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Basement-involved, shallow detachment faulting in the Bighorn Basin, Wyoming and Montana
Accepted Manuscript Basement-involved, shallow detachment faulting in the Bighorn Basin, Wyoming and Montana Gary Gray, Zuyue Zhang, Andres Barrios PII:
S0191-8141(18)30504-2
DOI:
https://doi.org/10.1016/j.jsg.2018.10.015
Reference:
SG 3766
To appear in:
Journal of Structural Geology
Received Date: 8 December 2017 Revised Date:
11 October 2018
Accepted Date: 21 October 2018
Please cite this article as: Gray, G., Zhang, Z., Barrios, A., Basement-involved, shallow detachment faulting in the Bighorn Basin, Wyoming and Montana, Journal of Structural Geology (2018), doi: https:// doi.org/10.1016/j.jsg.2018.10.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Basement-involved, shallow detachment faulting in the Bighorn Basin, Wyoming and
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Montana.
3 Gary Gray1*, Zuyue Zhang1, 2, and Andres Barrios1, 3
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Texas, 77005
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718500 3
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Current address: Schlumberger Oilfield Services, S.A., Jingbian, Shaanxi, PR China,
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Rice University, Department of Earth, Environmental and Planetary Sciences, Houston
Current address: Wells Fargo Securities, Houston, TX.77002 Corresponding author, [email protected]
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Key words: Detached structure; shallow basement detachment; Laramide province; Elk
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Basin; cross-section restoration; mechanical modeling.
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Abstract
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‘Laramide-style’ structures from the Laramide province of the western US involve the
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underlying crystalline basement. The basement is broken into blocks of varying scales
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that are generally bounded by moderately-to-steeply dipping reverse faults. This style is
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very pervasive, so that most structures are interpreted this way, even when field
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relationships suggest otherwise. The Elk Basin anticline, which lies in the northern
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Bighorn Basin is one of these structures. Basement is not elevated on the southwest side
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of the structure, however, which argues strongly for a detached origin. Balanced section
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restoration and forward finite-element modeling both support the interpretation that the
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underlying Elk Basin thrust is a detached, listric or ramp-flat style thrust fault. The
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detachment lies within the crystalline basement, ~0.5 km below the sediment-basement
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contact. This detachment is fundamentally different from the detachments in the deep
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crust interpreted by Erslev (1993) to be the underlying link between widely-separated
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basement arches within the Laramide province. We propose a new ‘shallow basement
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detached” class of structural style. Several candidate structures for this same style are
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present within the Bighorn basin, with both east and west vergence. These structures do
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not appear to form an interconnected detachment system, but rather are local ‘flakes’ of
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basement that may form due to bending of the underlying crust during basin-scale
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Laramide-age contraction and folding.
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1. Introduction
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The Laramide Orogeny was originally defined by Dana, (1895, as cited in Tweto, 1975)
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for the Laramie Formation, which was the name for localized Upper Cretaceous non-
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marine strata that overlie widespread marine Cretaceous beds throughout Wyoming,
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Colorado, and New Mexico. The change in depositional environment represented by the
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Laramie Fm. also marked a change from widespread to local basin depocenters related to
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the breakage of the smooth foreland into broad basement uplifts and intermontane basins.
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The term Laramide is in wide usage today, having both a time and a structural style
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connotation (e.g. Dickenson and Snyder, 1978). Dana (1895) defined a large ‘Laramide
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province’, extending from Montana to Mexico, characterized by similar basement-cored
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uplifts and their adjacent basins.
49 The geometry of Laramide uplifts was debated for quite some time (summarized in
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Stone, 1984), until long-record seismic data (e.g., Smithson et al., 1978), exploration
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drilling (Berg, 1962; Stone, 1993), and balanced cross section methods (Suppe, 1983;
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Brown, 1984; Suppe and Medwedeff, 1987; Narr and Suppe, 1994; Mitra and Mount,
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1998; Erslev, 1986, 1991, 1993) demonstrated the uplifts were underlain by relatively
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planar, moderately to steeply-dipping reverse faults at least within the upper crust. These
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faults cut the basement-sediment interface, and elevate the basement together with the
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overlying cover rocks. The involvement of much of the upper crystalline crust in these
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structures has led to the adjective ‘thick-skinned’ to describe this structural style.
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Contraction within fold-and-thrust belt environments, in contrast, does not generally
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elevate the basement rocks. Instead, shortening is accomplished by faults that displace the
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cover strata along and over the basement-sediment interface, with ramps that cut upward
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towards secondary detachments and/or the surface (Bally et al., 1966; Price, 1981;
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Coogan, 1992). This general lack of involvement of crystalline basement within these
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systems coupled with the relatively shallow fault depths has led to the adjective ‘thin-
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skinned’ to be applied.
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These basic differences in structural styles also differentiate the “Sevier Orogeny” (thin-
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skinned) (Armstrong, 1968), from the “Laramide Orogeny” (thick-skinned) in the
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western US, although the later stages of the Sevier Orogeny and the Laramide Orogeny
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are contemporaneous (Tweto, 1975; Gries, 1983; Dickinson et al., 1988; Kulik and
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Schmidt, 1988).
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This paper examines the Elk Basin anticline in the northern Bighorn basin, along the
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border between Montana and Wyoming (Fig. 1). Oil was discovered in this structure in
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1915 (Hein, 2017), and it has been extensively drilled as a consequence, which provides
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excellent depth constraints (Wetzel, 1954). Crystalline rocks of the Archean Wyoming
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province are present within the core of this anticline (Stone, 1993). 2D seismic data
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across the structure have been interpreted by Stone (1993) and Weitzel (1985) to indicate
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that the faults bounding this basement core are similar to the steeply dipping reverse
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faults associated with the major basement-cored, thick-skinned uplifts within the
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Laramide province. A simple test of fault geometry is to look at the elevation of basement
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in the hangingwall side versus the footwall side of a structure. This relationship is shown
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schematically in Fig. 2 for three different contractional fault configurations (see also
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Eichelberger et al., 2017). The configuration of both the hangingwall and footwall of Elk
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Basin is shown in Fig. 3. The hangingwall is interpreted to be elevated above a dipping
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ramp in the center of the structure, but it returns to the regional level further east. This
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geometry is most like example 2a, which suggests the Elk Basin fault is detached.
