Dynamics and implications of slab detachment due to ridge‐trench collision

The approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab. Previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism (e.g., in Baja California), but such a scenario has not been tested using fully dynamic numerical models. We use dynamic two‐dimensional models including a non‐Newtonian rheology to study the approach of a trench‐parallel spreading ridge to a subduction zone. We find that before the ridge approaches within 100 km of the trench, detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity. In models exploring effects of subducted slab length, distance of the ridge from the trench, shear zone strength, and lithospheric yield strength, we find the following: (1) a decrease in subduction velocity as the ridge approaches the trench, (2) a shrinking surface plate that maintains a uniform plate‐like subduction velocity, (3) ridge abandonment distances 100–275 km from the trench, and (4) slab gap distances 195–285 km from the trench. These results are consistent with observations in Baja California, where detachment of the Cocos slab may explain abandonment of observed segments of the East Pacific Rise 50–200 km outboard of the trench and the presence of nonarc related magmatism located 100–250 km inboard of the trench. Ridge subduction does not occur within any of these two‐dimensional cases of ridge‐trench collision, indicating that 3‐D geometric effects may be responsible for ridge subduction.