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Fold propagation and interference in a single multilayer unit
Tectonophysics, 34 (1976) T37-T42 o Elsevier Scientific Publishing Company,
T37 Amsterdam
- Printed
in The Netherlands
Letter Section Fold propagation and interference in a single multilayer unit
A. J. WATKINSON* Department (Submitted
of Geology, Washington State University, Pullman, Washington (U.S.A.) May 31; accepted
for publication
July 27, 1976)
ABSTRACT Watkinson, A.J., 1976. Fold propagation Tectonophysics, 34: T37-T42.
and interference
in a single multilayer
unit.
Model analogue experiments show that folds will often initiate at several discrete points along a layer or multilayer under compression. With continuing deformation, a characteristic fold geometry emerges of regular folds or periodic sets of folds interspersed by interference zones - zones where propagating fold sets have converged and are out of phase. Characteristic geometries in interference zones appear to be localized ‘dead’ zones of no folding, anomalously asymmetric folds or anomalous span-length folds. Some interference folds are unstable and hinge adjustments continue to occur at high fold amplitude and limb dips. An example of natural folds showing geometric features closely analogous to those observed in the models is illustrated.
INTRODUCTION
The localized initiation of folds, at either material or induced heterogeneities along the length of a layer under compression, has been predicted on theoretical grounds (e.g. Biot et al., 1961; Johnson, 1970; Cobbold et al., 1971; Watkinson and Cobbold, in preparation) and illustrated using model analogues (e.g. Willis, 1894; Cobbold, 1975; Price, 1975; Watkinson, 1975). Furthermore Cobbold (1975) has suggested that if localized fold complexes initiate at inhomogeneities which are closely spaced along the length of a layer, then as these folds propagate during a finite interval of deformation, there will often be a mismatch at the areas where fold complexes meet. This interference may cause localized zones of no folding (see also Biot et al., 1961, p. 1627) or of folding with irregular periodicity (Cobbold, 1976). This paper illustrates in a qualitative way some of the distinctive fold interference geometries formed in model analogue experiments. Some analogous fold geometries from a natural example are then illustrated. *Present
address:
Geology
Department,
Trinity
College,
Dublin,
Ireland.
T38
MODEL
ANALOGUES
Multilayer systems were constructed using modelling clay with non-linear rheologies and of differing competencies. The system was composed of a regular multilayer unit, set in a softer uniform matrix, and was deformed in a simple press using hydraulic pistons. An example showing initiation of folds, apparently at several discrete points along the multilayer unit, is shown in Fig. 1. This model consists of a multilayer unit composed of lubricated layers of equal thickness and competence. The unit was set into a uniform matrix of lower competence than the layers, at an initial angle of ten degrees to the principal compression direction. No obvious or intentional variations in thickness were set into the layers. The first obvious expressions of folding in the centre of the model (away from the end boundary folds) appear to be three discrete zones of localized folds (Fig. 1A) with a central, obviously asymmetric fold of higher amplitude. With further compression (Fig. lB), the folds amplify forming cusp/chevron/
raA.
I.
T39
concentric fold profiles - a function of the competence contrast between the unit and the matrix (e.g. see Johnson and Honea, 1975). It appears that at this initial stage of folding, no readjustment towards forming a periodic set of folds occurs. Note that the folds are not in phase, so during the continuing deformation, two interference zones between the folds become apparent. The first interference zone is a ‘dead’ zone (Fig. 1B) which develops to a double cusp/box fold geometry (Fig. 1C). In the other interference zone a cusp/chevron/concentric fold is developed between the
Fig. 1. Stages in the deformation of a multilayer unit. A. Three areas of initiating folds. Presumably the folds initiated at small heterogeneities along the layers. B. Two interference zones become apparent. C. Amplification of the initiating folds and the converging of the cusps in the interference zone. D. Convergence to a single cusp point in the interference zone. R = ‘regular’ span folds, S = small span folds, L = large span folds.