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Our work utilizes cross-section restoration and numerical modeling techniques to revisit
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these two significantly different structural interpretations of this anticline. In particular,
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we address whether the major underlying fault is planar to significant depth, or if it
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becomes parallel to the basement-sediment interface at relatively shallow depths within
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the crystalline basement. Understanding the structural style, particularly whether the
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structure is detached or basement-involved, is important for exploration in these
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provinces around the world. It can infer the presence or absence of reservoir within a
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particular structural trap, and the distribution of potential fractures within the structure.
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Examples of foldbelts with mixed structural styles where this can be important include
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large provinces such as the Zagros foldbelt in Iran (e.g. Alavi, 2007), down to relatively
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small examples such as the Yakima foldbelt in Washington State (Reidel, 1984; Blakey et
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al. 2011; Gomberg et al. 2012).
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2. Local Setting and Previous Work
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Elk Basin anticline lies about half way between the Beartooth uplift and the northern
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Bighorn Mountains (Fig. 1). These ranges are two very large, classic basement-involved
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‘arches’ characteristic of the Laramide province (Erslev, 1993). The Beartooth uplift is
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bounded on its NE side by a steeply-dipping reverse fault (Fig. 1) (Bevan, 1923; Wise,
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1963) that has been confirmed by drilling (Stone, 1993; Omar et al., 1994). Stone (1993)
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interprets the eastward displacement on the Beartooth fault to be approximately 4 km.
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The asymmetry of the Bighorn uplift also suggests it is an eastward-verging structure
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with reverse faulting near the middle of the range (Stone, 1993) (Fig. 1). Additional,
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smaller scale basement-cored uplifts occur in the Pryor Mts., about 20 km east of Elk
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Basin (Harding and Lowell, 1979).
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A cross section of the Elk Basin anticline shows the east-directed asymmetry of the
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structure, with the hangingwall displaced approximately 2 km eastward over the footwall
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along the Elk Basin thrust (Fig. 3). Crystalline rocks have been encountered in wellbores
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in the core of the anticline (Stone, 1984, 1993; Weitzel, 1985). Stratal geometries in the
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overlying Paleocene Fort Union Formation indicate the Elk Basin anticline was actively
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growing during the Paleocene, in accordance with evidence for synchronous activity on
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both the Beartooth uplift and Bighorn mountains, plus many other structures in the
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province (e.g., Stone, 1993; DeCelles et al., 1991).
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The 2D seismic data over the anticline were the subject of a modeling study by Weitzel
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(1985). She used ray-trace modeling and velocity analysis to test both shallow detached
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and planar reverse fault interpretations for the principle structure beneath the anticline
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(Fig. 4). She concluded that the main fault was most likely a planar reverse fault. This
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interpretation suggests the Elk Basin structure is similar to other thick-skinned Laramide
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structures in the region. Stone (1984, 1993) also examined the Elk Basin structure using
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physical modeling and area balance arguments. Stone (1993) also concluded that the
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anticline is most likely underlain by a planar reverse fault.
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This study employs 2D cross-section restoration and 2D mechanical models to provide a
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new assessment of these fault interpretations, as well as their resulting fold geometries.
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Stone’s (1993) idealized model with both a planar reverse fault and an alternative shallow
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detachment fault interpretation form the basis of our testing (Fig. 4).
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3. Cross-section restoration of the Elk Basin structure
140 A cross-section restoration, line- and area-balance analysis of the Elk Basin anticline was
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performed using the Lithotect(TM) software. This method tests the geometric compatibility
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of the hangingwall and footwall interpretations along a fault trace. A bedding-parallel,
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flexural slip mechanism was used to restore both the strata and the basement blocks. The
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restoration datum was the basement-sediment contact. These analyses use the footwall as
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a template into which the restored hangingwall is fit. A ‘perfect’ deformed-state
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interpretation can be restored so that all pre-tectonic strata achieve a flat initial state, and
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the hangingwall and footwall blocks match without gap or overlap along the fault trace
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(e.g., Woodward et al., 1989).
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The structural interpretation presented in Stone (1993) was modeled ‘as is’. No effort was
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made to improve any line-length or area mismatches in that interpretation, since the
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purpose of this study was to evaluate the first-order geometry of the underlying faults
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(Fig. 5).
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3.1 Cross-section Restoration Results
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Gaps occur on both restorations at the fault tip. The fault tip region is the same on both
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interpretations and contains a small mismatch of the hangingwall geometry relative to the
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footwall geometry. A box is shown on Figs. 5B and D to highlight where the restored
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interpretations diverge. The shallow basement detachment fault interpretation in Fig. 5A
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produces a restored hangingwall geometry that fits closely to the footwall geometry, with
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few gaps and no overlaps (Fig. 5B).
163 The planar fault interpretation, in contrast, produces a restored hangingwall geometry
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that does not fit well with the footwall geometry, and results in a large gap between the
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hangingwall and footwall (Fig. 5D). This large area of gap indicates that hangingwall and
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footwall interpretations for the planar fault are relatively incompatible compared to the
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small area of gap between the hangingwall and footwall for the listric fault interpretation.
169 4. Finite Element Model analysis
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The detached and planar fault interpretations were also tested using finite element
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forward modeling (Zhang et al., 2013; Gray et al., 2014). The purpose of this mechanical
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modeling was to test the two different fault geometries in a forward sense, from the
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undeformed to the deformed state, and see if either could reproduce the observed
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hangingwall geometry. Finite element (FE) modelling differs from restoration analyses in
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many ways, but in particular that volume (area in 2D) can change as defined by the
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constitutive laws employed (Crook et al, 2006a, 2006b; Gray et al, 2014). All FE
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modeling was undertaken using ELFEN(TM) software (supplementary materials).
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The model setup is described in detail within the supplemental materials. The constitutive
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law used in these models is a modified critical-state soil mechanics-type law developed
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by Rockfield (Crook et al. 2006a, 2006b; Thornton and Crook, 2014). This type of law
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includes elastic, poro-elastic, and plastic behavior depending upon the stress conditions
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during deformation (Crook et al., 2006a, 2006b; Thornton and Crook, 2014). The
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ELFEN(TM) code handles high-strain deformation through a process of adaptive remeshing,
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whereby the mesh is re-calculated once it deforms past pre-set threshold. Remeshing
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causes individual mesh polygons to become smaller, which further localizes strain and
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simulates the propagation of faults through the materials when stress conditions warrant
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(Thornton and Crook, 2014). Both models were displaced parallel to the faults, so as to
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avoid creating gaps or overlaps between the hangingwall and footwall blocks.
