly Strongly Moderately Weakly Deformed Deformed Deformed Deformed 400 m * ♦ ♦

200 m -- i 1 Thickness

1 1 1 2 km 4 km 6 km Distance North a. Mississippian Escabrosa Limestone

Very Strongly Strongly Moderately Weakly Deformed Deformed Deformed Deformed 600 m

1100 m

400 m

200 m

Thickness

2 km 4 km 6 km Distance North b. Pennsylvanian-Permian Naco Group 40

Figure 16. Contoured stereoplots of poles to bedding in the Mississippian Escabrosa Limestone, the Pennsylvanian-Permian Naco Group and the Cretaceous Bisbee Group, (a) Weakly deformed (213 readings), (b) Moderately deformed (113 readings), (c) Strongly deformed (72 readings), and (d) Very strongly deformed (52 readings) — Contours 1-5-9-12% per 1% area. 41

Figure 17. Contoured stereoplots of poles to foliation in the Hississippian Escabrosa Limestone, the Pennsylvanian-Permian Naco Group and the Cretaceous Bisbee Group, (a) Weakly deformed (62 readings), (b) Moderately deformed (107 readings), (c) Strongly deformed (285 readings), and (d) Very strongly deformed (409 readings) — Contours 1-5-9% per 1% area. 42 moderately deformed rocks is N46°W, 15°NE. In the weakly deformed rocks it is N48°E, 6°NW.

The angle between bedding and foliation remains fairly constant throughout the area, regardless of degree of deformation. Figure 18 shows the frequency distribution of the stereographlcally determined angle between bedding and foliation where both can be measured on a

single outcrop. For all domains, the magnitude of this angle was most

frequently between 20° and 30°.

Lineation. Lineation trends show a very puzzling pattern. In

the very strongly deformed rocks, where lineation is well developed,

there is a well-defined maximum concentration around 10o,S85°E. How­

ever, in the strongly deformed rocks, where lineation is poorly

developed, lineation trend swings progressively from east to northeast

to north with increasing distance northward (Figures 4, 5, 10a, and 19).

Geometry of the Structural/Metamorphic Transition

In general, the trend of the structural/metamorphic transition

zone as defined by the contacts between domains is east-west (Figures 4

and 5). It must be noted, however, that the gradational nature of the

contacts and the narrow east-west width of bedrock exposure in the Bueh-

man Canyon area precludes rigorous evaluation of the trends of these

contacts. Thus, the contacts may trend considerably northwest or north­

east. Interestingly, in the area where most of the contacts between the

domains are mapped, the formation contacts are, when generalized, also

east-west trending. By contrast, regional trends are north-south

(Figures 4 and 5). This local east-west trend is due to the large 43

5

d

e Figure 18. Bar graphs showing the frequency distribution of the stereographically determined angle between bedding and foliation in 5° increments, (a) Weakly deformed rocks, (b) Moderately deformed rocks, (c) Strongly deformed rocks, (d) Very strongly deformed rocks, and (e) composite plot — In each case, both bedding and foliation were measured on a single outcrop. Positive values for the X-axis denote that, when one looks north, foliation is counterclockwise from bedding. Negative values denote the opposite. The Y-axis is the frequency (number of readings). 44

2000 m-

1000 m -

Distance North of Contact Between Strongly Deformed and Very Strongly Deformed Rocks

Lineation Trend

Figure 19. Graph showing progressive swing of lineation trend — In the Pennsylvanian-Permian Naco Group from east to northeast to north with increasing distance north of the contact between strongly and very strongly deformed rocks. 45 number of west-dipping normal faults which distort the contacts. These faults may also be partly responsible for the east-west trend of the structural/metamorphic transition zone as mapped in the Buehman Canyon area.

Regionally, rocks south and west of the Buehman Canyon area are tectonites whereas those north and east of the area are not (Figure 3).

Thus the Buehman Canyon area is part of a regional transition in the

Santa Catalina Mountains: the gradational northern margin of a large zone of tectonites.

The projection of this margin below the surface cannot be deter­ mined on the basis of field evidence. However, it seems highly improbable that the margin is significantly discordant from foliation.

It is therefore probably shallowly to moderately east-dipping.

