9 First steps in overall
9.3 Report writing
Fig. 9.7 Curvimeter for measuring line lengths on maps and cross-sections.
fold plunge of the major structures);
(2) Determine the stratigraphic thicknesses in the area and construct a stratigraphic template (Fig. 9.8b).
(3) Establish a pin line — where there is no interbed slip and the beds are pinned together. The pin line is usually located in the foreland (Fig. 9.8b), where there is no deformation, or on the axial surface of the anticline.
(4) Measure away from the pin line producing a restored section at the same time as constructing the balanced section. The restored section should have no gaps or overlaps and the fault trajectories should be reasonable (Fig. 9.8c).
For staircase fault geometries the
ramp angles of faults should be less than or equal to 30°. The cross- section should end on a pin line as shown in Fig. 9.8c.
Fig. 9.8 Construction of a balanced cross-section through a simple fold-thrust structure.
(a) Field data is plotted on the topographic profile (using down plunge projections (Fig. 9.6) and apparent dips (Appendix 1) where required). Bedding dips, lithological and tectonic boundaries are shown together with the location of a measured section used to determine stratigraphic thicknesses.
(b) Balanced section constructed at the same time as the restored section — Fig. 9.8c. The stratigraphic template — Fig. 9.8c, was based on the measured section (Fig. 9,8a). The section has been balanced using line length balancing and the assumptions outlined in the text (section 9.2.3). The front pin line has been placed in the undeformed foreland and the end pin line has been placed in a convenient segment of stratigraphy at the western end of the section. Faults have been projected to depth using their surface dips.
(c) Restored section on the statigraphic template. The thrust fault trajectories and the front and end pin lines are shown.
149
6 Relationships to regional structures and a synthesis of the structural evolution of the area.
7 Stress and strain analysis based upon measurements of fracture patterns and deformed objects.
8 Kinematic analysis — tectonic transport directions and structural evolution (e.g. dominant move
ment patterns of thrust faults etc.).
Your report should be fully illustrated with maps, stratigraphic columns, cross-sections, fully annotated diagrams and photo
graphs.
Good luck!
References and further reading
References to take to the field
BARNES, J. W., (1981) Basic Geological Mapping. Geological Society of London Hand
book Series, 1. Open University Press, 112 pp.
BOYER, S., and ELLIOTT, D . , (1982) 'Thrust systems'. Bulletin of the American Association of Petroleum Geologists, 66, 1196-1230.
DAVIS, G. H., (1984) Structural Geology of Rocks and Regions. New York, Wiley, 492 pp.
FRY, N., (1984) The Field Description of Metamorphic Rocks. Geological Society of London Handbook Series, 3. Open University Press, 110 pp.
HANCOCK, P. L., (1985) 'Brittle microtectonics: principles and practice'. Journal of Structural Geology, 7, 437-458.
PHILLIPS, F. C , (1971) The Use of Stereographic Projection in Structural Geology. 3rd ed.
London, Edward Arnold, 90 pp.
RAGAN, D . M., (1985) Structural Geology: an Introduction to Geometric Techniques. 3rd ed., New York, Wiley, 393 pp.
THORPE, R. S., & BROWN, G. C , (1985) The Field Description of Igneous Rocks. Geological Society of London Handbook Series, 4. Open University Press, 162 pp.
TUCKER, M. E., (1982) The Field Description of Sedimentary Rocks. Geological Society of London Handbook Series, 2. Open University Press, 124 pp.
Further reading
ANDERSON, E. M., (1951) The Dynamics of Faulting. Edinburgh, Oliver and Boyd, 241 pp.
BADGELY, P. C , (1959) Structural Methods for the Exploration Geologist. N e w York, Harper, 280 pp.
BELL, A. M., (1981) 'Vergence: an evaluation', journal of Structural Geology, 3, 197-202.
BUTLER, R. W. H. (1982) 'The terminology of structures in thrust belts'. Journal of Structural Geology, 4, 239-45.
DAHLSTROM, C. D. A., (1969) 'Balanced cross sections'. Canadian Journal of Earth Sciences, 6, 743-57.
HOBBS, B. E., MEANS, W. D., & WILLIAMS, P. F., (1976) An Outline of Structural Geology.
New York, Wiley, 571 pp.
HUDDLESTON, P. J., (1973) 'Fold morphology and some geometrical implications of theories of fold development'. Tectonophysics, 16, 1-46.
PARK, R. G., (1983) Foundations of Structural Geology. Glasgow, Blackie, 135 p p .
151
P R I C E , N . J., (1966) Fault and Joint Development in Brittle and Semi-Brittle Rocks. Oxford, Pergamon, 176 pp.
RAMSAY, J. G., (1967) Folding and Fracturing of Rocks. New York, McGraw-Hill, 567 pp.
RAMSAY, J . G . , (1974) 'Development of chevron folds'. Geological Society of America, Bulletin, 85, 1741-54.
RAMSAY, J. G., (1980) 'Shear zone geometry: a review'. Journal of Structural Geology, 2, 83-99.
RAMSAY, J. G., (1982) 'Rock ductility and its influence on the development of tectonic structures in mountain belts', in Hsu, K. (ed.) Mountain Building Processes. London, Academic Press, 111-128.
RAMSAY, J . G . & G R A H A M , R. H., (1970) 'Strain variation in shear belts'. Canadian Journal of Earth Sciences, 7, 786-813.
RAMSAY, J. G. and HUBER, M. I., (1983) The Techniques of Modern Structural Geology.
Volume 1: Strain Analysis. L o n d o n , Academic Press, 307 p p .
SIBSON, R. H., (1977) 'Fault rocks and fault mechanisms'. Journal of the Geological Society of London, 133, 191-214.
SIMPSON, C. & SCHMID, S. M., (1983) 'An evaluation of criteria to deduce the sense of movement in sheared rocks'. Geological Society of America, Bulletin, 94, 1281-1288.
W I L S O N , G. (with COSGROVE, J.) (1982) An Introduction to Small-Scale Geological Structures.
Allen and Unwin, 128 pp.
WILLIAMS, G. D . , & CHAPMAN, T. J., (1979) ' T h e geometrical classification of non- cylindrical folds'. Journal of Structural Geology, 1, 181-186.
Appendix I
Fig. I.1 A nomogram for determining the true dip from an apparent dip and vice versa. Example:
on a cross-section the beds have an apparent dip of 40° and the line of the cross-section is at 30°
to the strike of the beds (as seen on the map)—on the nomogram mark off the difference in strike (30°) and the angle of apparent dip (40°), and then draw a straight line through these points to give the true dip — 57°.
153
Appendix II
APPARENT DIP VALUES (Solid lines) AND THICKNESS EXAGGERATION FACTOR FOR SECTIONS AT AN ANGLE TO TRUE DIP DIRECTION
Fig. II.l A graph to determine the apparent dip angles (solid curved lines) and the thickness exaggerations (dashed curved lines) for bedding in cross-sections which are constructed at an angle
to the true dip direction (i.e. not at 90•1 to the strike). Example: for bedding with a true dip of 45°
and for a cross-section oriented at 60* to the dip direction; from the graph the apparent dip is 27°
and the thickness of bedding will be 1.275 x the true thickness.