Hydrothermal heat flux through aged oceanic crust: where does the heat escape?

Recent publications suggest that most of the fluid flow in the upper oceanic crust is channelized through small volumes of rock and vented into the ocean. This implies that at flanks of generally thinly sedimented mid-ocean ridges, focused discharge at the seafloor should be concentrated most likely at outcrops, high-angle normal faults or seamounts. These vents should be associated with a significant heat flow signature. However, only few observations worldwide support this assumption up to now. On our quest for focused fluid exchange between young oceanic crust and the ocean we surveyed a 720 km long and 40-90 km wide off-axis portion of seafloor intersecting the East Pacific Rise near 14degrees14'S. A wealth of geophysical methods including high-resolution swath mapping bathymetry, single channel seismics, sediment echo sounding, magnetics and heat flow determinations were used, Heat flow data in the tectonic corridor cover crustal ages of 0.3-9.3 Ma. With respect to the conductive plate cooling model the data show the well-known pattern of low values close to the ridge, associated with vigorous hydrothermal circulation of cold seawater through the young upper crust. and a fast recovery to almost lithospheric conductive cooling values at a surprisingly young crustal age of 9.3 Ma. Although the sediment cover is fairly thin, measurements with a 3.6 in violin bow type heat probe were possible almost everywhere within the investigated area. A detailed survey between two large seamounts at 4.5 Ma revealed localized extremely high values of up to 618 mW/m(2) (275% of the expected heat flow) at the foot of the seamount. This is interpreted as a clear indication of focused discharge of hydrothermal fluid, If we, however, relate heat flow normalized by the expected conductive heat loss to the character of igneous basement, heat flow is highest in areas with an almost flat and sedimented basement, and lowest within similar to10-20 km of seamounts and other rough basement relief. We therefore hypothesize that the large number of seamounts covering the ocean floors governs a major amount of convective heat loss of aging oceanic lithosphere. (C) 2002 Elsevier Science B.V. All rights reserved.