Within the Nankai accretionary prism off Cape Muroto, Japan, the frontal thrust and thrusts within the imbricate thrust zone show consistent negative polarity fault plane reflections that decrease in amplitude landward. However, the protothrust faults lack a fault plane reflection in three-dimensional seismic data. Differences in velocity and density of the overthrust sections at the proto- and frontal thrusts vs. their underthrust sections is insufficient by an order of magnitude to explain the presence of the fault plane reflection at the frontal thrust. Forward modeling with synthetic seismograms shows that the best match of the logging-while-drilling data with the seismic data at Site 808 is a ~1-m-thick low velocity (1800 m/s) and low density (1.25 g/cm3) fault zone; however, a wider zone with higher velocity and/or densities is also consistent with the seismic data. High resistivity values across the frontal thrust suggest that the presence of free gas generates the negative polarity reflection. Correlation of the shallowest high-amplitude bright spots along the numerous fault plane reflections from the imbricate thrust zone with the gas hydrate–related bottom-simulating reflector suggests that free gas migrates up the fault zone to pool at the base of the hydrate stability zone and potentially increase concentrations of hydrate above the base of the hydrate stability field. These results suggest that free gas may play a role in generating negative polarity fault plane reflections elsewhere.
1Gulick, S.P.S., and Bangs, N.L.B., 2004. Negative polarity at the frontal thrust—is free gas the culprit?: insights from Nankai accretionary prism off Cape Muroto using seismic-logging integration. In Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/190196SR/353/353.htm>. [Cited YYYY-MM-DD]
2Institute for Geophysics, Jackson School of Geosciences, University of Texas, 4412 Spicewood Springs Road, Building 600, Austin TX 78759, USA. Corespondence author: sean@ig.utexas.edu
Initial receipt: 1 December 2003
Acceptance: 22 June 2004
Web publication:
20 August 2004
Ms 196SR-353