FG B4  ./ 4 +῎14 ,**+ wStructural Geology, No. ./, +῎1, ,**+x

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Recent Progress in Experimental Structural Geology *
Shun-ichiro Karato*
Abstract : Signiﬁcant progress has been made in the experimental structural geology
due mainly to the advancement of experimental technology. They include the development of new apparatus or techniques to conduct large-strain, shear deformation
experiments under high-pressure and temperature conditions and a technique ofhigh-resolution micro-fabric measurements such as EBSP. Signiﬁcant microstructural
evolution at high strain has been documented in both calcite and olivine associatedwith dynamic recrystallization which results in grain-size reduction and associatedrheological weakening. However, no evidence of shear localization was observed inthese experiments. Some thoughts to resolve this puzzling observation are presented
emphasizing the role of strong constraints on deformation geometry imposed in torsion
tests. Using newly developed technique of large-strain shear deformation at highpressures, new types of fabrics have been identiﬁed in olivine under high waterfugacity conditions. They provide a new interpretation of observed wide range of
fabrics in peridotites and the complicated pattern of seismic anisotropy in subduction
zones. Microscopic models for the origin of water-induced fabric transition are proposed in which the role of anisotropic Peierls stress is emphasized.
Key words : experimental structural geology, lattice preferred orientation, shear localization, seismic anisotropy
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* Department of Geology and Geophysics, University of
Minnesota, +*2 Pillsbury Hall, Minneapolis, MN //.//

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Conceptual framework of “Experimental Structural Geology”

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Three methods of deformation experiment. A large strain, non-coaxial deformation is possible by methods (b) and (c).
(a) A triaxial compression test
This method is most common. A sample undergoes coaxial deformation.
(b) A sandwich method
A thin sample is sandwiched between two strong pistons. Axial motion of pistons is transformed to (nearly) simple shear
deformation of a sample.
(c) A torsion test
A cylindrical sample is twisted by a rotational actuator. The displacement velocities, u, of materials along a line from thecenter follow u¸rw where r : distance from the center and w : angular velocity of rotation. Therefore the strain-rate of a¹material increases with distance r from the center as e¸rw/ (h : thickness of the sample).

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@3- Carter & Avé Lallemant #+31*& Fig. Dominant slip systems in olivine (modiﬁed after
Carter and Avé Lallemant, +31*). The conditions at which
dominant slip systems change correspond roughly toconstant stress values.

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scale). If the ﬂow law were the power law with the samestress exponent, then all the lines for di#erent slip
systems are parallel in this diagram and one would not
expect any changes in slip systems when stress magnitude is changed. However, when the Peierls mechanism
becomes important for certain slip systems in which a.
ῌ E**«Cs῎ operates, thenﬂow law of the type e ¹ exp «RT ῏
῍there will be a change in the dominant slip systems whenstress is increased.
Water could also modify theparameter C, leading to a water-induced fabric transition(s).

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Fig. /
Fabric diagram for olivine in the water content-stress space (modiﬁed after Jung and Karato, ,**+).
A, B, C, D correspond to various fabric types identiﬁed in our study. Dominant slip systems inferred foreach fabric are shown in the legend.

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Bai, Q., Mackwell, S. J. and Kohlstedt, D. L., +33+,
High temperature creep of olivine single crystals, +. Mechanical results of bu#ered samples,
J. Geophys. Res., 30, ,..+ῌ,.0-.
Bystricky, M., Kunze, K., Burlini, L. and Burg, J-P.,,***, High shear strain of olivine aggregates :¼ 6 ¼

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rheological and seismic consequences, Science,,3*, +/0.ῌ+/01.
Carter, N. L. and Avé Lallemant, H. G. +31*, Hightemperature ﬂow of dunite and peridotite,
Geol. Soc. Amer. Bull., 2+, ,+2+ῌ,,*,.
Fressengeas, S. and Molinari, A., +321, Instability
and localization of plastic ﬂow in shear at highstrain rates, J. Mech. Phys. Solids, -/, +2/ῌ,++.
Jung, H. and Karato, S., ,**+, Water-induced fabrictransitions in olivine, Science, in press.
Karato, S., +33/, E#ects of water on the seismic
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