Tephrochronological study for evaluation of fault activity

To evaluate fault activity, reliable dating methods for geological units (landforms or geological sediments) are important, as a capable tectonic source under the NRA regulatory guides is defined as the presence of surface or near-surface deformation of geological units of a recurring nature within the last 120­–130 kiloyears corresponding to the Late Pleistocene. Furthermore, the guides also state that if the evidence within the last 120­–130 kiloyears is obscured, the capable source should be evaluated taking into account the geologic evidence of past activity within the last 400 kiloyears corresponding to the latter half of the Middle Pleistocene. Therefore, the Regulatory Standard and Research Department, Secretariat of Nuclear Regulation Authority (S/NRA/R) has been carrying out a project to develop dating methods for Middle and Late Pleistocene landforms and geologic sediments.

Tephra (volcanic ash, pumice, etc.) is one of the important age indicators for landforms and sediments over long distances because tephra fallout is geologically instantaneous and provides an excellent isochronous surface. Tephras occurring in marine sediments could obtain a well-dated astronomically-tuned age by referring to the marine oxygen isotopic curve, and then their ages could be applied for their correlatives in terrestrial sediments.

We first select marine cores which are associated with a long-term oxygen isotopic curve covering the Middle and Late Pleistocene, then we detect tephra layers in the marine cores by core examination (see figure). A tephra horizon (depth) is correlated to a marine isotopic stage (MIS) by referring to the oxygen isotopic curves of the cores, then the MIS is converted to an astronomically-tuned age using the reference oxygen isotopic curve. For example, the WP tephra at a depth of 109 m in the JAMSTEC Chikyu Hole C9001C cores retrieved from Pacific Ocean floor is correlated with MIS 7b, then converted to 200~210 kiloyears ago. The resultant age is preferable to the ages of their correlatives in terrestrial sediments using other dating techniques (170~330 kiloyears ago). The well-dated tephra age is actually used for evaluation of fault and fault-related fold activity of the Middle and Late Pleistocene ages. Please also refer to the following papers in international scholarly journals for details.

Matsu’ura, T., et al., 2014. Quaternary Geochronology 23, 63–79.

Matsu'ura, T., et al., 2017. Quaternary Geochronology 40, 129–145.

Matsu'ura, T., Komatsubara, J., 2017. Quaternary International 456, 163–179.

 

Contact
Tabito Matsu'ura
Regulatory Standard and Research Department,
Secretariat of Nuclear Regulation Authority, Japan