Can Opal-A/Opal-CT BSR Be an Indicator of the Thermal Structure of the Yamato Basin, Japan Sea?

Abstract

One of the important results of the Japan Sea drilling by ODP is the detection of an opal-A/opal-CT transformation at all the drill sites. The transformation boundary is well recognized in physical property and downhole measurements as well as in lithologic descriptions. Because the boundary shows a marked change of density, the boundary appears in seismic reflection profiles as a BSR. The transformation of opal-A to opal-CT is closely correlated with temperature and age of host rock in the Japan Sea. We estimate and evaluate Japan Sea heat flow by using the opal-A/opal-CT bottom simulating reflector (BSR) observed in seismic reflection profiles. Then we estimated the age of host rock from available seismic profiles based on the assumption of constant sedimentation rate. Therefore, the temperatures at opal-A/opal-CT boundary were estimated from seismic profiles. The temperature data were converted to surface heat flow data by using the thermal conductivity data obtained by shipboard physical property measurements. The estimated BSR heat flow showed significant agreement with individual probe heat flow data. We confirmed the validity of BSR heat flow on a multichannel seismic profile. This method can provide large areal estimates of the surface heat flow. The opal-A/opal-CT BSR can contribute to defining the thermal structure of the Yamato Basin, Japan Sea.