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The listric, shallow basement detachment fault FE model is similar to the ‘detachment 2”
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model of Stone (1993), except that the radius of curvature of the fault bend was decreased
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slightly for ease of modeling (supplemental materials). This resulted in a more a flat-
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ramp type geometry, though it retains the primary feature of a long detachment parallel to
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bedding. The hangingwall block was displaced parallel to the ‘flat’, horizontal portion of
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the fault.
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The fault dip in the planar-fault model matches the 30o dip in Stone’s (1993) preferred
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basement faulting model. The resulting fold geometries were then compared to both Fig.
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2 and the observed folding of the Elk Basin anticline. The ‘goodness of fit’ for each of
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these models was assessed on the basis of the relative elevation of the hangingwall after
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movement.
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4.1 Finite Element Model Results
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The shallow detachment fault model creates a fold-thrust geometry that is very similar to
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the interpretation shown in Fig. 2A and the shallow detachment option on Fig. 3. The
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dotted line on Fig. 6B denotes the regional level of the basement. Basement is only
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elevated with respect to this regional surface where the fault changes from flat to ramp.
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There is a good overall fit between the interpreted Elk Basin hangingwall geometry and
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the detachment forward model (supplemental materials).
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The forward model with the planar fault is perhaps the most illustrative result of this
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study. Movement on the planar fault elevates the entire hangingwall to the same degree.
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Any movement on this fault results in a commensurate uplift of the hangingwall above
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the pre-faulting regional level as also shown in Fig. 2C. This produces the classic drape
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fold or monocline geometry common to many Laramide uplifts (Harding and Lowell,
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1979; Stone, 1984). The results of each forward model have been superposed over the
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Stone (1993) interpretations in the supplemental materials.
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5. Discussion
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The results from both restoration and forward modeling strongly favor a shallow
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detachment geometry for the Elk Basin Thrust. The detached interpretation restores with
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far less gap, and the detached forward model creates a reasonable facsimile without
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elevating basement throughout the hangingwall. The restoration and forward model with
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planar faults have problems when compared to the Elk Basin structure. The restoration
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has a large gap, and the forward model elevates the entire hangingwall. Neither of these
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results support the hypothesis that the planar fault better explains the observed geometry
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of the Elk Basin structure. Thus, we think the Elk Basin thrust has a listric, or even flat-
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ramp detachment-style geometry. Furthermore, this fault detaches within crystalline
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basement, approximately 2.2 km beneath the surface, or 0.5 km beneath the basement-
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sediment contact.
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Elk basin is apparently not the only anticline with an underlying shallow basement
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detachment. The Grass Creek anticline (Fig. 7) has been interpreted by Stone (1993) and
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Mitra and Mount (1998) to have a backlimb that returns to the pre-faulting regional level,
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very similar to the geometry of Figs. 5A and 6B. This geometry requires that the fault
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displacement be transferred along a detachment fault, parallel to the basement-sediment
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contact, approximately 600 meters below the basement-cover contact. The Garland
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anticline, immediately south of Elk Basin, Pitchfork on the far west side of the basin, and
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Bonanza, on the east side of the basin (Fig. 1; DeBruin, 1996) also have a similar
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backlimb geometry, as presented in Stone (1993).
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It is not clear why these structures detach so shallow within the basement. The
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stratigraphy of the Bighorn basin contains many shale-rich horizons that seem far more
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suitable as potential detachment horizons, especially near the base of the sedimentary
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section. The basal detachments for the Sevier belt further west occur in very similar
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stratigraphy (Royse et al., 1975; Coogan, 1992). One potential reason for detachments
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within the gneissic basement rocks may be that they are locally highly layered. Near
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Cody, Wyoming, this layering is imparted by granitic sills that are nearly parallel to the
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basement-sediment contact in the Shoshone River canyon (Fig. 8). The relatively small
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displacements of most of these detachments, plus the opposing vergence seen in the
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southern Bighorn basin, suggest these shallow detachment structures are local, and
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probably not part of a connected, thoroughgoing detachment system. Instead, they may
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be adjustments that formed in the uppermost crust due to the movement on deep crustal
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(~30 km) ductile ‘detachments’ and ramps as proposed by Erslev (1993), and Yeck et al
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(2014). As such, these shallow detachments could be small flakes displaced along the top
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of the crust as it moved laterally on deep faults. Regardless of the mechanism, this work
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documents that classical detachment-style faulting may also occur within basement
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lithologies, even in the classic province known for steeper-oriented thick-skinned
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structures.
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6. Acknowledgements
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We thank Halliburton and Midland Valley for providing Lithotect(TM) and MOVE(TM)
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structural restoration software to Rice University. Rockfield Global provided their
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ELFEN(TM) software. Thanks especially to Dr. Melanie Armstrong and Fen Paw of
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Rockfield for their technical support during the finite element modeling and writing
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process. Rick Groshong, Bill Dunne, Nur Schuba and an anonymous reviewer are
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thanked for their comments which significantly improved this manuscript. Eric Erslev is
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thanked for reviewing an earlier version of this work. This research did not receive any
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specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Stone, D.S., 1984. The Rattlesnake Mountain, Wyoming, debate: A review and critique of models. The Mountain Geologist, 21, 37-46.
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Stone, D.S., 1993. Basement-involved thrust-generated folds as seismically imaged in the
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subsurface of the central Rocky Mountain foreland. Geological Society of
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America Special Paper 280, 271-318.
Suppe, J., 1983. Geometry and kinematics of fault-bend folding. American Journal of Science, 283, 684-721.
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Suppe, J. and Medwedeff, D.A., 1990. Geometry and kinematics of fault-propagation folding. Eclogae Geologicae Helvetiae, 83, 409-454. Thornton, D.A. and Crook, A.J.L., 2014. Predictive Modeling of the evolution of Fault Structure: 3-D Modeling and Coupled Geomechanical/Fluid Flow simulation.
