Origin of a zone of anomalously high porosity in the subduction inputs to Nankai Trough

One poorly understood feature of the subduction inputs to the Nankai Trough subduction zone (SW Japan) is a stratigraphic interval with an anomalously high porosity zone (HPZ), which is up to 240 m thick and located within the clay- and volcanic ash-rich Shikoku Basin facies. To investigate the origin of their peculiar physical properties, we integrated logging-while-drilling (LWD) data, shipboard density measurements, and visual descriptions of core samples recovered from four drill sites of the Ocean Drilling Program and Integrated Ocean Drilling Program. We combined those observations with scanning electron microscopy (SEM) and laboratory consolidation tests on both HPZ samples and artificial mixtures of ash (glass shards) + smectite and vesicular pumice + smectite. LWD data indicate that the HPZ mudstones have a large proportion of dispersed volcanic ash (~ 20-30%). The consolidation tests show that the rate of porosity loss with increasing effective stress (consolidation behavior) is consistent among HPZ specimens and matches artificial mixtures containing up to 60% volcanic material. However, absolute values of porosity remain higher for HPZ samples compared to artificial mixtures, so processes in addition to the mechanical effects of volcanic clasts must be contributing. We suggest that hydration and partial dissolution turns clusters of volcanic glass into aggregates with distinct microfabric. SEM images confirm the presence of strengthened grain-to-grain contacts, which probably inhibit the collapse of the intra-aggregate pore space. The aggregates behave like grains, so that cohesive strength of the bulk sediment and rate of porosity loss remain nearly unchanged during burial. The two-step diagenetic process of dissolution and precipitation depends critically on a threshold abundance of fine-grained dispersed volcanic ash/pumice. Older units in the Shikoku Basin with only traces of dispersed ash show no such effects. HPZs should be expected in other subduction zones with similar compositional and diagenetic prerequisites.