Implications for Extensional Tectonics GEOSPHERE, V

Home , Avawatz Mountains, Ibex Pass, Nopah Range, Owlshead Mountains, Saddle Peak Hills, Sperry Hills

Research Paper

GEOSPHERE Superposition of two kinematically distinct extensional phases in southern Death Valley: Implications for extensional tectonics GEOSPHERE, v. 17, no. X Z.D. Fleming1,*, T.L. Pavlis1, and S. Canalda2 1University of Texas at El Paso, Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, USA https://doi.org/10.1130/GES02354.1 2El Paso Community College, 9050 Viscount Boulevard, El Paso, Texas 79925, USA

18 figures; 1 plate; 3 tables; 1 supplemental file ABSTRACT proposed to explain extension-parallel folding in many extensional terranes, CORRESPONDENCE: [email protected] and the geometry of the Ibex Hills is consistent with these models. Collec- Geologic mapping in southern Death Valley, California, demonstrates Meso- tively, the field data support an old hypothesis by Troxel et al. (1992) that an CITATION: Fleming, Z.D., Pavlis, T.L., and Canalda, S., 2021, Superposition of two kinematically distinct zoic contractional structures overprinted by two phases of Neogene extension early period of SW-NE extension is prominent in the southern Death Valley extensional phases in southern Death Valley: Implica‑ and contemporaneous strike-slip deformation. The Mesozoic folding is most region. The younger NW-SE extension has been well documented just to the tions for extensional tectonics: Geosphere, v. 17, no. X, evident in the middle unit of the Noonday Formation, and these folds are cut by north in the Black Mountains, but the potential role of this earlier extension p. 1–29, https://doi.org/10.1130/GES02354.1. a complex array of Neogene faults. The oldest identified Neogene faults primar- is unknown given the complexity of the younger deformation. In any case, ily displace Neoproterozoic units as young as the Johnnie Formation. However, the recognition of earlier SW-NE extension in the up-dip position of the Black Science Editor: Shanaka de Silva in the northernmost portion of the map area, they displace rocks as young as Mountains detachment system indicates important questions remain on how

Received 18 September 2020 the Stirling Quartzite. Such faults are seen in the northern Ibex Hills and con- that system should be reconstructed. Revision received 9 March 2021 sist of currently low- to moderate-angle, E-NE–dipping normal faults, which Collectively, our observations provide insight into the stratigraphy of the Accepted 21 May 2021 are folded about a SW-NE–trending axis. We interpret these low-angle faults Ibex Pass basin and its relationship to the extensional history of the region. as the product of an early, NE-SW extension related to kinematically similar It also highlights the role of transcurrent deformation in an area that has deformation recognized to the south of the study area. The folding of the faults transitioned from extension to transtension. postdates at least some of the extension, indicating a component of syn-extensional shortening that is probably strike-slip related. Approximately EW-striking sinistral faults are mapped in the northern Saddlepeak Hills. However, these ■■ INTRODUCTION faults are kinematically incompatible with the folding of the low-angle faults, suggesting that folding is related to the younger, NW-SE extension seen in the The Death Valley extensional terrane has long been recognized as a prime Death Valley region. Other faults in the map area include NW- and NE-striking, example for studying continental extension (e.g., Snow and Wernicke, 2000; high-angle normal faults that crosscut the currently low-angle faults. Also, a Miller and Pavlis, 2005) due to spectacular rock outcrops, abundant markers major N-S–striking, oblique-slip fault bounds the eastern flank of the Ibex Hills that can be used as pre-extensional piercing points and evaluate displacements, with slickenlines showing rakes of <30°, which together with the map pattern, and its relatively young age, allowing tight age constraints on deformation. suggests dextral-oblique movement along the east front of the range. Nonetheless, because any correlation, be it stratigraphic or structural, is an The exact timing of the normal faulting in the map area is hampered by the interpretation open to debate, there are questions that surround the nature of lack of geochronology in the region. However, based on the map relationships, the geologic structures that accommodated Neogene extension in the Death we find that the older extensional phase predates an angular unconformity Valley region. Understanding these structures has been a focus of numerous between a volcanic and/or sedimentary succession assumed to be 12–14 Ma studies (e.g., Wernicke, 1981; Wernicke et al., 1988; Topping, 1993; Miller and based on correlations to dated rocks in the Owlshead Mountains and overlying Prave, 2002; Miller and Pavlis, 2005; Pavlis et al., 2014), and there is no consen- rock-avalanche deposits with associated sedimentary rocks that we correlate sus on the kinematic history of extension across the region. The rolling hinge to deposits in the Amargosa Chaos to the north, dated at 11–10 Ma. model is a widely cited concept applied to this region, and this kinematic model The mechanism behind the folding of the northern Ibex Hills, including the is coupled with an inference of ~140 km of NW-directed extension across the low-angle faults, is not entirely clear. However, transcurrent systems have been Death Valley extensional terrane based on a suite of inferred offsets of Mesozoic and older markers (e.g., Snow and Wernicke, 2000). Inherent in the rolling hinge model is the interpretation of a single, regional detachment that underlies the Zachariah Fleming https://orcid.org/0000-0002-5172-9788 This paper is published under the terms of the *Present address: Occidental College, Department of Geology, 1600 Campus Road, Los Angeles, region. In contrast, other workers have emphasized alternative explanations for CC‑BY-NC license. California 90041, USA the region’s geology. Specifically they’ve suggested that displacement occurring

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along strike-slip faulting in the region, along with multiple, distinct detachment the Black Mountains detachment system, exposing the basement gneiss in horizons, result in the development of discrete pull-apart basins (e.g., Wright et the footwall of the detachment. This interpretation has been used to support al., 1991; Serpa and Pavlis, 1996; Fridrich and Thompson, 2011). These models the detachment fault model with an implication for significant denudation and typically suggest significantly less extension than the rolling-hinge family of subsequent exposure of the footwall in the western Black Mountains (Fig. 2) models, in most cases using many of the same pre-extensional markers but (Wernicke, 1981; Snow and Wernicke, 2000). Alternatively, Troxel et al. (1992) different interpretations of those markers (Topping, 1993; Serpa and Pavlis, 1996; recognized dikes and fault geometries immediately to the south of the Ibex Hills, Miller and Pavlis, 2005; Fridrich and Thompson, 2011; Renik and Christie-Blick, in the Saddle Peak Hills (Fig. 1), indicative of NE-SW extension that predated 2013). The refinement of these models for the areas within the Basin and Range NW-SE extension. This NE-SW extension is widely interpreted to be coeval Province, and the amounts of extension in them, are important for our broader with slip along the Kingston Range detachment immediately to the southeast understanding of continental extension and transtension. For example, in the (Davis et al., 1993), including the Miocene extensional basin system, the China case of Death Valley, the rolling-hinge model has been used to describe large- Ranch basin (Scott et al., 1988). The Sheephead fault is adjacent to the Ibex scale deformation of the Basin and Range region (e.g., Wernicke et al., 1988). Hills, just to the north, and has been cited as playing a role as a displacement Therefore, additional study is warranted to test alternative reconstructions. transfer structure in the extension (e.g., Wright et al., 1991; Serpa and Pavlis, The Ibex Hills are located in the southern part of the Death Valley exten- 1996). Nonetheless, the sense of movement on the Sheephead fault has been sional terrane and occupy a structural position that varies radically among debated with some authors supporting sinistral slip (e.g., Holm and Wernicke, different reconstructions. Two structural features in particular are especially 1990; Serpa and Pavlis, 1996), while the most recent work has interpreted the important in this context: the Black Mountains detachment and the strike-slip fault as an overall dextral slip system (Renik, 2010), consistent with the tectonic Sheephead fault (Fig. 1). Holm and Wernicke (1990), Holm et al. (1992), Top- model of Wright et al. (1991). ping (1993), and Fridrich and Thompson (2011) interpreted low-angle normal Given the Ibex Hills proximity to the Sheephead fault, along with its inter- fault systems in the Ibex Hills (Fig. 2) as the up-dip, easternmost, portion of preted relationship to the Black Mountains detachment, an understanding of

B.M.

G.V. R.S. CA B.D. S.H. Plate 1 N.R. NV Death V CA alley I.H.

K.D. I.P. K.R. S.P. S.T.

FFaultault TTypesypes DetachDetachmmentent NoNormal 0 35 70 km StriStrike-slip UnkUnknownown

Figure 1. Map showing the general locality of the study area along with nearby structures and geographic areas referred to in the text. Right portion shows the location of the Death Valley region within the context of California. The black inset shows the southern Death Valley region. On the left is southern Death Valley with major structures and localities identified. I.H.—Ibex Hills, I.P.—Ibex Pass, S.P.—Saddle Peak Hills, S.T.—Saratoga Hills, S.H.—Sheephead fault, G.V.—Grandview fault, R.S.— Resting Springs Range, N.R.—Nopah Range, K.R.—Kingston Range, K.D.—Kingston Range detachment, B.M.—Black Mountains, B.D.—Black Mountains detachment.

A A' 2.5 Black Mtns. BMD

km Ibex Hills

5 km

BMD(?) B.M. Ibex Hills G.V. A R.S. B.D. S.H. Plate 1 Normal fault related to N.R. SW-NE extension. Death

Valle I.H. y A' Figure 2. (A) Simplified cartoon showing a singular Black K.D. I.P. Mountain detachment (BMD) surface projected under the K.R. Ibex Hills. (B) Alternative scenario in which the faults associ- S.P. ated with the BMD lie structurally above older, normal faults S.H. related to SW-NE extension (red). (C) Location map; refer to Figure 1 for labels. This map differs, however, by showing FFaulltt TTypesypes Detachhmmentent potential SW-NE extensional structures in the Ibex Hills. The Normal StriStrike-se-sliplip Kingston Range detachment and portions of the faults in Unknown the Ibex Hills are colored red to indicate their relationship to SW-NE extension.

the geologic history of the range should provide better constraints on the tec- basin system, establish two phases of Mesozoic contractional deformation and tonics of the area. For example, the role of strike-slip faulting, as compared to two phases of extension. The Neogene structures are also clearly associated normal faults and their related detachment surfaces in the extensional history with at least two phases of kinematically distinct strike-slip systems that include of the Death Valley region has been a point of debate (e.g., Miller and Pavlis, the Sheephead fault system (Fig. 1). We then assess the regional correlation 2005). Despite the pivotal location of the Ibex Hills, most recent work (e.g., of these structures and discuss their significance and relation to the Black Corsetti and Kaufman, 2005; Petterson et al., 2011) has been limited to topical Mountains and Kingston Range detachments (Figs. 1 and 2). In doing so, we stratigraphic studies lacking a focus on the regional structure at the heart of add to the understanding of the tectonic history of the Death Valley region, reconstruction controversies. The most detailed published mapping in the which should guide future work and aid in assessment of extensional tectonic Ibex Hills is that of Wright and Troxel (1968); however, this mapping involved processes based on the region (e.g., Lutz et al., 2019, 2020). only a small portion of the range with a focus on talc deposits in the region. Therefore, an analysis of the structures of the Ibex Hills is long overdue and was the goal of the work we present herein. ■■ TECTONIC SETTING This study presents geologic mapping of the Ibex Hills, Saratoga Hills, northern Saddlepeak Hills, and the intervening topographic lowland, the Ibex The Death Valley region exposes rocks that range from middle Proterozoic to Pass basin, which provides clear indications of a complex structural history Quaternary in age. After the middle Proterozoic events that formed the crystalline (Plate 1). We begin with a description of the major structures and crosscutting basement for the region (Heaman and Grotzinger, 1992), the area experienced relationships that, together with stratigraphic relationships in the associated two cycles of sedimentary deposition: (1) middle Proterozoic deposition of the

