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Allmendinger, R. W., 1998, Inverse and forward numerical modeling of trishear fault propagation folds, Tectonics, v. 17, p. 640-656. Cardozo, N., 2005, Trishear modeling of fold bedding data along a topographic profile: Journal of structural Geology, v. 27, p. 495-502. Davis, G. H., Reynolds, S. J., and Kluth, C., 2012, Structural Geology of Rocks and Regions: New York, John Wiley and Sons, p. 414-419. Hardy, S., and Ford, M., 1997, Numerical modeling of trishear fault-propagation Hoin, D., Surpless, B., Mays, B., 2012, The role of mechanical stratigraphy in fault-propagation fold evolution: a case study: Abstracts with Programs, Southcentral GSA, Alpine, TX. Keesling, G., de Zoeten, E., Mercado, L., Surpless, B., 2013, Topographic Profile, Mechanical Stratigraphy, and interlayer Slip: Exploring Coupled Fold-Fracture Evolution of the Stillwell anticline: Trinity Summer Research Symposium, Trinity University, San Antonio, TX. Mays, B., Surpless, B.; Hoin, D., 2012, Kinematic development of The Stillwell anticline, west Texas: Abstracts with Programs, Southcentral GSA, Alpine, TX. Mitra, S., 1990, Fault-Propagation Folds: Geometry, Kinematic Evolution, and Hydrocarbon Traps (1): AAPG Bulletin, v. 74, p. 921-945. Mlella, M., Surpless, B., 2014, Implication of fracture characteristics of fluid flow within fold systems: a case study from the Stillwell anticline, west Texas:: Abstracts with Programs, Southcentral GSA, Vancouver, British Columbia Muehlberger, W.R., 1980, Texas Lineament Revisited: New Mexico Geological Society Guidebook, 31st Field Conference, Trans-Pecos Region, p. 113 - 121. Page, W., Turner, K., and Bohannon, R., 2008, Geological, Geochemical, and Geophysical Studies by the U.S. Geological Survey in Big Bend National Park, Texas, in Gray, J., and Page, W., eds.: U.S. Geological St. John, B.E., 1965, Structural geology of Black Gap area, Brewster County, Texas: Ph.D. Thesis, University of Texas at Austin, Austin, Texas, 200 p. Quiroz, K., and Surpless, B.E., 2010, Responses of different lithologies to similar stresses in Black Gap, Texas: Trinity University Summer Research Symposium. Suppe, J., and Medwedeff, D.A, 1990, Geometry and kinematics of fault-propagation folding: Eclogae Geologicae Helvetiae, v. 83, p. 409-454. St. John, B.E., 1965, Structural geology of Black Gap area, Brewster Country, Texas: Ph.D. Thesis, University of Texas at Austin, Austin, Texas, 200 p. Surpless, B.E., Mays, B., and Hoin, D., 2012, Kinematic Evolution the Stillwell Anticline System, West Texas: Implications for fluid Flow within Subsurface: American Geologic Society, Abstracts with Programs. The mechanisms for accommodating strain throughout the fold varied in intensity depending on structrual position. Our field observations suggest that the greatest strain is accommodated in the forelimb of the fold system, as predicted by trishear model results (Fig. 4; Section III). Shear strain has been accommodated by a range of mechanisms, including interlayer slip, fracturing, brecciation, and faulting. The series of images below provide examples of the mechanisms of strain accommodation at various scales in the Stillwell Anticline. On the largest scale, Figure 8 shows brecciation in the forelimb and hinge zone of exposure N3 (Section IV). Figure 9 depicts a close-up of a brecciated area similar to the one in Figure 8. Figures 10 and 11 display fault-bedding relationships on the forelimb. The final photographs (Figure 12 and 13) displayon bed-scale de- formation in the forelimb of the anticline. These observations and modeling results suggest that shear stresses were significantly higher in the forelimb than at other positions within the anticline. Figures 10 and 11 show fault-bedding relationships as exposed in Cretaceous units in the second location on central segment of the anticline (Fig. 5). The yellow lines indicate the well-exposed, steeply dipping, layers of bedding and the blue lines indicate faults. Faults commonly utilize bedding planes, so not all fault surfaces could be easily identified. Figure 10 shows the interactions between well-bedded, steeply-dipping limestone and significant faults. These faults display flat-ramp geometry on a multi-bed scale. Figure 11 focuses on a zone in the hinge-forelimb transition, where a significant bedding-parallel fault has affected adjacent beds. The zone of blue in figure 11 highlights shear deformation within a fault zone. These and other exposures support our view that flat-ramp geometries accommodate a significant percentage of the shear within the forelimb and hinge zone of the anticline system. Figure 12 displays an intra-bed flat-ramp fault geometries similar those shown in Figure 10. This image also shows duplexing associated with these intrabed faults. We suggest that planes of weakness (intra-bed lamination) have been activated during shear. These images support a deformational history that has been dominated by brittle deformation and bedding-parallel slip across a range of scales. Figure 13 highlights both slickenlines and the intense fracturing adjacent to a signifcant fault in the fore- limb of the Stillwell anticline. The intensity of fracturing is significantly greater adjacent to the more signifi- cant faults, such as those shown in Figure 10. Figure 8 shows deformation of Cretaceous units in the first location on the central segment of the anticline (Fig. 5). The image focuses on the hinge zone and forelimb of the anticline. The yellow lines indicate bedding, which is sub-horizontal in the midlimb and transitions to the steeply-dipping beds in the forelimb. The blue lines indicate faults, while the region of dashed white lines highlights a zone of intense brecciation. Figure 9 shows a close up photograph of a brecciated area similar to that found in the white dotted area in Figure 8. Fault-propagation folds directly link the consequences of fault movement to the deformation of stratified layers above the fault. In fault propagation folding, the geometry and deformation of the fold is controlled by the absorption of slip from the thrust fault below (e.g., Davis et al. 2012; Mitra and Mount, 1998). As the fault propagates upwards through the stratified layers, the layers become further deformed, resulting in an anticline or monocline (e.g. Davis et al. 2012). The amount of deformation depends on the competency of the rock unit, the distance of the unit from the propagating fault, and the velocity of the fault propagation. Deformation increases in intensity as the fault con- tinues to propagate (Davis et al. 2012). Suppe and Medwedeff (1990) developed a model for the progressive formation of a fault propagation fold associated with a flat-ramp thrust fault (Fig. 3). In their model, the initial stage of fold formation begins as the fault ramps upwards, propagating across stratigraphy. The initial movement of the fault displaces the units above to form a visi- ble forelimb (B-B’) which sharply dips towards the foreland, a midlimb which remains parallel to the original beds, and backlimb (A-A‘) which dips towards the hinterland (Fig. 3.I). As the fault continues to propagate, the fore- and backlimbs become more longer, and the midlimb decreases in length (Fig. 3.2). As the fault tip propagates further, the back- and forelimb lengths continue to increase, while the midlimb nearly disappears (Fig. 3.3). Cross-sectional exposures of the Stillwell anticline reveal geometries that can be compared to all 3 of the stages shown below (see Section IV). Trishear Model The Trishear model (e.g., Erslev, 