Convergent Margin Structure in High-Quality Geophysical Images and Current Kinematic and Dynamic Models

Abstract

Understanding the mechanics of convergent margins is fundamental to assessing risks from earthquakes and trans-oceanic tsunamis. Marine observations of the past decade have advanced that understanding. A once commonly inferred accreted wedge extending from trench axes to shelves is now resolved into 3 domains of different mechanics in space, that vary during an earthquake cycle. The frontal prism increases weight on subducting materials elevating pore fl uid pressure and reducing interplate friction. The middle prism is moderately stable and merges into the more stable margin framework of the inner prism beneath the upper slope and shelf. Signifi cant accretion occurs as material from the frontal prism is added to the middle prism. Accretion is common along thickly (>1 km) sedimented trenches and slowly converging margins. Rapid convergence enhances the effi ciency of sediment subduction and subduction erosion. The subduction channel on the lower plate accepts a fi nite amount of trench sediment and any excess is added to the frontal prism on the upper plate. Erosion beneath the middle slope contributes material to the subduction channel. Erosion and accretion can be coeval, for instance, subducted seamounts erode the upper plate as adjacent sediment accretes. The change in strain during interseismic locking that is released during coseismic slip, changes the dynam- ics of each segment in time. This helps explain extensional normal faults in a con- verging plate environment. Recent observations provide information for a unifying framework concept to aid interpretations of both accreting and eroding margins.