Assessment of heat flow in the Ulleung Basin, East Sea (Sea of Japan) and its relationship with the depth of gas hydrate stability zone

Abstract

The Ulleung Basin is one of three major basins in the East Sea (Sea of Japan), which occupies the southwestern part of the sea. it is believed to form through back-arc extension from the late Oligocene to middle Miocene. Marine heat flow studies have been conducted since 1960s, however, the scarcity of location and the reliability of data does not allow one to figure out heat flow distribution in the basin or to estimate the background heat flow from the asthenoshpere. According to previous studies, the heat flow in the basin varies from 51-157 mW/m2. However, there is no obvious correlation between the heat flow values and major topographical features of the basin. This raises an important question as whether the data obtained over several decades instruments represent the true background heat flow when considering the basin formation history. In June 2007, gas hydrate consisted of 99.9% methane was recovered for the first time in the Ulleung Basin several meters below the seafloor (mbsf). In the following month, using a Ewingtype marine heat probe we conducted marine heat flow survey at 10 new sites (HF01-10

Fig. 1. Left) including sites where bottom simulating reflectors were clearly detected in high-resolution seismic profile. According to the new measurements, the geothermal gradient ranges from 103-137 mK/m and in-situ thermal conductivity 0.82-0.95 W/m·K. The corresponding heat flow (84-130 mW/m2) appears to increase towards the center of the basin. Volcanic sills/flows emplaced within sediment do not seem to play a role as heat flow anomaly source. The validity of each heat flow measurement can then be tested by comparing the lower boundary of gas hydrate stability zone with the BSR depth. BSR depth was detected at 12 sites more among existing heat flow measurements based on densely-spaced high-resolution reflection seismic profiles obtained by Korea Institute of Geoscience and Mineral Resources. While there is little correlation b