1991) was created to explain observations of fold geometries in fault-propagation folds. The model grew in popularity over the kink kinematic model because it better explains differing fold geometries and takes into account varying amounts of shear within what is called the trishear zone (e.g. Cardozo, 2005). This zone is associated with the propagation of a fault and takes into account the varying velocities of particles in front of a propagating fault (Allmendinger, 1998). The distri- bution of shear in the Trishear zone is modeled with velocity vectors in a line perpendicular to the fault (Fig. 4). The greatest velocities are located closest to the hanging wall and the lowest velocities are closest to the footwall (e.g., Davis et al., 2012). With this in mind, the most significant deformation occurs in front of the fault tip, mostly affecting the forelimb of the anticline. Beasley et al. (2013) successfully modeled formation of the Stillwell anticline using trishear kinematics. While faults may be hidden below the surface, structural features, such as anticlines, reveal clues about their geometries and displacements. The surface geometries of the Stillwell anticline, located in west Texas, are defined by well-bedded Cretaceous units and a left-stepping en echelon pattern with three segments. Previous studies have revealed that this anticline is cored by a fault system that is responsible for most of the deformation documented along the fold. The Stillwell anticline is an ideal formation to study fold and fault interactions because erosional forces have exposed cross-sectional views of the fold at different locations along the anticline. These cross sectional expo- sures permit us to observe fold geometries and mechanisms of strain accommodation at each site. The variations in fold geometries along the anticline provide us the equiva- lent of temporal snapshots of fault propagation-fold evolution. During the late Mesozoic to early Cenozoic, the shallow subduc- tion of the Pacific Plate under the North American plate created a compressional stress field across the western United States (Fig. 2) (e.g., Muehlberger, 1980). This compressional event, the Laramide orogeny, reactivated previous northwest-trending zones of crustal weakness along the Texas lineament (Fig. 2) (e.g., Page et al., 2008). We focused on the Stillwell anticline, a fold subparallel to the trend of the Texas lineament. The Stillwell anticline features a left-stepping en echelon pattern with three main segments (see map in Section IV). The asymmetrical fold is more than 8 km long and ap- proximately 500 meters in width. At the central locations, the seg- ments have steep forelimbs and gently dipping backlimbs (e.g., Mays et al., 2012; Keesling et. al., 2013). This research has been supported by NSF RUI #1220235. Thanks to Travis Smith and the rest of the Black Gap Wildlife Management for their hospitality and logistical support, without which this research would not have been possible. Lastly, thank you to the Trinity University Department of Geosciences for logistical and financial support. Research Questions What do fold geometries reveal about displacements and geometries of sub-surface faults? What deformation mechanisms accommodate strain in the Stillwell anticline system? What part of the fold accommodates the most strain, and how does this relate to the fault movement below? Is deformation documented in the field consistent with the results of 2D and 3D kinematic computer models? How do the geometries of fault-propagation fold systems influence fluid flow? Figure 1. Stratigraphy of the Stillwell Anticline. Modified from St. John, 1965. Figure 2. Map with major Laramide-age faults and folds of the Big Bend region (distribution of faults and folds modified from Muehlberger, 1980; Muehlberger and Dickerson, 1989), with inset map displaying approximate distribution of Lar- amide-age deformation (modified from Miller, 1992). Abbreviations: SM = Santia- go Mountains; SDC = Sierra del Carmen; BBNP = Big Bed National Park; SA = Still- well anticline. Figure modified from Surpless and Quiroz (2010). To investigate these questions, we used the geologic history of the region and previous work on the relationship between subsurface faults and the Stillwell anticline fold system. Our research utilized field data and field photography to investigate these questions. As part of this study, we used photogrammetry to produce a 3D model of an ex- posed cross-sectional view of the fold in order to better analyze inaccessible outcrops. II. Tectonic History of the Big Bend Region and the SƟllwell AnƟcline III. Fault-propagaƟon Folding and the Trishear Model Figure 4. Diagram showing the trishear fault-propagation model. Modified from Hardy and Ford (1991). Figure 3. Sequential model of fault-propagation fold evolution. Modified from Suppe and Medwedeff (1990). Agisoft Photoscan photogrammetry software (agisoft.ru) transforms a series of 2D images into a 3D model. We captured 30 stan- dard 2D images using standard photography equipment and then used Agisoft Photoscan software, which utilizes object or feature point detection, 3D-morphing, and dense stereo construction to build a high-resolution 3D model of the exposure. In this case, we used the software to create a clear, high-resolution 3D epresentation of the entire outcrop. To building a high-resolution model, we captured images at exactly the same orientation and camera settings (focal length, F-stop, aperture, ISO) from many uniformly-spaced locations parallel to the outcrop face, ensuring significant overlap between each image. Figure 7 shows a close up of deformation in the hinge zone and forelimb of the outcrop in Figure 6. The yellow lines indicate exposed layers of bedding which dip steeply from the hinge to the forelimb. The blue lines indicate faults, which are at low angles to bedding in the forelimb. The Agisoft software provides a clear depiction of not only the location dip of the faults and beds, but also enables the manipulation of the outcrop in three dimensions, permitting the observation and documentation of areas that were previously inaccessible. The diverse geologic formations found in the Big Bend Region of west Texas are a result of tectonic events dating back to the Neoproterozoic (e.g., Muehlberger, 1980; Page et al., 2008). Rifting during this time produced northwest-trending faults which were the initial control for the Texas lineament (e.g. Page et al., 2008). The Late Triassic to Late Cretaceous rifting between North America and South America produced normal faults across the region (e.g., Muehlberger, 1980; Page et al., 2008). This rifting caused subsidence in the region, allowing for the sea to transgress and create a shallow marine environment. During this transgression, limestone, claystone, and siltstone units, includ- ing the Del Rio claystone, Santa Elena limestone, and Buda limestone, were deposited in the Big Bend region (e.g., St. John, 1965) (Fig. 1). I. IntroducƟon Figure 6. 