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________________________________________ ~O~C~E~A~N~O~L~O~G~IC~A~A~C~T~A~.~19~8~1~.~N°~S__ P ~~----- Mechanism of formation • of active margllls AClive margin s Ho t manlle Subduc tion Basal te-eclogitc transi tion Marges active, Manteau chaud Subduction T ransilion basalte-êclogitc E. V. Artyushkov (nstilute of PhySÎcs of the Earlh , Moscow, USS R ABSTRACT The upper mantle mate riaJ is strongl y healed benealh active rnargins as compa red 10 stable continental and occanle areas. The energy associa ted with ils a bno rmal hcating is by IWO o rders of magnitude hig he r than 3ny e nerge lic effc e ls of subduction. This hot material has becn Înjected from a large depth. most Iikel y from the core-mantle boundary. Low-velocity and low-denSÎl y ma ntle mate r ial is segregaled from the hOI upper mantle benealh ac tive margins. It forms a la yer severa l tens of kilo mete rs thic k bcneath the lithosphere. Oceanic lithos phe re is denser than anomalous man tle a nd il s inks into Ihe a nomalous manlle layer. Heavy e clogite is formins in the oceanic c rus t after it contac ts a nomalous mantle. This permits the s ubduc tio n to occur a lo ng an inc lined boundar y be tween nor mal mantle of high viscosity and hot lig ht material of lo w viscosity located beneath active marsins. Deep depressions of marginal seas are formed by the basall-eclogite tra nsition after the contac t of low.velocity mantle wi th the continental crust in a number of reg io ns. Local centers of sea floor spreading can arise in Ihese depressions when low-velocity ma ntle te mperat ure is ve ry high unde r the crus!. Oceanul. Acta, 1981 . Proceed ings 26'h Internatio nal Geological Congress, Geology of continenta l margi ns s ymposium, Paris , July 7-17, 1980. 245 ·250. RÉSUMÉ Mécanisme de formatio n des marges acti ves. Le manteau supérieur sous les marges aç ti ves est porté à une tempéra ture s upérieure à ceIJe régnant sous les zones océaniques ou les marges pa ssive~. L'énergie associée à ce réchauffcm e nt ano rm;11 est environ deUil fo is plus importanl que I·énergie assoc iée à la s u bduction. Le maté riel chaud est issu de niveaull très profond s, très probablement de la limite noyau-manteau. Sous les marges act ives, il y a ségrégation d·un m;tnteau à basse vi tesse sismique e t faible de nsité à partir du ma nteau supérieur anormalement récha uffé. Celui-ci fo rme une couche de quelques dizaines de kilomètres sous la lithosphère. La lithosphère océanique. plus dense. s'enfo nce dans le mantea u anor malement c haud. Il ya formmion d'éclogile au contac t c roûte océanique·mante au c haud. Ceci cond uit à une subduct ion le lo ng d'une limite inclinée entre le manteau normal fortement visque ull. e t le maté riel léger et c ha ud p lu ~ flu ide situé sous les ma rges a ctives. Les dépressio ns corresponda nt aux me rs marginales appa ra issent dans les domaines où la transilion basalte-éclogite a lieu après conta ct entre un manteau à basse vÎlesse s ismique et la croû te continentale. Lo rsque la tem pérature du ma nteau basse vitesse est s uffisamme nt é le vée, il y a for mation d' un centre d·accrétion océaniq ue. Oceanol. A t·ta, 1981. Aç tes 26< Congrès International de Géologie. colloque Géologie des marges conlinentales, Paris. 7· 17 juil. 1980. 245-250. 245 E. V. ARTYUSHKOV MA NTLE INHOMOGENEITIES BENEATH ACTIVE MARG INS ENERGY RELEASE BY SUBDUCTION Il is known that the subducting lithosphere cools the mantle (McKenzie . 