Formation of Horizontally Deflected Slabs in the Mantle Transition Zone Caused by Spinel‐to‐Post‐Spinel Phase Transition, Its Associated Grainsize Reduction Effects, and Trench Retreat

Abstract

Seismic observations reveal distinctly different morphologies of subducted slabs in the mantle transition zone (Goes et al., 2017). At some subduction zones (e.g., the North America and the Central America), slabs penetrate into the lower mantle and could reach the core-mantle-boundary (e.g., Grand et al., 1997;van der Hilst et al., 1997); while at other subduction zones (e.g., in the Honshu, Bonin, and Chile), slabs appear to be deflected and extend horizontally over a long distance in the mantle transition zone above the 670 km depth (French & Romanowicz, 2014Fukao & Obayashi, 2013;Ritsema et al., 2011) (note that the horizontally deflected slabs are sometimes referred to as "stagnant" slabs in literature, although the slabs are not stationary in the mantle transition zone). Various factors affect subduction zone dynamics and contribute to the formation of deflected slabs in the mantle transition zone including trench retreat, viscosity jump from the mantle transition zone to the lower mantle, the endothermic phase change of spinel-to-post-spinel at ∼670 km depth, slab age and rheology, and nonequilibrium pyroxene garnet transition (e.g.,

On the Origin of Small‐Scale Seismic Scatters at 660‐km Depth

“…These types of models with realistic thermodynamic properties are more physically self‐consistent compared with previous models with either simplified thermodynamic properties or purely statistical models (Nakagawa et al., 2010). For the short‐term (tens of Myrs) subduction models, these new models show similar dynamic patterns as previous studies (Christensen, 1996; Mao & Zhong, 2021; Zhong & Gurnis, 1995). One of the largest differences is the complex density and seismic velocity pattern in the mantle transition zone (Figures S8–S10 in Supporting Information ), which previous models could not capture.…”

“…Similar to previous computations of subduction zone dynamics, we varied trench retreat velocity to explore slab dynamics in the mantle transition zone. Our short‐term regional subduction models show similar dynamic evolution as in previous models with more simplified density (e.g., Christensen, 1996; Mao & Zhong, 2021; Zhong & Gurnis, 1995).…”