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R. asfi..Soc.{ i985) 81.47 -74
Geophys.,I.
Thetransition betweenthe ShebaRidgeand Owen
Basin:rifting of old oceaniclithosphere
Carol A, Stein* and James R. Cochran Lamont-Dohertv
GeologicalObsenatoryof Cofu bid Llnitertity, Palisades,
NY 10964,USA
6;in originalform 1984
1984September
13.Received
Accepted1984September
January30
Summary. Magnetic quiet zones are present along the marginsof the entire
length of the Gulf of Aden to the Owen fracture zone. This includes the
easternmost300km between the eastern edgesof Arabia and Africa to the
Owen fracture zone where old oceanic lithosphere was rifted to form the
ShebaRidge. Within this eastemmostregion the boundary betweenthe quiet
zone and the old oceadc lithosphere is marked by ridge complexes,the
Sharbithat Ridge Complex to the north and the Error Ridge Complex to the
south. These ridge complexes, which lack a magnetic signature,occupy a
structural position similar to the hinge zonesat the continental marginsto the
west. They appearto have formed early in the openingof the Gulf of Aden or
perhaps to have been pre-existing features.The boundary between Sheba
Ridge and the northem magnetic quiet zone is often marked by an abrupt
end of the Sheba Ridge seafloor spreadingmagnetic anomaly pattern and a
sharp basement deepening to the north. The boundary between the East
ShebaRidge and northern magneticquiet zone becomeslessdistinct near the
Owen fracture zone. This is also accompaniedby changesin the East Sheba
Ridge, specifically a decreasein magnetic anomaly amplitudes, increasein
'cooling-curve' ridge flank shape.This
ridge flank depths and a loss of the
may be the result of lower mantle temperaturesin the vicinity of the Owen
fracture zone. The sedimentswithin the magneticquiet zone can be divided
into a lower disturbed unit and an upper unit consisting of flatlying
reflectors. The disturbanceof the lower sedimentsmay haYeresultedfrom a
period of diffuse extension in the magneticquiet zone pdor to the establishment of the ShebaRidge spreadingcentre.
The similarity of the easternmostquiet zone to quiet zones at dfted
continental margins leadsto the suggestionthat theseregionswere formed by
diffuse extension of old oceanic lithosphere. Using a tu,olayer lithospheric
attenuatiol model and assuming extension during a period of 15Myr
followed by l0Myr of cooling, the basementdepths and heat flow measute'
*Formerly Cafol A. Geller.Now at: Depadmentof GeologicalSciences,
NodhwestelnUnivefity,
Illinois60201.USA.
Evanston.
48
C. A- Stein and J. R. Cochran
ments can be adequatelymatched.The modelling implies 45 per cent crustal
extension in the quiet zone. The amount of extension calculatedis compatible with documented motion between Arabia and Africa' The old oceanic
iithosphere thus must have been substantiallythinned to a thickness similar
to l0-14 Myr old ocean before seafloor spreadingwas initiated at the Sheba
Ridse.
Introduction
The rifting of continental lithosphere and subsequent seafloor spreading creates a
'passive'margins.Specifically,between
characteristicset of featurescommonly recognizedat
the clearly defined continental and oceanic lithosphereis a zone resultingfrom the margill's
rifting and subsequenteyolution, which is frequently called a magneticquiet zone because
of the generally low amplitude and lack of couelation of the magnetic anomalies.The
transition from undisturbed continent to the magneticquiet zone is marked by the hinge
zone, a major structural boundary acrosswhich there is a rapid basementdeepening.The
magnetic quiet zone often appearsto consist of thinned and faulted continental crust, as
found in the Bay of Biscay (de Charpel et al. 1978',Mofiadert et 41. 1979), the southen
margin of Australia (Talwani et al. 1979) and the Newfoundtand margin (Keen & Barrett
with lage-scaledyke intrusion
lgSl). At some margins,the dfting processis associated
Gulf of Aden and Red Sea
the
westernmost
as
found
in
and volcanic activity, such
(Beydoun 1970; Colemanet al. 1975,1979) and on the Outer Voring Plateau,Norway'
where layered yolcanic flows have been detected from seismicdata (Mutter, Talwani &
Stoffa 1982) and from drilling results (Talwani & Udintsev 1976). The magnetic quiet
zone oceanic lithosphere boundary is usually sharp, often defined by a basementdepth
dlscontinuity and magneticand gravity gradients(Talwani & Eldholm 1973).
The Gulf of Aden (Fig. 1) is a young oceanbasin resultingfrom the seParationo f Africa
and Arabia beginning in the late Oligocene or the earliest Miocene (Somaliland Oil
Exploration Co., Ltd 1954; Azzaroli& Fas i964; Beydoun 1982). The easternedgeof
'geographical'Gulf of Aden, is
continental rifting, which defines what we shall call the
marked by a line extending from the edge of the continental shelf near Ras Sharbithat
(Arabia) to the easternedge of the submergedpeninsulacontaining the island of Socotra
(Africa). However, from a tectonic point of Yiew the Gulf of Aden extends an additional
30okm east to the Owen fracture zone. Seafloor spreadingis occurring throughout the
Gulf of Aden from Afar to the Owen flacture zone at the ShebaRidge spreadingcentre
which is divided into the West and East ShebaRidge by the major Alula-Fafiak fracture
zone at 51"E. The East Sheba ridge is offset right'laterally by 300km from the Carlsbery
Ridge at the Owen fracture zone.
Ridge complexes extend eastward from both Ras Sharbithat and Socotra to the Owen
fracture zone: the Sharbithat Ridge Complex to the north, and the Error Ridge Complex
to the south. The ridge complexesseparatecrust associatedwith the ShebaRidge from older
oceanic basins. the Owen Basin (described by Whitmarsh 1979) to the north, and the
northern Somali Basin (describedby Bunce et al. 1967) to the south- At the intersectionof
both ridge complexeswith the Owen fracture zone, the trend of the fracture zone changes
from a nearly N-S trend along the boundariesofthe Owen and no hernmost Somali basins
to a NNE-SSW trend between the two ridge complexes.This direction is parallel to the
present Sheba Ridge spreadingdirection and the trend of both ridge compleies is perpendicular to the present ShebaRidge spreadingdirection. The topographic expressionof the
Owen fracture zone is subduednear the intersectionwith SharbithatRidge and immediately
(Fig.2).
to the southit is completelyburiedby sediments
Rifting of old oceanic lithosphere
60'
40"
50"
70"
60"
regions:
Figure1. Locationandbathymetryof the Gulf of Aden-EastShebaRidgeareaandsurrounding
study areais or! ineal.Bathymetriccontoursin metrcsfrom the GenefalBathymetricchart of the
bv the
ale rcpresented
Oeeans(GEBCO),19?5. Locationsof Sharbithatand Errcr Ridgecomplex€s
hatchedlines.