Relationship Between Lithology and Intensity of Ductile Deformation

In addition to the spatial variation in degree of ductile defor­ mation described above, there is also a wide variation as a function of lithology. The lithologies discussed above, the argillites, limestones and limestone-pebble conglomerates, are the most ductile of the various lithologies in the Buehman Canyon area. As noted in the section on

stratigraphy and lithology; quartz diorite, sandstone, diabase, dolomite

and quartzite have a weak, discontinuous tectonite fabric if they have

one at all, even when directly adjacent to very strongly deformed

lithologies. 46

Structures

Large-scale Structures

Normal Faults. Two distinct types of normal faults are common in the Buehman Canyon area, those which are subparallel to bedding and those which cut bedding at a high angle.

Two bedding-subparallel normal faults are located in very strongly deformed rocks in the southern part of the Buehman Canyon area

(Figures 4, 5 and 6c). The easternmost of these faults can be traced in the Buehman Canyon area for 1100 m. and continues southward for another

2500 m. until it is truncated by a high-angle fault (Creasy and Theo­ dore, 1975). Stratigraphic separation on this fault increases progressively southward, from no stratigraphic separation to a juxtapo­ sition of Hississippian Escabrosa Limestone against Cambrian Bolsa

Quartzite just south of the Buehman Canyon area, a stratigraphic 'separa­ tion of at least 200 m. (Creasy and Theodore, 1975). This east-dipping fault is curviplanar and dips eastward between 19° and 58° as determined from three-point calculations (Figures 4 and 5). The few exposures of this fault reveal it to be remarkably sharp and roughly parallel to bed­ ding; there is no sign of brecciation along it. It thus seems to be quite ductile and is probably contemporaneous with tectonization.

There are numerous minor bedding-subparallel faults throughout

the area. These faults are probably responsible for some of the thin­ ning of stratigraphic units discussed above.

The normal faults which cut bedding at a high angle commonly dip moderately to steeply westward; they uncommonly have steep eastward dips 47

(Figure 20). These faults cut the tectonite foliation, the folds (dis­ cussed in the next section), and the reverse fault (Figures 4 and 5).

Several normal faults cut earlier ones; in each case a steeper fault cuts a shallower one. The west-dipping normal faults are probably at least partly responsible for the predominantly eastward dip of strata.

Concentric Folds. Both bedding and foliation are folded locally into upright concentric folds on all scales. The largest one of these folds is a syncline in the southeast part of the area (Figures 4 and 5).

The stereographically determined axis to this fold is horizontal, N25°W; the axial plane is N25°W, 78°W (Figure 21).

Mesoscopic Structures

Shear Zones. Shallowly east-dipping mesoscopic shear zones are very common in the Late Precambrian diabase sills, especially just below the Barnes Conglomerate member of the Dripping Springs Formation

(Figures 4, 5, 22 and 23a). The shear zones are also present in quart­ zites within the Late Precambrian Apache Group and the Cambrian Bolsa

Quartzite. These units generally do not display a foliation outside of

the 0.5 to 2 cm thick shear zones. The shear zones dip moderately east­ ward. A stereoplot of poles to shear zone orientations reveals two cen­

ters of maximum concentration: N50°W, 15°NE and N4°E, 40°NE (Figure

23a). The shear zones commonly display a very well developed lineation which trends east, averaging 40°, N84°E, roughly parallel to the general

orientation of lineations in the mylonites (Figures 23b and 10b).

Foliation curves sigmoidally in the manner theoretically described by

Ramsay and Graham (1970), thus allowing determination of the 48

N

Figure 20. Stereoplot of poles to normal faults which cut bedding at a high angle. 49

Figure 21. Contoured stereoplot of 100 poles to foliation around major syncline in the southeast part of the area — Fold axis (B): horizontal, N25°W. Axial plane (AP): N25°W,78°W. Contours 1-3-5-8Z per 1% area. Figure 22. (a) Photograph of mesoscopic shear zones in Precambrian diabase sill — Upper and lower boundaries of shear zones shown by dashed lines. Photograph taken looking north. Shear sense is top-to-the-east (right). (b) Photograph of lineations on foliation surface in shear.zone 50

(b) 51

N N

Figure 23. Contoured stereoplots of (a) 31 poles to shear zone orientations and (b) 27 lineations in shear zones — Contours 1-5-12% per 1% area. 52 sense of shear. The curvature of the foliation in every shear zone observed indicated a top-to-the-east shear sense.

