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LECTURES 9 POLYPHASE STRUCTURES LECTURE PLAN 1) INTRODUCTION 2) CHARACTERISTICS OF POLYPHASE DEFORMATION 3) TERMINOLOGY USED IN POLYPHASE TERRAINS 4) FOLD INTERFERENCE PATTERNS 5) SUPERPOSED LINEATIONS 6) SUPERPOSED FOLIATIONS 7) SHEATH FOLDS 8) MAPPING OF SUB-AREAS Undeformed cover sedimentary rocks Folded basement metamorphic rocks Time 1 Folded basement metamorphic rocks are unconformably overlain by unfolded cover sedimentary rocks. A second phase of deformation folds the cover rocks and re-folds the basement rocks to form polyphase folds in the basement. The polyphase folds have folded axial planes. 1) INTRODUCTION In many continental terrains the crust may be divided into a basement which may contain folds, faults and fractures acquired during early orogenic phases, unconformably overlain by undeformed sedimentary or volcanic rocks which are termed cover. When such an area undergoes a new phase of orogenic activity, the cover rocks may develop relatively simple structures such as the faults, folds fracture systems described in earlier lectures. However, the development of new structures in the basement rocks which already contain structures leads to the development of complex structures known as polyphase structures such as superposed folds. The unconformity between the cover and basement rocks may represent substantial erosion which can only be produced during a long period of time. Thus, two tectonic phases that produce polyphase structures in basement rocks may be separated by relatively long periods of time (50 - >500Ma). Back to text Time 2 Course Homepage Lecture 1 2 3 4 Contact Staff 5 6 7 8 9 10 Practical 1 2 3 4 5 6 71 8 9 10 2) CHARACTERISTICS OF POLYPHASE DEFORMATION If an area has undergone only one phase of deformation, then poles to bedding are generally distributed about a great circle when plotted on a stereonet (see stereonet practicals). Minor fold axis are parallel to major fold axis, and cleavages are generally axial planar to folds. If an area has undergone more than one phase of deformation then more complex structures develop which contain:1) A wide distribution of bedding attitudes whose poles do not plot on a great circle on a stereonet. Poles to bedding fan around the fold ld Fo axi Poles Fold axis s Apparently random poles for a re-folded fold 2) Fold interference patterns 3) Foliations and lineations of an earlier deformation which are folded. Back to text 4) Superposition of later fabrics 3) TERMINOLOGY USED IN POLYPHASE TERRAINS Deformation 1st = D1 2nd = D2 3rd = D3 Fold Phase Axial-planar foliation Lineations F1 S1 L1 (L1 in So) F2 S2 L2 F3 S3 L3 Start of this Lecture 4) FOLD INTERFERENCE PATTERNS Type 1 Three basic end-member of a continuous series of interference patterns can be recognised. Type 1- This pattern arises when both F1 and F2 fold axial planes and fold axes are orthogonal to each other, with the fold axes originally co-planar. The F1 axial planes remain unfolded, but the F1 fold axes are themselves folded, running across domes and basins. Often called an egg box or dome and basin pattern. F fold axis 1 F fold axial plane 1 F fold axis Back to text 2 F fold axial plane Type 3 2 F fold axial plane These fold interference patterns only appear if the erosion surface through the rock is in a certain orientation. 2 F fold axis 1 F fold axis F1 fold axial plane Start of this Lecture Back to text Type 2 2 Type 2- This occurs when the F1 and F2 fold axial planes are orthogonal to each other, but the F2 fold axis lies within the F1 fold axial plane and at 90o to theF1 fold axis. The F1 fold axial plane is re-folded. Often called the mushroom pattern because some of the fold closures are attached to stalks unlike the completely closed forms of Type 1. Type 3- This occurs when the F1 and F2 fold axial planes are orthogonal to each other, but the F1 and F2 axes are parallel. F1 fold axial planes are folded. This is called the re-folded fold pattern. 2 F fold axial plane 1 F fold axial plane Back to text 5) SUPERPOSED LINEATIONS 6) SUPERPOSED FOLIATIONS Linear structures have a wide variety of orientations within polyphase structures as early lineations are folded and new lineations are formed. Remember that there are two classes of lineations related to folding:1) Lineations parallel to the fold axes (You can call this the F direction). 2) Lineations parallel to the stretching direction or X direction which is generally at right angles to the fold axes (You can call this the X direction). In areas which have been affected by numerous deformation phases, early foliations (S1) may become folded and new foliations (S2) may develop. The second foliation is usually a crenulation cleavage which intersects the first foliation to produce a crenulation lineation (L2 in S1). The crenulation foliation may also intersect bedding (S0) to produce a crenulation lineation (L2 in So). Both F and X lineations can be found related to F1 and F2 folds. - Lineations formed during the first folding (F1 and/or X1 directions) phase may show a curvilinear form as they are folded by F2 folds. - Lineations formed by the second phase of folding are orientated parallel to the F2 fold axes (F2 direction) or parallel to the stretching direction of the the D2 strain ellipsoid (X2 direction). In areas of only two phases of folding, the crenulation foliation S2, (approximately axial planar to the second phase of folds) will be fairly constant in orientation, whereas the orientation of L2 in S1 will vary according to the orientation of S1 within the first phase fold structures (i.e it will be different within the different parts of the same structure and between types of interference patterns). 