Fernando Martinez and Robert Dunn

School of Ocean and Earth Science and Technology,
University of Hawaii at Manoa

Abrupt along- and across-axis changes in crustal structure at the Eastern Lau Spreading Center imply rapid changes in mantle composition and mode of advection to the ridge.

The Eastern Lau Spreading Center in the Lau Basin exhibits large changes in crustal structure, morphology, geophysical, and geochemical characteristics as a function of its separation from the volcanic arc (Escrig et al., 2009; Jacobs et al., 2007; Martinez and Taylor, 2002; Martinez et al., 2006; Pearce et al., 1995). The along-axis changes have been well noted in the above cited studies, but less recognized are equally abrupt changes in crustal properties across the axes of individual spreading segments (Fig. 1). Across individual segments, depth, crustal thickness, morphology, geochemistry, and the nature of segmentation can abruptly change with only a few (<5 km) of incremental spreading. Even neighboring ridge segments separated by only ~8 km non-transform offsets can have long-lived contrasts in the nature of their crustal accretion. A seismic tomographic study (see Dunn and Martinez white paper) indicates that crustal velocity contrasts, interpreted as related to porosity and composition, closely follow the morphologic changes. The abrupt nature of these changes suggests correspondingly large and abrupt changes in the nature of the mantle wedge supplying melt to the axes. These observations suggest that distinct and contrasting mantle domains exist in close proximity and can separately supply even neighboring ridge axes leading to the distinct crustal domains recorded at the seafloor. The concept of broad triangular melting regimes as envisioned beneath mid-ocean ridge axes may not apply to arc-proximal back-arc ridges where mantle wedge viscosity contrasts may span several orders of magnitude as a result of varying water content and strongly influence the mode of advection to the ridge.

References

Escrig, S., Bezos, A., Goldstein, S.L., Langmuir, C.H., and Michael, P.J., 2009, Mantle source variations beneath the Eastern Lau Spreading Center and the nature of subduction components in the Lau basin–Tonga arc system: Geochem. Geophys. Geosyst., v. 10.

Jacobs, A.M., Harding, A.J., and Kent, G.M., 2007, Axial crustal structure of the Lau back-arc basin from velocity modeling of multichannel seismic data: Earth and Planetary Science Letters, v. 259, p. 239-255.

Martinez, F., and Taylor, B., 2002, Mantle wedge control on back-arc crustal accretion: Nature, v. 416, p. 417-420.

Martinez, F., Taylor, B., Baker, E.T., Resing, J.A., and Walker, S.L., 2006, 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: Earth and Planetary Science Letters, v. 245, p. 655-672.

Pearce, J.A., Ernewein, M., Bloomer, S.H., Parson, L.M., Murton, B.J., and Johnson, L.E., 1995, Geochemistry of Lau Basin volcanic rocks: influence of ridge segmentation and arc proximity, in Smellie, J.L., ed., Volcanism Associated with Extension at Consuming Plate Margins, Volume Special Publication No. 81: London, Geological Society, p. 53-75.

Fig 1. Bathymetry and Bouguer gravity anomalies in the area of the L-SCAN seismic experiment. A) Complied multibeam bathymetry shows rapid transition from shallow complex landforms on the ELSC flanks (Domain II) to a ~500 m deeper flat seafloor dominated by linear abyssal hills (Domain III). B) Bouguer gravity anomalies and isostatic calculations indicate a change in crustal thickness of ~1.9 km across the transition.  Domain I is an older terrain west of the pseudofault created by the southward propagation of the ELSC. Fine dashed lines denote the Domain II-III crustal boundaries and the narrow transition region between them.