蛇紋岩の力学的性質とそのテクトニックな意義

蛇紋岩の力学的性質とそのテクトニックな意義
平内健一*
片山郁夫**
Rheological Properties of Serpentinite and Their Tectonic Significance
Ken-ichi HIRAUCHI* and Ikuo KATAYAMA**
[Received 3 August, 2014; Accepted 22 May, 2015]
Abstract
Serpentine minerals (lizardite, chrysotile, and antigorite) are a major group of hydrous
phyllosilicates resulting from the hydrothermal alteration of mantle peridotite. Their distinct
rheological properties mean that serpentine minerals have a strong influence on the mechanical
and seismogenic behavior of faults and plate boundaries in both continental and oceanic settings.
In this paper we review the results of laboratory experiments performed to understand the frictional and mechanical properties, and deformation mechanisms of serpentinite. Frictional sliding
experiments at low slip rates show that antigorite exhibits velocity-strengthening behavior (a − b
> 0) over a wide range of temperature (25–400°C), while values of (a − b) for chrysotile become
negative as temperature increases (25–281°C). This indicates that the stability of slip along serpentinite-bearing faults depends on the serpentine species and fault depth. Frictional sliding of
antigorite at seismic slip rates leads to weakening by flash heating. Axial compression experiments at confining pressures of up to 4 GPa show that antigorite is stronger than lizardite by at
least a factor of two. The flow law for dislocation creep of antigorite based on stress values at ~15%
strain also predicts differential stresses that are substantially lower than those for the dislocation
creep of olivine at natural strain rates (10−10 to 10−14 s−1). This suggests that the viscosity of serpentinite promotes slab-mantle decoupling. However, the antigorite flow law should be used with
caution because antigorite starts to deform by semi-brittle flow after ~20% strain. Large-strain
simple-shear deformation of antigorite aggregates at high pressure (1 GPa) results in a strong
alignment of antigorite c-axes normal to the shear plane. This observation explains the
trench-parallel anisotropy beneath the Ryukyu subduction zone. Although dehydration embrittlement is considered a primary cause of intermediate-depth earthquakes, recent high-pressure
experiments on antigorite show stable sliding behavior or detect no acoustic emissions during
dehydration reactions. We emphasize that the presence of talc derived from the metasomatic alteration of serpentine further weakens and stabilizes the slab–mantle interface and promotes
long-lived (>1 Ma) detachment faulting.
1
Key words： decoupling, dehydration embrittlement, earthquake, detachment fault, friction,
mantle wedge, rheology, seismic anisotropy, serpentinite, subduction zone
キーワード：デカップリング，脱水脆性化，地震，デタッチメント断層，摩擦，マントルウェジ，
レオロジー，地震波異方性，蛇紋岩，沈み込み帯
*静岡大学大学院理学研究科地球科学専攻
**広島大学大学院理学研究科地球惑星システム学専攻
+現所属：静岡大学理学部地球科学科
*Department
of Geosciences, Graduate School of Science, Shizuoka University, Shizuoka, 422-8529, Japan
**Department
of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima Universi-
ty, ,Higashi-Hiroshima, 739-8526, Japan
+Present
address: Department of Geosciences, Faculty of Science, Shizuoka University, Shizuoka, 422-8529, Japan
2

Download Report