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doi:10.2204/iodp.proc.338.207.2017

Introduction

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) has grown over several years to become the most comprehensive ocean drilling transect in any subduction system (Ashi et al., 2009; Screaton et al., 2009; Tobin et al., 2009; Underwood et al., 2010; Expedition 333 Scientists, 2012a; Tobin et al., 2015; also see the “Expedition 338 summary” chapter [Strasser et al., 2014a]). Expeditions 333 and 338 of the Integrated Ocean Drilling Program (IODP) cored at Sites C0018 and C0021 (Fig. F1). Mass-transport deposits (MTDs) are common at both sites, which are located in a trench-slope basin immediately downslope of a megasplay fault in the accretionary prism (Fig. F2). Site C0021 is located ~2 km northwest of Site C0018 in a more proximal position for the MTDs (Fig. F2). The primary drilling objectives were to establish a Quaternary mass-movement event stratigraphy with good age control and to sample an exceptionally thick MTD complex for subsequent analyses of the sediments’ rheological behavior.

The dominant lithology at Sites C0018 and C0021 is clayey silt to silty clay (hemipelagic mud). Calcium carbonate contents are relatively high in the mud due to elevation of the slope basin above the calcite compensation depth. Depositional ages, based on nannofossil events and paleomagnetic datums, are younger than 1.46 Ma (Expedition 333 Scientists, 2012b; also see the “Site C0021” chapter [Strasser et al., 2014b]). Thin interbeds of silt- to sand-size volcanic ash are abundant, as are silty to sandy turbidites. A lithologic boundary between Subunits Ia and Ib was recognized based on the prevalence of sand beds (more sand in Subunit Ib). That boundary occurs at 190.65 meters below seafloor (mbsf) at Site C0018 and at 176.17 mbsf at Site C0021 (Fig. F3).

MTDs occur within Subunit Ia. Visual evidence from the split cores for mass-transport mechanisms (e.g., submarine slides and debris flows) includes convolute and tilted strata, intervals of remobilized mud clasts overlying tilted strata, and internal shear zones. Those observations were bolstered by 3-D analyses of X-ray CT scans. The thicker MTD intervals comprise a mixture of mud-rich debrites, intact hemipelagic layers, and convoluted banded deposits denoting extensive deformation within the interior of the failure.

This report documents the results of X-ray diffraction (XRD) analyses of 122 core samples from Sites C0018 and C0021. The report’s primary objective is to provide compositional data from the slope basin deposits and MTDs for comparisons with other sites and tectonostratigraphic units along the NanTroSEIZE transect (e.g., Guo and Underwood, 2012; Underwood and Guo, 2013).

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