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© 2006 Nature Publishing Group
Widespread active detachment faulting and core
complex formation near 138N on the Mid-Atlantic
Ridge
Deborah K. Smith
1
, Johnson R. Cann
2
& Javier Escartı
´
n
3
Oceanic core complexes are massifs in which lower-crustal and
upper-mantle rocks are exposed at the sea floor
1–3
. They form at
mid-ocean ridges through slip on detachment faults rooted below
the spreading axis
2–6
. To date, most studies of core complexes have
been based on isolated inactive massifs that have spread away from
ridge axes. Here we present a survey of the Mid-Atlantic Ridge
near 138N containing a segment in which a number of linked
detachment faults extend for 75 km along one flank of the
spreading axis. The detachment faults are apparently all currently
active and at various stages of development. A field of extinct core
complexes extends away from the axis for at least 100 km. Our
observations reveal the topographic characteristics of actively
forming core complexes and their evolution from initiation within
the axial valley floor to maturity and eventual inactivity. Within
the surrounding region there is a strong correlation between
detachment fault morphology at the ridge axis and high rates of
hydroacoustically recorded earthquake seismicity. Preliminary
examination of seismicity and seafloor morphology farther
north along the Mid-Atlantic Ridge suggests that active detach-
ment faulting is occurring in many segments and that detachment
faulting is more important in the generation of ocean crust at this
slow-spreading ridge than previously suspected.
The existence of low-angle faults extending deep below the axes of
mid-ocean ridges
7,8
has long been inferred from seafloor exposures of
deep-seated rocks. Direct evidence for such faults has only recently
come from bathymetry
1,2
and seafloor sampling and drilling
4,5,9
. The
faults are corrugated parallel to the spreading direction and cap
smooth topographic highs, termed oceanic core complexes, where
deep-seated rocks such as gabbros and serpentinized peridotites are
exposed
4,5,10,11
. Most core complexes have been identified towards the
ends of spreading segments where magma supply appears to be low,
but in places they extend for tens of kilometres parallel to the
axis
4,5,12–14
. In parts of some segments, extension by low-angle
faulting may have accounted for .50% of the total extension by
spreading
3,11,15
.
Because almost all core complexes identified to date are far enough
from the axis to be inactive, the nature of active detachment faults is
controversial. How do they initiate? How do they evolve as they
emerge from the ocean floor? How is active detachment faulting
accommodated along the length of the spreading axis? Is there a
seismic signature to detachment faulting? We answer these questions
using a new survey of the Mid-Atlantic Ridge (MAR) (D.K.S. & J.E.,
unpublished cruise report, RV Knorr Leg 182, June 2005) near 138N
together with existing multibeam bathymetry and hydroacoustically
recorded seismicity, and extend our conclusions to other mid-ocean-
ridge segments in the region.
A bathymetric map of the MAR between the Fifteen-Twenty and
the Marathon fracture zones (Fig. 1) shows an alternation in the
morphology of the spreading axis. Of the two major segments in this
area, that between 148350Nand138500N (labelled the 148N
segment) has a volcanic signature, with closely spaced volcanic ridges
on both flanks, tens of kilometres long, and cut by steep inward-
facing faults. The segment to the south between 138500N and
128400N (the 138N segment) has more chaotic topography. The
west flank, in particular, is distinctive, with widely spaced, narrow
LETTERS
Figure 1 |Location map and bathymetry near 138N on the MAR. a, Satellite
altimetry data
25
. The red stars indicate seven sections of the axis showing
persistent, high levels of hydroacoustically recorded seismic activity
22
. The
inset at the top shows the MAR axis. b, Multibeam bathymetry from refs 24
and 12 and D.K.S. & J.E., unpublished cruise report (RV Knorr Leg 182, June
2005). The ‘148N segment’ has faulted volcanic morphology, while the
segments to the north and south have irregular and blocky topography and
core complexes. The peridotite-hosted Logatchev hydrothermal vent field
26
is marked. Red dots show locations of 292 hydroacoustically detected events
for the period 1999–2005 with 1jerror bars
22
less than ^10 km.
1
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
2
School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK.
3
CNRS, Marine
Geosciences Group, IPGP Case 89, Paris 75252 cedex 05, France.
Vol 442|27 July 2006|doi:10.1038/nature04950
440
© 2006 Nature Publishing Group
ridges, typically ,20 km long, that have steep slopes dipping away
from the spreading axis, and are separated by areas of smoother sea
floor. As shown below, a large part of this smoother sea floor is
corrugated parallel to the spreading direction.
In well-explored oceanic core complexes, the existence of a
corrugated surface is correlated with detachment faulting and
exposures of deep-seated rocks
1–4,6
. Consequently, corrugations
have been used to identify other core complexes in regions with
limited data
2
. Corrugations have a wavelength of hundreds of metres
and an amplitude of tens of metres
1,3
, and can be seen on shaded relief
maps of multibeam bathymetry data. The origin of the corrugations
is still enigmatic. Direct observations have shown that they are not
produced by faults parallel to the spreading direction
15,16
.One
possibility is that they originate by continuous casting
17
of a ductile
footwall by irregularities in a strong and brittle hanging wall.
