Download Evidence for magma entrapment below oceanic crust

Evidence for magma entrapment below oceanic crust from deep
seismic reflections in the Western Somali Basin
Daniel Sauter1, Patrick Unternehr2, Gianreto Manatschal1, Julie Tugend1, Mathilde Cannat3, Patrick Le Quellec2,
Nick Kusznir4, Marc Munschy1, Sylvie Leroy5, Jeanne Mercier de Lepinay1, James W. Granath6, and Brian W. Horn7
Institut de Physique du Globe de Strasbourg, CNRS UMR 7516, Université de Strasbourg, 1 rue Blessig, 67084 Strasbourg cedex,
France
2
TOTAL–Exploration and Production, 2 place Jean Millier, La Défense 6, F-92078 Paris la Défense cedex, France
3
Institut de Physique du Globe de Paris, CNRS UMR 7154, Université Paris Diderot, 1 rue Jussieu, 75238 Paris cedex 05, France
4
Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 3BX, UK
5
UPMC Institut des Sciences de la Terre de Paris, CNRS UMR 7193, 4 place Jussieu, 75252 Paris cedex 05, France
6
Granath & Associates Consulting Geology, 2306 Glenhaven Drive, Highlands Ranch, Colorado 80126, USA
7
ION Geophysical, 2105 City West Boulevard, Houston, Texas 77042, USA
1
INTRODUCTION
Mantle upflow, decompression, and melting
beneath mid-ocean ridges result in the formation
of new oceanic crust. The extraction of magma
and formation of new oceanic lithosphere occurs
over a zone a few tens of kilometers wide at
the ridge axis. However, the processes controlling magma focusing and extraction, which
lead to the generation of the magmatic crust,
are still poorly constrained and debated (e.g.,
Kelemen et al., 2000). The main reason for this
uncertainty is that the mantle structure beneath
mid-ocean ridges is inaccessible to direct observations and our knowledge of melt generation,
and its delivery to the crust or retention in the
mantle, is mostly derived from field investigations of ophiolites and thus constrained by indirect observations.
Here we present a >200-km-long seismic
reflection section (ION BasinSPAN line; www​
.iongeo​.com​/Data​_Library/) located within the
Western Somali Basin offshore eastern Africa
(Fig. 1). This section images the sub-oceanic
lithosphere with high resolution beneath an
inferred extinct seafloor spreading center. The
aim of this paper is to describe the geometry of
strong reflections that can be imaged to depths of
>30 km below the top basement (>16 s two-way
traveltime, TWTT) and to discuss their nature
as well as the insights that they provide on the
magmatic plumbing system associated with the
generation of oceanic crust.
Ophiolites expose large sections that enable
the study of mantle-crust relationships. However,
even the best-exposed mantle section in Oman
does not extend deeper than ~10 km below the
Moho (Boudier and Coleman, 1981). There, the
crust-mantle transition zone (Moho transition
zone, MTZ) is characterized by dunitic-troctolitic layers as much as 1200 m thick which have
been interpreted as frozen melt channels along
the paleo–lithosphere-asthenosphere boundary
(Rabinowicz and Ceuleneer, 2005). The analysis
of troctolites in Integrated Ocean Drilling Program (IODP) Hole U1309D at the Mid-Atlantic
Ridge suggests that they were formed by continuous input of melt into a mushy reacting
matrix, similar to melt impregnation of mantle
38°E
40°E
42°E
44°E
46°E
48°E
Western
Somali
Basin
Kenya
4°S
8°S
Tanzania
6°S
10°S
bas
Kerim n
12°S grabe
M0y
M5o
M10y
Davie Ridge
ABSTRACT
Our understanding of melt generation, migration, and extraction in the Earth’s mantle
beneath mid-oceanic ridges is mostly derived from geodynamic numerical models constrained
by geological and geophysical observations at sea and field investigations of ophiolites, and
is therefore restricted to the oceanic crust and the shallow part of the mantle. Here we use a
>200-km-long, deep seismic reflection section to image with high resolution the sub-oceanic
lithosphere within the Western Somali Basin (offshore eastern Africa) where spreading ceased
at ca. 120 Ma. The location of the failed spreading axis is inferred from both seismic data and
gravity data. Several groups of strong reflections are imaged to depths of >30 km below the
top of the oceanic crust. We interpret the deepest reflectors, within the mantle, as resulting
from frozen melt bodies which may be relicts of a paleo–melt channel system located at the
base of the lithosphere and formerly feeding the failed ridge axis. Other reflectors within
the mantle may correspond to melt bodies injected into major shear zones along the Davie
fracture zone. Another group of reflectors, located below a 8–5-km-thick oceanic crust, is
interpreted as marking a fossil melt-rich crust-mantle transition zone as much as 3 km thick.
