FOREARC TECTONICS Arc-Trench Systems The overall geotectonic settings of arc-trench systems are highly varied (Fig. 1). Some volcanic arcs have oceanic lithosphere in the backarc region as well as in front of the trench. The maximum thickness of crust in such intraoceanic arcs is intermediate between standard oceanic and normal continental values. Some intraoceanic arcs display extensional deformation of lithosphere in the backarc region, as interarc basins there open by a type of seafloor spreading that occurs during the time of igneous activity in the arc. In other places, however, the oceanic lithosphere in the backarc is apparently older than any of the arc activity. Other arcs that he along the edges of continental blocks have crustal thicknesses comparable to or far in excess of those beneath cratons in continental interiors. Some of these continental-margin arcs display contractional deformation along fold-thrust belts in the backarc region. Others, however, do not. Some display extensional deformation either along or just behind the igneous belt, but reverse faults manifest compression affecting this zone in some arc-trench systems. In concentrating exclusively here on the forearc region, we are aware that some aspects of forearc evolution may be influenced strongly by geologic circumstances and styles of tectonism in the backarc region. The most important of such factors are probably those that bear on the crustal thickness in the arc massif, and on the rate and kind of sediment delivery to the forearc area. Nevertheless, the dominant controls on forearc evolution are seemingly related to the geodynamic effects of subduction. We are thus able to discuss forearc development as a separate and co