Variations in sediment thickness and type along the northern Philippine Sea Plate at the Nankai Trough

Abstract

We documented regional and local variations in basement relief, sediment thickness, and sediment type in the Shikoku Basin, northern Philippine Sea Plate, which is subducting at the Nankai Trough. Seismic reflection data, tied with ocean drilling program drill cores, reveal that variations in the incoming sediment sequences are correlated with basement topography. We mapped the three-dimensional seismic facies distribution and measured representative seismic sequences and units. Trench-parallel seismic profiles show three regional provinces in the Shikoku Basin that are distinguished by the magnitude of basement relief and sediment thickness: Western (<200-400 m basement relief, >600 m sediment thickness), Central (>1500 m relief,~2000 m sediments), and Eastern (<600 m relief,~1200 m sediments) provinces. The total thickness of sediment in basement lows is as much as six times greater than that over basement highs. Turbidite sedimentation in the Shikoku Basin reflects basement control on deposition, leading to the local presence or absence of turbidite units deposited during the middle Oligocene to the middle Miocene. During the first phase of sedimentation, most basement lows were filled with turbidites, resulting in smooth seafloor morphology that does not reflect basement relief. A second phase of turbidite deposition in the Eastern Province was accompanied by significant amounts of hemipelagic sediments interbedded with turbidite layers compared to the other provinces because of its close proximity to the Izu-Bonin Island Arc. Both regional and local variations in basement topography and sediment thickness/type have caused lateral heterogeneities on the underthrusting plate that will, in turn, influence lateral fluid flow along the Nankai accretionary prism.

IODP Expedition 333: Return to Nankai Trough Subduction Inputs Sites and Coring of Mass Transport Deposits

“…The three major seismic stratigraphic sequences identified in the northern Shikoku Basin are the lower and upper Shikoku Basin facies and local spill-over of Quaternary trench-wedge turbidites (Ike et al, 2008a). The lower part of the Shikoku Basin displays a complex geometry influenced by basement topography (Ike et al, 2008b). Basement highs are also imaged within the subduction zone (Kodaira et al, 2003;Dessa et al, 2004), where they influence margin structure (Lallemand et al, 1992;Le Pichon et al, 1996;Park et al, 1999;Mazzotti et al, 2002;Bangs et al, 2006;Moore et al, 2009) and seismicity (Kodaira et al, 2000a;Park et al, 2004).…”

Shear strength of sediments approaching subduction in the Nankai Trough, Japan as constraints on forearc mechanics

“…Site C0012 is located on the crest of a bathymetric high known as the Kashinosaki Knoll [Ike et al, 2008], while Site C0011 is located landward on the flank of the knoll (Figure 1b). Because of the topographic difference, the deposits are significantly condensed at Site C0012, where the entire sediment section was recovered and the basaltic basement was reached at 538 mbsf (meters below seafloor).…”

Reply to comment by Kawakatsu and Abe on “Nature of the seismic lithosphere‐asthenosphere boundary within normal oceanic mantle from high‐resolution receiver functions”

“…Coherence peaks that are evenly spaced in frequency are consistent with simple resonant behavior. We note that the stations that exhibit this behavior most strongly, T06, T07, and T09, are situated in the Shikoku basin where thick sediments have been documented [Ike et al, 2008]. Note that coherence peaks at these stations are not expressed at frequencies f > 2 Hz, suggesting a progressive breakdown in phase correlation.…”