This interpretation implies an inefficient extraction of melt out of the mantle, which is favored
by the combination of a slow spreading rate and a high magma budget.
Comoros
45 40 35 30 25 20 15 10
5 0 km
Figure 1. Crustal thickness map of the Western Somali Basin based on gravity inversion.
Blue colors indicate typical oceanic crustal
thicknesses. Crustal thickness of western
Somali Basin increases in ocean-continent
transition zones and toward oceanic islands.
Dashed black lines show gravity lineations
indicating spreading directions. White lines
indicate inferred location of extinct spreading
axis. Seismic section (ION BasinSPAN line;
www​.iongeo​.com​/Data​_Library/) is shown
by red line. Colored squares indicate marine
magnetic anomalies M0y–M10y (see the Data
Repository [see footnote 1]).
peridotites observed at the MTZ in ophiolites
(Drouin et al., 2009).
Seismic imaging has previously been used
to give insights into the deep structure of present-day spreading systems. At the Juan de Fuca
Ridge (offshore western North America), seismic reflection data show molten sills within the
lower crust (Canales et al., 2009) and frozen
melt lenses in a 2-km-thick MTZ (Nedimović
et al., 2005). Although very rare, in domains of
GEOLOGY, June 2016; v. 44; no. 6; p. 1–4 | Data Repository item 2016133 | doi:10.1130/G37747.1 | Published online XX Month 2016
©
2016 Geological
Society
America.
permission to copy, contact [email protected].
GEOLOGY 44 | ofNumber
6 For
| Volume
| www.gsapubs.org
1
GEOLOGICAL SETTING
Seafloor spreading began in the Western
Somali Basin in Jurassic time, separating Madagascar and India from Africa (Seton et al., 2012)
(Fig. 1). Spreading was slow (<~20 km/m.y. halfspreading rate) and ceased shortly after the time
of the M0 marine magnetic anomaly (Aptian)
(Seton et al., 2012). However, the location of
the isochrons and of the extinct spreading axis is
debated (see the GSA Data Repository1). Gravity lineations suggest a north-south direction of
spreading before the extinction and a NNE–SSW
direction during an earlier stage of spreading
(see also Fig. DR1 in the Data Repository).
The seismic section strikes NNW-SSE across
a small basin bounded to the west by the Tanzanian continental margin and to the southeast by
the Davie Ridge (Fig. 1). It crosses the northern
end of the Davie Ridge and of the Kerimbas
graben, a possible seaward extension of the East
African Rift System (Mougenot et al., 1986)
(Fig. 1). The Davie Ridge is inferred to be the
topographic expression left by the north-south
motion of Madagascar relative to Africa and is
known to have been reactivated several times
from the Late Cretaceous up to the present time
(Mascle et al., 1987).
NEW SEISMIC DATA
The time-migrated seismic section presented
in this paper (Fig. 2; Fig. DR3) shows thin crust,
interpreted as oceanic, beneath a thick sedimentary cover. Beneath the oceanic crust, within the
mantle, are several groups of laterally coherent
strong seismic reflections, which can be traced
without difficulty to at least 16 s TWTT (~30
km beneath the oceanic top basement).
