from Stereographic Projection Techniques for Geologists and Civil Engineers
Published online by Cambridge University Press: 05 June 2012
The various types of planar structure that occur in rocks are listed on p. 2. Cleavage is an example of a pervasive planar structure induced by tectonic strains (changes of shape) coupled with metamorphic modifications of a rock's fabric. An alignment of grains visible microscopically is often expressed on the scale of the hand specimen by the rock's ability to break along parallel planes. The direction perpendicular to these cleavage planes is the direction of greatest shortening strain. It is frequently observed that the shortening strains giving rise to cleavage also result in the folding of other planar structures such as bedding. Folding and cleavage are therefore often associated.
Cleavage is frequently parallel or sub-parallel to the axial surfaces of associated folds and in such cases is referred to as axial-plane cleavage (Fig. 26a). As a consequence the lines of intersection of folded bedding planes with the cleavage plane are parallel to the fold axis (Fig. 26b). Field measurements of cleavage provide therefore not only an indication of the orientation of fold axial planes but also, in combination with bedding, information on the plunge and plunge direction of fold axes. Bedding–cleavage intersection lineations can be measured directly in the field or constructed stereographically from the measured attitudes of bedding and cleavage (see p. 26).
Axial-plane cleavages are not often exactly parallel to fold axial surfaces. The strains around the fold are variable in amount and direction because of competence contrasts between the folded beds.
To save this book to your Kindle, first ensure [email protected] is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.