Deep-sea intraplate paleoseismicity recorded in tephrostratigraphy of ODP Site 881

Deep-sea cores are pivotal in constructing long tephrochronological frameworks. Correlations of tephra layers between cores improve their individual age models and enables the compilation of composite tephrochronological model using a Bayesian age modeling approach. Subsequently, tephra layers with new or refined age estimates may provide an age reference for previously undated deposits or events.

The Kuril Island Arc is the least studied volcanic belt of the northern Pacific because of its remoteness and the limited exposure of volcanic edifices above sea level. Our recent deep-sea tephrochronological studies in the NW Pacific, near the Kamchatka peninsula, have produced a 6.2 Ma chronology of large volcanic eruptions mostly from Kamchatka volcanoes. To the south, only one site of the Ocean Drilling Program (ODP) Leg 145 Site 881 provides a long sedimentary sequence likely recording volcanic eruptions of the Kurils. To construct the tephrochronological model for Site 881, visible tephra layers from Holes 881A to 881D were sampled, geochemically fingerprinted using single-shard EMPA and LA-ICP-MS analyses, correlated among the cores and then matched to the Detroit Seamount reference chronology.

Although studied tephra layers appear undisturbed, the depth interval between 92 and 122 m (depth below sea floor in core 881B) hosts four tephra sequences that contain sets of ash layers repetitively redeposited as coherent units from deeper levels (122-148 m). The original sequence and three redeposited intervals are sandwiched between undisturbed tephra layers correlating to the DS61 and DS69 tephras from the Detroit Seamount reference chronology with the ages of 1.27 Ma and 1.61 Ma, respectively. We interpret this previously unknown stratigraphic repetition as the result of stacking within a subaqueous landslide occurred at ~1.29 Ma.

The Site 881 is located in a footwall of a normal fault revealed by seismic profiling, approximately 800 m south of the fault scarp (Rea et al., 1993). Although the fault was previously considered inactive, an onboard seismic-lithological correlation permitted to trace fault plane through subunit 1B of presumed Late Miocene – Pliocene age. We consider a slip on this fault to be the most possible trigger of the landslide, indicating fault activity in the Quaternary.

These results allowed us to resolve the tephrostratigraphy of ODP Site 881 and to establish a reliable age model based on dated tephra layers. The refined model adjust the original age model within the redeposited interval by up to 300 ka. This case illustrates that detailed geochemical studies of tephra layers in marine cores can substantially improve age model accuracy and are crucial to avoid misinterpretation of chronology in sedimentary sequences.