Geneses of Two Contrasting Antigorite Crystal Preferred Orientations and Their Implications for Seismic Anisotropy in the Forearc Mantle

Abstract

Crystal preferred orientation (CPO) of antigorite is an important parameter for the interpretation of seismic anisotropies and related geodynamic processes in subduction zones. However, mechanisms for the development of various antigorite CPOs and their effects on seismic anisotropies in the forearc mantle still remain to be constrained. In this study, we investigated olivine and antigorite CPOs and calculated seismic anisotropies of the serpentinized peridotites from Val Malenco, Central Alps. The results show that antigorite in the olivine-rich (weakly serpentinized) layer displays an L-type CPO (i.e., (h0l) plane//foliation and [010] axis//lineation), whereas antigorite in the antigorite-rich (highly serpentinized) layer develops an LS-b-type CPO (i.e., (001) plane//foliation and [010] axis//lineation). The antigorite L-type CPO is most likely formed by diffusion and dislocation creep accommodated phase boundary sliding and substantial grain rotation at a higher temperature and smaller strain regime. In contrast, the development of an antigorite LS-b-type CPO requires diffusion and dislocation creep accommodated grain boundary sliding, smaller contribution of grain rotation, and significant role of dissolution-precipitation creep at a lower temperature and larger strain regime. In this context, we propose an antigorite CPO distribution model in the forearc mantle. Based on this model and under vertically incident teleseismic waves, if significant serpentinization and asymmetric shearing do indeed occur within serpentinized layers, then strong and weak trench-parallel seismic anisotropies can be expected for cold and warm moderate-angle subduction zones, respectively. This model may provide an alternative interpretation on the seismic anisotropy observations in some modern subduction systems.