Anomalous Heat Flow in the Northwest Atlantic' A Case for Continued Hydrothermal Circulation in 80-M.Y. Crust

A detailed study of a 60 x 150 km area at 60øW, 24øN at the eastern end of the Nares Abyssal Plain indicates that hydrothermal circulation is still active in the 80 m.y.B.P. oceanic crust. The 58 heat flow measurements made at five stations in the area have revealed (1) constant heat flow over the abyssal plain (56 mW m-2), (2) a cyclic heat flow over the abyssal hills (mean of 77 mW m-2), and (3) a large anomaly of 710 mW m -2 over one of several small domes which protrude from the abyssal plain. The domes are 0.5-1.0 km in diameter near the top and rise 50 m above the level of the abyssal plain. They are recognized from surface echo sounders by an abrupt disappearance in the abyssal plain subbottom reflectors, but on near-bottom pinger records they appear as steep-walled structures which are covered by --•10 m of sediment (compared to --•75 m on the surrounding abyssal hills). From analogy with active ridge crests, these features are probably small volcanoes. The heat flow anomaly over one of the domes is matched well by a finite element convection model with the following characteristics: (1) recharge at one basement outcrop and discharge at another, (2) 300 m of sediment fill between outcrops, and (3) permeabilities of 10 -lø cm 2 for basalt and 10 -13 cm 2 for sediment. In other words, we believe that there is very effective convective heat transfer within the crust and out of the relatively permeable, thinly sedimented basement dome, resulting in the local high heat flow. Overall, the results from the Nares survey vividly show the age independent muting effect of sediment on the surface manifestation of crustal convection. In our survey area the mode of heat transfer varies from purely conductive in the more thickly sedimented abyssal plain areas (--•300 m sediment cover) to moderate amplitude convection pattern beneath the abyssal hills (--•75 m sediment cover) to a very large thermal anomaly over the small dome or 'chimneylike' structure (--•10 m sediment cover). The domes are possibly active analogues to the presently inactive basement chimney drilled at DSDP site 417A.