Controls on Fault Geometry During Early Stages of Extension

Home , Brothers Fault Zone, Fault block

CONTROLS ON FAULT GEOMETRY DURING EARLY STAGES OF EXTENSION

IN THE LARKSPUR HILLS, NORTHWEST BASIN AND RANGE

______

A Thesis

Presented to

The Graduate Faculty

Central Washington University

______

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Geology

______

Diana Jean Strickley

May 2014

CENTRAL WASHINGTON UNIVERSITY

Graduate Studies

We hereby approve the thesis of

Diana Jean Strickley

Candidate for the degree of Master of Science

APPROVED FOR THE GRADUATE FACULTY

______Dr. Anne Egger, Committee Chair

______Dr. Jeff Lee

______Dr. Walter Szeliga

______Dean of Graduate Studies

ABSTRACT

CONTROLS ON FAULT GEOMETRY DURING EARLY STAGES OF EXTENSION

IN THE LARKSPUR HILLS, NORTHWEST BASIN AND RANGE

Diana Jean Strickley

May 2014

Detailed analyses of normal faults in the Larkspur Hills, CA-NV, northwest Basin and Range, offer insight into factors controlling normal fault initiation, growth, and distribution. N-trending faults in the southern portion of the study area share trends of major range-bounding structures and Pliocene linear volcanic vents; in contrast, NNW- and NNE- trending faults dominate further north and into south-central Oregon. Stress analyses and comparison with experimental and field data suggest that preexisting structures control faults in the northern Larkspur Hills, while faults form perpendicular to 3 in the southern hills. The change in fault orientations is abrupt, occurring across a major NNE fault. A regional transition is thus captured within the Larkspur Hills, suggesting they overlie a structural boundary at depth that separates isotropic crust from crust with a pre-existing NW-trending fabric. This has implications for better understanding of local and regional structural controls on subsurface hydrothermal flow paths.

iii

TABLE OF CONTENTS

Chapter Page

I INTRODUCTION ...... 1

II JOURNAL ARTICLE ...... 7

Introduction ...... 8 Background ...... 14 Methods ...... 23 Results ...... 27 Discussion ...... 44 Conclusions ...... 57 Acknowledgements ...... 59 References ...... 60

LIST OF FIGURES

Figure Page Page

1 Map of the Basin and Range. Simplified from Colgan (2013) ...... 9

2 Map of the northwest Basin and Range, with current and previous study areas noted ...... 10

3 Map of Larkspur Hills, northwest Basin and Range ...... 13

4 Block diagrams and associated map views illustrating linking of en echelon faults (4a & 4b) and the similarity in surface traces for branching faults vs. individual faults (4c) ...... 16

5 Close-up map of domain I. Legend for domain close-up map follows (5a) ...... 20

6 Rose diagrams showing fault segment orientations for each domain ...... 30

7 Graphs of one prominent fault per domain and domain boundary. Circles indicate maximum scarp height and azimuth for each segment, plotted at the midpoint of each segment ...... 31

8 Close-up map of domain II ...... 32

9 Close-up map of domains III and IV ...... 34

10 Close-up map of domain V ...... 37

11 Rose diagrams showing fault segment orientations for domain boundary faults ...... 39

12 Cross-sections A-A’ through E-E’ corresponding to section lines shown on Larkspur Hills map ...... 42

13 Comparison of Andersonian and orthorhombic fault geometry ...... 45

LIST OF FIGURES (CONTINUED)

Figure Page

14 Simplified fault maps comparing results of sandbox models of Bellahsen and Daniel (2005) and fault traces in the study area ...... 48

15 Simplified map showing drill hole locations (grey circles) and hot springs (white circles) relative to faults ...... 5

CHAPTER I

INTRODUCTION

A number of factors influence the formation and evolution of normal faults, including: the orientation of principal stresses (Anderson, 1951); changes in stress orientations over time (e.g. Henza et al., 2011; Giba et al., 2012); heterogeneities in the crust, such as differences in lithology, layering, or grain size (Ackermann et al., 2001;

Crider and Peacock, 2004; Schopfer et al., 2006); and pre-existing structures, e.g. veins, joints, and faults (Martel et al., 1988; Acocella et al., 2003; Crider and Peacock, 2004;

However, it can be difficult to determine the role of pre-existing structures during the early development stages in other extensional provinces, such as the North

American Basin and Range, where long-term and recent normal faulting can obscure earlier structures. The Basin and Range province is characterized by approximately N-S- striking, normal-fault-bound ranges up to 100 km in length, spaced ~30 km apart, and separated by similarly-scaled linear basins (Stewart, 1998). Sequential reconstructions of

1 2

McQuarrie and Wernicke (2005) indicate a total of ~235 km of extension in the Basin and Range province, with the amount of extension ranging from 200% in the central region to 50% in the northern region and initiating 15-25 Ma. Extension is most recent and continues along the eastern and western margins.

As in the rest of the Basin and Range, most of the extension in the northwest

Basin and Range is accommodated by major-offset (vertical offset ≥ 300 m) faults striking N to NNE (Pezzopane and Weldon, 1993), of which one of the more prominent is the Surprise Valley fault in northeastern California. Extension is relatively recent, with evidence that extension along the Surprise Valley fault occurred in two phases, the first between 14 and 8 Ma and the second phase of rapid uplift beginning at ~3 Ma (Colgan et al., 2008; Egger and Miller, 2011) and continuing until the present (Personius et al.,

2009). Several studies suggest that the percent of E-W-directed extension decreases northward along the margin, with a maximum of approximately 15% extension since the middle Miocene (Pezzopane and Weldon, 1993; Scarberry et al., 2010; Egger and Miller,

2011; Trench et al., 2012). Restored cross-sections indicate 6-7 km of extension across the 86 km-long Surprise Valley fault, which accommodates most of the 12-15% extension across the Warner Range region (Egger and Miller, 2011). Extension decreases northwards, from 3% along the Abert Rim fault (Scarberry et al., 2010) to 0.01% at the northern termination of the Hart Mountain fault system (Trench et al., 2012), and only

0.004% in the Brothers Fault Zone. Due to the comparatively recent onset and lower magnitude of extension, as well as the presence of late Miocene to Pliocene volcanic

3 rocks, the northwestern Basin and Range (NWBR) preserves the early stages of extension, thus is an excellent place to evaluate models for fault initiation and growth in extensional regimes.

Basin and Range extension and unextended area to the north (Lawrence, 1976). This interpretation was based on mapping lineations from satellite images and not measured offsets or fault studies in the field. Later field investigations of Pezzopane and Weldon

(1993) found that faults throughout the region were dominantly dip-slip, with negligible to minor oblique motion. Trench et al. (2012), working in southeast Oregon, also found abundant evidence for dip-slip motion on the NW-oriented faults, including those within the BFZ, and sparse evidence of minor strike-slip motion. Scarberry et al. (2010) observe that the NW-striking faults are more numerous along northern tips of NNE-striking faults throughout the region, so concluded that the NW-striking set in the Abert Rim area formed as result of splays along the northward propagating tip of the Abert Rim fault exploiting a pre-existing NW-oriented fabric. Trench et al. (2012) reached a similar conclusion by observing cross-cutting relationships between the two fault sets in

Oregon, where NNE-striking faults cut the NW-striking set in the south, but the two sets are mutually cross-cutting in the north. Trench et al. (2012) thus concluded that NW- striking faults first developed along the fault tips of NNE-striking faults, but the NNE- striking faults ultimately grew through and cross-cut the NW-striking faults; however, they also point out that a lack of major NNE-striking faults in or proximal to the Brothers

Fault Zone indicates that the NW-striking faults within that zone developed independently and were not influenced by NNE-striking structures. Scarberry et al.

(2010) suggest that in the Abert Rim area, the similarity between orientations of the two distinct middle to late Miocene (16-5 Ma) fault sets and those of early to middle

Miocene (23.8-16.4 Ma) dikes that trend 5-10° and 290-315° indicate that the development of both fault sets was influenced by a pre-existing fabric .

Unlike the rest of the Basin and Range, the NWBR also hosts a second set of pervasive, minor-offset (vertical offset < and often << 150 m) normal faults that strike

NW (Pezzopane and Weldon, 1993; Scarberry et al., 2010; Trench et al., 2012). The origin and nature of these structures is unknown; however, cross-cutting relationships and fault density along the larger-offset NE-striking structures (Scarberry et al., 2010) indicates that the NW-striking set may develop as a result of NNE-striking fault growth, with fault-tip splays reactivating a pre-existing NW-striking structures, similar to the formation of fault tip dilatational (horsetail) fractures (Trench et al., 2012).