Asymmetric Folds. Outcrop-scale asymmetric folds with axial planes subparallel to foliation are uncommonly present in the Buehman

Canyon area. They are especially concentrated near the southern boundary of the area. These folds are much more abundant to the south, where they were studied in detail by Schloderer (1974).

Granulation Cleavage. A crenulation cleavage is common in the southern part of the Buehman Canyon area, especially in the strongly deformed phyllites of the Cretaceous Bisbee Group (Figure 24). This cleavage generally dips steeply to the northeast (Figure 25). The crenulation cleavage affects both tectonite foliation and the aforemen­ tioned crack-seal veins in the phyllites (Figure 26). Interestingly, the crack-seal veins commonly control the position and orientation of the crenulation cleavage surfaces as the two are commonly parallel and/or coincident.

Microstructures

Microscopic examination of the tectonites reveals a remarkable consistency in the asymmetry of certain microstructures. All thin sec­ tions were cut parallel to lineation and perpendicular to foliation.

Long axes of small, dynamically recrystallized calcite grains in limestone and limestone-pebble-conglomerate samples are commonly oblique to the macroscopic foliation defined by color-banding and mineral segre­ gation. A similar observation was made by Schmid and others (1981) on deformed limestones in the Swiss Alps. Four of the samples from the 53

Figure 24. Granulation cleavage in very strongly deformed Cretaceous Glance Conglomerate. N

Figure 25. Contoured stereoplot of 51 poles to crenulation cleavage surfaces — Contours 1-10-20% per 1% area. 55

0.5 m m

Figure 26. Photomicrograph of crack-seal vein truncated by crenulation cleavage surfaces. 56

Buehman Canyon area show a consistently asymmetric relationship between the orientations of the long axes of calcite grains and the macroscopic foliation. The long axes of calcite grains are consistently counter­ clockwise from the macroscopic foliation when viewed looking north

(Figure 27a).

Another asymmetric microstructure present in the mylonites of the Buehman Canyon area is a late discontinuous foliation which cross­ cuts and crenulates the earlier foliation at a low angle. This feature is common in the pelitic units of the Late Precambrian Apache Group.

There is a consistently asymmetric relationship between the orientations of this secondary foliation and the earlier foliation. When viewed looking north, the secondary foliation is oriented consistently clock­ wise from the earlier foliation (Figure 28).

The kinematic implications of these asymmetric microstructures will be discussed in the next chapter. 57

0.5mm

1 infinitesimal strain ellipse 2 incremental strain ellipse 3 finite strain ellipse 4 shear plane

(b) 58

0.1 m m

Figure 28. Photomicrograph of shear bands in the Late Precambrian Pioneer Shale — East is to the left. The planar structures oriented obliquely to the long dimension of the grain are shear bands which indicate top-to-the-east (left) shear sense. CHAPTER 4

INTERPRETATION

The Buehman Canyon area can best be interpreted as the grada­ tional northern margin of a regionally extensive shear zone that is at least 1000 m. thick (the approximate structural relief exposed in the

Buehman Canyon area). The various types of tectonite foliation and lineation, the bedding-subparallel normal faults, crack-seal veins, mesoscopic shear zones, asymmetric folds, and asymmetric microstructures are all interpreted as being related to shear zone deformation. The reverse fault, the upright folds, the crenulation cleavage and the high- angle normal faults represent later superposed events.

Kinematics of the Shear Zone

Direction and Sense of Shear

The direction and sense of shear for the tectonites in the Bueh­ man Canyon Area can be deduced from several different structural ele­ ments: 1) lineation, 2) mesoscopic shear zones, 3) microfabrics, and 4) asymmetric folds. Lineation trends average around 10°,S85CE (Figure 10) and thus gives the direction but not the sense of shear. The mesoscopic shear zones, with their east-northeast lineations and sigmoidal folia­ tions (see previous section), directly indicate top-to-the-east shear.

