Download Oceanic corrugated surfaces and the strength of the axial

Earth and Planetary Science Letters 288 (2009) 174–183
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Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Oceanic corrugated surfaces and the strength of the axial lithosphere at slow
spreading ridges
Mathilde Cannat a,⁎, Daniel Sauter b, Javier Escartín a, Luc Lavier c, Suzanne Picazo a
a
b
c
Equipe de Géosciences Marines, CNRS-UMR7154, Institut de Physique du Globe, 4 place Jussieu 75252 Paris cedex 05 France
Ecole et Institut de Physique du Globe, CNRS-UMR 7516 5 rue Descartes 67084 Strasbourg cedex France
University of Texas, Institute for Geophysics, Jackson School of Geosciences, Austin, Texas 78759 USA
a r t i c l e
i n f o
Article history:
Received 24 February 2009
Received in revised form 29 August 2009
Accepted 9 September 2009
Available online 15 October 2009
Editor: R.D. van der Hilst
Keywords:
mid-ocean ridges
detachment fault
topography
corrugated surface
lithosphere strength
a b s t r a c t
We analyse the topography and gravity signature of 39 corrugated surfaces formed over the past 26 myrs in the
footwall of axial detachment faults at the eastern Southwest Indian Ridge. These corrugated surfaces appear to
have formed at a melt supply about half the global melt supply average for mid-ocean ridges, and we find that
their presently elevated topography, relative to adjacent non-corrugated seafloor, was mostly acquired at the end
of their formation, at the “termination stage”. This configuration, which also characterizes many off-axis
corrugated surfaces in other oceans, suggests that the plate flexural rigidity was very low during the formation of
the corrugated surface, and increased significantly at the termination stage. Following Buck (1988), we
hypothesize that stresses related to bending of the plate cause internal deformation and damage in the footwall of
the fault, which is associated with weakening. As a possible mechanism for enhanced footwall weakening while
corrugated surfaces form, we propose the formation of weak shear zones coated with hydrous minerals such as
talc, amphibole, chlorite and serpentine, in mantle-derived ultramafics next to gabbro intrusions. If this
hypothesis is correct, the amount of footwall weakening and roll-over along axial detachment faults at slow
spreading ridges may be controlled both by access to hydrothermal fluids in the footwall of the detachment, and
by the abundance and distribution of gabbros intrusions in exhumed ultramafics.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Oceanic corrugated surfaces are domal features characterized by
spreading-parallel undulations of the seafloor. They were first
discovered in the Atlantic (Cann et al., 1997), and interpreted as the
exposed inactive footwall of large offset axial normal faults, also called
detachment faults (Cann et al., 1997; Tucholke et al., 1998).
Corrugated surfaces have since been dredged, cored and sampled
with submersibles at a number of near-axis and off-axis locations,
yielding variably deformed rock suites, including gabbros, serpentinized peridotites, and lesser volumes of basalt (MacLeod et al., 2002;
Karson et al., 2006; Ildefonse et al., 2007; Dick et al., 2008). Deformed
samples from the corrugated fault zone typically display low
temperature to greenschist facies syntectonic minerals, and fabrics
are dominantly brittle, with intervals of plastically deformed talc,
amphibole, chlorite and serpentine (Escartin et al., 2003; Schroeder
and John, 2004). Higher grade sheared gabbro and peridotite
mylonites have also been sampled below exposed detachment
surfaces (Cannat et al., 1991; Dick et al., 2000 ; Schroeder and John,
2004). The presence of gabbros confirms that, in spite of a thin crust
inferred from seismic and gravity-data and suggesting reduced melt
⁎ Corresponding author.
E-mail address: [email protected] (M. Cannat).
0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2009.09.020
supply (Blackman et al., 1998; Canales et al., 2004), corrugated
surfaces form during magmatically active stages of ridge spreading
(Buck et al., 2005; Ildefonse et al., 2007).
Recent observations at 13°N in the Atlantic have shown corrugated
domes originating in the footwall of active axial normal faults, with
volcanic ridges in the hanging wall (Smith et al., 2006, 2008).
Assuming fault dips of 40° or more at depth (e.g., (deMartin et al.,
2007)), roll-over to dips less than 5° occurs within 5 km of footwall
emergence (Smith et al., 2006), indicating a very low footwall flexural
rigidity (Smith et al., 2006, 2008).
Corrugated surfaces have now been identified near most intermediate to ultraslow spreading ridges, predominantly, but not exclusively (Escartín and Cannat, 1999; Smith et al., 2008), in ridgetransform inside-corner settings. Their domal shape has been
reproduced in numerical models by flexure of the footwall of large
offset normal faults (Lavier et al., 1999). Numerical models of
symmetrical spreading predict that such faults are most likely to
form when about 50% of total extension is accommodated by
magmatic accretion in the hanging wall (Buck et al., 2005; Tucholke
et al., 2008). Corrugations, however, are not always present in the
footwall of large offset axial normal faults. Near-axis domal structures
devoid of corrugations, such as at the Trans Atlantic Geotraverse
(TAG) (deMartin et al., 2007), are also interpreted as the footwall of
detachments. It has been proposed that axial detachments, forming
M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
corrugated surfaces or not, are active along nearly 50% of the length of
slow-spreading ridges (Escartin et al., 2008b). This is consistent with
the estimated surface proportion of ultramafic and gabbroic seafloor
exposures in the Atlantic (~25% ;(Cannat et al., 1995)), because these
exposures form asymmetrically about the axis in the exhumed
footwall of these detachments.