The focus of this study is the Larkspur Hills in northeastern California, which is overlain by a series of late Miocene to Pliocene basalt flows, with average thicknesses of

~1-5 m per flow, which are interbedded with tuffaceous sediments, and cut by normal faults with strikes ranging from NNW to NNE. Total extension decreases northward from

~4-8% to ≤2%, thus faults are in various early stages of development. Over this relatively small area of about 550 km2, there are significant differences in fault orientations, from dominantly N-striking in the south to NNE- and NNW-striking in the north. This observation leads to the following questions: how do the local fault orientations relate to the regional stress field over time, and what controls the differences in orientations from south to north across the field area?

Better characterization of controlling structures in the Larkspur Hills may provide an analogue for nearby buried intra-basin faults which control locations of hot springs throughout the valley. Both the Surprise Valley and the southern Larkspur Hills have been the object of geothermal exploration (Barker et al., 2005; Benoit et al., 2005;

Bertani, 2007) and studies to define structural features in the subsurface (Egger et al.,

2010; Glen et al., 2013). Local studies (Benoit et al., 2005; Egger, et al. 2014) point to

6 the importance of fault controls on subsurface fluid circulation and the surface location of hot springs in the Surprise Valley (Glen et al., 2013); this is supported by other studies that emphasize the importance of fault controls in geothermal systems (Curewitz and

Karson, 1997; Faulds et al., 2010). While hot springs occur on both sides of the basin in the Surprise Valley, along the east side they do not appear to be located along faults; however, faults are known to be a primary control on fluid flow (Caine et al., 1996;

Rowland and Sibson, 2004; Faulds et al., 2010). Faults expressed at the surface in the adjacent Larkspur Hills may serve as an analog for faults buried in Surprise Valley, and may lend insight into their geometry and controlling structures.

CHAPTER II

JOURNAL ARTICLE

Controls on fault geometry during early stages of extension in the Larkspur Hills, northwest Basin and Range

1. Introduction

Crider and Peacock, 2004; Schopfer et al., 2006); and pre-existing structures, e.g. veins, joints, and faults (Martel et al., 1988; Acocella et al., 2003; Crider and Peacock, 2004;

Bellahsen and Daniel, 2005; Blenkinsop, 2008). Pre-existing structures may promote faulting, especially in the early stages of extension, by weakening the crust (Buck, 2009, and references therein). Combined geophysical and geological studies in young active rifts, such as the East African Rift system, are often able to discern the relative influences of modern stress orientations vs. pre-existing structures (e.g. Morely, 1995, and references therein; Modisi et al., 2000). However, it can be difficult to determine the role of pre-existing structures during the early development stages in other extensional provinces, such as the North American Basin and Range (Fig. 1), where long- term and recent normal faulting can obscure earlier structures.

The Basin and Range province is characterized by approximately N-S-striking, normal-fault-bound ranges up to 100 km in length, spaced ~30 km apart, and separated

Fig. 1. Map of the Basin and Range. Simplified from Colgan (2013).

by similarly-scaled linear basins (Stewart, 1998). Sequential reconstructions of

McQuarrie and Wernicke (2005) indicate a total of ~235 km of extension in the Basin and Range province, with the percent extension ranging from 200% in the central region to 50% in the northern region, initiating from 15-25 Ma, with the most recent extension along the eastern and western margins.

As in the rest of the Basin and Range, most of the extension in the northwest

Fig. 2. Map of the northwest Basin and Range, with current and previous study areas noted. Quaternary faults (USGS, 2006) that are discussed in the text are shown in bold and edited to show slip sense. Locations of hot springs in the Surprise Valley are shown with white dots.

11 between 14 and 8 Ma and the second phase of rapid uplift beginning at ~3 Ma (Colgan et al., 2008) and continuing until the present (Personius et al., 2009). Several studies suggest that the percent of E-W-directed extension decreases northward along the margin, with a maximum of approximately 15% extension since the middle Miocene

(Pezzopane and Weldon, 1993; Scarberry et al., 2010; Egger and Miller, 2011; Trench et al., 2012). Restored cross-sections indicate 6-7 km of extension across the 86 km-long

Surprise Valley fault, which accommodates most of the 12-15% extension across the

Warner Range region (Fig. 2) (Egger and Miller, 2011). Percent extension along single faults or fault zones decreases northwards, from 3% along the Abert Rim fault

(Scarberry et al., 2010) to 0.01% at the northern termination of the Hart Mountain fault system (Trench et al., 2012), and only 0.004% in the Brothers Fault Zone (Fig. 2). Due to the low magnitude of extension, as well as the presence of late Miocene to Pliocene volcanic rocks, the northwestern Basin and Range (NWBR) preserves the early stages of extension, thus is an excellent place to evaluate models for fault initiation and growth in extensional regimes.

Unlike the rest of the Basin and Range, the NWBR hosts a second set of pervasive, minor-offset (vertical offset < and often << 150 m) faults that strike NW; while NW-striking faults are not absent in the Basin and Range, in the NWBR they comprise a distinct set of faults that dominate in number, while the NE-striking faults are fewer but have the most offset (Fig. 2) (Pezzopane and Weldon, 1993; Scarberry et al., 2010; Trench et al., 2012). The origin and role that these NW-striking structures play

12 in accommodating strain is unclear; however, cross-cutting relationships and fault density along the larger-offset NE-striking structures (Scarberry et al., 2010) indicate that the NW-striking set may develop as a result of NNE-striking fault growth reactivating pre-existing NW-striking structures, similar to the formation of fault tip dilatational (horsetail) fractures (Trench et al., 2012).

The focus of this study is the Larkspur Hills in northeastern California (Figs. 2 and

3), which are made up of a series of late Miocene to Pliocene basalt flows interbedded with tuffaceous sediments cut by normal faults with strikes ranging from NNW to NNE.

The percent extension decreases northward from ~4-8% to ≤2%, thus normal faults are in various early stages of development. In addition, NW-oriented faults are absent in the southern Larkspur Hills but become dominant in the north (Fig. 3).

This study involves detailed mapping of faults and linear volcanic features, and numerical and graphical analyses of fault segmentation, fault and segment orientations, horizontal offset, and scarp heights, as well as analyses of cross-cutting or truncating relationships. The results of these analyses were used to determine possible controls, such as extension direction, change in extension direction, pre-existing structures, or heterogeneities (e.g. lithological differences), on the growth and development of normal faults during early stages of extension in this area. Better characterization of controlling structures in this region is of particular interest because the northwest Basin and Range is an area of active geothermal exploration, and fault geometries and interactions (e.g. linkage or overlap of faults) are a primary control on the locations and flow paths of

Fig. 3. Map of Larkspur Hills, northwest Basin and Range. Roman numerals denote fault domains. Buried basin faults from Egger and Miller (2011) are inferred from potential field data.

14 hydrothermal fluids in the subsurface (Caine et al., 1996; Rowland and Sibson, 2004;

Faulds et al., 2010).

2. Background

2.1. Normal fault initiation and growth

In an extensional environment, as finite strain accumulates over time it changes from being diffuse to more localized (e.g. Meyer et al. 2002). Some faults become inactive while others connect and evolve into larger, through-going structures

(Gawthorpe et al., 2003; Crider and Peacock, 2004; Moriya et al., 2005). As faults evolve, they undergo changes in growth mechanisms: isolated faults first grow through tip-line propagation and mode I (open) fracturing (Reches and Lockner, 1994; White and Crider,

2006; Rowland et al., 2007; Herman, 2009). As faults overlap, they begin to link (Fig. 4); linking is the dominant factor in the development of larger faults rather than growth along tip-lines (Ferrill et al., 1999; Mansfield and Cartwright, 2001; Crider and Peacock,

2004).

The linking process commonly involves the formation of a relay ramp within the zone of overlap between segments (Fig. 4a) (Peacock, 2002); further growth of the faults causes the ramp to fracture, and ultimately break (Soliva and Benedicto, 2004).

Initial breaching of the ramp can occur anywhere along its vertical extent, so faults can link at depth even when the ramp is intact at the surface (Trudgill and Cartwright, 1994;

Peacock and Parfitt, 2002; Long and Imber, 2012). Once the ramp is fully breached, the

15 two original faults connect to become segments of one longer fault, a process described in detail by Peacock and Sanderson (1994).