Map Legend Orientationsion_Filtered Fold AxialFold T raceAxial Trace B faultsfault anticlineanticline foliation antiforantifom rm bedding synclinesyncline overtuoverbedrned beds synformsynform Fault ContacContats cts Indeterminate strike-slip exposedexposed noNorrmalmal inferred Thrust inferred infeStrikre-sliprred speculaspeculative tive (Faulltsno arer solidmalapp where exposed,rox_p dasrojectedhed quaternary where inferred, dotted where speculative. quaternary Normal fault bar and thrust teeth on unconfounrmityconformity hangingwalls.) Map Units QaQa- Alluvium QoalQoal- older alluvium A' Tmsms- Stirling megabreccia Tmnmn- Noonday megabreccia Tmbmb- Beck Springs megabreccia Tmcsmcs- Crystal Springs megabreccia Tss- Fine sediment undivided Tmg2mg2- Upper granite megabreccia Tg2g2- Gravels within megabreccias Tmg1mg1- Lower granite megabreccia TifTif- Felsic Intrusives TirTir- Rhyolitic Intrusives TidTid- Dacite Intrusives TiaaTiaa- Altered Andesite Intrusives Tvava- Andesite Tvrvr- Rhyolite Tvupvup- Upper pyroclastics Tvwtvwt- White Tu C Tvbpvbp- Basaltic pyroclasts Tvpuvpu- Pyroclastics Undivided Tvmpvmp- Middle pyroclastics Tvgavga- Altered Andesite Tvlvvlv- Lower volcanic units Comet Mine TbgTbg- Basal Gravel A B' CzCz- Zabriski Quartzite CwcuCwcu- Wood Canyon Fm. upper CwclCwcl- Wood Canyon Fm. lower ZsuZsu- Stirling Quartzite upper ZsmZsm- Stirling Quartzite middle ZslZsl- Stirling Quartzite lower ZjuZju- Johnnie Fm. upper ZjmZjm- Johnnie Fm. middle Area of Figure 5 ZjlZjl- Johnnie Fm. lower ZkuZku- Kingston Peak upper ZkmZkm- Kingston Peak middle ZklZkl- Kingston Peak lower ZnuZnu- Mahogany Flats Member-Nonday Fm. ZnmZnm- Radcli Member-Noonday Fm. ZnlZnl- Sentinal Peak Member-Noonday Fm. ZbZb- Beck Springs Dolomite ZhtZht- Horse Thief Springs Fm. Ycsicsi- Intrusive Gabbro Ycsuscsus- Crystal Springs upper shale Ycsuccsuc- Crystal Springs upper carbonate Ycsl2csl2- Crystal Springs lower carbonate Ycsl1csl1- Crystal Springs lower quartzite Xg - Basement Gneiss

E' Plate 1. Geologic map of the Ibex Hills, Saratoga To view Plate 1 at full size, please visit https:// Hills, and Ibex Pass area. To view Plate 1 at full doi.org /10.1130 /GEOS .S .14642631 or access size, please visit https://doi.org/10.1130/ GEOS.S E the full-text article on www.gsapubs .org . .14642631 or access the full-text article on www .gsapubs.org.

Tbg

Tif Tvpu

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0 3 6 km

Crystal Spring Formation; and (2) Neoproterozoic to late Paleozoic strata that P

ZABRISKIE ALEOZOIC form a thick, miogeoclinal succession that is widely recognized as the passive QUARTZITE margin assemblage deposited during the breakup of Rodinia (Stewart, 1972; WOOD CANYON Wright et al., 1974; Corsetti and Kaufman, 2003). The latter produced the bulk of FORMATION the sedimentary sequence recognized in southern Death Valley; this sequence includes the Neoproterozoic Horse Thief Spring Formation through middle STIRLING Paleozoic units of the Wood Canyon and Zabriski Quartzite Formations (Fig. 3). QUARTZITE During the latest Paleozoic and throughout the Mesozoic, much of western North America underwent a series of contractional deformation events. The Mesozoic events are collectively known as the Sevier orogeny (Armstrong, 1968; Fleck, 1970; DeCelles, 2004). The Sevier orogenic belt is a zone of gen- JOHNNIE erally thin-skinned thrusts that extend from southern Canada into eastern FORMATION California (DeCelles, 2004). In the Death Valley area, the Sevier fold-thrust system is complicated and appears to change strike from northeast to northwest (Burchfiel and Davis, 1971; DeCelles, 2004). While some authors have NOONDAY NEOPROT E included the northwest-striking structures as part of the Sevier orogenic belt DOLOMITE (e.g., DeCelles, 2004), others have proposed they are, in fact, a younger set of

Laramide thrusts overprinting older Sevier structures (Miller, 2003; Pavlis et ROZOIC al., 2014). While previous work suggests that overall contractional deforma- KINGSTON tion in the Death Valley region is thought to have ended no later than 70 Ma PEAK (Fleck, 1970; Walker et al., 1995), Pavlis et al. (2014) challenged that view with FORMATION a hypothesis of significant Laramide-age deformation in the region. Following Mesozoic–Paleogene deformation, the Death Valley region was part of a Cordilleran highland analogous to the Andean Altiplano, commonly referred to as the Nevadaplano (Ernst, 2009). Erosion during this interval vari- BECK SPRING DOLOMITE ably exhumed the previously deformed miogeoclinal strata, recorded now as a widespread unconformity beneath Neogene strata with rocks beneath the HORSETHIEF unconformity, ranging in age from Proterozoic basement to Cambrian (e.g., SPRINGS FORMATION Pavlis et al., 2014). MESOPRO T Neogene extension associated with the Basin and Range began in the Miocene and continues today as active transtension (e.g., Burchfiel and CRYSTAL SPRING Stewart, 1966; Norton, 2011). Despite the consensus that the region is under- ERZOIC FORMATION going transtension at the present time, there are still disagreements about the kinematic histories of Neogene structures that make up the Death Val- ley extensional terrane (e.g., Holm and Wernicke, 1990; Holm et al., 1992; GNEISS Miller and Pavlis, 2005). The primary debate centers on the origin of low-angle BASEMENT

500 m detachment systems that are exposed along the eastern side of Death Valley in the Black Mountains (Figs. 1 and 2). In that area, the detailed mapping by Wright and Troxel (1984) formed the basis of some of the early hypotheses on the role of low-angle normal faults in extension (Wright and Troxel, 1973). Figure 3. Generalized stratigraphy of the southern Death Valley area. Modified from Mahon and Link (2013) and expanded upon from this work. Further studies (e.g., see review in Miller and Pavlis, 2005) revealed that successively deeper structural levels are exhumed from SE to NW across the Black Mountains, from high-grade metamorphic rocks in the Death Valley within the Death Valley turtlebacks in the central Black Mountains showed evi- turtlebacks to low-temperature brittle deformation in the SE. Although this dence of an exhumed, deeper-level detachment that formed prior to the main observation supports the concept of a single detachment system across the Black Mountains detachment system. Moreover, Miller and Prave (2002) and Black Mountains (e.g., Holm and Wernicke, 1990), several observations are at Miller and Pavlis (2005) argued against the occurrence of even a single detach- odds with this generalization. Serpa and Pavlis (1996) noted that structures ment, arguing instead for complex block rotations within a 3D, transtensional

environment. Additional complications include significant structural relief on Wernicke, 2000); yet there is little published mapping of the area. Published fault systems within the central Black Mountains, which include the antiforms mapping in the Ibex Hills is limited to localized mapping of talc deposits by L. of the Death Valley turtlebacks and curviplanar fault surfaces within the Amar- Wright in the 1960s (Wright and Troxel, 1968) and reconnaissance mapping for gosa chaos (Fig. 1). These curviplanar surfaces are often assumed to be primary the 1:250,000 Trona sheet (Jennings et al., 1962). These maps predate concepts corrugations in fault systems (Otten, 1976), but Mancktelow and Pavlis (1994) of low-angle normal faults with most normal faults shown as thrust faults; and Serpa and Pavlis (1996) presented evidence that these features are folds yet the faults exhume crystalline basement in their footwalls, consistent with that developed coincident with the extension due to distributed transcurrent significant extensional exhumation. The presence of these low-angle faults motions accompanying the extension. has resulted in the interpretation that they represent an up-dip extension of The Kingston Range detachment system (Fig. 1) represents the most signif- the Black Mountain detachment (e.g., Holm and Wernicke, 1990; Snow and icant extension that predates northwest-southeast extension, and it is exposed Wernicke, 2000), implying these faults represent the product of top-to-the-NW most clearly along the front of the Kingston Range and farther south in the extension younger than ca. 11 Ma (Fig. 2A). Halloran Hills (e.g., McMackin, 1992; Davis et al., 1993; Fowler and Calzia, 1999). To the south of the Ibex Hills, Troxel et al. (1992) recognized that top-to- Initial discussion of detachment faulting in the vicinity of the Kingston Range the-SW movement and low-angle normal fault systems dominated the structure placed such structures within the context of major northwest-southeast exten- of the Saddlepeak Hills. Recent mapping and compilations of that geology sion in the region (Burchfiel et al., 1983). However, later work on the Kingston (Mahon and Link, 2013; Mahon et al., 2014) confirm that conclusion, indicating Range detachment showed a top-to-the-southwest sense of movement (e.g., older extension related to the Kingston Range detachment continued to at least McMackin, 1992; Davis et al., 1993). The timing of movement along the Kinston the Saddlepeak Hills (Fig. 1) and presumably into at least the basinal deposits Range detachment is constrained locally by a 13.4 Ma hypabyssal sill, which is to the south of the Ibex Hills. Here we examine the hypothesis that this defor- cut by the detachment, and the 12.4 Ma Kingston Peak pluton, which crosscuts mation extended even farther north, into the Ibex Hills themselves (Fig. 2B). the detachment (Calzia, 1990; Fowler and Calzia, 1999; Calzia and Ramo, 2000). To the west and northwest of the Kingston Range lies a suite of Tertiary-aged basins that have been key in understanding the evolution of southern Death ■■ METHODOLOGY Valley (Fig. 1) (e.g., Wright, 1974; Scott et al., 1988; Topping, 1993; Prave and McMackin, 1999; Fridrich and Thompson, 2011). These basins include the China Geologic Mapping and Cross-Section Construction Ranch Basin (Wright, 1954; Scott et al., 1988) and the Dumont Hills Basin (Prave and McMackin, 1999) (Fig. 1); however, other work has envisioned these basins Field mapping was done electronically via a tablet computer running QGIS as part of a larger-scale feature, the Greater Amargosa-Buckwheat–Sperry Hills with a data structure similar to that described by Pavlis et al. (2010). Mapping Basin (GABS Basin) (Holm et al., 1994). In the latter context, the GABS Basin was aided by georeferenced topographic maps and orthophotography data has since undergone significant attenuation through a combination of the Black layers viewed in real time. Topographic maps were U.S. Geological Survey Mountain detachment and strike-slip on the Grand View fault (Topping, 1993; (USGS) digital raster graphics (DRGs) scanned from 7.5′ quadrangle maps, Holm et al., 1994). In general, these basins record the deposition of a sequence and orthophotography was from USGS, ArcGIS Online, Google maps, and of alluvial fans and fanglomerates, coarse sandstones, and fine sandstones Bing maps. Spatial positioning was aided by a Wide Area Augmentation Sys- and mudstones (e.g., Prave and McMackin, 1999). Understanding the evolution tem (WAAS)–corrected GPS that provided real-time positioning through the of this basin system is important for the competing models of Death Valley field application. The geologic map (Plate 1) is a compilation of mapping by extension. For example, interpretation of these Tertiary basins as half-grabens Canalda (2009), Pavlis (2004, 2011, 2015, 2017), and Fleming and Pavlis (2018) containing asymmetric basin facies is consistent with later extension along with the pre-2018 data incorporated into an ArcGIS geodatabase through a a detachment (e.g., Topping, 1993). Alternatively, other authors documented combination of direct copy to the master and redigitizing linework to produce evidence of at least two, more symmetric, source directions for the fanglom- a final map. In addition, a portion of the northern Ibex Hills mapping was erates of the basins, more consistent with a strike-slip–driven, “pull-apart” essentially a revamped version of the work of Wright and Troxel (1968) using model of basin development (Prave and McMackin, 1999). modern orthophotos and digital elevation models (DEMs), along with some The presence of such a suite of complex features in the southern Death recent reconnaissance mapping. Valley region suggests an intricate extensional history. In line with that sug- The contact lines and orientation data were exported from QGIS as shape- gestion, the structure of the area is complicated by a number of crosscutting files and were then imported into Move 2017 http://www.mve.com/software( ). relationships and apparent structural overprinting. However, the body of pub- A digital DEM was downloaded from the USGS National Map Download Client lished works concerning the southern Death Valley area is limited. For example, (viewer.nationalmap.gov/). A 1° × 1° DEM of the area was imported into Move the Ibex Hills, a focus of this study, have been mentioned in relationship to along with the geologic mapping data. In the process of the import, the DEM the Black Mountain detachment (e.g., Holm and Wernicke, 1990; Snow and was cropped to only the study area. Within the Move program, the contacts

