Kinematic histories of fold-thrust belts are typically based on careful examinations of high-grade metamorphic rocks within a salient. We provide a novel method of understanding fold-thrust belts by examining salient-recess junctions. We analyze the oft-ignored upper crustal rocks using a combined approach of detailed fault analysis with experimental sandbox modeling.
Within fold-thrust belts, the junctions between salients and recesses may hold critical clues to the overall kinematic history. The deformation history within these junctions is best preserved in areas where thrust sheets extend from a salient through an adjacent recess. We examine one such junction within the Sevier fold-thrust belt (western United States) along the Leamington transverse zone, northern Utah. Deformation within this junction took place by faulting and cataclastic flow. Here, we describe a protocol that examines these fault patterns to better understand the kinematic history of the field area. Fault data is supplemented by analog sandbox experiments. This study suggests that, in detail, deformation within the overlying thrust sheet may not directly reflect the underlying basement structure. We demonstrate that this combined field-experimental approach is easy, accessible, and may provide more details to the deformation preserved in the crust than other more expensive methods, such as computer modeling. In addition, the sandbox model may help to explain why and how these details formed. This method can be applied throughout fold-thrust belts, where upper-crustal rocks are well preserved. In addition, it can be modified to study any part of the upper crust that has been deformed via elastico-frictional mechanisms. Finally, this combined approach may provide more details as to how fold-thrust belts maintain critical-taper and serve as potential targets for natural resource exploration.
Fold-thrust belts are composed of salients (or segments), where the thrust sheets in adjoining salients are decoupled by recesses or transverse zones1,2,3. The transition from salient to recess may be markedly complex, involving a multifaceted suite of structures, and may hold critical clues to fold-thrust belt development. In this paper, we carefully examine a salient-recess junction, using a combination of multiscale field data and a sandbox model, in order to better understand how deformation can be accommodated within fold-thrust belts.
The junction of the Central Utah segment and the Leamington transverse zone is an ideal natural-laboratory for studying salient-recess junctions for several reasons (Figure 1). First, the rocks exposed within the segment continue, uninterrupted, into the transverse zone4. So, deformation patterns can be tracked continuously, and compared across the junction. Second, the rocks are essentially monomineralic, so variation in fault patterns are not a result of heterogeneities within units, but instead reflect the overall folding and thrusting within the study area4. Third, elastico-frictional mechanisms, such as cataclastic flow, assisted deformation throughout the field area, allowing for direct comparisons of mesoscale fault patterns4. Finally, the overall transport direction remained continuous along the length of the segment and transverse zone; therefore, variations in shortening direction did not influence the preserved deformation patterns4. All of these factors minimize the number of variables that may have affected the deformation along the segment and transverse zone. As a result, we surmise that the preserved structures formed primarily because of a change in the underlying basement geometry5.
Figure 1. Example of index map. The Sevier fold-thrust belt of western USA, showing major salients, segments, recesses and transverse zones. Figure 2 indicated by boxed area (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
Folding and thrusting within the Central Utah segment and Leamington transverse zone, took place at depths < 15 km, i.e., within the elastico-frictional regime, where deformation occurred primarily by outcrop-scale (< 1 m) faults and cataclastic flow4,6. Because transport and folding of the thrust sheet took place primarily by elastico-frictional mechanisms, we predict that a detailed fault analysis can provide further insight into the kinematic history of the Leamington transverse zone and the underlying basement geometry. In order to test this hypothesis, we have collected and analyzed fault patterns preserved in the rocks within the northern portion of the Central Utah segment and throughout the Leamington transverse zone (Figure 2).
Figure 2. Example of macroscale topographic map. Shaded-relief topographic map of boxed area in Figure 1. The 4 Regions are separated by solid white lines. Bedding contacts between the Proterozoic Caddy Canyon quartzite (PCc), Proterozoic Mutual quartzite (PCm) and Cambrian Tintic quartzite (Ct) are shown. Dashed lines show the trend of the mountains within this area. Site locations are shown with numbered black squares. First-order lineations are shown with solid gray lines (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
Sandbox experiments were carried out to compare against, and supplement, the fault data. A push-block sandbox model, with frontal and oblique ramps, was used to assist our analyses of the structures preserved in, and around, the Leamington transverse zone (Figure 3) 7. The objectives of this approach are four-fold: 1) determine if the mesoscale fault patterns are consistent, 2) determine if the sandbox model supports and explains the field data, 3) determine if the sandbox model provides more details on structures that are not observed in the field, and 4) evaluate whether this combined field-experimental method is useful and easy to replicate.