The asymmetric microfabrics described above also indicate top- to-the-east shear when interpreted in the following ways:

59 60

1) The obliquely oriented dynamically recrystallized calcite grains in

limestones and limestone-pebble conglomerates probably formed rela­

tively late during progressive shear deformation and thus only record

the final stages (Simpson and Schmid, 1983). If this assumption is

correct, the grains are aligned parallel to the flattening plane of

the last incremental strain component which is nearly parallel to the

flattening plane of the infinitesimal strain component. In pro­

gressive rotational shear deformation, this flattening plane is

oblique to the flattening plane of the finite strain ellipsoid,

represented by the macroscopic foliation in the rock (Simpson and

Schmid, 1983). The sense of shear can thus be deduced from the rela­

tionship between the orientations of the macroscopic foliation and

the elongated calcite grains (See Figure 27b). Applying this to four

samples with consistent asymmetries, they indicate a top-to-the-east

sense of shear.

2) The secondary foliation planes observed in the Apache Group pelites

closely resemble the shear bands described by Berthe and others

(1979) and White and others (1980). These shear bands are thought to

develop at very high shear strains when the flattening foliation has

been rotated to an orientation roughly parallel to the shear zone

walls (Simpson and Schmid, 1983). The shear bands indicate the over­

all sense of shear* Note that the shear bands in Figure 28 offset

the primary foliation planes in a consistent top-to-the-east manner.

This indicates a top-to-the-east shear sense for the rock as a whole.

The top-to-the-east shear sense determined from mesoscopic shear

zones and microfabrics in this study is supported by the work of 61

Schloderer (1974) on the asymmetric folds in an area immediately south

of the Buehman Canyon area. He employed the Hansen (1971) separation- angle method of fold analysis (Figure 29) and deduced a top-to-the-east

slip line orientation of S60°E.

The top-to-the-east shear sense is also supported by the wes­

terly dip of the foliation in the northernmost part of the area (Figure

17). This may represent the curvature of foliation expected at the mar­

gin of a shear zone: in a shallowly-dipping east-directed shear zone,

one would indeed expect the orientation of foliation on the upper and

lower margins of the shear zone to be shallowly west-dipping.

It can therefore be confidently stated that the shear sense in

the Buehman Canyon Area is top-to-the-east.

Orientation of Shear Zone Walls

Several structural elements indicate the orientation of the

shear plane in the Buehman Canyon area and therefore the orientation of

the upper and lower shear zone walls since the two are theoretically

parallel (Ramsay and Graham, 1970). The shallowly to moderately east­

dipping orientation of the mesoscopic shear zones gives a direct indica­

tion of the orientation of the shear plane. The N4°E,10°NE orientation

of the slip plane derived by Schloderer gives a second indication. In

addition, shear zone theory (Ramsay and Graham, 1970) indicates that the

shear zone walls must be oriented at an angle of 45° or less to the

N20°W, 20°NE average foliation orientation. All of the above indicate

that the shear plane and therefore the shear zone walls generally dip

shallowly to moderately to the east. 62

N

a S-folds • Z-folds

Figure 29. Hansen (1971) plot of S- and Z-folds by Schloderer (1974) from an area immediately south of the Buehman Canyon area — Slip line determined from this plot is S60o e , top-to-the-east. 63

Regional Implications of the Shear Zone Margin

The shear zone margin, as exposed in the Buehman Canyon area, helps elucidate the geometry of the regional shear zone as a whole. The contacts between domains of the structural/metamorphic gradient trend east-west, parallel to the regional lineation trend and perpendicular to the regional strike of bedding (Figures 4 and 5). Thus the part of the shear zone margin exposed in the Buehman Canyon area apparently trends parallel to the transport direction.

Based on these observations, figure 30 shows four possible interpretations for the position of the Buehman Canyon area relative to the regional shear zone. In summary, these interpretations are 1) the area is at the bottom wall of a shear zone which has a southerly compo­ nent of dip, 2) the area is at the upper wall of a shear zone which which has a northerly dip component, 3) the amount of displacement on

the shear zone tapers down to nothing towards the north, i.e., the shear

zone dies out in the Buehman Canyon area, and 4) the shear zone margin

actually trends north, perpendicular to the transport direction; it

appears to trend east because of disruption by normal faulting*

Any one of these could be accurate given the data available.

However, since there is evidence for disruption by normal faulting, and

because most of the structural elements do have a slight northerly com­

ponent of dip, a combination of 2 and 4 above seems the most reasonable.

Thrust-Slip or Normal-Slip Shear?