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Rock Mechanics and Rock Engineering 47, 1533–1549. doi: 10.1007/s00603-
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024-0589-6.
Tweto, O., 1975. Laramide (Late Cretaceous-Early Tertiary) Orogeny in the Southern
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Rocky Mountains. in, Curtis, B.F., ed., Cenozoic History of the Southern Rocky
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Mountains, Geological Society of America Memoir 144, 1-44.
378
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Weitzel, C.M., 1985. Seismic modeling and interpretation of Elk Basin field, Park
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County, Wyoming, and Carbon County, Montana [M.S. thesis]: Golden, Colorado
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School of Mines, 55 p.
Wetzel, J.H., 1954. Elk Basin Field – Carbon County, Montana, and Park County,
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Wyoming. Billings Geological Society: Guidebook, Fifth Ann. Field Conf., 112-
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116.
Wise, D., 1963. Overprinting of Laramide Structural Grains in the Clarks Fork Canyon
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Area and Eastern Beartooth Mountains of Wyoming. Geology of the Bighorn
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Basin, 34th Annual Field Conference Guidebook, Wyoming Geological
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Association, 77-87.
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Woodward, N.B., Boyer, S.E., and Suppe, J., 1989. Balanced Geological Cross-Sections:
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An Essential Technique in Geological Research and Exploration. American
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Geophysical Union, Short Course in Geology, 6, 132 p. Yeck, W.L., Sheehan, A.F., Anderson, M.L., Erslev, E.A., Miller, K.C., and Siddoway,
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C.S., 2014. Structure of the Bighorn Mountain region, Wyoming, from
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teleseismic receiver function analysis: Implications for the kinematics of
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Laramide shortening. Journal of Geophysical Research: Solid Earth, 119, 7028-
395
7042.
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Zhang, J., Morgan, J.K., Gray, G.G., Harkins, N.W., Sanz, P.F., & Chikichev, I., 2013.
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Comparative FEM and DEM modeling of basement-involved thrust structures,
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with application to Sheep Mountain, Greybull area, Wyoming. Tectonophysics,
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608, 408-417.
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Figure 1. Location map of the Bighorn basin in Wyoming and Montana. Geology draped
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over shaded topography, prepared with GeoMapApp. Precambrian basement exposures
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shown in pink. Mesozoic and Paleozoic rocks in green, and Tertiary rocks in light brown.
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Black lines mark the location of the Elk Basin and Grass Creek cross sections used in this
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analysis. Towns in the Bighorn basin: C, Cody; G, Greybull; T, Thermopolis. Trace of
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the Beartooth Thrust taken from Bevan (1923) and Wise (1963). g, p, and b refer to the
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Garland, Pitchfork, and Bonanza structures, respectively.
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Figure 2. Schematic diagram showing the interplay between fault dip and elevation of
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the hangingwall relative to the regional level. The regional level is shown as a dashed
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line. A) Main fault trace (with arrow) is parallel to bedding. Lateral displacement on fault
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causes frontal anticline to rise above regional level, but trailing part of the hangingwall is
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not elevated relative to regional. B) Non-planar fault trace is inclined steeper than
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bedding contact. Movement on fault causes both frontal anticline and trailing
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hangingwall to elevate above regional. C) Planar fault is inclined steeper than bedding
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contact. There is no frontal anticline due to planar nature of fault. Entire hangingwall is
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elevated above regional level, creating a monoclonal structure. Similar fault/cover
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geometries have been recently investigated by Eichelberger, et al. (2017).
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Figure 3. Profile view of the Elk Basin anticline modified from Stone (1993). Dashed
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lines show the location of selected wellbores used by Stone (1993) to constrain the
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geometry of the structure. Dotted line, surface topography, pC, preCambrian crystalline
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basement of the Wyoming province, lPz, Lower Paleozoic units, uPz, Upper Paleozoic
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units, Mz, Triassic through lower Upper Cretaceous units. The upper Upper Cretaceous
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and Tertiary units have been omitted for clarity. Note that the Elk Basin Thrust is
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interpreted as a planar, moderately dipping reverse fault. No vertical exaggeration.
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Figure 4. “Mature thrust-fold” model of Stone (1993). Light gray area is cover strata as
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in Figure 3. Dark gray is crystalline basement rocks. Stone’s two fault interpretations
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tested with the structural models, ‘Detachment 2’ and ‘Planar reverse fault’ are shown in
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the dark lines. The planar fault interpretation is favored by both Weitzel (1985) and Stone
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(1993).
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Figure 5. Flexural-slip restorations of the two faults shown in Fig. 4. A) The ‘detachment
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2’ fault is a listric geometry that becomes parallel to the sediment-basement contact at
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approximately 500 m below the unconformity. B) Restoration of the listric fault, datumed
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on the basal unconformity. C) Planar fault geometry, showing a folded hangingwall
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above a planar fault. D) Restoration of the planar fault interpretation, using the same
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datum and techniques as in B. The fault tip area on both interpretations is that of Stone
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(1993) and restores with an upward-increasing gap, and was not modified. The uppermost
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light gray unit was area-balanced, also due to complications with the fault tip. The inset
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box highlights the area tested by the alternate fault geometries. The restored hangingwall
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of the listric fault geometry (B) fits together with the footwall without overlap and only
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slight gaps. The restored hangingwall of the planar fault geometry (D) does not fit well
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against the footwall block. There is no overlap between the two blocks, but a wedge-
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shaped gap opens downward. This large gap is the consequence of a having a folded
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hangingwall above a planar fault. 1 and 2 denote changes in the angle of the fault in the
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hangingwall block not present in the footwall block
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Figure 6. Finite-element forward models of the same fault configurations as shown in
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Fig. 4 and supplemental materials. A) Starting configuration for the detachment fault
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model similar to the restoration in Fig 5B, but with both hangingwall and footwall fitting
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together perfectly. B) Ending state of the model in (A) after 5 km of fault displacement
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from the left (see also supplemental materials). C) Starting configuration for the planar
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fault model, which is similar to the restoration in 5D, but also without the gaps between
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the blocks. D) Final configuration of the model in (C), with 3 km of fault offset. The
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hangingwall in this model was displaced parallel to the fault, which prevents gaps or
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overlaps between the two basement blocks. Note the regional level shown by the dotted
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lines. Regional basement level is maintained in model A, but is elevated in the
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hangingwall of model C, as shown by the arrow.
