Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Prakash P. Roday ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar-470003, IN

Abstract

Two new parameters, continuity index (I) and continuity ratio (C/DC), are suggested as measures of anisotropy, which determine the initial instabilities and the types of displacement patterns produced in multilayer media. Both parameters have high significance for anisotropic media having small t/L values for individual competent layers and low values of mean spacing (S). In reality, the values of t/L are dependent upon the continuity index (I). The parameters seem to affect the finite displacement patterns of only the highly or moderately anisotropic media. For complexes of low anisotropy, the parameters are ineffective. The parameters appear to control the bulk displacements but on relatively smaller scales, t/L and S would continue to produce finite patterns characteristic of high anisotropy. Type 1 and type 3 patterns appear to be distinguishable from each other by the presence of double-hinged structures and second order fold pairs in the latter as these are developed under greater differential stress, accompanied by modification of original type 2 reverse conjugate kinks with progressive shortening between ends of layers.

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Prakash P. Roday ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar 470003, IN

Abstract

An analysis based on measurements made on the profile sections of several chevron folds developed in the biotitic limestone and calc gneiss units of the Delhi system of rocks around Biliawas in Central Rajasthan reveals that while folds in calc gneisses locked up after no great shortening across the layers had been accomplished and limbs had attained only moderate dips, those in biotitic limestones underwent considerable shortening normal to axial surfaces and locked up at much smaller interlimb angles. Estimates made of the ratios of viscosities between two sets of layers in each lithology indicate that the contrast between properties of two sets of layers is greater in biotitic limestones. The greater shortening in biotitic limestones than in calc gneisses is envisaged rather as a widely differing behaviour of different multilayer complexes than any likely finite strain variability across the area.

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Prakash P. Roday ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar 470003, IN

Abstract

An enormous nearly flat-lying isocline related apparently to the earliest recognisable deformational movement (F₁) in the Ajabgarh rocks around Anakhar in Ajmer district is superposed by numerous upright open or tight structures formed during a later event (F₂). Many minor F₂ structures have a westerly sense of movement. Not only are the axes of the two sets of structures parallel, they also possess high directional stability. Top half of the westerly closing isocline has been denuded in the northwestern part of the area. The lithologic map in this part is therefore an axial planar representation of the early fold. Heron's contention that 'the huge pegmatite dike at Anakhar cuts vertically through a recumbent fold' therefore, appears to be strongly supported.

The youngest structures are broad warps (F₃) lying at high angle to early structures but their axes have low directional stability owing to development of folds on surfaces of variable attitude.

Minor folds of all generations are rare in calc-schists but abundant in other lithologies. All the rocks in the area appear to have behaved as a multilayer complex during the first and second deformative phases with competent lithology sueh as calc schist controlling fold initiation and buckling.

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Prakash P. Roday ¹, Arun K. Bhat ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar 470003, IN

Abstract

The Bijawar rocks exposed at the Barmhanghat section of the Narmada Valley bear imprints of two folding episodes (D₂ and D₃) and a shearing movement (D₁) that predates folding in the rocks. The first fold structures (F_l related to D₂) evolved by a combination of tangential longitudinal strain and oblique flexural slip, the latter suggesting initial unsymmetrical disposition of rock layers in relation to principal stresses. Numerous slickensided fibres were produced as a result of this on layer surfaces, these are unrelated to any recognisable fault movements in the rocks examined. Cleavage contemporaneous with F₁ folds shows slight transection in a few minor folds. Three geometric variations of transected folds are described. In submacroscopic fold chains, the geometry of single or paired folds suggests their initiation in the north and gradual propagation to the south along the layering, newer folds probably appellring with the advancing stress front. The youngest structures (F₂, related to D₃) are broad flexural slip warps, lying at high angle to early folds.

The shearing movernent was accompanied by the intrusion of basic dikes and quartz veins. The youngest event in the area was the intrusion of a second set of basic dikes, devoid of any traces of deformation or metamorphism.

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Prakash P. Roday ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar 470003, IN

Abstract

Extension veins with sigmoidal crystal fibres affected by post-crystallization deformation and synkinematic with the F₂ deformational event are developed in the metamorphic rocks east of Barwaha in western Madhya Pradesh. Though the original vein textures have been modified by the effects of post-crystallization deformation such as bending, rupture and shearing, they are sufficiently well-preserved to conclude that the growth process was essentially of the' stretched crystal' or 'crack-seal' type.

The vein origin can be ascribed to a period of low grade metamorphism and associated deformation. Post-vein modifications in the matrix are not associated with any retrogressive metamorphic assemblages, suggesting that the modification process occurred under more or less the same conditions which prevailed during vein growth but under the greater influence of ductile deformation within the veins.