and orientation data were then projected vertically to the DEM to produce a TABLE 1. LOCATIONS FOR THE FAULT DATA USED IN THE 2.5-dimensional surface model of the geologic features. KINEMATIC ANALYSIS OF THE EASTERN BOUNDARY FAULT Prior to construction of cross sections, the stratigraphy of the study area was Y X Strike Dip entered into Move. These data included the names of the units, their thicknesses, (NAD83 11N) (NAD83 11N) and their ages. The ability to preload stratigraphic data into Move is a useful 3966403.494 556572.7964 31 72 feature which allows the user to draw a line in cross section, assign it a unit, and 3966346.883 556601.1087 20 72 then construct the units above and below automatically, based on the provided 3966132.49 556719.8179 10 78 3966132.963 556712.8839 150 75 stratigraphy. It is important to note, however, that the stratigraphic thicknesses 3966171.655 556780.5516 293 60 of some of the mapped units, namely the Beck Springs Dolomite, the Kingston 3966131.682 556715.8931 200 72 Peak Formation, and the Noonday Formation, change dramatically within the 3966463.129 556609.4744 267 64 study area. Therefore, the thicknesses entered into the Move program were not 3966463.026 556635.1416 327 84 always used in the projections of the cross sections. To counteract this issue, the 3966419.192 556683.1015 0 85 program allows for the change of unit thicknesses “on the fly” when constructing 3966419.433 556682.6189 204 78 3964806.744 556136.9463 251 60 different sections, thereby not affecting the thicknesses for other areas of the map. 3964804.021 556143.7516 57 48 3961284.477 553903.4329 40 70 3961555.062 553907.8042 222 60 Kinematic Analysis

To analyze the fault along the eastern flank of the Ibex Hills, herein referred also used in observations of the Kingston Range (Fowler and Calzia, 1999). The to as the Eastern Boundary fault, we used slickenside measurements within the reasoning for this method is that a NW-striking fault would have cutoff lines well-exposed fault zone to evaluate the kinematics (Table 1). In this study, we trending 90° from those of a NE-striking fault, assuming the beds they cut have use the modified odd-axis method of Bruhn et al. (2004), which is based on the a similar orientation. Therefore, if distinct groups of cutoff trends are observed, original method of Krantz (1988, 1989). The modified odd-axis method assumes they could suggest multiple fault orientations that may be associated with fault sets within a fault zone form a rhombohedral family of surfaces that slip different phases or extension directions. While the Ibex Hills experienced pre- in response to an incremental strain field. In this method, the fault planes, slip extensional deformation, evidence that we support in later sections of this paper, Supplemental Material 1: Bedrock Unit Descriptions vectors, and the poles to the movement plane for each fault (equivalent to the we believe assessing the geometry of normal faults in the Ibex Hills is a useful Pre-Extensional Rock Units: The Death Valley region has a well-known stratigraphy (Figure 2), but the Ibex Hills contain complex lateral facies changes in many of the units. Thus, B axis of a focal mechanism) are plotted on a stereonet. This method results in tool, especially when used in conjunction with the other evidence we present. lithostratigraphic units vary across the mapped area requiring some description of the stratigraphic units used in our mapping. clusters of poles to fault planes to which a best-fit pole of the clusters can be The pre-Neogene bedrock includes Mesoproterozoic basement and overlying Mesoproterozoic,

Neoproterozoic, and Paleozoic sedimentary rocks with several unconformities bounding sequences determined (e.g., Carvell et al., 2014). When applied to fault zone analyses, the within the stratigraphic assemblage (Figure 2). The most extensive and complex stratigraphic method typically yields either a simple cluster of B axes within the fault plane ■■ OBSERVATIONS AND RESULTS assemblage in the study area is the Pahrump Group which forms the base of the sedimentary cover across much of the region. The Pahrump Group is comprised of four formal lithostratigraphic units consistent with plane strain or a great-circle distribution of B axes that intersects (Figure 2): the Crystal Springs Formation, Horse Thief Springs Formation, Becks Spring Formation, and Kingston Peak Formation (Wright et al., 1974; Mahon and Link, 2013). Overlying the Pahrump the fault plane representing approximately uniaxial stress (Bruhn et al., 2004). Map Units: Stratigraphy of the Mapped Area group are the Neoproterozoic Noonday, Johnnie, and Stirling Formations followed by Ediacran- Cambrian strata of the Wood Canyon Formation and overlying Paleozoic rocks (Figure 2) (Wright et The odd axis is the pole to the best-fit plane of the B axes (Krantz, 1988, 1989). al., 1974). In general, these formal units are sufficiently thick that they are insufficient as map units for

the scale of this study. Thus, the units were subdivided into informal members described here. The odd axis is either contractional (maximum strain) or extensional (minimum Figure 3 shows a generalized stratigraphic section for the area. Formal

The Crystal Springs Formation is the oldest sedimentary unit in the mapped area and is made up

of carbonate and siliciclastic rocks intruded by mafic sills ranging up to 450-m in thickness (Wright et strain), which is then determined with slickenlines of a known slip sense. In both stratigraphic units for this area are too thick for routine mapping at scales al., 1974). The sills are medium to coarse grained diabase and gabbro which have been dated to 1.084 cases, the fault slip vector is simply 90° away from the B-axis maximum (plane- larger than ~1:100,000, and thus, like previous workers (e.g., Wright and Troxel, Ga, and these intrusive contacts have created talc deposits throughout the area (Heaman and Grotzinger, 1992). The Crystal Springs Formation has traditionally been divided into three informal strain case) or the fault plane/B-axis intersection, measured in the fault plane 1984), we divided the formal units into subunits for more detailed mapping. The members, the lower, middle, and upper units, in addition to the Mesoproterozoic intrusions (e.g. Wright 1 et al., 1974). Diabase intrusions are abundant in the lower and middle Crystal Springs members but (Bruhn et al., 2004). The intermediate and maximum (or minimum) extension details of how we divided the units can be found in the Supplemental Material , their presence in the upper member has been controversial. Calzia et al., (2008) cited intrusions within the upper Crystal Springs member in the Kingston Range, implying the upper member is a part of the directions are defined as the obtuse and acute bisectors of the movement plane but two features are important in evaluating our geologic map: same depositional sequence as the lower and middle members. In contrast, Mahon et al., (2014)

recently redefined what had been mapped as the upper member of the Crystal Springs Formation as a pairs (Krantz, 1988; Carvell et al., 2014). The advantage of this approach com- (1) Neoproterozoic rock units below the Johnnie Formation (Fig. 3) vary separate, distinctly younger rock unit, the Horse Thief Springs Formation which spans ~150 m. This pared to conventional kinematic analysis techniques (Marrett and Allmendinger, dramatically in thickness, facies, or both, across the mapped area. Thick- 1990) is that all slickenside measurements can be used in the analysis, even if ness variations are predominantly due to variable erosion across major only a few have known slip sense. In the case of the Eastern Boundary fault, we disconformities at the base of the Kingston Peak Formation, within 1 Supplemental Material. Description of map units of prefer the “odd-axis” method, given the well-exposed fault zones that contain a the Kingston Peak Formation, and the base of the Noonday Formation Ibex Hills and Saratoga Hills, as well as adjacent Ibex multitude of slip surfaces, which can then be analyzed as a whole. (Fig. 3), including syndepositional structure described in this paper. Lat- Pass basin. Please visit https://doi.org/10.1130 /GEOS .S.14642634 to access the supplemental material, and In addition to the odd-axis method, we also made use of fault bedding cutoff eral facies variations are prominent in the Kingston Peak and Noonday contact [email protected] with any questions. lines to assess the geometry of the normal faults in the Ibex Hills, a method Formations due to syn-rift deposition (Macdonald et al., 2013).

NEOGENE STRATIGRAPHIC SEQUENCES

Quaternary DOMAIN 3 Tg2 Tmn Tmb unconformity DOMAIN 2 Tg2 Tmb Tg2 Tg2 D Tmcs Domain 1 U Tmcs DOMAIN 1 Tg2 Tg2 Ts Tmb Tmg2 Ts base not exposed Tg2 fault? Tg2 Tmcs Tmg1 Tmcs angular unconformity Ts Upper Sedimentary Sequence Tg2 Tmg2 IBEX HILLS Tir S. PEAK HILLS

Tmb Tif Tvup DOMAIN 2 Tvr Tmg1 Tid Tvwt D Tva Figure 4. Comparison of the three basin domains U D intrusives Tiaa Tvbp Tvpu discussed in the text. The bottom right section U Tvwt Tvmp Tvpu is modified from the interpretations of Canalda Tvr Tvpu Tvga 0 750 1500m Tvlv (2009) for the Ibex Pass basin. For a detailed Tvpu explanation of the units discussed in the figure,

U Volcanic Sequence Lower D Tbg refer to Supplemental Material (text footnote 1). Tva angular unconformity pre-Neogene Tvga Tvlv

Tg2 ? Tmg1

? Tvbp D Tvwt ? D U Tbg Tif U ? Tvpu D U Tvup Tva Tvwt Tvpu Tvwt ? Tvpu X Tvbp U D U D Tiaa Tmg1

Tvwt

(2) Neogene units in the Ibex Hills sub-basin have a complex stratigraphy, sediments and landslide megabreccias. Based on our mapping, illus- but the stratigraphic sequence is recognizable in only three sub-areas, trated in Figure 4, Domain 1 is located in the northern Saddle Peak with each recording a distinct section subject to interpretation (Fig. 4). Hills–Ibex Pass area and is defined by a volcanic sequence overlain by Details are found in the Supplemental Material (footnote 1); but for the two megabreccias, themselves separated by a layer of gravels. Domain 2, purposes of this paper, the key observation is that this succession is to the west, contains similar gravels but also a sequence of fine-grained composed of two sequences separated by an angular unconformity: clastic rocks in the lower portion of the section, along with distinct mega- a lower volcanic sequence, tentatively correlated to the Wingate Wash breccias of Noonday and Beck Spring Dolomites (Fig. 4). Domain 3, to Volcanics exposed to the west and north (Luckow et al., 2005; Canalda, the north of Domains 1 and 2, again contains gravels overlying mega- 2009) and an upper sedimentary succession composed of interbedded breccias; but here the volcanic sequence, seen below the sediments of

