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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS001 Oral Presentation 1717
Investigation of extensional deformation of ductile rocks
Dr. Shayesteh Mehrabian
EARTH PHYSICS INSTITUTE OF GEOPHYSICS, UNIVERSITY OF TEHRAN IASPEI
Knowledge of the deformation and particularly ductile flow properties of both the crust and the mantle
is important for better understanding a wide range of geological and geophysical processes. Models of
earthquake distribution, extensional tectonics and convection all require appropriate flow laws for the
rocks in the regions of interest. Structural and microstructural studies of in situ rocks or of samples from
boreholes represent an important way for learning about natural rock deformation. Mechanical tests on
rocks, over a wide range of temperature, pressure and strain rate followed up by microstructural studies
of the deformed materials can provide additional information. Experimental deformation of rocks has
provided much of the information on which the choice of constitutive laws is based. One of the aims of
geological and geophysical studies is to gain a deeper insight into the physics of deformation
mechanisms in rocks in order to be able to extrapolate constitutive equations obtained in experiments to
inaccessible conditions of strain rate and time in nature. One problem of experimental techniques,
however, is the inaccessibility of microstructural data during the deformation processes. In this study
the main purpose is to simulate the behavior of a cylindrical marble rock sample of 10 mm diameter and
20 mm length in extension with necking under high temperature and pressure conditions and to
compare the model results with the observed behavior of the rock sample in real high
pressure/temperature laboratory experiments which performed by Rutter (1995). By using a numerical
approach based on the finite element method the deformation of the sample is modeled. The
discretised equations for a mesh of eight-node quadrilateral elements in the case of visco-plastic
behavior, including power law flow, which is expected to correspond to the real situation, are solved. In
order to be able to assess the capability of the model to simulate the ductile behavior, the geometry
and boundary conditions are appropriate to experimental studies. The validation process of the
numerical frame-work has been made by using the analytical calculations for a single element with
uniform deformation conditions. From the obtained results of the numerical calculations it appears that
responses of the program in terms of displacement-force curves are very similar to the experimental
results and the deformed shape corresponds to diffusive necking deformation along the sample length.
It can be concluded that the strain-dependent flow law together with a consideration of the temperature
gradient can be applied to establish a framework for the numerical modeling of the necking
phenomenon or potentially the evolution of localized shear zones. Hence this approach with a
constitutive model offers a capability to simulate some aspects of rock deformation in extension at least
up to 20% bulk strain, and can be used to set up more realistic and complex applications to a wide
range of conditions for comparison with the experimental results.
Keywords: deformation, visco plastic, constitutive flow law