Seafloor spreadingat the ShebaRidge axishasproduced correlatablemagneticanomalies
with NNE-trending fracture zones, identified by Laughton, Whitmarsh & Jones(1970) and
Cochran (1981) as indicating organized seafloor spreadingsince magnetic anomaly 5 time
(about 10Myr BP) through most of the Gulf of Aden. It has also been suggestedthat the
ied Sea and the Gulf of Aden opened in two stages,an initial seafloor spreadingphase
30-15 Myr gp and a secondphasebeginning5 Myr er (Girdler& Styles1978,l982lStyles
& Hall lg80lGirdler er a/. 1980)with oceaniclithosphere,in someplaces,ascloseas20km
from the coastline.We do nqt agreewith this interpretation (seeCochran 1981, 1982aand
Girdler & Styles 1982 for a completediscussion).
Inside the geographicGulf of Aden, magneticquiet zonesare located betweenthe oldest
identifiable magnetic anomaly (usually anomaly 5) and the steep continental margins,with
boundariesmarkedby basementdepth discontinuities(Cochran l98l) Magneticquiet zones
50
C. A. Stein and J. R. Cochran
Figure 2, Bathymetry of the study area (F€. l). Contou interval is 500m. Locatioi of ship tracks used
in corstructing this map is shou,n,contours from GEBco (1975) werc usedwhele data ale not available.
Circleswith dot itside show the location of DSDPsites.
exist not only along the continental margins of the geographical Gulf of Aden but also
extend 300km eastward to the Owen fracture zone with the position of the stable conti
nental crust replacedtry the oceanic crust of the Owen and Somali Basins(Cochran 1981).
The position of the continental hinge zones is occupied by the Shaftithat and Error Ridge
Complexes(Cocfuan 1981).
Rifting of old oceanic lithosphere
51
to thosefor
codespond
FiSure3. Locationof shiptracksandheaiflow stationsTrackidentifications
figures.
profilesin subsequent
of old
We propose that the easternmostmagnetic quiet zones were formed by rifting
purpose
The
ofAden'
the
Gulf
oceanic tithosphe,eduring the early stagesof the opening of
crust
of this paper is to stutly the boundary betweenthe ShebaRidge and the older oceanic
the
magnetic
*ith
zones
to the north and south. A cqmparisonof these oceanic transition
with the rifted continental magins should yield information on the
quiet zones assoQiated
proc"r, of lithospheric rifting, We will concentrate on examining the northern magnetic
there
quiet zone, the transition between the East ShebaRidge and the Owen Basin,because
is much better data coveragethere than in the south.
Northem magneticquiet zon€
ofAden is
The northern boundary of the magnetic quiet zone eastof the geographicalGulf
from the
extends
feature
which
continuous
long
a
300km
formed by the Shafuithat Ridge,
ofRas
rise
east
continental
the
Arabian
with
intersection
a
complex
Owen fracture zone to
to
sedibarrier
unbroken
a
relatively
act
as
to
appears
Ridge
Sharbithat (Fig. 2). Sharbithai
also
ridges'
lower
basement.
of
number
A
1981).
(Cochran
ment transpo; from the north
the
magnetic
present
within
are
also
(Fig.
2),
to
the
Ridge
Sheba
elongated perpendicular
and are
qui.l ,on. ,outh of the Sharbithat Ridge (Fig. 4), but these are not continuous
generallylessthan 40 km long.
of the East
Free-air gravity anomaly data contoured at l0mgal intervals from the areas
of the
features
dominant
5'
The
in
Fig.
presgnted
are
Sheba Ridge and the Owen Basin
anomalies
and
negative
positive
amplitude
large
gradients
and
gravity mJp are the steep
52
C. A- Stein and l. R. Cochran
TOPOGFIAPH Y
WE STE FN ZONE
SHARBITHAT BEG ION
5W
NE
Figure 4. (a) Topogmphicprofiles from the \Pesteinzone acrossthe SharbithatRidgeComplex Location
oiall profiles is shown in Fig. 3. Profileshave been projectedat N32'E and alignedalongthe Sharbithat
Ridge axis. The thin horizontal line through eachpiofile is the 4000rn level. The vertical hatched ling
marks the East shebaRidgeaxis. The dotted line repfesentsthe expectedlocation of1nagneticanomaly5
basealon l.scmyr_r spreaalingrate on the nofthern flank. The sho thin vertical line Iepresentsthe
bounatarybetweenShebaRidge and the magneticquiet zone and is dashedwherethe boundaly is not as
certain. Fracture zonesnamealaccotalingto cochran ( 1981).The small arrow indicatesthe location of the
with the northern flank of Sharbithat for ship tracks with gravity datafree air gravity low associateal
Note that the shapeof Sharbithat Ridge changesfrom track to track but the ridgecomplex is presenton
all profiles. (b) T;pographic profiles from the easternzone acrossthe Sharbithat Ridge Complex' See
caption of (a) for explanationoi symbols.ProfileshavebeenprojectedalongN32"E-
associated with the Owen fracturc zone. The Sharbithat Ridge ls marked by a relative
gmvity high, the magnitude varying with the height of the ridge. A large broad gravity low
is located on the northern flank of
with maximum amplitude of about -5Omgal
basement feature (The gravity
acoustic
Sharbithat Ridge and is not associated with any
gravity
minimum locations are shown by an arrow on the topography profiles in Fig 4 ) The
'bight'
projecting
two
ridges
between
low extends westward from 59oE to near 58"8 in the
from the continental margin. These ridges separate this Sharbithat gravity low from similar
'edge
effect' lows extending along the continental margins ofthe Owen Basin and
amplitude
Guif of Aden (Fig. 5) near the 3-3.5km
isobaths. East of 59"E a gravity low exceeding
Rifting of old oceanic lithosphere
53
T O P O GR A P H Y
E A S T ER N Z O N E
S H A RB I T H A T F I E G I O N
t-..,
v36 3
,
!t/-
^
\t
[.^
^ ^ 4 " \ , . { V Y Vl /\ A
\
/ \ ' \ , /
.-
Y
!\-
___.\
D.