Figure 3. Example of push-block model. Photograph of empty sandbox model. The southern frontal ramp (SFR), oblique ramp (OR), northern frontal ramp (NFR), and the four Regions (1-4) are labeled (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
1. Collection of Macroscale Field Data
2. Collection of Mesoscale Field Data
Figure 4. Example of a mesoscale outcrop. Bedding is highlighted with white dashed lines. Specific fault sets discussed in paper are highlighted with thin, solid white lines. m2 grid is shown (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
3. Collection of Microscale Data
Unit | Bed thickness (m) | Bedding fabric | Grain size (m) | X/Z Fry strain (Average Rf) | X/Y Fry strain (Average Rf) | Amount of overgrowth | Amount of iron oxide | Amount of impurities | Other characteristics |
Ct | 1,000 | Prominent, thick and thin bedded | Ave: 1.59 x 10-4 (Range: 3.6 x 10-6 to 3.31 x 10-4) |
1.15 | 1.12 | moderate, semi-connected in small patches | moderate, semi-connected in small patches | moderate, semi-connected calcite in small patches | Ridge former, white to grayish-pink, weathers tan to reddish brown |
PCm | 570-750 | Prominent, well-developed graded and cross-bedding | Ave: 1.48 x 10-4 (Range: 1.15 x 10-4 to 2 x 10-4) |
1.22 | 1.19 | major and well-connected | moderate and well-connected | minor calcite and poorly connected | Massive outcrops, purplish red-brown, weathers purple-black |
Table 1. Example of microscale morphology. Description of the Proterozoic Mutual (PCm) and Eocambrian Tintic (Ct) quartzite units. X/Z Fry strain is measured in a vertical section parallel to the transport plane, while X/Y Fry strain is measured in a vertical section perpendicular to the transport plane (modified from Ismat and Toeneboehn7). Please click here to view/download this table in Microsoft Excel format.
4. Plotting Mesoscale Fault Data
Figure 5. Examples of Equal-area plots. Equal-area plots of fault sets from two sites — site 41 is from Region 2 and site 5 is from Region 1. Fault sets are plotted as contoured poles (1% area contours). Average fault sets are determined from pole-concentrations and plotted as great circles. Maximum shortening directions, determined from conjugate-conjugate fault sets, are plotted as black dots. Fault-pole contours are colored according to percentage contribution at each site. Pole concentrations that contribute to >20% are colored red, between 15-19% are colored orange, 10-14% are yellow, 5-9% are green and <5% are colored blue. Red fault-pole contours are labeled as LPS (layer-parallel shortening), LE (limb extension), and HE (hinge-extension) (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
Site | Bedding | Shortening | Highest fault-pole | Fault sets(s) |
(dip, dip direction) | directions(s) | concentration(s) | (dip, dip direction) | |
(plunge, trend) | (plunge, trend) | |||
41 | 83, 268 | 79, 115 | 22, 064 | 68, 244 |
60, 345 | 30, 265 | |||
73, 276 | 17, 096 | |||
5 | 63, 265 | 67, 130 | 08, 343 | 82, 263 |
36, 247 | 54, 067 |
Table 2. Example of mesoscale fault data. Chart, showing just 2 of the 24 sites, documenting the following: bedding orientation, shortening direction(s), orientation of the highest fault pole concentration(s) and their corresponding fault set(s) (modified from Ismat and Toeneboehn7).
Figure 6. Example graph showing distribution of fault populations. Graph showing the percentage and type of the maximum fault sets (highlighted in red in Figure 5) for each site. Just sites within the Ct quartzite are shown here (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
5. Construction of the Push-block Sandbox Model
Figure 7. Example sandbox model diagram. Diagrams for the sandbox model, illustrated as plan and cross-sectional views. The southern frontal ramp (SFR), oblique ramp (OR) and northern frontal ramp (NFR) are labeled. Thin arrows drawn over the ramps illustrate potential direction of sand movement. See Figure 3 for a photograph of an empty sandbox model (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
Figure 8. Example threaded bar connection. Close-up view of the threaded bar and matching nut mounted to the frontstop. Please click here to view a larger version of this figure.