In detail, three sets of reflectors (R3N,
R3M, and R3S) can be identified between ~9 s
and ~12.5 s depth. R3N and R3M are dipping
1 GSA Data Repository item 2016133, supplementary methods for the acquisition parameters of the seismic section, thermal modelling, and gravity inversion,
is available online at www.geosociety.org​/pubs/ft2016​
.htm, or on request from editing​@geosociety​.org or
Documents Secretary, GSA, P.O. Box 9140, Boulder,
CO 80301, USA.
Two-way traveltime (s)
A2
100
125
150
175
200
225
250
275
300
325
350
375 km
N
S
4
6
8
10
12
14
16
B2
Two-way traveltime (s)
active convergence, dipping reflections are seen
to penetrate into the uppermost mantle (Carton et
al., 2014). These deep reflections are commonly
interpreted to be linked to serpentinization of
mantle rocks along deep fault planes (Carton et
al., 2014). Kilometer-scale structures linked to
migration and/or crystallization of melts have
not been imaged so far within the uppermost
oceanic mantle. However, numerical models predict that melt percolates along a thermal boundary layer at the base of the lithosphere toward the
ridge axis (Hebert and Montési, 2010) while the
off-axis portion of the distributed melt network
may remain largely trapped within the mantle
(Lizarralde et al., 2004).
4
courtesy of ION
distance along the profile (km)
seafloor
sediments
6
top basement
8
10
U1
U2
GI Moho
R1
R2
12
14
Davie Ridge
Extinct Ridge axis ?
R3N
R3S
R3M
R4
16
Figure 2. Seismic reflection time section in the Western Somali Basin (A) and corresponding
interpretation (B) (see Fig. 1 for location). U1–U2 and R1–R4 are units and groups of reflectors,
respectively, discussed in text. Dashed gray line marked “GI Moho” is gravity-inversed Moho
(see the Data Repository [see footnote 1]). Small red arrows indicate local onlap of sediments
on volcanic edifices. ION—www.iongeo.com.
northward while R3S is dipping southward. Both
R3N and R3S correspond to a series of bright
reflectors below 11 s depth while R3M is less
well defined. Between 120 km and 230 km distance along the profile, R4 is defined by a group
of strong reflectors, as much as 0.75 s thick. It
shallows from 16 s to 13 s to the south where it
almost joins R3N at 230 km distance. R4 and
R3N are separated by only a 0.5 s vertical offset.
The seismic section shows contrasting types
of basement relief. From 110 to 195 km distance
along the line, faults are almost absent and the top
basement is smooth and highly reflective. Southward, the top basement is less reflective with
increasing faulting to 250 km distance where
two large southward-dipping normal faults are
associated with >1.5-s-thick rotated blocks that
bear numerous strong reflectors. Further to the
south, the top basement quickly shallows across
a series of large north-dipping faults bounding
tilted blocks characterized by strong reflectors.
These conjugate south- and north-dipping faults
define a graben-type structure between 270 and
300 km distance.
Below the top basement, the shallowest
reflectors (R1) are bright, smooth, and continuous between 110 km and 180 km distance.
Although they can be observed southward, they
are attenuated and interleaved by 5–10 km gaps
of transparent crust. They define an upper unit
(U1), which is nearly transparent and decreases
in thickness from ~2.5 s TWTT at 120 km to ~1.5
s at 180 km distance. Between 180 and 250 km
distance the thickness of U1 does not change significantly. South of 250 km distance, U1 is barely
defined. Between 107 and 160 km distance, a
lower unit (U2) is clearly defined by a set of discontinuous reflectors (R2) with contrasting relief
and reflectivity. The strength of these reflectors
weakens progressively southward toward 250
km distance. R2 is located at constant TWTT
(11 s north of 140 km and 10.8 s south of 170
km) except between 140 and 170 km distance
where it shallows roughly in the same way as
R1, resulting in a mean thickness of ~1 s for U2.
