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Lineaments of the Oceanward and Landward Slopes
off Miyako
Lineament orientations, based on the 10-m contour interval maps, around the northern
Japan trench are very similar between the oceanward and landward slopes (Fig. 9 ).
Unusually, the two slopes apparently developed differently. As interpreted by
Hilde ( 1983 ), in the subduction (tectonic) erosion of the Japan trench landward
slope, the slope sediments erode into the trench, filling the graben structures of the
downgoing Pacific plate. As a result of this erosion, the landward slope is retreating
steadily (e.g., von Huene and Lallemand 1990 ; Clift and Vannucchi 2004 ). Each of
the grabens on the oceanward slope becomes totally filled by the collapse of the
landward slope toe. The volume of the retard region suggests that the toe retreats at
the rate of 5-6 km per 1 m.y. (Fig. 10 ). In the 10 m.y. since the middle Miocene,
the calculated distance of 50-60 km is a reasonable estimate of trench landward
advance caused by erosion of the Honshu arc.
Landward slope
Oceanward slope
39 00' - 39 40'
N = 53
N = 33
38 30' - 39 00'
N = 55
N = 45
37 50' - 38 30'
N = 36
N = 59
circle 30%
Fig. 9 Rose diagrams of lineaments on the oceanward and landward slopes, taken from the swath
maps (From Ogawa and Kobayashi 1994 )
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