Evolution of Sedimentary Environments in the Subduction Zone v of Southwest Japan: Recent Results from the NanTroSEIZE Kumano Transect

Abstract

The Nankai Trough is located off the coast of southwest Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The margin's Kii Peninsula or Kumano Basin corridor is the focal area for the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). This multi-disciplinary, multi-stage, multi-expedition project was designed to improve our understanding of seismogenic processes along the plate interface. Drilling Stage 1 of NanTroSEIZE included Expeditions 314, 315, and 316 of the Integrated Ocean Drilling Program. Coring and logging during those three expeditions generated a wealth of new information about the stratigraphic and structural evolution of the margin. Our summary descriptions are organized into three broad domains: the Kumano (forearc) Basin; the megasplay fault zone, which is a significant out-of-sequence thrust within the accretionary prism; and the frontal thrust zone of the accretionary prism. For the most part, drilling results validate structural interpretations of 3D seismic-reflection data, but they also place invaluable constraints on the timing of such events as the initiation of forearc-basin sedimentation and the history of slip on the megasplay fault. We see many types of interaction between tectonics and sedimentation. The frontal thrust zone is dominated by packets of sand and gravel that probably accumulated within trench-floor axial channel systems prior to their accretion in the early Pleistocene. The accretionary prism becomes progressively older toward land, and a time-transgressive unconformity separates the prism from the overlying slope apron. Turbidite sedimentation within the Kumano Basin did not begin until the early Pleistocene, after a succession of precursor events: formation of the underlying accretionary prism during the late Miocene and early Pliocene; erosion of an unconformity above the accretionary prism, with an associated hiatus of $1.2 My; a long phase of unusually slow hemipelagic sedimentation, lasting from $3.8 Myr to $1.67 Myr; acceleration of uplift along the megasplay fault at $1.55 Myr, which enhanced accommodation space; and organization of a robust sediment-delivery system, which was achieved through uplift and erosion of the hinterland and incision of canyons and gullies into the upper trench slope.

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

“…1). Similarly, igneous, metamorphic and sedimentary fragments reworked from the Inner and Outer Zones have been significant components of turbidites in the lower forearc (i.e., trench slope, trench-slope basins and trench) since the Miocene (De Rosa et al, 1986;Fergusson, 2003;Underwood et al, 2003;Underwood and Fergusson, 2005;Underwood and Moore, 2012;Clift et al, 2013;Usman et al, 2014). However, to date, constraints on the provenance of turbidites have been restricted to the modal composition of sandstones (e.g., Fergusson, 2003;Usman et al, 2014) and one zircon study (Clift et al, 2013).…”

“…is an erosional unconformity that marks a regional hiatus ∼5.6 to 3.8 Ma on top of the prism Underwood and Moore, 2012;Kameo and Jiang, 2013), whereas SB3 is a time transgressive surface marking the transition from slope sedimentation (Unit 1) to incipient basin infill (Unit 2). Unit 1 corresponds to IODP Units IIIA and IIIB at site C0009 (3.8 to 0.9 Ma) and IODP Unit III at site C0002 (3.8 to 1.6 Ma), which are composed of hemipelagic mud with few, thin beds of fine sand turbidites and ash deposits (Fig.…”

“…2). It is also characterised by a mud-rich condensed interval at site C0002 Underwood and Moore, 2012) and a newly recognised condensed interval at site C0009 that correlates to low PEF values (Figs. S1, 3).…”

“…The initiation of the Kumano Basin created a large accommodation space in the upper forearc, which is expected to have strongly modified sediment routing along the margin (e.g., Underwood and Moore, 2012;Takano et al, 2013;Usman et al, 2014). Determining the time of basin initiation, therefore, is critical in reconstructing sedimentation patterns in the forearc region.…”

“…The rate and mode of supply of detrital sediments from the volcanic front to the trench are first order controls on formation and development of accretionary complexes and the structural evolution of forearc regions (e.g., Melnick and Echtler, 2006;Kimura et al, 2008;Simpson, 2010). The transport of detrital sediments towards the trench is affected in turn by forearc deformation and development of basins and canyons that control sediment routing in the outer margin (Dickinson and Seely, 1979;Underwood and Karig, 1980;Dickinson, 1995;Underwood et al, 2003;Underwood and Moore, 2012). In this context, the formation of forearc basins is of particular significance because they form large sediment traps that can profoundly modify the transport and depositional patterns of sediments, with likely implications for the mechanical state of the margin and the formation of earthquakes (Fuller et al, 2006;Wang et al, 2010).…”