T40
two dominant folds, but it has a significantly smaller span than the dominant folds (Fig. 1C). With further deformation the initial folds, which now have a very close span length, continue to amplify (Fig. 1D). The double cusp structure is, however, unstable and starts to converge to form a single cusp point, creating two anomalously large span folds on either side of the now single cusp point (Fig. 1D). Note that the convergence of this double cusp structure occurs at finite fold amplitudes, showing that in this particular model some fold spans adjust even at high fold amplitudes and limb dips (see also Hara et al., 1975, p. 129). Concomitantly with the development of these interference patterns, new folds appear by lateral propagation from the end boundary folds, creating new zones of interference with the centre folds. Similar interference patterns were developed in many other model experiments. In other models, sometimes sets (or complexes) of several regular folds would develop before interference occurred (Fig. 2). This was due to the fact that the folds initiated at areas spaced further apart than in the example illustrated in Fig. 1. In experiments where the system was composed of more than one multilayer unit set in a softer matrix, the propagation history and interference geometry become more complex (Watkinson and Cobbold, in preparation). NATURAL
FONDS
It is emphasized at this point that although fold geometries analogous to those described from the models can be observed in natural folds, it is impossible to treat such geometries as unique evidence of interference by fold propagation. The purpose of the paper is to demonstrate one viable mechanism of creating such geometries, there may, of course, be others.
Fig. 2. Interference zone between two sets of regular folds.
T41 The anticlinal fold illustrated in Fig. 3 is from the north side of Bovisands Bay, S. Devon (Natl. grid ref. SX490507). It consists of a multilayer unit set in an apparently uniform shale matrix. There are several minor parasitic folds which have cusp/che~on/concen~ic profiles. In the hinge zone of the large fold there is a set of near-periodic folds with a central symmetric fold. In the limb zones, the minor folds are asymmetric. A double cusp/box fold form can be observed (Fig. 3B), which may be the expression of the propagation of folds from either side of this zone, converging to form an interference fold.
Fig. 3. A. Anticline from Bovisands, S. Devon.
B. Line drawing of the folds to show the discrete nature of the parasitic folds in the upper limb (small rings) and the box fold geometry in a possible interference zone (large ring).
T42
Note also that minor folds further along this upper limb are localized, nonperiodic folds, but have approximately equal span lengths. This again may be an expression of folds vitiating at separate discrete points along the layers although no obvious material inhomogeneities such as sedimentary nodules, which could have triggered the initiation of the folds, were discernible in the layers. CONCLUSIONS
Observations from multilayer experiments suggest that when propagating folds converge and are out of phase, certain distinctive fold geometries may result. A distinctive geometry is that of regular folds, or sets of folds, intersperse d by either: (a) ‘dead’ zones; or (b) anomalous small or large span folds; or (c) anomalously asymmetric folds. In some interference zones the folds may be unstable and, during further compression, adjustment by hinge migration will take place at high fold amplitude and limb dip. Some of these distinctive fold profile geometries can be observed in natural folds and the model analogues suggest one viable mechanism for the creation of such geometries. ACKNOWLEDGEMENTS This research was financed by the National Science Foundation, grant no. DES 74-05677. I would like to thank Mike Broth and Will Shaw for their enthusiastic help with the models, Peter Cobbold for his invaluable comments, and Dave Hobson for help in locating the Devon fold. REFERENCES Biot, M.A., Ode, H. and De Roever, W.L., 1961. Experimental verification of the folding of stratified viseoelastic media. Geol. Sot. Am. Bull., 72: 1621-1632. Cobbold, P.R., 1975. Fold propagation in single embedded layers. Tectonophysics, 27: 333-351. Cobbold, P.R., 1976. Fold shapes as functions of progressive strain. Philos. Trans. R. Sot. Lond., Ser. A, (in press). Cobbold, P.R., Cosgrove, J.M. and Summers, J.M., 1971. Development of internal structures in deformed anisotropic rocks. Tectonophysics, 12: 23-53. Hara, I,, Yokoyama, S., Tsukuda, E. and Shiota, T., 1975. Three-dimensional size analysis of folds of quartz veins in the psammitic schist of the Oboke District, Shikoku. J. Sci. Hiroshima Univ. C. 7(3): 125-132. Johnson, A.M., 1970. Physical Processes in Geology. Freeman-Cooper, San Francisco, Calif., 577 pp. Johnson, A.M. and Honea, E., 1975. A theory of concentric, kink and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers, III. Tectonophysics, 27: l-38. Price, N.J., 1976. Rates of deformation. J. Geol. Sot. Lond., 131: 553-676. Watkinson, A.J., 1975. Multilayer folds initiated in bulk plane strain, with the axis of no change perpendicular to the layering. Tectonophysics, 28: 7-11. Willis, B., 1894. Mechanics of Appalachian structure. 13th Ann. Rep. U.S. Geol. Surv., pp. 213-281.
