Top Banner
駿河湾奥部大陸棚の地質構造 誌名 誌名 東海大学紀要. 海洋学部 ISSN ISSN 13487620 巻/号 巻/号 62 掲載ページ 掲載ページ p. 1-14 発行年月 発行年月 2008年7月 農林水産省 農林水産技術会議事務局筑波産学連携支援センター Tsukuba Business-Academia Cooperation Support Center, Agriculture, Forestry and Fisheries Research Council Secretariat
15

駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Jan 02, 2020

Download

Documents

dariahiddleston
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

駿河湾奥部大陸棚の地質構造

誌名誌名 東海大学紀要. 海洋学部

ISSNISSN 13487620

巻/号巻/号 62

掲載ページ掲載ページ p. 1-14

発行年月発行年月 2008年7月

農林水産省 農林水産技術会議事務局筑波産学連携支援センターTsukuba Business-Academia Cooperation Support Center, Agriculture, Forestry and Fisheries Research CouncilSecretariat

Page 2: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

東海大学紀要海洋学部「海 自然と文化」 第 6巻第2号 1-14頁 (2008)Journal of The School of Marine Science and Technology, Tokai University, Vo1.6, No.2, pp.1-14, 2008

「駿河湾奥部大陸棚の地質構造」

梶 琢*1・根元謙次本2・山崎晴雄*3・生野静香料・松田健也判

“Geological Structure of the Continental Shelf in the N orthern Part of Suruga Bay"

Taku KAJI, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

Abstract

Suruga bay is a structural bay with a considerable number of geographical features. The Suruga Trough runs

from north to south, and the Fujikawa Fan Delta is in the northern part of Suruga Bay. From seismic data on the

continental shelf in the northern part of Suruga Bay, we clarified the formation processes of the geographical features

from detailed sea-beam data and determined the displacement of the basement in the Wurm ice age due to the

subduction of the Philippine Sea Plate. As a result, unique geographical features were found to exist on the continental shelf in the northern part of Suruga Bay. We distinguished some of the geographical features and assumed

a wavy surface and two gradual slopes. Moreover, we found that the base sank gradually from the region off Ueda

to that off Fuji River from seismic data. However, large displacement of the base was confirmed to the west of Fuji

River. This large displacement of base might show geographical features change from Wurm.

1. Introduction

The northern margin of the Philippine Sea Plate

subducts in Suruga Bay on the west side of the Izu

Peninsula, forming Suruga Trough toward north-north-

east, a deep, long, and slender valley more than 2000m

from N ankai Trough, off the southwestern coast of

J apan (Fig. 1). This area has been subject to numerous

investigations, including an investigation using a Shin-

kai 2000 submersible.

The base of the Izu Peninsula is exposed on the

eastern slope, but has sedimentary rocks that are not

highly agglomerated distributed on its western slope in

the Suruga Trough; and its geography comprises repeat田

ing steep slopes and flat plains. (Kato et al., 1983; Kato

and Yamazaki, 1985; Yamazaki and Kato, 1986; Kit-

azato, 1988; Koyama et al., 1992; Niizuma et al., 1992;

2008年 6月9日受理

etc.). Moreover, the topographic high of Senoumi-tai,

which is a part of the fold belt that continues from Udo

Hill to the south, enters along the west side of the trough

(Shiba et al., 1990). Senoumi-tai is thought to be an

outer ridge, while the Senoumi Basin is in the deep-sea

terrace to the west. Thus, the tectonic relief that

develops on the hanging side of the plate subduction

zone is admitted to the west side of the Suruga Trough.

However, the Fujikawa Fan and Ukishimagahara subsi-

dence regions are located on the land side of the Suruga

Trough. Moreover, the active fault zone (Fujikawa

fault belt) , where the maximum speed of displacement

occurs, is on the west side of the Fujikawa Fan. These

two active faults that run south-north are responsible

for the remarkable geographical features at the bound-

aries between the mountains, hills, and plains. These

mountains are composed of Fujikawa Group and Kan-

bara Conglomerate, which are thick sediments from the

*1 首都大学東京都市環境科学研究科地理環境科学研究科専攻 (TokyoMetropolitan University/urban environmental science/Geography) *2 東海大学海洋学部海洋資源学科 (TokaiUniversity/Marine Mineral Resources) *3 首都大学東京都市環境学部地理環境コース地形地質学教室 (TokyoMetropolitan University/urban environmental science/Geography) *4 ジオ ・サーチ株式会社 (GeoSearch CO., L TD.) *5 国際航業株式会社 (KokusaiKogyo CO., L TD.)

第 6巻第2号 (2008)

Page 3: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Taku KA]I, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

138・20'

35・∞'

s g '切'.

AMt. Fuj i

Seno湖 ibasin s (" ~も

ε、 ,1

守SeOOlJlli -ta i f

34・40'

Fig.l The location map of study area.

