Fault rocks of the Tanque Verde Mountain decollement zone, Santa Catalina Metamorphic Core Complex, Tucson, Arizona

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Authors Di Tullio, Lee Dolores

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Link to Item http://hdl.handle.net/10150/557972 FAULT ROCKS OF THE TANQUE VERDE MOUNTAIN

DECOLLEMENT ZONE, SANTA CATALINA

METAMORPHIC CORE COMPLEX,

TUCSON, ARIZONA

by

Lee Dolores Di Tullio

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

19 8 3 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 judg­ ment the proposed use of the material is in the interests of scholar­ ship. In all other instances, however, permission must be obtained from the author.

SIGNED:

GEORGE HERBERT DAVIS Professor of Geosciences

ii ACKNOWLEDGMENTS

I would like to sincerely thank my past and present fellow students: Carolyn Johnson, Ralph Rodgers, Debbie Caskey and Jean

Crespi; all were a constant source of advice, encouragement, and other more tangible forms of assistance. A special debt is owed to Bob

Krantz for his photographic and many other forms of help; Ann Bykerk-

Kauffman lent me her computer and drafting supplies and put up with me

in the final frantic stages of completion.

To my advisor, George Davis, I owe a special debt of gratitude

He suggested the thesis project, gave monetary and moral support, and

carefully edited the manuscript. Beyond this, I would like to thank him for his unflagging and contagious interest in and enthusiasm for

structural geology.

iii TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS...... v

A B S T R A C T ...... vii

INTRODUCTION ...... 1

PREVIOUS W O R K ...... 4

GENERAL GEOLOGIC FRAMEWORK ...... 6

C o r e ...... 6 Cover ...... 15 Decollement Z o n e ...... 17

FAULT R O C K S ...... 21

Terminology...... 21 Structural Stratigraphy ...... 24 Brecciated Mylonitic Gneiss Subzone ...... 24 Cataclasite Sub zo n e ...... 35 Internal Geometry ...... 38 Brecciated Gneiss— Saguaro National Monument East Study A r e a ...... 45 Cataclasite— Southwestern Tanque Verde Study Area .... 63

DISTRIBUTION OF FAULT ROCKS IN AND GEOMETRY OF THE DECOLLEMENT ZONE ...... 75

D I S C U S S I O N ...... 77

CONCLUSIONS...... 83

LIST OF R E F E R E N C E S ...... 85

iv LIST OF ILLUSTRATIONS

Figure Page

1. Location, geologic map, and downplunge view of the Santa Catalina-Rincon metamorphic core complex...... 2

2. Rock slabs of two typical varieties of mylonitic gneiss in the Santa Catalina-Rincon metamorphic core complex ...... 7

3. "S-C" foliations in mylonitic gneiss ...... 14

4• Well-developed lineation in augengneiss ...... 16

5. Two stereoplots showing the close correspondence in the folded nature of the core gneiss and the Catalina detachment f a u l t ...... 19

6. Reproduction of Higgins’ (1971) and Sibson’s (1977) classification schemes for fault rocks ...... 22

7. Schematic block diagram and column of structural stratigraphy of the decollement zo n e ...... 25

8. Series of photographs showing the progressive brecciation of gneissic fabrics and mineral grains in the brecciated gneiss subzone of the decollement zone . 26

9. View of the "knife-edge" contact between overlying cataclasite and brecciated gneiss in the decollement zone of the southwest Tanque Verde study a r e a ...... 36

10. Two views of the outcrop morphology of the cataclasite sub zo n e ...... 37

11. Three sets of photomicrographs in plane and polarized light of relict gneiss clasts in cataclasite showing the high degree of recrystallization of these clasts in polarized l i g h t ...... 39

12. Photomicrographs of c a t aclasite...... 42

v vi

LIST OF ILLUSTRATIONS— Continued

Figure Page

13. Close-up view of dark, fine-grained, chloritic cataclasite...... 43

14. Details of complex relationships between different varieties of c a t a c l a s i t e ...... 44

15. Maps of Saguaro National Monument East Study Area .... 46

16. Two views of locally developed small open folds and small dome and basin structures in brecciated gneiss subzone in Saguaro East Study A r e a ...... 50

17. Two views of monoclinal flexures in brecciated gneiss subzone— Saguaro East Study Area ...... 51

18. Macroscopic and microscopic kink folds in brecciated g n e i s s ...... 52

19. Stereoplots of fabric elements in Saguaro East Study Area divided into three domains based on degree of brecciation...... 53

20. Stereographic model to explain rotation of fabric elements in Saguaro East Study Area ...... 59

21. Stereoplot of fabric in brecciated gneiss subzone on the southern flank of Tanque Verde antiform ...... 62

22. Geologic map and cross sections of the extreme southwestern end of Tanque Verde antiform ...... 64

23. Instrusive-appearing relations in the cataclasite subzone...... 67

24. Three views of phenomena known as "streaking" ...... 71

25. Views in hand samples and thin section of faults in cataclasite...... 72

26. Three schematic models for the genesis of the Tanque Verde decollement z o n e ...... 79 ABSTRACT

The exceptionally well-exposed segment of the Santa

Catalina-Rincon decollement zone that bounds the western side of the

antiformal Tanque Verde Mountains affords a rare opportunity to study

the type, distribution and internal geometry of brittle fault rocks developed in a crustal-scale shear zone. Within this gently west­ dipping, curviplanar zone, quartzofeldspathic mylonitic gneiss of the footwall is abruptly transformed by brittle processes of brecciation, comminution, and cataclasis into varying amounts of two types of co­ hesive random-fabric fault rocks: brecciated mylonitic gneiss and sharply overlying cataclasite.

The overall distribution of fault rocks about the antiform is symmetrical but irregular: cataclasite is concentrated on the nose of the antiform; brecciated gneiss is thickest on the flanks; and meta­ morphosed sedimentary rocks occur only in the.keels of the two adjoin­ ing synforms. This distribution strongly suggests that the folded geometry of the "core" exerted a controlling influence on decollement- zone formation— and thus necessarily predates it.

vii INTRODUCTION

The Santa Catalina-Rincon metamorphic core complex located just north and east of Tucson, Arizona (Figure 1) is one of a series of distinctive terranes recently recognized in the Western Cordillera

(Crittenden et al. 1980). The more than twenty five known metamorphic core complexes define a northwest-trending belt from northernmost

Mexico to southernmost Canada (Coney 1980). Each complex is charac­ terized by a core of penetratively lineated mylonitic gneiss that is separated from a cover of brittlely deformed but generally unmeta­ morphosed allochthonous rocks by a sharp, low-angle structural dis­ continuity. This discontinuity is known variously as a dislocation surface, detachment fault, or decollement zone. In this paper decolle- ment zone is used, reserving detachment fault for the sharp upper boundary of the zone because in detail the structural interface between the ductilely deformed core and the brittlely deformed cover comprises a distinctive zone of cataclastic fault rocks.

Fault rocks of the decollement zone of the Santa

Catalina-Rincon metamorphic core complex are exceptionally well-exposed on the western flanks of the Tanque Verde Mountains. The purpose of this paper is to describe in detail the lithology and fabric of these fault rocks as well as their distribution and internal geometry.

These data provide constraints on the physical and kinematic conditons of deformation during the late-stage development of the complex.

1 Figure 1. Location, geologic map, and downplunge view of the Santa Catalina-Rincon metamorphic core complex.

a) Location and geological map of the Santa Catalina-Rincon metamorphic core complex. Note study areas.

b) Downplunge view of the Catalina detachment fault oriented 22 S60W. Note sedimentary rocks beneath detachment fault in keels of synforms. Figure la.Figure CORE COVER .. ges drvd any rm Tertiary from mainly derived gneiss .'.I / I L. - - r'*1 rntc rocks granitic ',*,1r - - /j/ ges drvd any rm Precambrian from mainly .-//■jV/J derived gneiss 1 rntc rocks granitic rcmra granodiorite Precambrian ...... x Ctln dtcmn fault detachment Catalina —x \ Santa 2 6 1 kilometers 10 8 6 4 2 O

2 Co PREVIOUS WORK

The Tanque Verde Mountains, as well as the entire Santa

Catalina-Rincon metamorphic core complex, have been the subject of much study over the years by geologists of the United States Geological

Survey and the University of Arizona. The first comprehensive study is by Moore et al. (1941), who divide the crystalline core of the com­ plex into dominantly granitic and dominantly gneissic components.