T37 Amsterdam
- Printed
in The Netherlands
Letter Section Fold propagation and interference in a single multilayer unit
A. J. WATKINSON* Department (Submitted
of Geology, Washington State University, Pullman, Washington (U.S.A.) May 31; accepted
for publication
July 27, 1976)
ABSTRACT Watkinson, A.J., 1976. Fold propagation Tectonophysics, 34: T37-T42.
and interference
in a single multilayer
unit.
Model analogue experiments show that folds will often initiate at several discrete points along a layer or multilayer under compression. With continuing deformation, a characteristic fold geometry emerges of regular folds or periodic sets of folds interspersed by interference zones - zones where propagating fold sets have converged and are out of phase. Characteristic geometries in interference zones appear to be localized ‘dead’ zones of no folding, anomalously asymmetric folds or anomalous span-length folds. Some interference folds are unstable and hinge adjustments continue to occur at high fold amplitude and limb dips. An example of natural folds showing geometric features closely analogous to those observed in the models is illustrated.
INTRODUCTION
The localized initiation of folds, at either material or induced heterogeneities along the length of a layer under compression, has been predicted on theoretical grounds (e.g. Biot et al., 1961; Johnson, 1970; Cobbold et al., 1971; Watkinson and Cobbold, in preparation) and illustrated using model analogues (e.g. Willis, 1894; Cobbold, 1975; Price, 1975; Watkinson, 1975). Furthermore Cobbold (1975) has suggested that if localized fold complexes initiate at inhomogeneities which are closely spaced along the length of a layer, then as these folds propagate during a finite interval of deformation, there will often be a mismatch at the areas where fold complexes meet. This interference may cause localized zones of no folding (see also Biot et al., 1961, p. 1627) or of folding with irregular periodicity (Cobbold, 1976). This paper illustrates in a qualitative way some of the distinctive fold interference geometries formed in model analogue experiments. Some analogous fold geometries from a natural example are then illustrated. *Present
address:
Geology
Department,
Trinity
College,
Dublin,
Ireland.
T38
MODEL
ANALOGUES
Multilayer systems were constructed using modelling clay with non-linear rheologies and of differing competencies. The system was composed of a regular multilayer unit, set in a softer uniform matrix, and was deformed in a simple press using hydraulic pistons. An example showing initiation of folds, apparently at several discrete points along the multilayer unit, is shown in Fig. 1. This model consists of a multilayer unit composed of lubricated layers of equal thickness and competence. The unit was set into a uniform matrix of lower competence than the layers, at an initial angle of ten degrees to the principal compression direction. No obvious or intentional variations in thickness were set into the layers. The first obvious expressions of folding in the centre of the model (away from the end boundary folds) appear to be three discrete zones of localized folds (Fig. 1A) with a central, obviously asymmetric fold of higher amplitude. With further compression (Fig. lB), the folds amplify forming cusp/chevron/
raA.
I.
T39
concentric fold profiles - a function of the competence contrast between the unit and the matrix (e.g. see Johnson and Honea, 1975). It appears that at this initial stage of folding, no readjustment towards forming a periodic set of folds occurs. Note that the folds are not in phase, so during the continuing deformation, two interference zones between the folds become apparent. The first interference zone is a ‘dead’ zone (Fig. 1B) which develops to a double cusp/box fold geometry (Fig. 1C). In the other interference zone a cusp/chevron/concentric fold is developed between the
Fig. 1. Stages in the deformation of a multilayer unit. A. Three areas of initiating folds. Presumably the folds initiated at small heterogeneities along the layers. B. Two interference zones become apparent. C. Amplification of the initiating folds and the converging of the cusps in the interference zone. D. Convergence to a single cusp point in the interference zone. R = ‘regular’ span folds, S = small span folds, L = large span folds.
