No part of this book may be reproduced in any way, or transmitted, or translated into a machine language without the written permission of the publisher. In all cases emphasis is placed on systematic field observations, accurate measurements of the east.
Bedding
Their fold axes are usually distributed in the plane of the sagging skin and recum. 4 The downthrown side of the fault is usually filled with a triangular wedge of sediments (Fig. 1.3a) which in some cases can be coarser grained than the surrounding sediments.
Basic References
Safety
2 Mapping techniques
Equipment
For structural mapping you need a clinometer with a compass that meets the following requirements; (1) accuracy, (2) reliability, (3) ease of operation, and, if possible, (4) one containing a bubble balance. For structural geology, the Freiberg compass has distinct advantages, as the lid on the hinges (Figure 2.1b) is placed opposite or along.
Stereographic projections The stereographic projection is a
In the field, this can be easily done using a counting dish, the Kalsbeek net (Fig. 2.2c, p. 20) (see Ragan (1985) for details on counting and contouring techniques). The equal area plot is placed on top of this grid and the points falling into each hexagonal segment (1% of the total area of the grid) are counted towards the total listed in the center of each hexagonal segment.
Method 1a: Strike and Dip Method 1 Find the strike line (horizontal line on the planar structure) using the Silva compass as a clinometer and establishing the zero dip direction on the plan (Fig. 2.5a). 2 Measure the azimuth of this line (ie its direction from the north) (Fig. 2.5b) - this is the strike of the plane.
2 Look along your compass and measure the azimuth or bearing of the aircraft (Fig. 2.9a). Place the m a p board along the fold axis and, keeping it vertical, measure the azimuth (direction) of the dip.
T h e field map and aerial photographs
Structural data is plotted at the bottom of the section so that the section can be more easily related to the map. Great care must be taken in locating yourself accurately and getting used to the scale of the photograph.
Field notebook
Record the observations of structural features and measurements of the dip of the bedding (Fig. 3.1). 1 On the map, symbols must be plotted directly on the map location for the outcrop from which the measurements were taken and not left 'floating' in areas of the map where no exposure is recorded.
Oriented samples
3 Data should be plotted on the map either using a protractor or using the Silva compass directly (Barnes, 1981, p. 60). Reference marks on the specimen indicate the upward direction and the strike and dip of the reference surface.
Photography
4 Measure the orientation of this surface, and with a waterproof marker write the pitch and dip on the reference surface, (Fig. 2.21). Record the data in your notebook and draw a sketch of the sample and its structural relationships.
3 Fold structures
- Basic fold nomenclature Basic fold nomenclature is outlined
- F o l d types
- Analysis and classification of folds
- folds: Folds with con
- folds: Folds with parallel dip isogons: similar folds
- folds: Folds with diver
- Symmetries of parasitic minor folds
- Vergence
- Strain in folds
- F o l d s associated with faults Many folds are geometrically related
- Kink bands
Note the concept of the enveloping surface, which is either tangential to the fold hinge lines, or passes through the inflection points (points of maximum slope) on the folded layers. A level of no stress (finite neutral surface—Fig. 3.12a) exists within the folded layer, but it moves down toward the inner arc of the fold as it tightens. Always analyze folds in the profile section looking down the dip of the fold axis — where possible photos and sketch are.
Measure the bedding around the fold - this will allow a better definition of the axis of the fold across the surface.
4 Foliations
- C o m m o n foliations
- Axial-planar foliations In most cases foliations are approxi
- Foliations and folds
- Mapping foliations
- Lineations associated with folding
- Mineral stretching and elongation lineations
- Lineations formed by boudins, mullions or rodding
- Lineations associated with faults
- M a p p i n g linear structures Linear structures are extremely
4 Pressure solution cleavage: A split fracture that produces a mineral compartment (often associated with microfolding) and dark layers of insoluble material that give a visible band to the rock (Fig. 4.1d). In the case of interbedded pelites (fine-grained mudstones) and psammites (coarse-grained sandstones) (Fig. 4.4a), in folded limbs the cleavage is at a low angle to bedding. In flexural strike-slip folds (Fig. 3.12b) the internal sliding of layers on top of each other produces fine ridges, grooves and/or mineral extension lines, all of which are approximately 90° to the fold axis.
Grooves are formed by reduction (grinding) and solution as the two fault surfaces slide over each other (Fig. 5.6d).
6 Faults and shear zones
Classification and description of faults
Fault planes that are concave upward and have a fault plane that flattens out at depth are called listric faults. To an observer standing on the fault plane, the hanging wall is the rock unit above the fault plane, and the footwall is the rock unit below the fault plane; (a) a dip slip extensional fault seen in cross section producing stratigraphic omission. As seen by the observer standing on the nearest fault block, the fault block on the other side of the fault plane has moved to the left;. d) dextral strike-slip or wrench fault seen from above.
As seen by the observer standing on the nearest fault block, the fault block on the opposite side of the fault plane has shifted to the right.
Fault displacements In many cases exact fault dis
As the observer can see, the hanging wall fault block has been displaced downslope relative to the footwall fault block; (b) dip-slip contraction fault as seen in a cross section producing stratigraphic overlap. As the observer can see, the hanging wall has moved upward relative to the footwall; 4 Rotation is usually difficult to assess in the field and requires knowledge of displaced points on both sides of the fracture plane.
