Testing models of fault propagation and damage zone development across the central Sevier fault zone, southern Utah

What: This five-student Advanced project focuses on the evolution of the Sevier normal fault zone in southern Utah, near Zion National Park. The Sevier normal fault, considered one of the most important structures in the Basin and Range province, is part of the Toroweap-Sevier fault system, which extends for more than 300 km from northern Arizona to southern Utah.

When: June 23-July 20, 2024

Where: Glendale, Utah (field work) and San Antonio, Texas (lab work)

Who: Five students and project director Dr. Ben Surpless, Trinity University, bsurples@trinity.edu

Prerequisites and Recommended Courses: Suggested (but not required) are upper level courses in the student’s geosciences major, including Structure/Tectonics and at least 2 other upper level geosciences lab courses. Students should have completed key cognate courses in Chemistry and Math. Experience at a field camp or in a field geology course is recommended but not required. We are particularly interested in applicants with an interest in structural geology, who have a high degree of comfort in outdoor settings, are flexible eaters, and who want to use this work to complete a senior thesis (or equivalent) in geology.

Expectations and Obligations:
1. Participation in field work and data analysis during the summer.
2. Write an abstract and present a poster at a national conference (GSA in Anaheim, CA; AGU in Washington, DC; or a spring GSA section meeting).
4. Write a short contribution (4-6 pages text + figures) to be published in the Proceedings of the Keck Geology Consortium 2024 Volume (first draft due Mid-February).
5. Expected but not required: Use this work for the completion of a senior thesis (or equivalent) at your home institution.

PROJECT DESCRIPTION

Figure 1. Development of a fault damage zone during fault propagation and displacement. A. damage zone development ahead of a propagating tip and parallel to the fault plane (adapted from Fossen, 2016). B. Damage zone architecture and terminology (adapted from Laio et al., 2020).

In addition, although researchers have long recognized that fault zones are segmented, as opposed to continuous, planar surfaces (e.g., Tchalenko, 1970; Schwartz and Coppersmith, 1984), researchers have made significant advances in the role that segmentation plays in overall fault system evolution (e.g., Long and Imber, 2011; Siler et al., 2018; Surpless and Thorne, 2021) as well as how interacting faults affect damage zone development (e.g., Kim et al., 2004; Choi et al., 2016). Where two adjacent normal fault segments interact, fracturing is commonly amplified, increasing the volume of rock damaged relative to two separate, isolated faults (e.g., Stock and Hodges, 1990; Hudson, 1992; Faulds, 1996).

For this project, we will focus our investigation on the central Sevier fault zone in southern Utah (Fig. 2), a well-studied, steeply west-dipping normal fault system with ~400 – 800 m dip-slip displacement (e.g., Scheifelbein, 2002). The sparse vegetation, range of fault segment geometries and interactions, and excellent land access provide opportunity to document a range of cross-sectional views of faults and damage zones; this is an ideal location to investigate the coupled evolution of fault propagation, displacement accumulation, and damage zone development across a range of spatial scales.

Figure 2. Physiographic context for the Sevier fault zone study area within the Basin and Range-Colorado Plateau transition zone (see inset). In combination with the Grand Wash, Hurricane, and Paunaugunt faults, the Sevier- Toroweap fault helps accommodate extension across the transition zone. Ball is on the hanging wall of the west-dipping faults. Detailed geology of the Sevier fault study area (boxed) is displayed in Figure 3. Digital shaded relief modified from Thelin and Pike (1991). Figure modified from Reber et al. (2001).

Development of the Sevier fault zone

The Sevier normal fault, considered one of the most important structures in the Basin and Range province (e.g., Davis, 1999; Lund et al., 2008), is part of the Toroweap-Sevier fault system, which extends for more than 300 km from northern Arizona to southern Utah (Fig. 2). The fault has accommodated extension across the transition zone from the Basin and Range province to the relatively stable Colorado Plateau since the Miocene (e.g., Reber et al., 2001; Lund et al., 2008), and previous workers have noted the potential of the fault to produce significant earthquakes (Anderson and Rowley, 1987; Doelling and Davis, 1989; Anderson and Christenson, 1989; Lund et al., 2008).