This same division appears on the most recent state geologic map

(Wilson et al. 1969) . Another pioneering study is that of Pashley

(1966) who, among other things, noted the close relationship between structure and physiography in the range. Detailed maps of the Rincon

Mountains specifically are provided by Drewes (1974, 1977). He has also published a regional map and synthesis (Drewes 1980, 1981). A somewhat different interpretation of the geology of the complex is presented by Lingrey, Davis, and Coney of the University of Arizona

(Lingrey 1982; Davis 1975, 1977, 1980; Davis and Coney 1979; Davis et al. 1981; and Lingrey and Davis 1982).

Detailed investigations of the crystalline rocks of the core include petrographic studies by Peterson (1968), Sherwonit

(1974) and Banks (1980) and radiometric dating studies by Damon et al.

(1963), Creasey et al. (1977), Shakel et al. (1977), Banks et al.

4 5

(1978), and Banks (1980). These are synthesized in the excellent summary paper by Keith et al. (1980).

Structural studies carried out specifically in the Tanque Verde

Mountains include Leger's (.1967) study of folding and jointing in the gneiss and studies by McColly (1961), Davis (1975) and Davis and Frost

(1976) in the folded and faulted cover rocks.

Especially pertinent to this study were discussions with George

Davis and a very detailed preliminary structure map by him in a wide area of brecciated gneiss within the decollement zone on the northwest flank of the Tanque Verde Mountains. His map appears in Davis et al.

(1981). The area was independently remapped by me as part of this study. GENERAL GEOLOGIC FRAMEWORK

The Tanque Verde Mountains, located in the northern Rincon

Mountains, are one of a series of broad, upright, gently west-southwest- plunging antiforms that characterize the Santa Catalina-Rincon meta­ mo r phi c core complex (see Figure 1). The complex contains the three key elements noted above: a core of mylonitic gneiss, a cover of generally unmetamorphosed, brittlely deformed rocks, and an abrupt decollement zone between the two. A brief description of these im­ portant structural packages follows. For more detailed descriptions of the overall geology of the complex, the reader is referred to the papers by Davis (1975, 1980), and Keith et al. (1980) and the maps by

Drewes (1974, 1977, 1980).

Core

The Tanque Verde Mountains are dominantly made up of a core of mylonitic gneiss. As noted by Moore et al. (1941), and others subsequently, there are two major types of gneiss: a dark, medium to coarse-grained biotite-rich augen gneiss and a leucocratic medium to fine-grained two mica-garnet-bearing equigranular gneiss (see

Figure 2).

The first unit is derived from porphyritic hornblende, biotite-bearing granodioritic to quartz monzonitic plutons the majority of which are 1.44 b.y. old (Shake! et al. 1977, Silver 1978, see Keith

6 Figure 2 (a-j) . Rock slabs of two typical varieties of mylonitic gneiss in the Santa Catalina-Rincon metamorphic core complex. - View is parallel to lineation and perpendicular to folia tion. Scale bar is two centimeters.

a) Porphyritic, biotite-rich augen gneiss. Note the recumbent fold.

b) Medium-grained variety of leucocratic garnet-bearing gneiss. Note two foliations at low angles to one another. 7 Figure 2.— Continued

(c-e). Three views of augen gneiss oriented:

c) parallel to lineation and perpendicular to foliation

d) perpendicular to lineation and perpendicular to foliation

e) perpendicular to foliation. 8 Figure 2.— Continued

(f~h). Three views of fine-grained equigranular gneiss oriented:

f) parallel to lineation and perpendicular to foliation

g) perpendicular to lineation and perpendicular to foliation

h) perpendicular to foliation. 9 Figure 2.— Continued

(i~j)• Photomicrographs of feldspar augen in mylonitic gneiss.

i) View in polarized light showing quartz growing in pressure shadows around clast in the plane of foliation

j) View in place light showing foliation, (here, defined by quartz ribbons), wrapping around feldspar. Note the subrounded nature of the porphyroclasts. Scale: in i, 1cm = .23mm; in j , 1cm = ,24mm. r 11 et al. 1980 for details of dating and correlation). This rock and its gneissic equivalents are mapped as Continental Granodiorite by

Drewes (1974, 1977) and appear as Oracle granite in the Santa Catalina

Mountains.

Petrographically, this gneiss contains from 30 to 40% quartz,

30% orthoclase and minor microcline, 20% plagioclase, 5 to 10% biotite and accessory zircon, sphene and pyrite (compositional data largely taken from Warner 1982, Sherwonit 1974 and Banks 1980). In hand sample, grain size of the matrix, which is mostly quartz, ranges from 3 to 4mm.

Microscopically, the quartz forms ribbons of 100 microns length and

50 microns width with serrated edges and subgrains of 10 to 30 microns.

Biotite commonly has dimensions of 1mm by 2mm with recrystallized trails (50 to 100 microns in grain size) in the plane of foliation.

The subangular to rounded feldspar augen range from .1 to 4cm and compose as much as 50% of the rock. The feldspar grains also display pressure shadows filled with equant recrystallized quartz (10 to 40 microns in grain size) in the plane of foliation (see Figure 2i-j).

The second gneissic unit is derived from fine-grained, equigranular, two-mica, garnet-bearing granite to quartz monzonite plutons of the Wilderness Suite as defined by Keith et al. (1980).

This unit was mapped by Drewes (1977) as Precambrian Wrong Mountain

Quartz Monzonite. A correlative unit has since been dated by Shakel et al. (1977) and Keith et al. (1980) at 44 to 47m.y. old, and all parties are now at least in partial agreement as to its Eocene age. 12

It should be noted that both gneisses give a K/Ar cooling age of

* 25my (Damon et al. 1963; Mauger et al. 1968; Marvin et al. 1978;

Creasey et al. 1977; Banks et al. 1978; Banks 1980; Keith et al. 1980).

Petrographically, this rock consists of about 20% quartz, 35% orthoclase

and microcline, 35% plagioclase, and up to 8% muscovite with minor bio-

tite and garnet. Macroscopically, this generally equigranular gneiss

has a grain size of 1 to 3mm. In thin section, the quartz has a re­

crystallized appearance similiar to that in the augen gneiss with a

grain size of 5 to 50 microns. Feldspar porphyroclasts average 1 to

1.5mm in grain size, whereas micas are from .5 to 1mm in grain size.

The boundary between the two gneisses is typically marked by a relatively thin band of fine-grained grey to black biotite-rich

schist. This rock often contains as much as 50% biotite and a few percent angular porphyroclasts of feldspar and porphyroblasts of muscovite in a matrix of very fine-grained (5 to 50 microns) strained quartz. Small intrafolial tight to isoclinal recumbent folds are common. Drewes (1977) has mapped this unit as Precambrian Pinal Schist.

However, Davis (1978, 1980), noting the strong mylonitic fabric and the consistent structural position, has reinterpreted this unit as a mylonitic schist developed in response to a ductility contrast at the boundary between the two deforming granitic bodies. In other places, the younger granite has intruded the older pluton in such a way as to give their mylonitized equivalents a distinctive lit par lit or 13

"banded gneiss" character. In addition, both gneisses locally contain abundant dikes and sills of unmylonitized, granitic pegmatite and aplite.

All mylonitic rocks of the core contain a well-developed foliation. Where porphyroclasts are abundant— i.e., especially in the augen gneiss, this foliation is seen to consist of two foliations at low angles to one another (Figure 3). The first foliation is defined by ribbons of quartz and by the long axes of platy minerals and is interpreted as a flattening foliation or "S" plane after the designa­ tion of Berthe et al. (1979). The second foliation is defined by dis­ crete shear planes and the bending of dynamically recrystallized por- phyroclastic "tails" into these planes. This foliation is designated the "C" foliation by Berthe et al. after the French word "cisaillement" for "shear." A discussion of the origin and kinematic significance of these foliations is given by White et al. (1980) and Simpson and Schmid

(1983) and is beyond the scope of this paper. Suffice it to say that in the Tanque Verde Mountains, S-C relationships consistently indicate a southwest sense of shear.

