Computer Science
Scientific paper
Apr 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992e%26psl.109..303l&link_type=abstract
Earth and Planetary Science Letters, Volume 109, Issue 3, p. 303-318.
Computer Science
17
Scientific paper
Fluid flow manifestations within the outer 20 km of the eastern Nankai accretionary wedge, between the water depths of 4000 and 2000 m, are concentrated near the outcrop of a shallow detachment fault immediately behind the frontal thrust near a depth of 3800 m and along a probable strike-slip fault, on the top of a 2000 m deep “pop-up” structure, 20 km inside the wedge. Using subbottom temperature measurements, we estimate Darcy upward flow velocities below clams colonies at the 3800 m site and show that one can associate a narrow range of velocities with the different types of colonies. Then, combining a statistical survey of the distribution of colonies with the velocity estimates, we compute a yearly outflow budget of 200 m3 per metre of width of the wedge in the toe area. This is forty times larger than the amount expected from steady-state compactive dewatering. We propose, however, that 80-90% of this flow is produced by shallow seawater convection. Because variations in sea-bottom temperatures were too large, we could not estimate fluid outflow budgets in the 2000 m venting area. However, 20,000 and 130,000 year old shells fossilized within widespread carbonate concretions indicate that fluid flow has been active in the 2000 m venting area since at least 130,000 years ago, probably in a sporadic fashion. Chemical evidence indicates that a significant part of the fluid is coming from a depth of at least several hundred metres. Subbottom temperature measurements over a period of two months in the 3800 m venting site show fluctuations of 20-30% in the venting velocity below an active colony that have an apparent period of two weeks. Although the fluid circulation appears to be controlled by active faults at least in the shallow part of the wedge, not every active fault corresponds to an aquifer. In the absence of good high-resolution seismic data and drill holes, we cannot describe this plumbing system in any detail and are not able to show why some active faults correspond to fluid outlets and some do not.
Kaiko-Nankai Scientific Crew
Kobayashi Koji
Le Pichon Xavier
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