A LITTLE STRUCTURAL HISTORY OF THE VALLEY OF FIRE STATE ... · A LITTLE STRUCTURAL HISTORY OF THE...

Stanford Rock Fracture Project Vol. 23, 2012 J-1

A LITTLE STRUCTURAL HISTORY OF THE VALLEY OF FIRE STATE PARK AND ITS IMMEDIATE SURROUNDINGS, SE NEVADA

Atilla Aydin Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United States

e-mail: [email protected]

Abstract This paper is a prelude to our field trip and includes

a summary of the contributions to the structural history of the Valley of Fire State Park and its surroundings.

Keywords:

Valley of Fire, Sevier Orogeny, Basin and Range, deformation bands, joints, faults

Introduction The southeastern corner of Nevada, northwest of Lake Mead, in which Valley of Fire State Park is situated, includes rocks of Paleozoic to Tertiary age (Bohannon 1983a, b). The Paleozoic sequences are

predominantly carbonates, whereas the Mesozoic and Tertiary sequences are predominantly detrital deposits (Fig. 1). The area has been subjected to two major deformation phases: The first one is the Sevier Orogeny that occurred between middle to late Cretaceous time (Suppe, 1984), at about 105-75 m. y. (Fig. 2). The younger one is known as the Basin and Range extension, primarily in the Miocene, about 25 to 5 MYA, and produced the characteristic landscape of the eastern Great Basin. The aeolian Jurassic Aztec Sandstone covers much of the area in the park, and its diagenesis (Fig. 3) and deformation (Figs 4 to 20) have been the focus of the Rock Fracture Project (RFP) over the last 20 years. The purpose of this contribution is to provide primarily a summary of RFP studies, which focused on the structure types, their properties, and the processes responsible for their formation.

Fig. 1. Geological map and cross sections of the Valley of Fire State Park and its surroundings. From Bohannon (1983b).


Fig. 2. A summary of geological events affecting the Valley of Fire State Park and its surroundings. Revised from a compilation by Flodin (2002).

Fig. 3. A summary of diagenetic events affecting the Aztec Sandstone at Valley of Fire State Park. From Eichhubl et al., (2004).


The Mesozoic or Sevier structures

Within the Valley of Fire Park and its immediate surroundings, the Sevier Orogeny manifests itself in the form of three major thrust faults: The Muddy Mountain, Summit, and Willow Tank faults (Fig. 1), altogether accommodating 40 to 60 miles of shortening. The Muddy Mountain thrust places the Cambrian carbonate sequences over the Triassic and Jurassic detrital rocks near the southern boundary of the park. According to Bohannon (1983b), the Muddy Mountain and Arrowhead faults are two parts of a single thrust, whereas the Summit and Willow Tank are parts of another thrust system, slightly older than the Muddy Mountain thrust. An alternate interpretation is that the Arrowhead fault is a reverse and left-lateral strike-slip fault, as suggested by smaller sub-parallel faults immediately west of the western entrance to the park (Carpenter and Carpenter, 1994; Taylor et al., 1997). The Willow Tank Thrust near the Tear Fault Mesa (Fig. 1, Figs. 4 and 5) is believed to be the lowest member of the Sevier thrust system (Bohannon, 1983b). A thin, distinctive conglomerate of lower Cretaceous age at the base, and the Baseline Sandstone which overlies it, were interpreted as foreland basin deposits

(Bohannon, 1983a, b) fed by the progressive movement and erosion of the overriding thrust sheets, which included the Aztec Sandstone and the Paleozoic carbonates. The Sevier thrusting has left a strong diagenetic imprint in the Aztec Sandstone outcrops in the park. According to Eichhubl et al. (2004), the Aztec Sandstone was reddened by hematite coating of quartz grains early in the deposition phase during the Jurassic. However, most of the fascinating alteration colors owe their existence to basinal fluids pushed through the Aztec Sandstone by the thrusting-related deformation (Fig. 3). Thus, the interaction between the various diagenetic alteration colors and structures provide relative timelines for the structures. For example, all deformational structures are younger than the reddening of the sandstone, but the Basin and Range structures are younger than the Sevier alteration products. However, the cross-cutting relationships may vary because of the presence of a younger phase of alteration in the Tertiary, related to the mixing of basinal and meteoric fluids (Fig. 3). We will refer to these temporal relationships as we discuss the structures within the Aztec Sandstone during the field trip.

