Download Simple Models of Diffuse Extension and the Pre

OC EANOLOG ICA ACTA , 1981, N' SP
Eaf-----
Con tine nt al margins
G ulf of Aden
Simple models of diffuse extension
and the pre-seafloor spreading
development of the continental
margin of the Northeastern Gulf of Aden
H eUI fl ow
Rift leClonics
Li thos phcric rif ling
Marges con ti ne'lIales
Go lfe d' Auen
Flux de chaleur
Tectonique des rift!;
Rifling lithosphé rique
J. R. Cochran
Lamont-Doherl y GcoJogical Observatory of Columbia Universit y, Palisades. New York
10964. USA .
ABSTRACT
The pre-seafJoor spreadi ng developme nl of the continental margin of the Northeas tern Gulf of
Aden is invcstigated us ing s im ple modcls for diffuse extension th ro ug h lîstric fa ulting and/o r
dike intrusio n. Il is found Ihill provided dike inlrusion makes up a s ignificanl par t o f the
extension mec hanism. the s imple model can adcquatcly cltplnin the obscrvcd heat fl ow and
basement depths without violating eonstrain ts placed o n the tota l amoun t of exte ns ion by
e rus tal th ic k nesses a nd pla te geome try ,
The infl uence of va rio us fa ctors o n the behavior of the models is inves tigated, and they are
round to be qui te sensiti ve to the c hoice of initial cr ustal parame ters and to the dura tion and
nat ure of the pre-seafloor s preading e x te nsion , T hus to adequa tely understand the rifting
process a nd the de velo pment of continental margins, it is neçessary to obtain data from well s
d rilled Înto the synrift and basement roc ks , and to stud y areas suc h as the Nor thern Red Sea,
which arc presently in the pre-scafloor spreadi ng phase of the ir e volution,
OcewlOl , At:/(J , 1981 , Proceedings 26,h Inte rna tio nal Geological CongreSs, Geology of
conti nental margins symposium , Paris, Jul y 7-17 , 1980, 155- 165,
RÉ SUMÉ
Modèle s s impl es d 'exte nsion diffu se et développemen t pré-accrétion océa nique de 1<1 marge co ntine ntale du Go lfe d 'A den nord-orie nta l.
Le dévelo ppement de la rn:lrge co ntinentrale du Go lfe d ' Aden no rd-orien tal pré cédant le s tage
d 'accré tio n océanique est étudié à l' aide de modèles s imples d'extens ion diffuse par fa illes
ou/et intrusion mag ma tique, S i l'o n ad met que l'intrusio n magma tique constitue une part
s ign ificulive du mécanisme d'extensio n, on observe q ue le modèle s imple expliq ue c orrecteme nt les flux de c haleur et les profo nde urs de s ubstratum mesurés, sans violer les contraintes
s ur l'cxte nsion tO lrale fo urnies par l'épa isseur de la çroûte ct la géométrie des plaques,
L'influence de nombreux facle urs sur le comporteme nt des modè les est eX:lminée, e t o n
montre quc ceux-ci sont sensibles a u c ho ix des paramètres c rus tau x initiaux, ains i qu 'à la
durée ct la nra ture de l'ex tens ion pré-accré tioll , Ainsi pour comprend re correc te me nt le
proçessus de d is tension et le dévelo ppement des m:uges co ntinen tales, il est nécessaire
d 'obtenir des do nnées de pUi lS foré s da ns les roç hes s yn-dis tens io n et celles d u subs tratum . ct
d'étudier les doma ines tels que la Mer Rouge se ptentrio nulc, qu i sont actuellement d;lOs la
phase pré-accrétion océlinique de le ur évolution ,
Oela/wl. Acw, 1981. Actes 26" Congrès InternatÎoml1 de Géologie , co lloque Géologie des
marges contÎnenta les, Pa ris . 7- 17 juil. 1980, 155-165,
155
J . A. COCHRAN
INTRODUCTION
been applied to several contine ntal margins (Royden. Kcen.
1980; Le Pichon. S îbuet. in press ; Steckler. Watts. 1980:
Cochran. in press). The purpose of Ihis paper is to app!y thb.
model 10 the young continen tal margin in the Gulf o f Aden
a nd , in doing sa. to examine how vario us parame ters affec t
the mode!. The conclusion is Ihat , al1bough the diffuse
extension model can fit the observed behavior of th îs <lod
other continental margins. there is enough uncertainty and
extrapolation involved in its application. thal a de tailed
understanding of the processes involved in continental
rifting. and the initiation of seafloor spreadins depends on
oblaining biostratigrap hi c dala from wells inlo the pre· and
syn·rift sed iments. which has nOI yet been donc and on
studying l-Irel-ls such as the Northern Red Sea. which appears
to still be in the diffuse extension stage of ilS evolution.
T hcrc has becn a grcal cmphasis during the past scveral
yea rs on slUdying the s tructure and evolution of continental
margins. Until recently, ve r y !iule was ac tually known of
this region where ocean and continent meCI, and Drake and
Burk (1974) were able \0 c haracterize continental margins as
"a helt of ignorance extending sînuo us1y aç ross the Earlh's
s urface··. Much of Ihis Jack of knowledge resuhed from Ihe
difficulty of obtaining re1iable data, since the sreat quantity
of sediments (as muc h as 15 km ) on the much siudied
margins of the Atlantic has prevented observai ion of the
base ment s lructure by seismic methods, as weil as
obviously preventing direci sam pli ng 10 determine the
nature of the basement rocks. Scismic refraClion dala are
a lso difficult 10 înterpret . giving a helerogeneous structure
usually with a layer ha ving a ve10city of l-Iboul 7.4 km/sec.
rather than normal mantle velocities as Ihe deepest refraclor
(Drake, Nafe. 1968).
