1-20 OF 207 RESULTS FOR

Spring Hill Synform

Results shown limited to content with bounding coordinates.
Save search
Follow your search
Access your saved searches in your account
Would you like to receive an alert when new items match your search?
Close Modal
Sort by
Journal Article

Resolving complexities associated with the timing of macroscopic folds in multiply deformed terrains: The Spring Hill synform, Vermont Available to Purchase

K.A. Hickey, T.H. Bell
Journal: GSA Bulletin
Published: 01 October 2001
GSA Bulletin (2001) 113 (10): 1282–1298.
DOI: https://doi.org/10.1130/0016-7606(2001)113<1282:RCAWTT>2.0.CO;2
...K.A. Hickey; T.H. Bell Abstract Determining the timing of macroscopic folds of bedding in multiply deformed terrains is difficult, especially for rocks that have undergone a succession of overprinting near-orthogonal deformations. The Spring Hill synform in southeast Vermont is an example...
FIGURES | View All (21)
View PDF for article title, Resolving complexities associated with the timing of macroscopic folds in multiply deformed terrains: The <span class="search-highlight">Spring</span> <span class="search-highlight">Hill</span> <span class="search-highlight">synform</span>, Vermont
Purchase
Add to Citation Manager for Resolving complexities associated with the timing of macroscopic folds in multiply deformed terrains: The <span class="search-highlight">Spring</span> <span class="search-highlight">Hill</span> <span class="search-highlight">synform</span>, Vermont
Image
Figure 3. Geological map of the Spring Hill synform with representative bedding measurements. Main lithological relationships and structural features are those of Ratcliffe and Armstrong (1995, 1996), with some modification based on our own mapping. Field localities are those referred to in text. The lithological units from youngest to oldest are: Yg—Mesoproterozoic metasedimentary, granitic, and amphibolitic basement gneisses; CZh—Hoosac Formation (quartz-mica-plagioclase ± garnet schist and amphibolite [black]); Cr—Rowe Schist (quartz-mica ± plagioclase ± garnet ± chlorite metasedimentary schist and amphibolite); Om—Moretown Formation (laminated quartz-mica-plagioclase, garnet metasedimentary schist, coticule and amphibolite [gray]); Ont—North River igneous suite (trondhjemitic, tonalitic, and amphibolitic intrusive rocks); Ochh—Cram Hill Formation (hornblende ± garnet amphibolite and biotite-quartz granofels); Ochg— Cram Hill Formation (carbonaceous and noncarbonaceous quartz-mica-garnet ± plagioclase metasedimentary schist and phyllite); Ochq—Cram Hill Formation (garnet-bearing conglomeratic quartzite); Ocha—Cram Hill Formation (hornblende-plagioclase amphibolite); Ochfs—Cram Hill Formation (plagioclase-mica ± garnet metasedimentary schist and granofels, and hornblende-magnetite coticule). TT—Townshend thrust. Cross sections A–A′, B–B′ and C–C′ are shown in Figure 14. Map coordinates in meters on UTM (universal transverse Mercator) projection (zone 18) using NAD27 datum

Figure 3. Geological map of the Spring Hill synform with representative bed... Available to Purchase

Published: 01 October 2001
Figure 3. Geological map of the Spring Hill synform with representative bedding measurements. Main lithological relationships and structural features are those of Ratcliffe and Armstrong (1995, 1996) , with some modification based on our own mapping. Field localities are those referred
Image
Figure 20. Sketches illustrating how the Spring Hill synform must have developed if it originated by means of boudinage of the Star Hill fold (Fig. 19). (A–C) Plan view of the Star Hill fold hinge. The Spring Hill synform developed from a tongue-like protrusion (A and B) of Cram Hill–Hawley rocks (gray) that became detached (C) during subhorizontal nappe formation. During dome-stage folding, the Spring Hill synform was rotated to a steeper orientation (D, adapted from Rosenfeld, 1968). This steepening should have caused folds and intersection lineations in bedding to have progressively steeper plunges into the core of the fold in a north-south longitudinal section (E). The latter geometry is not observed along the Spring Hill synform; see Figures 9B and 11B. SHS—Spring Hill synform; BHF—Butternut Hill fold; SHF—Star Hill fold

