Introduction to the Landforms and Geology of Japan


Tei Mélange and Muroto
  -- Shimanto accretionary complex

Cape Muroto

Turbidite and debris flow deposits are also exposed at Cape Muroto, but the formation is the Tsuro formation of the Nabae Group (Miocene). The almost perpendicular beds (alternating beds of sand and mud) are conspicuously deformed. Gabbro intruded into these beds.

View Tei melange and Muroto in a larger map

Turbidite at Cape Muroto
Photo 14 Turbidite at Cape Muroto [Another window]

Map of Cape Muroto
Fig. 6 Map of Cape Muroto


Exposed gabbro, which is spotted with white coarse grains, makes the landscape of part of the beach. The gabbroic complex is about 220 m in maximum width and 1800 m in length, crossing the southern end of Muroto peninsula northeast to southwest (Figure 6). Basaltic magma intruded concordantly into the horizontal alternating beds of sand and mud near the trench about 14 million years ago. The magma cooled down slowly and solidified to be gabbro (sill). The alternating beds of sand and mud with the gabbro sill inclined northward at a steep angle and emerged. As a result, the sill can be easily observed from the top through the bottom. Therefore, the gabbro in Muroto has been regarded as important to the study of magmatic differentiation processes (e.g. Hoshide et al., 2006b). Because the magma cooled down slowly deep under the ground, minerals crystallized in different timing according to their physical conditions for precipitation and settled in the lower part (crystallization differentiation). Accordingly, the various types of gabbro, such as olivne gabbro and augite gabbro, were produced in the sill, having different appearances. In addition, this igneous activity is an unusual event as it occurred near the trench because igneous activity in subduction zones generally arises at some distant from a trench. The sedimentary rocks that contact with the gabbro were metamorphosed by heat of the magma to be hornfels (contact metamorphic rock). 

Photo 15 Gabbro (near Spot 1) [Another window]

Texture of gabbro
Photo 16 Texture of gabbro (Spot 2) [Another window]
White grains are plagioclase and black ones are pyroxene.

Upper margin of the gabbroic sill
Photo 17 Upper margin of the gabbroic sill (Spot 1) [Another window]
A: Chilled margin (dolerite, fine-grained gabbro); B: Xenolith (mud); C: Blocks of chilled margin; D: Wall rock (hornfels). A white line (far left) shows the length of about 12 cm.

Basaltic dike
Photo 18 Basaltic dikes (Spot 3) [Another window]
Many basaltic dikes intruded into the sedimentary rock from the sill. This location is the northeastern end of the sill. A dike (red arrow) is about 15 cm wide. A white building near the top edge is Hotel Akenohoshi [ホテル明星]

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