Mylonitic foliation throughout the complex is deformed into a series of open, upright, gently west-southwest-plunging folds of which the antiformal culmination of the Tanque Verde Mountains is the best known. Leger (1967) describes this fold as being an "asymmetri­ cal, cylindrical fold with an axis trending S70W and plunging 19 degrees." The axial plane dips steeply to the southeast. In general. Figure 3. "S-C" foliations in mylonitic gneiss.

a) Close-up of Figure 2b, sense of shear is sinistral.

b) Photomicrograph of gneiss showing muscovite "fish." 001 cleavage in the mica defines the "S" or flatten­ ing plane; recrystallized trail of mica between the two porphyroclasts defines the "C" or shear plane. Sense of shear is dextral. Scale: 1cm = .27mm. 14 o 15 foliation attitudes in the core gneiss rarely attain dips of more than

20 or 25 degrees.

Lineation in the mylonitic rocks is pervasive and remarkably consistent in orientation trending S65W-N65E + 5 throughout the Tanque

Verde Mountains. This orientation is approximately colinear with the axis of the antiform. The lineation is defined by the long axes of inequant mineral grains and the rodding of quartz and porphyroclasts giving a "streaked out" appearance (see Figure 4). The mylonitic foliation has been interpreted to represent a flattening fabric with the direction of maximum extension being parallel to the penetrative mineral lineation (Davis 1977, 1980; Davis et al. 1975; and Davis and

Coney 1979).

Two joint sets are well developed in the complex (Pashley 1966;

Peterson 1968; Leger 1967). Dominant trends of these steeply dipping

to vertical joints are N20-40W and N60E for the first set and N5-20E and N80W for the second set. Subhorizontal joints are present as well and are commonly filled with thin quartz veins.

Cover

At low elevations around the southern and western gneissic

flanks of the Tanque Verde Mountains are scattered a variety of

allochthonous rocks ranging in age from Precambrian to Miocene (see

Figure 1). These rocks are at most weakly metamorphosed and are cut

by numerous low and high-angle faults with dominantly normal separa­

tion (Davis and Frost 1976; Davis 1980; Krantz 1983). In addition. Figure 4. Well-developed lineation in augen gneiss. — Note the rodding of quartz and "streaking out" of porphyroclasts.

17 most units display upright to recumbent folds, the majority of which verge to the west away from the core gneisses (McColley 1961; Davis

1975, 1983; Davis and Frost 1976). These rocks are juxtaposed with each other in a variety of stacking orders along numerous rotational

(listric?) faults that coalesce down-dip in the low-angle detachment fault that caps the decollement zone. A thin (usually a few centimeters thick) gouge zone is ubiquitously developed at the base of the cover rocks.

Decollement Zone

In this paper, the decollement zone is defined as that body of rock lying structurally above the non-brittlely deformed mylonitic gneisses of the core and below the detachment fault that lies at the base of the cover. In essence, this zone consists of variable amounts of pervasively altered and cataclastically deformed rocks of the core plus minor amounts of late undeformed pegmatite, aplite and micro- diorite dikes and very minor local occurrences of metamorphosed, silicified and contorted sedimentary rocks. All of these rock types are sharply truncated by the overlying polished and striated detach­ ment fault surface. The lower boundary with the core gneiss is less abrupt taking place within a few meters.

As illustrated in Figure la, the decollement zone of the Santa

Catalina-Rincon metamorphic core complex has a sinuous trace that forms a series of salients and recesses along the southern and western flanks of the complex. The zone is over 80km long stretching from 18

Pusch Ridge on the western edge of the Santa Catalina Mountains to the

southern end of the Rincon Mountains near Vail, Arizona. It is trun­

cated at both ends by high-angle Basin and Range faults.

The entire zone is curviplanar, closely mimicking the gently

west-southwest-plunging folds in the underlying core gneiss (see Figure

5). In detail, however, the zone truncates fabrics in the core. This

is the most dramatically seen in the downplunge view of the complex

(Figure lb) at Posta Quemada Ridge to the southeast of the Tanque Verde

Mountains. The best exposed part of the zone and the one discussed in

this paper is the 30km long segment that bounds the flanks of Tanque

Verde antiform.

The decollement zone was first documented by Davis (1977, 1980)

who described it as consisting of H. • . a distinctive crudely tabular

zone of crushed and granulated but strongly indurated mylonite, myloni-

tic gneiss, microbreccia and chlorite breccia." Pervasive chloritic

alteration and brecciation have converted pristine mylonitic gneisses

into a greenish-brown chaos that has justly been called by George

Davis and his students "junk rock." Alteration consists of both iron

and magnesium-rich varieties of chlorite replacing mafic minerals and

filling open spaces. Hematite and silica are also locally important

(Acker 1957; Davis 1980; Warner 1982; Krantz 1983). Due to brecciation

and alteration, rocks in the decollement zone weather more readily

than the footwall gneiss. Consequently, the zone is generally topo­

graphically low except for the thin layer of very resistant cataclasite Figure' 5. Two stereoplots showing the close correspondence in the folded nature of the core gneiss and the Catalina detachment fault. CATALINA FAULT GNEISS

N

. poles to foliation 86 pts poles to the fault surface * lineation 7 9 pts 36 pts 20 that forms a distinctive ledge (the microbreccia ledge of Davis 1977,

1979, 1980), immediately beneath the detachment fault.

The detachment fault was first mapped in detail by Pashley

(1966), who named it the "Catalina Fault." His data indicate that it is a gently southwest-dipping curviplanar surface with dips generally ranging from 20 to 25 degress but varying from 8 to 53 degrees. Early investigations considered the fault to be a thrust of Laramide age

(e.g., Moore et al. 1941). More recent workers (e.g., Davis 1975,

1980, 1983; Davis and Coney 1979; Krantz 1983), have emphasized the dominantly younger-over-older fault relationships and the down-dip vergence of the cover rocks that are anamolous in classical thrust terranes. These geologists consider the fault to be of Miocene age and to have accomodated down-dip or normal slip— thus the name "detach­ ment." Other geologists consider the fault to be a Laramide thrust with only minor Miocene normal-slip reactivation (Pashley 1966; Drewes

1977, 1981; and Keith 1983). FAULT ROCKS

Terminology

To clarify any possible confusion in the use of general terms,

I will use the following definitions from the AGI Glossary of Geology

(1972): cataclasis "rock deformation accomplished by fracture and ro­ tation of mineral grains or aggregates without chemical reconstitution," brecciation "crushing of a rock into angular fragments," and comminution

"the gradual diminution of a substance to a fine powder or dust by crushing, grinding, or rubbing."

The nomenclature used in this paper to describe the fault rocks of the Tanque Verde decollement zone is taken from two important overview papers on the classification of fault rocks: Higgins (1971) and Sibson (1977). To aid in following the discussion both classifi­ cation schemes are reproduced in Figure 6.

Higgins considers all fault rocks including mylonites as cataclastic rocks and classifies the cohesive varieties according to two primary divisions: the presence or absence of a foliation or fluxion structure and the degree of cataclasis versus neomineralization/ recrystallization.

However, the cataclastic basis of Higgins’ classification no longer seem valid. More recent work on microscopic deformation pro­ cesses (Bell and Etherridge 1973, White 1973) clearly indicate thJt the diagnostic reduction in grain size, (especially of quartz), in

21 Figure 6 . Reproduction of Higgins’ (1971) and Sibson’s (1977) classification schemes for fault rocks. See text for dis­ cussion. 2

T a b l e i : Textural classijicatinn o f fa u lt rocks

HArncr - fab ric F0UATr.D

rM.iT tn c c u 7 I ( v is ib le (recsee'S » X): el rxfc eeee > FMJLT COUCZ V (visible (rafeaets «W e( reek m#es)

rmmoTACKTLm 7 11 5 Z

C K S * UICCU (Iretw vts * 0.1 cel

r o n civs* uzccu (0 . Ic e « ( r s is . « O .V e )

c m s * MicvoutzmA (frseeeBce « 0.1 c»> [ [ 10! • II

norocAT aqju in rtOTtXTUMITl H i i i i g s 1 ! 7 CAT AH A im •rv o e n t . ; 1 a I E s 5 I rLnucATACvsm uvrtiwrrv*.m L i

7 ■ A s re m o e m

i i (from Slbson 1977)

Table 1.— Ctiisi/lcvfiOH of cataclaitic rock* ire rp ti rovlastlc. profoclnslle. dlnphlborltlc. pevudotncbrlltlf, poIjmMamerphlr. pol)carecli»tlc. phjllluc. and o(b»r t»rm« ar» u«wl am modlflf n. 8« text and gloaear) |

R o c k s w it h o u t R o c k s w it h prim ary cohesion prim arv cohesion Catarlasts dominant o>er Neominerahiation- neomineraliLation-recrysullizatFon recrv«talliiation dominant oxer cataclasis

Rock* » rith *m t Rocks » ilh Rocks I' Uh fluxion structure fluxion structure fluxion structure

laiNe t f Protomvlonite Mylunitv gneiss naked e\e 11 Fault breccia Microbreccia 1 my Ionite schist I l! Mylomtr II N I f "0 Z-m m i : Fault rouge r atacla«ite £ 7 | Blastnmi Ionite I Is Vltramylonitf Q . ___ Mi All rnrkm are rradational (from HigglnS 1971) 23 myIonites is due to dynamic recovery and recrystallization of highly i strained grains— and is not due to cataclastic processes.

