Origin and diagenetic priming of a potential slow-slip trigger zone in volcaniclastic deposits flanking a seamount on the subducting plate, Hikurangi margin, New Zealand

Slow-slip megathrust events in the Hikurangi forearc region may be related to seamount subduction. IODP Expedition 375 drilled the flank of Tūranganui Knoll, a Cretaceous seamount east of the subduction zone containing units analogous to intervals associated with subducted seamounts. Detailed investigations of the volcaniclastic Unit V at Site U1520, based on scanned core images, petrographic observations, point-counting, and μXRF mapping, reveal that it is a mafic volcaniclastic succession rich in clay minerals which are products of a multiphased history of diagenetic alteration. This unit is interpreted as resulting from downslope transport of volcaniclastic debris from submarine eruptions. In contrast, most volcanic clasts recovered at the nearby Site U1526, near the top of Tūranganui Knoll, are interpreted as products of subaerial eruptions reworked by wave action. This shallow water sedimentary package experienced minimal clay-mineral alteration and more extensive early carbonate cementation. Differences in the clast origins, depositional settings and diagenetic histories of these volcaniclastic units highlight the heterogeneity of volcanic systems on the Hikurangi Plateau. However, the presence of voluminous hydrous clays within thick, altered flanking volcaniclastic units at Site U1520 provides a ready source for excess pore fluids that may enable slow-slip events along the Hikurangi subduction zone.