Since the mid 1970·s, Ihere has been an inereased efforl 10
undersland the nature of continental margins. both because
of Iheir reSOUTce potential and because man y of the more
eas i1 y attacked problems related to the age and evolulion of
the deep ocean bl-lsins have been solved. This increased
interest. along with tec hnical advances s uc h as the de ve lopment of multichannel seismic methods and the increased
number of deep explora tory wells o n continent<ll s hel ves,
has 1ed 10 a beuer understanding of the behavior of
continental margins.
Bioslratigraphic data from exploratory wells showed that
much of the sedime nt pile is made up of shal10w water
sed iments (Jansa. Wade . 1975: G radstein et al., 1975).
which would not he expcctcd mercly (rom the fill ing of a n
exi~linl"l b:l~in . Steep (1971) correcle!! Ihe <;Llh~idence rale~
recorded in wells from the US coastl-ll plain for the effects
of sediment loading , and found that the remaining subsidence was both s ubs lantial , and s howed an exponential
decay in time with a time constant of about SU m.y. T his is
the type of response expected from the cooling of a heated
lithosphere (Sclater, Francheteau, 1970). and this observa·
tion led 10 a Great emphasis o n the role of thermall y
produced vertical motions at a continental margin. It was
ve ry quickly realized Ihl-lt li mcchani sm is needed to cause
the crust to subside below sea level since. if the litbosphere
is simplY healed, it will expand and then COniTact back to its
original position. Earl y models suggested that additiomtl
s ubside nce could bc caused by crustal thinning through
erosion of the therm<llly uplifted crust (Sleep, 1971 : Kinsman. 1975) or by densification of the lower erust through
thermally induced phase changes (Falvey, 1974). These
mode!s s uUer from the d iffi culties that al man y margins . the
initial dom ing stage which the y requiTe d oes not appear to
have occurred (for example. MontadeT! et al., 1979:
Cochran, 1981). and that they cannot reasonably produce
the required amount of subsidence (Steckler, Walls. 1978).
This is pl-lrticularJ y truc of the crustal erosion model. which
wo uld require 17 km of erosion to satisfy the observed
tectonic subsidence in the ij·2 weil (5teçk1er. Walls, 1978).
Tbese difficulties, along with the observation of bl-lsement
fealures which have been interpreted as rolated fault blocks
on MCS lines across some continental margins (de Charpel
et al. , 1978), have led to another class of models which
produce crustal and lithospheric thinning through diffuse
extension. Variations of this mode!. fir st proposed by
MeKenzie (978) to explain the behavior of areas of
extension and subsidence within a continent. have recently
MODELS FOR THE DE VELO PME NT OF A CONTI·
NENTAL MA RGIN
The s implest model which has been suggesled for d iffuse
extension and the s ubseque nt evolution of the extended
arel-l is tbe '·stretc hins· · or "atte nualion " model proposed
by McKenzie (1978). This model s tarts with fi unit le ngth of
lithos phere of th ic kness t with a crust of thîckness t. , over
an isothermal asthenosphere with a temperl-lture of the order
of 1 300' C-l 350"C. A simple linear tempe ra ture gradient is
llssumed in the lithosphere (Fig. 1 a). At time T = 0, the
lithos phere is instantaneously extended to a lenglh 13 in
some unspec ified ma nner (Fig. 1 b). In arde r to conserve
mass. the lithospheric thickness decre<lses to t Ip. and the
crustal thickness to t;f, while Ihe ast henos phere pl-lssively
rises in response to Ihe ]ithospberic Ihinning .
The firS I result o f the litbosphe ric thinning is an isostalic
adjustement, referred 10 liS the "f<lult bounded subsidence ,.
by Sclater and C hris lie (1980). which is due 10 the fac i that
the extension res ults in changing the mass in Il vertical
column. T his adjus tme m can be determ ined by simply
balancing m.. ss col umns ~tnd is given by
S.=
'1 )71,(,
(P. - P.
T" ) .T,p,]( ' -il')
- 0 'V -~
P.(I nT ,) p~
where Po .. nd Pc are the densities of the mantle and crusi
respectively, both at O~C. p ... is the density of sea water. « is
tbe coefficient of thermal expansion ass umed to be the same
in erusl and mantle. and T l is the asthenos pheric temperature .
Once the extcnsion has occurred. the lithos phere will start
to thicken , as heat is conducted to the surface. and with
time will return 10 Îts equilibrium thickness (Fig. 1 cl. which
is determined by a b::t lance of the heat being conducted to
the surface and the heal supplied 10 the base of the
lithosphere. Since the initial and final thermal s tates are
known. the temperature distribution and th us the surface
heat flow can be delermined as 11 fun ction of time.
Knowledge of the thermal structure and thus of densities as
a functio n of time also allow the s ubsidence history of the
extended res ion to be calculatcd (Mc Kenzie. 1978: Ro y·
den. Keen, 1980).
A second model, which has been proposed by Royden el al.
(1980). is bl-lsed on extension thro ugh dike injectio n. T hi s
mechanism results in a d iffcrent temperature distribu tion
than the stretching model (Fig. 1 a). with higher tempcralU'
Tes near the surface. This heat is quickly lost resulting in
156
PRE-SEAFLOOR SPREADING DEVELOPMENT OF THE GULF OF ADEN
INTRUSIQN MOOEL
(Royden et 011
STRETCHING MODEl
(McKenZII!.19781
A
A
B 1-0
B 1" 0
l. ,.