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Figure 7. Interpreted cross section across Grass Creek anticline in the southern Bighorn
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basin, after Stone (1993) and Mitra and Mount (1998). The hangingwall of this structure
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returns to a regional level (dotted line) to the east. It has opposite (westward) vergence to
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Elk Basin. The detachment level for this structure also appears to be within the crystalline
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basement, about 600 meters beneath the basal unconformity. No vertical exaggeration.
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Figure 8. Photographs of compositional layering in Archean basement rocks in Shoshone
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Canyon, west of Cody Wyoming. Layering is comprised of granitic sills cutting through a
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schistose, amphibolite host rock. A) View to the west, up-canyon towards Buffalo Bill
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dam, showing layering on a large scale. Cliff is approximately 200 m high. B) Close-up
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view of north side of canyon. The height of the center outcrop is approximately 10
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meters. The average dip of layering in this outcrop is about 20 degrees east, similar to the
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overlying strata which dip 15 degrees east. Both photos taken on the north side of the
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Shoshone River near the Buffalo Bill powerplant. Approximate Lat.: 44.508620; Long.: -
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109.173502.
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A new type of shallow detachment structure is recognized that involves crystalline basement The detachment is approximately 2.2-2.4 km below the basement sediment contact. This structure is found where only steeply-dipping basement faults had been believed to exist
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There are other examples like this detached structure that may have been mis-interpreted
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These detachments probably form due to large-scale bending of the basement
S0191-8141(18)30504-2
DOI:
https://doi.org/10.1016/j.jsg.2018.10.015
Reference:
SG 3766
To appear in:
Journal of Structural Geology
Received Date: 8 December 2017 Revised Date:
11 October 2018
Accepted Date: 21 October 2018
Please cite this article as: Gray, G., Zhang, Z., Barrios, A., Basement-involved, shallow detachment faulting in the Bighorn Basin, Wyoming and Montana, Journal of Structural Geology (2018), doi: https:// doi.org/10.1016/j.jsg.2018.10.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Basement-involved, shallow detachment faulting in the Bighorn Basin, Wyoming and
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Montana.
3 Gary Gray1*, Zuyue Zhang1, 2, and Andres Barrios1, 3
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Texas, 77005
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718500 3
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Current address: Schlumberger Oilfield Services, S.A., Jingbian, Shaanxi, PR China,
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Rice University, Department of Earth, Environmental and Planetary Sciences, Houston
Current address: Wells Fargo Securities, Houston, TX.77002 Corresponding author, [email protected]
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Key words: Detached structure; shallow basement detachment; Laramide province; Elk
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Basin; cross-section restoration; mechanical modeling.
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Abstract
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‘Laramide-style’ structures from the Laramide province of the western US involve the
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underlying crystalline basement. The basement is broken into blocks of varying scales
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that are generally bounded by moderately-to-steeply dipping reverse faults. This style is
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very pervasive, so that most structures are interpreted this way, even when field
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relationships suggest otherwise. The Elk Basin anticline, which lies in the northern
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Bighorn Basin is one of these structures. Basement is not elevated on the southwest side
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of the structure, however, which argues strongly for a detached origin. Balanced section
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restoration and forward finite-element modeling both support the interpretation that the
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underlying Elk Basin thrust is a detached, listric or ramp-flat style thrust fault. The
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detachment lies within the crystalline basement, ~0.5 km below the sediment-basement
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contact. This detachment is fundamentally different from the detachments in the deep
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crust interpreted by Erslev (1993) to be the underlying link between widely-separated
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basement arches within the Laramide province. We propose a new ‘shallow basement
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detached” class of structural style. Several candidate structures for this same style are
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present within the Bighorn basin, with both east and west vergence. These structures do
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not appear to form an interconnected detachment system, but rather are local ‘flakes’ of
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basement that may form due to bending of the underlying crust during basin-scale
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Laramide-age contraction and folding.
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1. Introduction
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The Laramide Orogeny was originally defined by Dana, (1895, as cited in Tweto, 1975)
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for the Laramie Formation, which was the name for localized Upper Cretaceous non-
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marine strata that overlie widespread marine Cretaceous beds throughout Wyoming,
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Colorado, and New Mexico. The change in depositional environment represented by the
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Laramie Fm. also marked a change from widespread to local basin depocenters related to
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the breakage of the smooth foreland into broad basement uplifts and intermontane basins.
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The term Laramide is in wide usage today, having both a time and a structural style
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connotation (e.g. Dickenson and Snyder, 1978). Dana (1895) defined a large ‘Laramide
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province’, extending from Montana to Mexico, characterized by similar basement-cored
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uplifts and their adjacent basins.
49 The geometry of Laramide uplifts was debated for quite some time (summarized in
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Stone, 1984), until long-record seismic data (e.g., Smithson et al., 1978), exploration
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drilling (Berg, 1962; Stone, 1993), and balanced cross section methods (Suppe, 1983;
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Brown, 1984; Suppe and Medwedeff, 1987; Narr and Suppe, 1994; Mitra and Mount,
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1998; Erslev, 1986, 1991, 1993) demonstrated the uplifts were underlain by relatively
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planar, moderately to steeply-dipping reverse faults at least within the upper crust. These
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faults cut the basement-sediment interface, and elevate the basement together with the
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overlying cover rocks. The involvement of much of the upper crystalline crust in these
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structures has led to the adjective ‘thick-skinned’ to describe this structural style.
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Contraction within fold-and-thrust belt environments, in contrast, does not generally
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elevate the basement rocks. Instead, shortening is accomplished by faults that displace the
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cover strata along and over the basement-sediment interface, with ramps that cut upward
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towards secondary detachments and/or the surface (Bally et al., 1966; Price, 1981;
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Coogan, 1992). This general lack of involvement of crystalline basement within these
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systems coupled with the relatively shallow fault depths has led to the adjective ‘thin-
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skinned’ to be applied.
