Changes

Modern theories of structural geology generally relate the formation of folds to accommodation on irregular fault surfaces.<ref name=Hamblin_1965>Hamblin, W. K., 1965, Origin of "reverse drag" on the downthrown side of normal faults: Geological Society of America Bulletin, v. 76, p. 1145-1164.</ref> <ref name=Dahlstrom_1970 />) Generally, the folds are more obvious on seismic sections than faults, but fortunately there are geometric rules that allow us to predict one shape from the other<ref name=Suppe_1983>Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283, p. 684-721.</ref> <ref name=Verrall_1982>Verrall, P., 1982, Structural interpretation with applications to North Sea problems: Geological Society of London.Course Notes No 3, JAPEC (UK).</ref> <ref name=Gibbs_1983 />; Williams and Vann, 1987<ref name=Williams_etal_1987>Williams, G., and I. Vann, 1987, The geometry of listric normal faults and deformation in their hanging walls: Journal of Structural Geology, v. 9, p. 789-795.</ref> <ref name=Groshong_1989a /> in both extensional and compressional examples. An example of a cross section solution explaining the relationship between extensional rollover and listric faults is shown in [[:Image:Drive-mechanisms-and-recovery_fig1.png|Figure 4]].  

Modern theories of structural geology generally relate the formation of folds to accommodation on irregular fault surfaces.<ref name=Hamblin_1965>Hamblin, W. K., 1965, Origin of "reverse drag" on the downthrown side of normal faults: Geological Society of America Bulletin, v. 76, p. 1145-1164.</ref> <ref name=Dahlstrom_1970 />) Generally, the folds are more obvious on seismic sections than faults, but fortunately there are geometric rules that allow us to predict one shape from the other<ref name=Suppe_1983>Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283, p. 684-721.</ref> <ref name=Verrall_1982>Verrall, P., 1982, Structural interpretation with applications to North Sea problems: Geological Society of London.Course Notes No 3, JAPEC (UK).</ref> <ref name=Gibbs_1983 />; Williams and Vann, 1987<ref name=Williams_etal_1987>Williams, G., and I. Vann, 1987, The geometry of listric normal faults and deformation in their hanging walls: Journal of Structural Geology, v. 9, p. 789-795.</ref> <ref name=Groshong_1989a /> in both extensional and compressional examples. An example of a cross section solution explaining the relationship between extensional rollover and listric faults is shown in [[:Image:Drive-mechanisms-and-recovery_fig1.png|Figure 4]].  

[[File:Drive-mechanisms-and-recovery fig1.png|thumbnail|'''Figure 4.''' Modeling extensional fault shapes from the rollover geometry. (a) the groshong<ref>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses oblique simple shear with a reference grid constructed with a spacing equal to the fault heave. Distance 2 from the rollover up to regional elevation of the same reference bed is transferred to 2&prime;; likewise, 2&prime; + 4 is transferred to 4&prime; and so on to complete the fault trajectory. Interpolation between these points is carried out using a half grid spacing. (b) fault trajectory reconstruction by the groshong<ref>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpubl. Data, 1988.)]]

[[File:Drive-mechanisms-and-recovery fig1.png|thumbnail|'''Figure 4.''' Modeling extensional fault shapes from the rollover geometry. (a) the Groshong<ref name=Groshong_1989a /> method uses oblique simple shear with a reference grid constructed with a spacing equal to the fault heave. Distance 2 from the rollover up to regional elevation of the same reference bed is transferred to 2&prime;; likewise, 2&prime; + 4 is transferred to 4&prime; and so on to complete the fault trajectory. Interpolation between these points is carried out using a half grid spacing. (b) fault trajectory reconstruction by the Groshong<ref name=Groshong_1989a /> method uses simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpubl. Data, 1988.)]]

===Balanced cross sections===

===Balanced cross sections===

Balanced cross sections are used to test the viability or admissibility of a cross section. The deformed cross section is redrawn on a template in the undeformed state so that the beds are unfolded and the offsets on the faults removed ([[:Image:Drive-mechanisms-and-recovery_fig2.png|Figure 5]]). Section balancing requires reference pin-lines and loose lines at opposite ends of the section from which measurements of bed lengths are made. Bed thicknesses and bed lengths are generally retained so that the deformed and undeformed cross sections have the same area. For an ideal restoration, there should be no gaps or overlaps between adjacent fault blocks.<ref name=Woodward_etal_1985 />)

