Effects of stress in the evolution of large silicic magmatic systems: An example from the Miocene felsic volcanic field at Kii Peninsula, SW Honshu, Japan

摘要

Miocene felsic volcanic field at the Kii Peninsula (MFVK) in southwest Japan allows us to reveal temporal and spatial change of a large felsic volcanic system in response to changes in the local and regional stress field. The MFVK was active episodically ejecting more than a few hundred cubic kilometers of magma in a span of hundred thousand to million years. Remnant of this large-scale volcanism is characterized by a broad area of deeply incised and complex cluster of collapse calderas in the Kii Peninsula. Major conduits for ash- flow tuff eruptions lie on the margins of the MFVK. Paleostress orientation of the σ_(Hmax) was inferred from lithologic distribution and structure in rocks in the MFVK. The optimal tectonic stress orientation at the time of the MFVK formation was NNE-SSW to the present azimuth. Approximately 1500 km~3 of magma were erupted from the MFVK during a duration of 0.1 million to 1.6 Myr. This suggests an area normalized eruption rate of 10~(-2) to 10~(-3) km~3/ka km~2. This long-term eruption rate of the MFVK is at the high end of rates for other large felsic volcanic fields (LFVFs), and ranks as one of the larger rates of LFVF worldwide. The large eruption rate of the MFVK and the formation of a large magma chamber is discussed in relation to tectonic stress. Although a compressional stress field favors the formation of a sill-like large magma chamber, a larger magma supply rate is more essential. We believe that a large rate of magma supply in combination with local and regional stress conditions in the early development of the MFVK promoted magma accumulation over eruption leading to the development of a large silicic magma chamber. We suggest that such a large magma chamber can generate excess pressure inside the chamber itself that results in a local tumescence and counteracts the tectonic compressional stress field, allowing magma to reach the surface. This scenario is the most plausible explanation for the large eruption rates of caldera volcanoes under a compressional stress field. The case of the MFVK is similar to the Toba caldera (Indonesia), which lies adjacent the geologic suture zone of wrench faults (the Sumatra faults).