Erselev, 1993. It is interpreted as having formed in response to bending stresses during folding. Now at: UMR 7193, ISTEP Institut des Sciences de la Terre de Paris, Campus Jussieu - Tour 46-45, 2ème étage, boîte courrier 129, 4, Place Jussieu, 75252, Paris Cedex 05, France. 2009). 5. [84] During LPS2, (σ1 − σv) and (σ1 − σh) increase from the backlimb to the forelimb (Figure 12). Abstract: Amelia Zhi Yi Mentor: Laramide foreland deformation in Wyoming represents a classic tectonic province, yet there are outstanding questions about the kinematic and mechanical processes responsible for the mountain systems seen today. 1990). During Laramide LPS2, both (σ1 − σv) and (σ1 − σh) increased from the backlimb toward the forelimb. Lampert S.A. Late stage fold tightening is marked by development of new mesoscale fault systems, calcite twinning and largely by shear reactivation of preexisting fracture sets. Geophysics, Biological Grelaud S. For instance, we favor the reactivation of the underlying SW dipping (normal) fault as a high‐angle thrust fault with a short cut accommodated by many splays in order to account for the relatively distributed deformation required for basement folding. This increase is well identified on cooling curves where the susceptibility is systematically much higher than those of the heating curves (Fig. The Sevier orogeny was the result of convergent boundary tectonic activity, and deformation occurred from approximately 160 million years (Ma) ago to around 50 Ma. Mouthereau F. Dinarès-Turell J.. Rathore J. Bedding-parallel slip, evidenced by slickensides along bedding surfaces in the Madison formation, tail cracks emanating from bedding surfaces in the Phosphoria formation, and polished bedding surfaces of the Phosphoria formation indicates that flexural-slip with slip directions approximately normal to the fold axis (Hennier & Spang 1983; Sanz et al. 6) is observed close to 580 °C supporting that magnetite is the dominant phase. Erselev, 1993. Calcite twinning from both matrix and veins in various formations from Sheep Mountain Anticline recorded three tectonic stages: (1) a prefolding compression parallel to fold axis, likely related to pre‐Laramide tectonism (Sevier orogeny LPS1 stage); this event appears to be of primary importance since fractures formed in this stress field were later reactivated during the fold‐related stages, thus … For each twin set measured, the average twin width, number of twins, grain width normal to twins, and the orientations of the c axis and the e‐twin plane are measured. Fry analysis indicates that the Tensleep and Amsden sandstones accumulated only small bedding-parallel shortening strains (Paterson and Yu 1994; Wetmore 2003). Mercier E. Despite some slight changes in stress orientations marked for instance by shear reactivation of earlier formed set II joints [e.g., Fiore, 2006] and stress permutations, these local effects have poorly influenced stress orientations during LPS and late stage fold tightening; in contrast, they greatly influence differential stress magnitudes. There have been varying interpretations regarding the direction(s) of shortening during Laramide deformation of the Rocky Mountains. 8b), Louis et al. The boundary conditions are set as follows: the three principal stresses increase with depth as a result of the effect of burial. We believe that these E-W trending structures are evidence for a change in Laramide shortening direction from NE-SW to N-S, similar to that describe by Bergh and Snoke (1992) in the Shirley Mountains … Faulting related to Laramide deformation has led to extensive fracturing and brecciation in Little Sheep Mountain anticline. Field data indicate that these joints occur in fewer locations in the forelimb than in the hinge and backlimb. [2] 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.g., De Sitter, 1956; Faill, 1973; Ramsay, 1974; Dahlstrom, 1990]. Impact of fracture stratigraphy on the paleo-hydrogeology of the Madison Limestone in two basement-involved folds in the Bighorn basin, (Wyoming, USA). Our study shows that: (1) when the magnetic directions are clustered, they are systematically related to the regional structures. Reported fracture orientations for sets I, II, and III are coaxial with bedding and unfolded. By comparison with other fabrics observed in the fold, this is interpreted as bed-parallel shearing, which appears to be more evolved here. and Petrology, Exploration Thicker Pre-Laramide deposits allow more secondary faults to form early during deformation, absorbing the horizontal shortening within the sedimentary layer. This suggests that NE–SW LPS2 was not a simple stress regime, varying both spatially and temporally throughout the fold from true NW–SE perpendicular extension to true NE–SW compression or a strike‐slip regime with NE–SW compression and NW–SE extension. 