UNLV Retrospective Theses & Dissertations
1-1-2002
Fault segmentation, fault linkage, and hazards along the Sevier fault, southwestern Utah
Ilsa M Schiefelbein University of Nevada, Las Vegas
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Repository Citation Schiefelbein, Ilsa M, "Fault segmentation, fault linkage, and hazards along the Sevier fault, southwestern Utah" (2002). UNLV Retrospective Theses & Dissertations. 1393. http://dx.doi.org/10.25669/pu4m-oa41
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62
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u m t
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FAULT SEGMENTATION, FAULT LINKAGE, AND HAZARDS ALONG
THE SEVIER FAULT, SOUTHWESTERN UTAH
by
Usa M. Schiefelbein
Bachelor of Science University of Wisconsin - Milwaukee 1998
A thesis submitted in partial fulfillment of the requirements for the
Master of Science Degree Department of Geoscience College of Sciences
G raduate College University of Nevada, Las Vegas August 2002
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thesis Approval IJNIV The Graduate College University of Nevada, Las Vegas
August 1 ^2002
The Thesis prepared by
Ilsa M. Schiefelbein
Entitled
Fault Segmentation. Fault Linkage, and Hazards along the Sevier
Fault. Southwestern Utah______
is approved in partial fulfillment of the requirements for the degree of
Master of Science
Examination Committee
Dean of the Crnduate Cotte^e
Examination Coniniitte^Member
Examination Cfimmittec Member
Graduate College faculty Representative
I’K i m - IX)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT
Fault Segmentation, Fault Linkage, and Hazards along the Sevier Fault, Southwestern Utah
by
Ilsa M. Schiefelbein
Dr. Wanda J. Taylor, Examination Committee Chair Associate Professor of Geology University of Nevada, Las Vegas
Theoretically, short normal faults link to form long faults and salients develop in
the linkage zones. This project provides new data and interpretations on (1) normal fault
linkage zones; (2) earthquake and landslide hazards; and (3) fold formation along the
active Sevier fault, Utah. Geologic tmq>ping, geometric analyses, geochronology,
landslide evaluation, and stream history led to five conclusions. (1) Six principal faults
linked to form the Orderville salient and connect the Mt. Carmel and Spencer Bench
segments. Four relay ramps formed between overlapping faults. This multipartite
linkage zone implies that simple models only imperfectly represent natural examples. (2)
Two relay ramps contain rarely analyzed fault-parallel folds. These folds acconunodate a
downward decrease in space between faults. (3) Fluvial deposits indicate three
downcutting stages of a south-flowing stream. Some slip along the Sevier fault occurred
after the first two stages and tilted the deposits. (4) 570 ka basalt is offset ~3 m. Thus,
the young slip rate is 0.018 mm/yr. (5) Mechanical weathering processes or seismicity
induced 14 landslides and similar future slope failures are likely.
Ill
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
ABSTRACT...... Hi
LIST OF FIGURES...... v
ACKNOWLEDGEMENTS...... vi
CHAPTER 1 INTRODUCTION...... 1
CHAPTER 2 CONCEPTUAL MODELS...... 3 Introduction ...... 3 Fault Linkage Models ...... 3 Displacement vs. Distance Diagrams ...... 9 Relay Ramps...... 11 Segment Boundaries ...... 14
CHAPTER 3 REGIONAL TECTONIC BACKGROUND...... 17 Introduction ...... 17 Evolution of the Transform Boundary ...... 18 Regional Tectonics of Southwestern U tah ...... 19 Mesozoic Sevier Orogeny ...... 19 Laramide Orogeny ...... 20 Cenozoic Volcanism ...... 20 Late Cenozoic Basin and Range Style Extension ...... 21 Definition of Provinces in Southern U tah ...... 23 Basin and Range Province ...... 23 Colorado Plateau ...... 23 Utah Transition Zone ...... 24 High Plateaus subprovince of the Colorado Plateau ...... 25
CHAPTER 4 STRATIGRAPHY...... 26 Mesozoic Stratigraphy ...... 26 Cenozoic Stratigraphy ...... 27
CHAPTER 5 STRUCTURAL DESCRIPTIONS...... 29 Introduction ...... 29 Sevier Fault ...... 29 Southern Domain - Orderville Relay Ramp ...... 30 Central Domain - Stewart Canyon Overlap Zone ...... 31 Spencer Bench Domain ...... 35
IV
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Folding within the Study Area ...... 38
CHAPTER 6 VIRGIN RIVER DEPOSITS ...... 39 Introduction ...... 39 Modem Fluvial Deposits ...... 40 Young Fluvial Conglomerate ...... 41 Older Fluvial Conglomerate ...... 42 Interpretations ...... 43 Discussion of the ^rgin River Deposits ...... 44 Faulting within the Quaternary Fluvial Deposits ...... 44
CHAPTER 7 STRUCTURAL INTERPRETATIONS ...... 47 Linkage ...... 47 Examples of Fault Linkage ...... 47 Fault Capture in the Orderville Area ...... 48 Overlapping Faults in the Stewart Canyon Area ...... 49 Eastern Stewart Canyon Overlap Zone ...... 49 Central Stewart Canyon Zone ...... 50 Western Stewart Canyon Zone ...... 51 Folds within Relay Ramps...... 52 Summary...... 53 Stages of Relay Ramp Development in the Stewart Canyon Overlap Zone ...... 53 Stages of Linkage Along the Central Sevier Fault ...... 54 Segments and Segment Boundaries ...... 56 Age Constraints for the Sevier Fault ...... 57 Slip R ates...... 57 Fault Scarps...... 59 Folding along the central Sevier fault ...... 60
CHAPTER 8 EARTHQUAKE AND LANDSLIDE HAZARDS...... 63 Earthquakes...... 63 Landslides ...... 65 Identifying and Categorizing Landslides ...... 65 Regional Slope Failures ...... 67 Triggering Mechanisms for Long Valley Landslides ...... 68 Classification of the Dry Wash Landslide ...... 70 Ethical Assessment of Hazards ...... 72
CHAPTER 9 REGIONAL TECTONIC INTERPRETATIONS...... 73 Introduction ...... 73 Northern Termination of the Sevier Fault ...... 73 Other Long Normal Fault Relationships ...... 74 Sevier, Laramide, or Cenozoic Regional Folding within the Study Area ...... 76
CHAPTER 10 PETROLEUM POTENTIAL...... 77 Normal Fault Linkage and Hydrocarbon Production ...... 77
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hydrocarbon Production near the Study Area ...... 78
CHAPTER 11 CONCLUSIONS...... 80
APPENDIX A METHODS...... 83 Field Mapping Techniques ...... 83 Cross Section Construction Techniques ...... 83 Stereographic Analyses...... 84 "Ar/^’Ar Methods ...... 85
APPENDIX B ARGON DATA TABLE...... 86
FIGURES...... 87
REFERENCES...... 120
VITA...... 132
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES
Figure 1 Location Map ...... 87 Figure 2 Models of Types of Linkage ...... 88 Figure 3 Stress Field Diagram ...... 90 Figure 4 Displacement vs. Distance Models ...... 91 Figure 5 Relay Ramp Models ...... 92 Figure 6 Segment Boundary Models ...... 93 Figure 7 Tectonic Map ...... 94 Figure 8 Lower Stratigraphie Column ...... 95 Figure 9 Upper Stratigraphie Column ...... 96 Figure 10 Simplified Geologic and Location M ap ...... 97 Figure 11 Basalt Isochrons ...... 98 Figure 12 Fault Trace Map ...... 99 Figure 13 Orderville Relay Ramp Fence Diagram ...... 101 Figure 14 Orderville and Glendale Relay Ramp Stereonets ...... 102 Figure 15 Geologic Map of Quaternary Units ...... 103 Figure 16 Stewart Canyon Overlap Zone Fence Diagram ...... 104 Figure 17 Photos of Different Aged Conglomerate ...... 105 Figure 18 Photos of Channels and Sandbars within the Conglomerate U nit ...... 106 Figure 19 Displacement vs. Distance Diagrams for the Central Sevier Fault ...... 107 Figure 20 Displacement vs. Distance Diagrams for the Overlap Zones ...... 108 Figure 21 Space Accommodation Diagram ...... 109 Figure 22 Stages of Linkage along the Central Sevier Fault ...... 110 Figure 23 Fault Trace Maps of the Stages of Linkage within the Stewart Canyon Overlap Zone ...... I l l Figure 24 Types of Landslides ...... 112 Figure 25 Landslide Map of the Area ...... 113 Figure 26 Photo (A) Glendale Landslide (B) Dry Wash Landslide ...... 114 Figure 27 Photos of Vegetation on the Dry Wash Landslide ...... 115 Figure 28 Photos of Vegetation within and Surrounding the Dry Wash Landslide... 116 Figure 29 Photos of the (A) Toe and (B) Head Scarp of the Dry Wash Landslide.... 117 Figure 30 Photo of Toe of Dry Wash Landslide ...... 118 Figure 31 Relay Ramp Models for Petroleum Accumulation ...... 119 Figure 32 Location Map of Oil Fields in Southwestern Utah ...... 120
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDEMENTS
First and foremost, I would like to thank my parents, Rick and Kathy
Schiefelbein, for their endless love, support, and encouragement and my brother for all
the humorous phone conversations and just making me laugh I would also like to thank
my advisor and friend. Dr. Wanda J. Taylor, for all her support, patience, confidence
boosting, understanding, thoughtfulness, and motivation. I could not have done it
without her. Thanks goes to my committee. Dr. Gene Smith, Dr. Terry Spell, and Dr.
Barbara Luke, for all their helpful reviews, encouragement, and meeting important
deadlines. Additionally, thanks to Dr. Gene Smith for assisting me with sample
collection, motivation in the field, and dealing with all my traumas throughout my final
semester.
I would like to acknowledge the following sources of funding which helped make
this project possible; American Association of Petroleum Geologists Grants-in-Aid,
Geological Society of America Research Grant, Sigma Xi Grants-in-Aid of Research,
Arizona/Nevada Academy of Science research grant, UNLV Graduate College Summer
Session Scholarship, Bemada E. French Scholarship, UNLV Graduate Student
Association Grant-in-Aid, Geological Society of America Travel Grants, Roy Shelmon
Meeting Awards, and a UNLV PIA grant to Dr. Wanda J. Taylor and Dr. Margaret Rees.
I would like to thank my field assistant. Treasure Bailey, for helping me keep my
sanity in the field for twelve weeks, input on geologically difficult areas, collecting
VI
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. samples, friendship, and companionship. Thanks to all of the landowners, especially, the
Spencer family, Barry and Judith Ford, the Sorensen family, the Lamb family, the
Chamberlain family, the Heaton family, the BrinkerhofT family, the Cox family, and
Norman Carroll for access to and help navigating their property. I would also like to
thank Ellen Lamb, her family, and staff the Glendale KOA for their friendship, allowing
us to camp at a very reasonable rate in an enjoyable camping environment, allowing us to
store equipment, use the phone, and watching over us. Thanks for everything Ellen.
You, your family, and staff made summer fieldwork logistics easier and very enjoyable.
I acknowledge all the help and advice on land ownership from the staff at the Kanab
Bureau of Land Management office.
Thanks to each faculty and staff member in the UNLV Geoscience Department
for all the contributions and creating an enjoyable environment to work in. I would
particularly like to thank Dr. Timothy Lawton at New Mexico State University for his
helpful contributions and discussions of the geology of the area. Thanks to Kathy Zanetti
for all the help with understanding the process of dating, dating my samples, and reading
the results in the Nevada Isotope Geochronology Lab
I would like to thank my friends and colleagues for creating an enjoyable leaming
environment. I particularly would like to thank my friends Robyn Howley, Melissa
Hicks, Amy Brock, John VanHoesen, and Jason Smith for all their support both in and
out of the office, helpful reviews and comments, good times, and friendship.
Last, but certainly not least, I would like to thank my loving boyfriend Rob
Kerscher, for all his love, support, patience, encouragement, and being the scale in my
field photos. I could have never done it without him. Thanks again.
VII
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1
INTRODUCTION
The purpose of this project is two-fold. First, conduct a detailed study of
multipartite normal fault (several faults that link and form on long fault) linkage zones
using the central Sevier fault, southwestern Utah as a case study (Fig. 1). Second,
evaluate the earthquake and landslide hazards in the region.
Most published data and interpretations focus on simple linkage zones (e.g..
Crone and Haller, 1991; Koukouvelas and Doutsos, 1996; Pavlides et al., 1998).
Although multipartite linkage zones have been mapped in the past (e.g.. Bowers, 1991;
Billingsley, 1993,1994; Barton et al., 1998; Zampieri, 2000; Ferrill and Morris, 2001),
these sites have not been analyzed in detail. Linkage zones contain information
important to understanding how long (lO's-lOO’s of km) faults form.
Linkage zones are also important to hydrocarbon exploration because active
exploration is common within them (e.g., Gulf of Mexico). Understanding multipartite
linkage zones will help determine where migration pathways and potential petroleum
traps exist.
The Sevier fault is an ideal place to study fault linkage because (1) it is located in
a relatively geologically simple region, (2) exposure is excellent, and (3) detailed work
has not been done along the central Sevier fault. The new data document a multipartite
linkage zone between Glendale and Orderville Utah, which separates two geometric
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 segments (Mt. Cannel and Spencer Bench segments). The linkage zone forms the
Orderville salient (bend). This salient contains the linkage sites of several faults and four
relay ramps (strain transfer zones). This number of faults and folds is greater than
predicted in simple linkage models. Two of the relay ramps contain fault-parallel folds in
one linkage area in addition to the ramp monoclines (Plates 1 and 2). Understanding the
structure within relay ramps is necessary to fully describe linkage zones. However, fault-
parallel folds within linkage zones are not commonly discussed because these folds are
typically suttle and not recognized.
The second problem addressed in this project is the earthquake and landslide
hazards for the region. Evaluating earthquake and landslide hazards is important because
the area is popular with summer tourists and is populated by small (<250 people) towns
and agricultural communities. However, little detailed work on seismic activity or
landslides has been done along the central Sevier fault. The data presented here show
that the central Sevier fault cuts -570 ka basalt and tilts Quaternary stream terraces.
Historic earthquakes have been felt and reported in the vicinity of the central Sevier fault
trace, which indicates the central Sevier fault may have been active in the Holocene. In
addition, many of the rock formations are weak and are prone to landslides. The
landslides may be either gravity or seismically induced and may pose a risk to the
residents.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2
STRUCTURAL CONCEPTUAL MODELS
Introduction
Long (lO’s-lOO’s km) normal faults cannot rupture as a whole during a single
earthquake because earthquakes can not produce enough slip to rupture lengths greater
than a few lO's of kilometers. This relationship suggests that long faults may have
formed by linkage of several shorter faults. Fault linkage forms geometric segments
(bends) that may or may not correlate to rupture segments (discussed later). Models for
fault linkage and structures formed within the linkage zones are described below.
Fault Linkage Models
Fault segment linkage is the mechanical interaction between two separate fault
segments (e.g.. Peacock and Sanderson, 1991; Trudgill and Cartwright, 1994; Cartwright
et al., 1995; Crider and Pollard, 1998; Crider, 2001; Ferrill and Morris, 2001). Linkage
affects the slip distribution on each of the faults because as the faults interact, slip is
transferred from one fault to the other fault (Crider and Pollard, 1998). The bounding
faults in linkage zones have approximately parallel strikes and display opposite
displacement gradients (Ferrill and Morris, 2001).
Two mechanically different styles of linkage are called “soft” and “hard” linkage.
Soft linkage occurs when the fault geometry changes only because of the stress field
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 interactions (Gupta and Scholz, 2000; Taylor et al., 2001). During soft linkage, strain is
transferred between faults through the rock. In contrast, during hard linkage the faults
physically connect (Gupta and Scholz, 2000; Taylor et al., 2001), and faults cut relay
ramps (strain transfer zones) or blocks between the faults resulting in a single fault with
multiple bends (Young et al., 2001). To elucidate fault linkage and the structures formed
within linkage zones, a definition and explanation of radial propagation, linkage, and the
stress fields at fault tips is given below.
Geometrically simple faults tend to grow by radial propagation. Radial
propagation typically occurs when an individual fault extends in length and width, where
the fault has zero slip at the fault tips and the maximum slip is located in the middle 1/3
of the fault (Fig. 2) (Cowie and Shipton, 1998). This type of growth is accompanied by
increasing displacement (or displacement gradient) and along strike propagation of fault
tips (Fig. 2) (Wu and Bruhn, 1994). With increased slip along the fault, stress builds up
at the fault tip. Eventually when the stress at the fault tips exceeds the strength of the
surrounding rock, the surrounding rock will fail and the fault will grow in length (Scholz,
1990; Cartwright et al., 1995). In this type of fault growth, the footwall is not deflected
(Cartwright et al., 1995). Faults that grow by radial propagation follow a single growth
path provided the materials have constant material properties when the fault propagates
(Cartwright et al., 1995).
Radially propagating faults may grow according to a relationship between the
fault length and the maximum displacement, which is perpendicular to the fault trace
(Cartwright et al., 1995; Cladouhos and Marrett, 1996; Cowie and Shipton, 1998). This
relationship requires the maximum displacement to be equal to critical shear strain
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 multiplied by the maximum length (Cartwright et al., 1995). This relationship is typically
used for faults with single growth paths because complex fault geometries will cause
scatter in the displacement and length data (Cartwright et al., 1995).
Isolated normal faults typically have a gently curved or "shovel shape" in map
view (Fig. 2). This geometry is caused by a variety of complex mechanisms (cf., Mandl,
2(K)0). When an individual fault propagates along strike or dip, the strike according to
the Mohr-Coulomb theory will be parallel to a? (intermediate principal stress) (Mandl,
2000). The shovel shape is caused by the curving of the Ot trajectories (Mandl, 2000).
The curved 0 2 trajectories may have existed prior to faulting or may have been induced
by the initial propagation of the fault and the interaction of local stress fields at the fault
tips (Mandl, 2000).
In contrast, long faults with salients (bends) tend to grow by radial propagation
followed by segment linkage (Fig. 2). The initial faults may be en echelon or non-en
echelon. Fault growth by segment linkage consists of propagation, local stress field
interaction, and linkage of segments with or without deflection of the footwall
(Cartwright et al., 1995; Ferrill and Morris, 2001).
Active normal faults have local stress fields located near the fault tips that are
different from the regional stress field. These local stress fields exist because of a
mechanical interaction between the fault tip and the surrounding rock. The local stress
field along a single dip-slip normal fault has the greatest shear strength near the fault tip.
The normal shear stresses increase beyond the local stress field and decreases near the
center of the fault. The fault is more likely to rupture in the increased shear stress field
than the decreased shear stress field (Fig. 3A) (Cowie and Shipton, 1998).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 Local stress fields interact when two or more faults propagate and the tips lie
close to each other. When the zones of relatively high shear stresses near two fault tips
touch, the local stress fields begin to curve toward each other and interact (Fig. 3B).
Crider and Pollard (1998) suggest that the local stress field at the fault tip is located in the
footwall of the faults when the increased shear stresses interact (Fig. 3B-D). With
continued slip, these stress fields will curve towards each other causing the faults to
curve. When the local stress fields interact soft linkage exists between the two faults.
The shear stresses also will increase in the future strain transfer zone (relay ramp) (Crider
and Pollard, 1998). With continued slip, the local stress fields and faults will ultimately
curve and physically connect (link), called hard linkage. The hard linkage is not pictured
in Fig. 3 because this type of linkage has not been numerically modeled.
Two major types of segment linkage occur where fault tips: (1) underlap or (2)
overlap. The overlapping-fault linkage type, contains two subtypes: (1) fault capture and
(2) breakthrough or cross faults. These types and subtypes are discussed below.
Underlapping faults or fault tips are two or more individual faults, either en
echelon or non-en echelon in map view, with the tips spatially separated such that they do
not lie across strike from each other (Fig. 2A). The greatest amount of slip is typically in
the center 1/3 of the fault and by definition the fault tips have zero slip. Toward the tip,
the size of the local stress field will decrease rapidly because faulted and unfaulted rocks
are adjacent to each other (Cowie and Shipton, 1998). This difference develops the local
stress field at the fault tips. With continued slip along the faults, soft linkage will occur
(Fig. 2A). This geometry can be viewed in the field as two or more subparallel faults
curved towards each other with underlapping fault tips. However, after physical hard
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 linkage occurs, only one fault with one or more salients will be apparent in natural
settings (Fig. 2A).
Overlapping faults are two or more individual subparallel faults with the tip of
one fault across strike from the other fault(s) (Fig. 2). In the basic type of overlapping
fault linkage, faults with a similar amount of slip propagate past each other and
interaction between the local stress fields near the fault tips requires the faults to curve
toward each other (Crider and Pollard, 1998; Ferrill et al., 1999; Crider, 2(X)1; Ferrill and
Morris, 2(X)1 ; Taylor et al., 2(X)1 ). A relay ramp, which accommodates strain transfer
between the faults, forms in the fault overlap zone (discussed in a following section)
(e.g., Larsen, 1988; Morley et al., 1990; Peacock and Sanderson, 1994; Trudgill and
Cartwright, 1994; Crider and Pollard, 1998; Ferrill et al., 1999; Moore and Schultz, 1999;
Walsh et al., 1999; Peacock et al., 2000; Zampieri, 2000; Crider, 2001; Ferrill and
Morris, 2001; Reber et al., 2001). As the two faults continue to slip, strain is transferred
between the faults and the faults may ultimately link into one long fault (Fig. 2B).
Fault capture is a subtype of linkage in which fault tips overlap. As the faults
propagate in the direction of strike, one fault has more or a greater rate of slip than the
other fault (Ferrill et al., 1999; Reber et al., 2001; Taylor et al., 2001). More slip along a
fault may be related to one fault having more frequent and/or larger earthquakes than the
other faults. Ultimately, the stress field developed around the tip of the more active fault
interacts with that of the other fault. Then, only the larger slip or faster moving fault
curves toward the other fault and the faults link. With additional slip, the faults
ultimately transfer strain with or without the formation of a relay ramp (Fig. 2D). In the
field, faults linked by fault capture are recognized by one curved fault that connects with
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 another fault with a comparatively straight trace. These faults, when linked, form a
single fault. The end of the captured fault that lies across strike from the capturing fault
(fault with more or more rapid slip) commonly is isolated in the hanging wall (or possibly
the footwall) and no further slip occurs along it. The captured end may also be buried by
the hanging wall-basin sediments. If a relay ramp formed in the overlap zone, it too may
be down dropped in the hanging wall or isolated in the footwall after linkage (discussed
in a later section).