3D model of anƟcline at C2 exposure in Figure 5. The blue squares show the posiƟons of 6 of 30 photo- graphs used to create the model. The central sphere enables the rotaƟon of the model in three dimensions. Fluid flow rates rely on the connectivity of fractures (Fig. 14), which varies depending on the location on the anticline (midlimb, forelimb, or backlimb). In the Stillwell anticline, our observations suggest that the most intense deformation is focused in the forelimb of the system, with most of this de- formation accomodated by faults and fractures, with fractur- ing occuring on a wide range of orienations adjacent to major faults. Based on these results, we suggest that if this system were in the subsurface, fluid flow should be anisotro- pic, focused parallel to the fold axis. Mlella et al. (2014) found that all fracture sets in the anticline were simultaneously open in the past, suggesting some period of significant flow prior to the precipitation of calcite that sealed most of the fracture network. The exposed fold geometries are similar to those predicted by fault propagation folding (Section III) and trishear modeling, with shortening of the midlimb and greater intensity of shear strain in the forelimb as the fault propagates. Field observations confirm trishear modeling results. The Stillwell anticline has experienced the greatest strain in the forelimb of the system. • Shear strain is accomodated by a combination of bed-scale ramp-flat fault geome- tries, intra-bed faulting, intense fracturing, and brecciation. Photogrammetry permitted us to make detailed observations at previously inac- cessible locations. In the past, fluid flow within the Stillwell anticline system was likely anisotropic, concentrated parallel to the fold axis within the forelimb. VIII. InterpretaƟons and Conclusions References Acknowledgments V. Photogrammetric 3D Modeling Figure 14. (A) shows low fracture connectivity, which would result in low fluid flow rates. (B) shows high fracture connectivity , which would lead to higher fluid flow rates. Modified from Keesling et al. (2013) and Gudmundsson (2011). VII. ImplicaƟons for Fluid Flow Figure 7. Close-up view of the hinge and forelimb zone in the 3D model displayed in Figure 6. Figure 10. Photograph of faulted area in the forelimb region. Figure 8. Southwest wall of N3 exposure (see Section IV), with zone of intense brecciation in hinge and forelimb. Image reflected for clarity. Figure 11. Photograph of faulted area in forelimb. Figure 9. Photograph of brecciated limestone . Lens cap for scale Figure 13. Photograph of slickenlines and intense fracturing. Veins are about 2-3 mm and approximately 20 cm apart. Lens cap for scale. Modified from Mays et al. (2012). Figure 12. Ramp-flat intra-bed fault geometries exposed in forelimb of the Stillwell Anticline of fault in contrast to bedding. Lens cap for scale. InterpretaƟons These cross-sectional exposures capture temporal snapshots of fault-propagation fold evolution. Perhaps the most inter- esting exposures are those near the southern end of the northern segment of the anticline, where: N2 exposures show long mid-limbs; N3 (northwest wall) shows a signficant- ly shorter midlimb; and N3 (southeast wall) shows no well-de- fined midlimb. Unlike the model, however, the midlimb in the first cross-sectional view shown for N2 dips towards the foreland instead of lying horizontal, suggesting a pre-existing monocline (Beasley et al., 2013). VI. Mechanisms of Strain AccommodaƟon in Hinge-zone and Forelimb IV. VariaƟons in Fold Geometry Fault-propagaƟon folds WNW ESE 042, 74 033, 79 042, 74 035, 73 051,76 NE SW Intact Limestone Brecciated Limestone Intact Limestone SW SW NE NE 8 meters 7 meters BBNP Mexico USA SA fold: Fault or monocline: AnƟcline: Marathon uplift (Pz strata exposed) N 0 25 km KEY: SM SDC TX Mexico Pacic Ocean N 500 km Laramide orogeny TL TL Figure 5 meters 3 meters Bedding faults Flat Ramp Flat Footwall Hanging wall Fault tip 2γ θ = 0 θ = γ u 1 = s v 1 = 0 u n = 0 v n = 0 u 2 = s 2 cos(θ) f Ɵe line Figure 1. Stratigraphy of the Stillwell Anticline. SCALE IN METERS PEN FORMATION BOQUILLAS FORMATION [Kb] BUDA LIMESTONE [Kbu] SANTA ELENA LIMESTONE [Kse] SUE PEAKS FORMATION [Ksp] DEL CARMEN LIMESTONE [Kdc] DIABASE SILL BLACK GAP BASALT FLOWS ALLUVIAL GRAVELS 200 300 400 500 600 GULFIAN COMANCHEAN CRETACEOUS TERT. MIOC. Quaternary alluvium, colluvium, landslide deposits TELEPHONE CANYON FM. [Ktc] 700 800 900 DEL RIO CLAYSTONE [Kdr] Hinterland Hinterland Foreland Foreland Foreland I II III B B B B’ B’ A’ A’ A’ A A A B’ Hinterland Brecciated Limestone Duplex Structure 30cm Bedding faults Bedding faults Figure 5. Preliminary geologic map of the Stillwell anticline within the study area. See stratigraphy in Figrue 1. This cross-sectional exposure is near the northern end of the central segment. This view shows a long, shallowly-dipping backlimb, no well-defined midlimb, and a steeply dipping forelimb (similar to Fig. 3.III). Image is reflected for clarity. SW NE C2 The northernmost exposure of the anƟcline shows gently sloping limestone layers. There are no sharply dipping limbs, indica Ɵng less displacement along the fault below (similar to Fig. 3.I in SecƟon III). NE SW C3 This cross-sectional exposure is near the northern end of the central segment. There is a short, poorly-defined midlimb and a medium length backlimb. The limestone units that form the forelimb dip steeply toward the northeast (similar to Fig. 3.II or 3.III). Image is reflected for clarity. This is the southernmost cross-sectional exposure in the southern segment of the anticline. In the photo, the backlimb dips moderately to the southwest, the midlimb dips gently towards the northeast, and the forelimb dips steeply towards the northeast (similar to Fig. 3.II). SW NE S1 This cross-sectional exposure is in the northern segment of the anticline system. This exposure, on the northwest side of the canyon, shows a moder- ately-dipping backlimb, a long, gently-foreland-dipping midlimb, and a steep- ly-dipping forelimb (similar to Fig. 3.I or 3.II). SW NE This cross-sectional exposure is located on the opposite wall of the canyon from the image shown above. There is no clear midlimb, the exposure reveals only a shallowly-dipping backlimb and a steeply-dipping forelimb (similar to Fig. 3.III). Image reflected for scale. SW NE N3 N2 Analysis of Finite Fold Geometry and Variations in Strain Based on Structural Position: A Case Study From the Stillwell Anticline, West Texas Nicola Hill ([email protected]), Benjamin Surpless, Rebecca Schauer, and Mark Mlella Trinity University Department of Geosciences, 1 Trinity Place, San Antonio, Texas 78212 This exposure is located on the SE wall of N2. The well-exposed unit shows a long, sub-horizontal midlimb. The backlimb dips shallowly toward the SW, and beds in the forelimb dip steeply to the NE (similar to Fig. 3.I or 3.II). Image re- flected for clarity. NE SW N2 SW NE This cross-sectional view is located in the northern most part of the central segment. In comparison to the previous cross-sectional views, the length of the midlimb has shortened significantly, but remained subhorizontal (similar to Fig. 3.II). The forelimb dips steeply. N3 SW NE Center Sphere N1 bedding faults Fi 5P li i l i f th Still ll ti li ithi th t d S Kdr Kbu Te Kb Kbu Kbu Kdr Qal Kbu Kdr Kbu Kb Te Kdr Kdr Kse Te Kdr Kbu Kb Kdr Kdr Kbu Kse Kse Kbu Kdr Kbu Kbu Kdr Kdr Te Kb Kb Kb Kb Te Te Te Kdr Kdr Te Te Kdr Kbu Kdr Kbu Kb Te Te Kse Kse Kse Kse Kse Kse Kse 702460m E 3277795m N 695610m E 3283395m N N1 N2 N3 C2 S1 C3 Qal Te Kb Kbu Kdr Kse Alluvium (Q) basalt ows (Tert.) Boquillas Fm. (K) Buda limestone (K) Del Rio clay (K) Santa Elena limestone (K) Unit contact Inferred contact AnƟcline LeŌ- lateral fault Legend NORTHERN SEGMENT SOUTHERN SEGMENT CENTRAL SEGMENT Fractures B A 0.5 meter 50 meters 30 meters 40 meters 40 meters 60 meters 60 meters 40 meters 30 meters 60 meters 3 meters
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Analysis of Finite Fold Geometry and Variations in Strain ... · Allmendinger, R. W., 1998, Inverse and forward numerical modeling of trishear fault propagation folds, Tectonics,