Figure 20. Sketches illustrating how the Spring Hill synform must have deve... Available to Purchase

Published: 01 October 2001
Figure 20. Sketches illustrating how the Spring Hill synform must have developed if it originated by means of boudinage of the Star Hill fold ( Fig. 19 ). (A–C) Plan view of the Star Hill fold hinge. The Spring Hill synform developed from a tongue-like protrusion (A and B) of Cram Hill–Hawley
Image
Figure 10. Approximate surface trace of S4 across topography superimposed on the surface geology shown in Figure 3. S4 transects the southern half of the Spring Hill synform, suggesting that it is a pre-D4 structure. The asymmetry of mesoscopic D4 folds observed in this study is also shown, as is the trace of a large D5 synform that overprints the Butternut Hill fold west of the Spring Hill synform (a smaller D5 fold is also developed at the southwest end of the Spring Hill synform). Although small macroscopic folds can be delineated on the basis of switching fold asymmetry, similar folds are not developed on the scale of the Spring Hill synform where D4 folds appear to have an overall near- symmetric geometry. The approximate trace of the axial plane to the Spring Hill synform is based on its interpreted pre-D4 timing and the distribution of lithological units. Tight D4 folding precludes a more accurate determination of its position

Figure 10. Approximate surface trace of S 4 across topography superimposed... Available to Purchase

Published: 01 October 2001
Figure 10. Approximate surface trace of S 4 across topography superimposed on the surface geology shown in Figure 3 . S 4 transects the southern half of the Spring Hill synform, suggesting that it is a pre-D 4 structure. The asymmetry of mesoscopic D 4 folds observed in this study is also
Image
Figure 19. Nappe- and dome-style fold model for the formation of the Spring Hill synform and the Chester and Athen domes presented by Rosenfeld (1968). See text for description. SHS—Spring Hill synform; BHF—Butternut Hill fold; SHF—Star Hill fold. Adapted from Rosenfeld (1968) and Rosenfeld et al. (1988). (Note: Section D–D′ in Fig. 14–1 of Rosenfeld [1968] appears to have been drawn incorrectly; it shows the Spring Hill synform folded around the Star Hill fold, in contradiction to Rosenfeld's interpretation [1968, p. 195] that the synform is a detached hinge of the Star Hill fold.)

Figure 19. Nappe- and dome-style fold model for the formation of the Spring... Available to Purchase

Published: 01 October 2001
Figure 19. Nappe- and dome-style fold model for the formation of the Spring Hill synform and the Chester and Athen domes presented by Rosenfeld (1968) . See text for description. SHS—Spring Hill synform; BHF—Butternut Hill fold; SHF—Star Hill fold. Adapted from Rosenfeld (1968) and Rosenfeld
Image
Figure 17. East-west cross sections constructed along the lines A–A′, B–B′, and C– C′ in Figure 3. Both the Spring Hill synform and the Butternut Hill fold are shown as steeply dipping structures overprinted by the D4 and D5 deformations. Note zone of relatively higher D5 strain between the two folds, where S4 has a steep, enveloping dip to the west. Analysis of foliation overprinting asymmetries suggests that the Spring Hill synform is not folded around any large macroscopic D4 or D5 structures and that it has a steep enveloping dip almost due east. The synform amplitude has been extensively flattened at the mesoscopic scale by tight D4 folds that transect the synform. Although mesoscopic D4 folds of bedding appear to be nearly symmetrical across most outcrops, the macroscopic geometry suggests that they should have an overall reversal in asymmetry across the Spring Hill synform as a consequence of the pre-D4 fold geometry. The large D4 folds in the Townshend thrust on the east limb of the synform (Fig. 10) owe their synform-to- east asymmetry to this same pre-D4 geometry. The Butternut Hill fold has a macroscopic geometry similar to that of the Spring Hill synform, but it has been partially overprinted by a steeply dipping D5 synform near to its axial plane. Lithological units as for Figure 3 (units in the Cram Hill formation, Och, have not been differentiated by name except for Ochmc and Ochq). The prominent variation in thickness of the North River igneous suite (Ont) along and across the Spring Hill synform is considered to be a primary intrusive feature. Positions for the structural relationships shown in Figures 4 to 8, 12, and 13 are also shown

