A Serial Cross-Section Analysis of the Lewiston Structure

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A SERIAL CROSS-SECTION ANALYSIS OF THE LEWISTON STRUCTURE,

CLARKSTON, WASHINGTON

MICHAEL ROBERT ALLOWAY

A Thesis submitted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE IN GEOLOGY

WASHINGTON STATE UNIVERSITY School of Earth and Environmental Sciences

December 2010

To the Faculty of Washington State University

The members of the Committee appointed to examine the thesis of MICHAEL ROBERT ALLOWAY find it satisfactory and recommend that it be accepted.

______A. John Watkinson, Ph.D., Chair

______Simon A. Kattenhorn, Ph.D.

______John A. Wolff, Ph.D.

ii ACKNOWLEDGMENTS

First and foremost I would like to thank Dr. A. John Watkinson not only for serving as chair on my committee, but for his patience, guidance, friendship, and mentorship throughout my graduate career. I am grateful to Dr. Watkinson for thorough edits and constructive criticism during the development of the final manuscript. I also thank committee members Dr. Simon A.

Kattenhorn and Dr. John A. Wolff for reviewing the initial manuscript and providing helpful suggestions. Special thanks to Dr. Kattenhorn for teaching me the most valuable lesson I have learned during my graduate education: to trust and have confidence in my intuition. Thanks to

Dr. Wolff for his help with the chemical analysis.

I would like to express extreme gratitude to Dr. Stephen P. Reidel for help in the field, help with chemical data, allowing me to borrow his personal fluxgate magnetometer, constant email correspondence, sponsoring my abstract for AGU, providing helpful references, and editing figures and tables.

Thank you Dr. Victor E. Camp, Dr. Peter R. Hooper, and Dean Garwood for helpful conversations, either through email or through the United States Postal Service. Also, thanks Dr.

Camp and Dr. Hooper for allowing the reanalysis of samples from your past projects in the area.

Thank you very much to the WSU Geoanalytical Laboratory staff for help preparing XRF samples. Special thanks to Rick Conrey for patience and kindness during the chemical analysis phase of the project in addition to teaching me how to use the fluxgate magnetometer. Also, thanks to Matt Engle for training me to prepare samples for XRF analysis. Thanks to Dr. Jure

Žalohar for giving me access to and helping me with the software T-TECTO 3.0.

iii A number of assistants helped in the field through helpful discussions and carrying samples. Thank you very much Rachel Brewer, Kevin Tarbert, and Brian Spall. Also, thanks to

Jenice Jim for allowing me to borrow her dog Kona to help carry samples down the hill.

Thanks to all of the landowners in the field area for allowing me access to their property, especially Pat McCann, and Gary and Cara Snyder. Very special thanks to Pat and Ryan

McCann for helping me during a case of heat exhaustion. Also, thanks to Jenice Jim and Rachel

Brewer for being there for me when I needed help as well.

I thank my parents for financial support during my graduate career, but most of all for emotional guidance. Also, this project would not have been possible without the financial support from the School of Earth and Environmental Sciences, Geology departmental grants which include the Apatite to Zircon, Inc. (A-Z) Award and the H. Walter and Jeanette Praetorius

/ Exxon Graduate Fellowship in Geology.

iv A SERIAL CROSS-SECTION ANALYSIS OF THE LEWISTON STRUCTURE,

CLARKSTON, WASHINGTON

Abstract

By Michael Robert Alloway, M.S. Washington State University December 2010

Chair: A. John Watkinson

The Lewiston Structure is located in southeastern Washington / west-central Idaho and is a generally E-W trending, asymmetric, non-cylindrical anticline in the Columbia River Basalt

Group (CRBG) that transfers displacement into the Limekiln fault system to the southeast and the Silcott fault system to the southwest. A serial cross-section analysis and 3-D construction of this structure shows how the fold varies along its trend and sheds light on the deformational history of the Lewiston Basin. Construction of the fold’s 3-D form shows that the fold’s wavelength increases and amplitude decreases near its eastern and western boundaries. Balanced cross-sections show approximately 5 shortening across the structure which is consistent with the Yakima Fold Belt (YFB). Although the structure is similar to the YFB, it does not form part of a belt and its local nature has been suggested to mark the North American continental margin of the Cretaceous

Discovery of an angular unconformity below the Grande Ronde Basalt – normal polarity unit 1 (GRB-N1) in addition to a variation of N1 unit thickness across the structure suggests that the fold was deforming before emplacement of N1. Analysis of structural data using the Gauss method for heterogeneous fault-slip data indicate N-S shortening prior to and after N1

v emplacement. Analysis of slip data for strain-inversion and specification of spatial-distribution patterns help identify the existence of a transpressional tectonic environment.

The nature of faulting associated with the Lewiston Structure is a topic of some debate, namely the presence of a reverse fault on the southern limb of the fold. The reverse fault under debate outcrops to the east of the field area and is GRB-R2 (reverse polarity unit 2) juxtapose with Pliocene (?) gravels. Better control on unit thicknesses and map contacts rule out the surface exposure of a reverse fault on the southern limb of the fold in the field area. This major fault dies out or becomes blind before reaching the ID-WA border and the change in flow attitude from the north side of the river to the south is interpreted to be accommodated by an abrupt fold hinge beneath the Snake River.

vi TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ...... iii

ABSTRACT ...... v

LIST OF TABLES ...... ix

LIST OF FIGURES ...... x

DEDICATION ...... xii

CHAPTER

1. INTRODUCTION ...... 1

Geologic Setting ...... 1

Structural-Tectonic Setting ...... 4

Stratigraphy ...... 7

Location of Field Area ...... 8

2. PREVIOUS INVESTIGATIONS ...... 10

3. ANALYSIS OF THE LEWISTON STRUCTURE ...... 16

Introduction ...... 16

Methods of Investigation ...... 19

Field Observations ...... 20

Chemical Analysis ...... 23

Results ...... 29

vii Discussion ...... 40

Conclusion ...... 53

REFERENCES ...... 55

APPENDICES

1. CHEMICAL ANALYSIS AND SAMPLE LOCATION ...... 59

2. STRUCTURAL DATA ...... 70

3. PARAMETERS AND EXPLANATION OF PARAMETER VALUES USED FOR

PALEOSTRESS INVERSION IN T-TECTO 3.0 ...... 72

4. REGIONAL STRATIGRAPHIC NOMENCLATURE AND ISOTOPIC AGES ...... 78

5. TECHNIQUES USED WITH FIELD FLUXGATE MAGNETOMETER ...... 80

GEOLOGIC MAP ...... Plate 1

viii LIST OF TABLES

Table Page

1.1 Stratigraphic section for CRBG units in field area ...... 8

A-1.1. Normalized major element chemistry ...... 60

A-1.2. Unnormalized trace element chemistry ...... 63

A-1.3. Normalized major element chemistry for samples within field area but not collected by

this study ...... 66

A-1.4. Unnormalized trace element chemistry for samples within field area but not collected by

this study ...... 68

A-2.1. Flow-plane attitudes ...... 71

A-2.2. Slickenside data ...... 71

A-2.3. Fault-plane attitudes ...... 71

A-3.1. Parameters and statistics: results of the Gauss method ...... 73

A-3.2. Parameters and statistics: results of the Gauss method for structural data from the western

domain...... 74

A-3.3. Parameters and statistics: results of the Gauss method for structural data from the eastern

domain...... 75

A-3.4. Explanation of parameters in T-TECTO 3.0 ...... 76

ix LIST OF FIGURES

Figure Page

1.1. Map of the Columbia River flood basalt province...... 3

1.2. Setting and extent of the Columbia River Basalt Group in the Pacific Northwest ...... 5

1.3. Location and physiographic map of the mapping area ...... 9

2.1. Structure map of the Lewiston Basin ...... 11

2.2. Photograph of Vista fault outcropping in the field area ...... 12

2.3. Photograph of the north-dipping reverse fault on the south limb of the structure ...... 13

3.1. Panoramas looking down the fold hinge ...... 18

3.2. Photograph slickenside-striae ...... 21

3.3. Picture of the southern limb where fracture zones are visible ...... 22

3.4. Photograph of the southern limb of the anticline showing an abrupt hinge and no fault ..23

3.5. P2O5 versus Cr and TiO2 and SiO2 versus MgO diagrams ...... 25

3.6. TiO2 versus Zr, MgO, and P2O5 diagrams ...... 27

3.7. Structure cross-sections ...... 32

3.8. Three-dimensional illustration of structural variation along trend ...... 36

3.9. Balanced cross-sections ...... 37

3.10. Panorama of the angular unconformity within the Grande Ronde Basalt R1 unit ...... 38

3.11. Stereonet diagrams from paleostress inversion analysis of structural data ...... 39

3.12. Stereonet diagrams from paleostress inversion analysis of structural data from both east

and west domains ...... 39

3.13. Basement-involved, fault-propagation genetic model of fold formation...... 46

3.14. Traditional flat-ramp-flat fault-bend fold geometry ...... 47

x 3.15. Comparison of thin- and thick-skinned models presented by Tozer (2002) with the model

presented in this study ...... 48

3.16. Kinematic analysis of the two structural domains ...... 51

3.17. Kinematic analysis assuming both structural domains reflect the same regional stress

field ...... 52

A-4.1. Stratigraphic nomenclature for the Columbia River Flood Basalts ...... 79

Dedication

This thesis is dedicated to my parents, I love you

both with all of my heart.

xii

CHAPTER ONE

INTRODUCTION

GEOLOGIC SETTING

The Columbia Plateau is located in the intermontane region between the Cascades and the Rocky Mountains and is a broad plain constructed from the Miocene Columbia River Basalt

Group (CRBG; Reidel et al., 1989). In the western and central areas of the Columbia Plateau, the CRBG is underlain by Tertiary continental sedimentary rocks which overlie crystalline basement (Reidel et al., 2002). In the east, the CRBG is underlain by thin (<100m) sedimentary rocks which overlie crystalline basement and is overlain by a thin (<10m) layer of late

Quaternary eolian sediments known as the ‘Palouse Loess’ (Reidel et al., 2002).

The CRBG cover a section of the Cretaceous continental margin which is delineated by the 87Sr/86Sr = 0.706 line and marks the boundary of the North American craton to the east

( 87 r 86 r > 0.706) and accreted island arc terranes to the west ( 87 r 86 r < 0.706; Figure 1.1;

Hooper et al., 2007). Subduction during the Cretaceous was responsible for the formation of the

Idaho batholith, uplift of the Rocky Mountains, and accretion of ocean island arc terranes from the west (Hooper et al., 2007). After the Cretaceous, subduction jumped to the western edge of the accreted terranes which initiated the Eocene Cascade volcanic arc (Hooper et al., 2007). The

87Sr/86Sr = 0.706 boundary has an erratic pattern throughout the intermontane zone (Figure 1.1;

Hooper et al., 2007). It runs south-north along the western edge of Idaho before abruptly changing direction 90 east-west along the north side of the Lewiston Basin and then changing back to the south-north trend to the northwest of the Lewiston Basin (Figure 1.1; Hooper et al.,

2007; Mohl and Thiessen, 1995). This margin is marked by east-west trending folds that include

the Blue Mountains anticline and adjacent Lewiston basin and Troy basin synclines, the

Lewiston Structure, and the Yakima folds (Figure 1.1; Hooper et al., 2007).

Figure 1.1. Map of the Columbia River flood basalt province (shaded) to show the location and nature of the 87Sr/86Sr = 0.706 boundary taken from Hooper et al. (2007) and modified after Camp and Ross (2004). SCF—Straight Creek fault; SB—Snoqualmie Batholith; OWL— Olympic-Wallowa Lineament; CE—Columbia Embayment; PB—Pasco basin; Y—Yakima fold belt; HF—Hite fault (down to the west, but with minor postbasalt left lateral displacement); BMA—Blue Mountains anticline; L—Lewiston and the Lewiston basin syncline; LF—Limekiln fault (down to the west, but with minor postbasalt left lateral displacement); TBS—Troy Basin syncline; CJ—Chief Joseph Dike swarm; LG—La Grande Graben; BG—Baker Graben; W— Wallowa Mountains horst; KBML—Klamath–Blue Mountains Lineament (Riddihough et al., 1986); BMU—Blue Mountains uplift; M—Monument Dike swarm; F—Farewell Bend on the Snake River; VF—Vale fault zone; SCR—southern Cascade rift; BFZ—Brothers fault zone; EDFZ—Eugene-Denio fault zone; MFZ—McLaughlin fault zone; HB—Harvey basin; MG— Malheur Gorge; MC—McDermitt caldera.

