Program of the Сourse BASES OF MECHANICS OF

Program of the Сourse BASES OF MECHANICS OF DEFORMATION AND DESTRUCTION OF SOLID BODIES SPECIAL FEATURES OF MECHANISM OF TECTONIC DEFORMATION RECONSTRUCTION OF TECTONIC STRAIN AND STRESS STRUCTURAL PARAGENESES

Lecture 4 2. SPECIAL FEATURES OF MECHANISM OF TECTONIC DEFORMATION (CONTINUATION) 2.3. Instability of viscous deformation of homogeneous medium Viscosity, that the same is friction, on one larger surface, is always less than summary viscosity on many small surfaces.

“Necks” due to tension of layers F  =  (1.3.17) S Determination of stress.  = 2έ (1.4.4) Law of viscous (Newtonian) flow for pure shear, i.e. lengthening- shortening. First, uniform lengthening of the layer occurs (Figs. a  b). However, ideally flat layers do not occur, a located section with slightly reduced thickness will always be. On it, in accordance with formula (1.3.17), the cross-sectional area is lowered, and therefore stress is increased. Therefore, according to formula (1.4.4), the strain rate is increased here, and thus very strain of horizontal lengthening and vertical shortenings is also increased (Figs. b  c). The greater is the stress, which increases in proportion to the decrease of cross-sectional area, the greater is the strain rate, and thus also the greater is the strain itself. Avalanche-type process. Tension joints In contrast to the “plastic” case, microscopic crack is born in the layer transverse to the latter. Therefore, in this cross section, the real cross section is less than in other sections of the layer. So, and in accordance with formula (1.3.17), the stress is increased here, and it will reach the ultimate strength more rapidly . As a result, the crack will begin to grow, the real cross section will become still less, and the stress, in turn, will grow even more greatly. Also an avalanche. The process of crack growth will end if and only if the crack reaches the boundaries of the layer.

Folds due to compression of layers First, layers are uniformly shortened, but then are bent, since bending is more advantageous energetically than uniform longitudinal shortening. Concentration of shearing strain by simple shear The formation of faults is the limiting case of deformational instability.

Strengthening the instability of strain in the case of inhomogeneous medium Until now, we considered the medium as uniform, but also in this case, the instability of strain is more advantageous energetically. In the case of inhomogeneous medium this is aggravated. The concrete expression of instability appears. Intra-grain, intergranular, and cataclastic strain Intra-grain strain: the weakened planes of cleavage of minerals are used. Intergranular strain: the sections of more yielding cement are used. Cataclastic strain: the already existing cracks and faults which cut the rock massive are used. The role of bedding Sequential formation of cleavage, folds and upthrusts by compression; the alternative: formation of duplex structure. Stages of longitudinal shortening of a bedded rock series: uniform longitudinal shortening of layers (with formation of cleavage or schistosity)  their bending and mutual slippage  general horizontal flattening  formation of upthrusts. And all this for one purpose: the horizontal shortening of the bedded rock series. More detail about this will be said in the spring semester. Just as about the alternative : formation of duplex structure.

Experiment of H. Ramberg

Boudinage due to tension Redistribution of stresses in the process of strain 2 versions : The 1st version. Layer is isolated and surrounded by air. Let us exert it tensile forces to. If the strength of layer is higher than the elastic limit (viscous case), i.e., there is a certain range of plasticity, then an orthogonal section with the smallest area will always be located; the talk about it already went. But if the strength limit of layer coincides with its elastic limit (brittle case), then the tension joint appears also in the area of the smallest orthogonal section; this case has already been discussed as well.

Thus, in both examined cases (viscous and brittle) stress is redistributed and is concentrated in the sole cross section of layer. But we observe entirely another in nature: boudinage of the layers, in each of which many “necks” or tension joints are formed. The 2nd version. Now we will have a talk about the influence of the distributed application of force on the process of tectonic deformation. s h ls =  0 Determination of distance between the adjacent tension joints, which cut a layer, by its tension, caused by distributed application of stretching force to it. Based on this example, it is very clearly: the relationship of forces and stresses. This is seemingly stretchable rubber, stuck on the layer and which evenly stretches layers; that is just some distributed application of force. Or as if layer extend ants, each pulling its section. We don’t know о, but we know s and h. о is one and the same, in the uniform structural situation, for example, on the wing of a fold. Depending on the relationship of e and s , either viscous or brittle cases will occur.

Another case of distributed application of force: tension joints disposed en échelon by simple shear. Direction of shear (“against the fur”) is the most frequent error at the examination.