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

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 corre late with the positions of subducted oceanic plateaus. Travel time tomograp hic images confirm the three-dimensional slab geometry and suggest a lithos pheric tear may bound the NW edge of the flat slab segment, with possible s lab detachment occurring down dip as well. Other fiat slab regions worldwid e are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, s outhern 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 obser ved between subducting oceanic plateaus and flat subduction, we conclude th at flat subduction is caused primarily by (1) the buoyancy of thickened oce anic crust of moderate to young age and (2) a delay in the basalt to eclogi te transition due to the cool thermal structure of two overlapping lithosph eres. A statistical analysis of seismicity along the entire length of the A ndes demonstrates that seismic energy release in the upper plate at a dista nce of 250-800 km from the trench is on average 3-5 times greater above fla t slab segments than for adjacent steep slab segments. We propose this is d ue to higher interplate coupling and the cold, strong rheology of the overr iding lithosphere which thus enables stress and deformation to be transmitt ed hundreds of kilometers into the heart of the upper plate.