The Orderville geometric bend

The study area is part of a particularly complex portion of the Sevier normal fault, termed the Orderville geometric bend (e.g., Reber et al., 2001), with a range of fault zone geometries exposed at different locations along the NNE-striking system. Figure 3 displays the fault network that accommodated extension along the Orderville bend. The bold yellow fault traces represent the three primary high-displacement faults across the study area, the Mt. Carmel, Spencer Bench, and Orderville segments, and white faults represent lower displacement normal fault systems that link and aid in the accommodation of extension, evolving within zones of strain transfer between the primary faults.

Figure 3. Fault map of the Sevier fault zone near Orderville, Utah. Yellow lines indicate normal faults that accommodate significant (>100 m) displacement or play an important role in fault linkage, and white lines indicate normal faults that play a role in strain accommodation but display lesser displacements. Ball symbols are on the hanging wall. Study locations are indicated by numbers in circles. Faults shown here are based primarily on Schiefelbein (2002) and Surpless and McKeighan (2022). See Figure 2 for location.

Recent studies (e.g., Davis, 1999; Schiefelbein and Taylor, 2000; Schiefelbein, 2002; Doelling, 2008) and results from the 2018-2019 and 2022-2023 Keck Advanced Projects (Hankla et al., 2018; Surpless and McKeighan, 2022; Nishimoto et al., 2022; Hayton et al., 2023; Jennings et al., 2023; Sharp et al., 2023) revealed locations within the Orderville geometric bend that preserve fault geometries related to different stages of fault propagation and damage zone development along the Sevier fault zone of southern Utah. These studies establish a well-defined structural framework for this year’s research efforts (see study locations displayed in Fig. 3).

Current state of research: fault propagation and damage zone development

As faults initiate and begin to lengthen and accumulate displacement, they propagate horizontally and vertically as the fault grows in an approximately elliptical shape (Fig. 4; e.g., Peacock, 2002; Long and Imber, 2011; Siler et al., 2018). However, researchers debate whether (1) a normal fault’s displacement (D) and length (L) grow at constant rates over time, with a relatively constant D/L ratio as the fault matures (the propagation model) (e.g., Cartwright et al., 1995; Reber et al., 2001), or (2) there is an early phase of rapid fault lengthening followed by a prolonged period of displacement accumulation without significant lengthening (the constant-length model) (e.g., Walsh et al., 2003; Childs et al., 2009; Nicol et al., 2016). Additionally, recent studies suggest that damage zones may not develop symmetrically on either side of the fault plane; instead, damage zone development may be asymmetric, with one side (hanging wall or footwall) displaying a greater thickness than the other (Fig. 1B) (Berg and Skar, 2005; Liao et al., 2020).

Figure 4. Diagrams revealing the 3D geometry of elliptical fault plane propagation. A. Stratigraphy deformed by an elliptical fault plane. If viewed in cross‐section (front of diagram), the fault trace and damage zone around it would be visible. B. View of the elliptical fault within the rock volume. Any two‐dimensional exposure of the fault would reveal a fault trace with tip points, where displacement is zero. C. isolated elliptical fault plane displaying how hypothetical displacement values vary based on position on the fault plane. Adapted from Fossen, 2016.

Research Questions

The questions below will help drive our field and laboratory research, but it is likely that these questions will evolve as we collect and analyze project data. Because each student will have a different project, it is likely that a given student will only address a few of these questions.

• Question 1: How did the complex fault system exposed at the Orderville geometric bend evolve in 3 dimensions?

• Question 2: How does total displacement accommodated by mapped faults vary along strike, and what does that tell us about seismic hazard associated with transfer zones?

• Question 3: How do local stress and strain states vary between fault segments as displacements increase and faults propagate and interact?

• Question 4: How do the characteristics of fractures and other subsidiary structures vary within the stratigraphy at different positions relative to mapped faults, and can these variations be related to primary or secondary characteristics of lithologies?