In this paper, we present a detailed description of the corrugated
surfaces identified in ultraslow-spreading seafloor of the Southwest
Indian Ridge (SWIR), east of the Melville Fracture Zone (Fig. 1). Our
study area covers 630 km of ridge axis, and extends to ages of 28 myrs
on both plates. Seismic data suggest that this eastern part of the SWIR
has a very low average melt supply, equivalent to a 3 to 4 km-thick
magmatic layer (Muller et al., 1999; Minshull et al., 2006). This is
about half of the average melt supply estimated for the global midocean ridge system (Chen, 1992). Corrugated surfaces occur throughout the mapped area (Fig. 1), although they cover only about 4% of the
total surface (Cannat et al., 2006). This region of the SWIR also
displays large expanses (~40% of the mapped area; Fig. 1) of non-
175
corrugated «smooth seafloor», which have been interpreted as mostly
mantle-derived ultramafic rocks exposed in the footwall of axial
detachments (Cannat et al., 2006). The rest of the seafloor (“volcanic
seafloor” in Fig. 1) displays volcanic cones, and spreading-perpendicular scarps. Many of these scarps have steep outward-facing slopes
and may represent tectonically-rotated volcanic surfaces, as proposed
by (Smith et al., 2008) for similar scarps formed by axial detachments
in the 14°N area of the Mid-Atlantic Ridge (MAR). The prevalence of
detachments in accretion processes at the eastern SWIR is thus
probably greater than at the MAR (Cannat et al., 2006).
Previous work in the eastern SWIR has shown that smooth seafloor
forms at minimal melt supply, and that the seafloor opposite the
corrugated surfaces and across the ridge axis (conjugate seafloor) is
primarily volcanic (Cannat et al., 2006). The eastern SWIR therefore
has two assets for a study of how corrugated surfaces form: (1) it
contains a large number of these surfaces in a relatively well defined
spreading and regional melt supply context; and (2) some of these
corrugated surfaces transition into seafloor that was formed at a
Fig. 1. Shaded bathymetric map of the eastern Southwest Indian Ridge, showing the location of 39 corrugated surfaces (modified after (Cannat et al., 2006). Red lines are corrugation
trends, white lines are calculated spreading vectors for 2 myrs-worth of plate spreading (plate rotation parameters as in (Cannat et al., 2006)). White contours separate seafloor with
numerous scarps and volcanic cones (volcanic seafloor) from seafloor with broad hills and no volcanic features (smooth seafloor; (Cannat et al., 2006). Thick dashed line: Central
Magnetic Anomaly. Thinner dashed lines are other magnetic isochrons (Sauter et al., 2008).
176
M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
minimal melt supply, with little to no volcanism. This is not the case in
the Atlantic, where corrugated surfaces are surrounded by seafloor
with morphological evidence for a volcanic carapace (e.g. (Smith et al.,
2006, 2008)). Based primarily on the eastern SWIR data, we
specifically discuss the topography and the conditions of termination
of oceanic corrugated surfaces. We then propose a conceptual model
in which apparent variations of the rigidity of the axial plate during
and after the formation of corrugated surfaces, are explained by
variable degrees of mechanical weakening of the footwall of axial
detachment faults.
2. Size, age and map distribution of eastern SWIR
corrugated surfaces
Bathymetric data used for this study were acquired with a
Thomson TMS 5265B multibeam system, and have a horizontal
resolution of 1 to 2% of seafloor depth. At depths of 4000 m and more,
these data therefore do not properly image seafloor structures less
than 100 m across. They are sufficient, however, to map the
corrugated surfaces, and to derive some basic characteristics of their
transition to non-corrugated seafloor.
Individual corrugated surfaces in the eastern SWIR cover areas up
to 880 km2 (Fig. 2a; Table 1). Many surfaces have similar along- and
across-axis lengths, yet a few large surfaces extend substantially more
along-axis (up to 74 km; Fig. 2b), than across-axis (up to 31 km;
Fig. 2b). Most surfaces appear to have formed in less than 2 myrs
(Fig. 2c), and corrugations are parallel to the spreading vectors (Fig. 1)
calculated using the rotation parameters listed in (Cannat et al., 2006),
which are within errors of the more recent regional plate velocity
model of (Patriat et al., 2008). Significant differences between
corrugations and calculated spreading vectors are only observed for
corrugated surfaces between magnetic anomalies 6 (A6) and 6C
(A6C). These differences are likely due to local variations in spreading
direction with respect to the regional plate velocity model.
Based on the magnetic anomaly pattern, the ages of corrugated
surfaces in Fig. 1 range between recent (Fuji Dome; (Searle et al.,
2003; Searle and Bralee, 2007), and older than 25 myrs. They are more
abundant, and commonly larger, in seafloor older than A6 (Fig. 2a).