Domains 1 and 2, is not observed. Instead, the gravels of the basin lie interpret this array of faults as a sinistral fault system, and the net slip across unconformably atop the tilted bedrock of the Ibex Hills (Fig. 4). the zone is ~1.5 km based on offsets of the Johnnie-Stirling contact. The relative age of this sinistral fault array is well established by crosscutting relationships with intrusives and the unconformity. Specifically, although Structure of the Ibex Hills Area some of the faults in the array are overlapped by the unconformity (Fig. 4), others cut the unconformity. Similarly, a small intrusive body cuts faults in the General Map Relationships array (Fig. 4), while just to the west, a different intrusive rock is fully involved in the fault array (Fig. 4). Finally, at the northwest tip of the Proterozoic expo- From the Saratoga Hills, northward to the central Ibex Hills (Plate 1), the sures in the northern Saddlepeak Hills (Plate 1), a low-angle normal fault places rocks are structurally coherent and dominated by steeply dipping rocks the upper Stirling Formation on middle Johnnie Formation but is cut by the composed primarily of the Proterozoic Pahrump Group. Bedding strikes are sinistral fault array. Because this low-angle normal fault shows cutoff lines generally NNE but curve from NE to NS across the Saratoga Hills, indicating with a NW trend, we conclude this fault records top-to-the-SW motion similar a large, steeply SE-plunging, open syncline. Additional structural complexity to faults seen just to the south in the core of the Saddlepeak Hills (Mahon and is indicated in the northern Saratoga Hills, where the Noonday and Johnnie Link, 2013). However, because of the mutual crosscutting relationships of the Formations lie in low-angle fault contact with the Crystal Springs Formation faults with the unconformity and igneous assemblage, we conclude that the and the Beck Springs Formation (Plate 1), a stratigraphic omission of at least sinistral fault array is also related to the SW-directed extensional event, pre- 500 m of Kingston Peak Formation, indicating that these are low-angle normal sumably as a displacement transfer structure. faults. Significantly, these low-angle normal faults are also cut by an array The unconformity seen at the base of the Neogene in the northern Saddle of higher-angle faults that strike both northeast and northwest (Plate 1). The Peak Hills is notably absent to the north in the northern Ibex Hills (i.e., Domain 1 Tertiary sedimentary rocks deposited in the adjacent basin are presumably of Fig. 4). Instead, in Domain 1 (Fig. 4), the Proterozoic is directly overlain by Ter- concurrent with the slip along these low-angle faults, but direct observation tiary gravels, and the volcanic sequence seen in both the southern Ibex Hills and of this relationship is obscured in the northern Ibex Hills by the presence of Saddle Peak Hills is absent. We suggest that this relationship necessitates a fault the Eastern Boundary fault, which cuts the basin deposits up to the youngest between Domains 1 and 3 (Fig. 4) structurally below the upper gravel sequence. megabreccias and is only overlain by the Quaternary units. A description of Incongruities between Domains 2 and 3 are also likely the result of faults within these units is provided in Plate 1. the basin (Fig. 4). These incongruities include differences in the thicknesses of In the northern Ibex Hills, the map pattern is significantly different and is volcanic sequences below a conspicuous tuff layer (Fig. 4). In the southern Ibex dominated by the Noonday and Johnnie Formations, along with small expo- Hills, Domain 2, the sequence below the tuff is significantly thinner than that seen sures of younger rocks, especially in the northeast corner of the mapped area in the northern Saddle Peak Hills, Domain 3 (Fig. 4). For a complete description (Plate 1). Structural complexity increases dramatically to the north from rela- of the Tertiary map units, see the Supplemental Material (footnote 1). tively simple, homoclinal rocks of the Saratoga Hills and central Ibex Hills to complexly faulted rocks in the northern Ibex Hills (Plate 1). The orientations of the rocks in the north also change at a variety of scales, primarily due to Eastern Boundary Fault and Ibex Pass Basin multiple faults with complex crosscutting relationships. Nonetheless, bedding dips generally are shallower than those to the south (Plate 1). The Ibex Hills are bounded to the east by the steeply to moderately dipping Eastern Boundary fault (EBF) (Plate 1), which separates the Ibex Hills from the adjacent Ibex Pass basin. In the south, the fault is moderately well exposed Sinistral Faulting at the Southern Margin of the Ibex Pass Basin in discontinuous exposures along the NE-trending, eastern flank of the Ibex Hills. Along this segment, the fault curves from a strike of ~015° near Ibex The northern Saddlepeak Hills expose the southern margin of the Ibex Pass Spring in the south to ~030° near the Giant Mine with relatively uniform dips of basin in a composite boundary that includes both extensive exposures of the 45°–60°. This geometry produces low-angle, fault-bedding intersections in the basal Neogene unconformity and fault contacts. Throughout this area, the Proterozoic rocks. Thus, despite clear evidence of significant down-to-the-SE basal unconformity is extensively exposed along a gently north-dipping contact slip based on the outcrops of Cenozoic rocks in the basin, the stratigraphic (Fig. 4; Plate 1) marked by either volcanic rocks lying directly on Proterozoic shifts across the fault are modest below the Tertiary unconformity (e.g., Crystal rocks or a 0–30-m-thick gravel lying between the volcanics and Proterozoic. Springs against Crystal Springs). Just to the south of the unconformity, however, is an array of ~EW-striking, In the northern part of the Ibex Hills, the EBF becomes complex with contra- subvertical faults that show subhorizontal slickenlines and have consistent dictory crosscutting relationships and complex structure in both the hanging sinistral offsets of contacts in the east-dipping Proterozoic rocks. Thus, we wall and footwall. From the Giant Mine to just south of the Eclipse Mine (Fig. 5;

Plate 1), the fault continues at a strike of ~030° but clearly truncates a series of moderate- to high-angle, N-NW–striking normal faults in both the hanging wall and footwall. On the map (Plate 1), these faults are obvious in the footwall but are partially obscured by younger sediments in the hanging wall, including the uppermost landslide megabreccias (Fig. 5; Plate 1). The most Fault E significant of these hanging-wall structures (labeled X in Fig. 5) show evidence of extended slip during the deformation with consistent, east-side-down slip. D That is, the unconformity beneath the Neogene gravels shows a small shift

U indicating east-side-down, but rocks beneath the unconformity show a much greater shift across the fault with Crystal Springs on the west and Johnnie Formation to the east (Plate 1). Three other NNW-trending faults show similar sub-unconformity shifts in this segment as well as a complex array of faults with smaller shifts. Clear relative age relationships are present along the seg- D ment of the fault between the Giant Mine and east of the Comet Mine where U a fault sub-parallel to the EBF cuts the younger gravels (Tg2); yet, the fault D and gravels are overlapped by the Beck Springs megabreccias (Tmb) (Sup- U Fault D plemental Material [footnote 1]) (Fig. 5; Plate 1). That landslide megabreccia, in turn, is also cut by the EBF. Thus, the EBF in this segment is a pair of faults; but slip on the main segment of the EBF outlasted slip on the fault to the SE. Just south of the Eclipse Mine (Plate 1), the EBF is associated with complex faulting that can be interpreted in several ways. In that area, the NE-trending U EBF to the south meets a fault system striking ~340° with a moderate (30°–50°) D east dip. This NNW-trending fault segment is either part of a curved EBF or Fault C represents a distinct fault system with mutual crosscutting relationships to

U the EBF. The curved fault hypothesis is supported by the NNW-trending fault D showing similar geomorphic expression as the EBF, although there are no clear displacements of Neogene rocks along this segment to confirm this Fault B D hypothesis. In contrast, the NNW-trending fault is clearly continuous with a Fault A U fault that continues through the bend to the south as an apparently continuous structure, placing Stirling Quartzite to the east against Johnnie and Stirling Formations to the west. Nonetheless, structures with trends similar to the

D southern segment of the EBF also continue NE from the bend. Finally, even farther north in the Eclipse Mine area, the NNW segment of the D U U EBF intersects a second zone of NE-striking faults. Like the bend to the south, U this zone displays contradictory crosscutting relationships and allows multiple

D interpretations. Nonetheless, traces of the NNW segments appear to be present up into the northernmost Ibex Hills and consistently place Stirling Quartzite X 0 500 1000 m and younger formations along the eastern flank of the range. Given this correlation, the Eastern Boundary fault appears to have an approximate heave of Figure 5. Portion of our geologic map, which highlights some of the faults, along with 4 km, based on the map distance between the Stirling Quartzite in the central key crosscutting relationships discussed in the text. Low-angle normal faults are shown Ibex Hills to that in the north (Plate 1). Complicating this correlation, however, by solid highlighted lines with boxes on the hangingwall. Moderate- to high-angle faults is the loss of the high-angled EBF in the northern Ibex Hills, which suggests are shown as dashed highlights with up-down markers along their trace. The Eastern that the fault may become part of the currently low-angle fault system (Plate 1). Boundary fault (EBF) is also shown along the eastern flank of the range as a solid highlight. Unit labels and colors follow that of Plate 1. Area of Figure 9 in the solid box. Kinematic analysis on the Eastern Boundary fault in the northern Ibex Hills and along the eastern front near the Wonder Mine (Plate 1) shows an ortho rhombic fault geometry within the fault zone (i.e., two sets of conjugate fault systems), albeit with a more dominant N-NE–striking set (Fig. 6). In general, the

slickenlines have rakes of 30° or less, mostly from the S-SW (Fig. 6). The B axes show a distinct cluster (Fig. 6B) plunging moderately to steeply to the north. With a master fault dipping steeply to the ESE, this B-axis cluster, together with the corresponding odd axis (Fig. 6B) and known stratigraphic shifts, indicate the odd axis is an extension direction, and the eastern boundary fault is an oblique, dextral-normal slip fault. This is consistent with the average orientation of the fault. Collectively, these observations suggest the EBF is a composite structure with the main fault to the south representing an oblique, dextral-normal fault that interacts complexly with a NNW-striking fault system to produce two apparent bends in the fault trace of the EBF. However, based on the kinematic analysis of the southern portion, south of the Comet Mine area, a component of dextral movement clearly displaces units as young as Stirling Quartzite. This fault relationship is maintained to the north, albeit with some complex crosscutting relationships. Figure 6. (A) Stereonet of fault planes and slickenlines along the Eastern Boundary fault (EBF). (B) Plot of the B axes and their best-fit plane, the pole to which is the “Odd-Axis.” Red plane indicates the map trace of the EBF with the rake of the slip vector also plotted. Younger Normal Faults

