Lineation (geology)

The trace of bedding on an intersecting foliation plane commonly appears as colour stripes generally parallel to local fold's hinges.

During deformation the rock was flattened and then stretched by movement along a ductile shear zone within which this outcrop resides.

The spherical conglomerate pebbles stretched along the direction of movement of this shear zone, attaining their current somewhat flattened cigar-shaped form.

The pebbles thus record important information on the orientation of the shear zone (subvertical) and the direction of movement of the shear zone, and the overall change in pebble shape from originally sub-spherical to presently elongate cigar-shaped, allows one to quantify the strain experienced by the rock mass in the geologic past.

Stretching lineations may also manifest as linear features upon pre-existing surfaces such as foliations within shear zones (see image at right, below).

In the case of the illustrated lineations within the sandstone, they do not record an earlier deformation event's foliation and cannot be used to infer orientation information for folds or original bedding.

Stretched pebble conglomerate L-tectonite illustrating a stretch lineation within a shear zone , Glengarry Basin, Australia. Pronounced asymmetric shearing has stretched the conglomerate pebbles into prolate (cigar-shaped) rods.
L-teconite mylonite formed from coarse-grained sandstone protolith, Glengarry Basin, Australia. This photograph illustrates a pronounced and prominent stretching lineation plunging steeply to the north, as a rake upon the main shear foliation parallel with the protractor. Stretching lineations may form in any faulting regime when conditions are such that rocks deform ductiley, including extensional, compressional, transpressional, and transtensional.