The linking process can also involve fault segments propagating towards each other, with tension at the tip lines causing the segment tips to at first diverge, then converge, and ultimately intersect the other fault trace, resulting in a hook shape in map view (Fig. 4b) (Pollard and Aydin, 1984; Acocella et al., 2000). This type of linking process also involves a ramp developing in the zone of overlap; however, the faults link directly rather than through breaching of a ramp (Acocella et al. 2000).

Pre-existing or precursory structures can also influence the orientation of normal faults and linking structures as they initiate and evolve (Giba et al., 2012; Bellahsen and

Daniel, 2005; Hus et al., 2005; Crider and Peacock, 2004; McClay et al., 2002). Crider and

Peacock (2004) define precursory structures as early structures formed in the same stress field as current faulting, and pre-existing structures as those which formed in a stress field unrelated to current faulting; later extensional events can reactivate such structures at depth, and the rupturing of a large structure through subsequent cover may appear as segmented normal faults at the surface (Giba et al., 2012) or as non- optimally oriented normal faults (Morely et al., 2004).

Without three-dimensional data at depth, it can be difficult to determine how large faults evolved in mature regions, as resulting surface traces can be the same for different growth mechanisms (Fig. 4c). For example, high-resolution 3D seismic data from the Taranaki Basin, New Zealand (Giba et al., 2012) and the Bonaparte Basin, NW

Australia (Long and Imber, 2012) have shown that faults have complex subsurface geometries associated with fault growth, linkage, and reactivation. In the absence of

Fig. 4. Block diagrams and associated map views illustrating linking of en echelon faults (4a & 4b) and the similarity in surface traces for branching faults vs individual faults (4c). Diagrams adapted from Ferrill et al. (1999), Acocella et al. (2000), and Giba et al. (2012).

17 seismic data, analogue and numerical models can provide insights into controls on fault geometries during initial stages of extension (e.g. Crider, 2001; McClay et al., 2002;

Schopfer et al., 2006; Kaven and Martel, 2007; Holland et al., 2011).

2.2. Field studies of the northwest Basin and Range

Most of the extension in the northwest Basin and Range is accommodated along large-offset N- to NNE-striking normal faults. In addition to major offset NNE-striking faults, the region hosts a pervasive set of NW to NNW-striking faults and fractures with little to no offset (Donath, 1962; Pezzopane and Weldon, 1993). An early interpretation of the NW-oriented faults, particularly in the Brothers Fault zone (BFZ) (Fig. 2), was that they were right-lateral strike-slip forming a transfer zone (Faulds and Varga, 1998) between the northern edge of Basin and Range extension and unextended area to the north (Lawrence, 1976). This interpretation was based on mapping lineations from satellite images, not measured offsets or fault studies in the field. Later field investigations of Pezzopane and Weldon (1993) found that faults throughout the region were dominantly dip-slip, with negligible to minor oblique motion. Trench et al. (2012) found abundant evidence for dip-slip motion on NW-oriented faults, including those within the BFZ, and sparse evidence of strike-slip motion.

Scarberry et al. (2010) observed that NW-striking faults are more numerous along northern tips of NNE-striking faults throughout the region, so concluded that the

NW-striking set in the Abert Rim area (Fig. 2) formed as result of splays along the northward propagating tip of the Abert Rim fault exploiting a pre-existing NW-oriented

18 fabric. Trench et al. (2012) reached a similar conclusion by observing cross-cutting relationships between the two fault sets in Oregon, where NNE-striking faults cut the

NW-striking set in the south, but the two sets are mutually cross-cutting in the north.

Trench et al. (2012) thus concluded that NW-striking faults first developed along the fault tips of NNE-striking faults, but the NNE-striking faults ultimately grew through and cross-cut the NW-striking faults; they also suggest that a lack of major NNE-striking faults in or proximal to the Brothers Fault Zone indicates that the NW-striking faults within that zone developed independently and were not influenced by NNE-striking structures. Scarberry et al. (2010) suggested that in the Abert Rim area (Fig. 2), the similarity between orientations of two distinct middle to late Miocene (16-5 Ma) fault sets and those of early to middle Miocene (23.8-16.4 Ma) dikes that trend 5-10° and

290-315° indicate that the development of both fault sets was influenced by a pre- existing fabric.

In northeastern California, two relatively isolated and poorly understood faults strike parallel to the regional NW-striking fabric. The Fandango fault (Fig. 2) borders a linear valley within the northern Warner Range in the footwall of the Surprise Valley fault (SVF) and appears to be cut by the SVF (Egger and Miller, 2011). It may be an accommodation zone between the east-dipping SVF and the west dipping faults along the Goose Lake graben (Surpless and Egger, 2006), but the exact timing of motion and amount of slip on the fault is poorly constrained. The Likely fault system (Fig. 2) has been mapped as a strike-slip fault in the USGS Quaternary fault and fold database

(2006), but evidence for this fault is minimal, based on references to early mapping by

Gay and Aune (1958), and observations by Jenkins (1959) of sag ponds and apparent dextral offset of some topographic features.

2.4 The Larkspur Hills

The Larkspur Hills study area, which lies east of Surprise Valley and north of the

Hays Canyon Range, straddles the border between California and Nevada (Figs. 2 and 3).

The area is characterized by low-relief hills formed by numerous normal faults, most of which record minor down-to-the-east offset (Fig. 3). The faults cut a series of Late

Miocene to Pliocence volcanic and volcaniclastic rocks, primarily a group of distinctive, 3

-8 Ma low-potassium olivine tholeiites (LKOTs) (Carmichael et al., 2006). Basalt flows are thin, with average thicknesses of ~1-5 m, and are interbedded with tuffaceous sediments. The thickness of the entire unit is not well constrained as there are no other units exposed within the study area. Where observed, the minimum thickness of the interbedded basalts and tuffaceous sediments is 80 m, but it likely varies throughout the study area (Egger and Miller, 2011). The LKOT basalts onlap the Oligocene unit along the

Hays Canyon range (Fig. 5) and cover the northern trace of the Hays Canyon fault, indicating that this fault has been inactive since the late Miocene to Pliocene (Egger and

Miller, 2011).

The study area is characterized by predominantly west-facing dip-slopes of basalt with east-facing normal faults; in the southern area, minor basins between these hills are filled with Pleistocene lake sediments of pluvial Lake Surprise (Figs. 3 and 5)

Fig. 5. Close-up map of domain I. Legend for all domain close-up maps follows (5a). Scarp heights for each fault are noted. Solid white line is pluvial highstand level (Irwin and Zimbelman, 2012). Dotted white line outlines an E-W magnetic low (Egger et al., 2009).

21 5a

Fig. 5a. Legend for all domain close-up maps.

(Egger and Miller, 2011). In contrast, the northern and eastern portions of the field area are characterized by primarily flat-lying basalts, fewer faults, and small playas in the intervening basins (Fig. 3), consistent with a lower magnitude of extension. Faults show no signs of activity in the Holocene: there are no Holocene fault scarps, and Pleistocene shorelines below the high-stand of pluvial Lake Surprise (Fig. 5) do not appear to be deformed.

Over this relatively small area of about 550 km2, however, there are significant differences in fault orientations, from dominantly N-striking in the south to NNE- and

NNW-striking in the north (Fig. 3). How do these local fault orientations relate to the

22 regional stress field over time, and what controls the differences in orientations from south to north across the field area?

These questions have relevance to the characterization of nearby buried intrabasin faults, which control locations of hot springs throughout the valley (Fig. 2). Both the Surprise Valley and the southern Larkspur Hills have been the object of geothermal exploration (Barker et al., 2005; Benoit et al., 2005; Bertani, 2007) and studies to define structural features in the subsurface (Egger et al., 2010; Glen et al., 2013). Local studies

(Benoit et al., 2005; Egger et al., 2014) point to the importance of fault controls on subsurface fluid circulation and the surface location of hot springs in the Surprise Valley

(Glen et al., 2013); this is in agreement with other studies that emphasize the importance of fault controls in geothermal systems (Curewitz and Karson, 1997; Faulds et al., 2010). While hot springs occur on both sides of the basin in the Surprise Valley, along the east side they do not appear to be located along faults; however, faults are known to be a primary control on fluid flow (Caine et al., 1996; Rowland and Sibson,

2004; Faulds et al., 2010). Faults expressed at the surface in the adjacent Larkspur Hills may serve as an analog for faults buried in Surprise Valley, and may lend insight into their geometry and controlling structures.