Balanced cross sections are used to test the viability or admissibility of a cross section. The deformed cross section is redrawn on a template in the undeformed state so that the beds are unfolded and the offsets on the faults removed ([[:Image:Drive-mechanisms-and-recovery_fig2.png|Figure 5]]). Section balancing requires reference pin-lines and loose lines at opposite ends of the section from which measurements of bed lengths are made. Bed thicknesses and bed lengths are generally retained so that the deformed and undeformed cross sections have the same area. For an ideal restoration, there should be no gaps or overlaps between adjacent fault blocks.<ref name=Woodward_etal_1985 />)

Balanced sections were first constructed for thrust belts, but Gibbs<ref name=Gibbs_1983>Gibbs, A. D., 1983, Balanced cross section construction from seismic sections in areas of extensional tectonics: Journal of Structural Geology, v. 5, p. 153-160.</ref>, Groshong<ref name=Groshong_1989a>Groshong, R. H., 1989a, Half graben structures--balanced models of extensional fault bend folds: Geological Society of America Bulletin, v. 101, p. 96-105.</ref>, and Rowan and Kligfield<ref name=Rowan_etal_1989>Rowan, M. G., and R. Kligfield, 1989, Cross section restoration and balancing as aid to [[seismic interpretation]] in extensional terranes: AAPG Bulletin, v. 73, p. 955-966.</ref> have successfully applied the method to extensional and salt-related structures. Extensional section balancing is more difficult than compressional balancing because of the bed thickness changes that occur across faults. The balancing template has to show these thickness changes accurately. Generally, computer-aided methods are essential because they can sequentially backstrip the section to remove tectonic as well as compaction strains. Examples of these are described by Rowan and Kligfield,<ref name=Rowan_etal_1989 /> Worrall and Snelson,<ref name=Worrall_etal_1989>Worrall D. M., and S. Snelson, 1989, Evolution of the northern Gulf of Mexico with emphasis on Cenozoic growth faulting and the role of salt, in A. W. Bally and A. R. Palmer, The Geology of North America--An Overview: Geological Society of America, v. A, p. 97-138.</ref> and Shultz-Ela and Duncan.<ref>Schultz-Ela, D., and Duncan, K., 1990, Users manual and software for Restore, version 2.0: The Univ. of Texas Bureau of Economic Geology, 75 p.</ref>

Balanced sections were first constructed for thrust belts, but Gibbs,<ref name=Gibbs_1983>Gibbs, A. D., 1983, Balanced cross section construction from seismic sections in areas of extensional tectonics: Journal of Structural Geology, v. 5, p. 153-160.</ref> Groshong,<ref name=Groshong_1989a /> and Rowan and Kligfield<ref name=Rowan_etal_1989>Rowan, M. G., and R. Kligfield, 1989, Cross section restoration and balancing as aid to [[seismic interpretation]] in extensional terranes: AAPG Bulletin, v. 73, p. 955-966.</ref> have successfully applied the method to extensional and salt-related structures. Extensional section balancing is more difficult than compressional balancing because of the bed thickness changes that occur across faults. The balancing template has to show these thickness changes accurately. Generally, computer-aided methods are essential because they can sequentially backstrip the section to remove tectonic as well as compaction strains. Examples of these are described by Rowan and Kligfield,<ref name=Rowan_etal_1989 /> Worrall and Snelson,<ref name=Worrall_etal_1989>Worrall D. M., and S. Snelson, 1989, Evolution of the northern Gulf of Mexico with emphasis on Cenozoic growth faulting and the role of salt, in A. W. Bally and A. R. Palmer, The Geology of North America--An Overview: Geological Society of America, v. A, p. 97-138.</ref> and Shultz-Ela and Duncan.<ref>Schultz-Ela, D., and Duncan, K., 1990, Users manual and software for Restore, version 2.0: The Univ. of Texas Bureau of Economic Geology, 75 p.</ref>

[[File:Drive-mechanisms-and-recovery fig2.png|thumbnail|left|'''Figure 5.''' Example of a balanced section through a complex thrust ramp structure showing both the deformed and undeformed sections.<ref name=Mitra_1986>Mitra, S., 1986, Duplex structures and imbricate thrust systems--geometry, structural position, and hydrocarbon potential: AAPG Bulletin, v. 70, p. 1087-1112.</ref>]]

[[File:Drive-mechanisms-and-recovery fig2.png|thumbnail|left|'''Figure 5.''' Example of a balanced section through a complex thrust ramp structure showing both the deformed and undeformed sections.<ref name=Mitra_1986>Mitra, S., 1986, Duplex structures and imbricate thrust systems--geometry, structural position, and hydrocarbon potential: AAPG Bulletin, v. 70, p. 1087-1112.</ref>]]