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We analyzed geotechnical and acoustic properties of sediments from Lau Basin backarc Sites 834, 835,838, and 839 to examine the large-scale heat and fluid circulation in the Lau Basin. We hypothesize that a feedback system exists in which sediment properties, particularly permeability, affect large-scale heat and fluid circulation and are, in turn, affected by the circulation of fluids. Sediments in the Lau backarc basin display none of the expected trends in physical and acoustic properties as a function of depth and are probably underconsolidated. The sediments can consolidate normally, however, as shown by measuring both compressionaland shear-wave velocity in a controlled laboratory triaxial setting. In-situ underconsolidation might occur because high sedimentation rates could prevent the sediments from dewatering and consolidating normally, or water lost during the consolidation process could be rapidly replaced by fluids circulating through the system convectively. Although initial sedimentation rates were extremely high in the Lau Basin, laboratory measurements indicate that the overlying clayey nannofossil oozes are sufficiently permeable to allow the passage of fluids through the sediments on the scale of several years; however, a comparison of vertical and horizontal components of flow indicates that horizontal fluid flow is 3 orders of magnitude greater than vertical flow. The highly permeable vitric silts and sands probably dominate the horizontal component of fluid flow. Measured vertical heat flow at these sites is much lower than would be predicted by a simple cooling model, suggesting that heat is flowing laterally. A large difference in measured heat flow exists between the lowermost stratigraphic units of Sites 834 and 835, indicating that lateral fluid flow may be occurring. The chemistry of pore fluids measured at Sites 834, 835, and 838 shows an increase and subsequent decrease in dissolved calcium with depth within the sediment column, again suggesting lateral fluid flow. The dissolved-calcium profile at Site 839 shows a seawater signature and no consistent trend downhole, indicating possible downwelling of bottom water throughout the entire sediment column. The volume of fluid required to (1) dissipate the excess heat expected but not measured and (2) prevent the sediments from consolidating normally in these Lau backarc sites is very small, about 8 m at Site 834. Very small gradients are required to replace this amount of fluid lost during the 5.5 m.y. of sediment deposition. In general, sites of downwelling are much more diffuse and cooler than sites of upwelling. Site 839 probably is a downwelling site. The remaining three sites possibly are located in a zone of lateral fluid flow within a large-scale, convective circulation cell. INTRODUCTION Physical and acoustic properties of marine sediments are indicators of large-scale geological processes taking place in situ. For example, heat and fluid exchange between seawater and basalt basement occurs easily when basalt is either exposed or covered with a thin veneer of sediment (Louden and Wright, 1988). However, as the sediment cover thickens and sediments become less permeable, convective heat and fluid exchange between basalt and seawater diminishes. The permeability, or ease of fluid flow, through the sediment limits convective seawater circulation and controls the rate of dewatering of the sediment and, therefore, the amount of consolidation that occurs. A number of sediment properties, including permeability, index properties, shear strength, and compressionaland shear-wave velocity, are dependent on the consolidation state of the sediment. We hypothesize that a feedback system exists in which sediment properties, particularly permeability, affect large-scale circulation and are, in turn, affected by the circulation of fluid. The objective of this paper is to present analyses of the geotechnical and acoustic sediment properties of Lau backarc Sites 834, 835, 838, and 839; and, from these results, to infer something about the effects of fluid flow on larger scale geological processes occurring in the Lau Basin. In normally consolidated sedimentary sequences, properties such as density and velocity increase with depth, and other Hawkins, J., Parson, L., Allan, J., et al., 1994. Proc. ODP, Sci. Results, 135: College Station, TX (Ocean Drilling Program). 2 Naval Research Laboratory, Code 7431, Stennis Space Center, MS 39529, U.S.A. 3 U.S. Geological Survey, Branch of Pacific Marine Geology, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A. properties such as porosity and water content decrease with depth. An unexpected finding in the backarc sites was that physical properties gradients within the sediments overlying basalt, particularly at Sites 834 and 835, were negligible. The variations observed in the physical properties measurements were related to distinct changes in lithology. One result of this interpretation of the physical properties values is that high fluid flow through the sediments may explain why normal gradients in physical properties were not found, that is, may explain the underlying reason for the lack of consolidation in the sedimentary column in the backarc basin. Background The Lau backarc basin is characterized by a number of small sedimented basins and bedrock highs formed as horsts and grabens. Sites 834 and 835 were drilled during Ocean Drilling Program (ODP) Leg 135 in two separate sub-basins 48 km apart, part of a north-trending basin with 200-400 m of relief, approximately 100 km east of the Lau Ridge and 80 km west of the Central Lau Spreading Center (CLSC; Fig. 1). Water depths at Sites 834 and 835 are 2692 and 2916 m, respectively. Sites 838 and 839 lie in the same basin in 2617 and 2322 m of water, but they are separated by a flexure that forms a low ridge of 400-600 m relief, approximately 55 km west of the East Lau Spreading Center (ELSC) and about 200 km east of the Lau Ridge axis. The total sediment thickness is 106 m at Site 834, 155 m at Site 835, 103 m at Site 838, and 100 m at Site 839. Scientists on Leg 135 found that a significant contribution to the sediments came from Lau Ridge and intrabasin volcanic constructions from mid-Miocene to mid-Pliocene time (Parson, Hawkins, Allan, et al., 1992). The minimum basement ages of Sites 834 and 835 are 5.6 and 3.5 Ma, re-

Geotechnical and Logging Evidence for Underconsolidation of Lau Basin Sediments: Rapid Sedimentation vs. Fluid Flow