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TECTONICS, VOL. 10, NO. 2, PAGES 455-473, APRIL 1991

THE ROLE OF DEFORMATION IN THE FORMATION OF METAMORPHIC GRADIENTS: RIDGE BENEATH THE

BradleyR. Hacker

Departmentof Geology,Stanford University, Stanford,

Abstract. Two tectonicscenarios have been proposed for simulationspoint out the importanteffect synmetamorphic genesisand emplacement of the Oman ophiolite. One suggests deformationcan have on the formationof metamorphicfield thatthe ophiolite was generated at a spreadingcenter, the gradients. In contractionalfault zones,metamorphic field othersuggests generation within an intraoceanicarc. An gradientsmay be normalor invertedand contractedor integratedthermal and kinematic model of thetemperature, extendedrelative to the initial thermobarometricgradient, stress,rock type, and displacement fields during early stages andadjacent fault zonerocks may display crossing PT paths. of theemplacement of theOman ophiolite was developed to Unusuallycontracted or extendedmetamorphic field testthese two possibilities.The thermalevolution was gradientsmay also developin the footwallsof normalfault calculatedby a finitedifference algorithm for heat zoneswhere portionsof the heatinghangingwall accrete to conduction,considering heats of metamorphicreactions, the uplifting, cooling footwall. deformationalheating, heat advection by flowingrock, mantleheat flow, and radioactiveheating. The stressand INTRODUCTION displacementfields were calculated by ananalytical model usinga velocityboundary condition, power law constitutive The mechanismsby whichslabs of oceaniclithosphere tens relations,and a brittle frictionalsliding relationship. Field of thousandsof squarekilometers in areaand as much as 20 observationsin Oman can be satisfiedwith the spreading km thickare emplacedonto less dense center model but not with the arc model. Moreover, remainobscure. Tethyan-type are emplacedonto simulationsindicate that the ophiolitewas probably <2 m.y. continentalmargins by subductionof the edgeof a continent old at the time of intraoceanicthrusting and that shear beneathoceanic [e.g., Moores, 1982], but the stressesof ~ 100 MPa were attainedduring thrusting. In evolutionof the deformationand metamorphismin sucha additionto satisfyingcurrently available field constraints, convergencezone is poorly understood.Many ophioliteslabs thesesimulations also indicate specific further work that are boundedbelow by soles,or basalfault zones,which will helpresolve the controversy over thevolution of this consistof high-temperaturemetamorphic rocks. Thesesoles uniqueorogen. Future work should be directedtoward (1) accreteto the baseof Tethyan-typeophiolites because the usingthermobarometry and thermochronology to constrain fault zone at the baseof the ophiolitemoves progressively the spatialvariation in PTt pathsof the metamorphicsole; downwardin responseto the cooling(and hardening)of the (2) datingthe igneous genesis of theophiolite; and (3) using ophioliteperidotite and the heating(and softening)of the quartziteand recrystallized grain size mafic rockssubducted beneath the ophiolite[Pavlis, 1986; paleopiezometersto infer stresses during thrusting. These Peacock,1987; Hacker, 1990]. They containinverted metamorphicfield gradients more extreme than 1000 K km-1, gradingfrom myloniticperidotite downward through Copyright 1991 granulite-,-, and -faciesrocks into by theAmerican Geophysical Union unmetamorphosedmaterial. Thesesoles provide the key to understandingophiolite emplacement because their Papernumber 90TC02779. metamorphicminerals and texturesrecord their metamorphic 0278-7407/91/90TC-02779510.00 and deformationalhistory. Hacker [1990] presented 456 Hacker:Ridge Subduction Beneath the Oman Ophiolite simulationsof the early stagesof the thermaland kinematic Bdchennecet al., 1988;Robertson et al., 1990a]. The ophiolite, historyof thesole of theOman ophiolite based specifically now disruptedinto a numberof relativelyintact blocks, on the ophioliteemplacement hypothesis of Lippardet al. exposesa classicophiolite pseudostratigraphy as recognized [1986]. It wasconcluded that Lippard et al.'s [1986]model, by thePenrose Conference Participants [ 1972]. The Batinah whichconsiders the Omanophiolite to haveformed in an Complexincludes m61ange believed to