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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
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 halfrate) 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, SSENNW trending magnetic anomalies have been mapped
whose origin is currently unresolved.
NE Seychelles margin:The 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
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.
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 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 subMoho reflection from around 60 km depth
(white inverted triangles) is also imaged.
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 > 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.
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).
•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.