Once the direction of movement of a fault is determined, the simple classification of faults can be refined to specify the direction of slip as illus.
Extension faults
Contraction faults
Staircase thrust fault trajectory showing the development of flats (f) where the thrust fault is parallel to bedding and ramps (r) where the thrust fault cuts up through bedding. In the hanging wall sheet there will be necessary folds geometrically connected to the ramps - e.g. frontal folds, oblique folds and lateral folds. e) Thrust sheets associated with strike-slip faults (tear faults). This is also reflected in smaller-scale structures (Fig. 6.15) where thrusts cut bedding at a high angle (90°), thus indicating that thrusting accompanied or post-dated folding.
3 Movement is normal for folds formed across face ramps or folds formed in the ductile leg in front of the thrust tip line.
Strike-slip or wrench faults Strike-slip or wrench faults are
1 'Bow-and-arrow rule': In plan, thrust faults are usually curved (Fig. 6.14a) and the direction of movement is generally normal to the 'string' formed by joining the ends of the 'arc', i.e. 5 Movement can be determined from the development of smooth sides and other lines on brittle fault planes. 7 In ductile thrust regimes, folds will initially form parallel to the thrust front, but then subsequent deformation will rotate them in parallelism with the direction of transport.
Reidel shear faults are the R1 synthetic and R2 antithetic systems (in most cases displacements are small on these faults).
Fault rocks
The nature of the matrix tectonic reduction in grain size dominates; grain growth by recrystallization and neomineralization, grain growth pronounced.
Geometry of shear zones Shear zones can form conjugate
Stress orientations and sense of shear orientations can be deduced from the pattern of Riedel shears and from the fabric in the shear hole (Fig. 6.19a). The pressure-solution cleavage (if developed) forms 90° to σ1 and the vein tips, but is rotated to parallelism with the shear zone walls in the cent. The ideal orientation of the σ1 and. of the σ3 stresses outside the shear zone are also.
Note the development of a schistosity at the shear zone margins, and the rotation of this schistosity in parallelism with the shear zone.
Structures in shear zones The orientation of structural
Linear Structures: Lineations Many shear zones develop a strong mineral stretching lineation parallel to the shear direction (eg Fig. 6.21a). The field criteria that can be used (with care) as kinematic indicators to derive sense of displacement are listed below. In addition to the mesoscopic matter elements that allow the determination of the sense of displacement, microscopic analysis can also allow the sense of displacement to be analyzed.
Shear zones or mylonitic foliations should therefore be sampled (oriented samples, Section 2.7) for laboratory analysis.
Mapping shear zones Where possible the factors listed in
7 Joints, veins and stylolites
- T y p e s of joints
- Joints in fold and fault systems
- Veins
- Stylolites
- Mapping joints, veins and stylolites
If you plot this on the stereographic projection (Fig. 7.2b), you will see that the intersection of the planes is the σ2 axis. Typically, T or H patterns develop (Fig. 7.1b) where the younger joint (the upright of the T or the crossbar of the H) abuts each other. Therefore, bent fibers in undeformed veins reflect the change in the orientation of the veins with respect to the σ3 axis (Fig. 6.20).
Cylindrical (Fig. 7.4) Non-cylindrical (Fig. 7.5) Analysis of fracture systems in relation to bedding and folded limbs.
8 Polyphase deformation
- Fold interference patterns Polyphase deformation is recognised
- Lineations i n polyphase terranes
- Sub-areas
- Mapping polyphase terranes When mapping in terranes that show
The orientation of the F2 fold axes will depend on the orientation of the F1 fold limbs. In this case, the orientation of line L1 changes as the layers slide over each other (the fold is parallel but the line maintains a constant angle to the fold axis F2, see Fig. 8.5). 3 Observe, map and record the structural relationships (Tables 5.1-5.3) of the lines developed in the map area.
Sub-zones are identified using the dip uniformity of F, minor fold axes, and L1 bedding/separation intersection lines (created by the F1 fold).
9 First steps in overall
- Map patterns and map interpretation
- c cleavage and lineation maps are combined) which shows the
- Cross-sections
- Report writing
Note the displacement of the axial fold trace upward from the surface termination on the map. The topographic surface is plotted on the cross section X-Y and the outcrop of the thrust plane T is marked. For a region of non-deep folds (Fig. 9.5), the cross-section line is placed perpendicular to the trend of the fold axes, where there is relatively abundant orientation information and interesting fold structures.
Structure Contours of the cross-sectional plane are drawn both on the map and on the projection.
Appendix III Strain measurements
2a) in the cleavage plane and on a joint plane normal to the cleavage
The joint plane is 90° to the shale cleavage plane and parallel to the long axis of the final stretch ellipsoid: i.e. in undeformed sediments these are circular in plane on the bed plane and the longitudinal axis of the burrow is usually 90°. . This axial ratio can be measured directly in the field, together with the orientation of the long axis of the strain ellipse and the axes of the worm tubes, which have been displaced in the bedding (Fig. III.3b), making it possible to determine the angular shear stress.
The parting trace defines the trace of the XY plane of the finite strain ellipsoid.