havebeen derived intraoceanicarc, is unableto explainseveral significant field from beneaththe ophioliteduring late-stage extensional observations in Oman. An alternative model wherein the faulting and allochthonousthrust sheets of sedimentary Omanophiolite formed at a mid-oceanspreading center, rocksthat slid onto the ophiolitelate in the emplacement presentedby Coleman[1981], Boudierand Coleman [ 1981], history[Woodcock and Robertson, 1982b; Robertson and Boudieret al. [1988] andMontigny et al. [1988],is considered Woodcock, 1983a]. in thispaper. For thepurposes of thispaper, a "metamorphic The presentlyexposed ophiolite extends more than 600 km field gradient"is a gradientof (peak)metamorphic north-south,and is up to 150 km wide and 5-10 km thick. It is temperaturesrecorded in rocks,and a "thermalgradient" inferredto havebeen 16-20 km thick (Figure2) prior to its refersto an instantaneousgradient of temperaturesin Earthat dismembermenton the cratonby extensionalfaulting. The any giventime [Spearand Peacock, 1989]. ophioliteconsists of a basalmetamorphic sole (150-200 m), peridotitetectonite (8-12 km), igneousperidotite and GEOLOGY OF TIlE OMAN OPHIOLITE (0.5-6.5 km), sheeteddikes (1-1.5 km), (0.5-2.0 km), and overlyingpelagic sedimentary rocks (<100 m) [Boudier MostTethyan ophiolites were emplaced onto the Afro- and Coleman,1981; Christensenand Smewing,1981; Arabian platform in Cretaceoustime and then further Manghnaniand Coleman,1981; Pallister and Hopson, 1981; deformedand metamorphosed during the continent-continent Lippardet al., 1986]. The metamorphicsole is the thrustzone collisionthat ended the Alpine [Lippard et al., 1986]. at the baseof the ophioliteand is composedof partially TheOman ophiolite, however, was spared the continent- melted(rarely) lower granulite/upperamphibolite facies continentcollision and constitutes a natural laboratory for rocksat the top that gradedownward into lower greenschist investigatingophiolites. Diverse field studiesof this faciesrocks. The majorityof the amphiboliteshave trace complexhave been published, including five journal issues or element abundances that are intermediate between normal volumesdedicated to theOman ophiolite [Glennie et al., mid-oceanridge and within-platetholeiites, similar to 1974;Coleman and Hopson, 1981; Lippard et al., 1986; volcanicrocks in the underlyingHaybi Complex[Searle and Boudierand Nicolas, 1988; Robertson et al., 1990b]. Malpas, 1980;Lippard et al., 1986]. The amphibolite-facies TheOman Mountains comprise a southwestdirected rocksare gneissicto schistose,and amphibole and plagioclase systemof oceanicthrust sheets (Figure 1) thatwere thrust crystalscontain crystal-plastic deformation features. over a Precambrianto Cretaceouscontinental autochthon in Greenschist-faciesrocks underlying the amphibolitesinclude Cretaceoustime and then overlapped by Maastrichtian and metamorphosedsandstone, , limestone, chert, and Tertiarysedimentary rocks. The autochthon includes volcanicrocks, probably derived from the Hawasina continentalshelf rocks of theArabian platform resting on Assemblageor the Haybi Complex. Initial metamorphismof earlyPaleozoic and possibly late cratonal rocks the greenschist-faciesrocks occurred during deformation, [Glennieet al., 1974;Gass et al., 1990;Pallister et al., 1990]. producinga penetrativeschistosity, and later mineralsgrew The thrustsheets are, from structurallylowest duringstatic conditions [Lippard et al., 1986]. The least (southwesternmost)to structurally highest disruptedexposures of the metamorphicrocks contain 60-100 (northeasternmost),theSumeini Group, the Hawasina m of amphibolite-faciesrocks formed at peak temperatures Complex,the Haybi Complex, the ophiolite, and the Batinah rangingfrom 500ø to 800øCand 80-120 m of greenschist- Complex.The Sumeini, Hawasina, and Haybi are continental faciesrocks formed at peaktemperatures ranging from slope,slope-basin, and seamount rocks, respectively, deposited -200ø(?) to 500øC [Ghentand Smut, 1981; Searleand in a MiddleTriassic to Late Cretaceous passive margin and Malpas,1982; Searle, 1990]. Thereare apparent oceanbasin northeast of theArabian [Glennie et al., discontinuitiesin metamorphicgrade within the solethat 1974;Woodcock and Robertson, 1982a; Lippard et al., 1986; indicatethe presenceof postmetamorphicfaults, but the