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These basic differences in structural styles also differentiate the “Sevier Orogeny” (thin-
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skinned) (Armstrong, 1968), from the “Laramide Orogeny” (thick-skinned) in the
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western US, although the later stages of the Sevier Orogeny and the Laramide Orogeny
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are contemporaneous (Tweto, 1975; Gries, 1983; Dickinson et al., 1988; Kulik and
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Schmidt, 1988).
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This paper examines the Elk Basin anticline in the northern Bighorn basin, along the
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border between Montana and Wyoming (Fig. 1). Oil was discovered in this structure in
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1915 (Hein, 2017), and it has been extensively drilled as a consequence, which provides
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excellent depth constraints (Wetzel, 1954). Crystalline rocks of the Archean Wyoming
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province are present within the core of this anticline (Stone, 1993). 2D seismic data
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across the structure have been interpreted by Stone (1993) and Weitzel (1985) to indicate
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that the faults bounding this basement core are similar to the steeply dipping reverse
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faults associated with the major basement-cored, thick-skinned uplifts within the
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Laramide province. A simple test of fault geometry is to look at the elevation of basement
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in the hangingwall side versus the footwall side of a structure. This relationship is shown
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schematically in Fig. 2 for three different contractional fault configurations (see also
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Eichelberger et al., 2017). The configuration of both the hangingwall and footwall of Elk
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Basin is shown in Fig. 3. The hangingwall is interpreted to be elevated above a dipping
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ramp in the center of the structure, but it returns to the regional level further east. This
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geometry is most like example 2a, which suggests the Elk Basin fault is detached.
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Our work utilizes cross-section restoration and numerical modeling techniques to revisit
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these two significantly different structural interpretations of this anticline. In particular,
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we address whether the major underlying fault is planar to significant depth, or if it
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becomes parallel to the basement-sediment interface at relatively shallow depths within
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the crystalline basement. Understanding the structural style, particularly whether the
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structure is detached or basement-involved, is important for exploration in these
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provinces around the world. It can infer the presence or absence of reservoir within a
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particular structural trap, and the distribution of potential fractures within the structure.
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Examples of foldbelts with mixed structural styles where this can be important include
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large provinces such as the Zagros foldbelt in Iran (e.g. Alavi, 2007), down to relatively
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small examples such as the Yakima foldbelt in Washington State (Reidel, 1984; Blakey et
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al. 2011; Gomberg et al. 2012).
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2. Local Setting and Previous Work
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Elk Basin anticline lies about half way between the Beartooth uplift and the northern
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Bighorn Mountains (Fig. 1). These ranges are two very large, classic basement-involved
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‘arches’ characteristic of the Laramide province (Erslev, 1993). The Beartooth uplift is
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bounded on its NE side by a steeply-dipping reverse fault (Fig. 1) (Bevan, 1923; Wise,
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1963) that has been confirmed by drilling (Stone, 1993; Omar et al., 1994). Stone (1993)
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interprets the eastward displacement on the Beartooth fault to be approximately 4 km.
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The asymmetry of the Bighorn uplift also suggests it is an eastward-verging structure
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with reverse faulting near the middle of the range (Stone, 1993) (Fig. 1). Additional,
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smaller scale basement-cored uplifts occur in the Pryor Mts., about 20 km east of Elk
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Basin (Harding and Lowell, 1979).
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A cross section of the Elk Basin anticline shows the east-directed asymmetry of the
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structure, with the hangingwall displaced approximately 2 km eastward over the footwall
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along the Elk Basin thrust (Fig. 3). Crystalline rocks have been encountered in wellbores
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in the core of the anticline (Stone, 1984, 1993; Weitzel, 1985). Stratal geometries in the
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overlying Paleocene Fort Union Formation indicate the Elk Basin anticline was actively
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growing during the Paleocene, in accordance with evidence for synchronous activity on
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both the Beartooth uplift and Bighorn mountains, plus many other structures in the
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province (e.g., Stone, 1993; DeCelles et al., 1991).
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The 2D seismic data over the anticline were the subject of a modeling study by Weitzel
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(1985). She used ray-trace modeling and velocity analysis to test both shallow detached
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and planar reverse fault interpretations for the principle structure beneath the anticline
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(Fig. 4). She concluded that the main fault was most likely a planar reverse fault. This
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interpretation suggests the Elk Basin structure is similar to other thick-skinned Laramide
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structures in the region. Stone (1984, 1993) also examined the Elk Basin structure using
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physical modeling and area balance arguments. Stone (1993) also concluded that the
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anticline is most likely underlain by a planar reverse fault.
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This study employs 2D cross-section restoration and 2D mechanical models to provide a
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new assessment of these fault interpretations, as well as their resulting fold geometries.
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Stone’s (1993) idealized model with both a planar reverse fault and an alternative shallow
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detachment fault interpretation form the basis of our testing (Fig. 4).
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3. Cross-section restoration of the Elk Basin structure
140 A cross-section restoration, line- and area-balance analysis of the Elk Basin anticline was
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performed using the Lithotect(TM) software. This method tests the geometric compatibility
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of the hangingwall and footwall interpretations along a fault trace. A bedding-parallel,
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flexural slip mechanism was used to restore both the strata and the basement blocks. The
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restoration datum was the basement-sediment contact. These analyses use the footwall as
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a template into which the restored hangingwall is fit. A ‘perfect’ deformed-state
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interpretation can be restored so that all pre-tectonic strata achieve a flat initial state, and
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the hangingwall and footwall blocks match without gap or overlap along the fault trace
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(e.g., Woodward et al., 1989).
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The structural interpretation presented in Stone (1993) was modeled ‘as is’. No effort was
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made to improve any line-length or area mismatches in that interpretation, since the
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purpose of this study was to evaluate the first-order geometry of the underlying faults
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(Fig. 5).
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3.1 Cross-section Restoration Results
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Gaps occur on both restorations at the fault tip. The fault tip region is the same on both
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interpretations and contains a small mismatch of the hangingwall geometry relative to the
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footwall geometry. A box is shown on Figs. 5B and D to highlight where the restored
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interpretations diverge. The shallow basement detachment fault interpretation in Fig. 5A
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produces a restored hangingwall geometry that fits closely to the footwall geometry, with
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few gaps and no overlaps (Fig. 5B).