A set of nearly orthogonal high-angle normal faults crosscuts the low-angle n = 47 normal faults of the northern Ibex Hills (Fig. 5). The dips of these faults range from ~50° to >80° (Fig. 7). The majority of the measured faults in the northern Ibex Hills strike to the northeast with a range of ~030º to ~070° with a second group striking predominately north-south to northwest-southeast, consistent with mapped faults (Plate 1). These N-S–striking faults generally dip at higher angles than the predominately NE-striking faults, with many near vertical (Fig. 7). Fault-bedding cutoff angles for the high-angle normal faults in the northern Ibex Hills are generally >60° with a few as low as 30° (Fig. 8). The range of cutoff angles is presumably due to the dip of beds before faulting. Cutoff lines for these faults also consistently trend NE-SW (Fig. 8), consistent with an origin as post–11 Ma, extensional structures developed during NW-SE extension. n = 22 Figure 8E shows the expected geometry of that scenario, in which east-dipping beds are cut by a NE-striking normal fault, thus producing NE-SW cutoff lines. Figure 7. (A) Equal-area stereographic projection plot of low-angle normal fault in the northern Ibex Hills with slickenlines plotted as red dots, poles to fault planes as black The influence of both the northeast- and northwest-striking normal faults dots, and best-fit “fold axis” in blue. (B) Equal-area stereographic projection of poles in the northern Ibex Hills can be seen most clearly in the southern portion of to the planes, Kamb contoured, of the moderate- and high-angle faults in the Ibex Hills. the area (Fig. 5), where they cut into basement rock and displace the Crystal Springs Formation and the overlying Horse Thief Springs Formation. The faulting in the area results in a sequence of blocks in which the hanging walls of the Low-Angle Normal Faults faults are being dropped successively down to the northwest (Fig. 5; Plate 1). Despite the similar orientations of the higher-angle normal faulting in the Observations. We recognize five sets of currently low-angle normal faults southern Ibex Hills to that in the northern Ibex Hills (Plate 1), the crosscutting in the northern Ibex Hills (Fig. 5; Plate 1). Faults A–D (Fig. 5) form a stack of relationships are somewhat different. In the southern Ibex Hills, a pair of promi- subparallel faults that is best displayed in the central part of the northern Ibex nent, NW-striking, high-angle faults clearly truncate the north-northeast– striking Hills (Plate 1). The structurally lowest fault in this sequence (Fault A, Fig. 5) faults (Plate 1). These N-NE–striking normal faults in general have a down-to- places the middle Crystal Springs Formation in the hanging wall against both the-NW sense of motion. However, some have a minor down-to-the-SE sense crystalline basement and the lower Crystal Springs in the footwall. In map of displacement. Farther south in the Saratoga Hills, the high-angle, W-NW– view (Plate 1), this fault, where it is exposed, is essentially bed parallel in the striking faults appear to be truncated by NE-striking normal faults that down hanging wall but has mostly basement rock in the footwall, with only a small drop the Johnnie Formation in the hanging wall (Plate 1). isolated lens of lowermost Crystal Springs Formation present. The continuation

Fault-Bedding Intersections (Low Angle Faults) 4

2 Count

0 0 10 30 50 70 90 n=14 Angle (Degrees) Figure 8. (A) Lines of fault-bedding intersec- Fault-Bedding Intersections (High Angle Faults) tions of the low-angle normal faults in the Ibex Hills. Light-gray lines indicate cutoffs of Faults 3 A and B. (B) Lines of fault-bedding intersections of the high-angle normal faults in the Ibex Hills. (C) Histogram of the angle of fault-bedding intersections of the low-angle normal faults in the 2 Ibex Hills. (D) Histogram of the angle of fault-bedding intersections of the high-angle normal faults in the Ibex Hills. (E) Hypothetical example Count of east-dipping beds cut by a NE-striking fault 1 and resulting cutoff trend. (F) Hypothetical example of east-dipping beds cut by a NW-striking fault and resulting cutoff trend. 0 0 10 30 50 70 90 n=12 Angle (Degrees) E

N SE Plunge N

NE Plunge

of this fault to the south is ambiguous because it is truncated by a series of unconformity in the north, through the Horsethief Spring in the south (Fig. 9), moderately NW-dipping normal faults that produce multiple repetitions of with a similar pattern in the footwall cutting downsection from uppermost the nonconformity beneath the Crystal Springs. This crosscutting relationship Crystal Springs to near the base of the upper member in the south (Fig. 9; demonstrates that Fault A is older than these NW-dipping faults; however, map Plate 1). The hanging wall of Fault A is partially obstructed by the offset of Fault geometry is ambiguous on the kinematics of Fault A. B, making cutoffs less clear than Fault B. Nonetheless, west of the Comet Mine Structurally above Fault A is a series of subparallel, low-angle faults that (Plate 1), Fault A cuts downsection to the south from the upper Crystal Springs carry Horse Thief Springs, the Mahogany Flats member of the Noonday For- to the lower Crystal Springs (Fig. 5; Plate 1). The hanging-wall cutoff relation- mation, and the Johnnie Formation in their hanging walls, labeled Faults B, ships are particularly clear at the arrow in Figure 4 where the lower-upper C, and D, respectively (Fig. 5). Faults C and D show distinct fault intersections Crystal Springs contact is cutoff along the fault. Collectively, these observations with branch lines plunging NE, whereas those of Fault B generally trend NW. of the orientations of bedding cutoff lines (Fig. 8A) and map-scale stratigraphic This sequence of stacked faults disappears to the northwest where rocks in the juxtapositions suggest that Faults A and B originated as NW-striking normal hanging wall of Fault D (Johnnie Formation) overlie fault slivers of the Noonday faults, recording an older phase of NE-SW extension. Either observation alone Formation, in the Fault C hanging wall; these slivers in turn lay on crystalline could be interpreted as an artifact of pre-extensional structure, but together, basement. The structurally highest low-angle fault—Fault E (Fig. 5)—carries they are difficult to reconcile as products of NW-SE extension. For example, Stirling Quartzite and Wood Canyon Formation in its hanging wall, juxtaposed regional relationships (Pavlis et al., 2014) and sub-Tertiary unconformity geol- on middle Johnnie Formation in the footwall. Fault E is only exposed in the ogy (Plate 1) suggest that prior to extension, the units dipped NE, and, thus, NE corner of the mapped area (Plate 1). a NE-striking normal fault, which would be expected to form from NW-SE The NE-striking faults discussed in the previous section crosscut all struc- extension, could produce the observed map-scale relationships. However, tural levels of the low-angle normal faults in the mapped area (Fig. 5). However, cutoff lines would trend N to NE, nearly 90° from measured cutoffs (Fig. 8). the timing is somewhat ambiguous because they crosscut the low-angle Faults The map-scale relationships are different for structurally higher faults. The A and B. Crosscutting relationships with Faults C and above were not clearly geometry of Fault C is partially obscured by down-to-the-northwest slip along observed (Fig. 5). In addition, the relationship between the low-angle faults northeast-striking normal faults as well as the cutoff by Fault D. However, Fault and the steep frontal fault to the east, the Eastern Boundary fault (EBF), is not C does appear to cut out a section of the Radcliffe Member of the Noonday entirely clear (Fig. 5). In the central Ibex Hills, the EBF crosscuts Fault B (Fig. 5). Formation toward the northwest, suggesting geometry comparable to Faults However, in the northern Ibex Hills, the exact trace of the EBF is difficult to A and B. Nonetheless, because of lateral stratigraphic changes of the Noonday decipher, and relationships to structural higher faults are unclear. (e.g., Corsetti and Kaufman, 2005), it is possible this juxtaposition is a combined Analysis of the cutoff angles of the low-angle normal faults shows that stratigraphic and structural effect. Fault D has a more clearly defined geometry. for the 14 faults analyzed, half of the faults have high-angle cutoffs >45°, In the east-central portions of the range, the oolite marker bed in the upper John- but 36% have cutoff angles of less than 20° (Fig. 8C). In contrast, all of the nie is placed directly against the Mahogany Flats Member of the Noonday along younger high-angle faults have cutoff angles greater than 30° (Fig. 8D). These Fault D (Fig. 9; Plate 1). Farther to the north and west, Fault D cuts downsection observations can be interpreted a number of ways. If bedding had rela- in the hanging wall, placing the middle member of the Johnnie Formation on tively low dips prior to extension, then approximately half of the low-angle Noonday (Plate 1). Faults C and D are interpreted to merge within the mapped normal faults nucleated as low-angle faults with the remainder nucleating area and thereby place the Johnnie and Noonday Formations onto basement in as high-angle faults. Given the map pattern and observation of variable northernmost portions of the mapped area (Plate 1). However, Quaternary cover angular discordance beneath the Tertiary unconformity, it is more likely in the wash in the north partially obscures the contact. In addition, there are these variations reflect differences in the initial bedding dips, prior to the high-angle fault surfaces at the northern and northwest extent of the Noonday extension. Nonetheless, the orientation of the cutoff lines for the low-angle Formation, suggesting the possibility of a younger structure that is responsible faults shows a general south trend, scattered along a low-dip great circle for juxtaposing the formations against basement rock. (Fig. 8A), which is distinct from the shallow NE-SW trends of cutoff lines Although low-angle normal faults are most prominent in the northern Ibex for the younger high-angle faults (Fig. 8B). Most significant, however, are Hills, low-angle normal faults are also present in the southernmost Ibex Hills SE-trending cutoff lines for Faults A and B (gray dots, Fig. 8A), which are and northern Saratoga Hills (Plate 1). The low-angle structures in this area distinct from the general NE-SE trend of other cutoff lines (Figs. 8A and 8B), carry the Johnnie Formation in the hanging wall and place it against the Crystal suggesting Faults A and B originally had NW-SE strikes. Figure 8F illustrates Springs Formation in the southern Ibex Hills and the Beck Springs Formation this concept by showing a NW-SE–striking fault cutting east-dipping beds; in the northern Saratoga Hills (Plate 1). Along the northern flank of the Sara- this fault produces SE-trending cutoff lines. toga Hills, this fault dips NW at ~45°, whereas the fault in the southern Ibex Faults A and Fault B both cut down stratigraphic section to the south Hills dips to the SE at a similar angle, although the dip of the fault appears to (Fig. 9). Fault B cuts down section in the hanging wall from the basal Noonday steepen to the north (Plate 1).

Figure 9. Portion of our geologic map in the F ault A northern Ibex Hills showing an area where Faults A and B cut up stratigraphic section. Arrows indicate the areas where unit contacts are faulted, contacts are highlighted with semi-transparent thick lines with blocks on the hanging-wall side. Unit labels follow the legend of Plate 1. Fault B

Interpretations. These field relationships suggest strongly that Faults A C, and A (Fig. 5). The stereonet plot (Fig. 7) for measured low-angle normal faults and B, and probably C, formed during NE-SW extension. This is supported by forms a steeply dipping great circle indicating a general fault curvature about an the stratigraphic cutoff lines that trend south to southeast (Fig. 8), consistent axis plunging shallowly to the southwest (Fig. 7). This pattern is consistent with with faults that cut downsection to the southwest (Fig. 9). Additionally, Fault B the general synformal map pattern of the northern Ibex Hills seen as both bed- carries the conspicuous basal unconformity of the Noonday Formation above ding dips and younger rocks surrounded by older rocks. Curiously, slickenlines a nearly absent Kingston Peak Formation (Plate 1). In contrast, the structurally for the same low-angle faults have rakes currently greater than 70° (Fig. 8) and higher low-angle faults, as well as NE-striking, high-angle normal faults, are scattered about a WNW-ESE axis (Fig. 7), which is inconsistent with the evidence interpreted as products of NW-SE extension based on oppositely oriented that Faults A and B are top-to-the-SW normal faults. Given the fault curvature, stratigraphic cutoffs (Figs. 8 and 9). Low-angle normal faults are also present however, it seems likely that the slickensided surfaces are recording an over- in the Saratoga Hills and have a top-to-the-NW sense of slip. Crosscutting printing related to folding of the fault surface or younger top-to-the-NW faulting. relationships indicate that the NE-striking normal faults are younger than Faults A and B, which have a top-to-the-SW sense of slip. Cross-Section Observations

Fault Kinematics Cross sections (Fig. 10) support the interpretation that the structurally lower Faults A and B represent earlier top-to-the-southwest motion, whereas Fault The northern Ibex Hills contain a few well-exposed fault surfaces that yield C and the structurally higher normal faults were the result of later top-to-the- kinematic information. The fault planes were measured primarily from Faults B, northwest movement. The projection of Fault B suggests that it crosscuts Fault

D D' A A' B - B' 2000 m -

Zb 1000 m - Zht Zju Qa Zkm Qa Zcsu Qa Zjm 0 m - Zj Zb Znl Znu Zju Xg Ycl Znm Zjm Ycu Znu Zh -1000 m - Ycu Znm Znm Ycl Zht Znl Zcsl Ycu Xg Xg Znl Xg -2000 m -

2000 m Tertiary Volcanic 4000 m

E' B B' E NNW SSE 2000 m- Figure 10. Cross sections created for the map area; refer to Plate 1 for Znu Znl Zh the trace locations. Unit abbrevia- Zj Zh Ycu Znm Zs 1000 m- tions follow those on the legend Ycu Ycl Zj Tmb Qa Tg2 of Plate 1. Section traces are also Znu Ycl Zkm shown on Plate 1. Xg Xg Ycu

Ycl 0 m- Xg Xg Xg Ycl

2000 m 2000 m

BasementLow Angle GneissFaults

C C' B - B' F F' 1000 m-

Zkl Znu Zj Qa 0 m- Zb Zkm Znm Zkm Zh Zkl Ycl Zb Xg Zb Xg Zh -1000 m- Ycu Ycl Xg

3000 m 1000 m

A along the western flank of the northern Ibex Hills, suggesting it was a younger SW-directed extensional structure that cut Fault A (A–A′ of Fig. 10). In addition, the current fault plane orientations (Fig. 7) suggest either folding about a SW-NW axis or reflect an original corrugated fault geometry, and this curvature was incorporated into the projection of faults A–D at depth (Fig. 10). The interpretation of Fault C as a NW-directed structure was based on the field relationships and projections of the structurally lower faults in cross section (Fig. 5), which suggested the fault crosscuts the structurally lower faults to the NW. In addition, sections were refined through the process of reconstruction, and the classification of Fault C as a NW-directed structure fit well within our models.

