The Aleutian arc is a ridge-trench geomorphic system associated with active volcanism, high seismicity, and a hypothetical transition zone from underthrusting to non-underthrusting approximately midway along its length. Two contrasting acoustic signatures can be recognized on seismic reflection profiles in the area, and these are correlated with informally designated early series and late series rock units recognized on the Aleutian Islands. The two series are separated by an unconformity on the summit and flanks of the Aleutian Ridge. The early series consists of altered and deformed sedimentary and volcanic rocks of Eocene to middle Miocene age (45 to 14 m.y.). The late series includes generally unaltered and undeformed sedimentary and volcanic units that range in age from late Miocene to present (10 to 0 m.y.). Reflection profiles show that as much as several kilometers of the late series underlies the Aleutian Trench, the Aleutian Terrace, and summit basins on the Aleutian Ridge; the series reflects sediment draping and basin infilling on an extensionally fragmenting ridge. Magnetic and refraction data and preferred gravity models demonstrate that the southern Aleutian Terrace and inner (northern) Aleutian Trench wall are underlain by low-velocity, low-density rock.
A study of the sedimentary fill and inner wall of the trench revealed no structural difference between the postulated underthrust and non-underthrust segments, and it cannot be concluded on this basis that underthrusting has occurred. However, the gravity data (and magnetic and refraction data) do allow the inference that sedimentary offscrapings could form the inner trench wall and underlie the Aleutian Terrace along the postulated zone of underthrusting. Recent JOIDES data from the North Pacific suggest a limited amount (less than 1,000 km) of underthrusting since the Eocene; this interpretation is most compatible with the model of discontinuous plate motion for the North Pacific deduced earlier by Pitman and Hayes. The tectonic, magmatic, and depositional histories of the arc also agree with their model and suggest an episode of active underthrusting in the early Tertiary, a slowing or cessation throughout the middle Tertiary, and a resumption of underthrusting in the late Cenozoic. The early Tertiary underthrusting is indicated by the rapid growth of the ridge during this time. Cessation or slowing in the middle Tertiary is apparently marked by uplift and folding, widespread epizonal plutonism (but no intense volcanic activity), and formation of the unconformity separating the rocks of the early and late series. Resumption of underthrusting in the late Cenozoic is correlated with the outbreak of the present episode of volcanism and fragmentation of the ridge crest to form numerous summit basins.