•
1
~
L------
1'
--.,--
C 1-00
Figure 1
!~~
Simple mode/l' for cruslal exlensioll, /11 voll, model.,. a a"il lell;:11I of lililospilere (A ) Is. af I;me T _ O. l'')'I/m/(meollsly exlellded la lell;:lh ~ (8)
rnlll/l,,/: 1" Il c l,(Ullie ;11 I/le l emperalllre l'trl/(·IU'/'. [" Ille Si relchlll;: model, Ihe eXIi'IIsiO/I is " ccomplislled vy Ihilillin;:llle crllSI tllld Iilliospilue If}
11~ of 1/1I!;r o ri[.l;lIal Il!Îcknn~. fi, Ille ill/fl/slOIl /IIodel. il Is a ccli/llpli~I,ed by /III! ill/mslo" of dikes of /IImllle /II(l/eri,,/ fmm Ille uSlllenosphere.
WilI, lime Ille Iitlws{)here Ih!ckells and Ihe Ilremwl ;:rIIdlelil relilms 10 ilS orl;:illol Slale ( e). Kll o"'/ed;:e (Jf Ille illililll will fillllllempefll!lIre /lm/lile
densilies of cruslill (Uld maltl/e materil/I.r ullo",s Ille lIeul fla", and .wrfllce elel'{,/;ml 1,/ ve culeltl/ll ed /lS /1 fll ne/;on of lime.
of extension is to cause a loss of heat and thus subsidence
prioT to the initiation of seafloor s preadi ng thus res uhing in
less subsidence and a slower subsidence rate during the post
seaflOOf spreading period.
Jarvis and Mc Ken zie (1980) in'lestigatcd the eHects of fin ile
extension times a nd concl uded that ··for most basins the
simple model Bives reasonably accuratc resulls provided the
dura tio n of s preading is less than 20 m.y.· · Their results are
used in Figure 2 to s how . as a function of 13. tbe maximum
time over which extension can oecur for the McKenzie
(1978) model 10 predict tbe post-stretching s ubsidence and
heal fJow to witbin 10 %. In general , Ihe subs idence. wbicb
is lhe mos t frequenlly used parame ter, will be predicted to
within that amount if the extensional evenl is of Icss Ihan
about 15 m.y . duration.
higher hellt fl ow a nd subs idence rates during the time
immediately following the exte nsional episode. Thus.
during Ihis lime, a lower val ue of t3 would be required 10
match observed s ubsidence rates. depths or heal flow . After
abo ut 10-1 5 m .y., however, it is probably not possible to
d istinguish between tbe two models.
Dike intrusion is a n important means of extension in Afar
(Barberi el 141., 1975 ; Ch ristiansen et al., 1975). which may
be a continental margin in formation. and intense dike
s warms into the continental Cru SI ha ve been reported from
areas as diverse as thc eastern margin of the Red Sea
(Coleman et (lI. , 1975), the Atlantic margin of Greenland
(Myers. 1980) and the margin of the proto-Athmtic ·· IapelUS" ocean in New Brunswick (Rast . 1979). In spiteof thesc
observations. the dike injection model has not been used in
su bsequent sl udies, probably in large measure beca usc of
the Great impac t made by tbe discovery of fealUre s
inte rpreled as rotated fault blot:ks and listric faults on the
marg!n of the Ilay of Biscay (de C harpel et (II" 1978).
FINITE EXTENSION TIMES
The two model s dist:ussed above botb assume Înstantaneous
extension. This ass umption makes the mathematical tTeatment much simpler by pro'liding simple and weil dcf incd
initial conditions, but does not correspond 10 what is found
in nature. The pre -seafloor spreading rifting his lory of
continenta l margins appcars to last from about 10 m.y . (for
example. Cochran . 1981 ) to perhaps as muc h as 50 m.y. for
tbe southern marg!n of Australia (Konig. TalwanÎ, 1977).
The final dep lh (corrected for sed iment loading) and the
total amounl of excess heat re leased arc both independent
of the time over which the extension occurred, since botb
depend so lely o n the pammeter 1). The effec! of a finite time
Figure 2
!-~~--~c-~~~--~.,~-c~",--•.,
r ..... tm.y)
M/lximum lime as II f llll C/ili li offi Q"t'r ",/lic/, e;rlen ûo n cali IICCllr
for l 'imple Sl re/chlnl.( m odellO pretllel Il.e pm'I .rlre/dûnl.( .wfnicfe ll ce
and /11'11/ fla ... 10 ... illlÎlI 1(} %.
157
J . A. COCHRAN
CA LIBRATION OF THE MODEL
Parsons a nd Scia ter (1977) used the observationaJJy
determined val ue of 3.900 m fo r ClIO obtain es ti mlltes of
3. t 1 X 1Q- ' cC - 1 and 3.28 x to - l oc 1 for a. the coefficient
of thermal expans ion . Ir th e revised Co rm is used, along with
the val ues given in Table 1 for the o lher paramelers. a valuc
o f 3.40 x 1Q- ' oC 1 is obtained. This fiu published value~
for olivine (Skinner . 1966) a s weil as does Parso ns and
Sclater 's estima le. The subs idence a nd heat flow determined for o ur model mid-ocean ridge wi th the parameters given
in Ta ble 1 is com pared with the empirical curves o f Parson~
and Sclmer (1977) in Figure 3.