SW NE

,1!i, ! i ! ! i ! i i i i i i i i i i i i i i i i i i i ======

Fig. 1. Schematicillustration of the thrustsheets in the Oman Mountains. Hacker:Ridge Subduction Beneath the Oman Ophiolite 457

Actual Model lithosphereof "normal" chemistryand thickness[Alabaster et al., 1982;Pallister and Gregory,1983; Lippardet al., 1986; Sheeted dikes Erneweinetal., 1988]. Eleven 206pb-238U ages on plagiogranitesin the intrusivesequence are in therange 95.4- .11.•..• Pillowlava r.'.'.' Plutonic 93.5 Ma, althoughthere are two that are slightlyolder (96.9 r.'.'.' and 97.3 Ma); all ageshave estimated uncertainties of +0.5 r.'.'.' and m.y. [Tilton et al., 1981]. The lowermost,oldest lavas may be r.*.*.* gabbro cogeneticwith the underlyingsheeted dikes, whereas younger lavas have trace element affinities with tholeiites [Lippardet al., 1986;Ernewein et al., 1988]. Pearceet al. [1981] and Alabasteret al. [1982] suggestthat the younger Peridotite lavaserupted above a subductionzone. Erneweinet al. [1988], D ½,",: PeridotRe however,point out that pelagic,not volcaniclastic, •,,½,,,,• sedimentaryrocks are interbeddedwith the youngerlavas and km prefer the interpretationthat the lavas inheritedtheir subductionzone componentfrom eithera regionalmantle chemicalanomaly or from second-stagemelting of their •Metamorph•c sole mantle source. This is a critical difference because Pearce et al. [1981] andLippard et al. [ 1986]interpret the Oman Quartzite ophioliteas part of an arc that formedabove an intraoceanic and shelf sediments subductionzone, whereasBoudier and Coleman [1981], Coleman[1981], Erneweinet al. [1988], Montignyet al. [1988], Nicolas et al. [ 1988], Boudieret al. [1988], and Thomas Fig. 2. A pseudostratigraphicsection of the Omanophiolite et al. [1988] believethat the ophiolitewas generated at a mid- and underlyingunits and the simplifiedsection used in the oceanspreading center. model. Sections measured in the field have 0.5-2.0 km of Hacker [1990] reportedsimulations of the formationof pillow , 1.0-1.5 km dikes, 0.5-6.5 km plutonic the soleof the Oman ophiolitebased on the "arc" model peridotireand gabbro,8-12 km peridotitetectonite, and proposedby Lippardet al. [1986]. The ageof the ophiolite 0-0.2 km of metamorphicrocks [Boudier and Coleman, 1981; was assumedto be 5 m.y. at the inceptionof subduction,and Christensenand Smewing,1981; Manghnaniand Coleman, the effectsof subducting5- to 100-m.y.-oldlithosphere 1981;Pallister and Hopson, 1981; Lippard et al., 1986]. The beneaththe ophiolitewere examined. The subductionzone pseudostratigraphicsection is a compositereconstruction was assumedto dip 45ø to a depthof~17 km, wherethe dip basedon numerousincomplete sections of variablethickness, changedto horizontal. None of the simulationswas able to and is inferredto representthe pseudostratigraphyof the reproduceeither the peak metamorphictemperatures inferred ophiolitebefore disruption. for the Oman sole [Ghentand Stout, 1981; Searle, 1990], or the wide rangeof K/Ar agesreported for the metamorphic sole[e.g., Montigny et al., 1988]. aforementionedthicknesses and temperatures imply One salientfeature of the previousstudy [Hacker, 1990] is metamorphicfield gradients of >1000K km-1. The that the accretionof metamorphicrocks at thebase of the amphibolite-and greenschist-facies rocks (Figure 3) yieldK- Oman ophiolitecan be explainedwith a transient,two- Ar agesof ~100-70 Ma [Allemanand Peters, 1972; Searle et al., 1980;Lanphere, 1981; Montigny et al., 1988]. Twofoliations have been mapped in theperidotite, (1) a pervasivecoarse porphyroclastic foliation, and (2) a fine- grainedporphyroclastic to myloniticfoliation in the lower 0Amphibole I 150-2000m of theperidotire, which overprints the pervasive foliationand increases in intensitydownward toward the ß M.us.covite I metamorphicsole [Searle et al., 1980;Nicolas et al., 1980; Boudierand Coleman, 1981; Boudier et al., 1988;Ceuleneer et al., 1988]. The pervasivefoliation is inferredto haveformed duringhigh-temperature, low-stress asthenospheric flow, and lllllllllllllllllllllllllllllll the myloniticfoliation is inferredto have formedat lower 100 9o 8o temperaturesand higher stresses during emplacement of the ophiolite[Boudier and Coleman, 1981]. Olivine m.y. before present microstincturesand two- thermometry have been usedto infertemperatures of mylonitizationof 750ø-1000øC [Boudierand Coleman, 1981; Lippard, et al., 1986;Ceuleneer Fig. 3. K-Ar agesfrom the metamorphic sole of theOman et al., 1988]. ophiolitefrom Allemanand Peters [1972], Lanphere [1981], Theperidotire tectonite, igneous peridotire and gabbro, and Montigny et al. [1988]. Mean valuesand lc• standard sheeteddikes, and lowermost lavas represent oceanic deviations(horizontal bars) are shown. 458 Hacker:Ridge Subduction Beneath the Oman Ophiolite dimensionalthermomechanical model using laboratory ophioliteand the lower subducted plate. Considera one- measurementsof the thermal and mechanical properties of dimensionalcolumn through both plates (Figure 4a). The rocks. Figure4 illustratesthe importance of this initial thermalgradient is a sawtooth,and deformationoccurs synmetamorphicfaulting, using a conceptuallysimplified at thebase of theperidotite where the peridotite deforms by one-dimensionalapproach for easeof illustration.It showsa powerlaw creepat stresseslower than those required for faultzone between two moving plates, the upper plate frictionalsliding. As thermalconduction occurs, the thermal

