A thermal event detected in a cataclastic seismogenic fault using paleomagnetic methods.

Paleomagnetism on fault zones will provide evidence to discuss deformations using paleomagnetic orientations and thermal events, such as frictional heating, using thermoremanent magnetization. We examined deformation history and thermal event on a fossil seismogenic fault zone in an exhumed accretionary complex, the Yokonami mélange in the Cretaceous Shimanto Belt, Southwest Japan. The fault zone is located at the northern end of the Yokonami mélange close to a coherent unit in the North. The rocks around the fault zone were classified into three; host rock mélange, cataclasite with pseudotachylyte, and sandstone in a coherent unit adjacent to the Yokonami mélange. Based on vitrinite reflectance, the paleo-maximum temperature was reported around 250±30 ℃. We have performed thermal and alternating field demagnetization experiments on these rocks.

Blocking temperatures obtained from the thermal demagnetization experiments is expected to indicate the maximum temperature at a re-heating event if there is in a case. In the results of experiments, the blocking temperature, which could be related to a re-heating event, of 300-350˚C higher than 250±30 ℃ was identified only in cataclasite. At the temperature, the direction of remanent magnetization changed from NE to SW with a counterclockwise rotation. The paleo-maximum temperature within the catalcasite was estimated to be about 324˚C using grain size of a dynamic recrystallization in a previous study. This is also higher than the paleo-maximum temperature from Vitrinite reflectance. And our results from the thermal demagnetization experiments are also consistent with the higher temperature in the cataclasite. Accordingly, we interpret that the cataclasite have a thermoremanent magnetization with change in paleomagnetic direction when this seismic fault activated, which suggests the high temperature was caused by a frictional heating.

Normally magnetic direction is useful to discuss about a regional tectonic rotation with age. It is, however, challenging to use the magnetic direction in the strongly deformed mélange zone and cataclasitic seismogenic fault. We will carefully examine the magnetic direction with considerations both regional tectonic history and local deformations.