3. Methods

3.1. GIS-based methods

Surface structures in the Larkspur Hills were mapped in the ESRI GIS program

ArcMap 10.1, using hillshade maps generated from two digital elevation models (DEMs)

23 in combination with Digital Orthophoto Quadrangles (DOQs) at 1 m resolution. One

DEM covers the southern part of the field area, and was generated from high resolution

(1 pixel = 0.5 m) LiDAR point cloud data collected from aerial swath mapping conducted in July 2012; data were processed at the National Center for Airborne Laser Mapping

(NCALM) at UC Berkeley. To extend the field area farther north, a DEM generated from digitized topographic maps with 3 m spacing was obtained for the northern part of the field area from the USGS National Elevation Dataset (Gesch et al., 2002; Gesch, 2007).

We initially identified fault traces by locating prominent cliffs on the hillshade model.

Where surface traces were not well defined, visual analysis was augmented by drawing profiles across the faults. This involved interpolating a line perpendicular to strike, and generating an elevation profile. This was repeated along strike to identify signs of fault offset, and where the offset tapered off at the tip-lines. Google Earth imagery provided

3D perspective, and analysis of the imagery aided fault analysis and selection of field sites.

The orientation of linking segments due to breaching of relay ramps (Fig. 4a) can be influenced by pre-existing structures (Trudgill and Cartwright, 1994). Also, faults can develop as linking structures when there is a change in extension direction (Henza et al.,

2011) or when a pre-existing fault set at an angle to the extension direction is reactivated (Bellahsen and Daniel, 2005). In order to test for common orientations in fault segments that may reflect the influence of either pre-existing structures or a change in extension direction, we determined the azimuth of individual segments by

24 manually drawing straight best-fit lines along mapped faults at the 1:24,000 scale. The term “segment” is commonly used to refer to fault segmentation at a wide range of scales and based on varying criteria from one study to the next (DePolo et al., 1991); for the purpose of this analysis, the term “segment” refers to these best-fit line segments, which varied in azimuth from adjacent segments by an average of 25° with a standard deviation of 15°. A plugin for ArcGIS, EasyCalculate 10.0 by ET Spatial Techniques, was used to calculate the azimuth for each segment, which was then exported to an Excel spreadsheet. Segments were normalized to total fault length, which was calculated by summing the lengths of the segment lines. Since dip direction is dominantly to the east and this analysis was not affected by dip direction, all azimuths were plotted in the upper compass quadrants, exported to Stereonet (Allmendinger et al., 2012), and plotted on rose diagrams in bins of 10°.

Segments were then analyzed according to percent of fault length. Hus et al.

(2005) found that faults interacted and formed linking structures when the overlap was

~1/8 of the combined fault lengths, which is ~12.5% of fault length. Linking structures that form as a result of change in extension direction (Henza et al., 2011) or the reactivation of pre-existing structures (Bellahsen and Daniel, 2005) do not have a predictable ratio to overall fault length, so grouping was also based on the average number of segments per fault. For the entire study area, the average was 5 segments per fault, so segments that made up a quarter, or more, of the total fault length were grouped on one diagram, as those segments tended to dominate overall fault

25 orientation and most were associated with shorter faults. Segments less than 25% of fault length were grouped on another, as they tended to be associated with very segmented faults, so were of relatively equal dominance in controlling the fault orientation. The faults lying along domain boundaries were plotted separately, but with all segments on one diagram for each boundary, since these faults were the most segmented. Mean segment orientation for each group, along with the corresponding σ3,

was also plotted. The likely paleostress orientation, σ3, for the study area was computed at 90° from the mean segment orientation for all segments within the study area.

Extension was calculated along topographic profiles from section lines (Fig. 3) generated in ArcGIS. The tilt of fault blocks was measured directly from the topographic profile where slopes represented dip-slopes of basalt, which are well preserved throughout the study area. The dips of faults, however, could not be measured due to scarp retreat, which we assumed to be significant due to the interbedded tuffs, prominent vertical fractures in the basalt, and large talus slopes at the base of cliffs. For extension calculations, dip was assumed based on the ~68° dip on the Surprise Valley fault as exposed in a trench (Personius et al., 2009) and the 60°-90° range of fault dips throughout the region as summarized by Scarberry et al. (2010). Extension was calculated using a minimum dip of 65° and a maximum of 80° in order to remain within the range for the region, and allow for uncertainty in the assumption. Fault dips on the cross-sections were corrected for apparent dip and amount of block tilt measured from the topographic profile. Extension was calculated by measuring the horizontal

26 displacement for each fault, and summing the displacements across the section line.

Percent extension was calculated across a distance of 10 km for each line for consistent comparisons across the study area. The values reported are minima due to the lack of correlation between capping flows of individual faults, and later flows may have filled in topographic lows (Ritzinger et al., 2013). Block tilt was reported after correcting for apparent tilt.

Scarp height was measured at the location of maximum elevation change along each segment, identified by using an overlay of USGS published topographic maps with contour lines at 20 ft intervals. Scarp height was correlated to fault segment data. For each fault, the scarp height of each segment was plotted at the midpoint of the segment

(so the spacing of the points would reflect the segment spacing along strike) using a circle that represents the segment azimuth, with larger empty circles indicating more westward trends and larger grey circles indicating more eastward trends. Graphing using this procedure allowed for comparisons of several parameters: maximum scarp height, segment length, segment azimuth, number and spacing of segments per fault, and variations in azimuth from segment to segment. Each fault was graphed for analysis.

3.2. Field methods

Field analyses focused on areas where the resolution of the aerial imagery and

DEMs was insufficient for distinguishing minor offset faults and possible volcanic features, and for examining cross-cutting and truncating relationships of faults, especially where NNW striking faults or fractures were identified in the footwalls or tip-

27 lines of N-NNE faults. Based on these targeted analyses, locations of fault traces, linkages, and linear volcanic features were revised or mapped.

4. Results

4.1 Larkspur Hills domains

The Larkspur Hills study area was divided into domains based on fault density and orientation (Fig. 3), and thus the results of these fault analyses are reported initially within domains, and summarized in Table 1. Larger scale maps of domains I-V have been included; Fig. 6 compares segment orientations across all domains; and Fig. 7 compares fault orientations, segmentation, and offsets across all domains. Domain boundaries occur primarily along major-offset NNE- and N-striking faults or basins (Fig. 3), and data from domain-boundary faults are treated separately.

4.1.1 Domain I

The primary characteristic of domain I (Fig. 5) is closely-spaced, N-S-striking fault blocks.

Domain I exposes 20 faults (Table 1) with overall northward trends, dip directions to the east, and lengths ranging from 0.7-11.4 km. Average fault spacing is 0.8 km; spacing is more regular in the western half of the domain than in the east. Fault blocks are tilted 5-

13° to the west, with the degree of tilt decreasing across the domain towards the east.

One fault has a scarp height of 224 m, the rest range from 18-128 m, with an average of

64 m.

Faults are segmented, with an average of 6 segments per fault and mean segment length of 0.5 km. Segments that make up ≥25% of fault length trend

Table 1. Summary of fault domain characteristics. Tilt measured from profile line and corrected for apparent tilt.

Domain Number Fault Mean fault Spacing Mean Tilt of Scarp Mean scarp Segments Number of Segment Mean

of Faults length length (km) (km) spacing fault blocks height height (m) per fault Segments length (km) segment

(km) (km) (westward) (m) length (km)

I 20 0.7-11.4 2.1 0.1-1.7 0.8 5-13° 18-224 64 7 132 0.08-1.9 0.5

II 24 1.0-8.4 2.4 0.2-3.6 1.3 0-6.5° 6-156 59 6 136 0.08-2.1 0.6

III 27 1.6-9.3 1.9 0.3-5.8 1.7 3-8° 5-186 42 6 80 0.08-3.3 1.7

IV 9 0.8-4.1 2.0 0.3-5.0 2.7 0-4.5° 17-183 48 2 20 0.4-2.4 1.8

V 6 1.4-4.5 4.1 1.2-6.2 3.7 0-2° 37-177 63 3 28 0.2-3.9 3.2

310°- 060°, with an mean trend of 359° and standard deviation of 23°; the segments form two groups with mean trends of 345° and 015° (Fig. 6). Segments <25% of fault length trend 300°- 070°, with a mean of 005° and standard deviation of 24°. Adjacent segments of the same fault vary in orientation by as much as 40° (Fig. 7); faults have arcuate or zig-zag traces. Fault traces in domain I display hook-shaped linking structures

(Figs. 4b and 5); this style of linkage is not observed in the fault traces in domains II-V.