Contrasting with U1, U2 is not transparent and
includes many small chaotic reflectors.
As much as ~5 s of thick sediments cover
the basement toward the north end of the line.
Southward their thickness decreases by half.
Local onlaps of sediments on volcanic edifices
indicate passive infill. The basal reflectors also
show a regional southward downlap onto the
top basement away from the Davie Ridge. This
regional downlap might reveal a southward progressively decreasing basement age.
DISCUSSION
The Top Basement
The change in polarity of the normal faults
at the top basement and the downlap geometry
2www.gsapubs.org | Volume 44 | Number 6 | GEOLOGY
A
N
150
0
10
depth (km)
Crustal Reflections
The position and nature of reflectors R1
and R2 are intriguing, as the Moho is usually
assumed to correspond to a sharp boundary at
the base of the crust. In order to test which of
the two reflectors may correspond to the Moho,
we determined crustal thicknesses by gravity
anomaly inversion (Figs. 1 and 2; Figs. DR4–
DR7). This method incorporates a thermal gravity anomaly correction, a parameterization of
decompression melting to predict volcanic additions, and a correction for sedimentary thickness
(Chappell and Kusznir, 2008). The gravityderived Moho, converted to the time domain,
falls between R1 and R2 (see also Fig. DR8 for
the depth-domain comparison of R1 and R2 and
the gravity-derived Moho). We therefore suggest
that unit U1 is made of igneous rocks and that
some additional low-density magmatic material is heavily embedded in unit U2. U1 could
then correspond to a magmatic crust and U2 to
a MTZ made of numerous trapped melt bodies
within the mantle, responsible for the reflective
character of U2.
Sequences as much as 1 s thick of laterally
discontinuous reflectors have been described
below slow-spreading crust and have been attributed to a complex MTZ with series of mafic and
ultramafic lenses producing alternating bands of
high and low reflectivity (Morris et al., 1993).
Bright subhorizontal reflections have also been
imaged below the oceanic Moho on the flanks
of the Juan de Fuca Ridge and have been interpreted as originating from frozen gabbro lenses
in the oceanic mantle (Nedimović et al., 2005).
In slow-spreading environments, melt is thought
to be emplaced at and above the lithosphereasthenosphere boundary within the mantle and
later partly extracted to form a magmatic crust
(Cannat, 1996; Lizarralde et al., 2004). The
detailed structure and composition of dunites of
the Oman ophiolites point to the development of
a dunitic layer by melt segregation (Rabinowicz
and Ceuleneer, 2005). The underlying harzburgites show large volumes of crystallization products of lateral extent reaching 1 km and more
(Rabinowicz and Ceuleneer, 2005).
We suggest that unit U2 could correspond to
a mantle section partly replaced by magmatic
products, which would explain the absence of
bands of reflectors and the chaotic pattern of
reflectivity within U2 (Fig. 3). This interpretation implies an inefficient extraction of melt
out of the mantle, which is favored by the combination of a slow spreading rate and a high
magma budget (Lizarralde et al., 2004). Such a
melt-rich environment is supported by the observation that reflector R2 is marked by stronger
reflectivity where unit U1 is the thickest and the
top basement is devoid of faults. We attribute
both the southward decrease in thickness of U1
and the weaker reflectivity of R2 to a smaller
magma budget. Assuming a crustal velocity of
6.5 km/s for both U1 and U2 results in a ~3 km
thickness for the MTZ (U2) while the crustal
20
175
distance (km)
200
225
250
275
S
Ridge axis
U1
R1
U2 R2
30
40
rising mantle
R4
magmatic crust
depleted mantle
melt bodies
flow lines
melt pathways along the lithosphereasthenosphere boundary
B
depth below top basement (km)
of the sediments observed on the northern part
argue for the location of an extinct spreading
axis between 250 and 300 km distance. There,
the seismic line crosses a lineated gravity low on
the free-air anomaly map, which is commonly
observed over extinct slow-spreading-rate ridges
(Fig. DR1). We therefore tentatively interpret
the different basement characters as resulting
from changes in seafloor spreading processes.
