GEOPHYSICAL INVESTIGATION OF A CONJUGATE PAIR OF RIFTED MARGINS

1
GEOPHYSICAL INVESTIGATION OF A CONJUGATE PAIR OF
RIFTED MARGINS FORMED AT HIGH EXTENSION
RATE LAXMI RIDGE NORTHERN SEYCHELLES BANK,
WESTERN INDIAN OCEAN Collier, J S1, Minshull, T
A3, Whitmarsh, R B3 ,Kendall, J-M2, Lane, C I3,
Sansom, V1, Rumpker, G4, Ryberg, T4 1Dept. of
Earth Science and Engineering, Imperial College,
RSM Building, Prince Consort Road, London, SW7
2BP United Kingdom 2School of Earth Sciences,
Leeds University, The University, Leeds, LS2 4DW
United Kingdom 3School of Ocean and Earth
Sciences, Southampton University, Southampton
Oceanography Centre, European Way, Southampton,
SO14 3ZH United Kingdom 4GeoForschungsZentrum,
Potsdam, Telegrafenberg, Potsdam, 14473 Germany
Introduction Although the majority of numerical
models of lithospheric extension recognise the
importance of strain rate 1-4, thermal
conditions 5-7 and temperature-dependent
rheology 8, 9 there is no consensus on which,
if any, is the dominant factor. Our objective was
to test the role of extensional strain rate in
the development of rifted continental
margins. Geophysical profiles and swath
bathymetry were acquired in January-February 2003
by RRS Charles Darwin across a pair of conjugate
rifted margins in the NW Indian Ocean (Fig. 1a)
which are thought to have formed under much
higher strain rates (59 mm/a half-rate) than
typical Atlantic margins. A single transect was
designed across the pair of margins avoiding
fracture zones and seamounts (Fig. 1b).
Laxmi Ridge margin Here, we collected wide-angle
and multichannel seismic (MCS) reflection data
along a 480 km profile from anomaly A27 to the
continental rise north of the Gop Rift. 32 OBS
recorded shots from a 6920 cu.in airgun source,
fired every 60s (Fig. 2). Coincident MCS profiles
were recorded with a 2.4km 96-channel streamer
fired every 30s (Fig. 5). The sediment thickness
is 2-3 km. The crustal thickness is 7 km at
anomaly A27, and reaches 15 km at the northern
end of the profile. Lower crustal velocities
reach 7-7.5 km/s on the seaward flank of Laxmi
Ridge (Fig. 6). Seaward-dipping intrabasement
reflections are seen for over 30 km south of the
crest of Laxmi Ridge. The northern edge of Laxmi
Ridge abuts the Gop Rift within which
high-amplitude, linear, SSE-NNW trending magnetic
anomalies have been mapped whose origin is
currently unresolved.
NE Seychelles marginThe Seychelles islands
consist principally of pre-Cambrian granite
surrounded by a carbonate platform. Wide-angle
and MCS data were obtained along a 300 km NNE
line extending from the main island Mahé to
undisputed oceanic crust (beyond anomaly A27) of
the eastern Somali Basin (Fig. 1). An additional
800 km of MCS data, including Line 13 (Fig. 4),
were also collected in the area. 32 OBS were
deployed along the main line and 21 land
seismometers were installed on the islands for
the wide-angle work (Fig. 3). On MCS profiles,
top oceanic basement and Moho are imaged at 6.5 s
and 8 s twt, respectively, indicating that the
early oceanic crust of this margin is anomalously
thin. This idea is supported by wide-angle
modelling results (Fig. 6). Seaward-dipping
reflectors are also evident. Three seamounts were
dredged and yielded basalts erupted in shallow
marine or subaerial environments.
Data from the wide-angle experiment
LR
Fig. 2
S
Fig. 3
Figure 3 (bottom) Vertical geophone record
section from station MSEY(04) on Mahé, Seychelles
(Fig. 1b). The record shows PmP arrivals between
80 and 170 km, and the mantle refraction (Pn) as
a first arrival only beyond 160 km (filter 5-25
Hz, no amplitude scaling) because of the thick
continental crust beneath the Seychelles Plateau.
A sub-Moho reflection from around 60 km depth
(white inverted triangles) is also imaged.
Figure 2 (top). Ocean bottom hydrophone (OBS03)
record section from the Laxmi Ridge margin (Fig.