A second subtype of overlapping fault linkage is breakthrough along connecting
faults. Breakthrough along connecting faults occurs when the tips of two or more
individual faults propagate past each other, overlap and new faults with a different strike
form in the overlap zone (Cartwright et al., 1996; Ferrill et al., 1999; Ferrill and Morris,
2001). Generally, the new breakthrough faults form parallel to each other and at
approximately 30° to the bounding faults. Strain is transferred from one fault strand to
the other fault strand along this newly formed fault set (Fig. 2D). The breakthrough fault
set forms because of an increased displacement gradient in the overlap zone, which
causes extension to increase (Ferrill et al., 1999). This extension produces additional
faults (called breakthrough or cross faults) that allow slip transfer between the
overlapping faults, and occurs where fault throw is disproportional to the fault tip
propagation (Ferrill et al., 1999; Ferrill et al., 2001). If breakthrough faults propagate
from both faults, then fault bounded blocks are formed within an anastomosing fault zone
(Ferrill et al., 1999). In the field, linkage via breakthrough faults (or cross faults) is
identified by two nearby faults with similar strikes, and between the two faults are one or
more faults that strike at a moderate to high angle (-30°) to the bounding faults. Upon
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 linkage, these faults become one fault. The breakthrough faults are unlikely to cross the
longer bounding faults, but relict fault tips within the overlap zone are likely to be seen
(Ferrill et al., 1999).
Spacing or the distance between faults is important in determining whether fault
linkage has occurred or will occur. If the faults are spaced too far apart, local stress fields
will not interact. Hence, fault linkage will not occur. Fault spacing at the surface and at
depth may differ because of the dips of the faults. For example, the steeper the fault dip,
the closer in space they need to be in order to link at shallow depths. Faults with
shallower dips can be spaced farther apart and still link at a shallow depth.
Displacement vs. Distance Diagrams
Displacement vs. distance diagrams are used to show the total amount of offset at
various positions along strike of a fault. Here, displacement vs. distance diagrams are
used to emphasize displacement variations within linkage zones (cf., Reber et al., 2001;
Taylor et al., 2001). Focusing on the linkage zones is important, because displacement
anomalies in these areas help diagram and indicate which type of linkage occurred. This
use is different from other studies (e.g. Cowie and Scholz, 1992; Cartwright et al., 1995;
Bohnenstiehl and Kleinrock, 2000) that use displacement vs. distance diagrams to model
the entire fault.
In the vicinity of salients or geometric bends that are convex toward the hanging
wall, on displacement vs. distance diagram can be used to evaluate whether linkage
occurred and the type of linkage providing two assumptions are valid: (I) The total
amount of slip on each individual fault before linkage was zero at the fault tip. (2) Since
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 the time of linkage, the slip rate was not markedly greater in the linkage zone than in the
middle 1/3 of each original fault. Thus, in a linkage zone, the displacement should
markedly decrease near the former fault tips. Variations in displacement vs. distance
were modeled along idealized faults to show patterns associated with underlapping faults,
overlapping faults, and fault capture types (Fig. 4) (Taylor et al., 2001). New models for
breakthrough hard linkage and soft linkage were created for this study and are described
below (Fig. 4). These models are used in this study to interpret fault linkage zones along
the central Sevier fault.
Along linked underlapping faults, the slip magnitude decreases sharply in the
center of the fault, which is where the slip or displacement would be expected to be the
greatest on unlinked faults (Fig. 4A). This decrease occurs at the linkage zone. After the
faults linked, additional slip may occur along the entire fault, along zones defined by the
geometry or along distinct earthquake rupture segments (discussed in segment boundaries
section).
On a displacement vs. distance diagram, a basic overlapping type linkage zone
shows less displacement across a broad bench at the site of linkage (Fig. 4B). The bench
forms because two faults propagated past each other, so the total amount of displacement,
added across both faults along strike in the linkage zone, is similar or the same. The
bench length is the same as the length of the overlap zone (Taylor et al., 2001).
However, during the soft linkage of overlapping faults, instead of a bench developing in
the zone of linkage, the displacement tapers to zero (Fig. 4E).
Fault capture subtype linkage zones produce a series of plateaus with decreasing
slip on a displacement vs. distance diagram (Fig. 4C). This decreasing series of plateaus
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. il occur because the fault with the greatest amount or rate of slip intercepts the fault with
the lesser amount of slip at a site with significant displacement. However, displacement
at that site may be less than the maximum displacement on the intercepting fault. The
fault with the lesser amount of slip is cut off and becomes a relict fault.
The breakthrough subtype linkage zone produces a pattern similar to the
overlapping fault linkage zone on a displacement vs. distance diagram but with small
amplitude perturbations within the plateau (Fig. 4D). If the displacement data are
sampled with a small spacing, then "steps" occur where the cross faults are located. No
cross faults exist at the tips of the bounding faults hence the total displacement abruptly
increases in slip outside the linkage zone, following the displacement gradient.
Where a relay ramp forms in the linkage zone, the faults that bound the relay
ramp have tapering slip as they enter the overlap zone (Crider and Pollard, 1998). These
geometries can be related to the displacement gradients at the fault tips (Peacock and
Sanderson, 1991).
Relay Ramps
Linkage of overlapping faults typically exhibit an interaction phase during which
a relay ramp forms. A relay ramp is an inclined panel of rocks between two overlapping
faults with smoothly curving tiplines (Fig. 5) (Larsen, 1988; Peacock and Sanderson,
1991 ; Crider and Pollard, 1998). In the relay ramp, the strike of bedding changes from
parallel to the bounding faults outside the ramp to nearly perpendicular to the bounding
faults within the ramp (Fig. 5) (e.g., Trudgill and Cartwright, 1994; Faulds and Varga,
1998; Ferrill et al., 1999; Moore and Schultz, 1999; Peacock et al., 2000: Ferrill and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12 Morris, 2001; Reber et al., 2001). Relay ramps can range in area from square meters to
lO's of square kilometers.
Relay ramps and the structures formed within the ramps are important to aid in
the understanding of (1) how segmented long normal faults link, (2) how faults grow, and
(3) how strain is transferred from one fault strand to another fault strand. Relay ramps
also are important because they allow fluid (hydrocarbons or water) migration from the
footwall to the hanging wall, which may form a fluid trap (Morley et al., 1990; Crider
and Pollard, 1998; Dawers and Underhill, 20(X)).
Relay ramps initiate and begin to form in soft linkage zones between two fault
tips. Relay ramps typically form along overlapping faults; including the subtypes of
breakthrough faults and fault capture (Fig. 5). The relay ramp models below will be used
to describe and interpret linkage zone structures exposed along the central Sevier fault.
Relay ramps connect the hanging wall of one fault strand to the footwall of
another fault strand, effectively transfer strain between the two strands, and ultimately
link two normal faults (Larsen, 1988; Morley et al., 1990; Peacock and Sanderson, 1991;
Peacock and Sanderson, 1994; Trudgill and Cartwright, 1994; Crider and Pollard, 1998;
Faulds and Varga, 1998; Ferrill et al., 1999; Moore and Schultz, 1999; Walsh et al., 1999;
Zampieri, 2000; Peacock et al., 2000; Ferrill and Morris, 2001; Reber et al., 2001). The
model or ideal relay ramp transfers strain from the hanging wall of one fault to the
footwall of the other fault by (1) tilting of the bedding within the ramp, (2) vertical axis
rotation, and (3) cutoff-parallel elongation without additional deformation to the relay
ramp or the surrounding rocks (Ferrill and Morris, 2001).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13 A relay ramp accommodates displacement gradients along the bounding faults.
These bounding faults typically have opposite displacement gradients (Ferrill and Morris,
2(X)1). Peacock and Sanderson (1991) suggest that a relationship exists between the
geometry of the relay ramp and the displacement variations of the bounding faults; a
more complex displacement gradient leads to a more geometrically complex relay ramp.
In addition, greater displacement gradients on the bounding faults form larger (vertical
vs. horizontal dimension) ramps (Peacock and Sanderson, 1991; Peacock et al., 2000).
As slip continues after the initial formation, relay ramps commonly expand or propagate
parallel to strike of the bounding faults (Fig. 5). Hence, the relay ramp must continue to
deform to keep the hanging wall and footwall connected (Ferrill and Morris, 2001).
Breached relay ramps are completely bound by two hard linked faults and may
contain cross faults or fractures (Peacock and Sanderson, 1994; Ferrill and Morris, 2001).
These ramps can be further subdivided into breached top and base ramps (Crider, 2001).
Breached top and base ramps occur when a bounding fault cuts across the ramp (hard
linkage site) at the structurally higher or lower parts of the ramp, respectively (Fig. 5)
(Crider, 2001). Cross faults in breached relay ramps may form because of tip interaction
prior to fault overlap (Ferrill and Morris, 2001). This may explain why the breakthrough
fault orientation is at an oblique angle to the relay ramp (Ferrill and Morris, 2001).
Relay ramps may contain a variety of structures. By definition, a relay ramp
contains a fold and is essentially a monocline and trends approximately normal to the
bounding faults (Fig. 5) (Larsen, 1988; Peacock and Sanderson, 1991; Peacock and
Sanderson, 1994; Faulds and Varga, 1998). In addition, other contractile structures may
develop within the relay ramp, including anticlines and synclines depending on the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 geometry of the bounding faults. These fold axes are approximately parallel to the
bounding faults and form in addition to the already existing ramp monocline. Structures
such as these have been rarely studied in the past.
Segment Boundaries
Fault segments and the boundaries between them can be subdivided into three
types: (1) structural, (2) geometric, and (3) earthquake (dePolo et al., 1991). It is
possible, therefore, to define segment boundaries based on surface observations of fault
geometry, scarps, footwall structures, kinematic indicators, and/or earthquake epicenters
(McCalpin, 1996; Stewart and Taylor, 1996). However, most segment boundaries are not
a single point, but rather a broad complexly faulted zone (Janecke, 1993).
Geometric segments are recognized by changes in fault zone morphology, fault
trace orientations (bends, step-overs or en echelon faults, separations), and a change in
fault separation or a gap in a fault zone (Fig. 6A) (dePolo et al., 1991; Keller and Pinter,
1996). Geometric segment boundaries typically exhibit a dramatic change in strike that
shape a salient. These changes in strike usually occur in zones of linkage. Geometric
segments may be consistently and objectively identified on aerial photographs or by
detailed geologic mapping.
Structural segment boundaries end at a structural discontinuity (Fig. SB) (dePolo
et al., 1991). Structural discontinuities can be older faults or folds that strike at a high
angle to the segmented fault. A change in the geologic material (e.g., changing from
coherent rock to fault gouge back to coherent rock) crossed by the fault may be a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15 characteristic of a structural segment boundary (Fig. 6B) (Janecke, 1993; Keller and
Pinter, 1996).
Earthquake segments are portions of a fault zone that rupture as a unit during an
earthquake (Fig. 6) (dePolo et al., 1991; McCalpin, 1996). McCalpin (1996) further
defines earthquake segments as "discrete portions of faults that have demonstrably
ruptured to the surface two or more times". However, in large earthquakes more than one
segment may rupture. Earthquake segments are separated by earthquake rupture
terminations that are a zone that typically contains several fault splays and high amounts
of fractured, crushed, and faulted rocks (Janecke, 1993). An earthquake segment
boundary or earthquake discontinuity, is therefore defined as "a portion of a fault where
at least two rupture zones have ends" and can potentially arrest earthquake ruptures
(Wheeler, 1989; dePolo and Slemmons, 1990). The most reliable method of
documenting earthquake segments is paleoseismological evaluation and fault behavioral
data from historic earthquakes (Zhang et al., 1991). Determining earthquake segments is
the most important concept for evaluating earthquakes and their associated hazards.
However, earthquake segment boundaries do not always stop all propagating ruptures
(Crone and Haller, 1991). For example, a large (M 7+) earthquake can rupture more than
one earthquake segment boundary. Hence, earthquake hazard evaluation needs to be
done beyond a determined earthquake segment boundary.
A segment boundary can be any one type, or a combination of geometric,
structural, and earthquake boundaries (Fig. 6). For example, a structural boundary where
an older fault strikes at a high angle to the segmented fault can also serve as an
earthquake rupture boundary. An example would be the 1914 Pleasant Valley, central
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 Nevada earthquake. This earthquake was terminated by a structural discontinuity, and
thus, is also an earthquake segment boundary (Zhang et al., 1999). Another example is a
dramatic change in strike, a geometric segment boundary, that may also serve as an
earthquake rupture boundary. An example of this type is the Lost River and Lemhi faults
in east central Idaho. These faults have geometric bends that are also earthquake segment
boundaries (Janecke, 1993). Hence, segment boundaries may have the physical
characteristics of one or more type of boundary.
Some studies in seismology and fracture mechanics indicate that fault geometry
can be important in the generation of earthquakes and rupture patterns (Bruhn et al.,
1987; Menges, 1990). Indeed, studies along the Hurricane fault document that some
recent earthquake rupture breaks are limited to geometric segments (Stenner et al., 1999a,
1999b). In contrast, dePolo et al. (1991) suggest that geometric segments in map view
may not have a significant effect on earthquake ruptures with a normal sense of
displacement during large (7.0+) magnitude earthquakes. However, smaller (e.g., M 3.0)
earthquakes typically only rupture a single segment and the segment boundary will arrest
the propagating earthquake. The size of the segment boundary that arrests an earthquake
appears to scale with the length of the ruptured fault and the amount of displacement
during the rupture (Zhang et al., 1999).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 3
REGIONAL TECTONIC BACKGROUND
Fault linkage models are motivated by and tested using natural examples (for this
study, the central Sevier fault in southwestern Utah). To understand fault linkage models
and the development of fault segments, the regional tectonics must be understood. Also,
the regional deformation represents the regional stress field within which the faults and
local stress field at the fault tips formed.
Introduction
Throughout the history of the western United States plate interactions along the
western plate margin have influenced tectonism. Through the Paleozoic and into the
Tertiary, subduction dominated. From the Jurassic to at least the late Cretaceous, the slab
dip was steep (60°) (Atwater, 1970; Sveringhaus and Atwater, 1990; Atwater and Stock,
1998). However, from late Cretaceous through the Cenozoic, the slab dip changed to flat
or shallow (<30°). As a result, two orogenies occurred near the western edge of the
Colorado Plateau between latest Jurassic and Eocene: the Sevier and Laramide orogenies.
These orogenies occurred because of shallow slab subduction. Folds and thrust faults of
these orogenies can occur throughout the Utah Transition Zone to the High Plateaus
subprovince of the Colorado Plateau. At -23 Ma, a transform boundary began to form
along the western edge of North American (Sveringhaus and Atwater, 1990). This event
17
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18 is recognized by the replacement of the time-transgressive sequence of arc magmatism by
the San Andreas transform fault (Atwater, 1970; Sveringhaus and Atwater, 1990). The
length of the transform boundary has increased to nearly 1,125 km (700 mi) since its
inception (Sveringhaus and Atwater, 1990).
Evolution of the Transform Boundary
At -23 Ma the San Andreas transform boundary was initiated when the East
Pacific Rise made contact with the North America Plate (Atwater, 1970). This change
from subduction along the plate margin to a transform boundary had a great effect on the
tectonic evolution of western North America. Dickinson and Snyder (1979) suggested
that the formation of a transform boundary created a slab-window beneath part of western
North America. However, Sveringhaus and Atwater (1990) and Atwater and Stock
(1998) suggest that this slab-window is more of a slab-gap, meaning a completely
slabless area exists underneath North America. The slabless area is a sub-rectangular
shape rather than a "triangle" under North America. The formation of the slab-free zone
relates to several geologic events including regional uplift of the Colorado Plateau,
central Basin and Range extension, formation of the San Andreas fault system, and the
end of arc magmatism.
Today most of the central Basin and Range, Utah Transition Zone, and Colorado
Plateau (Fig. 1) overlie this slab-free zone. Basin and Range style normal faults have cut
into the western margin of the Colorado Plateau in southwestern Utah. These faults
probably initiated in the Miocene.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 Regional Tectonics of Southwestern Utah
During the Phanerozoic, southwestern Utah was a site of (1) deposition of a thick
cratonal Mesozoic and Cenozoic sedimentary sequence; (2) the Sevier Orogeny, mainly
during the Mesozoic; (3) the Laramide orogeny mainly during the early Tertiary, (4)
Cenozoic volcanism and sedimentation; and (5) Cenozoic extension. The following
section provides a simplified tectonic overview of southwestern Utah including the
southeastern Basin and Range Province, Utah Transition Zone, High Plateaus
subprovince of the Colorado Plateau, and the western edge of the Colorado Plateau.
Mesozoic Sevier Orogeny
During the Jurassic into the Eocene, the Sevier Orogeny formed a contractile belt,
the frontal part of which extends from southeastern California, through Utah, and into
northern Canada (Armstrong, 1968). The onset of the Sevier Orogeny was probably
caused by the subduction of the Farallon plate with accelerated plate convergence
(Armstrong and Ward, 1991). During the last decade, most workers have agreed that the
majority of deformation associated with the Sevier orogeny in southwestern Utah is
bracketed between late Albian time and late Campanian (DeCelles, 1995; Lawton et al.,
1997). Sevier thrusting resulted in tectonic thickening, development of an extensive
foreland basin east of the thrust front, and the development of broad folds and uplifts
(Armstrong, 1968; Axen et al., 1990; Burchfiel et al., 1992; Wannamaker et al., 2001).
The foreland basin lies within the vicinity of and extends to the east of the present
location of the Sevier fault. Surface breaking thrusts are thought to have only extended
as far east as Cedar City (Fig. 1) (Hintze, 1988; Axen et al., 1990).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20
Laramide Orogeny
Shortly after the Sevier orogeny, the Laramide orogeny occurred between 75 and
35 Ma (Dickinson et al., 1988; Bird, 1998). During the early portion of the Laramide
orogeny (75-65 Ma), the Kula Plate subducted under North America (Bird, 1998). After
65 Ma, the Farallon Plate began to subduct (Bird, 1998). At -50 Ma, the subducted
Kula/Farallon Plate transform boundary passed under the present western edge of the
Colorado Plateau (Bird, 1998). Because of the plate configurations, this orogeny had
driving mechanisms different from the Sevier orogeny. The Laramide orogeny was
driven by basal traction of flat slab subduction, not by motions at the plate margin (Bird,
1998). Evidence of this orogeny (gentle folds) is visible to the east and south of the
Paunsaugunt fault (Fig. 1) (Davis, 1999; Wannamaker et al., 2001). The Laramide
orogeny formed gentle folds such as the monoclines in the eastern High Plateaus east of
the study area (i.e., the Kaiparowits and Waterpocket folds). These folds are thought to
overlie high-angle Precambrian normal faults, which were reactivated as reverse faults
(Davis, 1978; Dumitru et al., 1994).
Cenozoic Volcanism
Volcanism in southern Utah began at -30 Ma (Rowley, 1975; Best et al., 1980)
and continued into the Holocene. Most of the basalt flows in southwestern Utah range
from 10.8 to 0.3 Ma in age (Best et al., 1980). The basalt flows are generally younger to
the east (Best et al., 1980; Nelson and Tingey, 1997). Basalt flows along the Sevier fault
are -0.5 Ma (Best et al., 1980; this study).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 Along the latitude of the southern boundary of Utah (37“N), the effects of
subduction of the Farallon Plate ended at -10 Ma indicating that the post 10 Ma volcanic
rocks in southwestern Utah are not related to active plate subduction (Best et al., 1980;
Severinghaus and Atwater, 1990; Atwater and Stock, 1998). However, the volcanism in
southern Utah may be related to Basin and Range style extension (Best et al., 1980;
Nelson and Tingey, 1997). Cenozoic volcanic units crop out (1) near the Hurricane fault,
(2) near Navajo Lake, (3) in the Marysvale volcanic field, and (4) along and near the
central Sevier fault (Figs. 6 and 7) (Rowley, 1975; Best et al., 1980; Smith et al., 1999).
Late Cenozoic Extension in Southern Utah
Near St. George, the Hurricane fault defines the structural boundary between the
Transition Zone and the High Plateaus. The Hurricane fault is an active, segmented long
normal fault zone (Stewart and Taylor, 1996; Stewart et al., 1997; Taylor et al., 2(X)1).
Movement along the Hurricane fault probably initiated during the Miocene or Pliocene
and continues today (Stenner et al., 1999; Davis, 1999; Taylor et al., 2001). Several
earthquakes and Holocene fault scarps have been located along the Hurricane fault
(Arabasz and inlander, 1986; Christenson and Nava, 1992; Stewart et al., 1997; Lund et
al., 2002). A M 5.8 earthquake is thought to have occurred along the Hurricane fault in
1992 (Christenson and Nava, 1992).
The Sevier-Toroweap fault, a segmented long normal fault, lies -65 km (-40 mi)
to the east of the Hurricane fault. I will use the name Sevier fault when referring to the
Utah portion, Toroweap fault when referring to the Arizona portion, and Sevier-
Toroweap fault when referring to the entire fault. The Sevier-Toroweap fault can be
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22 traced -195 km (-120 mi) from the Grand Canyon in Arizona north into the Miocene
Marysvale volcanic field in Utah (Davis, 1999). The Sevier fault probably initiated
during the Miocene and may be active today (Smith and Arabasz, 1991; Hecker, 1993;
Davis, 1999). The central Sevier fault has not been observed cutting Holocene
sediments. However, Jackson (1990) and Hecker (1993) suggest that evidence for recent
activity may be lacking or obscured because of active colluvial slopes and thick
vegetation. The Sevier fault cuts Quaternary basalt and sediments suggesting at least
Quaternary activity (Christenson and Nava, 1992; this study). A few earthquakes have
been recorded in the vicinity (<5 km) of the Sevier fault (Arabasz and Inlander, 1986)
suggesting that this fault may have been active in the Holocene.
The Paunsaugunt fault is the easternmost long normal fault in the High Plateaus.
The fault trace extends -65 km (-40 mi) (Davis, 1999). The Paunsaugunt fault is thought
to be the oldest of the three faults because the hanging wall is topographically higher than
the footwall. This topographic reversal may be caused by a more resistant lithologie unit
capping the hanging wall than the footwall, thus allowing the footwall to erode faster
than the hanging wall. Such topographic reversal takes a significant time to form,
supporting the suggestion that the Paunsaugunt fault has not ruptured recently
(Schiefelbein and Taylor, 2000). The balanced rocks (hoodoos) of the Tertiary Claron
Formation in Bryce Canyon National Park (Fig. 1) also may suggest a lack of large recent
seismic events (Bruhn and Wu, 1993; Brune, 1996, Davis, 1999).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23 Definitions of Provinces in Southern Utah
In southern Utah, the Transition Zone is generally further defined by three
different criteria: (1) structural geology, (2) physiography, and (3) geophysics. The
structural definition is based on the structural geology of the area. The physiographic
definition includes geographically defined boundaries. The geophysical definition is
based on crustal thicknesses of the region.
Basin and Range Province
The Basin and Range province is characterized by a thin crust, high heat flow,
magmatism, and north-south trending valleys and mountain ranges (Parsons, 1995;
Wannamaker et al., 2001). These valleys and ranges are bounded by north-south striking
long normal faults. The region is seismically active. Prior to Cenozoic extension, rocks
in the Basin and Range province underwent several episodes of deformation (contraction
and extension).
The region also had significant volcanism since its first initiation -35 Ma
(initiation time varies with latitude) and is seismically active. At the latitude of the
Utah/Arizona border (37"N), the eastern boundary is defined as the Hurricane fault
(Stewart et al., 1997).
Colorado Plateau
In southern Utah, the Colorado Plateau is defined as a physiographic and tectonic
province that lies to the east of the Paunsaugunt fault. The Colorado Plateau is
topographically high and contains generally gently dipping strata, broad regional folds,
and lacks major thrust and normal faults. This province is underlain by as much as 50 km
(31 mi) of crust and is considered relatively tectonically stable (e.g., Beghoul and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24 Barazangi, 1989; Nelson and Harris, 2001; Wannamaker et al., 2001). It has undergone
little deformation since the Proterozoic (Wannamaker et al., 2001). However,
contraction, volcanism, and extension have been repeated through geologic time along
the flanks.