The smooth and highly reflective top basement
devoid of faults between 110 and 195 km suggests a high magmatic budget. By contrast, to
the south, the less reflective, more rough and
fault-controlled top basement indicates a lower
magmatic budget. There, the alternating strong
and weak reflectors within the highly rotated
blocks could correspond to volcano-sedimentary sequences (hyaloclastites interlayered with
turbidites and hemipelagic sediments). Alternatively, this change in the top basement faulting
pattern may indicate a later overprint along the
Davie fracture zone.
0
150
200
225
250
275
C
700°
°C
10
0
110
20
30
175
ector
R4 refl
40
Figure 3. Schematic illustration of magma
transport and entrapment and/or extraction
up to seafloor in western Somali Basin (A) and
isotherms calculated using a half-space cooling model of oceanic lithosphere compared
to R4 group of reflectors (B). U1 and U2 are
units defined by reflectors R1 and R2. Blue
and green lines show isotherms calculated
with spreading axis located at 270 km and
290 km distance, respectively, along seismic
reflection profile.
GEOLOGY | Volume 44 | Number 6 | www.gsapubs.org
thickness (U1) varies from ~8 to ~5 km, leading
to a dramatic variation of the total melt thickness
from ~11 km to ~5 km although gravity inversion suggests less variation. Part of this change
in the magma budget can be the result of the
southward approach of the seismic section to
the Davie Ridge, as thinner crust occurs at segment ends close to transform faults. Focusing of
melt delivery at slow-spreading segment centers
may also result in crustal thickness variations
of as much as 5 km (e.g., Niu et al., 2015). The
southward magma budget decrease can also be
partly attributed to the extinction of spreading
(Louden et al., 1996).
Deep Reflections within the Mantle
Either magmatic or tectonic processes could
be responsible for the formation of deep mantle
reflections. We first discuss the possibility that
reflectors R3 and R4 are produced by an accumulation of frozen melt. Because they underlie
oceanic crust, it is unlikely that they represent
a pre-breakup inherited structure. Because they
shallow toward the inferred frozen ridge axis,
we suggest that they are genetically related to
the spreading processes in the Western Somali
Basin. The cooling of the oceanic lithosphere
after the extinction of spreading, which is
enhanced close to the transform margin, could be
responsible for the preservation of the pathways
along which melt migrated toward the paleo–
ridge axis. Numerical models suggest that melt
travels from >20 km depth in the mantle up to
the crust along the base of the lithosphere, which
acts as a melt-impermeable freezing boundary
(e.g., Hebert and Montési, 2010). We therefore
explore the geometry of the mantle reflectors and
examine if they could be explained by a sloped
isotherm becoming shallower toward the spreading axis. We use a simple half-space cooling
model to calculate the 1100 °C isotherm at the
time of cessation of spreading, i.e., the paleo–
base of the lithosphere. This type of model of
conductive heat transfer cannot account for the
thermal structure of the axial region of spreading ridges where the oceanic crust is strongly
cooled by hydrothermal circulation. We therefore restrict our analysis to the deepest mantle
reflections and show that R4 lies close to the
calculated paleo–~1100 °C isotherm (Fig. 3).
Although this geometry is rather persuasive, the
fit is approximate and hence the interpretation of
R4 as melt channels along the base of the lithosphere may be based on circumstantial evidence.