Fig. 4. Photograph showing the Willow Tank Thrust Fault at the Valley of Fire State Park.


Bands within the Aztec Sandstone

The first unambiguous field observation of what we today know as compaction bands was Hill’s (1989) analysis of the bands in the Aztec Sandstone in the Valley of Fire State Park, Nevada. In this regard, the park is a type locality for compaction bands. Hill interpreted the compaction bands he studied as closing-mode structures and attributed them to the Sevier age contractional deformation. His observations of the volumetric deformation of the bands and their relative ages were supported by Çakir and Aydin (1994) and Taylor et al. (1997). Also relevant are a pair of publications by Antonellini et al. (1994a and 1994b) who presented observations and data that included deformation bands dominated either by shearing and or volumetric deformation. Aydin et al. (2006) synthesized these data and concluded that deformation bands may occur in a continuous range of kinematics, and that compaction and dilation bands are the end members of this spectrum. The next effort is that by Sternlof et al. (2005) who developed an anticrack inclusion model for compaction bands, determined their closing displacement distribution from a combination of their thicknesses and the porosity reduction within them with respect to the host rock. Sternlof et al. (2006) also investigated how a certain category of compaction bands in the park may influence fluid flow. Aydin and Ahmadov (2009) reported the occurrence of bed-parallel compaction bands and the possible role of shear-enhanced compaction localization in their

formation. Eichhubl et al. (2010) reported that compaction bands occur in a wider range of orientations and confirmed a possible role of shearing in their formation. Finally, Deng and Aydin (in this volume, paper in review) documented localization of compaction bands of various orientations in dunes and proposed that the nature of cross-beds and their orientations influence the presence and orientation of compaction bands thereby causing extensive compartmentalization of compaction bands.

There is little doubt that shear and compaction

bands are highly complicated structures, and they pose many intriguing questions (Holcomb et al., 2007). We here emphasize their age relationships. Figs. 6a, b, and c show these relationships. In (a), a thick band in N-S configuration is a compaction band and is being offset by two relatively thin diagonal shear bands. Many shear bands in the aeolian sandstone in the park occur along cross beds with an approximately east-directed overthrust, as seen in (b). Also interesting is the fact that, in some cases, shear bands occur right next to bed-parallel compaction bands, as shown in (c).

Photographs in Figs. 4 and 5 show the Willow Tank

Thrust exposed at the Tear Fault Mesa, described briefly under “Thrust Faults”. Having now introduced the shear bands, we can now point out the fact that these photographs show very extensive occurrence of shear bands. Although the fault zone has been altered by diagenetic processes, some of which most likely postdate the thrusting, the images show that the deformation mechanism during the Sevier Orogeny was

Fig. 5. Details of the fault zone deformation. Notice many shear bands within the fault core and the adjacent sandstones.


probably shear-banding-based faulting, when the fault zone were confined within the Aztec and Baseline sandstones.

The Tertiary or Basin and Range structures

Normal faults

The earliest Basin and Range structures are a series of normal faults (Fig. 7), which can only be identified unambiguously at cross-sectional views of the outcrops (de Joussineau and Aydin, 2007a). These usually are low-angle faults, accommodate little slip (less than about 10 meters), and are highly segmented (Fig. 8). Typically, each segment has numerous splay

fractures (Figs. 8 and 9a). The splays are generally at an acute angle to the fault traces, and sometimes they have trace geometry with a slight curvature (Fig. 9a). In map view, the traces of the splays are sub-parallel to the traces of the normal faults (Fig. 10a), but they are very distinct in cross section with their steeper dip angles (Figs. 8, 9a, 10a).

