Development of oblique- slip basement- cored uplifts: insights from the Kaibab uplift and from physical models
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Authors Tindall, Sarah Elizabeth
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Link to Item http://hdl.handle.net/10150/558760 DEVELOPMENT OF OBLIQUE-SLIP BASEMENT-CORED UPLIFTS:
INSIGHTS FROM THE KAIBAB UPLIFT AND FROM PHYSICAL MODELS
by
Sarah Elizabeth Tindall
Copyright © Sarah Elizabeth Tindall 2000
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
2000 2
THE UNIVERSITY OF ARIZONA ® GRADUATE COLLEGE
As members of the Final Examination Committee, we certify that we have read the dissertation prepared by S a r a h f.t i T - i n ^ n ______entitled Development of Oblique-Slip Basement-Cored Uplifts:______
Insights from the Kaibab Uplift and from
Physical Models
t it be accepted as fulfilling the dissertation requirement /fo ree of Doctor of Philosophy
Date
Date
If ~ 3 ' Z-cTi/'L.
Final approval and acceptance of this dissertation is contingent upon the candidate's submission of the final copy of the dissertation to the Graduate College.
I hereby cefrtijfy that I have read this dissertation prepared under my direction^and recommend that it be accepted as fulfilling the dissertation requirement /
4- //- Go Dissertatibn^Director Date 3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements of an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are 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 copyright holder.
Signed: 4
ACKNOWLEDGMENTS
The people who have influenced my personal and professional experiences, not only during graduate school but throughout my life, are too numerous to name. If I were to attempt to make a list anyway, I would have a very difficult time deciding who should occupy the first position, and the last, and every one in between. So let me put it this way:
A GIANT GRIN for most of you
a small scowl for others but each of you has played an important role, and I thank you for it. 5
TABLE OF CONTENTS
LIST OF ILLUSTRATIONS...... 8
LIST OF TABLES...... 10
ABSTRACT...... 11
CHAPTER 1: INTRODUCTION...... 12 Statement of Problem...... 12 Previous W ork...... 15 Explanation of Dissertation Format...... 19
CHAPTER 2: PRESENT STUDY...... 22 Major Findings...... 22 Behind The Scenes...... 28
REFERENCES...... 33
APPENDIX A: MONOCLINE DEVELOPMENT BY OBLIQUE- SLIP FAULT-PROPAGATION FOLDING: THE EAST KAIBAB MONOCLINE, COLORADO PLATEAU, UTAH...... 42 Abstract...... 42 Background...... 43 Monoclines as Drape Folds...... 43 Monoclines as Fault Propagation Folds...... 44 Importance of Oblique Deformation...... 46 Geologic Setting...... 48 Structural Data and Observations...... 55 Domain 1 ...... 55 Domain 2 ...... 61 Domain 3 ...... 64 Domain 4 ...... 69 Summary of Field Observations...... 73 Discussion...... 76 Conclusions...... 84 Acknowledgements...... 85 References...... 86
APPENDIX B: THE COCKSCOMB SEGMENT OF THE EAST KAIBAB MONOCLINE: TAKING THE STRUCUTRAL PLUNGE...... 90 Abstract...... 90 Introduction...... 91 6
TABLE OF CONTENTS - Continued
Background...... 95 Regional Setting...... 95 The Cockscomb...... 99 Structural Roots...... 100 A Visual Tour...... 102 Structural Observations...... 108 Northward Plunge...... 110 Down-Plunge Viewing...... 112 Fault Tip Zone...... 114 Oblique Deformation...... 116 Fault Slip Gradient...... 121 Fault-Propagation Folding...... 123 Summary...... 126 Conclusions...... 127 Acknowledgments...... 130 References...... 131
APPENDIX C: DECIPHERING RIEDEL PATTERNS IN OBLIQUE SHEAR ZONES...... 135 Abstract...... 135 Introduction...... 136 Riedel Shear Zones...... 136 Riedel Geometry and Oblique Deformation...... 140 The East Kaibab Shear Zone...... 144 Geologic Setting...... 144 Oblique Shear Zone...... 146 Riedel Geometry...... 150 Application...... 155 Conclusions...... 157 References...... 159
APPENDIX D: BASEMENT-INVOLVED OBLIQUE SHORTENING STRUCUTRES: PHYSICAL MODELS AND FIELD EXAMPLES...... 163 Abstract...... 163 Introduction...... 164 Modeling M ethods...... 167 Modeling Material...... 169 Scaling...... 170 Data Collection...... 171 Physical M odels...... 172 Strike-Slip...... 173 Review of Literature...... 173 7
TABLE OF CONTENTS - Continued
Experimental Design...... 176 Results...... 178 Dip-Slip Shortening...... 180 Review of Literature...... 180 Experimental Design...... 184 Results...... 186 Oblique Shortening...... 190 Review of Literature...... 190 Experimental Design...... 193 Results...... 195 Discussion of Model Results...... 199 Field Example: The East Kaibab Monocline, Utah Background...... 207 Major Structural Features...... 209 Comparison with Model Results...... 213 Conclusions...... 215 References...... 218
APPENDIX E: CATALOG OF PHOTOGRAPHS OF PHYSICAL ANALOG M ODELS...... 226 Introduction...... 226 Strike-Slip...... 227 Dip-Slip Shortening With Growth...... 237 Dip-Slip Shortening Without Growth...... 242 Oblique Shortening With Growth...... 245 Oblique Shortening Without Growth...... 248 Oblique Shortening With Strain Circles...... 257
APPENDIX F: STRATIGRAPHIC SEPARATION DIAGRAM 260 8
LIST OF ILLUSTRATIONS
Figure 2-1. Location of the East Kaibab monocline and its segments...... 23 Figure 2-2. Sample of detailed geologic mapping along the East Kaibab shear zone...... 29 Figure 2-3. Photographs of physical analog models...... 31 Figure 2-4. Schematic diagram of map view and cross section fault and fold interpretations from the oblique shortening model...... 32
Figure A l. Location of the East Kaibab monocline...... 49 Figure A2. Structure contour map of the northern East Kaibab monocline...... 50 Figure A3. Simplified geologic map of the northern East Kaibab monocline...... 51 Figure A4. Generalized stratigraphic column...... 53 Figure A5. Structural relationships in the Grand Canyon...... 54 Figure A6. Geology of Structural Domain 1...... 56 Figure A7. Photograph of faults in Domain 1...... 59 Figure A 8. Geology of Domain 2 ...... 62 Figure A9. Geology of Domain 3 ...... 65 Figure A10. Photograph of faulting between Domains 2 and 3 ...... 67 Figure All. Geology of Domain 4 ...... 70 Figure A12. Cross section sketch of faulting in Domain 4 ...... 71 Figure A13. Summary of structural and stratigraphic relationships...... 74 Figure A 14. Calculated shortening and extension axes...... 78 Figure A15. Projections of fault orientations, strain axes, and the strain ellipsoid... 80 Figure A16. Difficulties associated with fault reactivation...... 83
Figure B l. Location of the East Kaibab monocline...... 92 Figure B2. Cross section sketch of a Colorado Plateau monocline...... 93 Figure B3. Regional maps of Mesozoic and Cenozoic structures of the Colorado Plateau and surrounding geographic provinces...... 96 Figure B4. Sketches of Laramide and post-Laramide plate tectonic setting...... 98 Figure B5. Fault-fold relationships in the Grand Canyon...... 101 Figure B6. Photographs of the landscape along the East Kaibab monocline...... 104 Figure B7. Geologic map of the East Kaibab monocline in Grand Staircase- Escalante National Monument...... 109 Figure B 8. Diagrams of monoclines with horizontal and plunging fold axes...... I l l Figure B9. Down-plunge viewing...... 114 Figure B10. Upward migration of a basement-rooted fault tip...... 117 Figure B l 1. Riedel shear geometry...... 119 Figure B 12. Lateral variations in fault and fold offset...... 122 Figure B13. Diagrams of drape folding and fault-propagation folding...... 124 Figure B 14. Block diagrams of the Cockscomb in Grand Staircase-Escalante National Monument...... 128 9
LIST OF ILLUSTRATIONS - Continued
Figure CL Elements of right-handed and left-handed Riedel shear zones...... 138 Figure C2. Stresses responsible for Riedel shear geometry...... 143 Figure C3. Location of the East Kaibab monocline...... 145 Figure C4. Cross section of fault and fold relationships in the Grand Canyon...... 147 Figure C5. Fault relationships along the East Kaibab monocline in southern Utah...... 148 Figure C6. Orientations of synthetic and antithetic faults, and calculation of stress directions...... 152 Figure C7. Maps and stereographic projections of fault and slickenline Orientations in map view and in CJ1-O3 plane view...... 156
Figure D l. Fault patterns associated with ideal strike-slip systems...... 174 Figure D2. Strike-slip apparatus...... 177 Figure D3. Overhead view of strike-slip deformation...... 179 Figure D4. Representative strike-slip cross sections...... 181 Figure D5. Strike-slip fault pattern in the subsurface...... 182 Figure D6. Dip-slip shortening apparatus...... 185 Figure D7. Overhead view of dip-slip shortening without growth...... 187 Figure D8. Representative dip-slip shortening cross sections...... 189 Figure D9. Dip-slip shortening fault pattern in the subsurface...... 191 Figure DIO. Oblique shortening apparatus...... 194 Figure Dll. Overhead view of oblique shortening without growth...... 196 Figure D12. Representative oblique shortening cross sections...... 198 Figure D13. Oblique shortening fault pattern in the subsurface...... 200 Figure D14. Summary block diagrams of model results...... 201 Figure D15. Location of the East Kaibab monocline...... 208 Figure D16. Fault patterns on the crest of the East Kaibab monocline...... 210 Figure D17. Fault patterns within the steep limb of the East Kaibab monocline..... 211
Figure FI. Stratigraphic separation diagram of the East Kaibab monocline...... 260 10
LIST OF TABLES
Table 2-1. References for selected areas of continental interior deformation or oblique deformation worldwide...... 27
Table D l. Summary of important parameters applied to strike-slip, dip-slip shortening and oblique shortening models...... 168
Table D2. Summary of major fault and fold characteristics observed in physical models of strike-slip, dip-slip shortening and oblique shortening deformation...... 202 11
ABSTRACT
Detailed structural geologic mapping of the East Kaibab monocline in southern
Utah, combined with results of physical analog modeling, demonstrate that basement- cored structures that form by oblique deformation exhibit a number of distinctive, characteristic features. Map relationships and stress inversion techniques reveal that
Laramide growth of the East Kaibab monocline involved a previously unrecognized component of right-handed offset accommodated by reverse-right-lateral fault-
propagation folding. Evidence for this new interpretation lies in a monocline-parallel
shear zone that changes character with changing structural and stratigraphic level.
Structural character and fault orientations within this shear zone resemble a ‘fault-tip
process zone' ahead of an upward- and northward- propagating basement-rooted fault
tip. Moreover, fracture patterns in the fault-tip deformation zone resemble Riedel shear
geometry normally identified with the map view perspective of pure strike-slip systems.
Analysis of fault and slickenline orientations and recognition of the relationship of
Riedel fracture geometry to principal stress and strain directions allow an estimate that
the strike-slip to dip-slip ratio in the East Kaibab shear zone was between 2:1 and 6:1.
The revised estimate of the sub-regional shortening direction during Laramide
deformation is 078° or more easterly, as opposed to the 065° shortening direction
calculated in previous studies of the Grand Canyon region.
Results of a new physical analog modeling technique complement field-based
observations of structural features associated with oblique shortening. Physical analog
models were constructed of wet clay overlying rigid basement forcing blocks in order to 12 simulate cover deformation above strike-slip, dip-slip reverse, and oblique-reverse faults in underlying basement. The experiments were used to identify basic structural patterns characteristic of oblique shortening deformation. Features associated with oblique shortening experiments include basement-rooted faults in the cover section that strike obliquely to the fault boundary between underlying forcing blocks. These faults accommodate reverse-oblique slip in the cover section, and resemble the Riedel
geometry mapped along the East Kaibab monocline. The most distinctive features
formed by oblique shortening models are outer-arc extensional faults (on the crest of a
basement-cored uplift) that strike obliquely to the trend of the uplift, the underlying
basement fault orientation, and the horizontal shortening direction. A similar system
faults has been mapped in detail by other geologists working along the crest of the East
Kaibab monocline. The correspondence between physical model results and secondary
structures mapped along the East Kaibab monocline indicates that these and other
structural features provide convenient tools for recognizing and analyzing oblique
deformation in other natural examples of oblique-slip basement-cored uplifts. 13
CHAPTER 1: INTRODUCTION
Statement of Problem
The purpose of the present study is to identify structural patterns characteristic of oblique deformation associated with basement-involved shortening structures, or basement-cored uplifts. Characteristics generally identified with basement-cored uplifts are folding and faulting of sedimentary strata above basement blocks that have been uplifted along thrust or reverse faults rooted deeply in basement (Mitra and Mount,
1998). These uplifts, sometimes called foreland basement-involved structures, are most commonly identified with foreland deformation ahead of active fold-thrust belts. The
Laramide-age uplifts of the Colorado Plateau and Rocky Mountain foreland in the western United States are the best-studied examples of this deformation style.
However, structures with similar characteristics also form during deformation of continental interiors and in regions of basement fault reactivation (Vially et al., 1994;
Beauchamp et al., 1996,1999; Marshak et al., 1998,1999).
Interpretation of the structural development of basement-cored uplifts such as those of the Colorado Plateau and Rocky Mountain foreland traditionally has been fraught with confusion stemming from the variable structural geometries and wide range of orientations associated with these uplifts (e.g. Schmidt and Perry, 1998;
Schmidt et al., 1993). Observations including faults that steepen with depth (Prucha et al., 1965), faults that flatten with depth (Stone, 1993), and both deformed and undeformed hanging wall basement wedges (Wise, 1963; Erslev and Rogers, 1993;
Schmidt et al., 1993) seem to indicate very different deformation mechanisms. To 14 explain the observations, authors have called upon a range of deformation mechanisms and fault-fold relationships from drape folding (Prucha et al., 1965; Steams, 1971;
Reches and Johnson, 1978) to fold-thrusting (Berg, 1962), upthmsting (Foose et al.,
1961; Berg, 1962; Prucha et al., 1965) and fault-propagation folding (Blackstone, 1940;
Erslev, 1991). Some geologists proposed that the orientations of basement-cored uplifts
in the western United States resulted from vertical uplift tectonics (e.g. Steams, 1971,
1978; Matthews, 1978) rather than horizontal compression (Kelley, 1955; Davis, 1978;
Reches, 1978; Cries, 1981; Huntoon, 1981; Lowell, 1983; Stone, 1984; Anderson and
Bamhard, 1986; Bergerat et al., 1992; Tindall and Davis, 1999). Others speculated that
a rotating stress field could be responsible at least for the variable orientations of uplifts,
if not for their apparently disparate structural styles (Chapin and Gather, 1983; Cries,
1983; Livaccari, 1991; Bird, 1998).
A few authors have suggested that the variations in orientations and structural
styles of basement-cored uplifts may result from varying degrees of oblique
deformation (Kelley, 1955; Stone, 1969; Barnes, 1974,1987; Pearce, 1998).
Unfortunately, recognition of general features characteristic of oblique deformation, in
contrast with dip-slip or strike-slip motions, has been complicated. Oblique
deformation produces fault and fold patterns that are easily mistaken for strike-slip or
dip-slip deformation (e.g. Stone, 1969). For this reason any systematic relationship
between oblique motion along basement faults and the resulting structural features and
patterns in overlying sedimentary cover has gone unrecognized. 15
This dissertation presents evidence that basement-involved oblique deformation generates structural geometries and patterns in sedimentary cover that can be distinguished from those created by strike-slip and reverse-slip deformation. Insights from a new physical analog modeling technique complement detailed structural mapping and kinematic analysis of data from the East Kaibab monocline, a reverse- right-lateral basement-cored uplift. Results indicate that apparently disparate processes such as fault-propagation folding (a mechanism associated with thrust or reverse faulting) and Riedel shearing (typical of strike-slip systems) can occur together during basement-involved oblique deformation. Furthermore, oblique slip at the level of basement generates patterns of structures in sedimentary cover that are different from those produced by strike-slip or dip-slip motions. The manuscripts appended to this thesis describe data, observations and experiments that allow recognition of numerous structural hallmarks of basement-involved oblique shortening structures.
Previous Work
The research presented in this dissertation builds on previous work from a wide range of subdisciplines of structural geology, including studies of physical and numerical modeling, field investigations in the Colorado Plateau and Rocky Mountain foreland, methods of kinematic analysis, and theoretical consideration of faulting and folding in many tectonic environments. However, the four studies presented in the
Appendices of this dissertation share a common intellectual challenge: recognition and analysis of oblique deformation related to basement-cored uplifts. 16
The papers contained in the first three appendices build most directly on more than a century of work on Colorado Plateau monoclines. The earliest explorers of
Colorado Plateau geology, Powell (1873), Dutton (1882), and Walcott (1890), recognized a relationship between the broad, low monoclinal folds characteristic of the
Plateau and their underlying cause - apparent reverse faulting in Precambrian basement.
Similar features in the Rocky Mountain foreland province on the north and east sides of the Colorado Plateau, although much more developed, seemed to demonstrate the same mode of origin (Matthews, 1978). However, the lack of consistency in the orientations and structural styles of these uplifts has remained a problem.
The East Kaibab monocline on the western margin of the Colorado Plateau is one of the largest and best-studied of the Plateau uplifts. Detailed investigations and mapping of geomorphology, structure, and stratigraphy along the East Kaibab have been completed by Gregory and Moore (1931), Babenroth and Strahler (1935), Sargent and Hansen (1982), Doelling et al. (1989), Stem (1992), Mollema (1994), Mollema and
Aydin (1997) and Rosnovsky (1998), but none concentrated on specific evidence for basement-rooted oblique deformation. Stone (1969) suggested the possibility that oblique motions played an essential role in the development of Colorado Plateau and
Rocky Mountain uplifts. He hypothesized that the variable orientations of major Rocky
Mountain structures resulted not from vertical uplift or from a rotating horizontal shortening direction, but from a single horizontal shortening direction acting on pre existing weaknesses to cause varying degrees of oblique deformation (Stone, 1969).
Kelley (1955) noted en-echelon folding as evidence for oblique deformation within the 17
Colorado Plateau and Barnes (1974,1987), McCormack (1989) and Pearce (1998) examined structural evidence for reverse-oblique slip along the Coconino and Black
Point segments of the East Kaibab monocline in northern Arizona. These previous studies of oblique deformation associated with basement-cored uplifts contributed importantly to interpretations presented in this dissertation.
Davis and Tindall (1996) pointed out the first compelling evidence for a significant lateral component of slip along the East Kaibab in southern Utah. Davis was responsible for noticing peculiar structural patterns in a geologic map by Sargent and
Hansen (1982), and these patterns are discussed in detail in Appendix 1. At the beginning of the project, the reverse-oblique fault-propagation fold interpretation was not obvious, and other possibilities were suggested to explain structural observations.
At first glance, the fault pattern in southern Utah resembles a rotated, low-angle normal- slip detachment fault system rooted in Jurassic or possibly Triassic rocks. However, detailed mapping revealed no evidence for progressive rotation of the fault zone. Major fault surfaces maintain strike orientation and dip direction as they cross bedding with changing dip angles. The same evidence argues against the possibility that all large and small fault surfaces represent conjugate outer-arc extensional features, with some rotated to display apparent reverse offset.
Other interpretations of structural complexity along the East Kaibab monocline have been published in the literature as well. For example, detailed studies by Stem
(1992) and Rosnovsky (1998) attributed complicated fault patterns and right-lateral slip indicators in the steep limb of the monocline to steps or bends in the underlying 18 basement fault. Observations for both of these investigations concentrated within sub- areas of the East Kaibab monocline where the trend of the fold changes. However, it is impossible to determine based on surface evidence whether the changes in fold trend are related to basement fault geometry, or are simply accommodation features within the sedimentary cover. The interpretation by Stem (1992) and Rosnovsky (1998) of oblique faulting at the bends along the monocline is well supported, but evidence for oblique deformation is in fact abundant along the entire 60 km trace of the monocline in southern Utah. Therefore the right-lateral component of slip is not restricted to the areas examined by Stem (1992) and Rosnovsky (1998).
The manuscript in Appendix 4 not only draws on the literature surrounding oblique deformation and basement-cored uplifts, but also builds on an enormous background of knowledge about physical analog modeling. Familiarity with structural features associated with strike-slip deformation (e.g Riedel, 1929; Tchalenko, 1970;
Reading, 1980; Sylvester, 1988) and dip-slip reverse faulting (Stone, 1993; Mitra and
Mount, 1998) allows recognition of a different suite of features diagnostic of oblique deformation. Only a few researchers have modeled oblique simple shear similar to the basement-involved deformation investigated here, including Richard and Cobbold
(1990), Richard (1991) and Richard et al. (1995). The intent and scope of the physical modeling experiments presented here differ from those of previous workers, but the ideas presented in Appendix 4 grew out of familiarity with the work of these and other geologists. 19
Explanation of Dissertation Format
Four manuscripts contained in the following appendices represent the body of
this dissertation. The common theme shared by these papers is recognition of structural
features characteristic of oblique deformation during development of basement-cored
uplifts. Each paper is written for publication in a separate journal or book, which
necessitates that basic introductory and background material appears in each
manuscript. ‘Chapter 2: Present Study’ contains a review of the major findings of this
dissertation and a description of the significance of the work. The following paragraphs
summarize the important conclusions presented in each of the four Appendices.
The first paper, Monocline development by oblique-slip fault-propagation folding: The East Kaibab monocline, Colorado Plateau, Utah [Tindall and Davis]
presents structural and kinematic evidence that the northernmost, northeast-trending
segment of the East Kaibab monocline formed by fault-propagation folding rather than
by drape folding. In addition, orientations of secondary structures indicate that a
considerable component of right-lateral slip occurred during Laramide growth of the
uplift. This paper was published in The Journal o f Structural Geology in October, 1999.
The second manuscript titled The Cockscomb segment of the East Kaibab
monocline: Taking the Structural Plunge [Tindall] supports an oblique-slip fault-
propagation fold origin for the East Kaibab monocline using qualitative observations
and sub-regional map relationships. The scientific conclusions presented in this paper
are similar to those in Appendix A; however, the second paper includes different lines
of evidence for this conclusion, and introduces concepts that help explain questions left open by the manuscript in Appendix A. The manuscript included in Appendix B has been accepted for September, 2000 publication in the Utah Geological Association
Millennium Guidebook.
The third manuscript, Deciphering Riedel patterns in oblique shear zones
[Tindall], points out that the fault-tip fracture zone exposed along the East Kaibab monocline fits expected orientations of Riedel geometry in oblique deformation systems. This observation strengthens the oblique-slip interpretation of the origin of the uplift. The manuscript also demonstrates that this Riedel geometry can be used to estimate the ratio of lateral to dip-slip offset in oblique shear systems. Stress inversion of the Riedel fracture array also permits an accurate estimate of the sub-regional
shortening direction. This paper will be submitted to The Journal of Structural
Geology.
The final manuscript, titled Basement-Involved Oblique Shortening Structures:
Physical Models and Field Examples [Tindall, Eisenstadt, Withjack, and Schlische]
details the results of physical models of strike-slip, dip-slip reverse, and oblique-reverse
deformation in cover sediments overlying reactivated basement structures. The dip-slip
and oblique-reverse models are analogous to basement-cored uplifts like those of the
Rocky Mountain foreland and Colorado Plateau. Comparison of secondary structures
formed in these idealized models allows identification of certain patterns that are
diagnostic of oblique shortening, including oblique outer-arc extensional faults,
oblique-reverse offset, and en-echelon fault traces at angles greater that 15° from the
principal displacement zone. These features are identifiable not only on the East 21
Kaibab monocline but are associated with other basement-involved oblique-shortening structures worldwide. This final paper will be submitted to American Association of
Petroleum Geologists Bulletin.
The research involved in this dissertation and summarized in these manuscripts was done entirely by the author. However, co-authors on two of the manuscripts
(George H. Davis, Appendix A; Gloria Eisenstadt, Martha Withjack, and Roy
Schlische, Appendix D) provided initial research topics, support both moral and technical, and invaluable guidance without which the research could not have been completed. Financial support for research related to the first three manuscripts largely
was provided through NSF#EAR-9406208 (G.H. Davis - Principal Investigator). Some
of the field work for the second manuscript was supported by the Dr. H. Wesley Peirce
Scholarship, Department of Geosciences, University of Arizona. All research related to
the fourth manuscript was supported by Mobil Technology Company in Dallas, TX. 22
CHAPTER 2: PRESENT STUDY
Detailed accounts of field and analytical methods, data, observations and conclusions appear in a series of manuscripts appended to this thesis. A summary of the major findings and significance of the dissertation research is presented here, followed by a behind-the-scenes account of field and modeling work completed during the course
of dissertation research.
Major Findings
The East Kaibab monocline in northern Arizona and southern Utah is the best
studied of the Laramide-age basement-cored uplifts of the Colorado Plateau
physiographic province. The structure is approximately 240 km long and trends
generally north-northeast from the San Francisco volcanic field in Arizona to near
Bryce Canyon National Park in Utah (Figure 2-1). Permian through Cretaceous rocks
are exposed along the fold, with progressively higher stratigraphic units incorporated in
the steep east-dipping limb from south to north along its surface trace. This exposure
results from the slight (30-5°) northward plunge of the structure, which in turn reveals
deeper structural levels toward the south. In addition, excellent exposures of
Precambrian through Permian rocks are visible where the East Kaibab monocline
crosses the Grand Canyon. There, a steep, west-dipping fault (the Butte fault) with
west-side-up, reverse separation underlies the east-vergent monoclinal fold.
Traditionally the origin of the East Kaibab and other Colorado Plateau
monoclines has been attributed to reverse-slip reactivation of underlying basement (Palisades, Grandview, Coconino, Black Point) in northern Arizona and southern and northerninArizona Point) Black Coconino, Grandview, (Palisades, Utah. Figure 2-1. Location of the East Kaibab monocline and its branching segments branching its and monocline Kaibab East the of Location 2-1. Figure
KAIBAB GRAND VIEW San Peaks Francisco PALISADES IOCONINO BLACK POINT
23 24 faults, resulting in passive drape folding of the overlying Paleozoic and Mesozoic sedimentary cover. Detailed examination of secondary fault patterns and kinematic indicators along the East Kaibab monocline, as described in this thesis, demonstrates instead that this uplift formed by oblique deformation and fault-propagation folding.
A 60 km-long shear zone parallel to the trend of the monocline provides compelling evidence for this kinematic re-interpretation. Map-scale and outcrop-scale faults and slip surfaces within the shear zone occupy distinct northeast-striking and northwest-striking orientations, both oblique to the strike and dip of the associated shear zone. These faults accommodate significant components of right-lateral and left-lateral offset, respectively. The fault sets represent synthetic and antithetic Riedel shears within a reverse-right-lateral zone of deformation occupying the steep monoclinal limb.
The Riedel nature of the fault pattern along the East Kaibab monocline lends itself to stereographic analysis of stress and strain directions. For example, in typical
Riedel shear systems (associated with pure strike-slip deformation) the Gz direction
(intermediate principal stress) parallels the intersection between synthetic and antithetic shears. Consequently, rotating a stereographic projection of East Kaibab shear zone fault orientations so that the intersection of antithetic and synthetic faults is vertical provides a view of the shear zone in the plane containing maximum and minimum principal stress directions. Within this C|-(?3 plane it is possible to measure the angle between synthetic and antithetic Riedel shears, calculate the ai direction (maximum principal stress) and estimate generally the ratio between strike-slip and dip-slip deformation. This analysis reveals that an ENE-directed horizontal shortening direction was responsible for Laramide deformation along the NE-trending segment of the East
Kaibab monocline in southern Utah. Orientations of the finite strain ellipsoid axes are consistent with reverse-right-lateral shear within the steep monoclinal limb.
The East Kaibab shear zone also exhibits variations in structural style from north to south along its trend. These changes correspond with migration of the shear zone to
progressively lower structural and stratigraphic levels toward the south. Deformation is
marked by discontinuous antithetic and synthetic fault segments at the north end of the
monocline, but these give way to long, continuous, monocline-parallel faults southward.
This spatial transition from discontinuous en-echelon faults to through-going faulting at
deeper structural levels is analogous to the progressive development of Riedel shear
zones on the upper surface of physical models of strike-slip deformation. In such
models, arrays of synthetic and antithetic fractures are gradually replaced by basement-
rooted, shear zone-parallel faults as strain increases. By analogy, the changes in
structural style exposed along the East Kaibab monocline represent the frozen final
moment of upward and lateral propagation of a basement-rooted, reverse-right-lateral
Riedel shear zone within the steep limb of the growing East Kaibab flexure. This
fortuitous exposure reveals that the East Kaibab, and possibly other Colorado Plateau
monoclines, are more accurately described by a fault propagation fold origin than by a
drape fold model.