Utah Transition Zone
The Utah Transition Zone lies between the Basin and Range province and the
Colorado Plateau. The Transition Zone changes in width from north to south. This
transitional region is suggested to be an uplifted rift shoulder in the north along the
Wasatch Front near Salt Lake City, Utah and its southern extension (Wannamaker et al.,
2001). In southwestern Utah, the transition occurs across an -62 mi (-100 km) wide
zone (Wannamaker et al., 2001).
The initiation and boundaries of the Transition 2kne are still not completely
understood. The Transition Zone has characteristics of both the Basin and Range
province and the Colorado Plateau. It contains a gradual change in geologic
characteristics from Basin and Range style deformation to the less deformed tectonic
style of the Colorado Plateau. It is thought that the Transition Zone is not related to
Pacific and North American plate boundary interactions because the plate boundary
stresses could not propagate this far into the continental interior (Wannamaker et al.,
2001). Wannamaker et al. (2001) suggest that the Transition Zone has been an evolving
structure since 25-30 Ma. This timing is based on the onset of extensional events, which
create the present day Transition Zone (Wannamaker et al., 2001).
The physiographic and structural distinctions place the western boundary of the
Transition Zone at the mouth of the Virgin River Gorge in Arizona and the eastern
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25 boundary at the Hurricane fault. Structurally, the Transition Zone has Basin and Range-
type normal faults. Geophysically, the transition zone is defined by crustal thickness and
generally lies between the Hurricane and Paunsaugunt faults. The Transition Zone crust
is generally between 19-22 mi (30-35 km) thick (Wannamaker et al., 2001).
High Plateaus Subprovince of the Colorado Plateau
The High Plateaus subprovince of the Colorado Plateau is located between the
Transition Zone and the Colorado Plateau. This subprovince generally lies between the
Hurricane and the Paunsaugunt faults (Fig. 1) and incorporates the Grand Staircase. The
High Plateaus are divided by generally north-south oriented normal faults: the Hurricane,
Sevier-Toroweap, and Paunsaugunt faults, from west to east (Fig. 1). This region has
characteristics of the Colorado Plateau as well as the Transition Zone. The crustal
thickness in the High Plateaus is generally 22-25 mi (35-40 km), which is between the
thickness of the Transition Zone and the Colorado Plateau (Wannamaker et al., 2(X)1).
The high-angle normal faults resemble those in the Transition Zone. Hence, the High
Plateaus are variously grouped with either the Transition Zone or the Colorado Plateau.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4
STRATIGRAPHY
Stratigraphie units with many well-defined thin members are exposed in the High
Plateaus subprovince and along the Sevier fault (Gregory, 1951; Hintze, 1973; Sargent
and Philpott, 1987; Hintze, 1988; Stokes, 1988; Doelling, 1989). These thin members are
ideal for documenting small stratigraphie separations and allow relatively small
uncertainties in the calculations o f stratigraphie separation along the Sevier fault and
related strands and splays.
The stratigraphy exposed along the central Sevier fault consists of a thick
succession of late Triassic to Cretaceous carbonate and siliciclastic rocks, which are
imconformably overlain by Cenozoic volcanic and sedimentary units (Fig. 8). No
metamorphic or intrusive units crop out in the study area. A detailed description of the
lithologie units is presented on Plate 3. Units and thicknesses that are Triassic and older
on the cross sections, but not exposed in the map area are from Hintze (1988) and
Doelling et al. (1989) (Fig. 8). The units are dominantly limestone, dolostone, and
sandstone (Fig. 8).
Mesozoic Stratigraphy
The exposed Mesozoic section is approximately 2,300 m (7,546 ft) thick, gently
folded, and generally dips to the northwest and northeast (Fig. 9) (Gregory, 1951; Hintze,
26
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27 1973, 1988; Sargent and Philpott, 1987; Stokes, 1988; Doelling et al., 1989; Nelson and
Tingey, 1997). The Late Triassic through Cretaceous units in the study area dominantly
consist of sandstone and shale with a minor amount of limestone of the Jurassic Carmel
Formation (Fig. 9). Three unconformities exist in the Mesozoic section: the J-1, J-2, and
K-1 (Hintze, 1973; Hintze, 1988; Marzolf, 1988; Stokes, 1988; Doelling et al., 1989)
(Fig. 9).
Cenozoic Stratigr^hy
Tertiary and Quaternary volcanic and sedimentary units unconformably overlie
the Mesozoic succession (Fig. 9). The Cenozoic section is approximately 400 m (1,312
ft) thick (Fig. 9) (Gregory, 1951; Hintze, 1973,1988; Sargent and Philpott, 1987; Stokes,
1988; Doelling et al., 1989; Nelson and Tingey, 1997). The Tertiary succession consists
of a fresh water unit with mostly fluvial and lacustrine conglomerate and sandstone
(Rowley et al., 1975; Stokes, 1988; Doelling et al., 1989; Taylor, 1993: Goldstrand,
1994). The Tertiary rocks generally dip to the northeast.
Late Cenozoic basalt is common in the region (Nelson and Tingy, 1997; Smith et
al., 1999, Downing et al., 2001) and a flow crops out in the mapped area. This basalt is
olivine rich with olivine phenocrysts up to 3 mm (0.11 in) in diameter. The ground mass
is fine, crystalline and dark gray. Basalt flows near Spencer Bench have been dated by
K-Ar techniques at 0.56 Ma +/- 0.07 Ma (Fig. 10 and Plate 1) (Best et al., 1980). Best et
al. (1980) interpreted this flow to be a hawaiite. Using ‘*°Ar/^^Ar dating techniques
(Appendices A and B), two samples (BRC-1 and BRM-2) from the northern portion of
Spencer Bench and Black Mountain were dated (Figs. 10 and 11). One sample was
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28 collected from the hanging wall at Black Rock Canyon, and the other sample was
collected from the footwall of the Sevier fault at Black Mountain (Fig. 11 and Plate 1).
The sample from Black Rock Canyon yielded a ^Ar/^^Ar isochron age o f0.564 +/- 0.02
Ma (Fig. 11). The sample from Black Mountain yielded an ‘'^Ar/^^Ar isochron age of
0.58 +/- 0.05 Ma (Figs. 10 and 11). These ages are similar to the ages determined by
Best et al. (1980) for volcanic rocks in Black Rock Canyon (Fig. 10).
Late Cenozoic sedimentary units include spring deposits, stream terraces, slope
failures, colluvium, sinter-type spring deposits, and alluvium. The older part of the
Quaternary succession contains (now consolidated) fluvial terrace deposits that are
overlain by basalt. The most recent deposits are unconsolidated fluvial alluvial and
surficial debris (Hintze, 1973; Hintze, 1988; Doelling et al., 1989). These late Cenozoic
units unconformably overlie units ranging from Jurassic through the early Tertiary.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTERS
STRUCTURE DESCRIPTIONS
Introduction
The Sevier fault brittlely deforms the nearly flat-laying to gently folded strata of
the High Plateaus. Fault splays, strands, and separate normal faults were identified by
fault breccia, gouge, and offset units. The sense of separation on each fault was
determined by ages (inferred by lithology and fossils ages) of juxtaposed strata. The fault
with greatest stratigraphie separation is referred to as the main strand. Because of the
lack of kinematic indicators, the exact net-slip direction was not determined for the faults.
New data from this study show that four relay ramps and a geometric bend, or salient,
occur along the central Sevier fault. This section describes the newly collected data and
the geometries of structures exposed in the study area are described below.
Sevier Fault
The Sevier/Toroweap fault generally strikes ~N30“E and dips 70-85“W along a
trace length of ~3S0 km (220 mi) between the northern tip line (northern most point) in
Miocene volcanic rocks near Richfield and the southern tip south of the Grand Canyon in
Paleozoic rocks (Fig. 7) (Gregory, 1951; Doelling et a l , 1989; Davis, 1999). The Sevier
fault from the Arizona/Utah border to the northern termination is 260 km (158 mi) in
length.
29
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 For the purpose of this study, the central Sevier fault will be subdivided into three,
separate domains: the southern domain, central domain, and Spencer Bench domain (Fig
12A). Each of these domains contains different fault strands and splays. The strike of
these strands varies between north-south and east-west, a 90° variation about the general
~N30°E strike. The "main strand" of the Sevier fault is defined here as the fault with the
greatest stratigraphie separation. The fault that is the main strand changes along strike.
The strikes vary from N5°E to N80°E.
Southern Domain - Orderville Relay Ramp
The southern domain extends from the southern boundary of the map area to ~1
km (-0.6 mi) north of the community of Orderville (37°15'50"N to 37°16'54"N) (Fig.
12A). This domain contains the Orderville relay ramp on which Davis (1999) made
preliminary structural and stratigraphie observations. The rocks exposed in this domain
range from late Triassic to (Quaternary in age (Plate 2). Rocks as young as Cretaceous in
age are faulted, but no Quaternary units are offset in this domain.
Two strands of the Sevier fault bound the Orderville relay ramp: A on the west
and B on the east (Figs. 12A and 12B). Fault A, the main strand o f the Sevier fault has a
curved fault trace, but generally strikes N10°E and dips 76°W. The fault strike changes
from approximately north to south in the south to northeast in the north (Figs. 12A and
12B). Fault A juxtaposes the Cretaceous Tropic Shale against Jurassic Navajo Sandstone
for -760 m (-2,500 ft) of stratigraphie separation. Fault B generally strikes N40“E and
dips 79“W (Plate 4). From south to north, the strike of fault B curves from northeast to
north, back to northeast and finally to north-south. This fault juxtaposes the Jurassic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31 Navajo Sandstone and the Jurassic Co-op Creek Member of the Carmel Formation for
-140 m (-450 ft) of stratigraphie separation (Plate 4).
Between faults A and B within the Orderville relay ramp (Fig. 13), map patterns,
cross sections, and stereo plots show that the stratigraphie units form a plunging syncline
oriented 17°, N7°W (Fig. 14A and Plate 4). The fold within the Orderville relay ramp is a
gentle, subhorizonal, upright fold (cf., Ramsey, 1967). However, to the west of fault A
and to the east of fault B, the strata are generally flat lying to gently west dipping.
Faults A and B connect at the surface at the southern end of the relay ramp (Figs.
12A and 12B and Plate 2). Cross section construction and retrodeformation indicate the
interpretation that these faults also connect in the subsurface (Fig. 13 and Plate 4).
In the subsurface, the faults in this region are interpreted to have different
geometries. Fault A has a relatively simple and planar geometry (Plate 4). Fault B is
planar at the surface and becomes curved by a depth of-1,220 m (-4,000 ft). This fault
is required to be curved in order to restore cross sections (Fig. 13 and Plate 4).
The depth to the base of the relay ramp is interpreted to increase from south to
north. Retrodeformation cross sections suggest that these faults coruiect in the southern
portion of this region at a depth o f670 m (2,200 ft). However, in the northern portion of
the relay ramp, the ramp probably initiates at a depth of -2,135 m (7,000 ft) (Fig. 13 and
Plate 4).
Central Domain - Stewart Canyon Overlap Zone
The central domain extends from -10 km (6 mi) northeast of the community of
Orderville to -2 km (-1.2 mi) south of the community of Glendale (37°16'54"N to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32 37°19^9”N) (Figs. 12A and 12B). The southern part of this domain is here named the
Stewart Canyon overlap zone (Figs. I2A and I2B). The Stewart Canyon overlap zone
exposes rocks that range in age from Jurassic to (Quaternary (Plate 1). The total
stratigraphie separation in this domain ranges from 770 to 1,045 m (2,525 to 3,425 ft).
Rocks as young as Cretaceous are faulted. However, no (Quaternary offset is documented
in this section.
The main strand of the Sevier fault within the Stewart Canyon overltq) zone
(Figs. 12A and 12B), has strikes that vary from N35°E to N65°E, and dips from 76-84°W.
In this region a portion of the main strand of the Sevier fault is buried by Quaternary
alluvium (Figs. 12A and 12B and Plate 1). Retrodeformable cross section construction
geometrically requires the main strand of the fault to be under the alluvium, near the
modem East Fork of the Virgin River (cross sections F-F' and E-E' on Plate 4) (Fig. 15).
To the south, near cross section F-F', the main strand of the Sevier fault has 435 m (1,425
ft) of offset and places the Jurassic Windsor Member of the Carmel Formation next to
Jurassic Navajo Sandstone (Fig. 16 and Plate 4). However, to the north near cross section
E-E’ the main strand of the Sevier fault has 390 m (1,275 ft) of stratigraphie separation
(Fig. 16 and Plate 4), where it places Jurassic Navajo Sandstone against the Jurassic
Windsor.
The Stewart Canyon overlap zone contains eleven fault strands with three
different fault set orientations; striking northwest, northeast, northeast-east and dips of
69°-84° to the north and west (Figs. 12A and 12B). Two of these faults (faults D) strike
~N55°E but bend northward to strike nearly east-west (Figs. 12A and 12B). These
strands are interpreted to connect at depth (cross section F-F' on Plate 4). At the surface.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33 the entire group o f faults cuts the Jurassic Navajo Sandstone through the Cretaceous
Dakota Sandstone in exposures. At cross section F - F', the Sevier fault zone has 770 m
(2,525 ft) of stratigraphie separation (Fig. 16 and Plate 4). In cross section F-F’ the
westerly splay is interpreted to connect to the main strand at a depth of 1,280 m (4,200 ft)
(Fig. 16 and Plate 4). The center fault (fault C) is interpreted to connect at depth with the
western strand (fault B) at a depth o f960 m (3,150 ft) (Figs. 12A, 12B, and 16 and cross
section F-F on Plate 4).
Three relay ramps occur within the Stewart Canyon overlap zone; the Glendale,
Stewart Canyon, and Elkheart Cliffs relay ramps (Figs. 12A, 12B, and 16). The ramps
have steeply dipping bounding faults that either connect or appear to connect at depth.
The differences among these three relay ramps are described below.
The Glendale relay ramp (Figs. 12A, 12B, and 16) is bounded by two faults that
and interpreted to connect at a depth of 2,135 m (7,000 ft) to the north and a depth of
-2,745 m (9,000 ft) to the south (Figs. 12 A, 12B, and 16 and Plate 4). The bounding
faults are steeply dipping and planar. These bounding faults connect at the surface at the
structurally lowest point of the ramp suggesting the ramp is base breached. A cross fault
cuts the relay ramp and is bounded by the ramp bounding faults (Fig. 16 and Plate 4).
This straight and planar cross fault is interpreted to connect with the easterly bounding
fault at the surface as well as at depth. The connection is interpreted to occur at a depth
o f930 m (3,050 ft) along cross section L-L’ (Fig. 16 and Plate 4).
Within the Glendale relay ramp, map patterns and cross sections show a north
plunging anticline oriented 3°, N59°E (Figs. 14B and 16 and Plate 4). This fold is
classified as a gentle, subhorizonal, upright fold. However, to the east and west of the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34 Stewart Canyon overlap zone, the stratigraphie units are gently west dipping (3-7°)
suggesting that the anticline is restricted to the ramp. Ramp-restricted folds, such as this
one, have rarely been described because detailed studies of multipartite linkage zones
with relay ramps has not be done.
The Stewart Canyon relay ramp is a breached ramp with both a base and a top
breach. A top breach means that the bounding faults of the relay ramp coimect at the
structurally high part of the relay ramp. The two bounding faults appear to connect at the
surface on both the north and south (Figs. 12A and 12B). However, the western
bounding fault is buried by Quaternary alluvium. The faults are interpreted to be planar
and appear to connect at depth (Fig. 16 and Plate 4). Two cross faults also cut the ramp
(Figs. 12A and 12B). These faults terminate at the bounding faults. No fold in addition
to the ramp monocline was observed within this relay ramp, but the entire relay ramp is
not exposed.
The easternmost portion of the Stewart Canyon overlap zone contains three faults
(Figs. 12A and 12B). The two eastern and subparallel faults have attitudes of
approximately N40°E, ~85°NW (faults U’ and T’ on Figs. 12A and 12B). These faults
juxtapose Jurassic Navajo Sandstone and the Jurassic Co-op Creek Member of the
Carmel Formation at the surface for a total of-100 m (-330 ft) of stratigraphie separation
(Plate 2). The easternmost faults connect in map view and are interpreted to connect in
cross section Y-Y’ at a depth o f 762 m (2,500 ft) (Fig. 16 and Plate 4). The third fault is
orientated at a high angle to the main strand of the Sevier fault; it strikes -N30°W and
dips -75°NE. This fault places the Jurassic Crystal Creek Member of the Carmel
Formation next to the Jurassic Winsor Member with 183 m (600 ft) of stratigraphie
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 separation (cross section E-E' on Plate 4). This fault is interpreted to connect with the
Sevier main strand at a depth of 335 m (1,100 A) on cross section E-E’ (Fig. 16 and Plate
4). The total amount of stratigraphie separation across the Sevier fault zone near cross
section E to E' is 1,044 m (3,425 ft) (Plate 4).
Faults U’ and T’ overly in map view with the western fault (V’) of the Glendale
relay ramp (Figs. 12A and 12B). Between these faults is a relay ramp here named the
Elkheart Cliffs relay ramp. The bounding faults do not link in map view, however, they
may link at depth (Figs. 12A, 12B, and 16). No fold besides the ramp monocline was
observed within this ramp.
Map patterns indicate that the faults in the central domain connect at the surface
(Plate 2 and Figs. 12A and 12B). Cross section construction and restoration suggests that
faults within the Stewart Canyon overlap zone are interpreted to connect in the
subsurface (Fig. 16 and Plate 4). However, the eastern two faults do not connect in map
view with the western faults. Cross section analysis reveals that these faults are
interpreted to appear to connect at depth (Fig. 16 and Plate 4).
Spencer Bench Domain
The Spencer Bench domain is located between the conununity of Glendale and
the northern boundary of the mapped area (37°19'36"N to 37°23'29"N) (Fig. 12A). This
domain exposes rocks that range in age from Cretaceous to (Quaternary (Plate I). An
offset (Quaternary (-570 ka) basalt flow is exposed on and near Black Mountain (Plate 1
and Fig. 12A). However, no (Quaternary sediments or sedimentary rocks are offset (Plate
1).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36 In contrast to the other domains, in the Spencer Bench domain the Sevier fault is
generally a single strand fault with hanging wall faults, not a complex splay zone. The
main strand of the Sevier fault, labeled fault E on Figure 12 A, strikes -N40°E and dips
76-81°W. The total amount of stratigraphie offset across the four cross sections varies
between 472 and 869 m (1,550 and 2,850 ft) within the Spencer Bench domain (cross
sections A-A* through D-D' on Plate 4). The greatest amount of total stratigr^hic
separation across the Sevier fault zone is within the splay zone near cross section C-C'
(Fig. 12A and Plate 4). Here the central Sevier fault zone has 518 to 640 m (1,700 to
2,100 ft) of stratigr^hic separation. The stratigraphie separation decreases to the north
and south from C-C’.
Eight faults, fault group C, are exposed to the northeast of Glendale (Fig. 12 A).
The faults within group C can be divided into a north-northwest striking fault set and a
north-northeast fault set with one cross fault. The dips are generally 58-81 °W. The total
amount of stratigraphie offset across these faults is 543 m (1,780 ft) near cross section D-
D’ (Plate 4). The western splay connects with the main strand of the Sevier fault at the
surface (cross section D-D' on Plate 4). However, the westernmost fault in the group,
called fault C , dips east. Fault C strikes N12°W and dips 73°E. It is interpreted to
connect at depth with a west-dipping fault (cross section D-D' on Plate 4). All the faults
are generally planar. Cross cutting fault relationships can be seen on the fault trace map
(Fig. 12A). Fault E, the main strand, is cut by fault R’ (Fig. 12A).
Fault set D lies just to the west of a bend in the main strand of the Sevier fault and
comprises five hanging wall faults: three west-dipping, one east to southeast dipping, and
one non-planar fault (inferred to restore cross sections) with northwest and north-dipping
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37 sections (O') (Fig. 12 A). The east-dipping fault and north-dipping fault section are buried
by Quaternary alluvium (Fig. 12A and Plate 1), but are geometrically required to explain
outcrop patterns. The west-dipping faults generally strike N2S**E and dip between 79**
and 8S**W. The total stratignq)hic separation along these faults is 175 m (575 ft) (cross
section C-C on Plate 4). The three western faults are interpreted to connect at depth
(cross section C-C' on Plate 4). The western faults also appear to connect with fault D’ at
the surface (Fig. 12A and cross section C-C' on Plate 4). In map view, the generally
north-south striking faults qxpear to terminate at the generally east-west striking fault
(Fig. 12A). Thus, this east-west striking fault may be a transfer fault. The fourth fault
from the west. O’, is an east-dipping fault that is buried, but has an inferred attitude of
N25**W, 80**E. It has a stratigraphie separation of 130 m (425 ft). This fault is interpreted
to connect at a depth with the west-dipping strand to the east (cross section C-C on Plate
4). Set D faults offset units as young as the Cretaceous Kaiparowits/Wahweap at the
surface (cross section C-C on Plate 4). The total stratigraphie separation for fault group
D is 503 m (1,650 ft).
The main strand of the Sevier fault is the easternmost fault near fault set D, fault
E (Fig. 12A and cross section C-C on Plate 4). Between cross section D-D’ and C-C’,
the Sevier fault forms a large bend with strikes, which vary from south to north; N70°E,
N20**E, and N5‘*E. The stratigraphie separation is 549 m (1,800 ft). The main strand of
the Sevier fault is interpreted to connect at depth with both the west-dipping and east-
dipping faults of fault set D at (cross section C-C on Plate 4). The total stratigraphie
separation o f fault set D and the main strand is 869 m (2,850 ft). North of fault set D, the
Sevier fault strikes ~N35‘*E and dips 76®W and continues this pattern north to fault set F.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 In this domain, the Sevier fault cuts Quaternary (-570 ka) basalt Four fault
strands, including the main strand, cut the Black Moimtain basalt flow (Plate 1 and Fig.
12A). Three of these faults, called fault set F, strike N44®E. One dips 82®SE and the
other two dip -80® NW. The fourth fault is a cross fault with an attitude of N82®E, 80®N
and -1 meter (-3 feet) of stratigraphie separation. The total amount of post basalt
stratigr%q)hic offset across set F faults is -3 m (-10 feet).
Folding within the Study Area
Along the central Sevier fault, the pre-Claron Formation units have a general
north-northwest dip (1-19®) except in fault blocks or close to faults. Cross section
construction and restoration requires that both the hanging wall and footwall to have
undergone some normal fault drag. The cross sections (A-A', B-B', and C-C) show an
anticline in the hanging wall of the main strand of the Sevier fault. In addition to this
anticline, several regional folds exist near the study area (Fig. 7). In map view, the
Claron Formation generally dips between 3 and 8® NE. However, this unit also has
undergone some fault drag (cross section A-A'). The (Quaternary conglomerate units all
have dips o f <10®E to NE.