6. Running the Push-block Sandbox Model
Figure 9. Example of undeformed sand in sandbox model. Partial plan-view of undeformed sand in sandbox model. Note grid indentation and square cross-pins. The southern frontal ramp (SFR), oblique ramp (OR), northern frontal ramp (NFR), and the four Regions (1-4) are labeled (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
Figure 10. Example of deformed sand layers. Plan-view of the end-result deformation from the sandbox model. Select cross-pins labeled with blue dots showing dextral offset. Folded cross-pins highlighted with yellow lines. Thrust faults are highlighted with thin, black lines. The four Regions (1-4) are labeled (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
7. Collecting Samples from the Sandbox
Figure 11. Example of metal dividers. Plan-view, showing 2 metal dividers, one through a frontal ramp and one through the oblique ramp, in the deformed sand. The metal divider along the oblique ramp is filled with epoxy. Note tape measure for scale (Modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
Figure 12. Examples epoxied samples from sandbox model. Epoxied samples from the (a) northern frontal ramp and the (b) oblique ramp within the sandbox model. Shown samples are cut perpendicular to the trend of the ramps. Layers are highlighted with thin, white lines. Solid white lines mark reverse faults, dashed white lines mark strike slip faults (modified from Ismat and Toeneboehn7). Please click here to view a larger version of this figure.
Aerial photographs were used to subdivide the field area into four Regions (1-4), based on the trend of the modern mountain ridge crest (Figure 2). Multi-scale fault data is compared between these four Regions. Assuming that these trend changes reflect the underlying basement geometry, the oblique ramp is positioned within Regions 2 and 3, where the mountains trend oblique to the Sevier fold-thrust belt. Throughout the four Regions, we found that the mesoscale faults preserve a deformation fabric that is penetrative and homogeneous at the mesoscale (i.e., cubic meter of rock) and are representative of areas larger than the cubic meter sites (Figure 4) 4,16. In addition, microscale variations, shown in Table 1, are not reflected in the collective character of fault patterns. So, the mesoscale fault sets can be directly compared throughout all four Regions (Figure 5). In brief, we found that the mesoscale faults sets can be defined as conjugate-conjugate sets and rotate with bedding, which entail that the shortening directions do, too. This pattern suggests that the mesoscale faults formed early, and different combinations of fault sets are used to assist the fold shape. In more detail, we found that the fault patterns are unique within each of the four regions — Regions 2 and 3, and Regions 1 and 4, are similar to each other (Figure 6). This pattern supports the macroscale assumption that the oblique ramp underlies Regions 2 and 3, and suggests that our conjugate-conjugate fault analysis is reliable. Beyond that, however, this method of analysis is not any more illuminating. Because of this, we further analyzed the fault data by examining the Equal-area net fault-pole concentrations (Figure 5). This approach is used to track which of the youngest sets were most dominant during deformation. These patterns also suggest an oblique ramp underlying Regions 2 and 3, and unlike the conjugate-conjugate fault analysis, reveal a sharp break between these two Regions. Therefore, we interpret that this pole-concentration analysis is reliable and potentially elucidates subtle structures that may not be clear from the conjugate-conjugate fault method.
Similar to previous models, based on finite element modeling (FEM) we have assumed that the oblique ramp is continuous17. The sharp break in bedding and fault patterns across the boundary between Regions 2 and 3 can be explained by differential motion over a continuous oblique ramp. Alternatively, the discontinuity in bedding and fault patterns across Regions 2 and 3 may reflect a break in the underlying basement. Here, we compare our field data to our sandbox model results in order to test these two hypotheses. We found that a break in the overlying thrust sheet formed even though there was no break in the basement (Figure 10). Interestingly, the location and orientation of the break is comparable to the position and orientation of the boundary between Regions 2 and 3 on the macroscale maps. Therefore, the break observed in the overlying thrust sheet may have simply formed via a complex interaction of an eastward moving thrust sheet over an oblique ramp. In other words, deformation preserved in thrust sheets may not directly mirror the underlying basement geometry. So, this sandbox experiment successfully replicates, and potentially explains, fault patterns preserved in the field.
The epoxied sandbox samples were analyzed from the sandbox model to observe the internal structure of the deformed sand, and compare these structures against field observations. Two representative samples were analyzed — a sample from the frontal and oblique ramps (Figure 12). In general, the reverse faults and folds preserved in the epoxied samples from the frontal ramp accommodate transport to east, and those from the oblique ramp accommodate transport to the southeast. The strike-slip faults in all the samples accommodate dextral motion. This kinematic record along the frontal and oblique ramps supports previous models17-19, as well as the mesoscale fault data. These hand samples are novel way to analyze internal structures that may not be accessible in the field.
The Central Utah segment of the Sevier fold-thrust belt, and its northern boundary, the Leamington transverse zone serves as an ideal natural laboratory for studying salient-recess junctions (Figure 1). Along this junction, the transport direction remains constant and the thrust sheets are uninterrupted across the junction, so the only variable is the underlying basement geometry5.
