OCEANIC BASEMENT STRUCTURE, SEDIMENT THICKNESS, AND HEAT-FLOW NEAR HOLE 504B

A new seismic reflection survey around Hole 504B, the deepest borehole in ocean crust, reveals active faulting, possible volcanic centers, a nd a lateral change in the relationship of heat flow and basement stru cture near the borehole. Migration of single channel and multichannel seismic profiles collected in a 25 by 25 km grid with a 1 km line spac ing significantly improved the resolution of basement structure and se diment thickness. West of Hole 504B, heat flow is high above east-west lineated basement ridges, whereas heat flow to the east is normal abo ve ridges and high above two buried basement knells. The difference is probably due to lateral variations in sediment thickness. Small, buri ed basement knells are common and may have been point sources for lava flows. Hole 504B lies in a flat-floored basin that slopes gently upwa rd to the west. A recently active fault 1.1-1.2 km south of Hole 504B is indicated by sediment reflector discontinuities that extend up to t he seafloor. The fault strikes east-west and crosses a buried volcanic knell where Holes 678B and 896A were drilled. Regionally, basement re lief north of Hole 504B is 100 to 150 m lower than to the south, which we attribute to an increased spreading rate obtained from dating publ ished local magnetic anomaly patterns with a recent timescale. We find at least five graben structures resembling failed rifts which may hav e formed in response to asymmetric spreading or to the change in tecto nic stress accompanying the spreading rate change. South facing scarps on basement ridges are as common as north facing scarps. Sediment thi ckness is highly correlated to basement depth due to preferential depo sition in topographic lows when the crust was 1-2 Ma old and to later winnowing.