Figure 17. East-west cross sections constructed along the lines A–A′, B–B′... Available to Purchase

Published: 01 October 2001
Figure 17. East-west cross sections constructed along the lines A–A′, B–B′, and C– C′ in Figure 3 . Both the Spring Hill synform and the Butternut Hill fold are shown as steeply dipping structures overprinted by the D 4 and D 5 deformations. Note zone of relatively higher D 5 strain
Image
Figure 8. (A) Hinge of a small D4 fold showing axial plane D4 crenulations overprinting well-developed S3 foliation subparallel to S0. Sample v452B on the west limb of the Spring Hill synform (Fig. 3). Vertical section; strike indicated. Plane polarized light. (B) Inclined isoclinal D4 fold of S0 and S3 overprinted by a steeply dipping S5 crenulation cleavage with a synform-to-west asymmetry. Sample v443A from the west limb of a mesoscopic D5 fold on the west limb of the Spring Hill synform (Fig. 3). Vertical section; strike indicated. Plane polarized light. (C) Subhorizontal isoclinal folds of S0 and S3 overprinted by D5 crenulations with steeply dipping axial planes in a carbonaceous phyllite. Sample v262 west of the Spring Hill synform (Fig. 3). Vertical section; strike indicated. Plane polarized light

Figure 8. (A) Hinge of a small D 4 fold showing axial plane D 4 crenulat... Available to Purchase

Published: 01 October 2001
Figure 8. (A) Hinge of a small D 4 fold showing axial plane D 4 crenulations overprinting well-developed S 3 foliation subparallel to S 0 . Sample v452B on the west limb of the Spring Hill synform ( Fig. 3 ). Vertical section; strike indicated. Plane polarized light. (B) Inclined isoclinal
Image
Figure 9. (A) Map of S4 and Sm (matrix foliation of unknown age, but in most cases likely to represent fully differentiated S4). S4 typically has a moderate west to northwest dip across the Spring Hill synform. Farther west near the nose of the Butternut Hill synform, S4 has a moderate to steep northeast dip. (B) Map of S4 intersection lineations with S0 and S3. Note reversal of plunge along the Spring Hill synform and the presence of a more westward plunge direction on its eastern limb

Figure 9. (A) Map of S 4 and S m (matrix foliation of unknown age, but in... Available to Purchase

Published: 01 October 2001
Figure 9. (A) Map of S 4 and S m (matrix foliation of unknown age, but in most cases likely to represent fully differentiated S 4 ). S 4 typically has a moderate west to northwest dip across the Spring Hill synform. Farther west near the nose of the Butternut Hill synform, S 4 has a moderate
Image
Figure 21. Sketch illustrating the progressive development of the Butternut Hill and Spring Hill folds in an east-west section, looking north. (A) Folds at the end of D3 time shown as tight upright structures with S3 parallel to their subvertical axial planes. Note, however, that as shown in Figure 1B, D3 need not have formed the fold couplet. The east limb of the Spring Hill synform had a shallower dip than the west limb did. (B) During D4, both folds were overprinted by pervasive microscopic and mesoscopic folding during subvertical bulk shortening without development of large macroscopic D4 folds on the same scale as the Spring Hill synform. (C) Subsequent east-west–directed subhorizontal bulk shortening during D5 produced tight mesoscopic folding and crenulation cleavage development (S5). At the macroscopic scale, D5 rotated the S4 reference surface to a moderate west-northwest dip across the Spring Hill synform, a zone of relatively higher strain developing on its west limb. A large macroscopic D5 synform in S4 developed immediately east of the Butternut Hill fold. Note that D5 lineations reverse plunge along the synform (Fig. 11B), but are slightly oblique and are shallower plunging than D4 intersection lineations (Fig. 9B). This suggests that the doubly plunging nature of folded bedding, though partially a consequence of heterogeneous D5 strain, originated during or before D4. Och— Cram Hill Formation