The other folds are located within relay ramps, which are interpreted to be fault
controlled. The following interpretations are based on combining the structure contour
and map data from Doelling et al. (1989) and Davis (1999) and field data collected for
this study.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6
VIRGIN RIVER DEPOSITS
Introduction
The objective of this section is to provide data and interpretations of the fluvial
deposits and how they relate to the central Sevier fault. These fluvial deposits provide
information to help (1) understand the evolution of the East Fork of the Virgin River, and
most importantly, (2) place age controls on faulting or tectonic deformation in the area.
From here on, I will refer to the East Fork of the Virgin River as the Virgin River.
Three Quaternary fluvial deposits of different ages crop out along the central
Sevier fault (Plates 1 and 2). The relative ages were based on the stratigraphie position of
the deposits in the landscape. Assuming the stream has downcut through time, older
deposits are exposed at higher elevations 1,939 - 1,963 m (6,360 - 6,440 ft). The oldest
deposits are conglomerates that crop out 8 km (5 mi) north of Glendale (Qcgl). This
conglomerate is older than the -570 ka basalt, which unconformably overlies the
conglomerate. The intermediate-aged Quaternary fluvial unit (Qcg) is conglomerate that
crops out only south of the Hidden Lake spring system (Plates I and 2). This unit is
located topographically lower, 1,707 - 1,865 m (5,600 - 6,120 ft), than the Qcgl, but
higher than the modem alluvium and stream channel (Fig. 15). The Qcg is also
unconformably overlain by a basalt flow southwest of Glendale (Fig. 15 and Plates 1 and
2). The modem channel is subparallel to the Sevier fault and lies along the fault trace
39
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 south of Glendale and -0.5 km (-1,500 ft) west of the fault, north of Glendale. The
Quaternary alluvium and modem channel are younger than the basalt because of the lack
of basalt flows on top of any of the deposits. The deposits contain basalt clasts and
grains.
Modem Fluvial Deposits
The mapped modem Virgin River channel deposits have clasts smaller than four
centimeters in diameter and are dominantly sand and gravel with few sandbars. The
channel deposits become finer grained to the south. However, some large (>1 meter)
clasts occur locally in the south. These clasts are probably evidence of flooding. The
Sevier fault zone does not visibly offset the modem fluvial deposits.
The flood plain of the Virgin River is -610 m (-2,000 ft) across in the south near
37“I5’50"N to 37®20’30"N and <152 m (<500 ft) in the north near 37°20'30"N to
37°23'36"N (Fig. 15). This difference in flood plain width may be caused by the change
in the gradient of the stream, a change in bedrock lithology, or an increase in discharge
from the north to the south. The gradient of the Virgin River is steeper in the north than
in the south (Fig. 15). North of Dry Wash, the gradient of the Virgin River is 0.026, and
to the south of Dry Wash the gradient is 0.011. The bedrock lithology changes from the
more resistant Cretaceous Kaiparowits/Wahweap Sandstone to the north to the less
resistant mudstones and shales of the Tropic Shale, Dakota Sandstone, and Carmel
Formation to the south. The amount of discharge is greater in the south than the north
particularly because the Dry Wash channel contributes to the Virgin River north of
Glendale (approximately 37°20'30"N) (Fig. 15 and Plates 1 and 2). Therefore, changes in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41 gradient, bedrock, and discharge may all impact the width of the Virgin River flood plain.
Knowing and being able to predict the width of the flood plain is important because
several people live along the Virgin River.
Young Fluvial Conglomerate
The young fluvial conglomerate (Qcg) has a lithologie make-up, which differs
from the modem stream deposits (Fig. 17A). The conglomerate clasts comprise
limestone, dolostone, chert, sandstone, basalt, and petrified wood. Generally, the clasts
are well rounded and range from one centimeter to greater than a meter in diameter and
graded bedding is present locally. However, the petrified wood is generally angular to
subangular. The petrified wood is dark brown to black and appears to be agatized. The
matrix consists of an indurated lithic sand. Discontinuous layers of lithic sandstone also
occur throughout the conglomerate unit (Fig. 18A). The discontinuous layers of
sandstone contain cross beds of the type (simple trough cross-stratification) seen in
sandbars or migrating ripples (Prothero and Schwab, 1996). The cross beds, graded
bedding, and channels (Fig. 18B) suggest that the Quaternary conglomerate has a fluvial
origin.
From north to south, the gently dipping surfaces of the outcrops become
topographically lower. These surfaces could have formed by (1) erosion, (2) as part of a
floodplain, or (3) as stream terraces. (1) These surfaces may or may not have formed by
erosion. If the surfaces were formed by erosion, the surrounding bedrock would have the
same erosional characteristics. The bedrock contains no evidence of this type of erosion.
(2) The surfaces did not form during flooding because floodplains typically are composed
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42 of fine-grained sediments such as muds and silts. The deposits under these surfaces are
sand to boulder sized. (3) Stream terraces form when the base level of an active stream
drops. As the base level drops the steam begins to down cut leaving the old streambed at
a higher elevation than the present stream elevation. The older terraces become inactive
benches (Keller and Pinter, 1996; Prothero and Schwab, 1996). I interpret these
conglomerate units to be old stream terraces, which may have been eroded. The paleo-
Virgin River must have flowed from the north to the south primarily because the terraces
become topographically lower toward the south (Fig. 15).
Older Fluvial Deposits
The older conglomerate (Qcgl) crops out in one location east of U.S. 89 and in
three locations west of U.S. 89 (Fig. 15 and Plate 1). This may be because the older
channel is farther west of the U.S. 89 or other outcrops have been eroded or not
preserved.
Limestone, dolostone, chert, sandstone, and basalt are the dominant clasts in the
conglomerate. Generally, the clasts are well rounded and range from one centimeter to
greater than a meter in diameter and graded bedding is present locally. The matrix
consists of a sandy siltstone.
The main lithologie difference between the older fluvial deposits and the younger
fluvial deposits is the absence of petrified wood clasts in the older conglomerate. The
absence of the petrified wood may be due to the fact that this unit unconformably overlies
the Kaiparowits/Wahweap undivided Formation (Kkw) (Fig. 9). Therefore, the Virgin
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43 River at the time of the deposition of the older conglomerate did not erode layers
containing petrified wood within the Kkw.
Three additional differences between the older conglomerate and the other
conglomerates help to distinguish the units. First, the older conglomerate is generally 73
m (240 ft) above the modem stream channel (Fig. 15), whereas the younger conglomerate
(Qcg) is typically found between 12 - 37 m (40 - 120 ft) above the present Virgin River
channel (Fig. 15). Another difference is that Qcgl is not on the same grade as the Qcg
indicating that they are separate deposits. Lastly, the older conglomerate has a finer
grained matrix with 0.6 m (2 ft) clasts generally of basalt (Fig. 17B).
Both the older and the younger conglomerates are capped by basalt flows (Fig. 15
and Plates 1 and 2). The older conglomerate is capped by the -570 ka basalt flow
indicating that the conglomerate is older than 570 ka. In addition, both conglomerates
contain clasts of basalt from other flows.
Interpretations
The bedding within the conglomerate and the original slope was used to
determine if this unit was tectonically tilted. Beds formed by a south flowing stream
generally dip gently to the south. The top surfaces of the exposures dip gently to the
northeast and east (<10°NE and <10°E) indicating that they may be tectonically tilted.
Two possible sources of the petrified wood clasts in the Qcg are the Cretaceous
Kaiparowits/Wahweap formations undivided or the Triassic Chinie Formation. Petrified
wood was found within the Kaiparowits/Wahweap formations in the mapped area. The
flow of the paleo-Virgin River was most likely from north to the south. Exposures of the
Triassic Chinie Formation occur only to the south of the outcrops of Qcg and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44 Kaiparowits/Wahweap Formation is exposed near the outcrops and to the north. Also the
petrified wood found in the conglomerate is darker in color and highly agatized. The
petrified wood found in the Chinie Formation is generally a lighter color and not as
agatized. However, petrified wood crops out in the Chinie Formation near Zion National
Park, but is darker in color and highly agatized. I suggest that the petrified wood in the
Qcg is derived from the Kaiparowits/Wahweap formation.
Discussion of the Virgin River Deposits
Modem Virgin River deposits have an overall smaller grain size than Qcg. This
suggests that the present day flow is slower than the flow in the past. However, the grain
size of the Qcgl and the modem fluvial deposits are relatively similar indicating that the
flow of the river that deposited Qcgl and the modem flow rates are similar. Both are
faster flow rates than the flow that deposited Qcg. Additionally, the original surface of
the conglomerates was slightly to the south.
Faulting within the Quatemary Fluvial Deposits
None of the Quatemary conglomerate units within the mapped area are cut by a
fault at the surface. This observation suggests that either surface mpturcs are older than
the conglomerate units along this section of the fault, or these deposits are not located on
the faults. The later scenario is more probable because none of the fault strands in the
area of interest project under the conglomerate. However, the 570 ka basalt is cut by
faults and several of the conglomerate outcrops are tilted. The tilting was probably
caused by faulting.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 The active channel and floodplain of the Virgin River are located in the hanging
wall just west of the main strand of the Sevier fault in the southern part of the area (Plate
2). This geometry fits the Leeder and Gawthorpe (1987) model of a continental half-
graben with axial trough drainage. Both the model and the Sevier fault have footwall
uplands with incised drainages. In both the model and the study area these drainages
seem to flow nearly perpendicular to the main channel and the fault trace. Both have
meandering streams or rivers that flow approximately parallel to the fault trace. The
model suggests that as the faults continues to slip, sediments progressively onlap hanging
wall units during extension as the stream migrates closer to the fault trace (Leeder and
Gawthorpe, 1987). The southern portion of the Virgin River and Sevier fault fit this
model. No abandoned meander belts occur in the modem alluvium to suggest
progressive onlap and tilting during fault movement. However, all of the Qcg surfaces
dip approximately 10°NE to 10“E. Because the fault dips west, when it moves, the
sediments in the hanging wall are tilted toward the fault or to the east. These data suggest
possible tilting during fault movement because the Sevier fault zone is generally to the
east of the Virgin River.
In the northem portion of the area, the Virgin River and the Sevier fault lie farther
apart. Near Glendale, the main strand of the Sevier fault bends to the northeast and the
Virgin River continues to flow south and does not follow the trace of the fault. However,
north of Glendale, a large tributary. Dry Wash Canyon (Plates 1 and 2,) to the Virgin
River follows the Sevier fault as far north as Black Mountain (Fig. 1). The Quatemary
alluvium within Dry Wash Canyon covers a possible transfer fault of set D (Fig. 12).
Several of the Qcg outcrops are exposed in Dry Wash Canyon suggesting that this canyon
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46 was a tributary to the Virgin River during the time of Qcg deposition (Plates 1 and 2 and
Fig. 15). This implies that the Virgin River followed the trace of the Sevier fault in Dry
Wash Canyon in the past, but a new drainage has formed to the west.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER?
STRUCTURAL INTERPRETATIONS
Two different types of linkage formed near the Orderville relay ramp and within
the Stewart Canyon overlap zone. Within these linkage sites, relay ramps with fault
parallel folds formed. Together these linkage sites form the Orderville geometric
boundary, or salient. The following sections will discuss (1) the linkage zones and the
structures formed within them, (2) stages of linkage, and (3) slip rates along the central
Sevier fault.
Linkage
Examples of Fault Linkage
Originally isolated faults are interpreted to have linked between Orderville and
the Stewart Canyon overlap zone to form the central Sevier fault (Figs. 12A and 12B).
Total displacement vs. distance diagrams were created from where the central Sevier
fault is generally a single strand in the north to the southern boundary of the mapped area
(Fig. 19). The total displacement decreases within the linkage zones (Fig. 4). Within
these linkage zones, two different styles of linkage were identified. Near Orderville, fault
capture is suggested. In the Stewart Canyon overlap zone, linkage of overlapping faults
is suggested. However, several fault tips overlap, and thus this fault zone is multipartite.
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48 more complex than typically described (i.e., Ferrill et al., 1999). The two different styles
are discussed below.
Fault Capture in the Orderville Area
Near Orderville, the central Sevier fault linked by fault capture. On displacement
vs. distance diagrams the total displacement decreases near the Orderville relay ramp
(Fig. 20A). Displacement decreases are expected to occur near linkage sites because
linkage zones are typically located near original fault tips where the displacement is less.
The pattern on the displacement vs. distance diagram for the Orderville area closely
resembles the pattern for the model of fault capture (Fig. 4C). Both diagrams have a
plateau in the area where the faults overlap. In this type of linkage, one fault has more or
more rapid slip than the other fault. The fault with more or more rapid slip is the fault
with the most displacement (Fig. 4C). In addition, the map fault trace pattern (Figs. 12A
and 12B) closely resembles the fault trace pattern for the model of fault capture (Fig. 4C).
Faults A and B (Figs. 12A and 12B), have nearly the same surface fault trace pattern as
the model surface fault trace pattern (Fig. 4C).
Different stages of linkage were identified by the mapped fault trace pattern. For
example, stage one, the least advanced stage of linkage, is indicated by two faults that are
separate and isolate. Stage two is indicated by the linkage of two faults at one linkage
site. Stage three, or the most advanced stage of linkage, occurs where two faults link at
two linkage sites. One linkage site is buried under the Quatemary alluvium to the north
of Orderville (Figs. 12A and 12B). The other linkage site is located where faults A’ and
B’ connect at the surface to the south (Figs. 12A and 12B). The Orderville area is in an
advanced stage of linkage via fault capture because the faults have two linkage sites.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49 I suggest that the faults in the Orderville area linked by fault capture because their
initial fault spacing and orientations did not allow the fault tip stress fields to interact
until the fault with the greater amount of slip, the western fault, propagated to
approximately the center 1/3 of the eastern fault (Figs. 12A and 12B).
In the fault capture linkage area near Orderville, the Orderville relay ramp formed
(Figs. 12A and 12B) (Davis, 1999; this study). The Orderville relay formed in a fault
capture linkage situation, which is a type of hard linkage. The attitudes within the
Orderville relay ramp reveal the typical ramp monocline (Fig. S) as well as an additional
fold (discussed later). This ramp is also breached at both the top and bottom at the two
linkage sites (base and top breaches) (Figs. 12A and 12B).
Overlapping Faults in the Stewart Canyon Area
The Stewart Canyon overlap zone may be a strain transfer zone, as indicated by
four major linked overlapping faults. The Stewart Canyon overlap zone is more complex
than simple overlapping fault tip linkage. It is a strain transfer zone that involves at least
twelve faults, four sites of linkage, and three relay ramps. The zone is subdivided into
three domains: eastern, central, and western (Figs. 12A and 12B).
Eastern Stewart Canyon Overlap Zone
The eastern most fault (T’) is interpreted to be a splay of the next fault to the west
(U’) (Figs. 12A and 12B). To the north near cross section E-E , the northeast dipping
fault (Q’) is also interpreted to be a splay of fault U’ (Figs. 12A and 12B).
Farther to the north, faults U’ and S’, which connect in map view (Figs. 12A and
12B), form a fault slice. Fault S’ was once the main strand, however, the stress fields
changed and a new fault (U”) propagated, which became the main strand.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 I suggest that the eastern portion of the Stewart Canyon overlap zone is a relay
ramp and herein name it the Elkheart Cliffs relay ramp (Figs. 12A and 12B). This relay
ramp formed between overlapping fault tips. On displacement vs. distance diagrams, the
plotted data (Fig. 20D) closely resemble the shape predicted by the model for soft linkage
via overlapping faults (Fig. 4E). Additionally, the bounding faults do not link at the
surface suggesting soft linkage. However they may link at depth suggesting a hard
linkage situation (Plate 4). Thus, both hard and soft linkage occur along these two faults.
Central Stewart Canyon Overlap Zone
In the central portion of the Stewart Canyon overlap zone, the faults that form the
Glendale relay ramp are linked overlapping faults for several reasons (Figs. 12A and
12B). First, the displacement vs. distance profile (Fig. 20B), generally resembles the
overlapping fault tips model (Fig. 4B). However, an anomaly exists within the linkage
zone along the northern part of the linkage site (Fig. 20B). The location of the anomaly
corresponds to the location of fault D, a breakthrough or cross fault. Therefore, the
anomaly is most likely a direct result of that cross fault. Second, the mapped patterns for
the central Stewart Canyon overlap zone closely resemble the model fault trace map
patterns for overlapping faults (Figs. 4B and 20B). Both have overlapping faults that
connect at the surface. The faults in the central Stewart Canyon overlap zone and in the
models in map pattern overlap and connect (Figs. 4B, 12A, 12B, and 12B). In this case,
hard linkage occurred because the ramp is breached on the basal end. However, this area
also contains cross faults, which, require a combination of the overlapping and cross fault
models (Figs. 4B and 20B). The central Stewart Canyon overlap zone mapped fault trace
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51 patterns are interpreted to have formed by a combination of the overlapping faults
(Taylor et al., 2(X)1) and breakthrough fault linkage models (Fig. 4).
The Glendale relay ramp (Figs. 12A and 12B) formed between overlapping faults
(V’ and W’) within the central portion of the Stewart Canyon overlap zone, with hard
linkage and a base breach (physical connection of the faults at the structurally lowest
point of the relay ramp). The attitudes of bedding within the ramp reveal the typical
ramp monocline as well as an additional fold (discussed later). The Glendale relay ramp
also contains a cross fault that may ultimately break down the relay ramp by continued
slip along the faults.
Western Stewart Canyon Overlap Zone
The western faults linked overlapping faults with hard linkage at two sites, one to
the north and one to the south (Figs. 12A and 12B). The pattern for the western portion
of the Stewart Canyon overlap zone has a lens shape, like in the overlapping fault tips
model. The displacement vs. distance diagram for the pattern of the Stewart Canyon
relay ramp area (Figs. 12A and 12B) most closely matches the model of overlapping fault
tips (Fig. 20B). However, the fault trace map shows cross faults. In addition, an
anomaly or step is present on the displacement vs. distance diagram (Fig. 20B). I suggest
that these faults formed after linkage because of the cross cutting relationships of the
faults (Figs. 12A and 12B). The cross faults terminate sharply at the bounding faults with
a high angle of intersection rather than the 30° typical of cross faults. Therefore, in this
area late cross faults and overlapping faults, combined, formed the linkage site.
I suggest that these faults linked across overlapping fault tips because of fault
spacing and fault tip stress field interaction (Chapter 2). The fault spacing was small
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 enough to allow both stress fields to interact with each other at the same time. Linkage
by overlapping faults may also be an issue of propagation rates. For example, both faults
have similar propagation rates and displacement, so when the stress fields interact, the
faults link via overlapping type of linkage rather than fault capture.
Folds Within Relay Ramps
The Orderville relay ramp contains a fault-parallel syncline and the Glendale
relay ramp contains a fault-parallel anticline (Figs. 13 and 14). The rocks outside of the
bounding faults are generally flat lying or gently west dipping. If these folds formed by
drag along the fault, the beds would "roll-over " and dip more steeply towards the fault or
exhibit normal drag and dip steeply away from the fault. If the folds are fault
propagation folds, the beds in the footwall would be deformed. However, neither
geometry is the case. I suggest that the folds formed as a result of a downward decrease
in space between the bounding faults (Figs. 13 and 16).
A downward decrease in space between two relay ramp bounding faults occurs
because the faults merge at depth in the linkage zone (Fig. 21). At the moment when the
ramp-bounding faults link, the rocks within the relay ramp fill the space between the
faults (Fig. 21). However, if fault B or both fault A and B move (Fig. 21), the rocks
within the fault-bounded wedge-shaped block are dropped down into a smaller space.
This decrease requires the rocks to shorten normal to the bounding faults, which can be
accomplished through a fault-parallel fold (Fig. 21). The more the faults slip, the tighter
the fold becomes within the fault block. Therefore, fault-parallel folds within relay
ramps suggest that faults bounding the relay ramp approach each other and/or merge with
depth and the bounding faults have had significant slip after formation of the relay ramp.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53 The Glendale relay ramp contains a generally fault-parallel anticline. On Figs.
12A and 12B, notice that the axial trace of the fold bends and changes strike. This
change suggests that the east-bounding fault is not planar at depth, but changes strike
and/or dip with depth causing the axial surface of the fold to bend.
Because most of the Stewart Canyon relay ramp is buried under Quatemary
alluvium, it is not know whether or not it contains a fold. Where the relay ramp is
exposed, attitudes were difficult to collect because the Cretaceous Dakota Sandstone is a
poorly exposed dominantly muddy shale. The attitudes that were collected are on coal
seams. Based on mapping and cross section construction analyses, I suggest that the
Stewart Canyon relay ramp is a zone of fault linkage.
Summary
The Orderville, Glendale, Stewart Canyon, and Elkheart Cliffs relay ramps have
formed between overlapping faults. However, the fault capture subtype of overlapping
linkage occurred near the Orderville relay ramp (Fig. 20). Breakthrough cross faults are
important in the Glendale and Stewart Canyon relay ramps. These relay ramps were all
identified by fault patterns and/or the attitudes of bedding between the bounding faults.
The unique pattern of faults and relay ramps in the Stewart Canyon overlap zone may
represent a triple ramp linkage situation where one ramp forms and with continued slip,
another ramp forms while the first formed relay ramp becomes both base and top
breached. Later another ramp forms, while the second ramp becomes base breached.
Stages of Relay Ramp Development in the Stewart Canyon Overlap Zone
The Stewart Canyon relay ramp is interpreted to have formed first, next the
Glendale relay ramp, and finally the Elkheart Cliffs relay ramp. These stages of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54 development are based on (1) cross cutting fault trace patterns, (2) whether the relay
ramp is breached or not (if the ramp is breached at how many sites), and (3) the number
of cross faults, which ultimately lead to the break down the ramp. Breaching of the relay
ramp suggests that the ramp is in its final stage of development and will begin to
breakdown (Ferrill et al., 1999).
The Stewart Canyon relay ramp probably formed first because it has both a head
and top breach. No anticline or syncline was found within the ramp suggesting that the
ramp formed prior to the other ramps (i.e., the ramp couldn’t have formed through a pre
existing ramp with a fault parallel fold). However, there may be a fold, but most of the
ramp is covered by Quaternary alluvium. The Glendale relay ramp probably formed
next. This ramp has a base breach and a fault-parallel anticline. With additional slip
along the bounding faults, the ramp will probably form a top breach and the fault-parallel
anticline will become tighter. I suggest that the Elkheart Cliffs relay ramp formed last.
This is interpreted because (1) the ramp is not breached, (2) there is no evidence of a
fault-parallel fold, and (3) the bounding faults may connect at depth but not at the
surface.
Stages of Linkage along the Central Sevier fault
The new data and analyses provided here allow recognition of five stages of
linkage along the central Sevier fault.
During stage 1, the central Sevier fault inititated as six isolated faults: Spencer
Bench segment (U’), X’, V’, W’, Y’, and Mt. Carmel segment (Z’) (Fig. 22A). A
possible relay ramp developed between faults X’, V’, and +/- W’ (Figs. 22A and 23A).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55 This relay ramp is a combination of the Glendale and Stewart Canyon relay ramps. This
stage of development is soft linkage.
During stage 2, near the community of Orderville, fault Y’ continued to propagate
south and fault Z (Mt. Carmel segment) continued to propagate north (Fig. 22B). As
these faults overlapped, they formed the Orderville relay ramp. After this ramp formed,
hard linkage occurred. The relay ramp is double breached with a top and base breach.