The veins cross-cut the regional schistosity or a statically recrystallised hornfelsic fabric and therefore they postdate the F₁ movement and the post F₁ dyke, but predate the end of regional ductile deformation F₂. Extension veins are themselves cross-cut by shear veins or the shearing is restricted to the walls of extension veins suggesting significant change in orientation of the maximum principal compressive stress towards the late stages of vein dilation. The drusy fabrics of quartz crystals in nontectonic veins developed in the Lameta sandstones are also deformed, as are those of secondary calcite in basalt flow cavities in the area to the east of Sanavad, presumably related to the rifting and volcanism of late Mesozoic-early Cenozoic period. The effect of this later tectonic activity was found in the older syntectonic veins, this being the infilling of zeolite in the late' saddle-reef' type dilation sites between the bent crystal fibres.

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Prakash P. Roday ¹, L. P. Chourasia ¹, Shailendra Chaudhari ²

Affiliations
1 Department of Applied Geology, The University, Sagar 470003, IN
2 Department of Applied Geology, Indian School of Mines, Dhanbad 826004, Bihar, IN

Abstract

The morphology of ductile shear zones in the granitic basement of Bijawar rocks is described. The dextral shear zones are more ubiquitous than their sinistral counterparts and the obtuse angle between the two sets faces the direction of maximum shortening in rocks, the exact value of the angle presumably controlled by the mean stress. The maximum displacement vector of shear zones is generally subhorizontal. The negative dilatation in some shear zones has caused the overall defoi marion in them to be of the flattening type. The orientations of the principal stresses are calculated. The cleavage trajectories do not sometimes exactly bisect the acute angle between the conjugate zones, presumably due to rotation of principal strains with progressive shear or to the more accentuated development of one set of zones.

The sinusoidal and conjugate folds of monochnic or triclinic symmetry (D-1) in the Bijawar rocks are shown to be related genetically and from the point of view of orientation of principal compressive stresses to the ductile shear zone development in basement granite. The brittle-ductile shear zones in the basal carbonate unit associated at places with pressure solution or tectonic stylolitic structures whose formation was accompanied by about 14 to 24 per cent volume reduction formed at the same time.

There was a renewed activity during post-Vindhyan time along pre-existing shear zones in the older rocks but in the opposite sense under very Jow P-T conditions which produced ductile D-2 structures in Bijawar rocks, culminating in small or moderate scale strike-slip faults. The youngest movements along these faults were, however, with a vertical component under a subvertically oriented maximum principal compression. It appears that most shear zones have an minal period of strain softening, followed by rather sudden strain hardening or loss of material contmuity which causes either the termination of such zones or their passing into model ate scale wrench faults.

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Prakash P. Roday ¹, Yashwant B. Katpatal ¹

Affiliations
1 Department of Applied Geology, The University, Sagar 470003, M.P., IN

Abstract

All methods of computing Rs. the tectonic strain ratio from the various means (of axial ratios of ellipsoidal particles that undergo deformation homogeneously with the matrix-the passive particles) such as arithmetic (̅Rf). geometric (Rg) or harmonic (H) fail to adequately express the real values of Rs. Using the data given by Lisle (1979) for simulated conglomerates, it is shown that ̅Rf_H = H+̅Rf÷2-(H+̅Rf)^-1 is closest to Rs if Rs > Ri. For Ri > Rs (2φ > 90° in a given principal plane), R' (given by ̅Rf_a-̅Rf_a^-2/̅Rf_b-̅Rf_b^-1 or by ̅Rf_a-̅Rf_a^-1/̅Rf_b-̅Rf_b^-2, depending upon whether Ri_b > RS_b or Ri_a > RS_a respectively) gives the value of k and the values of a and b that are closest to that of the superimposed Rs. The data are tabulated and also plotted on a conventional Flinn diagram.

Keywords

Simulated Conglomerates, Passive Elliposidal Particles, Tectonic Strain Ratio, Initial Axial Ratio, Mean Shape.

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Prakash P. Roday ¹, Yashwant B. Katpatal ¹

Affiliations
1 Department of Applied Geology, Dr. H. S. Gour University, Sagar - 470 003, IN

Abstract

Any λ1 λ3 principal sectional boudin shape is considered to depart from the ideal ellipse and its overall shape could be described in terms of the average four values of the exponent, p, one for each quadrant. Based on this, some principal shapes and their geometrical variations are characterised and possible mechanical and kinematic significance of each shape outlined. The values of the exponent, p, were calculated using a digital computer.

Keywords

Boudins, Ellipses, Kinematics, Structural Geology.

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Prakash P. Roday ¹, P. Diwan ¹, Amiya Pal ¹

Affiliations
1 Department of Applied Geology, Dr. H.S. Gour University, Sagar 470 003, IN

Abstract

The rocks described as esmeraldites in the southern part of Centra1 Indian Bundelkhand granitic massif are suggested to the typically those formed by heterogeneous simple shear and by development of retarded runway shear instabilities in host granites.