Figure 21. Sketch illustrating the progressive development of the Butternut... Available to Purchase

Published: 01 October 2001
in Figure 1B , D 3 need not have formed the fold couplet. The east limb of the Spring Hill synform had a shallower dip than the west limb did. (B) During D 4 , both folds were overprinted by pervasive microscopic and mesoscopic folding during subvertical bulk shortening without development of large
Image
Figure 16. Maps showing the trend (and plunge where measured) of foliation inflexion or intersection axes (FIAs) in porphyroblasts determined from samples collected across the Spring Hill synform (Bell et al., 1998). Geometrically, an FIA is the axis of inclusion trail curvature within a porphyroblast. Most porphyroblasts around the Spring Hill synform have inclusion trail curvature that is described by a single, constantly oriented axis. However, in many garnets, the axis of curvature (i.e., the FIA) changes orientation outward from the core to the rim of the porphyroblast (multi-FIA porphyroblasts). Bell et al. (1998) found that there was a consistent pattern to the relative timing (from core to rim) with which FIA trends formed in multi-FIA porphyroblasts, and this enabled them to differentiate four populations of differently oriented FIAs, which they called FIA sets 1–4. The distribution of set 1 (solid line) and 2 (dashed line) FIAs across the Spring Hill synform is shown in A. FIAs from sets 3 and 4 are shown in B and C, respectively. Note that the FIAs maintain a consistent orientation across the synform. See text for discussion

Figure 16. Maps showing the trend (and plunge where measured) of foliation ... Available to Purchase

Published: 01 October 2001
Figure 16. Maps showing the trend (and plunge where measured) of foliation inflexion or intersection axes (FIAs) in porphyroblasts determined from samples collected across the Spring Hill synform ( Bell et al., 1998 ). Geometrically, an FIA is the axis of inclusion trail curvature within
Image
Figure 7. (A) Generally shallowly dipping differentiated crenulation cleavage, S4 (at stage 3 of crenulation cleavage development), orthogonally overprinting well-developed S3 (at stage 5 to 6 of cleavage development), which has a steep enveloping dip. S4 has been deformed into a slightly sigmoidal shape by the effects of D5. Sample v201 from the west limb of the Spring Hill synform (Fig. 3). Vertical section with strike indicated. Plane polarized light. (B) Tight, moderately west dipping, D4 crenulations of well-developed S3. Sample v449B from the east limb of the Spring Hill synform (Fig. 3). Vertical section with strike indicated. Plane polarized light. (C) Well- developed, fully differentiated S4 foliation (stage 5 to 6 of cleavage development) in a sample collected from lower limb of a tight, northwest-dipping, mesoscopic D4 synform. S3 is preserved in D4 plagioclase porphyroblasts. Sample v664 from east limb of Spring Hill synform (Fig. 3). Inclined thin section; strike and dip of section indicated in top right corner. Plane polarized light

Figure 7. (A) Generally shallowly dipping differentiated crenulation cleav... Available to Purchase

Published: 01 October 2001
into a slightly sigmoidal shape by the effects of D 5 . Sample v201 from the west limb of the Spring Hill synform ( Fig. 3 ). Vertical section with strike indicated. Plane polarized light. (B) Tight, moderately west dipping, D 4 crenulations of well-developed S 3 . Sample v449B from the east limb
Image
Figure 12. (A) Steep S5 at stage 3 of crenulation cleavage development overprinting S4 at stage 5. Note synform-to-west asymmetry with which S5 overprints S4. Sample v204 on the east limb of the Spring Hill synform. Vertical section; strike indicated. Plane polarized light. (B) Relatively weak S5 that overprints a well-developed S4 foliation, which has a moderate to steep enveloping dip to the west. Individual S5 folia are discontinuous, curving into the S4 foliation in a manner suggesting that D5 shortening was accommodated primarily by the reactivation, rather than the crenulation, of S4 (e.g., Bell, 1986; Fig. 14 here). Sample v443A from the west limb of a mesoscopic D5 fold on the west limb of the Spring Hill synform (Fig. 3). Vertical section; strike indicated. Plane polarized light