1b). For clarity, only every third trace is
plotted. The section shows clear sedimentary,
crustal and mantle arrivals, both reflections and
refractions (filter 5-25 Hz, amplitude
proportional to offset). Mode conversions from
the top of the basement are also observed. The
offset at which the mantle reflection (PmP) is
first seen varies from 15 km at the southern
(oceanic) end of the Laxmi profile, to 70 km at
the northern (continental) end not shown,
reflecting the variation in crustal thickness
across the margin.
Figure 1 (a) inset Satellite gravity field for
the NW Indian Ocean12 showing the Seychelles (S)
and Laxmi Ridge (LR) margins in their present
position. Laxmi Ridge is marked by a well-defined
gravity low (blue/green). The two roughly
triangular regions were reconstructed in (b), red
line cruise track. (b) Reconstruction of the
conjugate survey areas at A27 time, using a Chron
27ny pole13, showing magnetic anomalies and depth
contours (1000, 2000m GEBCO 2003). The white
line at the join roughly follows the Chron 27ny
picks of Miles et al.14. Offshore, symbols and
lines mark OBS/H deployments and shooting tracks
red Seychelles margin, black Laxmi Ridge
margin. Black arrow spreading direction at A27
- A26 time13. Hollow blue arrows fast
direction for teleseismic split S-wave arrivals
at land stations.
MCS profiles
A27
Figure 5 Example of an MCS profile (unmigrated)
from the Laxmi Ridge margin and Gop Rift (Line 5
Fig.1b). SDRS seaward dipping reflector
sequence, individual reflector lengths reach 12
km, lateral extent of the wedge is gt 35 km (CDPs
11800-15000), and the estimated wedge thickness
is 5 km. The profile shows a relatively thick (up
to 2.6 s) sequence of Indus basin sediments. The
Gop Rift extends between CDPs 3000 and 9500 and
magnetic anomaly A27 is positioned approximately
145 km to the south of CDP 15000.
A27
Preliminary wide-angle models
Figure 4 Example of an MCS reflection profile
(unmigrated and fk-dip filtered) from the
Seychelles margin (Line 13 Fig.1b). SDRS
seaward dipping reflector sequence, individual
reflector lengths reach 6 km, lateral extent of
the wedge is approximately 30 km (CDPs
7750-10000), and the estimated wedge thickness is
3.5 km. Possible sill intrusions (black arrows)
are indicated. White arrows indicate Moho
reflections. The grey arrow marks the position of
magnetic anomaly A27. Sediments are thinner than
over the conjugate Laxmi Ridge margin (Fig. 5).
Summary The seaward parts of both margins are
similar in the presence of upper crustal
seaward-dipping reflector sequences and
indications of coincident modest underplating in
the lower crust. This suggests that both margins
formed in a magma-rich (volcanic?) environment.
Basalts dredged from three seamounts on the NE
Seychelles margin appear to substantiate this
interpretation. Although the seismostratigraphy
of the post-rift sediment successions over the
Gop Rift and the oceanward side of the Laxmi
Ridge appear similar, the crustal structures
under these two regions appear quite different.
This suggests that the Gop Rift may be a
pull-apart basin that formed at a late stage of
continental break-up. Laxmi Ridge is underlain
by thinned continental crust up to 8 km thick
under our transect. In broad terms the crustal
structure along the reconstructed transect is
asymmetrical with a region of broad extension
across the Laxmi Ridge margin and a relatively
abrupt transition from continental crust to
oceanic crust over the Seychelles margin. Work
in progress will analyse and date the dredge
samples, interpret the MCS profiles and refine
the seismic crustal structure. Finally the
crustal structure will be simulated using
numerical models of extension.
Figure 6. Preliminary P-wave velocity models of
the conjugate margins. Contours are every 0.25
km/s, and bold lines are model boundaries. The
models were obtained by fitting observed travel
times of sediment and crustal arrivals with the
ray-tracing inversion program Rayinvr15.
Approximately half the instruments have been
modelled. Additional constraints on basement
structure were taken from the coincident MCS
reflection profiles.
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Acknowledgements We thank the Master and crew of
cruises RRS Charles Darwin 134B, 144 and 149 for
their assistance in collecting the data presented
here. We also thank Bramley Murton for collecting
rocks at Dredge Site 3 and Ernst Flueh for
providing access to the Geomar pool of OBS. The
Seychelles National Oil Company (SNOC) and
Patrick Joseph provided invaluable help and
advice in the Seychelles.