ABSTRACT

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.

摘要

Hikurangi前弧区的慢滑大逆冲事件可能与海山俯冲有关。IODP 远征 375 在 Tūranganui Knoll 的侧翼钻探，这是一个位于俯冲带以东的白垩纪海山，包含类似于与俯冲海山相关的间隔的单元。基于扫描岩心图像、岩相观测、点计数和 μXRF 测绘，对 Site U1520 火山碎屑单元 V 的详细调查表明，它是一个富含粘土矿物的镁铁质火山碎屑序列，是多期成岩蚀变历史的产物. 该单元被解释为来自海底喷发的火山碎屑的下坡运输。相比之下，大多数火山碎屑是在附近的 U1526 站点发现的，靠近 Tūranganui Knoll 的顶部，被解释为由波浪作用改造的地下喷发的产物。这个浅水沉积包经历了最小的粘土矿物蚀变和更广泛的早期碳酸盐胶结作用。这些火山碎屑单元的碎屑成因、沉积环境和成岩历史的差异突出了 Hikurangi 高原火山系统的非均质性。然而，在 U1520 站点的厚的、改变的侧翼火山碎屑单元中大量含水粘土的存在为过量孔隙流体提供了现成的来源，这可能会导致沿着 Hikurangi 俯冲带的缓慢滑动事件。这些火山碎屑单元的沉积环境和成岩历史突出了 Hikurangi 高原火山系统的非均质性。然而，在 U1520 站点的厚的、改变的侧翼火山碎屑单元中大量含水粘土的存在为过量孔隙流体提供了现成的来源，这可能会导致沿着 Hikurangi 俯冲带的缓慢滑动事件。这些火山碎屑单元的沉积环境和成岩历史突出了 Hikurangi 高原火山系统的非均质性。然而，在 U1520 站点的厚的、改变的侧翼火山碎屑单元中大量含水粘土的存在为过量孔隙流体提供了现成的来源，这可能会导致沿着 Hikurangi 俯冲带的缓慢滑动事件。