Figure 12. (A) Steep S 5 at stage 3 of crenulation cleavage development o... Available to Purchase

Published: 01 October 2001
Figure 12. (A) Steep S 5 at stage 3 of crenulation cleavage development overprinting S 4 at stage 5. Note synform-to-west asymmetry with which S 5 overprints S 4 . Sample v204 on the east limb of the Spring Hill synform. Vertical section; strike indicated. Plane polarized light. (B) Relatively
Image
Figure 2. Location map and regional geology of southeast Vermont showing major lithological units and position of Spring Hill synform. Modified from Doll et al. (1961), Thompson et al. (1990), Ratcliffe et al. (1992), and Ratcliffe and Armstrong (1995, 1996). Yg—Mesoproterozoic basement gneiss of the parautochthonous Green Mountain Massif and the allochthonous Chester and Athens domes; CZh—allochthonous Neoproterozoic to Early Cambrian Hoosac Formation; Cpors—Cambrian Pinney Hollow Formation, Ottauquechee Formation, Rowe Schist; Ordovician and Cambrian Stowe Formation; Omncb—Ordovician Moretown Formation, North River igneous suite, Cram Hill Formation, Ordovician to Silurian Barnard Gneiss; DSnw—Devonian and Silurian Northfield Formation and Waits River Formation. SHS—Spring Hill synform; BHF—Butternut Hill fold; SHF—Star Hill fold. Map coordinates in meters on UTM (universal transverse Mercator) projection (zone 18) using NAD27 datum

Figure 2. Location map and regional geology of southeast Vermont showing m... Available to Purchase

Published: 01 October 2001
Figure 2. Location map and regional geology of southeast Vermont showing major lithological units and position of Spring Hill synform. Modified from Doll et al. (1961) , Thompson et al. (1990) , Ratcliffe et al. (1992) , and Ratcliffe and Armstrong (1995, 1996) . Yg—Mesoproterozoic basement
Image
Figure 18. (A) Porphyroblasts that grew during the development of a synform should have a predominance of clockwise asymmetries on the western limb and counterclockwise asymmetries on the eastern one. The actual distribution of inclusion trail asymmetry across the Spring Hill synform for each of the four FIA sets is shown in A and B. FIA sets 1, 3, and 4 have trends at a moderate to low angle to the synform, and asymmetries are given looking north. FIAs from sets 1 and 4 show no significant difference in the distributions of asymmetries across the fold. Set 3 FIAs show a change in the relative proportions of each asymmetry across the synform, but they have the opposite sense to that shown in A. These results suggest there is no genetic relationship between the fold and inclusion trails defining FIA sets 1, 3, and 4. Set 2 FIAs have a predominantly counterclockwise asymmetry (looking west) on the west limb of the fold, and an equal number of both asymmetries on the east limb. We consider it unlikely that the Spring Hill synform formed at the same time as the set 2 FIAs, because it would have developed with a west-east trend, and the regional fold geometry shows no evidence for the large-scale refolding necessary to produce its present geometry

Figure 18. (A) Porphyroblasts that grew during the development of a synform... Available to Purchase