Pleistocene to Lower Pleistocene. Yamazaki (1992)

thought that the terrigenous sediments that had filled

the trough were added to the west side of the subduction

zone. Moreover, the active fault was an imbricate thrust

that diverged from the plate boundary fault to the

hanging side. An accretionary complex appears on the

land surface in the northern part of Suruga Bay. This is

a peculiar region, with a geology that is not commonly

seen worldwide. However, the geographical features of

the active structure are indistinct due to the large

amount of deposit supplied by the Fuji River that forms

the abyssal fan (Sho et al., 1995) on the continental

shelf. The depth of the shelf edge is an index of the

diastrophism, because the continental shelf is a coastal

plain created by the invasion or sedimentation of land

during the period of the maximum decrease in the sea

level during a past ice age. It is known that the eastern

(Izu) side of Suruga Bay is 70土50mdeeper than the

western (umaezaki) side (N akamura et al., 1989).

However, the depth of the shelf edge, which is subject to

local sedimentation and the process of erosion, does not

2

show the same period of the formation. The flat reflec-

tion surface distributed from the neritic region to the

shelf edge across the entire area of the continental shelf,

which is seen in the seismic data off Udo Hill, was

defined as reflection L (N emoto et a人 1988).We

defined same surface as reflection L to planation surface

of Wurm maximum, the distribution was pursued, and

the amount of diastrophism was clarified quantitatively

in this research. The purpose of this research was to

clarify the distribution of the planation surface from-

detailed bathymetry and seismic data on the continental

shelf in the northern part of Suruga Bay, and to clarify

the amount of diastrophism.

2. Research method

Fig. 1 shows the study area in the northern part of

Suruga Bay. The continental shelf is not well developed,

and numerous small submarine canyons exist on the

narrow continental shelf from the coast off Fujikawa to

Tagonoura (Misawa, 1993, 1994). Misawa (1993)

東海大学紀要海洋学部「海一自然と文化」

Page 4: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Geological Structure of the Continental Shelf in the N orthern Part of Suruga Bay

(8)

35・08'

35・07'

35・06'

Fig.2 (a): The bathymetric map off Hara-Tagonoura. (b): The topographical cross section off Hara-Tagonoura.

defined a gulley as a valley where the toe of the slope

disappears at the middle of the continental slope (-400

~-600m depth), and a channel as where several valleys

join and continue to the Suruga Trough. These gullies

and channels developed as roads that supply sediment

from Fujikawa to the ocean. Moreover, the upper flat

surface (0~-30m) and lower flat surface ( -50~-120m)

are identified on the continental shelf in the eastern part,

off the coast of Ueda (Fig. 2a). The edge of the lower

flat surface corresponds to the continental margin.

Misawa (1994) divided the deposit on the continental

shelf in this region into four layers by using seismic

reflection data; these corresponded to the upper part of

the alluvium, the central part of the alluvium, and the

region under the alluvium. The distribution of the ero-

sion surface is also known in the northern part of

Suruga Bay: it exists at 130m off Sagara (Ishii and

Nemoto, 1995), 130:t10m off Ookuzure (Yamamoto et

al., 1998), 110土10moff Udo Hill (Nemoto et al., 1998;

Ishii and Nemoto, 1998), 105:t5m in Uchiura Bay

(Nemoto et al., 1989). We compared reflection L with

the unconformity surface formed during the Wurm

glacial maximum, because this surface is a smooth

erosion surface distributed at about -100m overall. This

erosion surface was formed in the vicinity of the coast-

line during the period of relative stability in the sea level

at the Wurm glacial maximum (20000 BP); it is found

第 6巻第2号 (2008)

worldwide (Motegi, 1970). Therefore, it is thought that

the edge depth of the erosion surface is the same as that

when the erosion sUrface was formed. In other words,

the amount of diastrophism that has taken place since

the last ice age can be presumed by c1arifying the depth

distribution of the erosion surface.

In this research, we used bathymetric data collected

by Chubu Regional Bureau, Ministry of Land, Infras-

tructure, Transport and Tourism, from November 2001

to February 2002. This data was obtained with an

EM3000 multibeam sounding device, which can collect

very detailed data at a resolution of a few meters; the

traverse line interval is 100~500m. We also used seismic

reflection data collected by the J apan Coast Guard in

Suruga Bay in 1977; the traverse line interval is about

1km

3. Resu1t

3-1. Geographical features

We divided the area of study into two segments

based on the differences in their geographical features:

the eastern and western parts of Tagonoura (Fig. 1) •

• Continental shelves off Hara-Tagonoura

We created a grid with a lattice interval of 4m from

the multi fan-beam data, and also plotted a bathymetric

map with a contour interval of 10m (Fig. 2a). Cross一

3

Page 5: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Taku KAJ!, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

¥ Hara

Tagonoura

ー12伽 ¥

H 6km 吋

Fig.3 The relief map off Tagonoura-Hara.

sections of the geographical features with a fineness

ratio of 1 : 3 (VE x 3) were also created from the grid

(Fig. 2b). The cross-sections were labeled BP-1 (Bath-

ymetric Profile-l) through to BP-14 from east to west

(Fig. 2a). In addition, the relief map off Hara-

Tagonoura viewed from the southwest is shown in Fig. 3.