Through funding and support from Mobil Technology Company, physical
models were created by deformation of a clay cover sequence over strike-slip, dip-slip
shortening and oblique shortening basement faults, simulating basement-involved 26 deformation of the crust. Results of physical experiments demonstrate that several distinctive structural patterns observed along the East Kaibab monocline are generally characteristic of basement-involved oblique shortening. Strike-slip and dip-slip shortening experiments developed features commonly observed or interpreted in basement-involved strike-slip and reverse-slip environments, respectively. Structures formed in these models were consistent with results of model studies and field observations documented in the literature, allowing generalizations to be made about typical strike-slip and reverse-slip structural patterns. In turn, the oblique shortening models exhibited a number of features quite different from those formed in strike-slip and dip-slip deformation. For example, the presence of Riedel-type fractures with oblique-reverse offset within the steep limb of a major uplift is distinctive of basement- involved oblique shortening. In addition, outer-arc extensional faults on the crest of the uplift displayed orientations oblique to both the basement fracture trend and the horizontal shortening direction. These features correspond well with oblique structures mapped or observed during field studies of the East Kaibab monocline.
The conclusions presented in this dissertation will prove useful in identifying
oblique deformation associated with other structures in the Colorado Plateau and Rocky
Mountain foreland provinces, as well as in areas of basement-involved deformation
worldwide (Table 2-1). Where patterns of secondary faulting similar to those formed in
physical models can be identified in the steep limbs of other basement-cored uplifts, the
angular structural relationships can be used to identify the presence and magnitude of
oblique deformation. Furthermore, analyses used to determine the orientation of the 27
STRUCTURE ! LOCATION i FEATURES REFERENCES Owl Creek Mountains Wyoming, USA oblique normal faults Paylor and Yin, 1993; Wise, 1963 West Sussex-Dugout Wyoming, USA oblique normal faults King, 1969 Oil Field Rio Puerco fault zone New Mexico, USA oblique normal faults Slack and Campbell, 1976 Nacimiento Uplift New Mexico, USA oblique normal faults Baltz, 1967; Giral, 1995 Terlingua Uplift Texas, USA oblique normal faults Erdlac Jr., 1989,1990
Durmid Hill California, USA oblique normal faults Burgmann, 1991
Newport-lnglewood California, USA R and R' orientations and Harding, 1973,1974 Trend oblique normal faults associated with major uplift T ucuman T ransfer Argentina | transpression oblique to De Urreiztieta et al., Zone plate margin 1996 Sierras Pampeanas Argentina compressional basement Jordan and uplifts with variations in Allmendinger, 1986 orientation Northern Calcareous Austria synthetic and antithetic Linzer, Ratschbacher Alps lateral faults, thrusts and Frisch, 1995 Carboneras Fault Spain transpression with Riedel Keller, Hall, Dart and shear geometry McClay, 1995; Keller, Hall and McClay, 1997 Macizo de Nevera Spain joblique reverse/lateral faults Rondeel, Weijermars and Van Dorssen, 1984 Saharan Atlas | Algeria | oblique normal faults Vially et al., 1994
Table 2-1. Interpretations and conclusions developed in this dissertation are applicable to oblique basement-involved/continental interior features worldwide. This table presents a short list of structures and areas that display distinctive oblique-shortening features similar to those described in the following appendices, or that lie in tectonic settings likely to develop such features. finite strain ellipsoid and the directions of local principal paleostresses for the East
Kaibab monocline are applicable to data sets from areas where local kinematics and regional tectonic history are poorly understood.
Behind the Scenes
Structural geologic mapping of the East Kaibab monocline spanned four field seasons. The most detailed work was carried out in southern Utah where the northward plunge of the East Kaibab exposes a monocline-parallel shear zone in Mesozoic strata.
Figure 2-2 is an example of the detailed mapping carried out along the 60 km long exposure of the East Kaibab in southern Utah; additional maps appear in Appendix 1.
In addition, reconnaissance mapping and collection of joint, cleavage, fault, and slickenline orientations continued southward from the Arizona-Utah border to the
Grand Canyon. These data have been incorporated into interpretations and conclusions presented in Appendices 1 and 2. During two trips to the Butte fault exposure in the
Grand Canyon (one with a team from The University of New Mexico) kinematic data were collected in both Proterozoic and Phanerozoic rocks along several branching segments of the Butte fault. This information has contributed significantly to interpretations and conclusions presented in this dissertation, and has provided a basis for future publications. Fault slip data also were collected from the Grandview,
Coconino and Black Point segments of the East Kaibab monocline south of the Grand
Canyon. Interpretation of this information is still underway, but the work contributed to ideas and interpretations presented in the following appendices. 29
1 km Topographic Contours Cottonwood Road (interval - 200 ft) Faria River and Cottonwood Creek ------Lithologic Contacts
y/«4 Data stations Faults
Figure 2-2. Example of density of data collected during detailed geologic mapping. Data collected at each station are too numerous to represent on the map, and include strike and dip of bedding, measurements of major and minor fault surfaces, sense of offset, and joint and cleavage orientations. 30
Numerous physical analog models were constructed and/or examined during research for Appendix 4. Because the modeling medium was wet clay, each layered clay model required 4-7 weeks of drying before examination and data collection.
Model runs included a layered strike-slip model without syntectonic growth layers, a layered dip-slip shortening model with syntectonic growth layers, a non-layered dip-slip shortening model without syntectonic growth layers, and layered oblique shortening models with and without growth (Figure 2-3). All model cross sections and overhead photographs were considered during interpretation of model results (Figure 2-4). In addition, non-layered dip-slip shortening models were run at various strain rates (1 cm/h, 2 cm/h, 3 cm/h, 4 cm/h) to determine the effect of strain rate, if any, on fault orientations. Appendix 4 presents initial interpretations of model structures; these complement observations of oblique shortening indicators from the East Kaibab monocline. Additional models will be carried out in the future, and will answer questions not addressed in Appendix 4. (C)
Figure 2-3. Photographs of the oblique shortening model without growth, (a) Overhead view of the upper surface of the model immediately after deformation, (b) After drying, vertical cuts perpendicular to the deformed zone and to the basement fault trend revealed serial cross sections, (c) A representative cross section slice of the oblique shortening model without growth. Photographs o f all models examined in the course of this dissertation are contained in Appendix E. 32
Figure 2-4. Schematic diagram showing interpretation of the oblique shortening model without growth. Model results are discussed in detail in Appendix 4. 33
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lnif PcfSHiion Ncftli-t lolUiul I Xwcrpta McJica 41 JOURNAL OF STRUCTURAL GEOLOGY PERCAMON Journal ol'Structural GcoIol1) 21 l I't'Wi I.-03-I320 ~ — el>cvicr.nl locale jrtrugeo Monocline development by oblique-slip fault-propagation folding: the East Kaibab monocline, Colorado Plateau, Utah Sarah E. Tindall*. G.H. Davis Department of Ceufeiences. The L'nieertttv nl Arizona Tut san. AZ HS72I. L'SA Revened IX May IW8. accepted 2S March 1'WV Abstract Fault relationships along a 50-km stretch of the East Kaibab monocline in southern Utah suggest that Late Cretaceous early Tertiary development of the structure involved a significant component of right lateral strike-slip displacement, accommodated by basement-rooted faulting and fault-propagation folding. Evidence of oblique slip is provided mainly by pervasive map-scale .* • and outcrop-scale faults that define a shear zone occupying the steep east-dipping limb of the monocline for at least its northernmost 50 km. Dominant fault orientations are synthetic and antithetic to the shear zone, and accommodate reverse-right- lateral and reverse-left-lateral slip, respectively. Structural style within the shear zone changes character and increases in intensity with progressively lower structural and stratigraphic levels in the fold, suggesting that the shear zone propagated upward from a basement-rooted fault during monocline formation. We conclude that horizontal. ENE-directed. Laramide compression drove reverse-right-lateral slip on the basement fault zone beneath the developing East Kaibab monocline. The resulting transpressional fault-propagation fold is marked in southern Utah by 1600 m of reverse displacement and possibly 8000 m of right-lateral displacement across the shear zone and associated monoclinal flexure, t IW ) Elsevier Science Ltd. All rights reserved. I. Background dipping basement faults; these faults formed during Precambrian time, and underwent reverse reactivation during Late Cretaceous early Tertiary (Laramide) de /./. Monoclines as drape folds formation (Walcott. 1890; Maxson. 1961; Huntoon. The formation of regionally significant monoclines 1969. 1971. 1974; Huntoon and Sears. 1975; Reches. like those on the Colorado Plateau of the western 1978). United States has most often been explained as the Many authors have proposed that Colorado Plateau result of drape folding of a sedimentary rock sequence monoclines formed by drape folding, defined as the above near-vertical, normal or reverse faults in under passive response of a sedimentary cover sequence to lying basement. Early explorers of the Colorado faulting in the basement beneath (Sanford. 1959; Plateau and Grand Canyon regions described mono Prucha et al.. 1965). Stearns (19711 discussed the devel clinal flexures, and recognized a relationship between opment of drape folds in the Rocky Mountain fore folding in the Paleozoic and Mesozoic sedimentary land province, and extended his observations to the sequence and faulting at depth (Powell. 1873; Dutton. Colorado Plateau monoclines. In his descriptions, 1882; Walcott. 1890). Most Colorado Plateau mono drape folds occur where faulting is the primary defor clines exposed in the Grand Canyon (e.g. East Kaibab. mation mechanism in the basement, but folding domi West Kaibab. Hurricane. Grandview) lie above steeply nates in the sedimentary cover, i.e. faulting in the sedimentary cover is of minor importance (Stearns. 1971). The transition from fault to fold is accom * Corresponding author Fax: -► 1-520-621-2872. plished by detachments and thinning in the above E-mail uthlress set u u.arizona cdu IS E Tindall) basement sedimentary sequence, particularly aided by UIUMMI W $ . see front mailer < l*N Elsevier Science Ltd All nght> reserved Ml snI*>l-8 14 1( iodoxv-v 42 APPENDIX A: MONOCLINE DEVELOPMENT BY OBLIQUE-SLIP FAULT-PROPAGATION FOLDING: THE EAST KAIBAB MONOCLINE, COLORADO PLATEAU, UTAH Abstract Fault relationships along a 50-km stretch of the East Kaibab monocline in southern Utah suggest that Late Cretaceous/early Tertiary development of the structure involved a significant component of right-lateral strike-slip displacement, accommodated by basement-rooted faulting and fault-propagation folding. Evidence of oblique slip is provided mainly by pervasive map-scale and outcrop-scale faults that define a shear zone occupying the steep east-dipping limb of the monocline for at least its northernmost 50 km. Dominant fault orientations are synthetic and antithetic to the shear zone, and accommodate reverse-right-lateral and reverse-left-lateral slip, respectively. Structural style within the shear zone changes character and increases in intensity with progressively lower structural and stratigraphic levels in the fold, suggesting that the shear zone propagated upward from a basement-rooted fault during monocline formation. We conclude that horizontal, ENE-directed, Laramide compression drove reverse-right-lateral slip on the basement fault zone beneath the developing East Kaibab monocline. The resulting transpressional fault- propagation fold is marked in southern Utah by 1600 m of reverse displacement and possibly 8000 m of right lateral displacement across the shear zone and associated monoclinal flexure. 43 Background Monoclines as Drape Folds The formation of regionally significant monoclines like those on the Colorado Plateau of the western United States has most often been explained as the result of drape folding of a sedimentary rock sequence above near-vertical, normal or reverse faults in underlying basement. Early explorers of the Colorado Plateau and Grand Canyon regions described monoclinal flexures, and recognized a relationship between folding in the Paleozoic and Mesozoic sedimentary sequence and faulting at depth (Powell, 1873; Dutton, 1882; Walcott, 1890). Most Colorado Plateau monoclines exposed in the Grand Canyon (e.g. East Kaibab, West Kaibab, Hurricane, Grandview) lie above steeply-dipping basement faults; these faults formed during Precambrian time, and underwent reverse reactivation during Late Cretaceous/early Tertiary (Laramide) deformation (Walcott, 1890; Maxson, 1961; Reches, 1978; Huntoon, 1969, 1971,1974; Huntoon and Sears, 1975). Many authors have proposed that Colorado Plateau monoclines formed by drape folding, defined as the passive response of a sedimentary cover sequence to faulting in the basement beneath (Prucha and others, 1965; Sanford, 1959). Steams (1971) discussed development of drape folds in the Rocky Mountain foreland province, and extended his observations to the Colorado Plateau monoclines. In his descriptions, drape folds occur where faulting is the primary deformation mechanism in the basement, but folding dominates in the sedimentary cover, i.e., faulting in the sedimentary cover is of minor importance (Steams, 1971). The transition from fault to 44 fold is accomplished by detachments and thinning in the above-basement sedimentary sequence, particularly aided by extreme thinning of weak sedimentary rocks immediately overlying basement (Steams 1971). Reches and Johnson determined that the Palisades Creek branch of the East Kaibab monocline in the Grand Canyon region resulted from a combination of buckling and drape folding above a near-vertical fault (1978). According to Reches (1978), the mechanism of deformation of the drape-folded cover is virtually independent of the type of basement deformation (e.g. faulting, igneous intrusion, or local steepening of layers). Monoclines as Fault-Propagation Folds According to Suppe (1985), a fault-propagation fold represents deformation immediately in front of a propagating fault tip. By this broad definition, drape folds might be considered a subset of fault-propagation folds. However, implicit in fault- propagation fold models is the idea that fault-accommodated offset progressively gives way to fold-accommodated offset with higher structural and stratigraphic levels, and that with continued deformation the fault will propagate through the fold (Suppe and Medwedeff, 1984; Jamison, 1987). In drape folding, fault offset simply dies out just above basement in the sedimentary cover. Although fault-propagation fold models originally were developed to analyze ‘thin-skinned’ fold-thrust belt geometry (Suppe and Medwedeff, 1984), the term fault- propagation fold has been extended to include folding associated with basement-cored uplifts like those of the Rocky Mountain foreland of the western Unites States (e.g. 45 Erslev, 1991; Erslev and Rogers, 1993; Stone, 1993; Mitra and Mount, 1998). Stone (1984,1993) proposed that use of the term fault-propagation fold be reserved for application to areas of thin-skinned fold-thrust belt structures, and that the term thrust fold be adopted for basement-involved deformation like that seen in the Rocky Mountain foreland. Admittedly the two structural styles differ, but the term ‘thrust fold’ implies dip-slip kinematics, a connotation that might result in confusion if applied to areas of wrench faulting or oblique deformation. The broad definition of fault- propagation fold captures the simultaneous development of fault and fold for both thin- skinned and basement-cored structures without specifying slip direction. Following Erslev (1991), Erslev and Rogers (1993) and Mitra and Mount (1998), the term fault- propagation fold is used here to describe folding associated with basement-involved deformation in the study area. Colorado Plateau monoclines have not been excluded from basement-cored fault-propagation fold models, but they have not been cited as prime examples of fault- propagation folding. One reason may be that offset across Colorado Plateau structures is small compared to structural relief in the Rocky Mountain foreland, so that fault- propagation fold characteristics (if present) are less developed. Also, in Grand Canyon exposures of the monoclines basement-rooted faulting gives way to unfaulted folding very low in the Paleozoic (above-basement) section (Huntoon, 1971,1993; Reches, 1978; Reches and Johnson, 1978; Huntoon and others, 1996), a characteristic that seems to support the drape fold model. 46 It is important to note, however, that Grand Canyon exposures do not necessarily coincide with locations of greatest structural relief, or greatest offset, on the monoclines and their associated faults. For example, the East Kaibab monocline exhibits 1600 m of structural relief in southern Utah, but only 800 m of vertical relief in the Grand Canyon (Babenroth and Strahler, 1945). In a fault-propagation fold, basement-rooted faulting should extend higher into the sedimentary cover in areas of greater structural relief than in areas of lesser offset. Map relationships in southern Utah, where the East Kaibab monocline has its greatest structural relief, demonstrate that the structure developed through fault-propagation folding, not drape folding. Importance of Oblique Deformation Drape-fold and fault-propagation fold models are usually presented in vertical cross-section. This view indirectly encourages the assumption that principal stress and strain directions are exactly parallel and perpendicular to the plane of the cross section. For the sake of simplicity, oblique movement of material relative to the cross section plane is seldom considered. Such simplified constructions may produce reasonable interpretations when applied to individual structures, but can lead to confusion in interpretation of regional kinematics. For example, basement-cored uplifts tend to occupy a wide range of orientations, with no clear regional sense of vergence (e.g. Colorado Plateau monoclines, Rocky Mountain foreland uplifts and Ancestral Rockies). No single compression direction seems capable of producing reverse reactivation of structures 47 with such variable trends. Steams (1978) promoted the idea that vertical uplift, perhaps caused by a vertically-oriented greatest principal stress (at), accounted for the variable orientations and steeply-dipping basement faults associated with Colorado Plateau and Rocky Mountain uplifts. Since then, several authors have shown that the Laramide stress that drove basement reactivation and monoclinal folding on the Colorado Plateau was horizontal and compressive, not vertical (Reches, 1978; Huntoon, 1981; Anderson and Bamhard, 1986). Given a horizontal compressive stress, Chapin and Gather (1983) hypothesized two stages of Laramide deformation marked by a change in the compression direction to explain the disparate trends of the uplifts. Despite the apparent difficulty of reactivating a steeply-dipping fault with a horizontal compressive stress, most studies of Colorado Plateau uplifts imply that Laramide compression produced reverse, dip-slip motion on the basement faults (e.g. Huntoon, 1971,1981,1993; Davis, 1978; Reches, 1978; Steams, 1978). Among the exceptions are studies by Stone (1969) and Davis (1978) that pointed out that pre existing basement fractures of many orientations could be reactivated by a horizontal compressive stress. This would account for the wide range in structural trends, and would also result in oblique deformation on some structures. The possibility of basement-rooted oblique motion across Colorado Plateau monoclines has been suggested in studies by Barnes (1974,1987), Ohlman (1982) and Karlstrom and Daniel (1993), but detailed field documentation is lacking. Fault relationships discussed here not only suggest basement-rooted fault-propagation folding, but also indicate that a significant component of right-lateral slip took place during Laramide formation of the 48 East Kaibab monocline in southern Utah. This in turn opens up new possibilities for interpreting the Colorado Plateau monoclines as a system. Geologic Setting The Kaibab Uplift of northern Arizona and southern Utah is a north-south trending, asymmetrical anticline near the western margin of the Colorado Plateau. The moderately to steeply dipping east limb of the uplift, the East Kaibab monocline, meanders for approximately 180 km from near Bryce, Utah to just north of Flagstaff, Arizona (Fig. A l). The 50-km long Utah segment of the monocline, which is the subject of this paper, shows structural relief of 1600 m between the anticlinal crest of the uplift and the synclinal trough of the monocline based on structural contouring of the base of the Dakota Sandstone (Gregory and Moore, 1931) (Fig. A2). The East Kaibab monocline trends N20°E from the Arizona-Utah border to Bryce, where the monocline and the Kaibab Uplift die out; both structures plunge approximately 5° northward. The slight northward plunge of the Utah segment of the East Kaibab monocline creates an insightful perspective of the structure in map view (Fig. A3). The steep limb of the fold occupies progressively older strata when followed from north to south. Cretaceous units form the steep limb in the north near Grosvenor’s Arch, Jurassic rocks are intensely deformed at Paria Canyon, and Triassic and Permian rocks define the steep limb where Highway 89 crosses the fold. Although up to 2000 m of folded and faulted 49 112#45* 1121)0' 11115' Table \ \ Cliff x v V Plateau ^ ^ — -V. Escalante STUDY AREA ARIZONA © H ig h w a y s / Rivers e T o w n s Steep limbs of monoclines kilometers 35D O '-^— 112*45' Figure A l. Location and geologic setting of the East Kaibab monocline. The southern Utah study area is outlined, and the location of Fig. 5A (in the Grand Canyon) is shown. 50 Figure A2. Structure contour map of the northern East Kaibab monocline. Structure contours, in feet, drawn on the base of the Cretaceous Dakota Sandstone. Modified from Gregory and Moore (1931). Contour Interval = 250 feet 51 Figure A3. Simplified geologic map of the study area. Permian, Triassic, Jurassic and Cretaceous rocks are shaded differently to emphasize the slight northward plunge of the monocline. Note Groivenor's that faulting and folding in the 5 ^ L / Arch steep limb move from older stratigraphic units in the south into higher stratigraphic units 1) 0 northward. The relatively thin M Cretaceous Dakota Sandstone is A shaded black to highlight left- I N lateral separations on faults at the north end of the monocline. 1 Geographic features and structural domains discussed in the text are labeled. Pump I) Canyon 0 Spring M A 1 N 2 D 'Paria 0 Canyon M A I N 3 1) 0 M A I N 4 0 10 Proterozoic and Paleozoic sedimentary rocks lie between crystalline basement and the Kaibab Limestone, these are not exposed in the study area (Fig. A4). The timing of monocline formation is poorly constrained. At the north end of the structure, Cretaceous Wahweap and Kaiparowits Formations have been eroded from the crest of the uplift but are exposed on its flanks, where dips range from 40° near Grosvenor’s Arch to 0° in the vicinity of Table Cliff Plateau. These Late Cretaceous rocks clearly were deposited before folding. Paleocene rocks between Grosvenor’s Arch and Table Cliff are synclinally folded, probably a result of Laramide deformation as well (Sargent and Hansen, 1982). Eocene strata lie unconformably on the Late Cretaceous units at Table Cliff (Gregory and Moore, 1931; Bowers, 1972) but have been stripped from the folded edges of the Kaibab Uplift (Sargent and Hansen, 1982). The Eocene rocks may or may not have been affected by folding; their presence does not provide an upper time limit for monocline formation. Deep exposures in the Grand Canyon reveal that a steeply west-dipping (70°) basement fault zone underlies the East Kaibab monocline. Grand Canyon outcrops provide clear evidence that the basement structure originally formed as a normal fault in Precambrian time (Walcott, 1890; Maxson, 1961; Huntoon, 1969, 1993; Huntoon and Sears, 1975) but that the only Phanerozoic deformation on the fault resulted from Laramide compression (Fig. A5). This episode produced reverse separation across the fault at the level of the Proterozoic/Phanerozoic unconformity in the Grand Canyon and formed the broad, asymmetrical Kaibab Uplift in the Paleozoic and Mesozoic cover 53 FORMATION TH (m) LITHOLOGY Q Surficial deposits 0-200 d - X Id Claron Formation 350-600 TKc fresh-water limestone H interbedded sandstone Kaiparowits Fm. 350-1000 Kk £ and siltstone \ interbedded sandstone 300-500 Kw £Wahweap Formation / and siltstone U StraightCliffs \ interbedded sandstone 400-600 Kk g Formation V and siltstone ; Tropic Shale 200-400 Kt coal-bearing shale Dakota Formation 30-100 t e d (sandstone, conglomerate poorly-consolidated Entrada Sandstone 150-270 sand and silt u Carmel Formation 150-300 q shale, gypsum, limestone aeolian sandstone Page Ss Member 5-50 (Carmel Fm member) < B6 Ja 3 Navajo Sandstone 350-650 aeolian sandstone “i J Kaventa Formation 40-150 Jit » sandstone, siltstone u Moenave Formation 25-150 Trm* J sandstone, siltstone Chinle Formation 175-250 Trt shale, sandstone, 8 V/V/v a conglomerate Moenkopi Formation 350-550 shale, siltstone, limestone i % gypsum z Kaibab Limestone 40-100 W limestone, dolomite < Iti g Toroweap Formation 40-150 Pi anhydrite a< Coconino Sandstone 80-150 Pc sandstone ti Hermit Shale 100 Ph shale ■vxzv Supai Group 180-400 shale, siltstone, sandstone (A Surprise Canvon Fm 0-130 Mk siltstone, sandstone i Redwall Limestone 100-250 Mr cherty limestone DEV Temple Butte Ls 0-100 uliT limestone 5BB Muav Limestone 100-300 Cm limestone c Bright Angel Shale 75-200 < b i shale u Tapeats Sandstone 0-100 sandstone pf Grand Canvon Series 275+ pt Proterozoic sediments Figure A4. Generalized stratigraphic column for the East Kaibab monocline in southern Utah. Shaded units are used as markers to highlight structural relationships on maps of Domains 1 through 4. Stratigraphy compiled from Hintze (1988). sw NE 2500 — ■— 2500 2000— —2000 1500 p - 1 5 0 0 —KT 1000- e 1000 500 — - 5 0 0 0 —0 Figure A5. The East Kaibab monocline and underlying Butte fault in the Grand Canyon. Lower Proterozoic and Cambrian rocks are shaded to emphasize the apparent normal offset at the level of Precambrian sedimentary rocks, and reverse separation at the Proterozoic-Phanerozoic unconformity. Location of the cross section is shown in Fig. A I. After Huntoon et al. (1996). LA4^ 55 (Huntoon and Sears, 1975; Huntoon, 1993). Although the Grand Canyon provides the only exposure of the basement fault underlying the East Kaibab monocline, the fault (or a network of similar faults) is assumed to underlie the fold for its entire length (Davis, 1978; Stem, 1992). Structural Data and Observations Examination of the northern 50 km of the East Kaibab monocline has revealed a continuous, N20°E-trending, monocline-parallel zone of intense deformation expressed at map scale by abundant, systematic faulting within the steep limb. Map-scale and outcrop-scale structures in the deformed zone indicate a significant component of reverse-right-lateral offset. When followed south from Grosvenor’s Arch to the Arizona-Utah border, this narrow zone of faulting ‘steps’ progressively southwestward and stratigraphically downward through Cretaceous, Jurassic, and Triassic strata. Structural style within the zone changes from north to south as well, allowing subdivision of the Utah portion of the East Kaibab monocline into four domains based on style of deformation and stratigraphic interval (Fig. A3). The fault pattern in each domain and in the transitions between domains provides evidence for oblique-slip fault- propagation folding, as discussed in the following sections. Domain 1 Structural Domain 1 begins near Grosvenor’s Arch in Grand Staircase-Escalante National Monument, and extends about 15 km toward S20°W to Pump Canyon Spring 56 Figure A6. Geology of Structural Domain 1. Short, northwest-striking, northeast dipping faults offset Cretaceous Dakota Sandstone (dark shading) in an apparent left- lateral fashion. At the northern and southern boundaries, northeast-striking, northwest dipping faults accommodate reverse, right-lateral offset. East of (and stratigraphically above) the northeast-striking faults at the north and south ends, the right-lateral offset results in broad, z-shaped bends in the contact between Cretaceous Wahweap and Kaiparowits Formations. Equal-area plots summarize structural data: (a) Plot of poles to planes. Poles to faults are shown in black; poles to outcrop-scale slip surfaces are shown in grey. (b) Kamb contour plot of poles to faults and slip surfaces. Shades represent 2a contour intervals. White areas indicate fewer poles at contouring grid points than would be found in a uniform distribution minus la; light grey shading indicates grid points with number of poles within + la of that found in a uniform distribution; slightly darker grey shading indicates grid points with numbers of poles l-3 a more than in a uniform distribution, etc. (c) Slickenline orientations. (d) Kamb contour plot of slickenlines, emphasizing their low plunge and southeast trend. 57 Figure A6. 58 (Fig. A6). A 10 km-long, monocline-parallel zone of short, closely spaced, northwest- striking faults occupies the stratigraphic interval of Jurassic Page Sandstone through Cretaceous Tropic Shale. Strata within the zone are offset by meters to tens of meters in apparent left-lateral fashion and are rotated clockwise by the northwest-striking faults. More than 75 of these northwest-striking, northeast-dipping faults are visible in Domain 1 at 1:12,000 scale. Trace lengths of the largest faults are on the order of 0.5 to 1 km. The faulting is pervasive in outcrop as well, with sub-map-scale faults evident on the graded surface of the dirt road, where they offset steeply east-dipping, thin-bedded shales and evaporites in the Carmel Formation (Fig. A7). Average strike and dip of map-scale and outcrop-scale faults are N50°W, 58°NE with slickenlines (found on fault surfaces preserved in Dakota Formation and Page Sandstone member of Carmel Formation) that rake 20°SE. Fault and slickenline orientations suggest that at least the latest slip along these short faults was left-lateral with a small reverse component. To the north, near Grosvenor’s Arch, the monocline-parallel zone of northwest- striking faults ends abruptly at two northeast-striking faults, each with a trace length of about 3 km. These faults accommodate apparent right-lateral separation of Jurassic Carmel through Cretaceous Wahweap Formations. Strike-parallel offset of Cretaceous Dakota Sandstone is on the order of 1 km across each fault, but appears to decrease to the northeast (stratigraphically upward) into the Straight Cliffs and Wahweap Formations. Where preserved, the fault surfaces strike N65°E and dip 65°NW. Slickenlines rake 15o-20oSW, disclosing at least a late-stage episode of reverse, right- 59 Figure A7. Southwest-directed photograph of the graded road surface in Domain 1 (outcrop location is circled on Fig. A6. Northwest-striking faults offset northeast- striking, east-dipping shales and evaporites of the Carmel Formation. Geologist is Pilar Garcia. 