The origin of the CRBG (i.e. plume versus nonplume model debate; see Hooper et al.,

2007) is a topic of debate and is outside the scope of this project; however, within the scope of this project, is the magnitude of the CRBG emplacement and the tectonic setting associated with it. The tholeiitic flood basalt eruptions of the CRBG took place between 6.6 Ma and 6 Ma with over 98 of its volume being extruded within the first two million years (Hooper et al., 2007;

Swanson et al., 1971; Tolan et al., 1989). In total, the CRBG cover an area greater than

200,000 km2 with an estimated volume of 234,000 km3 (Hooper et al., 2007; Camp et al., 2003).

Many of the flows have been correlated to feeder dikes and traced over the Columbia Plateau by their constant chemical compositions (Hooper et al., 2007). Eruptions began around Steens

Mountain in east-central Oregon and then moved north along the north-northwest trending Chief

Joseph Dike Swarm in northeastern Oregon and southeastern Washington (Hooper et al., 2007;

Swisher et al., 1990; Camp et al., 2003). By 6 Ma the youngest flows were erupting in southeastern and central Washington (Hooper et al., 2007).

STRUCTURAL-TECTONIC SETTING

The Columbia Plateau covers four structural-tectonic regions, each with different structural styles: the Yakima Fold Belt (YFB), the Palouse slope, the Blue Mountains, and the embayments along the eastern margin (Figure 1.2; Reidel et al., 1989). The Lewiston Structure is found in the Clearwater Embayment (Figure 1.2); however, local deformation of the structure is more characteristic of the YFB because of its high-amplitude and short-wavelength.

Figure 1.2. Setting and extent of the Columbia River Basalt Group in the Pacific Northwest Columbia Basin. The five structural-tectonic provinces are bounded by black lines and labeled. Taken from Reidell et al. (2002).

Regional structures can be divided into three classes: 1) Mesozoic northeast-trending basement shear zones (Murphy, 2007), 2) Miocene east-west trending folds and faults within the CRBG, and 3) pre CRBG (Eocene ?) northwest trending zones of deformation best represented by the Olympic-Wallowa Lineament (OWL) (Reidel et al., 2002). Only the Miocene east-west-trending faults and folds are present in the field area. The northeast-trend of the

Lewiston Structure in the western and eastern parts of the fold may be a reflection of reactivated

Mesozoic basement shear zones. These basement shear zones are exposed at Granite Point just north of the field area (Murphy, 2007).

Most of the Columbia Plateau is characterized by a general north-south compression and east-west extension which formed zones of anticlines that trend approximately east-west (Reidel et al., 1989). The Palouse slope is located in east-central Washington and extends into part of west-central Idaho. It is described by Reidel et al. (1989) as the least deformed region in the

Columbia Plateau. The Palouse slope is characterized by very low-amplitude and long- wavelength folds that deform the gently southwest dipping paleoslope (Reidel et al., 1989).

Structures on the Palouse slope appear to have formed under the same generally north-south early compressional regime that formed the YFB; however, deformation is on a more localized scale because in the Palouse slope the basalt overlies and is possibly anchored to crystalline basement material (Reidel, 1984, Reidel et al., 2002). In contrast, Basalt of the YFB overlies, in some areas, more than 6 km of Eocene and Oligocene sediments and is not anchored to basement material (Reidel, 1984, Reidel et al., 2002). The sediment underlying the YFB allows the basalt to slip during deformation and deform independent of basement material (Reidel, 2010, personal communication). At surface or outcrop scales the Lewiston Structure has a similar structural style to the highly deformed folds of the YFB, which causes it to stand out from the surrounding deformation consistent with the Palouse slope and Clearwater Embayment. The folds in the

YFB are described by Price and Watkinson (1989) as being generally asymmetric, non- cylindrical, and some have overturned limbs. Although the Lewiston Structure does not meet all of the general regional patterns of the Yakima folds, it does have some striking similarities, namely its 5 shortening (Reidel et al., 2002). Camp and Hanan (2008) suggest the presence

of a ‘Yakima’-like fold on the Palouse slope is localized by the cratonic boundary of North

America which agrees with the conspicuous bend in the r 0.706 line (Foster et al., 2006).

STRATIGRAPHY

Appendix 4 presents regional stratigraphic nomenclature for the CRBG. Although all five regional formations are found in the field area, only those members involved in folding and faulting were of interest. These include all members of the Imnaha Basalt formation, R1, N1, and

R2 units of the Grande Ronde Basalt (GRB) formation, the Priest Rapids Member (Lolo flow) of the Wanapum Basalt formation, and the Asotin and Wilbur Creek Members of the Saddle

Mountains Basalt formations which are undifferentiated on the map (Plate 1). Other flows in the

Wanapum Basalt and the Saddle Mountains formations are not present in the field area. Pomona and Elephant Mountain Members are in the field area, but are intracanyon flows and do not help define the structure.

Due to the nature of deformation within the field area, local lithological descriptions are rarely applicable to the rocks in the mapping area. Within the field area, some attributes of the flows were used for mapping specific contacts (Table 1.1). The Imnaha-GRB-R1 and the GRB –

N1-R2 contacts were the only contacts that could be mapped based on field observations. The

Imnaha Basalt commonly has plagioclase phenocrysts that range in length from 0.5 to 2.5 cm

(Camp, 1976). Also, the Imnaha Basalt tends to weather into a coarse granular sand that is unlike weathering characteristics of other flows (Camp, 1976). Furthermore, this contact is made more obvious in the field by the tendency for the GRB-R1 to form colonnade in the lowermost members. The GRB-N1-R2 contact is locally marked by an intense zone of flow-top breccia.

This contact was not mapped based on field observations; however, after chemical analysis and a

field analysis of remnant magnetization with a fluxgate magnetometer, results show that this characteristic can be used to map the N1-R2 contact locally. This contact is discussed with greater detail in Chapter 3. The rest of the mapping requires chemical and paleomagnetic analysis in order to separate flows.

Table 1.1. Stratigraphic section of CRBG units within mapping area. FORMATION POLARITY MEMBER THICKNESS INTRAFLOW STRUCTURE (m) Lower Monumental Intracanyon flows with well- Elephant Mountain 50-70 SADDLE MOUNTAINS exposed colonnade 20-40 BASALT Pomona Wilbur Creek 30-80 Hackly entablature; vesicle zones Asotin WANAPUM BASALT Priest Rapids 70 Basal colonnade; entablature Meyer Ridge Grouse Creek Flow-top breccias; hackly R2 240 Wapshilla Ridge entablature; zones of vesicles Mt. Horrible Cold Springs Ridge GRANDE Hoskin Gulch Intense zone of flow-top breccia RONDE N1 China Creek 180 marks N1-R2 contact BASALT Frye Point Downy Gulch Center Creek Basal colonnade lies conformably Rogersburg R1 170-180 over Imnaha; entablature; zones of Teepee Butte vesicles Buckhorn Springs Many outcrops have plagioclase IMNAHA BASALT unknown phenocrysts 0.5-2.5 cm; tops weather to granular sand

LOCATION OF FIELD AREA

The mapping area is an approximately 60 km2 section just north of the Snake River and south of the Uniontown Plateau in Clarkston, WA (Figure 1.3). The study area extends east of

the Idaho-Washington border, but because Garwood and Bush (2005) mapped the structure to the east, mapping for this project was focused toward the westerly continuation of the structure into

Washington; however, the structural analysis and synthesis incorporates work done by Garwood and Bush (2005) east of the mapping area. The study area covers the borders of Whitman,

Garfield, and Asotin Counties in Washington and extends into Nez Perce County, Idaho. The confluence of the Clearwater and Snake Rivers is within the study area and marks the eastern boundary of the mapping area.

Figure 1.3. Location and physiographic map of the mapping area with the locations of cross- sections A-Aʹ through G-Gʹ. Modified after Camp (1976).

CHAPTER TWO

PREVIOUS INVESTIGATIONS

Although this area has been studied by various geologists over the past century, investigations done by Camp (1976), Swanson et al. (1980), Hooper et al. (1985), Jones (2003), and Garwood and Bush (2005) are the more relevant studies. This study attempts to map the structure at a local scale to achieve a higher level of detail. Because of the local and complex nature of the structure, previous regional investigations have left some questions unanswered.

Faulting associated with the fold is a topic of debate. The Vista fault is a high-angle reverse fault on the northern limb of the anticline and is well-documented (Camp, 1976;

Swanson et al., 1980; Hooper et al., 1985; Figure 2.1). This fault is well-exposed in the field area (Figure 2.2); however, Garwood and Bush (2005) indicate that it dies out or becomes blind eastward along the structure. Most of the discrepancy revolves around the presence of a second reverse fault on the southern limb of the fold that Camp (1976) referred to as the ‘Wilma’ fault.

Camp (1976) and Swanson (1980) have mapped this fault as a surface feauture conspicuously hidden by the Snake River. Hooper et al. (1985) has not mapped this fault at all and Hooper

(2010, personal communication) is skeptical that the fault exists. Garwood and Bush (2005) have mapped the fault east of the mapping area and have documented the location of its one outcrop where the GRB-R2 unit is thrust over Pliocene gravels (Figure 2.3). Garwood (2010, personal communication) does believe this is a major fault and suggests that it extends westward along the structure through the mapping area.

Figure 2.1. Structure map of the rhombohedral-shaped Lewiston Basin showing bounding faults and folds. The Lewiston Basin is bound to the east by the Limekiln fault and to the west by the Silcott fault. Modified after Tolan and Reidel (1989). LA – Lewiston Anticline; LS – Lewiston Syncline; WF – Wilma Fault; SF – Silcott fault; VF – Vista fault.

Figure 2.2. Picture of the Vista fault where a feeder dike for the Weissenfels Ridge Member used it as a conduit.

Figure 2.3. East-west-trending, north-dipping reverse fault located on the southern limb of the structure mapped by Garwood and Bush (2005). GRB-R2 (right of fault) is thrust over Pliocene gravels (left of the fault). Kevin Tarbert (person) and Kona Jim (canine) for scale. Outcrop is exposed in a small cut in the middle of section 26, T.36 N, R. 5 W (Garwood and Bush, 2005).

Work done by Camp (1976) is the foundation of this study. Camp’s (1976) genetic model suggests that the geometry of the fold is controlled by the Wilma fault and that the Vista fault is a back-thrust that post-dates the formation of the fold. Camp (1976) also compiled a number of detailed stratigraphic sections of which the ‘Moses iding’ section was the most important reference for this study. In compiling this stratigraphic section, Camp (1976) collected and chemically analyzed samples collected along the section. Basalt samples from Camp’s

( 976) ‘Moses iding’ section were re-analyzed using the newer x-ray fluorescence (XRF) equipment obtained by the Washington State University (WSU) Geoanalytical Laboratory in

2005 to gain better control on unit thicknesses. Re-analysis of Camp’s ( 976) ‘Moses iding’

samples also yielded contributions to a local database for GRB chemistry. From this database sample chemistry identified as GRB using the Clarkston database could be matched to one of the three magnetostratigraphic units within the GRB based on chemistry as opposed to paleomagnetic data. Chemical analyses are discussed in greater detail in Chapter 3.

The map of Hooper et al. (1985) was the starting point for making the map for this project (Plate 1). This map illustrates how the structure varies along its trend and shows how the structure in the western part of the mapping area is an anticline-syncline pair trending northeast that abruptly changes to a roughly east-west trend (Hooper et al., 1985). In addition, the map shows the structure changing from a high-amplitude, short-wavelength anticline to a monocline and back to an anticline but with much less shortening observed (Hooper et al., 1985). Detailed study of outcrop patterns indicate one, or some combination of the following three things: 1) that the structure and its associated faulting is more complex than illustrated on the map; 2) that the outcrop patterns are locally incorrect; or 3) that unit thicknesses vary greatly along the structure’s trend. This study attempts to resolve which of the possibilities listed above are accurate.

Jones (2003) did a regional kinematic analysis of the Lewiston Basin and found more of a northwest-southeast shortening direction. This paleostress environment was derived from an inversion analysis of structural data in concert with feeder-dike orientations (Jones, 2003).