The syncline formed just before or after hard linkage. To the north, faults X and V’
continue to propagate ultimately linking by hard linkage to form a breached relay ramp
(Figs. 22B and 23B). This breached ramp is the combined Glendale and Stewart Canyon
relay ramps. While faults X and V’ link, fault W’ propagated and divided the area
between faults X’ and V’ into the Glendale and Stewart Canyon relay ramps (Figs. 22B
and 23B). An anticline also formed within the Glendale relay ramp. Soft linkage
occurred between faults X and W’ as well as W’ and V’ (Figs. 22B and 23B). To the
northeast, fault U’ (Spencer Bench segment) continued to propagate south. Between
faults U’ and V’, the Elkheart Cliffs relay ramp developed soft linkage (Figs. 22B and
23B).
During stage 3, fault W’ propagated to the north and linked by hard linkage with
fault V’ (Figs. 22C and 23C). This hard linkage site is a base breach of the Glendale
relay ramp and isolates the Stewart Canyon relay ramp. In this stage the Stewart Canyon
relay ramp is bound by three faults X’, V’ and W’ (Figs. 22C and 23C). Cross faults
continued to develop in the Stewart Canyon relay ramp (Figs. 22C and 23C). To the east,
fault U’ continued to propagate to the south and the Elkheart Cliffs relay ramp developed
further by soft linkage (Figs. 22C and 23C).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56 During stage 4, a cross fault developed and cut the Glendale relay ramp and
anticline (Figs. 22D and 23D). Fault W’ propagated and linked with fault Z to form a
top breach on the Stewart Canyon relay ramp (Figs. 22D and 23D). In addition, a cross
fault cutting the Stewart Canyon relay ramp formed. These cross faults have an atypical
orientation of -90° to faults X and V’ and subparallel (20°) to V’ (Figs. 22D and 23D).
This non-standard orientation probably formed because the Stewart Canyon relay ramp is
has three bounding faults rather than the typical two bounding faults. To the east, fault
U' continues to propagate farther south (Figs. 22D and 23D). Fault U’ may possibly link
at depth with fault V’ to form a hard linkage site (Figs. 22D and 23D). Farther to the
east, fault T’ begins to splay from fault U’ and propagates north (Figs. 22D and 23D).
During the final stage (stage 5), cross and breakthrough faults continued to
develop and fault T’ propagated farther northward (Figs. 22E and 23E). Stage 5 is also
the modem fault trace map. Several of the faults have been buried by Quatemary
alluvium.
Segments and Segment Boundaries
Typically geometric salients and segment boundaries form at linkage sites or
along linkage zones between reentrants. The salient along the central Sevier fault
between Orderville and Glendale is a large geometric bend containing linkage zones
called the Orderville salient (Fig. 22E). This salient contains four linkage sites and the
Orderville, Glendale, Stewart Canyon, and Elkheart Cliffs relay ramps. A salient or
geometric segment boundary is a zone rather than a particular point (Fig. 22E). The
Orderville salient lies between the Mt. Carmel segment to the south and the Spencer
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57 Bench segment to the north. Both the Mt. Carmel and the Spencer Bench segments are
reentrants to the Orderville salient (Fig. 22E).
Age Constraints and Slip Rates on the Sevier Fault
Different portions of the Sevier fault have different ages of last known surface
rupture. The northem portion is geographically defined from Panguitch south to Black
Mountain and is called the Spencer Bench geometric segment. The central portion is
defined from Black Mountain south to Orderville and is called the Orderville salient. The
southem portion is defined from Orderville south to Yellow Jacket Spring called the Mt.
Carmel geometric segment (Fig. 1).
Slip Rates
Preliminary slip rate studies have been done for the northem portion of the
Spencer Bench geometric segment (Hecker, 1993). Hecker (1993) states that the slip rate
is 0.360 mm/year (0.014 in/year) near Red Canyon (Fig. 1). This slip rate was calculated
for the last 560 ka using the post Red Canyon andésite slip and the age of the andésite
(Best et al., 1980).
No slip calculations have been published for the central and southem portions of
the Sevier fault. This study introduces new data and calculations of slip rates along the
central portion of the Sevier fault.
Pre- and post-basalt slip rates for the central Sevier fault are useful to estimate (I)
the age of initiation, (2) whether the slip rate has been constant through time, (3) possibly
whether the region is at seismic risk in the future. To determine the age of initiation and
pre-basalt slip rate, the post-basalt slip rate must be determined.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58 Post-basalt slip rates were calculated in the Spencer Bench domain near Black
Mountain. Here, the Sevier fault displaced 570 ka basalt 10 meters (33 ft) (Best et al.,
1980; this study) (Figs. 12A and 12B). Assuming the slip rate was constant since 570 ka,
an average yearly slip rate for the Spencer Bench segment can be calculated from the
equation; total offset of the basalt divided by the age of the basalt. Using this formula the
slip rate for the last 570 ka is 0.0180 mm/year (0.0007 in/year).
After calculating the post-basalt offset the initiation of the Sevier fault can be
determined by using the formula r = d/t where t = time, d = distance or stratigraphie
separation, and r = post-basalt offset slip rate. The d used was the greatest amount of
stratigraphie separation along the Spencer Bench segment of the main strand of the
central Sevier fault. This calculation assumes that the fault has had a constant slip rate
throughout its entire history. By using this formula, central Sevier fault initiated 44 Ma.
However, the 44 Ma age is older than expected because entire Tertiary Claron Formation
is cut by the Sevier fault. Furthermore, the Claron Formation does not contain any
evidence of syn-faulting deposition. Therefore, it is required that the initiation of the
central Sevier fault is post-Claron. The top of the Claron Formation is -30 Ma based on
a date for a tuffaceous sandstone within the upper Claron (Lundin, 1989).
The pre-basalt slip rate was calculated based on cross section construction and
using the formula r = d/t. Where d = stratigraphie separation and t = age of the top of the
Claron Formation (30 Ma). The total offset was calculated on the top of the Jurassic
Navajo Sandstone. This method of determining the total offset assumes that the slip rate
has been constant through pre-basalt time and that slip initiated immediately after the
cessation of deposition of the Tertiary Claron Formation. Thus, the minimum slip rate
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59 for the pre-basalt offset is 0.0229 mm/year (0.0009 in/year). If slip initiated significantly
later than when deposition of Claron Formation ceased, the rate would be faster.
So, based on the above calculations, two interpretations can be made. (1) The
southem section of the central portion (Spencer Bench segment) of the Sevier fault has
become less active through time. (2) The Red Canyon area is a separate geometric
segment from the Spencer Bench segment with a different slip rate. Either interpretation
agrees with calculations of decreasing slip rates along the Hurricane fault with time
(Lund et al., 2002).
The post-basalt slip rate along the Spencer Bench segment is significantly less
than the Quatemary slip rates for the northem portion of the Sevier fault. This suggests
that the northem portion of the fault has been more active in the last 570 ka. These slip
rates only suggest that the central Sevier fault has been active recently and may produce
seismicity in the future.
Fault Scarps
Several fault scarps occur along the Sevier fault. In the northem portion, near
Red Canyon, Hecker (1993) documented scarps, which based on scarp morphology may
be as young as Holocene in age (Fig. 1 ). However, at the westem entrance to Red
Canyon, the Sevier fault has a total of -900 m (-2,950 ft) of offset (Lundin, 1989). Here
a 560 ka andésite is juxtaposed against the Tertiary Claron Formation with -200 m (-660
ft) of offset (Best et al., 1980; Hecker, 1993). Christenson and Nava (1992) reported
fault scarps in Quatemary alluvial fans to the east of Panguitch (Fig. 1). Along the
central Sevier fault, no faults cut the Quatemary sediments. However, the 570 ka basalt
on Black Mountain is offset (Figs. 12 and 1). Stream terraces of late Tertiary/Quatemary
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60 age are tectonically tilted. Along the southem portion of the Sevier fault, south of Mt.
Carmel Junction, Doelling et al. (1989) mapped alluvial gravels, eolian sands, and mixed
eolian sands and alluvium cut by strands of the Sevier fault. The above fault scarp data
suggest that seismic activity along the Sevier fault is youngest to the north and oldest in
the central portion.
Both the slip rate and the ages of scarps suggest that the most recently active
portion of the Sevier fault is the northem portion followed by the southem portion then
the central portion. However, further studies and additional mapping will help constrain
these relationships.
Folding along the Central Sevier Fault
By using the collected field data, I suggest that the folds in question do affect the
Tertiary Claron Formation and therefore are a result of early Laramide with additional
Cenozoic normal fault folding. The trace of the axial surface of the westem syncline
(Fig. 7) (Doelling et al., 1989) would be approximately along U.S. 89 (Fig. 7). The field
area lies in the east limb of this fold, thus dips in the area should be north to northwest.
The trace of the Harris Mountain anticlinal axial surface to the south (Doelling et al.,
1989) crosses the trace of the Sevier fault near Orderville and continue north in the
footwall (Fig. 7). The structure contours and attitudes of bedding suggest that these fold
pattems are possible.
The Tertiary Claron Formation has an ~10°NE dip and the Quatemary
conglomerate units have <10°E dips. This strata rotation was probably formed by
tectonic tilting or fault drag along the Sevier fault. This warping deformed the already
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61 existing Laramide folds and the Tertiary Claron Formation. This interpretation helps
explain the north to northwest dipping pre- Tertiary Claron units and the east dips of the
Tertiary Claron and post- Tertiary Claron units.
Cross section restoration suggests that the units near the faults are folded into the
fault. This fold is parallel to the fault and is interpreted to be caused by Cenozoic normal
fault drag along the Sevier fault.
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62
This reproduction Is the best copy available.
UMI‘
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 8
EARTHQUAKE AND LANDSLIDE HAZARDS
Earthquakes
The central Sevier fault is located in the southern part of the Intermountain
Seismic Belt (ISO), a tectonically active region with large (M 7.0+) earthquakes (Fig. 1).
This belt extends from northwestern Montana to southern Nevada and northern Arizona
and is characterized by shallow (<15 km focal depths) earthquakes (Smith and Sbar 1974;
Christenson and Nava, 1992; Mason, 1996). Large prehistoric earthquakes of M 7.0 -
7.5 occurred in southwestern Utah, and thus, may possibly occur in the future
(Christenson and Nava, 1992). Earthquakes that occurred prior to July 1962 are based on
felt reports (shaking felt and reported by humans) whereas earthquakes occurring after
July 1962 are based on instrumental data (Christenson and Nava, 1992). Historical
earthquakes in southwestern Utah have reached magnitudes of 6.0 - 6.5 (Christenson and
Nava, 1992). Some examples are the 1902 M 6.5 Richfield, 1902 M 6.3 Pine Valley, and
1921 M 6.3 Elsinore earthquakes (Christenson and Nava, 1992). Approximately 2,300
earthquakes >M 2.0 occurred in southwestern Utah since 1850 (Christenson and Nava,
1992), and approximately 20 earthquakes greater than M 4.0 occurred during this century
in southwestern Utah and northwestern Arizona (Christenson and Nava, 1992). A M 5.5
- 5.7 eanhquake was recorded in 1959 southeast of Kanab (Doelling et al., 1989;
Christenson and Nava, 1992). Christenson and Nava (1992) document an earthquake of
63
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 Mercalli Intensity VII (Richter magnitude of -5.7) near Kanab in 1887 and several M 2.0
- 4.0 earthquakes near the trace of the Sevier fault. However, it is difficult to determine
whether these earthquakes occurred along the Sevier fault zone because earthquake
locations and focal depth resolutions are poor. On January 1, 1924, a Mercalli Intensity
III earthquake was felt in the town of Orderville (Fig. 1) (Doelling et al., 1989). From
January 1, 1924, through November 27, 1927, fourteen aftershocks (Mercalli Intensity of
II to III) were also felt in Orderville (Doelling et al., 1989). Doelling et al. (1989)
suggest the January 1, 1924 earthquake and aftershocks were caused by movement along
the Sevier fault zone. However, no Holocene fault scarps or ground ruptures are
documented along the Sevier fault zone near Orderville, however deep earthquakes may
not cause surface rupture. The proximity of these reported earthquakes to the Sevier
fault, suggests that this fault may have Holocene activity.
The Sevier fault lacks evidence for Holocene surface rupture, but evidence for
late Quaternary surface rupture does exist (Jackson, 1990; Christenson and Nava, 1992;
Hecker, 1993). The Sevier fault offsets -570 ka basalt near Black Mountain (Plate 1)
(Best et al., 1980; this study), indicating surface rupture during the Quaternary. The lack
of documented Holocene surface ruptures suggests recurrence intervals are greater than
10 to 30 ka (Christenson and Nava, 1992). However, Jackson (1990) and Hecker (1993)
state that evidence for recent activity may be lacking or obscured because of active
colluvial slopes and thick vegetation. Also, erosion by the Virgin River may have
removed scarps. Christenson and Nava (1992) suggest relatively high late Quaternary
slip rates of 0.30 - 0.47 mm/year (0.012 - 0.019 in/year) that seem to contradict the lack
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65 of evidence for Holocene surface rupture. Little information is known about the average
earthquake recurrence intervals along the Sevier fault in southwestern Utah.
Earthquakes of M 4.0 and higher generally cause a variety of earthquake and
associated hazards. Some of the hazards in the region include ground shaking, surface
rupture, liquefaction, flooding, and slope failures (rock falls, landslides, debris flows,
avalanches, etc.) (Christenson and Nava, 1992; Harty, 1992; Hecker, 1993; Lund, 1997;
Stewart et al., 1997a, 1997b; Reber et al., 2001). This study will focus on slope failures.
Landslides
A landslide is the downward movement of earth material along a distinct failure
surface. Active landslides can range in velocity from one meter per day to as fast as 300
km/hour (186 mi/hour) (Rahn, 1996). Landslides that no longer move are considered
inactive or stable (Rahn, 1996). This study provides new information on 14 landslides.
Identifying and Categorizing Landslides
Most of the landslides documented in southern Utah are at elevations greater than
1,829 m (6,000 ft) above sea level (Schroder, 1971) and are derived from the from the
Cretaceous Tropic Shale, Dakota Sandstone, and Jurassic Carmel, Triassic Chinle, and
Moenkopi Formations (Schroder, 1971; Doelling et al., 1989; Christenson and Nava,
1992; Harty, 1992; Lund, 1997; Stewart et al., 1997; Reber et al., 2(X)1). These
formations contain expandable clays such as montmorillonite (Mulvey, 1992). When
wet, these clays can expand up to 2,(X)0 times their original dry volume causing
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66 landslides to occur within these high clay content collapsible soils and rock units
(Mulvey, 1992).
One natural trigger for many landslides is earthquakes. Earthquake induced
landslides typically occur on steep slopes (>25°) that have a relief greater than 152 m
(500 ft) (Keefer, 1984; Rahn, 1996). Typically, such slopes can be found along streams,
washes, or young faults. Failure occurs when the material is dislodged by earthquake
shaking. After failure, the slopes are then typically undercut by streams or active washes
(Keefer, 1984; Rahn, 1996). The amount of undercutting is dependent on how much and
what type of material fell. Also, the amount of undercutting is dependent on whether or
not the debris fell into the stream.
Naturally occurring landslides can also form by gravitational failure along
bedding planes of weak units (i.e., shales and mudstones) or clay rich soils. The
weakness and potential for failure can be increased by prolonged rain and freeze-thaw
cycles (Harty, 1992; Rahn, 1996).
Human induced landslides are also common. Two ways in which humans induce
landslides are ( 1 ) clear-cutting vegetation and (2) adding weight (i.e., building a
structure) on or above a potential or existing stable slide. Clear-cutting vegetation causes
a loss of roots, which lowers the shear strength of the soil or rock unit allowing it to
potentially fail (Rahn, 1996). Adding weight to the potential slide or a stable pre-existing
slide can accelerate or trigger movement of the slide because the angle of repose has been
changed. Humans also cause landslides by providing a triggering mechanism for slope
failure (Rahn, 1996). An example is cutting the toe of an existing landslide to build a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67 roadway or structure. Cutting the toe causes the slide to become unstable and begin to
move.
Landslides are categorized into three types based on the lithology in which the
landslide occurred; (1) bedrock, (2) soils, and (3) unconsolidated materials (Rahn, 1996).
This study focuses on the landslides that occur in bedrock. Bedrock landslides are further
categorized into four types: (1) rockfall, (2) rotational slump, (3) planar block slide, and
(4) rockslide (Rahn, 1996) (Fig. 24). Rockfalls generally occur along steep cliffs and
may occur in M 4.0 and greater earthquakes (Keefer, 1984; Harty, 1992). Along the
central Sevier fault, fourteen rockfalls and/or rotational slumps were documented (Plates
1 and 2). The rockfalls are located near the high cliffs of the Navajo Sandstone and the
Co-op Creek Member of the Carmel Formation. The rotational slumps are derived from
the weak shale and mudstone units (Tropic Shale, Dakota Formation, Carmel Formation)
near Glendale (Plates 2 and 3).
Regional Slope Failures
Landslides are common in southwestern Utah because the region experiences
relatively high precipitation and has high elevation, steep slopes, earthquakes, and
unstable geologic formations (Doelling et al., 1989; Harty, 1992). Many of these
landslides have damaged infrastructure, homes, and businesses. However, these slides
are relatively unstudied. Below are possible triggering mechanisms and classification of
the fourteen landslides along the central Sevier fault.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68 Triggering Mechanisms for Long Valley Landslides
Landslides are common within the vicinity of the Sevier fault (Schroder, 1971;
Doelling et al., 1989; Harty, 1992). Characteristic features of landslides include: (1)
steep topography, (2) arcuate to linear scarps, (3) benched or hummocky topography, (4)
bulging toes, (5) ponded or re-routed drainages, and (6) immature vegetation (compared
the vegetation around the landslide) on the landslide (McCalpin, 1996). All of the
fourteen landslides in the study area may have been natural slope failures in weak units
triggered by high rainfall and/or the freeze-thaw cycle. Three landslides, labeled A, B,
and C, were identified along the Glendale Bench Road (Plate 2 and Fig. 25) (Harty, 1992;
this study). One of these landslides, slide A, has either been triggered or reactivated by
humans (Fig. 25). The center of the landslide has been cut for road construction (Fig.
26A). Several failure precaution techniques have been applied to this landslide to prevent
future failure. The first technique is to cut back the slope (above the road) of the existing
landslide to a lower angle to prevent the landslide from sliding over the road (Fig. 26A).
The second technique is to reinforce the landslide topographically below the road to
prevent a new landslide from forming within the old slide (Fig. 26A). The other
landslides, slides B and C, along the Glendale Bench Road may or may not have been
human induced. No stabilization techniques have been performed on these slides.
In contrast, the landslide in the eastern portion of Dry Wash, called the Dry Wash
Landslide, may have been triggered by ground shaking from a historical earthquake
(Plate I, Figs. 25 and 26B). To state that a landslide is seismically induced the landslide
must be dated and associated with earthquake shaking (Keefer, 1994; McCalpin, 1996;
Bamhardt and Kayen, 1999; Papadopoulos and Plessa, 2000).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 To demonstrate whether an earthquake triggered a landslide is difficult because
only a few studies have been done on seismically induced landslides. Therefore, the
interpretations vary widely. Rahn (1996), McCalpin (1996), and Bamhardt and Kayen
(1999) summarize several basic criteria to determine whether a landslide has a seismic
origin. These techniques are used in this study. First, is the regional analysis of
landslides. Commonly, in a seismically active zone, a group of landslides will occur
simultaneously (Nikonov, 1988). However, single large landslides can also be triggered
by earthquakes (Nikonov, 1988). Second, the landslide morphology must be analyzed.
The hillslope from which the slide was derived must be at a steep angle with some
topographic relief. In regions where the topographic relief is due to streams downcutting,
the landslide is typically undercut by a stream. These landslides also may contain
characteristics such as liquefaction features. Seismically induced landslides typically
have a more blocky shape whereas slides that form because of intense rainfall typically
have a more spread out and less blocky shape (Perrin and Hancox, 1992; Bamhardt and
Kayen, 1999). Lastly, the historical records of reported earthquakes and landslides must
match, keeping in mind that earthquake-triggered landslides may have a three to five day
delay in movement (Rojstaczer and Wolf, 1992). This delay in movement may be caused
by increased groundwater flow locally or time-dependent manifestations of increased
pore water pressure due to ground shaking during an earthquake (Rojstaczer and Wolf,
1992). Papadopoulos and Plessa (2(XK)) and Keefer (1984) suggest that earthquake
triggered landslides usually occur in weak materials such as soils, weathered, sheared,
heavily fractured, jointed, or saturated rocks.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 Classification of the Dry Wash Landslide
By using the above stated techniques, the Dry Wash Landslide can be classified
and its triggering mechanism can be evaluated. I suggest that the Dry Wash Landslide is
a rotational slump (Fig. 24) and may have been triggered by the 1924 Orderville
earthquake and/or associated aftershocks.
Evidence to support the Dry Wash Landslide as a rotation slump include steep
topography, an arcuate to linear scarp, and hummocky topography (Fig. 26A). The Dry
Wash Landslide also contains blocks that were tilted or rotated during failure. Several of
the trees incorporated in the landslide are also rotated (Fig. 27A). A vegetation
difference is notable between the landslide and the surrounding topography. Little to no
new vegetation exists on the deposit and what new growth exists is sparse (Fig. 27A).
The new growth consists of -0.9 m (-3 ft) tall pine trees and immature manzanita, sage
brush, yucca, and tall grass (Fig. 27B). This vegetation is less mature than the same types
of vegetation on surfaces surrounding the landslide (Fig. 28A). However, the dead
vegetation within the deposit is similar in maturity to the live vegetation surrounding the
feature. Most of the vegetation incorporated within the landslide is highly decayed and
lacks foliage (Figs 28B and 29A). This vegetation is rotated and incorporated within the
landslide. However, some of incorporated vegetation survived the landslide and is
growing at an oblique angle to its original growth direction giving the trunks of the trees
a bent or hook shape (Fig. 27A). The original surface as well as vegetation before the
failure is preserved both above and below (in the scarp graben) the head scarp of the slide
(Figs. 24 and 29B). The toe is bulging and has ponded and rerouted drainages (Fig.
30A).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 71 The age of the Dry Wash Landslide was determined by using historical methods,
vegetation analysis, and geomorphic analysis. Historical methods include reports of
events from local inhabitants. Local inhabitants of the region state that the slide occurred
approximately 75 years ago. However, no written documentation of when the landslide
occurred is available. Geomorphic analyses suggest that the landslide occurred relatively
recently perhaps in the last century. The scarp of the Dry Wash Landslide is very steep
(nearly vertical) with little degradation (Fig. 26B). The slip surface is on a steep (nearly
vertical) slope with 37 - 46 m (120 - 150 ft) of relief. The surface of the landslide has a
blocky to angular and hummocky shape with relatively few (1-2) stream channels cutting
the landslide indicating little erosion has occurred on the slide. However, a stream
undercuts the toe (Fig. 30A). Vegetation analysis also suggests that the landslide
occurred recently, perhaps within the past century because the maturity of the vegetation
incorporated within and living within the landslide is -<100 years old. By the above
analyses, I suggest that the Dry Wash Landslide is young (<100 years old).
