Exothermic events in a fossil seismogenic fault acquiring thermoviscous remanent magnetization in an exhumed accretionary complex

Abstract

Detecting exothermic events within fault rocks is key to understanding fault behavior. While paleo-temperature studies of fault rocks have been conducted via vitrinite reflectance or magnetic analysis, secondary remanent magnetization caused by exothermic events within fault rocks has not been examined directly. Direct detection of the secondary remanence has the advantage of revealing both paleo-temperature constraints and the paleomagnetic direction, which can help assess the relationship between secondary remanence and deformation. We conducted thermal and alternating-field demagnetization analyses and other rock–magnetic experiments on a fossil seismogenic fault zone and the surrounding host rocks in an exhumed accretionary complex. The fault zone comprises a few cataclastic zones, including thin-slip zones. The blocking temperature in the secondary magnetization site was detected at 300–360 °C only in cataclasite with magnetite or pyrrhotite as carriers. The paleomagnetic direction of the secondary magnetization is characteristic in the cataclasite, and the blocking temperature was higher than the paleo-maximum temperature of the host rocks. Therefore, the blocking temperature records a local exothermic event in the cataclasite. Thermoviscous remanent magnetization (TVRM) were suggested as candidate mechanisms for the blocking temperature.