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Mortera-Gutierrez, C.A., 1996

Seaward slope faults in the Pacific plate along the Aleutian trench

Bibliographic Reference

Mortera-Gutierrez, C.A., 1996, Seaward slope faults in the Pacific plate along the Aleutian trench: College Station, Texas, Texas A&M University, Ph.D. dissertation, 218 p., illust., maps.


Side-scan sonar images of the seaward trench slope along the Aleutian Trench provide constraints in the stress-field in the uppermost part of the oceanic lithosphere. The relationships between faults and shallow earthquakes are examined to define the stress-field along a convergent-transform plate boundary. GLORIA images show two trends of fault patterns in the Aleutian seaward trench slope (165 degrees E - 165 degrees W). Between Amlia F.Z. (AFZ) and Rat F.Z. (RFZ), most faults strike nearly parallel to the trench; whereas west of RFZ and east of AFZ, faults strike oblique to the trench (13-26 degrees). West of AFZ, earthquake nodal-planes of normal faults in the seaward trench slope have strikes nearly parallel to the trends of seaward slope faults; while east of AFZ, the nodal-planes are not concordant with the fault-trends. Most epicenters in the oceanic lithosphere are confined no farther than 2/3 of the trench-outer rise distance. Gravity data show that the outer-rise extends from Alaska Peninsula to Stalemate Ridge. West of RFZ, faults intersect the magnetic lineations at 52-74 degrees; whereas east of RFZ, faults strike nearly parallel to the magnetic lineations. Different orientations of fault-trends and nodal-planes indicate three strain-stress regions to the south of the trench. Western-region (169 degrees E - 179 degrees E), trench-oblique faults are 37 degrees from the fault-trends to the east of 179 degrees E. This change occurs where the trench-parallel slip component of convergence is larger than the trench-perpendicular component, implying that the lateral dextral shear-couple along this boundary redirects the stress in the oceanic lithosphere. Fault-trends and nodal-plane orientations are best explained by a stress-field near the trench that results from superimposing shear-stresses on bending-stresses. Central-region (179 degrees E - 173 degres W), fault-trends and nodal-planes are parallel to the trench and magnetic lineations. It is difficult to discern whether bending or the superimposed stresses control the fault-trends in this region. Eastern-region (173 degrees W - 165 degrees W), nodal-planes are aligned with the bending axis in the upper-part of the lithosphere; while faults are oriented parallel to pre-existing faults associated with seafloor spreading. Fault-trends are discordant with the nodal-planes by 9-47 degrees. This suggests that fresh-faults associated with earthquakes at 10-40 km depths result from pure bending; while pre-existing faults at depths <10 km are reactivated by bending.

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