Asymmetric melt sills and upper crustal construction
beneath overlapping ridge segments: Implications for
the development of melt sills and ridge crests
C. H. Tong 1, J. W. Pye, P. J. Barton, R. S. White, M. C. Sinha 2, S. C. Singh 3, R. W. Hobbs
Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley
Road, Cambridge, CB3 0EZ, United Kingdom.
S. Bazin 3, A. J. Harding, G. M. Kent, J. A. Orcutt
Cecil and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of
Oceanography, University of California, San Diego, CA 92093, USA.
To be submitted to Geology
Corresponding author: [email protected]
Present addresses: 1 T. H. Huxley School, Imperial College of Science, Technology and
Medicine, Prince Consort Road, London, SW7 2BP, United Kingdom.
2
School of Ocean and Earth Science, University of Southampton, Southampton, SO14
3ZH, United Kingdom.
2
3
Laboratoire de Géosciences Marines, Institut de Physique du Globe de Paris, 75252 Paris
Cedex 05, France.
Abstract
A new 3-D tomographic velocity model and depth-converted image of the melt sills beneath
the 9°03'N overlapping spreading center (OSC) on the East Pacific Rise shows that the
upper crustal construction at this ridge discontinuity is highly asymmetric with reference to
the bathymetric ridge crests of the overlapping limbs. Despite the similarly curved ridge
crests, the asymmetries are markedly different under the two limbs, and appear to be related
to the contrasting evolutionary history of the limbs. The overlap basin is closely related to
the propagating eastern limb in terms of its seismic structure. By contrast, the selfdecapitating western limb forms a distinct morphological region that displays little structural
relationship with the adjacent overlap basin and other relict basins. The observed structural
asymmetries may arise because the rotational effects of the stress field in the overlap region
act on the upper, brittle part of the crust, while the melt sill geometry is controlled by the
deeper, ductile behavior of the plate boundary. As the OSC is migrating southward, the
differential development of melt sills and ridge crests may be inferred from the results of this
study. Ridge propagation appears to involve two major processes: the advancement of the
melt sill at the ridge tip, and the development of ridge crest morphology in the northern part
of the overlap basin region near the existing propagating limb. The eastward displacement of
the melt sill with respect to the bathymetric ridge crest of the self-decapitating limb suggests
that the rotational motion of the ridge crest is not primarily driven by the melt sill, but may
be associated with the rotational stress field in the overlap region.
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Introduction
Our understanding of the evolution of the geometry of overlapping spreading centers,
commonly found on fast- and intermediate-spreading mid-ocean ridges (Macdonald and
Sempere, 1984; Macdonald et al., 1991), has hitherto been based on their morphological
expression at the seafloor. Crack propagation theory has been applied to explain the
formation of the curved shape of overlapping en-echelon ridge segments (e.g. Pollard and
Aydin, 1984; Baud and Reuschle, 1997). Numerical modeling has successfully described the
evolution of OSCs based on magnetic and bathymetric data (e.g. Wilson, 1990; Carbotte and
Macdonald, 1992; Cormier et al., 1996), suggesting that many OSCs are migrating relative to
their nearest transform offsets. Generally, one of the ridge segments is propagating, while
the other segment repeatedly cuts itself off from the main ridge axis and is rafted off to the
flank of the plate boundary. This process is known as self-decapitation (Macdonald et al.,
1987). Although crack propagation theory predicts the observed configuration of OSCs (e.g.
Macdonald and Sempere, 1984), it does not explain how these overlapping “cracks” are
formed at crustal levels. Furthermore, no crustal constraints have been placed on the
kinematic reconstruction of the evolution of ridge segments.
With extensive geophysical studies conducted in its vicinity (e.g. Carbotte and
Macdonald, 1992; Kent et al., 1993, 2000; Harding et al., 1993), the 9°03'N OSC on the East
Pacific Rise is one of the best places to investigate the crustal structure related to ridge
propagation and self-decapitation. The northern ridge segment of this OSC is propagating
southward, while the southern ridge segment has undergone repeated episodes of selfdecapitation, leaving relict basins and relict ridge segments to the west of the ridge axis
(Carbotte and Macdonald, 1992). We here use refraction and reflection seismic data to
4
construct a 3-D tomographic velocity model and a depth-converted image of the melt sills to
investigate the upper crustal construction at the OSC.
Method
Our 3-D tomographic analysis used 69,796 crustal first arrivals from seismograms collected
by an array of 19 ocean bottom seismometers (Fig. 1) in the ARAD experiment (Singh et al.,
1999). The maximum source-receiver offset is ~10 km and the mean pick uncertainty is 28
ms. We used a travel-time inversion algorithm that jointly minimizes travel-time misfits and
model roughness (Hobro et al., in press). The velocity model is densely parameterized with a
horizontal node spacing of 500 or 494 m and a vertical spacing of 195 m.