The four conversion points are clearly seen in the

cross-sections (Fig. 2a): their depths are -10, -30, -50,

and -120m. Thus, two flat surfaces exist in this area: one

is the slope from -30 to -50m with an average inclination

of 100/1000, and the other is from -50 to -120m with an

average inclination of 60/1000. The slopes from the

shore line to -10m and from -30 to -50m have an inclina-

tion of 270/1000 and 830/1000, respectively. The shelf

edge is at a depth of -120m, beyond which is the slope of

the continental shelf. The two flat surfaces ( -10~-30m

and -50 ~ -120m) are remarkable off Hara (Fig. 2b;

BPl-6), but the lower flat surface does not develop off

Ueda in the western part of Hara (Fig. 2b; BP7-14). The

upper flat surface ( - 10~ -30m) and lower flat surface

( -50~ー120m) correspond to the upper layer and lower

layer, respectively, in Misawa's (1993) classification.

The upper flat surface is also seen in the relief map as

a gradual surface (Fig. 3); it has a width of 200m off

Hara, 200m off Ueda, and 150m off Tagonoura. The

edge of the continental shelf can be seen to have a wavy

surface at the depth of -120m. The edges of the upper

flat surface off Hara, Ueda, and Tagonoura are at

depths of -20, -30, and -20m, respectively. The widths of

the lower flat surface off Hara and Ueda are 530 and

225m, respectively. In the western part off Ueda, the

lower surface disappears, and instead, a wavy surface,

4

seen as a repetition of the valleys and ridges in the

bathrymetric map, develops (Fig. 2a). The valleys and

ridges of the waηT surface have a strike that is aligned

north-south. The ridges have a width (EW) of 200~

300m, amplitude of 250~300m, and height of 1O~30m.

They develop from depths of -30m off Ueda CBP-6) , and

increase in scale and amplitude toward the west (Figs.

2a and 3). These wavy surfaces are seen as ridges

(BP-7 and 10) and valleys CBP-9, 11, and 13) in the

cross-sections. However, these geographical features

are not seen off Tagonoura, where the continental shelf

does not develop at all.

• Continental shelves off R. Fuji

The bathymetric map with a contour interval of

10m (Fig. 4) , the cross-section of the north-south strike

(Fig. 5), and relief map (Fig.6) are shown. The relief

map (Fig. 6) shows the different characteristic to the

east and west of the boundary off Samejima and the Fuji

River. The wavy surface is continuous off Samejima

from Tagonoura, and its ridges have a maximum length

of 1. 3km, width (EW) of 400m, and height of 30~50m -

(Fig. 4). The valleys between the ridges develop from

depths of -30m off Samejima (Fig. 5). However, the

forms of the ridges are greatly different in the western

part off Samejima. An uneven surface that has three

large valleys and ridges exists on the continental shelves

and slopes off Fuji River. The ridge forms the triangular

pyramid type that the top part is sharpened (Fig. 5; BP

-19), and tl}e valley on both sides of it is gentle (Fig.5;

BP-21). This uneven surface is distributed from -70 to

-250m, and has a larger ridge with a maximum width

(WE) of 600m and a maximum height of 120m. A

東海大学紀要海洋学部「海一自然と文化」

Page 6: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

G巴ologicalStructure of the Continental Sh巴lfin the N orthern Part of Suruga Bay

350 08'

R.Fuji

~ q-.... . -.. 込

-1v,

350 06' 。 2km

countor interval:1Om

Fig.4 The bathymetric map off Fuji River.

伽a

-50

ー1∞

也 150

明 2∞

2仮lO

r ..-

15∞

/"

1000

+ Slooe break 一一-Off SlI鵬JI欄 (BP15-18)

--Off Fuji (BPI9-21l 一一一“ OffFuk j agenoh蜘 a(BP2Z)

5∞ 0・Fig.5 The topographical cross section of off Fuji.

swollen surface larger than the wavy surface is seen

between the gentle valleys on the continental shelves off

Fukiagenohama. This is distributed from -30 to -250m,

and its ridge has a maximum width of 1.2km and a

maximum height 120m. The uneven surface forms the

triangular pyramid type that has sharpened top (Fig. 5;

BP-19) off Fuji river ,but swollen surface forms the top

which is smoothness off Fukiagenohama in the relief

第 6巻第 2号 (2008)

map (Fig. 6).