Here, we present a method to analyze this type of salient-recess junction by combining multi-scale fault data collected in the field with a push-block sandbox model, which replicates the large-scale geometry of the field area. The sandbox model experiment represents a longer time period of deformation than the mesoscale fault sets — we assume that the youngest fault sets accommodated the observed fold geometry. So, the sandbox model, in conjunction with the faults sets, can be used to track thrust sheet deformation and determine details of the underlying basement geometry.
In order for this combined approach to be successful, the following critical steps need to be taken in the field and sandbox experiment. For the field portion, it is critical to determine the scale of fault homogeneity — fault sets that are not preserved at equivalent scales cannot be directly compared. In addition, a large population of faults (≥ 30 fault sets) need to be measured in order ensure statistically reliable data sets9. Moreover, faults should be measured away from heterogeneities, such as bedding contacts, in order to avoid local strain variations. Even microscale variations, such as impurities, a range in grain size and large amount of strain (Fry > 1.8) may influence mesoscale fracture development by creating foliation planes and other heterogeneities. For the experimental portion, the sandbox model must mimic the field geometry as closely as possible. It is recommended that the box be constructed at a larger scope than the field area, in order to avoid edge-effect complications. The macroscale Regions were also enlarged, for the same reason. It is important that the grain size of the sand mimics Coulomb behavior20 — an average grain size of ~0.5 mm is recommended21. Finally, once the experiment is being run, it is critical that the large scale faults and folds form in the same orientations and order (e.g., forward breaking, backward breaking, etc.) as observed in the field. Otherwise, the structures formed in the model cannot be compared to the field data, even if they look similar.
The results from this study are comparable to, and support, previous work conducted in this area based on FEM17,22, and provides more details to the kinematic history. This suggests that detailed fault data, measured in areas that have deformed by elastico-frictional mechanisms, can be used to develop more detailed kinematic models than some computer models. Although fault data collection and analyses is laborious and time consuming, this method may be more accessible than computer and analog modeling, and is less expensive. Fractures and faults are often overlooked23 — many geologists view upper crustal deformation as minor and void of patterns. However, a large portion of the crust — the upper ~15 km — deforms by faulting and other elastico-frictional mechanisms. This work suggests that a significant amount of geologic history is stored in the upper crust and is readily available for analysis.
We demonstrate that even in the simplest cases, such as examined here, the structures preserved in the upper crust do not necessarily mimic the underlying basement geometry. Detailed fault analyses can reveal subtleties that may not be revealed with map patterns, standard conjugate fault studies and/or computer models, such as FEM. Using a sandbox model can help explain why some of these subtle patterns exist. This method presented here is simple, reliable and easy to replicate. It can potentially change how many geologists perceive the role of faults and cataclastic flow, and what they can tell us. This method can be used to re-examine, and uncover more kinematic details, of underexplored field areas, and can be easily modified to accommodate geologic settings other than fold-thrust belts. This approach has far reaching implications in terms of tracking fracture controlled fluid flow in the upper crust as well as how fold-thrust belts maintain critical taper at salient-recess junctions.
The main weakness of this approach is that sandbox modeling may not be able to replicate complex geologic histories. For example, in cases where there are variable shortening directions, the timing and direction of events should be carefully tracked in the field and then replicated with different push-blocks in the sandbox model. However, the sand will likely not preserve these various directions of shortening because the sand will flow and bedding layers will not be maintained. This problem may be resolved by adding oil or petroleum jelly to the sand, to make the sand more cohesive. But, then the sand will not behave as a Coulomb material and thus, may not model deformation in the upper crust. Further work is required to unravel more complex natural systems, such as situations where the basement geometry not the only variable.
The authors have nothing to disclose.
We thank Erin Bradley and Liz Cole for their assistance in the field. Field work, thin-section preparation and material for the sandbox model was supported by Franklin & Marshall College’s Committee on Grants.
fiberboard | Any | NA | |
finishing lacquer | Any | NA | |
epoxy | Epoxy technology | Parts A and B: 301-2 2LB | best if warmed to 80º – 125º. If warming is not possible, it will cure fine, it will just take 1 week, rather than 1 day. |
ramp wood-pine | Any | NA | |
painters tape | Any | NA | |
rabbit joints | Any | NA | |
countersunk fasteners | Any | NA | |
sand paper | Any | NA | |
play sand | Any | NA | best if homogenous grain size, ~0.5mm |
food coloring | Any | NA | best to use one color and a dark color |
plastic mesh/grid | Any | NA | |
square cross oins | Any | NA | |
crank screw | Any | NA | |
crank handle | Any | NA | |
sheet metal | Any | NA | |
dividers bars | Any | NA |