Recognizing this, Sibson's classification dispenses with the division based on cataclasis versus neomineralization/recrystallization and emphasizes the presence or absence of a foliation which he considers to be of great mechanical significance. Sibson divides rocks into those with a foliation— the mylonite series, and' those with random fabric— the cataclasite series. According to him, the first set of fault rocks develop in a quasi-plastic regime and the second in a elastico-frictional regime. The boundary between the two regimes is taken to be the point at which extensive crystallographic fabrics develop in quartz. This point corresponds to conditions at the lower greenschist facies of metamorphism or depths of roughly 10 to 15km.

My terminology is mainly taken from Sibson's classification for cohesive random fabric fault rocks and includes microbreccia and the cataclasites. As a result, much if not all of what I call cata- clasites (fine-grained breccias with greater than 10% matrix) has been called microbreccias by previous authors (Davis 1980 for example) following Higgins' classification. The rock referred to as "chlorite breccia" by Davis I will refer to as "brecciated mylonitic gneiss" following Higgins' suggestion for combining terms when overprinting has occurred. This unit also includes minor microbreccia and proto- clasite. 24

Structural Stratigraphy

The thickness of the zone varies from a few tens to a few

hundreds of meters. A general structural stratigraphy can be applied

to the decollement zone and is illustrated in Figure 7. The stacking

consists, from structurally lowest to highest, of: mylonitic gneiss of

the core, lower decollement zone boundary, brecciated mylonitic gneiss,

cataclasite, detachment fault (= upper decollement zone boundary), and

finally the cover rocks with their thin basal gouge zone.

Brecciated Mylonitic Gneiss Subzone

In an abrupt and locally dramatic transition over the space of

a few meters or less, pristine mylonitic gneiss of the core is progres­

sively transformed by brittle processes of brecciation, cataclasis, and

comminution into mildly to strongly-brecciated to micro-brecciated gneiss. This progressive transition from mylonitic gneiss to micro­ breccia is illustrated in the sequence of photographs in Figure 8 . In this process of progressive brecciation, grain size is reduced, fabric is obliterated, and the degree of alteration increases.

In contrast to mylonitic rocks in the core, deformation within this subzone is characterized by the brittle behavior of quartz as well as feldspar. The protolith gneiss is fractured into angular to rounded fragments of gneiss, quartz and feldspar that range in size from 3cm down to less than .1mm in the matrix material. In thin section, the ^ -U N E A T IO N

'b'SURFACE •VSURFACE

— COVER ROCKS

FAULT GOUGE ^ D E T A C H M E N T FAULT X^CATACLASITE /X metasedimentary V ROCKS BRECCIATED GNEISSr

CORE GNEISS

to L n Figure 8 (a-p). Series of photographs showing the progressive brecciation of gneissic fabrics and mineral grains in the brecciated gneiss subzone of the decoilement zone. Scale bar is two centimers.

a) Close-up of mildly brecciated gneiss with only a few joints and well-preserved foliation and line- ation.

b) Close-up of extremely brecciated gneiss with no preserved foliation and lineation. 26 Figure 8.— Continued

(c-d). c) Close-up view showing faulted and disrupted nature of relict foliation in extremely brecciated gneiss. Lens cap bottom center for scale.

d) Close-up of nearly obliterated relict lineation in extremely brecciated gneiss. 27 Figure 8.— Continued

e) Spectacular example of chaotic nature of extremely brecciated gneiss. Note the rotation of fabric within fault-bounded blocks. 28 Figure 8.— Continued

(f-g). Two views of moderately brecciated gneiss oriented:

f) parallel to lineation and perpendicular to foliation

g) perpendicular to lineation and perpendicular to foliation. 29 Figure 8.— Continued

(h-j). Three views of strongly brecciated gneiss oriented:

h) parallel to lineation and perpendicular to foliation

i) perpendicular to foliation and perpendicular to lineation

j) perpendicular to foliation. 30 Figure 8.— Continued

(k-1). Two views of microbreccia.

k) Coarser-grained variety

l) Finer-grained variety including protocataclasite. 31 Figure 8.— Continued

(m-n). Photomicrographs in polarized light showing cracked, broken, and even kinked feldspar.

m) Feldspar in brecciated gneiss. Note cracks crosscutting quartz grains in upper left of m. Scale: in m, 1cm = 215 microns, in n, 1 cm = 270 microns,

Figure 8.— Continued

(o-p). Photomicrographs of feldspar (polarized light).

o) With an apparent stylolite in the plane of foliation between two grains (scale: 1cm = 190 microns) ,

p) Feldspar invaded by recrystallizing quartz (scale: 1cm = 205 microns).

34

quartz displays undulose extinction and numerous cracks and fractures.

Feldspars too are pervasively cracked, broken and even kinked (see

Figure 8 m-p).

As the degree of cataclasis increases, the proportion of matrix material (mainly fine-grained recrystallized quartz, feldspar and chlorite) increases from a few percent to nearly half the rock thus

spanning a range of terms from "crush breccia" to "protocataclasite" in

Sibson’s terminology (refer to Figure 6 ). The numerous fractures are cemented with a matrix of fine-grained (.5 to 5mm) recrystallized quartz and feldspar, chlorite, and locally hematite giving the rock its indurated character.

In outcrop and on fresh surfaces, rocks of this subzone are usually tan, brown, or brownish-green depending on the original color and on the degree of alteration. Alteration is dominantly chloritic, with variable but generally minor amounts of epidote, hematite, magne­ tite, pyrite and manganese oxides. Alteration of feldspars to clay is also common. Chlorite is present both as a replacement of biotite and as an open-space-filling (Figure 10). Warner (1982) studied the chloritic alteration in the decollement zone in the eastern Rincon

Mountains and found that biotite and potassium feldspar are replaced by quartz, chlorite and sericite with the attendant release of iron;

This type of reaction may account for local concentrations of hematite, magnetite and pyrite. As noted previously, rocks of this zone are generally less resistant than either the underlying mylonitic gneiss or the overlying cataclasite and consequently usually occupy relative topographic lows. 35

Cataclasite Subzone

A resistant ledge of cataclasite is developed in abrupt knife-edge contact (see Figure 9) above the brecciated gneiss and just below the actual detachment fault. The cataclasite truncates mylonitic fabric in the underlying unit but on a map scale is concordant to the gently dipping attitudes of the footwall gneiss and overlying detach­ ment fault. This subzone varies in thickness from a few centimeters up

to thirty meters in exceptional cases and averages 20 to 30cm.

This very fine-grained, random-fabric fault rock represents a

further degree of cataclasis from the underlying brecciated gneiss. The

cataclasite is separated from the brecciated gneiss subzone on the basis

of its distinctive igneous appearance in outcrop and its extremely sharp

contact relationships (Figure 14). This rock may be greenish-white,

tan, plum, dark green, or black depending on the dominant alteration

minerals and grain size.