The two flat surfaces (upper and lower flat sur-

faces) exist in the eastern part off Ueda; however, a

wavy surface develops instead of the lower flat surface

in the western part off Ueda. The lower flat surface is

seen in all the regions; and the wavy surface also exists

in all regions except the regions off Tagonoura and the

eastern part off Samejima. The continental shelves are

5

Page 7: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Taku KAJI, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

Wavy surface

Swelled surface Uneven surface

2.5km

i噌 6km

Fig.6. The relief map off Fuji River-Fukiagenohama.

not developed at all off Tagonoura. The uneven surface

consists of the triangular pyramid type ridge with a

sharpened top and a gentle slope off Fuji River. The

swollen surface consists of the swollen ridge and gentle

slope off Fukiagenohama.

3-2. Geological structure of the continental shelves

A strong reflection is observed in the seismic reflec-

tion data for the layers of the continental shelves. This

reflection continues smoothly over the whole area of

Suruga Bay. 1t is thought that this strong reflection is an

erosion surface because it forms a smooth surface in the

vicinity of the shelf edge marked by the sea level during

the Wurm ice age (20000 BP), and it is compared with

reflection L (Nemoto, 1988). 1n this research, we

assumed this erosion surface to be an acoustic basement

and especially examined the changing depth of the

basement edge. The sediment on this acoustic basement

is defined as the continental shelf sediment in this study.

The positions of the recordings are shown by the solid

lines in Figs. 3 and 6, and by SP ①~SP ⑮ on the solid

line corresponding to the number of seismic reflection

recordings.

• Continental shelves off Hara-Tagonoura

The six records (SP-1 ~SP-6) of the seismic reflec-

6

tion and its interpretation charts are shown in Fig. 7. A

distinct reflection is seen in the vicinity of -110~-120m

in all sections, and this is assumed to be the acoustic

basement. The seismic reflection records are shown

from the vicinity of -30~-40m, and the structure of the

upper flat surface seen in the geographical cross-section

is not shown. The arrow in the figure indicates the edge

of the acoustic basement (Fig. 7). The depths of the

basement edges off Hara (SP1, 2) and Ueda (SP3) are

-115m and -120~-125m , respectively. Continental shelf

sediment reaching a maximum thickness of 90m is

distributed on the acoustic basement. A strong reflection

that follows the acoustic basement is seen at -80m in the

continental shelf sediment; this is divided on the bound-.

ary of -80m into Layer 2 (!ower layer) and Layer 1

(upper layer). Layer 2, with a thickness of 40m, forms

the lower flat surface in SP-1 and SP-2 from the coast

off Hara to Ueda; Layer 1 is 30m in thickness and is

piled above Layer 2. However, as Layer 1 reaches 40

~50m above Layer 2, its sediments continue to the

continental slope (Fig. 7; SP4-6).

• Contine!ltal shelves off Fuji River

Seven seismic reflection records and the interpreta-

tion charts for SP-13 (off Fukiagenohama) to SP-7 (off

Samejima) are shown in Figs.8 and 9. The spatial

東海大学紀要海洋学部「海 自然と文化J

Page 8: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

"

第 6巻第 2号 (2008)

Geological Structure of the Continental Shelf in the N orthern Part of Suruga Bay

" om "

"

持ゐ鴨

s

s

仁コ Layer2

仁コLayerl

聞 Bas鰍 nt

総Om

Fig.7 Seismic profiles and interpretations off Hara-Tagonoura (SPl-6).

s

7

Page 9: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Om締

8

Taku KAJI, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

s " s

仁コLayer2

EコLayerl

.Bas鵬 nt

Fig.8 Seismic profiles and interpretations off Samejima-Fuji (SP7-10).

東海大学紀要海洋学部「海 自然と文化j

Page 10: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Geological Structure of the Continental Shelf in the N orthern Part of Suruga Bay

" s

仁コ Layer2

.Sas側 ent

日g.9 Seismic profiles and interpretations off Fukiagenohama (SPll-13).

distribution pattern of the sediments is quite different

from that in the eastern part of SP-6, and the continen-

tal shelves are wider in the western part of SP7 than in

the eastern part of Tagonoura. Layer 2 (20-40m in

thickness) and Layer 1 (20m in thickness) pile up to the

continental margin on the flat basement that is distribut-

ed widely off Samejima. Layer 1 is very thick, and

reaches the continental margin with the maximum layer

in the seismic profile having a thickness of 60m (SP-9,

10) off Fuji River. The wavy surface and uneven surface

are seen on the continental slope in the seismic profile

(SP-9~ 13). However, the reflection from the inside of

Layer 1 is hardly seen (SP-9~ 11), because the reflec-

tion from the surface strengthens off Fuji River.

The wavy surface off Samejima (SP-7, 8) and the

uneven surface off Fuji River (SP-9, 10) have different

第 6巻第2号 (2008)

geographical features and scales in the seismic profiles

of the coast off Fuji River from Samejima (Fig. 8; SP

7~ 10). The uneven surface off Fuji River (SP-9, 10) is

largest than the wavy surface off Samejima (SP-7, 8).