It is important not to pick the model pa ramcter~ cas ually. A
s mall c hange in the c rustal parameters c an have a relatively
large effec i on the ini lial "faull bounded " subsidence. a s
can be seen on Figure 4. The range of crustal d ensi tj e~
c ho sen by various învestigators range (rom 2,78 glcm J
(Le Pic ho n. Sibuet . in press) 10 2.9 glc m l ( Royden . Keen .
1980). This smaJJ d ifference c hanges the ctllcu lated "fa ult
bound ed" su bs idence by 700 fi l fo r ~ = 2 and by over 1 km
for ~ = 4. S imilarl y. various investigators have used crustal
thiekness of 30 o r 35 km . causing estima tes o f the initinl
subsidence to vary by 600 m (or ~ == 2 and 900 m for j3 == 4.
The parameters used in th e modelling (Ta ble 1) we re chose n
give an internally consistent. isostaticalJ y balanced system. Values of 2.8 g/cm l a nd 3.33 glc m J were chosen fo r the
O' C densities of the CruS I and mantle respeçtively and a
mid-ocean ridge crest. dcfined as a 5 km th ick cru st under
2.5 km of water, und e rlain by 1.333°C man tle. was c hosen
as a rcference Îsostlllic se c tion. This implies that. for a
125 km thic k lithosphere and a linea r thermal gradi e nt , the
pre-extension continental crust, ass umed 10 be al sea leve!.
has a thickness of 31.2 km.
10
Table 1
Values of mudd
puram~lt'r~
P.. rameter
Symbol
Value
lithosphcric thickness
continental crustal thicknes,
crustal dcnsity (ifC)
mantl ... densilY (O' C)
walcr dcnsily
çoeffiçicnl of thermal exp:lnsion
aSlhenospheric temperalure
thermal diffusivit y
thermal condUcl ivity
t
l,
p,
Po
p~.
125 km
31.2 km
2,8 Slcm'
3,33 t:lcm '
1,03 g/cm'
k
3.40 x tO "'c
1 33J"C (ide:l])
t 350' C(Sheba Ridge)
.(IOS cml/,cc
k
.0075 cal/deg c m sec
(l
T!
APPLICATI ON OF DIFFUSE EXTE NSION MO DE L TO
TH E G ULF OF ADE N
The Gulf of Ade n is a yo ung oceunie basin formed by the
rifting of Arabia away (rom Africa ( Fig. 5). I I posse sse s a
steep continental margin. small ab yssal basins and an a ctive
mid·ocean ridge s pread ing cente r (S heba Ridge) c haraclerized by rough topography. a rift valley and fra ctur e zones.
Riftin g beg:.n 20-25 m.y.b.p, in the late Oligocene or earlies t
Miocene ( ~omal!land U il Explo ration Co .. 1'11:$4 : Azzaroli,
Fo is, 1964 ; Beydoun , 1966) and Ihe o ldes t magnetic anomaly over Sheba Ridge is Anom:tly 5 (1 0 m.y.b.p,)
(La ughton el al., 1970; Cochran. 1981 ). Of particular
Înlcrcst 10 this st ud y i~ Ihat Iho::re b a n ar<!u !lpproximately
80 km wide belween Anomaly 5 and the coast , much of it
deep wateT (Fig. 6). in wh ich the magnelic field is eilher fiat
1 h,we also sJîghtly modified the expression for Parsons a nd
Sclfller's (1977) pflmmeter C l' whic h gi ves the total thermal
expansion of the lithosphere at the mid-ocean r idge . The y
assume th at th e asthcnospheric dens it y is iden lical 10 Po' the
O"C density o f manlle malerio\f. In fact. fo r Po = 3.33 glcm j •
the densi ty al l ,333'C will be 3. t8 gh:m l . which is signi fi cantly different.
The revised form for C, is
2(p<>{l
Po tu: T ,
a T ,)
P.. )·
" EliT FLOW
Figure 3
Hi'ai f1"w ,,,,d d.".uûrm us li flUlctitm of li",e fo,
•
Il,,, n!vdd ",id·m'l'dll rid/{f uSI'd in l/lis slIIdy.
rompu'cd ,,'ilh Iht l'tl/f};'icM rltr"t)· vf PI/n'VI!S
il/Id Sr/ote, (1977). M odl'l rid/{e cr/!SI ('OIlSiSIS of
........ _ 5<10", '''1
/l" '"
km a/ut
(li! /I.Wht'JUllplle'e (1/ I.JJJ<C
A li'!I?a, /I!rmwl /{'udit'III ü uuu", ,'d i" Il,,,
rr/MI of Il,e ,id!:c cresl.
_
1/ .{
'.
,
,
, ••0
Figutj: 4
1/!illal . "feulll" bmmded
--- -------"
, ,
., '
, , •
"
,
158
., '
,
, •
subsid~nre os i l
fl mct;on of 13 fur "arjous '·l.Imf"S /lf rms·
/allhidm~ss (A) ,md crus/al densily (B ).
Shadtd /lre/l rep'esenl.i ran;:.'/! of .'nilles
ustd b)' "UriOIlS in" I'Sligalvrs. Da)'''td
/jllt ;n (b) r~prtsetJlS l'olue of 2,81irm l
and 3/.2I1m lIud in lhis sllldy.