4.1.2. Domain II

The primary characteristic of domain II (Fig. 8) is closely spaced, NNW-striking faults and fault blocks. There are 24 faults (Table 1) with overall NNW trends and NE- facing scarps. The average fault spacing is 1.3 km. Spacing is more regular in the southern portion of the domain; the northern portion has closely spaced faults along the basin margin, and a wide (4.6 km) un-faulted section in the NE. Fault blocks are tilted 0-7° to the west, with the degree of tilt decreasing across the domain towards the north. Fault lengths range from 1.0-8.4 km. One fault has a scarp height of 151 m, the rest range from 6-98 m, with an average of 59 m.

Faults are segmented, with an average of six segments per fault and mean segment length of 0.6 km. Segments ≥25% of fault length have trends ranging from

300°- 040°, with an mean of 347° and standard deviation of 26° (Fig. 6). Segments <25% of fault length have trends ranging from 300°- 030°, with a mean trend of 344° and standard deviation of 24°. Most adjacent segments of the same fault vary in orientation by about 10°-20° (Fig. 7); the fault traces are more linear than in domain I.

Fig. 6. Rose diagrams showing fault segment orientations for each domain. Segments are normalized to fault length and plotted in 10° bins. For each plot, n=number of segments displayed. Length of petals represents percent of n segments. Segments >25% of fault length are likely dominant structures, while segments <25 % of fault length are likely linking structures or of relatively equal influence on overall fault orientation. Heavy grey line shows mean segment orientation; dashed grey line shows least compressive stress perpendicular to mean segment orientation.

Figure 7. Graphs of one prominent fault per domain and domain boundary. Circles indicate maximum scarp height and azimuth for each segment, plotted at the midpoint of each segment along strike. Grey circles represent orientations 001°-090°, and open circles 270°-359°. The size of the circle indicates the magnitude of deviation from north, with larger grey circles for more eastern azimuths, and larger open circles for greater western azimuths, as indicated by key (above).

Figure 8. Close-up map of domain II. See figure 5a for legend. Scarp heights for each fault are noted. Solid white line is pluvial highstand level (Irwin and Zimbelman, 2012).

Domain II has five linear vents with trends that are subparallel the normal faults.

Three are proximal and appear to be cut by a fault along the same trend.

4.1.3. Domain III

Domain III (Fig. 9) is characterized by fewer faults than domains I and II, and several fissure vents that parallel faults. There are nine faults (Table 1), with overall trends to the N and the NNW; unlike domains I and II, fault dip directions vary. While the dominant dip direction is eastward, two faults dip to the W. This domain also has 12 linear volcanic vents that parallel fault trends. Fault lengths range from 1.6-9.3 km, with an average length of 1.9 km. Scarp heights range from 5-108 m, with an average height of 42 m. Fault spacing ranges from 0.3-5.8 km, with an average spacing of 1.7 km. Fault spacing is more variable than in domains I and II, and while most faults have similar spacing as other domains, there are also wide un-faulted areas between the domain boundaries (up to 5.8 km wide). Fault blocks are tilted 3-8° to the west, with the degree of tilt decreasing across the domain towards the east.

Faults average six segments per fault, with a mean segment length of 0.6 km.

Segments ≥25% of fault length trend 300°-030°, with a mean trend of 347° and standard deviation of 25°; there are two modes at 330° and 20° (Fig. 6). Segments <25% of fault length trend 270°-070°, with a mean of 003° and standard deviation of 26°; this plot also has two modes but at slightly different orientations at 340° and 000° (Fig. 6). In contrast to the other domains, segment orientations in domain III vary more along longer faults

Figure 9. Close-up map of domains III and IV. See figure 5a for legend. Scarp heights for each fault are noted. Boundary faults are labeled.

(Fig. 7) than along the shorter faults (Fig. 9); long faults are more arcuate and shorter faults are relatively linear.

Numerous fissure vents parallel the faults throughout the domain, and some relief is caused by the exposed margins of basalt flows rather than minor offset faults or vents. In the southern part of domain III, a vent with a NNE stike parallels the major- offset NNE-striking fault along the domain boundary (Fig. 9). Other vents in the southern part of the domain parallel a N-striking fault that may cross-cut the boundary fault. The combination of little block tilting, the significant playa sediments along Crooks Lake, and relief due to primary volcanic features complicated the offset calculations for faults in this domain, as well as adding uncertainty in measurements of scarp heights.

4.1.4. Domain IV

Domain IV (Fig. 9) is characterized by a relatively few faults. There are eight faults (Table 1) that trend predominantly NNW. Five of the faults are in the footwall of a major-offset fault along the eastern boundary. Fault lengths range from 0.8-4.1 km.

Scarp heights range from 17 to ~48 m; though determining maximum topographic relief on four of the faults is complicated by cross-cutting relationships (Fig. 9). Seven of the faults dip E to NNE, and one dips S to SSE. Westward slope of 1°-5° along the eastern margin is likely due to footwall flexure of the major offset fault along Long Valley rather than to overall tilting of fault blocks. Fault spacing is irregular, with sets of 2-3 closely spaced (0.3-0.9 km) faults separated by wide (2-5 km) un-faulted areas.

Faults in this domain are the least segmented, with an average of two segments per fault and average segment length of 0.9 km. Segments ≥25% of fault length trend

310°-070°, with an average trend of 340° and standard deviation of 29°, and segments

<25% of fault length trend 310°-350° with an average of 324° and standard deviation of

27° (Fig. 6). Segments of the same fault have little variation in orientation (Fig. 7), though there is variation between different faults; faults have relatively linear traces.

4.1.5. Domain V

Domain V (Fig. 10) is characterized by a narrow zone of faults and wide un- faulted areas. There are six faults (Table 1) that trend predominantly north, and dip east and west. Fault lengths range from 1.0-3.4 km. Scarp heights range from 37-115 m, with an average of 63 m. Fault spacing is irregular, with 4 of the 6 faults spaced <2 km apart along one narrow zone, and most of the domain consists of un-faulted areas over 6 km wide. The footwall of one major offset fault slopes ~2° to the east, likely a reflection of footwall flexure rather than tilting of the fault block.

Faults are less segmented than domains I-III, with an average of only three segments per fault and average segment length of 1.2 km. Segments ≥25% of fault length trend 300°-020°, with an average trend of 345° and standard deviation of 31°; segments <25% of fault length have trends from 280°-060°, with an average trend of

335° and standard deviation of 41° (Fig. 6). Fault segments have little variation in azimuth along strike (Fig. 7), so have relatively linear traces.

There is little relief in domain V. While examination of the DEM and Google earth imagery indicated the possibility of minor offset faults, field observations confirmed much of the minor topography is created by margins of individual basalt flows, which in some cases appear steep due to drainages that follow this topography.

There are six linear vents that trend consistent with the faults, and five appear to have been subsequently offset by faults (Fig. 10).

Fig. 10. Close-up map of domain V. See Fig. 5a for legend. Scarp heights for each fault are noted. Inset map shows close-up of structurally controlled linear vent, with the fault trace dashed where it offsets the vent; see text for discussion.

4.2. Major boundary faults

Differences between the fault characteristics among the domains aided determination of the boundary locations, and faults lying along boundaries were analyzed separately from the domain faults using the same methods. The eastern boundary of domain II (Fig. 9) consists of three faults with overall NNE trends and eastward dips. Westward tilting of fault blocks is 12° on the southernmost fault, decreasing to 4° on the next fault, and no tilt was noted on the northern-most fault.

The faults are highly segmented, with an average of 12 segments per fault. Fault lengths range from 10-11.4 km, and segment lengths average 0.6 km. Two of the faults have maximum scarp heights of 185 m and 100 m. The third fault may be linked to the fault along Crooks Meadow (Fig. 9), and this fault likely cuts an older NW trend fault in the northern part of the domain. The scarp height is ~43 m. Segments trend 270°-080°, with a mean of 022° and standard deviation of 28° (Fig. 11); the faults have arcuate to zig-zag traces, with the greatest variation of segment orientations along strike (Fig. 7). The NNE- striking fault, D2B-1, that makes up the southern part of the boundary (Fig. 9) has minor offsets in the footwall which trend NNW, especially along northern segment of the fault.