The maximum thickness (60m) of the sediment layer

off Fuji River on the basement is more developed than

off Samejima (20~40m in thickness; Fig. 8). The depth

of the basement margin is -105~-125m off Samejima

(SP-7, 8) and -105m off Fuji River (SP-9, 10; Fig. 8).

However, the depth of the basement margin off

Fukiagenohama (SP-12, 13) is -90m, and its distribution

is shallower than that in the eastern part. In addition,

the fault that displaced the basement and a part of

continental shelf sediment are seen off Fukiagenohama

in SP-13 (cross-section of ridge) .

9

Page 11: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Taku KA]I, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

4. Discussion

• The topography of the continental shelf and the

sediment structure

The topographical features are obviously different

to the east and west of the boundary of Tagonoura in

this research. The features in each region are collective-

ly shown in Table 1: the edge depth of the acoustic

basement and the distance (that is assumed to be width

of the continental shelf) from the edge of the acoustic

basement to the present shore lines are shown in Fig. 10.

The width of the continental shelf is different to the east

and west of the boundary of Tagonoura (Fig.10 and

Table 1). The width of the continental shelf is narrow

(500~800m) to the east of Tagonoura, while, the west

of Tagonoura has a continental shelf that is consider-

ably wider (700~ 1050m). The upper flat surface and the

lower flat surface of the bathymetric map and cross

section off Hara (Figs. 2a and b) can respectively be

compared with Layer1 and Layer 2 in the seismic reflec-

tion record (Fig. 7; SP1 and 2). Moreover, the lower flat

surface disappears, and instead, the wavy surface

develops from the coast off Ueda to Tagonoura. The

sediments (Layer1) develop to the shelf edge in that

region (Fig. 7; SP3-6). In addition, the width of the

continental shelf is very narrow from the region off

Ueda to Tagonoura (Fig. 10). In short, the lower flat

surface (Layer 2, -50~-120m) is not seen, the upper

surface of the sediment is contiguous with the continen-

tal slope when the width of the continental shelf is

narrow (SP4-SP6), and the continental shelf sediment

(Layer1) on the basement develops toward the shelf

edge. On the other hand, the lower flat surface develops

when the width of the continental shelf is wide, and

continental shelf sediment (Layer 1) does not develop

toward the edge of the continental shelf (Fig.7, SP-1,

and SP-2). This sediment (Layer 1) that is contiguous

with the continental shelf slope is shown in Fig. 7 (SP5

and 6), and the sediment with the irregular reflection

can be seen on the continental shelf slope. The sediments

in SP-5 and 6 are thought to be the slump sediments that

slipped down to the continental shelf slope by landslides.

The topography of wavy surface with repeated valleys

and ridges is possibly formed by landslides. Therefore, a

lower flat surface remains in -50~-120m because it is

.60epth(m) -40 -50

明 60

-70

-80

-90

-1∞ 叩 110

申 120

申 130

由 140

-150

35・08'

.同ト司 I.A レイ 、

凡JII"1--'" ¥ 国

ν ~ I ./ ~

」 区 S8I1 ~JI Ina ¥ ノV""I

ー ー¥ l¥』 ~ / J H !-.I

¥.. 1企 a‘ "'¥ A 、α c 1./ '‘ F

R.I UJI 、‘ 企... 同同嶋崎

~

Ta onc ura iUed~ Haral

。 5 10 Oistance(km) frαn R.Fuji

m

州問側∞∞∞∞∞∞∞∞∞

is--41hMMnHM司

tnnvFDaaτ

向。?h

・s・hu

W

E

U

Fig.10 Distribution of the acoustic basement (Wurm maximum) margin and width of continental shelves

.... : Acoustic basement margin ・Widthof continental shelves

10 東海大学紀要海洋学部「海一自然と文化J

Page 12: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Geological Structure of the Continental Shelf in the N orthern Part of Suruga Bay

Table 1 Conclusion of bathymetric feature and distribution.

肝f Off Off Off Off 肝fFukiagenohama R~Fuj i S鵬 ji胴 Tagonoura Ueda Hara

Feature of Swelled Uneven Wavy One step Wavy Two step topography surface surface surface surface surface surface

