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Abstract Antigorite is a hydrous sheet silicate with strongly anisotropic seismic and rheological properties. Hydrous minerals such as antigorite have been invoked to explain numerous geologic observations within subduction zones including intermediate‐depth earthquakes, arc volcanism, the persistent weakness of the subduction interface, trench‐parallelSwave splitting, and episodic tremor and slip. To understand how the presence of antigorite‐bearing rocks affects observations of seismic anisotropy, three mylonites from the Kohistan palaeo‐island arc in Pakistan were analysed using electron backscatter diffraction. A fourth sample, which displayed optical evidence for crystallographically controlled replacements of olivine, was also investigated using electron backscatter diffraction to identify potential topotactic relationships. The resulting data were used to model the bulk seismic properties of antigorite‐rich rocks. The mylonitic samples exhibit incredibly strong bulk anisotropy (10–20% for the antigorite + olivine). Within the nominally undeformed protomylonite, two topotactic relationships were observed: (1) (010)ant//(100)ol with [100]ant//[001]ol and (2) (010)ant//(100)ol with [100]ant//[010]ol. However, the strength of a texture formed by topotactic replacement is markedly weaker than the strength of the textures observed in mylonitic samples. Since antigorite is thought to be rheologically weak, we hypothesise that microstructures formed from topotactic reactions will be progressively overprinted as deformation is localised in regions with greater percentages of serpentine. Regions of highly sheared serpentine, therefore, have the potential to strongly influence seismic wave speeds in subduction settings. The presence of deformed antigorite in a dipping structure is one explanation for observations of both the magnitude and splitting pattern of seismic waves in subduction zones.
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This content will become publicly available on March 1, 2026
Exploring microstructures and anisotropies of serpentinites
Abstract Serpentine minerals have received a lot of attention because of their unique crystal structures, their wide occurrence in orogenic belts and their potential role in contributing seismic anisotropy in subducting slabs. Several studies have investigated crystal preferred orientation (CPO) in high temperature antigorite serpentinites from Japan, the Alps, Spain, Cuba and Tibet, documenting significant crystal alignment. However, only a limited number of lower grade serpentines have been explored to date. Mainly because of submicroscopic microstructural heterogeneities CPO cannot be measured with conventional methods such as optical microscopy and EBSD. In this study 15 serpentinites from different tectonic settings in California, the Central Alps and Northern Spain have been investigated, mainly with high energy synchrotron X-ray diffraction, to quantify bulk crystal alignment. We find that CPO is strong on sheared surfaces of fractured blocks and secondary veins but the bulk of most serpentinite samples, except high-grade recrystallized antigorite serpentinite, show only weak crystal alignment. Correspondingly calculated seismic anisotropy based on CPO is not very significant. This is supported by very heterogeneous microstructures as documented with SEM and TEM analyses.
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- PAR ID:
- 10598872
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Contributions to Mineralogy and Petrology
- Volume:
- 180
- Issue:
- 3
- ISSN:
- 0010-7999
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Using numerical models, we compute the evolution of the mantle flow field and the crystal preferred orientation (CPO) of mineral aggregates in the mantle wedge of generic subduction systems from their nascent to mature stage and investigate shear wave splitting (SWS) through the forearc mantle wedge corner and overriding crust. Upon subduction initiation, the maximum depth of slab‐mantle decoupling (MDD) is relatively shallow (∼20 km depth), resulting in mantle flow and CPO development in the wedge corner. As subduction continues, the MDD deepens, the wedge corner cools and stagnates, and the olivine CPO becomes frozen‐in. In the cool wedge corner, antigorite can form if water is available. In non‐deforming mantle, antigorite CPO develops relative to the host olivine CPO through topotactic growth. We calculate splitting parameters of synthetic local S waves based on the model‐predicted A‐ and B‐type olivine CPOs and topotactically grown antigorite CPO that replaces A‐type olivine CPO in the wedge corner. The fast direction is trench‐normal for A‐type olivine and antigorite CPOs and trench‐parallel for B‐type. When the delay times are long enough (>0.1 s), we find them positively correlated with the thickness of the mantle wedge corner. In NE Japan, where the results of detailed analyses on the spatial variation of the SWS parameters are available, such correlation is not observationally reported. However, the addition of an anisotropic overriding crust provides delay times (∼0.1 s) and trench‐normal fast directions that are consistent with the local SWS observations.more » « less
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