Northwest-southeast compression is contrary to the tectonic model of a more north-southerly compression as a result of clockwise rotation of the Cascade arc which in turn is caused by the oblique subduction of the Juan de Fuca and Gorda plates (Wells et al., 1998; McCaffrey et al.,

2000; Hooper et al., 2007). Although this model explains the presence of a north-south compressional tectonic setting, the Columbia plateau is surrounded by complex tectonic features and the reasons for north-south compression are almost certainly more complex. For example,

northward migration of the Sierra Nevada microplate during the Neogene has been suggested to have compensated for the 20-25% of total relative plate motions accommodated by dextral offset along the San Andreas fault (Cashman et al., 2009). With the nature of tectonic settings surrounding the region, the north-south compression observed is likely a complex combination of forces acting on the region.

CHAPTER THREE

ANALYSIS OF THE LEWISTON STRUCTURE

INTRODUCTION

The Lewiston Structure is an asymmetric, non-cylindrical anticline that varies from an asymmetric anticline to a south-facing monocline. The structure is asymmetric in that its south limb is steeply dipping ( 45 ) and its north limb is more gentle ( 6 ). The structure marks the northern boundary of the structurally-bound, rhombohedral-shaped Lewiston Basin in southeastern Washington and west-central Idaho (Figure 2.1). The basin is bound to the east by the Limekiln fault/Waha escarpment, to the west by the Silcott fault, and to the south by more east-west folds and faults (Figure 2.1; Camp, 1976). The Waha escarpment marks the northeastern extension of the Waha and Limekiln fault systems (Figure 2.1; Jones, 2003). Camp and Hanan (2008) suggest the presence of this structure reflects the North American cratonic boundary of the Cretaceous and Hooper (2007) and Reidel (2010, personal communication) agree.

The Lewiston Structure mostly trends approximately 075 and is made up of complexly folded and faulted basalt flows of the Columbia River Basalt Group (CRBG). In the western part of the field area the structure trends approximately 035 and changes orientation just to the northeast of Chief Timothy State Park. East of the field area the structure takes on the same northeast trend again (Figure 2.1). These northeast-trending sections of the structure seem to be a reflection of northeast trending basement shear zones observed at Granite Point (Murphy,

2007) and suggests reactivation of basement structure. Basement shear zones with this trend show oblique normal and left-lateral sense of shear with the down-dropped block consistently toward the west (A.J. Watkinson, 2010, personal communication).

The Lewiston Structure stands out geologically and visually because of its intense deformational style and dramatic topography. Figure 3.1A and B are panoramic photographs looking along the fold hinge from two different angles: A) looking west along the fold hinge in the center of the field area and B) looking toward the northeast in the western part of the field area from Chief Timothy State Park. The extreme nature of the fold from the north side of the

Snake River to the South side has caused past researchers to infer a reverse fault underneath the river. Camp (1976) referred to this fault as the Wilma fault. The southern limb of the anticline dips up to 55 north of the Snake River and the flows are approximately flat-lying to the south.

The Uniontown plateau sits topographically 400 m above the Lewiston basin.

Figure 3.1. Panoramic photographs of the fold hinge. A) Looking west from the center of the field area through the McCann Ranch. B) Looking northeast, from south of the Snake River and Chief Timothy State Park.

METHODS OF INVESTIGATION

Field mapping was done at a scale of : 2,500 and then transferred onto an office map with a scale of :24,000. Because flows in the CRBG generally look the same, identification of stratigraphic units was mostly accomplished by x-ray fluorescence (XRF) chemical analysis of major and trace elements by the Washington State University (WSU) Geoanalytical Laboratory.

Samples were collected along the fold hinge in multiple series of generally north-south sections throughout the field area. Preliminary contacts were mapped in the field using a fluxgate magnetometer as well as general field observations. The fluxgate magnetometer was used to separate the magnetostratigraphic units within the Grande Ronde Basalt (GRB). Within the mapping area there are three magnetostratigraphic units that the GRB can be separated into: R1,

N1, and R2 from oldest to youngest, respectively (Table 1.1.). There is an N2 unit regionally; however, that flow is absent in the field area as a result of regional uplift that controlled emplacement of upper R2 and N2 (Camp and Hooper, 1981). A minimum of three geographically-oriented samples were measured at each outcrop for reverse or normal remnant magnetization (Camp, 1976). Red oxidized flow-tops give the most reliable results (Camp,

1976). Because magnetic overprinting can occur, results using the fluxgate magnetometer are to be interpreted with care.

Samples were collected for every flow that outcropped in each section. Where exposure was good, flow-guides were drawn in, not as contacts, but lines on the map to use for drawing contacts once the samples were chemically analyzed and contacts could be inferred. Each sample’s coordinates were recorded using a field GP . In all, 72 samples were collected for chemical analysis. Samples collected by Hooper et al. (1985), Camp (1976), and Reidel (2010,

unpub.) were analyzed and re-analyzed by XRF and for use in the making of the final map (Plate

1).

FIELD OBSERVATIONS

Structural features such as tectonic breccias, shear zones, and faults observed within the

Lewiston Structure were observed measured and mapped. Because the basalts are somewhat chaotic up close, most attitudes were better measured from some distance. Thus, all attitudes are technically apparent. Slickenside orientations were measured and possible sense of slip was recorded where possible on fault surfaces, including bedding-plane parallel faults. Sub- horizontal slickensides were observed on the fold hinge indicating the possibility of a transpressional tectonic environment (Figure 3.2). Slickenside data are scarce because in many cases the fault surface cannot be conclusively determined to be in situ and also because fault surfaces can be extremely difficult to find within the CRBG without clean road-cuts. When polished surfaces are found, sense of slip is often difficult to assess confidently.

There is a zone of intense brecciation that separates two regimes of flow-plane attitudes located on the southern limb of the anticline. In air-photos, this zone appears to be the westward extension of the major east-west-trending, north-dipping reverse fault mapped by Garwood and

Bush (2005; Figure 2.3). The breccias zone separates one zone of flow-planes dipping approximately 45 south, north of the zone, and another dipping approximately 0 south, south of the zone. The abrupt change in attitude in concert with the zone of breccias is conspicuous and typical of limb-intersecting faults in the YFB (Reidel, 2010, personal communication); however, field mapping in this zone indicated that it is a zone of intense flow-top breccia and seems to be characteristic of the GRB-N1-R2 contact. Measurements of remnant magnetization support this conclusion as well as chemical analyses of samples taken across this zone.

Figure 3.2. Slickenside surface with sub-horizontal slickenlines on fault outcrop near the hinge zone of the anticline.

Deformation of the CRBG appears to be scale-dependant as fold deformation appears to be continuous or ductile from far away and brittle up close. The fold-hinges are accommodated by series of brittle fracture zones that from afar appear to be smooth folds (Figure 3.3; Figure

3.4). One fold hinge that may appear curved from afar will typically consist of approximately

2-4 kink-like folds with fracture zones at each fold (Figure 3.3). Fracture zones within abrupt hinges contain through-going bedding-plane-parallel fault surfaces that typically die out before reaching the adjacent hinge zone. Flow-plane attitudes typically change dip by approximately

0 for each fold-hinge fracture zone, but it is not rare to see up to 30 changes. This zone of flow-top breccias happens to coincide with a tectonic fracture zone on the synclinal hinge

(Figure 3.3). Because this contact is a tectonically-fractured flow top breccia, it is easy to mistake as a fault zone.

Figure 3.3. Picture of the southern limb of the fold where it is monoclinal (F-Fʹ; Figure 3.7). Up close, the kink-like folding is clearly accommodated by series of fracture zones that form at the hinges.

Figure 3.4. Photograph of the southern limb of the anticline and the synclinal hinge. Picture illustrates the scale-dependency for how deformation can be interpreted. From afar, the fold looks continuous, but up-close, the fold hinges are clearly brittle fracture zones. This picture was taken looking northeast across the Snake River from underneath the Red Wolf Crossing bridge in Clarkston, WA.

CHEMICAL ANALYSIS

In all, 72 samples were collected and analyzed for major and trace elements with XRF by the WSU Geoanalytical Laboratory. The complete chemical analysis in addition to sample locations and results from the field fluxgate magnetometer are presented in Appendix 1. Most of the samples collected were within the GRB formation and the main purpose of the chemical analysis was to separate the magnetostratigraphic units within. Before this is possible, the non

GRB samples needed to be identified.

Using the Clarkston Database compiled by Schuster (1993), separation of the four formations within the CRBG as well as separation of specific members within the Wanapum and

Saddle Mountains Basalts could be accomplished (Table 1.1). Because the Imnaha formation has limited outcrop in the Lewiston Basin, data from Hooper (2000) were used to supplement

Imnaha data points. P2O5 versus TiO2 and Cr and SiO2 versus MgO diagrams were used to separate formations within the field area (Figure 3.5). Other major and trace elements can be used to accomplish this goal; however, data from this study clearly separate with those mentioned above. Overlapping chemistry of the Elephant Mountain and Pomona Members with other formations is not a problem because they are both intracanyon flows which can be separated using field observations and are thus not plotted in Figure 3.5.

Separating flows within the GRB is more complicated. Because GRB chemistry varies regionally, a database of local GRB chemistry needed to be compiled. Camp (1976) collected two sections of samples called ‘Moses iding’ and ‘ choolhouse Draw’ which were used to form a database of known local GRB chemistry. The only exception to this local database was that of the Cold Springs Ridge Member within the N1 unit, which has been defined since Camp’s ( 976) study (Reidel, 2010, personal conversation). With this control, TiO2 versus Zr, P2O5, MgO, and

Cr diagrams were constructed to identify samples with GRB chemistry into the three magnetostratigraphic units (Figure 3.6). The TiO2 versus Cr diagram was used specifically to separate the Cold Springs Ridge Member from the Meyer Ridge Member of the R2 unit which has higher Cr values.

Most samples collected within the GRB were able to be separated using this analysis.

Samples with more complicated chemistry were able to be identified by stratigraphic location, remnant magnetization data, and field observations. The Lewiston Structure could not be analyzed without an adequate map of the structure, and this chemical analysis was specifically used to complete the field map (Plate 1).

3.5

Tpr 3.0

2.5 (wt%) 2 2.0 Tgr Tpr

TiO Ti 1.5 Tgr Taw Ti 1.0 0.1 0.3 0.5 0.7 0.9

P2O5 (wt%)

350 Tpr 300 Tgr Taw Ti 250 200

150 Ti

Cr (ppm) Cr 100 Tpr

50 Tgr 0 0.1 0.3 0.5 0.7 0.9

P2O5 (wt%)

Figure 3.5. P2O5 versus Cr and TiO2 and SiO2 versus MgO diagrams were used to separate formations in the field area. Polygons for characteristic chemical compositions were formed using data from Schuster (1993). Taw = Asotin/Wilbur Creek Member, Tpr = Priest Rapids Member, Tgr = Grande Ronde formation, Ti = Imnaha formation (Table 1.1).

9 Tpr 8 Taw Tgr 7 Ti Tpr 6 5 Ti 4 Tgr

MgO (wt%) MgO 3 2 1 48 50 52 54 56 58

SiO2 (wt%)

Figure 3.5. (continued)

230

210 Tgn1 Tgr1 190 Tgr2

170 Zr (ppm) Zr Tgr2 150 Tgn1 Tgr1 130 1.5 2.0 2.5

TiO2 (wt%)

6.0 5.5 Tgr2 5.0 Tgr1 4.5 Tgn1 4.0

3.5 MgO (wt%) MgO Tgr2 3.0 Tgn1 2.5 Tgr1 1.5 2.0 2.5

TiO2 (wt%)

Figure 3.6. Titanium variation diagrams versus Zr, P, and Mg for samples identified to be part of the Grande Ronde formation. Polygons for characteristic chemical compositions were formed using data from the Moses Siding section from Camp (1976).

120 Tgr2 - MRM Tgr2 100 Tgn1 - CSRM Tgr2 Tgn1 80 Tgr1 Tgr1 60

Cr (ppm) Cr 40 Tgn1 20 0 1.5 2.0 2.5

TiO2 (wt%)

Figure 3.6. (continued)

RESULTS

One of the most important results of this project is the detailed geologic map of the

Lewiston Structure (Plate 1). This map allowed the construction of a series of structure-sections

(Figure 3.7) which, in turn, allowed the fold’s three-dimensional (3-D) form to be constructed

(Figure 3.8). A balanced-section analysis and the fold’s 3-D form illustrates how the fold varies along its trend. Structure-sections were constructed at a scale of :24,000 with no vertical exaggeration so as to preserve true angular relationships between fault and unit planes.