60 lateral displacement. These northeast-striking faults occupy a higher stratigraphic interval than do the northwest-striking faults between Grosvenor’s Arch and Pump Canyon Spring. Northeast of the faults themselves, in Cretaceous Wahweap and Kaiparowits Formations, lateral displacement is accommodated by a broad, z-shaped folding of the trend of the monocline, suggestive of right-handed shear (see contact between Kw and Kk, Fig. A6). Faults at the southern termination of Domain 1 are similar to the northeast- striking faults at the northern end, but occupy a lower stratigraphic interval. Near Pump Canyon Spring, the zone of northwest-striking faults ends abruptly near a northeast- striking fault in Page Sandstone. Its polished surface strikes N55°E, dips 60°W, and displays grooves raking 20o-30°SW. The geometry again indicates reverse, right-lateral slip. This outcrop marks the north end of a lineation traceable on topographic maps and air photos for at least 4 km toward S40°W into gently dipping Navajo Sandstone. The fault-controlled lineation and preserved fault surface occupy upper Navajo and Page Sandstones, and Jurassic Carmel through Cretaceous Wahweap Formations immediately to the east form another broad, z-shaped bend in the trace of the monocline. As a whole, the map- and outcrop-scale faulting in Domain 1 defines a narrow, monocline-parallel zone of intense deformation that constitutes a shear zone. From the north to the south end of Domain 1 the shear zone occupies progressively lower stratigraphic intervals within steeply east-dipping beds. Fault and slickenline orientations at the north and south ends of Domain 1 indicate a large ratio (up to 5:1) of 61 right-lateral strike-slip to dip-slip offset across the long, northeast-striking, west-dipping faults; the northwest-striking faults between Grosvenor’s Arch and Pump Canyon Spring also record a 5:1 ratio of left-slip to reverse-slip. Although slickenlines typically preserve only the latest slip vector on a fault surface, the observed orientations along this 15 km stretch of the monocline are consistent with interpretation as synthetic (NE- striking) and antithetic (NW-striking) conjugates in a zone of oblique (i.e. reverse right- lateral) displacement. Domain 2 Structural Domain 2 begins immediately south of the northeast-striking fault surface at the southern end of Domain 1 (Fig. A8). This 12 km-long interval is marked by several map-scale northeast-striking faults in lower Carmel Formation, Page Sandstone, and upper Navajo Sandstone; the deformation has moved again to a slightly lower stratigraphic interval. Map-scale faults in Domain 2 have trace lengths on the order of 1 km. Map view reveals apparent right-lateral offset on the order of tens to a few hundred meters, and where canyons incise the faults their reverse separation is evident. Average orientation of the northeast-striking faults in Domain 2 is N41°E, 46°NW, again with slickenlines raking about 30°SW. These faults are similar in orientation to those found at the north end of Domain 1 and between Domains 1 and 2, but with shorter trace lengths and offset on the order of only a few meters to tens of meters. 62 Figure A8. Geology of Domain 2. Northeast-striking, northwest-dipping faults offset the Page Sandstone (light shading) in reverse-right-lateral fashion. Dakota Sandstone, intensely faulted in Domain 1, is unaffected by faulting in Domain 2. Representative orientations of slip surfaces and deformation bands depict structural features too small to show a map scale. (a) Equal-area plot of poles to faults (black) and slip surfaces (grey). (b) Kamb contour plot of poles illustrates tight clustering of northeast-striking, northwest-dipping fault and slip surface orientations. (c) Equal area plot of slickenline orientations. (d) Kamb contour plot of slickenline orientations. Slickenlines plunge gently toward the southwest, disclosing reverse-right-lateral slip on northeast-striking, west-dipping faults. ■ I 63 N=53 A 1 km Representative orientations of slip surfaces and deformation bands Map-scale faults Down-thrown * side of fault Figure A8. 64 At outcrop scale, the Page and Navajo Sandstones in Domain 2 are intensely fractured. Minor fault surfaces (slip surfaces) and deformation bands show two primary orientations: a prominent northeast-striking, northwest-dipping set and a secondary northwest-striking, northeast-dipping set (Fig. A8). Deformation in Domain 2 is still consistent with interpretation as a reverse, right-lateral shear zone, but long, en echelon synthetic faults rather than short, closely-spaced antithetic faults dominate Domain 2. Domain 3 Domain 3 begins at Paria Canyon, and is distinguished by evidence for continuous, through-going faulting in the Navajo Sandstone and Kayenta Formation (Fig. A9). The mouth of Paria Canyon exposes a northeast-striking, northwest-dipping fault surface similar to the one at the boundary between Domains 1 and 2, again with slickenlines and grooves that rake 30°SW. The cross-sectional view at the mouth of the canyon reveals Navajo and Page Sandstones in the hanging wall above reverse drag- folded Carmel Formation redbeds in the footwall (Fig. A 10). Jurassic Entrada Sandstone through Cretaceous Wahweap Formation east of the fault surface (up- section) again form a broad, z-shaped fold in map view. The fault surface exposed at Paria Canyon marks the north end of a series of linear valleys that trend S20°W across N10°E-striking, steeply east-dipping Navajo Sandstone. Evidence for through-going faulting is found in the valleys as fault gouge and breccia, intensely fractured Navajo Sandstone, and several exposures of northeast-striking, steeply west-dipping polished fault surfaces. Because the strike of the fault zone nearly parallels the strike of bedding 65 Figure A9. Geology of Domain 3. Linear valleys, gouge, breccia, and exposures of polished fault surfaces reveal long, continuous faulting in Navajo and Kayenta Formations. Slip surfaces and deformation bands occupy both northeast-striking and northwest-striking orientations. (a) Equal-area plot of poles to faults (black) and slip surfaces (grey). (b) Kamb contour plot of poles to faults and slip surfaces, showing a primary set of northeast-striking, northwest-dipping surfaces and a secondary set of northwest- striking, northeast-dipping surfaces. (c) Equal-area plot of slickenline orientations. Kamb contour plot of slickenline orientations. Southwest-plunging slickenlines lie on northeast-striking faults, and southeast-plunging slickenlines lie on northwest-striking faults. 66 N=106 Map-scale faults Down-thrown side of fault Representative orientations of slip surfaces and deformation bands Figure A9. 67 Figure A 10. North-directed photograph of the northeast-striking, northwest-dipping fault surface at the mouth of Paha Canyon. Page Sandstone member of the Carmel Formation on the hanging wall lies in fault contact above stratigraphically higher Carmel Formation redbeds in the footwall. Stratigraphic relationship and drag folding in Carmel redbeds indicate a reverse component of faulting. Slickenlines on the fault surface (not visible) rake 20o-30°SW, disclosing a significant right-lateral component of slip. Geologist is Bill Abbey. 68 in the Navajo, the zone of deformation crosses the Navajo Sandstone at a very low angle; the steeply west-dipping fault requires 8 km of strike length to cross the (approximately) 400 m thick, east-dipping sandstone. As a result of this geometry, a large amount of right lateral displacement across the fault zone theoretically is possible without causing a noticeable disruption of the surface trace of the Navajo Sandstone. Map-scale fault surfaces measured in the Navajo in Domain 3 yield an average orientation of N36°E, 59°NW, with slickenlines raking 30°SW. At the south end of Domain 3 the fault zone offsets Triassic/Jurassic Kayenta and Triassic Moenave Formations in apparent right-lateral fashion on the east side of Fivemile Valley before it disappears beneath alluvium and colluvium on the valley floor. Although at map scale northeast-striking (synthetic) faults are prevalent in Domain 3, a few short northwest-striking, northeast-dipping faults similar to those found in Domain 1 offset the Navajo Sandstone east of the through-going synthetic faults. These antithetic faults accommodate meters to tens of meters of reverse, left- lateral offset within the Navajo Sandstone. Oblique slip is expressed by slickenlines that rake 18°E. Outcrop-scale deformation bands and slip surfaces also show a bimodal distribution of synthetic and antithetic orientations (Fig. A9). Deformation in Domain 3 is concentrated in the Navajo Sandstone, with some fracturing and deformation bands affecting the Page Sandstone; however, no map-scale faults offset the Page south of Paria Canyon. The most intense deformation has again moved strati graphically down- section, from the interval of upper Navajo/Page/Carmel in Domain 2 into Kayenta/Navajo within Domain 3. 69 Domain 4 In Domain 4 the shear zone moves southward and down section into Triassic Chinle and Moenkopi Formations in Fivemile Valley (Fig. A ll). These shaley Triassic units are sandwiched between resistant Permian Kaibab Limestone on the west side of the valley, dipping 25° to 35° east, and a ridge of 650-85° east-dipping Moenave, Kayenta, and Navajo Sandstones on the east side. Most evidence for the continuation of the shear zone is hidden beneath alluvium and colluvium on the valley floor, but a few key outcrops allow it to be traced southward almost to the Utah-Arizona border. Location 1 is a northeast-striking, steeply west-dipping, remarkably planar slope of Navajo, Kayenta, and Moenave Formations along strike with the linear valleys described in Domain 3. Near the base of the slope, a sliver of Triassic Moenkopi Formation shale several tens of meters long rests against Triassic Moenave sandstone; the Triassic Chinle Formation, which should separate the two, is missing. This older- on-younger relationship could be produced by faulting with a reverse component of offset. The exposure of interest at Location 2 follows a drainage that provides a transect into the ridge of Moenave, Kayenta, and Navajo Formations on the east side of Fivemile Valley (Fig. A12). Near the mouth of the wash, several outcrops of overturned Moenave Formation beds are visible, striking N15°E and dipping 52°NW. Toward the east along the wash, dips gradually steepen to vertical over the course of tens of meters in Moenave and Kayenta Formations. Within 200 m of the overturned outcrops at the mouth of the wash bedding is upright, striking N10°E and dipping 65°SE. The attitudes 70 Map-scalc faults, / dashed where inferred N— Down-thrown side ™ of faults Representative orientations of slip surfaces and deformation bands Outcrops discussed in text N = 32 N=23 Figure All. Geology of Domain 4. Through- going faulting is inferred based on four key outcrops described in the text (locations circled). Intense deformation is obscured by alluvium in the valley, (a) Equal area plot and (b) Kamb contour plot of poles to slip surfaces in Navajo, Kayenta and Moenave Formations. (c) Equal area and (b) Kamb contour plot of slickenline orientations. 71 W E 0.5 km Trmv Figure A 12. Cross sectional sketch based on outcrops visible in the canyon at Location 2 (above dashed line) and inferred subsurface structure (below dashed line). The inferred west-dipping fault with reverse separation accounts for overturned bedding at Location 2 and the apparent absence of Chinle Formation in this part of Domain 4. The sliver of Chinle Formation shown in the sketch just below the surface is a reverse- (and right-lateral?) fault-bounded block caught in the shear zone, representing the relationship shown at Location 3. describe an overturned syncline that may be the result of drag folding of beds immediately in the footwall of the shear zone assumed to lie beneath alluvium on the valley floor. At Location 3, an isolated hill of northwest-striking, steeply east-dipping sandstone and conglomerate of the Chinle Formation (Shinarump Member) protrudes from the valley floor. Triassic Moenave sandstone and shale on the east side of the valley, only a few tens of meters away, strike northeast. Triassic Moenkopi and Permian Kaibab Formations on the west side of the valley also strike northeast. A northeast-striking, near-vertical fault surface with southwest-raking slickenlines is preserved in the isolated Shinarump sandstone block. The outcrop is likely a sliver of Chinle Formation caught in the fault zone, which itself is obscured on the valley floor. Evidence for faulting at Location 4 is similar to that at Location 3. A wedge of distinctively-striped Moenkopi shale striking northwest is truncated at its southern edge by a ridge of Kayenta Formation striking northeast; Chinle Formation is absent between the two. Like the Chinle ridge at Location 2, the wedge of strangely oriented Moenkopi Formation here may be a sliver of material caught in a reverse fault zone. The fault contact between the two units is evident and the missing stratigraphic section discloses at least a reverse component of offset; a right-lateral component also may be present. These outcrops make it possible to track the presence of the shear zone almost to the Utah-Arizona border, south of which exposure is completely obscured by alluvium. 73 Summary of Field Observations In southern Utah the steep, east-dipping limb of the East Kaibab monocline hosts a narrow zone of intense deformation marked by pervasive map-scale and outcrop-scale faulting. This ‘shear zone' is seen to move progressively down-section through steep, east-dipping Mesozoic strata from north to south, and the character of deformation changes with each new stratigraphic interval affected (Fig. A 13). At the northern termination of Domain 1, northeast-striking, steeply west-dipping faults offset Jurassic Carmel through Cretaceous Straight Cliffs Formations in reverse-right-lateral fashion. Slickenlines on fault surfaces rake 15o-20°SW. Cretaceous Wahweap and Kaiparowits Formations east of (and stratigraphically above) these faults are bent in map view into a broad, z-shaped fold. Domain 1 deformation occupies a slightly lower stratigraphic interval: Jurassic Page Sandstone through Cretaceous Tropic Shale. Northwest-striking, northeast dipping faults with 20°SE-raking slickenlines accommodate reverse-left-lateral offset and clockwise rotation of intervening strata. Faults lie in a right-stepping en echelon pattern and define a narrow deformation zone that trends N20°E, parallel to the trend of the monocline. Between Domains 1 and 2 another long, northeast-striking fault lies just west of (and stratigraphically below) a broad, z-shaped bend in steeply east-dipping Jurassic and Cretaceous strata. Southward, deformation in Domain 2 affects the upper Navajo Sandstone, Page Sandstone, and Carmel Formation redbeds. Map- and outcrop-scale, northeast-striking, steeply west-dipping faults accommodate reverse-right-lateral 74 Figure A13. Summary of structural and stratigraphic evidence for an oblique shear zone on the steep limb of the East Kaibab monocline. Progressively higher stratigraphic intervals are affected by intense deformation from south to north, and structural style changes from continuous, through-going faulting in the south to disjointed but pervasive fractures northward. Along the entire shear zone, northeast-striking, northwest-dipping synthetic faults accommodate reverse-right-lateral slip, and northwest-striking, northeast-dipping antithetic faults accommodate reverse-left-lateral slip. The progression in structural style and stratigraphic level combined with consistent slip indicators suggests transpressive fault-propagation folding. 75 Figure A 13. DOMAIN 1 Kt Pervasive NW-striking, Kd NE-dipping faults Slickenlines rake SE Je Small reverse, left- Jc lateral offsets Jcp DOMAIN 7 NE-striking, NW-dipping faults dominate Slickenlines rake SW Reverse, right-lateral linei=36 offsets planci-53 DOMAIN 1 - Continuous, NE-striking, NW-dipping fault(s) - A few NW-striking, NE- dipping faults DOMAIN 4 f r -Shear zone obscured Faults mv by alluvium V -Isolated outcrops A Slip reveal a NE- [Tr Surfaces mo striking fault zone * Slickenlines liner plane »=32 5 km 76 displacement of intervening strata. Slickenline orientations on large fault surfaces average 30°SW, implying a 3:1 ratio of strike-slip to dip-slip on the northeast-striking faults. Domain 2 faults are left-stepping and slightly oblique to the trend of the monocline, but again define a N20°E-trending, monocline-parallel zone of deformation. Beyond yet another prominent northeast-striking fault surface and z-shaped bend at the southern end of Domain 2, Domains 3 and 4 display evidence for reverse- right-lateral displacement on a single northeast-striking, west-dipping fault or series of long, continuous relay faults. In Domain 3 intense deformation is concentrated in the Navajo Sandstone and Kayenta Formation. Major fault surfaces have an average strike and dip of N36°E, 59°NW with southwest-raking slickenlines. Evidence for continuous, through-going faulting continues to the south in Domain 4, moving down- section into Triassic Moenave, Chinle, and Moenkopi Formations. Outcrops in these valley-forming shales are scarce, but several key exposures reveal the presence of a northeast-striking fault with at least a reverse component of separation. Discussion The continuous, narrow zone of deformation described above is interpreted as a brittle to semi-brittle shear zone occupying the steep limb of the East Kaibab monocline. Northeast-striking faults are synthetic to an overall reverse-right-lateral sense of shear, and northwest-striking faults are antithetic to the shear zone. Orientation and sense of offset on map-scale and outcrop-scale structures are consistent with a reverse-right-lateral sense of shear for at least the northernmost 50 km of the monocline. 77 Although slickenline orientations typically record the slip vector of only the latest episode of movement on a fault, the close agreement o f fault attitudes and slickenline orientations observed at map and outcrop scale over a full 50 km distance strengthens the argument that a right-lateral component of slip operated throughout shear zone development. Fault-slip data were used to calculate the orientations of shortening and extension axes using the method described by Marrett and Allmendinger (1990). East Kaibab monocline fault-slip data included strike and dip of faults and slip surfaces, rake of slickenlines (representing the slip vector), and sense of slip. Shortening and extension axes were calculated for each of 168 faults for which all of the above information was known. Orientations and Kamb contour plots of the axes are shown in Figure A14. The average shortening axis trends 271.2° and plunges 3.4°, and the average extension axis plunges 47.3° toward 177.0°. The near-horizontal, east-west orientation of the shortening axis is consistent with Laramide, ENE-directed, compressive stress determined by Reches (1978) and Anderson and Bamhard (1986). Attitudes of synthetic and antithetic faults within the shear zone were used to determine the orientation of the finite strain ellipsoid. Three assumptions were made concerning fault geometry: first, that the line of intersection of synthetic and antithetic faults is the intermediate stretch axis (S2) of the ellipsoid; second, that the S2-S3 plane bisects the acute angle between the fault sets; and third, that faults did not rotate considerably during progressive deformation. Based on these assumptions, orientations of the principal axes of the finite strain ellipsoid in the deformed zone are (trend, 78 Shortening Axes N = 168 Average trend Kamb contour and plunge of plot shortening axes: 271.2,° 3.4° Extension Axes N = 168 Average trend Kamb contour and plunge of plot extension axes: 177.0° 47.3' Figure A 14. Equal-area and Kamb contour plots of shortening (S 3 ) and extension (Si) axes calculated for 168 faults and slip surfaces using the kinematic analysis described by Marrett and Allmendinger (1990). The average shortening axis is consistent with ENE-WSW horizontal compression, and the orientation of the extension axis indicates reverse-right-lateral slip given a N20°E-trending shear zone (the trend of the East Kaibab monocline). 79 plunge): 171,41 (Si); 261, 1 (S3); and 350,48 (S2) (Fig. A15). These values are remarkably similar to the shortening (S3) and extension (Si) directions found using the Marrett and Allmendinger method. The geometric solution also yields a minimum stretch (maximum shortening) axis that is horizontal with an ENE trend, generally parallel to the direction of Laramide contraction. Although the relative magnitudes of the stretch axes have not been determined, the orientation of Si implies reverse-right- lateral offset across the zone. The sense of offset indicated by the strain ellipsoid is consistent with the sense of offset demonstrated by slickenline orientations observed on fault surfaces, which themselves imply a ratio of up to 5:1 of right-lateral slip to reverse slip across the shear zone. Despite the similarity in fault and slickenline orientations along the northern 50 km of the East Kaibab monocline, deformation mechanisms are partitioned from one domain to the next. At map scale, en echelon antithetic faults are favored in Domain 1, en echelon synthetic faults dominate Domain 2, and through-going faulting is preferred in Domains 3 and 4. The reasons for the changes in style are unclear. Different structures may result from different mechanical responses of the stratigraphic intervals involved, since both structural style and stratigraphic interval change from north to south. It is also possible that the changes are related to structural position within the fold. The slight northward plunge of the monocline creates an extremely elongated down-plunge view of deformation, such that each step toward the southwest exposes a deeper structural level, closer to the basement fault. Considered in this way, it is relevant that evidence for through-going faulting is present in the structurally lower 80 SYNTHETIC FAULTS ANTITHETIC FAULTS A A A' N10W S10E Orientation of S| = 171°, 41° 5 2 = 350°, 48° 5 3 = 261°, 1° Figure A15. (a) Lower hemisphere equal-area projection showing the orientations of maximum, minimum and intermediate stretch axes (Si, S3 and S2, respectively) and principal planes for a finite strain ellipsoid in the shear zone. Axes were calculated from average orientations of synthetic and antithetic faults, (b) Map-view projection of a strain ellipsoid with the calculated Si and S3 orientations within a N20°E-trending shear zone. Map traces of synthetic and antithetic faults are shown. Relative lengths of Si and S3 axes have not been calculated, but the orientation of the horizontal strain ellipse shows right-handed shear, (c) Cross section view of the strain ellipsoid along A- A ’, parallel to the S1-S2 plane. Orientation of the Si axis with respect to the shear zone displays reverse, right-lateral shear. 81 southern part of the study area but gives way to more distributed deformation toward the north at higher structural levels. The progression from continuous faulting at depth to distributed fracturing at shallower levels is consistent with fault-propagation folding (Suppe and Medwedeff, 1984; Suppe, 1985; Jamison, 1987; Erslev, 1991). In the case of the East Kaibab monocline, basement-rooted faulting has propagated upward through Paleozoic and Mesozoic strata to the level of the Navajo Sandstone. En echelon faults in Domains 1 and 2 may represent fractures immediately ahead of the propagating fault tip that would have joined and extended the basement-rooted fault if deformation had continued. In the down-plunge perspective provided by the map, these fractures are exposed over a distance of 25-30 km, whereas in vertical cross-section they would occupy a stratigraphic thickness of less than 1500 m, possibly making them difficult to recognize and measure. The down-plunge view of the monocline and shear zone exposed in southern Utah invites interpretation of the East Kaibab monocline as a basement-rooted fault- propagation fold. Grand Canyon exposures show that the fold form of the monocline widens upward from basement, consistent with trishear fault-propagation fold models (Erslev, 1991). However, the brittle shear zone exposed along the steep limb in southern Utah remains narrow as it propagates up-section through the steep limb of the fold. The shear zone represents a frozen moment in the progressive development of fault and fold: it preserves intense deformation that formed directly ahead of the fault tip as the propagating fault overtook the developing fold. The right-lateral component of slip in the shear zone is likely tied to Laramide right-lateral displacement on the 82 underlying basement fault. Thus the origin of the East Kaibab monocline should be considered in the context of transpressional fault-propagation folding rather than reverse-slip drape folding. The regional tectonic implications of these findings are significant. Literature on Colorado Plateau monoclines has commonly emphasized the role o f reverse-slip reactivation of Precambrian fault zones (e.g., Huntoon and Sears, 1975; Davis, 1978; Huntoon, 1993). However, as seen in cross-section, a horizontal compressive stress acting perpendicular to a near-vertical fault results in a high magnitude of normal stress on the fault plane, making reverse reactivation difficult to achieve. This limitation largely disappears when the perspective of viewing changes from cross-sectional to map view (Fig. A 16). A northeasterly-directed horizontal compressive stress acting on a N20°E-striking, steeply west-dipping Precambrian fault is suited ideally to reactivate the fault in right-handed strike-slip fashion, with a component of reverse motion resulting from the steep westward dip of the fault. This is what we believe has occurred along at least the northern 50 km of the East Kaibab monocline, and possibly across other basement-cored uplifts with structural trends oblique to the regional shortening direction. 83 Cross-Sectional Trace of —• Basement Fault Map-View Trace of z Basement fault Figure A 16. The apparent difficulty o f reactivating a steeply dipping basement fault with Laramide horizontal compressive stress as seen in cross section (a) largely disappears when the perspective changes to map view (b). An ENE-oriented horizontal compressive stress is ideally directed to cause right-lateral reactivation of a N10o-20°E striking, steeply dipping basement fault such as the one underlying the East Kaibab monocline. 84 Conclusions A long, narrow zone of concentrated map-scale and outcrop-scale faulting defines a brittle to semi-brittle shear zone on the steep limb of the East Kaibab monocline. The character of deformation in the shear zone varies from south to north: through-going faulting offsets older strata at the south end of the study area, and more distributed, discontinuous deformation affects progressively younger strata to the north. A down-plunge view of the northern 50 km of the north-plunging monocline resembles a fault-propagation fold in which the discrete fault rupture has propagated through Triassic strata into Jurassic Navajo Sandstone. Intense deformation directly ahead of the fault tip is seen in stratigraphically higher Jurassic and Cretaceous strata. The orientations of fault surfaces exposed in the southern part of the study area closely parallel the orientation of the underlying basement fault exposed in the Grand Canyon, leading to the assumption that the shear zone roots into the basement fault. Orientations of faults and slickenlines within the shear zone record at least a late-stage episode of reverse-right-lateral slip. Northeast-striking and northwest- striking faults are interpreted as synthetic and antithetic, respectively, to a N20°E- striking, steeply west-dipping shear zone, parallel to both the monocline and the Grand Canyon exposure of the underlying basement fault. Inversion of fault and slickenline data yields a finite strain ellipsoid with an orientation consistent with reverse-right- lateral slip. The maximum shortening axis of the ellipsoid coincides with the northeast- directed horizontal compressive stress determined for Laramide deformation on the Colorado Plateau. 85 Oblique displacement in the shear zone involved a ratio of up to 5:1, strike-slip to dip-slip. If this ratio characterizes the slip vector throughout formation of the monocline (during initial folding and late-stage faulting in Mesozoic strata), the observed structural relief of 1600 m would correspond to a right-lateral offset of up to 8000 m between the structural crest of the Kaibab Uplift and the adjacent Kaiparowits Basin. Acknowledgments We acknowledge the valuable input received from Charles F. Kluth and William G. Higgs, both of Chevron, Inc., with whom we discussed map relationships early in the project. Conversations with the structural geologists at Mobil Oil in Dallas, TX were instrumental in clarifying many of the concepts presented here. We gratefully acknowledge the field assistance and input provided by Seth Gering, Shari Christofferson, Danielle Vanderhorst, Pilar Garcia, William Abbey, and Jessica Greybill. Thanks to Karl Karlstrom, Ken McClay, Richard Allmendinger, Don Fisher, and Scott Wilkerson for valuable input on early versions of the manuscript. Field work was supported through funding by the National Science Foundation, namely through NSF#EAR-9406208. 86 References Anderson, R.E. and Bamhard, T.P. (1986) Genetic relationship between faults and folds and determination of Laramide and neotectonic paleostress, western Colorado Plateau-Transition Zone, central Utah. Tectonics 5, 335-357. Babenroth, D.L. and Strahler, A.N. (1945) Geomorphology and structure of the East Kaibab monocline, Arizona and Utah. Geological Society of America Bulletin 56,107-150. Barnes, C.W. (1974) Interference and gravity tectonics in the Gray Mountain area, Arizona. In Geology of Northern Arizona with Notes on Archaeology and Paleoclimate — Part 2, Area Studies and Field Studies, eds T.N.V. Karlstrom, G. A. Swann and R.L. Eastwood, pp. 442-453. Geological Society of America Rocky Mountain Section. Barnes, C.W. (1987) Geology of the Gray Mountain area, Arizona. In Geological Society of America Centennial Field Guide 6, ed S.S. Beus, pp. 379-384. Bowers, W.E. (1972) The Canaan Peak, Pine Hollow and Wasatch formations in the Table Cliffs region, Garfield County, Utah. United States Geological Survey Bulletin 1331-B. Chapin, C.E. and Gather, S.M. (1983) Eocene tectonics and sedimentation in the Colorado Plateau - Rocky Mountain area. In Rocky Mountain Foreland Basins and Uplifts, eds J.D. Lowell and R.Gries, p. 33-56. Rocky Mountain Association of Geologists. Davis, G.H. (1978) Monocline fold pattern of the Colorado Plateau. In Laramide folding associated with basement block faulting in the western United States, ed V. Matthews III, pp. 15-233. Geological Society of America Memoir 151. Dutton, C.E. (1882) Tertiary history of the Grand Canyon district. United States Geological Survey Monograph 2. Erslev, E.A. (1991) Trishear fault-propagation folding. Geology 19, 617-620. Erslev, E.A. and Rogers, J.L. (1993) Basement-cover geometry of Laramide fault- propagation folds. In Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States, eds C.J. Schmidt, R.B. Chase and E.A. Erslev, pp. 125-146. Geological Society of America Special Paper 280. 87 Gregory, H.E. and Moore, R.C. (1931) The Kaiparowits region: a geographic and geological reconnaissance of parts of Utah and Arizona. United States Geological Survey Professional Paper 164. Hintze, L.F. (1988) Geologic History of Utah. Brigham Young University Geology Studies Special Publication 1. Huntoon, P.W. (1969) Recurrent movements and contrary bending along the West Kaibab fault zone. Plateau 42,66-74. Huntoon, P.W. (1971) The deep structure of the monoclines in eastern Grand Canyon, Arizona. Plateau 43, 148-158. Huntoon, P.W. (1974) Synopsis of Laramide and post-Laramide structural geology of the eastern Grand Canyon, Arizona. In Geology of Northern Arizona, Part 1 - Regional Studies, eds R.L. Eastwood, T.N.V. Karlstrom and G.A. Swann, pp. 317-335. Northern Arizona University, Flagstaff, Arizona. Huntoon, P.W. (1981) Grand Canyon monoclines; vertical uplift or horizontal compression? In Rocky Mountain Foreland Basement Tectonics, eds D.W. Boyd and J.A. Lillegraven, pp. 127-134. University of Wyoming Contributions to Geology 19. Huntoon, P.W. (1993) Influence of inherited Precambrian basement structure on the localization and form of Laramide monoclines, Grand Canyon, Arizona. In Laramide basement deformation in the Rocky Mountain foreland of the western United States, eds C.J. Schmidt, R.B. Chase and E.A. Erslev, pp. 243-256. Geological Society of America Special Paper 280. Huntoon, P.W., Billingsley, G.H., Sears, J.W., Eg, B.R., Karlstrom, K.E., Wiliams, M. L., Hawkins, D., Breed, W.J., Ford, T.D., Clark, M.D., Babcock, R.S. and Brown, E.H. (1996) Geologic map of the eastern part of the Grand Canyon National Park, Arizona. Grand Canyon Association. Huntoon, P.W. and Sears, J.W. (1975) Bright Angel and Eminence Faults, eastern Grand Canyon, Arizona. Geological Society of America Bulletin 86,465-472. Jamison, W.R. (1987) Geometric analysis of fold development in overthrust terranes. Journal of Structural Geology 9,207-220. Karlstrom, K.E. and Daniel, C.G. (1993) Restoration of Laramide right-lateral strike slip in northern New Mexico by using Proterozoic piercing points: tectonic implications from the Proterozoic to the Cenozoic. Geology 21, 1139-1142. 