Width of 17'仮初1 1似冷n 1∞伽

得)13伽 (ff) 2側冒 0022伽first flat surface (日15伽 (E)2倫明 (E) 12伽

Edge of 命2~

側ト2伽t 側)ー3~ (wト2伽1 得)-2~ (W)ー3伽first flat surface (E)そ伽 (Eト2伽 (Eト2伽 (E)ー3伽 (日ー2伽

Width of Nothing Nothing Nothing Nothing

00 Noth i ng 0034伽second flat surface (E)225m (E) 77伽

Edge of Nothing Nothing Nothing Nothing 情)蜘thing 側)-12伽

second flat surface (Eト121伽 (Eト121伽

Depth of bas蜘 ent -9伽1 ベ05m(wト105m

申 125m得)ー12伽

情 115mmargln (E) -125m (E)ー115m

W i dth of basen鳩nt 105~ 7側冒008ぬ官

Unknown 情)50011

8倫明(E) 105伽 (E) 7ぬn

Block Unknown Smoothness 鈎間thness 加 thnessI加 othness

F… 陶 抑 制 付 属 . h抑制r'齢・ Itvy制r伽知・ T岡町楠s・I,1叫鴻

T-@) 『・③ 1・<'b T-(j)

網目障脇町伽f04".'.eヨ...,., 庇il]lrIot.Cコ齢制...,., , ...

not buried so much by the continental shelf sediment on

the basement off Hara. However, off Ueda to Tagonour.

a, the continental shelf is narrow and sediment (Layer

1) on the basement continues to the continental slope,

while a part has slipped down: this forms wavy surface.

This wavy surface was not formed by present processes,

but by the sea level rise during the postglacial age,

because it is thought that the present sedimentation is

the uppermost layer from the coastline to -10m that is

shallower than wave base (Fig. 2b). The sediments

(Layer 1) on the continental shelf reach from the conti.

nental shelf edge to the continental slope in the seismic

reflection profiles of the off Samejima and off R. Fuji

(Fig. 8). It is thought that the sediments on the continen.

tal shelf reach the continental slope because R. Fuji that

is the supply source of the sediment is near though the

width of the continental shelf is overall wide compared

with eastern part of Tagonoura. The continental sedi-

ments reach the continental slope and piles up thick

(Fig. 10, Table. 1), because the width of the continental

shelves decreases to off R. Fuji from Samejima (Fig.8;

SP-9, 10). The ridge of the triangular pyramid type on

the slope of the continental shelf where this is shown in

SP9 and 10 as a result is formed. In addition, the gentle

第 6巻第2号 (2008)

valley between the ridge and the ridge is thought that is

the route to transport the deposit from Fujikawa to the

trough bottom, and the cause of unclearness an internal

structure by diffused reflection of the seismic reflection

profile (SP9-11) is a large amount of gravel supplied

from Fuji River. The swelled surface was admitted off

Fukiagenohama (Fig. 4). The existence of the fault that

cuts continental sediments (Layer 1) on the continental

shelves and the acoustic basement like the block can be

admitted by using the seismic reflection profile that cut

from east to west (Fig. 9; SP-13). In a word, it is

thought that the acoustic basement was displaced by

some diastrophism. Table1 (T①~④) shows the rela-

tionship between the continental shelf sediments

(Layers 1and 2) and the width of the continental

shelves.

• Relation between width of continental shelf and

displacement of acoustic basement

The tendency of distribution of basement depth and

the width of the continental shelves turns on the bound-

ary of Tagonoura. The width of the continental shelves

has narrowed from Hara to Tagonoura. The distribu-

tion of basement depth gradually becomes deep from

Hara to Tagonoura, and it is the deepest at Tagonoura.

11

Page 13: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Taku KAJI, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

In a word, Tagonoura where the continental shelves don'

t develop at all is a region that subsides most. The

basement depth becomes gradually shallow in the west.

ern part of Tagonoura except a part of Samejima. In

addition, the width of the continental shelves is wider in

western part of Tagonoura than eastern part of

Tagonoura. The relation between the width of continen.

tal shelves and the displacement magnitude is admitted,

because the width of continental shelves is wide at

upheaved region, on the other hand the width of conti-

nental shelves is narrow at subsided region. The cause is

not clear though it is thought that the difference of a

regional environment is a cause in a part of exception

reglOn

• The distribution depth of the basement and displace-

ment magnitude

The depth of the edge of the acoustic basement

become gradually deeper from Hara (-115m) to Same-

jima (-125m). However, the west side of R. Fuji and

Fukiagenohama become gradually shallower from -105

to -90m. The displacement is 35m, if depth of the base-

ment edge compared Fukiagenohama (-90m) with

Samejima (-125m). It is thought that this value obvious-

Iy contains the amount of diastrophism. The displace-

ment is a possibility of the sea side extension fault of

Fuji River Fault Group including the Iriyamase Fault,

because Iriyamase Fault that elevates the west side on

land of the shallowest basement off Fukiagenohama.

However, displacement magnitude (1.75m/ky) of 35m

every 20ky might be too smaller than displacement

magnitude (7m/ky) of Iriyamase fault, if it is thought

the continuation of Iriyamase fault. Therefore, two

possibilities are thought as a cause of those differences.

