Distribution and depth of bottom-simulating reflectors in the Nankai subduction margin

Akihiro Ohde,Hironori Otsuka,Juichiro Ashi,Arata Kioka

doi:10.1186/s40623-018-0833-5

Abstract

Surface heat flow has been observed to be highly variable in the Nankai subduction margin. This study presents an investigation of local anomalies in surface heat flows on the undulating seafloor in the Nankai subduction margin. We estimate the heat flows from bottom-simulating reflectors (BSRs) marking the lower boundaries of the methane hydrate stability zone and evaluate topographic effects on heat flow via two-dimensional thermal modeling. BSRs have been used to estimate heat flows based on the known stability characteristics of methane hydrates under low-temperature and high-pressure conditions. First, we generate an extensive map of the distribution and subseafloor depths of the BSRs in the Nankai subduction margin. We confirm that BSRs exist at the toe of the accretionary prism and the trough floor of the offshore Tokai region, where BSRs had previously been thought to be absent. Second, we calculate the BSR-derived heat flow and evaluate the associated errors. We conclude that the total uncertainty of the BSR-derived heat flow should be within 25%, considering allowable ranges in the P-wave velocity, which influences the time-to-depth conversion of the BSR position in seismic images, the resultant geothermal gradient, and thermal resistance. Finally, we model a two-dimensional thermal structure by comparing the temperatures at the observed BSR depths with the calculated temperatures at the same depths. The thermal modeling reveals that most local variations in BSR depth over the undulating seafloor can be explained by topographic effects. Those areas that cannot be explained by topographic effects can be mainly attributed to advective fluid flow, regional rapid sedimentation, or erosion. Our spatial distribution of heat flow data provides indispensable basic data for numerical studies of subduction zone modeling to evaluate margin parallel age dependencies of subducting plates.

Highlights

The development of gas hydrates, the guest molecules consisting of almost pure methane (e.g., Kvenvolden 1988), has been confirmed in marine sediments using seismic reflectors, sediment cores, and downhole logging data (e.g., Shipley et al 1979; Kvenvolden and McDonald 1985; Cook et al 2010)
Distribution of bottom-simulating reflectors (BSRs) We confirmed the existence of extensive BSRs in the Nankai Trough (Fig. 3), ranging from the forearc basins to the prism slope, which was consistent with previous studies (Ashi et al 2002; Baba and Yamada 2004)
BSRs had been thought to be absent from the prism toe off Tokai as well as the trough floor, slope basins, steep slope, and submarine canyons

Summary

Introduction

The development of gas hydrates, the guest molecules consisting of almost pure methane (e.g., Kvenvolden 1988), has been confirmed in marine sediments using seismic reflectors, sediment cores, and downhole logging data (e.g., Shipley et al 1979; Kvenvolden and McDonald 1985; Cook et al 2010). The range in depth of the methane hydrates has been determined by subseafloor temperatures and pressures (e.g., Shipley et al 1979; Dickens and Quinby-Hunt 1994). The presence of methane hydrate can be used to obtain subseafloor thermal information by taking advantage of the hydrate’s known stability characteristics under low-temperature and high-pressure conditions. The base of methane hydrates has been generally confirmed from acoustic reflectors called bottom-simulating reflectors (BSRs), which, in seismic reflection images, is characterized by high-amplitude reverse-polarity waveforms paralleling the seafloor (e.g., Markl et al 1970). Hyndman and Wang (1993) provided constraints on the seismogenic zone with a thermal model that utilized heat flow values from BSR, probe, and borehole measurements

Methods

Results

Discussion

Conclusion