There are, broadly speaking, two categories of cataclasite:

a generally light-colored, coarser-grained variety and a finer-grained,

dark green or black variety. The light-colored variety, which locally

includes protocataclasite, consists of 20 to 50 percent angular to

rounded clasts (from .1 to 15mm and averageing 1 to 5mm in diameter)

floating in a very fine-grained (1 to 5 microns) matrix. Optically,

the mineralogy of the matrix is unidentifiable but X-ray diffraction

reveals that it consists of recrystallized quartz, feldspar (both

plagioclase and potassium feldspars), chlorite and sericite. This Figure 9. View of the "knife-edge" contact between overlying cataclasite and brecciated gneiss in the decollement zone of the southwest Tanque Verde study area. 36 Figure 10 (a-b). Two views of the outcrop morphology of the cataclasite subzone.

a) View looking northwest in the Saguaro East study area at resistant ledge of cataclasite (former microbreccia ledge of Davis— see text) immediately underlying the Catalina detachment fault.

b) View, looking north, of the "mullion-like" morphology of cataclasite at the tip of Tanque Verde antiform.

38 matrix composition is very similar to that of cataclasite from the

Whipple Mountains of California (Phillips 1982).

Clasts in both varieties of cataclasite consist of fragments of gneiss, quartz, feldspar, and older cataclasite— the last indicating more than one episode of cataclasite formation. As in the brecciated gneiss, visible grains of quartz are strained and feldspars are cracked and all clasts display fracturing, faulting and recrystallization

(Figure 11).

The darker finer-grained variety of cataclasite (Figure 13) consists of 70 to 95 percent matrix and locally includes ultracatacla- site. As in the lighter-colored variety, clasts are angular to rounded and range in size from 1 to 3mm. The matrix differs from that of the coarser-grained cataclasite in that it contains no potassium feldspar and greater amounts of chlorite.

As in the brecciated gneiss, the dominant alteration mineral in both types of cataclasite is chlorite. The chlorite is very fine­ grained (5 to 10 microns) and is disseminated throughout the matrix.

A few percent magnetite, sometimes euhedral and of about 1mm in size, is ubiquitous. Numerous small veins of quartz, chlorite, hematite and locally calcite are also present. Rare thin brown (clay-filled?) styo- lites are present in thin section (Figure 12).

Internal Geometry

The internal geometry of the two subzones was examined in detail in two study areas that contain excellent exposures of the Figure 11 (a-f). Three sets of photomicrographs in plane and polarized light of relict gneiss clasts in cata- clasite showing the high degree of recrystallization of these clasts in polarized light. Note the highly fractured nature of the clasts and the variable size of the matrix. Scale: in a and b, 1cm = .14mm; in c and d , 1cm = 30 microns; and in e and f , 1cm = 245 microns. 39

d 40 41

l Figure 12 (a-b). Photomicrographs of cataclasite.

a) Stylolite in very fine-grained cataclasite (plane light, scale: 1cm = 222 microns).

b) Hematite-filled vein in coarse-grained cataclasite (plane light, scale: 1cm = 295 microns). 42 Figure 13. Close-up view of dark, fine-grained, chloritic cataclasite. Vs. Figure 14 (a-b). Details of complex relationships between different varieties of cataclasite.

a) Sealed fractures and faults juxtapose white, black, and green varieties of cataclasite.

b) Detail of abrupt but diffuse contact between white and black cataclasites.

45

brecciated gneiss and the cataclasite respectively. Maps and

detailed descriptions of each of these structurally complex areas are

presented below.

Brecciated Gneiss— Saguaro National Monument East Study Area

Description. Within the brecciated gneiss subzone on the northwest flank of the Tanque Verde Mountains, relict orientations of

foliation strike dominantly east-west in marked contrast to the

typically north-northeast strikes in the footwall gneiss. This 500

to 1000 meter wide band of anomalously oriented gneiss extends for roughly 10km from the head of Tanque Verde Valley to the nose of

Tanque Verde antiform. A small well-exposed part of this exceptionally wide band of brecciated gneiss in Saguaro National Monument East was mapped at a scale of 1:1200 (Figure 15). As noted above the same area was mapped in preliminary fashion by Davis and his map appears in

Davis, et al. 1981.

In this area, the lower boundary of the subzone is very abrupt. Often within less than a meter, one goes from relatively pristine mylonitic gneiss to extremely brecciated and altered gneiss.

Consequently, the lower boundary, here, probably represents a distinct fault and is so shown on the maps in Figure 15. As evidenced by its outcrop pattern, this fault, like the detachment fault above it, dips gently to the southwest. The upper boundary of the subzone consists of a 30cm thick ledge of cataclasite and overlying, allochthonous

Paleozoic strata of the cover. Figure 15 (a-c)• Maps of Saguaro National Monument East Study Area.

a) Structural geologic map of the study area.

b) Derivative map showing domain boundaries of consistent fabric orientation along with small folds and possible faults.

c) Derivative contour map based on four arbitrary and gradational degrees of brecciation. See text for discussion. 46

M ^-'y ifCx^gl WY'/MU

V. ^ ^rr'-Axr

T VT">

\ b ?:> "v2 a • f-S1 J” iVr

Lx6

-V 1 8 8 LEGEND

V STBIKE AND DIP O ' F O L IA tlO N P TBtNO a no plunge of lin c a tio n

X STWPE AND DIP OF BEOOINO

« / FAULT COVER

=Z=j LIMESTONE DECOLLEMENT ZONE

CATACLASITE

^ BBECCIATEO M TLONITIC CNE SS CORE

^ MTLONITIC GNEISS

Figure 15a. 47

J

I

m

LEGEND

■flf *IWK FOLDS

OUTCROP TREND OP *' EXTREMELr BRECCip t e d O NtlSS

s t r ik e and o i» o r rAuLT 7 t r e n d and p lu n g e or s t r ia e

X STei" t 4R0 Dip or rotiATiON T T**»0 * no plu n g e o r l in e a t .o n

r *uLr

OOUAIN BOUNDARY

SMALL rAULTS

Sm a l l rOLDS

Figure 15b. Figure 15c. 49

Structures present both in this subzone and in the footwall include locally developed small, gentle, upright folds with wavelengths of from 1 to 3 meters and amplitudes of less than 1 meter. These gentle undulations trend both northwest and west-southwest and locally interfere to form small dome and basin structures (see Figure 16).

Other monoclinal warps as well as macroscopic and microscopic kink folds locally occur in the more brecciated rocks (Figures 17 and 18).

Numerous small faults cut the zone and have a wide variety of orienta­ tions but are generally parallel to the dominant joint attitudes noted in the section on general geologic framework.

These minor structures produce some variability in fabric orientations in the subzone but cannot alone account for the wide and systematic rotations of attitude seen in Figures 15a, 15b and 19.

Figure 15a is a structural geologic map of the Saguaro East Study

Area which shows the jumbled and apparently random distribution of fabric orientations there. Figure 15b is a derivative map showing domains of relatively consistent attitudes of both foliation and line- ation. All domains contain at least one mapped anomalous attitude.

However, they are generally accurate and further subdivision would render the figure useless.

As shown in Figure 15 most foliation orientations fall into three broad categories: 1) striking roughly east-west and dipping north, 2) striking roughly east-west and dipping south, and 3) striking north-northeast and dipping west. Figure 16. Two views of locally developed small open folds and small dome and basin structures in brecciated gneiss subzone in Saguaro East Study Area. 50 Figure 17. Two views of monoclinal flexures in brecciated gneiss subzone— Saguaro East Study Area.

Figure 18. Macroscopic (a), and microscopic (b), kink folds in brecciated gneiss. 52 Figure 19. Steroplots of fabric elements in Saguaro East Study Area divided into three domains based on degree of brecciation— see corresponding outcrop photographs. See text for discussion. Figure 19b) Outcrop appearance of three degrees of brecciated gneiss. 53

FOLIATION LINEATION

2 2 3 PTS 111 PTS

196 PTS 196 PTS,

4 5 PTS 4 7 PTS. SAGUARO NATIONAL MONUMENT - EAST Figure 19a. 54

Figure 19b. 55

Orientations of lineation are more variable, but where preserved, are

generally more northerly or southerly in trend than those of the foot-

wall gneiss. It is important to note that attitudes of both foliation

and lineation in the study area are commonly quite steep— especially in

the most brecciated rocks. This relationship suggests that boundaries

between these domains are faults. In other cases, particularly where

dips are gentle, domain boundaries most likely represent roll overs in

foliation due to warping.