Since the complete shear zone is not exposed, displacement of

marker horizons cannot be used to determine whether the shear zone is Figure 30. Four possible interpretations for the position of the Buehman Canyon area relative to the regional shear zone — In each case the Buehman Canyon area is represented by the small surface projected into the block diagram: (a) The bottom margin of a top-to-the-east shear zone with a southerly component of dip. (b) The top margin of a top-to-the-east shear zone with a northerly component of dip. (c) The tapering lateral margin of a top-to-the-east shear zone, (d) The top margin of an east-dipping top-to-the-east shear zone. 64 65 thrust-slip or normal-slip. Therefore, indirect methods must be employed.

Figure 31 shows the five possible types of top-to-the-east slip on the shear zone; horizontal, low-angle normal (30°), high-angle nor­ mal (60°), thrust (30°), and reverse (60°). The resulting geometries are shown after shear strains (Y) of 1 (45° angular shear) and 2 (65° angular shear). For simplicity, it is assumed that bedding was origi­ nally horizontal and that the deformation was by simple shear only. It

is also assumed that foliation is parallel to the flattening plane of

the finite strain ellipsoid.

The orientation of the flattening plane was calculated using;

2 (derived from tan 2ct = — Nicolas and Poirier, 1976, p. 19)

where Y denotes shear strain and a denotes the angle between the flat­

tening plane (foliation) and the shear plane. The orientation of

bedding was calculated using;

cot 8* = cot 8 + y (from Ramsay, 1967, p. 88)

where 8 denotes the original angle between bedding and the shear plane

and 8*denotes the final angle between bedding and the shear plane.

Five major observations can be used to evaluate the applicabi­

lity of the above five types of slip to the Buehman Canyon area;

1) bedding and foliation dip east, 2) foliation dips less steeply than

bedding; the angle between the two is 20 ° to 30°, 3) bedding remains

upright, 4) stratigraphic units are thinned and/or extended and ‘Figure 31. Schematic cross-sections of five possible types of top-to-the-east slip on the shear zone — East is to the right. Assuming originally horizontal bedding, top-to-the-east shear sense, and simple shear only. Resulting orientations of bedding and foliation are shown after shear strains (y) of 1 and 2. (a) parallel to bedding, (b) shallowly east-dipping (normal-slip), (c) steeply east-dipping (normal-slip), (d) shallowly west-dipping (thrust-slip), (e) steeply west-dipping (reverse-slip). Solid line denotes bedding. Dashed line denotes foliation. /t e 67

5) bedding orientation remains relatively constant with increasing deformation.

Low angle normal-slip shear fits the four above observations very well. The other four types of slip are inapplicable if it is assumed that the deformation was by simple shear only. However, if there is a large component of pure flattening perpendicular to the shear zone walls and/or pure extension in the shear direction and if the shear zone was rotated eastward after the cessation of shear deformation (as a result of rotational normal faulting), horizontal— slip shear and very low angle thrust—slip shear could also account for the above observa­ tions. Since the relative contributions of pure and simple shear to the total deformation has not as yet been evaluated, these possibilities cannot be ruled out. In addition, it is also possible that bedding was originally west-dipping. In this case, if the shear zone walls of a thrust-slip zone dipped less steeply to the west than bedding did, the relative orientations of bedding and the shear plane would be the same as those for low-angle normal shear with horizontal bedding (tilt Figure

31b to the left to visualize this). Again, rotation after shear defor­ mation would be needed to bring it to its present orientation.

Since low-angle normal-slip shear is the simplest explanation and because it holds whether or not there was significant pure shear and whether or not there was rotation after the shear deformation, it is favored here 68

Fault-Rock Terminology

Since the ductile deformation in the Buehman Canyon area is related to a shear-zone, fault-rock terminology could be applicable to the metamorphic tectonites. The terminology used here is that suggested by Sibson (1977) and Tullis and others (1982). They suggest that ’cata- clasite1 and related terms be reserved for fault rocks which have undergone brittle deformation by cataclasis and that 'mylonite' and related terms be reserved for rocks which have undergone ductile defor­ mation, generally within a shear zone. By contrast, Higgins (1971) interpreted both cataclasltes and mylonites as brittle cataclastic fea- - tures.