Pre-Extensional Structures

Scattered exposures show mesoscopic folds and bedding-cleavage relation- Figure 11. Field photograph showing the tight to isoclinal folds in the ships indicative of older, Mesozoic structure, and one small exposure of a thrust Radcliffe Member of the Noonday Formation. system was recognized in the northern Ibex Hills (Plate 1). Nonetheless, in most of the mapped area, extensional structures are sufficiently complex that resolution of pre-extensional structure is difficult. An exception is a site in the Noonday Formation (Figs. 11 and 12) that shows a complex fold geometry. The outcrop lies within a small valley with highly folded upper Radcliff member beds bounded N to the southwest and east by down-to-the-east normal faults (Fig. 5). The fault to the southwest cuts out lower Radcliff member, placing the folded upper beds against the Sentinel Peak member, which lacks the complex mesoscopic folding due to its massive bedding. To the northeast, the Mahogany Flats member of the Noonday lies in stratigraphic continuity with the Radcliff member (Fig. 5) but also lacks the conspicuous folding seen in the underlying Radcliff member. Fleming and Pavlis (2016) described this outcrop in the context of 3D outcrop development for the site, but its significance to the regional structure was only briefly considered. The outcrop exposes a system of non-cylindrical folds with mesoscopic folds showing curved hinges and changes in fold axis orientation within ~1 m. The folds range from open to sub-isoclinal with tighter folds typically within the interbedded shales (Fig. 11). The main-phase folds are associated with a pressure-solution cleavage that is locally axial planar but is typically a divergent cleavage fan (Fig. 12). The folding is most conspicuous in the thinly bedded, shale-rich units, but the cleavage is present in both carbonates and slates. Small-scale, steeply dipping faults, with ~1–3 m of apparent offset, Figure 12. Field photograph annotated to show the cleavage (red lines) permeate the outcrop and complicate simple interpretation of bedding traces. present in the Radcliffe folds. Black lines show bedding traces. To better understand the geometry of these non-cylindrical folds, separate domains of the outcrop were analyzed, and their fold axes were determined from groupings of two or more planar measurements in each domain. These domains but orientations of the second fold generation are unclear from Figure 13 alone. were determined based on perceived changes in fold orientation while in the Cleavage orientations from both the outcrop as well as nearby exposures of the field as well as analyzing the point cloud models. In addition to the field data, Radcliffe Member (Fig. 14) show significant scatter but are broadly consistent measurements from the point cloud were also plotted with the field data (Fig. 13). with a re-folding about a NE-trending axis. We suggest the scatter in Figure 13 The fold axes (Fig. 13A) and bedding-cleavage intersections (Fig. 13B) indicate reflects an original fanning cleavage developed along folds with an ~NS to NW F1 axes scattered around an ~E-W–striking, steeply dipping great circle. This trend; these folds were re-folded about NE-trending fold axes to produce both scatter is consistent with the observed curved fold axes and indicates re-folding, the curved fold axes and dispersion of bedding-cleavage intersections (Fig. 13).

Cross-Section Reconstruction Saratoga Hills

To develop a reasonable model for the structural history of the Ibex Hills and the Saratoga Hills, estimates of fault displacement were reconstructed using cross sections made throughout the ranges (Fig. 10; Plate 1). Figures 14–16 show the restorations, and Table 3 summarizes the total amounts of heave and stretch for sections A–A′, B–B′, D–D′, and F–F′. Restoration of cross sections from the Saratoga Hills (sections D and F, Figure 13. (A) Fold axis measure- Fig. 15) uses a simple approach of realigning contacts that were cut by the faults ments from the Ratcliffe outcrop in in question (Fig. 15). Using the move on fault module in Move, marker beds the Ibex Hills plotted on equal-area stereographic projections; fold were selected on either side of the fault and restored to pre-faulting condition, axes determined from direct mea- assuming cross sections are parallel to the slip direction of the faults (Fig. 15). N 18 surement. (B) Field measurements The structural geometry in the Saratoga Hills is much simpler than that seen of bedding-cleavage intersections to the north in the Ibex Hills (Fig. 10). The most significant difference is the lack of in the outcrop. Note that (A) and (B) are indistinguishable and con- curved, low-angle faults such as those seen in the northern Ibex Hills (Fig. 15B; sistent with curved fold axes seen Plate 1). There is, however, a low-angle fault in the Saratoga Hills that places in the field, with warping about a the Kingston Peak Formation atop the Beck Springs Dolomite (Figs. 6 and 15). NS– to N-NE–trending axis. This low-angle fault has clearly been cut by the higher-angle normal faults and exhibits the same relationships seen to the north in the Ibex Hills but with less ambiguity on initial fault geometries (Figs. 6), simplifying the restoration process (Fig. 15B). Therefore, analyzing the restored section of the Saratoga Hills provides a template for the more complex structure of the northern Ibex (Fig. 15B). The cumulative heave calculated from the restoration of cross section D–D′ in the Saratoga Hills is ~3 km (Fig. 15B). After reconstruction of the extension, a northwest-trending fold pair remains within the units and is interpreted to N 20 represent a pre- to syn-extensional fold in the Saratoga Hills. We interpret this fold system as a Mesozoic, pre-extensional structure because it predates both the low- and high-angle normal faults, and the pelitic layers in the Crystal Springs Formation are slates with bedding-cleavage relationships consistent with this fold geometry. In addition to fault restoration, the effects of range tilt were also restored using the orientations of Neogene volcanic deposits, and Figure 15B shows the results of this tilt restoration. Following this rotation and after the restoration of the moderate- and high-angle normal faults, the restored heave is ~1.7 km. However, a low-angle fault surface remains with Kingston Peak Formation and younger rocks in its hanging wall (Fig. 15B; Table 3). Based on the projections Figure 14. Stereonet of poles to of the restored section, the heave of this low-angle surface is 1.3 km. foliation measured in the Radcliffe Member in the Ibex Hills. Great Restoration of section F–F followed the same workflow as the other sec- ′ circle is the best fit to the data, tions and shows a total heave of ~800 m and a throw of ~135 m (Fig. 15D; and the large pole indicates the Table 3). Given that section F–F′ is oriented E-W and therefore oblique to the interpreted fold-axis of F2. regional NW- and SW-directed transport directions, it is not surprising that it has the lowest amount of extension, although it does cross one significant normal fault (Fig. 15D; Table 3). The syncline of the fold pair recorded in cross

section D–D′ also projects into F–F′ and constrains the orientation of the fold n 54 as trending northeast-southwest (Fig. 15).

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Figure 15. (A) Cross section D–D′ in the Saratoga Hills showing the current geometry of the range. (B) After the fault movement has been restored across the range. (C) Cross section F–F’ in the Saratoga Hills showing the current geometry of the range. (D) After the fault movement has been restored within F–F’.

Ibex Hills to B–B′, was the lack of the Kingston Peak and Beck Springs Formations, which are not present below the Noonday Formation in ranges north of the Ibex Hills Restoration in the northern Ibex Hills was more complex because our (Wright et al., 1974). Restored movement along Faults A and B totaled ~2.4 km interpretation of two phases of extension, along different trends, indicates a of heave (Fig. 17E). The effects of the high-angle fault restoration were mini- three-dimensional process (Figs. 16 and 17). Figure 15 shows sequential restoration mal; therefore, the NE-SW extension of the range is primarily accommodated of section B–B′. The restoration first restores motion along the high-angle normal by the low-angle faults (Fig. 17) with a net horizontal displacement of ~2.4 km faults (Fig. 16B) revealing the earlier geometry of the low-angle normal faults. At across the mapped area (Table 3). this stage, however, range tilt has not been restored and low-angle normal faults The summary of total heave and throw along the faults in the northern retain their easterly dip (Fig. 16B) with apparent thrust motion but younger-on-older Ibex Hills is provided in Table 3. In general, the majority of displacement is stratigraphic juxtaposition. Thus, the same tilt restoration as the Saratoga Hills accommodated by the low-angle normal faults, in both the NW-SE and NE-SW is used. Note that the origin of this tilt is unconstrained in the mapped area but extension directions. The total extension in the NW-SE (as shown by section presumably represents a deeper detachment as envisioned by Serpa et al. (1988), B–B′) direction is more than twice that of the NE-SW directions, and ~10% of relatively young folding across the range (e.g., Miller et al., 2007), or both. None- that displacement is along the younger, high-angle normal faults. In compar- theless, the geometry of the Tertiary geology must be considered in the restoration. ison, in the NE-SW direction (as shown by section A–A′), the displacement Figure 16C also includes projected beds in the footwall of Fault C, although along high-angle normal faults was found to be negligible. the orientations of these beds are not well constrained because they are currently covered by the Tertiary basin to the east of the Ibex Hills (Fig. 1; Plate 1). Therefore, the footwall geology assumes the simple interpretation of homo- ■■ DISCUSSION clinal bedding based on regional range tilting and not by any more complex structure. For example, none of the folding and faulting present in the Ibex Overprinting Deformation in Southern Death Valley Hills is included in the footwall interpretation. The presence of faulted Johnnie Formation atop much of the northern Ibex Older Structures Hills (Plate 1) presented a space problem when attempting to restore the unit along Fault D. To address this, two inferred faults were projected into the res- The geology of the southern Death Valley region records a multi-phase toration at the point of Figure 16C. Given the history of late NW-SE extension deformational history that includes at least two phases of contraction and two in the region, this inference of NE-striking faults in the Fault D hanging wall phases of extension. The contractional history is best displayed at the focus site is a reasonable way to accommodate the space problem. where we used a high-resolution 3D outcrop model to resolve fold geometry. Because Fault D is the youngest low-angle normal fault based on map pat- In this area, early NW-trending (modern coordinates) folds record an intense, tern, the next step, after correction of range tilt, was to restore Fault D, with ductile deformational event associated with a conspicuous cleavage. Folds the inferred faults of its hanging wall (Fig. 16D). This results in an estimated of this generation range from close to sub-isoclinal. These early fold systems horizontal displacement of ~1.4 km (Fig. 16D). Following this, Fault C (Fig. 16E) were then overprinted by NE-trending open folds that produced curvature in was restored with a heave of ~3.2 km. The total heave along Faults C and D is the F1 fold axes as well as folding of the pressure-solution cleavage. Note that ~4.6 km (Fig. 16), and the total heave of the high-angle normal faults across this contractional history is geometrically distinct from overprints recognized the section was 550 m (Fig. 16B). Taken together, this restoration indicates just to the east in the Resting Spring and Nopah Ranges, where Pavlis et al. 5.1 km of NW-SE extension for the northern Ibex Hills (Table 3). (2014) documented early NE-trending structures associated with a low-grade The restoration of section A–A′ followed the same initial steps as that of cleavage overprinted by NW-trending fold and thrust systems. This suggests B–B′ (Figs. 17A–17C). Fault E was also contained in A–A′ and was restored either the Ibex Hills area experienced a distinctly different kinematic history after restoration of the higher-angle normal faults (Fig. 17B) because it is inter- than the area to the east, or the Ibex Hills have experienced a rigid body, ver- preted as a top-to-the-NW structure. The hanging walls of Faults C and D were tical axis rotation of ~90°—a problem that is discussed further below. removed for the rest of the restoration because they were restored out of the line of section from the previous steps (i.e., Fig. 16). As in the A–A′ restoration, the footwalls and hanging walls of the faults were projected upwards using Evidence for SW-NE Extension the known stratigraphy to constrain the reconstructions (Fig. 17C). That is, because younger faulting and erosion have excised part of this footwall, we The earliest extensional phase of the region is recorded in the Ibex Hills, assume a simple, continuous section was present prior to faulting—a reason- Ibex Pass basin, and northern Saddlepeak Hills. In the Ibex Hills, we suggest able assumption unless unrecognized thrust systems were present prior to that NE-SW extension is recorded as top-to-the-SW movement along Faults A extension. A key difference in the projected stratigraphy for A–A′, as opposed and B (Figs. 5 and 9; Table 2; Plate 1). Map observations support this hypothesis