Tectonic processes could also produce mantle reflections. In this case either serpentinization
of mantle rocks or injection of melt along deep
fault planes could generate the seismic reflectivity within the uppermost mantle. However, it is
unlikely that fluids would serpentinize mantle
rocks at 30 km depth below top basement (Qin
and Singh, 2015). We therefore suggest that a
magmatic event, posterior to the extinction of
3
seafloor spreading, could have resulted in the
injection of melt into major shear zones. Volcanic intrusions have been observed on seismic
reflection profiles of the Davie Ridge south of
9°S (Mougenot et al., 1986). We therefore suggest that R3 reflections could have been produced by Cretaceous volcanic activity similar to
that observed in western Madagascar and along
the Mozambique Channel (Bassias and Leclaire,
1990). The reactivation of north-south fracture
zones in the Somali and Mozambique Basins
during Late Cretaceous times has been related
to the onset of motion between India and Madagascar (Mascle et al., 1987). The reactivation of
a major transform fault such as the Davie fracture zone may have tapped a connection to deep
lithospheric fissures and thus channel magma
from the top of the asthenosphere up to the crust
within the fracture zone.
We finally suggest that both magmatic and
tectonic processes are not mutually exclusive:
the deepest reflectors R4 may result from frozen
melt bodies which may be relicts of a paleo–melt
channel system located at the base of the lithosphere and feeding the failed ridge axis, while
the R3 reflectors may correspond to melt bodies
injected into major shear zones along the Davie
fracture zone.
ACKNOWLEDGMENTS
We thank ION Geophysical for permitting publication of the seismic line. Funding was partly provided
by TOTAL and support by the Institut National
des Sciences de l’Univers–CNRS, the Modeling of
Margins MM3 Consortium, and the University of
Strasbourg. Constructive reviews by Juan Pablo Canales, Dieter Franke, and Tim Minshull are gratefully
acknowledged.
REFERENCES CITED
Bassias, Y., and Leclaire, L., 1990, The Davie Ridge
in the Mozambique Channel: Crystalline basement and intraplate magmatism: Neues Jahrbuch
für Geologie und Paläontologie: Monatshefte,
v. 2, p. 67–90.
Boudier, F., and Coleman, R.G., 1981, Cross section
through the peridotite in the Samail ophiolite,
southeastern Oman Mountains: Journal of Geophysical Research, v. 86, p. 2573–2592, doi:​
10.1029​/JB086iB04p02573.
Canales, J.P., Nedimovic, M.R., Kent, G.M., Carbotte,
S.M., and Detrick, R.S., 2009, Seismic reflection
images of a near-axis melt sill within the lower
crust at the Juan de Fuca ridge: Nature, v. 460,
p. 89–93, doi:​10​.1038​/nature08095.
Cannat, M., 1996, How thick is the magmatic crust at
slow spreading oceanic ridges?: Journal of Geophysical Research, v. 101, p. 2847–2857, doi:​
10.1029​/95JB03116.
Carton, H., Singh, S.C., Hananto, N.D., Martin, J.,
Djajadihardja, Y.S., Udrekh, Franke, D., and Gae­
dicke, C., 2014, Deep seismic reflection images
of the Wharton Basin oceanic crust and uppermost mantle offshore Northern Sumatra: Relation
with active and past deformation: Journal of Geophysical Research, v. 119, p. 32–51, doi:​10​.1002​
/2013JB010291.
Chappell, A.R., and Kusznir, N.J., 2008, Three-dimensional gravity inversion for Moho depth at rifted
continental margins incorporating a lithosphere
thermal gravity anomaly correction: Geophysical
Journal International, v. 174, p. 1–13, doi:​10.1111​
/j​.1365​-246X​.2008​.03803​.x.
Drouin, M., Godard, M., Ildefonse, B., Bruguier, O.,
and Garrido, C.J., 2009, Geochemical and petrographic evidence for magmatic impregnation in
the oceanic lithosphere at Atlantis Massif, MidAtlantic Ridge (IODP Hole U1309D, 30°N):
Chemical Geology, v. 264, p. 71–88, doi:​10​.1016​
/j​.chemgeo​.2009​.02​.013.