Published: 01 October 2001
Figure 18. (A) Porphyroblasts that grew during the development of a synform should have a predominance of clockwise asymmetries on the western limb and counterclockwise asymmetries on the eastern one. The actual distribution of inclusion trail asymmetry across the Spring Hill synform for each
Image
Figure 4. Patterns of overprinting matrix foliations in the vicinity of the Spring Hill synform. (A) Three near-orthogonal overprinting foliations preserved in a garnet-bearing carbonaceous phyllite from the southern end of the Spring Hill synform (sample v437A, Fig. 3). A subvertical, north-south striking crenulation cleavage, S5, is the youngest fabric developed in this sample and has evolved to stage 3 to 4 of Bell and Rubenach's (1983) model of crenulation cleavage development. An earlier crenulation cleavage, S4, is preserved within the S5 microlithons where it has a subhorizontal dip (the orientation of fabrics in this and other samples was determined from the variation in foliation pitch and asymmetry of curvature in as many as eight differently oriented thin sections). S4 is rotated into a steep westerly dip as it passes into the S5 folia. S4 crenulates an earlier foliation, S3, which in S5 microlithons has a subvertical enveloping dip between spaced S4 folia. Vertical section striking 095 (in this and subsequent photomicrographs the strike direction is shown by half arrow in top right-hand corner) oriented subperpendicular to the intersection of S3, S4, and S5. Plane polarized light. (B) Garnet-mica schist collected from the hinge of a mesoscopic D5 fold on the west limb of the Spring Hill synform (sample v450A, Fig. 3). A weakly developed subvertical crenulation cleavage, S5, symmetrically overprints an earlier crenulation cleavage, S4, which has a subhorizontal enveloping dip. S4 crenulates an earlier foliation, S3, which has a subvertical attitude within microlithons between the well-developed S4 folia. In areas unaffected by S5, S3 is rotated toward a subhorizontal dip as it passes into the S4 folia. Sample v450A. Vertical section (strike given in top right-hand corner) oriented subperpendicular to the intersection of S3, S4, and S5. Plane polarized light

Figure 4. Patterns of overprinting matrix foliations in the vicinity of the... Available to Purchase

Published: 01 October 2001
Figure 4. Patterns of overprinting matrix foliations in the vicinity of the Spring Hill synform. (A) Three near-orthogonal overprinting foliations preserved in a garnet-bearing carbonaceous phyllite from the southern end of the Spring Hill synform (sample v437A, Fig. 3 ). A subvertical, north
Image
Figure 13. The overprinting effects on a pervasive S5 foliation of local weak D6 crenulations with subhorizontal axial planes and top-to-the-east asymmetry. Note that the shallow-dipping (in three dimensions) inclusion trails in the two garnet porphyroblasts are truncated by the matrix, S5. Sample v436A from the east limb of the Spring Hill synform. Vertical section; strike indicated. Plane polarized light

Figure 13. The overprinting effects on a pervasive S 5 foliation of local... Available to Purchase

Published: 01 October 2001
5 . Sample v436A from the east limb of the Spring Hill synform. Vertical section; strike indicated. Plane polarized light
Image
Figure 11. (A) Map of S5. The surface trace of S5 across topography has been superimposed on the surface geology shown in Figure 3. Direction to D5 synform was determined from asymmetry with which S4 curves into S5 folia. Late, steep crenulation cleavages mapped by the U.S. Geological Survey (Ratcliffe and Armstrong, 1995, 1996) and thought to be equivalent to S5 in this study, are also shown. D5 structures have a predominantly synform-to-west asymmetry across the Spring Hill synform. (B) D5 intersection lineations across study area. Note reversal of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(L_{5}^{0}\) \end{document} and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(L_{5}^{4}\) \end{document} plunge direction along the synform

Figure 11. (A) Map of S 5 . The surface trace of S 5 across topography has... Available to Purchase

Published: 01 October 2001
Survey ( Ratcliffe and Armstrong, 1995, 1996 ) and thought to be equivalent to S 5 in this study, are also shown. D 5 structures have a predominantly synform-to-west asymmetry across the Spring Hill synform. (B) D 5 intersection lineations across study area. Note reversal of \batchmode
Image
Figure 5. (A) Sketch of tight D3 folds of S0 and a bedding-parallel foliation, S2, overprinted by open, symmetrical D4 folds. Note subvertical and shallow enveloping dips of S3 and S4, respectively. Structural measurements are given as dip and dip direction. Field locality v227, in the Cambrian Rowe Schist on the east limb of the Spring Hill synform (Fig. 3). (B) Detail of D3 fold hinge overprinted by D4 folds with shallow northwest-dipping axial planes. D3 and D4 axial planes are shown with small and large dashes, respectively. The tight upright folds are identified as D3 because they are overprinted by a set of symmetric folds with shallowly dipping axial planes. In surrounding outcrops, these folds become tighter and develop an intense axial plane foliation, S4 (Fig. 7C), that maintains a relatively consistent dip into areas where it is overprinted by a steeply dipping crenulation cleavage, S5