_
-.--*lt
Eo5l St€bo R dqe
(b)
-
'MOZ
NE
Fignre4 - continued
-40 mgal is also associatedwith Sharbithal Ridge, but is here located south ofthe ridge
thicknessesin the
comple; over a deep trough which parallels it to the south' Sediment
gradientsthan
steeper
trouih exceed 2s in places.This gravity low is narrower with much
-10 to
(about
gravity
low
tn" gr""lty low to the west. Theie is also a lower amplitude
-20 mgal) over the magnetic quiet zone south of Shaftithat Ridge in the western zone
where sedimentthicknessesaverage0.5 s.
hasthe appearThe grayity pattern over the magneticquiet zone in someprofiles(Fig' 6)
between the
boundary
the
near
of u broad low 100 l50km wide ixtencling from
"n"" Ridge aid magnetic quiet zone to well north of Sharbithat Ridge The graYityhigh of
Sheba
to the gravity
the Sharbithat Ridge is superimposedon the broad low This is similar
show that lbr
calculations
simple
anomaly pattern res"ultingf.om flexural loading.However,
the gravity
ligidities,
lithosphelic
of
a
range
feasonabl;basementand sedimentdensitiesand
(Fig' 6)'
loading
from
flexural
result
simply
anomaliesobservedover the quiet zone cannol
'edge
isostatic
thermal
from
a
result
cannot
Calculationsalso show that the gravity anomaly
1982) predicts
effect' (Karner & Watts 1982)' Specifically,thermal isostasy(Karner & Watts
and a longer
obsewed'
is
not
which
negative,
a positive anomaly of similar amplitude to the
wavelengthanomaly than is observed.
northern flank ofthe
The highly stratified sedimentsof the Owen Basin abut againstthe
7)' This observation
(Fig'
disturbance
tectonic
without any evidenceof
Sharbithai Ridge
-,hut
beginning of rifting
predates-the
part
in
at
least
the Sharbithat Ridge
h",
,ugg.r,,
quiet zone, pdor to
"i
ofthe
development
in
the
inihe tate Otigoceneor that it formed early
54
C. A. Stein and J. R. Cochran
contoured at l0mgal intervals Areasof
F8uie 5. Free-airgravity anomaliesIn the study area(Fig l)
points
are shown as fine dots Anomales ale
-30mgal
Data
ale
shaile'l
than
iess
g,fuity
=
"no.utl",
ier"rr.d to th" 1930 International Ellipsoid (flattening l/29?)
within the magnetlc
the depositionofthe upper secondofOwen Basinsedimentssediments
into two units The
divided
be
can
Ridge
ofSharbithat
Oui.i ,on" and on the southern flank
and in places
ofdisturbance
signs
show
Jna"rtying sediments,which can be up to 0.6 s thick,
layers
these
upturned
places
in
some
ar" upiuri"d on to Sharbithat RidgeaFig.T). Although
Rifting of old oceanic lithdqhere
V35'3
55
Free_otrq.ovtrY
re'24
Te'5
V35-3
F.eeonq.ovnY
'-::]
s
#
N
magnetic quiet zone and
Figure 6. Obsetved anal calculated free_at glavity anofialies acrcssthe nofihern
v36-3 (seeFig' 3 for
and
profiles
v35-3
fot
computed
are
sharbithat Ridge. Free-airgtavlty anomafiJs
crust-mantle boundarv due to the
the
of
d;flection
plates
the
with
elastic
krn
25
tor
i
and
Lcatlong
due to the load of
i."i' .i if," sedimentsanat topographv included' For prcfile V35-3 'leflections
For
elasticthicknessesrespectively_
25km
5
and
for
the
and
1km
2
to
about
ale
up
Ridge
Sharbithat
5
and
for
the
0
6
km
and
up
to
I
are
Ri'lge
of
sha$ithat
;;11" n:63 i"nections due to the load
of the Sharbithat Ridge is not
flank
northem
on
ihe
gravity
low
the
plate,
Note
that
i5km elastic
reproducedin the modelling.
throughout the quiet zone
on the ridge may be the result of draping, the general app€arance
to.0'5 s thick' are chatac,ugg.rt, tJ"toni" disturbance.The uppei sediments,which-.areup
althoughthere are a few
general
undisturbed
in
tefieO Uy flat-lying reflectors' They
"pp""t of tne sedimentsinto two units may be a rcflecminor faults wiih small offsets.Ttre Oivision
reflects the present relative
tion of the tectonic history of the quiet zone' The upper unit
may be the result of an
stability of the quiet zone, white the disturbanceof the lower unit
ofthe GulfofAden'
earlieractivetectonic period associatedwith the early oPening
quiet zone is well defined
the
magnetic
and
Ridge
Sheba
The boundary between the East
(westernzone) and
for all profiles more than about 130km west ofthe Owen fracture zone
in the magnetic
i"
uri.trrir.a by an increasein basementdepth (Fig' 8a) and.a decrease
anomalies
"t
The
magnetic
(Fig7a)'
5
anomaly
just
of
magnetic
north
anomaly amplitude
than magnetic anomalies
within the magnetic qulet zone are generally lower amplitude
C-A- SteinandJ. R. Cochran
56
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57
Rifting of old.oceaniclithosphere
ZONE
WESTERN
J
, " ]
-
z
E A S T E R NZ O N E
? o'r
: _ l
:
l
<
- 5 ool
v36- 3
_ ]
- 3
-
o
z
N
opprox.IOOKM
Ftgdtrei - continued
on the ShebaRidge' andcanonly be traced
formedin the last 10Myr by seafloorspreading
profilessH375 and
Lei*rrn pront* where th€ tracks are closetogether(for example,
up of ihe N-S
V35-3, Fig. 8a). The far westernsectionof ih; SharbithatRidge,made
with it' The
,iOse on the continental slope has no magneticanomaly-associated
trenft
gamma)
magnetic
(<
150
mainpJrtioi of th" ri6geis characterizidby a broad low amplituile
anomaly.
systematicallyeast'
The'amp[rude of the seafloorspreadingmagneticanomaliesdecrease
199kmwestofthe
about
;ith Plofile DIS'3,
waril towardthe Owenfracturezonestart-ing
is clearlydefined
Owen frrcture zone. Near the Owen fracturezone' only the axial anomaly
58
C. A. Stein 6ndI. R. C.ochrun
MAGNEIICS
ZONE
WESTERN
SHARBIThATREGION
t-
-l
W\i"ilr-"*"
al
I
-*WWw",.
i r l Ji i t
i , { i l}
I
Fi8ue 8. (a) Total intenrity magnetic anomaly profiles acrossthe Sharbithat Ridge Complex ftom the
western zone. Locatlon of all prcfiles shown in Fig. 3. All profiles have been plojected along N32'E.
Theoretical seafloor spieading anomaly sequenc€ is al$o shovn generated using LaBrecque, Kent &
C^nde (1977) rcversaltime-scaleand inclination 17', declination 0'. Symbols are the sameas io F8. 4.
(b) Total intensity magnetic anomaly profile acrcss the Sharbithat Ridge Complex from the eastern
zo'le.
on profile V36-l
located about 90km
west of the Owen fracture zone (Fig.8b).
Topo-
graphic control from cross-linesclearly shoi thal this Une is fiot situated in a fracture zone
(Fig.2).