163 The planar fault interpretation, in contrast, produces a restored hangingwall geometry
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that does not fit well with the footwall geometry, and results in a large gap between the
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hangingwall and footwall (Fig. 5D). This large area of gap indicates that hangingwall and
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footwall interpretations for the planar fault are relatively incompatible compared to the
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small area of gap between the hangingwall and footwall for the listric fault interpretation.
169 4. Finite Element Model analysis
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The detached and planar fault interpretations were also tested using finite element
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forward modeling (Zhang et al., 2013; Gray et al., 2014). The purpose of this mechanical
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modeling was to test the two different fault geometries in a forward sense, from the
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undeformed to the deformed state, and see if either could reproduce the observed
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hangingwall geometry. Finite element (FE) modelling differs from restoration analyses in
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many ways, but in particular that volume (area in 2D) can change as defined by the
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constitutive laws employed (Crook et al, 2006a, 2006b; Gray et al, 2014). All FE
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modeling was undertaken using ELFEN(TM) software (supplementary materials).
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The model setup is described in detail within the supplemental materials. The constitutive
181
law used in these models is a modified critical-state soil mechanics-type law developed
182
by Rockfield (Crook et al. 2006a, 2006b; Thornton and Crook, 2014). This type of law
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includes elastic, poro-elastic, and plastic behavior depending upon the stress conditions
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during deformation (Crook et al., 2006a, 2006b; Thornton and Crook, 2014). The
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ELFEN(TM) code handles high-strain deformation through a process of adaptive remeshing,
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whereby the mesh is re-calculated once it deforms past pre-set threshold. Remeshing
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causes individual mesh polygons to become smaller, which further localizes strain and
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simulates the propagation of faults through the materials when stress conditions warrant
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(Thornton and Crook, 2014). Both models were displaced parallel to the faults, so as to
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avoid creating gaps or overlaps between the hangingwall and footwall blocks.
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The listric, shallow basement detachment fault FE model is similar to the ‘detachment 2”
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model of Stone (1993), except that the radius of curvature of the fault bend was decreased
194
slightly for ease of modeling (supplemental materials). This resulted in a more a flat-
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ramp type geometry, though it retains the primary feature of a long detachment parallel to
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bedding. The hangingwall block was displaced parallel to the ‘flat’, horizontal portion of
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the fault.
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The fault dip in the planar-fault model matches the 30o dip in Stone’s (1993) preferred
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basement faulting model. The resulting fold geometries were then compared to both Fig.
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2 and the observed folding of the Elk Basin anticline. The ‘goodness of fit’ for each of
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these models was assessed on the basis of the relative elevation of the hangingwall after
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movement.
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4.1 Finite Element Model Results
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The shallow detachment fault model creates a fold-thrust geometry that is very similar to
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the interpretation shown in Fig. 2A and the shallow detachment option on Fig. 3. The
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dotted line on Fig. 6B denotes the regional level of the basement. Basement is only
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elevated with respect to this regional surface where the fault changes from flat to ramp.
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There is a good overall fit between the interpreted Elk Basin hangingwall geometry and
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the detachment forward model (supplemental materials).
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The forward model with the planar fault is perhaps the most illustrative result of this
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study. Movement on the planar fault elevates the entire hangingwall to the same degree.
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Any movement on this fault results in a commensurate uplift of the hangingwall above
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the pre-faulting regional level as also shown in Fig. 2C. This produces the classic drape
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fold or monocline geometry common to many Laramide uplifts (Harding and Lowell,
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1979; Stone, 1984). The results of each forward model have been superposed over the
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Stone (1993) interpretations in the supplemental materials.
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5. Discussion
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The results from both restoration and forward modeling strongly favor a shallow
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detachment geometry for the Elk Basin Thrust. The detached interpretation restores with
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far less gap, and the detached forward model creates a reasonable facsimile without
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elevating basement throughout the hangingwall. The restoration and forward model with
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planar faults have problems when compared to the Elk Basin structure. The restoration
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has a large gap, and the forward model elevates the entire hangingwall. Neither of these
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results support the hypothesis that the planar fault better explains the observed geometry
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of the Elk Basin structure. Thus, we think the Elk Basin thrust has a listric, or even flat-
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ramp detachment-style geometry. Furthermore, this fault detaches within crystalline
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basement, approximately 2.2 km beneath the surface, or 0.5 km beneath the basement-
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sediment contact.
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Elk basin is apparently not the only anticline with an underlying shallow basement
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detachment. The Grass Creek anticline (Fig. 7) has been interpreted by Stone (1993) and
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Mitra and Mount (1998) to have a backlimb that returns to the pre-faulting regional level,
238
very similar to the geometry of Figs. 5A and 6B. This geometry requires that the fault
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displacement be transferred along a detachment fault, parallel to the basement-sediment
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contact, approximately 600 meters below the basement-cover contact. The Garland
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anticline, immediately south of Elk Basin, Pitchfork on the far west side of the basin, and
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Bonanza, on the east side of the basin (Fig. 1; DeBruin, 1996) also have a similar
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backlimb geometry, as presented in Stone (1993).
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It is not clear why these structures detach so shallow within the basement. The
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stratigraphy of the Bighorn basin contains many shale-rich horizons that seem far more
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suitable as potential detachment horizons, especially near the base of the sedimentary
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section. The basal detachments for the Sevier belt further west occur in very similar
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stratigraphy (Royse et al., 1975; Coogan, 1992). One potential reason for detachments
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within the gneissic basement rocks may be that they are locally highly layered. Near
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Cody, Wyoming, this layering is imparted by granitic sills that are nearly parallel to the
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basement-sediment contact in the Shoshone River canyon (Fig. 8). The relatively small
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displacements of most of these detachments, plus the opposing vergence seen in the
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southern Bighorn basin, suggest these shallow detachment structures are local, and
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probably not part of a connected, thoroughgoing detachment system. Instead, they may
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be adjustments that formed in the uppermost crust due to the movement on deep crustal
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(~30 km) ductile ‘detachments’ and ramps as proposed by Erslev (1993), and Yeck et al
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(2014). As such, these shallow detachments could be small flakes displaced along the top
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of the crust as it moved laterally on deep faults. Regardless of the mechanism, this work
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documents that classical detachment-style faulting may also occur within basement
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lithologies, even in the classic province known for steeper-oriented thick-skinned
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structures.