This may relate to a decrease in spreading rate with time, from 30 km/
myr prior to A6C, to ~16 km/myr between A6 and A6C, and to
~14 km/myr since A6 (Sauter et al., 2008; Patriat et al., 2008). The two
largest groups of corrugated surfaces (numbers 29 to 37, and 9 to 13;
Fig. 1) initiated on opposite sides of the ridge soon after the onset of
SWIR spreading at the Rodriguez Triple Junction. At that time the
spreading rate was ~ 30 km/myr (Patriat et al., 2008). Surfaces
younger than 10 myrs (magnetic anomaly 5) are small (<50 km2;
Table 1), with the exception of surfaces number 5, 14 and 21. Finer
resolution Towed Ocean Bottom Instrument (TOBI) data on surfaces
15 (Fuji Dome; (Searle et al., 2003)) and 16, show that they represent
two portions of a single, larger, corrugated surface (Searle and Bralee,
2007). Surfaces 17 and 18, on the northern axial valley wall, may also
belong to a single, faintly corrugated surface. Surfaces 25 to 28
delineate corrugated areas on the southern flank of a blocky
topographic high (Fig. 1), previously described as the fossil footwall
of a large axial fault (fault block h9 in (Cannat et al., 2003); also
discussed in (Searle and Bralee, 2007)).
3. Gravity and topographic signature of eastern SWIR
corrugated surfaces
Processing of the gravity data and calculation of residual topography for the study area are presented in (Cannat et al., 2006). Modelling
of both residual seafloor depths and gravity anomalies involves a
correction for plate cooling with age, which is most pronounced and
probably more inaccurate, in near-axis regions. Because the near axis
region in Fig. 1 also comprises few corrugated surfaces, we have
limited our study to the gravity and topographic signature of crust
formed prior to magnetic anomaly 3A (Figs. 1 and 2a). The residual
Fig. 2. Characteristics of the 39 eastern Southwest Indian Ridge corrugated surfaces shown in Fig. 1 (Table 1). Open circles : corrugated surfaces accreted since the time of magnetic
anomaly 3A. Grey circles : corrugated surface with area <200 km2; black circles : area > 200 km2. Ages for magnetic anomalies 3A, 6, 6C and 8 in (a) as in (Cande and Kent, 1995).
Estimated duration in (c) is maximum across-axis extension of corrugated surface divided by half-spreading rate at time of formation. It is a maximum because it assumes
symmetrical spreading during the formation of corrugated surfaces, which may not be the case (Searle and Bralee, 2007; Baines et al., 2008; Dick et al., 2008). Residual topography in
(d) is calculated by difference with the square root of age subsidence model also used for gravity data correction (Cannat et al., 2006). Crustal thickness in (d) is modelled from
residual mantle Bouguer gravity anomalies (RMBA), with a reference crustal thickness of 3.4 km, and constant density crust and mantle (Cannat et al., 2006). This crustal thickness
model is consistent with available seismic refraction data (Minshull et al., 2006). Line is (d) is best linear fit for corrugated surfaces >200 km2; it has a slope of 0.43 (with a
correlation coefficient R2 of 0.42). The predicted slope for isostatic balance (for crustal density of 2700 kg/m3, and mantle density of 3300 km/m3) would be 0.26. Dashed arrows in
(d) link corrugated surfaces formed along the same flow line as shown in Fig. 4.
177
M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
Table 1
Characteristics of the 39 eastern Southwest Indian Ridge corrugated surfaces shown in Fig. 1.
Number
Area
Length
Width
Azimuth
(km2)
(km)
(km)
(°)
31
66
130
511
450
105
700
358
280
198
880
156
31
180
39
35
46
16
132
37
134
41
169
13
26
40
46
9
58
36
70
302
289
542
828
265
67
32
25
4
8
14
23
20
13
20
29
12
17
31
13
4
23
6
9
9
6
13
8
15
6
11
5
7
13
8
4
7
5
9
20
7
10
18
9
3
5
6
7
9
13
24
25
14
58
14
23
14
45
19
10
8
8
4
8
4
11
7
12
5
22
4
4
5
7
3
12
8
14
19
40
42
74
35
20
8
7
189
183
187
185
183
183
181
187
186
184
188
185
181
175
179
179
179
180
180
182
180
178
181
182
180
183
180
180
188
181
183
182
181
181
189
185
184
184
179
Corrug.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
Longitude
Latitude
Res.
Crustal
Term.
E
S
Topo
Thickness
Fault
av.
Heave
(myr)
av.
av.
(m)
(km)
(m)
Before/after
West/east
26.6
25.3
28.0
20.1
8.4
25.3
24.1
26.6
22.8
20.1
20.1
22.8
24.7
9.3
2.6
2.6
1.3
1.3
10.9
9.3
8.4
10.9
14.6
14.6
9.3
9.3
9.3
9.3
28.5
28.0
28.0
26.6
26.6
25.1
24.1
22.1
22.1
17.8
2.9
61.261
61.534
62.537
62.115
62.261
61.510
61.698
62.514
64.718
64.614
65.038
64.190
64.107
65.300
63.782
63.671
63.362
63.422
64.592
64.551
64.676
63.727
63.818
63.611
64.150
64.215
63.955
64.042
63.251
63.413
63.563
63.304
63.905
63.577
63.629
64.176
63.853
65.138
65.484
− 30.935
− 30.720
− 30.423
− 29.837
− 28.079
− 27.414
− 27.524
− 26.701
− 26.375
− 26.645
− 26.610
− 26.476
− 26.348
− 28.345
− 28.015
− 28.023
− 28.006
− 27.953
− 27.245
− 27.350
− 27.434
− 28.537
− 28.753
− 28.783
− 28.448
− 28.469
− 28.417
−28.429
− 30.171
− 30.029
− 29.991
− 29.761
− 29.725
− 29.564
− 29.412
− 29.251
− 29.293
− 26.712
− 27.852
587
436
724
961
826
− 268
738
454
388
1137
354
723
352
924
–
–
–
–
114
409
1279
269
689
109
629
134
668
1121
− 111
62
85
568
185
103
159
225
135
233
–
2.9
3.7
2.7
3.8
3.8
2.8
3.3
3.8
3.4
4.3
3.5
4.5
3.9
3.3
–
–
–
–
3.6
3.2
4.1
3.1
3.2
3.0
3.0
3.7
3.8
3.3
2.2
2.9
2.8
2.8
2.6
3.2
3.0
2.9
3.3
2.7
–
0
100
1435
996
1417
0
1332
869
700
1642
727
886
0
0
0
0
0
0
0
1213
1005
615
1801
285
0
0
0
0
808
1019
804
1294
824
785
788
416
336
0
815
?
