Paleotemperature of the Nankai accretionary prism estimated by vitrinite reflectance of carbonaceous materials retrieved during the IODP Expedition 348

Abstract:

Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 348 took place from 13 September 2013 to 29 January 2014. During the Exp. 348, cuttings, mud gas, and logging data were collected from Holes C0002N and C0002P down to 3058.5 mbsf. Cores were collected from 2163 to 2218 mbsf of Hole C0002P. Three lithologic units were identified at Site C0002 based on geological and geochemical characteristics of core and cuttings samples: Unit III (875.5-975.5 mbsf in Hole C0002N), Unit IV (975.5-1665.5 mbsf in Hole C0002N), and Unit V (1665.5-2325.5 mbsf in Hole C0002N, and 1965.5-3058.5 mbsf in Hole C0002P) (Tobin et al., 2015).

To evaluate whole thermal structure of the Site C0002, we performed vitrinite reflectance analysis for cuttings samples collected every ~100 m, and for borehole core samples collected every ~10 m of Hole C0002N and C0002P. Vitrinite reflectance (Ro) is an indicator to estimate maximum paleotemperature, which has been widely applied to reveal tectonic evolution of on-land accretionary complex in Southwest Japan (e.g. Underwood et al., 1992; Ohomori et al., 1997) and thermal anomalies along fault slip zones reflecting frictional heating due to seismic slip (e.g. Sakaguchi et al., 2011). This is the first study that applied vitrinite analyses systematically to the deep portion of modern accretionary prisms.

In this presentation, we report preliminary results of vitrinite reflectance analysis. Ro values are 0.15 to 0.20 in Unit III (forearc basin strata), 0.21 to 0.27 in Unit IV (accretionary prism strata), and 0.26 to 0.38 in Unit V (hemipelagic sediment), respectively. In general, Ro values tend to be gradually and continuously increasing with depth. Estimated paleotemperatures are ~67°C in Unit IV and ~77°C in Unit V. Estimated paleotemperatures are lower than estimated modern temperatures based on borehole temperature measurements and their downward extrapolations (Sugihara et al., 2014). Gaps on Ro values are showen at ~1400-1500 mbsf, ~1900-2000 mbsf and ~2400-2500 mbsf. Ro gap at ~1400-1500 mbsf is correspondent to an age boundary indicated by nannofossils (1435 mbsf), whereas Ro gap at ~2400-2500 mbsf is associated with lithology change where clay-size content partly increases in the silty claystone (start from 2360.5 mbsf).