A B C D E F G H I J K L M N O P Q R S T U V W X Y Z All
Shah, Jyoti
- Mesoscale Fractures as Palaeostress Indicators: A Case Study from Cauvery Basin
Authors
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Pondicherry University, Pondicherry - 605 014, IN
3 Reliance Industries Ltd, Navi Mumbai - 470 110, IN
4 Department of Geology, National College, Tiruchirapalli - 620001, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 70, No 4 (2007), Pagination: 571-583Abstract
This paper presents the results of field studies, and palaeostress analyses of the mesoscale fractures and veins in Cauvery basin. It shows that different sedimentary sequences (119-64 Ma) are cut by tensile structures that belong to two successive phases of post-Palaeocene brittle tectonics (I) an early phase during which bedding parallel fractures and veins were developed due to horizontally directed maximum compression and vertical extension, and (II) a late phase of hydraulic fracturing in a tectonic regime of vertically directed maximum compression. Dynamic analyses imply triaxial and axial compression deviatoric states of palaeostress during first and second phases of fracturing, respectively. Very high pore-fluid pressure and low differential stress, during the second phase of fracturing, resulted into hydraulic brecciation as a consequence of simultaneous extension in different orientations. We suggest that the development of the mesoscale extensional (mode I) fractures in Cauvery basin is due to reactivation of large-Scale normal faults in the basement rocks.Keywords
Extensional Fracturing, Hydraulic Fracturing, Pore-Fluid Pressure, Cauvery Basin, Tamil Nadu.
- Limitations of Conjugate Fractures and Scope of Reactivated Faults in Palaeostress Analysis
Authors
1 Department of Earth Sciences, IIT Roorkee, Roorkee 247 667, Uttarakhand, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 71, No 3 (2008), Pagination: 303-312Abstract
This article reviews the conditions that must be satisfied for application of the Anderson's law to decipher the orientations of the principal stresses from conjugate pairs of shear fractures/faults. It shows that the reactivated faults and fractures are more powerful stress indicators than the Andersonian fractures, because the former can reveal not only the orientations of the principal stresses, but also the relative magnitudes of the principal stresses in terms of the shape of the stress ellipsoid. A few commonly used graphical and numerical methods for palaeostress analysis from the reactivated faults are evaluated critically. Finally, the significance of relative magnitudes of the principal stresses is demonstrated with an example of the striated faults cutting through the Amritpur granite in the Kumaun Lesser Himalaya.Keywords
Conjugate Faults, Anderson's Law, Reactivated Faults, Striae, Stress Tensor, Stress Ratio.- Strain Estimation from Single Forms of Distorted Fossils - A Computer Graphics and MATLAB Approach
Authors
1 Schlumberger Asia Services Limited, Goregaon(E), Mumbai-400067, IN
2 Department of Earth Sciences, IIT Roorkee, Roorkee-247 667, IN
3 Department of Physics, IIT Roorkee, Roorkee-247 667, IN
4 Shell Technology India, Bangalore-560 048, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 75, No Spl Iss 1 (2010), Pagination: 89-97Abstract
Most of the existing methods of strain analysis can estimate strain in a single form of distorted brachiopod, or trilobite provided independent evidence, such as the association of the fossil with cleavage and/or stretching lineation is available for inferring the direction of maximum principal strain. This article proposes a simple computer graphics based method and its MATLAB code that determine the minimum amount of strain in a single distorted fossil form even if data for inferring the maximum principal strain direction are lacking. Our method is a rapid computer-graphics alternative to some of the existing analytical methods.
In a distorted fossil form of original bilateral symmetry, the relative senses of angular shears along the hinge line and the median line are mutually opposite to each other. It follows, therefore, that the maximum principal strain direction lies within the acute angle between the hinge and the median lines in the plane of the fossil. Using this principle, our method performs several simulations such that each simulation retrodeforms the distorted fossil by assuming a particular orientation, lying within the acute angle between the hinge line and the median line, as the potential direction of the maximum principal strain. Each simulation of retrodeformation yields a potential strain ratio. The distribution of all the potential strain ratios, obtained by assuming different orientations as the potential directions of the maximum strain, is typically a parabola-like curve with a distinct vertex that corresponds to the minimum amount of strain in the distorted fossil. An entirely computer graphical approach is somewhat time-intensive because it involves a large number of retrodeformational simulations. We, therefore, give a MATLAB code, namely, the Minstrain, that rapidly retrodeforms the fossil and determines the minimum strain with precision.
Keywords
Strain, Distorted Fossil, Lineation, Cleavage, MATLAB.References
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