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Figure 16. (A) Cross section of the current geometry along section B–B′. (B) Section B–B′ after restoration along the high-angle normal faults. (Continued on following page.)

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Figure 16 (continued). (C) Section B–B′ after a 30° rotation about a horizontal axis to restore range tilt. Also note the projected geology in the hanging wall in order to restore the low-angle fault movement. (D) After the restoration of Fault D. (E) After the restoration of Fault E.

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Figure 17. (A) Cross section of the current geometry along section trace A–A′ (B) Cross section of A–A′ after the restoration of movement along high-angle normal faults. (C) Cross section A–A′ after rotation of 30° about a horizontal axis to correct range tilt. Also note the lack of the structurally higher faults D and E and the projection of footwall rocks in order to restore the interpreted SW extension. (Continued on following page.)

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Figure 17. (continued). (D) After the restoration of Fault B. (E) After the restoration of Fault A.

TABLE 2. SUMMARY TABLE OF THE MAJOR FAULTS DISCUSSED IN THE TEXT INCLUDING THEIR SENSE OF MOTION AND APPROXIMATE TIMING Fault A B C D E High-angle normal faults Eastern boundary fault Interpreted sense of slip Top to the SW Top to the SW Top to the NW Top to the NW Top to the NW Down to the NW Dextral oblique–down to the E Age range >10 Ma >10 Ma Ca. 10–5.3 Ma Ca. 10–5.3 Ma Ca. 10–5.3 Ma Ca. 10–5.3 Ma (post Fault E) Ca. 10–5.3 Ma (post Fault E) Interpreted setting NE-SW extension NE-SW extension NW-SE extension NW-SE extension NW-SE extension NW-SE extension Transtension Note: “Top to the” language is used for Faults A–E to emphasize their low-angle nature.

because the traces of both Faults A and B cut upsection to the northeast (Fig. 9; Hills, we believe that is unlikely given the lack of exhumation in the footwall Plate 1). Cross-section construction and restoration support this conclusion between the two ranges. That is to say, if the low-angle faults of the northern with movement along Faults A and B leading to an estimated stretch of ~1.54 Ibex Hills extended south, we would expect the presence of deeper structural in the range (Table 3). levels in the footwalls of the Saratoga Hills; however, such a presence is not Evidence for the timing of this extension, as well as direct connection to observed. Instead, we infer that the southern and northern Ibex Hills represent previously recognized NE-SW extension, is illustrated by the structure of the two distinct fault systems. Ibex Pass basin and northern flank of the Saddlepeak Hills. In the Ibex Pass Importantly, although a period of NE-SW extension has long been known area, the volcanic rocks have been interpreted as part of the 14–12 Ma volca- to have occurred in the Saddlepeak Hills (Troxel et al., 1992), no previous map- nic assemblage based on correlations to the Wingate Wash assemblage (e.g., ping has documented any related structures farther north into the Ibex Hills. Luckow et al., 2005; Canalda, 2009) and one unpublished K-Ar date (Calzia Recognizing these older extensional structures in the Ibex Hills is important and Rämö, 2005). In the Ibex Pass area, these volcanics dip NE, consistent because previous models have either ignored its geology or included the with tilting during a SW-directed extensional event, but are overlain along structures as part of the Black Mountain detachment fault (Fig. 2) (e.g., Holm a nearly flat-lying angular unconformity with distinctive megabreccias that and Wernicke, 1990; Snow and Wernicke, 2000). We note, however, that this Topping (1993) correlated to rock-avalanche deposits now exposed to the conclusion has further implications for strike-slip offsets in the system. Stud- north, in the Amargosa Chaos. Topping (1993) showed that these granitic ies of the Kingston Range detachment (e.g., Davis et al., 1993) have generally megabreccias were sourced from the ca. 14 Ma Kingston Peak pluton and only projected the detachment a few km north of the Kingston Range (Fig. 1). were deposited between ca. 10–8 Ma. Taken together, the data suggest that Projecting that northern limit to the SW is consistent with observed extensional the majority of the volcanic deposits of the Ibex Hills area were deposited structures in the Saddlepeak Hills (Fig. 1) but not the Ibex Hills. However, our between ca. 14–12 Ma with the angular unconformity developed between work in the Ibex Hills indicates the presence of top-to-the-SW faulting occurred 12 and 10 Ma. Thus, deformation had begun with some tilting prior to the ~15 km to the northwest of the Saddlepeak Hills. In order to explain this, either development of the unconformity. Similarly, in the northern Saddlepeak Hills, the northern edge of this extensional belt has been offset ~15 km, or NE-SW mutual crosscutting relationships among EW-trending sinistral faults and the extension was present farther north into the Nopah and Resting Spring Ranges igneous succession as well as truncation of earlier low-angle normal faults by and later displaced to the SW (Figs. 1 and 2). This potential offset of ~15 km that fault array (Fig. 4) place the top-to-the-SW faults within the time windows can at least partly be explained by our estimates of ~5 km of heave along the of the 14–12 Ma igneous event. Finally, in parts of the Ibex Hills basin (lower younger NE-striking faults (Table 3). The remainder of the offset is probably insert, Fig. 4), a NW-striking, east-dipping fault is clearly cut by NE-striking due to dextral slip on the Grandview fault (Fig. 1), consistent with Topping’s faults associated with younger, NW-SE–directed extension. However, this (1993) hypothesis based on other evidence. Thus, future studies need to con- NW-striking fault and another fault to the SE also cut the angular unconformity, sider this more complex kinematic history. indicating the NE-SW extension continued after development of the unconformity with the basal megabreccias (Table 2). This timing is consistent with TABLE 3. SUMMARY OF THE TOTAL previous studies that place the transition to the younger, NW-SE extension in STRETCH (LF/LO) AND HEAVE RECORDED the 12–10 Ma time interval (Fridrich and Thompson, 2011). IN THREE OF THE RESTORED SECTIONS PRESENTED IN THIS STUDY The presence of potential NE-SW extension in the Saratoga Hills is ambiguous, but an older, currently low-angle fault surface is exposed in the northern Section Stretch Heave (km) Saratoga Hills. That fault is cut by top-to-the-NW normal faults (Figs. 6 and 15). Thus, based on similar crosscutting relationships to the north, this low-angle A–A’ 1.52 2.4 fault is probably part of older, NE-SW extension. While it is possible that these B–B’ 2.36 5.1 D–D’ 2.12 3 low-angle faults in the Saratoga Hills are coeval with those seen in the Ibex

Overprinted Normal Faults behind as deformation progressed to the northwest. Moreover, the presence of the thick megabreccias derived from Kingston Range (unit Tmg) and the Crosscutting Faults A and B in the northern Ibex Hills are Faults C and D, presence of Kingston Range granite in younger gravels (Tg2) (see Supple- interpreted here as top-to-the-NW low-angle normal faults (Fig. 16; Table 2). mental Material [footnote 1] for their full description) support some original These faults record ~4.6 km of heave to the NW and place the upper Noonday continuity between the Amargosa Chaos and the Ibex Pass basin, prior to the Formation and younger rocks against crystalline basement (Plate 1). More potential NW-directed movement along the detachment. Ultimately, while the recent top-to-the-NW structures are also present in the Ibex Hills as well as Ibex Hills have no doubt been transported along other regional structures (for farther south in the Saratoga Hills (Figs. 15 and 16; Plate 1). This extension example, the Sheephead fault), Grandview fault, some portion of the Black is represented by moderate- to high-angle normal faults that clearly crosscut Mountain–Amaragosa fault system, and/or faults within the Ibex Pass area the currently low-angle Faults A–E (Table 2). The presence of these structures (Canalda, 2009), a single structure cannot account for the multi-phase defor- is important because it does indicate this region was overprinted by top-to- mation recorded in the range. the-NW structures, supporting the hypothesis that this region also contains In addition to enhancing our understanding of the local Death Valley geol- the up-dip equivalents of the Amargosa–Black Mountain fault system, or sim- ogy, refining models, such as those mentioned in the previous paragraph, also ilar structures. improves our broader understanding of continental extension. The Basin and The most recent faulting in the map area occurred along the Eastern Bound- Range Province is one of the most well studied areas of continental extension ary fault (Fig. 5; Table 2; Plate 1). The presence of the Eastern Boundary fault on Earth (Cemen et al., 2002), and Death Valley provides insight into many is interpreted here as related to the most recent phase of transtension in the extensional processes (Miller and Pavlis, 2005; Hussein et al., 2007; Lima et Death Valley region because it crosscuts earlier normal faulting in the Ibex Hills al., 2018). To that point, recent models that seek to understand strain partition- (Fig. 5). Although the trace of the Eastern Boundary fault is obscured in the ing in extensional regions rely on a number of data sets, including piercing northern portion of the study area, the fault displaces the Johnnie and Stirling lines and Cenozoic basin history (Lutz et al., 2019, 2020). While our work here Formations ~4 km to the southeast relative to the footwall positions of these represents only a small piece in a larger puzzle, it nonetheless lies in a key rocks (Plate 1). In addition, the gravels that overlie the Tertiary sections of the position and will provide an updated interpretation of one of southern Death basin, and are interbedded with the granitic megabreccias (Fig. 4), presum- Valley’s extensional basins. ably continue to the north and lap onto the flank of the Ibex Hills and contain clasts of the Noonday and Beck Springs megabreccias (Plate 1). Given that the megabreccias of the Noonday and Beck Springs Formations are presumed to Genesis of Curved Fault Surfaces in the Ibex Hills be coeval with the latest phases of extension and transtension in the range, the interbedded upper gravels and megabreccias of the Tertiary Ibex Pass At large scale, the northern Ibex Hills is a broad synformal structure as basin are also coeval with those events. Active extension and transtension indicated by the map traces of units and faults (Plate 1). Stereonet analysis of continue to the west of the Ibex Hills, but faulting in the study area ended by low-angle fault orientations in the area supports this map-scale observation the Pliocene (Table 2). Remaining activity in the area is now dominated by and suggests a curvature of the fault surfaces about a northeast-southwest– contraction and transpression (Menges et al., 2005). directed axis equivalent to the orientation of the synform axis (Fig. 7). While some of this curvature is likely due to different initial geometries of the vari- ous low-angle normal faults mapped in the area (e.g., Fault A vs. Fault D), the Implications for Extensional Models of the Region curved geometry is observed at all structural levels. Thus, because the faults are all Neogene extensional structures, both the synform and the fault systems Previous workers have associated the faulting in the northern Ibex Hills are presumed to be syn-extensional products. The phenomenon of fault cur- with the Black Mountain detachment fault (e.g., Holm and Wernicke, 1990; vature is common in extensional terranes and can originate as primary fault Snow and Wernicke, 2000), and one of the goals of this study was to test this curvature and/or corrugations with axes parallel to extension (e.g., Fowler and hypothesis. Based on our interpretations, the data are not consistent with a Calzia, 1999), as long-wavelength folds with axes perpendicular to extension single detachment surface and instead indicate a more complex history with related to isostatic rise of a detachment footwall (Wernicke and Axen, 1988), at least two extensional systems superimposed. However, one variant on the a result of folding of fault planes in transtension (e.g., Mancktelow and Pavlis, detachment model that is allowable from the data is that the Ibex Hills, and 1994; Serpa and Pavlis, 1996), or some combination of these processes. specifically Faults C and above, represent a piece of the extensional detach- Primary corrugations parallel to extension have been proposed as a com- ment system that was close to, or at, the breakaway for the Black Mountains mon manifestation of large-scale extension (e.g., Spencer, 2000; Spencer et detachment. That is, motion on the fault at the structural level of Fault C and al., 2019) and have been well documented within the Kingston Range detach- higher is limited to ~5 km, which could be because these rocks were left ment system just to the south of the study area (McMackin, 1992; Fowler and

