Middle Miocene volcaniclastic dikes at Kukedo, Shimane Peninsula, SW Japan: fluidization of volcaniclastic beds by emplacement of syn-volcanic andesitic dikes

Abstract

At Kukedo, northern Shimane Peninsula, SW Japan, volcaniclastic (peperite) dikes extend parallel to andesitic dikes in volcaniclastic turbidites of the middle Miocene Takashibiyama Formation. They mainly comprise a poorly sorted mixture of volcaniclasts derived from the host volcaniclastic turbidites. Andesitic dikes in direct contact with the volcaniclastic turbidites have fluidal contacts, locally protruding into the volcaniclastic turbidites. Volcaniclastic beds are disorganized in a zone up to 1 m wide and are partly welded within 1–30 cm from the contacts. One major volcaniclastic dike 0.5–1.5 m wide extends upwards from the lower segment to the upper segment of a laterally offset andesitic dike (1.5–3 m wide). Coarse lithic clasts are locally concentrated along the wall of the dike, and juvenile clasts derived from the andesitic dike are sparsely set in the poorly sorted volcaniclastic mixture. Juvenile clasts are poorly vesicular and microcrystalline, and mainly polyhedral in shape. Sparse fluidal juvenile clasts are fractured in situ into small clasts by quench cracks. Fluidization of host volcaniclastic turbidites occurred to form volcaniclastic dikes during emplacement of andesitic dikes, driven by the excess pore fluid pressure imparted from the stress-concentration at the advancing magma head and heating of pore water in the host volcaniclastic beds. The volcaniclastic dike extending directly from the andesitic dike was produced by small-scale dynamic mixing of magma and watery volcaniclastic beds upon offsetting of the andesitic dike. Welding was favored by the increased water vapor pressures and high temperatures in direct contact with andesitic dikes.