KINK-BANDS, CONTRACTIONAL OR REVERSE

The difference between the normal and reverse kink bands is shown in figure accompanying this text. If the compressive stress acts parallel to the layering, conjugate kink-bands or box folds are produced but if it acts at an angle to the layering, a single set of kink-bands are developed more or less at the exclusion of the other. Patterson and Weiss (1966) produced conjugate and single kink-bands experimentally and one of their significant conclusions has been that under large shortening, kink-bands are converted into chevron folds. Dewey (1966) made significant contribution towards our understanding of the nature and origin of kink-bards and Arvid Johnson and his coworkers have presented through a series of papers (see 1976 numbers of Tectonophysics) the origin of kink-bands by extensive theoretical treatment of the problem of their origin and geometry.

Such kink-bands are called reverse or contractional kink-bands. Conversely, if the orthogonal thickness within the kink-band is systematically reduced than that in the un-deflected layering the kink-bands are called normal or extensional kink-bands. The difference between the normal and reverse kink bands is shown in figure. The axial plane normally bisects the angle between kinked and unkinked sectors but this is not true if there is dilation within the kinked zone (D ) so that the angles b₁  and b₂   are not equal(see Figure below). In this case the amount of orthogonal thickness is increased by an amount  dt.  The dilation D or d t/t is given by :  

       D =[(sin  b₂)/(sin  b₁)]-1                     

If b2> b1, then there is reduction in volume within the kink-band and dilation will take negative value.  Positive dilation however is more common in naturally developed rocks.