Structure-section G-Gʹ was modified from Garwood and Bush (2005) to match the color scheme and unit thicknesses of the other sections (Figure 3.7). Structure-sections A-Aʹ, C-Cʹ, F-Fʹ, and

G-Gʹ were used to construct a 3-D form of the structure which illustrates how the structure varies along its trend (Figure 3.8).

Structure-sections B-Bʹ, F-Fʹ, and G-Gʹ were used to construct balanced cross-sections and estimate shortening across the 5.4 km sections (Figure 3.9). Because the purpose of the balanced sections was to estimate shortening and because subsurface structure is unknown, structure geometries are not extrapolated to depth. The use of surface geometry to infer structure at depth is analyzed in the discussion section. Balanced-sections B-Bʹ, F-Fʹ, and G-Gʹ yielded a shortening of 6.6 , 8.9 , and 3.4 respectively. After emplacement of N1 shortening estimates for B-Bʹ, F-Fʹ, and G-Gʹ are 3.8%, 4.3%, and 1.3% respectively. Shortening is at a maximum near the middle of the structure and at a minimum near the ends indicating partitioning and transferring of strain into bounding fault systems. Furthermore, detailed geologic mapping rules out the necessity to postulate surface exposure of a reverse fault on the southern limb of the anticline. Analysis of the fold’s geometry indicates that this fault is for the

most part blind, and its involvement in the formation of the fold is analyzed in greater detail in the discussion section.

Discovery of an angular unconformity at the the GRB-N1-R1 contact (Figure 3.10) in the western section of the mapping area in addition to varied N1 unit thickness throughout the field area suggests that the structure was deforming before N1 emplacement. N1 unit thickness varies up to 70 m in some areas and is thinnest across the fold hinge (Figure 3.7). Because thickness is not well-constrained in the basin, the N1 unit is assumed to be its maximum observed thickness in the Lewiston Basin (Figure 3.7). Maximum N1 unit thickness is observed above the northwestern syncline in section A-Aʹ where the structure trends northeast (Figure 3.7). In this location, the N1 unit is approximately 330 m thick, where elsewhere along the fold hinge unit thickness is approximately 90 m thick.

Structural data (Appendix 2) were used for paleostress analysis of heterogeneous fault- slip data with the software T-TECTO 3.0 (Žalohar and Vrabec, 2007, 2008, 20 0). Parameters as well as an explanation of parameter values are available in Appendix 3. The Gauss-method for analyzing heterogeneous fault-slip data indicate a paleostress environment of essentially north-south ( : 35 2) shortening (Figure 3.11) which is consistent with most current tectonic models for the region (Wells et al., 1998, McCaffrey et al., 2000, Hooper et al., 2007). It is important to note that the shortening direction is just west of north-south (~350°) and correlates with regional feeder-dike orientations that trend approximately north-northwest (Jones, 2003).

Because the structure trends northeast ( 035 ) in the western part of the field area and trends approximately east-west ( 075 ) in the eastern part of the field area, the structural data were separated into two structural domains, a west and an east. Although both of these structural domains have been interpreted by this study to be syntectonic, data from each domain are clearly

different and formation from separate paleostress environments cannot conclusively be ruled out.

Again using the Gauss method for heterogeneous fault-slip data, paleostress analysis of structural data indicate a shortening direction of approximately northwest-southeast ( : 28 2) and approximately north-south ( : 74 2) for the west and east domains respectively (Figure 3.12).

A Aʹ

32 Bʹ B

Figure 3.7. Series of structure-sections from A-Aʹ (west) to G-Gʹ (east). Sections have no vertical exaggeration. Note that G-Gʹ is a section of cross-section B-Bʹ from Garwood and Bush (2005).

C Cʹ

33 D Dʹ

Figure 3.7. (continued)

E Eʹ

F Fʹ

Figure 3.7. (continued)

35 G Gʹ

Figure 3.7. (continued)

Figure 3.8. 3-dimensional construction of the structure using structure-sections A-Aʹ, C-Cʹ, F-Fʹ, and G-Gʹ. G-Gʹ is modified from Garwood and Bush (2005).

B-Bʹ

F-Fʹ

G-Gʹ

Figure 3.9. Balanced cross-sections from B-Bʹ, F-Fʹ, and G-Gʹ constructed to estimate a total shortening within the 5.4 km sections of 6.6 , 8.9 , and 3.4 respectively. Shortening of B- Bʹ, F-Fʹ, and G-Gʹ after N1 emplacement was estimated to be 3.8%, 4.3%, and 1.3% respectively. Hinge lines are dashed and contacts are dashed where inferred as a result of eroded topography.

Figure 3.10. Picture of the angular unconformity discovered by Steve Reidel (2010, personal communication) within GRB-R1 unit proves deformation was occurring during R1 time. Picture was taken looking north across the Snake River in the western part of the mapping area.

Figure 3.11. Paleostress analysis for heterogeneous fault-slip data using the Gauss method in the software T-TECTO 3.0 (Žalohar and Vrabec, 2007, 2008, 2009). Bedding plane orientations are represented by bold-black-dashed lines, slickenside data are represented by solid-grey lines, and fault plane orientations are represented by dashed-grey lines. A) Bedding and fault plane orientations (n=53). B) Slickenside-striae (n= 0); arrow indicates direction of slip was known. C) Paleostress analysis of all available structural data.

Figure 3.12. Paleostress analysis for heterogeneous fault-slip data using the Gauss method for all available structural data for the east and west domains in the software T-TECTO 3.0 (Žalohar and Vrabec, 2007, 2008, 2009). Bedding plane orientations are represented by bold-black- dashed lines, slickenside data are represented by solid-grey lines, and fault plane orientations are represented by dashed-grey lines. Structural data are separated into east and west domains.

DISCUSSION

The angular unconformity at the GRB-N1-R1 contact and varied N1 unit thickness across the structure indicates that the structure was deforming before N1 emplacement. Shortening estimates are at a maximum in the center of the structure and minimum at its boundaries indicating partitioning of strain with bounding fault systems, the Silcott and Limekiln faults

(Figure 2.1). Fold growth during emplacement dictates a genetic model similar to that presented by Reidel (1984) for the Saddle Mountains anticlinal ridge. This model shows flows being thin over the anticlinal hinge and thick above the synclinal basins (Reidel, 1984).

The bounding fault system to the east is the northeast-trending Limekiln fault and Waha fault systems and the bounding fault system to the west is the northeast trending Silcott fault

(Figure 2.1). These faults are understood to be oblique dip-slip faults (Camp, 1976; Jones,

2003). Under north-south compression, these faults would be sinistral oblique reverse faults but because sense of shear is hard to assess in basalt, a demonstrable sense of shear is difficult

(Camp, 1976). Sub-horizontal slickensides are observed south of the Vista fault zone on the hinge of the anticline where it trends 070 -080 ; however, sense of shear is unknown. Horizontal slickensides have also been observed by Hooper et al. (1985) on the northeast trending Silcott fault, but were not found in this study. Horizontal slickensides on east-west trending fault surfaces under north-south (~350°) shortening indicate a local transpressional tectonic environment.

The northeast trend of the Silcott fault and the Lewiston Structure in the western part of the field area is not likely to be coincidental. The northeast ( 035 ) structural trend is very similar to Mesozoic ductile shear zones observed at Granite Point (Murphy, 2007). The

Lewiston Structure is Miocene in age and for the most part trends approximately east-west

( 075 ). The conspicuous change in orientation near the Silcott fault zone by Chief Timothy

State Park is likely to be a reflection of reactivated basement structure.

Balanced cross-sections

The balanced cross-sections oversimplify the nature of the fold hinges. The sections show each hinge as being a single-kink fold whereas in reality the fold hinges are accommodated by a series of kink-like folds that from a distance do appear to be smooth (Figure 3.9).

Simplifying the structure in this way emphasizes geometries associated with specific models of formation. Although detailed mapping ruled out surface exposure of a reverse fault on the southern limb of the fold, the geometry of the fold indicates that the major reverse fault (Wilma fault) mapped by Garwood and Bush (2005; Figure 2.3) may be responsible for the formation of the fold. If this is true and the fault did control the geometry of the fold, then the fault must be blind within the field area. The steep ( 45 ) southern limb and gentle (0- 6 ) northern limb are strikingly similar to geometric models of basement-involved, fault-related folds that imply reactivation of basement structures (Suppe, 1983; McConnell, 1994; Suppe et al., 1992).

Simple models

A hypothesis for a genetic model of fold formation for the Lewiston Structure is presented herein (Figure 3.13). This model breaks the stages of fold development down into four sequences of events, t0, t1, t2, and t3, where t3 is the current fold geometry of section B-Bʹ (Figure

3.9; Figure 3.13). At time zero, the initial CRBG flows, the Imnaha Basalt formation and the

GRB – R1 unit, are emplaced. Time one is at the time of GRB – N1 emplacement and after the inferred ~3.8% shortening as a result of the propagation of the Wilma fault has occurred. Time two illustrates the continued propagation of the Wilma fault and increased shortening. It is

important to note that time two is not at the moment of fault rupture on the Vista fault, and more volume rock is transferred through the hinge before time three.

Figure 3.13 illustrates a genetic model for a basement-involved, fault-propagation fold where the north limb is the backlimb and the south limb is the forelimb. In this model the Wilma fault is the geometry-controlling fault and the Vista fault is an antithetic reverse fault that post- dates fold formation (Figure 3.13). It is important to note that the backlimb is not parallel to the fault in this model because deformation is assumed to be thick-skinned and there is no postulated décollement at the depth of the section (Tozer et al., 2002). Jones (2003) inferred that a regional décollement exists at the depth of the brittle-ductile transition zone; however, this study does not extrapolate subsurface structure to such a depth. Because the concept of balancing cross-sections was developed to help describe thrust belts where shortening is typically 40-50% (Dahlstrom,

1969), traditional methods for calculating a depth-to-décollement might not be applicable to regions such as the Lewiston Basin where shortening is less than 10%.

Traditional models of fault-propagation folds have backlimb geometries parallel to the fault surface (Medwedeff and Suppe, 1997). The reason this model does not conform to this geometry is that the hinterland (the Uniontown Plateau) is topographically higher than the foreland (the Lewiston Basin) by approximately 400 m (Figure 3.7). In order for the model to accommodate observed structural relief and observed shortening (Figure 3.9), the Wilma fault has to be at an angle of approximately 47 (Figure 3.13). This fault-attitude approximation is close to its observed dip of 40 and with the amount of observed variation in fold shape, it is likely that the fault dip also varies along the trend of the structure as well. Furthermore, the right-stepping en echelon nature of the fold hinge traces (Plate 1), where the structure trends approximately east-west (070° - 080°), may be an expression of segmentation of the Wilma fault.

In traditional fault-propagation folds with ramp-flat geometries, strata in the foreland and hinterland are at the same elevation (Figure 3.14; Medwedeff and Suppe, 1997; Johnson and

Johnson, 2002). Models that show topographic relief from the hinterland to the foreland include ramp-nucleated fault-propagation folding (McConnell et al., 1997), multibend fault-bend folding

(Medwedeff and Suppe, 1997), and trishear folding (Allmendinger and Shaw, 2000).

Accounting for all available structural data, the model presented herein is the best working hypothesis at this time (Figure 3.13). This model was chosen because it is the simplest explanation for all of the observed surface features (Figure 3.7; Figure 3.13). The model explains the geometries observed and, because the transport direction is toward the south, it also explains why flows on the north side of the structure sit at a topographically higher elevation than the same flows south of the structure (Figure 3.13).

Since this study was focused on mapping surface observables and because subsurface structural data are at a minimum, a number of alternative genetic models can be hypothesized.