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Page 1: Analysis of Finite Fold Geometry and Variations in Strain ... · Allmendinger, R. W., 1998, Inverse and forward numerical modeling of trishear fault propagation folds, Tectonics,

Allmendinger, R. W., 1998, Inverse and forward numerical modeling of trishear fault propagation folds, Tectonics, v. 17, p. 640-656.Cardozo, N., 2005, Trishear modeling of fold bedding data along a topographic profile: Journal of structural Geology, v. 27, p. 495-502.Davis, G. H., Reynolds, S. J., and Kluth, C., 2012, Structural Geology of Rocks and Regions: New York, John Wiley and Sons, p. 414-419.Hardy, S., and Ford, M., 1997, Numerical modeling of trishear fault-propagationHoin, D., Surpless, B., Mays, B., 2012, The role of mechanical stratigraphy in fault-propagation fold evolution: a case study: Abstracts with

Programs, Southcentral GSA, Alpine, TX.Keesling, G., de Zoeten, E., Mercado, L., Surpless, B., 2013, Topographic Profile, Mechanical Stratigraphy, and interlayer Slip: Exploring Coupled Fold-Fracture Evolution of the Stillwell anticline: Trinity Summer Research Symposium, Trinity University, San Antonio, TX.Mays, B., Surpless, B.; Hoin, D., 2012, Kinematic development of The Stillwell anticline, west Texas: Abstracts with Programs, Southcentral GSA,

