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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 i':bEfl iHEF! b E E F i -1 svl|l,'iv9 o o a7E z 6 - .E 6E ds ii V 9 ll E E . EF . + s0Nocrs o 6 r ( l ) l frE3.s5 N l l l l o.: i o o d T r d o 5 E . 3s s s z E E i - e R H R E € i ! c d t Hsg€E* * . E9 . s8 € gEB::E ;IE!#I ; F . , . 9 9 E E:iilc f ig!i $ :85 t . E Fj: F E d S € : * : :.s"frEiE ieei TI q EE: E r,1 ^<! I x | ^ - . d I FgE;H€5 _ L; N LL = o C bJ z N z E t! t-a .{ -4 UJ o ; , E o . / l - v a a a o FeSE;g 5I!;57 F 6.i 9!€ 6 ;E€.EEE3I8 .: E 3 ; s ' ;g ; g€i€Eg : 3 . HE{ i E-;F E ;E€ j ; F O | ! l s - 3 5 fi$;'5; E 6- c >6 v F < 5 E;;;Eq E ; :S E € E 3 F ' i 3 . - - ! = v t ! 4 6 ' ; : ! i i : ,EiiE;e I!:IE; ."$EEgBF 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. Reference the East Pacific rise between Anclerson,R. N. & Sclate!, f, G., 19'72.Topograptryand evolutjon of 5'S and 20'S, talt, planet. Sci.Lett-, 14'433-4ll' betweenvadations R. N., Spariosu,D. J., Weissel,J' K. & Hayes,D' E-, 1980 The interrelation Anderson, ' the Southeast along chemistty and magnetizatioo in 'nrgn"ti"_ unomaly amplitudes anal basalt geophyt. 85, 3883-3898' Rer., t. lndizn Ridge, Hom of Afuica'22nd int' Azz^toh,A. A fas, V., tSg+. Geologicaloutlines of the northe end of the gr. 293-314. Part lV, ReP., Co GeoL geoloey' PhiI. Ttlns R Soc' Beydoun. z. R., 1970. Southem Atabia analnortheln somaiia: comPalative -292 . 267 26',7 A, , The oceat B|titls and Margins: Beydoun, z. i., fggZ. fn" Gulf of Aden and Northwest Arabian Se , n the IndiLn Ocean,edsNaim, A. E' M & Stehli, F G , PlenutnPress'New York' western Somali Basin"I' Bunce,E. T., Langseth,M. G., Chase,R. L., Ewidg, M' 196T Structure of the geophys,Res,,72,25 4'l -255 5 . history ol the Southwest Cana",-S.C., tt"rton, E. M. & Han, B. R., 1982. The Early Cenozoictectonic Pacilic,Estth pldnet. Sci.Lett., 57' 63-'7 4 young oceanbasin and continental cochran. J. i.. fSgi. Th" Gulf of Aden: structureanatevolution of a -288 . maqin, t. geoqhYs. Res., 86, 263 Cochran,J. R., 1982a.F.eplv J. geophys. Res.,a7,6765-67'10' jn the easternGulf of Aden: loplications for the early cocfriua, l. n., t9e2b. ftre magneticquiet zore developmentof the continentalmargin' Geophys-f' R' ^str' Soc , 68,17^1-201' of sedimentarybasins,'Edltlt Cochran. J. R., 1983. Effects ot fioite riting timei on the alevelopment pl4net, Sci.Lett-, 66,289 -302, rocks of southwestSaudi Cotmt, n. G., Fleck, R. J., Hodg€,C. E. & Ghent, E D', 19?5 The volcanic Resour'' Aiabia and the opening of the Red Se ,in RedSedRese,rch 1970-1975' Bull' Mi\eL Arabia Jiddah' Saudi Resouices, 22, D1-D30, DiJectorateGenera!ofMineral The MioceneTihama Coleman,R. G., Hadtey,D. G, Fleck, R. C,, Hedge,c T &Donato'M M ' 1979 and Mineralization of ttt Evolutiotl Sea' of the F(ed opening A.t Optiotite antl its beadng on the 186' Institute the Arabian-NubiLn snt€ld, ed. Al-Sh;ti' i v s , nult tnst' Appl Geoi' 3' 173 Jiddah University, of Applied G€ology,King Abdulaziz ctustal attenuation and de Charpal, O., Guedooc,P., Montaalert,L. & Roberfs, D G', 19?8 Rifting ' 275,706-'11l Nature in the Bay ofBlscny, ' subsialence CanadianHvdrographic Service' Getleral Bathymetric Aart of the Oceans(CssiO),5th edn, 19?5 O(lawa,Canada of the westernmost Girdler, R. W., B.o*n, C., Noy, D J. M. & Styles,P', 1980 A g€ophysicalsuryev Culf of Aden, PriL Trans.R, Soc'A,298'l-43 Gulf of Aden' Ndtwe' 271' Girdler, R. W. a Styles, P., 1978. Seafloor spreadingin the westen 615-617. and evolution of a young Gkdler, R. W. & Styles,P., 1982. Commentson "The Gulf of Ailen: Sttucture geophys Res''87'676I-6763' JR.Cochr?rn'l by oceanbasin analcontinentalmatgin" fid9es'I' geophys' Hey, R., Duennebier,F. K. & I'torgai, w. i., 1980' Propagatilg rifts on mid-ocean R e r . .8 5 . 