Other possible models include the compressional reactivation of an extensional fault-propagation fold (or monocline), any variety of thin- and thick-skinned compressional fault-related fold models, or an anticline that formed independent of basement structure. The extensional fault- propagation fold model is of interest because the Lewiston Basin has been assumed to be a synclinal basin formed under compression (Camp, 1976). Similar styles of deformation are observed in extensional settings and, with little subsurface knowledge of the Lewiston Basin, should not be ignored. For example, in pre-Suez rift sediments, propagation of normal faults during rifting form broad rift-facing monoclines (Sharp et al., 2010). Geometries of basement- involved fault-propagation monoclines that form in extensional settings can have geometries similar to that of the Lewiston Structure and can also have antithetic-secondary reverse faults

(Sharp et al., 2010; Tozer et al., 2002). Such a model would have the structure begin forming as a basement-involved fault-propagation monocline where the basement fault later reactivates as a reverse fault that propagates through the CRBG and controls the geometry of the basement- involved, fault propagation anticline. This model would have the transport direction being toward the north, making the Vista fault the geometry-controlling fault and the Wilma fault the antithetic secondary reverse fault. Detailed mapping of the surface structure reveals striking similarities to fault-related folds in both compressional and extensional tectonic settings and is the reason for the genetic geometric models of fold formation presented herein (Figure 3.13;

Suppe, 1983; McConnell, 1994; Suppe et al., 1992; Sharp et al., 2010).

Work done by Tozer et al. (2002) interpreted the geological structure of the Central

Apennines, Italy using both thin- and thick-skinned tectonic models (Figure 3.15). Fold geometry as well as the lack of subsurface structural data are strikingly similar to what is observed in the Lewiston Structure (Figure 3.15; Tozer et al., 2002). They showed that both models fit outcrop and well data and can be restored, which ultimately implied the two models being amongst a series of possible explanations for the structure (Tozer et al., 2002). The thick- skinned interpretation from Tozer et al., (2002) is of particular interest because it involves reactivation of a normal fault during compression (Figure 3.15). It is also important to note that the backlimb is not parallel to the fault surface in the thick-skinned model (Figure 3.15; Tozer et al., 2002).

Without sufficient subsurface structural data it is meaningless to be dogmatic about style of deformation. However, understanding the different tectonic environments in which certain structural geometries can form has both professional and academic applications. Tozer et al.

(2002) warns petroleum geologists of the risk in using strategies based on the thin-skinned

models to find repetition of suitable reservoir rocks. Understanding the varieties of mechanical models that result in specific surface geometries can lead to better judgement of future research and thus better allocation of resources. It is important to keep multiple working hyoptheses until more discriminating evidence is available.

Figure 3.13. Basement-involved, fault-propagation genetic model of fold formation. This model illustrates Wilma fault as the fault responsible for fold geometry. The Vista fault post-dates the fold. Final fold geometry (t3) is that of section B-Bʹ (Figure 3.9).

Figure 3.14. Traditional flat-ramp-flat fault-bend fold geometry. Modified from Medwedeff and Suppe (1997). Take note of the backlimb geometry parallel to the fault surface in addition to equal elevations of strata in the hinterland and foreland.

Figure 3.15. Comparison of two models from Tozer et al. (2002) and the model presented in this paper. A) Thin- and thick-skinned models that both agree with all available data for the same structure (Tozer et al. (2002). Figure was modified after Tozer et al. (2002). B) Model presented in this paper suggests thick-skinned deformation.

Kinematic analysis of structural data

The general north-south (~350°) shortening result of the paleostress inversion analysis for all available structural data (Figure 3.11) align with the currently accepted tectonic model for the region (Wells et al., 1998; McCaffrey et al., 2000; Hooper et al., 2007). Because the kinematic data separate into two domains of homogeneous data, a paleostress inversion analysis was completed for the separate domains (Figure 3.12). The results were such that a northwest- southeast shortening may have been responsible for the west domain and a more north-south shortening for the east domain (Figure 3.12). Originally, this analysis was run because the hypothesis that the structure formed from separate paleostress environments could not be ruled out; however, it is entirely possible that the kinematic data from the west domain formed as a result of a regional north-south stress field and that the northwest-southeast shortening is a local stress field resulting in the northeast-trending folds and faults. If the northeast trending surface structures in the CRBG are a reflection of reactivated basement structure, the kinematic data in the west domain could be a result of strain partitioning resulting in stress perturbations around the basement fault tip.

Because the Lewiston Structure trends northeast in the western part of the structure, east- west in the middle, northeast in the eastern part of the structure, this study interprets the structure to be the result of interacting basement faults. Figure 3.16 illustrates the structures that are expected to form from the stress fields from each domain: A shows expected structures forming from a regional north-south shortening in the east domain, and B shows expected structures from the regional northwest-southeast shortening in the west domain. Jones (2003) hypothesized that the basin formed from a regional northwest-southeast shortening. If this were true, the areas of maximum shortening would have to occur on the northeast trending structures, but this is not

observed in the Lewiston Basin (Figure 3.17). Folds covering northeast-trending basement structures in a north-south compressive stress environment are expected to accommodate less shortening than those that cover east-west trending basement structures and have a parallel strike-slip component in order to maintain strain compatibility. The east-west section of the

Lewiston Structure accommodates far greater amounts of shortening than the northeast-trending sections that bound the structure to the east and west. The kinematic data that result in a northwest-southeast shortening orientation are best explained as a local stress field caused by the partitioning of displacement with a northeast-trending fold that covers basement strike-slip fault that is right-stepping and assumed to be sinistral (Figure 3.17). According to this hypothesis, the

Lewiston Structure forms in a regional stress field of approximately north-south shortening in the zone of compression that links the two interacting structures (Figure 3.17). The local northwest- southeast shortening orientation is interpreted to form from strain partitioning with the northeast- trending basement fault (Figure 3.17).

Figure 3.16. Kinematic analysis of the two structural domains assuming the data reflects regional stress fields and that the surface structures are a reflection of reactivated basement structures. A is the east structural domain with a shortening orientation of approximately 351°. B is the west structural domain with a shortening orientation of approximately 128°. The schematic is an illustration of the structures expected to form in such a regional stress r environment and is not a reflection of what is observed in the Lewiston Basin σ1 – Maximum principal remote stress (compression positive sign convention). LS – Lewiston Structure. SF – Silcott fault.

Figure 3.17. Hypothesis explaining the occurrence of two structural domains forming under one regional stress field. Kinematic data from the west domain is interpreted to be a result of a local stress perturbation located at the tip of a reactivated basement fault. The Lewiston Structure and the data from the east domain are interpreted as a result of basement fault interaction. This hypothesis assumes that both structural domains formed from the same regional stress field. LS r – Lewiston Structure. SF – Silcott fault. σ1 - Maximum principal remote stress (compression l positive sign convention). σ1 - Maximum principal local stress (compression positive sign convention).

CONCLUSION

The purpose of this study was to construct an accurate map of the Lewiston Structure west of the Washington-Idaho border and to determine the possible existence of the Wilma fault on the southern limb of the fold. Detailed mapping of surface geology reveals no fault exposed within the field area and thus, the Wilma fault to the east mapped by Garwood and Bush (2005) either dies out or becomes blind before reaching the field area from the east; however, geometric fold models suggest that this fault is responsible for the formation of the fold, in which case the fault must become blind within the mapping area.

Past researchers studying much larger areas of the region disagree on the nature of the faulting associated with the fold. Visually the fold is striking, especially in the western part of the field area where the river covers the synclinal hinge on the southern limb of the fold. On the north side of the river, the southern limb dips up to 55 and on the south side of the river the flows are flat-lying. Looking along the hinge of the anticline, it is easy to assume the presence of a fault given the extreme changes in attitudes. The intense nature of the structure’s visual appearance makes it easy to declare the westward extension of the Wilma fault documented to the east of the field area (Garwood and Bush, 2005). In the end, abrupt hinges that form without faults seem to be constant throughout the structure regardless of whether the hinge is synclinal or anticlinal. A fault is not necessary to accommodate this amount of shortening as there is sufficient space for the synclinal hinge to fold all of the basalt beneath the Snake River. The previously perceived problem of insufficient space to accommodate the thickness of the basalt is solved by the new recognition of thinning geometry across the fold hinge. The fault to the east mapped by Garwood and Bush (2005) outcrops once approximately 0 km away from the field area. This is enough of a distance for the fault to die out and it cannot be assumed that the fault

continues through the field area. Discovery of the angular unconformity within the field area in addition to thinning of the GRB-N1 unit across the fold hinge indicate that the surface expression of the Wilma fault is not necessary to accommodate the amount of observed shortening in the

Lewiston Structure; however, geometric modeling suggests the subsurface expression of the

Wilma fault is responsible for fold formation and that the Lewiston Structure is a fault- propagation fold.

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58 APPENDIX ONE

CHEMICAL ANALYSIS AND SAMPLE LOCATIONS

Table A-1.1. Normalized major elements (wt%), sample locations and remnent magnitization. † Sample Location (UTM) Date Polarity Unit name SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Total Zone Northing Easting (mm/dd/yyyy) MA01 11T 5144644 488318 09/23/2009 - Grande Ronde R2 54.68 2.308 13.70 12.77 0.199 3.74 7.08 3.40 1.68 0.456 100 MA02 11T 5144373 489989 09/30/2009 - Grande Ronde R2 55.04 2.138 14.20 11.26 0.204 4.21 8.06 3.16 1.31 0.417 100 MA03 11T 5144524 489996 09/30/2009 - Grande Ronde N1 54.19 1.991 13.57 12.37 0.215 4.31 8.41 3.11 1.47 0.381 100 MA04 11T 5144692 490012 09/30/2009 - Grande Ronde N1 54.19 1.786 14.84 11.16 0.177 4.93 8.78 2.99 0.89 0.268 100 MA05 11T 5144289 489443 09/30/2009 - Grande Ronde N1 55.62 1.802 14.36 10.35 0.197 4.54 8.31 2.99 1.53 0.302 100 MA07 11T 5142841 486274 10/28/2009 - Grande Ronde N1 55.00 1.732 14.35 11.25 0.198 4.62 8.40 3.01 1.18 0.265 100 MA08 11T 5142572 486740 10/28/2009 - Grande Ronde N1 56.33 1.937 14.14 10.70 0.188 3.93 7.61 3.20 1.70 0.273 100 MA06 11T 5143784 489400 09/30/2009 - Grande Ronde N1 55.56 2.083 14.24 11.45 0.185 3.94 7.51 3.13 1.58 0.326 100 MA10 11T 5144714 488350 03/15/2010 - Grande Ronde R2 54.28 1.721 14.55 10.71 0.207 4.98 9.06 2.87 1.34 0.285 100 MA11 11T 5144832 488411 03/15/2010 - Priest Rapids 49.89 3.274 13.87 13.62 0.225 4.99 9.44 2.70 1.20 0.790 100 MA12 11T 5144198 488695 03/15/2010 - Grande Ronde N1 56.95 2.228 13.62 11.66 0.186 3.20 6.78 3.11 1.93 0.339 100 MA13 11T 5143789 488746 03/15/2010 - Grande Ronde N1 57.38 2.228 13.60 11.24 0.189 2.97 6.61 3.29 2.17 0.329 100