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). The mechanisms active during the LPS stage for sandstone lithologies have been partly elucidated [Frizon de Lamotte et al., 2002; Saint‐Bezar et al., 2002; Sans et al., 2003; Robion et al., 2007]. SHE 32 and SHE 14 are magnetite-rich sample, SHE6 is a mix of hematite, magnetite and pyrrhotite, SHE 38 is a mix of hematite and magnetite. 1994; Louis et al. The mixed populations of reverse faults and strike‐slip faults probably represent protracted deformational events where, because of slight changes in relative horizontal stress magnitudes, the vertical stress component at some times and places represents σ2 and at other times and places within the fold represents σ3. [2006a], and Ahmadhadi et al. Critical state finite element models of contractional fault-related folding: Part 2. Inversion of calcite twin data for paleostress orientations and magnitudes: A new technique tested and calibrated on numerically-generated and natural data. Dalziel I.W.D.. Lacombe O. Vilasi N. In sandstones, strain within the bedding plane is assumed to be a proxy for layer-parallel tectonic shortening, whereas strain tends to occur perpendicular to bedding for pre-tectonic compaction (Burmeister et al. Frizon de Lamotte D.. Sagnotti L. In the backlimb and in the hinge, relatively higher values of the anisotropy after saturation can be explained with a more anisotropic matrix. (σ1 − σv) increases from nearly 20 MPa in the backlimb to a value ∼60 MPa in the forelimb while (σ1 − σh) evolves from 10 MPa in the backlimb to ∼50 MPa in the forelimb. compression/shortening. Gries [1983, 1990] specifically related variation in orientation of Rocky Mountain foreland uplifts to the suggested temporal changes in compression direction. Jeff Hennier, John H. Spang. The expected relaxation of the stress perturbation at fault tip during folding leads to a presumable slight decrease of σv, even in the absence of significant synfolding erosion, which implies a small decrease of σ1. In contrast, late or postfolding twin sets should yield two horizontal stress‐strain axes and one vertical one (assuming that the regional stress‐strain field is in that orientation). Calcite twin analysis reveals three main tectonic stages (Amrouch et al. Concerning the latter ones, one notes a significant variation of Pj, where the anisotropy drops from 1.16 to values lower than 1.03 whereas in the backlimb the decrease is less pronounced and the shape parameter T values are systematically positive and increasingly planar toward the forelimb (Fig. Magnetic mineralogy investigations on representative samples. 12). Control of Cambrian evaporites on fracturing in fault-related anticlines in the Zagros fold-and-thrust belt. [1996], who argued that the SMA fault is a SW dipping back thrust of an older NE dipping thrust. These results are in good agreement with independent analyses of widespread joints and veins as well as striated microfaults measured throughout the fold. Poisson A. Part II. 1, Fig. Salas R.. Parés J.M. Transverse jointing in foreland fold-and-thrust belts: a remote sensing analysis in the eastern Pyrenees. Because of a poor exposure of carbonates at the hinge, only the forelimb and the backlimb were investigated in the Phosphoria formation (Fig. First, Laramide LPS2‐related set II veins clearly abut on the set I veins, as already reported by Bellahsen et al. Part I, The velocity of compressional waves in rocks to 10 kilobars, Part II, Stress direction history of the western United States and Mexico since 85 Ma, Magnetic susceptibility, petrofabrics and strain, Correlation of strain with anisotropy of magnetic susceptibility (AMS), Interpreting anomalous magnetic fabrics in ophiolite dikes, Tectonic applications of magnetic susceptibility and its anisotropy, The influence of deformation mechanisms on magnetic fabric in weakly deformed rocks, Comparison of Fry strain ellipse and AMS ellipsoid trends to tectonic fabric trends in very low-strain sandstone of the Appalachian fold–thrust belt, Rock texture and magnetic lineation in dykes, a simple analytical model, Fault-propagation folds above thrusts with constant dip, The problem of strain-marker centers and the Fry method, Elasticity of high-porosity sandstones: theory for two North Sea datasets, Relative motion between oceanic and continental plates in the Pacific basin, Normalized center-to-center strain analysis of packed aggregates, Thrusts, back-thrusts and detachment of Rocky Mountain foreland arches, Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States, The