Hebert, L.B., and Montési, L.G.J., 2010, Generation
of permeability barriers during melt extraction
at mid-ocean ridges: Geochemistry Geophysics Geosystems, v. 11, Q12008, doi:​10​.1029​
/2010GC003270.
Kelemen, P.B., Braun, M., and Hirth, G., 2000, Spatial distribution of melt conduits in the mantle
beneath oceanic spreading centers: Observations
from the Ingalls and Oman ophiolites: Geochemistry Geophysics Geosystems, v. 1, 1005, doi:​10​
.1029​/1999GC000012.
Lizarralde, D., Gaherty, J.B., Collins, J.A., Hirth, G.,
and Kim, S.D., 2004, Spreading-rate dependence
of melt extraction at mid-ocean ridges from
mantle seismic refraction data: Nature, v. 432,
p. 744–747, doi:​10​.1038​/nature03140.
Louden, K.E., Osler, J.C., Srivastava, S.P., and Keen,
C., 1996, Formation of oceanic crust at slowspreading rates: New constraints from an extinct
spreading center in the Labrador Sea: Geology,
v. 24, p. 771–774, doi:1​ 0.​ 1130/​ 0091-​ 7613(​ 1996)​
024​<0771:​FOOCAS>2​.3​.CO;2.
Mascle, J., Mougenot, D., Blarez, E., Marinho, M., and
Virlogeux, P., 1987, African transform continental
margins: Examples from Guinea, the Ivory Coast
and Mozambique: Geological Journal, v. 22,
p. 537–561, doi:​10​.1002​/gj​.3350220632.
Morris, E., Detrick, R.S., Minshull, T.A., Mutter, J.C.,
White, R.S., Su, W., and Buhl, P., 1993, Seismic
structure of oceanic crust in the western North
Atlantic: Journal of Geophysical Research, v. 98,
p. 13,879–13,903, doi:​10​.1029​/93JB00557.
Mougenot, D., Recq, M., Virlogeux, P., and Lepvrier,
C., 1986, Seaward extension of the East African
Rift: Nature, v. 321, p. 599–603, doi:​10​.1038​
/321599a0.
Nedimović, M.R., Carbotte, S.M., Harding, A.J., Detrick, R.S., Canales, J.P., Diebold, J.B., Kent, G.M.,
Tischer, M., and Babcock, J.M., 2005, Frozen
magma lenses below the oceanic crust: Nature,
v. 436, p. 1149–1152, doi:​10​.1038​/nature03944.
Niu, X., Ruan, A., Li, J., Minshull, T.A., Sauter, D.,
Wu, Z., Qiu, X., Zhao, M., Chen, Y.J., and Singh,
S., 2015, Along-axis variation in crustal thickness at the ultraslow spreading Southwest Indian
Ridge (50°E) from a wide-angle seismic experiment: Geochemistry Geophysics Geosystems,
v. 16, p. 468–485, doi:​10​.1002​/2014GC005645.
Qin, Y., and Singh, S.C., 2015, Seismic evidence of a
two-layer lithospheric deformation in the Indian
Ocean: Nature Communications, v. 6, 8298, doi:​
10​.1038​/ncomms9298.
Rabinowicz, M., and Ceuleneer, G., 2005, The effect of
sloped isotherms on melt migration in the shallow
mantle: A physical and numerical model based
on observations in the Oman ophiolite: Earth and
Planetary Science Letters, v. 229, p. 231–246,
doi:​10​.1016​/j​.epsl​.2004​.09​.039.
Seton, M., et al., 2012, Global continental and ocean
basin reconstructions since 200 Ma: Earth-Science Reviews, v. 113, p. 212–270, doi:​10​.1016​/j​
.earscirev​.2012​.03​.002.
Manuscript received 1 February 2016
Revised manuscript received 3 April 2016
Manuscript accepted 6 April 2016
Printed in USA
4www.gsapubs.org | Volume 44 | Number 6 | GEOLOGY