60 MA14a 11T 5142880 488588 03/15/2010 - Grande Ronde 56.07 2.019 13.91 11.59 0.197 3.47 7.32 3.25 1.82 0.356 100 MA15 11T 5142748 488629 03/15/2010 - Grande Ronde R1 53.40 2.341 13.72 12.94 0.224 4.23 8.13 3.29 1.27 0.451 100 MA16 11T 5142590 488615 03/15/2010 - Grande Ronde R1 53.72 2.304 14.13 11.73 0.182 4.66 8.72 2.96 1.25 0.345 100 MA17 11T 5142589 488603 03/15/2010 - Grande Ronde R1 53.26 2.362 13.78 12.96 0.222 4.25 8.21 3.31 1.21 0.449 100 MA18 11T 5142269 488699 03/15/2010 - Grande Ronde R1 54.09 2.551 13.77 12.48 0.204 3.96 7.70 3.17 1.68 0.395 100 MA19 11T 5144326 491771 03/17/2010 - Grande Ronde N1 54.58 1.709 14.58 10.85 0.182 4.91 8.82 3.05 1.07 0.259 100 MA20 11T 5143830 491864 03/17/2010 - Grande Ronde N1 54.49 2.061 14.13 12.27 0.196 4.04 8.04 3.04 1.33 0.401 100 MA21 11T 5143255 491914 03/17/2010 - Grande Ronde N1 56.29 2.067 13.96 11.22 0.195 3.68 7.38 3.23 1.63 0.339 100 MA22 11T 5143162 491898 03/17/2010 - Grande Ronde N1 56.46 2.096 13.93 10.36 0.191 3.89 7.87 3.13 1.78 0.306 100 MA23 11T 5142860 491768 03/17/2010 - Grande Ronde N1 54.81 2.403 13.82 11.82 0.196 3.96 7.74 3.11 1.73 0.401 100 MA24 11T 5142693 491746 03/17/2010 - Grande Ronde N1 55.82 2.066 14.22 10.82 0.185 3.96 7.58 3.18 1.82 0.349 100 MA25 11T 5142492 491815 03/17/2010 - Grande Ronde N1 56.24 2.078 14.03 10.59 0.189 3.95 7.76 3.09 1.78 0.304 100 MA26 11T 5142424 491836 03/17/2010 - Grande Ronde N1 55.76 2.064 13.81 11.97 0.199 3.67 7.34 3.14 1.71 0.336 100 MA27 11T 5142341 491849 03/17/2010 - Grande Ronde N1 55.38 1.796 14.41 11.01 0.182 4.44 8.04 3.23 1.22 0.299 100 MA28 11T 5142212 491888 03/17/2010 - Grande Ronde N1 54.70 2.224 13.81 12.35 0.215 3.80 7.74 3.29 1.46 0.405 100 MA29 11T 5142151 491893 03/17/2010 - Grande Ronde N1 55.13 1.793 14.25 11.17 0.198 4.60 8.31 2.92 1.34 0.297 100 MA30 11T 5142100 491848 03/17/2010 - Grande Ronde N1 54.23 2.186 13.63 13.13 0.223 3.89 7.61 3.20 1.50 0.396 100 Table A-1.1. (continued) † Sample Location (UTM) Date Polarity Unit name SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Total Zone Northing Easting (mm/dd/yyyy) MA31 11T 5141958 491845 03/17/2010 - Grande Ronde N1 53.87 2.096 13.91 12.90 0.223 4.27 7.91 3.20 1.23 0.400 100 MA32 11T 5141807 491796 03/17/2010 - Grande Ronde N1 54.66 2.019 13.78 12.40 0.216 4.02 7.69 3.37 1.45 0.381 100 MA33 11T 5141794 491751 03/17/2010 - Grande Ronde N1 55.63 2.074 13.98 13.01 0.166 3.46 7.32 2.83 1.10 0.431 100 MA34 11T 5141757 491692 03/17/2010 - Grande Ronde N1 54.58 2.020 13.81 12.45 0.216 4.06 7.78 3.34 1.36 0.382 100 MA35 11T 5141681 491632 03/17/2010 - Grande Ronde N1 54.31 2.051 14.07 13.03 0.216 4.04 7.67 3.04 1.16 0.394 100 MA36 11T 5142033 488861 03/19/2010 - Grande Ronde N1 54.53 2.451 13.68 12.38 0.202 3.79 7.59 3.20 1.76 0.414 100 MA37 11T 5141948 488912 03/19/2010 - Grande Ronde N1 55.47 2.098 14.15 11.29 0.191 3.93 7.58 3.31 1.62 0.364 100 MA38 11T 5141874 488898 03/19/2010 - Grande Ronde N1 55.51 2.072 14.28 11.17 0.188 4.04 7.64 3.19 1.56 0.349 100 MA39 11T 5142307 485855 06/03/2010 N Grande Ronde N1 54.64 1.742 14.45 11.29 0.197 4.85 8.58 2.81 1.19 0.264 100 MA40 11T 5142239 486558 06/03/2010 N Grande Ronde N1 56.55 2.144 13.72 11.40 0.215 3.42 7.15 3.22 1.85 0.325 100 MA41 11T 5142172 486638 06/03/2010 N Grande Ronde N1 55.00 2.323 13.42 12.99 0.205 3.17 7.04 3.41 2.01 0.426 100 MA42 11T 5142133 486707 06/03/2010 N Grande Ronde N1 55.96 2.039 14.19 10.97 0.185 3.92 7.47 3.29 1.66 0.321 100

61 MA43 11T 5142040 487067 06/03/2010 N Imnaha 51.14 2.542 14.23 13.42 0.218 4.95 9.11 3.17 0.82 0.401 100 MA44 11T 5141579 487252 06/03/2010 N Imnaha 51.65 2.726 13.66 13.73 0.227 4.66 8.85 3.26 0.78 0.462 100 MA45 11T 5141521 487323 06/03/2010 R Grande Ronde R1 53.26 2.351 13.70 12.95 0.223 4.34 8.10 3.32 1.31 0.449 100 MA46 11T 5141777 486331 06/08/2010 R Grande Ronde N1 55.43 2.116 14.26 11.05 0.194 4.08 7.71 3.14 1.66 0.369 100 MA47 11T 5141425 486227 06/08/2010 R Grande Ronde R1 54.43 2.356 13.72 12.36 0.195 3.99 7.65 3.17 1.75 0.387 100 MA48 11T 5141404 491691 06/08/2010 R Grande Ronde R2 55.68 2.334 13.90 12.28 0.197 3.28 6.89 3.07 1.93 0.440 100 MA49 11T 5141234 491891 06/08/2010 R Grande Ronde R2 54.79 2.404 13.99 12.64 0.202 3.48 7.22 3.26 1.54 0.478 100 MA50 11T 5142720 490238 06/26/2010 - Grande Ronde R1 52.33 2.329 14.36 11.98 0.183 5.26 9.36 2.88 0.99 0.330 100 MA51 11T 5143053 490560 06/26/2010 R Grande Ronde N1 56.14 2.029 14.22 10.71 0.182 3.92 7.48 3.28 1.73 0.312 100 MA52 11T 5143217 490748 06/26/2010 N Grande Ronde N1 56.25 2.083 13.95 10.68 0.189 3.93 7.71 3.18 1.72 0.301 100 MA53 11T 5143768 494406 07/02/2010 N Grande Ronde N1 54.85 2.105 14.35 11.42 0.180 4.25 8.22 3.13 1.15 0.340 100 MA54 11T 5143522 494405 07/02/2010 N Grande Ronde N1 54.62 1.775 14.57 10.97 0.185 4.74 8.64 2.98 1.23 0.288 100 MA55 11T 5142994 494316 07/02/2010 N Grande Ronde N1 55.29 1.764 14.35 10.66 0.201 4.69 8.45 3.03 1.28 0.293 100 MA56 11T 5142723 494358 07/02/2010 - Grande Ronde N1 57.66 2.239 15.13 10.92 0.131 2.58 6.02 3.31 1.58 0.431 100 MA57 11T 5142662 494380 07/02/2010 N Grande Ronde N1 55.28 1.783 14.31 10.73 0.203 4.55 8.34 3.05 1.46 0.294 100 MA58 11T 5142728 494408 07/02/2010 Grande Ronde N1 55.76 2.075 13.91 11.76 0.208 3.71 7.42 3.10 1.71 0.342 100 MA59 11T 5142439 494750 07/02/2010 N Grande Ronde N1 54.78 1.976 13.88 12.38 0.204 4.09 7.74 3.29 1.28 0.377 100 Table A-1.1. (continued) † Sample Location (UTM) Date Polarity Unit name SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Total Zone Northing Easting (mm/dd/yyyy) MA60 11T 5142185 494948 07/02/2010 R Grande Ronde R2 56.20 2.223 13.89 11.42 0.197 3.44 7.01 3.18 2.02 0.426 100 MA61 11T 5141902 489376 07/09/2010 R Grande Ronde N1 53.84 2.415 14.04 13.35 0.200 3.51 7.26 3.45 1.50 0.433 100 MA62 11T 5141910 489380 07/09/2010 R Grande Ronde N1 55.08 2.301 13.43 12.79 0.210 3.50 7.12 3.48 1.65 0.434 100 MA14b 11T 5142880 488588 07/09/2010 N Grande Ronde 56.57 2.030 13.93 11.99 0.160 3.14 6.50 3.44 1.86 0.377 100 MA63 11T 5142016 489527 07/09/2010 - Grande Ronde N1 55.00 2.116 14.27 11.73 0.191 4.07 7.68 3.12 1.46 0.367 100 MA64 11T 5142089 489516 07/09/2010 - Grande Ronde N1 54.03 2.252 14.41 11.70 0.177 4.18 8.49 3.14 1.27 0.352 100 MA65 11T 5142160 489508 07/09/2010 R Grande Ronde N1 54.39 2.429 13.66 12.60 0.199 3.90 7.61 3.22 1.59 0.410 100 MA66 11T 5141967 489521 07/09/2010 R Grande Ronde N1 55.25 2.334 13.53 12.83 0.203 3.31 6.91 3.52 1.69 0.420 100 MA67 11T 5142461 489499 07/09/2010 - Grande Ronde R1 52.24 2.308 14.36 12.00 0.182 5.35 9.44 2.87 0.92 0.328 100 MA68 11T 5142813 489492 07/09/2010 - Imnaha 50.73 2.509 14.59 13.05 0.197 5.40 9.50 3.08 0.57 0.380 100 MA69 11T 5142984 488600 07/09/2010 - Grande Ronde R2 56.15 2.148 14.23 11.50 0.168 3.14 6.88 3.23 2.19 0.360 100 MA70 11T 5142390 489522 07/09/2010 R Grande Ronde R1 54.17 2.533 13.86 12.33 0.194 3.95 7.69 3.32 1.57 0.389 100

62 MA71 11T 5143088 491405 07/09/2010 R Grande Ronde N1 55.10 2.326 13.45 13.10 0.188 3.23 7.09 3.44 1.66 0.428 100 MA72 11T 5143180 491428 07/09/2010 N Grande Ronde N1 57.14 2.236 13.57 11.48 0.183 3.02 6.73 3.33 1.97 0.341 100 † Remnent Magnetization measured in the field with a fluxgate magnetometer. Table A-1.2. Unnormalized trace elements (ppm). Sample Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd U MA01 22 6 33 380 699 46 320 192 40 14.0 23 39 135 9 25 57 3 31 2 MA02 13 10 37 371 673 34 339 174 38 11.9 22 32 130 8 24 48 2 27 2 MA03 14 10 36 376 624 34 328 169 38 11.0 20 46 125 7 18 45 3 27 2 MA04 13 15 35 320 502 21 343 156 33 10.7 22 28 111 7 17 39 3 24 1 MA05 9 17 36 307 567 38 332 166 34 11.5 21 11 120 7 18 45 3 26 1 MA07 10 20 36 316 487 34 322 155 33 10.4 21 12 116 7 20 39 3 22 1 MA08 11 8 33 329 601 43 338 178 35 11.3 22 21 119 8 20 48 5 25 2 MA06 13 9 30 349 638 44 370 200 37 13.1 22 37 122 8 26 52 4 28 2 MA10 34 93 38 330 490 33 315 152 34 11.0 20 59 113 6 20 45 3 23 2 MA11 46 91 41 375 519 28 299 198 47 16.6 22 47 144 5 26 55 2 33 2 MA12 8 6 32 356 753 53 332 213 40 14.7 23 18 133 11 22 52 5 31 3 MA13 6 4 31 359 704 57 320 216 40 14.6 22 15 132 10 26 57 6 31 1 MA14a 9 6 31 325 702 110 336 184 35 12.5 21 15 123 9 25 59 4 28 3

