Sediment flow routing during formation of forearc basins: Constraints from integrated analysis of detrital pyroxenes and stratigraphy in the Kumano Basin, Japan

Abstract

Editor: A. Yin Keywords:Kumano forearc basin turbidity current pyroxene drill cuttings canyon megasplay fault The evolution of sediment flow routing during complete evolution of the Kumano forearc basin is determined through integration of stratigraphic and sediment provenance analyses in the upper Nankai forearc. A new approach uses the compositional variability of detrital clinopyroxenes and orthopyroxenes collected at eight major rivers in Japan and three drill sites in the basin and nearby slope environment, including the first drill cuttings retrieved by the Integrated Ocean Drilling Program (IODP). Joint interpretation of these datasets reveals that the sedimentation history of the basin is characterised by three main phases separated by newly-recognised time-transgressive boundaries. We show that the Kumano Basin initiated as a trench-slope basin in the early Quaternary (∼1.93 Ma) and that it progressively evolved towards an upper slope environment with increased turbidite confinement and influence from climatic forcing. Basin initiation was broadly synchronous with development of the Nankai megasplay fault, suggesting a causal relationship with construction of the Nankai accretionary prism. Unlike preceding studies documenting long-distance longitudinal transport of clastic material along the lower Nankai forearc, only limited longitudinal transport is documented by detrital pyroxenes in the upper forearc. These results suggest that transverse canyons are a major control on the sediment flow routing during maturation of forearc basins and that long-distance longitudinal flows along convergent margins are principally restricted to near-trench environments, even in the presence of large forearc basins.

Evolution of tectono-sedimentary systems in the Kumano Basin, Nankai Trough forearc

“…Unlike sites farther seaward and downslope (e.g., C0001, C0018, C0006), no evidence exists for longitudinal transport into Kumano Basin from the collision zone between the Honshu and Izu-Bonin arcs system (e.g., Taira and Niitsuma, 1986;Fergusson, 2003) or via oblique transport from the Tenryu River through the Tenryu Canyon (Kawamura et al, 2009). Thus, routing of sandy turbidites into lower slope basins and the trench wedge has been decoupled during the Pleistocene from routing into the distal Kumano Basin (Usman et al, 2014;Buchs et al, 2015). Sediment provenance for the proximal turbidites of Astumi Knoll remains undocumented, but that supply was probably rerouted through Tenryu Canyon into the trench by uplift of the knoll.…”

“…3), but patterns of sediment dispersal seem to have shifted over time. To constrain the detrital provenance, several tracers (e.g., bulk modal percentages, heavy minerals, pyroxenes) have been used to compare sands from Sites C0002 and C0009 with those from modern river mouths on Honshu (Usman et al, 2014;Buchs et al, 2015). Those comparisons indicate that the Outer Zone of SW Japan (Shimanto Belt) has been the main sediment source, with additional input from higher-grade metamorphic rocks of the Inner Zone.…”

Giant caldera in the Arctic Ocean: Evidence of the catastrophic eruptive event

“…Therefore, wide distribution of omnipresent sedimentary layers of volcanogenic origin is expected. Such layers can be identified by increased concentration of monoclinic pyroxenes and opaque ore minerals141516, and by higher values of magnetic susceptibility and residual magnetization caused by higher concentration of magnetite and titanomagnetite.…”

Possible climate preconditioning on submarine landslides along a convergent margin, Nankai Trough (NE Pacific)

“…1 and 2). There is no direct sediment source into the basin and the main depocenters nearby are located in the Kumano basin and downslope in the Nankai Trough (Buchs et al 2015;Usman et al 2014). There is possibly sediment input from the NNE, from the Tenyu or Fuji rivers (Usman et al 2014), but as the sediments deposited in the studied basin are very fine grained, this represents only the very distal to pelagic part of the supplied sediment.…”