determination of the elastic field of an ellipsoidal inclusion and related problems, Etude des états de contraintes en tectonique cassante et simulation de déformations plastiques (approche mathématique), Corrigendum to: Fluid control of localized mineral domains in limestone pressure solution structures, Strain partitioning of deformation mechanisms in limestones: examining the relationship of strain and anisotropy of magnetic susceptibility (AMS), 3D characterization and mechanics of brittle deformation in thrust fault related folds, Structural interpretation of Sheep Mountain Anticline, Bighorn Basin, Wyoming, Cinématique du plissement et déformation interne des roches; l'exemple du pli de Lagrasse (Aude France), Early record of tectonic magnetic fabric during inversion of a sedimentary basin. To determine the stress fields associated with fold evolution in the investigated area, we have carried out a quantitative inversion of distinct families of fault slip data determined at each individual site, using the method proposed by Angelier [1990]. [30] The optimal tensor is obtained when (1) the maximum of twinned planes are taken into account; (2) the maximum of untwinned planes are taken into account; and (3) the f value is minimal (in practice, one can authorize a weak percentage, 10%–15%, of untwinned planes receiving a RSS larger than τa′ because of measurement uncertainties and local heterogeneities at the grain scale). Two interpretations can however be proposed. Although this approach has proven to be useful to propose consistent geometrical models of folding, it is mainly macroscopic, and the successive deformation mechanisms that accommodate internal strain within folded strata at the meso- to microscopic scale remain to be properly linked to such geometrical models. Jeff Hennier, John H. Spang. Thus, they might be interpreted as due to local extensional stresses, layer‐parallel and in a plane close to vertical. Set II joints striking 045° (after unfolding) and observed mainly in the hinge and the backlimb are interpreted as related to the NE-oriented Laramide compression (Dickinson and Snyder 1978; Engebretson et al. 2009). [51] A postfolding stage is also preserved by twinning in the veins and matrix from sandstones of the Tensleep Formation and carbonates from the Madison and Phosphoria formations, whatever the orientations of the veins from which measurements were taken. [52] Extensional stress tensors were also identified (Figure 10). The Upper Pennsylvanian Tensleep Formation (29 m thick) is composed of interbedded thin sandstones, shales, and carbonates in its lower part and thicker beds of cross‐bedded quartz arenites in its upper part. The two southernmost ridges, Sheep Mountain and Delaney Butte, are … Four fracture sets (sets I to IV) were identified in SMA based on orientation data and deformation mode. Fig. Near Buffalo, Wyoming, two high-angle dextral faults striking N. 10° E. cut both the upturned sedimentary rocks and the crystalline basement. Whereas LPS2 is rather marked by stylolites and reverse microfaults, this event is instead marked by mixed reverse and strike‐slip faults. Evans & Elmore 2006; Latta & Anastasio 2007). Martín-Crespo T. [80] Figure 13 shows that the most compressive principal stress (horizontal, perpendicular to the fault in strike) is higher above the fault tip. This explains that in the pre-folding stage, maximum differential stresses recorded by calcite twins in the backlimb are much lower than in the forelimb (Fig. Fracture network geometry was identified as a major control on fluid migration within the structure that has provided conduits for episodic hydrothermal fluid flow. 14, section A—A'). In July, we will collect oriented samples in the Wyoming foreland, and will concentrate on individual fold systems (Thermopolis anticline, Derby Dome, and Sheep Mountain areas). 6). 1999). [8] The Rocky Mountain uplifts consist of isolated basement highs and folds in the foreland of the Sevier thrust belt, associated to high‐angle thrust faults rooted deep in the basement [Stone, 1993; Erslev, 1993] (Figure 1). 1985; Forster et al. Strain factorization and partitioning in the North Mountain thrust sheet, central Appalachians, U.S.A. Corrigendum to: Fluid control of localized mineral domains in limestone pressure solution structures, Strain partitioning of deformation mechanisms in limestones: Examining the relationship of strain and anisotropy of magnetic susceptibility (AMS), Kink band folding. At each measurement site, all fractures were measured in areas typically a few tens of meters on a side. Frizon de Lamotte D. Further work is needed to discuss these hypotheses. 