The degree of brecciation or cataclasis in the study area is

variable and does not show a linear increase towards the detachment

fault as might be expected. Figure 15c is a contour map of the de­

gree of brecciation and is based on four arbitrary divisions of the

gradation from the unbrecciated core gneiss to extremely brecciated

gneiss. These divisions are based on qualitative outcrop appearance,

including the density of fractures, the degree of alteration, and the

relative preservation of fabric. Accordingly, "unbrecciated gneiss"

is basically the equivalent of the footwall core gneiss and has an

appearance like that of the rock in Figure 2 with just a few joints

and no alteration. The "mildly brecciated gneiss" has an average of about 25 large fractures per linear meter, is weakly altered and re­

tains a clearly recognizable foliation. "Strongly brecciated geniss"

contains on the order of forty or more fractures per linear meter with significant alteration and rapidly deteriorating fabric expression.

"Extremely brecciated gneiss" is riddled with hundreds of large and 56

small fractures on the scale of a hand specimen that usually completely

obliterate the lineation and seriously mask the foliation. Alteration

in this rock is at its zenith with pervasive chlorite, minor epidote,

and iron and manganese oxides. In Figure 15c, the last division, in

the interest of completeness, includes the cataclasite subzone where

it occurs immediately beneath the detachment fault (see Figure 15a).

As illustrated in Figure 15c, a narrow band of extremely

brecciated gneiss marks the lower bounding fault between the footwall

gneiss and the decollement zone. From this boundary to the overlying

cataclasite, the width of the zone varies from 100 to 500 meters and

the thickness from 50 to 200 meters.

Within this area, the distribution of different degrees of brecciated gneiss is complex. A body of less deformed gneiss is pre­ served on a topographic high in the widest part of the subzone. On

the basis of the intersection of the outcrop pattern with topography, this mass of relatively undeformed gneiss has the general form of a slightly curviplanar, gently west-dipping concordant layer within more brecciated gneiss. The presence of this "sandwich-like" relationship, like the presence of the decollement zone itself, suggests the existence of anastomizing faults within and perhaps bounding the decollement zone.

Figure 19 contains a series of stereoplots of poles to foliation and lineation divided, here, into only three domains based on the degree of brecciation. Corresponding photographs, illustrating the outcrop appearance of the different categories of brecciated gneiss, are included in Figure 19. Attitudes of relict foliation and 57 lineation show a non-random progressive scattering as brecciation increases. Foliation orientations show a progressive scattering from a tight west-northwest-dipping cluster in the footwall to a wide zone of orientations that approximate a rough pi girdle distribution with a gently west-northwest-plunging pi axis. Lineation orientations scatter from a gently west-southwest-plunging cluster to low-plunging orientations throughout the northeast and southwest quadrants. Please note, as illustrated in Figure 19, that not all areas of the gneiss, regardless of the degree of brecciation, show the same amount of, or indeed any, rotation. This relationship indicates that differential rotation was accommodated on a variety of fracture orientations.

Kinematic Model. To explain the systematic rotations of fabric elements in Saguaro East, a basic assumption is made that the gneiss in this subzone was derived from the immediately underlying core rocks along a S60W line of transport. This assumption is sup­ ported by the work of many authors such as Davis (1980, 1981), Drewes

(1977, 1980) and Lingrey and Davis (1982). The evidence consists of:

1) fold and fault slip lines, 2) the interpretation of the mylonitic lineation as representing a "stretching" or extension direction, and

3) suggested palinspastic reconstructions (Reynolds 1981 pers. comm., and Krantz 1983) that would restore allochthonous upper plate (cover) units such as the Cretaceous Cat Mountain Rhyolite and the Precambrian

Rincon Valley Granodiorite to source areas roughly 40km to the east- northeast. 58

The simplest and most satisfying model to explain the observed rotations in the study area is presented in Figure 20. A key constraint in the derivation of any model for this area is the orientation of lineation throughout the zone. It is important to note that the gentle to moderate plunges of lineation in Figure 19 may be due to its seldom being preserved in the generally steeply-dipping extremely brecciated gneiss. Despite this bias, preserved lineation orientations still seem to require rotations, of approximately 50 degrees, about a vertical axis— mainly in a counter-clockwise sense. This type of rotation is illustrated in the model in Figure 20. Some of the resulting orienta­ tions are then modified by small clockwise (or down to the southwest) rotations about a northwest horizontal axis— perpendicular to the inferred transport direction. For such a model to be viable, it must also explain rotations of foliation attitudes and significantly, this model is able to do so for the majority of data points. Additional scatter can be accounted for by complex rotations on numerous fractures in steeply-dipping fault zones such as those that form some east- ■ northeast-trending domain boundaries.

Geologically, rotations about a vertical axis prove problematic but possible if one can assume that the subzone of brecciated gneiss, and/or domains within it, were free to pivot about a vertical axis between the two gently-dipping bounding faults. This concept can be extended to include gently to moderately-dipping domain-bounding faults as well. Otherwise, one is forced to invoke a Figure 20. Stereographic model to explain rotation of fabric elements in Saguaro East study area (see Figure 19 and text for discus­ sion) . — Shaded area represents new position of fabric after the rotation in that frame is accomplished. Note (Figure 19) that not all are rotated the amount shown.

Figure 20b. 61 series of rotations that become so numerous, complex, and essentially random, that they are impossible to model in any meaningful way. I would envision then a zone or zones of sinistral shear along the north­ west flank of Tanque Verde antiform that accomodate horizontal clock­ wise rotations of varying magnitude and even local dextral shear

(Figure 20b). The gneiss now dragged into roughly east-west orienta­ tions is subsequently— or concurrently— subjected to additional breccia- tion and vertical rotations consistent with its position in the developing decollement zone.

The dominant east-northeast trend of domain boundaries is parallel to the lineation direction in the core and the inferred trans­ port direction of the complex. Faults may have developed in this dominant orientation because it represents a pre-existing direction of weakness— i.e., a well-developed joint set that was exploited either during differential shear strain and/or during extension (in the sense of a "B" joint) during arching of Tanque Verde antiform.

Interestingly, the brecciated gneiss subzone on the south

flank of Tanque Verde antiform, mapped only in reconnaissance (1:

24,000), does not appear to possess the same complexities as the northwest flank. There, a much thinner zone of brecciated gneiss displays much less severe disruptions in orientation. Attitudes are

easily explained geometrically, kinematically, and geologically by

rotations to the southwest about a horizontal N30W axis with a com­ ponent of tilting to the south due to late-stage arching of the

antiform (see Figure 21). This difference in size and geometrical Figure 21. Stereoplot of fabric in brecciated gneiss subzone on the southern flank of Tanque Verde antiform. 62

A - L 1 9pts. • = S 1 38pts. 63 complexity of the brecciated gneiss subzone strongly suggests the presence of a significant zone of brittle sinistral shear along the northwest flank of the Tanque Verde Mountains.

Cataclasite— Southwestern Tanque Verde Study Area

On the extreme southwestern tip of Tanque Verde antiform, just east of where it plunges under late-Tertiary cover, is a remarkable exposure of exceptionally thick cataclasite. Here, as much as 25 meters of this strikingly igneous-appearing rock overlies only a few meters of brecciated gneiss. The outcrop appearance of this cataclasite is also notable for its distinctive mullion-like morphology and its locally developed, brownish-red-weathering, knobs adorned with ancient petroglyphs.

To better understand the origin and internal structure of this intriguing area, I prepared a detailed (1:1200) map and cross sections (Figure 22) of the study area and made a preliminary petro­ graphic examination of the cataclasites. Mapping revealed that there are at least four apparently successive generations of cataclasite that form apparently tabular bodies that locally overlie one another

(see cross sections in Figure 22). These cataclasite layers are generally concordant to the arched morphology of the underlying core gneiss, but locally truncate it (see especially cross section B-B1).

