Pure shear is a coaxial deformation. During coaxial
deformation the principal stretch directions of the strain ellipse do not
rotate. Therefore lines that parallel directions of shortening will always
shorten, lines that parallel directions of extension will always extend
during the same progressive coaxial deformation event. In order to have
shortening followed by extension, as was the case for the small quartz
vien, and invoke only pure shear coaxial deformation would require two
deformation events for which the orientation of the stress field are significantly
different. Thus, the strain ellipses associated with each deformation event
would be significantly different with respect to orientation of S1
and S3 on the outcrop. In this way you shorten an object (fold
it) during the first deformational event (parallels S3
), then extend it (pull it apart into boudins) during the second deformation
event (parallels S1).
10.) On figure 3 draw and label sN
, sS , and determine the angle q.
(5 points)
The angle q is the acute angle between the plane of interest and the stress vector.
11.) Calculate the magnitude of sN and sS. (5 points)
The angle q = 60°;
sN= s
* sinq = 100 MPa * (.866) = 86.6
MPa.
sS= s
* cosq = 100 MPa * (.5) = 50 MPa.