DSDP legs 69 and 70. Gradients of porosity and density with depth where sediment layers are thin (Sigma 50 m thick) are anomalously high compared with those of other areas. Good correlation was found between the anomalous porosity and density gradients and the present-day heat flow. High gradients may be due to active hydrothermal circulation through a thin sediment cover, which is inhibited by a thicker sediment layer; pattern of hydrothermal circulation may be essentially fixed with the moving plate. Interpretation that a threshold thickness of about 50 m would inhibit direct diffuse discharge or recharge of hydrothermal flow through the undisturbed sediment layer yields an average permeability of the underlying basement layer of about 3-6 X 10 (super -14) m (super 2) (30-60 mdarcies).--Modified journal abstract.