Structural evolution of a segmented normal fault transfer zone, Sevier fault, southern Utah

Overview: This four-student 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 (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. 1). 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).

 

Geologist sketching an outcrop in the Navajo sandstone, southern Utah

For this project, we plan to examine the transfer zone between two major segments of the Sevier fault system, where a network of subsidiary faults and fractures has developed during fault propagation and slip. Because these structural features are so well exposed in the spectacularly-crossbedded Jurassic Navajo and Temple sandstones, we can use those features to shed light on how the stress field has affected the intensity of fracturing and faulting both horizontally and vertically in relation to the major fault segments.

Students projects will focus on the evolution of this major normal fault transfer zone, using a combination of field data collection, petrographic analysis of sandstones and fracture fill, 3D computer modeling of fault evolution, fracture intensity and propagation analysis, drone-based image analysis, and 3D computer modeling of stress-strain fields during fault propagation.

When: June 15-July 13, 2018

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

Who: Four students and project leader Dr. Ben Surpless, Trinity University, [email protected]

Prerequisites and Recommended Courses: Suggested (but not required) are core courses in the Geology major: Historical Geology, Structure/Tectonics, Stratigraphy, Mineralogy, and Petrology. 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 (June 15-July 13, 2018)
2. Write an abstract and present a paper for the Geological Society of America Cordilleran Section Meeting in Portland, Oregon (abstracts due Feb. 7, 2019; conference is May 15-17, 2019).
4. Write a short contribution (4-6 pages text + figures) to be published in the Proceedings of the Keck Geology Consortium 2019 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.

Figure 1: 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 2. Digital shaded relief modified from Thelin and Pike (1991). Figure significantly modified from Reber et al. (2001).

PROJECT DESCRIPTION

Geologic Overview

The study area (Fig. 2) 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 transfer zone geometries exposed at different locations along the NNE-striking, west-dipping system.  Figure 2 displays the fault network that accommodated extension along the Orderville bend, from the Elkheart Cliffs area in the south to the area north of Stewart Canyon. The bold yellow fault traces represent the dominant, high-displacement faults across the study area.  The interaction of these fault systems is responsible for the formation of the relay ramps shown adjacent to Stewart Canyon and Red Hollow Canyon, and white faults represent lower-displacement normal fault systems that helped accommodate extension, likely evolving within the perturbed stress field associated with the transfer zones between dominant faults.

Recent studies (e.g., Davis, 1999; Schiefelbein and Taylor, 2000; Schiefelbein, 2002; Doelling, 2008) and reconnaissance work by Trinity University undergraduates (Mathy et al., 2016; Simoneau et al., 2016) have revealed locations within the Orderville geometric bend that preserve fault geometries related to different stages of transfer zone evolution along the Sevier fault zone of southern Utah.  These studies have also established a well-defined structural framework for detailed analysis of the subsidiary structures (e.g., relay ramps, minor faults, fractures, deformation bands) that form as synthetic normal fault segments propagate and link (Fig. 3).

Figure 2: Fault map of the Sevier fault transfer 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, and relay ramps within the transfer zone are labeled with red text. Faults shown here are based primarily on Schiefelbein (2002) and on the results of summer 2016 field research. See Figure 1 for location.

Figure 3: Diagrams displaying relay ramp development, fault capture, and damage zone classification. A) In all diagrams, the dark gray shaded area represents the magnitude of displacement, and the lines represent the slip direction as the hanging wall drops relative to the footwall. 1. A system of small‐displacement faults develops to accommodate upper crustal stresses. The dotted lines represent the future propagation of the faults. 2. As displacement increases across the system, two faults (1 and 2) become dominant, both lengthening in map view and displaying increasing total displacement. A relay ramp forms in the zone of overlap between the faults. 3. Faults 1 and 2 link as a new cross‐fault connects them. A future cross‐fault may form where indicated, fully breaching the relay ramp. B) Map‐view of a segmented fault system (bold lines are faults), with 1 and 2 representing progressive stages of fault segment linkage and capture. In 1, the three fault segments overlap but are only soft linked, and in 2, one of the segments has “captured” displacement from the other segments, isolating the overlapped portions of segments (red lines) that are no longer active. The segments are now hard linked and act as a single, corrugated fault. Arrows show the direction and relative magnitude of fault slip projected onto plan view. C) Schematic map view diagram of damage zone types associated with a segmented normal fault system (bold lines) with ball symbols on the hanging wall. Figure A. adapted from Peacock (2002) and Long and Imber (2011), Figure B. adapted from Reber et al. (2001), and Figure C. adapted from Kim et al. (2004).