The metamorphic tectonites have undergone ductile deformation and are therefore not cataclasltes. The very strongly deformed lime­

stones are foliated and lineated and there is evidence for grain-size

reduction, all characteristics which are essential to the definition of

'mylonite.' Therefore, the term, 'mylonite' does seem appropriate, at

least to the very strongly deformed limestone tectonites of the Buehman

Canyon area.

Strain Significance of Pebble Deformation

Because of the strong ductility contrasts between the limestone

pebbles and the chert and sandstone pebbles in the Cretaceous Glance

Conglomerate,'the shapes of the deformed pebbles do not give an absolute

evaluation of the magnitude of strain. However, the pebbles do give an

indication of the shape of the strain ellipsoid and an estimate of the

magnitude of strain. 69

Interestingly, the moderately and strongly deformed rocks are predominantly flattened in the plane of foliation but not significantly elongated. On the other hand, the very strongly deformed rocks are pre­ dominantly elongated parallel to lineation and not significantly flattened beyond that observed in the strongly deformed rocks. Thus, the strain near the margin of the shear zone is predominantly in the flattening field while the strain closer to the center of the zone is in the constriction field.

An estimate of the magnitude of strain can be obtained from the aspect ratios of the deformed pebbles. The ratio between two principal axes of the finite strain ellipsoid can be obtained from

where r* and r denote the average aspect ratios of deformed and unde­ formed pebbles, respectively. and X% denote the quadratic elongation parallel to two of the principal axes of the strain ellipsoid.

Combining the ratios between two pairs of the three axes of the strain ellipsoid yields an estimate of the 3-dimensional axial ratio of the strain ellipsoid. In each case the original aspect ratio (r) is assumed to be 2:1. The results are as follows:

Pebble-measuring site #2. Xi^iXg * 9:9:1

Pebble-measuring site #3. Xi:X21X3 ■ 42:42:1

Pebble-measuring site #4. X^Xa^Xg * 900:64:1 70

Post-Shear Deformation

At least two stages of deformation followed the shear zone deformation in the Buehman Canyon area: 1) Upright folding, reverse faulting and crenulation cleavage formation and 2) predominantly west­ dipping normal faulting. The upright folds, reverse faults and crenulation cleavages all accommodated layer-parallel shortening in a roughly east-west direction. The later normal faulting, on the other hand, accommodated layer-parallel extension in an east-west direction.

The Buehman Canyon area represents the northern margin of a regional low-angle normal-slip shear zone that is at least 1000 m thick.

The walls of this shear zone dip east-northeast. The most strongly deformed limestones in the area can be considered myIonites. Estimates of strain ratios based on deformed pebble shapes are as high as 900:64:1 in the most strongly deformed rocks.

Shear zone deformation was followed by both layer-parallel com­ pression and layer-parallel extension in an east-west direction. CHAPTER 5

REGIONAL SIGNIFICANCE

The general structural evolution of the Buehman Canyon area is similar to that of other parts of the Santa Catalina-Rincon metamorphic core complex: ductile low-angle shear deformation overprinted by brit­ tle west-dipping normal faults (Davis, 1980; Lingrey, 1982). However, the Buehman Canyon Area is unique in that the shear sense is top-to-the- east. In the southern and western parts of the complex, the sense of shear in the mylonitic tectonites is top-to-the-southwest, with the same sense as the inferred movement on the detachment faults (Lingrey, 1982).

In addition, upright folds and crenulation cleavages affecting the tec-

tonite foliation are not commonly reported in the complex.

Clearly, the ductile deformation in the Santa Catalina-Rincon metamorphic core complex is quite complicated and the sense of shear is

not consistent throughout the complex.

There are several possible explanations for the different shear

senses within the same complex. The first is that they represent

totally different events. However, both appear to accommodate regional

extension. Another possible explanation is that the two shear systems

are conjugate (Ramsay, 1980a), with the top-to-the-southwest shear

system probably being dominant.

In any case, the low-angle normal-slip east-directed shear of

the Buehman Canyon area does not neatly fit any of the three models

71 72 proposed (east-directed thrusting, west-directed thrusting, and west- directed normal faulting) to explain the ductile structures of the Santa

Catalina-Rincon metamorphic core complex. Any new models proposed must take into consideration the complexities reported here in the nature of the ductile deformation in the complex. REFERENCES

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