• Question 5: How do the characteristics of fault damage zone architecture vary with accumulated displacement, rock type, or structural position?

• Question 6: Do fracture intensities and orientations outside of damage zones vary in relation to mapped faults and major joint systems?

• Question 7: Can the relative development of a normal fault segment boundary be used as a predictor for subsidiary structure characteristics?

• Question 8: Do our results shed light on the debate about normal fault initiation, lateral propagation, and accumulation of displacement (i.e., the propagation model vs. the constant-length model?

Research Methods

In the field

The primary 2024 study sites reveal different structural contexts that students can leverage to address a range of research questions. Near location 1 (Fig. 3), all displacement associated with the Sevier fault zone is accommodated by a single fault (the Mt. Carmel Segment). Locations 2A and 2B, along the Spencer Bench segment (Fig. 3), display very different accumulated displacements, including the region in front of the propagating fault tip (Fig. 1A). Location 3 is located within a more complex zone, where strain is transferred and shared between several faults. At each location, erosion has exposed continuous outcrop both parallel and perpendicular to strike, permitting us to observe rocks affected by faults at different stages of development. While in the field, students will use a range of methods to document fault and subsidiary structure characteristics. All students will be involved in the collection of all types of field data, and all field data will be compiled in a GIS database, using location data from handheld Trimble GEO XH units.

Students will use classic geologic mapping techniques to locate map-scale unit contacts and faults, and they will document bedding dips as well as fault orientations and senses of slip. Students will also collect data related to outcrop-scale subsidiary structures, including orientations of fractures, minor faults, and deformation bands. Students will document important field relationships using a combination of field sketches and photography. At key locations, students will also document fracture attributes, including fracture position along a measured scanline, fracture orientation, fracture morphology, and crosscutting relationships. Students will also collect hand samples of map units.

Because of the rugged topography in some parts of the study area, we will use UAV-based photography and structure-from-motion (SfM) 3D model construction (e.g., Johnson et al., 2014) to supplement classic field-based structural mapping methods (model construction will take place in the lab).

In the lab

Here, I describe a range of approaches that a given student may use to analyze normal fault evolution. I assume that all 5 students will use field data, but I also assume that each student will use at least one 3D computer modeling approach to analyze data and test hypotheses. While at Trinity, every student will work through software tutorials and begin initial model construction prior to departure. In addition, all students will participate in cutting billets for thin sections, which will be sent to Spectrum Petrographics, with a return time that will allow petrographic analysis to begin during the academic year. Only one or two students will be involved with petrographic analysis.

To supplement outcrop field data, multiple students will use Agisoft Metashape Professional, which permits construction of 3D models from UAV photographic data of inaccessible exposures. The software uses feature-matching algorithms that allow for changes in scale or perspective (e.g., Lowe, 2004), making it easier to produce high-quality virtual outcrop models from UAV photography. Geologists have proven the viability of this type of modeling to document geology in sparsely-vegetated field areas (e.g., James and Robson, 2012; Fonstad et al., 2013; Corradetti et al., 2018), similar to the cross-sectional exposures in the Stewart and Red Hollow canyons of the Orderville geometric bend.

Students using this method can focus on a range of topics, including: horizontal and vertical variation in fracture characteristics relative to mapped faults, changes in fracture characteristics associated with rock strength, the influence of primary structures (cross-beds, bed contacts, or map-unit contacts) upon fracture formation and propagation, or changes in fracture characteristics based on structural position (relative to the mapped fault network). Importantly, Agisoft Metashape Professional is very user friendly, and I have budgeted for two licenses that non-Trinity students can install on their computer or on a computer at their home institution. Students can confidently use this software, following a workflow that I’ve developed, in just a few days.

