EPS-ELSI Winter School “Evolution of Earth & Life” (EEL2019)

Field Excursion Guide

Convergent Plate Boundary & Geology of Island Arc in Tanzawa area, Japan

Excursion Leader: Yuichiro Ueno Guidebook Editor: Tomohiko Sato, Shigenori Maruyama

Dec. 2nd – Dec. 3rd, 2019

1 1. Introduction Japan is a unique place to study plate tectonics because the island located at the convergent plate boundary. In order to understand the collisional orogeny and crustal evolution, we plan to visit Tanzawa Mountain in the Izu-Hakone region, where we can observe how the Philippine Sea Plate subducted below the island arc in the field. Through the geology of the convergent plate boundary, we are able to consider how the continental crust evolve through the history of the Earth.

2. Geology of Japan Four plates meet around Japan island arc along the subduction zone The subduction of the Pacific Plate under the Philippine Sea Plate creates a Izu-Bonin-Mariana Arc, where many volcanic islands are distributed. The Philippine Sea Plate is now moving 3 ~ 5 cm/year to the north and is subducted under Eurasian and North American Plates, which create a main Japan island (Honshu Arc). The famous Mt. Fuji is located near the triple junction of the three plates, where the oceanic islands collided with the Honshu Arc at Izu-Hakone region (Red box in Fig.2). Also, the back arc basins (Japan Sea and Okinawa Trough) are spreading since about 20 million years ago.

Fig. 1. Plate boundaries around Japan. Red box is Izu-Hakone area, very rare region on the Earth having triple junction where the boundaries of three tectonic plates meet.

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Fig. 2. Present model of volcanism around Japan island arc. Volcanic front forms along plate subduction zone where huge amount of water carried into deep mantle in the form of hydrated minerals. Dehydration of hydrated mineral causes earthquake.

Fig. 3. Schematic cross section of subduction zone. Since 600Ma, hydrated oceanic plate has been subducting to carry surface water into mantle in the form of hydrated minerals. Some part of water return to the surface area through dehydration of slab (slab=subducting oceanic crust).

3 3. Geology of Izu-Hakone region The Izu-Mariana arc is quite different from the rest of arcs in Japan islands. Izu-Mariana arc is so- called intra-oceanic arc. Formation process of Intra-oceanic arc is different from that of island arc which is formed along subduction zone at active continental margin. Intra-oceanic arc is originally transform fault on the sea floor, which is the boundary between oceanic crusts. With time, plate motion changes. As a result, transform fault turns to subduction zone. The Izu-Mariana arc has been originally located near Borneo where the arc appeared under intra- oceanic environment. Then, the arc rotated clock-wise, and it moved northwards since 17Ma. Finally, the arc collided with Honshu island to form mountains.

Fig. 4. General tectonic map of Izu Collision zone (Hirata et al., 2010).

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Fig. 5. NS cross section of the central Japan including Izu area (Hirata et al., 2010).

Fig. 6 Geologic map of Tanzawa area. 5

Fig. 7 Map of Tanzawa area.

6 Stop 1: Kosugesawa (Pillow Basalt)

Pillow basalt and hyaloclastite of Tanzawa Group (17-5 Ma) can be seen. These structure are formed when the lava erupted under the water. In the Tanzawa Group, limestone with coral fossils occur with the basalts. This indicate the Tanzawa Group originally emplaced as a tropical sea-mount, which then travel to the north and collided into the Honshu arc.

Fig. 8. Pillow basalt and coral fossil of Tanzawa group.

7 Stop 2: Nakagawa River (Greenschist/Amphibolite)

Metamorphosed basalts (greenshicst and amphibolite) occur along the Nakagawa River. The basalt of the Tanazawa Group is intruded by Granitoids (Tonalite), where the basalts are metamorphosed over 300˚C and deformed. The biotite Ar-Ar age of the granite is about 5 Ma.

Fig. 9. Metamorphic Facies.

Fig. 10. Crustal structure of Izu Arc. Based on seismic tomography, middle crust of the arc may consist of granitic rock (Tonalite).

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Stop 3: Kawachi River (Ashigara Group)

Ashigara Group consists of clastic sediments mainly conglomerate and sandstone. The age of the Group is from 2 to 0.5 Ma. Thousands meters’ thick coarse clastic pile deposited as “molasse” in front of rising mountain.

Fig. 11. Conglomerate beds of Ashigara Group along Kawachi River containing clasts of granite and metamorphosed basalt.

Fig. 12 Depositional model of the thick and coarse clastic rock in Ashigara Group.

9 Stop 4: Minase River (Kannawa Fault)

Kannawa Fault represents boundary between North American and Philippine Sea Plates.

Fig. 13 Field photograph of the Kannawa Fault.

Fig. 14. Paleogeographic map of Izu-Ogasawara arc (Hirata et al., 2010).

10 Apendix

Classification of igneous rocks.

11 Reference

Taira A. et al. (1998) Nature and growth rate of the Northan Izu-Bonin (Ogasawara) arc crust and their implications for continental crust formation. The Island Arc 7, p. 395-407.

Kawate S and Arima M (1998) Petrogenesis of the Tanzawa plutonic complex, central Japan: Exposed felsic middle crust of the Izu-Bonin-Mariana arc. The Island Arc 7, p. 342-358.

Ito M (1987) Middle to Late Miocene foredeep basin successions in an arc-arc collision zone, northern Tanzawa Mountains Central Honshu, Japan. Sedimentary Geology 54, p. 61-91.

Ogawa Y et al. (1985) Collision of the Izu arc with Honshu and the effects of oblique subduction in the Miura-Boso Peninshulas. Tectonophysics 199, p. 349-379.

Hirata D, Yamashita H, Suzuki K, Hirata T, Li YB, Kon Y Collision Accretion Tectonics of the Proto- Izu–Mariana Arc. Journal of Geography (Chigaku Zasshi) 119, 6, 1125-1160 (2010)

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