Origin of Lineation Deformed pebbles Oolites and Mineral

Origin of Lineation Deformed pebbles, Oolites and Mineral grains The orientation of the lineation relative to the major structure and the origin depend, of course, on the kind of lineation. In general, the lineation is systematically related to the major folds, but this is not necessarily the case if the lineation is the result of stresses independent of those that produced the folding. The attitude of deformed pebbles, oolites and grains of such minerals as quartz are generally related in some systematic way to the associated folds. But the relations may differ in different areas and in different parts of the same fold. Department of Geology, University of Dhaka Elementary Structural Geology Slide 1 of 7 ©SHA

Origin of Lineation Deformed pebbles, Oolites and Mineral grains The attitude of deformed oolites in Maryland Pennsylvania are illustrated in the following figure. The fold axes are essentially horizontal, whereas the axial planes of the fold and the slaty cleavage dip to the southeast. The long axes (l) and intermediate axes (i) of the ellipsoidal oolites lie in the plane of the slaty cleavage. The long axis of each oolite plunges southeast essentially down the dip of the cleavage. It is apparent that in this case, because of a couple acting along northwest –southeast lines, the thick pile of sediments was elongating upward toward the northwest, partly by folding but especially by rock flowage. Department of Geology, University of Dhaka Elementary Structural Geology Slide 2 of 7 ©SHA

Origin of Lineation Deformed pebbles, Oolites and Mineral grains Near Newport, Rhode Island, the carboniferous sediments contain some very coarse-grained conglomerates. Many of the boulders are from one to three feet long. The fold axes plunge gently south. The long axes of the deformed boulders also plunge gently south. The intermediate axes are essentially vertical, parallel to the axial plane of the major fold; the short axes are almost horizontal, striking E-W, and are thus perpendicular to the axial plane of the major fold. Under E-W compression the sediments were thrown into folds striking N-S; the easiest relief was N-S, and thus the sediments elongated in that direction. Department of Geology, University of Dhaka Elementary Structural Geology Slide 3 of 7 ©SHA

Origin of Lineation Deformed pebbles, Oolites and Mineral grains In much of eastern Vermont the long and intermediate axes of the pebbles on the limbs of folds lie parallel to a bedding cleavage (Fig. C). The fold axes are essentially horizontal, and the long axes of the pebbles plunge directly down the dip. This type of deformation is the result of stretching of the limbs of the folds, accompanied, of course, by considerable thinning of the beds. Near the crest of the folds, however, the long axes of the pebbles are parallel to the fold axes (Fig. D). here the easiest relief was parallel to the fold axes. In western Hampshire the long axes of stretched pebbles are oriented this way. Department of Geology, University of Dhaka Elementary Structural Geology Slide 4 of 7 ©SHA

Origin of Lineation Elongate Minerals Lineation shown by elongate minerals, such as hornblende needles or flat oval flakes of micas, likewise show various orientations. In these case, however, the mineral was not plastically deformed into its present shape, but grew in that direction because it was the easiest direction of growth. Department of Geology, University of Dhaka Elementary Structural Geology Slide 5 of 7 ©SHA

Origin of Lineation Intersection of Bedding and Cleavage A lineation that is the result of the intersection of bedding and an axial plane slaty cleavage is parallel to the fold axis. A consideration of the geometry of folds shows that this must be the case. Similarly, the intersection of bedding and of fracture cleavage that is related to the folding is parallel to the fold axes. The same is true of the intersection of fracture cleavage with slaty cleavage that is parallel to the axial planes of the folds. Department of Geology, University of Dhaka Elementary Structural Geology Slide 6 of 7 ©SHA

Origin of Lineation Crinkles If a bedding cleavage is thrown into small crinkles the axes of the crinkles are likely to be parallel to the major fold axes. This is because the crinkles are essentially drag folds. After an axial plane cleavage has been produced, further compression may cause slippage along this cleavage toward the major fold axes. The axes of any crinkles will likewise be parallel to the major fold axes. In many cases, however, the rocks may be so tightly compressed that the different movements parallel to the bedding or cleavage are unrelated to the major fold axes; consequently the crinkles may be oriented in most any direction. In some areas it is not uncommon to find several sets of differently oriented crinkles. They were produced in succession as the rocks slipped in different directions parallel to the cleavage or bedding. Department of Geology, University of Dhaka Elementary Structural Geology Slide 7 of 7 ©SHA