Potential Student Projects

Project 1: Structural evolution of the Orderville geometric bend, Sevier fault zone. This student would use published maps and new map data to construct accurate structural cross-sections across the Orderville geometric bend. Although a previous researcher (Scheifelbein, 2002) developed cross-sections across the fault system, the subsurface interpretations displayed in those sections are, in, some cases, problematic. This student can then construct a hypothetical 3D model of the fault system from surface to depth. An important part of this work would be to shed light on how faults in similar systems propagate and link both horizontally and at depth. If the student has access to the Move2020 software suite at their home institution, they can use the Fault Response Module to test their hypothesis(es). However, this project does not require computer modeling to be successful. This project would build on the initial model constructed by Demi Durham (2023-2024 Keck Advanced Project).

Project 2: Predicting changes in strain accommodation associated with increasing fault zone complexity. This student would use a combination of field data, structural analysis of faults and fractures, cross-section construction, and, possibly, 3D modeling using the Move2022 Fault Response Module to investigate how total fault displacement changes along strike within the Orderville geometric bend. They would build 3 primary models, constrained by field and existing map data, to evaluate the characteristics of predicted fracture systems that develop in fault systems of varying complexity. When the student compares model results with field data and mapped fault patterns, they can develop hypotheses for strain accommodation and permeability development in rocks affected by fault systems of varying complexity. This project would require access to Move2022 during the academic year and will build on Audrey Jenning’s modeling of isolated normal fault initiation and evolution (2023-2024 Keck Advanced Project).

Project 3: Predicting fault damage zones associated with lateral propagation of overlapping normal faults. This student would use a combination of field data, mapped faults, fracture analysis, and SfM modeling (which permits constructions of a spatially-accurate virtual outcrop model). This student would also likely use thin section petrography to document changes in both lithologic characteristics and fracture network characteristics in relation to faults with well-constrained kinematics. In one case, fault segment displacement decreases from 10s of meters of displacement to 0-m displacement (at the fault tip) over 100s of meters along strike (easternmost fault in Red Hollow Canyon, Fig. 2). Importantly, the rocks ahead of the fault tip are well exposed in the field, permitting students to evaluate damage in the fault tip zone during fault propagation. A student investigating these topics should be able to develop a model of fault propagation and fracture network development. This would likely build on the work started by Morgan Sharp, who focused on single-fault systems (2023-2024 Keck Advanced Project).

Project 4: Changing stress-strain fields within a complex normal fault transfer zone. This student would use a combination of field data, mapped faults, fracture analysis, and 3D computer modeling using the Move2022 Fault Response Module to investigate how different fault geometries and overlaps affect the stress field and create subsidiary structures (mostly fractures). This student would choose several of the better constrained en echelon fault geometries (e.g., no overlap, some overlap, significant overlap, hard-linked) to build and test 3D models of fracture development. This student would also likely use thin section petrography to document changes in both lithologic characteristics and fracture fill textures and compositions. These model results and petrographic analysis results could then be compared to field data, permitting the student to address how stress and strain vary in different cases of synthetic normal fault interactions.

Project 5: Fault damage analysis within a complex normal fault transfer zone. This student would use a combination of field data, mapped faults, fracture analysis, and 3D computer modeling using Metashape Professional to investigate how fracture networks vary in cross-sectional exposures of the fault network exposed in Stewart and Red Hollow Canyons (Fig. 2). These 3D models will be the virtual outcrops that this student can use to analyze changes in fracture intensities and orientations both horizontally (relative to mapped faults) and vertically (relative to changes in lithology). This student would also perform some petrographic analysis, especially investigating changes in lithology that might impact fracture development. The results of this student’s analysis would inform our understanding of fluid-flow potential associated with similar fault networks in the subsurface.

Project 6: The impact of primary structures on fracture network evolution. This student would use a combination of field data and SfM constructions of georeferenced virtual outcrop models. They would investigate how primary structures, including bed contacts, crossbeds, and crossbed-set contacts affect the initiation, propataion, and termination of fractures. Because the models are georeferenced, they can directly measure distances to perform quantitative comparisons between different outcrops at different structural positions across the field area. The results of their investigation would shed light on the interplay of a developing fracture network in the context of both primary structures and structural position within a complex fault zone.