Figure 5. (A) Sketch of tight D 3 folds of S 0 and a bedding-parallel fo... Available to Purchase

Published: 01 October 2001
, in the Cambrian Rowe Schist on the east limb of the Spring Hill synform ( Fig. 3 ). (B) Detail of D 3 fold hinge overprinted by D 4 folds with shallow northwest-dipping axial planes. D 3 and D 4 axial planes are shown with small and large dashes, respectively. The tight upright folds are identified as D 3
Image
Figure 6. (A) Sketch of a steeply dipping asymmetric D5 fold on west limb of Spring Hill synform. Field locality v212 (Fig. 3). S5 has a subvertical east-west dip and is the youngest foliation developed in this outcrop. D4 folds of S0 and S3 are preserved in the shallow east-dipping short limb where S5 is relatively weakly developed. S4, the axial plane foliation to D4 folds, is progressively rotated from a shallow east dip, through the horizontal, toward a steep west dip as is passes into areas of more intensely developed S5. D4 folds of S0 and S3 are tight and near symmetrical in profile in areas where S5 is relatively weakly developed. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(L_{4}^{0}\) \end{document}\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(L_{4}^{3}\) \end{document} (lineations formed by the intersection of S0 and S4, and S3 and S4, respectively) are approximately parallel to the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(L_{5}^{4}\) \end{document} intersection lineation. (B) Detail of tight, shallow, east-dipping D4 folds of S0 and S3 overprinted by rare, weakly developed, subvertical S5 crenulation cleavages. Note how pre-S5 foliations are rotated toward a steep dip as they pass into the S5 folia (white arrows). Sample v212C. Vertical section (strike given in top right corner) oriented subperpendicular to the intersection of S0/S3, S4, and S5. Plane polarized light

Figure 6. (A) Sketch of a steeply dipping asymmetric D 5 fold on west limb... Available to Purchase

Published: 01 October 2001
Figure 6. (A) Sketch of a steeply dipping asymmetric D 5 fold on west limb of Spring Hill synform. Field locality v212 ( Fig. 3 ). S 5 has a subvertical east-west dip and is the youngest foliation developed in this outcrop. D 4 folds of S 0 and S 3 are preserved in the shallow east-dipping
Image
Frontispiece. Aerial photograph with view to the northeast, emphasizing an asymmetric anticline (Beer Mug Anticline; cored by Pennsylvanian Tensleep Ss. and flanked by Permian Goose Egg Fm.) thrust atop a less obvious, synformal anticline (Ellis Ranch Anticline of Taylor, 1996; cored by Triassic Red Peak Fm. and stratigraphically overlain by the white, more clearly defined, Triassic Alcova Ls.). The steeply north-dipping thrust fault between the anticlines (Indian Spring Fault) is exposed only near the photo's lower-left corner, and it strikes northeastward from there, hidden by the slopes of erosional debris. The camera is above the northern edge of the eastern Hanna Basin, and the general view is across southern parts of the adjacent Freezeout Hills. This image was selected for use as the Frontispiece jointly because of its clarity and for being emblematic of the extraordinary degree of Laramide structural complexity characteristic of the Hanna Basin area — and particularly so along the basin's northern and southern margins. For structural details, see Taylor (1996, p. 31–50 and, especially, fig. 7). Photograph courtesy of Dan Hayward (©Copyright 2011; HaywardPhoto.com, image taken July 30, 2011).

Frontispiece . Aerial photograph with view to the northeast, emphasizing an... Open Access

Published: 01 July 2015
Frontispiece . Aerial photograph with view to the northeast, emphasizing an asymmetric anticline (Beer Mug Anticline; cored by Pennsylvanian Tensleep Ss. and flanked by Permian Goose Egg Fm.) thrust atop a less obvious, synformal anticline (Ellis Ranch Anticline of Taylor, 1996 ; cored by Triassic