vv
vv
sv
vv
vv
vv
vv
sv
vv
sv
sv
sv
vs
vv
vv
vs
vs
vv
vv
vs
vv
vv
vv
vv
vv
vv
vv
vv
vv
?
?
ss
vv
vv
ss
ss
vv
vv
ss
vv
v?
ss
ss
?v
?s
vv
vs
vv
vs
vs
vs
vv
sv
ss
vs
ss
vv
ss
ss
vv
vs
vv
vv
vs
vv
vv
?
ss
ss
?s
sv
?s
vs
ss
ss
ss
vv
Estimated
age
Transition
Length is along-axis extension, width is across-axis extension. Longitude and latitude are mean values for each surface. Age is estimated using the plate rotation parameters in
(Cannat et al., 2006), which differ only marginally from those of Patriat et al. (2008). Residual topography and crustal thickness (from gravity-derived model in (Cannat et al., 2006)
are averages for all grid points which belong to the surface. Standard deviation values range between 100 m and 500 m for residual topography, and between 50 m and 500 m for
crustal thickness. Termination fault heave is maximum value measured along termination scarp. Last two columns correspond to types of seafloor (v : volcanic ; s : smooth) accreted
along the same flow line before and after the corrugated surface, or along the same isochron to the west and east of the surface. Question marks correspond to ambiguous
morphologies in older, sediment-covered seafloor.
topography is the difference between actual topography, and the
square root of age subsidence model also used for gravity data
correction (Cannat et al., 2006). Our gravity-derived crustal thickness
model is consistent with available seismic refraction data (Minshull
et al., 2006).
Collectively, corrugated surfaces shown in Fig. 1 have a positive
average gravity signature, and an anomalously elevated residual
topography. The conjugate corrugated-volcanic seafloor pairs (Cannat
et al., 2006) are characterized by extreme topographic and gravimetric asymmetry (Fig. 3). Corrugated surfaces stand on average 500 m
above the predicted thermal subsidence level, while volcanic seafloor
accreted simultaneously in the opposite plate has near zero average
residual topography (Fig. 3b). Higher mantle Bouguer gravity
anomalies on average (Fig. 3a) also suggest thinner average crustal
thickness for corrugated surfaces than for the conjugate volcanic
seafloor. However, the mean gravity signature for the corrugatedvolcanic conjugate pairs is close to the regional average (Fig. 3a).
Seismic data suggest that this regional average is equivalent to a 3 to
4 km-thick magmatic layer (Muller et al., 1999; Minshull et al., 2006).
Corrugated surfaces of the eastern SWIR thus appear to have formed
at a melt supply about half the global melt supply average for midocean ridges (Chen, 1992). Gravity anomalies in the eastern SWIR also
suggest that melt supply to the ridge was less focused during the
formation of the older seafloor of our study area (Cannat et al., 2006),
resulting in a larger proportion of the seafloor being formed at the
average regional melt supply. This is consistent with the observation
that corrugated surfaces are more common in this older seafloor
(Fig. 2a).
Individually, corrugated surfaces show a wide range of topographic and gravity signatures (Fig. 2d). Corrugated surfaces have gravityderived crustal thickness ranging between 2.1 km and 4.4 km
(Fig. 2d) and many have a gravity-derived crustal thickness lower
than the reference value of 3.4 km used for the inversion (Cannat
et al., 2006). Mean residual topography, by contrast, is positive in all
surfaces but two, with values up to 1200 m, and shows a weak overall
positive correlation with gravity-derived crustal thickness (Fig. 2d). In
four pairs of corrugated surfaces which formed on the same flow line
within 2 to 5 myrs (numbers 6 and 7, 9 and 10, 13 and 12, and 19 and
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M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
surfaces do culminate near the termination, with outward-facing
slopes up to 12° (Fig. 6). This is very similar to the outward-facing
topography of many corrugated surfaces in the Atlantic (Tucholke et
al., 1997; Escartin et al., 2003; Smith et al., 2008); Fig. 7). This suggests
that uplift was, for a good part at least, achieved at the termination
stage, and that the corrugated surface exposed at the seafloor has
been flexurally tilted away from the axis. Heave values vary
substantially along each termination scarp (as can be seen in the
examples shown in Fig. 6), which probably explains the large scatter
in Fig. 5 (in which only maximum heave is shown for each scarp).