T40
two dominant folds, but it has a significantly smaller span than the dominant folds (Fig. 1C). With further deformation the initial folds, which now have a very close span length, continue to amplify (Fig. 1D). The double cusp structure is, however, unstable and starts to converge to form a single cusp point, creating two anomalously large span folds on either side of the now single cusp point (Fig. 1D). Note that the convergence of this double cusp structure occurs at finite fold amplitudes, showing that in this particular model some fold spans adjust even at high fold amplitudes and limb dips (see also Hara et al., 1975, p. 129). Concomitantly with the development of these interference patterns, new folds appear by lateral propagation from the end boundary folds, creating new zones of interference with the centre folds. Similar interference patterns were developed in many other model experiments. In other models, sometimes sets (or complexes) of several regular folds would develop before interference occurred (Fig. 2). This was due to the fact that the folds initiated at areas spaced further apart than in the example illustrated in Fig. 1. In experiments where the system was composed of more than one multilayer unit set in a softer matrix, the propagation history and interference geometry become more complex (Watkinson and Cobbold, in preparation). NATURAL
FONDS
It is emphasized at this point that although fold geometries analogous to those described from the models can be observed in natural folds, it is impossible to treat such geometries as unique evidence of interference by fold propagation. The purpose of the paper is to demonstrate one viable mechanism of creating such geometries, there may, of course, be others.
Fig. 2. Interference zone between two sets of regular folds.
T41 The anticlinal fold illustrated in Fig. 3 is from the north side of Bovisands Bay, S. Devon (Natl. grid ref. SX490507). It consists of a multilayer unit set in an apparently uniform shale matrix. There are several minor parasitic folds which have cusp/che~on/concen~ic profiles. In the hinge zone of the large fold there is a set of near-periodic folds with a central symmetric fold. In the limb zones, the minor folds are asymmetric. A double cusp/box fold form can be observed (Fig. 3B), which may be the expression of the propagation of folds from either side of this zone, converging to form an interference fold.
Fig. 3. A. Anticline from Bovisands, S. Devon.
B. Line drawing of the folds to show the discrete nature of the parasitic folds in the upper limb (small rings) and the box fold geometry in a possible interference zone (large ring).
T42
Note also that minor folds further along this upper limb are localized, nonperiodic folds, but have approximately equal span lengths. This again may be an expression of folds vitiating at separate discrete points along the layers although no obvious material inhomogeneities such as sedimentary nodules, which could have triggered the initiation of the folds, were discernible in the layers. CONCLUSIONS
Observations from multilayer experiments suggest that when propagating folds converge and are out of phase, certain distinctive fold geometries may result. A distinctive geometry is that of regular folds, or sets of folds, intersperse d by either: (a) ‘dead’ zones; or (b) anomalous small or large span folds; or (c) anomalously asymmetric folds. In some interference zones the folds may be unstable and, during further compression, adjustment by hinge migration will take place at high fold amplitude and limb dip. Some of these distinctive fold profile geometries can be observed in natural folds and the model analogues suggest one viable mechanism for the creation of such geometries. ACKNOWLEDGEMENTS This research was financed by the National Science Foundation, grant no. DES 74-05677. I would like to thank Mike Broth and Will Shaw for their enthusiastic help with the models, Peter Cobbold for his invaluable comments, and Dave Hobson for help in locating the Devon fold. REFERENCES Biot, M.A., Ode, H. and De Roever, W.L., 1961. Experimental verification of the folding of stratified viseoelastic media. Geol. Sot. Am. Bull., 72: 1621-1632. Cobbold, P.R., 1975. Fold propagation in single embedded layers. Tectonophysics, 27: 333-351. Cobbold, P.R., 1976. Fold shapes as functions of progressive strain. Philos. Trans. R. Sot. Lond., Ser. A, (in press). Cobbold, P.R., Cosgrove, J.M. and Summers, J.M., 1971. Development of internal structures in deformed anisotropic rocks. Tectonophysics, 12: 23-53. Hara, I,, Yokoyama, S., Tsukuda, E. and Shiota, T., 1975. Three-dimensional size analysis of folds of quartz veins in the psammitic schist of the Oboke District, Shikoku. J. Sci. Hiroshima Univ. C. 7(3): 125-132. Johnson, A.M., 1970. Physical Processes in Geology. Freeman-Cooper, San Francisco, Calif., 577 pp. Johnson, A.M. and Honea, E., 1975. A theory of concentric, kink and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers, III. Tectonophysics, 27: l-38. Price, N.J., 1976. Rates of deformation. J. Geol. Sot. Lond., 131: 553-676. Watkinson, A.J., 1975. Multilayer folds initiated in bulk plane strain, with the axis of no change perpendicular to the layering. Tectonophysics, 28: 7-11. Willis, B., 1894. Mechanics of Appalachian structure. 13th Ann. Rep. U.S. Geol. Surv., pp. 213-281.