Opposing trends in crustal thickness and spreading rate along the back-arc Eastern Lau Spreading Center: Implications for controls on ridge morphology, faulting, and hydrothermal activity

Along back-are ridges located near the arc volcanic front, crustal thickness is observed to vary systematically with arc proximity independent of spreading rate. This effect is especially well expressed by the similar to 400-km-long Eastern Lau Spreading Center (ELSC) where, approaching the arc volcanic front (from similar to 100 to 40 km), crustal thickness nearly doubles (similar to 5.5-9 km) as spreading rates decrease by more than half (97-39 mm/yr). The crustal thickness variations at the ELSC appear to result primarily from changes in mantle wedge composition caused by subduction. We investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology, faulting, and hydrothermal activity as part of the first phase of RIDGE2000 Integrated Studies in the Lau back-are basin. We used deep-towed side-scan sonar instruments (DSL120A and IMI30) to continuously map the near-axis region within broader-coverage ship multibeam bathymetry and acoustic imagery swaths. An array of optical sensors (MAPRs) concurrently surveyed the near-bottom water for hydrothermal plume anomalies. Hydrocasts made at identified plume sites confirmed their hydrothermal origin. The data show that as spreading rates increase and crustal thicknesses decrease along the ELSC: (1) morphology transitions from shallow peaked volcanic highs to a deeper flat axis; (2) faults become larger and more widely spaced; (3) hydrothermal activity, as measured by plume incidence, increases and appears to reach levels higher than the global mid-ocean ridge trend with spreading rate. The observations indicate that crustal thickness (magma supply) has a greater control on ridge morphology and faulting than spreading rate, even at fast rates where the thermal lithosphere should be thin. However, heat input from mantle advection proportional to spreading rate, perhaps in combination with increased fault permeability, appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge. Although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the ELSC, the observed effects may have generic implications for controls of seafloor spreading characteristics at mid-ocean ridges. (c) 2006 Elsevier B.V. All rights reserved.