-
PRE-SEAF LOOR SPREADING DEVELOPMENT OF THE GULF OF ADEN
Gu lf of Aden
Bolhymelry in lallloms
TopaIlraphy in leel
(COII'O"" oJ,., l""91"011 " '" 0970 )
Figure 5
Loca/ion and ba/hyme/ry of the Gulf of Aden. Wide rulinR marks orea shol/ower rhon 1,000 lo/honts. Fine rulinR marks areos deeper rhan
1.OOOfa/homs. Borhymetry is alter LauRhton etai. (1970). S/udy art!u is Qu/lined.
•
""......
""•
~
_-_.,
_
. .... so,o ...
so'........" ...... LIoo •
.. ...- ...
~.-.
a-oz...._,
DS . D.~
f ..
50"
'_Z....
+
-,
o
Fillure 6 u
Bathyme/ric und tec/ollic chart of the eos/em portion of th e srudy ureu. Bmh ymtlric C(Jr, /(mf.r ure ufler Laughton (unpublished). llIudilied by
additiollol dlllo shawn us fine dou. FWCfUU lu ne ulld nWJ,:nelic anomaly idelll,jicmions are .. f/er Cac'lIron (1981).
159
J. A. CQCHRAN
Soi ..... ~.".CI'"
.....
.... '. ..',"..'....
".
FiGure 6 b
Bul hymelric alld uctmrie clrart of Ih~ ..·ts/~rn portion of
/ht sl udy artu. Bathymelrie COlltaurs art af/tr l.aughlOlI
(u llpublishtd), modi{itd b)' addiliollal da/u sIIO"'n as {i1l~
do/s. Frac/urt =ont und rnu,llntric allomal)' idtnll{i~a/iQ/u
art a/ur Cochrun (198 1).
or consiSls of .. nom:llies whit'h cannot be correlated over
d istances gre ater than about 20 km . The nature of this
"magnetic quiet zone'· is obviously of greal importance,
s ince it is situaled between the demonstrably oceanic crust
of the Sheba Ridge a nd demonstrably contÎ nen tal crust on
lohore and is separaled from both by disti nct boundaries.
The landward boundary is an esearpmenl made up of a
series of nor mal faults, and which extend from 2-3 km
bclow sealevel to 1,500 m above sealevel. The.c;e esearpments whieh are round ail a long both sides of the Gulf of
Aden are probably cquivalent to the ·· hÎnge zo ne ·' round at
mature continental margin~ (J !In S!l, Wade. 1975). T he sea·
ward boundary is an increase in depth of a fe w hundred
meters 10 a lmost 1 km, which is found about 10 km
land ward of Anomaly 5 (Cochran, 1981). The quiet zone is
characlerized by moderately rough lopography al a Cairly
constant depth withou t the topographie grad ient of a
mid-ocean ridge fCochran. in press (Fig. 7)). It has a
helerogeneous c rustal struc ture and, where Moho depths
have been determined (Laughlon. Tramontini. 1969). the
crus tallhickness is s lightly greale r than that of Sheba Ridge.
l n previous studies which have utilized the "~Ire t c hing ··
mode!, the subsidence hilolOry as determined by da ta from
exploratory wells has been used as the major const raint on
Ihe modelling (for example . Sclater . Christie. 1980 : Steckler, Watts, 1980 : Royden, Keen. 1980). S inee the G ulf of
Aden is a ··stltr ved " margin. s uc h s ubsidence data is not
available and other conStraÎnts must he used.
Total motion belYief!n Arabia IInd Alrica
Cochran (1981) eSlimated th:.t 80 km more mOlion has
occurred betwee n Arabia a nd Africa in the Easte r n Gulf of
Aden {han can he aeco unted for by the seafloor ~ preading
magnetic anomalies. This estima te is based on the 105 km of
motion which has occ urred a long the Dead Sea Rift (Quennell. 1958 ; Freund n al .. 1970) and a n estimate o f 30 km of
ope l'li ng in the Gulf of S uez. This estimale was d iloc ussed by
Cochran (in press) who eoncluded {hal whi!e 80 km WII S
probably Il good '·belot e~tÎmate'·. the non seafloor s preading opening could vary from 45 10115 km without contradicting any observations. allhough it is more likely 10 he
near the center than the extremes of that range .
lit ai floYi
Seven heat flow s tations we re occupied in the Eastern Gulf
of Aden during Vema crui.c;e 35 in 1978 along two lines from
the magnetiç q uiet zone onto Sheba Rid ge (Table 2). The
160
PRE-SEAFLOOR SPR EAOING DEVELOPMENT OF THE GULF OF ADEN
QUIET ZONE
EASTERN GULF OF ADEN
location of Ihe stations is s hown in Figure 6. The heat flow
measure menlS in the quiet zone are generally high , and
indicale a high temperature i radie nt and relative ly thin
lithosphere. The measuremenlS are in the range of wh,lI
would be expected fo r 15 1024 m.y. 011.1 seafloor. usi ng
Parsons and Sclater's (1977) age-heat fl ow relationship. The
measureme nts were oblained with a multipenetration appa·
TalUS. so that severa! measuremenlS approximately 1 km
apart were made at each station. The small scaller in the
meas uremenlS at each statio n indicates Ihat Ihey are representati ve. The env iro nme ntal evaluatio n {Table 2) is based
on the system of Sclaler et al . (1976).
Il was ass umed in Ihe model calculatio ns Ihat the heat flow
in Ihe undisturbcd continenlal crust is 58.5 m w/m 2
(1.4 h.f.u .) based on four val ues obtai ned from bottom hole
temperatu res in wells in Northeastern Somalia (E va ns,
Tammemagi, 1974). Il was also assumed Ihat Ihe heal flow
from the as thenosphere is 33.4 m w/m1 (0. 8 h.f.u.) (Parsons,
Sclaler. 1977; Lachenbruch, 5as5,1978), with 25.1 m w /m 1
(0.6 hJ.u .) contributed by rad ioacti ve elements in the conti·
nental crusl , and that the mdioactive elements are uniforml y
dis tri buted through the crust. 50 that the ir cont rÎbutio n 10
the heal flow is inve rsely proportionallo crustal thickness.
