STRUCTURAL FACING (SHACKLETON'S RULE): A DISCUSSION AND CAUTIONARY TALE
CZECK, Dyanna M. and HUDLESTON, Peter J., Department of Geology & Geophysics, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455 firstname.lastname@example.org
Shackleton's Rule ("structural facing") is a useful tool for elucidating structural relationships and structural history in complexly folded terranes. It utilizes stratigraphic facing within the plane of the cleavage. For a single deformation involving folding and development of axial planar cleavage, structural facing is constant across a region, whereas stratigraphic facing will vary around folds. Problems regarding the use of structural facing arise when cleavage is not approximately axial planar or contemporaneous with folding. An additional use of structural facing is to interpret multiple stages of deformation. Systematic variations in structural facing typically indicate a history of superposed deformations. Additionally, when bedding and cleavage are nearly parallel, a situation that is common throughout many of the world's granite-greenstone terranes, small variations in cleavage or bedding may cause Shackleton's Rule to fail when applied to multiple deformation events. We provide an example from North America's Superior Province that highlights such a problem. Without cautious application, spurious results can be obtained resulting in an interpretation of non-existent folds. With care, useful information can still be obtained about structural history in such situations.
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