4. Transition of eastern SWIR corrugated surfaces to adjacent
volcanic and smooth seafloor
Fig. 3. Gravity and topographic signature for volcanic–volcanic, corrugated–volcanic,
smooth–volcanic, and smooth–smooth pairs of conjugate eastern Southwest Indian
Ridge seafloor (seafloor inferred to have formed simultaneously on each side of the axial
valley ; (Cannat et al., 2006). Grey line links mean values for conjugate pairs, and vertical
dashed lines link corresponding mean values for each type of seafloor. Plots are
restricted to crust accreted in the area of Fig. 1, prior to magnetic anomaly 3A (5.89 myr ;
(Cande and Kent, 1995). (a) Mean residual mantle Bouguer anomaly (RMBA). The
average RMBA value over the study area is 20 mGal. The crustal thickness used as a
reference for gravity modelling based on available seismic data (Muller et al., 1999) is
3.4 km (Cannat et al., 2006). (b) Mean residual topography (see caption for Fig. 2).
Standard deviation values are large in both diagrams :~8 mGal in (a), and ~500 m in (b).
21; Fig. 1), we also observe that the most recent surface has a thicker
gravity-derived crust and a higher residual topography (Fig. 2d).
Most corrugated surfaces described in the Atlantic and other
oceans are characterized by one or several isochron-parallel ridges
which define the breakaway zone, where the fault initiated, and by a
‘scarp and valley’ structure, also isochron-parallel in most cases, at the
termination of the fault (Tucholke et al., 1998). Terminations of some
Atlantic corrugated surfaces, however, are not associated with a scarp,
but the detachment simply plunges into the seafloor. This is the case
for the northern part of the Atlantis Fracture Zone core complex (Cann
et al., 1997; Blackman et al., 1998). By contrast, the southern part of
this core complex is elevated more than 3000 m above the adjacent
rift valley floor by a large termination fault scarp (Cann et al., 1997;
Blackman et al., 1998 ).
Corrugated surfaces that presently form at 13°N at the MAR have
sharp breakaway ridges, interpreted as rotated extrusives (Smith et al.,
2006). Ridges similar to these breakaway structures, but that do not
necessarily occur at breakaway, have been interpreted as tilted blocks
of hanging wall material (mostly volcanics) rafted on the footwall. This
is demonstrably the case for the volcanic ridge identified at the SWIR
Fuji Dome (Searle et al., 2003), and, at the MAR, along part of the
Atlantis Fracture Zone detachment (Cann et al., 1997).
Many corrugated surfaces in the eastern SWIR do not have a
breakaway ridge (in Fig. 4 it is for example present only in corrugated
surface number 19), and thirteen surfaces (Table 1) do not show a
termination scarp (for example surfaces number 6 and 19 in Fig. 4).
Most of these thirteen surfaces are small (<50 km2; Table 1), four are
very recent and located on, or near the walls of the axial valley (numbers
15, 16, 17 and 18), and four (numbers 25 to 28) are interpreted as
corrugated portions of a single fault block (Cannat et al., 2003; Searle
and Bralee, 2007). Their residual topography is variable (Fig. 5).
Among those corrugated surfaces that do have a termination scarp
(26 out of 39; Table 1), we observe a correlation between the
maximum heave measured along this scarp, and the surface's mean
residual topography (Fig. 5). In map view, it is apparent that these
Corrugated surfaces typically form at the transition between
volcanic domains and smooth seafloor domains. Fig. 1 shows that only
seven corrugated surfaces (number 22, 24-28 and 39) occur within
wide expanses of volcanic seafloor, and none within a wide expanse of
smooth seafloor. This is consistent with the intermediate gravity
signature of corrugated-volcanic conjugate pairs (Fig. 3a), which
suggests that they formed at intermediate melt supply, in the low
regional melt supply context of the eastern SWIR (Cannat et al., 2006).
Seismic data (Muller et al., 1999; Minshull et al., 2006) suggest that
this intermediate melt supply corresponds to about half the global
melt supply average for mid-ocean ridges (Chen, 1992).
Along flow lines, almost two thirds (24 out of 39) of the corrugated
surfaces transition to and from volcanic seafloor, while transitions to
and from smooth seafloor are observed in only 3 cases (Table 1).
Along isochrons, the (lateral) transition occurs in equivalent proportions to volcanic or to smooth seafloor (Table 1). Spreadingperpendicular scarps and volcanic ridges in volcanic seafloor domains
about the edges of corrugated surfaces, curving in some cases but not
in a systematic manner (see corrugated surfaces number 10 and 19 in
Fig. 4). Similar relations are observed in the Atlantic (Smith et al.,
2006). By contrast, the lateral transition to smooth seafloor is
commonly diffuse: large corrugations (with a wavelength of 500 m
or more) disappear, but there are hints of smaller wavelength
undulations, which are not resolvable in the shipboard bathymetric
data.
Another common characteristic feature is that corrugated surfaces
with no termination scarp, or portions of corrugated surfaces located
away from the termination scarps, lie at a greater depth than adjacent
contemporaneous seafloor, be it volcanic, or smooth. This can be seen
in Fig. 4c, where corrugated surface number 19 lies 300 to 600 m
below the volcanic seafloor to the east and west, and in Fig. 4d, where
the eastern part of corrugated surface number 23 lies 700 m below
contemporaneous smooth seafloor to the east.