63 MA15 23 15 39 414 509 29 309 224 51 14.9 22 111 135 6 22 51 3 30 0 MA16 26 59 34 330 466 38 346 190 35 14.7 22 47 122 7 21 47 3 28 2 MA17 24 15 39 421 525 28 307 223 50 14.2 20 105 136 5 17 49 2 30 1 MA18 12 9 34 370 566 46 351 224 39 15.8 22 37 136 9 27 54 3 31 1 MA19 15 16 35 310 581 26 346 148 33 9.8 21 32 109 5 20 37 2 22 1 MA20 13 10 37 352 662 34 342 170 38 11.8 20 29 123 7 23 53 4 29 1 MA21 12 5 34 354 662 43 328 201 40 13.4 22 31 130 9 20 48 4 30 1 MA22 14 8 33 343 617 45 330 189 37 13.0 21 45 120 9 17 51 5 28 0 MA23 23 17 34 362 598 45 334 229 43 15.5 22 98 132 8 24 59 4 32 3 MA24 14 8 32 351 603 47 367 199 37 12.7 22 42 123 9 25 51 4 27 2 MA25 14 8 32 336 627 45 328 189 35 13.2 22 41 122 10 21 45 3 27 2 MA26 11 5 34 354 618 42 313 200 39 14.2 22 32 125 8 23 53 3 28 1 MA27 10 18 36 310 538 31 325 163 33 11.8 21 12 118 6 18 40 4 22 3 MA28 16 17 36 390 585 35 325 173 36 11.4 21 21 129 6 23 51 4 26 0 MA29 9 17 36 307 560 36 329 164 33 11.5 21 13 113 6 22 40 2 23 2 MA30 13 17 37 386 585 36 321 169 36 12.2 22 19 127 7 22 52 4 26 2 MA31 13 12 36 360 581 29 325 168 36 11.8 20 31 128 6 21 49 4 30 1 Table A-1.2. (continued) Sample Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd U MA32 14 11 37 389 584 38 326 179 39 12.6 21 41 131 8 25 48 4 26 1 MA33 14 11 38 387 569 23 319 175 40 12.2 21 36 125 7 21 43 2 28 0 MA34 14 8 37 387 585 39 332 177 39 12.0 21 45 131 6 19 43 4 27 2 MA35 12 13 37 371 558 34 334 181 38 12.3 20 34 132 7 22 48 5 28 2 MA36 23 16 35 372 606 53 313 232 45 15.7 22 114 135 8 23 53 4 31 0 MA37 14 10 34 343 580 46 362 194 37 12.6 23 41 123 7 20 50 3 27 3 MA38 12 10 32 344 593 43 364 196 36 12.8 24 37 121 7 24 55 5 28 1 MA39 11 20 36 314 441 31 323 154 33 10.7 21 12 113 6 19 39 2 21 2 MA40 8 5 31 344 681 46 326 202 38 12.8 22 18 127 10 24 51 6 26 3 MA41 17 9 34 272 685 39 305 220 43 15.2 21 21 133 7 25 54 4 32 2 MA42 13 9 32 351 618 44 365 198 35 13.2 22 37 124 9 24 48 4 29 2 MA43 65 69 38 393 375 19 334 205 45 14.0 23 165 129 4 19 43 1 26 3 MA44 44 43 40 399 404 23 319 230 49 15.7 21 157 140 5 24 54 1 32 -1

64 MA45 25 16 38 413 504 31 305 223 50 15.4 22 110 135 5 25 56 2 32 0 MA46 17 11 31 341 588 43 368 198 38 12.8 22 45 125 7 25 51 4 27 2 MA47 22 21 34 354 583 48 324 224 41 16.2 22 92 131 9 22 55 3 31 2 MA48 10 5 32 338 750 53 333 218 40 15.0 22 15 137 9 31 61 6 33 2 MA49 10 9 33 347 713 34 339 209 41 14.4 23 14 139 9 27 58 6 32 2 MA50 47 114 36 344 408 28 352 184 35 13.4 22 92 121 4 20 47 2 29 1 MA51 12 8 32 354 609 45 364 196 36 13.2 21 30 120 10 22 51 4 28 1 MA52 13 8 34 335 645 44 326 190 37 13.0 21 37 120 8 22 48 5 28 2 MA53 12 13 37 373 558 27 344 159 34 10.4 21 20 128 8 16 38 3 22 1 MA54 13 16 35 325 474 32 334 155 33 10.8 20 34 113 7 23 40 3 23 2 MA55 10 17 34 304 540 31 336 160 33 11.9 21 11 116 7 21 38 4 23 2 MA56 9 3 34 333 672 42 328 209 38 14.2 24 27 129 8 26 48 4 29 2 MA57 10 17 36 311 524 37 328 162 34 11.9 21 12 117 6 20 41 3 24 0 MA58 10 3 33 357 642 50 318 200 40 13.7 23 29 125 7 22 52 4 29 3 MA59 12 11 35 376 582 40 326 174 39 11.9 20 42 128 8 23 45 3 24 3 MA60 8 11 33 338 736 56 362 193 36 13.9 21 13 128 9 29 57 7 33 2 MA61 17 10 33 396 661 21 311 228 45 16.3 22 42 139 6 23 54 5 31 1 Table A-1.2. (continued) Sample Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd U MA62 17 8 32 376 652 49 305 223 45 15.7 21 44 135 8 29 56 4 34 1 MA14b 7 4 30 308 725 53 317 193 36 13.8 22 15 122 10 25 56 5 30 2 MA63 13 8 33 341 565 41 361 195 37 13.4 22 33 122 8 23 51 4 28 1 MA64 22 33 34 346 531 35 352 198 38 14.0 22 74 119 8 19 51 4 30 4 MA65 22 16 35 367 588 45 317 230 43 15.5 21 105 133 8 27 56 3 31 4 MA66 16 9 33 391 645 46 298 223 43 15.1 22 54 136 7 25 56 4 30 3 MA67 49 114 35 341 402 25 350 180 34 13.6 21 88 119 5 16 42 2 23 2 MA68 76 79 36 372 330 12 342 192 42 14.2 21 162 128 4 19 44 3 27 1 MA69 8 7 32 348 712 58 323 183 35 14.0 22 16 128 8 27 52 5 29 2 MA70 13 10 34 372 560 42 346 220 39 15.1 23 36 133 7 25 50 4 31 2 MA71 17 10 33 339 675 31 306 215 43 15.4 20 46 134 8 26 52 3 31 1 MA72 8 4 30 359 715 51 327 214 40 15.4 23 18 129 10 27 54 6 30 2

65 Table A-1.3. Normalized major elements (wt%) for samples within the field area but not collected by this study. † Sample Unit name SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Total HSI-02 Priest Rapids 49.48 3.22 13.58 14.47 0.25 4.84 9.50 2.79 1.05 0.82 100 HSI-04 Grande Ronde R2 51.79 1.66 14.78 10.67 0.18 6.85 10.77 2.44 0.63 0.23 100 HSI-05 Elephant Mountain 51.19 3.57 13.03 15.02 0.22 4.32 8.54 2.36 1.21 0.56 100 HSI-06 Elephant Mountain 51.68 3.71 13.02 15.35 0.21 3.53 8.19 2.42 1.31 0.59 100 HSI-07 Elephant Mountain 51.54 3.62 12.98 15.04 0.22 4.16 8.32 2.29 1.27 0.57 100 HSI-08 Weissenfels Ridge 54.28 2.54 13.38 13.20 0.27 3.04 6.96 3.25 2.14 0.95 100 HSI-12B Priest Rapids 50.09 3.29 13.81 13.74 0.23 4.88 9.50 2.64 1.04 0.79 100 HSI-14 Priest Rapids 51.80 1.64 15.28 10.50 0.19 6.53 10.01 2.51 1.20 0.34 100 HSI-19 Pomona 53.03 1.66 14.79 11.49 0.18 5.32 9.62 2.78 0.86 0.27 100 HSI-33 Elephant Mountain 51.61 3.68 13.02 14.70 0.20 3.80 8.60 2.49 1.32 0.58 100 HSI-46A Grande Ronde R2 53.74 1.84 14.38 12.01 0.21 4.73 8.93 2.94 0.93 0.29 100 HSI-54 Grande Ronde R2 53.60 1.73 14.47 11.44 0.21 5.08 9.25 2.74 1.20 0.28 100 CLA-9A Asotin/Wilbur Creek 50.40 1.43 16.32 9.93 0.15 7.63 11.17 2.29 0.50 0.17 100

66 CLA-10 Weissenfels Ridge 50.17 3.18 13.54 13.58 0.22 5.19 9.97 2.49 0.95 0.71 100 CLA-11 Asotin/Wilbur Creek 56.55 2.09 15.91 8.77 0.12 2.58 8.64 3.01 1.80 0.54 100 D08-1 Grande Ronde R2 52.99 2.24 13.56 14.11 0.24 4.68 7.88 2.43 1.53 0.35 100 D08-2 Grande Ronde R2 53.77 2.21 13.69 13.21 0.20 4.05 7.81 3.12 1.55 0.39 100 D08-3 Grande Ronde R2 54.21 2.23 13.69 13.18 0.20 4.05 7.82 2.51 1.74 0.38 100 D08-4 Grande Ronde R2 53.91 2.21 13.78 13.05 0.20 4.02 7.93 3.07 1.44 0.39 100 D08-5 Grande Ronde R2 54.16 2.22 13.63 13.16 0.21 3.87 7.79 2.90 1.65 0.40 100 D08-6 Grande Ronde R2 53.39 2.21 13.92 13.17 0.22 4.03 8.14 3.05 1.47 0.39 100 D08-7 Grande Ronde R2 54.63 2.18 13.69 12.96 0.16 3.87 7.21 3.30 1.58 0.42 100 D08-9 Grande Ronde R2 53.89 2.21 13.74 13.14 0.21 3.92 7.83 3.10 1.55 0.40 100 D08-10 Grande Ronde R2 55.50 2.08 13.94 11.81 0.19 3.72 7.53 3.15 1.72 0.37 100 D08-11 Grande Ronde R2 55.29 2.08 13.81 12.32 0.19 3.68 7.47 3.07 1.74 0.35 100 D08-12 Grande Ronde R2 55.20 2.08 13.81 12.34 0.20 3.69 7.37 3.18 1.78 0.36 100 D08-13 Grande Ronde R2 53.54 2.20 13.69 13.35 0.20 3.92 7.98 3.07 1.66 0.39 100 D08-14 Grande Ronde R2 52.99 2.20 13.81 13.34 0.20 4.23 8.29 3.15 1.40 0.37 100 D08-15 Grande Ronde R2 53.75 2.19 13.66 13.23 0.20 3.92 8.00 3.09 1.59 0.39 100 D08-16 Grande Ronde R2 53.80 2.24 13.71 13.25 0.20 3.94 7.84 3.03 1.61 0.39 100 Table A-1.3. (continued) † Sample Unit name SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Total D08-17 Grande Ronde R2 54.11 2.20 13.65 13.23 0.21 3.87 7.75 2.83 1.74 0.41 100 10-DC1 Grande Ronde R2 55.88 1.94 14.24 10.70 0.25 3.91 7.89 3.23 1.62 0.34 100 † Samples beginning with HSI and CLA were collected by Hooper et al. (1985) and the rest were collected by Reidel (2010,

67 Table A-1.4. Unnormalized trace elements (ppm) from within the field area but not collected by this study. Sample† Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd U HSI-02 47 95 36 362 581 23 303 194 47 19 23 49 148 6 27 60 2 38 1 HSI-04 53 105 35 281 240 16 236 138 30 13 20 54 98 4 20 36 1 21 1 HSI-05 19 18 32 409 524 33 236 269 51 28 23 27 157 9 34 79 5 39 2 HSI-06 21 21 33 405 630 36 242 281 52 27 23 28 157 10 35 75 4 39 2 HSI-07 20 21 32 405 520 34 236 276 52 27 22 27 160 10 34 74 6 39 3 HSI-08 7 7 36 188 1026 48 342 268 61 22 21 26 171 11 39 82 4 49 2 HSI-12B 45 99 39 366 889 29 309 195 46 17 21 51 154 6 28 60 3 36 0 HSI-14 93 182 30 265 529 22 266 176 33 14 19 67 105 8 28 53 3 27 0 HSI-19 38 122 38 320 493 25 331 142 32 12 20 75 113 7 17 45 2 26 3 HSI-33 17 21 33 418 529 34 243 285 52 28 22 27 160 8 38 78 5 41 1 HSI-46A 18 43 36 321 510 24 318 159 33 12 20 30 116 6 18 40 4 22 2 HSI-54 28 88 38 334 455 28 316 146 32 12 19 55 112 5 17 35 2 20 1 CLA-9A 127 298 32 255 266 10 249 108 29 9 19 92 83 5 17 27 1 22 2