In most outcrops, the splays of the normal faults are often sheared, leading to the initiation of strike-slip faults in orientations (Fig. 10b) similar to those of the larger-scale strike-slip faults in the park. This interplay between the normal splays and strike-slip faults will be discussed later following a brief description of the formation and evolution of the strike-slip fault network.

Fig. 6. Shear band-compaction band relationships in the Aztec Sandstone. (a) Two thin shear bands offsetting a thick compaction band. From Aydin and Zhong (in prep.). (b) A shear band along a cross bed offsetting a high-angle compaction band. From Aydin and Deng (this volume). (c) Thin shear bands occurring at the edge of a bed-parallel compaction band. From Aydin and Ahmadov (2009).


Fig. 7. A series of low-angle normal faults and their splays on a cliff face, which were proposed to be precursors of the strike-slip faults in the park. From de Joussineau and Aydin (2007a).

Fig. 8. A series of normal fault segments and the associated splays From de Joussineau and Aydin (2007a).


Fig. 9. A series of splays associated with a small normal fault (a), and a normal fault surface with slickenlines (b). These are used to determine the normal nature of slip along the faults. (b) From de Joussineau and Aydin (2007a).

Fig. 10. Horizontal shearing of the normal fault splays (a) Cross-section and map view. (b) Sheared splay fractures mapped on the top surface of a sandstone bed, and (c) the orientation of the resulting right- and left- lateral faults. From de Joussineau and Aydin (2007a).


Joints and joint zones Perhaps the most ubiquitous structures in the Aztec Sandstone within the park are joints (Fig. 11). They play an important role in the initiation and growth of a majority of the faults in the park, as presented below.

Strike-slip faults

An extensive system of strike-slip faults in the park (Fig. 12) has been mapped by various authors (Myers and Aydin, 2004; Flodin and Aydin, 2004, de Joussineau and Aydin, 2007b; de Joussineau et al., 2007). These authors determined that the strike-slip faults in the park initiated from shearing of initial discontinuities believed to be older joints or joint zones (Figs. 13). They identified various geometries of initial joint zones with consequences for the architecture of the resulting fault zones. We here highlight a number of interesting results from these studies. First, strike-slip faulting occurs by a frictional sliding mechanism, splay

fracturing and subsequent shearing of these splays. We argued that the many joints in the park originated by normal faulting and the shearing of the related splay fracturing (de Joussineau and Aydin, 2007a). However, this does not rule out the occurrence of isolated joints or joint zones away from the normal faults but induced during the same extensional deformation period. In any case, the strike-slip growth process involves incorporating a highly fractured and consequently

weakened inner damage zone into the fault core for widening (Fig. 14), and linkage of neighboring faults for lengthening (Fig. 15). The underlying mechanism for both is the weakening of rock adjacent to the fault core and within relay zones (Aydin and Berryman, 2010), caused by extensive localized fracturing (Fig. 16). In addition, there is a sequential shearing and splaying process that results in a system of what we referred to as apparent conjugate faults, as observed in the field (Figs. 17a, b) and conceptualized in a schematic diagram shown in Fig. 17c (Myers and Aydin, 2004; Flodin and Aydin, 2004).

Fig. 11. A slightly sheared joint system in its original configuration. Note the reddish staining along the joint surfaces indicating fluid flow along them.


Fig. 12. Fault map of the Valley of Fire State Park showing the Willow Tank Thrust (top) and strike-slip fault network. From Flodin and Aydin (2004). Scanlines and related statistics in de Joussineau and Aydin (2007b).


Fig. 13. Incipient strike-slip faults in increasing slip magnitudes, all of which initiated from shearing of joint zones of various configurations. Note the splay joints at an angle to the sheared joints and localization of the splays at the tip areas and relays. Panels (a and b) from Myers and Aydin (2004), and (c) from de Joussineau and Aydin (2007b).