The decreasein the amplitude of the magneticanomaliestoward the Ov/en fracture zone
is associatedwith an infiease in the depth of the ridge crest and, betweenthe Owen fracture
region and apprcximately 130km to the west (in the eastem zone) a loss ofthe charac.
teristic mid"ocean ridge shape.In this region the East Sheba Ridge does not display the
Rifting of old oceonic lithosphere
59
"4r.^,*"'
if
Edshbtue
Fignret
HJ:i^
continued
regular increasein depth away from the ridge crest typical of mid-ocean ridge spreading
centres, but rather consistsof a zone of rcugh bathymetry with a fairly constant average
depth of about 3500m. Sheba Ridge does, however, retain a well developed rift valley
(Fig. 4b, profiles V34-l and V36-l) up to the intersection with the Owen fracture zone
which is marked by Wheatley Deep. This steep-sidedtectonically produced trough with
water depths exceeding 5600m (Matthews, Williams & Laughton 1967), is similar to the
depressionscommonly observedat the intersection offracture zonesand ridge crests.These
obseryationsindicate that, despitethe lack of a well developedmid-ocean ridge morphology,
seafloor spreadingat a localized spreadingcentre is occurring at the present time to the
e a s t e r en n do t t h e E a s tS h e b aR i d g e .
In the easternzone, the boundary betweenthe East ShebaRidge and magneticquiet zone
is not well defined. The ridge and trough ShebaRidge morphology doesstill terminate at a
down-to-the-north increasein depth, but this depth increaseis located significantly north of
the predicted location of the 10 Myr isochron (Figs 4b and 8b). The changein morphology
also appearsto be offset left laterally acrossfracture zoneswhile the ridge crest is offset
right laterally (see Fig. 2). Thus, near the Owen fracture zone it becomesdifficult to distin"
guish 'Sheba Ridge' crust from 'magneticquiet zone' crust except in the vicinity of the ridge
crest.
Southem magretic quiet zone
Far fewer data are available for the southem flank of the ShebaRidge. A magneticquiet
zone and a ridge complex, Error Ridge, occupy analogouspositions to the quiet zone and
Sha6ithat Ridge to the north. The Error Ridge complex is made up of two parallel ridges
separatedby a 30 km wide trough (Fig. 2) containing up to I km of sediment.The double
60
C. A. Stein andI. R Cochran
IVIAGNETICS
ERRORREGION
Error Ri'lge ComPlexProfileshavebeen
Fiqure9. Total intensitymagneticanomaliesactossthe
the
line throu€heachprofilerepresents
heary
shownin Fig 3 The
;;;il'bcatioos
lilt
ilil",j
Ridge
of
Error
ridges
betweenthe two
locationof the gravity*ini-t-'rn u"o"iutil *"ith the trough
Complex.
Error' a large guyot cappedby limestone
ddge structure terminates in the southeastat Mt
1966), which reachesto within 200m ofthe seasurface'
iii"tnt*
quiet zone' nor the northem Somali
Neither Error Ridge, the southern magnetic
(Fig' 9)' The northem somali Basin
Basin is associatedwith significant magnetic-anomaliis
the southern Somali Basin where
consideredto be a northe'rn continuation of
;;;;;;"liy
havebeen identified by Rabinowitz'
*"gnril" uto*"u* M25 to M9 (oxfordian-Hautervian)
anomalies hav-1b;en identified in the
Coffin & Falvey (1983). Howeu"', no magnetic
the two long N-S piofiles (v36-l and
northern somali Basin and non. "r, "pp"r.-nt in
Basin has significantly deeper basement
V:-Sa ,ft"*t in Fig. 9. Also the northein Somali
pa (Bunce et al' 1967)'
unJ ,rior. ,"ai*.nt u'"cumulationthan the southern
values-lessthan -l30mgal is associated
with
low
anornaly
'Rtdge
A prominent free-air gravity
(Fig 5) and appearsto be of a greaterampli
with the trough in the middle of Error
gravity low is' however' unlike
u. explained simply by u""il"nt tttiti' This
i"i" irt""
"becauseit is dile,ctly associatedwith a
the low associatedwith the northern quiet zone
that associatedwith the northern flank of
basementfeature. A broad gravrty low similar to
The regional level of the free-air gravrty
irr"-ifr"rut rt* Ridge is noifound at Error Ridge
--o mgat t the northern Somali Basin about 50km
anomaly field increasesfrom uuoui
magneticquiet zone'
,outft of ettot niage to about 0 mgal in the southern
Rifting of oM oceaniclithospherc
61
between the southern
There are insufficient data to define the nature of the boundary
the
southernmagnetic
whether
magneticquiet zone and the ShebaRidgeor to dete{mine
to
the
northern quiet
similar
a
manner
qul"at ,on" changescharacter from west to east in
exist'
may
a
change
such
that
,'on". fto*.urr, clomparisonof the bathymetry suggests
Model of lithosphericrifting
of the ShebaRidgeare
The magneticquiet zonesand ridge complexesat the easternend
'hingezones'to the west
and
zones
quiet
in the s'amestructuralposition as the magnetic
Gulf of Aden, formed by the rifting of the continentallithosphere
within the geographical
inside the
of Arabia and Africa. The similarity of the t'eaturesnear the continental margins
Ridge
East
Sheba
between
found
Gulf of Aden and other rifted marginsto the structures
the
from
resulted
region
latter
the
and the oceanicbasinsto the noith and south suggest
rifting of oceaniclithosPhere.
quiet zones in the East
Tlie basic similarity between the morphology of the magnetic
Gulf of Aden where
the
withln
ShebaRidge where oceanic lithospherehas been rifted and
the composition of
or
thickness
the
continentai lithospherewas ifted suggeststhat it is not
of
the maBin and the
development
the originalcrust that is the major factor in the overall
of the entire
structure
th€rmal
and
resultingstructuralelementsas much as the thickness
litho'
nf
old
oceanic
lor
lithosphlere.Thus the conditions and processesrequired
-ttlllnt
A
comparison
lithosphete
sphere must be similar to those for the rifting of continental
lithosphele thus should give
between the rcsults obtained liom oceanic and continental
mechanisms'
additional information on and insight into the rifting
MuchoftheSubsidenceofriftedcontinentalmarginsappealstobeduetothelmal
proposedto
(Sleep l97l : Watts & Ryan 1976).A number of modelshave been
processes
elevation
flow
and
heat
distribution,
explain this observation which predict temperature
result
as
the
motions
the
vertical
explains
vriifr,irn.. The simplest modet (McKenzie 1978)
gradienl
This
temperature
linear
initial
an
of horizontal extension of a lithosphere with
po
lon
toP
the
with
extension,
two-layered
simple model has sincebeen modified to include
by
dyke
also
extension
and
below
than
factor
fitfrotpn"r" stretched by a different
oiii"
resultingin a
intrusion (Rtyden, Sclater& von Herzen 1980r Royden & Keen 1980)
different tempetaturedistribution.