First, the displacement magnitude on the sea side is

smaller than the land side. Secondarily, there is a possi-

bility that great fluctuations have occurred also in the

western part of this research region. These upheaval

tendency of the basement is same as a thrust structure

like stairs by sinking of Philippine Sea Plate on the west

side of the trough, and it is displayed as the displace-

ment of the superficial geographical features.

5. Summary

1. The geographical features of the continental shelves

in the northern part of Suruga Bay can be divided

into four kinds (upper flat surface and lower flat

surface off Hara, upper flat surface and wavy sur-

face of the section from Ueda to Samejima, upper

12

flat surface and wavy surface off Fuji River, upper

flat surface and swollen surface off Fukiageno-

hama). These geographical features on the continen-

tal shelves (upper flat surface, lower flat surface,

wavy surface, swelled surface) were formed by the

relation between the change in the thickness of the

continental sediment (Layers 1 and 2) on the base-

ment and the width of the continental shelves.

2. A relation between the width of continental shelves

and the displacement magnitude is observed. In

short, the width of the continental shelf is greater at

the upheaved regions, while it is narrow at the subsid-

ed regions.

3. The erosion surface (20000 BP) of the Wurm maxi

mum inclines gradually from east (off Hara) to off

Samejima ( -115~ー 120m); it upheaves up to -90m in

the maximum on the west side of Samejima. These

features are the same as the reverse fault structure

on the land side and the stair structure on west side

of the trough, which are caused by plate sinking.

Thus, the fault structures of Fuji and the stair struc-

tures on the west side of the trough are also seen on

the continental shelf in the northern part of Suruga

Bay. This displacement is a result of the east-west

compression due to the subsidence of the Philippine

Sea Plate.

6. Acknowledgments

We appreciate the generosity of the ]apan Coast

Guard in making the seismic data available to us. We

also thank our colleagues at the Chubu Regional Bureau,

Ministry of Land, Infrastructure, Transport and Tour-

ism for their support with the multibeam data. We wish

to express our deep gratitude to the abovementioned

organization and individuals.

References

Ishii, R and K. N emoto (1995): Variation in Water Depth

of Erosional Terraces on Continental Shelf Edges off

Western Coast of the Suruga Bay, Central Japan. Jour.

nal of the School of Marine Science and Technology,

Tokai University, 39, 159-171 (in Japanese with English

abstract) .

Ishii, R and K. Nemoto (1998): Basement Structure off

UDO_ Hills, Western SURUGA Bay. Journal of the

School of Marine Science and Technology, Tokai Uni.

versity, 46, 87-105 (in Japanese with English abstract).

Kato, S and H. Yamazaki (1985): Submarine Topographi.

東海大学紀要海洋学部「海一自然と文化」

Page 14: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay

cal and Geological Survey by the Manned Submergence

Research Vehicle SHINKAI 2000 in the Suruga Trough

Technical bulletin on hydrography, 3, 16-21 (in J apanese

with English abstract).

Kennett, J. P. (1982): Marine Geology. Prentice-Hall. N.

J., 813pp.

Kitazato, H (1988): Geology of the Axial Part of the

Suruga Trough Diving Survey Reports of the Submersi-

bl巴 Shibkai2000, Dive No.309. Technical reports of

Japan Marine Science and Technology Center, 8, 145-

161 (in J apanese with English abstract).

Misawa, Y. (1993): A Description of the Geological Struc-

tures in the N orthern Part of Suruga Bay through

Acoustic Profiles. J ournal of the School of Marine

Science and Technology, Tokai University, 14, 37-44 (in

Japanese with English abstract).

Misawa, Y. (1994): Sedimentary Structure of the Continen-

tal Shelves in the Northern Part of the Suruga Bay.

Journal of the School of Marine Science and Technol-

ogy, Tokai University, 38, 171-184 (in J apanese with

English abstract)

Mogi, A. (1970): [Geological dictionary] (The Associa-

tion for the Geological Collaboration in Japan). Heibon-

sha Limited, Publishers, Tokyo, 1852pp (in Japanese).

Nakamura, K. (1989): [The volcano and Plate TectonicsJ.

In, Kobayashi, Y., M, Ishida. et al., University of Tokyo

Press, Tokyo, 323pp (in Japanese with English

abstract) .

Nemoto, K., T. Sato, E. Akai, S. Horiuchi, Y. Kinoshita, F.

Mural王ami,K. Ikehara, T. K, Vuivau, L. K, Dehira

(1988): Submarin巴 Topography,Bottom Material, and

Geological Structure of the Continental Shelf off恥1t.

Udo, Western Suruga Bay. Journal of the School of

Marine Science and Technology, Tokai University, 27,

61-82 (in J apanese with English abstract)

Nemoto, K., T. Sato, A. Miyazaki and A. Nakama (1989):

Echo Character of the Uchiura Bay, Shizuoka, and Its

Relation to Sedimentary Structure and Processes. Jour-

nal of the School of Marine Sci巴nceand Technology,

Tokai University, 28, 95-118 (in J apanese with English

abstract) .