1969; Toksoz el al.. 1971). This is hecausc its upper boundary is at a temperature T- O"C before the su bduction. As a res ult the amount of energy necessary to heat the oceanic lithosphere up 10 the temperature of the surroundi ng mantle appears to he lower than the release in the potentiaJ energy by subduction. Hence s ubduction cannot he the cause of formation of the above mentioned inhomogeneities. Let us neve rtheless compare the amount of heat energy stored in the m with the release of energy by subd uction. Suppose (hat the lithospheric s lab of the thickness d is subducted down to a deplh 0 ,tI an angle q; 10 a horizonlal plane. Then the re lease of t he potential energy is According 10 Ihe recenl ideas ( Mc Kenzie, 1969) the oceanic lithosphere Îs heavier than the underlying asthenosphere aU over the oceans except at the mid-ocean ridges regions. This is considered to be a cause of the lithosphere s ubduction on the active margins. The position of a heavier layer on a lighter one is convectivel y unstable . Hence subduction s hould arise in many regions from the above point of view. Actually it takes place only in the regions of large and strongly pro nounced inhomogeneities in t he uppe r manlle. This difficulty is usuaUy avoided by assuming that the inhomogeneities themselves a re produced by subduction. Let us consider if it is possible or not. Seismic wave velocities are considcrably dec reased and their al1e nuation is strongly incrcased beneath active mar· .!lins as compared to s table cont inental a reas and 10 the adjacent oceanic regions (Baraza ngi . lsacks. 1971). Sud properties of the mantle are normally observed from the Benioff zo nes tothe conti nen ts for a d istance - 1000 km in a hori zontal direct ion and from the Moho boundary d own 10 a dep lh - 250-300 km in a vertical direct io n. Thus the volume of the inhomogeneities is very large. A ty picaJ c rust and mantle structure on an active margin is shown in Figure 1. As it foUows from t his section P·wave ve!ocities are strongly reduced henea th the sea of Japan as compared to the mantle on the oceanic sidc of the Benioff zone. The minimum V values are - 7.7 km/sec. in a layer located at a depth - 100-200 km whic h is by - 1 km/sec. lowcr than in stable areas. d C\L=lJ.p . g · - - ·D , cos 1{> (1) per unit vertical section of the slab. Here!J.p is the density contrast between the slab and the mantle. g is the gravity. Let us neglect the heating of the slab itself and assume that one half of the encrgy is spent on the heating of the mantle below the sJab and the other half of eoergy prod uces the heating of the mantle above the slab. Then the colomn of mantle of the height 0 above the s Iab is heated by lJ.T = !J. p.g.d 2C. cosl{>· (2) where C~ is the heal cap,lCilY under constant pressure. Taki ng the ave rage value of !J. p a long the slab as 0.1 g/cm ' . d = 50 km . Cf = 45 CI' = 1 eni/cm' we find IhM Q , (3) T his quantity is by 1.5-2 order!; of magnitude lo wer than the add itional healing .6. T- 500·C of the manlle in the regions under consideration. Hence this heating cannol reprcsent the resull of subduction and it s hould he of qu ite another origin. T HE CAUSE OF INCLINED SU BDUCTION If the lithosphe re is subducting ioto a lighler mantle which is origioally uniforrn. it tends to bend down to subside vertically. Large deformations of the lithosphere -y;;t Hr - 1 occur beneath the deep·sea tre nchcs where the lithosphere cha nges its position from horizonlal to an inclined one. In this case elaSlic stresses would he a - M-y where M is the shear modu lus. M - 4.1W bar/c m1. For the above value of "y the stresses are a - l 00 kbar which is far heyond the limit of elastic ity. Hence the lithos phere should be broken under the deep·sea trenc hes a nd il s hould s ubside into the mantle as a set of blocks. Sud a Ii thos pheric slab has a very 10w rigidity which cannOI be re sponsible for an indined subduction . Figure 1 Selfemmit: seÎsmic cross·sectÎOl! (llolIg tire profile Jupan Se(l • H O/flll· Pucific oce(ll! (Alexul'. R)"(lho}". 