5. Discussion
5.1. Fault topography and the rigidity of the axial plate
Dome-shaped corrugated surfaces that form presently at 13°N in
the Mid Atlantic Ridge show evidence for a large rotation of the
detachment fault footwall (at least 35°) over remarkably short
distances (less than 5 km; Smith et al., 2006). This requires a very
low flexural rigidity (equivalent to an elastic thickness <1 km; (Smith
et al., 2006, 2008)). Very low rigidity is also indicated by the subdued
topography of some off-axis corrugated surfaces in our SWIR study
area, relative to surrounding volcanic or smooth seafloor (Fig. 4c
and d). We interpret this adjacent uncorrugated (volcanic or smooth)
seafloor as fault blocks or exhumed fault surfaces, that have
undergone a lesser roll-over and record higher fault uplift.
Termination scarps of the SWIR corrugated surfaces accommodate
substantial uplift, have an overall dip of 15° to 22°, and cut the
adjacent corrugated surfaces at angles of 15° or more (Fig. 6). These
M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
179
Fig. 4. Detail maps of residual topography for selected eastern Southwest Indian Ridge corrugated surfaces, numbered as in Fig. 1. Arrows point toward ridge axis. Detail maps of
corrugated surfaces number 14, 32, 34 and 35 may be found in (Cannat et al., 2006). Outlines of corrugated surfaces (dashed lines) are conservative : because ship tracks also trend
north south, faint corrugations could be artefacts in the multibeam data and have not been considered.
termination scarps show evidence for mass-wasting in the form of
cuspate landslides head scarps (visible in the bathymetric maps of
Fig. 4). The dip of the termination fault as it emerged from the axial
valley seafloor was therefore larger than the present dip of these
scarps. Corrugated surfaces of the MAR commonly show a similar
relationship between the flat, or outward-facing slopes of old
corrugated surfaces, and the steeper, inward-facing termination
scarps with evidence for mass wasting (Figs. 7, and 1 in (MacLeod
et al., 2002)). This indicates that axial normal faults at the termination
stage proceeded with less footwall flexure, creating a larger
topography than during the formation of the corrugated surface.
From this, we deduce that the rigidity of the plate increased at the
termination stage.
Spatial and temporal changes in axial plate rigidity at slow
spreading ridges may be due to changes in the overall thermal
structure, related to along-axis or temporal variations in melt supply
(Chen and Morgan, 1990; Shaw and Lin, 1996). However, a thin
thermal plate and hot thermal regime is inconsistent with observa-
tions made at the actively forming corrugated surfaces of the MAR.
These MAR surfaces are associated with high levels of seismicity
(Smith et al., 2006, 2008), indicating that the axial brittle plate has a
substantial thickness. In addition, earthquakes as deep as 8 km below
the seafloor have been detected at the Trans Atlantic Geotraverse
(TAG) detachment, which does not expose clear corrugations, but
appears to undergo a large roll-over (~50° ; (deMartin et al., 2007)).
The alternative to a thin thermal plate, as proposed by (Buck,
1988), is that stresses related to bending of the detachment footwall
cause internal deformation, forming a weaker domain in an
otherwise thick mechanical lithosphere. Very large roll-over and
subdued topography would then be consistent with maximum
weakening of the footwall at detachments that form corrugated
surfaces (Fig. 8a). Incidentaly, this very weak footwall domain could
also be able to cast the irregularities of the hanging wall in the form
of corrugations, as proposed by (Spencer, 1999). We now use
available observations, from this and previous studies, to discuss
what mechanisms could enhance the mechanical weakening of the
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M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
Fig. 5. Maximum heave of termination scarps as a function of average residual
topography (Table 1) for eastern Southwest Indian Ridge corrugated surfaces formed
prior to magnetic anomaly 3A (Fig. 1). Same symbols as in Fig. 3. Dashed line is best
linear fit for corrugated surfaces of area >200 km2 (slope is 1.47, with a correlation
coefficient R2 of 0.52). Dashed arrows as in Fig. 2d.
footwall of axial detachments during the formation of corrugated
surfaces.
5.2. Enhanced mechanical weakening of the detachment footwall
Our study of the eastern SWIR indicates that corrugated surfaces
there do not form when the melt supply is so low that there is no, or
very little axial volcanic activity (Cannat et al., 2006). In such very low
melt supply conditions, the new seafloor is not corrugated, but
smooth, and structured in axis-parallel fault blocks (Fig. 4d) with
substantial relief (>500 m) suggesting that the detachment's footwall
had a somewhat larger rigidity than during the formation of
corrugated surfaces. Although sampling of these smooth seafloor
domains is still scarce, the proportion of gabbros in each available
dredge is less than 10%, with over 90% serpentinized ultramafics
(Seyler et al., 2003). This observation, from the low melt supply end of
the mid-ocean ridge spectrum, points to a link between the
weakening mechanism we are trying to identify for detachments
that do form corrugations, and a significant volume of gabbros in the
footwall of the detachment fault. Such a link is also indicated by
sampling evidence for kilometer-sized gabbro bodies beneath MAR
corrugated surfaces, next to outcrops of serpentinized peridotites
(Ildefonse et al., 2007; Dick et al., 2008). Atlantis Bank at the SWIR,
where 2 km of gabbro have been drilled (ODP site 735) next to
exhumed mantle-derived ultramafics, may also have once had a
corrugated upper surface, now eroded (Dick et al., 2000).
