HR: 1330h
AN: B12A-0728    [PDF]
TI: Along-Axis Crustal Structure of the Lau Back-Arc Basin From Multichannel Seismic Observations
AU: * Jacobs, A M
EM: amjacobs@ucsd.edu
AF: Institute for Geophysics and Planetary Physics, Scripps Institution of Oceanography 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Harding, A J
EM: aharding@ucsd.edu
AF: Institute for Geophysics and Planetary Physics, Scripps Institution of Oceanography 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Kent, G M
EM: gkent@ucsd.edu
AF: Institute for Geophysics and Planetary Physics, Scripps Institution of Oceanography 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Collins, J A
EM: jacollins@whoi.edu
AF: Woods Hole Oceanographic Institution, Clark Laboratory, Woods Hole, MA 02543 United States
AB: The Lau basin is a back-arc, intermediate-rate spreading environment comprised of two complementary southward propagating ridge systems, the Central Lau Spreading Center (CLSC) and the Eastern Lau Spreading Center/Valu Fa Ridge (ELSC/VFR).ÿ Multichannel seismic images reveal systematic variations in axial crustal structure related to spreading rate, proximity to the volcanic arc, morphology, and petrology.ÿ Upper crustal refraction data selected from all seven along-axis seismic lines shot during this study were modeled in the t-x and tau-p domains to create velocity-depth models, providing validation and detail to the seismic reflection observations.ÿ As the spreading rate decreases southward from about 90 mm/yr to 40 mm/yr, several overall trends are apparent: 1) The average crustal depth of the axial magma chamber (AMC) increases from $\sim$1.3 km along the CLSC to $\sim$2.9 km along the VFR, excluding the northern section of the ELSC which lacks an axial high and shows no AMC reflector, 2) Seismic layer 2a thickens by $\sim$700 m, from $\sim$300 m in the northern extent of the basin to just over 1 km at the southern extent, and 3) The upper-crustal basement velocities decrease from $\sim$2.8 km/s to $\sim$1.9 km/s to $\sim$1.6 km/s along the CLSC, ELSC, and VFR, respectively.ÿ A closer look at the along-axis layer 2a thicknesses and AMC depths reveal discrete jumps within the overall trends that coincide with changes in the axial morphology; the ELSC, for example, transitions from axial high morphology and consistent layer 2a and AMC values of 900 m and 2.7 km, respectively, to the more blade-like morphology of the VFR whereupon the corresponding values become fairly fixed at 1.1 km and 2.9 km, respectively.ÿ Another key to understanding the Lau basin is noting the significant differences between the CLSC and the ELSC.ÿ The thinner layer 2a, shallower AMC, faster crustal velocities and axial morphology of the CLSC are usually indicative of fast-spreading environments, suggesting variations in spreading character within the basin.ÿ The ELSC and VFR were modeled with a thin ($\sim$250 m), lower-velocity layer that comprises the upper portion of layer 2a, thereby inferring a more porous, densely-fractured near-surface crust than at the CLSC.ÿ Along the VFR, the average velocity and depth values defining this low-velocity layer, in addition to the average AMC depth and crustal velocities, correlate well with values presented by {\it Turner et al.} in their 1999 analysis of a wide-angle seismic survey of the southern basin.
DE: 0930 Oceanic structures
DE: 3025 Marine seismics (0935)
DE: 7220 Oceanic crust
SC: Biogeosciences [B]
MN: 2003 Fall Meeting