2002; Evans et al. Bedding dips are between 10° and 40° to the SW in the backlimb and can be locally overturned in the forelimb. The presence of weak shale layers in the sedimentary section allows numerous small faults to form, and limits the depth of all the faults. This obliquity is characteristic of this part of the fold. In the latter case, the cracks appear mainly to follow non-isotropic stress conditions or loading/unloading episodes (Louis et al. The result is a better-defined strain ellipse. The Sheep Mountain anticline (Wyoming, USA) is a well-exposed asymmetric, basement-cored anticline that formed during the Laramide orogeny in the early Tertiary. Sheep Mountain Anticline is located on the northwest flank of the Bighorn Basin, west of the Bighorn Mountains (Figure 1). This phase is assumed to be hematite. [27] Several methods for determining stress or strain components from calcite twin lamellae have already been proposed (see review by Burkhard [1993]). 2005; Evans & Elmore 2006; Graham 2006). [2008], stresses that develop during folding likely show a complex pattern and evolution in space and time since they are potentially affected by the interaction between deforming rock layers with different mechanical properties [e.g., McConaughy and Engelder, 2001; Bourne, 2003], the interfacial condition of the layer boundaries and the presence or absence of bed‐parallel slip [e.g., Couples and Lewis, 2000; Johnson and Johnson, 2000; Cooke and Underwood, 2001; Guiton et al., 2003a, 2003b]. 7), we can see that the distribution of the magnetic axes orientation is not very pronounced, suggesting that the anisotropy of the rock is weak relatively to other parts of the fold. A vertical, compression-parallel joint set apparently formed at Sheep Mountain Anticline (SMA) during the early Laramide orogeny, prior to significant folding. Physics, Solar Start studying Chapter 11 Crustal Deformation and Mountain Building. [4] In contrast, description of internal deformation of folded strata and characterization of controlling mechanisms at the microscopic scale have received less attention, so the mechanical response of rocks during folding and the distribution of strain within the fold remain poorly understood. In the absence of positive evidence of successive distinct Laramide compressional trends in our new results, we follow Varga's [1993] conclusion that around the Bighorn basin, the dominant Laramide compressional trend remained almost NE–SW and that local deviations may have arisen from strain partitioning with zones of dominantly compressional deformation and zones of dominantly strike‐slip faulting along structures strongly oblique to the overall Laramide compression. In both rock matrix and veins, calcite twins recorded three different tectonic stages: the first stage is a pre‐Laramide (Sevier) layer‐parallel shortening (LPS) parallel to fold axis, the second one is a Laramide LPS perpendicular to the fold axis, and the third stage corresponds to Laramide late fold tightening with compression also perpendicular to the fold axis. [93] Our study interestingly supports that microstructures and macrostructures recorded at the same time the combination of far‐field orogenic stresses and of the local sources of stress perturbation such as effect of the underlying basement fault, bedding rotation, bed‐parallel slip and reactivation of early formed fractures. In south-central Yellowstone, the Paleozoic and Mesozoic sedimentary rocks were tightly folded into three anticlines separated from one another by synclines and faults (fig. The stress regime is compressional (. Robion P. Closer to the coast, right-lateral strike-slip deformation in the dying Sierran arc reflects some obliquity to convergence at the plate margin. "Left-slip evolution of the North Owl Creek fault system, Wyoming, during Laramide shortening", Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States, Christopher J. Schmidt, Ronald B. Related to Geologic Time, Mineralogy [35] The CSGT [Groshong, 1972, 1974] allows computation of the strain ellipsoid. 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. Figure 1 (previous page). Arlegui L.E. Mechanisms for Deformation of Sedimentary Strata at Sheep Mountain Anticline, Bighorn Basin, Wyoming. 1998; Pares et al. The craters are exposed on the northeast flanks of Sheep Mountain and Sage Hen Anticline. 1990). 2000) to get information on microstructures (Louis et al. Several lines of evidence support the formation of this set before Laramide folding. 8). The large fold seen in the sedimentary rocks at Sheep Mountain was created by crystalline basement rock uplifted along faults that parallel the long axis of the structure along the steeper east flank.