In an accretionary complex, pelagic sediment commonly overlies layers of midocean ridge ba- salt, but in some complexes, oceanic pelagic beds are absent ...
Double ridge subduction recorded in the Shimanto accretionary complex, Japan, and plate reconstruction Soichi Osozawa Institute of Geology and Paleontology, Faculty of Science, Tohoku University, Sendai 980, Japan
ABSTRACT Combining a quantitative model relating ridge subduction and radiolarian biostratigraphical data from the Shimanto belt including accreted midocean ridge basalt, the plate configuration for the western North Pacific since 83 Ma can be reconstructed. The Kula-North New Guinea and North New Guinea-Pacific ridges passed along the Japan arc. Assuming a constant plate motion, the half-spreading rates, angles at which ridges entered the trench, convergent rates and angles, and migration rates of triple junctions can be calculated. INTRODUCTION In an accretionary complex, pelagic sediment commonly overlies layers of midocean ridge basalt, but in some complexes, oceanic pelagic beds are absent, and hemipelagic and terrigenous sediment overlie intercalated basalt. In the Setogawa complex of the southern Shimanto
belt of Japan (Taira et al., 1988; Fig. 1), the youngest basalt lacks a cover of pelagic sediment. Systematically older basalt, however, is overlain by pelagic sediment, and the duration of pelagic sedimentation lengthens in both landward and seaward directions. These relations, noted by Osozawa et al. (1990), constitute
I propose here a simple quantitative model related to ridge subduction and apply it to the Shimanto accretionary complex. Calculations are made for the migration rate of a triple junction, the ridge's spreading rate and angle at which it entered the trench, and the convergent rate of each oceanic plate and its convergent angle relative to the trench.
Figure t . Location map. Horizontal ruling = northern Shimanto belt; vertical ruling = southern Shimanto belt. Locations 1b, 1c, 2e, 2g,3c, 4b, 4e, and 5a are basalt lacking cover of pelagic sediment, and correspond to locations in Figure 5.
MODEL Where an active midocean ridge is being subducted under an ocean margin (Fig. 3), the age of oceanic sea floor relative to its spreading center (i4,) at any point (Fig. 3, a) along the trench is a function of time (t) and is given by the linear equation:
triple junction^ seaward limit of hemipelagic sedimentation trench .landwc .accretionary .complex ^ - ^ j j d b s i ^ v v^oungestv
-ridge seaward v-^maan^
(la)
__
pelagic sediment
Figure 2. Diagram of ridge subduction event, showing relations of basalt and overlying sediment.
[±V\ sin (0r + 0,) - Vr] A _
A, = L sin0r —
tv V V V
hemipelagic^ terrigenous sediment x p e | a gj c sediment sediment
A,= L sin0r
b a s a l t
GEOLOGY, v. 20, p. 939-942, October 1992
evidence for the partial accretion in the Setogawa complex of the upper part of the igneous crust of a subducted, actively spreading, midocean ridge (Fig. 2). If the trench-trench-ridge triple junction migrated, other parts of the Shimanto belt could also be expected to preserve a similar record of relative basalt age and of occurrence and age span of pelagic beds. If this is so, we may be able to reconstruct a precise plate configuration for the western North Pacific from these data.
J, (lb)
[±V2 sin (0r + 0 2 ) + K1 A yf
J'
where in ±, plus signifies the case when the ridge moves to the left in Figure 3, minus signifies the case when it moves to the right, a (in Fig. 3) = the intersection of the trench and Vr, b (in 939
counted clockwise. T h e terms V\ a n d V2 also satisfy the equations V{ sin (0 r + 0 , ) = V2 sin (0 r + 0 2 ) ± 2 Vn
(lc)
and
(2)
V\ cos (0 r + 0 i ) = V2 cos (0 r + 0 2 ).
