Axial crustal structure of the Lau back-arc basin from velocity modeling of multichannel seismic data

Located west of the Tonga trench, the Lau back-arc basin is a prime environment for studying the interplay between oceanic spreading systems and an active subduction zone. Within the basin lie two complementary, intermediate-rate spreading systems, the Central Lau Spreading Center (CLSC) and the Eastern Lau Spreading Center/Valu Fa Ridge (ELSC/VFR), that are positioned 170 to 40 km away from the active volcanic arc, respectively. Multichannel seismic (MCS) images of both systems reveal systematic variations in axial crustal structure primarily related to the proximity of the volcanic arc, but also related to spreading rate, morphology, and petrology. Upper crustal refraction data selected from the along-axis seismic lines collected during the 1999 MCS survey were modeled in both the time-versus-range (t–x) and intercept time-versus-slowness (τ–p) domains to provide validation and detail to the seismic reflection observations. The results show that as both the proximity to the arc and the spreading rate decrease southward: 1) seismic layer 2A thickens by 0.6 km between the CLSC and VFR, from 0.4 km to 1.0 km, 2) the average depth of the axial magma chamber (AMC) increases from 1.5 km at the CLSC to 2.8 km at the southern VFR, excluding the northern section of the ELSC that shows no continuous AMC reflector, but does show an isolated melt sill, 3) the upper crustal basement velocity decreases from 2.1 km/s at the CLSC to 1.8 km/s at the VFR, and 4) the velocities of both layer 2A and 2B decrease between the CLSC and northern VFR from 3.2 to 2.5 km/s and 5.0 to 3.9 km/s, respectively. Along with a broad axial high morphology, these features of the CLSC — thinner layer 2A, shallower AMC, and faster crustal velocities — correlate best with the fast spreading East Pacific Rise and intermediate spreading Juan de Fuca Ridge. Conversely, the structural characteristics of the central ELSC/VFR have no known counterpart in the global mid-ocean ridge system. We attribute this primarily to the volatile concentration in the magmas coming from the Tonga subduction zone.