Normal moveout correction and CDP stacking (with trace-offset range of 0-3 km)
were applied to the 3-D seismic reflection data collected using a 3.1 km streamer with 124
channels. After two-pass Kirchhoff time migration, the resulting data were interpolated to a
final bin size of 25 m x 25 m with amplitude compensation and filtering applied. The twoway travel-times of the horizons which were interpreted as the melt sills were picked. The
velocity model obtained from tomographic inversion was used for depth–converting the
horizons by integrating along vertical ray paths. Due to the complex local bathymetry, depth
conversion using vertical rays is superior to ray-tracing for producing a sharper image of the
reflectors (Tong, 2000). Details of the tomographic and reflection analyses can be found in
Tong (2000).
Results
The 4.5 km/s velocity contour, which generally coincides with the depths of sharpest change
in velocity gradient (Fig. 3: A2, B2 and C2), is chosen to represent the layer 2A/2B
5
boundary. This is compatible with definitions used in previous studies (e.g. Harding et al.,
1993). The shallowest points of the 4.5 km/s contour under the spreading limbs are
identified as the neovolcanic axis (Fig. 2c), which is a good proxy for the locations of highest
magmatism (Perfit and Chadwick, Jr., 1998). (The precise choice of velocity contour has little
effect on the results.) The lateral resolution of the velocity model is estimated as ~2.5-5 km,
and the uncertainty in the depth of the melt sill is about ±110 m (Tong, 2000). There are
significant differences in geometry between the depth-converted melt sills and those
observed on the time-migrated seismic sections (Fig. 3: A1–C1, A4–C4). This highlights the
importance of velocity control in the depth-conversion of reflectors. Compared with the
central parts of ridge segments well removed from major discontinuities (e.g. Toomey et al.,
1994; Hooft et al., 1997), a relatively thick layer 2A and deep melt sills are imaged in this
study. These results are consistent with those reported in previous investigations near major
ridge discontinuities (e.g. Tolstoy et al., 1997).
The asymmetric crustal structures beneath the limbs are likely to be related to their
evolutionary history. The thick low-velocity layer under the overlap basin and the tip of the
propagating eastern ridge crest may be caused by lateral injection of melt beyond the tip of
the melt sill (Fig. 3: C1), a mechanism proposed in similar geological settings elsewhere (e.g.
Cormier et al., 1996). The rotational effect of the stress field in the overlap region (Perram
and Macdonald, 1990) may lead to the inwardly curved ridge tip seen in the bathymetry and
the thick low-velocity layer 2A that covers the eastern ridge tip and overlap basin. The
“unrotated” plunging tip section of the propagating eastern melt sill may provide a reference
against which the extent of rotation can be measured. Although the thick layer 2A may
represent a frozen lava pond (Bazin et al., in press), it is also possible that the tensional
component of the rotation induces fractures, leading to the observed negative velocity
6
anomaly and the shallow layer with distinct lower velocity-gradients (Fig. 3: C2 and C3). The
relatively extensive region of low velocity-gradient layer 2B (Fig. 3: C2) may be caused by
fractures or by a positive thermal anomaly created by the propagating tip of the melt sill,
where a high-magnetisation zone (Fe-Ti basalts) is found (Sempere and Macdonald, 1986;
Bazin et al., in press).
By contrast, the high velocity-gradient region that characterizes layer 2A at the tip of
the western limb is localized under the ridge crest as it forms an isolated morphological
region (Fig. 3: A2). The northern part of the overlap basin to its east forms part of the
eastern limb system with its continuous crustal and melt sill constructions (Fig. 3: A2), and
the relict basins to its west display structures similar to those found under the central and
southern parts of the overlap basin (Fig. 3: A3–C3). Our results confirm the magmatically
depleted nature of the western ridge tip (Sempere and Macdonald, 1986). The thick layer 2A
at the western limb tip does not show the positive velocity anomaly observed above the
western melt sill (Fig. 3: A3 and C3). This positive velocity anomaly may reflect the thin onaxis layer 2A, similar to those observed along mid-segment regions (e.g. Toomey et al., 1994).
The lack of this positive anomaly at the ridge tip may indicate the pattern of on-axis
hydrothermal alteration (Sohn et al., 1996), and possible faulting induced by the dwindling
melt supply. If the depth of the melt sill is related to the heat balance of the crust (e.g.
Henstock et al., 1993), the comparable depths of the melt sills may imply that the apparent
difference in melt supply between the limbs has little effect on their bulk thermal structure.
Implications
As the OSC is migrating southward, we may regard the along-axis variations of the
asymmetric melt sills and the upper crustal layers as snapshots detailing the stages of the
7
construction of the fuller and wider ridge crest that is observed away from major ridge
discontinuities. The advancement of the melt sill may be accompanied by lateral melt
injection at the ridge tip. As the OSC migrates further south, the plunging tip section of the
eastern melt sill may eventually acquire a geometry and dimensions similar to that of the
same melt sill imaged further north. A shallower, elongated region on the melt sill may
eventually appear, marking the location of the neovolcanic axis. The effect of the rotational
stress field on the upper, brittle part of the crust is significant, resulting in the “rotated”
extrusives observed beyond the ridge tip.