An inactive, corrugated core complex near 308N (ref. 1) at the
MAR (Fig. 2) shows other characteristics common to core complexes.
First, a steep (25–308) slope faces away from the axis at the older edge
(scarps at slow- and intermediate-spreading ridges typically face
towards the axis
18
). Second, a narrow, linear ridge parallel to the axis
caps the outward-facing slope
1,2
and extends beyond the corrugated
surface. Dredge samples from the linear ridge in Fig. 2 show that it is
composed of basalt
1
and thus is probably volcanic sea floor created at
the axis. Finally, a steep normal fault dips towards the spreading axis,
truncating the corrugated surface on its younger side.
In the 138N segment, corrugated surfaces and outward facing
steep slopes dominate the western flank of the spreading centre,
extending for 75 km along axis and .100 km across axis (Fig. 3a).
The feature centred at 138100N, 45800 0W (labelled ‘1’ on Fig. 3a) is
typical of many of those observed off-axis, and has a similar
morphology to that shown in Fig. 2. The morphology and spatial
scale of this and other corrugated surfaces and ridges in the 138N
segment indicate that the evolution of the western flank involves
repeated detachment faulting and core complex formation.
Two core complexes north of complex ‘1’ at 13820 0N, 44855 0W
(labelled ‘2’ on Fig. 3a, b) and 13830 0N, 44855 0W (labelled ‘3’) have
a different morphology from the core complexes that lie off-axis.
Each extends ,10 km along-axis and is close to the spreading axis.
The corrugated surfaces are domes that meet ridges to the west, and
curve over to the east, dipping at 158to meet the axial valley floor.
Instead of being truncated by later faults, as with the off-axis core
complexes, the slope at the east end of each of the domes intersects
the median valley floor along a curved line ,5 km from the volcanic
axis (Fig. 3). These features suggest that the domes are the surfaces of
detachment faults actively emerging from the valley floor.
We also identify a larger and apparently compound core complex
extending for 30–40 km parallel to the axis (labelled ‘C’ on Fig. 3a,
stretching from 128550Nto138100N). This compound core com-
plex includes complex ‘1’ and another similar complex to the south
(labelled ‘4’ in Fig. 3a) linked by a third complex located ,5kmto
the east. This third complex terminates in a boundary with the
volcanic morphology of the valley floor that is convex towards the
east. A corrugated surface is present over a large part of the linking
complex.
Because many of the core complexes in this segment have straight
narrow ridges at their outer sides, we consider linear ridges as
possible precursors to the emergence of the detachment faults.
Such ridges occur in several places close to the spreading axis
(labelled ‘R’ on Fig. 3a, b), each backed by a deep basin (Fig. 3).
The outer slopes of the ridges dip at 15–208away from the axis and
show a hummocky volcanic morphology similar to those of the
ridges farther from the spreading axis. Presumably they form by
rotation of sections of volcanic sea floor away from the axis. We infer
that the ridges mark the breakaway zones of detachment faults.
The strong morphologic evidence for active detachment faulting led
us to search for associated seismic activity. Traditionally, seismically
active MAR segments have been identified by locating foci of earth-
quakes of magnitude .4.5 detected by land seismometers, but their
locations are poorly constrained, and these earthquakes are few in
number. Between 1999 and 2005 an autonomous hydrophone array
in the North Atlantic
19
recorded the hydroacoustic energy from
thousands of earthquakesof smaller magnitude (.2.5). Hydroacoustic
events are better located than those detected teleseismically
19–21
,and
clearly show the currently active sections of MAR segments
22
.Two
sections of ridge between the Fifteen-Twenty and Marathon fracture
zones (Fig. 1b) show relatively high and persistent seismicity
23
, and
are separated by the seismically quiet 148N segment. Core complexes
have been identified in the active segment just south of the Fifteen-
Twenty fracture zone
24
. The other seismically active section, the 138N
segment, is active along its entire length, with no obvious spatial or
temporal clustering of events. Location errors (Fig. 1b) are large
enough, though, that events cannot be associated with individual
detachments, nor with possible seismic sources associated with core
complex formation.
The seismic evidence supports the morphologic evidence that a
75-km-long chain of detachment faults on the western side of the
axial valley is active. Some of the detachment faults are mature
(complexes ‘1’ and ‘4’ on Fig. 3a), some are in the early stages of
development (‘2’ and ‘3’), and some are in an incipient stage of
evolution, as shown by the narrow ridges close to the spreading axis
(‘R’). As they evolve and spread away from the axis, the detachments
become inactive and new detachments initiate within a few kilometres
of the axis.
There are a number of core complexes at different stages of
evolution, so we can reconstruct the evolution of a core complex
from initiation to maturity to inactivity (Fig. 4). The first stage is the
subsidence of a basin within the axial valley floor, coupled with the
emergence of a narrow basaltic ridge (Fig. 4a). The basin lies
Figure 2 |Bathymetry map of a well-explored, extinct core complex south
of the Atlantis fracture zone on the MAR
3
.Map contour interval is 100 m.