Project 7: The geometry of vertical fracture propagation within fracture corridors. This student would use a combination of field data and SfM constructions of georeferenced virtual outcrop models. Fracture corridors are vertically-oriented, tabular zones of high fracture intensity that may have a profound effect on the migration of fluids in the subsurface. In the study area, these corridors are commonly subparallel to the dominant fault segments but display no shear displacements. Our work during the 2022 Keck Advanced Project revealed many examples that should be easily analyzed by this student. The results from this study should permit the student to develop hypotheses about fracture corridor development on the outcrop scale as well as how these features might impact fluid flow in the context of transfer zone fault and fracture geometries.

PROJECT LOGISTICS

Students will fly to San Antonio on June 18 and check in to Trinity dorm rooms. Surpless will take students to purchase food for their initial time in San Antonio. In the Geosciences department, we will spend the first 4 days building basic field- and lab-focused skills and knowledge development. Surpless will also guide students in reading key peer-reviewed articles that should improve students’ understanding of what we know about the structural evolution of the study area as well as the concept of fault damage zones. On June 23, students and Surpless will depart San Antonio in a Trinity Suburban (with a trailer), staying at a hotel in Albuquerque, NM, on the night of the 23rd.

On the 24th, we will drive to Bauer’s Canyon Ranch RV park and Campground in Glendale, Utah. We will stay at Bauer’s Canyon Ranch RV Park from the night of the 24th until the night of July 1, performing fieldwork in the nearby canyons (7 nights at the RV park). We will purchase food in Orderville and prepare all meals in the RV park (which also has shower and laundry facilities). During our time in the field, we will visit Zion National Park, where the same rock units are spectacularly exposed. Students will have access to power and wifi throughout our time at the RV park, so students can perform significant database building and field data analysis while in the field.

Students will be expected to carry medium-sized field packs with field equipment, lunches, and water every day in the field. Although hiking will be involved, we will not be hiking in rugged topography where safety is an issue. We will never be more than 2 miles from our field vehicle, and in most cases, we will not be more than 3 miles from a city or town. Although we will be performing fieldwork in late June, the students and I will perform most fieldwork starting at 7 am and ending at about 2 pm, in order to avoid the hottest temperatures (commonly in the low- to mid-90s). In the afternoon, we will compile field data and discuss our findings in the context of the research questions.

On July 2 – July 3, we will drive back to San Antonio, TX, staying in Roswell, NM for one night. Students would work in the Geosciences department from July 5– 16, staying in Trinity’s dorms with other summer research students. Most dorms have kitchen facilities, so meals can be prepared on campus. Many on-campus (non-Keck) research students will have access to cars, or alternatively, there is an on-demand hourly or daily car-rental system), so students can leave campus to purchase food. During this two-week period, all 4 students would focus on analyzing field data, learning to use the 3D computer modeling software necessary for their project, and developing a research proposal and timeline for the rest of the academic year. Near the end of their time at Trinity, students will collaborate to write and submit at least two abstracts (1 or 2 students per abstract) for posters to be presented at a meeting in either fall 2024 or spring 2025 (see Professional Development, below). Students will depart San Antonio on July 16.

PROFESSIONAL DEVELOPMENT

In fall 2024, students will produce posters for either the annual GSA conference in Anaheim, California, the AGU Fall Meeting in Washington, DC, or at a spring GSA section meeting. I will work with home research advisors to help students build their posters. While at the meeting, students will present the results of their research to that point in the academic year, and the Project Team will meet to talk about the current state of each student’s research, to discuss future collaboration plans (where applicable), and to discuss students’ post-graduation plans. Students will also participate in student-focused activities at the conference, many of which encourage students to engage in clarifying their educational and career plans.

All students are required to complete a 4-6 page paper (short contribution) that will be published in the Proceedings of the Keck Geology Consortium 2024 Volume (see examples from previous years). The first draft of this paper will be reviewed by your research advisor at your home institution sometime in late February, 2024, with the revised version sent to the project directors by March 1. Final versions of your paper and figures will be submitted to the Keck Office in Mid-March.

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