68 CLA-10 37 80 42 391 718 17 256 284 53 29 22 55 157 8 45 96 1 48 2 CLA-11 16 50 30 268 961 45 319 277 49 19 21 31 147 12 48 89 7 43 2 D08-1 29 19 35 429 491 32 274 174 36 14 20 46 129 6 22 45 5 26 0 D08-2 28 20 37 434 628 51 332 177 37 14 20 47 129 9 22 50 4 29 1 D08-3 27 18 35 435 588 41 315 169 35 12 21 45 125 8 20 44 3 26 3 D08-4 29 18 36 435 591 46 331 175 35 14 21 49 128 7 27 44 3 27 3 D08-5 24 17 35 429 603 39 315 174 35 14 21 46 127 8 25 54 3 29 1 D08-6 26 21 36 439 611 34 338 175 36 14 21 47 130 8 24 53 4 28 2 D08-7 22 15 33 403 648 41 316 183 37 15 21 45 127 9 23 54 4 30 3 D08-9 23 17 35 429 635 59 333 179 37 14 22 46 133 8 28 50 4 27 2 D08-10 13 9 33 349 699 37 332 181 37 13 21 24 128 8 24 52 6 28 3 D08-11 13 8 33 351 650 47 325 178 35 13 21 23 124 7 26 50 5 26 2 D08-12 12 8 33 356 667 59 324 180 35 14 20 23 126 8 26 58 4 30 1 D08-13 28 18 35 433 636 48 334 174 35 13 20 39 126 7 20 47 4 28 0 D08-14 28 18 36 452 601 31 336 167 35 12 20 50 122 7 22 52 3 28 2 D08-15 24 17 36 433 596 63 332 174 36 12 21 45 128 7 22 55 3 29 0 D08-16 27 15 35 440 602 69 333 176 36 13 22 48 126 6 23 50 3 27 2 Table A-1.4. (continued) Sample† Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd U D08-17 26 18 36 431 634 44 323 175 36 14 21 47 127 6 21 50 4 28 0 10-DC1 16 14 35 347 707 45 334 173 36 11 21 30 121 8 23 54 4 29 2 † Samples beginning with HSI and CLA were collected by Hooper et al. (1985) and the rest were collected by Reidel (2010,

69 APPENDIX TWO

STRUCTURAL DATA

Table A-2.1. Flow plane attitudes. Table A-2.2. Slickenside data. Domain Strike Azimuth of Dip Dip Domain Strike Azimuth of Dip Dip Rake Quadrant (°) (°) West 217 307 4 East 240 330 64 67 SW West 040 130 14 East 030 120 25 45 SW West 037 127 3 East 000 090 58 5 SW West 060 150 46 East 062 152 90 8 NE West 060 150 55 East 194 284 80 20 SW West 060 150 20 East 070 160 33 76 SW West 060 150 27 *East 045 135 80 10 N West 060 150 46 *East 180 270 75 40 S West 035 125 16 *East 035 125 65 0 N/A East 073 163 53 *East 215 305 60 0 N/A East 057 147 26 * Data aquired by Peter hooper (unpublished data). East 059 149 39 East 055 145 45 East 060 150 45 East 050 140 30 Table A-2.3. Fault plane attitudes. East 047 137 37 Domain Strike Azimuth of Dip Dip East 250 340 7 (°) East 250 340 16 East 290 020 56 East 250 340 5 East 030 120 27 East 080 170 28 East 080 170 70 East 087 177 45 East 260 350 40 East 070 160 36 West 037 127 70 East 070 160 27 East 070 160 40 East 070 160 43 East 070 160 7 East 070 160 3 East 080 170 4 East 080 170 35 East 070 160 25 East 080 170 5 East 070 160 30 East 070 160 30 East 070 160 26 East 070 160 42 East 070 160 45 East 070 160 35 East 080 170 10

71 APPENDIX THREE

PARAMETERS AND EXPLANATION OF PARAMETER VALUES USED

FOR PALEOSTRESS INVERSION IN T-TECTO 3.0

Table A-3.1. Parameters and statistics: results of the Gauss method for heterogenious fault-slip data. Direction of principal strain axes: Controlling parameters in the inversion method:

ε1: 351 / 12 Type of analysis: 3

ε2: 145 / 76 Parameter q1: 60 (Deg)

ε3: 260 / 6 Parameter q2: 20 (Deg) Maximum misfit angle: 60 (Deg) Parameter D = 0.5 Standard deviation of angular misfit: 22 (Deg) Parameter s: 30 (Deg) Microrotation parameter C = 0 Parameter d: 60 (Deg) Andersonian regimes: Yes Relative values of principal strains: Density of grid points: Low 0.46 : -0.06 : -0.4 Permutation of strain axes: No Micropolar kinematic method: No Relative values of principal stresses: Symmetrical stress (b=0): Yes 0.93 : 0.41 : 0.07 Asymmetrical stress (b< >0): No Stress parameter: 2 Parameters in the constitutive law: Mohr-selection method: No Parameter p: 0.47 Weighting factors used in analysis: No Parameter a: 1 User defined strain axis: No Parameter b: 0

Faults with multiple-lineation systems: Total number: 0

1 relative age index (max.=1)

Compatible systems: N= 0 [ N (%)= 0

73 Table A-3.2. Parameters and statistics: results of the Gauss method for western domain. Direction of principal strain axes: Controlling parameters in the inversion method:

ε1: 128 / 12 Type of analysis: 3

ε2: 218 / 2 Parameter q1: 60 (Deg)

ε3: 317 / 77 Parameter q2: 20 (Deg) Maximum misfit angle: 30 (Deg) Parameter D = 0.4 Standard deviation of angular misfit: 9 (Deg) Parameter s: 9 (Deg) Microrotation parameter C = 0 Parameter d: 30 (Deg) Andersonian regimes: Yes Relative values of principal strains: Density of grid points: Low 0.47 : -0.06 : -0.41 Permutation of strain axes: No Micropolar kinematic method: No Relative values of principal stresses: Symmetrical stress (b=0): Yes 0.95 : 0.42 : 0.07 Asymmetrical stress (b< >0): No Stress parameter: 2 Parameters in the constitutive law: Mohr-selection method: No Parameter p: 0.48 Weighting factors used in analysis: No Parameter a: 1 User defined strain axis: No Parameter b: 0

Faults with multiple-lineation systems: Total number: 0

0 relative age index (max.=1)

Compatible systems: N= 0 [ N (%)= 0

74 Table A-3.3. Parameters and statistics: results of the Gauss method for eastern domain. Direction of principal strain axes: Controlling parameters in the inversion method:

ε1: 174 / 2 Type of analysis: 3

ε2: 283 / 84 Parameter q1: 60 (Deg)

ε3: 084 / 6 Parameter q2: 20 (Deg) Maximum misfit angle: 30 (Deg) Parameter D = 0.4 Standard deviation of angular misfit: 9 (Deg) Parameter s: 15 (Deg) Microrotation parameter C = 0 Parameter d: 30 (Deg) Andersonian regimes: Yes Relative values of principal strains: Density of grid points: Low 0.41 : 0 : -0.41 Permutation of strain axes: No Micropolar kinematic method: No Relative values of principal stresses: Symmetrical stress (b=0): Yes 0.88 : 0.47 : 0.06 Asymmetrical stress (b< >0): No Stress parameter: 2 Parameters in the constitutive law: Mohr-selection method: No Parameter p: 0.47 Weighting factors used in analysis: No Parameter a: 1 User defined strain axis: No Parameter b: 0

Faults with multiple-lineation systems: Total number: 0

0 relative age index (max.=1)

Compatible systems: N= 0 [ N (%)= 0

75 Table A-3.4. Explanation of parameters in T-TECTO 3.0 Parameter s This is the dispersion parameter of the distribution of angular misfits between the predicted and actual direction of movement along the faults. In the case the stress/strain field at the time of faulting was highly inhomogeneous, choose a large value of this parameter, for example, 30 degrees or higher.

Parameter d This is the Δ parameter in the Gauss method. This parameter defines the threshold for the value of compatibility measure for some fault-slip datum to be compatible with a given stress/strain tensor or not. In the case the stress/strain field at the time of faulting was highly inhomogeneous, choose a large value of this parameter, for example, 60 degrees or higher.

Type of analysis 2 – the polarity of the fault-slip data will be used only for faults of C reliability. 3 – the polarity of the fault-slip data will be used for faults of P and C reliability. 4 – the polarity of the fault-slip data will be used for faults of P, S and C reliability.

Parameter q1 This is the slope φ1 of the tangent of the largest Mohr circle on the Mohr diagram.

Parameter q2 This is the angle φ2 of friction along the faults.

Stress parameter This parameter defines the importance of friction along the faults. Its default value is 2 ( = 20 in the program), but you can choose a different value. Maximum value is 50 ( = 500 in the program). Use a high value of this parameter to find mechanically acceptable results.

Density of grid points The grid points on the stereographic projection define possible orientations of stress/kinematic axes. High or low density of the grid points influences the accuracy of the solutions. Low density is used by default. However, if needed, also a high density may be selected, leading to longer time of calculation of the optimal solution.

Andersonian regimes When Andersonian stress regimes are expected, the “Andersonian regimes” should be set to "Yes" in order to perform as fast calculation of the results as possible. However, this option is only reasonable when analyzing low number of fault-slip data.

76 Table A-3.4. (continued) Symmetrical stress (b=0) If one is to find mechanically acceptable results, the “Symmetrical stress” should be set to "Yes." In this case, the ratio between the shear and the normal stress along the faults is considered in the inverse method as well. It is supposed that the results should be in agreement with the Amontons's frictional law.

Asymmetrical Stress (b< >0) This function is used in the Cosserat method. The user should specify the values of the three parameters in the constitutive law; p, a and b. The value of the parameter p is calculated by the program itself, however, the values of a and b should be defined by the user. Since a + b = 1, only the b value should be set. Use the following options: b = 0 for symmetrical stress tensor, b > 0 for asymmetrical stress tensor, b = 1 for antisymmetrical stress tensor. For all other information regarding the T-TECTO 3.0 software, see Žalohar and Vrabec (2007. 2008, 2010).

APPENDIX FOUR

REGIONAL STRATIGRAPHIC NOMENCLATURE AND ISOTOPIC

Isotopic Ages Magnetic Series Group Formation Member (Ma) Polarity Lower Monumental 6.0 N Ice-Harbor 8.5 N, R, N

Buford R UPPER SADDLE Elephant Mountain 10.5 N, T Pomona 12.0 R MOUNTAINS Esquatzel N BASALT Weissenfels Ridge N Asotin 13.0 N Wilbur Creek N Umatilla N Priest Rapids 14.5 R WANAPUM Roza T, R Shumaker Creek N MIDDLE BASALT † Frenchman Springs 15.12 ± 0.38 N, E Eckler Mountain †15.70 ± 0.34 N Sentinel Bluffs Winter Water Umtanum * Fields Spring 16.27-15.98 N2 Kendall Monument Ortley Slack Canyon

MIOCENE Meyer Ridge

Grouse Creek * 16.30-16.27 R2 GRANDE Wapshilla Ridge Mt. Horrible RONDE Cold Springs Ridge BASALT COLUMBIA RIVER COLUMBIA BASALT GROUP Hoskin Gulch * China Creek 16.47-16.30 N1 Frye Point Downy Gulch

LOWER Center Creek Skeleton Creek

Rogersburg * 16.54-16.47 R1 Teepee Butte Birch Creek Buckhorn Springs

IMNAHA * 16.72-16.54 N0 BASALT

Figure A-4.1. Stratigraphic nomenclature for the Columbia River Basalt Group. Modified after Reidel et al. (1989). † indicates ages from Barry et al. (2010). * indicates ages from Jarboe et al. (2008).

AGESAPPENDIX FIVE

TECHNIQUES USED WITH FIELD FLUXGATE MAGNETOMETER

Because techniques using a fluxgate magnetometer seem to be scarce in the literature, methods of increasing data reliability are documented as follows: orient the sample in situ with a pen indicating magnetic north; aim the north-arrow at the probe, tilt the sample approximately

(top toward you) to adjust for angle of inclination for earth’s dipole field at the time of crystallization, and send the sample toward the probe. After measuring the geographically oriented sample, make sure that the sample is actually oriented correctly by making small adjustments in how you are holding the sample up to the probe. If the needle remains constant, rotate the sample (i.e. send the geographic south end of the sample toward the probe) to make sure the needle changes. If the needle does not remain steady while making small adjustments to orientation or if the needle does not change while sending the geographic south end of the sample toward the probe, discard the sample completely. It is important to note that this is far from a user’s manual however some of these techniques may not be intuitive. Also, based on the cooling process of flow-top breccias, it is easy to make the mistake that as the solid clasts are swept up in the lava the final orientation of the magnetic minerals will be random, rendering those flow-tops useless for paleomagnetic analysis with a fluxgate magnetometer.

Because the Curie point is approximately for magnetite ( e ) and for hematite

( e ), which is well below the temperature that the lavas crystallize (typically - ), the clasts in the flow-top breccias are in situ by the time the magnetic minerals align with the earth’s magnetic field (Fowler, 2005; Reidel, 2010, personal communication).