extensionrs
The initial changein elevation from isostatic readjustmentafter lithospheric
density,
average
the
incleases
that
thinning,
of
clustal
effects
due to the cornbination of the
and
ofthe
thermal
Models
density
average
the
decreases
and to lithospheric heating, that
thick
km
a
30-35
assumed
have
typically
margins
mechanicaldevetopmentof continental
lithospheric thickness'
continental crust, which makes up approximately a quafer of the
ple-Iift
clust of this thick'
a
containing
lithosphere
continental
uniform extension of the
is roughly half this
nessalwaysproducesinitial subsidence.However,if the crustalthickness
and
the result is inilial
amount, heuilng effects will be more significant than crustal thinning
and oceaniclithouplift. ihus on" diff"r"n". between the uniform extensionof continental
becausethe
latter'
sihere, accordingto these models, is that initial uplift-will re^sulifor the
and litho'
crustal
oceanic crustal thickness is typically 5-7km. Regardlessof the injtial
and at
time
with
spheic thickness after rifting, heat flux and elevation will decrease
before
than
deeper
be
equilibrium (assumingthe crust is thinned), an extended area will
riiting sincethe equilibrium depth dependsonly on the crustal thickness'
by Steckler
Th'e proceduri used in the theimal modelling follows that developed
conducheat
of
lateral
(1981, 1'985)and Cochran(1983). It explicitly includesthe effects
and
(steckler
l98l)
tion ."ross the horizontaltemperaturegradientsset up by the rifting
of an extendedrather than instantaneousrifting event (Cochran 1983) '
62
C. A. Stein and J. R. Cochran
Most models assumeinstantaneousrifting; however, geologicalevidence suggeststhat
rifting of the continents and initiation of seafloor spreadinggenerallyoccurs over tens of
millions of years. A finite length period of rifting is specifically included in our numedcal
calculationsduring which the extension parameterp increaseslinearly. The €ffects of finite
periods of slow extension have been discussedby Jarvis & McKenzie (1980) and Cochran
(1983). During and just after a period of extension lateral heat flow will significantly
redistribute heat and change the temperature distribution, especiallynear the boundaries
of the extendedlithosphere(Steckler& Watts 1980, 1981).
Also, most models of rifted continental lithosphere have assumedan initial' linear
temperature distribution with depth. This greatly simplifies the mathematical treatment.
Howeyer, the method which we use permits an initial temperature gradient appropdate
to the oceaniclithosphereof a given ageto be assumedat the beginningof rifting The values
of constantsused for thesenumericalcalculationsare given in Table l.
Table l. Valuesof constantmodel parafieters.
Parameter
Value
Lithosphericthickness
Oceaniccrustalthickness
CrustaldeNity (0"C)
Mantledensity (0"C)
Water d€nsity
Coefficientof thermal €xpansion
Asthenosphericiemperature
Thermal conductivity
lzJxm
5km
2 . 8g c m - '
-'
3 . 3 3g c m
1 . 0 3 gc m - 3
3 . 4x t 0 - 5 ' c ' '
1333"C
3 . 1 4W m " C '
Model constraints
BASEMENT DEPTHS
The most easily observablequantity which dependson the specific mechanismof rifting is
the basement depth within the rifted region. At a well sedimented passiYecontinental
margins,the subsidencehistory can be determineduslng biostratigraphicdata obtained from
deep wells or by extrapolation of known stratigraphy to the study area using seismic
reflection records.Thesesort of data are not aYailablefor the East ShebaRidge quiet zone'
However, the presentdepths can be used as a constraint providing one point on the subsi
dencecurve l0Myr after the end ofthe lifting event.
The basement depths used to constrain the modelling must be correct€d ibr sediment
loading. The sediment cover north of anomaly 5 on the East ShebaRidge in the quiet zone
and Owen Basin generally rangesfrom 0.5 to 2 s (Fig. l0). Sonobuoy data coll€cted during
V3617 and published data from Whitmarsh(1979) were used to determine sedimentseismic
velocities and estimated densitieswith depth in the sediment column (Table 2) for these
calculations. An Airy type compensation was considered adequate because of the
moderatelysmall thicknessand evendistribution of the sediment.
The height and appearanceof the Sharbithat Ridge complex vades substantially from
profile to profite along its length. It consistsof a fairly continuous ddge with a number
of large peaks spacedalong it and hasthe appearanceof a volcanic ridge extruded on top of
the crust, although it lacks the large magnetic anomaly often associatedwlth rapidly
constructed volcanic features. Thus, its rclief would not be predicted flom the simple
thermal models. Therefore, no attempt will be made to match the specific shape of the
ridge,but rather a smooth surfacethrough its basewill be assumedas a datum'
Rifting of old oceaniclithosphere
63
of two-way travel time frofi northern part of study area Area
Figure 10. SedimentisoPachsin seconats
by sedimentlesi than 0.5 s thick is stippled. Light lines show location of Neismicreflection and
co-vered
Numbers
stars show locations of heat flow measurements.
refraction alata usealto consttuct the map.
_'.
rnW
m
next to s(arsaie heat flow valuesin
Table 2. Sedimentparametersusedto calculateunloadedbasementdepths
Depth (In)
0-600
600-1750
>1750
-')
Velocity(km s
Density (g cm-3)
1.85
'r7
3.2
HEAT FLOW
The secondobseryablequantity which dependson the exact nature of the dfting mechanism
and temperature is the surface heat flow. This is a particularly s€nsitiveparameler fot
locations, such as the ShebaRidge, where the rifting has occurred recently' Three heat flow
64
C. A- Stein and J. R. Cochran
Table3. Heatflow stations.
c.adre.c
n'c/n
{ea! nqd
n /n'
! e s ! e m r l a s n e t r .a u l e t z o n e
D
71.4
4ra9
30.1
16.1
r7.3
Lr07
4037
4o6t
t?.2
F
G*
H
56.1
1600.7
03.6
a*
l7'25.0',N t9"04-r'E3653
l66t
21.9
00.t
59.2
3667
Fasrern xscnetrc auret bne1 5 " 4 3 . 3 ' , N5 3 " 1 r , 7 ' E 4 0 3 5
( - 0.951 t:lts
/ '"c
- 1.016 salcs / fr"c
*Instrumenttilt between8" and40".
profiles in the northern magnetic quiet zone and one in the Owen Basin were obtained
havebeen published
dviag Vema cruise 3617 in 1980 (Table 3). No previousmeasurements
for this region. The locations of the heat flow valuesare indicated on the sedimentisopach
were madewith the Lamont-Doherty Geological
map (Fig. lO). Temperaturemeasurements
Observatory digital heat flow instrument, employing five thermistors mounted on a 5-5 m
spear with a sixth thermistor placed on the core head to measurebottom water temperatures. Temperatures,water prcssureand instrument tilt are recorded every 30s in digital
form and stored on magnetictape in the instrument aswell astransmitted acousticallyto the
surfaceusing a l2kHz pinget.Instrument tilt is indicated if the angle is greater than 8' or
4Oo, Thermal conductivity valueswere determined from piston cores taken during cruise
V3617 using the needle probe technique (Von Herzen & Maxwell 1959) and corrected for
rn situ conditions (Radcliffe 1960).