V3!H8
V ~ I~
•
1
1
,,,..
VJ~·09
•
•
!
...
",
•
Present basemenl depth
Alt hough the lack of sediments prevents determ inatio n of
the subsidence his tory of the Easlern Gulf of Ade n. one
point of the s ubsidence curve, the present basement depth .
can be dete rmined. Sediment thickness and seismic velocÎlY
information fro m Cochran (in press) were used to obtain
base ment depths in the Eastern G ulf of Aden. The sediment
was then removed , and the basement allowed to respo nd
isostalically resulling in the basemenl depth shown in
Figure 8. ln preparing Figure 8, basement depths were
Figu re 7
Seismic rtf/retion profilrs from Iht Ea.llun Gulf of Aden. Hea~y
~rrtlcal UntS ma rk Iht bounda ry btl"'un Iht Shtba Ridgt f/ank ond
Ih r magntlic q uitl tont. Shtba Ridgt rifl "allly con bt sUn in
prof/lu VJ! -OB. V35-09 and V35-1O. Profi/ts tnd 10 mi/ts (37 km)
from shon.
Table 2
Vtm/l J5 Iif/ii
SUl/ion
V353· 1
/10'" JWlio 'l s
flil
1
2
)
V3.S 4-2
1
2
,
)
V35 6·4
1
2
)
V357·5
1
2
)
Vl5 &-6
1
2
)
V3.'l9-7
1
2
)
V35 10-8
1
2
ill the Gillf
Luliludt
14°32.9'
14"30.8'
14"29.7'
14"21.9'
14"22.2'
14"22.5'
14"22.3'
13· 54.4'
11"54.6'
13°56.2'
11"26.4'
13"25 .6 '
13"25.4'
1)°08.9'
13"08.2'
13°07.8 '
12"58.6 '
12"58 .0 '
12' 57.4 '
12°42.1 '
12°4) .4'
0/ Ai/tn
LOIIJ.:Ï1udt
SO"30.H'
50"31.7 '
SO"32.6'
50"26.9'
50"27.) '
50"27.6 '
50"27.7'
50"14. 1'
5<M).O'
50"13.0'
47"51.9'
47"50. 1'
47"411.11 '
47"37.8'
47"36.6 '
47"35.7'
47"27. 4'
47"26.6 '
47"211 .11 '
47"29.1 '
47"29.11'
Dtplh
(ml
l .n6
1.874
1.914
2.001
1.992
1.984
1.998
2.460
2.457
2.455
1.912
1.932
1,930
2.0·12
1.971
2.011
1.880
1.856
1,1154
2.193
2.202
Sedimtnl
thickntss
stc.lm
1.5
1.700
1.6
1.1165
.6
'''''
1.0
1.080
A'
'10
.S>
520
.,
'"
16 1
N .T.
,,
,,
,,
,,,
,,
,,
,,,
,,,
,,
Enl'iron .
tvo/.
A
A
A
A
A
A
A
B
"
B
A
A
A
A
A
A
B
0
B
D
0
Gr/lditnl
. 102
.11 9
Cflllducti.'ily
.Im'C
.938
.112
.126
. 116
.1111
.125
.155
.163
.143
.123
. 123
.12 1
. 128
. 119
. 1)2
.1,.
. 146
. 147
.178
. 182
.938
.938
.934
.,..
.934
.934
Q
m ..·/"r
95.7
111 .6
105.t
11 8.2
l08.M
110.7
117 .3
145.4
152.9
134. 1
114.9
114.9
112.1
119.6
111.1
123.3
127.0
136.4
137.)
166.3
170.0
J . R. COCHRAN
Figure 8
tI
SmIHJl h~d . btll·tm~nl dtpllrs in Ih/,' l'Qstem ponÎon of rhl' sfUdy arl'U (uluT C /KhTtln . in IIf~U). C/J nWIlTS liTt În ml'rt'TS.
i"."
~·~"~=="""""",,~"F=..................
avcraged over a hour of ships trac k (about HI km) to smooth
o ut the locli i rOllghne~~ and make the General !evcl more
e videnl.
Va riatio ns ln r idge crest d epth
."
T
."
;
LJ
-
T hcre is anothc r d ifficulty with mode! calculations in the
G ulf of Ad en, and perhaps also Ilt o t her margins such as
those in the North Atlantic, where the adjacent mid·ocean
ridge has significant varia tions in ridge crest d epth. Specifi.
ca11y, the model was cal ibrated using a mid-ocean ridgc
c rest depth of 2.500 m, which is the worldwide standard
(Parsons. Sclaler , 1977). w hile the ridge crest depth on
Sheba ridge is about 1,800 m in the eastern portion of the
study area and about 1,300 m in the wes tern portion (Fig. 6).
The shallow ridge c rest depths are the rcsult of a generlll
s hallowing o f the Gulf of Aden loward the Afar hot spot.