T h e graph of equations l a a n d l b is s h o w n in Figure 4 a n d is symmetric with reference to
t=tx
(tx is derived f r o m substituting 0 for A, of the a b o v e equations). F o r simplification, I assumed constant plate motion, symmetric spreading, a n d n o transform faulting. Figure 3. Subducting midocean ridge; t v Iq,= age chrons; A, = r , -
See text (or discussion. DATA FROM THE SHIMANTO
D a t a o b t a i n e d f r o m the S h i m a n t o belt are
ridge (the triple junction), L = the distance between
plotted in Figure 5 . 1 divide the S h i m a n t o belt into five regions, K y u s h u (1), S h i k o k u (2), Kii (3), T o k a i (4), a n d Boso (5), a n d use the center
a a n d b w h e n / = 0, Vr= the half spreading rate of t h e ridge, V], Figure 4. Graph relating
/
\
BELT
Fig. 3) = the intersection of the trench a n d the
the t r e n c h convergent rate of
oceanic plates 1 a n d 2, respectively, 0 r = the angle between the trench a n d the ridge, measured counterclockwise, a n d 0 t , 0 2 = the angle between the trench a n d V\ a n d V2, respectively,
of each region as the reference point a s h o w n in Figure 3. Distances between the regions are 300, 2 0 0 , 3 0 0 , a n d 2 0 0 k m , respectively. T i m e t = the basal age of the terrigenous sediment, b u t t h e
Figure 5. Data and graph from five reference regions of Shimanto belt. Graph is Ce Tu C o - C a m p partly drawn by extrapolation. Age of hemipelagic and terrigenous sediment 100 M a of accretionary complex becomes younger seaward in order of letters in each region, and no regions have undergone major strike-slip faulting. Radiolarian biostratigraphical data are from Teraoka et al. (1990) and Osozawa (1991a) for Kyushu; Taira et al. (1980,1988), Kumon (1981), Ishikawa (1982), Suyari and Y a m a s a k i (1987), and Osozawa (unpublished data) for Shikoku; Matsushima et al. (1982), Kimura (1986), and Kishu Shimanto Research Group (1986,1991) for Kii; and Kawabata (1984), Kano (1984), Muramatsu (1986), and Osozawa et al. (1990) for Tokai; and Saito (1992) for Boso. Data are reexplained by work of Sanfilippo and Riedel (1985), Sanfilippo et al. (1985), and others. Correlation of absolute age and radiolarian biostratigraphy is after Haq et al. (1987). Rb-Sr age by Hamamoto and Sakai (1987) is also used. Age of basalt at location 2f is plotted as early Eocene but probably ranges down to Paleocene. This basalt was previously thought to form by ridge subduction (Taira et al., 1988) but is covered by pelagic sediment, and basalt lacking cover of pelagic sediment is at landward location 2e and seaward location 2g. Area near locality 2f is affected by severe thermal imprint (Underwood et al., 1992); thus, its meaning is unknown. Shimanto belt represents double ridge subduction, and southern Shimanto belt in Kyushu (1), Tokai (4), and Boso (5) clearly records second event. In Shikoku (2), location 2g is considered to be only site proposed for second ridge subduction, even if ridge is also considered Shikoku basin spreading ridge (Hibbard and Karig, 1990). Geology of location 2g and especially 4e (part of Setogawa complex; Osozawa et al., 1990) is similar and exhibits similar event. Possibility that Setogawa complex is manifestation of subduction of Shikoku basin spreading ridge is small because Shikoku basin was separated from Japan margin when Setogawa complex was formed (Hibbard and Karig, 1990; Osozawa, 1991b).