Calzia, 1999). Thus, the primary corrugation hypothesis is seemingly allow- One potential model for the folding of the Ibex Hills is shown in Figure 18 able; yet, we argue here it is highly unlikely for the Ibex Hills. The problem and relies on the assumption of a left-lateral Sheephead fault. This interpre- with this explanation is that while the curvature observed in the Ibex Hills is tation of the Sheephead fault, when paired with the right-lateral EBF of the oriented along a northeast-southwest axis, consistent with corrugations of top- Ibex Hills, would generate compression in and around the northern Ibex Hills to-the-SW extension, it is difficult to envision a process that would preserve (Fig. 18). Similar cases of crustal shortening have been documented to the this geometry given the intensity of the extension that clearly overprints the south at the intersection of the Garlock fault and the southern Death Valley older structures. For example, low-angle faults C–E (Fig. 5) all show cutoff fault zone (Brady, 1984; Serpa and Pavlis, 1997). This model also hinges upon relationships suggestive of an origin as top-to-the-NW normal faults; yet they timing of fault systems, with a requirement that the Sheephead fault and the share the curvature as underlying faults and bedding—an observation difficult EBF are contemporaneous. to rationalize as a primary corrugation. Alternatively, if the Sheephead fault is a dextral fault, the model of Manck- Alternatively, syn-extensional folding may have occurred via an isostatic telow and Pavlis (1994) potentially is applicable to the Ibex Hills (Fig. 1). That response to northwest extension in the Ibex Hills. The suggestion of large iso- is, a NE-trending fold could result from wrench folding along an EW-trending static responses in the region is a key component of the rolling-hinge model dextral fault. Given the evidence for older, top-to-the-SW faulting in the Ibex (Buck et al., 1988; Wernicke and Axen, 1988; Snow and Wernicke, 2000) since Hills, this model would provide another explanation for the WNW kinematics the deformation front propagates forming a long-wavelength fold in its wake. Combined with the fact that previous work has included the Ibex Hills as part of the up-dip portion of the Black Mountain detachment fault (e.g., Holm and Wernicke, 1990), the rolling-hinge model provides an attractive explanation. Nonetheless, although this process is well documented at the large scale, in regional detachments, the wavelength of the folding seen in the Ibex Hills is Sheephead only 2–3 km, which is inconsistent with most models of isostatic flexure (Buck et al., 1988; Wernicke and Axen, 1988). Fault Given these complications, we suggest that the most likely scenario is that the curved fault surfaces were folded after their initial movement. This folding would have to have occurred during, or after, the development of the U D NE-striking normal fault systems but prior to the Quaternary when the range became part of a large, NW-trending anticline (Menges et al., 2005; Miller et al., 2007). The young folding about a NE-trending axis is broadly coaxial with Souther the re-folding of older F1 fold axes within the Noonday Formation (Fig. 7), sug- U gesting correlation; yet, the small-scale folds are very different in style than the D map-scale structure. That is, the regional structure is a broad, relatively open n Death synform with a wavelength of ~4 km, whereas the structures in the Noonday

Formation are tight, mesoscopic structures producing complex interference V patterns in outcrop. Thus, although they may be related, it is much more alley F likely that they are structures of different age and are coincidentally coaxial. The driver for the young, NE-trending folding is not immediately obvious. ault Given its relative age, it seemingly is related to the modern, transtensional kinematics in the Death Valley region (e.g., Topping, 1993; Serpa and Pavlis, 1996; Norton, 2011). Thus, some form of transcurrent-motion–related folding seems likely. Nearby strike-slip systems that are relevant to this hypothesis include: (1) the NW-trending dextral southern Death Valley system to the

west; (2) the dextral-normal eastern boundary fault of the Ibex Hills described 10 km above; and (3) the enigmatic Sheephead dextral (or sinistral) fault to the north. The sense of offset along the Sheephead fault has been debated with some Figure 18. Conceptual model of a possible explanation of the “Ibex Fold.” Note that in this model, the Sheephead fault is interpreted as left-lateral, authors supporting a right-lateral fault (e.g., Renik and Christie-Blick, 2013), thus creating a potential zone of compression when paired with the while others suggest a left-lateral sense of motion (e.g., Topping, 1993; Serpa right-lateral movement of the Eastern Boundary fault (EBF) of the Ibex and Pavlis, 1996) along the fault. Hills. U and D stand for the upthrown block and downthrown block of the fault, respectively.

shown in Figure 14D, in addition to the effects of regional vertical axis rotation is, that before ca. 10 Ma, the deformation in the Ibex Hills and Ibex Pass area (e.g., Serpa and Pavlis, 1996). was dominated by NE-SW extension. This was then followed by the NW-SE Clearly distinguishing dextral versus sinistral slip on the Sheephead fault extension recorded throughout the region (Table 2) (e.g., Holm et al., 1992). is critical to evaluating this local tectonic problem. In addition, however, abso- The multi-phase deformation of the Ibex Hills also highlights the transition lute timing may be critical. It is clear the folding is relatively young in the Ibex from simple extension to transtension that occurred in the southern Death Hills with good evidence folding occurred after the low-angle faulting, but Valley region. We propose that the folding of the low-angle faults is related before the most recent, high-angle normal faulting (e.g., Fig. 8). Nonetheless, to this shift to transcurrent deformation, perhaps combined with regional although they lie within the same relative time window, it is not clear the block rotation, most likely tied to the nearby Sheephead fault (Fig. 18). Such folding is contemporaneous with slip on the Sheephead, EBF, or both. Qua- polyphase deformation as seen in the Ibex Hills is important to consider for ternary, syn- to post-extensional, folding in the southern Death Valley region future study of the region since previous work has primarily emphasized the has been recognized in the Tecopa Basin to the east (Miller et al., 2007), and NW-SE extension (e.g., Holm et al., 1994) and failed to recognize the presence strikingly similar geometries of folded low-angle normal faults have been of NE-SW extensional structures west of the Saddle Peak Hills (Topping, 1993). mapped in the Amargosa Chaos area to the northwest (Castonguay, 2013). The complexity of the northern Ibex Hills is not seen farther south in the However, in both cases, the fold axes are trending to the northwest, nearly study area where the structure is dominated by steeply dipping normal and 90° to the synform of the Ibex Hills. The variance in the geometry of young oblique faulting (Plate 1). Down-to-the-NW normal faults typical of the southern fold axes in the southern Death Valley region may be explained by vertical Death Valley region are well exposed in the boundary between the Saratoga axis rotation related to transtension, which may differ between these areas. Hills and the southern Ibex Hills (Plate 1). Younger, generally right-lateral to Nonetheless, this rotation is unconstrained in many places (e.g., Serpa and oblique faults strike to the NW and are interpreted to crosscut some of the Pavlis, 1996; Guest et al., 2003). normal faults in this area (Plate 1). These high-angle, oblique to strike-slip Collectively, these relationships indicate the need for further clarifications faults are most obvious in the Saratoga Hills, where they are clearly exposed of both timing and vertical axis rotations in this region. Vertical axis rotations and are the dominant structure, especially in the central and southern portions could be easily tested in the Tertiary volcanics of the Ibex Pass basin, and timing of the range (Plate 1). could be further clarified by dating those rocks. Similarly, thermochronology Future work in the area of the Ibex Hills and Saratoga Hills could help to could help resolve timing issues by clarification of footwall exhumation, and resolve some of the ambiguities in the observations presented herein. Most studies in progress may provide some clarification. Nonetheless, more work pressing might be to time the initiation of extension in the Ibex Hills because is clearly needed on both of these issues before we can fully understand the this will provide new constraints on the period of SW movement in the region. significance of the fold system in the northern Ibex Hills. Also, dating the volcanics at the southern Ibex Hills, along with those of Ibex Pass, would be a big step in our understanding of the basin evolution and ■■ syn-extensional volcanism of southern Death Valley. In doing so, we may be CONCLUSIONS able to shed light on the kinematics of regions that transition from continental extension to transtension. For example, if the oldest low-angle normal faults of The northern Ibex Hills expose a structurally complex sequence of normal the Ibex Hills are in fact related to the Kingston Range detachment, this would faults, many of which are currently low-angle, placing units as young as the indicate that the range was displaced significantly along some combination of Johnnie Formation against Precambrian basement rock (Fig. 4; Plate 1). The the Grandview and Sheephead faults. Such a conclusion would further highlight low-angle normal faults appear to be folded about a NE-SW axis, consistent the importance of strike-slip faulting in the evolution of extensional provinces. with the broader map pattern of the area. The fact that the low-angle faults are folded, but the younger normal faults are not, supports a folding event

younger than the low-angle normal faulting but which preceded the younger ACKNOWLEDGMENTS faulting in the Ibex Hills. This work was supported by National Science Foundation grant EAR-1250388 to Pavlis and a Geo- The low-angle normal faults are cut by other normal faults that exhibit logical Society of America student grant to Fleming. We would like to thank Midland Valley Ltd., down-to-the-NW motion in their hanging walls (Figs. 4 and 8). In addition, Agisoft, Inc., for software donations that made this work possible. In addition, Jim Rukofske was there is compelling evidence for top-to-the-SW movement on the low-angle a vital part of completing the field work necessary for this study. We would also like to thank the two anonymous reviewers whose comments helped to improve this manuscript. normal faults in the northern Ibex Hills. This SW displacement is also consistent with the offsets seen in the Ibex Pass area and the northern Saddlepeak Hills, where NW-striking, low-angle faults cut a ca. 12–10 Ma unconformity. The REFERENCES CITED evidence for top-to-the-SW movement along faults in the Ibex Hills supports Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah: Geological Society of America our assertion of a previously unrecorded multi-phase extensional history—that Bulletin, v. 79, p. 429–458, https://doi.org/10.1130/0016-7606(1968)79[429:SOBINA]2.0.CO;2.

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