FOLD AXIAL TRACE
All hinge points could be contained in a single plane in a fold profile and the line formed by joining them in called the axial trace or axial plane trace. It is the trace of axial plane which contains hinge lines on all surfaces in a folded multilayer complex. The axial surface may be planar if all the hinge lines are straight as in case of cylindrical folds. It is also planar in case the fold is (a special kind of ) plane noncylindrical i.e. the axial plane is perfectly planar but the hinge lines contained within it are curvilinear. It is however in non-plane noncylindrical folds, curviplanar in nature, that is the hinge lines of the folds have nonplane curvatures, none of the hinge lines can be contained in a single perfectly planar surface. The axialdirections of major folds on the maps are also called axial traces. In general, an axial trace is rectilinear if the folds are cylindrical, else ir is curvilinear.
If every surface in fold structure contains a straight line, at least one, parallel to the hinge line, the fold is known as cylindrical and the line as fold axis and its trend the axial direction. In non-cylindrical folds, none of the folded surfaces contains a straight line parallel to the hinge line. If the surfaces in a non-cylindrical fold are folded in such a way as to form a cone either circular or elliptical they are called conical folds.
The line joining points of greatest curvature on a folded surface is called the HINGE of a fold. The hinge can be observed in naturally developed outcrop sized folds. Hinge and axis are not the same as is explained in other section.
Axial Direction : A straight or nearly straight line representing mean attitude of fold axes in a given area. This is usually drawn on maps on the basis of attitudes plotted or analysed by employing projection techniques.