Even though the Dry Wash Landslide fits the above stated criteria to be
categorized as a seismically induced earthquake (cf., Rahn, 1996; McCalpin, 1996), I
suggest that the evidence is lacking to be certain whether the Dry Wash Landslide was
seismically induced. First, only an approximate date of when the landslide occurred was
documented. No written historical records were found. Therefore, it is difficult to say
whether the landslide occurred approximately at the same time as the 1924 Orderville
earthquake swarm. Secondly, the vegetation and geomorphic descriptions of the Dry
Wash Landslide also fit the descriptions of landslides that were not triggered by
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 earthquakes. Therefore, by using the above criteria and data the Dry Wash Landslide
may or may not be seismically induced.
Ethical Assessment of Hazards
Natural disasters such as earthquakes and landslides are unavoidable and can
threaten life and property. Investigations of landslides and potentially active faults
involve good science as well as ethical scientific practices (Cronin and Sverdrup, 1998).
Thus, great care should be taken when locating new sites for structures. Areas above,
below, as well as on the weak shale and mudstones should be avoided. Potential
earthquake and landslide hazards should be analyzed before new structures are built.
Land and building owners should be made aware of any potential earthquake and
associated hazards. A geoscientist must follow a code of ethics to ensure the reduction of
life and property loss during a natural disaster (Cronin and Sverdrup, 1998). Therefore,
with better earthquake building codes, understanding of landslides and their locations,
ethical scientific practice, and increased public awareness, the possible life and property
loss from such natural hazards can be minimized.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 9
TECTONIC INTERPRETATIONS
Intrcxluction
Additional data and analysis may help resolve some of the controversial topics
about the evolution of the western edge of the Colorado Plateau. Discussed below are the
northern termination of the Sevier fault, relationships of the Basin and Range style faults
in southwestern Utah, and the age of folding within and near the study area.
Northern Termination of the Sevier Fault
Controversy about where the Sevier fault terminates to the north is found
throughout the literature. Most workers agree that the Sevier fault ends near or within the
Marysvale Volcanic field (Fig. 7). Determining the northern termination is important to
determine the seismic potential of the Sevier fault as well as to determine if the Sevier
fault connects to the Wasatch fault zone to the north.
Previous workers (i.e., Eardley and Beutner, 1934) mapped the Sevier fault as a
single strand cutting the Marysvale Volcanic Field and place the northern termination
near Central, Utah (Fig. 7). However, the Utah state geological map shows the Sevier
fault ending in the middle of the Marysvale Volcanic Field. Later mapping (Rowley et
al., 1981) suggested that the Sevier fault has several splays and strands cutting the
Marysvale Volcanic Field up to 5 km (3.1 mi) north of Piute Reservoir and possibly
73
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 74 farther north. Hecker (1993) showed Quaternary faulting along trend of the Sevier fault
to Richfield. However, Hecker (1993) showed no Quaternary scarps between Panguitch
and the southern portion of the Piute Reservoir (Fig. 7). This suggests that either the
Sevier fault does not exist in this region or its last movement is pre-Quatemary. However
D. Simon (personal communication 2002) documented Holocene events near the Piute
Reservoir. Davis (1999) suggested that the Sevier fault lost its characteristics within the
Marysvale Volcanic Field. However, Rowley (personal communication 2002) observed
scarps indicating that the Sevier fault does cut the Marysvale Volcanic Field and
terminates near Richfield. These observations suggest that the Sevier fault does not
connect with the Wasatch fault, and terminates near Central or Richfield (Fig. 7).
Other Long Normal Fault Relationships
The Hurricane, Sevier, and Paunsaugunt faults have the same general structural
characteristics (i.e., down-to-the-west, steeply dipping, segmented, etc.). However, their
relationships to each other may provide an insight to the evolution of the western margin
of the Colorado Plateau.
First, the greatest amount of seismicity is along the Hurricane fault with the
largest recorded earthquake in the last decade (M 5.8) (Arabasz et al., 1992; Pechmann et
al., 1992). Some seismicity occurs near the Sevier fault, but less than along the
Hurricane fault (Davis, 1999). Two earthquakes have been recorded along the trace of
the Sevier fault in the last century (Doelling et al., 1989; Davis 1999). The Paunsaugunt
fault has had no recorded seismic events within the last 100 years (Doelling et al., 1989;
Engdahl and Rinehart, 1991; Davis, 1999).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 75 Second, offset units become younger to the west. For example, the Hurricane
fault offsets Holocene units and several Quaternary sedimentary deposits (Stewart et al.,
1997). Quaternary basalt is also offset (Stewart and Taylor, 1996; Stewart et al., 1997).
The Sevier fault offsets Quaternary basalts (Best et al., 1980) and Quaternary sediments
(Doelling et al., 1989; Hecker, 1993). The Sevier fault also tectonically tilts stream
terrace deposits near Orderville and Glendale (Fig. 1). However, along the northern
portion, near the Piute Reservoir, as many as four events have occurred in the Holocene
(Fig. 7) (D. Simon personal communication, 2002). Mapping by Bowers (1991) along
the Paunsaugunt faults shows only one location, south of Bryce Canyon National Park,
with fault scarps that cut Quaternary units. The two scarps are <1 meter (<~3 ft) high and
are located in Pliestocene to possibly Holocene pediment surfaces (Fig. 7).
Third, the Paunsaugunt fault has reversed topography (the hanging wall is
topographically higher than the footwall) (Doelling et al., 1989; Bowers, 1991). This
indicates either the units in the hanging wall are more susceptible to erosion than the
footwall units or the fault has not had recent movement. Along the Paunsaugunt fault,
erosion, little to no movement along the fault, or both options are probable. Using the
above previously published data and new data from this study, 1 agree with the previous
suggestion that the faults are more active or younger to the west (i.e., Doelling et al.,
1989; Davis 1999; Schiefelbein and Taylor, 2000).
Sevier, Laramide, or Cenozoic Regional Folding within the Study Area
Broad regional folds can be found from the vicinity of Zion National Park east
through the Colorado Plateau (Doelling et al., 1989; Davis, 1999). From structure
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76 contour maps Doelling et al. (1989) suggested that the Mt. Harris anticline lies in the
hanging wall of the Sevier fault south of Mt. Carmel Junction (Fig. 7). They also
suggested that the synclines lie in the hanging wall north of Black Mountain and in the
footwall east of Black Mountain (Figs. 7 and 1).
These folds may have been formed during the Sevier orogeny, Laramide orogeny,
or by Cenozoic normal faulting. Davis (1978) and Wannamaker et al. (2001) suggested a
Laramide age for these folds. However, Davis (1999) suggested that several folds
located near the normal fault traces formed by fault drag. Data from this study suggest
that these folds were formed during the Laramide orogeny, Cenozoic normal faulting,
and/or both.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 10
PETROLEUM POTENTIAL
Normal Fault Linkage and Hydrocarbon Production
The hanging walls and footwalls linked by relay ramps as well as structures
within the relay ramp can form migration pathways and/or traps for hydrocarbons
(Morley et al., 1990; Peacock and Sanderson, 1994; Dawers and Underhill, 2000; this
study). Relay ramps can be important for finding hydrocarbon traps because fault-
parallel folds within the ramps, in addition to the ramp monoclines may be traps (Morley
et al., 1990; Peacock and Sanderson, 1994). The bounding faults of and/or breakthrough
faults within relay ramps can ( 1 ) form barriers to hydrocarbon migration, (2) act as
hydrocarbon flow pathways, or (3) be permeable such that the hydrocarbons can pass
through the fault. Below, relay ramps will be discussed as potential traps and/or
migration pathways for hydrocarbons.
Relay ramps with different internal structures trap hydrocarbons in different
locations. Relay ramps that do not have associated fault parallel folds, but have
impermeable faults typically trap hydrocarbons on the up-dip side of the ramp (Fig. 31 A).
Relay ramps that contain fault parallel anticlines, such as those recognized in this study,
trap hydrocarbons within the structurally highest portion of the anticline (Fig. 3 IB).
Relay ramps with fault parallel synclines should trap hydrocarbons along the limbs of the
fold near the bounding faults, if the faults are impermeable (Fig. 31C). With
77
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78 breakthrough faults, hydrocarbons may accumulate within fault bounded blocks
depending on whether the faults are permeable (Fig. 3ID). These hydrocarbons can
either migrate from the source rock up along a non-permeable fault or through a
permeable fault and into the topographically highest part of the ramp below the cap rock
(Fig. 31 ). The amount of hydrocarbon accumulation depends upon the source rock.
In addition to relay ramps, physical fault linkage may be important for
hydrocarbon exploration. For example, if two faults with overlapping fault tips link at
one site and a fault-parallel anticline develops within the relay ramp, then four-way-
closure can form. The four closures are; (1) the linkage site, (2) the ramp monocline, and
(3 and 4) the two limbs of the anticline. However, if the faults link via overlapping fault
tips along breakthrough faults, then a relay ramp may or may not form. However, a
structure to trap the hydrocarbons can still form (Ferrill and Morris, 2001).
Hydrocarbon Production Near Study Area
Hydrocarbon producing fields located near the central Sevier fault include the
Virgin oil field. Upper Valley field, and Anderson Junction field (Fig. 32). The source
rocks for these fields include the Pennsylvanian Hermosa Formation, Permian Hermit and
Kaibab Formations as well as the Virgin Limestone and Timpoweap Members of the
Triassic Moenkopi Formation (Fig. 8) (Peterson, 1974; Hintze, 1988; Van Kooten, 1988;
Doelling et a!., 1989; Harris, 1994; S.J. Reber, personal communication 2001). The
structures that have been explored include folds associated with the Laramide and Sevier
orogenies and Cenozoic normal faults. For the Anderson Junction and Virgin oil fields
the Hurricane fault was considered to be a migration pathway (Peterson, 1974; Harris,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79 1994). In the Anderson Junction field, the oil was trapped either on an anticlinal closure
of the Kanarra anticline, a fault wedge of the Hurricane fault, or the Virgin anticlinorium
(Peterson, 1974; Harris, 1994). In the Upper Valley field, hydrocarbons were trapped
and pumped out of the Upper Valley anticline. The total number of barrels produced in
southwestern Utah between the first oil production in the state in 1907 (Virgin field) and
1988 is approximately 26 million barrels (26 MMBO) (Peterson, 1974; Van Kooten,
1988; Doelling et al., 1989). However, the total amount of oil produced in the entire state
of Utah is -900 million barrels (900 MMBO).
The High Plateaus subprovince near the Sevier fault may be a region for future
hydrocarbon exploration. Today, the number of oil producing fields are minimal in the
area. However, four exploration holes with shows of oil and/or gas have been drilled in
the footwall near Mt. Carmel, and one hole with no shows of oil or gas was drilled in the
footwall near Orderville (Fig. 32) (Doelling et al., 1989). Four relay ramps have been
identified along the Sevier fault (Davis, 1999; this study). These relay ramps and
associated faults fit the above models for hydrocarbon traps and/or migration pathways.
By understanding relay ramps and the types of linkage zones along the Sevier fault, these
structures may be used as analogs for potential traps and/or migration pathways in other
oil producing extensional regimes (e.g.. North Sea, Gulf of Suez, and the East African
Rift).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER II
CONCLUSIONS
Recognition of large convex geometric bends, or salients, along faults coincides
with concepts that long faults form by linkage of two or more shorter faults. Linkage
typically creates salients along the trace of the fault that geometrically segment the faults.
Segmented long faults can form by a number of different types of linkage However,
models of segment linkage generally do not address multipartite linkage zones. Such
studies of active linked or linking faults have several applied significances Fault linkage
zones are important to the petroleum industry to understand migration pathways and
where potential traps exist, and important to the public to understand the potential
earthquake and landslide hazards for the region. This study focuses on multipartite
linkage zones and the structures formed within these zones using the central Sevier fault
as a case study
Two main types of fault linkage and several subtypes are currently recognized;
overlapping tips, simple underlapping tips, fault capture, and breakthrough faults.
Within overlapping fault linkage zones, relay ramps typically form. Understanding the
relay ramps and the structures within is necessary to fully describe linkage zones
However, fault-parallel folds within relay ramps have not been discussed at length.
80
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81 The Sevier fault is a segmented long normal fault that probably initiated in the
Miocene (IS -12 Ma) (Davis, 1999). The Sevier fault is generally a N30°E striking,
steeply (>75°) west dipping, multi-strand normal fault.
Two subtypes of fault linkage were documented along the central Sevier fault.
The first is a fault capture situation in the Orderville relay ramp area (Fig. 12). The
second is a series of overl^ping faults with cross faults in the Stewart Canyon overly
zone (Fig. 12). This overlap zone contains three relay ramps (Glendale, Stewart Canyon,
and Elkheart Clififs) (Fig. 12). Relay ramps are a structure that allows strain transfer
between two faults (Fig. 5). The Stewart Canyon relay ramp is different from the simple
relay ramp model; it is bound by three faults.
Along the central Sevier fault, two structures were identified within the relay
ramps. In the Orderville relay ramp and the Glendale relay ramp, fault parallel folds are
exposed within the relay ramp. In addition, in the Glendale relay ramp cross or
breakthrough faults were identified. These faults cut the fault parallel fold. The
multipartite nature of the region between Orderville and Glendale has a series of linkage
sites that form a large salient across which the Mt. Carmel segment and Spencer Bench
segment linked.
The Sevier fault is possibly an active, at least active in the Quaternary, fault. It
offsets ~570 ka basalt and tilts yotmg (Pleistocene?) river deposits. Earthquakes have
also been felt and recorded near the Sevier fault. The area of interest also has landslide
hazards. Several landslides which maybe seismically induced have been identified in the
area.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 82 In conclusion, this study provided (1) new natural examples of multipartite
normal fault linkage zones, (2) an evaluation of the earthquake and landslide hazards
along the central Sevier fault, and (3) new data and interpretations for the regional
tectonics of southern Utah.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A
METHODS
The techniques of data collection and analyses include field mapping, cross-
section construction, stereographic analyses, and ^Ar/^^Ar isotopic dating. An overview
of each technique is presented below.
Field Mapping Techniques
Geologic mapping was the primary technique used for data collection.
Approximately 30 km^ (19 mi^) were mapped in Kane County, southwestern Utah during
the summer o f2000. Mapping was done using standard geologic techniques at a scale of
1:12,000. The topographic base was enlarged from the Orderville, Glendale, and Long
Valley 7.5’ U.S.G.S. quadrangles printed from the MapTech topographic map computer
program. Color aerial photographs were used to aid in locating geologic features,
structures, and formations. However, all mapping was based on direct field observation.
Cross Section Construction Techniques
Retrodeformable cross sections were constructed from structural and stratigraphie
data and relationships observed and mapped in the field. All cross sections were drawn
approximately perpendicular to the strike of the faults in order to analyze along strike
variations in splay zones and along single strands o f the fault. All fault attitudes were
83
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 84 calculated by three point problems and/or structure contours (Marshak and Mitra, 1988;
Rowland and Duebendorfer, 1994) because no faults in the study area yielded a
measurable surface. Apparent dips were calculated by using the alignment diagram for
solving iq)paient dip problems (Rowland and Duebendorfer, 1994). The cross sections
were constructed under the assumptions that plane strain occurred, the volume o f rock
remained constant, and the bedding thickness remained constant. The bed lengths
balance and no loss of area occurred. Bed lengths in both restored and deformed cross
sections can be and are within the standard uncertainty of 5-10% (Rowland and
Duebendorfer, 1994). Constant unit thickness is based on (1) the calculated thickness
from the map pattern of exposed units in the study area and (2) the published thickness of
subsurface units (Figs. 8 and 9) (Hintze, 1988; Stokes, 1988; Doelling et al., 1989).
The geometries o f the faults are based on the surface data and the ability to be
retrodeformed along each cross section. Standard cross section retrodeformation
techniques were used for the faults (e.g., Gibbs, 1983; Davison, 1986; Williams and
Vann, 1987; Vendeville, 1991; White and Yielding, 1991; White, 1992; Kerr and White,
1994; Song and Cawood, 2001).
Stereographic Analyses
Several stereographic analyses were done on structures within the study area.
Stereonets were created to show the attitudes of folds within the relay ramps. All
stereographic analyses were done using the computer program GeOrient.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 85 ^Ar^^Ar Methods
Two Quaternary basalt samples from the same flow were collected for ^Ar/^^Ar
isotopic dating as part of this study. BRC-I was collected from the hanging wall and
BRM-2 was collected from the footwall of the Sevier fault. Approximately 2.25 kg (5
lbs) of each sample was collected and trimmed of weathered surfaces in the field. Thin
sections were made and analyzed from each sample to ensure that the samples could be
isotopically dated. The samples were crushed and seived to uniform sizes. The seive
size chosen was the size that yielded the largest possible individual crystals that lacked an
adhering matrix. Olivine and volcanic glass were removed from the samples by mineral
separation methods including heavy liquids and hand picking. All mineral separations
were completed by the author at the UNLV Mineral Separation Lab.
Approximately 0.1 oz (250 mg) of basaltic groundmass firom each sample were
sent to the TRIGA Reactor at the University of Michigan for irradiation. Both samples
were then dated using the ^Ar/^^Ar dating techniques at the Nevada Isotope
Geochronology Lab under the direction of Dr. Terry Spell. Methods of ^Ar/^®Ar dating
are described in McDougall and Harrison (1988).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■o I 8 Q.
■D CD
BRC-1, Hangingwall baaatt J = 0.0009860 +/- 0.5% step 36Ar 37Ar 38Ar 39Ar 40Ar %40Ar‘ % 39Ar Had Ca/K 40Ar*/39ArK Age(ka) lad. 1 1.168 21.043 7.099 46.681 347.471 4.5 3.2 3.02204972 385.3463 581 45 2 0.373 21.399 4.397 65.996 127.538 21.7 4.5 2.17319728 474.9860 693 18 3 0.399 34.899 8 4.783 137.595 157.175 33.7 9.5 1.69969812 437.4950 647 14 ■D 4 0.463 50.267 4.95 228.484 201.768 40.6 15.7 1.47420945 408.8139 611 14 5 0.459 52.0 4.278 247.133 211.537 43.1 17.0 1.40992569 415.1381 619 14 6 0.452 44.595 3.282 199.718 192.101 37.5 13.7 1.49624894 400.9480 601 15 7 0.631 62.979 3.666 3 210.743 243.816 30.9 14.5 2.00282796 403.3032 604 18 CD 8 0.523 48.595 2.555 106.222 183.313 25.0 7.3 3.06702336 493.2572 715 21 9 0.648 59.985 2.684 86.579 206.163 16.7 6.0 4.64703833 450.4271 663 26 10 2.409 3. 561,781 6.719 107.924 612.525 7.7 7.4 35.2344524 533.9035 763 142 3" 11 0.962 53.545 0.949 12.408 274.831 CD 3.4 0.9 29.1574354 974.6847 1215 159 12 0.998 12.953 0.371 3.255 288.36 0.7 S 0.2 26.6692229 790.8199 1040 545 ■o Total gas age = O 644.26 15.01 CQ. steps 4-7 Plateau age = 609.62 15.01 g. BRM>2, Footwall Isasalt J = 0.0009920 +/-0.5% o step 36Ar 37Ar 38Ar 39Ar 40Ar %40Ar* 3 %39Arrlsd Ca/K 40Ar*/39ArK Aae(ka) lad. 1 4.661 ■o 35.768 9.069 73.269 1334.228 1.6 5.5 3.33034434 338.2579 522 123 o 2 1.152 44.009 8.491 102.421 363.437 11.5 7.6 2.93099522 488.6136 713 25 3 1.15 52.307 10.25 187.277 386.931 17.2 14.0 1.90460093 414.9916 622 18 CD 4 1.15 49.131 Q. 9.457 241.597 409.738 21.5 18.0 1.38651649 429.2611 640 19 5 1.133 41.9 6.166 203.626 388.245 17.4 15.2 1.40205111 379.1655 576 21 6 1.01 35.703 3.478 142.427 338.564 15.6 10.6 1.70928902 429.2611 640 29 7 1.223 47.142 3.528 143.132 404.517 14.7 10.7 2.24617964 498.6021 725 26 "D 8 1.038 38.102 2.812 CD 75.708 322.777 9.9 5.6 3.4334775 502.7836 730 52 9 1.437 59.372 3.47 75.865 426.994 5.9 5.7 5.34216699 381.4743 579 49 (/) 10 3.555 488.233 C/) 6.421 82.936 952.688 4.1 6.2 40.610412 597.7710 640 181 11 1.022 68.762 1.164 13.183 307.063 9.9 1.0 35.9321625 7036.4809 3747 283 Total gas age = 680.50 20.70 steps 3-6 Plateau age = 618.51 20.70 Appendix B. ^"Ar/^^Ar step heating data for basalt with olivinie and glass removed. Only nine steps are used in each isochron calculation (Fig. 11). * 87
Panguitch
Red Canyon H urricaneT Sevier Bryce Canyon Fault Fault National Park St. George Grand W ash Glendale Orderville Mt. Carrne YeSi, VRSure*» Paunsauaun G Jacket -pEoraS ARIZONA Sevier Orogenic Belt & Thrust Front /
^ Washin^tonj
/ \ ’■ / Approximate ISB Southern Boundary COLORADO PLATEAU
Kilometers
Figure I. This location map shows the Hurricane fault, the Sevier-Toroweap fault, the Paunsaugunt fault and their locations relative to the Basin and Range province. Transition Zone, and the Colorado Plateau. The ISB (Intermountain Seismic Belt) is located north of the labeled dashed line. The study area is indicated by the dark gray polygon.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7] ■DCD Q.O C g Q.
■D CD
C/) C/) A. Linkage of Underlapping B. Linkage of Overlapping C Fault Capture Along D. Linkage of Overlapping Type Fault Segments Type Fault Segments Connecting FauHs Subtype witti Breakttvougl) Faults Subtype
8
(O'
3.3" CD
CD T3 O Q. Ca o3 T3 O
CDQ.
Figure 2. Stages of fault propagation and linkage, with stages 1-3 in chronological order. Diagram A is
T3 the three general steps for linkage of underlapping type faults. Diagram B (overlapping type fault tips), C CD (fault capture subtype), and D (breakthrough fault linkage subtype) show the three types of linkage that (/) (/) occurs in overlapping fault tips. Boxes labeled I show faults growing by radial propagation. Boxes labeled 2 show the second stage of linkage. The faults and stress fields have interacted with each other but have not linked. Boxes labeled 3 show the final phase of linkage. The shaded areas and arrows represent the total fault displacement. The heavier lines represent the normal faults. This diagram is modified from Reber et al. (2001) and Taylor et al. (2001). % 7] ■DCD O Q. C 8 Q.
■D CD
C/) C /i
A. Single Propagating B. Stress Field Interaction c . Continued Stress Field D. Stress Field Interaction Fault with Underlapping Interaction witti Under- with Overlapping Fault ■D8 Faults lapping Faults and Relay Ramp Development
CD
3. 3" CD ■DCD i O Q. C a if 3O "O O
CD Q.