3 6 4 7 - 3 5 5 8 . 73 of otd oceanic lithosphere and Newfoundland' Jansa,L. F. & Wade,J. A , 1975.Geologyof the continentalmargin^offNova.scotia zt 5i -1u)' geot /4-JU Sur'tr' fup Canad|' of Eastern n\ Offshore Geologl ftles' Earth fi' P., 1980. Sedimentarybasin formation with finite extension ftl&"n#, l"ruir, c.-t-a ptdnet.Sci. Lett., 48,42-52. geophys. Res., & watts, A. 8., 1982. On isostasyat Ailantic-type continental margins,"/. rurrr"i-c.'i. 2948. 87,2923 the ifted margilrol easterncarlada: x""n, i.'s. & narrett, D. C., 1981. Thinned and st'bsidedcrust on clustalstlucturc,therma]evolutionan.lsubsiilence:n]rgtory,Geophys.I.R.astr.Soc''65' 443-465. anomalvand bathvmetry Klitgord, K. J. & Mammerickx, J., 1982. Northern East Pacific rise: magnetic 6'150 87 6125 geophys Res., ' fiafiework,l. polaitv time scalefor Late LaBrecque,l. I-., f"nt, D. V. & Cande, S. C, 197? R€vise'l magnetic 330-335' Tirne, Geolog!,5, Cretaceousaril Cenozoic Bet]!.lgSe^'I' geophys ReJ'' 85' Langseth,M. G., Hobait, M. A. & Horai, f', fggO' H"ut flow in the Rifting 3140-3150. in the lndian ocean'l' geophys' l-uocr"irr, t{. G. & Taylol, P. 1., 1967. Recent heat flow measurements -6260 . Res.,12, 6249 l Laughton,i. s., t9eo. The Gulf ofAden,Pftil- Trans.k.Soc A, 259'.150-17 . Gulfof Aden' Phil' Trons' ofthe evolution The 19?O ll T', B.alon"s, R a. s., Wltamarsh, i""Eii"", R. Soc.A,267 '22'l -266. M' 1975 Morphology and tectonic Mamme ckx, J., Anderson, R N., Menarit, H w & smith, s 111-118' Soc Am',86' evolution of the east-centrrrl9^ctfic,BuIl. Seol fossil spEadingridgesin the souiht\to fol Evidence lgSb L., & Dorman, E. Mammerickx,J., Herron, eastPacific,Butl. geol. Soc,Art. ,91'263 27\' evolution from 25 m v'B P to the Mammerickx,J. a fttgora, K. D., 1982. Northern East Pacific Rise: pftsent,J. geophys.Res.,87'6'751-6759' Matthews'D.H.,wiliams,c.A.&Laughton'A.S.,196T.Mid.oceanddgeinthemouthoftheGulfof Aden, Ndture , 215, 1052 -105 3, bzsins' Earth phnet' Sci' McKenzie, D. P,, 19?8. Some remarKson the tlevelopmentof sedimentary Lett-,40,25-32. Indian Oceansincethe late Cretaceous' McKen;ie. b. f. & S"lnt"r, J. G., 19?1. The evolution ot the -528' goc,,25,43'7 Geophts.L R. ,str. J C ' 19?9,'NortheastAtlantic passive Montaalert,L., de Chalpat,o , Roberts,D', Guennoc,P & Sibuet' Dtitlittg Results in the Atlantic Deep processes' in 'utsia"n"" continent.l ftaryins: rifting "ni ocean:ContinentalMdfginsdndPaleoenyifonmeflfu,edsTalwani'M',Hey,w.&Ryan,w.B.F., Am. geophys.utl- M, E ing Sefies,3' 154-186' reflectorsin oceamccrust off frfott"i,-r.'i., iuiru*i, M. & Stoffa, P. t., tl8Z, O'gio of t""*ard-dipping ihe Norwegianmarginby "subaerialseafloorspft di'n|",Geo,bgy 10'.353-357' dike complex of the somali ophiolite near llbra' oman' Pallistef. J. S.. 1981. Structure ot tne Srreeted J. geophls. Res.,86,2661-267 2. Ln analysisof ocean floor bathymetry and heat flow with age''L Pa$ons. B. & Sclater, J G., lgll geophys.Res., 82, 803-821. of Madagascatand Atuica,scie,ce, RabinJwitz, p. D., ioffin, M. F. & Falvey, D., 19g3. The s€paration 22O,61,69. ofocean se'liments'l' &eophys Res" 65' 1535-1541' Radcliffe,;. H., 1960.The thermal conaluctivities of the continentalmaGin of Royden, L. & Keen, C. E,, 1980. Rit'ting processesand thelmal evolution gci' Lett'' 5l' 343-361' pl,net' coves' Earth sribsidence tiom easternCanadadetermined subsidelrceand heat flow: Royden, L., Sclater, J. G. & von Helzen, R. P ' lg80 Continental margin Am Ass Petrol' GeoI'' 64' Bull' hydrocarbons' petobum of in tbtmation parameters impofiant r73-18'1. and evolution of the central Sclatel, J. G., Ande$on, R. N. & Bell' M' L,, 19?l' Elevation of rllges eastefiP^crfrc,I. geophys.Res., 76,'l888-? 