Deformation of Antigorite and Its Geological Implications. Issue 6 (18th June 2021)

• Record Type: Journal Article
• Authors: Shao, Tongbin; Zhou, Yongsheng; Song, Maoshuang; Ma, Xi; Zhang, Lei; Yao, Wenming; Dang, Jiaxiang; Li, Jianfeng
• Is Part Of: Journal of geophysical research. Volume 126:Issue 6(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-18
• Subjects: Antigorite -- deformation geometry -- earthquakes -- semibrittle flow -- slab‐mantle wedge decoupling -- subduction zones; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB021650
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: The deformation model of antigorite and its relationship to intermediate‐depth earthquakes are still controversial. Here, we conducted deformation experiments on foliated iron‐poor antigorite at confining pressures of 1, 100–1, 500 MPa, temperatures of 400–600°C, and axial displacement rates of 3.75 × 10 −6 –7.24 × 10 −4 mm/s using a solid‐medium Griggs‐type apparatus. As the temperature and/or pressure increases, the stress exponent decreases, and all samples show stable fault slip. Microstructural observations show ductile behavior indicated by kinking and bending of the originally straight foliation, and brittle characteristics, such as Riedel shear, comminution, and microcracking. Therefore, antigorite deformation is controlled by a semibrittle regime. Antigorite decomposed to forsterite and enstatite in samples sheared at temperatures ≥550°C but not in samples triaxially compressed at 600°C. This observation, combined with phase boundary information, implies that the deformation geometry (e.g., triaxial compression or shear) may change the phase stability of antigorite. Following dehydration, samples still show stable fault slip, indicating that dehydration does not directly trigger earthquakes. In cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces seismic activity by producing water and talc (or a talc‐like phase) in the process of shear friction because such phases within the fault zone may reduce the rate dependence, thereby inducing unstable slip. The comparison of the semibrittle strengths of antigorite and dunite and the weak strain rate dependence of stress suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge. Plain Language Summary: As one of the most important hydrous minerals in subduction zone, antigorite has many geodynamic significances. However, low dehydration temperatures and complex crystal structure have long prevented good characterization of its peculiar rheological properties and its relationship to intermediate‐depth earthquakes. In this study, we deformed foliated iron‐poor antigorite. We found that all samples show stable fault slip dominated by a semibrittle regime and thus dehydration does not directly trigger earthquakes. Combined with the phase boundary information determined by phase equilibrium experiments under static compression, our results suggest that deformation geometry (such as compression, simple shear, and the direction of foliation plane with respect to the maximum compression axis) may change the phase stability of antigorite. As water or talc or a talc‐phase phase was evidenced previously to likely reduce the rate dependence, thus we proposed that in cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces earthquakes by producing such phases during shearing. In the semibrittle regime, small difference in strength between antigorite and dunite ( σ (dunite)/ σ (antigorite) < 1.6) suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge. Key Points: The strength of antigorite has weak strain rate and temperature dependences, as well as moderate pressure dependence Dehydration cannot directly trigger earthquakes and the deformation geometry may change the phase stability of antigorite A thin antigorite layer is not sufficient to cause significant decoupling between the slab and the mantle wedge