Nov 10, 2000 - The seismic velocity models are consistent with gravity data which show weak residual .... Our seismic source was a 10-gun, 71 L tuned air gun.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. Bll, PAGES 25,809-25,828, NOVEMBER
10, 2000
Crustal structure of the Southwest Indian Ridge at the
Atlantis
II
Fracture
Zone
M. R. Muller • Bullard Laboratories, Department of Earth Sciences,University of Cambridge Cambridge, England, United Kingdom T. A. Minshull Schoolof Ocean and Earth Science,Southampton OceanographyCentre Southampton, England, United Kingdom
R. S. White Bullard Laboratories, Department of Earth Sciences,University of Cambridge Cambridge, England, United Kingdom
Abstract. The Southwest Indian Ridge is a slow spreading end-member of the mid-ocean ridge system. The deepestborehole penetrating the lower oceanic crust, Ocean Drilling Program hole 735B, lies on the eastern transverse ridge of the Atlantis II Fracture Zone at 57øE. A wide-angleseismicsurvey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous.To the east of Atlantis Bank, on which hole 735B is located, the crust consistsof a 2-2.5 krn thick high-velocity-gradientoceanic layer 2 and a 1-2 krn thick low-velocity-gradient layer 3. The transform valley has a 2.5-3 km thick crust with anomalously low velocities interpreted to consistlargely of highly serpentinized mantle rocks. The seismicallydefined crust is thickest beneath the borehole, where layer 2 is thinner and the lower crust is inferred to contain 2-3 krn of partially serpentinized mantle. The seismic velocity models are consistentwith gravity data which show weak residual mantle Bouguer anomaliesbecausethe regions of thinner crust have lower crustal
densities.
Stress variations
deduced from mass balances between
the
transform valley floor and the adjacent transverseridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically. The variation of crustal thicknesswith spreading rate definedby data from the SouthwestIndian Ridge and elsewhereis consistentwith modelsof melt generationin which the upwellingmantle is cooledby conductive heat loss at very slow spreadingrates, resulting in reduced melt generation under the spreading axis. Large segment-scalevariations in crustal thicknesssuggest subcrustal along-axismigration of melt toward segmentcenters. 1. Introduction
Triple Junction[Chuand Gordon,1999].The spreading
The SouthwestIndian Ridge extends 7200 km from the Bouvet Triple Junction in the South Atlantic to the RodriguesTriple Junction in the central Indian Ocean and marks the boundary between the Nubian and Somalian plates in the north and the Antarctic plate in
rate has not been significantlyhigher sinceabout 20 Ma
[Patfiat and $egoufin,1988]. The seismicstructureof oceanic crust formed at such slow spreading rates remains rather poorly characterized, but several recent
studies[Minshulland While, 1996;Muller et al., 1997, 1999; Klingelhoeferet al., 2000] indicate that crustal
the south (Figure 1). The spreadingrate variesfrom thicknessis substantially reducedfrom the 6-7 km com16.5 mm/yr in the west, near the Bouvet Triple Juncmonly found at faster spreadingridges. tion, to 11.3 mm/yr in the east near the Rodrigues The morphologyof the SouthwestIndian Ridge varies markedly along its length, with portions of the ridge 1Now at GeophysicalServicesDepartment, Anglo Amer- lacking large offsets(e.g., 15ø-25øE [Grindlay et al., ican Corporation, Marshalltown, South Africa. 1998]) contrastingwith regionsof large-offsetfracture zoneswith up to 6 km of relief (e.g., 52ø-60øE). The Copyright 2000 by the American GeophysicalUnion. gravity signature of these large-offsetfracture zones commonly can be followed out to the trace of the RoPaper number 2000JB900262. 0148-0227 / 00/ 2000JB900262509.00 driguesTriple Junction on the Somalianand Antarc25,809
25,810
MULLER
ET AL.:
CRUSTAL
STRUCTURE
OF THE
SWIR
18
32øS
33øS
34øS I/
ß
56øE
57øE
58øE
59øE
............... :......... • .................................... ..,,:"• :•.,•,.,::•...•.•}•.'. i:;•F"---':-•"•.-• '•:•.. -200
-160
-120
-80
-40
0
40
80
120
mGal
Figure 1. Satellitegravityimage[Sandwelland Smith,1997]of the SouthwestIndian Ridgenear the Atlantis II Fracture Zone. Contour interval is 20 mGal. NTD is a nontransform discontinuity. Box indicates the area of Figures 2, 12, and 15. Inset shows location of study area on the
SouthwestIndian Ridge(SWIR), as well asthe CentralIndian Ridge (CIR) and SoutheastIndian Ridge (SEIR). Bold linesshowridgeaxesand magneticisochrons6 and 18, and dashedlinesshow tracesof the RodriguesTriple Junction(RTJ) [Patfiat and Segoufin,1988].
tic plates, indicating that the morphologicaldifferences may reflect the behavior of the triple junction, which has apparently switched several times between a stable ridge-ridge-ridge configuration and an unstable ridge-
lantis Bank (Figure 2), has been the focus of inten-
the survey reported here, offsetsthe Southwest Indian
735B on Atlantis Bank penetrated the largest sectionof in situ oceanic lower crustal material yet recovered and hence has had a considerableimpact on our understanding of magmatic processesin the lower oceanic crust. The results of these sampling programs must be interpreted in the context of the large-scalecrustal structure of the correspondingridge segment and its evolution
sive geologicalsampling work over the last 15 years, in-
cludingdrilling on OceanDrilling Program (ODP) legs
118 [Robinsonet al., 1989]and 176 [Dick et al., 1999], ridge-faultconfiguration[Mitchell and Parson, 1993]. and a recent dredging, rock coring and ROV experiThe Atlantis II Fracture Zone, which is the focus of ment [MacLeodat al., 1998]. The ODP drilling at Site
Ridgeby 200 km at 57øE (Figure 1). At this longitude, magnetic anomaly data indicate that the ridge has been spreadingasymmetricallysinceat least 11 Ma [Dick et al., 1991a],so that the transformoffsetis growingby -•4 mm/yr. The transform width is unusuallylarge (-•38 km betweenthe crestsof the flanking transverse ridges)perhapsdue to an episodeof transtensionbetween17-20 Ma and 6-9 Ma [Dick et al., 1991a]. The Atlantis II Fracture Zone, and in particular an
uplifted block on its easterntransverseridge called At-
overtime. Muller et al. [1997]discussed in detail the seismic structure of the crust beneath Site 735B; here we present constraints on the crustal structure of the fracture zone and the spreading segment extending to
MULLER
ET AL.:
CRUSTAL
STRUCTURE
OF THE
SWIR
25,811
the next nontransform discontinuity to the east of it, derived from seismicand gravity data acquired on RRS
clock offsetswere checkedbefore and after deployment, and a linear drift was assumed between these times; Discoverycruise208 in 1994 (Figure 2). the resulting timing uncertainty is in all cases10 21 Pa s, whichis much increasefrom 3.2-3.7 km/s at the top of oceaniclayer 2 higherthan the valueof