Niizuma, N., K. Otsuka, K. Kano, H. Wada, R. Sato, T,

Shibutani, S. Takeuchi, T. Yoshida and M. Oourasaka

(1990): Submersible observations of plate relations in

the subduction zone of the Suruga Trough. Technical

reports of J apan Marine Science and Technology Cen-

ter, 6, 261-276 (in J apanese with English abstract).

第 6巻第2号 (2008)

Otsuka, K. and N. Niizuma (1985): Sedimentary Geology

and Tectonic Features of Suruga Trough, off Matsuzaki

-The Results of Dive 86 by the Submersible “SHIN-

KAI2000"ー.Technical reports of Japan Marine Science

and Technology Center, 1, 45-57 (in Japanese with

English abstract).

Shiba, M., K. Nemoto, Suruga Bay Collaborative Research

Group and Research Group of Continental Shelf off the

Udo Hill (1990): Geological Structure of the Udo Hill

and its Adjacent Continental Shelf, Western Suruga

Bay. Journal of the School of Marine Science and

Technology, Tokai University, 30, 47-65 (in Japanese

with English abstract).

Shiba, M., K. Yumiya and K. N emoto (1993): Age of the

Acoustic Basement of Uchiura Bay, the Northeastern

Part of Suruga Bay. J ournal of th巴 Schoolof Marine

Science and Technology, Tokai University, 14, 55-63 (in

Japanese with English abstract)

Suzuki, T (1965): [The history of Yoshiwara port], The

Society for the history of Y oshiwara region, 100pp (in

Japanese) .

W onn Soh, Takeo Tanaka and Asahiko Taira (1995)

Geomorphology and sedimentary processes of a modern

slope-type fan delta (Fujikawa fan delta), suruga

trough, J apan. Sedimentary Geology, Vol. 98, Issues 1-4,

79-95.

Yamamoto, G., K. Nemoto, R. Ishii, T. Okazaki (1998)

Submarine Geology of the off Ohkuzure Coast in Shizuo-

ka Prefecture.Journal of the School of Marine Science

and Technology, Tokai University, 45, 83-99 (in

J apanese with English abstract)

Yamazaki, H and S. Kato (1986): [Onshore extension of

the Suruga trough -A result of submersible survey of

“Shinkai,2000"一].The earth monthly, 8, 2, 74-78 (in

Japanese) .

Yamazaki, H and S. Kato (1988): Submarine Topography,

Geology and Tectonic Movement in the North巴rnPart

of the Suruga Trough. Technical reports of Japan

Marine Science and Technology Center, 4, 67-81 (in

Japanese with English abstract)

Yamazaki Haruo (1992): Tectonics of a Plate Collision

along the N orthern Margin of Izu Peninsula, Central

Japan. Bulletin of the Geological Survey of Japan, 43

(10), 603-657.

13

Page 15: 駿河湾奥部大陸棚の地質構造駿河湾奥部大陸棚の地質構造 誌名 東海大学紀要. 海洋学部 ISSN 13487620 巻/号 62 掲載ページ p. 1-14 発行年月

14

Taku KAJI, Kenji NEMOTO, Haruo YAMAZAKI, Shizuka SHONO and Takeya MATSUDA

要 旨

駿河湾は,フィ リピン海プレート北縁のプレー ト沈み込み境界である駿河トラフが存在する構造性の湾である.駿河湾

の海底地形については潜水船をはじめ多くの研究がなされてきた.その結果,駿河トラフの東側斜面には伊豆半島の基盤

が露出するのに対し,西側斜面には団結度の高くない堆積岩が分布し,急斜面と平坦面が繰り返すスラスト構造が認めら

れる.また,駿河 トラフ西側には石花海堆,石花海海盆といった外縁隆起体や前弧海盆がみられる.このように駿河トラ

フを境に東西の地質構造が異なることが知られている.同様にして,駿河トラフ延長の陸側の構造も調査されており,富

士川周辺地域には南北走向の断層帯が認められている.しかし,富士川前面の大陸棚から大陸棚斜面にかけては,富士川

からの大量の堆積物供給による富士川扇状地が発達しており,駿河 トラフと陸側との接続部の地質構造については良く知

られていない.本研究では,シービームによって得られた詳細な海底地形データと音波探査データをもとに,駿河湾奥部

の大陸棚から大陸棚斜面にかけての地質構造を明らかにした.特に音波探査断面より WUrm最終氷期の浸食平坦面の分

布を明らかにし,WUrm以降の構造運動を明らかにした.その結果,富士川沖以西で音響基盤が隆起していることが認

められた.これは駿河トラフ西側の階段状構造や富士川河口断層帯の構造と調和的である.また,海底地形データも用い

ることにより,堆積構造と地形の関係についても考えた.

東海大学紀要海洋学部「海 自然と文化」