1976). Strong decrease in P·wave veloôties lakes place when mantle tempe rature approaches the melting po int of basic inclusions ( Magnitsky, 1967). This temperature is T-t300' C for the above mentioned depth . T he temperature in stable regions is estimated as T- 700-800°C fo r the same depth. Hence an additio nal heating by .6. T - 500°C (or by several hundred degrees at least) in a very big volume is necessary to form the mantle inhomogeneilies beneath the aClive margins. Let us consider now. whether the oceanic lithosphere can be sustained in an inclioed posilion without considerable bending by visco us forces in the mantle. The rate of movement along the slab v. (Fig. 2) is controlled by viscous stresses at the slab boundarics : a., 246 _ (av. ~) a)' + ax . -TI (4) MECHANISM OF FORMATION OF ACTIVE MAAGINS ord er of magnitude at least. The upper man tle temperature is by A T -300-S00· higher abovc Ihe Be nioff zones than a l the same d e pth in stable a reas. He nce the viscosity should be lower by 3-5 orders o f magni tude a t le ns t above the Benioff zones as compared to the above mentioned values. '" If the viscosity li is much higher below the slab tha n above it. the vy-co mponenl s hould be two times greater below t he s lab as compared to the value de termined from (8). He nce r -v , _ L\p .R . d.cosl/' . L 4" (lO) Fiiure 2 ln o rd e r 10 prevenl a cons iderable bcnding o f the s lab, V y s ho uld be much le ss Ihan V •• The lalter quantity is no rmall y - j-l0 c mlyea r. Taking V r '" 1 cm/ year . the ave rage v11lue o f de nsit y contraSI p _ 0.1 gicm J , d o. 50 km , L ... 1000 k m we find from (10): Grlll'Ïly !orCt actillg 011 Iht tltment of Iht Iilhosphuic pl/llt subduf l ing inlo Ihe mlltll/e II/Id visCOUJ .f/rusu 1111 il S bOllndariu. T he rate o f bending of the s lab \Ir in the d irect io n norm al 10 the slab is controlled by viscous s tress Cl rr and il can be eSlima led from the fOllowing rela tion u,,· 2~ l'la y' li ~ (5) 5 _ Yr. CI , (Ix L ' !!t _ ~ L ' CI, L - 2'1 (2T+ v" Y...) -!1p . g , d.cos/(' - 4'1 L 2V 'L ' J poise. (11) C HEMICA L CO NVECTION IN T H E MANTL E T he formatio n of the manlle inho mogeneities on the ac tive margins can he ex plained by c hemical con vection (A rt yushko v, 1968; 1970). This convection results from a density diffe rentiat ion a l t he core-mande bou nda r y. The lowe r man lle conta ins Ihe outer core mate rial in a form of inclus io ns in a solid Slale . This mate rial (most likely FeO (Dub rovsky. Pankov. 1972) is in li Jiquid s la le in the oute r core. The isot herm of solid dielectric - Jiquid high condu ctin g phase transition ro ughly coinc ide s with the core -mantle boundar y. (6) The (J"y-com ponents acti ng on the upper a nd lo wer s lab boundaries s ho uld compensale on the average the fo rce F" .. L\ p .g.d.sinlf' acting alo ng the x-ax is on a un il col umn of Ihe s lab which is perpendicular to Ihe x-axis. Sim ilarly Ihe (J y-components o n the upper 1lnd lo we r bound a ries of the s r.'\b sho uld balance the force Fr .. L\ p.g. d.cosq: acti ng o n t he same column along the y-ax is. Then using (4)-(6) wc fin d • p . g . d • Sln/(' " 1er Il is the same estimate aS lhat fo r the upper manlle viscosi t y below the as the nosphere in stoble a reas. Thus Ihe ma ntle s ho ulJ indeeJ ha ve qu ite d irrerent properties below a nd above Ihe BenioH zones on Ihe ac tive margins. The componenl o f the mantle velocit y v~ is constant a l the sla b boundaries whe re v~ _ V. and ". is variable wit hin the manlle. The componen! 'Ir varies both aJo ng the slab and in the normal direction. The flow induccd by subduction in Il homogeneous ma nlle is maioly concentrated in the area locll ied al a d ista nce - U 2 from the sJab, w here Lis the lenglh o f the slab. Hence tak ing the charac tcrist ic s cale of v~ as V y we have : Clv, _ 2V •. 3. The pressure of the trans ition s ho uld de c rease a s the lempera ture Tise s ( Magnits ky. 1967). T he lower ma nt le tempera lu re increase s wilh lime bec,lUse of radi oact ive healing. As a res ult Ihe phase bound ary ascends in the lower mantle . The solid core malcrial bccomes liquid bclow Ihis boundary prod uc ing a d ensity differentia tion of the lower mantle ma lter. In the process o f differenliation Ihe heavy core material sinks a nd joins Ihe core. The residual mixt ure o f subst!l nces !lccumulates unde r the lower m:tntle. This mi xture - light male r ial - is lightc r tha n the overlying ma ntle whie h still has a normal conce ntrat ion o f hea vy core matcrial. (7) (8) The le fl -hand s ides o f (7). (8) are a pproxim1ltcJy equal w hen q;- 4.'j °. He nce the right-hand s idcs o f (7). (8) s hould ha ve close values too. which gives (9) As a res ull o f conve c ti ve ins ta bility light ffi;lterial intr udes t he lower mantle a nd il ascends up to Ihe uppe r ma nt le Ihrough the lowe r mantle . This movemenl is associa ted wi th a hlrge releas e in t he potentinl encrgy : This mcans that the velocity-components of the slab are of the !lame order of magnit ude both ulong the slab and no rm all y to il. He nce an o riginn lJ y straigh t slab will mpidl y bend in ra homogeneous mantle ta kin g li position close to a ve nical one . Therdore an indined subsidence is possible o oly w hen the man tle below the slab has a muc h hig he r viscosit y than above il. The lIs the nosphe re viseos ity is ,,- 10\'- lcro poise in s tab le continental lu eas (Artyushko v, 1966 ; 1971 a). The upper m1lOtie unde r the a sthenos phcre has a viscos it y lI0!!3. 10 u poise. An increase in t he mantle tempe ratu re by o ne hundre d degrees results in a d ecrease o f ils viseosity by o ne U - A p.g. H . (12) per unil volume o f lig ht mate ria l. Here A p is the densil Y contras l between the lower man tle a nd the light mate rial. H is the lower mantle th ic kness. Taking .6 p = 0 .2 g/cm l , li _ 2000 km we find Chut U - 1000 kalfc m' . 247 (13) E.V. ARTYUSHKOV 6. T - 1ooo" . ~_ ~ t Figure 3 Scheme {JI Cru SI a/Id . r~. ~ • • marr/le SIrI/cl l/rt Îlr I/rt regiO lr /11 SIIbdru·/ iIJII (II Ille malll/ .. o n the <letÎ I·t margilr. tire plaie 01 O"ea"ie lilllfMp /Jere ÎIilO considerably lower than thal of a normal mantle under the c r ust p - 3.35 g/cm ' . Dceanie lithosphe re is usually composed of a thick mantle peridotite layer a nd li thin oeeanic c rust near continental margins. The mean !ithosphere den· s ity is p = 3.27 glcm ' when its thickness is 50 km. this layer including basallic crust of the thickness 7 km and of the mean density p '" 2,8 g/cm J . Anomalous mande is lighler than the oceanic lithosphere. When it cornes to the lithosphere, subduction of the oceanic lithosphere arises tnto the low-velocity mantle layer (the possibil ity of subduc tion into li light material wilho ut indication of its oriSin was earlier s uggested by G. Lo mnit z and C. Lo mni tz). (14) A certain amount of heat is spenl on the s pinel-olivine transition in the upper mantle cooling the light material by a few hund red degrees. Eventually the light material should reach the upper layers of the upper mantle heated by 6. T - 500_700" . , , ..... " , , , , , ____ ... , , , , , _-..rf,· , • .,.. __ , , , , , , , , , r T his e ne rgy is spent on the ovcrcoming of visco us friction and it is event ually transfor med into heat. Rock viscosity slrongly decreases with temperature. After the uplift of a large block of light material through the lower mantle a zone of decreased viscosity appears. An uplift of new blocks bccomes easier along this zone s ubsequently decreasing it s viscosity. Th is is a kind of a Slrong instability concent rating an ascending movement of light material in a relative!y small amounl of pipes of very slrong!y reduced viscosity. It can be s hown that their diameter s hould be - 100 km (Artyushkov. 1979). Because of the low temperalure conduclivity of the lower man tle X - 10- 1 cml/scc. Ihe heat cannot penetra te far through the walls of the pipe. For example the regions are heated a l a distance of ~ 50 km from the walls during the lime of - 50 M . Y. The main portion of the potential energy is released in a relatively small volume of a pipe and il is spe nt on Ihe heating of the lower ma nt le substance and of the light material in the pipe. When the vol ume of light material passing through the pipe is much greater than that of the pipe, the main portion of the heat release is consumed by light material. Taki ng t his quantity as (13) and Cp - 1 kal/cm l wc fi nd that a n increase in tempera HJ re of light matcrial after passi ng th rough the lower mantle should be The thickness of the low-vclocity mantle layer dues not usually exceed a few tens of ki lometers. The underlying mantle shou ld have the de nsity typical of normal mantle. uCl>l'ite it il> sll"UIlgly hcalcu. Otllcrwise <In isu.