Cross sectional reconstructions indicate that the lowest tan cataclasite ranges in thickness from 5 to 25 meters and averages Figure 22. Geologic map and cross sections of the extreme southwestern end of Tanque Verde antiform GEOLOGIC MAP AND CROSS SECTIONS OF SOUTHWESTERN TANQUE VERDE ANTIFORM

LEGEND

V STRIKE AND DIP o r rO U A T ION 7 * TREND ANO PtUNOE OE UNEATION

^ STRIKE ANO DIRECTION Of DIP O f STEEP JOINTS

\ TREND O f STREAKS O f WHITE CATACLASITE

. y / " QEOIOOIC CONTACT

n OAEEN m ■LACK CATACLASITE □ MIXED □

BAECCIATEO M riO NiTlC ONEISS

ED M v io w m c GNEISS

co*TOuwm«e*v«i »•»«*» 65 about 10 meters. Structurally above this, the mixed white and black

or green cataclasite is roughly 8 meters thick. Above this a very

thin 5 meter layer of black cataclasite is preserved. Finally, the

structurally highest dark green cataclasite has a preserved thickness

of 10 meters.

While the four types of cataclasite distinguished in Figure 22

are distinctive in outcrop, they are not so easy to discriminate in

thin section. This is especially true of the green and tan varieties.

Both have very fine-grained (5 to 15 micron) matrices of recrystallized

quartz and feldspar with angular to rounded (30 micron to 2mm in grain

size) porphyroclasts of strained quartz and feldspar. The tan cata­

clasite locally contains protocataclasite (i.e., slightly more clasts

and coarser grain size), but the fundamental difference between the

two varieties of cataclasite is that of color which is due to greater

amounts of chlorite in the matrix of the latter. X-ray studies indi­

cate that the rocks possess a chemical difference in that the tan

cataclasite contains microcline as well as plagioclase.

In outcrop, the white cataclasite consists of 2mm blebs of

greenish-white milky quartz. In thin section, the rock is seen to

have a very fine-grained (5 micron) matrix of recrystallized quartz

and chlorite with 30 to 40% of 1 to 2mm in diameter clasts of strained

quartz and feldspar.

The black cataclasite probably owes its color to its extremely

fine grain size. The matrix of .5 to 5 micron grain size comprises

80 to 95% of the rock and thus locally is an ultracataclasite. The 66 black color, aphanitic luster, conchoidal fracture, and chert-like appearance of this rock suggest that it may contain pseudotachylite— a fault rock that has melted due to frictional fusion. However, no glass, spherulites, or microlites such as have been reported in other occurrences of pseudotachylite (for example Philpotts 1964; Allen 1979;

Sibson 1980; and Passchier 1982), have been found here. Nonetheless, intrusive-appearing contacts do occur among all the cataclasites and even into the underlying brecciated gneiss (see Figure 23). These sharp, interfingering, and cross-cutting relationships persist to the microscopic scale and seem to require that the rock, while perhaps not molten, was in a state of fluidized flow. A possible mechanism for this behavior might resemble that for the fluidized flow of sediments, only in this case we have finely pulverized rock in a dynamically re­ crystallizing matrix. Such finely pulverized and injected rocks, without glass, have been noted by Philpotts (1964) and Wenk (1978), but even those rocks are isotropic in thin section. The possibility remains that the cataclasites on the tip of Tanque Verde antiform once contained glass which is no longer identifiable.

Minor structures that occur in the cataclasites include numerous joints and faults. The previously noted joint sets of the

complex are especially well-developed here. Three orientations

control the morphology of the outcrops. They are a steep N60E-

striking set, a steep N30-50W-striking set and a subhorizontal set.

The first set may control the distinctive "mullion" shape of many

outcrops (see Figure 10). The subhorizontal and especially the Figure 23 (a-f). Instrusive-appearing relations in the cataclasite subzone.

a) Bleb of black cataclasite cross-cutting gneissic foliation. Pencil points to small vein of this cataclasite that may represent a generation sur­ face in the fashion of pseudotachylite.

b) Complex faulted and interfingering contact relations between different cataclasites. 67 Figure 23— Continued

(c-d). Very intrusive-like, irregular and interfingering contacts between cataclasites. 68

c

d

■ Figure 23— Continued

(e-f). Microscopic view in plane, (e), and polarized, (f), light of one of these interfingering contacts. Note very fine grain size of the dark cataclasite. Scale: 1cm = .4mm. 69 70 northwest-striking set may control faulting. In particular, numerous small, sealed, dominantly northwest-striking faults sharply juxtapose different varieties of cataclasite in outcrop and thin section (as illustrated in Figure 25) and probably are responsible for the phenomena referred to below as "streaking."

The Tanque Verde study area displays an enigmatic, macroscopic fabric element which I informally describe as "streaking." Streaking

is pervasive in the cataclasite, especially the tan and green varieties

and has also been noted locally in the underlying brecciated gneiss.

The "streaks" consist of pale greenish-white milky quartz or white

cataclasite that averages 1cm in width and 25 to 40cm in length (see

Figure 24). They are regularly spaced about 25cm apart and maintain

a consistent N40W+10 trend throughout the map area. Their third

dimension is planar to curviplanar and of variable dip where visible.

A consistent planar attitude for the streaks (and other types of

layering— notably color banding), while suggested, was not confirmed.

As to what causes the uniform northwest trend of the streaks, it is

postulated that the streaks represent a repeatedly offset layer of

white cataclasite along a series of closely-spaced, northwest-striking,

presumably normal faults. Or, they may represent some type of bleach-

ing or alteration along such faults. Figure 24 (a-c). Three views of phenomena known as "streaking." — Note that streaks are white, very regular in trend of longest dimension and often fault-bounded.

c) View looking southeast at one of the faults. 71 Figure 25 (a-g). Views in hand sample and thin section of faults in cataclasite. — Scale bar is two centimeters.

a-c) Views in hand sample, note offset banding in b. 72 Figure 25.— Continued

(d-e). d) Microfaults in dark cataclasite continuing into underlying protocataclasite. — Again note the layering. (Specimen is approximately 5cms across).

e) Thin section view, (plane light), of faults in d. Scale: 1cm = .2mm. 73 Figure 25.— Continued

(f-g). Faults in thin section.

f) Reverse microfault in plane light. Scale: 1cm = 125 microns.

g) Triple junction of three types of cataclasite, (polarized light). Scale: 1cm = 390 microns. Note the extremely sharp contacts.

DISTRIBUTION OF FAULT ROCKS IN AND GEOMETRY OF THE DECOLLEMENT ZONE

From the above descriptions, it should be clear that the distribution of various fault rock types is irregular within the Tanque

Verde decollement zone. Cataclasite is thickest on the culmination of

Tanque Verde antiform and brecciated gneiss is thickest on the flanks

of the antiform. Finally, metasedimentary rocks are locally developed

in the keels of the two adjoining synforms.

These metasedimentary rocks consist of metamorphosed,

contorted, and highly silicified Paleozoic strata. In Tanque Verde

Valley on the north side of the Tanque Verde Mountains, a thin (100 by

600 meter) slice of probably Mississippian Escabrosa Formation is

located well to the east of (1 to 2km— see Figure 1), and therefore

below, the detachment fault. This sliver, consisting of foliated

limestone and sparse sandstone, is situated to the east of a large

hill of shattered granite and gneiss that is also contained, albeit

in an unusual position, within the decollement zone. On the other

side of the antiform in the head of Rincon Valley, a much larger area,

about 600 by 2000 meters, is covered with a highly contorted siliceous

rock of doubtful affinity. Remnants of Cambrian Bolsa Quartzite are

locally distinguishable and it is thought that this lithology may be

the source of much of the silica. A thin (10 to 15 meter thick) sub­

zone of brecciated gneiss underlies both these slivers, but very little

75 76

to no cataclasite was observed directly above the metasedimentary

rocks in the keel of either synform. It should be noted that abundant

rocks of this type are also present in Happy Valley, beneath a detach­ ment fault, on the eastern side of the Rincon Mountains (Lingrey 1982) .

Thus, not only is the distribution of fault rocks within the decollement zone irregular about Tanque Verde antiform; it is also symmetrical. This distribution strongly suggests that the folded geom­ etry of the core exerted a controlling influence on the decollement

zone deformation. This relationship is best seen in the downplunge view in Figure lb.