Alpine, TX. Mitra, S., 1990, Fault-Propagation Folds: Geometry, Kinematic Evolution, and Hydrocarbon Traps (1): AAPG Bulletin, v. 74, p. 921-945.Mlella, M., Surpless, B., 2014, Implication of fracture characteristics of fluid flow within fold systems: a case study from the Stillwell anticline, west Texas:: Abstracts with Programs, Southcentral GSA, Vancouver, British Columbia

Muehlberger, W.R., 1980, Texas Lineament Revisited: New Mexico Geological Society Guidebook, 31st Field Conference, Trans-Pecos Region, p. 113 - 121. Page, W., Turner, K., and Bohannon, R., 2008, Geological, Geochemical, and Geophysical Studies by the U.S. Geological Survey in Big Bend National Park, Texas, in Gray, J., and Page, W., eds.: U.S. Geological St. John, B.E., 1965, Structural geology of Black Gap area, Brewster County, Texas: Ph.D. Thesis, University of Texas at Austin, Austin, Texas, 200 p.Quiroz, K., and Surpless, B.E., 2010, Responses of different lithologies to similar stresses in Black Gap, Texas: Trinity University Summer Research Symposium.Suppe, J., and Medwedeff, D.A, 1990, Geometry and kinematics of fault-propagation folding: Eclogae Geologicae Helvetiae, v. 83, p. 409-454.St. John, B.E., 1965, Structural geology of Black Gap area, Brewster Country, Texas: Ph.D. Thesis, University of Texas at Austin, Austin, Texas, 200 p.

Surpless, B.E., Mays, B., and Hoin, D., 2012, Kinematic Evolution the Stillwell Anticline System, West Texas: Implications for fluid Flow within Subsurface: American Geologic Society, Abstracts with Programs.

The mechanisms for accommodating strain throughout the fold varied in intensity depending on structrual position. Our field observations suggest that the greatest strain is accommodated in the forelimb of the fold system, as predicted by trishear model results (Fig. 4; Section III). Shear strain has been accommodated by a range of mechanisms, including interlayer slip, fracturing, brecciation, and faulting. The series of images below provide examples of the mechanisms of strain accommodation at various scales in the Stillwell Anticline.