The ridge crest dep t h decreases from about 2.500 m tO
sealevel in Afar. The gravit y fie ld does not reflecl this
shalJowing (Talwani . Kahle . 1975 : Cochran, in press) and
both heat flow values on the ridge flank and s ubsidence
away from the ridge crest are close to the values predicted
by the empirical curves o f Parsons and Sclater (1977). The
average unloaded basement depth in the magnetiç quiet
zone al liO vary from East 10 Wes t (Fig. 8) with depths of
about 3.200 m round eas t of Fracture Zone C and 2,5002.600 m round west of FraclUre Zone F. Thus the depth of
the quiet zone below the ridge crest remains constant a s the
rid ge crest shaJlows.
Cothran (in press) considered severa! methods of modelling
the shallow depths . and concluded that the most satisfactory is to slightly inc rease the thermal gradient in the mantle
down to d epths of several hundred kilometers. This model
whjçh was suggested by Zielinski (1977 ; 1979) to explain
."
~
Figure Il h
SmlHJ/lted un/utldl,'d ba stml'n/ deplhs in Ihl' ..·tSIUn p ortio n 01 thl
Sil/if)' arf a (aIrer CochrQn . in prl'S5). Coltlour~ are in m~I~n.
'62
PAE-SEAFLOOA SPREAOING DEVELOPMENT OF THE GULF OF ADEN
A
shalJow depths in the Nor wegian-G reenland Sea, res ults ln
shalJow depths wÎtho ut significanlly altering the ridge s ubsidence rate or heat flow. Thus it is assumed that as the Afar
hot spot is approached, the thermal gradie nt in the mantle
becomes progressively steeper. In the Eastern Gulf of
Aden. it is o nl y neceiisary to inc rease Ihe temperalUre at
125 km by 20"C.
PRE-RifT
MODEL CALCULATrO NS
·i~~4
Profile V35 -10(Fig. 7) was c hosen as a typical profile across
the magnetic quiet zone for modelling purpoiies. This c hoice
was based on the faci that it is nearly paralleJ 10 the fl ow
lines weil away from fractu re zones, has a clear magnetie
pattern and is less Ihan 15 km from the heat flow stations on
profile V35-11 , so that Ihey can easily be projected onto il
(no usable magnetic or seiiimic renection data are available
from profile V35-t l ). The bathymetric profile was continued onto shore us ing Ihe Saudi Arabian Ministry of
Petroleum and Mineral Resources topograp hie map of the
Arabian Peninsula (US Geo[ogica[ S urvey, 1972).
•
,.
The three model constraints discussed in the previous
seClion should be adequate 10 tes t the crus lal extension
model and, if it proves satisfactory , to define t he behavior
of the lithos phere dur ing Ihe pre-seafJoor s preading s lage of
ils de velopment. The basement depths are dete rmined by a
combination of s ubsidence due to c ruslal thinning and uplift
due to thermal ell:pansion caused by heatins due 10 lilhospherie Ihinning: Ihe heal fl ow is primarily determined by
the lithospheric Ihinning : and we have limits on reasonable
amounts of cruSlal extension. Thus the a mo unt of crus lal
extension and lithospheric thinning can be varied 10 matc h
the observed unloaded basement deplhs, and the heat flow
and c rustal ell:lension constraints used to determine whether
the val ues oblained are reasonable.
,. ·I~ ~
• " l " ., "
~I
~
u' _ _
1
1
-
L...!!....!" ..,
Figure 9
10
.,.
Model for Ih e developmen/ of Ihe cunline" ",1 margin in Ille Gulf of
Aden /Ising sire/('hing model. A 725 km liliek Iilhosphere Wilh a
31.2 km Ihiek eruSI (A)is t:lunded as .d'l'''''' in Ihe Jowerpart o[(B).
Fine lines show "boxes" uud in modet ealelllOlions. Numbers show
~ values for crusl und li/hosphere. Upper diagram of (8) sho,.,!
morph%gy o[ Ihe rifl ~alley 01 Ille en d of the ex/tl/sion plJllse. A
mid-ocean ridge is assumed 10 de"e/op ill Ille eenlerof IIII! rifl .'ulley
(dashed /ine).
DISCUSS ION
The res ults of the model calculations are s hown in Figu res 9
a nd 10. Figure 9 s hows the amounl of pre-seafloor s preading extension and Iithosphe ric thinning across the G ul f of
Ade n rift at the lime of initiation of seafloor spreading.
Figure 10 shows the situation 10 m.y. later, and compares
the caJcuJated res ults with those observed in the Eastern
Gulf of Ade n. It can be seen in Figure 10 that the "stretChing" model is capable of explaining the observed deplhs
and hea t fl o w. However, a great deal of ell:tension is
required , primarily within Ihe deep portio n of the magnetic
quiet zone. The lotal amount of ell:lensio n ac ross the 280 km
wide rifl in Figure 9 is 115 km. 95 km of which occurs in Ihe
deep central 120 km of the rift. The amount of exte nsion
req uired in this region implies cruSla[ thicknesses of 5.2 10
7.8 km , wh ich is only slightly grealer than the thickness of
oceanic crust in the model , a nd s lighlly less than the
observed c rus tal thickness of 7.2 to 9.7 km (Laughto n.
Tramontini. 1969). The observed crustal thickness ca n he
used to pUI a n upper bound on the amount of ell:tensiOn t ha t
has occurred in the quiet zone. If an average crustal
thickness of 9 km is assumed, then fi value o f 3.5 is implied
for the parame ter 13. a nd the tolal amo unt of extensio n
occurring wilhin Ihe deep, central 120 km of the rift valley
in Figure 9 is 85 km o r 10 km less than deduced above . It
should also he remembered t hat Ihis represents a ma ximum
estima te as the average crustal Ihickness is liable to he
greater than that deduced from the Ihree stations whieh
recorded mantle arrivais (Talwani et al., 1979).
