OUTER SLOPE FAULTING ASSOCIATED WITH THE WESTERN KURIL AND JAPAN TRENCHES

Elongated fault escarpments on the outer slopes of the western Kuril a nd Japan trenches have been investigated through detailed swath bathym etric mapping. Numerous horsts and grabens formed by these escarpments were identified. Distinct N70 degrees E linear alignment of the escar pments, parallel to the magnetic anomaly lineations, was revealed on t he outer slope of the western Kuril Trench. In the Japan Trench north of 39 degrees 00'N, most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations. A zig-zag pattern of fau lting exists south of 39 degrees 00'N. Each topographic profile was de composed by computer analysis into two curves representing (1) the smo othed long-wavelength slope of the subducting ocean-crust surface and (2) the short-wavelength (< 10 km) roughness of plateaus and valleys e dged by outward- and inward-facing fault escarpments. Throughout the s urveyed areas, escarpment heights increase from the crest of the trenc h outer swell down to a depth of about 6000 m on the slope of the oute r trench wall, but with no distinct increase below that depth. No sign ificant difference is recognized in fault throws towards and away from the trench. It can be concluded that these elongated escarpments orig inate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis, w hich is caused by downward bending of the subducting lithosphere. The relationship of escarpment height to escarpment length is similar to t hat obtained from normal fault escarpments in the East Pacific Rise cr est. The maximum length and height of escarpments are small in the Kur il Trench compared with those in the Japan Trench, implying a differen ce in mechanical strength depending on the fault orientation. The crus t is weakest along the inherited spreading fabric, second weakest prob ably along the non-transform offset direction and strongest in directi ons very oblique to these orientations. Seamounts appear to be more ri gid than normal ocean crust, with no particular weak orientations, res ulting in fewer but larger faults along the axis of plate bending, as most clearly represented in the subducting Daiichi-Kashima Seamount.