On the largest scale, Figure 8 shows brecciation in the forelimb and hinge zone of exposure N3 (Section IV). Figure 9 depicts a close-up of a brecciated area similar to the one in Figure 8. Figures 10 and 11 display fault-bedding relationships on the forelimb. The final photographs (Figure 12 and 13) displayon bed-scale de-formation in the forelimb of the anticline. These observations and modeling results suggest that shear stresses were significantly higher in the forelimb than at other positions within the anticline.

Figures 10 and 11 show fault-bedding relationships as exposed in Cretaceous units in the second location on central segment of the anticline (Fig. 5). The yellow lines indicate the well-exposed, steeply dipping, layers of bedding and the blue lines indicate faults. Faults commonly utilize bedding planes, so not all fault surfaces could be easily identified. Figure 10 shows the interactions between well-bedded, steeply-dipping limestone and significant faults. These faults display flat-ramp geometry on a multi-bed scale. Figure 11 focuses on a zone in the hinge-forelimb transition, where a significant bedding-parallel fault has affected adjacent beds. The zone of blue in figure 11 highlights shear deformation within a fault zone. These and other exposures support our view that flat-ramp geometries accommodate a significant percentage of the shear within the forelimb and hinge zone of the anticline system.

Figure 12 displays an intra-bed flat-ramp fault geometries similar those shown in Figure 10. This image also shows duplexing associated with these intrabed faults. We suggest that planes of weakness (intra-bed lamination) have been activated during shear. These images support a deformational history that has been dominated by brittle deformation and bedding-parallel slip across a range of scales. Figure 13 highlights both slickenlines and the intense fracturing adjacent to a signifcant fault in the fore-limb of the Stillwell anticline. The intensity of fracturing is significantly greater adjacent to the more signifi-cant faults, such as those shown in Figure 10.

Figure 8 shows deformation of Cretaceous units in the first location on the central segment of the anticline (Fig. 5). The image focuses on the hinge zone and forelimb of the anticline. The yellow lines indicate bedding, which is sub-horizontal in the midlimb and transitions to the steeply-dipping beds in the forelimb. The blue lines indicate faults, while the region of dashed white lines highlights a zone of intense brecciation. Figure 9 shows a close up photograph of a brecciated area similar to that found in the white dotted area in Figure 8.

Fault-propagation folds directly link the consequences of fault movement to the deformation of stratified layers above the fault. In fault propagation folding, the geometry and deformation of the fold is controlled by the absorption of slip from the thrust fault below (e.g., Davis et al. 2012; Mitra and Mount, 1998). As the fault propagates upwards through the stratified layers, the layers become further deformed, resulting in an anticline or monocline (e.g. Davis et al. 2012). The amount of deformation depends on the competency of the rock unit, the distance of the unit from the propagating fault, and the velocity of the fault propagation. Deformation increases in intensity as the fault con-tinues to propagate (Davis et al. 2012).

Suppe and Medwedeff (1990) developed a model for the progressive formation of a fault propagation fold associated with a flat-ramp thrust fault (Fig. 3). In their model, the initial stage of fold formation begins as the fault ramps upwards, propagating across stratigraphy. The initial movement of the fault displaces the units above to form a visi-ble forelimb (B-B’) which sharply dips towards the foreland, a midlimb which remains parallel to the original beds, and backlimb (A-A‘) which dips towards the hinterland (Fig. 3.I). As the fault continues to propagate, the fore- and backlimbs become more longer, and the midlimb decreases in length (Fig. 3.2). As the fault tip propagates further, the back- and forelimb lengths continue to increase, while the midlimb nearly disappears (Fig. 3.3). Cross-sectional exposures of the Stillwell anticline reveal geometries that can be compared to all 3 of the stages shown below (see Section IV).

Trishear Model The Trishear model (e.g., Erslev, 1991) was created to explain observations of fold geometries in fault-propagation folds. The model grew in popularity over the kink kinematic model because it better explains differing fold geometries and takes into account varying amounts of shear within what is called the trishear zone (e.g. Cardozo, 2005). This zone is associated with the propagation of a fault and takes into account the varying velocities of particles in front of a propagating fault (Allmendinger, 1998). The distri-bution of shear in the Trishear zone is modeled with velocity vectors in a line perpendicular to the fault (Fig. 4). The greatest velocities are located closest to the hanging wall and the lowest velocities are closest to the footwall (e.g., Davis et al., 2012). With this in mind, the most significant deformation occurs in front of the fault tip, mostly affecting the forelimb of the anticline. Beasley et al. (2013) successfully modeled formation of the Stillwell anticline using trishear kinematics.

While faults may be hidden below the surface, structural features, such as anticlines, reveal clues about their geometries and displacements. The surface geometries of the Stillwell anticline, located in west Texas, are defined by well-bedded Cretaceous units and a left-stepping en echelon pattern with three segments. Previous studies have revealed that this anticline is cored by a fault system that is responsible for most of the deformation documented along the fold. The Stillwell anticline is an ideal formation to study fold and fault interactions because erosional forces have exposed cross-sectional views of the fold at different locations along the anticline. These cross sectional expo-sures permit us to observe fold geometries and mechanisms of strain accommodation at each site. The variations in fold geometries along the anticline provide us the equiva-lent of temporal snapshots of fault propagation-fold evolution.

