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struct - 1 Geologic Structures Types of geologic...

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1 Geologic Structures Types of geologic structures: (1) Primary structures: those which develop at the time of formation of the rocks (e.g. sedimentary structures, some volcanic structures, .... etc.). (2) Secondary structures: which are those that develop in rocks after their formation as a result of their subjection to external forces. (3) Compound structures : form by a combination of events some of which are contemporaneous with the formation of a group of rocks taking part in these "structures". Stress : is the force applied over a given area of the rock mass. It is of three different kinds: (1) Compressional stress which tends to squeeze the rock (2) Tensional stress , which tends to pull a rock apart (3) Shear stress , which results from parallel forces that act on different parts of the rock body in opposite directions. Strain : Is the change in the shape or size of a rock in response to stress. A rock is said to deform elastically if it can return to its original size once the stress is removed. Plastic deformation on the other hand, results in permanent changes in the size and shape of the rock, even after the stress is removed. Plastic deformation of a rock is also known as ductile deformation. After deforming plastically for some time, a rock which continues to be subjected to stress may finally break, a behaviour known as brittle deformation. Factors affecting how a rock deforms: 1. Depth: Lithostatic pressure + heat 2. Time: 3. Composition 4. Fluids Therefore, a rock may undergo ductile deformation when subjected to stress at certain depths within the earth where pressures and temperatures are relatively high, or if fluids are abundant, but the same rock may undergo brittle deformation at shallower depths. Measuring geological structures: Strike : (direction) Dip : (direction & angle) A- Secondary structures Types of secondary geologic structures: (a) folds , which are a form of ductile deformation, and (b) fractures , represented by faults and joints which generally result from the brittle behaviour of rocks in response to stress. I- Folds
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2 Folds are bends or flexures in the earth's crust, and can therefore be identified by a change in the amount and/or direction of dip of rock units. Most folds result from the ductile deformation of rocks when subjected to compressional or shear stress . In order to understand and classify folds, we must study their forms and shapes, and be able to describe them. The following definitions are therefore essential for the description of a fold: 1- Hinge line : Is the line of maximum curvature on a folded surface. The hinge line almost always coincides with the axis of the fold defined as a line lying in the plane that bisects a fold into two equal parts. 2- The axial plane is an imaginary plane dividing the fold into two equal parts known as limbs . It is therefore the plane which includes all hinge lines for different beds affected by the same fold.
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