Another observation is that, in 29 out of 39 eastern SWIR
corrugated surfaces, the termination coincides with a transition to a
volcanic mode of spreading (Table 1). In this mode, seafloor with
volcanic edifices is accreted to the two diverging plates, and gravity
suggests systematically thicker crust than in the other two main
modes of spreading identified in this ridge region (corrugatedvolcanic and smooth-smooth ; (Cannat et al., 2006)). This is consistent
with the observation made by (Tucholke et al., 1998, 2008) that
gravity-derived crustal thickness appears to increase at the termination of many MAR corrugated surfaces. (Canales et al., 2008) recently
confirmed this with seismic data for a subset of corrugated surfaces
near the Kane Fracture Zone. Moreover, the study of over 40
corrugated surfaces in the 13°N region of the Atlantic indicates that
they also generally have a thinner gravity-derived crust than the
surrounding volcanic seafloor (Smith et al., 2008). Thus, despite the
fact that gravity-derived crustal thickness models are non-unique and
do not in most cases have the horizontal resolution to study small
Fig. 6. Flowline-parallel bathymetric sections for selected Southwest Indian Ridge
corrugated surfaces. Sections are located in Fig. 4. Arrows point toward ridge axis. Bold
lines above topography : corrugated surfaces. Bold topography : termination scarps.
Scale is the same in all sections, with a vertical exaggeration of 4.5. Values in meter unit
refer to height of the principal scarps, values in degree unit refer to slopes.
scale variations at and around corrugated surfaces, observations
converge to indicate that most corrugated surfaces do terminate in a
thicker crust and presumably higher melt supply regime. The
weakening mechanism we are looking for is therefore probably not
that melt-rich zones, or hot and ductile gabbros, deform easily (as
demonstrated for example by the ductile shear zones drilled in
gabbros at the ODP Site 735B (Cannat et al., 1991; Dick et al., 2000))
because it would then be difficult to explain why this deformation
should be less extensive at termination, when it occurs in conditions
of higher melt supply.
The third observation that we view as relevant concerns the
prevalence of talc, chlorite, serpentine and amphibole-bearing
ultramafic schists in sheared horizons which have been sampled so
far at, and below, corrugated surfaces (Escartin et al., 2003; Schroeder
and John, 2004; Boschi et al., 2006; Dick et al., 2008; Boschi et al.,
2008). Spinel relicts in these schists shows that their protolith is at
least partially mantle-derived, while talc, amphibole and lesser
chlorite reveal the addition of silica, calcium, and aluminium. This
addition is thought to be controlled by hydrothermal fluids derived
through seawater–gabbro interactions (Escartin et al., 2003; Schroeder and John, 2004; Boschi et al., 2006; McCaig et al., 2007; Boschi
et al., 2008). Talc, chlorite, serpentine and tremolite–actinolite schists
M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
181
Fig. 7. Bathymetric map of the Mid-Atlantic Ridge in the 13°N area (from (Smith et al., 2006), with flowline-parallel sections and location of Achadze hydrothermal vents. Sections
are drawn at scale similar to that in Fig. 5. Values in meter unit refer to height of the principal scarps, values in degree unit refer to slopes. Sections 1 and 2 cut through the actively
forming corrugated surfaces described by (Smith et al., 2006). Fault uplift varies along-strike, from virtually no topography in Section 1, to about 700 m in Section 2. Section 3 cuts
through an older corrugated surface, now uplifted by a total of about 1800 m along the termination scarp. NVR : Neovolcanic Ridge.
have also been described next to gabbro veins, in mid-ocean ridge
ultramafic outcrops located away from major faults (Cannat and
Casey, 1995; Karson et al., 2006; Boschi et al., 2006, 2008; Dick et al.,
2008). This spatial association indicates that the growth of these
alteration products is directly related to the distribution of gabbroic
intrusions in exhumed ultramafics.
Fig. 8. Conceptual model showing the two contrasted axial configurations inferred from our study of corrugated surfaces. Across-axis sections. (a) A corrugated surface forms. A very
weak domain (grey) develops in the detachment footwall. The roll-over is large, and the fault uplift is small. The geological interpretation we propose for this configuration involves
gabbros (darker grey) as 1 to 2 km-sized bodies, and as dikes and veins (dashed lines) intrusive into ultramafic rocks. (b) Termination. Rock formations in the upper part of the
footwall plate have become stronger, the roll-over is less, and the fault uplift is greater. The geology proposed for the footwall at this stage is not sketched. It could comprise either
more, or less gabbros than in stage a (discussed in text). Seafloor topography is drawn to scale in (a) from an active corrugated surface at the axis of the Mid Atlantic Ridge (Section 1
in Fig. 7), and in (b) from Southwest Indian Ridge (SWIR) corrugated surface #7 (Section a-1 in Fig. 5). Base of brittle lithosphere (inferred to correspond to 750 °C isotherm ; (Hirth
et al., 1998)) on axis is drawn at about 8 km depth, from seismicity studies at the SWIR (Yamada et al., 2002) and Mid-Atlantic Ridge (Toomey et al., 1988; Wolfe et al., 1995;
deMartin et al., 2007). The base of the brittle lithosphere off-axis, and of the ductile lithosphere, are inferred. We have used recent seismic constraints of (Canales et al., 2007), and
(deMartin et al., 2007) on the TAG (Trans-Atlantic Geotraverse) region to sketch the detachment fault in (a). Our inferred very weak footwall domain is sketched to coincide with
their upper domain of low seismicity.