915' ofoceanic heat averazes'f'geophys' Sclater,J. G., Crowe, J. & Anoerson,R. N , 19?6' On the reliability Res.,a1,2991-3006. and contine[tal crust and sclater, J. i., iaupart, c. & Galson,D.' 1980 The heat flow through^oceanic ihe heat lossofthe Earth,Rev' Geophys'SpocePhys', 18'269-3ll' magneticljneations i ,"Norion'l &Meinke'L'.1979 Mesozoic sirnpsol E. s. w., sctater,J. G., Parsons, ' 260-264 in the MozambiqueBasin,Edrth plLnet. Sci Lett', 43' continentalmalginsby continentalbreakSleep,N. S., 19?l, Thermal eflects of the formation of Atlantic lp,Geophys. J. R. dstr. Soc.,24'325-350 74 C. A. Stein and I- R. Cochran depoits of Somalilancl Oil Explotation Co , Ltd, 1954. A geological reconrl4isvnce of the sedimentary Londonfor the Colonies' Agents Ctown the Ptutect;rute of Britlsh Som4lilInd, continental margins'P&' Steckler. M. S.. 1981. The thermal and me;hanicalevolution of Allantic-type York. New tftesit,ColumbiaUniversity, consiilerationsirl the subsidenceof continental margins' '/ src"k;, M. i., tg8s. t*o-a."ttsional geophYs. Res., in Prcss. young continental matgin, ste*re-i, i,l.'s. a watts, a. B., 1gg0. The Gulf of Lion: subsidenceof a 429 Natwe, 287, 425 of Atlantic-tvpe contiSteckler, M. 3. & Watts, A. 8., 1981, Subsidencehistory and tectonic evolution 184-196' Anericdn 6' Series' Geodrnaftics Maryirrs' of Passive i Dynemics nental margins, GeophysicalUnion, Washington,DC. of the westernGulf of Styles, P. e Ha[, S. A., 1980, A comparisonof the seafloorspteadinghistories AdenandthecentlalRedsea,ilGeodynafiicEvolutio,loftheAfrc.AfdbianRiftSystem' pp, 587-606, AccademiaNazunaliDei Lincei, Rome' basementat the margin famani. tU. A nnnofn, O , 1973,Th€ boundarybetweencontinentaland oceanic -330. 241,325 of rifted continents,ly'4turc, evolution ofthe area Talnrani,M., Mutter, J., Houtz, R. & Kdnig, M., 1979 The crustal structure and in Geologiealaad Australia' of south margin quiet on the zon; unO"ityirrg ttt" magnetic L- & Dickerson' Montadert' J S eds watkins, ' G-piyriiot tnv"rtigotlons of continentarMal8?4 p. w., pp. tS I - tZS, ,l*erican Associationfor PetroleumGeology,Tulsa'oklahoma_ 38, 1213-1242, Talwani, M. i udintsev, c., 1976.Tectonic synthesis,in _Ir!ir.Rsp. Deep seaDitl. Proi., DC' edsTalwani,M., Uditrtsov,G. et 4t , US GovemmentPlintillg Office, washington' of the Somaliophiolite' Tilton, G. R., Hopson,c. A & Wright, J. E., 1981. Uianium{ead isotopic ages 6' Oman,vith applicationsto Tethyan OceanRidge tectonics'l' geophys' Res' 86'2163-217 sediments of deep_sea conductivity of thelmal A E., 1959. The measuFment Von Herzen.R. & Maxwe11, by a needle-probemethod,f. geophys, Rel" 64' 155?-1563' b^sltrs'Tectotlo' watts, A. B. & Ryan, w. B, F., 1976. Flexure ofthe lithosphereand continentalm^tgin phrs-,36'24-44. implications' weissui,i. i. & Hayes,D. E., 1977.'fhe AustralianAntarctic Discordance,oew resultsand J. geophys.Res.,19' 2519- 2587. and the evolution of the Whitmarsh. i.'g., fgfg, 1'n" Owen Basinoff the south-eastmalgin of Ambia Owen FractureZone, Geophys.I. R 48tr,Soc., S8'44I-47O 23' 29f-3ll ' whitmarsh, weser & Ross et at., lg"l4a. Slte 223. 1i lnit' Results Deep Sea dtill Ptoi ' eds 3upko, R. R. & Wesel,o. E., US GovernmentPdnting Office, Washidgton'Dc' 'tt ltttt. RetultsDeep Seqdrill' P/oi'' 2!' 383-395' Whitmarsh,Wesei & Ross ?t aI., l9?4b, Site 224, eds3upko, R. R. & weser,O. E., US GovemmentPrinting Office, washiigton' DC'