Seismic section L5-7 crosses the outer forearc of the central Aleutian Ridge south of Adak Island, roughly along 175°W longitude. In this region the direction of underthrusting of the Pacific plate beneath that of the ridge is approximately 30° northwest of a direction normal to the regional trend of the Aleutian Island arc and its paralleling trench. Geomorphically, the ridge includes two provinces: the cresting massif of the island arc and the more deeply submerged forearc region about 4 km deep (see idealized structural section, p. 12). The forearc comprises the broad platform of the Aleutian Terrace and the flanking landward slope of the adjacent Aleutian Trench.

The Aleutian Ridge is constructed of three principal rock sequences. The ridge's igneous basement—the arc massif—consists mainly of extrusive and intrusive masses and related coarse volcaniclastic rubble of Eocene age (Scholl et al., in press; Vallier et al., in press). These rocks constitute the ridge's lower series. Regional information implies that lower series rocks began to accumulate between 55 and 50 Ma (early to middle Eocene). Rapid magmatic growth of the massif waned between about 45 and 40 Ma, and by at least earliest Oligocene (37 Ma) the submerged flanks of the ridge began to be buried by a thickening blanket of volcaniclastic and pelagic deposits, sandy and silty strata that constitute the ridge's middle series. Middle series sediment continued to accumulate until roughly the end of the Miocene (5 to 6 Ma), when the structural basin of the Aleutian Terrace began to