t (age of basal hemipelagic sediment)
940
GEOLOGY, October 1992
age of the basal part of the hemipelagic sediment is more accurately determined, and I use it alternatively as time. This means that the reference is not the trench of terrigenous sedimentation, but the seaward limit of hemipelagic sedimentation (Fig. 2). At = the age of the midocean ridge basalt with respect to the spreading ridge, or the age of the basalt determined by the basal age of overlying sediment minus the age of the basal hemipelagic sediment, or the duration of the pelagic sedimentation alone. Most of the accretionary complex of the Shimanto belt contains basalt overlain by pelagic sediment. In some areas, however, basalt bears no pelagic sediment (Sakai and Kanmera, 1981; Yanai, 1983; Osozawa et al., 1990; Osozawa, 1991a and unpublished data; Kiminami et al., 1990; Kiminami and Miyashita, 1992); hence, A, = 0. Some of these basalt locations underlie hemipelagic sediment and occupy the lowermost stratigraphic position in an accretionary complex (Fig. 5, locations lb, 2e, 3c, 4b, lc, 4d, and 5a). At other exposures, basalt extrudes over and intrudes into hemipelagic and terrigenous sediment, in the middle and upper stratigraphic positions (lb, 2e, 3c, lc, 4e, and 2g). Chilled contacts and xenoliths of hemipelagic and terrigenous sediment are commonly observed. The Shimanto belt consists mainly of a Cretaceous northern and a Tertiary southern track of rocks. Locations lb, 2e, 3c, and 4b are from the northern belt, and lc, 5a, 4e, and 2g are from the southern belt. Considering the duration of pelagic sedimentation from the other locations (Fig. 5; e.g., 2a, 2b, 2c, 2d, and 2f) as well, a plot of At = 0 from each belt represents the ridge subduction event. First, a spreading ridge migrated from Kyushu (1) and Shikoku (2), through Kii (3), to Tokai (4), as suggested by Kiminami et al. (1990). Then a second ridge migrated in the opposite direction from Boso (5), through Tokai (4), to Shikoku (2), but not Kyushu (1). PLATE RECONSTRUCTION I consider that the first ridge is the KulaNorth New Guinea and the second is the North New Guinea-Pacific. The contribution of the Kula plate to the first ridge is not in doubt (e.g., Engebretson et al., 1985). The North New Guinea plate was predicted and named by Seno and Maruyama (1984). The Philippine Sea plate made no contribution before 15 Ma; this plate was situated farther south of the Japan arc (Hibbard and Karig, 1990; Osozawa, 1991b). With respect to the Kula-North New Guinea ridge, the six plotted data points (Fig. 5, 4c-4h) for the Tokai region (4) are symmetric with reference to t = 23 Ma, and the graph has a gradient of 10. This gradient is extrapolated to the other regions. With respect to the North New GuineaPacific ridge, however, the graph is asymmetric. The graph of Shikoku (2) has gradients of 6.57 GEOLOGY, October 1992
from the five plotted data points 2a to 2e, and of 0.42 from the two data points 2e and 2f (Fig. 5). This asymmetry with reference to 83 Ma is probably explained by the severe shift of plate motion at this time (e.g., Engebretson et al., 1985), and the assumption of constant plate motion is not applied before 83 Ma. Even if the plate motion is slightly changed after 83 Ma, especially at 43 Ma, its effect on the gradient is considered to be negligible. The gradients of 6.57 before 83 Ma and 0.42 after 83 Ma are extrapolated to the other regions (Fig. 5). The Japan arc had the same trend of N35°E, along with the present Ryukyu arc (Fig. 6), before the opening of the Japan Sea, as shown by paleomagnetic studies (e.g., Otofuji and Mat-
suda, 1983). As related to the North New Guinea-Pacific ridge, Vj and 0 2 of the Pacific plate near Japan are 10 cm/yr and 70°, respectively, at least up to 37 Ma (Maruyama and Seno, 1986). Then, using equations 1 and 2, Vr = 0.6 cm/yr, 0, = 77°; Vx = \\ cm/yr, 0, = 66°, and the southwestward migration rate of the triple junction = 6.2 cm/yr, at least between 26 Ma (Fig. 5, 5a) and 15 Ma (2g). The angle is consistent with the convergent angle inferred from the trend of clastic dikes from 50 to 36 Ma (Taira, 1984). As related to the Kula-North New Guinea ridge after 83 Ma, Vr = 5.9 cm/yr, 0t = 96°; Vx = 13.5 cm/yr, = 123°, and the northwestward migration rate of the triple junction = 2.5 cm/yr,
Figure 6. Reconstruction of plate configuration at 40 and 30 Ma. Double line = midocean ridge; heavy solid line = Japan arc. Numbers correspond to five reference regions (see Fig. 5). Subducted oceanic plates are restored to horizontal. Cessation of Kula-Pacific spreading at 43 Ma (Lonsdale, 1988) is not considered, according to assumption.
50 Ma Eurasia
^ Vv \ \
% tCV?
North America 60 °N-
y Kula
/
