Fault trends on the seaward slope of the Aleutian Trench: Implications for a laterally changing stress field tied to a westward increase in oblique convergence

Abstract

Normal faults along the seaward trench slope (STS) commonly strike parallel to the trench in response to bending of the oceanic plate into the subduction zone. This is not the circumstance for the Aleutian Trench, where the direction of convergence gradually changes westward, from normal to transform motion. GLORIA side‐scan sonar images document that the Aleutian STS is dominated by faults striking oblique to the trench, west of 179°E and east of 172°W. These images also show a pattern of east‐west trending seafloor faults that are aligned parallel to the spreading fabric defined by magnetic anomalies. The stress‐strain field along the STS is divided into two domains west and east, respectively, of 179°E. Over the western domain, STS faults and nodal planes of earthquakes are oriented oblique (9°–46°) to the trench axis and (69°–90°) to the magnetic fabric. West of 179°E, STS fault strikes change by 36° from the E‐W trend of STS where the trench‐parallel slip gets larger than its orthogonal component of convergence. This rotation indicates that horizontal stresses along the western domain of the STS are deflected by the increasing obliquity in convergence. An analytical model supports the idea that strikes of STS faults result from a superposition of stresses associated with the dextral shear couple of the oblique convergence and stresses caused by plate bending. For the eastern domain, most nodal planes of earthquakes strike parallel to the outer rise, indicating bending as the prevailing mechanism causing normal faulting. East of 172°W, STS faults strike parallel to the magnetic fabric but oblique (10°–26°) to the axis of the trench. On the basis of a Coulomb failure criterion the trench‐oblique strikes probably result from reactivation of crustal faults generated by spreading.