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CHEVRON FOLDS, SADDLE REEFS

Sometimes hinge collapse areas are produced due to change in incompetent as well as competent layer thickness. Hinge collapse differs from bulbous hinge zone in the mode of displacement of competent layer. In hinge collapse the displacement is towards the inner arc while in the bulbous hinge zone, it is away from the inner arc. The hinge collapse is more pronounced if tl/t2 is high so that less competent material is available for accommodation of shape changes (E in fig) and the competent layers tend to curve inwards to take care or compensate for the dilation sites. The hinge collapse is insignificant if tl/t2 is low since large amount of competent material is available to accommodate the shape changes (F in Fig). While extension fractures may be observed in the hinge area of a competent layer, a fine or incipient cleavage may be noticed in the inner arc regions. Tension gashes may be developed in the incompetent layers and less commonly in the competent layers as well. Incompetent layers which show class 3 geometry, typical of flexural flow, also occasionally show sigmoidal cleavage trajectories (see Fig) typical of shear zones suggesting inhomogeneous flow of the material. Thus for any given amount of shortening or amount of limb dip in chevron folds the increments of shear are greater for folds with high t/l ratios and this leads to locking of a chevron fold at high interlimb angle or less 

amount of  shortening between ends of layers. Thus lower the t/l ratio, smaller is the interlimb angle at which folds lock up. Subsequent compression will change the geometry of chevron folds so that more competent layers will begin to take on subclass 1C geometry while the degree of divergence of isogons in the less competent class 3 layers will be diminished, as limbs come closer and closer.

It is the competent layers that control the dominant limb dip in the system. The shape changes in competent layers could be adjusted if sufficient incompetent material is available. The ratio t1/t2 is most significant in this respect where t1 is the competent layer thickness and t2 is the incompetent layer thickness. Saddle reef structures (C in Fig) are usually developed in the hinge areas of incompetent layers, essentially due to dilation. If t1/t2 is high then the amount of incompetent material available for space filling is insufficient, dilation spaces are therefore large (C in figure). If t1/t2 is low, the amount of incompetent material available for space filling is large so that the dilation spaces are insignificant (D in Figure). Sometimes hinge collapse areas are produced due to change in incompetent as well as competent layer thickness.

The hinge collapse is more pronounced if tl/t2 is high so that less competent material is available for accommodation of shape changes (E in fig) and the competent layers tend to curve inwards to take care or compensate for the dilation sites. The hinge collapse is insignificant if tl/t2 is low since large amount of competent material is available to accommodate the shape changes (F in Fig). While extension fractures may be observed in the hinge area of a competent layer, a fine or incipient cleavage may be noticed in the inner arc regions. Tension gashes may be developed in the incompetent layers and less commonly in the competent layers as well. Incompetent layers which show class 3 geometry, typical of flexural flow, also occasionally show sigmoidal cleavage trajectories (see Fig) typical of shear zones suggesting inhomogeneous flow of the material.