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5
Comparison with the Miura-Boso Accretionary Prism and
Formation Processes and Mechanisms of the Dark Bands
5.1
Characteristics of the Dark Bands of the Miura-Boso
Accretionary Prism
We previously classified the dark bands in the rocks of the Miura-Boso accretionary
prism into four major types on the basis of distribution, crosscutting relations, and
internal texture: 1-1 (independent particulate flow; Figs. 4 and 6 ), 1-2 (flexural-
slip faults; Figs. 4 and 6 ), 2 (sliding planes caused by submarine landslides; Figs. 5
and 6 ), and 3 (thrust faults; Figs. 5 and 6 ). The occurrence, internal structure, and
formation mechanism of each type were explained in detail by Michiguchi and
Ogawa ( 2011 ) (Fig. 9 ). They are briefly described below.
Type 1-1 dark bands developed parallel to bedding planes (Figs. 4a-e ) without
any crushed or deformed grains within the bands (Fig. 4e ). They are not original
sedimentary structures, unlike the sedimentary laminae. Instead, they show evi-
dence of independent particulate flow caused by the generation of excess pore-fluid
pressure, which occurred within the bed just after sedimentation (Fig. 6 ). Type 1-2
developed parallel to bedding planes, but the constituent grains in the band are
aligned oblique to those in the host rock (Fig. 4d , f ). Many bands of types 1-2 are
found in fold limbs and in sheared strata exhibiting thrust-sense dislocation. These
indicate flexural-slip faults formed during folding (Fig. 6 ).
Type 2 is distributed immediately beneath a submarine landslide body and
includes ductilely deformed grains (Fig. 5a ), which are similar to those of an S-C¢
structure (Fig. 5b, c ). Type 2 bands are observed in the slide planes of submarine
landslides (Fig. 9 ). Type 3 is characterized by cataclastic grains (Fig. 5d, f ) and is
developed along thrust faults formed during accretion (Fig. 9 ).
1.sedimentation
(Type 1-1)
4. thrust movement
(Type 3)
2. slump & slide
(Type 2)
3. fault and fold
(Type 1-2)
offscraping
migration pathway of sediment
Fig. 9 Four stages of formation of the dark bands (Modified from Michiguchi and Ogawa
( 2011 )). Type 1-1 shows evidence of independent particulate flow caused by high pore-fluid pres-
sures that occurred during sedimentation (stage 1). Type 1-2 shows flexural-slip faults formed
during folding in the accretionary prism (stage 3). Type 2 shows slide planes formed during sub-
marine landslides (stage 2). Type 3 shows thrust faults formed during accretion (stage 4)
 
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