88 Marrett, R. and Allmendinger, R.W. (1990) Kinematic analysis of fault-slip data. Journal of Structural Geology 12, 973-986. Maxson, J.H. (1961) Geologic map of the Bright Angel quadrangle, Grand Canyon National Park, Arizona. Grand Canyon Natural History Association. Mitra, S., and Mount, V.S. (1998) Foreland basement-involved structures. AAPG Bulletin 82,70-109. Ohlman, J.R. (1982) A structural analysis of the Butte-Eminence fault system, eastern Grand Canyon, Coconino County, Arizona. Mater’s Thesis, Northern Arizona University. Powell, J.W. (1873) Exploration of the Colorado River of the West and its tributaries explored in 1869-1872. Smithsonian Institution, Washington, D C. Prucha, J.J., Graham, J.A. and Nickelsen, R.P. (1965) Basement-controlled deformation in Wyoming province of Rocky Mountain Foreland. American Association of Petroleum Geologists Bulletin 49, p. 966-992. Reches, Z. (1978) Development of monoclines: Part I, Structure of the Palisades Creek branch of the East Kaibab monocline. Grand Canyon, Arizona. In Laramide folding associated with basement block faulting in the western United States, ed V. Matthews III, pp. 235-273. Geological Society of America Memoir 151. Reches, Z., and Johnson, A.M. (1978) Development of monoclines: Part II, Theoretical analysis of monoclines. In Laramide folding associated with basement block faulting in the western United States, ed V. Matthews III, p. 273-311. Geological Society of America Memoir 151. Sanford, A.R. (1959) Analytical and experimental study of simple geologic structures. Geological Society of America Bulletin 70,19-52. Sargent, K.A. and Hansen, S.E. (1982) Bedrock geologic map of the Kaiparowits coal- basin area, Utah. United States Geological Survey Miscellaneous Investigations Map I-1033-I. Steams, D.W. (1971) Mechanisms of drape folding in the Wyoming Province. In Symposium on Wyoming Tectonics and Their Economic Significance, ed A. R. Renfro, pp. 125-143. Wyoming Geological Association 23rd Field Conference Guidebook. 89 Steams, D.W. (1978) Faulting and forced folding in the Rocky Mountain foreland. In Laramide folding associated with basement block faulting in the western United States, ed V. Matthews in, p. 1-38. Geological Society of America Memoir 151. Stem, S. (1992) Geometry of Basement Faults Underlying the Northern Extent of the East Kaibab Monocline, Utah. Master’s Thesis, University of North Carolina at Chapel Hill. Stone, D.S. (1969) Wrench faulting and Rocky Mountain tectonics. The Mountain Geologist 6,67-79. Stone, D.S. (1984) The Rattlesnake Mountain, Wyoming, debate: a review and critique of models. The Mountain Geologist 21, 37-46. Stone, D.S. (1993) Basement-involved thrust-generated folds as seismically imaged in the subsurface of the central Rocky Mountain foreland. In Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States, eds C.J. Schmidt, R.B. Chase and E.A. Erslev, pp. 271-318. Geological Society of America Special Paper 280. Suppe, J. and Medwedeff, D.A. (1984) Fault-propagation folding. Geological Society of America Abstracts with Programs 16,670. Suppe, J. (1985) Principles of Structural Geology. Prentice-Hall, Englewood Cliffs, New Jersey. Walcott, C D. (1890) Study of line of displacement in the Grand Canyon of the Colorado in northern Arizona. Geological Society of America Bulletin 1,49-64. 90 APPENDIX B: THE COCKSCOMB SEGMENT OF THE EAST KAIBAB MONOCLINE: TAKING THE STRUCTURAL PLUNGE Abstract The East Kaibab monocline in northern Arizona and southern Utah is a north- to northeast-trending fold in Paleozoic and Mesozoic sedimentary rocks on the eastern margin of the Kaibab uplift. The east-dipping monoclinal fold developed above a west dipping fault in underlying Precambrian basement rocks between 80-50 million years ago (Ma). Erosion has since carved the monocline into a narrow series of ridges and valleys of colorful, candy-striped layers of rock, the most spectacular of which lie in Grand Staircase-Escalante National Monument. A sequence of processes including folding, fault growth, uplift, and erosion formed the breathtaking and variable landscapes visible along this ‘Cockscomb,’ and left clues helpful to unraveling the three-dimensional geometry and growth history of the monocline. The fold plunges gently to the north, exposing different stratigraphic levels, fault patterns, degrees of folding, and topographic and structural relief along its surface trace. Some of the changes in surface geology indicate variations in fault and fold geometry at depth, but some simply reflect the effects of erosion and exposure level. Analysis of these characteristics based on map relationships and field observations leads to the conclusion that the East Kaibab monocline formed by gradual upward propagation of a basement-rooted oblique-reverse fault, and its associated ‘fault 91 tip fracture zone,’ within the core of the growing fold. This paper describes visualization techniques, conceptual models, and geological arguments that support an oblique-reverse fault-propagation-fold interpretation of the Cockscomb segment of the East Kaibab monocline. Introduction The Cockscomb in Grand Staircase-Escalante National Monument is one of the most spectacular geologic features of the Colorado Plateau, and the elegant details of its structural growth through time are exposed in outcrops stretching from the Grand Canyon in northern Arizona to Table Cliff Plateau in Utah. The steeply inclined, candy-striped layers of rock along the Cockscomb are part of an abrupt fold, the East Kaibab monocline, that interrupts the otherwise flat-lying sedimentary rock sequence (Fig. B l). Changes in the form of the fold and the surface fault pattern along its 60 km trace in southern Utah are evidence of the changing character of the underlying fault and fault-fold relationships. Subtle differences in the stratigraphy and structural geology exposed at the surface are clues to the complicated interactions among folding, faulting, uplift, and erosion that created this stunning geologic feature. Early explorers in the Grand Canyon area described the region’s monoclines and speculated that they formed by simple bending of sedimentary strata over differentially uplifted basement blocks (Dutton, 1882; Powell, 1873) (Fig. B2). This kinematically simple explanation is sufficient to describe the form of the Cockscomb at any one location, but differences in the surface expression of the monocline along its northeast- 92 GRAND VIEW BLACK I POINT / Figure B l. Location of the East Kaibab monocline and its branching segments (Palisades, Grandview, Coconino, Black Point) in northern Arizona and southern Utah. Shaded area is the Grand Canyon. Line A-A' is the location of the cross section in Fig. B5. 93 Figure B2. Simple cross section sketch of a Colorado Plateau monocline. Basement faulting at depth has caused folding of overlying sedimentary strata. 94 southwest trend are the result of more complicated processes. Tindall and Davis (1999) presented quantitative data and analyses to demonstrate that the Utah segment of the fold formed by oblique motion (a combination of strike-slip and reverse-slip offset) on the underlying basement fault, and propagation of oblique faulting into high structural and stratigraphic levels of the fold during its growth. In fact, the map pattern of the Cockscomb itself exposes equally compelling evidence for this interpretation when certain concepts of structural geology and geologic map interpretation are applied. Because development of the Cockscomb involved interaction of many processes, understanding the structure’s complexity requires incorporation of a number of techniques and ideas. Structural geologists use maps and measurements of folds, fractures and rock types exposed at the Earth’s surface to build a thorough understanding of the three-dimensional geometry of rock units both buried beneath the surface and removed by erosion from above. This task comes naturally to some geologists accustomed to filling in the missing puzzle pieces through application of concepts derived from simplified models of geologic structures. However, even under the best circumstances some important map clues are easy to overlook. This paper describes the changes in surface expression of the East Kaibab monocline north of the Grand Canyon, and presents models and diagrams of basic structural concepts as a tutorial for interpreting the three-dimensional geometry of the Cockscomb in Grand Staircase-Escalante National Monument. 95 Background Regional Setting The Colorado Plateau geographic province o f the western United States occupies parts o f Utah, Arizona, New Mexico, and Colorado. It is a region o f relatively undeformed Phanerozoic sedimentary rocks surrounded by highly deformed rocks o f adjacent tectonic provinces - - the Rio Grande Rift on the east. Rocky Mountains on the east and north, and the Basin and Range province on the west and south (Fig. B3). The Rocky Mountains are an expression o f the Laramide tectonic event that affected western North America between 80-40 Ma (Brown, 1988). This mountain-building event was driven by east-directed subduction o f the Farallon tectonic plate (ancient floor o f the Pacific ocean) beneath the western margin of North America (Fig. B4). Interaction of the Farallon and North American plates transmitted horizontal compressive stress thousands o f kilometers eastward into the North American continent (Coney, 1976). Compression caused differential uplift o f crystalline basement blocks and overlying sedimentary rocks on the east and north sides o f the relatively rigid Colorado Plateau, and formed a belt o f folded and thrusted sedimentary rocks to the west and south o f the Plateau. More recently, extensional tectonics and crustal thinning affected the regions that were previously compressed and uplifted; tensional forces dissected the Rocky Mountain uplifts and formed the distinctive Rio Grande Rift and Basin and Range extensional provinces (Windley, 1995) (Fig. B4). Thinning o f the crust began soon after the end o f Laramide subduction and is still active in the Basin and Range and Rio Grande Rift today (Wernicke, 1992). Given the intense tectonic deformation expressed 96 Figure B3. Regional maps o f Mesozoic and Cenozoic structures o f the Colorado Plateau and surrounding geographic provinces. Shaded area on the Colorado Plateau is Grand Staircase-Escalante National Monument. Deformation o f the Plateau region has been minimal compared to that in surrounding areas throughout Phanerozoic time, for reasons that are still poorly understood. (a) Mesozoic and early Tertiary compressional tectonic events caused uplift o f the Cordilleran thrust belt and Mogollon highlands on the west and southwest edges o f the Colorado Plateau, and formation o f enormous Rocky Mountain uplifts to the north and east. Deformation affected the Colorado Plateau region only mildly, resulting in broad, low uplifts bounded by monoclinal folds. Colorado Plateau uplifts include the Circle Cliffs (CC), Defiance (D), Echo Cliffs (EC), Kaibab (K), Lucero (L), Monument (M), San Juan (SJ), San Rafael (SR), Uncompahgre (U), White River (WR) and Zuni (Z). (b) More recently, Tertiary and Quaternary extension o f the western United States has dissected the Cordilleran thrust belt, Mogollon Highlands and Rocky Mountain uplifts to form the modem Basin and Range and Rio Grande Rift. The Colorado Plateau remains largely unaffected by recent extensional tectonics. 98 Figure B4. (a) Simple sketch o f the plate tectonic setting of western North America during the Laramide orogeny (80-40 Ma). Shallow-angle subduction o f the Farallon Plate beneath the North American Plate transmitted horizontal compressive stress thousands of kilometers eastward into continental North America, forming the Rocky Mountain uplifts. The Colorado Plateau was only slightly affected by Laramide deformation, (b) After the end of Laramide subduction, a right-lateral transform boundary developed between the North American and Pacific Plates (the San Andreas fault). Areas previously subjected to compressive stress began to collapse due to gravitational forces and crustal extension, forming the Basin and Range province and the Rio Grande Rift, and dissecting the Rocky Mountain uplifts. The Colorado Plateau remains relatively unaffected by recent extensional tectonics. For the sake of simplicity, tectonic provinces far from the Colorado Plateau (for example the Cascade Mountains, Columbia Plateau, and Coast Ranges) are not shown. 99 in rocks of these bordering regions, it is remarkable that the sedimentary rock layers of the Colorado Plateau have remained so undeformed. Within the Colorado Plateau, the effect of Laramide deformation is expressed in the landscape by broad, low uplifts separated from vast shallow basins by erosional cliffs or low-amplitude folds in Paleozoic and Mesozoic sedimentary rocks, and evidence of recent extension is almost entirely absent. The Cockscomb The Kaibab uplift in northern Arizona and southern Utah and its steep eastern limb, the East Kaibab monocline, are examples of Colorado Plateau structures formed during the Laramide orogeny. The landscape expression of the northern part of the East Kaibab monocline is often called the Cockscomb because erosion of the steep, east dipping sedimentary layers has exposed strike-parallel ridges of near-vertical red and white rock that resemble a rooster’s comb. The most visually stunning parts of the Cockscomb lie in Grand Staircase-Escalante National Monument, extending from near Kodachrome Basin State Park in Utah to the Arizona-Utah border. This stretch coincides with the area of greatest structural relief (vertical separation between anticlinal hinge and synclinal trough), ranging from 1,200 m to 1,600 m in most of the Monument. The East Kaibab monocline actually continues southward into Arizona and across the Grand Canyon to near Flagstaff, bifurcating in places to form several branching segments (for example the Grandview, Palisades, Coconino, and Black Point segments) (Fig. B l). Structural offset decreases southward from the Monument to 800 100 m in the Grand Canyon, 700 m at Coconino Point, and 150 - 300 m along the Black Point segment (Babenroth and Strahler, 1945). The total trace length of the monocline is approximately 240 km, making it one of the largest of the monoclines on the Colorado Plateau (Reches, 1978). Structural Roots The East Kaibab is one of the best studied of the Colorado Plateau monoclines, in part for its enormous trace length and considerable vertical offset. Perhaps more importantly, the Grand Canyon offers a deep cross-sectional exposure that reveals the nature of deformation in Paleozoic and underlying Precambrian rocks. This cross- sectional exposure reveals that a steep (60o-70°) west-dipping fault zone in Precambrian basement rocks, the Butte fault, underlies the folded Paleozoic and Mesozoic rocks that constitute the East Kaibab monocline (Fig. B5). West-side-down stratigraphic offsets in the Precambrian sedimentary sequence of the Grand Canyon Supergroup show that the Butte fault first became active in Precambrian time, long before the deposition of Paleozoic and Mesozoic sediments that now make up the Cockscomb (Walcott, 1890; Maxson, 1961; Huntoon, 1969, 1993; Huntoon and Sears, 1975). Beginning at ~600 Ma (Bond, 1997; Timmons and others, 2000) Paleozoic and Mesozoic sediments accumulated to a thickness of at least 3,500-4,000 m during a time of tectonic quiescence (Hintze, 1988). Laramide compression initiated at about 80 Ma in this region and reactivated the ancient ‘basement’ fault, causing the west side to move up relative to the east. Over millions of years the gradual, earthquake-by-earthquake fault 101 — 2500 — 2000 3 — 1500 s 3 — 1000 — 500 — sea level Figure B5. Cross section showing East Kaibab monocline fault-fold relationships in the Grand Canyon. A west-dipping fault in Precambrian and lower Paleozoic rocks underlies the east-dipping monoclinal fold in upper Paleozoic strata. Dashed lines represent Paleozoic rocks that have been removed by erosion. Overlying Mesozoic rocks also have been stripped away by erosion. Note that the lowest Precambrian layer shows normal (west side down) offset, indicated by white arrows. Normal faulting occurred before deposition of Paleozoic sedimentary rocks. After deposition of the Paleozoic and Mesozoic sedimentary section, reverse offset along the same fault (black arrows) formed the East Kaibab monocline. The magnitude of reverse offset must have been smaller than the magnitude o f ancient normal offset, because normal separation is still preserved at the Precambrian level. Cross section location is shown on Fig. B l. 102 movement at depth formed the broad, asymmetrical Kaibab uplift and East Kaibab monocline in the overlying Paleozoic and Mesozoic cover (Huntoon and Sears, 1975; Huntoon, 1993). Although the Grand Canyon provides the only exposure of the basement fault underlying the East Kaibab monocline, the fault (or a network of similar faults) is assumed to underlie the fold for its entire length (Davis, 1978; Stem, 1992; Rosnovsky, 1998). This exposure and other Grand Canyon exposures of fault-cored monoclines (for example the Palisades Branch, Grandview, and Hurricane) are the basis for the widely accepted assumption that similar reactivated basement faults underlie other Colorado Plateau uplifts. A Visual Tour Both early and more recent studies of the East Kaibab monocline have focused on outcrops in and near the Grand Canyon because of their spectacular exposure of the underlying basement fault. Although the Grand Canyon outcrops have helped build a basic understanding of the deep structure associated with the Kaibab uplift, they offer only a limited view of the changes in structural character along the trend of the East Kaibab monocline. That is, the deep Grand Canyon outcrops offer only one perspective in one location along the 240-km fold. Outside the walls of the Grand Canyon the gradual changes in rock types, topography, and scenery along the Cockscomb offer additional evidence for the changing structural geometry of the fold at the surface and at depth. This evidence does not contradict basic models of monocline development, but rather adds an appreciation for the complexity of these regionally significant features. 103 Systematic variations in stratigraphy and structural style along the Cockscomb in northern Arizona and southern Utah provide the observations necessary for interpreting the growth history of the East Kaibab monocline. Both obvious and subtle features in the photographs of Figure B6 contain clues for deciphering underlying structural relationships. Figure B6a begins the visual tour at the bottom of the Grand Canyon where the steep, west-dipping Butte fault juxtaposes Proterozoic sedimentary rocks (right side) and volcanics (left side). At this location the folded Paleozoic and Mesozoic rocks of the Cockscomb have been stripped away by erosion along the Colorado River. However, the overlying strata are preserved nearby in tributaries of the Grand Canyon, as shown in Figure B6b. There the west-dipping fault terminates beneath the surface, but its west-side-up offset has generated an east-dipping monoclinal fold in Mississippian Redwall Limestone. Together, views B6a and B6b (Fig. B6) show that fault offset changes to fold-accommodated offset low in the Paleozoic stratigraphic section in the Grand Canyon. The point at which the discrete fault plane or fault zone disappears upward into folded strata is known as the fault tip (Fig. B2). At the location of photograph 6b, the tip of the Butte fault propagated upward through the stratigraphic section only to the level of Mississippian rocks before Laramide deformation ended. At the stratigraphic level of upper Paleozoic rocks. House Rock Valley stretches from the north rim of the Grand Canyon northward toward the Arizona-Utah border (Fig. B6c). East-dipping Kaibab Limestone forms the western slope of the valley, and 104 Figure B6. North-directed photographs of the variable landscape, stratigraphy and geologic exposures along the Cockscomb from the Grand Canyon to Table Cliff Plateau. Photographs a through h progress from south to north; locations are shown on an oblique perspective map o f the Kaibab uplift. Interesting features of each photograph are discussed in the text. Continued on the next page. 105 d e 106 the flat-lying, red Moenave and Kayenta Formations compose the Vermilion Cliffs to the east. It is possible to imagine that folded, east-dipping Moenave and Kayenta Formations capped the east-dipping slope of the Kaibab uplift millions of years ago, as the Kaibab Limestone does today, but their folded and faulted layers along the crest and in the steep limb of the monocline have since been removed by erosion. The yellowish beds of Kaibab Limestone in the foreground dip gently to the east, parallel to the present edge of the Kaibab uplift in the background. Sediments on the floor of House Rock Valley obscure east-dipping Triassic strata in the synclinal hinge of the monocline. Figure B6d is a view of the Cockscomb near the Arizona-Utah border. Brick- red and grey strata (left center) belong to the Triassic Moenkopi Formation, and the brighter red rocks on the right side are Triassic-Jurassic Moenave and Kayenta Formations. Erosion has not dissected the monocline as deeply here, so that folded Kayenta and Moenave are preserved in the steep limb. The purplish unit in the right center is a narrow, fault-bounded sliver of Triassic Chinle Formation (faults are not obvious in this picture). From area B6c to B6d (Fig. B6), two obvious changes have occurred in the landscape. First, the steep limb of the fold is exposed in higher stratigraphic units at the location of Figure B6d; that is, Moenave and Kayenta Formations are involved in the monoclinal fold at B6d (Fig. B6), but these were flat- lying on the east side of the fold at B6c (Fig. B6). Secondly, the dip of strata in the east-dipping monoclinal limb is much steeper at B6d than at B6c (Fig. B6); this reflects the gradual increase in structural relief between the two photo locations. 107 In Figure B6e, just southeast of Paria, steeply dipping Jurassic Carmel and Entrada Formations mark the continued up-section exposure of deformation toward the north. To the northeast, in the right-hand background of the photograph, flat-lying Cretaceous rocks (Tropic and Straight Cliffs formations) compose the high cliffs. The topographic expression of the cliffs is the result of erosion by the Paria River, which flows nearby in the synclinal trough of the East Kaibab monocline. Like the cliffs of flat-lying Moenave and Kayenta Formations in Figure B6c, erosion has removed the folded and deformed portion of the Cretaceous strata from the crest of the monocline here, leaving eastward-receding cliffs of undeformed rock. Where the Paria River crosses the steep limb of the Cockscomb, the canyon mouth exposes a west-dipping reverse fault in Navajo and Carmel Formations (Fig. B6f). Fault movement has placed a stratigraphically lower sandstone layer (white, left side) above stratigraphically higher Carmel Formation redbeds (right side). The fault is approximately parallel to the trend of the monocline, dips steeply west, and displays a west-side-up sense of offset, similar to but much smaller than the basement fault exposed in the Grand Canyon. Several west-dipping reverse faults are exposed along the monocline in the vicinity of Figure B6f. Still farther north, as shown by Figure B6g, tan and grey stripes of Cretaceous Dakota, Tropic, and Straight Cliffs Formations are preserved in the steep fold limb. In the background, white Navajo Sandstone occupies the crest of the monocline, dipping less steeply than the Cretaceous rocks in the foreground. Finally, Figure B6h is an oblique aerial photograph of the northern end of the East Kaibab monocline. Flat-lying, 108 white Navajo Sandstone forms the crest of the monocline on the west, and flat-lying Cretaceous strata of the Kaiparowits basin make up the desolate landscape on the east. Northward along the fold, dips gradually die out until the monocline disappears near Table Cliff Plateau (barely visible in the left background). The photographs in Figure B6 offer a representative sample of the changes in scenery, stratigraphy, fold form, and fault expression visible in different areas along the Cockscomb. These changes present clues about the geometry of the Cockscomb at depth, and how this geometry changes both vertically (with depth) and horizontally (along the monocline). Surface evidence can be integrated through the use of geologic maps, visualization techniques, and conceptual models in order to decipher the three- dimensional geometry and growth stages of the Cockscomb. Structural Observations The northward changes in landscape along the Cockscomb in southern Utah correspond to structural patterns and stratigraphic clues in the geologic map (Fig. B7). Understanding the structural implications of the map-view expression of the Cockscomb requires several conceptual tools. These include geometry of plunging folds, down-plunge viewing, Riedel fracture development, fault-slip gradient, and fault- propagation folding. The following sections contain a general description of each concept, and application of the concepts to interpreting patterns of faulting and folding exposed along the Cockscomb. 109 Figure B7. Geologic map o f the East Kaibab monocline in Grand Staircase-Escalante National Monument. Stratigraphic column includes Precambrian and Paleozoic rocks that are not exposed in the Monument but are visible in the Grand Canyon. Cross sections A -A \ B-B' and C-C refer to Fig. B 14. 110 Northward Plunge When leveled off by erosion, the geometry of a plunging fold creates interesting geometric forms in horizontal map-view exposures. Figure B8 presents two schematic block diagrams of north-trending, east-vergent monoclinal flexures. The similarity in fold form is obvious in the cross-section view of each diagram. Note that in the side view of the block in Figure B8a the sedimentary layers are flat-lying where not involved in the steep limb of the monoclinal flexure. This pattern is typical of folds that have horizontal fold axes. In Figure B8b the side view shows that sedimentary layers dip toward the north, reflecting the fact that the fold axis plunges north. This northward plunge produces a very different pattern in horizontal map view (representing the eroded ground surface) compared with the pattern created by erosion of a non-plunging fold. In the map view of the plunging structure (Fig. B8b) older stratigraphic layers (lower in the vertical cross section) are exposed up-plunge, toward the south; in the non-plunging example (Fig. B8a) the same stratigraphic units are exposed along the entire length of the structure. As a result, the map view of a plunging fold (like Fig. B8b) resembles a distorted version of the cross section, and therefore contains information about structural geometry at depth. In southern Utah the East Kaibab monocline plunges gently (3°-50) northward, exposing structural relationships in map view that relate to fold and fault geometry below the surface. The horizontal map view displays progressively lower stratigraphic and structural levels toward the south: Cretaceous rocks are folded in the steep limb near Grosvenor’s Arch, giving way southward to Jurassic and Triassic rocks at Pari a, I ll cross section Figure B8. Diagrams of monoclinal folds with horizontal (a) and plunging (b) fold axes. Note that a monocline with a horizontal fold axis would expose the same stratigraphic units along its trend in a horizontal map view. In this non-plunging fold, a single bed intersects a hypothetical vertical plane along a horizontal line - - a horizontal fold axis. On the other hand, the plunging fold exposes progressively younger stratigraphic layers at the surface in the down-plunge direction. The stratigraphic sequence o f layers can be seen in the front and side views o f the block diagram. In this case, a single bed intersects an imaginary vertical plane along a line that plunges away from the viewer - - a plunging fold axis. 112 and eventually Permian rocks near Buckskin Gulch (Fig. B7). This map pattern is a natural consequence of the northward plunge, and is ideal for down-plunge viewing. Down-Plunge Viewing Geologists use the down-plunge view technique to synthesize complicated map relationships into meaningful cross sections (Mackin, 1959). As described in the previous section, the map view of a plunging fold exposes an elongated but distorted view of fold geometry. Down-plunge viewing creates ‘foreshortening’ of the plunging map view, thus removing distortions imposed by the elongated horizontal perspective. The end product can be a properly scaled and accurate structural cross section based on map relationships rather than on speculation. Figure B9 depicts application of the down-plunge viewing strategy to the map of the East Kaibab monocline. Map data from Figure B7 indicate that the fold axis plunges about 5° to the north. Placing the map flat on a table and looking at it from 5° above the horizon creates a profile view of the structure (a cross section perpendicular to the fold axis). This perspective visually foreshortens the map view to create an apparent cross section of structural relationships at the level of strata shown on the map. An accurate profile view of the Cockscomb results from applying the down- plunge viewing technique to the full map in Figure B7. (Small irregularities of the lithologic contacts in the map view are caused by topography, and should be smoothed when visualizing the down-plunge cross section.) The down-plunge view reveals an abrupt monoclinal fold separating otherwise flat-lying strata, and a narrow zone of MAP VIEW u> 114 faults within the steep limb. However, an accurate cross section does not always provide the most informative view o f a structure. Because the Kaibab uplift plunges at such a low angle, down-plunge viewing turns the 9-inch-long structural map into a cross section less than one inch high. The down-plunge view effectively blurs and obscures structural relationships that are only visible in the elongated and distorted map view. For example, the individual small fractures along the steep limb o f the monocline visible in the plunging map view o f Figure B7 would not be evident in an actual vertical exposure like the Grand Canyon, or in a map view that exposed only a single structural level. With careful measurement and observation, and an understanding o f shear fracture geometry, these details reveal more about lateral structural changes along the Cockscomb than does the down-plunge view. Fault Tip Zone Erosion o f the Kaibab uplift to the level o f the Mesozoic rocks in Grand Staircase-Escalante National Monument has exposed a monocline-parallel pattern o f dense fracturing at the surface. Because the monocline plunges north the fault pattern, like the pattern o f sedimentary rock layers, presents progressively deeper structural levels toward the south. Examination o f the changes in fault orientations from south to north reveals a spatial and temporal sequence o f fault development from deeper to shallower levels in the core o f the monocline. As shown in Figure B7, faults south ofParia have long, continuous traces parallel to the surface trend o f the fold. These faults accommodate apparent right lateral 115 offset, shown by truncation and displacement of Chinle, Moenave, and Kayenta Formations. Between Paria and Pump Canyon Spring the continuous, monocline- parallel fault trace gives way to a disjointed series of faults that show right-lateral separation of Navajo and Carmel Formations. These are ‘synthetic’ faults because their apparent sense of offset is the same as that on the long, continuous faults farther south. The left-stepping, en-echelon synthetic faults strike about 20° clockwise from the trend of the monocline, but define a monocline-parallel zone of deformation. Between Pump Canyon Spring and Grosvenor’s Arch the fault pattern consists of short, northwest- striking faults with apparent left-lateral offsets in Entrada, Dakota, and Tropic Formations. The left-lateral offset on these faults is antithetic to the sense of offset on the continuous fault surfaces south of Paria. North of Grosvenor’s Arch the monocline- parallel fault zone disappears, indicating that deformation north of that location was accommodated entirely by folding rather than by a combination of folding and faulting. The changing fault pattern along the East Kaibab monocline may represent the sequence of secondary fault development in a narrow zone of intense deformation directly ahead of the upward-propagating basement-rooted fault tip. A similar sequence of secondary fault growth has been observed in physical analog models of strike-slip (lateral offset) deformation. Models of strike-slip faulting typically develop a pattern of synthetic and/or antithetic faults on the upper surface preceding the appearance of long, continuous, shear zone-parallel faults (Tchalenko, 1970; Naylor and others., 1986; Sylvester, 1988; McKinnon and de la Barra, 1998). In such models the synthetic and antithetic faults, although discontinuous on the surface, link with the basement fault at 116 depth. In effect, they accommodate strains that are slightly too great to be taken up by folding, but with continued deformation a discrete, shear zone-parallel fault is required to accommodate larger strains. The same sequence o f deformation took place along the developing East Kaibab monocline as Paleozoic and Mesozoic cover rocks folded and faulted in response to movement on the reactivated basement fault. The process of folding, development of discontinuous fractures, and eventual growth of a through-going fault began at depth near the basement-cover interface, and continued upward through the core of the East Kaibab monocline as the structure grew. The cross sections in Figure B 10 are exaggerated sketches of fold development and simultaneous up-section migration of the fault tip deformation zone. The same structural relationships are visible in map view along the East Kaibab monocline because of the northward plunge of the fold axis. The small, discontinuous faults seen along the East Kaibab monocline represent synthetic and antithetic fractures that formed in higher structural and stratigraphic levels ahead of the upward-propagating tip of the basement fault. Oblique Deformation The orientations of fractures in the fault tip zone also contain clues about the vertical and lateral movements involved in growth of the East Kaibab monocline. It is simplest to imagine that movement on the basement fault was pure reverse-slip, with the west side moving up relative to the east. In fact, most early studies of Colorado Plateau monoclines assumed that the structures formed by this dip-slip reverse fault motion 117 Figure BIO. (a) Up-section migration of the basement-rooted fault and its fault tip deformation zone began near the basement-cover interface during initial increments of basement fault offset and overlying fold growth, (b) With continued deformation, the fault tip and associated fracture zone propagated upward through the core of the East Kaibab monocline as the fold grew. From diagram (a) to diagram (b) basement fault offset (1) increases, and the basement-rooted fault tip (2) andits fault tip defor mation zone (3) migrate upward. The fracture pattern exposed along the north- plunging East Kaibab monocline in southern Utah preserves the final increment of this fault propagation and fault tip fracture formation in horizontal map view (Fig. B7). 