During the late Mesozoic to early Cenozoic, the shallow subduc-tion of the Pacific Plate under the North American plate created a compressional stress field across the western United States (Fig. 2) (e.g., Muehlberger, 1980). This compressional event, the Laramide orogeny, reactivated previous northwest-trending zones of crustal weakness along the Texas lineament (Fig. 2) (e.g., Page et al., 2008).

We focused on the Stillwell anticline, a fold subparallel to the trend of the Texas lineament. The Stillwell anticline features a left-stepping en echelon pattern with three main segments (see map in Section IV). The asymmetrical fold is more than 8 km long and ap-proximately 500 meters in width. At the central locations, the seg-ments have steep forelimbs and gently dipping backlimbs (e.g., Mays et al., 2012; Keesling et. al., 2013).

This research has been supported by NSF RUI #1220235. Thanks to Travis Smith and the rest of the Black Gap Wildlife Management for their hospitality and logistical support, without which this research would not have been possible. Lastly, thank you to the Trinity University Department of Geosciences for logistical and financial support.

Research Questions• What do fold geometries reveal about displacements and geometries of sub-surface faults?• What deformation mechanisms accommodate strain in the Stillwell anticline system?• What part of the fold accommodates the most strain, and how does this relate to the fault movement below?• Is deformation documented in the field consistent with the results of 2D and 3D kinematic computer models? • How do the geometries of fault-propagation fold systems influence fluid flow?

Figure 1. Stratigraphy of the Stillwell Anticline. Modified from St. John, 1965.

Figure 2. Map with major Laramide-age faults and folds of the Big Bend region (distribution of faults and folds modified from Muehlberger, 1980; Muehlberger and Dickerson, 1989), with inset map displaying approximate distribution of Lar-amide-age deformation (modified from Miller, 1992). Abbreviations: SM = Santia-go Mountains; SDC = Sierra del Carmen; BBNP = Big Bed National Park; SA = Still-well anticline. Figure modified from Surpless and Quiroz (2010).

To investigate these questions, we used the geologic history of the region and previous work on the relationship between subsurface faults and the Stillwell anticline fold system. Our research utilized field data and field photography to investigate these questions. As part of this study, we used photogrammetry to produce a 3D model of an ex-posed cross-sectional view of the fold in order to better analyze inaccessible outcrops.

II. Tectonic History of the Big Bend Region and the S llwell An cline

III. Fault-propaga on Folding and the Trishear Model

Figure 4. Diagram showing the trishear fault-propagation model. Modified from Hardy and Ford (1991).

Figure 3. Sequential model of fault-propagation fold evolution. Modified from Suppe and Medwedeff (1990).

Agisoft Photoscan photogrammetry software (agisoft.ru) transforms a series of 2D images into a 3D model. We captured 30 stan-dard 2D images using standard photography equipment and then used Agisoft Photoscan software, which utilizes object or feature point detection, 3D-morphing, and dense stereo construction to build a high-resolution 3D model of the exposure. In this case, we used the software to create a clear, high-resolution 3D epresentation of the entire outcrop.

To building a high-resolution model, we captured images at exactly the same orientation and camera settings (focal length, F-stop, aperture, ISO) from many uniformly-spaced locations parallel to the outcrop face, ensuring significant overlap between each image. Figure 7 shows a close up of deformation in the hinge zone and forelimb of the outcrop in Figure 6. The yellow lines indicate exposed layers of bedding which dip steeply from the hinge to the forelimb. The blue lines indicate faults, which are at low angles to bedding in the forelimb. The Agisoft software provides a clear depiction of not only the location dip of the faults and beds, but also enables the manipulation of the outcrop in three dimensions, permitting the observation and documentation of areas that were previously inaccessible.

The diverse geologic formations found in the Big Bend Region of west Texas are a result of tectonic events dating back to the Neoproterozoic (e.g., Muehlberger, 1980; Page et al., 2008). Rifting during this time produced northwest-trending faults which were the initial control for the Texas lineament (e.g. Page et al., 2008). The Late Triassic to Late Cretaceous rifting between North America and South America produced normal faults across the region (e.g., Muehlberger, 1980; Page et al., 2008). This rifting caused subsidence in the region, allowing for the sea to transgress and create a shallow marine environment. During this transgression, limestone, claystone, and siltstone units, includ-ing the Del Rio claystone, Santa Elena limestone, and Buda limestone, were deposited in the Big Bend region (e.g., St. John, 1965) (Fig. 1).

I. Introduc on

Figure 6. 3D model of an cline at C2 exposure in Figure 5. The blue squares show the posi ons of 6 of 30 photo-graphs used to create the model. The central sphere enables the rota on of the model in three dimensions.

Fluid flow rates rely on the connectivity of fractures (Fig. 14), which varies depending on the location on the anticline (midlimb, forelimb, or backlimb). In the Stillwell anticline, our observations suggest that the most intense deformation is focused in the forelimb of the system, with most of this de-formation accomodated by faults and fractures, with fractur-ing occuring on a wide range of orienations adjacent to major faults. Based on these results, we suggest that if this system were in the subsurface, fluid flow should be anisotro-pic, focused parallel to the fold axis. Mlella et al. (2014) found that all fracture sets in the anticline were simultaneously open in the past, suggesting some period of significant flow prior to the precipitation of calcite that sealed most of the fracture network.

