Principles of Rock Deformation
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The prefolding fault system consists of dominant reverse microfaults clearly tilted within the strata Figures 5c and 5d. In the backlimb, the NE compression is marked by numerous stylolitic peaks oriented within the bedding and well observable in the canyon of the Bighorn River within the Madison Formation Figures 5 and 8. This prefolding faulting event corresponds to LPS2. In some sites, the low bedding dip precludes unambiguous recognition of the prefolding character of faulting site X5 ; we chose however to relate them to LPS2. This late fold tightening stage seems to be better represented in the forelimb, but this may also be due to more favorable outcropping conditions due to bedding dip.
These reverse faults are consistent with previous observations by Hennier and Spang  and Forster et al. It is marked by steeply dipping normal faults striking parallel to SMA axis. Well documented by calcite twinning, this event is instead poorly marked by microfaults. Only 2 sites Figure 7 show evidence for microfaulting to mesofaulting related to this compression trend Figure 5k. The set I fractures defined by Bellahsen et al. In the field, they are observed in many places as predating all the other fractures. These fractures were subsequently sheared during Laramide times [ Bellahsen et al.
The N—S set was not interpreted by Bellahsen et al. The N—S fractures, more abundant in the forelimb than in the backlimb, were observed in other places around the Bighorn basin [ Callot et al. Callot, personal communication, , and likely predate the Laramide folding event. Their tectonic significance still remains unclear.
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As set II fractures probably initiate at the very beginning of folding prefolding to early folding LPS2 , these latter fractures probably occurred during the folding event. Finally, samples Kal. However, for the NW—SE directed compression LPS1 , the regime is dominantly strike slip in term of stress and dominantly compressional in terms of strain.
Comparisons between Laramide stress regimes derived from calcite twins with those derived from fault slip data shows a good agreement in both stress orientations and regimes. Some samples have also preserved stress tensors related to the Laramide LPS2 and to late stage fold tightening, which could be revealed only using Etchecopar's technique e. The late stage fold tightening was recorded in both the matrix and the veins irrespective of their orientation Figure 9.
Principles of Rock Deformation
Note that a vein attributed to the latest set IV by Bellahsen et al. A similar interpretation was made by Silliphant et al. Therefore, the mean calcite twinning strain in our samples reliably reflects the relative amount of regional deformation related to these two tectonic phases in the area of interest. Our results suggest that set II fractures are also present in the forelimb and that LPS2 in both the forelimb and the backlimb was also marked by reverse faulting and stylolitization.
Several lines of evidence support the formation of this set before Laramide folding. Finally see section 4.
These chronological observations lead us to conclude that set I joints and veins likely predate Laramide tectonism. Two interpretations can however be proposed.
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According to the first one, postfolding normal faults and prefolding calcite twin sets, both related to NE extension perpendicular to fold axis, are related to overall outer rim extension also marked by set III veins. Single slip events or a series of events are considered with a complete stress relaxation occurring between each event.
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Linear elasticity is well suited for our study since we want to focus on the very first steps of deformation caused by the early motion along the thrust fault. The boundary conditions are set as follows: the three principal stresses increase with depth as a result of the effect of burial. The most compressive stress is perpendicular to the fault about MPa at m deep and the least compressive one is vertical about 60 MPa at m depth. The intermediate principal stress is about 75 MPa at m depth. Young's modulus is set to 10 GPa, Poisson's ratio to 0.
These stress perturbations occur both in orientations and magnitudes, in agreement with previous works [e. However, because the model is basically elastic, the magnitudes of the perturbations of both stress orientations and values cannot be directly compared to our results. In the following we only focus on the increasing or decreasing trends of differential stress magnitudes predicted by the model, to be compared with increasing or decreasing trends of differential stress magnitudes revealed by calcite twinning paleopiezometry Figure The least vertical and intermediate compressive stresses are higher in the fault hanging wall.
This result suggests that SMA backlimb was located in the hanging wall of above the reactivated fault and that the forelimb is located within its footwall very close above the fault tip Figure 3. It therefore supports the idea that the increase of prefolding differential stresses from the backlimb toward the forelimb could be related to the stress perturbation induced in the overlying cover by the tip of the underlying thrust fault that did not propagate at least at the very beginning within the cover.
In contrast, differential stress magnitudes keep on increasing in the backlimb, but the simple model used is unable to model stresses during fold evolution. As mentioned in Section 5. Note finally that in this scenario, the pore fluid pressure is not taken into account. A striking point of this scenario is that the maximum differential stress increases in the backlimb from prefolding to postfolding stage.
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To account for this counterintuitive result, one can propose either significant hardening of the backlimb during folding or a decrease in pore fluid pressure during folding in response to fracturing of overlying impermeable formations such as the shaly Chugwater Formation by the development of the prefolding and synfolding fractures. Further work is needed to discuss these hypotheses. In addition, stylolitization supports widespread pressure solution processes. There are thus limited evidence of synfolding faulting and twinning; although this may be partly due to possible reorientation of stresses by bedding anisotropy during folding, which may mimic true prefolding structures, we suggest that internal strain is mainly achieved during the early and late stages of folding, probably during two peaks of stress which seem to predate immediately folding and to prevail after fold development fold tightening stage [ Onasch , ].
At the hinge where high curvature occurs, set III joints and veins that do strike parallel to the trend of the anticline accommodated bending stresses [ Bellahsen et al. Amrouch et al. These observations are in agreement with the simulation results of Sanz et al. In addition, even though set II joints and veins striking perpendicular to the fold axis mainly formed during LPS2 [ Bellahsen et al. Laramide LPS2 was accommodated by stylolites, mesoscale reverse faulting and calcite twinning and was accompanied by mode I fracturing set II from the scale of the outcrop to the scale of the thin section.
The stress field at that time was rather complex, at least in terms of magnitudes, because of stress perturbations caused by the underlying basement thrust fault. Late stage fold tightening is marked by development of new mesoscale fault systems, calcite twinning and largely by shear reactivation of preexisting fracture sets. Despite some slight changes in stress orientations marked for instance by shear reactivation of earlier formed set II joints [e.
These results are in good agreement with independent analyses of widespread joints and veins as well as striated microfaults measured throughout the fold. Erslev and P. Laurent, for their constructive comments on the manuscript. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors.
Any queries other than missing content should be directed to the corresponding author for the article. Volume 29 , Issue 1. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username. Open access. Tectonics Volume 29, Issue 1. Free Access. Olivier Lacombe E-mail address: olivier.
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Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Introduction and Aim of the Study  Folding in sedimentary rocks results from two major mechanisms: buckling due to lateral tectonic compression and slip on thrust faults in the underlying strata [e. Geological Setting 2. Figure 1 Open in figure viewer PowerPoint. Arrows indicate Laramide and Sevier compression trends reconstructed by different authors: 1, Laramide compressional trend from Craddock and van der Pluijm  ; 2, Sevier compressional trend from Craddock and van der Pluijm  ; 3, Laramide compressional trend from Varga  ; 4, Laramide compressional trend from Neely and Erslev .
Figure 2 Open in figure viewer PowerPoint. Stratigraphic section [after Ladd , ] and pictures of formations that crop out at SMA. Figure 3 Open in figure viewer PowerPoint. Methods for Characterizing Stress and Strain Patterns in SMA  In order to investigate the stress record during folding at both macroscopic and microscopic scales, we carried out a study of striated microfaults and calcite twins. Figure 4 Open in figure viewer PowerPoint.
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Photographs of twinned grains from veins and matrix from studied samples. The best tensor solution is searched as to minimize the function f , ideally equal to 0, defined as.