In addition, the entire decollement zone is probably thickest in the keels of the synforms and thinnest on the culmination of the antiform. This observation is taken from map relationships which indicate a thickness for the zone of from 400 to 750 meters in Tanque

Verde Valley, about 750 meters in Rincon Valley, a maximum of 400 meters in Saguaro East, approximately 10 to 20 meters on the southern flank, and a preserved thickness of only 35 meters on the culmination of the antiform. These figures are calculated from map widths

(Pashley 1966; Drewes 1977; and my Figures 13 and 20) assuming a dip of 22 degrees for the decollement zone. Unfortunately, it is not possible from surface geology to determine the true thickness of the decollement zone on the tip of Tanque Verde antiform. DISCUSSION

The Tanque Verde decollement zone juxtaposes rocks that have

undergone deformation under radically different conditions. Sherwonit

(1974) found mylonitic gneisses in the adjacent Santa Catalina Mountains

to be of amphibolite grade and suggested that they formed at tempera­

tures of 650 to 730 degrees centigrade and pressures of 4 to 6 kilobars.

In contrast, cover rocks display only slight, locally developed folia­

tion, primarily in carbonate units, and were deformed under dominantly

brittle conditions at no more than 2 or 3 kilometers depth. Trans­

formation from mylonitic gneiss to cataclasite within the decollement

zone indicates a change from ductile or crystal plastic deformation

processes to brittle or cataclastic ones. Such a change indicates a

decrease in temperature and pressure. Warner (1982) found the chloriti-

cally altered brecciated mylonitic gneisses of the eastern Rincon

Mountains reflect the albite-epidote subfacies of greenschist meta­ morphism, or temperatures of 300 to 400 degrees centigrade and

pressures of 1 to 2 kilobars. Sibson (1977) concluded that mylonites

form at depths of 10 to 15 milometers (his quasi-plastic regime), and

cataclasitic rocks form between 4 and 10 kilometers (his elastico-

frictional regime).

Thus, the entire Santa Catalina-Rincon metamorphic core

complex shallowed and cooled significantly between the time of forma­

tion of the mylonitic gneisses in Eocene/Oligocene? time and the

77 78

development and breaching of the fault by mid-Miocene time. The

chronology of events is controlled by the fact that Eocene Wilderness

Suite rocks (and even 25my old plutons in the nearby Tortillita Moun­

tains— see Keith et al. 1980), are mylonitized; Miocene sediments are

cut by the detachment fault ( Pashley 1966; Drewes 1977; Davis 1980), and clasts of chloritic brecciated gneiss are found in the post-20my old Rillito Formation (S. Reynolds 1982, pers. comm.).

In addition, the distinctive suite of fault rocks exposed within the decollement zone of the Tanque Verde Mountains displays a complex but geologically interpretable geometry. As has been noted, the distribution of fault rocks about Tanque Verde antiform is irregu­ lar but symmetrical, strongly suggesting that the folded core rocks exerted significant control on the development of the decollement zone.

Extreme cataclastic deformation was concentrated on the culmination of

Tanque Verde antiform while in the keels of the synforms, on its structurally lowest points, the narrow zone of brittle deformation cut up section trapping Paleozoic strata beneath it.

This geometry can be explained most easily by envisioning a folded gneissic basement with its overlying cover rocks truncated by a "flatter," or less curved fault. Two possible models for this scenario are presented in Figure 26. In the first model, the decolle­ ment zone is considered to have formed as a nearly flat front or boundary, perhaps an abrupt thermal boundary, of brittle deformation that truncates an already folded gneissic core. As time progresses, Figure 26. Three schematic models for the genesis of the Tanque Verde decollement zone. — Time progresses from A to C. In model one, the core gneiss is slightly folded before being trun­ cated by a flat detachment fault which truncates the tops of the antiformal culminations and traps sedimentary rocks beneath it in the keels of the synforms. Folding continues, slightly warping the detachment fault. In model two, the detachment fault follows the folded from of the underlying gneiss and flattens with time to reduce frictional resis­ tance. As it flattens, by cutting up-section in the synforms and down section on the antiforms, it produces the same relationships as model one. Model three, which best fits the data, has the detachment fault forming generally parallel to the underlying folded gneiss but locally truncating it in a series of anastomozing faults that trap slivers of sedi­ mentary rock beneath the "master" detachment fault. See text for discussion. CORE GNEISS

\o 80 arching of the footwall gneiss and the overlying, now formed, decollement zone continues producing the existing relationships. In model two, the decollement zone forms in an arched geometry paralleling and exploiting the foliation in the footwall gneiss. As time progresses, the zone migrates to assume a slightly flatter morphology in an attempt to minimize frictional energy loss.

Geologically, there are problems with both of these models. The structural relief on the decollement zone is from three to five kilo­ meters as measured in the downplunge view in Figure 1. In contrast, the folds in the core gneiss have amplitudes of only 2km to 3km. Thus, model one must be incorrect because the zone could not have folded more than the footwall gneiss. Model two has similiar inconsistencies.

First, while it, like model one, allows the fault zone to truncate the gneissic fabric, it, like model one, does not allow this to happen in the observed sense— i .e., with the limbs of the folds in the decolle­ ment zone steeper than the limbs of the folds in the gneiss. Second, and this follows from the previous objection, the model also does not allow the fault zone to be more curved than the gneissic foliation.

Unfortunately the suggestion that the decollement zone originated in its present curviplanar morphology truncating the more gentle gneissic foliation at a higher angle implies that one would find sedimentary rocks below the detachment fault on the top of the antiform and mylonitic gneiss above the fault in the synforms; such relationships are not observed in the Tanque Verde decollement zone. 81

To resolve this dilemma, one must bear in mind two key points:

1) the decollement zone probably consists of a mesh of anastomizing faults; and 2) folded detachment faults occur even where there is no underlying gneissic core to provide inviting planes of weakness. It has been noted by numerous workers (.Coney 1980; Cameron and Frost 1981;

Frost et al. 1982; Otton 1981; Dokka et al. 1982; Frost et al. 1982;

Spencer 1982), that detachment faults, with or without an underlying gneissic core, are warped, commonly on wavelengths of as little as 1 kilometer, about axes both parallel and perpendicular to the inferred transport direction (lineation where present). Spencer (1982) and

Frost et al. (1982), have explained these folds, with some degree of success, as being crinkles due to the inability of the crust to extend in all directions due to the strongly anisotropic stress field imposed during the formation of these extensional terranes. Whatever the cause, detachment faults are warped regardless of the presence of an underlying "core." The presence of folds in the footwall gneiss is therefore an enhancing but not a controlling influence on the resultant decollement zone shape.

In the Tanque Verde Mountains, a decollement zone easily could have been formed in rough concordance with, but locally truncating, the underlying already folded gneissic fabric (see model three). As in model two, the fault zone might flatten due to energy considera­ tions, leaving slivers of metamorphosed and contorted sedimentary rocks behind in the keels of the synforms and concentrating repeated pulses of cataclasis on the structurally high culmination of the 82 antiform. The apparently high geothermal gradient at the time of decollement zone formation, together with the large structural relief between the crest of the antiform and the troughs of the synforms, could account for the more brittle deformation on the crest of the antiform. Geometric relationships in brecciated gneiss on the flanks of the antiform, that indicate outward tilting due to minor late-stage arching, are not excluded by this model. CONCLUSIONS

Fault rocks developed in the Tanque Verde Mountain decollement zone indicate a large degree of brittle deformation. Cataclasis and comminution of mineral and rock fragments strongly suggest simple shear on a microstructural scale. Water was abundant as evidenced by exten­ sive fracturing and alteration. Extreme brittle deformation is restricted to a thin (j. to 25 meter) zone of cataclasite. The cata- clasite is cut by a series of faults, some of which are demonstrably normal (see Figure 25). Thus, this zone of cataclasite does not represent the last episode of movement along the detachment fault. In addition, a thin (1 meter) zone of gouge is ubiquitous above the detach­ ment fault at the base of the cover rocks.

Parallelism of kinematic indicators in the core and cover indicate a continuum of southwest-directed shear from approximately

45 to 15my, as proposed by Lingrey and Davis (1982) for the complex as a whole. The internal geometry of fault rocks in the Tanque Verde decollement zone is consistent with this model: sinistral, probable southwest-directed shear, in the brecciated gneiss subzone is indi­ cated on the northwest flank of the Tanque Verde Mountains and rotations about northwest axes are indicated in this subzone on both flanks of the antiform. In addition, brecciation and veining indicate overall dilation for this zone in accord with its development in an

83 8 4 extensional regime. Finally, the distribution of fault rocks within the decollement zone requires that the core gneiss was arched prior to the zone’s development. LIST OF REFERENCES

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