Several of these were also observed in the field, but were below the resolution of the

DEM and the scale of the mapping; they are noted here due to their consistent trend with other footwall faults. The offset on most of these footwall faults tapers off quickly from the main fault, but a few appear to be continuous with NNW-striking domain II faults. These NNW-striking footwall faults are not continuous across the fault in the

39 hanging wall, thus supporting the designation of a boundary along the major NNE- striking fault.

Similarly, the eastern boundary of domain III (Fig. 9) consists of five faults with overall NNE trends and eastward dips. Westward tilting of fault blocks is 8° on the southern-most fault, and there is no tilt noted on the rest of the boundary faults. The faults are less segmented than the eastern domain II boundary, with an average of five

Fig. 11. Rose diagrams showing fault segment orientations for domain boundary faults. Segments are normalized to fault length and plotted in 10° bins. For each plot, n=number of segments displayed. Length of petals represents percent of n segments. Each plot shows all the segments for the boundary. Solid grey line shows mean segment orientation; dashed grey line shows least compressive stress perpendicular to mean segment orientation.

segments per fault. Fault lengths range from 0.7-8.9 km, and segment lengths average

0.8 km. Scarp heights range from 13-145 m (Fig. 9). Segments trend 340°-070° (Fig. 11), with a mean of 023° and a standard deviation of 23°; the fault traces are arcuate, but segment orientations vary less along strike than along the eastern domain II boundary

(Fig. 7).

Fault D3B-1, a NNE-striking fault along the southern part of the boundary (Fig. 9) has prominent fractures which appear in the field as broad “notches” along the top of the scarp that extend from the scarp edge and taper off into rubble-filled prominent fractures that trend roughly NNW. These fractures were below the resolution of the

DEM and the scale of the mapping, but are noted here due to trends consistent with the faults. Along the northern portion of the fault, there are a few minor NNW-striking footwall faults. Overall, the fault is shorter with less offset than the major offset NNE fault in the domain II eastern boundary, and it appears to be cut by a N-striking fault at its southern tip, though this is difficult to constrain due to a major drainage, volcanic vent, and landslides that complicate identification of the D3B-1 fault trace.

4.3 Extension direction and magnitude

Averaging the mean least compressive stress orientation (3) for the domains

(Fig. 6) resulted in an average orientation of 263°. While the average σ3 for the boundary faults is oriented 292°, when the boundary faults are grouped with the domain faults, the resulting orientation is 267° ±3°. This is consistent with ~E-W principal stress direction for the northwest Basin and Range region determined by

Crider (2001), and 267° for south eastern Oregon determined by Pezzopane and Weldon

(1993). This ~E-W least compressive stress orientation is assumed to be consistent with the regional extension direction, based on Anderson’s classification of tectonic stress

(Anderson, 1951) and the seismic and geologic evidence for regional extension in an E-

W to ENE-WSW direction (Crider, 2001). Section lines for extension calculations were

41 drawn due E-W to remain within the range of uncertainty and also minimize the differences between apparent and actual dip on the majority of faults along the section lines.

Extension was calculated along five EW-striking cross-section lines to compare the magnitude and percent of extension from south to north across the study area (Figs.

3 and 12). Cross-sections are included to show dominant style of faulting; however, were not restored due to lack of correlation of units. The percent of extension was calculated over 10 km for each section line for purposes of comparison, though the faults along section lines A and B are concentrated along a shorter distance. Since the magnitude of extension was calculated assuming a minimum dip of 65° and maximum of

80°, the following results are reported as a range. The magnitude of extension across section line A is 222-424 m across a narrow zone of 6 down-to-the-east bookshelf style faults, resulting in 2.4-4.6% extension. Extension may be slightly greater along section line B, with 3.5-6.8% extension calculated across the same zone of faults. Extension of

237-569 m (2.4-6%) across section line C is accommodated primarily along four east- dipping faults, with one additional minor offset fault. Extension decreases across line D, with 151-327 m (1.5-3.4%) distributed across 9 faults, including one west dip fault. All but two of the faults have only minor offset. Only 70-166 m (0.4-0.9%) of extension was accommodated across line E, mainly on one major offset west-dipping fault. All of these values for extension should be considered minima, as later basalt flows may have filled in basins, decreasing the apparent offset along individual faults.

Fig. 12. Cross-sections A-A’ through E-E’ corresponding to section lines shown on Larkspur Hills map (Fig. 3). Cross-sections show dominant down-to-the-east bookshelf faults with assumed original dips of 65°. Lines are organized moving northwards from line A to line E.

4.3 Timing of faulting and extension rates

Calculating extension rates is complicated by lack of correlation between flows

(Ritzinger et al., 2013), the range of possible horizontal offset values due to uncertainties in fault dip, and a lack of constraint on the period of fault activity. All faults are likely younger than 3 Ma, but we cannot definitively rule out the possibility that some faults were active during the period of magmatism from 3-8 Ma. Faults in the

Larkspur Hills show no signs of activity in the Holocene: there are no Holocene fault scarps, and the Pleistocene shorelines do not appear to be disturbed. Assuming the faults were active from the start of the period of extension beginning ~3 Ma (Colgan et al., 2008) until just before the Pleistocene shorelines were developed ~ 0.02 Ma (Ibarra et al., 2014), the extension rates (in mm-yr-1) across each line are: 0.14-0.07 for section line A, 0.21-0.11 for line B, 0.19-0.08 along line C, 0.11-0.05 along line D, and 0.06-0.02 across line E. This results in an average of 0.1 mm-yr-1 across the field area.

5. Discussion

5.1 Controls on fault geometry

The analysis described above suggests that the main factors that influence the geometry of normal faults in the Larkspur Hills are extension direction and pre-existing structures. The relative importance of each of these is discussed below, followed by a brief discussion of the two other controls that were considered, a change in stress orientation and effect of differences in lithology.

5.1.1 Extension direction

Normal faults in a homogeneous medium form perpendicular to the least compressive stress, 3, according to assumptions outlined in Anderson (1951) (Fig. 13a).

Non-Andersonian mechanics have been theorized and tested both in the laboratory

(Reches and Dieterich, 1983) and in the field (Aydin and Reches, 1982; Krantz, 1988;

Krantz, 1989), and result in separate coeval fault sets (Fig. 13b).

Fig. 13. Comparison of Andersonian and orthorhombic fault geometry. The symbol σ represents stress, and ε represents the associated strain, with 1 indicating maximum stress/strain, 2 intermediate, and 3 minimum. Fig. 13a shows conjugate faults that form by plane strain as predicted by Anderson (1951). Fig. 13b shows two conjugate pairs that form by non-planar strain as illustrated by Reches (1983). Arrows indicate slip directions. Diagram adapted from Reches (1983).

Assuming Anderson mechanics (Anderson, 1951), where the maximum compressive stress in an extensional regime is vertical, and the minimum and intermediate stresses are both horizontal and unequal (Fig. 13a), the faults in the

Larkspur Hills are all favorably oriented to slip with a minimum stress oriented approximately E-W, or slightly to the N or S of this orientation, assuming some oblique component of motion on the NNW- and NE-striking faults. While it is not possible to measure stress directly at the time when the faults formed, we infer an E-W paleostress orientation based on:

1. The dominantly N-S orientation of faults in domain I (Figs. 3 and 5),

which are consistent with orientations of the proximal major structures,

the Hays Canyon fault and Surprise Valley fault (Fig. 2). Additionally, the

domain I faults have hook-shaped linking structures (Fig. 4b). This form

of linking structure results from interaction of tensions at fault tips

during fault growth, and is not observed in analogue models where pre-

existing structures ruptured through subsequent cover (e.g. Bellahsen

and Daniel, 2004). The seismic study of Giba et al. (2012) indicated that a

large structure at depth can rupture through subsequent cover, resulting

in en echelon segments at the surface, which are characterized by relay

structures in the zones of overlap (Figs. 4a and 4c); however, hook-

shaped structures were not evident (Giba et al., 2012). This suggests that

the faults traces in domain I resulted from individual N-S segments that

grew along tip-lines and connected, rather than resulting from

reactivation of pre-existing structures at depth.

