Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin

TLDR

Flat subduction occurs in about 10 % of modern convergent margins and is observed in regions such as central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. The study investigates the causes and geodynamic impacts of flat subduction. The authors examine the 1500 km Peru flat‑slab segment using seismicity and tomographic imaging. Seismicity and tomography of the Peru flat slab reveal two morphologic highs linked to subducted oceanic plateaus, a 3‑D slab geometry with a possible lithospheric tear and detachment, and show that flat subduction—driven mainly by buoyant, thickened oceanic crust and delayed basalt‑to‑eclogite transition—occurs in about 10 % of convergent margins and produces 3–5 × more upper‑plate seismic energy 250–800 km from the trench due to enhanced interplate coupling and a cold, strong overriding lithosphere.

Abstract

The cause and geodynamic impact of flat subduction are investigated. First, the 1500 km long Peru flat slab segment is examined. Earthquake hypocenter data image two morphologic highs in the subducting Nazca Plate which correlate with the positions of subducted oceanic plateaus. Travel time tomographic images confirm the three‐dimensional slab geometry and suggest a lithospheric tear may bound the NW edge of the flat slab segment, with possible slab detachment occurring down dip as well. Other flat slab regions worldwide are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. Flat subduction is shown to be a widespread phenomenon, occuring in 10% of modern convergent margins. In nearly all these cases, as a spatial and temporal correlation is observed between subducting oceanic plateaus and flat subduction, we conclude that flat subduction is caused primarily by (1) the buoyancy of thickened oceanic crust of moderate to young age and (2) a delay in the basalt to eclogite transition due to the cool thermal structure of two overlapping lithospheres. A statistical analysis of seismicity along the entire length of the Andes demonstrates that seismic energy release in the upper plate at a distance of 250–800 km from the trench is on average 3–5 times greater above flat slab segments than for adjacent steep slab segments. We propose this is due to higher interplate coupling and the cold, strong rheology of the overriding lithosphere which thus enables stress and deformation to be transmitted hundreds of kilometers into the heart of the upper plate.