3D view of the F1 axial planar cleavage (S1) intersecting the F2 axial planar cleavage (S2) F1 axial planar cleavage (S1) B 2 F axial planar 2 cleavage (S ) in this orientation F2 fold axial plane A The L2 intersection lineation in S2 changes in orientation spatially. It is almost horizontal at A and steeply dipping at B. Back to text In this Type 3 re-folded fold, L1 linear structures at right angles to the fold axis have been folded, whilst those parallel to the fold axis have not. Back to text Start of this Lecture 7) SHEATH FOLDS Where strains vary across a shear zone, layers can become stretched into tube-like shapes known as sheath folds. They occur in high strain rocks within shear zones. Strictly, they are not polyphase folds because they usually form during a single phase of deformation with spatially varying strain. The fold axes are curved, having been rotated almost parallel to the sides of the zone of high strain during progressive deformation. Sheath folds can be recognised from their sub-circular outcrop patterns when viewed down the transport direction 8) MAPPING OF SUB-AREAS B) Cross-section A) 3D view of a sheath folds Plane of cross-section A distance B A distance C) Graph showing how strain varies across the shear zone B strain Back to text Formation of a sheath fold Time 2 Time 1 In areas of superimposed folding the division of structurally homogeneous subareas is essential in order to analyse the overall structure. Time 4 fold axis Time 3 The division should be made whilst in the field so that you can find exposures with which to test your sub-divisions. Back to text Sheath folds can be recognised from their sub-circular outcrop patterns when viewed down the transport direction Start of this Lecture Back to text Division into sub-areas is based on:- Examples:- 1) Areas where there is a constant orientation of a particular generation of lineation. 1) Lineations deformed by flexural slip folds retain a constant angle between L1 and F1. This occurs because flexural slip folding does not strain the folded layers (the layers do not change shape and retain internal angles). 2) Areas where there is a constant orientation of a particular foliation. 3) Areas where there is a constant fold axis orientation. 4) Areas distinguished by the axial surfaces of the various fold structures, i.e. vergence boundaries. The sizes of the sub-areas will be constrained by the spacing of fold axial planes. 2) Lineations deformed by similar folds produced by tangential longitudinal slip (TLS Folds) with high strains, do not retain a constant angle between L1 and F1. This occurs because the folded layer becomes strained (i.e. it changes its shape so that internal angles are altered. The effects of folding mechanisms on the deformation of lineations (A cautionary tale concerning stereographic rotation of lineations to their pre-folding orientation) - The style of folding can influence the orientation of lineations within folds. If the beds are significantly strained then the angular relationships between between lines on the originally un-folded surface are changed. This is important when considering palaeo-current data from folded strata. If the layers are highly strained (e.g. isoclinal similar folds produced by the TLS folding mechanism) then simple stereographic rotation of lineations on bedding surfaces (e.g. flute casts) will produce erroneous palaeocurrent directions. Therefore only use simple stereographic rotation of palaeo-current data in rocks which have low strains. Start of this Lecture FURTHER READING AVAILABLE FROM THE ELECTRONIC LIBRARY S. K. Ghosh, S. Hazra and S. Sengupta 1999. Planar, non-planar and refolded sheath folds in the Phulad Shear Zone, Rajasthan, India, Journal of Structural Geology, 21, 1715-1729 G. I. Alsop and R. E. Holdsworth, 1999. Vergence and facing patterns in large-scale sheath folds, Journal of Structural Geology, 21, 1335-1349 F2 fold axis At Badcall, N.W. Scotland, Lewisian Gneisses have a foliation that has a vertical dip, but the strike changes from WNW-ESE to NW-SE. This large fold therefore has a vertical fold axis. The gneissic layers are composed of an earlier set of isoclinal folds. The overall structure is a Type 3 re-folded fold geometry. The vertical fold axis (green dashed line) is the F2 axis. One of our students, Ron Million in the red coat and flat cap, has spent many years mapping this structure and the surrounding area. F1 Folds in second photo View towards the WNW View looking ESE These are F1 folds at Badcall. Ron is standing close to where he was in the first photo F1 folds curving around from WNW to the NW due to the larger F2 fold. View towards the WNW Can you spot the F1 fold? View looking NW. Can you spot the F1 folds at the feet of another student, Ioannis Papanikolaou? View to the NW, with more F1 folds My Gn loni eis tic se s Next photo Thrust-sense shear zone containing mylonitic gneisses within less-deformed Lewisian Gneisses, Scotland. Steve Hirons for scale. The sheared foliations are again isoclinal folds of light (quartz and feldspar rich) and dark (amphibole-rich) gneiss. The shear zone folds the F1 folds into a Type 3 refold. A mafic layer in the core of an F1 fold is re-folded by a shear zone that formed in D2. A smaller D2 shear zone occurs above the lens cap. The sense of refolding is Type 3. Mylo nites A mafic layer in the core of an F1 fold is re-folded by a shear zone that formed in D2. The sense of re-folding is Type 3. Mylo nites M ylo ni te s A mafic layer in the core of an F1 fold is re-folded by a shear zone that formed in D2. A smaller D2 shear zone occurs within the larger shear zone. The sense of re-folding is Type 3. View of a sheath fold within Lewisian gneissic layering, N. W. Scotland Sketch Note closed or “Bulls eye” pattern indicating a sheath fold Another view of the sheath fold. A view of two closed outcrop patterns of gneissic foliation defining sheath folds (an antiform and a synform). The stretching lineation (green lines) in these metamorphic rocks is parallel to the elongation of the sheaths. Synform Antiform = dip of the foliation in this map view photograph