2. The average σ3 direction of 267° determined by a least compressive

stress perpendicular to average segment orientations (Fig. 6 and 7). As

indicated in Wang (2000), the movement of several faults can

approximate the regional stress, so while there are variations in the

inferred σ3 among the different fault domains and domain boundaries

due to the variations in fault orientations (Fig. 3, 6, and 7), it is

reasonable to infer that they work together to accommodate an overall

motion consistent with a single regional stress direction; however, in the

absence of sense-of-slip indicators for the faults in the study area, this

cannot be determined conclusively. Strain partitioning along faults with

varying orientations and pure dip-slip motion would also be more

consistent with non-planar strain (Fig. 13b) than plane strain (Fig. 13a).

3. The N-S trend of linear vents. Those that have NNW-SSE and NNE-SSW

orientations are at tip-lines of, or have been subsequently offset by,

faults along the same trend, so likely reflect the influence of pre-existing

structures (discussed in section 5.1.2), while many of the N-S oriented

vents are not proximal to faults so are interpreted as responding to

extension direction. Since dikes intrude along planes perpendicular to

the least principal stress (Anderson, 1951), dikes and linear volcanic

features (e.g. aligned cinder cones, elongated volcanic edifices) can be

used as good geologic indicators of the regional orientation of the least

principal stress (Nakamura, 1977; Zoback et al., 1994).

Our conclusion that extension during fault activity was oriented E-W is supported by previous work. Crider (2001) used a variety of methods and regional data to determine a minimum principal stress direction for the northwest Basin and Range ranging from 263°-269°. She concluded that an extension direction due E-W (270°) or slightly north of east (264°) would result in sinistral slip on NNE-striking faults, and dextral slip on NW-striking faults, which is consistent with field evidence for minor to negligible oblique motion on some faults in the region (Pezzopane and Weldon, 1993;

Trench et al., 2012); evidence of minor oblique motion was largely absent along the surface traces throughout the northwest Basin and Range, but detected in trenches

(Pezzopane and Weldon, 1993). Other faults, such as the Surprise Valley fault, show no evidence of lateral motion (Personius et al., 2009).

5.1.2 Pre-existing structures

Lacking seismic data at depth in the Larkspur Hills, comparison with sandbox models provides a means to assess the role of pre-existing structures. In particular, sandbox models that tested extension direction with varying orientations of pre-existing weaknesses provide insight. The surface traces of the NNW-striking faults in domain II compare favorably to those formed in sandbox models of Bellahsen and Daniel (2005)

(Fig. 14a and 14b). In the model, the size, dip, spacing, and orientation of pre-existing

48 faults was controlled by inserting a card through a sand layer. Pre-existing faults oriented 70° to the extension direction were reactivated and continued as the dominant fault set due to the high angle of the pre-existing structures to extension direction.

Assuming the same plane strain conditions of the model, the pre-existing NNW-striking faults in the study area could therefore have evolved under E-W extension; however, a component of oblique motion would be required. While sense of slip indicators are not observed in the Larkspur Hills, minor oblique motion on other faults in the region has been observed (Pezzopane and Weldon, 1993; Trench et al., 2012), even where evidence of minor oblique motion was largely absent along the surface traces it was detected in trenches (Pezzopane and Weldon, 1993).

Fig. 14. Simplified fault maps comparing results of sandbox models of Bellahsen and Daniel (2005) (a and c) and fault traces in the study area (b). Diagram 14a shows model with pre- existing faults at a high angle to extension, and diagram 14c shows pre-existing structures at a low angle to extension. Grey line: fault trace. Dashed line: pre-existing structure - high angle. Black line: pre-existing structure - low angle. Pre-existing structures for diagram 14b are inferred. See text for discussion.

The major offset NNE fault traces along the eastern boundaries of domains II and

III (Figs. 3 and 10) also have similarities to the sandbox models of Bellahsen and Daniel

(2005), in this case where pre-existing faults were oriented 45° to the extension direction (Fig. 14c). In this model, due to the highly oblique angle of the structures to extension direction, the faults were reactivated as linking structures between newly formed N-striking segments. The surface traces in the field have long segments oriented

10-20° (Fig. 14b) rather than directly N as seen in the model, perhaps due to differences in length, spacing, or lower angle of pre-existing structures. An alternative explanation is that the NE-striking structures are more developed than the NNW-striking set. This may also be why the NE-striking faults truncate the NNW-striking set, as well-developed pre- existing faults can be barriers to fault rupture, which has been observed both in analogue models (Henza et al., 2011) and in field studies (Machette et al., 1991; Milano and Di Giovambattista, 2011).

NNW-striking faults and fractures are present in the footwalls of major offset N-

NNE-striking structures in domains II-IV, especially along the northern segments of the longer faults. The NNW-striking structures may have reactivated initially as dilatational fractures along fault tips as the larger-offset NNE-striking faults propagated through a pre-existing fabric, as has been interpreted in the Abert Rim area (Fig. 2) (Scarberry et al., 2010). This would account for the consistently NNW trend of the footwall faults and fractures. The NNW-striking faults likely only dominate as an independent fault set in domain II due to a lack of N-NNE structures, similar to faults in the Brothers Fault Zone

(Fig. 2) (Trench et al., 2012). Scarberry et al. (2010) noted that mid-Miocene dike alignments in the Abert Rim area shared the same trends as the two fault groups, and concluded that both the dikes and the late Miocene faults were exploiting a pre-existing fabric. Similar evidence for exploitation of pre-existing fabrics may be seen in the linear volcanic features in the Larkspur Hills as well.

The trend of the NNW and NNE linear vents in the study area is sub-parallel to fault and fracture strikes suggesting that magma likely exploited both to reach the surface (Silver et al., 2004; Valentine and Krogh, 2006; Gaffney et al., 2007; Thomson and Schofield, 2008). In domains II and V, linear vents are proximal to or cut by faults along the same orientation (Figs. 9 and 11), suggesting that they exploited the same pre- existing structures as the faults, while in domains I and III, most of the NS-striking linear vents are not faulted, but parallel the faults (Figs. 5 and 10), suggesting they responded to extension direction (as discussed in 5.1.1 above) rather than pre-existing structures.

5.1.2 Change in extension direction

A mid-Miocene rotation of extension direction from 245°-250° to the current direction of 295° in the Basin and Range has been proposed (Zoback and Thompson,

1978; Zoback et al., 1994), with the rotation attributed to the influence of right-lateral shear resulting from the evolution of the San Andreas fault system. Colgan (2013) argues that long term Basin and Range extension has been oriented 290°-300°, with only a temporary rotation to the northeast in response to the impingement of the

Yellowstone Hotspot ~16 Ma, with the overall long-term spreading occurring as a result

51 of gravitational collapse perpendicular to the axis of thickest crust resulting from the

Sevier fold and thrust belt (Figure 1 of Colgan, 2013). The timing of the events proposed by these studies pre-dates the recent period of faulting in the Larkspur Hills, which are located in an area that has not experienced a change in its tectonic setting since the mid-Miocene (Egger and Miller, 2011). Differences in fault orientations in the study area as a result of rotation of the stress field during faulting should be consistent throughout the field area, but this is not observed; while, the possibility of a local rotation cannot be completely ruled out, it is unlikely and the cause of rotation would be only speculation.

It is also worth noting that the orientation of the axis of thickest crust, as illustrated and discussed in Colgan (2013), is consistent with ~E-W extension at the latitude of the northwest Basin and Range.

5.1.2 Lithological differences

While there are studies examining the effect of differences in lithology on fault initiation and growth in normal fault systems, both along the down-dip extent (e.g.

Shopfer et al., 2006) and on surface traces (Ackermannn et al., 2001), any relationship between the fault geometries in the Larkspur Hills and the nature or the thicknesses of the underlying layers would be merely speculation, due to lack of constraint on thicknesses and composition of the units in the study area, which likely vary throughout the field area. The exposed basalt flows vary in thickness, but most have an average thickness of ~1-5 m, and the flows are interbedded with tuffs and tuffaceous sediments.

