Geophysical Research Letters
Supporting Information for
Seismic observation of an extremely magmatic accretion at the ultraslow spreading
Southwest Indian Ridge
Jiabiao Li1, Hanchao Jian2,3, Yongshun John Chen2, Satish C. Singh3, Aiguo Ruan1, Xuelin Qiu4,
Minhui Zhao4, Xianguang Wang2, Xiongwei Niu1, Jianyu Ni1, and Jiazheng Zhang4
1Second
Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China. 2Institute of Theoretical
and Applied Geophysics, School of Earth and Space Science, Peking University, Beijing 100871, China. 3Laboratoire
de Géosciences Marines, Institut de Physique du Globe de Paris, 1 rue Jussieu, 75238 Paris Cedex 05, France. 4South
China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China.
Contents of this file
Figures S1 to S6
Introduction
Here we present some examples of OBS record, details of tomographic inversions and
comparison of our result and previous seismic observations. Fig. S2 shows the data examples
with hand picked arrivals, whose location is shown in Fig. S1. Fig. S3 compares the normalized
travel time residuals before and after inversions, whose resolution is assessed by checkerboard
tests shown in Fig. S4. As an addition, in Fig. S6, we show the anomaly from the 2D
tomography. Fig. S5 compares our result about the thick crust with previous seismic
observations at global mid-ocean ridges indicating the excess melt supply in our study area.
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Fig. S1. Bathymetric map. All used OBSs are shown as filled circles, and those shown in Fig. S2
are both number and color labeled. The thin lines mark the shooting profiles shown in Fig. S2,
with the same color that fills the corresponding OBS. CV, central volcano. Other geological
features are the same as in Fig. 1.
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Fig. S2. OBS Data. Receiver gathers for OBS (a) 34, (b) 36, (c) 4, (d) 23 and (e) 26. The locations
of OBSs and shooting profiles are shown in Fig. S1. The red and black lines indicate the Pg and
PmP arrivals used in the 3D tomography, respectively. The cyan lines indicate the first arrivals
used in the 2D tomography. A water-path correction, a water-wave muting and a band-pass
filter are applied to all data. (a) and (b) are scaled with absolute travel times and linear moved
out with a velocity of 6.5 km/s, and (c-e) are scaled with a trace balance operator and linear
moved out with a velocity of 8 km/s. Please note the larger moveout for rays traversing around -
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40 km along-axis distance, suggesting a higher velocity beneath as the water-path correction
has been applied.
Fig. S3. Normalized residuals. Normalised residuals versus source-receiver offsets for starting
model and final model for (a) Pg arrivals used in the 3D tomography and (b) PmP arrivals in the
3D tomography and (c) first arrivals in the 2D tomography. The normalized residual is the
difference between observed and predicted travel times divided by the picking uncertainty. The
RMS of normalized residuals is close to unity for the final model for each phase.
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Fig. S4. Checkerboard tests. (a) Input pattern of the perturbations added to the smoothed best
fitting model of the 3D tomography. The numbers along the contours indicate velocity
perturbation in %. (b) The recovered pattern by inverting the synthetic data. The input patterns
(c) I; (e) II and (g) III are added to the smoothed best fitting model of the 2D tomography.
Recovered patterns (d) I; (f) II and (h) III derived by inverting the synthetic data. The contours on
the recovered pattern are extractrd from the corresponding input pattern. The 3D test indicates
that the 3D tomography is able to recover anomalies with size of about 5 km horizontally by 2
km vertically at the center of the model. The 2D tests indicate that the 2D tomography can
recover anomalies of about 16 km by 2.5 km at most part of the model except at the lower crust
between -10 km and 10 km along-axis distance due to poor ray coverage.
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Fig. S5. Seismic crustal thickness versus spreading rates. The black line shows the average
crustal thickness determined with seismic experiments on mid-ocean ridges with from fast to
slow spreading rates [Chen, 1992], whereas the symbols indicate maximum crustal thickness
from seismic observations at current ultraslow and slow spreading centers. The green triangles
are the data from the Western Volcanic Zone, Sparsely Magmatic Zone and Eastern Volcanic
Zone of the Gakkel Ridge [Jokat and Schmidt‐Aursch, 2007]. The blue triangle is from the
Logachev Seamount on the Knipovich Ridge [Jokat et al., 2012]. The cyan diamond is the data
from the SWIR near 66E [Minshull et al., 2006]. The black squares are two examples of seismic
results at the slow spreading MAR at 33.5°-35° N with a full spreading rate of ~ 21 mm/yr [Hooft
et al., 2000] and at 33°S with a full spreading rate of ~36 mm/yr [Tolstoy et al., 1993]. Our result
is shown as the red diamond. At the bottom, the class of spreading is roughly indicated, where
the dashed lines indicate the transitions.
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Fig. S6. 2D along-axis velocity anomaly. The anomalies are calculated with respect to the
average of the 2D best fitting velocity model (Fig. 4). The thick black curves (solid, dashed and
dotted) represent the Moho interface, and have the same meaning as those in Fig. 4. The
contours (thin black curves) have an interval of 0.25 km/s and are labeled in km/s. The green
triangles indicate the OBSs used in the 2D tomography. Segments of the ridge axis are also
shown on the top.
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