Figure 3. These diagrams show the local stress field location and interactions. The + signs represent the locations ■D where the faults may be located aficr the next rupture. The - signs are areas that are not likely to fail during the next CD rupture. The contour lines represent the numerically modeled shear stress with darker gray fill for higher values. The
C/Î C/) enclosed shapes represent the local stress field for each fault. Diagram A is a representation of radial propagation. B is underlapping faults. C is continued slip along the faults in diagram B (underlapping faults). D is overlapping faults. In diagram D, a relay ramp (strain transfer zone) has formed between the overlapping fault tips. Ball and bar are on the hanging wall of fault. Diagram A is modified from Cowie and Shipton (1998). Diagrams B - D are modified from Crider and Pollard (1998) and Crider (2001). * 90
Underlapping Faults LEQEND U = Up-thrown block Map View D = Down-dropped block — ' = Normal fault S /fe o f —» = Magnitude of offset Unlage, Figure 4. This diagram shows model displacement 100 200 Distance Along Strike vs. distance diagrams for two types (A and B) and two subtypes (C and D) o f B Overlapping Faults fault linkage. The distance Map View is measured along the strike of a piece of a fault across the linkage zone. The data density for diagram D was «■gio doubled within the linkage zone to show the "step" 100 200 pattern. The ball and bar Distance Along Strike are on the hanging wall. The shaded area and arrows Fault Capture represent the amount of Map View displacement along the fault. The axes are unitless because the models are Plateau independent o f scale. S l4 0 Diagrams A through C are a 1 20 modified from Taylor et al. (2001). 100 200 300 Distance Along Strike
Breakthrough Faults u Map View
^ -^30
100 200 300 400 Distance Along Strike
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Soft Linkage Map View
U»
100 200 Distance Along Strike
Figure 4 cont. This diagram (E) shows model displacement vs. distance diagrams for soft linkage in an overlapping fault linkage situation.
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Footwall
Relay Ramp
Hanging-wall
Upper Fault Fpotwall Top Breach
Relay Ramp
Hanging-wall Lower Fault
Footwall Relay Rami
Hanging-wall
Figure 5. The diagramed faults overlap in A, link by fault capture in B, and link by breakthrough faults in C. A relay ramp formed between the overlapping faults in each diagram. Diagram B is an example of a top breached ramp. Notice the changes in the strikes of the beds in the ramp. The different shades of gray represent different strata. The arrows represent the amount of displacement along the fault. The heavy lines represent the normal faults.
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Geometric Segment Boundary
Geometric Segment Boundary Structural Segment Boundary
Geometnc Segment Boundary
Propagation of earttiquake rupture Earthquake Segment Boundary
Figure 6. This figure shows three types of segment boundaries: (A) a geometric segment boundary, (B) a combined structural and geometric segment boundary, and (C) a combined earthquake and geometric segment boundary at which the earthquake rupture terminated. Earthquakes may also initiate at boundaries. The dark gray shaded regions and thin arrows represent the total amount of offset along the fault. The heavier weight lines represent normal faults, and the thicker arrows represent motion along the fault.
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Figure 7. This location Richfield _ . map shows the Sevier Central/^* fault, axial surfaces, and the approximate northern Monroe. ' ' termination. Axial surface data are from Doelling et al. (1989) and Bowers (1991). Fault Marysvaf^ trace data are from Hecker(1993). The northern termination of Reseivoir the Sevier fault was Kingston; • i determined using literature. The approximate location fo the Marysvale Volcanic Field is represented by the light gray shaded area. Panguitch Canyon National
Cedar City Cedar Breaks ^National Monument y ^ m r / ,i
Bryce Canyon Navajo Lake Anticline Legend Paunsaugunt Syncline Normal Fault Mt. Carmel Syncline Jet. \//TOrdefville 4 - - f - Anticline — Roadways • Towns ' Kanab Miles Hams Mt. 15 30 Anticline m m l^AH ARIZONA Kilometers 20 40
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Age Unit Thickness Rock (ft) Type Tennv Canvon Tongue Lamb Point Tongue 400 (aventa Fm 300 .L Mostly fluvial and flat coastal plain deposits with aeolian Moenave Fm sandstone and shallow marine limestone layers. Chinie Fm 700 — % Petrified wood occunrs in the Chinie Formation. Moenkopi Fm 1200 ri-fn
Kaibab Ls 370 Toroweao Fm 450 Hermit Fm 650 r .' T A Cedar Mesa (Queantoweap) Ss 620 Dominantly marine limestone, Hermosa Fm 590 shale, and sandstone. Some limestone layers contain chert. Redwall Ls 840
Ouray Ls 150 Temple Butte Ls _LUL "Supramuav" 630 Muav Ls 590 Bright Angel Fm 340 Tapeats Ss 330 pG Grand Canyon Series 820+ Crystalline basement rock.
Figure 8. This stratigraphie column represents units in the subsurface of the study area. Standard symbols are used for rock types; wavy bands are used for volcanic flows, black ovals are used for coal, triangles are used for gypsum, open angles with lines represent cross beds, filled circles are used for chert, asterisks represent Precambrian basement rocks. The triangles with the crosses outside the stratigraphie column represent hydrocarbon producing units. Data from Gregory (1951), Hintze (1973, 1988), Stokes (1988), and Doelling et al. (1989).
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Age Unit Thickness Rock (ft)Type Surficial deposits <100 Basalt flows <100
Fresh water unit. Consists a» of mostly fluvial and •I Claron Formation 800 ,9 lacustrine conglomerate, sandstone and limestone.
Kaiparowits Largely terrigenous rocks and Wahweap 1500 derived from the Sevier Formations Orogenic Belt. Deposited undivided along a coastal plane where coal-bearing non-marine ÜI sandstone and Straight Cliffs SS 240 conglomerate interfinger Tropic (Mancos) with marine shale of mid 760 Shale continental seaway. Dakota SS 320
Paria River Mbr 240 ICrystal Creek Mbr 170 n r r Co-op Creek Mbr 400 J-1 Represent two distinct paleoenvironments (1 ) Navajo sandy desert - Navajo Sandstone (main body) 2000 Sandstone and (2) shallow marine invasion - Carmel Formation.
Figure 9. This stratigraphie column represents mapped units. Standard symbols are used for rock types; wavy bands are used for volcanic flows, black ovals are used for coal, triangles are used for gypsum, and open angles with lines represent cross beds. Data from Gregory (1951), Cashion (1967), Hintze (1973), Rowley et al. (1975), Geesaman and Voorhees (1980), Hintze (1988), Marzolf (1988), Stokes (1988), Doelling et al. (1989), and Goldstrand (1994).
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112"37'45 12°32'10 BRC-1 Black Rock Canyon
-37°23'35
W
Dry Wash Virgin River Landslide
37®20’30
Dry Wash Canyon Glendale Bench Glendale Road
Legend
□ Quaternary
Orderville HI Tertiary o 0.5 1.0 Cretaceous Kilometers I Jurassic 0 1500 3000 Stewart Feet Canyon Figure 10. The diagram is a simplified geologic map of the study area. Geographic locations and sample collection sites are indicated. The heavy weight lines are faults. The Virgin River is shown by a lighter weight line. 37°15'50 I
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0.0036
BRC-1 BasaK (black) 0.0032 Age = 564 +/- 20 ka «Ar/36Ar = 288.1 +/- 3.9 0.0028 MSWD = 0.48 Steps 1-9 0.0024 BRM-2 Basalt (gray) Age = 580 +/- 50 ka ‘0Ar/36Ar = 291.7+/-3.85 < 0.0020 MSWD = 0.62 Steps 1-9 < 0.0016
0.0012
0.0008
0.0004
^^A r/^A r
Figure 11. The above diagram shows the isochron plots for the Black Rock Canyon basalt and Black Mountain basalt. Nine steps for each sample (shown by ellipses indicating uncertainty) were used to determine the isochron age. The Black Rock Canyon basalt is shown in black and the Black Mountain basalt is shown in gray.
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Fault Set F Black Mountain 37°23'30
fl) m Fault E 8
COI
Fault Set D
37°20'30"-
Glendale " Fault Group C
Stewart Canyon. Relay Ramp .g,
Elkheart Cliffs Relay Ramp N
Orderville Stewart Canyon A Fence Diagram Stewart Canyon w / Overlap Zone ^'Glendale Kilometers Relay Ramp 0 1500 3000 Orderville % Relay I Ramp Feet
‘Orderville Fence Diagram 37°15’50 112°37’45” 112°32'10 _L Figure 12A. This diagram represents the fault traces in the Sevier fault zone. The ball and bar symbols are on the hanging wall. The labeled cross sections, fault domains, sets, and strands are discussed in the text. The main strand is colored gray. The light gray dashed lines are where the displacement vs. distance diagrams were constructed.
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Glendale #
Spencer Bench Segment
Elkheart Cliffs Stewart Canyon / Relay Relay Ramp ' Ramp
‘ Glendale Relay Ramp Orderville Salient
Orderville Relay Ramp
0.5 Order/ille Kilometers 3000 Mt. Carmel Segment Feet Figure 12B. This diagram represents the fault traces in the Stewart Canyon and Orderville areas. The ball and bar symbols are on the hanging wall. The labeled cross sections, fault domains, sets, and strands are discussed in the text. The main strand is colored gray.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD "D O Q. C g Q.
■D CD (/) C/) L
CD ■D8
(O' 3 1 Wash 37°20’20"- ndslide
3"3. CD ■DCD O Quatemary/Tertiary Q. C □Alluvium (Qa) aO Landslide (Qls) 3 □Gong. (Qcg) ■D O □Basait (Qb) □Sinter (Q/Ts) Cretaceous CD Q. □Kaiparowits/Wahweap (Kkw □straight Cliffs (Ks) □Tropic Shale (Kt) Kilometers □Dakota SS (Kd) ■D Jurassic Carmel Fm CD 37“19’20 - □Windsor Mbr. (Jew) □Paria River Mbr. (Jcp) (/) □Crystal Creek Mbr. (Jcc) □Co-og^reek^brJJco)
112°38’0" 112°36’0" Figure 25. The geologic map of the central portion of the study area shows both the Dry Wash Landslide and the landslides along the Glendale Bench Road. Notice that the landslides occur in the Cretaceous Tropic (Kt) and Dakota (Kd) Formations. 101
Figure 13. The fence diagram shows the fault parallel ^ clin e within OnJervMe the Orderville rday Relay Ramp , ramp. Notice that the strata outside the relay ramp are nearly flat lying. Also, notice to the south, the area along N-hT between the faults at the surface and at depth decreases as the m space decreases. The / heavy weight lines represent normal faults. \ The white represents bedding planes within / the syncline with light weight lines representing structure / contours. The stratigraphy and stratigraphie symbols are described on Plate 3. Cross sections on Plate 4 were used to constrain the shape o f the syncline. The unlabeled vertical lines are tie lines. However, several cross sections were removed to show the syncline shape.
No Vertical Exaggeration
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 102 A. Orderville Relay Ramp
22 R s.
B Glendale Relay Ramp N
3®. N59“E
21 R s.
Figure 14. These beta plots show the attitudes o f bedding within two relay ramps and fold axes orientations. A shows beds and the synclinal axis orientation within the Orderville relay ramp B shows the attitude o f beds and anticlinal axis within the Glendale relry ramp.
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112°37*46
37°23’36
Black Mountain
virgin River Qcg 37®20‘30 0 1.0 Dry Wash Canyon Kilometers
Glendale 0 1500 3000 Feet
□ Alluvium (Qa) ■ Younger Conglomerate (Qcg) Older Conglomerate (Qcg1) □ Other Quaternary Units Orderville Basalt (Qb) □ Bedrock (brx)
Figure 15. This diagram is a Quaternary geologic map of the central Sevier fault. The faults are in the heavy-weight solid, dashed, and dotted lines indicating a certain location, an approximate location, and a buried fault respectively. The Virgin River is shown with a heavy gray solid line. 37=15'50"
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Figure 16. The fence Elkheart CMS Relay Ramp diagram shows the > V* fault parallel anticline within the Glendale relay ramp. Notice that the strata outside the relay ramp is Glendale Relay Ramp nearly flat lying. However, the strata within the ramp form a fault paialld anticline. Stewart Canyon . The faults appear to Relay Ramp ^ connect at depth hence there is a decrease in space within the relay ramp. Notice within the ramp there are / cross fuilts which cut through the relay / ramp The heavy weight lines represent normal faults. The white represents the bedding planes showing the anticline with the light lines representing structure contours. The stratigraphy and \ stratigraphie ^m bols are described on Plate 3. Cross sections on Plate 4 were used to contrain the shape o f the anticline. The unamed vertical lines represent tie lines. Several cross sections No Vertical Exaggeration were removed to show the anticline shape.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■o Û.o c g û.
■o CD
(£(/) 3d
8 -Ov< ë '
3 CD
Cp. 3" CD ■OCD O Q. C Oa 3 ■O O
CD Q.
■D CD
C/) C/) Figure 17. Photo A shows the younger conglomerate (Qcg). Notice the variation in size and composition o f the clasts. In the lower left comer is a sandbar. Photo taken facing north. Rockhammer for scale. The outcrop faces south. Photo B is the older conglomerate (Qcgl). This unit is overall finer grained. Notice the large basalt clasts. Person (Robert Kerscher) for scale is 1.75 meters (S'9”) tall. 106
Figure 18. Photo A is a roadcut along U.S. 89 o f the Cretaceous Dakota Sandstone unconformably overiain by the younger conglomerate. The black unit in the left o f the photo is one o f the coal seams in die Dakota Sandstone. The photo was taken facing west. Photo B is a sandbar within the young conglomerate. This photo was taken facing north.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7} ■DCD O Q. C g Q.
■D CD
C/) C/î
800
400 ■D8
CD
600 3. 3" CD 400 ■DCD 200 O Q. C 800 a 3o 600 T3 400 O 200
(D Q. Stewart Canyon Orderville c Relay Ramp 1200 Northern Portion of Data Set Overlap Zone 800
T3 (D 400
(/) C/) Distance (km) Figure 19. This diagram shows the displacement vs. distance profiles (A - D) for each major fault. The vertical scale differs among the plots. A is the easternmost fault, is the westernmost fault, and (E) the central Sevier fault as a whole. Displacement was plotted along even increments of distance for each fault. Distance was measured along the fault. 108
Displacement vs. Distance Profiles A 800, Orderville Relay Ramp r 600
5 200 Orderville
0.5 1 1.5 2.5 Distance Along Strike (km)
B N 8 500 Glendale Relay Ramp 1 400 I 300 8 Cross Fault S 200 S' Q 100
0.51 1.5 2 2.5 Distance Along Strike (km)
. Stewart Canyon Relay Ramp ^ 600
1 1.5 2 Distance Along Strike (km) D 400
~ 300 Elkheart Cliffs Relay Ramp 8 200
5 100
0.1 0.2 0.3 0.4 0.5 Distance Along Strike (km) Figure 20. The diagram shows the displacment vs. distance profiles along potential linkage zones in the Sevier fault. A is near the Orderville relay ramp. B is the Glendale relay ramp. C shows the Stewart Canyon relay ramp area. D is the Elkheart Cliffs relay ramp. The relay ramps are located in the center of the diagrams. The displacement is the total stratigraphie offset of the bounding faults.
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Relay Ramp Fault B Fault A
Legend Linkage Site Shale
Sandstone Relay Fault A Ramp Fault B Limestone
Fault
Linkage Site
Figure 21. These diagrams show the evolution of a fault bounded panel of rocks with the formation of a fault-parallel syncline within the relay ramp. The faults are the heavy weighted lines and the relay ramp is labeled.
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Glendale* Glendale Spencer Bench, f Segment ~-/U' Kilometers 0 IKK) 3000 K -eet
Stewart Orderville. Canyon Overlap Zone
jDrdervllle z / ~ ~ Mt. Carmel Segment
Glendale Glendale
w:
Orderville. Stewart Orderville. Stewart Canyon Canyon Overlap Overlap Zone Zone
E Glendale < Figure 22. This diagram shows the stages Spencer Bench Segment - of linkage for the central Sevier fault. A is Elkheart the oldest and E is the modem fault trace map. Boxes B - D could be in any order. Stewart Canyon' There are no timing constraints of which Relay Ramp ; linkage happened first, second, or third. The black lines represent faults. The gray lines in the last diagram represent the main Orderville , • V Glendale strand. The fold symbols show the V; Relay Ramp approximate timing of folding within the relay ramps. Orderville /Orderville Relay Ramp / Salient />Mt. Carmel Segment
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■D CD
(/)
Stewart Canyon C. Stewart Canyon, ' Breached j Breached Beginning Stages Relay Ramp / Relay Ramp 8 of Linkage ■D !^ase U' Breach (O'
i X Base Breach Legend ' Glendale 3"3. Relay Ramp CD 2000 Feet Glendale Breached CD W Relay Ramp ■D W 1000 -Top O Breach CQ. Meters a 3O ■D Figure 23. The fault trace map of the O Stewart Canyon, E. Stewart Canyon / Breached Breached J / Stewart Canyon overlap zone shows the Relay Ramp. /Relay Ramp/ '' CD stages of development. Diagrams A - E Q. U’ are in chronological order with A representing the youngest time and E representing the oldest time. The gray ■D CD lines represent the faults with relatively //r unconstrained timing and the black lines (/) VEIkheart indicate faults with moderate to well \ Cliffs /f , v^^Elkheart // Z' /A / Cliffs constrained timing y / // y Relay Ramp Glendale Breached Relay Glendale Breached Relay Ramp With Cross Faults Y Ramp With Cross Faults 112
Rockfall Joint
(0 75
Rotational Slump Planar - Block Slide %
Scarp Graben o Q) Rupture S Surface JO
Extremely Slow to Moderate Moderate Scarp Face Rocksllde % Dip Slope i i Q Very Slow to Extremely Rapid
Figure 24. This diagram shows the different types of landslides. The landslides found in the study are are rockfall and rotational slump types. From Vames (1958) and Rahn (1996).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7) "OCD O Q. c 8 Q.
■D CD
C/) C/)
■D8 SfVyash 37°20'20" ndslide
3. 3" CD ■DCD O Û. Quaternary/Tertiary C EX Alluvium (Qa) Oa Landslide (Qls) 3 □Cong. (Qcg) ■o QBasalt (Qb) o □Sinter (Q/Ts) Cretaceous CD Q. □KaiparowitsA/Vahweap (Kkw □straight Cliffs (Ks) □Tropic Shale (Kt) Kilometers □Dakota SS (Kd) ■D 37°19'20- Jurassic Carmel Fm CD □Windsor Mbr. (Jew) (J) □Paria River Mbr. (Jcp) C/Î O □Crystal Creek Mbr. (Jcc) 3 □C o-op Creek Mbr. (Jco) 1 T 112°38'0" 112°36'0" Figure 25. The geologic map of the central portion of the study area shows both the Dry Wash Landslide and the landslides along the Glendale Bench Road. Notice that the landslides occur in the Cretaceous Tropic (Kt) and Dakota (Kd) Formations. u> 114
Figure 26. Photo A shows the Glendale Bench Roadlandslide. Notice remediation techniques used to stabilize the landslide. This photo is facing northeast. Photo B is a view o f the Dry Wash Landslide. This landslide has a steep and arcuate shape scarp. The photo is facing northeast.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 115
Figure 27. A = Young pine tree incorporated into the Dry Wash Landslide. The pine tree survived the landslide and is now growing at an oblique angle to its original growth path. Most o f the landslide has little to no new growth Photo is facing north-northeast B = New growth on the Dry Wash Landslide. The dark green bushes are manzanita. The steep slope in the background is the head scarp o f the landslide. The gray to black layers near the skyline are coal seams. The photo is fiidng southeast. The person (Robert Kerscher) is 1.75 m (5'9") tall.
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Figure 28. A = Sparse and immature vegetation on the landslide (in the foreground). Along the skyline the vegetation is denser and more mature. Photo A is facing southeast. B = The dead vegetation incorporated witftin the landslide has no foliage and is highly decayed Photo B is facing south-southeast. The person (Robert Kerscher) is S'9" (1.75 m) tall
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 117
a
Figure 29. A = Wood from a tree incorporated into the landslide. Rock hammer for scale. B = The scarp and scarp graben o f the Dry Wash Landslide. The scarp graben is the topographically lower area with the large trees. These trees were once at the same elevation as the trees on the skyline. The photo faces southeast.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 118
Figure 30. The Dry Wash Landslide is rerouting streams. Person (Robert Kerscher) is standing in one o f the rerouted streams Robert is 1 75 m (S'9") tall The steep slope behind him is the toe of the landslide.
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Figure 31. These cross sectional models show hydrocarbon accumulation within relay ramps in extensional linkage zones. A=classic relay ramp. B=fault parallel anticline within the ramp. C=fault parallel syncline within the ramp. D=breakthrough faults within the ramp. The large filled shapes are potential hydrocarbon accumulation B positions within the relay ramp.
Legend \ I Shale or cap rock
Sandstone or reservoir rock
Limestone or source rock
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114“W 113° 112° 38°N Iron Co. Garfield Co. Hurricane Fault'*i Sevier Fault
Upper Valley Field ^ Wastiington Co.i Kane County Carmel Anderson Jet. 'Orderville Field Virgin Oil Field St. George •Kanab UTAH ARIZONA
100
Kilometers
Figure 32. This map of southwestern Utah shows the oil fields in the region and the exploration holes along the Sevier fault. The solid circles represent communities. The open circles represent dry exploration holes. The half filled circles represent exploration holes that have shows of oil and/or gas. The gray filled shapes represent the oil fields in the region. Data are from Doelling et al. (1989).
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REFERENCES
Arabasz, W.J., Pachmann, J.C., and Nava, S.J., 1992, The St. George (Washington County), Utah, earthquake of September 2, 1992: University of Utah Seismograph Stations, Preliminary Earthquake Report, 6 p.
Arabasz, W.J. and Julander, D R., 1986, Geometry of seismically active faults and crustal deformation within the Basin and Range-Colorado Plateau transition in Utah: Geological Society of America Special Paper 208, p. 43-74.
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Young, M.J., Gawthorpe, R.L., and Hardy, S., 2001, Growth and linkage of a segmented normal fault zone; the Late Jurassic Murchison - Statfjord North Fault, northern North Sea: Journal of Structural Geology, v. 23, p. 1,933-1,952.
Zampieri, D., 2000, Segmentation and linkage of the Lessini Mountains normal faults. Southern Alps, Italy: Tectonophysics, v. 319, p. 19-31.
Zhang, P., Slemmons, D.B., and Mao, F., 1991, Geometric pattern, rupture termination, and fault segmentation of the Dixie Valley-Pleasant Valley active normal fault system, Nevada, U.S.A.: Journal of Structural Geology, v. 13, p. 165-176.
Zhang, P., Mao, F., and Slemmons, D.B., 1999, Rupture terminations and size of segment boundaries from historical earthquake ruptures in the Basin and Range Province: Tectonophysics, v. 308, p. 37-52.