All thermal gradients were linear (Fig. 11) within the measurement error of the
thermistors (approximately 10.005"C). The averagestandarddeviation of the temperature
gradients fits is approximately a few thousandthsof a degreeCelsiusper metre, a few per
cent of the measuredgradient.The quality of eachheat flow measurementis evaluatedusing
a zero (bad) to lO (excellent) scale(Langseth& Taylor 1967). Only those greateror equal to
six are listed in Table 3. The local sedimentaryenvironment near each measurementhasb€en
eyaluated using the catagoriessuggestedby Sclater, Crowe & Anderson (1976). For this
cruise, stations were in only two (A and B) of the categoiies.Type A environmentsare on
thick and relatively uniform sedimentedregions with all basement relief covered and no
outcrops within 10km. Type B environments are similar to A but have outcrops within
10 km of the station. Most of the heat flow valuesin Table 3 are type A regionsHeat flow valuesin the quiet zone tend to decreaseslightly northward towards Sha$ithat
Ridge and also tend to decreasesubstantiallytowards the Owen fracture zone. The western-
Rifting af old oceanic lithosphere
65
DELTATEMPERATURE..c
"l*quietzoneandstation
prcfiles.Stations66,68 and69 arein the magnetic
Figure11.Temperaturc-depth
6? is in the O$/enBasir(seeFig.10andTable3 for locations).
In thisligure,the depthsof theindividual
t€mperature
Fadingshavenot beencolrectedfor the angleof penetration
ofthe spear.
most measurements(station 66) weretaken very closeto profile V35-3 (Fig. 3) located in the
western zone. Measuredvaluesmnge from 83 to I 18 mW m-'. There is an averageof a halfsecond of sediment cover over very rough basementwith some nearby outcrops. The two
heat flow profiles (stations 68 and 69 in the eastem zone, near seismiclines V36-3 and
V36-i respectively, have significantly lower heat flow averagingabout 75mwm-2. The
easternmoststation is located about 100km from the Owen fracture zone over the narrow
grabenlike feature with sedimentthicknessgrcaterthan 2 s which was describedpreyiously.
Station 68, about 70 km further to the west, is on top of datiyely smooth basementwith
somewhat more than 1s of sediment cover. The averagedifference in heat flow between
station 66 and the two stationsto the eastis about 20 mW m-2. It might be expectedthat the
thermal blanketing effect of the sedimentsmay account for some of the differerce in the
ayerageheat flow between stations 66 and 68-69. Assumingthat all of the sedimenthas
been depositedin the last 25 Myr, the correction,using the techniqueof Langseth,Hobert &
Horai (1980), will be at most a 5 per cent increasein the heat flow valuesfor the westem
stations and 15 per cent increasefor the easternstations. Thesecorrectionsare insufficient
to closethe measuredgap.
Station 67 is located just north of the end of seismicprofile V36-3 (Fig.7) in the Ow€n
Basin.The measurementsare 130km north ofthe Sharbithat Ridge Comptex axis and these
measurementswere taken to establisha heat flux for the Owen Basin to comparewith the
quiet zone. The basement is relatively smooth with at least about l.5s of sediment.The
heat flow measurements(with the exception of 67F) are all much lessthan heat flow in the
magnetic quiet zone, but higher than theoretically expected for old oceanic lithosphere
@arsons& Sclater 1977). However, the valuesare near world-wide heat flow ofold regions
of about 52 mW m-'? (Sclater,Jaupart& Galson 1980).
D U R A T I O NO F R I F T T N G
Seafloor spreadingat the East ShebaRidge began about l0Myr nr. A number of types of
data suggestthat the dfting event beganabout 25 Myr Bp. The uplift and westwardtilting of
3
66
C, A. Stein and L R. Cochran
the high ridge of the Owen fracture zone beganduring the late Oligoceneor earliestMiocene
time (DSDP sites 223 and 224, Whitmarsh,Weser& Rosselal. 1974a,b). Large-scalefaulting resulting in the uplift of the Arabian plateau beganin Late Oligocene(Beydoun 1970),
westwardfrom RasFartak (Beydoun 1982)'
and in the earliestMiocenethe seastransgressed
(starting at 25Myrrr near the
of
extension
Thus we will assumea lsMyr leriod
ofcooling.
by
10
Myr
boundary),followed
Oligocene-Miocene
INITITAL THERM AL STRUCTURE
The age of the Owen Basin has not been determined. Whitmarsh (1979) reported the
presenceof ENE-trending magnetic anomaliesnear 19'N in the Owen Basinwhich havethe
appearanceof seafloor spreadingrnagneticanomalies.It has not. however, been possibleto
identify a specific sequence.Two DSDP sites (223 ^nd 2241 seeFrg.2 for locations) drilled
in shall0w areasnear the owen fracture zone reachedbasementwhich in both l0cations
consistsof early Tertiary (57 and 51.5Myr respectively)mafic volcanic rocks (Whitmarsh,
Weser& Rosset al- 19':'4a,b). These rocks are similar in age to the early Teftiary seafloor
te the east in the Arabian Sea and presumably reflect events related to spreadingon the
CarlsbergRidge rather than the creation of the Owen Basin.
The Owen Basln is probably not related to the Oman Basin (Fig. l), to the north'
structural trends in the oman Basin, as indicated by the samail ophiolites (Pallister 1981;
Tilton, Hopson & Wright 1981), are different from those observed in the Owen Basin
(Whitmarsh 1979). A more likely explanation is that the Owen Basin was formed by the
breakup of Gondwanaland.Thus, it would be expectedto be of a similar age10 the southern
Somali Basin and Mozambique Basin, where Late Jurassicand Early Cretaceousmagnetic
anomaly sequenceshave been identified (Simpson et al. 1979;Rabitowitz, Coffin & Falvey
1983).
Thus one would expect thick cold lithosphere to underly the Owen Basin. However,
geophysicalparametersfrom the Owen Basin away from the uplifted podions of the Owen
fracture zone are not consistentwith these conclusionsand indicate a thinner lithosphere.
Specifically the basement depths, about 4700m after correction for the sedimentloading,
and the heat flow values (Table 3) are what would be expected from a region about
40-70 Myr old.
Thus two sets of conditions will be used as €nd members to represent the possible
extremes of the initial temperature-depth relationship at the initiation of rifting
25Myr ar. Modet A is a thin lithosphere with the initial thermal structure of 15Myr old
oceanic lithosphere.Model B has a thicker lithospherewith the initial thermal slructule of
125Myro1d oc€aniclithosphere,consistentwith a late Jurassicorigin ofthe Owen Basin'
Since the present data set only includes th€ present depths and heat flow and not the
changesin these parametersthrough time we sha1lnot attempt to distinguish between the
two initial conditions.
Model calculations and results
Profile V35-3 (Fig. 7) was chosento mod€l becauseit is a typical profile acrossthe region in
which the various features of interest are present and well developed.The profile is also
quite close to heal flow measurements(stations DHF 66), is far away from any possible
complication effects of the Owen fracture zone, and is also not near East Sheba Ridge
fracture zones.