Fig. 14. Photographs and maps showing progressive fracturing of damage zones of two left-lateral strike-slip faults with 14m and 25m offsets. The fault cores eventually widen by incorporation highly fractured adjacent rock. (a and b) from de Joussineau and Aydin (2007b), and (c) from Aydin and Berryman (2010).


Fig. 15. Evolution of strike-slip faults by splay fracturing and linkage of neighboring faults. Panels (a and b) are schematic expressions of how mean segment length and mean damage zone width increase while number of fault steps (or relays) decreases with increasing fault offset. (c) Schematic illustration of the fault growth process. From de Joussineau and Aydin (2007b).

Fig. 16. Plots showing how two sets of fractures with various intersection angles weaken rocks with increasing fracture density. It is believed that this weakening leads to cataclastic deformation and formation of through going slip surfaces. From Aydin and Berryman (2010).


Summary and implications

There are a number of interesting results in regard to the history of the deformation that we would like to highlight. One is that the Basin and Range faults involve both normal and strike-slip faults. It appears that the normal faults with small offsets are precursors of the strike-slip faults, as conceptualized in a series of diagrams in Fig. 18. Note that if this is true, then a

major reorientation of the regional stresses is required between the two faulting events (Figs. 18a, b and c, d). Just to entertain an alternative model for the relationship between normal faults and strike-slip faults, we present one of the most popular cases (Fig. 19) , in which strike-slip faults in the Basin and Range are envisioned as transform faults or as accommodation zones between regions of uneven extension (see, for example, Faulds et al., 1988 among many others). However, a casual observer may realize that, in this case, the strike-slip faults are not perpendicular to the normal fault system.

Fig. 17. Field photographs (a and b), and a conceptual model (c) for the evolution of a strike-slip network with an apparent conjugate pattern. Panels (a and b) from Myers and Aydin (2004), (c) from Flodin and Aydin (2004).


Another interesting issue in the context of the changing deformation mechanisms in the aeolian sandstone is the cause of the apparent transition from compaction banding to shear banding, and eventually, from localized banding to discrete jointing and ensuing frictional sliding deformation mode (Fig. 20a, b). It is likely that there are multiple parameters playing a role in this switch, some of which are alluded to in Figs 21a,

b, c, such as porosity and confining pressure. Unfortunately, I have run out of time to discuss the possibilities here; we will have to revisit the issue during the workshop and the field trip!

Fig. 18. Conceptual model showing low-angle normal faults and their splays (a), and strike-slip fault formation from shearing of the normal fault splays (b through d). From de Joussineau and Aydin, 2007a).


Fig. 19. Strike-slip fault formation as accommodation zones in an extensional tectonic regime. From Faulds et al. (1988).

Fig. 20. Overprinting deformation mechanisms in the Aztec Sandstone. (a) A sheared joint-based fault offsetting a compaction band zone, (b) A sheared joint fault parallel to a set of compaction bands (CB) and the splays (SP) cross-cutting the compaction bands. Panel (a) from Aydin (2006) and (b) from Zhou et al. (2012, in press).


References Antonellini, M., Aydin, A., Pollard, D. D., and D'Onfro, P.,

1994. Petrophysical study of faults in sandstones using petrographic Image Analysis and X-ray Computerized Tomography. Pure and Applied Geophysics 143, 181-201.

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Aydin, A., and Berryman, J. G., 2010. Analysis of the growth of strike-slip faults using effective medium theory. Journal of Structural Geology, 32, 1629-1642, DOI: 10:1016/jsg.2009.11.007.

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Fig. 21. Petrophysical properties [(a) p-wave velocity, and (b) porosity] of the three alteration members (lower, transition, and upper zones) of the Aztec Sandstone in the park. Panel (c) shows a relative abundance of compaction and shear bands in the three zones. From Flodin et al. (2003).


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Zhou X., Karimi-Fard, M., Durlofsky, L. J., Aydin, A., 2012. Fluid flow through porous sandstone with overprinting and intersecting geological structures of various types. Geological Society, London, Special Paper on Advances in the Study of Fractured Reservoirs. (Geological Society Special Publ., in press)

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