Origin of a magnetic lineation on Kyushu Island, Japan

Abstract

Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of -1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.

Volcanic magma reservoir imaged as a low-density body beneath Aso volcano that terminated the 2016 Kumamoto earthquake rupture

“…The shaded gray area in (c) is outside the modeled zone. Density distributions are plotted using linear interpolation (Okubo and Shibuya 1993;Okubo et al 1997). Thus, the high temperature of the magma reservoir results in the aseismicity of the SLDB.…”

“…Central Kyushu, where Aso volcano is located, is characterized by a high heat flow (Ehara, 1984). Magnetic anomaly data suggest the existence of a high-temperature zone (T > 500 °C) around Aso (Okubo and Shibuya 1993;Okubo et al 1997). Temperatures below 1500 m depth are estimated to be >200 °C in the Yunotani region (New Energy and Industrial Technology Development Organization 1996).…”