It is assumedin the modelling that the amount of extensionincreasesacrossthe magnetic
quiet zone south towards the East ShebaRidge. The crustal stretching is the samefor bolh
o/
Rifting of old oceaniclithosphere
HEAI FLOW
;
{
B DISTRIgUTION
lvodelAAB(Crusl).
t:
B',1
B Dislributionfor
l,,lodelB subcruslol
;l
P31
SW
I
50
r0o
lnitf.l""::,:Tl:
_-_$:::l3ii[-'llffii:""
of slov extension
Filure 12, Comparisonof observedanalmodelled depths and heat flow after tsMyt
observedbaseto t!Iy. or post-nlft cooling, unloaded basemelt atepthswele calculatedby coltecting
Ridge
that
Sharbithat
"ni
assufies
The
model
compensation
type
Aiiy
ment forieOiment foaOingassuming
isavolcanicadilitionontopoftheclustandmatchesalinethroughitsbasemthertha.rtheexactshaDe
depths and
of rialge,which variesgreatly fiom profile to profile. Both models A and B result in similar
model B' with
heat fiow. For model A, the thin tithosphereanalcrust ate extendedby sameamount For
of the lithosphere
thick tithosphere,the crust is strelchedby the sameamount asfo' model A but the rest
is tlunned much morc in order to match the observeddata.
qxiet zone/East
end member model calculations, reaching a maximum of P=1'9 at the
thinned in
Sheba Ridge boundary. The original 5 km oceanic crust ir the quiet zone is thus
not
recorded
the model to a minimum of only 2.6 km. The quiet zone crustal thickness was
from sonobuoy refraction experiments. For model A, the thin lithosphere column was
For model B considerably more subcrustal thinning was necessary to
stretched unifomly.
by
matsh the geophysical parameters in the quiet zone. Extension of the lovr'er lithosphere
This
below'
a greater amount than the crust is used to simulate additional heating from
to
rodrl ulro requires a 100 per cent thinning of the Owen Basin's subcrustallithosPhere
heat
flow
match present-daydepths and heat flow. Both models result in similar calculated
and depths which are closeto the observedvalues(Fig. 12).
the
Fig. 13 shows calculations for both models when only the subcrustalportion of
flow
lithosphere is thinned by an amount similar to the calculationsin Fig' 12' The heat
as
valuespredicted by the two models are similar to each other and to the obseryedYalues'
the
one would expect since the thinning of the crust does not significantly change
temDeraturedistribution. HoweYer,the depths in the quiet zone are 100-400 m too high
68
C. A. Stein andI. R. Cochran
s
/
5
9
'
-
(Jnooded bo$meir deplhs
--
ModerS rhtrr
P
hosphtu€
Figure 13. Comparisonof obseryealand modelled
depth and heat flow resulthg tiom 15 Myr of slow
exlension and 10Myi of post-rift cooling, Only the
subcrustalportions ot_tfre lthosphere have been
thinned, by amounts similar to subcrustalthidning shown
in Fig. 12. Not"-tlr"t tt" rn"rp oUseryedbase_
ment deepeningis not rcpmducedand the predicteddepths
ale t;o shdio; in the quiet zone.
sharp.basement
flo1 th9 Sheba tudge inlo rhe quiet zone is nor rcprc1ii Ti,*"
_drop
duced.
-lhu,scrustal tfunning (or equivalently an increasein the averagecrustal
densityfis a
significant factor in matching the observedbasementdeDthsThe amount of extension in the easternmostGutf of Aden estimated
from plate reconstructions can be used as a check on the amount of extensionpredicted
from oui modelling.
The amount of total opening estimatedin that manner (Cochran
19gt,1982b)priorto the
initiation of seafloor spreadingat the latitude of profile
V35_3 is about 95 t 35km. The
amount of extension for both models (Fig. 12) acrossthe northern quiet
zone is 56km or
45 per cent of the original width. If the amount of extension in
the southernquiet zone is
similar this implies a totar of 112 km of opening prior to l o
Myr Bp. The two estimatesare
in reasonableagreement.
Lithospheric thickness and rifting
The calculatedtemperatureswith depth at the southernend ofthe
nofihern magneticquiet
zone just at the initiation of seafloor spreadingare shown in Fig.
14. since both ofthe .end
member' casesconsideredhad to match the sameelevationsanJheat
flow following lOMyr
of cooling it is not suryrising that their temperaturestructurcs
at the time of initial seafloor
spreadingare similar. The temperatureprofile at the time of initiation
ofthe seafloorspreading conespondsto the temperature profile for oceanic lilhosphere
between l0 and l4Myr
old. The old oceanic [thosphere of the Owen Basin was greatly
thinned before seafloor
sprcading began. similar amounts of lithospheric thinnini have
been deduced at other
Rifting of old oceaniclithosphere
69
Temperolure
fC)
E
;
-
[,4odelA lhin lilhosphere
--- Model B lhick liihosphere
of tempe$turesttuctulefor modelsA aad B (ftotn Fig l2) at EastShebaFigure 14. Compadson
strucwith temperatule
comPared
quiet
zoneboundaryafter15Myr of slowextension
nolrthernmagnetic
turefo! lO and14Myroceaniclithosphere.
continental margins where extensional models haYe been applied' Thus thin lithosphere
is requfuedbefore the generationof new oceaniclithospherecan commence'
New spreaclingcenires are alsofolmed on oceaniccrust through the processof ddge crest
Iriany such exampleshave been documented around the world, the most notable
iumps.
"being
the rcorganization of the East Pacific Rise system (Sclater, Anderson & Bell 1971;
Andirson & Sclater 1972: Cande,Herron & Hall 1982i Mammerickx & Klitgord 1982)'
Almost without exception these :jumPs' occur into lithosphere no more than 10Ma
(Table 4). The typical morphology associatedwith xheboundary betweenthe new crust and
otO ut
riag" jump is much sharper than for Sheba Ridge and resemblesthat normally
"
et dl 1982)' Also the time
associatedwilh'a fracture zone (Anderson& Scl'rltel 19'721,Cande
the
required for the new spreadingcentreto developappearsto be very shofi comparedwith
at
reported
been
have
l5ivlyr required at ShebaRidge or times approaching50Myr which
observations
Both
1979)'
some coniinental margins (Jansa& Wade 19751Talwani et al'
'ridge crest jumps'
reflect the already thin lithosphere near existing ridge crestsinto which
normally occur.
Both of the situations mentioned above,the rifting of old lithosphereto form a magnetic
quiet zone and ridge crest jumps, sharea common denominator, the need for a thin lithoqphere. The features found in our study are (Fig. 1) are a direct result of the necessity
begin at
substantially to thin the initially thick lithosphere before seafloor spreadingcan
'70
C. A. Stein and,L R. Cochran
Tlble 4, Ridgecrestjumps.
rtft€d
Abandoned spr*drn3
atheullcr,n
(n.r.)
drrt.s
cenler
seanounB (soulh)
(northl
,tnb.ctrc-Pacrf
lc rpr.adlnC
cen!€r northvatd propdga!1o.
b€tveen Tub 6 Hunboldt
calap.sos spreadtns
(lrdsE cres! junos and propaeartnE
rlfld)
References
aMammerickxe/ cl. (1975) and Mammerickx,Herron & Dorman (1980).
bMammerickx& Klitgoid (1982) and Klitgord & Mammerickx(1982).
ccandeer al. t1982),
dHey, Duennebier& Morgan(1980).
