A new approach to modeling perpendicular fabrics in porphyritic plutonic rocks using the finite element method |
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| Authors: | Jiří Žák Petr Kabele |
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| Institution: | 1.Institute of Geology and Paleontology, Faculty of Science,Charles University,Prague,Czech Republic;2.Czech Geological Survey,Prague,Czech Republic;3.Department of Mechanics, Faculty of Civil Engineering,Czech Technical University in Prague,Prague,Czech Republic |
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| Abstract: | A growing body of field evidence indicates that hypersolidus fabrics preserved in syntectonic plutons are likely to have formed
in highly crystallized ‘rigid sponge’ magma. This paper demonstrates that such magma could be idealized as a rheological solid
and that the development of non-coaxial fabrics in plutonic rocks can very conveniently be modeled in the framework of solid
mechanics. Using the finite element method (FEM), we modeled two strain regimes of small magnitudes (plane-strain horizontal
simple shear with the shear strain γ of up to 0.30 and plane-strain pure shear of up to 15% shortening) superposed onto vertically
oriented and variously spaced elastic phenocrysts set in the viscoelastic matrix. In the simple shear regime, the phenocrysts
slightly rotate toward the shear plane, while the principal strain directions in the matrix are instantaneously oriented at
an angle of about 45° or less to the phenocryst fabric. Simple shear thus can only lead to the formation of oblique phenocryst
and matrix fabrics. By contrast, the vertical phenocryst fabric is maintained in the pure shear regime, and a new horizontal
fabric can develop almost instantaneously in the matrix even for small amounts of superposed shortening (5% shortening after
10 ky in our model). We conclude that such a mechanism can easily produce perpendicular hypersolidus fabrics in plutonic rocks
and that only a very short time span (first thousands of years) is required to develop magmatic fabric in a pluton for ‘normal’
rates (10−15 to 10−13 s−1) of tectonic deformation. |
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