The complex has spread ,30 km east of the axis. Its morphological
characteristics include an outward-facing slope with a volcanically
constructed ridge capping the core complex on the older (outer) side.
A normal (inward-facing) fault scarp cuts the feature on its younger (inner)
side. Corrugations running parallel to the spreading direction cap the
shallow-dipping top. Serpentinized peridotite was recovered by dredging the
corrugated surface
7
. Mass wasting of the massif is indicated by the scoop on
its northwest corner.
NATURE|Vol 442|27 July 2006 LETTERS
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© 2006 Nature Publishing Group
immediately behind the ridge, and probably forms by outward
footwall rotation as a new fault initiates. The inner (eastern) side
of the basin (and hence the outer side of the ridge) has the volcanic
morphology of the axial valley floor, tilted at 158–258. As the ridge
evolves, the tilt of the outer slope increases to 208–308. A fault scarp
dipping at 158–258emerges on the inner side of the ridge.
The next stage is the emergence of a domal corrugated fault surface
that intersects the valley floor along a line convex towards the
spreadingaxis(Fig.4b).Thecorrugatedsurfacedipsat,158
where it plunges into the valley floor. The dip of the surface gradually
flattens as it emerges due to flexure of the footwall, and by ,5km
from its emergence the crest is nearly horizontal. Below the median
valley floor, the detachment fault may continue at a low angle, but the
early rotation of the footwall to form the basin suggests that the fault
curves and steepens below the sea floor, so that the tip of the fault may
lie several kilometres down.
The resulting mature core complex (Fig. 4c) becomes extinct when
it is cut off by a normal fault. At that stage the domal surface flattens
until it is close to horizontal. In some places, the core complex may
extend as a single elongate unit from tens of kilometres to over a
hundred kilometres from the spreading axis, as observed in the
Australian–Antarctic Discordance
10,11
and the Parece Vela basin
13
.In
our study area, the small spacing between breakaway faults indicates
a short life for any individual complex and regular nucleation of new
detachment faults in the median valley.
Because of the apparent correlation of persistent earthquake
seismicity and core complex morphology, we examined a larger
region of the MAR (248–158N) to look for additional sites of active
core complex formation. We identified five ridge sections with high
levels of hydroacoustically recorded seismic activity
22
(Fig. 1a). The
seismicity is persistent and not triggered by large earthquakes.
Preliminary examination of available bathymetry from these and
other segments in this larger area indicates a correlation between core
complex morphology and seismicity. We suggest that detachment
faulting may be more common than previously suspected in this part
of the MAR. As much as 35% of the spreading axis may be
experiencing detachment faulting and thus, .15% of the new
seafloor accretion may be dominated by core complexes. We also
suggest that the evolution of core complexes we have identified in our
study area and the associated seismicity may be applicable to under-
standing other regions of active detachment faulting both in the
Figure 3 |Multibeam bathymetry showing detachment faults and core
complexes in the 138N segment. a, Map contour interval is 50 m. Linked
and active core complexes extend ,75km along the axis. The dashed line
indicates the spreading axis. R, topographic ridges, inferred to be
breakaways for new detachment surfaces. Numbers, complexes discussed in
the text. C, compound core complex composed of complexes ‘1’ and ‘4’ and a
third linking complex. b, Three-dimensional perspective view of complex ‘2’
with no vertical exaggeration. The corrugated surface and its intersection
with the sea floor are marked.
Figure 4 |Schematic three-dimensional oblique view of the evolution of a
core complex by detachment faulting. Thick grey lines, sea floor (except for
the detachment surface). Thin black lines, surface of the detachment fault
and steeper normal faults, both exposed on the sea floor and below it. Thick
dashed line, spreading axis. a, A breakaway ridge with a basin behind it. The
basin is floored by volcanic sea floor, tilted up towards the breakaway ridge.
The initially steep subsurface normal fault at the breakaway has already
rotated to a shallower angle. A downward-curved detachment has started to
form linked to the breakaway fault. Both faults are below the sea floor. b, The
detachment has emerged. The thin dashed line marks the line of emergence.
The fault has warped into a dome, the ridge has become arched, and little
further rotation of the initial ridge has occurred. c, The detachment fault has
been cut off by a later normal fault, perhaps of the breakaway of the next
detachment. The dome has flattened, and the detachment fault is inactive.
LETTERS NATURE|Vol 442|27 July 2006
442
© 2006 Nature Publishing Group
oceans and on land, where the faults are more accessible but where
erosion severely hinders their interpretation.
Received 9 April; accepted 2 June 2006.
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Acknowledgements We are grateful to the captain and crew of the RV Knorr
(Leg 182, 2005). We are also grateful to P. Lemmond, H. Schouten and M. Tivey
for their help in collecting the data at 138N. We had fruitful discussions with
R. Searle, H. Schouten, M. Tivey and R. Sohn. We also thank J. Goff for
constructive comments on the manuscript. This work was supported by the
National Science Foundation. This is an IPGP contribution.
Author Information Reprints and permissions information is available at
npg.nature.com/reprintsandpermissions. The authors declare no competing
financial interests. Correspondence and requests for materials should be
addressed to D.K.S. (dsmith@whoi.edu).
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