The Role of Sediment Accretion and Buoyancy on Subduction Dynamics and Geometry. Issue 20 (26th October 2021)

• Record Type: Journal Article
• Authors: Brizzi, S.; Becker, T. W.; Faccenna, C.; Behr, W.; van Zelst, I.; Dal Zilio, L.; van Dinther, Y.
• Is Part Of: Geophysical research letters. Volume 48:Issue 20(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-10-26
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL096266
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Subducted sediments are thought to lubricate the subduction interface and promote faster plate speeds. However, global observations are not clear‐cut on the relationship between the amount of sediments and plate motion. Sediments are also thought to influence slab dip, but variations in subduction geometry depend on multiple factors. Here we use 2D thermomechanical models to explore how sediments can influence subduction dynamics and geometry. We find that thick sediments can lead to slower subduction due to an increase of the megathrust shear stress as the accretionary wedge gets wider, and a decrease in slab pull as buoyant sediments are subducted. Our results also show that larger slab buoyancy and megathrust stress due to thick sediments increase the slab bending radius. This offers a new perspective on the role of sediments, suggesting that sediment buoyancy and wedge geometry also play an important role on large‐scale subduction dynamics. Plain Language Summary: At subduction zones, an oceanic plate dives into the mantle below another plate. The downgoing plate is usually covered by sediments. These sediments can be carried down to depth along the interface and/or scraped off the top of the downgoing plate and appended to the edge of the upper plate, forming an accretionary wedge. Sediments subducted to depth act as a lubricant, influencing the shear resistance of the interface, and in turn, downgoing plate speed. However, natural data show that slow subduction can be associated with thick sediments. Sediments are also thought to affect the dip angle of the downgoing plate, but subduction geometry is also influenced by other factors. We conducted a numerical modeling study to understand the effect of sediment thickness and density on the downgoing plate speed and dip. We observe that thick sediments on the downgoing plate lead to a slower subduction and a shallower dip, due to the decrease in slab pull and increase of stress along the contact interface associated to a bigger accretionary wedge. Our findings suggest that the effect of sediments might be not limited to the lubrication of the contact interface, but buoyancy and accretionary wedge size also play a role. Key Points: We conduct 2D thermomechanical models of subduction with variable sediment thickness and density Thick sediments can increase resistance along the subduction interface and decrease slab pull leading to a slower subducting plate Sediments can act as a lubricant for long‐term subduction, but buoyancy and accretionary wedge development are also important