Study Area

The excellent vertical and lateral exposure of the Jurassic Navajo and Temple Cap sandstones at the three primary study sites (Fig. 2) provides the opportunity to directly observe faults, fractures, and deformation bands within these well-studied lithologies (e.g., Rogers et al., 2004; Schultz et al., 2010; Solom et al., 2010), especially within the context of the mapped transfer-zone fault networks.  The Elkheart Cliffs exposure at location 1 (Fig. 2) displays the simplest fault geometry because all E-W extension is accommodated by a single fault.  In contrast, location 2 (Red Hollow Canyon) displays a more complex fault system with a single relay ramp, and location 3 (Stewart Canyon) displays the most complex fault network, including 3 relay ramps (Fig. 2).  This spatial variation in fault complexity allows us to treat these locations as “snapshots” in time, capturing “moments” of growing fault zone complexity, permitting us to evaluate how the evolution of different fault geometries and complexities damages adjacent rock volumes (Fig. 3).

Goals and Significance of the Project

Our findings will shed light on how stress and strain in fault transfer zones vary spatially and evolve temporally across a range of scales.  Because rocks in the study area are so well exposed, we can use results from this research to address the 3D evolution of rock volumes associated with similar fault zones in the subsurface.  Brittle features, when formed in the subsurface, impact on the flow of fluids, including water and oil.  Additionally, this research may reveal how strain is diffused at fault segment boundaries during earthquake events, thus impacting the assessment of seismic hazard in similar systems elsewhere.  Related goals of this project include:  1) development of retro-deformable 2D and 3D models of the fault transfer zone, based on both published and new data, which we can test using industry-grade computer modeling software; 2) assessment of stress and strain fields, in the context of field data, that evolved during the vertical and lateral propagation of major fault segments using 3D computer modeling software; and 3) construction of a viable model of subsidiary structure formation in the context of rock properties and relative to the major fault segments mapped in the field.  This project allows students to perform structural field research and computer modeling of a major, segmented normal fault in the regionally important transition zone between the eastern Basin and Range Province and the Colorado Plateau, and the ideas developed during this project will inform our understanding of the complex evolution of normal fault transfer zones worldwide.

Potential Student Projects

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. When combined with existing cross sections, this student can then perform 3D computer modeling in Move2017 to build and test a complete 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.

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 3D modeling using the Move2017 Fault Response Module to investigate how total fault displacement changes along strike within the Orderville geometric bend (location 1 vs. location 2 vs. location 3). They would build 3 primary models, constrained by field and existing map data, to evaluate the characteristics of 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.

3. Predicting fault damage zones associated with horizontal propagation of overlapping normal faults. This student would use a combination of field data, mapped faults, fracture analysis, and 3D computer finite-element modeling using Abaqus to investigate two of the best-constrained (i.e., precisely mapped, with measurable displacements and well-documented fractures) faults within the system. This student would also likely use thin section petrography to document changes in both lithologic characteristics and fracture fill textures and compositions in relation to the faults they test with modeling. In several cases, fault displacements decrease from 10s of meters of displacement to 0-m displacement (at the fault tip) over 100s of meters along strike, permitting the student to develop a model of coupled lateral fault propagation-fracture network development.

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 Move2017 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.

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 Photoscan 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 become 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 LOGISTICS

Field Work: Our tentative field dates are June 15 – June 29, 2018.  Students will fly to San Antonio on June 15, and on June 16, the project team will depart San Antonio in 2 Trinity field vehicles, staying for one night in Albuquerque, NM, arriving at Bauer’s Canyon Ranch RV Park and Campground in Glendale, Utah, on the night of June 17.  We will stay at Bauer’s Canyon Ranch RV Park from the night of the 17th until the night of June 27, performing fieldwork in the nearby canyons (11 nights at the RV park).  We will purchase food in Orderville and prepare all meals in the RV park (which 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 June 28 – June 29, we will drive back to San Antonio, TX, staying in Albuquerque, NM for one night.

Lab and Analytical Work: Students would work in the Geosciences department at Trinity University from June 30 – July 12, 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 will focus on analyzing field data, cutting billets for thin sections, 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.  Students will depart San Antonio on July 13.

PROFESSIONAL DEVELOPMENT

We plan to take all participants to the GSA Cordilleran section meeting in Portland, Oregon (15–17 May 2019). We hope most students will be first author on one paper, and probably secondary authors on others due to the collaborative nature of the project.

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 2019 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, 2019, 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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