Abstract
New estimates of long-term consumption of Japanese and Peruvian continental forearc margins together with recent drilling data (Ocean Drilling Program) on some other erosional island arcs such as Izu-Bonin, Mariana, or Tonga allow the calculation of a mean rate of 7 6 3 km/m.y. landward migration of the arc-trench system on both east and west sides of the Pacific basin during Cenozoic time. Such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the Atlantic Ocean. Retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the Pacific basin. Mass transfers in subduction zones must be reconsidered in the light of these new estimates, especially the balance between net growth and loss of continental crust in recent geologic time. A 50 m.y. period is long enough to erode the initial width of the volcanic-arc and fore-arc massif of any subduction zone,assuming a mean trench–volcanic-arc landward migration of 5 km/m.y. and a mean trench–volcanic-arc distance of 250 km. This circumstance thus raises the question of whether the Izu-Bonin-Mariana and Tonga arcs are older than their reported Eocene age.