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6. Acknowledgements
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We thank Halliburton and Midland Valley for providing Lithotect(TM) and MOVE(TM)
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structural restoration software to Rice University. Rockfield Global provided their
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ELFEN(TM) software. Thanks especially to Dr. Melanie Armstrong and Fen Paw of
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Rockfield for their technical support during the finite element modeling and writing
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process. Rick Groshong, Bill Dunne, Nur Schuba and an anonymous reviewer are
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thanked for their comments which significantly improved this manuscript. Eric Erslev is
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thanked for reviewing an earlier version of this work. This research did not receive any
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specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Figure 1. Location map of the Bighorn basin in Wyoming and Montana. Geology draped
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over shaded topography, prepared with GeoMapApp. Precambrian basement exposures
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shown in pink. Mesozoic and Paleozoic rocks in green, and Tertiary rocks in light brown.
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Black lines mark the location of the Elk Basin and Grass Creek cross sections used in this
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analysis. Towns in the Bighorn basin: C, Cody; G, Greybull; T, Thermopolis. Trace of
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the Beartooth Thrust taken from Bevan (1923) and Wise (1963). g, p, and b refer to the
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Garland, Pitchfork, and Bonanza structures, respectively.
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Figure 2. Schematic diagram showing the interplay between fault dip and elevation of
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the hangingwall relative to the regional level. The regional level is shown as a dashed
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line. A) Main fault trace (with arrow) is parallel to bedding. Lateral displacement on fault
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causes frontal anticline to rise above regional level, but trailing part of the hangingwall is
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not elevated relative to regional. B) Non-planar fault trace is inclined steeper than
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bedding contact. Movement on fault causes both frontal anticline and trailing
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hangingwall to elevate above regional. C) Planar fault is inclined steeper than bedding
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contact. There is no frontal anticline due to planar nature of fault. Entire hangingwall is
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elevated above regional level, creating a monoclonal structure. Similar fault/cover
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geometries have been recently investigated by Eichelberger, et al. (2017).
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Figure 3. Profile view of the Elk Basin anticline modified from Stone (1993). Dashed
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lines show the location of selected wellbores used by Stone (1993) to constrain the
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geometry of the structure. Dotted line, surface topography, pC, preCambrian crystalline
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basement of the Wyoming province, lPz, Lower Paleozoic units, uPz, Upper Paleozoic
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units, Mz, Triassic through lower Upper Cretaceous units. The upper Upper Cretaceous
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and Tertiary units have been omitted for clarity. Note that the Elk Basin Thrust is
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interpreted as a planar, moderately dipping reverse fault. No vertical exaggeration.
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Figure 4. “Mature thrust-fold” model of Stone (1993). Light gray area is cover strata as
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in Figure 3. Dark gray is crystalline basement rocks. Stone’s two fault interpretations
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tested with the structural models, ‘Detachment 2’ and ‘Planar reverse fault’ are shown in
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the dark lines. The planar fault interpretation is favored by both Weitzel (1985) and Stone
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(1993).
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Figure 5. Flexural-slip restorations of the two faults shown in Fig. 4. A) The ‘detachment
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2’ fault is a listric geometry that becomes parallel to the sediment-basement contact at
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approximately 500 m below the unconformity. B) Restoration of the listric fault, datumed
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on the basal unconformity. C) Planar fault geometry, showing a folded hangingwall
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above a planar fault. D) Restoration of the planar fault interpretation, using the same
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datum and techniques as in B. The fault tip area on both interpretations is that of Stone
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(1993) and restores with an upward-increasing gap, and was not modified. The uppermost
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light gray unit was area-balanced, also due to complications with the fault tip. The inset
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box highlights the area tested by the alternate fault geometries. The restored hangingwall
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of the listric fault geometry (B) fits together with the footwall without overlap and only
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slight gaps. The restored hangingwall of the planar fault geometry (D) does not fit well
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against the footwall block. There is no overlap between the two blocks, but a wedge-
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shaped gap opens downward. This large gap is the consequence of a having a folded
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hangingwall above a planar fault. 1 and 2 denote changes in the angle of the fault in the
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hangingwall block not present in the footwall block
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Figure 6. Finite-element forward models of the same fault configurations as shown in
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Fig. 4 and supplemental materials. A) Starting configuration for the detachment fault
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model similar to the restoration in Fig 5B, but with both hangingwall and footwall fitting
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together perfectly. B) Ending state of the model in (A) after 5 km of fault displacement
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from the left (see also supplemental materials). C) Starting configuration for the planar
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fault model, which is similar to the restoration in 5D, but also without the gaps between
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the blocks. D) Final configuration of the model in (C), with 3 km of fault offset. The
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hangingwall in this model was displaced parallel to the fault, which prevents gaps or
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overlaps between the two basement blocks. Note the regional level shown by the dotted
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lines. Regional basement level is maintained in model A, but is elevated in the
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hangingwall of model C, as shown by the arrow.
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Figure 7. Interpreted cross section across Grass Creek anticline in the southern Bighorn
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basin, after Stone (1993) and Mitra and Mount (1998). The hangingwall of this structure
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returns to a regional level (dotted line) to the east. It has opposite (westward) vergence to
465
Elk Basin. The detachment level for this structure also appears to be within the crystalline
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basement, about 600 meters beneath the basal unconformity. No vertical exaggeration.
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Figure 8. Photographs of compositional layering in Archean basement rocks in Shoshone
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Canyon, west of Cody Wyoming. Layering is comprised of granitic sills cutting through a
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schistose, amphibolite host rock. A) View to the west, up-canyon towards Buffalo Bill
473
dam, showing layering on a large scale. Cliff is approximately 200 m high. B) Close-up
474
view of north side of canyon. The height of the center outcrop is approximately 10
475
meters. The average dip of layering in this outcrop is about 20 degrees east, similar to the
476
overlying strata which dip 15 degrees east. Both photos taken on the north side of the
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Shoshone River near the Buffalo Bill powerplant. Approximate Lat.: 44.508620; Long.: -
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109.173502.
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A new type of shallow detachment structure is recognized that involves crystalline basement The detachment is approximately 2.2-2.4 km below the basement sediment contact. This structure is found where only steeply-dipping basement faults had been believed to exist
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These detachments probably form due to large-scale bending of the basement