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-exten- sional 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 © 2021 The Authors GEOSPHERE | Volume 17 | Number X Fleming et al. | Superposition of extensional phases in southern Death Valley Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/doi/10.1130/GES02354.1/5387230/ges02354.pdf 1 by guest on 29 September 2021 Research Paper 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.