The development of shear zones appears to have occurred under two different stress fields. The early stress field was subhorizontal and nearly equatorial with low values of ESE trending σ₃ and low ¯σ in statistically isotropic material (elastic response). This produced positive dilational shear zones marked by intrution of pegmatite, filling en echelon tensile fissures. The early shear zones were converted into regular ductile shear zone at a later stage under a subhorizontal meridional stress field within highly compressive NNE trending σ₃ and high ¯σ (viscous response within previously weakened zone). This produced a subvertical or steeply dipping ESE trending fabric. Only this two-stage model is able to explain the development of these zones. Finite strain determinations are made from deflected basic dykes, deformed quartz veins and based on the angular relationship between C and S surfaces. The dilation during early episode is indeteminate because of rotation during latershearing. The shear along fabric suggests that the foliation failed to rotate with rotating λ₁ λ₂ planes of the simple shear. The finite strains computed on the basis of angular departures between fabric and zone walls are therefore slightly underestimated.

Keywords

Structural Geology, Shear Zone, Uttar Pradesh.

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Yashwant B. Katpatal ¹, Prakash P. Roday ²

Affiliations
1 Dept. of Civil Engg., V.R.C.E., Nagpur - 440 011, IN
2 Department of Applied Geology, Dr. H.S. Gour University, Sagar - 470003, M.P., IN

Abstract

Under large finite tectonic deformation and under a condition of low viscosity contrast, the axial orientational fluctuation method based on sine functions alone underestimates the values of a and b measured in the field. It is found that a better estimate of a and b and therefore of Rs could be obtained by using both sine and cosine functions. The parameters l, m and n introduced never equalise a, b and k respectively except when k = n = 0 (I = 1, m = α, k = n = 1 (1 = m but both greater than unity) and k = n = α (1 = α, m = 1). Usage of this method enhances the latitude of the ratios based exclusively on sine functions, thereby giving higher values of a and b despite small degree of orientational fluctuation of axes of ellipsoidal particles.

Keywords

Tectonics, Finite Strain Determination.

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Prakash P. Roday ¹

Affiliations
1 Department of Applied Geology, Dr. H. S. Gour University, Sagar - 470 003, Madhya Pradesh, IN

Abstract

Software for preparing plots to display the finite strain data such as (i) Flinn diagram (a against b), (ii) Ramsay's Diagram (a"1 against b"1), (iii) Logarithmic Flinn diagram (In a against In b or £,- £₂ against £₂-£₃), (iv) Burns and Spry triangular diagram (log x/r, log y/r, log z/r), (v) Plot of £s against Lode's parameter, v (Hossack Diagram) and finally (vi) Rf/plot (of Dunnet) have been written by the author and the zipped or cabinet (.ZIP or .CAB) files of the Setup or Install programs of this software can be downloaded from the URL: http://www.structural-geology-portaI.com/ downloads.html. The user has an option of superimposing Vx and V7 or r and p curves on the first three of these plots. The curves of equal a.b, equal a⁴.b⁴ or equal (In a).(In b) can also be superimposed by the user. The user can enter a caption for the plot and change its font and font attributes and colour; and the colour of labeling, colour of coordinates on graph and the background colour of the graph. The data is plotted as hollow or solid squares but in a recent version not available for downloading yet, the user can choose the symbols such as cross, plus, star (asterisk), circle (filled or hollow) etc. An object custom control captures the exact graph in a separate window where it can be COPIEd, SAVEd under a valid filename; PRINT PREVIEWed by using another object custom control and finally PRINTed by using a third object control. Apart from these six plots, a single composite software which can be used for using any of the six plots is also written, and its zipped Setup or Install file, the sixStrain. zip can be downloaded from the URL mentioned above. This brief paper is intended to give the details of this software bunch.

Keywords

Software, Structural Geology, Visual Basic 5 and 6, Finite Strain, Object Custom Controls.

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Prakash P. Roday ¹, Manish K. Purohit ², K. K. Prajapati ²

Affiliations
1 7, Vastu Vihar, Khajuri Kalan Road, Piplani, Bhopal - 462 022, IN
2 Department of Applied Geology, Dr. H. S. Gour University, Sagar - 470 003, IN

Abstract

Fry method enables rapid estimate of finite strain from deformed aggregates such as clastic grains, fossil colonies, oolitic or pisolitic aggregates, prophyroblastic minerals or phenocrysts. It has an advantage over the other methods of finite strain analysis in its very quality of enabling rapid estimation with a reasonable degree of accuracy. Details of the software to prepare a plot using Fry method are outlined. This program has an advantage over other computer based programs on the world wide web in its aesthetic getup, small size, user friendliness and a help file.

The program is a freeware and can be downloaded by following the links on a software website http://www.alstructural-geology-software.com.FryPlotSetup.zip or http://www.al-structural-geology-software.com/FryProg/ FryPlotProgSetup.exe.

Keywords

Fry Method, Centre-To-Centre Technique, Finite Strain, Computer Program, Visual Basic, Deformed Aggregates.

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