Ductile notes
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Scroll down these notes or select from the icons opposite. When rocks deform in a ductile fashion they can generate beautifully complex structures that can bamboozle even the most experienced structural geologist. So we need some strategies to describe these rocks and analyse them.

To start with, we need to establish the difference between different types of rock structure. Part of the problem is to establish how the features in a rock are organised - their dimensionality. Even this isn't enough. For example, a rock can contain several planar structures - e.g. bedding and cleavage. We need a way of describing these in terms of what is defining the fabric. A good way is to use language that is non-genetic. Is the fabric defined by aligned grain shapes or by the organisation of the location of different grain types (shape vs location fabrics). Shape fabrics are almost always created by deformation. Location fabrics can be produced by sedimentary (bedding), metamorphic (gneissic banding) or igneous processes (cumulate layering), even working in tandem with deformation (e.g. pressure solution seams).

Once we've established that deformation has occurred in a rock, it is useful to describe it in 3D. An important consideration is the 3D shape of the strain ellipsoid - the 3D extension of the strain ellipse concept (link to strain ellipsoid in 2D strain). Another, rather classic way, of understanding complex deformation is to unravel distinct phases of deformation.

A feature of some high strain shear zones is the development of sheath folds. These structures form by amplification of originally weakly curvilinear folds into very strongly curvilinear ones. The hinge lines rotate with intense deformation to align sub-parallel to the stretching lineation.

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