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Graduate College University of Nevada, Las Vegas
Hsa M. Schiefelbein
Address: UNLV Department of Geoscience 4505 Maryland Pkwy Box 454010 Las Vegas, Nevada 89154-4010
Degrees: Bachelor of Science, Geology, 1998 University of Wisconsin - Milwaukee
Special Honors and Awards: AAPG Research Grant, GSA Research Grant, Sigma Xi Research Grant, Arizona/Nevada Academy of Science Research Grant, UNLV Graduate Student Association Research Grant, UWM Travel Grant to the GSA Cordilleran Section Meeting, Two GSA Cordilleran Section Student Travel Grants, UWM Athletic Scholarship, Indiana University Field Camp Scholarship, UWM Field Camp Scholarship, ARCO Undergraduate Research and Travel Grant, Elk’s Club Outstanding Student Scholarship, Jefferson School Parent/Teacher Outstanding Student Scholarship, UNLV Geoscience Dept. Scholarship, UNLV Graduate College Summer Session Scholarship, Two GSA Engineering Geology Division Shlemon Awards
Publications: Schiefelbein, I.M., Buck, B.J., Lato, L., and Merkler, D., 2002 in press. Salt micromorphology in arid soils of southern Death Valley, California, USA: Proceedings in the 17"’ World Congressional Soil Conference, Bangkok, Thailand. Schiefelbein, I.M., 2002, Fault segmentation, linkage, and earthquake hazards along the Sevier fault, southwestern Utah: M.S. Thesis, University of Nevada Las Vegas. Reber. S., Taylor, W.J., Stewart, M E., Schiefelbein, I.M., 2001, Linkage and reactivation along the northern Hurricane and Sevier faults, southwestern Utah: The Geologic Transition, High Plateaus to Great Basin - A Symposium and Field Guide, Utah Geological Association Publication 30 and Pacific Section AAPG Guidebook GB 78, p. 379-400. Schiefelbein, I.M. and Taylor, W.J., 2001, Development of fault-parallel folds in relay ramps within overlapping normal fault linkage zones, southwestern Utah: AAPG and Society of Exploration Geophysists Student Expo Abstracts.
133
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Schiefelbein, I.M. and Taylor, WJ., 2001, Fault geometry and linkage along normal faults using the Sevier fault, southwestern Utah, as a case study: GSA Abstracts with Programs, v. 33, p. A25. Schiefelbein, I.M. and Taylor, WJ., 2001, Fault segmentation, linkage, and earthquake hazards along the Sevier fault, SW Utah: GSA Abstracts with programs, v, 33, p. A-38. Taylor, WJ., Criscione, JJ., Gilbert, J.J., Justet, L., Kula, J.L., Schiefelbein, I.M., Sheely, J.C., and Stickney, E.K., 2001, Geometry and neotectonics of the northern California Wash fault, southern Nevada: GSA Abstracts with programs, v. 33, p. A-58. Schiefelbein, I.M., 2000, Fault segmentation, linkage, and earthquake hazards along the Sevier fault, southwestern Utah: AAPG Bulletin, v. 84, p. 1873. Bailley, T.L., Gilbert, J.J., Howley, R.A., Rees, M.N., Schiefelbein, i.M., Taylor, W.J., and Van Hoesen, J.G., 2001, Geologic guide to Red Rock Canyon: Integrating research and education: Arizona-Nevada Academy of Science Abstracts with Programs, v. 45, p. 19. Schiefelbein, I.M. and Taylor, W.J., 2000, Fault development in the Utah Transition Zone and High Plateaus Subprovince: GSA Abstracts with Programs, v. 32, p. 194. Bass, C., Schiefelbein, I.M., Cronin, V S., and Dill, R.F., 1999, Near-shore seismic survey along eastern Malibu coastline, southern California: GSA Abstracts with Programs, v. 31, p. 76. Schiefelbein, I.M., Walsh, K., Boldt, J., Riedel, J., Cronin, V S., and Dill, R.F., 1998, Looking for unmapped strands of the Malibu Coast Fault, southern California; GSA Abstracts with Programs, v. 30, p. 46.
Thesis Title: Fault Segmentation, Linkage, and Earthquake Hazards Along the Sevier Fault, Southwestern Utah
Thesis Examination Committee: Chairperson, Dr. Wanda J. Taylor, Ph.D. Committee Member, Dr. Eugene I. Smith, Ph.D. Committee Member, Dr. Terry Spell, Ph.D. Graduate Faculty Representative, Dr. Barbara Luke, Ph.D.
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Surficial Deposits
Quaternary Landfill Q lf Kane County Landfill. Actively used.
Quaternary Alluvium Qa Weakly indurated to nonindurated deposits in active washes and floodpiains. Deposit types v from sandstone (yellow, pinkish orange, white), limestone (yellowish gray to gray, pink), and sii (brown) to sand, gravel, and developed soils. Clast (within the deposit) sizes range from silt to boulder and shapes vary from rounded to angular. Much of this unit is occupied by humans o actively cultivated. The contacts are sharp. Thickness of beds or unit is unknown.
Quaternary Alluvium Second Qas Weakly indurated to nonindurated deposits in active washes and floodpiains. Deposit is a developed soil with active washes. The parent material is basalt. Clast (within the deposit) size range from silt to pebble and shape is well-rounded. The pebbles are derived from the basal Tertiary Claron Formations. Much of this unit is occupied by humans or actively cultivated. Th< contacts are sharp. Thickness of beds or unit is unknown.
Quaternary Landallde Qls Includes all slope failures. Size ranges from 228.6 - 243.8 m (750 - 800 ft) In diameter. Associate with steep slopes or weakly indurated units, internal blocks range from silt size to greater than m (3 ft). Clasts are generally rotated, and trees and bushes are uprooted creating hummocky topography. Classified as a rotational slump.
Quaternary Colluvium 1 Qcl Quartz sand. Color is pink to white. Grain size is medium to fine. Unconsolidated. Eroded fronr adjacent Navajo Sandstone cliffs.
Quaternary Colluvium 2 Qc2 Unconsolidated clasts of platy limestone. Color Is gray to pale yellow on both the fresh and w eathered surfare«. Clasts are 5 - 10 rm 10 - A ini ufiHe and —9 rm r~n 7S inl thlrlr AnmiUr Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. escriptîons of Map Units
Cretaceous KaiparowitsyWahvreap Formations Undivided Kkw The unit consists of alternating sandstone, mudstone, siitstone, and conglomerate color is pale yellow to white on both the fresh and w< cemented. Contains quartz, mafic crystals, chert, feldspar, and clay. < • 12.7 cm (2 - 5 in) thick. Also contains ripple marks. The siitstone to color on both fresh and weathered surfaces. Thickness varies from ( alternating with the sandstone and siitstone to mudstone are congl conglomerate layers are 0.2 - 0.9 m (05 - 3 ft) thick. Layers are disco )sit types vary lenses. Generally clast supported. Clasts are very well-rounded and nk),and sllt diameter. Clasts are dominantly chert, but also limestone and doios rom sllt to Dominantly a cliff forming unit. However, some of the mudstones a humans or Total thickness = 457 m (1500 ft).
Cretaceous Straight Ciifte Formation Ksc The upper rock type in the unit Is a lithic sandstone. Colors are pale bsit is a drab yellow on the weathered surface. Coarse to medium grained ieposit) sizes well as shrimp burrows, cephlapods, pelecypods,oysters, and shark the basal in) iron concretions. Upper contact is sharp. Forms steep cliffs. Roc Itlvated. The lower unit are siitstone interbedded with coal. The siitstone is drab weathered surfaces with dark gray to black coal beds. The coal bee -305 cm (1 ft) thick. The lower contact is gradational. Poorly lithifi Total thickness = 73.2 m (240 ft).
>r. Associated Cretaceous Tropic Shale Kt Mudstone, siitstone, and sandstone. The top unit is sandstone. Coli jreater than 0.9 weathered surfaces. Grain size is medium to fine. The lower part o ummocky mudstone and siitstone. Olive gray to drab green on both fresh an coarsens upward. The upper and iower contacts are gradational, poorly exposed 05 m (15 ft) thick bed of septrian nodules in a yel Nodules range from 10.2 - 30.5 cm (4 -12 in) in diameter and the o Some of the nodules are fractured. The interior contains yeliow ca Eroded from size. The formation is poorly lithified and weathers to form slopes m (760 ft).
Cretaceous Dakota Sandstone Kd Limestone, mudstone, and shale. The upper part of the unit is lime both fresh and weathered surfaces. Abundant oysters. The upper resh and bed Is sharp. Generally forms float covered surfaces. The lower pa \nauiarto fin é t Alt kAth fm ch anH uüaathArarl ci iifarat f*nntainc tum di« Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Plate 3
ndivided Itstone.and conglomerate. Sandstone and Fresh and weathered surfaces. Calcite ir,and clay. Cross beds are low angle and 5.1 siitstone to mudstone is pink to purple in raries from 0.9 - 3.7 m (3 -12 ft) thick. Also ie are conglomerate layers and lenses. The fs are discontinuous and commonly form )unded and 13 - 25 cm (05 -1 in) in e and dolostone. Contacts are sharp, ludstones and siltstones weather to a slope.
ors are pale yellow on the fresh surface and m grained. Contains low angle cross-beds as ,and shark teeth. Contains 2.5 - 5.1 cm (1 - 2 3 cliffs. Rock types of the poorly exposed 3ne is drab gray on both fresh and coal beds are splintery, thinly bedded, and Dorly lithified and weathers to form a slope.
stone. Color is yellowish gray both fresh and /ver part of the unit is alternating layers of h fresh and weathered surfaces. Grain size iational. 183 m (60 ft) from the base is a es in a yellowish gray marine mud matrix, and the outer parts are light gray limestone, yellow calcite crystals up to 1.3 cm (05 in) in m slopes and valleys. Total thickness = 231.7
nit is limestone. Yellowish green to gray on he upper and lower contacts of the oyster lower part is shale to mudstone. Color is 1 C tu r n r llc m n tim irki ic m a l h a H c T k a ^ n a l Ic [ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Legend
Lithologie contact. Dashed where approxim; where concealed.
1 _ Fault contact. Dashed where approxiamtely I concealed. ? where the fault existance is une the hangingwall.
7,0 Strike and dip of bedding.
A. 20 G" 35 A = Anticline showing trace of axial surface a Syncline showing trace of axial surface and p
Age Correlation of Map Units
Surficial Deposits Qlf Qa Qls Qcl Qc2 Qas H olocene 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Legend
Lithologie contact. Dashed where approximately located. Dotted where concealed.
Fault contact. Dashed where approxiamtely located. Dotted where concealed. ? where the fault existance is uncertain. Ball and bar are on the hangingwall.
70 Strike and dip of bedding.
B. 35 A = Anticline showing trace of axial surface and plunge of axis. B = Syncline showing trace of axiai surface and plunge of axis.
Age Correlation of Map Units
)eposlts Qcl Qc2 Qas H olocene 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Quaternary Colluvium 2 Qc2 Unconsolidated clasts of platy limestone. Color Is gray to pale yellow on both the fresh and weathered surfaces. Clasts are 5 -10 cm (2 - 4 in) wide and ~2 cm (-0.75 in) thick. Angular t< subangular. Transport direction is downsiope movement. Source is the nearby Co-op Creek Member of the Carmel Formation.
Long Valley Quaternary Spring Deposit Qs Spring deposit, tan to brown on both fresh and weathered surfaces. Fine grained. Cemented with silica and locally small amounts of cartxmate. Millimeter to centimeter nearly planar beds of silica possibly opaline. Weathers to a resistant high. Associatec springs. Based on field observations is classified as thin-bedded opacline sinter facie; (Jarvis et al., 1998; Campbell et al., 2(X)1 ; Braunstein and Lowe, 2001 ). Total thicknes 12.2 m (0 -4 0 ft).
Young Quaternary Conglomerate Qcg Clast supported conglomerate. Framework Is composed of poorly sorted consolidated conglomerate and sandstone. Clast lithologles Include limestone (yellow, gray, pink), sandstc (yellow to gray), basalt (dark gray to brown), and petrified wood (light to dark brown and bla Clasts are rounded to angular. Size of clasts ranges from sand to greater than 15 m (5 ft). Co 305 cm (1 ft) thick layers of medium to coarse-grained llthic sandstone. Color Is tan on weat surfaces and light tan to yellow on fresh surfaces. Sandstone locally contains low-angle cros Upper and lower contacts are sharp. Weathers to rubbly slope or cliff former. Total thickness 9 m (3-30 ft).
Old Quaternary Conglomerate Qcgl Clast supported conglomerate. Framework Is composed of poorly sorted consolidated sand Clast lithologles Include limestone (yellow,gray, pink), sandstone (yellow to gray),and basalt gray to brown). Clasts are rounded to angular. Size of clasts ranges from sand to -0.5 m (-1. Contains 305 cm (1 ft) thick layers of medium to coarse-grained llthic sandstone. Color Is tar weathered surfaces and light tan to yellow on fresh surfaces. Locally contains low-angle cro< Upper and lower contacts are sharp. Weathers to rubbly slope or cliff former. Outcrops topographically higher than Qcg. Total thickness Is -12 m (-40 ft).
Bedrock
Quaternary Basalt Qb Basalt, brown to black on weathered surfaces and dark gray on fresh surfaces. Fine crystallln Phenocrysts of olivine -5 mm (0.125 In), plagloclase -2.5 mm (0.0625 In), and pyroxene. VesI bottom 0.9 -1 3 m (3 - 6 ft), dense middle 4.6-13.7 m (15-45 ft), and vesicular top 0.9 -1.8 nr 6 ft). Weathers to a resistant cap unconformably overlying Quaternary,Tertiary, and Cretacec rocks. Unit thickness Is 6.1 -183 m (20 - 60 ft).
Tertiary Claron Formation Tc Limestone, conalomerate, and sandstone to siitstone. Subdivided Into two members - uooe Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cretaceous Dakota Sandetome Kd Limestone, mudstone, and shale. The upper part of the unit Is llm both fresh and weathered surfaces. Abundant oysters. The uppe fresh and bed Is sharp. Generally forms float covered surfaces. The lower p Angular to olive gray on both fresh and weathered surfaces. Contains two d -op Creek dark gray to black on both fresh and weathered surfaces, splinter contains selenite gypsum. Gypsum crystals occur In 25 - 5.1 cm I size from 25 - 5.1 cm (1 - 2 In) In length. The unit Is generally poc forms valleys. The lower contact Is sharp and the upper contact I: lined. m (320 ft). sntimeter thick ssociated with Iter facies Jurassic Carmsi Formation - Winsor Member Jew I thickness 0 - Conglomerate, silty fine-grained sandstone, and siitstone. The up Pale yellow on both fresh and weathered surfaces. Generally mal sand. Clasts are well-rounded, gravel size, and composed of chert fine-grained, yellow to white sandstone with well-rounded grain; of alternating beds of silty fine-grained sandstone and siitstone. brown. Beds are -0.9 m (-3 ft) thick. Upper and lower unit cont* ated weathers to a slope or forms valleys. Total thickness = 853 m (28 k), sandstone m and black). 1 (5 ft). Contains Jurassic Carmel Formation • Paria River Member n on weathered Limestone, gypsum, shale, and siitstone. The upper unit Is fine-gr angle cross beds, surface and very light gray fresh surface. Abundant pelecypods. thickness Is 0.9 - poorly exposed. Lower unit has three alabaster gypsum layers se siitstone. The gypsum Is dirty white on the weathered surface an fresh surface. Has a sugary texture. Weathers to blocky ledges. E shale and siitstone are greenish gray and reddish brown on both vary In thickness from 2.5 - 7.6 cm (1 - 3 In). Contacts between thi ated sandstone, sharp. Upper and lower contacts are gradational. Weathers to sk md basalt (dark thickness = 73.2 m (240 ft). 0.5 m (-15 ft), lolor Is tan on angle cross beds, Jurassic Carmsi Formation - Crystai Creek Member reps Jcc Fine-grained siitstone, sandy shale to sandstone, and limestone. 1 siitstone with sandy shale to sandstone Interbeds. Colors range fi gray both fresh and weathered surfaces. Weakly Indurated. Abur stringers. Forms valleys and slopes. Upper and lower contacts an thick-bedded, fine-grained limestone. Colors are dark yellow on v on fresh surfaces. Cliff to ledge former. Total thickness = 51.8 m (
! crystalline, Jurassic Carmsi Formation - Co-op Creek Member xene. Vesicular Jco Sandy siitstone to siitstone and limestone. The upper unit Is lime: 0.9-1.8 m (3- and form small cliffs. The basal 13 m (6 ft) are a sandy siitstone tc i Cretaceous and red on both fresh and weathered surfaces. The bedding layei thick. Forms slopes. Under the upper limestone Is a fine grained I yellow on both fresh and weathered surfaces. The limestone Is di of reddish siitstone. Lower limestone layers are platy with calcite between. The bedding thickness Is -2 5 cm (-1 In). Upper conta^ Is sharp. Pentacrlnus, bryozoans, and oysters are common. WeatH ers - upper white thickness = 121.9 m (400 ft). I Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. unit Is limestone. Yellowish green to gray on The upper and lower contacts of the oyster ie lower part Is shale to mudstone. Color Is alns two discontinuous coal beds. The coal is is, splintery, thinly bedded, and typically > - 5.1 cm (1 - 2 in) beds. The crystals range in lerally poorly lithified, weathers to a slope, and r contact is gradational. Total thickness = 975
ie. The upper unit Is a conglomerate layer, lerally matrix supported. Matrix Is clay to fine ;d of chert and quartzlte. The middle unit Is a ded grains. The lower unit rock types consist siitstone. Colors range from brick red to unit contacts are gradational. Member 153 m (280 ft).
r t Is fine-grained limestone. Gray weathered ecypods. Forms a resistant ledge. Unit Is 1 layers separated by layers of shale and surface and white to locally motteled on the ledges. Each layer Is -0.9 m (-3 ft) thick. The n on both fresh and weathered surfaces. Beds îtween the gypsum, shale and siitstone are thers to slopes and rounded hills. Total
bar nestone. The upper unit Is a alternating rs range from red, brown, yellow, and greenish ted. Abundant gypsum talus and light gray intacts are gradational. The bottom of unit Is ellow on weathered surfaces and pale yellow = 51.8 m (170 ft).
er nit Is limestone. Beds are 0.9 m (3 ft) thick iltstone to siitstone. Colors are grayish green Iding layers vary from 25 - 91.4 cm (1 - 36 In) ? grained limestone. Color Is pale yellow to stone Is divided by a 10.2 cm (4 In) thick layer ith calcite layers 1.3 cm (05 In) thick 3er contact Is gradational and lower contact on. Weathers to a platy talus slope. Total
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Surficial Deposits
Qlf Qa Qls Qcl Qc2 Qas Holocene
Qcg1 Qb Pleistocene
Unconformity
Bedrock
Tc
Unconformity
Kkw
Ksc
Kt
Kd
K-1 Unconformity
Jew
Jcc
Jco
J-1 Unconformity __ j Jn I 1 I Triassic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Deposits
Qcl Qc2 Qas Holocene Q uaternary
Pleistocene
nform lty drock 3 Tertiary nform lty
3 C retaceous 3 3 conformity 3 Jurassic 3 Ç] onform lty : ] Triassic-Jurassic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bedrock Quaternary BawK Basalt brown to black on weathered surfaces and dark gray on fresh surfaces. Fine crystalll Phenocrysts of olivine ~5 mm (0.125 in), plagioclase -2.5 mm (0.0625 in), and pyroxene. Ve bottom 0.9-13 m (3-6 ft), dense middle 4.6 -13.7 m (15-45 ft), and vesicular top 0.9 -1.8 6 ft). Weathers to a resistant cap unconformably overlying Quaternary, Tertiary, and Cretacc rocks. Unit thickness is 6.1 -183 m (20 - 60 ft). Tertiary Claron Formation Limestone, conglomerate, and sandstone to siitstone. Subdivided into two members - upp and lower pink. The white member is a thick to medium bedded fresh water limestone. Co gray on the weathered surface and chalky white on the fresh surface. Contacts are sharp. ( cavities [25 - 7.6 cm (1 - 3 In) In diameter] filled with calcite. Thickness is 91.4 m (300 ft). Th the lower pink unit Is a sandy limestone. Pink to red on the weathered surface and light pir the fresh surface. The lower part of the unit contains scattered pebbles of Precambrian to Paleozoic quartzlte, chert, limestone, dolostone. Pebbles are very well-rounded. The lower | member Is fluvlal-lacustrlne In origin (HIntze, 1988; Doelling et al., 1989). Contacts are shaq; Thickness Is 152.4 m (500 ft). Unit weathers to hoodoos, steep cliffs, or steep slopes. Total tl = 243.8 m (800 ft). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. auinger». rorms vaiieysana smpes. upper ana lower contacts « thick-bedded, fine-grained limestone. Colors are dark yellow or on fresh surfaces. Cliff to ledge former. Total thickness = 51.8 rr e crystalline, Jurassic Carmsi Formation - Co-op Crssk Mambsr ixene. Vesicular Jco Sandy siitstone to siitstone and limestone. The upper unit is lim ) 0.9 -1.8 m (3 - and form small cliffs. The basal 13 m (6 ft) are a sandy siitstone id Cretaceous and red on both fresh and weathered surfaces. The bedding lay thick. Forms slopes. Under the upper limestone is a fine graine* yellow on both fresh and weathered surfaces. The limestone is < of reddish siitstone. Lower limestone layers are platy with calcit between. The bedding thickness is -2 5 cm (-1 in). Upper conti is sharp. Pentacrinus, bryozoans, and oysters are common. Weat >ers-upper white thickness = 121.9 m (400 ft). stone. Color is e sharp. Contains 100 ft). The top of Jurassic Navajo Sandstone d light pink on Jn Well-rounded, well-sorted, fine-grained, friable, quartz sandston* brian to and/or calcite. Red, orange, tan, to white on both fresh and weal he lower pink Bedding thickness ranges from 0.9 - 9.1 m (3 - 30 ft). Upper and are sharp. Contains 1.3 -25 cm (0.5 -1 in) red to orange oxidation spots or :s. Total thickness Forms cliffs. Total thickness = 609.6 m (2000 ft). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Kontacts are gradational, rne Dotcom or unit is I yellow on weathered surteces and pale yellow Is = 51.8 m (170 ft). nber r unit is limestone. Beds are 0.9 m (3 ft) thick y siitstone to siitstone. Colors are grayish green >edding layers vary from 25 - 91.4 cm (1 • 36 in) ine grain ^ limestone. Color is pale yellow to nestone is divided by a 10.2 cm (4 in) thick layer with calcite layers 1.3 cm (05 in) thick Jpper contact is gradational and lower contact I mon. Weathers to a platy talus slope. Total z sandstone cemented with hematite, silica, ih and weathered surfaces. Cross-I^ded. Upper and lower unit contacts are sharp, on spots on both fresh and weathered surfaces. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Jcc Jco J-1 Unconformity Jn ] Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Jurassic iJcc Jco iconformlty Jn Tnassic - Jurassic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NOTE TO USERS Oversize maps and charts are microfilmed in sections in the following manner LEFT TO RIGHT, TOP TO BOTTOM, WITH SMALL OVERLAPS UMI Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Plate 4a 8000 6000 5000 4000 3000 2000 1000 Reproduced with Pemiission of the copyrnght o'vner. Further reproduction prohibited without permission. 3 - Cross Sections A’ 8000 B 8000 Qb Qb 7000 7000 Qa 60 0 0 Jew 6 0 0 0 Jco 5000 500 0 4000 4000 3000 3000 2000 Jm 2000 1000 1000 permission of the copyright owner. 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