Paleomagnetic evidence for hot pyroclastic debris flow in the shallow submarine Shirahama Group (Upper Miocene‐Pliocene), Japan

Abstract

The cooling history of coarse andesite blocks erupted and deposited proximally in the shallow marine environment has been investigated by paleomagnetic studies and heat conductivity calculations. Six to 13 samples were cored and measured from six essential andesite blocks 1–5 m in diameter. Thermal demagnetization techniques were used to detect the directional change of remanence through demagnetization steps, each representing different blocking temperatures (Tb). Each sample has three components of remanent magnetization (C1, C2, and C3). C1 is characterized by a Tb of 250°C and an orientation coincident with the present geomagnetic field. C2 has a Tb of 250°–500°C and shows a consistent direction with reversed polarity. C3 has a Tb of 500°–600°C, and its orientation is nearly consistent from samples collected from nearby parts of the same clast and are random from samples from different clasts. On the basis of the remanence direction of each component, the differing Tb, and the petrography of the samples, C1 is interpreted as secondary viscous remanence, whereas C2 and C3 are the partial thermoremanent magnetizations obtained during and immediately after emplacement. C3 was probably acquired while the blocks were rolling or sliding downslope, and C2 formed during continued cooling of the blocks after they came to rest. Heat conductivity calculations suggest that the blocks were emplaced less than an hour after being erupted. If the eruptive vent was only 2–3 km distant, as suggested by field relations, a downslope transport rate of less than 36 km/h is implied.