118 (Powell, 1873; Walcott, 1890; Steams, 1971; Reches, 1978). However, the angular relationships between the monocline and the synthetic and antithetic faults in southern Utah suggest that right-lateral slip occurred in addition to reverse slip, causing the west side of the monocline and fault to move northward relative to the east side. The angular relationships between synthetic and antithetic faults described in the previous section resemble a characteristic surface fault pattern recognized in physical modeling experiments and field studies of strike-slip fault systems (for example Riedel, 1929; Tchalenko, 1970; An and Sammis, 1996; Reading, 1980; Sylvester, 1988). This characteristic ‘Riedel shear’ pattern is easiest to describe using an example. Figure B lla shows the typical Riedel pattern that forms as a result of right-handed strike-slip offset. The shear fracture array that develops at the surface consists of Riedel or R- shears at an angle of 15° to the basement shear direction, Riedel-prime or R' shears at about 75° to the basement zone, and faults that are parallel to the shear zone (Y-shears). In a right-lateral shear zone the R and Y shears accommodate right-lateral offset, synthetic to the shear direction; R' fractures are antithetic to the shear zone, accommodating small left-lateral offsets. In physical models and in natural fault systems, synthetic and/or antithetic fractures can develop independently or together, producing map patterns similar to Figure B llb , B ile , or B lld . Note that in Figure B7 the fault orientations in the fault-tip deformation zone strongly resemble Riedel fracture geometry. North of Paria, faulting in the steep limb takes the form of northeast-striking, left-stepping, en-echelon faults, and northwest- striking, right-stepping, en-echelon faults. These faults accommodate reverse-right- 119 Y (parallel to shear zone) Figure B 11. Riedel shear geometry, (a) In a right-handed system, synthetic faults at 15 to the shear zone accommodate right-lateral offset and antithetic faults at 75 to the zone accommodate left-lateral offset Synthetic and antithetic fractures can form independently or together, creating fault patterns that resemble (b), (c) or (d). 120 lateral and reverse-left-lateral offset, respectively. The fault pattern indicates that the steep limb of the East Kaibab acted as a shear zone during deformation, with small synthetic and antithetic faults accommodating reverse-right-lateral shear in the steep fold limb ahead of the advancing basement-rooted fault tip. The right-handed component of offset within the shear zone in southern Utah is demonstrated by the orientations of striae on fault surfaces and by small right-handed offsets of stratigraphic layers. The reverse, west-side-up component of movement is expressed by the 1,600 m, west-side-up structural relief of the Kaibab uplift as a whole. The Mesozoic-level shear zone and its underlying cause, the reactivated basement fault, therefore resulted from reverse-right-lateral, oblique deformation. The Riedel-type fault tip fracture pattern therefore provides two key pieces of information: it leads to recognition of the presence of an upward-propagating fault tip deformation zone, and permits interpretation of the oblique motions involved in fault and fold development. In this context, the disappearance of the shear zone north of Grosvenor’s Arch represents the expected up-section transition from faulting to folding; that is, the dying out of the basement-rooted fault tip and associated Riedel fractures. However, the absence of the fault pattern south of the Arizona-Utah border raises other questions (see Figs. B6b, B6c, and B6d). Erosion has exposed the same rocks along the monocline in northern Arizona as in southern Utah, but evidence for basement-rooted faulting at the surface disappears to the south. In fact, in the Grand Canyon basement- rooted faulting has propagated only as high as the Mississippian Redwall Limestone (Fig. B5). If the Riedel type fracture pattern in southern Utah represents growth of 121 basement-rooted faulting toward the surface, why is it not visible to the south, at lower structural and stratigraphic levels? Knowledge of the three-dimensional nature of fault surfaces and fault offset can explain the discrepancy. Fault Slip Gradient Fault surfaces are often roughly elliptical, with offset decreasing from the center of the fault plane toward the lateral terminations of the elliptical fracture (Barnett and others, 1987). The block diagram in Figure B 12a contains a segment of a reverse fault on which displacement gradually decreases. On the surface of the diagram, fault displacement is greatest at point X and decreases northward to zero at point Y. Gentle folding of the rock mass in the vicinity of the fault accommodates the change in fault slip along strike. In reality not all faults propagate to the surface of the Earth. Often fault offset at depth gives way to fold-related offset toward the surface. Figure B 12b presents the same geometric relationship shown in Figure B 12a, with fault displacement dying out from point X to point Y. However, cover strata in Figure B12b are folded in response to the fault offset. Figure B12b clearly shows that fold displacement at the surface, like fault displacement, decreases from X to Y. In complex natural structures like the East Kaibab monocline, fold profiles can vary considerably along a structural trend, indicating changes in fault offset at depth. The East Kaibab monocline obtains its maximum structural relief of 1,600 m in southern Utah (Gregory and Moore, 1931). This structural relief decreases gradually 122 (b) Figure B12. Faults and folds can accommodate different amounts of offset at different locations, (a) Offset on a reverse fault diminishes from point X to point Y. (b) The fault dies out beneath the ground surface. Above the fault tip, fold offest diminishes from point X to point Y. 123 south of the Utah-Arizona border toward the Grand Canyon where offset is only 800 m, likely reflecting variations in fault offset at depth. The magnitude of fault slip not only affects the fold form and degree of structural relief in overlying strata, but also determines the prevalence of faulting within the fold. Greater fault slip at depth results in more extensive faulting in the overlying rocks. The fault-propagation fold model describes the elegant interplay of faulting and folding. Fault-Propagation Folding Formation of monoclines traditionally has been explained by drape folding. In this conceptual model, fault displacement at depth gives way abruptly to fold- accommodated displacement in the sedimentary cover. The transition is accomplished by thinning and stretching of the lowest sedimentary rock layers over the displaced fault blocks, and as a result faulting does not play a major role in above-basement deformation (Steams, 1971; Reches and Johnson, 1978) (Fig. B13a). The drape-fold model was applied to Colorado Plateau monoclines for good reasons. Firstly, the surface form of monoclines tends to support a drape-fold origin. The monoclines are broad folds in above-basement sedimentary cover, and show little evidence for basement-rooted fault offset at the surface (as opposed to Rocky Mountain foreland uplifts, where faulting is of major importance; for example Schmidt and Perry, 1988; Schmidt and others, 1993). Secondly, deep Grand Canyon exposures of monoclines show that fault offset changes to fold offset at very low levels in the post- 124 Figure B13. Schematic diagrams of drape folding (a) and fault-propagation folding (b). The drape fold model suggests that most deformation in the above-basement rocks is accommodated by stratigraphic thinning; faulting does not play a major role in above basement deformation. In the fault-propagation fold model, increased fault offset in basement translates to increased fault development in sedimentary cover Stratigraphic thinning may still take place during fold development, but the propagating fault accommodates increasing displacement in the sedimentary cover as the fault and fold grow. 125 Proterozoic sedimentary section (Figs. B2 and B5). Where the Grand Canyon incises the East Kaibab monocline the transition from fault to fold is accomplished by obvious thinning in the lower Paleozoic rocks, allowing higher stratigraphic units to fold without obvious faulting. Although fault-accommodated offset gives way to fold- accommodated offset very low in the above-basement section in the Grand Canyon, basement-rooted faulting is apparent at much higher structural and stratigraphic levels of the fold in southern Utah. The drape-fold model fails to account for the prevalence of basement-rooted faulting along the East Kaibab monocline in Grand Staircase- Escalante National Monument. The fault-propagation fold model proposes that a fault at depth progressively overtakes and displaces its overlying fold. Geologists have applied the term ‘fault- propagation fold’ to various structural settings because it describes succinctly the intimate relationship between faulting and fold growth. Despite some resulting confusion about the term’s meaning (for example Mitra and Mount, 1999; Stone, 1999), one consistent feature of the model is that a fault tip at depth propagates upward, progressively offsetting higher levels in the overlying fold as displacement increases (Suppe, 1985; Davis and Reynolds, 1996) (Fig. B13b). The model implies that a fault with greater offset at depth will have propagated to higher levels in the overlying folded strata. In the case of the East Kaibab monocline, the observation that faulting gives way to folding at a low stratigraphic level in the Grand Canyon simply reflects the relatively small amount o f offset on the basement fault at that location. That is, if the 126 East Kaibab monocline is a fault-propagation fold, greater vertical fault slip in the Grand Canyon would have resulted in propagation of faulting higher into the overlying strata. The Riedel shear pattern exposed in southern Utah represents initiation of basement-rooted faulting in the Mesozoic rocks in an area with twice as much structural relief as is present in the Grand Canyon (1,600 m versus 800 m). In a sense, the Cockscomb reached a more ‘mature’ stage of development in southern Utah than in the Grand Canyon. Summary The geologic concepts of plunging structure, down-plunge viewing, Riedel shear geometry, fault-slip gradient, and fault-propagation folding combine to illuminate the processes involved in the Laramide development of the East Kaibab monocline. The gentle northward plunge of the Cockscomb in southern Utah exposes structural relationships across several stratigraphic levels. Because of the northward plunge, lateral changes in landscape and map relationships along the structure correspond to different fold and fault geometry at depth. The down-plunge viewing technique offers a quick and accurate method for visualizing cross-sectional structural relationships at the stratigraphic levels exposed on the map. Geometry of fractures exposed in the steep monoclinal limb indicates fault-tip fracture propagation related to a basement-rooted oblique fault zone. The absence of the shear fracture pattern south of the Arizona-Utah border is explained by fault-propagation folding combined with a decrease in fault slip to the south. 127 The concepts described in the preceding sections allow interpretation of the East Kaibab monocline’s oblique-slip fault-propagation fold origin and visualization of its resulting fault-fold geometry at depth. Figure B 14 presents block diagrams of the Cockscomb that summarize key relationships exposed by or inferred from the relationships exposed in Grand Staircase-Escalante National Monument. Conclusions Interpreting the three-dimensional geometry and mode of formation of the Cockscomb involves several conceptual steps. Firstly, reconnaissance of the structural changes along the East Kaibab monocline forms the basis for noticing clues to underlying structural relationships. Secondly, the special north-plunging map view provides the opportunity for down-plunge viewing of an accurate profile section. Distortions imposed by the elongated map pattern bring into view a deformation zone associated with the basement-rooted fault tip, and within this tip zone secondary fault orientations indicate oblique deformation. Changes in structural relief along the trend of the monocline indicate displacement variations along the basement fault at depth. These features together reveal the role of oblique fault-propagation folding in formation of the Cockscomb. Finally, simplified block diagrams can be constructed based on these essential techniques and concepts to summarize the three-dimensional geometry created by faulting, folding, uplift and erosion. The northward plunge of the East Kaibab monocline in Grand Staircase-Escalante National Monument provides all the necessary evidence for formulating an oblique-slip fault-propagation-fold interpretation 128 Figure B14. Sequential block diagrams o f the Cockscomb in Grand Staircase-Escalante National Monument, showing surface geology and its relationship to structural geometry at depth. Features visible in the plunging map view, such as the change in stratigraphic interval from north to south and the orientations o f fractures in the monocline-parallel shear zone, make possible the interpretation o f the changing form on the East Kaibab monocline at depth and along its trend. 129 N Figure B14. Caption is on the previous page. 130 o f the Kaibab uplift (Tindall and Davis, 1999). The exposure o f structural and stratigraphic relationships in the monument is unique, offering insight into the formation mechanisms o f Colorado Plateau uplifts that can be found nowhere else. For this reason, the importance o f oblique deformation and progressive fault-fold development in formation o f this, and possibly other, Colorado Plateau uplifts merits further investigation. Acknowledgments Many field assistants contributed to field work on the East Kaibab monocline, including Erin Colie, Scott Grasse, Nate Shotwell, Jessica Greybill, William Abbey, Pilar Garcia, Seth Gering, Shari Christofferson, and Danielle Vanderhorst. Thanks to Tom McCandless and George Davis for valuable reviews o f the manuscript. Research was supported by National Science Foundation grant NSF#EAR-9406208 and the Dr. H. Wesley Peirce Scholarship, Department o f Geosciences, University o f Arizona. 131 References An, L.J., and Sammis, C.G., 1996, Development of strike-slip faults: shear experiments in granular materials and clay using a new technique: Journal of Structural Geology, v. 18, no. 8, p. 1061-1077 Babenroth, D.L. and Strahler, A.N., 1945,Geomorphology and structure of the East Kaibab monocline, Arizona and Utah: Geological Society of America Bulletin, v. 56, p. 107-150. Barnett, J.A.M., Mortimer, J., Rippen, J.H., Walsh, J.J., and Watterson, J., 1987, Displacement geometry in the volume containing a single normal fault: American Association of Petroleum Geologists Bulletin, v. 71, no. 8, p. 925- 937. Bond, G.C., 1997, New constraints on Rodinia break-up ages from revised tectonic subsidence curves: Geological Society of America Abstracts with Programs, v. 29, no. 6, p. 280. Brown, W.G., 1988, Deformational style of Laramide uplifts in the Wyoming foreland, in Schmidt, C.J., and Perry, W.J. Jr., editors, Interaction of the Rocky Mountain foreland and the Cordilleran thrust belt. Geological Society of America Memoir 171, p.1-26. Coney, P.J., 1976, Plate tectonics and the Laramide orogeny: New Mexico Geological Society Special Publication 6 - Tectonics and mineral resources of southwestern North America, p. 5-10. Davis, G.H., 1978, Monocline fold pattern of the Colorado Plateau, in Matthews, V. Ill, editor, Laramide folding associated with basement block faulting in the western United States: Geological Society of America Memoir 151, p. 215-233. Davis, G.H., and Reynolds, S.J., 1996, Structural geology of rocks and regions: New York, John Wiley & Sons Inc., 776 p. Dutton, C.E., 1882, Tertiary history of the Grand Canyon district: United States Geological Survey Monograph 2. Gregory, H.E., and Moore, R.C., 1931, The Kaiparowits region - a geographic and geological reconnaissance of parts of Utah and Arizona: United States Geological Survey Professional Paper 164. Hintze, L.F., 1988, Geologic History of Utah: Brigham Young University Geology Studies Special Publication 7,202 p. 132 Huntoon, P.W., 1969, Recurrent movements and contrary bending along the West Kaibab fault zone: Plateau, v. 42, p. 66-74. Huntoon, P.W., 1993, Influence of inherited Precambrian basement structure on the localization and form of Laramide monoclines. Grand Canyon, Arizona, in Schmidt, C.J., Chase, R. B., and Erslev, E.A., editors, Laramide basement deformation in the Rocky Mountain foreland of the western United States: Boulder, Colorado, Geological Society of America Special Paper 280, p. 243- 256. Huntoon, P.W., and Sears, J.W., 1975, Bright Angel and Eminence Faults, eastern Grand Canyon, Arizona: Geological Society of America Bulletin, v. 86, p. 465- 472. Mackin, J.H., 1959, The down-structure method of viewing geologic maps: Journal of Geology, v. 58, no. 1, p. 55-72. Maxson, J.H., 1961, Geologic map of the Bright Angel quadrangle, Grand Canyon National Park, Arizona: Grand Canyon Natural History Association. McKinnon, S.D. and de la Barra, I.G., 1998, Fracture initiation, growth, and effect on stress field: a numerical investigation. Journal of Structural Geology v. 20 no. 12, p. 1673-1689. Mitra, S., and Mount, V.S., 1999, Foreland basement-involved structures: reply: American Association of Petroleum Geologists Bulletin, v. 83, p. 2017-2023. Naylor, M.A., Mandl, G., Sijpesteijn, C.H.K., 1986. Fault geometries in basement- induced wrench faulting under different initial stress states: Journal of Structural Geology 8,737-752. Powell, J.W., 1873, Exploration of the Colorado River of the West and its tributaries explored in 1869-1972: Smithsonian Institution, Washington, D. C. Reading, H.G., 1980., Characteristics and recognition of strike-slip fault systems: Spec. Publ. Int. Ass. Sediment, v. 4, p. 7-26. Reches, Z., 1978, Development of monoclines: Part I, Structure of the Palisades Creek branch of the East Kaibab monocline, Grand Canyon, Arizona, in Matthews, V. Ill, editor, Laramide folding associated with basement block faulting in the western United States: Geological Society of America Memoir 151, p. 235-273. 133 Reches, Z., and Johnson, A.M., 1978, Development of monoclines: Part II, Theoretical analysis of monoclines, in Matthews, V. Ill, editor, Laramide folding associated with basement block faulting in the western United States: Geological Society of America Memoir 151, p.273-311. Riedel, W., 1929, Zur Mechanik geologischer Brucherscheinungen: Centralbl. F. Mineral. Geol. U. Pal., v. 1929 B, p. 354-368. Rosnovsky, T.A., 1998, Variation in joint and fault patterns along the East Kaibab and Waterpocket monoclines, Colorado Plateau: Stanford University, M.S. thesis, 58 p. Schmidt, C.J., Chase, R.B., and Erslev, E.A., editors, 1993, Laramide basement deformation in the Rocky Mountain foreland of the Western United States: Geological Society of America Special Paper 280. Schmidt, C.J., and Perry, W.J. Jr., editors, 1988, Interaction of the Rocky Mountain foreland and the Cordilleran thrust belt: Geological Society of America Memoir 171. Steams, D.W., 1971, Mechanisms of drape folding in the Wyoming Province, in Renfro, A.R., editor, Symposium on Wyoming tectonics and their economic significance: Wyoming Geological Association 23rd Field Conference Guidebook, p. 125-143. Stem, S., 1992, Geometry of basement faults underlying the northern extent of the East Kaibab monocline, Utah: Chapel Hill, University of North Carolina, M.S. thesis. Stone, D.S., 1999, Foreland basement-involved structures: discussion: American Association of Petroleum Geologists Bulletin, v. 83, p. 2006-2016. Suppe, J., 1985, Principles of structural geology: Prentice-Hall, Inc., Inglewood Cliffs, New Jersey, 537 p. Sylvester, A.G., 1988, Strike-slip faults: Geological Society of America Bulletin v. 100, p. 1666-1703. Tchalenko, J.S., 1970, Similarities between shear zones of different magnitudes: Geological Society of America Bulletin v. 81 p. 1625-1640. 134 Timmons, M.J., Karlstrom, K.E., Dehler, C.M., Geissman, J.W., and Heizler, M.T., 2000, Proterozoic multistage (-1.1 and -0.8 Ga) extension in the Grand Canyon Supergroup and establishment of northwest and north-south tectonic grains in the southwestern United States: Geological Society of America Bulletin, in press. Tindall, S.E., and Davis, G.H., 1999, Monocline development by oblique-slip fault- propagation folding: the East Kaibab monocline, Colorado Plateau, Utah: Journal of Structural Geology, v. 21, no. 10, p. 1303-1320. Walcott, C D., 1890, Study of line displacement in the Grand Canyon of the Colorado in northern Arizona: Geological Society of America Bulletin, v. 1, p. 49-64. Wernicke, B., 1992, Cenozoic extensional tectonics of the U.S. Cordillera. In B. C. Burchfiel, P. W. Lipman and M. L. Zoback (Eds.), The Geology of North America, The Cordilleran Orogen: Conterminous U. S.: Geological Society of America V. C-2, pp. 553-581. Windley, B.F., 1995, The Evolving Continents: Chinchester, John Wiley and Sons, 526p. 135 APPENDIX C: DECIPHERING RIEDEL PATTERNS IN OBLIQUE SHEAR ZONES Abstract A reverse-right-lateral shear zone along the East Kaibab monocline in southern Utah offers an opportunity to examine Riedel shear geometry associated with oblique deformation. In pure strike-slip systems, R- and R'-shears form in the horizontal O1-O3 principal plane of the stress ellipsoid, and normally occupy expected 15° and 75° orientations with respect to the imposed shear direction. Riedel fracture orientations are not obvious within the oblique East Kaibab shear zone because the G1-G3 plane is not horizontal. However, orientations of synthetic and antithetic fractures in the steep limb of the East Kaibab monocline may be used to define the strike and dip of the shear zone boundaries and the G1-G3 plane. When viewed in this plane the orientations of synthetic and antithetic faults, slickenlines and the shear zone boundaries resemble Riedel shear orientations in a right-handed strike-slip system. Deviations from the expected 15° (R) and 75° (R') orientations are caused by a component of oblique simple shear or transverse shortening across the deformed zone. Comparison with published results of oblique-shortening and transpressional physical models allows the broad estimate that the ratio of strike-slip to dip-slip offset within the East Kaibab shear zone exceeded 2:1, but was less than 6:1. Angular relationships among fault orientations and local principal stress directions indicate that the azimuth of Laramide shortening in this region was no less than 78°. Recognition of Riedel shear geometry is not as 136 straightforward in oblique-slip settings as it is in pure strike-slip deformation; but where the Riedel pattern can be identified, fracture orientations offer a useful tool for determining local stress directions and strain ratios in oblique shear zones. Introduction Riedel Shear Zones Physical analog modeling and field studies of strike-slip faults have led to recognition of characteristic fracture arrays, known as Riedel shear zones, associated with the surface expression of strike-slip systems (e.g. Cloos, 1928; Riedel, 1929; Tchalenko, 1970). Cloos (1928) and Riedel (1929) first documented Riedel fracture geometry in experiments of clay cake deformation above a single, vertical strike-slip fault. Later shear experiments have produced Riedel fractures in a variety of materials and under a wide range of boundary conditions. When subjected to shear deformation, sand (Naylor et al., 1986), limestone (Bartlett et al., 1981) and mechanically heterogeneous sequences of varied granular or granular and viscous materials (Schreurs, 1994; An and Sammis, 1996) generate the same surface fracture pattern as the clay used by Cloos (1928), Riedel (1929), Morgenstem and Tchalenko (1967a) and others. Boundary conditions ranging from discrete, horizontal simple shear above a single ‘basement’ fault (Cloos, 1928; Riedel, 1929; Richard et al., 1991) to basal or gravity- driven distributed shear (Schreurs, 1994; An and Sammis, 1996; An, 1998) to shear-box experiments (Hvorslev, 1937; Morgenstem and Tchalenko, 1967a) also produce recognizable Riedel shear fracture orientations. The consistency of the structural 137 geometry formed in discrete simple shear and distributed shear models in a variety of materials has led to the conclusion that Riedel geometry characterizes the surface fault pattern of strike-slip systems during early stages of deformation (Reading, 1980; Sylvester, 1988). With continued strain, Riedel fractures lengthen and curve to create a tangled, anastomosing surface fault pattern (Naylor et ah, 1986; Sylvester, 1988). At early stages of deformation Riedel shear zones display one or more distinctive sets of discontinuous, en-echelon fractures oriented obliquely to the principal displacement zone (the direction of imposed shear). Figure C l depicts elements of typical right-handed and left-handed Riedel shear zones. In pure strike-slip settings R- shears strike ~15° oblique to the principal displacement zone, R'-shears strike -75° from the shear zone, and Y-shears parallel the displacement zone boundaries (Morgenstem and Tchalenko, 1967a; Tchalenko, 1970). Whether fault orientations deviate clockwise or counterclockwise from the imposed shear direction depends on the sense of shear, as shown in Figure C l. The R and Y fractures accommodate offset synthetic to the shear direction, and R'-shears are antithetic to the shear zone. Synthetic P-shears and extensional T-fractures oriented 10° and 45° from the imposed shear direction, respectively, also form in Riedel shear zones, but are not as prevalent as the R-, R'- and Y-shears (Tchalenko and Ambraseys, 1970). Physical models demonstrate that the R and R' fractures develop at the surface before principal displacement zone- parallel Y-shears, implying a spatial as well as temporal sequence of fracture development in which early-formed oblique faults (R and R') progressively give way to long, continuous Y-shears with increasing deformation (Tchalenko, 1970; Wilcox et al.. 138 Figure C l. Elements of ideal right-handed (a) and left-handed (b) Riedel shear zones. In pure strike-slip settings synthetic R and P shears form at 15 and 10 from the principal displacement zone, respectively. Antithetic R* shears form at 75° from the shear zone. Y shears develop later during deformation and parallel the direction and sense of shear of the principal displacement zone. Tensional T fractures may form at 45 from the shear zone, parallel to the direction. 139 1973; Naylor et al., 1986; Richard et al., 1995). McKinnon and de la Barra (1998) observed that slight changes in initial boundary conditions determined whether R or R' fault arrays were first to form in numerical models, but still the faults occupied the expected 15° and 75° orientations with respect to the principal displacement zone. The predictable nature of Riedel fracture orientations in ideal strike-slip systems encourages the use of Riedel shear geometry as an indicator of stress or strain ellipsoid axis orientations. Specifically, the direction of greatest principal stress (at) bisects the angle between the synthetic and antithetic shears (Davis and Reynolds, 1996), and the line of intersection of the R- and R'-shears defines the intermediate stress direction (02) (Anderson, 1951; Ramsay, 1967). For infinitesimal strain or small increments of finite strain, the axes of the strain ellipsoid nearly coincide with principal stress directions. The characteristic Riedel fracture array is most often identified with strike-slip systems simply because strike-slip deformation presents the G1-G3 plane (containing the greatest and least principal stresses) in horizontal map view. In fact, Riedel geometry often can be observed in the G1-G3 plane of many systems that accommodate normal, reverse, or oblique shear as well (the G1-G3 plane is vertical for normal or reverse shear zones). Davis (1999) noted Riedel shears associated with thrust faults in southern Utah, and Davis et al. (2000) documented a conjugate Riedel fracture geometry for both normal-offset and strike-slip deformation bands in the Navajo Sandstone of the Colorado Plateau. Morgenstem and Tchalenko (1967b) recognized Riedel geometry in the failure zones of landslides. The association of Riedel geometry with strike-slip 140 deformation results from the widespread accessibility, in map view, of the appropriate G1-G3 plane exposure of the fracture pattern. Riedel Geometry and Oblique Deformation Recently the results of reverse-oblique simple shear models and transpressional experiments have been interpreted in the context of Riedel geometry. Oblique simple shear involves movement of material parallel to shear zone boundaries; no transverse shortening or extension takes place (e.g. Jones and Holdswortn, 1998). Richard et al. (1995) performed oblique simple shear of sand and fault gouge above a 45°-dipping basement fault using varying ratios of strike-slip to dip-slip offset. A strike-slip to dip- slip ratio of 0.6 produced slightly en-echelon reverse faults, and a ratio of 6 produced nearly typical strike-slip Riedel geometry. However, a strike-slip to dip-slip ratio of 1.9 resulted in en-echelon R-shears at more highly oblique angles (>15°) to the imposed shear direction (Richard et al., 1995). Transpression differs from oblique simple shear in that a component of pure shear (motion perpendicular to shear zone boundaries) occurs in addition to shear zone-parallel lateral and/or vertical offset. Although the boundary conditions of transpression differ from those of oblique simple shear (compare Sanderson and Marchini, 1984, with Jones and Holdsworth, 1998), transpressional deformation models have yielded similar results. Richard et al. (1995) and Naylor et al. (1986) employed an experimental design that applied non-uniform horizontal compression throughout a deforming sand package by the stretching and relaxing of basal rubber sheets. This distributed transverse shortening accompanied 141 lateral motion on a discrete, vertical strike-slip fault in ‘basement’ beneath the modeling medium. The resulting transpression produced synthetic R-shears at an average angle of 37° from the basement fault and the imposed shear direction (Richard et al., 1995; Naylor et al., 1986). Schreurs and Colletta (1998) modeled transpression by applying transverse shortening to the sides of models deforming by basal distributed shear. En- echelon (stepping) Riedel shears formed at angles greater than 15° to the imposed shear direction in response to the shortening component of strain, with a maximum angle of 280-37° for a strike-slip to shortening ratio of 3.6 (Schreurs and Colletta, 1998). Both oblique simple shear models and transpression models produce similar high-angle R- and R'-shear orientations, just as discrete strike-slip and distributed shear experiments produce identical Riedel geometry. In theory, simple mechanical principles adequately explain the development of high-angle R and R' faults in response to oblique compressional deformation. R- and R'-shears, like other Mohr-Coulomb fractures, typically form at angles of 450-(|>/2 with respect to the local +/-30° from CTi. In strike-slip settings, the local Ot responsible for development of R- and R'-shears is the vector sum of a shear-induced Oi (45° from the principal displacement zone) and sub-regional Ci (e.g. Mandl, 1988; Davis et al., 2000). In idealized strike-slip systems, both shear-induced and sub-regional Oi lie at 45° to the shear zone, leading to the expected 15° and 75° Riedel fault orientations as measured 142 from the principal displacement zone (Fig. C2a). In reverse-oblique or transpressional settings, however, sub-regional o , may form an angle greater than 45° from the principal displacement zone (Fig. C2b). For this reason local Gi, which is the sum of the 45° shear-induced C| and a >45° sub-regional Gi, will form R- and R'-shears at angles greater than 15° and 75° in oblique deformation settings. Davis et al. (2000) described field examples of local Oi rotation in systems of conjugate Riedel deformation band shear zones on the Colorado Plateau. Riedel geometry is seldom noted, much less used to determine stress directions and strain ratios, in natural examples of transpression or oblique simple shear. Most field studies of oblique deformation have concentrated on plate boundary-scale or regional-scale systems (e.g. Fossen and Tikoff, 1998; Teyssier et al., 1995; de Urreiztieta et al., 1996; Pubellier and Cobbold, 1996; Linzer et al., 1995) in which Riedel patterns are likely altered by pre-existing structure or by superposed deformation events. Moreover, prolonged deformation during a single tectonic event effectively obscures or overprints Riedel geometry developed during the onset of deformation, as evidenced by the progressive development of Riedel strike-slip systems into disorderly zones of anastomosing faults and shear lenses (Naylor et al., 1986; Sylvester, 1988). To complicate matters further, the CT1-G3 plane of the stress ellipsoid is not necessarily vertical in cases of oblique deformation, so that typical Riedel geometry, even if present, is not exposed in horizontal map view. A local-scale shear zone along the steep limb of the East Kaibab monocline in southern Utah provides an unparalleled example of Riedel fracture geometry developed 143 sub-regional a X \ >(451 X - shear-induced cr, m principal displacement zone (imposed shear direction) rz m easu red from local p principal emplacement zone (imposed shear direction) 45' ^ 12 m easu red from local fr Figure C2. The local o, responsible for Mohr-Coulomb development of R and R1 shears is the vector sum of sub-regional o, and shear-induced a,, (a) In pure strike-slip, shear-induced a, and sub-regional o, lie at 45 to the principal displacement zone, so local o, also lies at 45°. R and R* shears develop at 45 -4>/2 as measured from o,, resulting in angles o f 15 (R) and 75° (R1) with respect to the principal displacement zone, (b) In oblique shortening or transpressional settings, shear-induced cr, still lies at 45 from the principal displacement zone, but sub-regional O] may lie at a higher angle. Therefore local a, can also lie at a greater angle from the principal displacement zone. R and R’ shears still form at 45 -4>/2 from local a,, resulting in R and R* faults at angles greater than the expected 15 and 75 from the direction of imposed shear. 