Geological structures of planar type

Abstract

Structures in rocks with a linear, as opposed to a planar, character also occur in a great variety of forms. Linear sedimentary structures These structures are primary and develop during sedimentation. Figure 3a shows an example of linear features on a steeply dipping bedding plane at the base of a sandstone bed. These are the crests of ripples on a tilted bedding plane in sandstone. These linear structures (aligned approximately along the present strike of the bedding plane) allow the original current direction to be inferred. Once corrected for the tilting that the bed has undergone since deposition, the measurement of the direction of these structures can be used to deduce ancient currents. Linear structures of tectonic origin Fold hinge lines (Fig. 3b), the lines of maximum curvature of folded surfaces, are examples of a linear structure of tectonic origin. The hinge lines are tilted or plunge at about 10° towards the left of the photograph. Other tectonic lineations are mineral lineations (linear alignments of minerals in metamorphic tectonites) and stretching lineations defined by strained objects with elongated, cigar-like shapes. Figure 3c shows a deformed conglomerate with a lineation defined by drawn-out pebbles. Slickenside lineations (Fig. 3d) are formed on fault planes during the motion of the two walls. These lineations are measured in the field so as to provide information on the direction of fault movement. Figure 3e shows a linear fabric in a metamorphosed granite. Deformation of this rock has occurred so that feldspar aggregates have been stretched out into cigar shapes.