*Spreading began on the East Pacific Rise 8.2MyrBP, concutrcnt to slow
spreadingat the GalapagosRise.
the East Sheba Ridge. The thickness ofthe lithosPhele at the initiation of seafloor spreading
is similar to the maximum tfuckness into which ddge crest jumps have occurred. This
corespondence suggeststhat there is a maximum lithosphericthicknessinto wbjch seafloor
sprcadingcan occu. This maximum thickness (about 40-45 km; Fig. 14) correspondsto
thermal structure of about 10Ma lithosphere.
Structulal changesnear the Owen fracture zone
The models which we have discussedin the preYioussections stictly apply only to the
western zone. As the Owen fracture zone is approached,the structures obseryed on the
Sheba Ridge and in the quiet zone change.The ridge flank depths increaseand the ridge
'cooling curve'shape. Instead it takes the form of a region of rough
loses its characteristic
bathymetry with a relatively constant mean depth of about 3500m (Fig.7, profile V36-l).
The changein the morphology of the ridge is accompaniedby a decreasein the amplitude
of the magnetic anomaliesbeginning at about l30km from the Owen fracture zone. Also,
as th€ Owen fracture zone is approached,the sharp topographic brcak between the ddge
flant and the quiet zone disappearswith the result that the ddge and trough morphology
extends further north and the distinction between ridge flank and quiet zone becomesless
clear.
The Australian-Antarctic Discordanceon the SoutheastIndian Ridge (120'E-128'E) is
another section of the mid-ocean ddge characterizedby greater than normal depth, lossof
the charactedsticmid-ocean ridge shape and poorly developedmagnetic anomalies.These
observationshave been interyrcted as indicating that the Discordanceresulted from downward asthenosphericflow with low crystallization temperatures for the oceanic crust
(Weissel& Hayes 1977;Ande$on et al. 1980). Although there is not an exact analogy,in
part becausethe Australian-Antarctic Discordancecovers a much larger area, the obser-
'7L
Rifting of old oceanic lithosphere
vations on the East Sheba Ridge also suggesi the presenceof lowered upper mantle
tempeiaturesnear the Owen fracture zone. This conclusion is suppofied by the decreasein
heat flow valuesin the magneticquiet zone toward the Owen fracturc zone.
One mechanismwhich might be expectedto produce lower upper mantle temperatures
is lateral conductive heat flow acrossthe Owen fracture zone. The lithosphereto the eastin
the Arabia Sea is about 50Myr older than the adjacentShebaRidge lithosphere and thus
there will be a significant temperature contrast acrossthe Owen fracture zone. However,
simple calculations show that this effect will only significantly decreaseheat flow and
increasedepths within about 5okm west of the fracture zone, not enough to account for
the valuesat stations 69 and 68 which are 100 and 170km respectivelyto the west ofthe
Owen fracture zone. Although the specific mechanismis not clear, it appearsthat the
normal convectiveflow to the mid-oceanridge is not aswell developedat the easternend of
Sheba Ridge as further to the west resulting in the lower than normal asthenosphericand
lithosphedctemperatures.
Conclusion
Magneticquiet zones are located throughout the Gulf of Aden to the north and south of the
Sheba Ridge from Afar to the Owen fracture zone. This includes the eastenmost region
where oceanic lithosphereof the Owen and Somali Basinswas rifted to form the East Sheba
Ridge. In this area the boundary between the quiet zonesand the old oceaniclithosphereis
marked by ddge complexes,the Sharbithat and Error Ridges(to the north and south respectively) which have no significant magnetic anomaly signatureand are associatedwith laBe
negatiyegravity anomaliesthat cannot be explainedby basementfeaturcs.The quiet zones
are characterizedby low and uncorrelatablemagnetic anomaliesand deeperbasement(by
about 1km) than the adjacent lOMa East ShebaRidge crust. In the westernzone the boundary betweenthe East ShebaRidge and no hern quiet zone is marked by a sharpincreaseto
the north in the basementdepth.
The well-stratified sedimentsof the Owen Basin abut againstthe northern flank of the
Sharbithat Ridge with no signsofdisturbance. This suggestseither that the Sharbithat Ridge
was formed early in the dfting history prior to the deposition of most of the sedimentsof
that it pre-datesthe opening. On some profiles across the magnetic quiet zone and the
southern flank of Sharbithat Ridge the lower half of the sediment cover appearsto be
disturbed and in some casesis upturned on to the ridge flank. This indicates possible
tectonic activity before the deposition of the relatively flat'lying upper layers. Thus the
major phaseof tectonic activity responsiblefor the develoPmentofthe magneticquiet zone
appeanto have occurredduring the early stagesof developmentof ShebaRidge.
The magnetic quiet zone regionsappearsto havebeen produced by the diffuse extqnsion
of old oceanic lithosphere. They formed during the opening of the Gulf of Aden and the
fomation of the magneticquiet zones along the continental marginsdudng a fifting event
from 25 to loMyrBP. Using a variation of stretching models developed by McKenzie
(1978), Royden, Sclater & von Herzen (1980), and Steckler (1981), which permits finite
length dfting events,it is possibleto match observeddepths and heat flow values'The model
results imply that during the dfting event the crust of the magneticquiet zone was extended
about 45 per cent. During extension the lithosphere was thinned to a thermal thickness
similar to that of l0-l4Ma oceaniclithosphere. Examination of ridge crest jumps suggest
the maximum age of lithosphere into which these jumps occur is about 10Myr. These
observations,as well as the large amounts of tfunning deducedat rifted continental margins,
indicate that lithosphere must be thianed to a thickness similar to that of 10-14Myr or
younger oceaniclithospherebefore seafloorspreadingis initiated.
C. A. Stein and L R. Cochran
72
Acknowledgments
of the R/V Vema for their help
We thanl< Captain H. C. Kohler and the officers and crew
and
Michael steckler who helped
Roach
thank
David
V"-a 3617. We also
dudng the
"rui"e
and interyreting the heat
analysing
in
aicled
,o g"il.rt ,ftt shipboard data. Michael Hobart
Sciences for providing
ocean
of
Institute
the
of
floi dat". we th;nk Dr Robert Whitmanh
reYiewer for their
an
anonymous
and
P
Styles
and
Dr
us with data from the RIV Shackteton
Tony Watts
Morgan,
w.
Jason
Hobart,
Michael
Alderson,
comments and suggestions. Roger
by the National
weisJJl reviewed ihe manuscript This work was supported
irrrr"y
Research contract
"nJ
of
Naval
Office
afi
7919241
grant
OCE
Science Foundation
Geological Obseryatory Contribution
NOoO14-80-C-0098 Scope HH. Lamont-Dohefy
No.3747.
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