144 in a reverse-oblique shear system. The Riedel nature of the fracture system is not immediately apparent in map view because the G1-G3 plane is not horizontal, the controlling shear zone is not vertical, and as a result the angular relationships between synthetic and antithetic faults and the shear zone boundaries are not obvious. When viewed in the G1-G3 plane the shear zone essentially becomes vertical, and synthetic and antithetic elements of the shear zone match expected orientations of Riedel fractures formed in oblique deformation experiments. The East Kaibab shear zone example demonstrates that (1) Riedel fault geometry is recognizable in some zones of oblique deformation; (2) comparison with physical models provides a broad estimate of the ratio of strike-parallel to strike-perpendicular deformation; and (3) oblique Riedel orientations allow reasonable estimates of local and sub-regional principal stress directions when considered in the appropriate G1-G3 plane. The East Kaibab Shear Zone Geologic Setting The East Kaibab monocline, located in Arizona and Utah near the western edge of the Colorado Plateau, marks the eastern margin of the Laramide-age, basement-cored Kaibab Uplift (Fig. C3). The monocline consists of approximately 240 km of branching, east- and north-dipping segments that separate the elevated Kaibab Uplift on the west from basins and low structural platforms to the east. Up to 1600 m of west- side-up structural relief separate the anticlinal crest of the Kaibab Uplift and the 145 Figure C3. Location of the East Kaibab monocline in northern Arizona and southern Utah. Shaded area in southern Utah contains the East Kaibab shear zone study area. Shaded area in Arizona is the Grand Canyon. Line A-A* refers to the cross section in Figure C4. 146 synclinal trough of the East Kaibab monocline in southern Utah (Babenroth and Strahler, 1945). Erosion by the Colorado River in the Grand Canyon has exposed a steep, west-dipping fault in the core of the monocline with west-side-up stratigraphic separation at the level of Phanerozoic sedimentary rocks (Reches, 1978; Huntoon et al., 1986; Huntoon, 1993). The fault is a reactivated Precambrian structure as evidenced by normal fault offset preserved in Proterozoic strata, but Paleozoic and Mesozoic strata were undeformed before Laramide, west-side-up reactivation and growth of the overlying East Kaibab fold (Huntoon, 1971, 1981,1993; Huntoon et al., 1986; Reches, 1978) (Fig. C4). Oblique Shear Zone Recent geologic mapping by Tindall and Davis (1999) documented the presence of a narrow zone of faulting along the steep limb of the East Kaibab monocline in southern Utah. The surface trace of the east-dipping monoclinal limb and the associated fault zone (the East Kaibab shear zone) is about N20°E. Figure C5 presents a general geologic map and summary of fault relationships along the Utah segment of the monocline. Orientations of synthetic (reverse-right-lateral) and antithetic (reverse-left- lateral) faults within the shear zone indicate that Laramide structural development involved reverse-right-handed movement rather than dip-slip reverse offset within the steep, northeast-trending fold limb (Tindall and Davis, 1999). Northwest-striking antithetic faults constitute the shear zone between Pump Canyon Spring and Grosvenor’s Arch, and northeast-striking synthetic faults are prevalent to the south 147 — 2 5 0 0 — 2000 I — 1 5 0 0 5> 3 — 1000 — 5 0 0 — sea level Figure C4. Sim plified cross section of East Kaibab fault and fold relationships exposed in the Grand Canyon. Stratigraphic separation at the Proterozoio Phanerozoic unconformity displays Laramide-age, west-side-up reverse offset responsible for monocline formation in overlying Paleozoic and M esozoic sedimentary rocks. Precambrian strata beneath the unconformity preserve west-side-down normal separation, evidence that the fault in the core of the East Kaibab m onocline is an inverted Precambrian normal fault. 148 Figure C5. Map o f fault relationships within the N20°E-trending East Kaibab shear zone. Northwest-striking faults (N50°W, 58°NE) accommodate small, reverse-left- lateral offsets within the shear zone between Grosvenor’s Arch and Pump Canyon Spring. Northeast-striking faults (N41°E, 46°NW) with reverse-right-lateral offsets occupy the steep monoclinal limb between Pump Canyon Spring and Paria. Reverse- right-lateral, monocline-parallel faults dominate the map pattern south o f Paria. Synthetic and antithetic fault orientations do not particularly resemble Riedel fracture geometry in map view. 149 STRATIGRAPHY Grosvenor's 40-150 C a n y o n tn Rg C7 PERMIAN # Locations mentioned in text Monocline axis with plunge Faults; arrows indicate relative lateral offset Figure C5. Caption is on the previous page. 0 5 10 kilometers 150 between Paria and Pump Canyon Spring (Fig. C5). Northeast- and northwest-striking fractures accommodate reverse-right-lateral and reverse-left-lateral offset, respectively, and occupy average angles 20° clockwise and 110° clockwise from the trend of the monocline. Average orientations of the synthetic and antithetic faults and fault striae in the study area are (strike, dip, rake) N41°E, 46°NW, 30°SW and N50°W, 58°NE, 20°SE, respectively (Tindall and Davis, 1999). These orientations are depicted stereographically in Figure C6a. South of Paria a long, reverse-right-lateral fault offsets east-dipping strata in the steep monoclinal limb (Fig. C5). Riedel Geometry Although the synthetic and antithetic orientations and offsets on faults of the East Kaibab shear zone are evident in map relationships, the Riedel nature of the fracture pattern is not apparent. As noted above, typical Riedel geometry is associated with the Ci-Gs plane of shear systems, but this plane does not necessarily coincide with the horizontal map view of structures formed by oblique movements. However, common geometric aspects of ideal strike-slip systems should be recreated in the O1-G3 plane of oblique structural systems if Riedel shearing is indeed the mode of deformation. By seeking the fault plane and stress axis orientations of typical Riedel shear zones in the O1-G3 plane of the oblique East Kaibab shear zone, it is possible to confirm the presence of Riedel geometry in this structural system. In ideal strike-slip shear zones (like those created in physical models above a vertical basement fault) the 02 direction is vertical, parallel to the shear zone 151 boundaries. Synthetic R and antithetic R' fractures are also vertical, and their intersection parallels the 02 direction. Assuming that these relationships apply to the East Kaibab shear zone, 02 must therefore plunge 43° toward N16°W, parallel to the intersection of synthetic and antithetic faults. The planar orientation of shear zone boundaries will contain both the <52 direction and the N20°E map-view trace of the monocline, constraining shear zone strike and dip to N20°E, 57°NW. Combined with the assumption that ct| bisects the acute angle between R- and R'-shears, this logic also permits determination of the C1-G3 plane. Stress ellipsoid axes calculated through this logical approach (C| = 78°, 5°; 0 2 = 344°, 43°; G3 = 172°, 47°) accompany synthetic and antithetic faults and the orientation of the shear zone boundaries in Figure C6a*. Evaluating the possible Riedel nature of faults in the East Kaibab shear zone requires observing fault orientations in the G1-G3 plane. In Figure C6b, the projection from Figure C6a has been rotated 43° counterclockwise around a N74°E azimuth so that the O2 axis is vertical and the O1-O3 plane is horizontal. In this view the angular relationships between synthetic faults, antithetic faults and the shear zone boundaries are comparable to expected Riedel fracture orientations in an ideal vertical, strike-slip shear zone, as shown in Figure C6c. Particularly encouraging is the observation that slickenlines, and therefore fault slip vectors, rotate nearly into the expected left-handed antithetic and right-handed synthetic orientations with respect to the right-handed shear 1 Tindall and Davis (1999) followed the same logic to determine principal strain directions in the East Kaibab shear zone, but they also assumed that the Oj/Sj direction was horizontal. For this reason, stress directions presented here differ slightly from their previously reported principal strain directions. 152 (a) F ig u r e C 6 . (a) The average orientations o f synthetic and antithetic faults along the East Kaibab shear zone were used to calculate the shear zone strike and dip and directions o f a , , ct2 and o3. M ethods for determining shear zone orientation and stress directions are described in the text. Unlabeled dots are average slickenline orientations. (b) Elem ents in Fig. 6 a are rotated so that o 2 is vertical and the 0 , - 0 3 p la n e is horizontal. (North is no longer at the top o f the projection.) Synthetic and atithetic faults lie 2 (F and 87 from the shear zone. Slickenlines rotate to near-horizontal orientations expected in the o,- 0 3 plane of pure strike-slip systems, (c) Expected R and R' relationships and stress directions in an ideal strike-slip shear zone. The similarity between the o,- O3 plane view of strike-slip features in (c) and the o r o 3 plane view of oblique-slip features in (b) is compelling. However, synthetic and antithetic shears (R and R') lie at slightly greater angles from the principal displace ment zone in oblique deformation (b) than in pure strike-slip (c). 153 zone as viewed in the 01-03 plane. However, important differences between ideal strike-slip orientations and the O1-O3 plane view of the oblique system must be explained. R- and R'-shears in a pure strike-slip environment typically form angles of 15° and 75°, respectively, with the shear zone boundaries (Fig. C6c). Synthetic and antithetic faults in the East Kaibab shear zone lie at angles of 20° and 87°, notably greater angles than expected in a pure strike-slip Riedel zone. A likely explanation is that the East Kaibab shear zone resulted from oblique deformation, with lateral offset accompanying components of vertical uplift (creating oblique simple shear, as in Jones and Holdsworth, 1998) and possibly transverse shortening (resulting in transpression, e.g. Sanderson and Marchini, 1984). Based on Grand Canyon exposures of a west dipping fault in the core of the monocline (Fig. C4), the East Kaibab example most closely resembles reverse-oblique simple shear deformation of cover strata above a discrete, non-vertical basement fault as modeled by Richard et al. (1995). In their oblique simple shear models the angle of R-shears relative to the shear zone increased as the ratio of dip-slip to strike-slip offset increased. A ratio close to 0.0 (pure strike- slip) formed the usual Riedel pattern, and a ratio of about 0.5 formed R-shears highly oblique to the imposed shear direction. In dip-slip-dominated deformation reverse faults, approximately parallel to the basement fault strike, formed at the model surface rather than R-shears (Richard et al., 1995). Drawing upon the structural similarities between the East Kaibab field example and the reverse-oblique deformation experiments of Richard et al. (1995), it is likely that the ratio of right-handed to reverse offset in the East Kaibab shear zone exceeded 2:1. The prevalence of R'-shears instead 154 of R-shears along different segments of the shear zone might result from slightly different boundary conditions (such as material properties, basement fault dip, structural or stratigraphic level, or initial stress state) along each segment of the monocline. The rotated (G1-O3 plane) perspective of the oblique shear zone also creates a convenient view of angular relationships between faults and stress directions. For example, in the O1-G3 plane the angle between the shear zone and the local Oj direction (bisecting R and R' shears) is 55° (Fig. C6b). According to Mandl (1988) and Davis et al. (2000) the local Gj responsible for R and R' development equals the vector sum of shear-induced and sub-regional stresses. Shear-induced stress lies at 45° to the principal displacement zone, but because the magnitudes of local and shear-induced stresses are not known the sub-regional Oi direction cannot be calculated definitively. However, in order to generate a local Gi of 55° with respect to the principal displacement zone, and to form the resulting highly oblique R- and R'-shears, sub regional Gi must also lie at least 55° from the shear zone when projected into the G1-G3 plane. Rotating the East Kaibab shear zone back to its original, west-dipping orientation yields an estimate that sub-regional Gi trends at least 78° east of north in horizontal map view. This calculation agrees generally with other geologists’ estimates of a N60oE-N70°E Laramide shortening direction on the Colorado Plateau, but is slightly more easterly (e.g. Reches, 1978; Anderson and Bamhard, 1986). 155 Application Figure C7 demonstrates application of the procedures described above to a small data subset from a 4 km long, N35°E-trending segment of the East Kaibab shear zone near Pump Canyon Spring. Slickenline orientations were measured on eleven fault surfaces in the area. The strike and dip of faults and trend and plunge of slickenlines are shown in map pattern and on a stereographic projection in Figure C7a. Strikes and dips of individual faults vary considerably from the average orientations discussed above, but the synthetic and antithetic fault sets are easily recognized and slickenline orientations indicate reverse-right-lateral and reverse-left-lateral slip, respectively. In this small data subset, the intersection of synthetic and antithetic faults (the local 0 2 direction) trends about N15°E and plunges 46°, a difference of 22° from the direction of G2 determined for the entire shear zone (344°, 43°). The discrepancy may result from random heterogeneity inherent in this small data subset, or may represent a local stress perturbation in the area of Pump Canyon Spring as described by Rosnovsky (1998). In Figure C7b the map of fault orientations and the stereographic projection have been rotated so that the local 02 direction is vertical. In the resulting o l-o 3 plane ‘map view’ of Figure C7b the apparent trends of fault traces differ only slightly from fault strikes in Figure C7a, retaining synthetic and antithetic relationships with respect to the shear zone boundaries. Along this segment of the East Kaibab shear zone the synthetic R-shears lie about 20° from shear zone boundaries when viewed in the O1-CT3 plane, and R'-shears form an angle of about 90° with the shear zone. Again, these highly oblique Riedel orientations likely reflect components of vertical uplift or 156 Figure C7. (a) Map view and stereographic projection of fault traces, fault dips, and slickenline trend and plunge along a N35 E trending segm ent o f the East Kaibab shear zone near Pump Canyon Spring (see Fig. C5 for location). Despite natural scatter in the data, synthetic and antithetic fault orientations are apparent. Black squares on the stereographic projection indicate approximate orientations of a, and ct2 . (b) Both the mapped fault features and the stereographic projection have been rotated 44 counterclockwise around a 105 azimuth to present view s of fault orientations in the a,- ct3 plane ((^vertical). Fault traces retain synthetic and antithetic orientations with respect to shear zone boundaries in the 0 , - 0 3 plane, but fault dips approach vertical and slickenlines becom e horizontal in this perspective. 157 transverse shortening across the shear zone. Despite the natural scatter in the data, average fault dips rotate closer to vertical and slickenlines approach the horizontal orientations expected in the O1-G3 plane of Riedel shear systems. With the O1-O3 plane horizontal, the a, direction is defined by the horizontal bisector of R and R' fault planes. For this data subset, local Oi makes an angle of 36° with average R and R' shears, and lies 55° clockwise from the shear zone orientation in the O1-O3 plane. Rotated back to the original horizontal coordinate system, the local Gi for this stretch of the shear zone trends 274° and plunges 10°. Because local Oi is the sum of shear-induced Gi (in this case, N80°E) and sub-regional Ci, this result might imply that the azimuth of Laramide sub-regional Gi was closer to S86°E than to N78°E. However, the N78°E shortening direction was calculated using a larger data set, and therefore represents a more reliable sub-regional shortening direction than the S86°E result. Conclusions The East Kaibab shear zone in southern Utah displays a surface pattern of synthetic and antithetic faults associated with reverse-right-lateral strain during growth of the. Fast Knihah monocline When viewed in the CT1-G3 plane of the stress ellipsoid, > resemble R- and R'-shear orientations observed in the xike-slip Riedel shear zones. Along the East Kaibab f R- and R'-shear orientations from the typical strike- 158 slip Riedel pattern diminish in the GrO] plane view, and result from the combination of lateral offset with oblique slip or transverse shortening. Through comparison with published results of physical models, these angular variations measure the ratio of lateral offset to vertical offset or transverse shortening within the shear zone. R- and R'- shear orientations also allow a revised estimate of the sub-regional Laramide direction of N78°E (or more easterly). If these relationships can be recognized in further field studies and physical models, Riedel geometry may offer a reliable tool for interpreting obliquity of slip and calculating principal stress directions in oblique or transpressional/transtensional settings. 159 References An, L.-J., 1998. Development of fault discontinuities in shear experiments. Tectonophysics 293,45-59. An, L.-J., Sammis, C.G., 1996. Development of strike-slip faults: shear experiments in granular materials and clay using a new technique. Journal of Structural Geology 18 (8), 1061-1077. Anderson, E.M., 1951. The Dynamics of Faulting. Oliver and Boyd, London. Anderson, R.E., Bamhard, T.P., 1986. Genetic relationship between faults and folds and determination of Laramide and neotectonic paleostress, western Colorado Plateau - Transition Zone, central Utah. Tectonics 5,335-357. Babenroth, D.L., Strahler, A.N., 1945. Geomorphology and structure of the East Kaibab monocline, Arizona and Utah. Geological Society of America Bulletin 56,107- 150. Bartlett, W.L., Friedman, M., Logan, J.M., 1981. Experimental folding and faulting of rocks under confining pressure, Part IX: Wrench faults in limestone layers. Tectonics 79,255-277. Cloos, H., 1928. Experimenten zur Inneren Tektonik. Centralbl. f. Mineralogie U. Pal. V. 1928B, 609-621. Davis, G.H., 1999. Structural geology of the Southern Utah Parks and Monuments Region, Colorado Plateau, with special emphasis on deformation band shear zones. Geological Society of America Special Paper 342,158 p. Davis, G.H., Bump, A., Garcia, P.E., Ahlgren, S., 2000. Conjugate Riedel deformation band shear zones. Journal of Structural Geology 22, 169-190. Davis, G.H., Reynolds, S.J., 1996. Structural geology of rocks and regions. New York, John Wiley and Sons, 776 p. de Urreiztieta, M., Gapais, D., Le Corre, C , Cobbold, P.R., Rossello, E., 1996. Cenozoic dextral transpression and basin development at the southern edge of the Puna Plateau, northwestern Argentina. Tectonophysics 254,17-39. Fossen, H., Tikoff, B., 1998. Extended models of transpression and transtension, and application to tectonic settings. In: Holdsworth, R.E., Strachan, R.A., Dewey, J.F. (Eds.), Continental Transpressional and Transtensional Tectonics. Geological Society of London, Special Publications, 135, pp. 15-33. 160 Huntoon, P.W., 1971. The deep structure of the monoclines in eastern Grand Canyon, Arizona. Plateau 42,66-74. Huntoon, P.W., 1981. Grand Canyon monoclines; vertical uplift or horizontal compression? In: Boyd, D.W., Lillegraven, J.A. (Eds.), Rocky Mountain Foreland basement Tectonics, 19. University of Wyoming Contributions to Geology, pp. 127-134. Huntoon, P.W., 1993. Influence of inherited Precambrian basement structure on the localization and form of Laramide monoclines. Grand Canyon, Arizona. Geological Society of America Special Paper 280,243-256. Huntoon, P.W., Billingsley, G.H., Sears, J.W., Hg, B.R., Karlstrom, K.E., Williams, M. L., Hawkins, D., Breed, W.J., Ford, T.D., Clark, M.D., Babcock, R.S., Brown, E.H., 1996. Geologic map of the eastern part of the Grand Canyon National Park, Arizona. Grand Canyon Association. Hvorslev, M.J., 1937. Uber die Festigkeitseigenschaften gestorter bindiger Boden. Ingeniorvidenkabelige Skrifter, Ser. A, no. 35. Jones, R.R., Holdsworth, R.E., 1998. Oblique simple shear in transpression zones. In: Holdsworth, R.E., Strachan, R.A., Dewey, J. F. (Eds.), Continental Transpressional and Transtensional Tectonics: Geological Society, London, Special Publications, 135, 35-40. Linzer, H. G., Ratschbacher, L., Frisch, W., 1995. Transpressional collision structures in the upper crust: the fold-thrust belt of the Northern Calcareous Alps. Tectonophysics 242,41-61. Mandl, G., 1988. The mechanics of tectonic faulting; models and basic concepts. In: Zwart, H. J. (Ed.), Developments in Structural Geology: Elsevier, Amsterdam, 407 p. McKinnon, S.D., de la Barra, I.G., 1998. Fracture initiation, growth, and effect on stress field: a numerical investigation. Journal of Structural Geology 20 (12), 1673- 1689. Morgenstem, N.R., Tchalenko, J.S., 1967a. Microscopic structures in kaolin subjected to direct shear. Geotechnique 17, 309-328. Morgenstem, N.R., Tchalenko, J.S., 1967b. Microstructural observations on shear zones from slips in natural clays. Geotechnical Conf. Oslo Proc. V. I p. 147-152. 161 Naylor, M.A., Mandl, G., Sijpesteijn, C.H.K., 1986. Fault geometries in basement- induced wrench faulting under different initial stress states: Journal of Structural Geology 8,737-752. Pubellier, M., Cobbold, P.R., 1996. Analogue models for the transpressional docking of volcanic arcs in the Western Pacific. Tectonophysics 253, 33-52. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York. Reading, H.G., 1980. Characteristics and recognition of strike-slip fault systems. Spec. Publ. Int. Ass. Sediment. 4,7-26. Reches, Z., 1978. Development of monoclines: Part I, structure of the Palisades Creek branch of the East Kaibab monocline, Grand Canyon, Arizona. In: Matthews, V. (Ed.), Laramide Folding Associated with Basement Block Faulting in the Western United States, Memoir 151. Geological Society of America, pp. 235- 273. Richard, P., Mocquet, B., Cobbold, P.R., 1991. Experiments on simultaneous faulting and folding above a basement wrench fault. Tectonophysics 188, 133-141. Richard, P.D., Naylor, M.A., Koopman, A., 1995. Experimental models of strike-slip tectonics. Petroleum Geoscience 1,71-80. Riedel, W., 1929. Zur mechanik geologischer Brucherscheinungen. Centralblatt fur Mineralogie, Geologic, und Paleontologie., 1929B, 354-368. Sanderson, D.J., Marchini, W.R.D., 1984. Transpression. Journal of Structural Geology 6 (5), 449-458. Schreurs, G., 1994. Experiments on strike-slip faulting and block rotation. Geology 22, 567-570. Schreurs, G., Colletta, B., 1998. Analogue modelling of faulting in zones of continental transpression and transtension. In: Holdsworth, R.E., Strachan, R.A., Dewey, J. F. (Eds.), Continental Transpressional and Transtensional Tectonics. Geological Society of London, Special Publications, 135, pp. 59-79. Sylvester, A.G., 1988. Strike-slip faults. Geological Society of America Bulletin 100, 1666-1703. Tchalenko, J.S., 1970. Similarities between shear zones of different magnitudes. Geological Society of America Bulletin 81,1625-1639. 162 Tchalenko, J.S., Ambraseys, N., 1970. Structural analysis of the Dasht-e Bayaz (Iran) earthquake fractures. Geological Society of America Bulletin 81,41-60. Teyssier, C , Tikoff, B., Markley, M., 1995. Oblique plate motion and continental tectonics. Geology 23 (5), 447-450. Tindall, S.E., Davis, G.H., 1999. Monocline development by oblique-slip fault- propagation folding: the East Kaibab monocline, Colorado Plateau, Utah. Journal of Structural Geology 21 (10), 1303-1320. Wilcox, R., Harding, T., Seely, D., 1973. Basic wrench tectonics. American Association of Petroleum Geologists Bulletin 57,74-96. 163 APPENDIX D: BASEMENT-INVOLVED OBLIQUE SHORTENING STRUCTURES: PHYSICAL MODELS AND FIELD EXAMPLES Abstract Scaled physical models and field studies indicate that secondary features characteristic of both dip-slip shortening and strike-slip deformation form in basement- involved oblique shortening structures. Features and patterns associated with basement-cored deformation previously attributed to pure dip-slip reverse faulting or pure strike-slip faulting are actually the result of oblique movement, possibly along a reactivated fault. Physical analog models were constructed to simulate deformation in sedimentary cover above reactivated strike-slip, dip-slip shortening, and oblique shortening basement faults. Model features associated with dip-slip shortening and strike-slip faulting are easily distinguished, and correspond well with features observed in previous modeling and field studies of strike-slip systems and dip-slip reverse fault systems. However, oblique shortening models display secondary structures characteristic of both strike-slip and dip-slip deformation. In map view, oblique shortening structures include a fault-parallel asymmetrical anticline, outer-arc normal faults oblique to both the basement trend and the shortening direction, and oblique- reverse faults with Riedel shear orientations. In cross-section secondary faults in sedimentary cover display reverse stratigraphic separation characteristic of dip-slip 164 reverse faulting but show braided fault traces and unmatched hanging wall and footwall cutoffs more characteristic of strike-slip systems. The fact that oblique deformation exhibits characteristics similar to those in both reverse-slip and strike-slip systems contributes to the difficulty of recognizing field examples of oblique deformation. Oblique shortening features observed in physical models also occur in field examples of oblique shortening structures. The East Kaibab monocline in southern Utah, a basement-cored uplift recently interpreted as the result of oblique reverse reactivation of a steeply-dipping basement fault, resembles the oblique shortening models in map pattern. The most distinguishing oblique-shortening characteristics of the monocline include normal faults on the crest of the fold that nearly bisect the angle between the fold trend and the shortening direction, and en-echelon reverse faults in the steep limb that accommodate a component of right handed offset. Introduction The term ‘basement-cored uplift’ refers to a deformation style marked by uplift of basement blocks and overlying sedimentary cover along a fault or several faults that are reactivated or nucleated in basement (Matthews, 1978; Schmidt et al., 1993). The style is commonly associated with deformation in the foreland of continental margin fold-thrust belts, and differs from ‘thin-skinned’ deformation in the extent to which crystalline basement is included, or exposed, in the uplifted blocks. Basement-cored uplifts are common in orogenic belts around the world (e.g. Sierras Pampeanas in Argentina, Atlas Mountains in Morocco), but by far the best-studied structures are 165 those of the Rocky Mountain foreland and Colorado Plateau of the western United States. There fold form and fault geometry vary significantly from one uplift to the next (Brown, 1988). Among the various hypotheses developed to explain the features associated with basement-cored uplifts are faults that steepen with depth (Prucha et al., 1965), faults that flatten with depth (Stone, 1993), deformed and undeformed hanging wall basement wedges (Wise, 1963; Erslev and Rogers, 1993; Schmidt et al., 1993), drape folds (Prucha et al., 1965; Steams, 1971; Reches and Johnson, 1978), fold-thrust structures (Berg, 1962) and fault-propagation folding (Blackstone, 1940; Erslev, 1991). Decades of debate focused on whether the underlying tectonic cause of basement-cored uplifts in the western United States was vertical uplift (e.g. Steams, 1971,1978; Matthews, 1978) or horizontal compression (Davis, 1978; Reches, 1978; Huntoon, 1981). Proponents of vertical uplift argued that a single horizontal shortening direction could not explain the varied orientations of uplifts, or observations of steep fault dips near the surface. Despite these arguments, compelling evidence for horizontal compression came from bore hole and seismic data showing thrust and reverse faulting beneath the surface (Cries, 1983; Lowell, 1983; Stone, 1984). These studies are confirmed by local calculations of Laramide stress directions based on various kinematic indicators (Reches, 1978; Anderson and Bamhard, 1986; Bergerat et al., 1992; Davis, 1999; Tindall and Davis, 1999). However, the varied orientations of Rocky Mountain foreland and Colorado Plateau uplifts remain a topic of research, with many geologists hypothesizing a rotating stress field during Laramide deformation (Chapin and Gather, 1983; Cries, 1983; Livaccari, 1991; Bird, 1998). 