Supplementary Information for “Mantle wedge control on back-arc crustal accretion”
Removal of across-basin gravity gradient due to slab effects.
Bathymetry and gravity data were compiled in three-dimensional (longitude, latitude,
depth/gravity) digital grids in the area from 23°-15°S and 180°-172°W. Swath bathymetric
coverage within ~40 km of the spreading centers is essentially complete. Farther off axis
complete swath coverage across the basin exists from ~17°-18°45′S. Outside these areas swath
bathymetry is limited and is supplemented by single beam data and in a few areas by a satellite
gravity-derived bathymetric prediction1. Gravity data include complete shipboard along-axis
coverage south of ~18°S and systematically spaced (<9 km) E-W across-axis lines from ~17°18°45′S. These data are supplemented by more widely spaced shipboard and satellite altimetryderived gravity data2 elsewhere in the basin. The shipboard free air gravity data were
individually cross-over corrected and the ensemble average was found to agree to within 1 mGal
with the satellite-derived gravity.
Gravity reductions were applied in stages to separate different contributions to the gravity
field and isolate the crustal component near the spreading centers. Gravity reductions employed
Fourier-domain calculation of the Bouguer and MBA anomalies following standard methods and
densities3. In back-arc basins, long wavelength gradients are associated with subduction and are
superimposed on the crustal gravity signal. As the spreading centers are located at varying
positions across the basin we first need to estimate and remove the long wavelength gradient
associated with subduction effects in order to examine the local crustal signal. Free air gravity
anomalies include the entire gravity signal but are dominated by the large density contrast at the
seafloor (see Fig. 1a&b). We successively calculate the Bouguer anomaly, which removes the
effect of the seafloor relief from the free air anomaly (Fig. 1c); the mantle Bouguer anomaly
(MBA) in which the effect of the density contrast at the Moho is removed from the Bouguer
anomaly assuming a uniform 6-km-thick crust (Fig. 1d); and an isostatic anomaly (Fig. 1e) in
which the relief of the Moho is determined from the bathymetry following an Airy model of
compensation and is removed from the Bouguer anomaly. In Airy compensation the relief of the
Moho varies inversely with the topography and is proportional to the ratio of the density
contrasts at the seafloor and Moho. The absolute crustal thickness is not determined, however,
by this model and we use seismic refraction studies4 of the VFR to establish this level. The
isostatic anomaly accounts for predictable effects associated with topographic variations and
their crustal roots. Inspection of this anomaly in map and profile view (Fig 1e) shows that most
of the short wavelength signal within the back-arc basin area is accounted for by this model and
that a simple monotonic long-wavelength gradient remains. We attribute this remaining gradient
to the subducted slab and related effects. We stack this anomaly parallel to the volcanic front
within the back-arc basin and fit a polynomial curve to it to remove short wavelength variations.
The resulting curve is shown in Figure 1e. We use the smooth curve to remove the gradient from
the MBA and refer here to this anomaly as the adjusted mantle Bouguer anomaly (aMBA).
Having removed the gradient produced by the subducted slab, aMBAs are comparable to MBAs
at MORs.
Calculation of the axial cross-sectional area
The cross-sectional area or “inflation” calculation is adapted from Ref. 5 as described
below. At 1 km intervals along the ridge axis, profiles are generated by projecting in the
spreading direction the gridded bathymetry data from a 16x1km box centered on the spreading
center with its long axis in the spreading direction. The area beneath this profile is calculated
with respect to the average depth of 0.5-Myr-old seafloor flanking the ridge axis. Because of
sedimentation from the arc we use the western flank 0.5 Myr depth within the spreading
propagation boundary. We apply the inflation calculation uniformly even where the ridge axis
forms a low, in which case the negative value provides a measure of the area of the near-axis
valley. We have not attempted to exclude anomalous terrain that may be within the sampling
box, so that in a few areas, such as near the southern propagating tip of the CLSC, the area is
artificially high because there the sampling box includes shallow flanking seafloor not formed at
that axis. Also, because crustal production at the back-arc spreading centers is not uniform in
time, but varies with distance of the spreading center from the volcanic front, the area calculation
is not completely equivalent to that at mid-ocean ridges.
References and Notes
1.
Smith, W. H. F. & Sandwell, D. T. Global sea floor topography from satellite altimetry
and ship depth soundings. Science 277, 1956-1962 (1997).
2.
Sandwell, D. T. & Smith, W. H. F. Marine gravity anomaly from Geosat and ERS1
satellite altimetry. J. Geophys. Res. 102, 10,039-10,054 (1997).
3.
Kuo, B.-Y. & Forsyth, D. W. Gravity anomalies of the ridge-transform system in the
south Atlantic between 31 and 34.5°S: Upwelling centers and variations in crustal
thickness. Mar. Geophys. Res. 10, 205-232 (1988).
4.
Turner, I. M., Peirce, C. & Sinha, M. C. Seismic imaging of the axial region of the Valu
Fa Ridge, Lau Basin--the accretionary processes of an intermediate back-arc spreading
ridge. Geophys. J. Int. 138, 495-519 (1999).
5.
Scheirer, D. & Macdonald, K. C. Variation in cross-sectional area of the axial ridge along
the East Pacific Rise- Evidence for the magmatic budget of a fast spreading center. J.
Geophys. Res. 98, 7871-7885 (1993).
Figure 1. Bathymetry and gravity anomaly profiles across the basin at latitude 20°S
sampled from the gridded maps. Dashed vertical line denotes the position of the
spreading center in these profiles. A) Bathymetry with Lau Ridge (LR), Tonga Ridge
(TR), Tonga Trench (TT), volcanic front (V), and sea-level (SL) indicated. B) Free air
gravity anomaly. C) Bouguer anomaly. D) mantle Bouguer anomaly (MBA). E) Isostatic
anomaly with smooth polynomial fit to the subduction gradient. I) Isostatic anomaly with
subduction gradient removed.