The melt supply from the melt sill beneath the overlap basin gradually transforms the
northern part of the basin region into part of the fuller ridge crest that extends from the
existing curved limb. The development of a new neovolcanic axis under the overlap basin
may lead to the abandonment of the current neovolcanic axis under the central part of the
eastern limb, where the neovolcanic axis is closely related to the similarly curved ridge crest.
The observed eastward displacement of the western melt sill from the bathymetric
high of the ridge axis may imply that the surficial expression of self-decapitation is
influenced primarily by the rotational effect of the stress field in the overlap basin, rather
than being driven by the melt sill. Equivalently, the geometry and location of the melt sill
may lag behind the anti-clockwise rotational motion of the western limb that forms the initial
stages of self-decapitation.
In conclusion, the asymmetric upper crustal features under the two limbs presented
in this study are closely related to the OSC evolution. Change in plate motion is suggested as
one of the causes of plate boundary re-orientation (e.g. Pockalny et al., 1997), and our
constraints on the upper crustal evolution at the OSC offer new insights into the
investigation of its underlying mechanisms.
8
Acknowledgement
We thank the Natural Environment Research Council (U.K.), National Science Foundation,
and Newton Trust for funding the ARAD experiment. Department of Earth Sciences,
University of Cambridge Contribution no. ES.6413.
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Figure captions
Figure 1. Location of the 9°03'N overlapping spreading center and the data acquisition
geometry. Inset indicates location of study area. 160,000+ shots were fired within the box
shown in solid lines and recorded by 19 ocean bottom seismometers (stars). Broken lines
denote limits of the 3-D reflection survey. The origin of the local coordinate system is
indicated by orthogonal arrows, which correspond to the x- and y- axes. Cross-sections in
Fig. 3 are located by lines A, B and C.
Figure 2. a) Depth variation of the melt sills below the sea surface, and b) depth below
seafloor. The ridge crests of the eastern limb (EL) and western limb (WL) are indicated by
the solid contour of 2700 m. The overlap basin (OB) and relict basins (RB) (Carbotte and
Macdonald, 1992) are shown by the dotted 2900 m contour. Cross-sections in Fig. 3 are
located by lines A, B and C. c) Detailed bathymetry of the OSC. Heavy black lines mark
limits of the melt sills; thick grey lines show locations of the neovolcanic axes (see text for
details). Note that the neovolcanic axis and the melt sill under the western limb are offset
toward the eastern side of the ridge crest, and the neovolcanic zone is shifted toward the
central part of the ridge crest, where the melt sill vanishes (Fig. 2c). The other melt sill
13
extends to the northern part of the overlap basin from the eastern limb and consists of an
elongated shallow region near the outward edge of the ridge crest (Fig. 2a). Fig. 2b shows a
similarly elongated shallow structure on the western side of the melt sill under the overlap
basin. Noticeable increase in the depth of the melt sill in this region is restricted to its
western edge. A gradual decrease in width and an increase in depth of the melt sill are
observed toward its tip, where the neovolcanic axis deviates from the curved ridge crest (Fig.
2a).
Figure 3. A1, A2, A3 show the velocity, velocity-gradient, and velocity anomaly variations of
line A (Figs. 1 and 2). Regions that are not constrained by rays are shown in lighter shades.
Velocity-gradient at each velocity node is calculated by the velocity difference over the
vertical interval of the node spacing divided by the node spacing. Velocity anomaly plot
shows velocity deviations from the mean velocity at a given depth below seafloor. A4 shows
the time-migrated seismic section of line A with the picked horizon (melt sill) indicated by
the red line a. TWTT is two-way travel-time. The depth-converted horizon is represented by
the black lines in A1, A2 and A3. The red and blue lines in all plots show the locations of the
ridge crests of the western limb and eastern limb, respectively, based on the 2700 m contour.
B1–B4, and C1–C4 show the results corresponding to lines B and C, respectively. Note that
a thicker layer 2A, characterized by negative near-seafloor velocity anomalies and high
velocity-gradients, is observed in the tip region of the western limb (Fig. 3: A2 and A3). A
thicker layer 2A that extends from the overlap basin to the western part of the ridge crest is
imaged in the tip region of the eastern limb (Fig. 3: C1). A relatively extensive region of low
velocity-gradients in layer 2B is also found there (Fig. 3: C2). Also note the continuity in the
characteristics of upper crustal structures in the study area: a continuous layer 2A underlies
14
the northern part of the overlap basin and the eastern limb (Fig. 3: A2). By contrast, the
central and southern parts of the overlap basin separate the continuous high velocitygradient layers found under the adjacent ridge crests and off-axis regions (Fig. 3: B2 and C2).
A negative velocity anomaly is observed under the relict basins and the central and southern
parts of the overlap basin (Fig. 3: A3, B3 and C3).