166 Some of the variation observed in the structural style of basement-cored uplifts may be due to the effects of varying amounts of oblique deformation. This concept is not new, and has been explored by Stone (1969), Kelley (1955) and other geologists investigating the Rocky Mountain foreland and the Colorado Plateau. However, recognition of general features characteristic of oblique deformation, in contrast with dip-slip or strike-slip motions, has been complicated. From the work of early explorers like Powell (1873) and Walcott (1890) to the recent kinematic models of Erslev (1991) and Mitra (1993), geologists have represented basement-cored uplifts in vertical cross- section, indirectly promoting the assumption that principal stress and strain directions are parallel and perpendicular to the plane of the cross section. Oblique movement of material relative to the cross section plane is seldom considered. Such simplified constructions produce reasonable interpretations when this assumption is valid, but can lead to confusion in interpretation of local and regional kinematics in areas of oblique deformation. The mental hurdle of visualizing movement in and out of the cross section plane presents only one of many obstacles in recognizing oblique deformation. Perhaps the greatest difficulty is that oblique deformation produces fault and fold patterns that are easily mistaken for strike-slip or dip-slip deformation. Field studies and physical models offer geologists a familiar suite of features commonly identified with the end-member cases of strike-slip and dip-slip reverse fault systems. For example, strike-slip settings often develop flower structures, Riedel fault geometry, or lateral changes in fault dip direction and sense of vertical offset (Reading, 1980; Sylvester, 1988; Tchalenko, 1970; Naylor et al., 1986). Basement-cored reverse 167 faulting typically forms large asymmetrical uplifts and reverse faults that propagate through the steep limb of the fold (Mitra and Mount, 1998). Unfortunately these suites of characteristic features may represent the rare end-members on a continuum of oblique deformation that produces complex structural geometries that are quite common difficult to recognize. The purpose of the present study is to identify structures or suites of structures characteristic of basement-involved oblique shortening. We compare results of strike- slip, dip-slip shortening, and oblique shortening models using similar model setup and boundary conditions in order to identify features diagnostic of oblique deformation. Having identified features characteristic of oblique deformation in the models, we compare the model results with a detailed field example of basement-involved oblique shortening. Modeling Methods Models constructed during the course o f this study were designed to resemble sedimentary cover rocks deforming above basement faults subjected to strike-slip, dip- slip reverse, and oblique-reverse offset. The models include strike-slip deformation without syntectonic growth layers, dip-slip shortening both with and without growth layers, and oblique shortening with and without growth layers. Strike-slip, dip-slip shortening and oblique shortening cases each required different apparatus configurations, and these will be discussed separately. However, Table D1 presents a brief summary of relevant parameters used in each model for quick reference and easy 168 DIMENSIONS DEFORMATION TOTAL BASEMENT EXPERIMENT TYPE THICKNESS RATE OFFSET 60 x 60 cm no vertical strike-slip 4 cm (13 pre 2.0 cm / hr, basement fault growth layers) ri^it lateral uplift, 2.75 cm 40 cm long right lateral 30 x 60 cm 4 cm (13 pre 2.0 cm / hr, dip-slip shortening 2.0 cm vertical (without growth) basement fault growth layers) horizontal 60 cm long shortening uplift 4 cm (13 |xe- 2.0 cm / hr, dip-slip shortening % x 60 cm growth layers) basement fault horizontal 2.0 cm vert cal (with growth) plus 4 growth layers shortening uplift 60 cm long (thickness varies) 60 x 60 cm 4 cm (13 pre 2.0 cm / hr, oblique shortening basement fault growth layers) horizontal 2.0 cm vert cal (without growth) 57 cm long shortening uplift 60 x 60 cm 4 cm (13 pre oblique shortening 2.0 cm / hr, basement fa lit growth layers) horizontal 2.0 cm vert cal (with growth) 57cm long plus 4 growth layers shortening uplift (thickness varies) Table D l. Summary of basic parameters for each of tire five physical analog models constructed for this study. Under DIMENSIONS, the basement fault length refers to the main fault segment accommodating offset relevant to each experiment (strike-slip, dip-dip, or oblique-dip). Materials, scaling and boundary conditions are discussed in detail in the text. 169 comparison. These parameters are described in more detail in the ‘Experimental Design’ sections. Modeling materials, scaling, and methods for data collection and analysis were the same for each model run, and are discussed in the following sections. Modeling Material Both wet clay and dry sand are commonly used to model deformation of sedimentary rocks in the crust (e.g. Riedel, 1929; Horsfield, 1977; Ellis and McClay, 1988; An and Sammis, 1996). Clay and sand are granular materials that display Coulomb behavior and have shear strengths that result in reasonable scaling factors with respect to crustal-scale processes (Hubbert, 1951). Sand, however, has almost no cohesive strength; this prevents formation of steep fold limbs and decreases the likelihood of fault reactivation (because a fault surface is no less cohesive than the surrounding material). The advantage of using wet clay is that the cohesive strength of clay allows fold formation and fault reactivation in the models, two processes commonly observed in basement-cored uplifts. For this reason, clay was selected as the modeling material for this series of experiments. The modeling clay used in this study consisted of mixed kaolinite, powdered nepheline-syenite, and powdered flint, with 50% water by weight. The density of the prepared clay mixture was approximately 1.6 g/cc with shear strength (measured by controlled-stress rheometer) between 40 and 65 Pa (Sims, 1993). 170 Scaling In order to produce features that are comparable to real-world structures, physical models must be properly scaled with respect to earth materials (Hubbert, 1937). Specifically, material strength and the length/width/depth dimensions of the model must be reduced by the same factor, and the density of the modeling material must be comparable to the density of the earth material being modeled. The shear strength of sedimentary rocks in the crust varies greatly depending on factors such as composition, consolidation, confining pressure, and pore fluid pressure. Assuming an average shear strength of 5 MPa for brittle rocks in the upper crust, the scaling factor associated with use of wet clay is between 10"4 and 10"5 (Hubbert, 1951). Applying the same factor to spatial dimensions, 1 cm of model length or depth equals between 100 m and 1 km of distance or depth in the Earth’s crust. Within an order of magnitude, the density of the modeling clay is comparable to the density of sedimentary rocks. Additional consideration must be given to the possible effects of strain rate. In theory, deformation is not dependent on strain rate in Mohr-Coulomb behavior (Naylor et al.y 1986; Richard and Cobbold, 1990). The shortening rate of the experiments therefore should have a negligible effect on the resulting structural geometries, and strain rate and spatial dimensions can be scaled independently (Hubbert, 1937). Nevertheless, for the sake of consistency all models were deformed at a rate of 2.0 cm (basal horizontal displacement) per hour, a factor of 105 faster than a reasonable continental interior deformation rate of 2-20 mm/yr. 171 Despite reasonable size and strength scaling, the physical models presented here are not perfectly scaled with respect to natural processes. The grain size of the clay was not considered quantitatively, and the clay is homogeneous and isotropic, lacking mechanical stratigraphy or pre-existing fractures that are ubiquitous in most crustal rocks. The purpose of these simplified models is to identify major features that occur in nature, rather than to reproduce perfectly a natural system. Data Collection Overhead photographs of the top surface of each model were taken after every 0.25 cm increment of basal horizontal offset in order to document the progressive development of structures on the model surface. After deformation, models were allowed to dry, and then were sliced at approximately 2 cm intervals perpendicular to the fault zone to reveal vertical cross sections. Drying the models had two unavoidable effects. First, drying resulted in a variable amount of cracking within the models, often concentrated in the deformed zone. Cross section interpretation was therefore limited to vertical model exposures in which cracking was minimal. Also, the drying process resulted in a 35% reduction in the thickness of the modeling clay - that is, a clay package 4 cm thick at the time of deformation dried and compacted to a thickness of 2.8 cm. It is unclear whether the effects of drying and compaction were uniform throughout each model. Fault trajectories in vertical sections of the strike-slip and dip- slip shortening models do not display significant dip changes with depth, implying that drying and compaction affected the models uniformly. However, fault trajectories in 172 the profile views of oblique shortening models do exhibit noticeable dip changes with depth. This geometry may be a characteristic of oblique shortening, but could also result from non-uniform dewatering and compaction. For this reason the models were not artificially ‘decompacted’ before interpretation. It is reasonable to assume that dewatering and compaction of sediments occur in natural systems to some extent; however this aspect of the models was not considered quantitatively with respect to natural processes. Most structures visible at the Earth’s surface have undergone some erosion or denudation, exposing deep structural levels. Portions of the dip-slip shortening (with growth), oblique shortening (with growth), and strike-slip models were shaved horizontally in 1 mm increments to reveal the map pattern of faulting at depth. Structural interpretations of each model are based on observations of fault and fold patterns on the upper surface, in vertical cross-sections, and in horizontal map view at depth. Physical Models The size of the literature on physical modeling is overwhelming, and a comprehensive review is beyond the scope of this paper. However, it is useful to summarize the methods and results of selected experiments for the sake of comparison with techniques and interpretations presented here. Methods and results of strike-slip, dip-slip shortening, and oblique shortening models are discussed in the following sections, with a brief review of literature relevant to each deformation style. Although 173 numerous physical experiments of strike-slip, dip-slip shortening and oblique shortening have been conducted by previous workers, no systematic series of experiments has attempted to isolate sets of features characteristic of each deformation style. In the present study, results of each of these deformation styles are compared in order to verify the presence of structures commonly associated with strike-slip faulting and dip-slip shortening, and then to isolate features distinctive of oblique shortening. Strike-Slip Review of Literature Models of strike-slip deformation encompass a wide variety of materials and experimental designs, but display remarkable consistency of major results. In classic experiments constructed by Riedel (1929) and repeated by Tchalenko (1970), simple shear of a clay cake overlying a pre-existing, vertical cut in rigid blocks resulted in development of synthetic and antithetic fractures in the clay with orientations oblique to the saw cut and shear direction, followed by through-going faults parallel to the shear direction (Fig. Dla). This characteristic strike-slip fracture geometry constitutes a ‘Riedel shear zone’ in which synthetic (R) fractures occupy angles of 15° with respect to the basement trend, and antithetic (R') fractures lie at angles of 75° (Fig. D l). At higher strains P shears may form at 10° from the shear direction, and eventually all offset occurs on long, continuous faults parallel to the shear zone boundaries (Y-shears; see Fig. D l). Experiments have shown that variations in initial boundary conditions determine whether R or R' shears develop first on the upper surface of the model 174 Figure D l. (a) Typical elements associated with a right-handed Riedel shear system, (b) Riedel shear fracture arrays commondly develop into complicated zones of anastomosing faults and shear lenses with increasing deformation. Antithetic (R1) shears are not always prevalent in Riedel shear zones, and are not represented in (b). 175 (McKinnon and de la Barra, 1998). Furthermore, the oblique R, R' and P shears may develop independently, together, or not at all. With increased displacement the basic Riedel shear pattern matures into a braided zone consisting of shear lenses bounded by anasomosing Riedel-type fractures (Fig. D lb) (Tchalenko, 1970; Sylvester, 1988). Riedel shear zones have been reproduced by simple shear experiments using various modeling materials including clay (Cloos 1928,1955; Wilcox et al. 1973), sand (Naylor et al. 1986) and limestone under confined shear (Bartlett et al. 1981) as well as in numerical investigations (McKinnon and de la Barra,1998). Despite the use of different modeling materials and boundary conditions, all of these discrete simple shear experiments produced a surface fault pattern with Riedel fault geometry. Distributed simple shear experiments (without a discrete pre-existing fault) have been used to study fault nucleation and growth in homogeneous materials (e.g. An and Sammis 1996; Schreurs 1994). Such experimental designs may employ basal rubber sheets or numerous parallel slipping blocks beneath the modeling medium, and therefore are not analogous to reactivation of a single basement fault. However, even these distributed shear experiments develop Riedel fracture orientations (An and Sammis, 1996; Schreurs, 1994). Based on the consistent results of a wide variety of strike-slip or horizontal simple shear models, and observations of similar features in nature, it has been concluded that Riedel shear geometry is characteristic of the surface fault pattern in strike-slip systems (Reading, 1980; Sylvester, 1988). Cross sectional or three dimensional exposures of strike-slip deformation reveal additional characteristic structures. In the subsurface faults typically dip steeply. 176 change dip direction both with depth and along strike, and accommodate small normal or reverse separations that vary along strike. The presence of en echelon faults and folds or simultaneous development of shortening and extension structures may also indicate strike-slip faulting (Sylvester, 1988; Reading, 1980). Reading (1980) noted that although the main fault movement is lateral, the only noticeable fault offset in cross- section may be normal or reverse separation. Experimental Design The experimental design used to model strike-slip deformation is similar to that of the classic Riedel experiments. The apparatus and modeling clay simulate strike-slip motion on a pre-existing crustal weakness and the resulting deformation of overlying, previously undeformed cover sediments. The model apparatus is enclosed on four sides by Plexiglas walls (Fig. D2a). Three walls are stationary, and the fourth is affixed to a computer-operated stepper motor that applies horizontal motion perpendicular to the moving wall. The base of the apparatus (Fig. D2b) consists of two thin, flat metal plates separated by a fault with three segments. The main fault segment is perpendicular to the moving wall and undergoes strike-slip offset during deformation. Two short segments extend from the ends of the main segment parallel to the moving wall, allowing one base plate to slip under the other (Fig. D2b). This geometry generates strike-slip offset along the main fault segment with no vertical, convergent, or divergent component of slip. Before deformation several colored clay layers are deposited on the basal metal plates. Spacing bars of fixed thickness are stacked along the edges of the model during construction to ensure the uniform thickness of the clay clay layers (cut away to ' show base plates) motor spacing bars - Figure D2. Diagrams of the strike-slip experimental apparatus. Two flat, basal metal plates are separated by a 'fault* with three segments (a, b). The main segment is perpendicular to the moving wall of the apparatus, and udergoes right-lateral strike-slip offset as one basal plate moves laterally past the other. Two short fault segments parallel to the moving wall allow the basal metal plates to slide over / under one another so that no vertical motion occurs along the main fault segment. (Deformation along these two fault segments is not strike-slip, and therefore was not analyzed.) Spacing bars allow 13 uniform layers of colored, wet clay to be constructed on the basal metal plates before deformation (a). 178 layers. The strike-slip model contains 13 layers of wet clay, about 0.3 cm thick each, for a total of 4 cm of clay cover overlying the basement fault. According to the calculated scaling factor, this package represents between 400 m and 4 km of upper crustal sedimentary rocks. After construction of the cover section the stepper motor applies horizontal offset to the moving wall and the underlying metal plate at a rate of 2.0 cm per hour, generating strike-slip offset along the main basement fault segment. The final strike-slip model represents a total of 2.75 cm of right lateral offset with no vertical uplift or transverse shortening at the level of the basement plates. Results Map view and cross section fault patterns produced in the strike-slip model resemble characteristic strike-slip features discussed in the literature. Figure D3 illustrates an overhead view (a) and fault interpretation (b) of the upper surface of the strike-slip model after 2.75 cm of right-lateral offset. The fault zone visible at the surface displays a pattern of braided fault segments separating fault-bounded shear lenses. Within the braided fault zone two dominant fault trends are evident. One fault set strikes about 15° oblique to the basement fault trend, similar to R-shears in the classic Riedel pattern. These faults show right-lateral offset on the surface, and their discontinuous traces form a left-stepping en echelon pattern. A second prominent fault set (labeled Y) parallels the basement fault orientation; these fractures represent incipient through-going Y-shears. Connecting the R- and Y-shears are short fault segments in an apparent P-shear orientation (compare with Fig. D l). This experiment did not develop R' shears during deformation. 179 'shear shear | lenses ! direction 5 cm lateral offset basement / faults markers 1 trend i Figure D3. Overhead photograph (a) and fault interpretation (b) o f the strike-slip model after 2.75 cm of right-lateral offset. Surface faults occupy R-, P- and Y-shear orientations typical of Riedel geometry in pure strike-slip systems (compare with Fig. Dla). 180 In cross section (Fig. D4) faults dip steeply but change dip direction with depth, creating a braided pattern in cross section as well as in map view. Small normal and reverse separations are visible across several faults, but the sense of vertical offset also changes with depth along some faults. Although the magnitude of vertical separation is trivial compared to the amount of lateral separation, lateral offset is not evident in cross-section. Besides small normal and reverse separations preserved at the upper surface of the model, no significant surface uplifts or folds developed in the strike-slip experiment. A 4 cm long segment of the strike-slip fault zone was progressively shaved in 1 mm increments to reveal changes in the map view fault pattern with depth. The pattern changes gradually, such that faults become laterally continuous (in particular the R shears) and the individual R, Y and P shear orientations are less distinct at depth. Figure D5 shows the fault pattern at a level approximately 14 mm above basement. The braided fault pattern is still apparent, but individual R, Y and P shear orientations are less obvious at depth than on the upper surface (compare Fig. D5 and Fig. D3). Dip-Slip Shortening Review of Literature Most studies of basement-involved thrust or reverse faulting have been carried out through field mapping, seismic interpretation, or kinematic modeling rather than with physical analog models. Numerous kinematic models have been developed to examine the growth of thrust and reverse faults and their associated folds (e.g. Suppe 181 (b) ______ 2 c m Figure D4. Representative strike-slip cross sections with 2.75 cm of right-lateral offset. Faults dip steeply and can change dip direction with depth. Small normal and reverse separations are evident, but these also change along individual fault traces at depth. The result is a narrow zone of faults that are braided in cross section as well as in map view (Figs. D3 and D5). 182 basement ^ trend 2 c m Figure D5. Block diagram of the strike-slip model eroded to a depth o f 10 mm above basement. The subsurface pattern consists of braided fault traces separating fault-bounded shear lenses. The pattern resembles late stages of strike-slip deformation; compare with Fig. Dlb. 183 and Medwedeff, 1984; Suppe, 1985; Chester and Chester, 1990; Erslev, 1991; Narr and Suppe, 1994; Allmendinger, 1998). These present descriptions of and predictions for general fold and fault form, but are not intended to predict patterns of secondary faulting in sedimentary cover. Davy and Cobbold (1991) performed shortening of a model of the continental lithosphere, but the scale of their experiment prevented emphasis on upper crustal secondary structures that might be recognized as diagnostic of dip-slip reverse faulting. Stone (1993) combined subsurface reflection seismic and borehole data and surface geologic mapping in the Rocky Mountain foreland with clay modeling to constrain geometric and kinematic features associated with basement- involved thrust-generated folds. Although seismic data show that basement faults in the Rocky Mountain foreland dip 20° to 35° at depth, they typically steepen upward within basement to angles of 45° or more at the basement-cover interface. Overlying sedimentary rocks are progressively offset by upward-propagating thrusts or reverse faults within the steep forelimbs of the growing anticlines (Stone, 1984,1993; Berg, 1981). Mitra and Mount (1998) summarized features commonly associated with foreland basement-involved structures based on seismic and borehole data, surface mapping, and balanced geometric and kinematic models, noting asymmetrical anticlinal uplifts, a widening-upward deformed zone within the forelimb of the uplift, and fault slip that decreases up-section. In addition, shallow crestal extensional faults parallel to the long axis of basement uplifts have been noted in the field by Bartram (1929), McCabe (1947) and French (1985). Whereas typical strike-slip structural patterns are easily recognized, common features of basement-involved shortening are not as clearly understood. A major factor contributing to this confusion is the considerable structural variability displayed by basement-cored uplifts like those of the Rocky Mountain foreland. Part of this variability results from the fact that both dip-slip shortening and oblique-shortening structures occupy foreland areas, and these structures display different basic characteristics. The difficulty of differentiating oblique-slip features from dip-slip features in such settings has complicated identification of the distinctive structural styles displayed by dip-slip and oblique-slip basement-cored uplifts. Modeling of these two deformation styles reinforces awareness of the differences between these types of basement-involved deformation. Experimental Design The dip-slip shortening experimental design is similar to the strike-slip setup except in the configuration of the basement blocks. The base of the dip-slip apparatus consists of two metal blocks separated by a fault that strikes parallel to the moving wall and dips 45° toward the moving wall (Fig. D6). The footwall basement block is fixed and the hanging wall rests between the stationary footwall and the moving wall of the apparatus. When horizontal shortening is applied to the moving wall the mobile hanging wall basement block is translated toward the fixed rear wall of the apparatus and simultaneously slides up along the basement fault. This configuration produces pure reverse, dip-slip offset along the basement fault, with components of horizontal shortening (Fig. D6b) and vertical uplift (Fig. D6c). 185 clay layers (cut away to " show base blocks) • spacing bars -- basal metal blocks (b) (c) Figure D6. Diagrams of the dip-slip shortening apparatus. Basal metal blocks are separated by a 45 dipping fault that strikes parallel to the moving wall (a). Spacing bars allow 13 layers of wet, colored clay (each 0.3 cm thick) to be constructed above the 'basement1 fault. A computer-controlled stepper motor then drives the moving wall toward the stationary footwall basement block (perpendicular to basement fault strike), causing the mobile hanging wall to translate toward the footwall (horizontal shortening) and up the fault plane (vertical uplift). The resulting deformation is illustrated schematically in map view (b) and in cross section (c). 186 Two dip-slip shortening experiments were run. The first consisted of 13 layers of clay (4 cm total thickness) deposited on basement before the onset of deformation. The model was subjected to horizontal shortening at a rate of 2.0 cm per hour to a total of 2.0 cm of horizontal offset (this also produced 2.0 cm of vertical uplift). Overhead photographs taken during deformation show the development of surface faults in the absence of syntectonic sedimentation. The second dip-slip shortening experiment consisted of the same 13 pre-growth clay layers deposited before the onset of deformation, and was deformed at 2.0 cm per hour for a total of 2.0 cm horizontal shortening. However, during deformation an additional clay layer was added to the upper surface of the model after every 0.5 cm increment of horizontal shortening (equal to 0.5 cm of vertical uplift), for a total of 4 syntectonic growth layers. Each growth layer was 0.25 cm thick over the uplifted hanging wall and 0.75 cm thick over the footwall, producing a flat upper surface before deformation resumed. The dip-slip shortening model with growth was examined in cross section and in horizontal map view with depth. Results Figure D7 shows an overhead photograph (a) and fault interpretation (b) of the upper surface of the dip-slip shortening model without growth. In the photograph, low- angle lighting from the right highlights the steep limb of a monoclinal uplift. The steep limb dips to the right toward the down-dropped footwall block. A long, continuous, monocline-parallel reverse fault is exposed at the surface in the hinge of the synclinal footwall fold. The reverse fault parallels the trend of the basement fracture. In 187 hanging wall normal I faults 11 reverse shortening direction /reverse /normal Z faults / faults Figure D7. Overhead photograph (a) and fault interpretation (b) of the dip-slip shortening model (without syntectonic growth layers) after 2.0 cm of horizontal shortening (resulting in 2.0 cm of vertical uplift). Lighting in the photograph (a) is from the right, and reflects brightly from the steep limb of a monoclinal uplift that faces toward the right. The underlying basement fault strikes perpendicular to the shortening direction and dips toward the left. A basement-rooted reverse fault offsets the synclinal hinge of the surface uplift and strikes parallel to the underlying basement fault trend. Outer-arc extensional faults on the crest of the monoclinal uplift strike parallel to the basement trend and the fold axis. 188 addition, normal faults with offsets on the order of millimeters or less form on the crest of the monoclinal uplift. These faults also parallel the basement fault orientation and the monocline axis. Normal faults dip toward both the right and left edges of the photograph (bright faults dip right, shaded faults dip left). Figure D8 contains representative cross sections of dip-slip shortening with syntectonic growth layers. In this experiment the basement-rooted reverse faults have not propagated to the surface of the model. Several basement-rooted reverse faultsaccommodate reverse separation at each cross section location. In Figure D8c fault offset is distributed somewhat equally on six independent reverse faults that originate near the tip of the hanging wall basement wedge, whereas in Figure D8b a single basement-rooted reverse fault accommodates the majority of reverse slip. In each cross section most reverse faults propagate up section from the tip of the hanging wall basement wedge; only a few occupy the clay layers beneath the uplifted hanging wall basement block. The zone of faulting and folding in the clay cover section is wedge-shaped, widening upward from basement and tilted over the footwall. The reverse faults are only moderately curved (convex upward) and they diverge up-section so that no braiding or anastomosing is evident in cross section. Outer-arc extensional faults are discernible in cross section as well. Clusters of short, small-offset faults in the pre-growth layers of each cross section appear to lose displacement downward. Although their apparent offset is reverse, the faults occupy the steep limb of the monoclinal uplift; if this limb is rotated back to horizontal these faults accommodate small apparently normal displacements. The faults likely formed 189 growth layers crack growth layers crack 2 cm Figure D8. Representative cross sections of dip-slip shortening with growth. Faults with reverse offsets occupy a widening-upward zone of deformation within the steep fold limb. Basement-rooted reverse faults diverge up-section. Outer-arc extensional faults (blue) lose displacement down-section, and have rotated to shallow dips within the steep limb. Cracks beneath the hangingwall basement wedge develop when the models are removed from the apparatus, and are not related to deformation. as normal faults at early stages of deformation and subsequently were rotated to their current positions. A 4 cm long segment of the dip-slip shortening model with growth deposits was shaved horizontally in 1 mm increments to reveal the nature of the fault pattern below the surface. Figure D9 is a block diagram of this model section eroded to 10 mm above hanging wall basement. Map view faults are dashed where inferred. The cross section of this model segment again reveals a wedge-shaped zone of deformation in which several basement-rooted reverse faults diverge toward the surface. The reverse faults are difficult to trace across the eroded surface because they do not interrupt the traces of dipping clay layers. Slight changes in the thicknesses of the folded clay layers could be attributed to primary variations in thickness (resulting from model construction) just as easily as fault offset. The consistent lateral continuity of the clay layers indicates that the basement-rooted reverse faults are for the most part parallel to the monocline axis and to the basement fault trend. Oblique Shortening Review of Literature The models of oblique deformation conducted in this study are examples of oblique simple shear, with boundary conditions that differ from those of transpression. Most physical and theoretical models of transpression assume that the far-field displacement vector lies in the horizontal plane (e.g. Sanderson and Marchini, 1984; Harland, 1971). Jones and Holdsworth (1998) presented a mathematical framework 191 Figure D9. Block diagram o f the fault pattern created by dip-slip shortening at a subsurface level ~10 mm above hangingwall basement. In cross section, basement-rooted faults accommodate reverse offset within a widening-upward zone o f deformation in the steep fold limb. In map view no lateral offset of clay layers is evident. Basement-rooted reverse faults strike parallel to the fold trend and to the basement fault orientation. 