There are no other units exposed within the study area, and the thickness of the entire

52 unit is only measureable along part of the eastern boundary (Fig. 9), where the major normal fault along Long Valley exposes LKOTs directly overlying an Oligocene volcanic unit (Egger and Miller, 2011). The approximate thickness of the LKOT unit at that location is 80 m. The LKOT basalts onlap the Oligocene unit along the Hays Canyon range to the south (Fig. 3). Logs from an exploratory drill hole located in the basin adjacent to the southern portion of domain I indicates the presence of dense altered shaley-sand at a depth of 1784 m (depth of hole), while drill-hole logs on the other side of the basin at approximately the same latitude indicates lake sediments to a depth of at least 1998 m (depth of hole) (Fig. 15). The presence of pillow basalts along the western margin of the study area supports the likelihood of lake sediments directly underlying the basalt unit along the valley margin, but again, the thickness of the basalt unit is unconstrained along this margin, as is the lateral extent of underlying sedimentary layers. Due to the lack of constraint on underlying units, the control they may exert on the surface traces is undetermined.

5.2 Evidence for a structural or lithological boundary separating fault domains

Although pre-existing structures appear to control fault development in much of the field area, namely the NNW- and NNE-striking faults in domains II-V, they do not appear to occur everywhere in the subsurface, especially in domain I where faults strike primarily N-S, and are located north of the Hays Canyon fault splays inferred from potential field data (Egger and Miller, 2011) (Figs. 3 and 5). Specifically, the change in fault orientations from domain I to domain II is abrupt (Figs. 3, 7, and 8), occurring along

Fig. 15. Simplified map showing drill hole locations (grey circles) and hot springs (white circles) relative to faults. See Figs. 2 and 3 for detailed maps of faults. an approximately E-W trend that coincides with the major NNE-striking boundary faults

DB2-1 and DB3-1 (Fig. 9).

The dominantly N-strike of domain I faults indicates E-W extension, which is the same as along the Surprise Valley and Hays Canyon faults (Egger and Miller, 2011).

While NNW and NNE-striking segments are present in domain I, the NNW-striking

54 segments dominate in domain II and NNE-striking segments dominate the boundary faults. These two groups become the primary fault orientations north of the field area, in south-central Oregon. The N-striking faults, including the inferred termination of the major Hays Canyon fault (Egger and Miller, 2011), die out as a dominant trend across the two boundary structures. The abrupt change in orientations between domain I and domains II-IV thus suggests a structural or lithological boundary at depth, indicating a border between relatively isotropic crust in the south to heterogeneous crust in the north.

While the primary line of evidence for a boundary is the abrupt change in fault orientations, there are two other items worth noting that may indicate domain I has different crustal characteristics than domains II-V. First, during a recent aeromagnetic survey using low-flying UAV, an E-W oriented magnetic low was detected extending from the basin into the southern portion of domain I (Glenn et al., 2013; Egger et al.,

2010). This magnetic anomaly lies along the same trend as a few minor E-W striking footwall faults in the southern part of domain I (Fig. 5). These E-W faults are not present elsewhere in the study area, but there are similar features in the footwall of the Surprise

Valley fault (Egger and Miller, 2011); there are no other prominent E-W striking anomalies indicated by the magnetic data collected in the basin. The E-W low anomaly separates N-S striking magnetic highs in the basin, which have been interpreted as dikes, at least one of which may have exploited a pre-existing fault plane (Glenn et al., 2013).

The magnetic low may represent a structural feature that resulted from the formation

55 of the dikes, or which subsequently altered them after emplacement (Glenn et al.,

2013); alternatively, it may represent a structural feature that pre-dated, and therefore influenced, the dike intrusions.

Another feature worth noting that is particular to the fault geometries in domain

I, but absent in other domains, is a pronounced but relatively short arc in an otherwise fairly linear fault trace. While these may be due erosional effects related to the hook style of linkage (Pollard and Aydin,1984; Acocella et al., 2000) (Fig. 4b), this pattern in the fault traces correlates from one fault to another along ~E-W lines, one of which corresponds to the magnetic low noted above. This pattern bears a marked similarity to one of the analogue sandbox models of McClay et al. (2002) where abrupt, nearly 90°- changes in the orientation of surface fault traces reflected the locations of lateral offsets in the basement. Again, while the similarity between the fault traces in the field and those in the model is speculative and may be the result of different processes, it is nevertheless another possible indication that the crust underlying domain I differs from that underlying domains II-V.

5.3 Implications for geothermal fluid flow

While the purpose of this study is to examine controls on fault geometries in the

Larkspur Hills, those faults are useful analogues for the geometries of buried intrabasin faults, and so the results of this study has implications for projects to better understand geothermal fluid pathways and barriers in this area (Barker et al., 2005; Benoit et al.,

2005; Glen et al., 2013; Egger et al., 2014). Faulds et al. (2010) noted that while

56 geothermal fields can be present along major range bounding faults, they are also found along less prominent fault zones. This is the case in the Surprise Valley (Figs. 2 and 15), where hot springs are located along both sides of the valley. Areas of fault intersection, interaction, and termination create areas of concentrated stress resulting in highly fractured, thus more permeable, regions (Curewitz and Karson, 1997; Faulds et al.,

2010). The size and shape of these areas are controlled by differences in fault geometry and kinematics (Curewitz and Karson, 1997), so it is important to understand the controls on the fault geometries in a particular area in order to predict the subsurface geothermal potential related to hot springs at the surface. Geochemical differences exist in the valley hot springs (Barker et al., 2005; Cantwell and Fowler, 2014) and this may be explained by the structures controlling the distribution of those springs. Helium isotope ratios measured in hot springs at the north end of the valley range from 2.28-

2.58 Ra (where Ra is 3He/4He ration in the atmosphere), differing from the 0.95-1.49 Ra measured in hot springs further south, which indicates a trend of increasing mantle influence to the north, possibly due to higher permeability in the north allowing for transportation of heat and mantle volatiles from depth (Barker et al., 2005). Cantwell and Fowler (2014) measured lower concentrations of B and Cl in the northern springs than those to the south, and interpreted this as the result of colder meteoric water mixing with hot thermal waters. They support this interpretation by silica polymorph geothermometry, which also indicates a hot fluid at depth diluted by cold water.

Cantwell and Fowler (2014) speculated that the Surprise Valley fault was the main

57 conduit for fluid flow, but that the Fandango fault may be contributing to meteoric water circulation. This seems unlikely, as the Fandango fault is south of the hot springs.

It may be that NNW-striking structures exposed in the hills to the east are present in the basin as well, and that these may be influencing the hot springs to the north. Springs to the south, along the intrabasin high (Fig. 15) and southward are likely influenced by N-S striking faults, which is supported by potential field data (Egger et al., 2010; Glen et al.,

2013) indicating basin structures share the same trends as faults in domain I.

6. Conclusions

Based on detailed analyses of surface fault traces using a variety of remote sensing data, coupled with field observations and comparisons to existing models, we conclude:

 In the Larkspur Hills, preexisting structures in rocks underlying late Miocene-

Pliocene units exert a strong control on surface fault geometries.

 NNW-striking structures are favorably oriented to slip under the current

stress field; NNW-striking faults therefore have longer segments and less

arcuate fault traces.

 NE-striking structures are less favorably oriented to slip under the most

recent stress field and thus formed primarily as linking structures; these

NE-striking structures link newer segments that form faults with an

overall orientation that is optimal to slip.

 Orientations of linear volcanic vents that range in age from 8-4 Ma (Carmichael

et al., 2006) are consistent both with the orientations of inferred pre-existing

structures and with E-W extension, suggesting that extension direction has

remained relatively constant since at least 8 Ma.

 North of the field area, NNW- and NE-striking faults are dominant, while N-

striking faults are not present. This regional transition is captured within the field

area between domain I and domains II- IV, suggesting that the Larkspur Hills

overlies a significant structural or lithological boundary at depth.

Acknowledgements

Funding for this project was provided as part of NASA grant 10-UAS10-0021 awarded to: Jonathan Glenn, USGS; Anne Egger, CWU; and Corey Ippolito, NASA Ames.

We thank Dr. Heezin Lee of the National Center for Airborne Laser Mapping (NCALM) at

UC Berkeley for processing LiDAR as well as providing instruction on processing methods. Brent Ritzinger’s help in the field with logistics and conversations about field and data interpretations is much appreciated. Lisa Ely provided useful insights regarding geomorphic features, especially drainages and landslides where they complicated interpretations of fault terminations or interactions. D.S. would also like to thank to

Diane and Ken Austin for offering a sounding board for ideas and for input on initial drafts of figures, and to Ken for discussions of geodetic data and geologic interpretations.

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