The s imple "strelching" model appears to require slightl y
more extension t hlln can he reaso nably accounted for from
geological and geometricaJ arg ue ments. There are, however, several factors not acco un ted for in the model which
would result in a n overeslimate of the a mo unt of extension.
One factor. whic h was mentioned above. is the fact that the
model assumes tha! the extension occurred insta nlaneous ly ,
while in reality it occurred over a period of 10-15 m.y. in t he
Gulf of Ade n. The efrect of a finite time of extension is to
tmnsfer pa rt of the subsidence from the post-rift to the
syn-rift stage of development whic h, for times Ilot too long
aft er rifting, will result in grealer basement dept hs than
predic ted by the instantaneous mode!. The effecl of a finile
ell:tens ion time on the basement depths can be invesligated
us ing ;In error funclion formulatio n and constanl ell:lension
rates (Haxby. in prep.) with the result Ihal if the ell:lension is
assu med ro occur over a period of 12 m.y., the ins tantaneous model predicts depths that are about 250 m too
shal low al 10 m. y. alter Ihe extens ion ceases for values o f t3
grealer than abou t 2. Thus. in Ihe dee p part of t he quiet
zone, less extensio n is necessary to produce the observed
basement deplhs a nd the 13 val ues of 4, 5, and 6deduced for
the centra l Ihree "boxes" (Fig. 9) can be reduced to 3.4, 4
a nd 4.8 respec tively . T his reduces Ihe average 13 in the deep
quiet zone from 5 (400 % exte ns ion) 10 aboul4 (300 %
ell:tension).
163
J . R. COCHRAN
HEAT FLOW
:.~.
.,
, œ
,
,
••
"•
1
1
1
OOSERVED DEPTH
1
UNLOAOED OEPTH
1
Figure 10
!~~---=:~.~
'~I ~&. -rl.-·----)(--:
!
~
oct:ANIC CRUS!
1
1 ' QUIET ZONE"
- l '~'"·
Comp(lrisou of model reStllls or 10 m. y. (Ifter initimion of
uaflaor l prtading ... Itlr obse"'ed ,'alues fram tire Ea stem Gulf
o{ Aden. Pre-uaflaar spreading exrensian is assumed as il!
Figure 9. Solid heul {Io", Clm.·e givts model caJcul(l/i01l s and
apen circlel oblU\'ed hem flo .... Solid circ/es f[il'tr hear fla ...
corrected for Ihemtal blankeling. Ali meaSU ffm ents ar i'ach
.</(/Iion are ...itlriu rhe circli'. Obsen'cd b(lume," deprhs m l
profile Vj5-IO ...ere co rrected for si'dim enr laading la Rit·€"
u"loadi'd depths.
-~
However , another observatio n that can he drawn Crom this
study is that in the gulf o f Aden. where scafloor spreading
has been occurr ing for 10 m.y., wc are rcachi ng the maximum time at which information can be obtained concerning
t he nature of the pre-seafloor spreading development of the
margin , as transient effects related to the exact mechanism
of rifting hlwc nearl y dissipaled. Ii is precisely the nalu re
and du ra tion of this pre-seafJoor spread ing development of
t he margin which is important in determ ining the mechanism
and causes of rifting and the development of the oceancontinent transition. Il is th u5 imporrant to obtain data from
the syn-rifl sediments when st ud yi ng the su bsidence history
of the margin. These sed iments are, unfortunately. se ldom
drilled. lt will nlso be im portant to s tud y the struct ure and
tectonic processes at work in the few areas, 5uch as the
No rlhern Red Sen, which presently appeaT to he in the
pre-seafloor spread ing stage of their development.
Anothe r fac tor which will result in an ovcreslimate of the
amoun t of extension is that it is quite likely that part of the
extension occurred t hrough diffuse di ke injection rather
than ro ta lional fauhing. Volcanic ac ti vity within the quiet
zone is indicated by the recovery of a 13 m.y . old basait at
DS DP s ile 231 in the Southern Gulf o f Aden (Shipboard
Scie ntific Pa rty , 1974). Extension Ih rough dike injection
results in a diffcrent temperature dis trib utio n tha n the
s tretc hing model (Fig. 1). with highcr temperatures found
near the s urface. This heat is quic kly lost resulting in higher
subs idence ra tes immedialely a fter the stretching episode
(Royden et al .. 1980) , 50 that during thÎs time a lower i3
value is needed to produce a given depth. T hus, if the
extension occurred entirely through d iffuse dike injec tion ,
at 10 rn.y . pos t extens ion, approximately the same depths
and heat flow are obtained wilh a i3 of 3 as wil h a i3 of 4
us ing the "stretc hi ng" model.
It is difficult to estimale the relative cOntribut Îo ns of dike
injec tio n and rO lational fauhing to the fo rmation of the
magnetic quiet zone. but if dike injection is a major
component, the amo unt of extension requ ired 10 produce
the obseTved depths, and heat flow is within the bounds set
by geographic constraims and observed crustal thicknesses.
We are thus able to state Ihat the diffuse extension model
can explain the observed geological and geophysÎCal features o f t he magnetic quiet zone in the G ulf of Aden .
Acknowledgements
This work was supported by Natio nal Science Foundation
grants OCE-77-20098 a nd OCE-79-19241. The ma nuscript
was reviewed by A. B. Watts and M. S. Steckler. LamontDoherty
Geological
Obse r vatory
Contribution
Numher 3110.
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