\oIa lb; upli!! many ki lometers high. o r extremely intense negative isoslatic anomal ies would arise on the surface. Hence wilhout an increase in its density the oceanic lithosphere cannot subs ide decply below the low-veloc ity mantle layer. (15) with respect tO the adjacent stable areas. This resull coincides weil with the above estimates o f the mantle tempe rature on the active margins . Intrusions of large masses of light material from the core-mantle boundary into the upper m<lntle OCCUf in many regions of the Earth : bcneath the mid-ocean ridges. high plateaux on the continents . geosyncline belts and continenlai margins. The oceanic lithosphere includes basaltic c rustal layer. Basait is unstable under:1 pressure p ;;?: 10 kbar and it s hould he transformed inlo edogite when the temperature Îs T - 700-904rC (11 0, Kennedy. 1970). AI such a lemperature the rate of phas e transition becomes very high (Sobolev. 1978). As a resull a heavy edogîte of the density p - 3.s5 glcm1 can be formed afte r a contact with the low-velocity mantJe during the time t ~ 1 M. Y. Eciogite is heavier than the normal mantle. Hence the mean dens ity of the lithosphere becomes higher than in Ihe normal mantle thus permitting the developmenl of s ubduction into deeper Jayers . A hOI low-velucity manlle substance is segregating from light ma terial in the asthenosphere a nd it ascends to the lithosphe re and c rustal bottom. Interaction of th is s ubs tance with the crust a ppenrs to be the main cause of tec tonic movements both horizontal and vertical (Artyushkov. 1979). The· low-velocity man tle lenses exist under the crust in mOSI regions of high teClonic activity. The best known examples are the Basin and Range province (Cook, 1966) and t he mid-ocean ridge s (Talw<lni el al .. 1965). Decreased seismic velocities are also observed under active marBins (Yoshii. Asa no. 1971 : James. 1971). Phase transitions were earlier suggested a~ the cause of subduction (R ing wood, 1%9). However thi s phenomenun cannot itse[f produçe subduçtion. Otherwise subdUc tion would take place in any region. An impulse is necessary to subside the lithosphere for a few ten kilometers 10 put the gabbro-eclogite tra nsition into operation. Thi s impulse is provided by a contact of the low-velocity mantle with the lithosphcre . MECHAN ISM OF SUBDUCTIO N U~ing the above sc heme of the structure of the upper mantle beneath active nUlrgins the mechanism of subduction can be presented as follow s. Large masses of light and strongly heated material arc injected into Ihe upper m<lntlc below sorne continental margins as a consequence of density differenliation on the corc-mantle boundary ( Fig. 3). Low-velocity mantle is segregated from this material. The low.ve!ocity mantle density i~ p - 3.1-3,25 glcm J which is An inçrease in density and subsidence of the oceanic lithosphere into the mantle s hould he accompanied by ilS bending under the ocean. The wid th of a zone of a surficial subs idence s hould be a few ti mes greater than the lithosphere thickness (WaJcott. 1970). As a result deep-seil trenches arise in the re gions of subduction on the surface. 248 MECHANISM OF FORMATION OF ACTIVE MARGINS acting on the oceanic lit hosphere fro m subduction when the fo rce resuhi ng from the spreading of Ihe ridges is known (Artyushkov, 1979): The bounda ry between a strongly heated light material and a cool nor mal manlle o n t he oceanic s ide of the aClive margins can be of a rather arbitrar y shape . No rmal mantle viscosity is at leas l several o rders of mag nilude higher than that of t he light ma terial. Oceanic lithosphere s ubsiding into Ihe mantle fro m the s urface meels the above boundary and begins 10 move a long it. Thus il is separating the afeas of hot light material and cool no rmal ma ntle. L = (0.3-0.7).109 bar. c m . (16) T he latter quanlity is [j few times s maller th[jn the compressional force acting o n the oceanic bas ins f rom the ridges. Separation of a portion of oceanic cruSI from the s ubsiding lithosphere al a depth - 150-200 km may explain the island a rcs volcanis m o bser ved on the surface. The real density contras t between Ihe subd uc ling plates a nd the surro und ing mantle is a very uncertain q ua nli ty. Deep-focus earthquakes occur in some regions like Pa mirsHindu- Kush zone where s ubduction of oceanic lithosphere from the s urface should have been terminated a long lime ago. In this case oceanic lilhosphere would be expected to s ubside very deeply under a high dens ity contrac!. Hence its density may appear to be very c lose to that of the sur rou nding medium . FORMA TlON O F MARGINAL SEAS The main po rtion of the te rritory of active margins is occupied by ma rgi nal seas . They include both deeply s u bme rged continen tal bloc ks and the regions underla in the oceanic crus t. Formation of marginal seas can be expJained as folJo ws (Art y ushkov. 1979). Ah er the 10w-veloCÎty mamie cornes to the crus t on continen tal margins, gabb ro-eclogite phase trans ition occurs in the basaltic layer of the continental crust in ma ny regio ns. Heavy eclogite tears away of the c rust a nd si nks into the mam ie . This results in crustal thinning and s ubsidence. Interac tion of a hot low-velocity mande wilh the upper.• gra nitic" geophysical layer of the crust can strongly increase seismic velocities in it up to Ihose typical o f the basaltic laye r in ce rtain cases. Large lenses of a 10w-veloCÎty ma ntle o f a lemperalu re T - 1200"C ma y then [jrise under a strongly atte nua ted crust in ma rginal seas. They produce the same force in the lithosphere as that crealed by the mid-ocean ridges. In th is case marginal seas can become local centers of sea-fl oor spreading. As a res ull the regions underlain by typical oceanic crust can a rise wi thin t he basins produce d by ve rtical subsidence of Ihe continental cr ust. Thus the main processes taking place on the active margins are associated wit h the injection of light heated male rial from the core-manlle boundary into t he upper mantle. More exactly continental margins beco me active aller this injec· tion occurs beneath them . S ubduction is a phenomenon of a minor energetic scale as compared 10 the injection of light material. It is o nly one of t he conseque nces of t he injec tion. This conclus io n is opposite to a popuJar idea that s ubd uc tio n is the main cause of d ynamic processes on the active ma rgins. DRI VING FORCE FRO M S UBDUCTION A nu mber of fa ults are formed in the oceanic lithosphere wh il e ilS be nding under a deep-sea trenc h before subduction. Hence even a large gravit y force (if il exists) ac ti ng o n a long subduc ting plate in Ihe upper mantle cannot be tra nsfe rred 10 t he lithosphere [ocated o n the surface. Thus the tensile force fro m s ubduc tion pulling the oceanic plates canno t be ve ry large . The other major fo rce acting in Ihe oceanic Iithos phere results fro m the spread ing of the lo w-velocity ma nlle beneath the mid-ocean ridges (Art yus hkov. 1971 b ; 1973). T his force is compressio nal and it can be calculated, when the crust and the low-ve locity mantle s truc tu re is known beneath the ridge. In this case the above force can be represe nted as a function of the relief of the ridge. The data o n Ihe focal mechanisms on the ridges s how that tens ile force is a cting in the lithosphere in their axial zones (S ykes, Sbar, 1973). It c hanges to compressio nal fo rce towards the margins of the ridges betwe en the magne tic a no malies 5 a nd 10. These data permit to find the fo rce L REFERENCES Artyushkov E. V., 1973. Stresses in the lithosphere caused by crustal thickness inhomogencÎties. J. Geophys. Res .. 18. 7675·7708. Artyushkoy E. V.• 1919. Geody"amics. Na uka.. Moscow. 328 p. (in Russi:'"J. Barua n!!:l M. , ISIIek5 B., 1911. 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