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M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183
Talc is a very weak material, with a strength and coefficient of
friction which are lower than those of lizardite (Morrow et al., 2000;
Escartin et al., 2008a). Chlorite displays mechanical properties that are
intermediate between those of talc and serpentinite (Murrell and
Ismail, 1976; Moore and Lockner, 2004). The mechanical properties of
tremolite have not been properly studied experimentally, but the
intense deformation of tremolite schists, as well as deformation
microstructures similar to those of talc and serpentinite schists, also
suggests a weak nature that promotes strain localization. Furthermore, fault rocks are commonly composed of a mixture and
intergrowth of several of these minerals (e.g., (Escartin et al., 2003;
Schroeder and John, 2004; Hirose and Hayman, 2008). Consequently,
the overall rheology of such fault or alteration zones are likely to be
controlled by the weakest mineral, even if present in small quantities.
Our preferred mechanism for footwall weakening during the
formation of corrugated surfaces is therefore the following: we
propose that weakening of the detachment footwall is primarily
achieved with the presence of sheeted minerals (mainly phyllosilicates) that result from alteration and fluid circulation in mantlederived ultramafics next to gabbro intrusions (Fig. 8a). After they have
cooled into the brittle regime, these gabbro intrusions are likely to be
rheologically stronger than serpentinized ultramafics, and the
contrast in rheology may help localize footwall deformation along
lithological boundaries (Ildefonse et al., 2007), where the growth of
weak hydrous minerals will be most favored. The domain in which
this would occur would be controlled by access to hydrothermal
fluids. In Fig. 8a, we have tentativelly drawn it as 2 to 3 km-thick, and
parallel to the main detachment. As sketched in Fig. 8a, this weak
footwall domain could be equivalent to the low seismicity domain
detected by (deMartin et al., 2007) in the footwall of the TAG
detachment fault. We hypothesize that successive gabbro intrusions
are progressively exhumed through this weak domain. Each intrusion
is inferred to be surrounded by veined ultramafics, in which
hydrothermal alteration will favor serpentinization, and the local
crystallization of talc, amphibole and chlorite, which will in turn
facilitate deformation.
5.3. Termination
Termination of a corrugated surface probably follows a change of
the mechanical behavior of the master detachment fault and may
occur in two configurations, which have recently been explored by
(Smith et al., 2008): 1) A new master fault initiates, which is most
likely to cut inward into the hanging wall plate (Buck, 1988 ; Tucholke
et al., 1998; Smith et al., 2008). This new master fault cuts into
material which would for the most part not have been deformed and
mechanically weaken by bending stresses yet and would thus initially
have a higher rigidity. And 2) the detachment continues on the same
master fault, but the roll over is less, and the main fault cuts at a
steeper angle through the upper crust, capturing a small wedge of the
hanging wall plate.
This second configuration is represented in Fig. 8b, in which we
assume that this captured wedge of dominantly volcanic rocks is
dismantled by mass wasting of the termination scarp. In this sketch
we propose that the alteration of the mechanical behavior of the
master detachment fault which leads to termination can occur: 1) if
gabbroic intrusions in the footwall are so frequent that they form a
coherent skeleton of strong brittle rocks ; and 2) if gabbroic intrusions
are so rare that shear zones coated by talc, amphibole, chlorite and
serpentine become too scarce and disconnected to effectively weaken
the footwall. More generally, we would infer that the strength of the
footwall at mid-ocean ridge detachments, the extent of roll over, and
the fault topography, would be influenced by the abundance of
ultramafic rocks in the hydrothermally altered domain of the footwall,
and by the size and distribution of gabbroic intrusions in these
ultramafic rocks. We would predict that corrugated surfaces would
have the optimum proportion of gabbros to insure that most faults
and fractures that accommodate footwall flexure may be coated by
weak assemblages of talc, chlorite, serpentine and amphibole.
6. Concluding remarks
In this paper, we document the location, size, approximate age,
gravity signature and residual topography of 39 corrugated surfaces in
the melt-poor eastern region of the SWIR. We find that these
corrugated surfaces formed at a melt supply about half the global
melt supply average for mid-ocean ridges (Chen, 1992). This is
consistent with the model of (Buck et al., 2005), which was recently
refined by (Tucholke et al., 2008). In this model, which does not
address asymmetric spreading during the formation of corrugated
surfaces (Searle and Bralee, 2007; Baines et al., 2008; Dick et al.,
2008), diking in the upper plate accommodates half of the plate
separation, the other half being accommodated by displacement along
the detachment fault. Our main focus in the discussion is to
understand why axial detachment faults at slow and ultraslow ridges
appear to accommodate very large footwall roll-over during the
formation of corrugated surfaces, and why the roll-over appears to be
less, in most cases, after these corrugated surfaces terminate. We
conclude that the formation of corrugated surfaces requires enhanced
weakening of the detachment footwall. The mechanism we propose as
a way to achieve such enhanced weakening is the coating of cracks
and faults zones associated with footwall flexure with weak hydrous
minerals (talc, amphibole, chlorite, and serpentine). This hypothesis,
although consistent with available observations on rock samples from
slow and ultraslow ridges, and with geophysical constraints on the
melt supply to the ridge during the formation of corrugated surfaces,
is not tested at this point, and should be viewed as a working
hypothesis for future investigations.
Acknowledgments
We warmly thank our three reviewers for their extremely helpfull
comments. Most data we used are from the «SWIR 61–63» cruise of RV
Marion Dufresne (2003), and processing was partly supported by a
research grant from CNRS-INSU. This is IPGP publication number 2545.
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