• The exposed fold geometries are similar to those predicted by fault propagation folding (Section III) and trishear modeling, with shortening of the midlimb and greater intensity of shear strain in the forelimb as the fault propagates.

• Field observations confirm trishear modeling results. The Stillwell anticline has experienced the greatest strain in the forelimb of the system. • Shear strain is accomodated by a combination of bed-scale ramp-flat fault geome-tries, intra-bed faulting, intense fracturing, and brecciation.

• Photogrammetry permitted us to make detailed observations at previously inac-cessible locations.

• In the past, fluid flow within the Stillwell anticline system was likely anisotropic, concentrated parallel to the fold axis within the forelimb.

VIII. Interpreta ons and Conclusions

ReferencesAcknowledgments

V. Photogrammetric 3D ModelingFigure 14. (A) shows low fracture connectivity, which would result in low fluid flow rates. (B) shows high fracture connectivity , which would lead to higher fluid flow rates. Modified from Keesling et al. (2013) and Gudmundsson (2011).

VII. Implica ons for Fluid Flow

Figure 7. Close-up view of the hinge and forelimb zone in the 3D model displayed in Figure 6.

Figure 10. Photograph of faulted area in the forelimb region.

Figure 8. Southwest wall of N3 exposure (see Section IV), with zone of intense brecciation in hinge and forelimb. Image reflected for clarity.

Figure 11. Photograph of faulted area in forelimb.

Figure 9. Photograph of brecciated limestone . Lens cap for scale

Figure 13. Photograph of slickenlines and intense fracturing. Veins are about 2-3 mm and approximately 20 cm apart. Lens cap for scale. Modified from Mays et al. (2012).

Figure 12. Ramp-flat intra-bed fault geometries exposed in forelimb of the Stillwell Anticline of fault in contrast to bedding. Lens cap for scale.

Interpreta ons These cross-sectional exposures capture temporal snapshots of fault-propagation fold evolution. Perhaps the most inter-esting exposures are those near the southern end of the northern segment of the anticline, where: N2 exposures show long mid-limbs; N3 (northwest wall) shows a signficant-ly shorter midlimb; and N3 (southeast wall) shows no well-de-fined midlimb. Unlike the model, however, the midlimb in the first cross-sectional view shown for N2 dips towards the foreland instead of lying horizontal, suggesting a pre-existing monocline (Beasley et al., 2013).

VI. Mechanisms of Strain Accommoda on in Hinge-zone and ForelimbIV. Varia ons in Fold Geometry

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Figure 1. Stratigraphy of the Stillwell Anticline.

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Figure 5. Preliminary geologic map of the Stillwell anticline within the study area. See stratigraphy in Figrue 1.

This cross-sectional exposure is near the northern end of the central segment. This view shows a long, shallowly-dipping backlimb, no well-defined midlimb, and a steeply dipping forelimb (similar to Fig. 3.III). Image is reflected for clarity.

SW NE C2

The northernmost exposure of the an cline shows gently sloping limestone layers. There are no sharply dipping limbs, indica ng less displacement along the fault below (similar to Fig. 3.I in Sec on III).

NESW C3

This cross-sectional exposure is near the northern end of the central segment. There is a short, poorly-defined midlimb and a medium length backlimb. The limestone units that form the forelimb dip steeply toward the northeast (similar to Fig. 3.II or 3.III). Image is reflected for clarity.

This is the southernmost cross-sectional exposure in the southern segment of the anticline. In the photo, the backlimb dips moderately to the southwest, the midlimb dips gently towards the northeast, and the forelimb dips steeply towards the northeast (similar to Fig. 3.II).

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This cross-sectional exposure is in the northern segment of the anticline system. This exposure, on the northwest side of the canyon, shows a moder-ately-dipping backlimb, a long, gently-foreland-dipping midlimb, and a steep-ly-dipping forelimb (similar to Fig. 3.I or 3.II).

SW NE

This cross-sectional exposure is located on the opposite wall of the canyon from the image shown above. There is no clear midlimb, the exposure reveals only a shallowly-dipping backlimb and a steeply-dipping forelimb (similar to Fig. 3.III). Image reflected for scale.

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Analysis of Finite Fold Geometry and Variations in Strain Based on Structural Position: A Case Study From the Stillwell Anticline, West TexasNicola Hill ([email protected]), Benjamin Surpless, Rebecca Schauer, and Mark Mlella Trinity University Department of Geosciences, 1 Trinity Place, San Antonio, Texas 78212

This exposure is located on the SE wall of N2. The well-exposed unit shows a long, sub-horizontal midlimb. The backlimb dips shallowly toward the SW, and beds in the forelimb dip steeply to the NE (similar to Fig. 3.I or 3.II). Image re-flected for clarity.

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This cross-sectional view is located in the northern most part of the central segment. In comparison to the previous cross-sectional views, the length of the midlimb has shortened significantly, but remained subhorizontal (similar to Fig. 3.II). The forelimb dips steeply.

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