192 and description of transpression in which horizontal pure and simple shear combine with a vertical simple shear. Thus in the situation described by Jones and Holdsworth (1998) the far-field displacement vector is oblique to all three Cartesian axes. One can in turn imagine a situation in which no horizontal pure shear occurs within the deforming zone - instead, components of horizontal and vertical simple shear combine to create oblique simple shear. In the case of reactivation of a basement structure, the orientation of the basement fault imposes components of horizontal shortening, vertical uplift, and lateral shear on the developing shear zone in the overlying sedimentary column. In essence, the three spatial components of deformation (fault-parallel, fault- perpendicular, and vertical uplift) can be thought of as a net oblique simple shear along the basement structure. Several authors have modeled oblique deformation with a component of basement uplift. A simplistic but effective model by Baltz (1967) involved vertical and lateral motion of flat boards covered with moist flour. The cover developed an asymmetrical zone of convex-up faults over the down-thrown footwall, and crestal normal faults oblique to the basement fault trend. Bartlett et al. (1981) modeled oblique deformation by applying vertical and lateral slip along a vertical discontinuity. This oblique motion generated an asymmetrical wedge of cover deformation tilted over the down-dropped forcing block. Richard et al. (1995) related the ratio of dip-slip versus strike-slip movement to the degree of obliquity of the pre-existing fault strike relative to the shortening direction. Richard (1991) and Richard and Cobbold (1990) created oblique-reverse slip on a moderately dipping basement fault and developed 193 obliquely oriented faults with oblique slip in the overburden. Like in other oblique shortening experiments, Richard and Cobbold (1990) noted that basement-rooted reverse oblique faults were convex-up and formed an asymmetrical wedge above the down-thrown block. Richard et al. (1995) performed oblique simple shear of sand and fault gouge above a 45°-dipping basement fault and found that a strike-slip to dip-slip ratio of 1.9 resulted in en echelon R-shears at angles greater than 15° from the imposed shear direction (Richard et al., 1995). However, none of these oblique deformation experiments systematically compared results of oblique deformation with results of strike-slip and reverse-slip faulting in order to isolate results characteristic of oblique slip. Experimental Design The oblique deformation apparatus is illustrated in Figure DIO. The base of the modeling deck consists of two metal blocks separated by a basement fault that strikes 45° from the horizontal shortening direction (45° from the moving wall of the apparatus) and dips 45° toward the moving wall. The footwall basement block is attached to the stationary edges of the apparatus, and the mobile hanging wall rests between the footwall and the moving edge of the modeling deck. During horizontal shortening the basement fault accommodates a component of right lateral offset imposed by its oblique strike relative to the applied horizontal shortening (Fig. DlOb), and vertical uplift imposed by its dip (Fig. DIOc). Two oblique shortening models were constructed. The first model consisted of 13 pre-deformation layers of clay, each 194 clay layers (cut away to " / show base blocks); spacing bars basal ^ metal r- blocks Figure DIO. Schematic diagrams o f the oblique shortening apparatus. Metal basement blocks are separated by a pre-existing 'fault' that strikes 45 from the moving wall (45 from the shortening direction) and dips 45 toward the moving wall (a). During deformation the stepper motor translates the moving wall of the apparatus toward the fixed basal footwall block. As a result, the mobile hanging wall moves toward the footwall and up the fault plane. This produces a 1:1 ratio o f right-lateral (b) to reverse (c) offset. Before deformation, layers o f colored wet clay are constructed between metal spacing bars on top of the basal metal blocks. 195 approximately 3 mm thick, for a total clay thickness of 4 cm. The basement blocks were subjected to 2.75 cm of horizontal shortening at a rate of 2.0 cm/hr, producing 2.0 cm of vertical uplift (the same amount of uplift produced in the dip-slip shortening experiments). The second oblique-shortening model also consisted of 13 layers of clay deposited before deformation, but a syn-tectonic growth layer was added to the top of the model after every increment of 0.5 cm of vertical uplift. Each growth layer was 0.25 cm thick over the hanging wall and 0.75 cm thick over the footwall (producing a flat upper surface). The oblique shortening model without growth was used to investigate fault and fold patterns at the model surface, and the growth model was examined in cross section and in map view at depth. Results Figure D ll contains an overhead photograph (a) and fault interpretation (b) of the oblique shortening model without growth at the conclusion of deformation. In the photograph, low-angle lighting from the right emphasizes a monoclinal flexure that dips toward the upper right comer of the photograph. An irregular zone of faults with apparent reverse offset cuts across the model surface in the synclinal hinge of the fold. Compared with the reverse fault zone that formed on the surface of the dip-slip shortening model (Fig. D7), the oblique-reverse fault zone appears wavy and irregular in trend. Small-offset outer arc extensional faults have formed on the crest of the monocline, and dip both toward and away from the steep fold limb. However, these faults have strike orientations that are oblique to both the basement fault trend and the shortening direction. Extensional faults strike 20° from the trend of the fold axis. 196 reverse faults hanging wall normal faults outer-arc reverse normal faults faults shortening direction Figure D ll. Overhead photograph (a) and fault interpretation (b) of the surface of the oblique shortening model (without growth). In the photograph, lighting from the lower right reflects from a monoclinal fold that faces the upper right comer. The underlying basement fault extends from the upper left to lower right in each figure. A zone of irregular reverse faults offsets the synclinal hinge of the fold. Outer-arc extensional faults strike obliquely to both the fold trend and the horizontal shortening direction. 197 Figure D12 contains three representative cross sections of the oblique shortening model with growth. At first glance the fault pattern is very similar to that of the dip-slip shortening model. Several faults diverge upward from the tip of the hanging wall basement wedge, and form a widening-upward zone of deformation. Only a minor amount of faulting is evident in the clay layers beneath the uplifted hanging wall basement block. Stratigraphic separation on basement-rooted faults is reverse, and clusters of outer-arc extensional faults have formed in the steep limb of the monoclinal fold below the upper surface of the model. However, several subtle differences are worth noting. First, the majority of reverse faults in the oblique shortening model actually root into basement several millimeters back on the upper surface of the hanging wall block (rather than at the tip of the hanging wall basement wedge). It is possible that early-formed basement-rooted faults became inactive and were translated passively toward the hanging wall as deformation progressed. Also, the basement-rooted faults show a tendency to braid in cross section, whereas reverse faults diverged up-section in the dip-slip shortening model. This tendency toward braided fault traces, both in map view and in cross section, is a characteristic observed in the strike-slip model. Finally, the basement rooted faults in the oblique shortening model initiate at steep angles at the basement- cover interface and show a more exaggerated convex-up form than the faults in the dip- slip model. The steep angle of faults near the basement-cover interface is similar to fault geometry in the strike-slip model. 198 crack 2 cm Figure D12. Representative cross sections of the oblique shortening model with growth. Basement-rooted faults have convex-up trajectories, resulting in braiding within an upward-widening zone o f deformation. Apparent offset is reverse in cross section view. 199 Figure D13 shows a block diagram of a portion of the oblique shortening model eroded to 14 mm above hanging wall basement. Again, the cross section view reveals a braided zone of basement-rooted faults with apparent reverse offset. In map view, fault segments that are oblique to the fold axis and basement fault trend offset the steeply dipping clay layers in apparent right lateral fashion. These fault segments average 20° oblique to the basement fault trend, forming a left-stepping en echelon pattern indicative of right-lateral shear. The right-lateral segments resemble R-shears in a Riedel shear zone, but their 20° angle is slightly greater than the usual 15° orientation expected in pure strike-slip systems. It is possible that the upper surface of this model also includes reverse or oblique-reverse faults parallel to the basement trend (similar to Y-shears in strike-slip systems) but that these are not evident because they do not accommodate lateral offset of clay layers. Discussion of Model Results Major results of each deformation style (strike-slip, dip-slip shortening and oblique shortening) are summarized in Figure D14 and in Table D2. Each block diagram in Figure D14 contains a representative cross section from one of the models combined in three-dimensional perspective with a view of the upper surface of the model (without growth) at the end of deformation. Basement-rooted faults and outer- arc extensional faults are interpreted on each block diagram. In general, strike-slip and dip-slip shortening models developed features traditionally recognized as characteristic of these end-member deformation styles. The primary and secondary structures formed 200 Figure D13. Block diagram showing the fault pattern produced by oblique shortening at a depth o f —14 mm above hangingwall basement. In cross section, anastomosing basement-rooted faults display reverse separations. In map view, several fault segments oblique to folded clay layers (and oblique to the basement fault strike) lie in a left-stepping pattern and accommodate apparent right-lateral offsets. 201 Figure D14. Block diagrams illustrating major features of strike-slip (a), dip-slip shortening (b) and oblique shortening (c) models. al 2 Summary Tablecharacteristicof D2. features noted in thestrike-slip, dip-slip combination of features observed strike-slipin and dip-slipdeformation. shortening and oblique Oblique shorteningshortening models. exhibits a MAJOR FEATURES al offset fault outer - arc arc - outer basem ent ent basem od form fold ae ent basem extension emetry geom rooted rooted rooted fault aeet trend basement aeet trend basement aall to parallel strike-slip biu to oblique tiesi reverse strike-slip braided none none + + EOMTO STYLE DEFORMATION aeet trend basement ieg upwarddiverge aeet trend basement shortening om l faultsnormal dip-slip aall to parallel a all to parallel ooln monocline monocline + aeet trend basement aeet trend basement aeet trend basement om l faultsnormal shortening biu to oblique aall to parallel biu to oblique oblique strike-slip braided reverse + + + 202 203 in our models closely resemble features documented in previous field and modeling studies of strike-slip and dip-slip reverse faulting, indicating that modeling techniques employed here produce reasonable structural styles. In turn, this allows confident interpretation of characteristic features associated with oblique shortening based on the results of the oblique-slip models, which were constructed using the same techniques. The strike-slip model produced a fault zone on the upper surface with left- stepping R-shears 15° oblique to the basement shear direction connected by shear zone- parallel Y-shears and right-stepping, 10° oblique P-shears. These distinctive Riedel shear fracture orientations have been documented in innumerable strike-slip and distributed shear experiments (e.g. Riedel, 1929; Morgenstem and Tchalenko, 1967; Tchalenko, 1970; Mandl et al., 1977; Richard et al., 1995; An and Sammis, 1996; McKinnon and de la Barra, 1998) as well as in field examples of strike-slip faulting (Tchalenko, 1970; Harding and Lowell, 1979; Sylvester, 1988). At depth within the strike-slip model, Riedel fracture geometry gave way to a complex zone of braided fault segments separating fault-bounded shear lenses. This fault pattern is typical of more mature strike-slip zones with large lateral offset (Tchalenko, 1970; Naylor et al., 1986; Sylvester, 1988). Strike-slip faults accommodated predominantly lateral offset, but minor normal and reverse separations were evident in cross section. In natural strike-slip environments where evidence of lateral offset is difficult to distinguish, such small vertical separations are often the only evidence of fault offset (Reading, 1980). Furthermore, faults in the strike-slip model were steeply dipping, and in some cross sections changed dip direction and sense of vertical separation with depth. No major surface uplift developed during pure strike-slip deformation. The structures produced in dip-slip shortening models resembled characteristic features documented in physical models and field examples of basement-cored uplifts. Models formed a major monoclinal uplift with basement-rooted reverse faults propagating through the steep fold limb. This fault-fold relationship is evident in schematic cross sections, seismic interpretations, and even physical models described by Berg (1981), Brown (1988) and Stone (1984, 1993). Dip-slip shortening models produced basement-rooted faults with strikes parallel to the fold axis and the basement fault strike both at the model surface and at depth. The basement-rooted fault zone widened upward from the tip of the hanging wall basement wedge and was tilted over the footwall, a common characteristic noted by Mitra and Mount (1998). In addition, outer-arc extensional faults developed on the crest of the uplift parallel to the fold axis and perpendicular to the horizontal shortening direction. Similar outer-arc extensional faults are present on the crests of basement-cored uplifts in the Rocky Mountain foreland (Stone, 1993); the Elk Basin anticline in Wyoming offers a premier example of outer-arc normal faults parallel to the uplift axis and perpendicular to the shortening direction (Bartram, 1929; McCabe, 1948; French, 1985). The most distinctive features developed in the oblique shortening models were outer arc extensional faults oriented obliquely (20°) to both the basement trend and the shortening direction. Oblique outer-arc normal faults have not been noted in previous models of oblique shortening, but are present along the Owl Creek mountains in 205 Wyoming (Wise, 1963), the Naciminento uplift in New Mexico (Baltz, 1967), the Saharan Atlas Mountains (Vially et al., 1994) and other examples of basement-cored structures suspected to have oblique-slip origins (see Paylor and Yin, 1993; Karlstrom and Daniel, 1993; Vially et al., 1994 for evidence of oblique deformation associated with the three areas named above). It is interesting to note that dip-slip shortening resulted in outer-arc normal faults parallel to the long axis of the uplift and perpendicular to maximum compressive stress, not oblique to both. The fact that outer- arc normal faults developed at an oblique angle to both the basement trend and the shortening direction is therefore characteristic of oblique shortening deformation, at least in this series of models. The obliquity of slip may dictate the angle between the basement trend and the outer-arc extensional faults; this is a topic for future investigation. However, the left-stepping, en-echelon pattern created by these faults resembles the surface pattern of basement-rooted R-shears associated with strike-slip systems. Therefore oblique shortening can only be recognized definitively if the small- offset, en-echelon normal faults occupy the crest of a major uplift. Strike-slip, dip-slip shortening and oblique shortening experiments developed very distinctive subsurface map patterns. Basement-rooted reverse faults in the dip-slip shortening models maintained strike orientations parallel to the basement fault, whereas both strike-slip and oblique shortening models contained Riedel-type faults with apparent lateral offset. However, in oblique shortening these R-shears were confined within the steep limb of a major uplift, whereas no large uplift formed in strike-slip deformation. This distinction makes it possible to distinguish field evidence for strike- 206 slip and oblique-reverse deformation in map view (assuming that strata were horizontal before deformation). The en-echelon Riedel shears noted in our oblique shortening experiment are similar to those formed in oblique-reverse models by Richard et al. (1995). The dip-slip shortening model, on the other hand, contained a zone of fold- parallel faults within the steep limb of the basement uplift. For this reason, it should be possible to distinguish dip-slip uplifts from oblique-slip uplifts even in the map view of a deeply eroded structure. Although examination of model cross sections did not reveal individual features diagnostic of oblique shortening, several interesting patterns were noted. Basement- rooted faults showed reverse separation similar to faults formed by dip-slip shortening, but originated farther back on the hanging wall basement block. Basement-rooted faults in the oblique experiments displayed a more pronounced convex-upward geometry than faults formed by dip-slip deformation, possibly due to steeper fault dips near the basement-cover interface. Also, faults in the oblique models showed a tendency to braid in cross section. Such steep fault dips and anastomosing fault traces were observed in cross sections of the strike-slip model, but not in the dip-slip shortening experiments. If observed in seismic data these structural patterns might offer evidence for oblique deformation, but their presence has not yet been noted or confirmed in good field examples of oblique shortening. The cross section characteristics of oblique basement-cored uplifts merit further study. 207 Field Example: The East Kaibab Monocline, Utah Background The Kaibab uplift of northern Arizona and southern Utah is a Laramide-age, basement-cored uplift in the Colorado Plateau province of the western United States (Fig. D15). The eastern margin of the uplift is marked by an abrupt, east-vergent fold, the East Kaibab monocline, in Paleozoic and Mesozoic sedimentary rocks. The fold was created by west-side-up reactivation of a steep, west-dipping fault in Precambrian basement (Reches, 1978; Huntoon et al., 1996). Along the 60 km long, northeast- trending segment of the fold in southern Utah, structural relief between the anticlinal crest and the synclinal trough of the fold is as much as 1600 m (Babenroth and Strahler, 1945). Presently the thickness of Paleozoic and Mesozoic sedimentary rocks overlying the basement fault is on the order of 2 km, but Cenozoic uplift and erosion of the Colorado Plateau has removed at least 1-2 km of additional sedimentary rocks from the area since Late Cretaceous - early Tertiary development of the monocline (Hintze, 1988). Kinematic and theoretical analyses have demonstrated that horizontal compressive stress, rather than vertical uplift, was responsible for west-side-up reactivation of the Butte fault during the Laramide orogeny (Reches, 1978; Anderson and Bamhard, 1986). In the Grand Canyon region, the Laramide shortening direction was about N65°E (Fig. D15), 45° oblique to the trend of the monocline (and assumed basement fault) in the southern Utah study area (Reches, 1978) (Fig. D15). Tindall and Davis (1999) used this angular relationship between stress direction and pre-existing 208 Laramide shortening Figure D15. Location of the East Kaibab monocline in southern Utah. Areas of Figs. D16 and D17 are shaded. 209 structure, along with field mapping and fault slip data, to determine that a significant component of right-lateral offset accompanied the west-side-up, reverse motions responsible for growth of the Kaibab uplift and East Kaibab monocline (1999). The monocline presents an excellent field analog for physical models of oblique shortening conducted in this study because it represents deformation of 2-4 km of upper crustal sedimentary rocks above a steeply-dipping, reactivated basement fault, with an angle of 45° between the horizontal shortening direction and the pre-existing structure. Major Structural Features The structural relationships exposed along the East Kaibab monocline in southern Utah are complex (Sargent and Hansen, 1982; Doelling et al., 1989; Rosnovsky, 1998; Tindall and Davis, 1999). The monocline plunges gently (3-5°) northward, so that progressively deeper structural levels and older stratigraphic units are exposed at the surface toward the south. A zone of intense faulting occupies the steep limb of the monocline, and structural orientations and senses of slip on secondary fault surfaces change from north to south within the zone. Near the northern termination of the monocline, short, northwest-striking, northeast-dipping faults lie at high angles to the trend of the monocline (80-90°) and accommodate small reverse-left- lateral offsets (Fig. D16). Farther south, several northwest-dipping faults strike about 20° oblique to the trend of the monocline and lie in a left-stepping, en-echelon pattern. These faults display reverse-right-lateral offsets (Fig. D17). Farther south still, the steep 210 Kaiparowits Formation Wahweap Fprmation Straight Cliffs Formation Tropic Shale Dakota Sandstone Entrada Formation Carmel Formation (with Page Sandstone member shaded) Navajo Sandstone 2 k m Figure D16. Map o f structures near the northern end o f the East Kaibab monocline (location shown in Fig. D15). Northwest-striking faults represent an R'-shear orientation that was not observed in our models of oblique shortening or of strike- slip faulting (see Fig. Dla). However, outer-arc extensional features in Jurassic Navajo Sandstone strike obliquely to both the fold axis and the horizontal shortening direction. These features are stunningly similar to crestal normal faults formed in the oblique shortening model without growth (Fig. D ll). 211 co 14 § Ksc Straight Cliffs Formation o Kt Tropic Shale E l Dakota Sandstone Entrada Formation Carmel Formation (with Page Sandstone member shaded) Navajo Sandstone Laramide shortening i t Figure D17. Geologic map of a segment of the East Kaibab monocline showing oblique- reverse faults in a left-stepping, en-echelon pattern within the steep fold limb. Faults strike about 20c oblique to the fold trend and dip steeply west. Compare the fault pattern with that of the oblique shortening model in Fig. D13. 212 monoclinal limb displays pervasive brecciation, fault gouge, and evidence for through- going, monocline-parallel faults with reverse-right-lateral offset (Tindall and Davis, 1999). Recent interpretations of this monocline-parallel shear zone point out that northwest-striking and northeast-striking faults represent antithetic and synthetic orientations, or R'- and R-shears, within a reverse-right-lateral shear zone (Tindall and Davis, 1999; Tindall, submitted). Furthermore, the south-to-north changes in structural style are consistent with fault-propagation folding within the steep limb of a basement- cored uplift, and therefore represent an early stage of deformation similar to that observed in the Rocky Mountain foreland (Tindall and Davis, 1999). Davis et al. (2000) also reported the presence of pervasive conjugate extensional deformation band shear zones (strain-hardened brittle faults) littering exposures Jurassic Navajo Sandstone where it forms the crest of the East Kaibab monocline. Figure D16 depicts representative orientations of these structures on the stripped structural surface of Navajo Sandstone. The deformation band shear zones dip both east and west and accommodate small normal offsets apparently necessary to allow outer-arc extension of the brittle sandstone during folding and uplift (Davis, 2000). Interestingly, the bands strike consistently N30o-50°E, oblique to both the fold axis (N20°E) and the horizontal shortening direction (N65°E). Where these faults have rolled over within the steep, east-dipping fold limb, they display shallower dip angles and apparent reverse offset of east-dipping sedimentary rock layers, simply as a result of progressive rotation after they formed. 213 Comparison with Model Results Naturally the structural relationships observed in any field example are more complex than those produced in a relatively homogeneous, isotropic modeling medium deformed under controlled boundary conditions. However, comparison of physical models and field examples serves to both verify the validity of model results and to distinguish which features of the models may serve as useful diagnostic tools. The structures within the steep limb of the East Kaibab monocline at its northernmost end (Fig. D16) do not resemble structures formed in oblique shortening models. The short, northwest-striking faults represent R'-shears typically associated with strike-slip deformation (e.g. Riedel, 1929; Tchalenko, 1970) and therefore strongly imply that a component of lateral offset may have occurred. However, R' faults were not observed in our models. It is possible that these faults were present but were too small in trace length or in magnitude of offset to be detected, or that the 1:1 ratio of strike-slip to dip-slip offset generated in the models is insufficient to form R' shears. Furthermore, numerical models by McKinnon and de la Barra (1998) demonstrated that slight variations in initial boundary conditions could determine whether R or R' shears formed first (or at all) in strike-slip settings. In any case, this example demonstrates that physical models cannot replicate exactly the details formed in natural systems. However, the deformation band shear zones on the crest of the Navajo Sandstone are an outstanding analog for outer-arc extensional faults observed in the oblique shortening model without growth. Comparison of Figure D16 with Figure D ll 214 brings into clarity the remarkable similarity between these features. Outer-arc faults in both cases accommodate small normal offsets, dip both toward and away from the steep fold limb, and lie at angles oblique to both the shortening direction and the underlying basement fault strike. In fact, the deformation band shear zones on the crest of the East Kaibab monocline display steeply southwest-raking slickenlines, evidence that they accommodated slight right-lateral offsets (Davis et al., 2000). Furthermore, the deformation bands rotate with the steep fold limb into low-angle positions, just as outer-arc normal faults do in the cross sections of Figure D12. Most remarkable is the close agreement demonstrated by the orientations of these faults with respect to basement trend and shortening direction. In both cases (model and field example) the shortening direction lies at 45° to the basement fault strike, and the outer-arc extensional faults average 20° oblique to the basement fault trend. Given the similarity of these features in physical models and in the field example, we suggest that oblique outer-arc extensional faults are a diagnostic indication of oblique shortening. The area shown in Figure D17 is located directly south of Figure D16, where the monocline-parallel shear zone occupies a slightly deeper structural level. At this location several northeast-striking, northwest-dipping faults form a left-stepping pattern within the steep fold limb. These faults are reminiscent of synthetic R-shears associated with Riedel shear zones, but the surface traces average about 20° oblique to the trend of the monocline. This represents a slightly greater angle than the typical 15° orientation expected for R-shears formed by pure strike-slip motions. In fact, R-shears 215 at angles greater than 15° have been reproduced in several experiments of oblique shortening or transpression (e.g. Richard et al„ 1995; Schreurs and Colletta, 1998), so their highly oblique orientation associated with this oblique-slip basement-cored uplift is not surprising. Apparent offset across the R-shears is right-lateral in map view, and vertical exposures in cross canyons reveal a component of west-side-up separation on each fault. In addition, slickenlines on northeast-striking fault surfaces rake toward the southwest, disclosing reverse-right-lateral slip (Tindall and Davis, 1999). The similarity in orientation of these oblique-reverse faults with those exposed at depth in the oblique shortening model is remarkable. Comparing Figure D17 with the map view of Figure D13, the position of oblique-reverse faults within the steep fold limb, the map-view expression of right-lateral offset, and the left-stepping pattern of short, oblique fault segments are identical features. If observed within the steep limb of other major folds or uplifts, this fault pattern provides compelling evidence for oblique- reverse deformation. Conclusions Comparison of physical analog models of cover deformation above strike-slip, dip-slip shortening and oblique shortening basement faults led to identification of several features that are characteristic of oblique basement-cored uplifts. Both oblique experiments and dip-slip shortening models developed a major monoclinal uplift and formed a widening-upward zone of basement-rooted reverse-separation faults in cross section. However, outer-arc extensional faults on the crest of the uplift in the dip-slip 216 model were oriented parallel to the fold axis and the underlying basement fault trend, and perpendicular to the shortening direction. Outer-arc normal faults in the oblique shortening model formed at an angle of 20° from the fold axis and underlying basement fault orientation. Furthermore, model results indicate that a combination of structures, some characteristic of strike-slip offset and some typical of dip-slip deformation, may be used to distinguish oblique shortening. The oblique experiments contained a major monoclinal uplift and an asymmetrical (tilted over the footwall) zone of faulting and folding that widened upward from basement; these features were also associated with the dip-slip shortening experiments. However, the oblique models also demonstrated braiding of basement-rooted faults in cross section and an en-echelon pattern of oblique faults with apparent lateral offset in map view at depth; these features were observed in the strike-slip model. In the absence of oblique outer-arc extensional faults it is possible to distinguish basement-involved oblique deformation by the presence of a combination of features typically associated with both strike-slip and dip-slip deformation. The East Kaibab monocline in southern Utah, an example of an oblique-slip basement-cored uplift, contains outer-arc extensional faults on the stripped structural crest of Navajo Sandstone that also strike 20° oblique to the trend of the monocline. These oblique outer-arc extensional faults are the most distinctive features associated with our study of oblique shortening. The steep limb of the monocline also contains oblique faults similar to Riedel shears that display a combination of reverse and lateral 217 offset; these faults are nearly identical to oblique-slip faults formed in the steep limb of the oblique shortening experiments. This field example verifies that distinctive oblique-shortening features formed in the physical analog models can be recognized in some naturally occurring oblique-slip basement-cored uplifts. 218 References Allmendinger, R.W., 1998, Inverse and forward modeling of trishear fault-propagation folds: Tectonics, vol. 17, no. 4, p. 640-656. 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The models include strike-slip, dip-slip shortening with growth, dip-slip shortening without growth, oblique shortening with growth, oblique shortening without growth, and two oblique shortening experiments with strain circles placed on the model surface before and after deformation. Only select photographs are presented as figures in the manuscript contained in Appendix 4, but all photographs and sections of the models were considered in forming interpretations of the physical model results. The photographs are catalogued in this Appendix because they represent relevant materials not contained in other portions of the dissertation. 227 228 Strike-Slip Without Growth - Overhead Photographs of Deformation 229 230 Strike-Slip Without Growth - Overhead Photographs of Deformation 2.25b 231 Strike-Slip Deformation Index of vertical cross sections 233 Strike-Slip Cross Sections 8b 234 Strike-Slip Cross Sections 15b 235 Strike-Slip Cross Sections T 18b 19b 20b 236 Strike-Slip C ross Sections 237 Dip-Slip Shortening With Growth Index o f vertical cross section slices Dip-Slip Shortening With Growth - Cross Sections (la - unusable) D ip-Slip Shortening W ith G row th - C ross Sections 240 Dip-Slip Shortening With Growth - Cross Sections 241 Dip-Slip Shortening With Growth - Cross Sections 19a (19b - unusable) Dip-Slip Shortening Without Growth - Overhead Photographs o f Deformation 0.75 cm 1.0 cm 243 Dip-Slip Shortening Without Growth - Overhead Photographs o f Deformation 1.75 cm 2.0 cm Dip-Slip Shortening Without Growth - Overhead Photographs o f Deformation 245 Oblique Shortening With Growth - Index o f Cross Sections 246 Oblique Shortening With Growth - Cross Sections 247 Oblique Shortening With Growth - Cross Sections 1 1 a lib 248 249 Oblique Shortening Without Growth - Index o f Cross Sections O blique Shortening W ithout G row th - C ross Sections O blique Shortening W ithout G row th - C ross Sections O blique Shortening W ithout G row th - C ross Sections O blique Shortening W ithout G row th - C ross Sections O blique Shortening W ithout G row th - C ross Sections 255 O blique Shortening W ithout G row th - C ross Sections