Sedimentary evidence relating to the tectonic evolution of the Lau Basin, SW Pacific, from ODP Sites 834-839 (ODP Leg 135)

Six sites were drilled during Ocean Drilling Program Leg 135 in the western Lau backarc basin in the southwest Pacific (Sites 834–839). These sites are all located in basins within a horst and graben terrain and form an approximate transect across the rifted arc basement onto crust considered to have been formed at the East Lau Spreading Centre. The sedimentary sequences recovered from these backarc sites range in age from the late Miocene to the Holocene; they consist primarily of a lower succession of volcaniclastic sediment gravity-flow deposits interbedded with hemipelagic clayey nannofossil oozes and nannofossil clays, overlain by a distinctive upper succession of hydrothermally stained hemipelagic, and locally redeposited, clayey nannofossil oozes. The volcaniclastic sediment gravity-flow deposits are predominantly massive, proximal, vitric gravels, sands and silts, that are mainly locally derived from adjacent basement ridges and intrabasin seamount volcanoes. At Site 835, which was drilled in a small extensional sub-basin in the oldest part of the Lau Basin, the upper clayey nannofossil ooze sequence is anomalously thick and rigorous sedimentological analysis shows that much of this sequence is redeposited. Thick clayey nannofossil ooze turbidite muds are identified that closely resemble the enclosing clayey nannofossil ooze hemipelagites. These thick turbidite muds are associated with mudclast conglomerates, interpreted as muddy debris-flow deposits, and a number of coherent rafted blocks of older hemipelagic sediment. These allochthonous deposits testify to several episodes of instability in the sub-basin that may be related to large-scale tectonic activity caused by the southward passage of ocean-ridge propagator tips past the latitude of the drillsite. Episodes of increased sediment deposition, due to increased frequency of turbidite emplacement, are evident at all of the Lau Basin backarc drillsites, and these correlate moderately well with the closest approach of the propagating ridge tip to each sub-basin. The research presented illustrates how turbidites, once identified, can be used to decipher complex tectonic histories, and demonstrates the importance of local tectonic controls on sediment redeposition in backarc basins