17 km Peridotite Lower

! !

Plate ' deformation in a Peridotite. peridotite at .base of upper Lower plate Plate Basalt

Temperature Stress

deformation in narrower zone b of peridotite at baseof upper plate

_ Temperature Stress

deformation chiefly in c metamorphosed basalt at top of lower plate

Temperature Stress Fig.4. One-dimensionalcartoon ofhow conductive decay of a sawtooth-shapedthermal gradient can producelocalized deformation atprogressively deeper levels, resulting in ofmetamorphic rocksto thebase of theophiolite. (a)-(c) Steps in a timesequence. Note that the simulations are not one- dimensionalwith a sawtooththermal gradient but are two dimensional andconsider time-dependent evolutionof temperatureand faulting. Hacker:Ridge Subduction Beneath the Oman Ophiolite 459 gradientrelaxes, and the upperportion of the lowerplate TECTONIC EVOLUTION OF THE OMAN OPHIOLITE basalticlayer is metamorphosedto greenschistfacies (Figure 4b). The stresssupported during power law creepof Thefield observations described above suggest the greenschistis greaterthan the stressfor frictionalsliding, so followingtectonic evolution of theOman ophiolite. In Late deformationcontinues only in the upperplate peridotire, Permiantime a passivemargin formed on theArabian althoughcooling of the upperplate causes the deformationto platform[Blendinger et al., 1990]. Riftingof thebasin and be concentrated in a narrower band. At a later time the lower subsequentcreation of newoceanic lithosphere resulted in platereaches amphibolite-facies conditions and is thenwarm terrigenousturbidire deposition succeeded by MiddleJurassic enoughfor powerlaw creepto occurat stresseslower than calcareousturbidire and chert deposition [Robertson and thosefor frictionalsliding and lower thanthose for power Searle,1990]. In LateJurassic to EarlyCretaceous time, slow law creepof peridotire(Figure 4c). In thesethree steps, the pelagicsedimentation was dominant. Radical change occurred zoneof deformationmoved progressively downward into the in Albian to Cenomaniantime, when intraoceanicsubduction warminglower plate, and upperparts of the lower plate wasinitiated between -101 and95 Ma. Metamorphismin the basalticlayer successively accreted to thebase of the ophiolitesole spanned Albian to Campaniantime (101-70 ophiolite. Ma); theamphibolite-facies metamorphism occurred between This paperaddresses the possibilitythat the Oman 101and 89 Ma, andthe greenschist-facies rocks developed ophioliterepresents the northeastflank of a mid-ocean from 87 to 76 Ma. Duringthis interval, the allochthonous spreadingcenter. The importantdifferences between this plateswere assembledand faultedonto the Arabian studyand the previousone [Hacker,1990] are thatthe continentalmargin. By Maastrichtiantime (70-65 Ma) all possibilitiesof subductionof very youngcrust and theplates had been partially eroded and transgressed by subductionat a shallowangle (10 ø) areconsidered. The shallow-watercarbonate rocks. These parts of theophiolite simulationssuccessfully reproduce both the peak developmentare agreed upon by mostauthors [Lippard et al., metamorphictemperatures inferred for the Oman soleand the 1986;Boudier and Nicolas, 1988]. However,as mentioned widerange of K/Ar agesreported for theOman sole. They previously,there is disagreementregarding the petrogenesis suggestthat the Omanophiolite likely originatedin oceanic andtectonic implications of theyounger lavas. Lippard et al. lithosphereless than 2 m.y. old, andthat shear stresses of [1986;p. 155]suggest that the ophiolite evolved on the -100 MPa were attained within the subduction zone. southwestflank of a backarcspreading center above a

Spreading protoOm7nophiolite ======:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::...... '•"":'"=•-.'•"• - _- - _

Intra-oceanicthrusting

======• -'-'-'.'-'-'-''-''-'.'.'.'.'-'.'-'-'' '-"' .•...•.•.•...... •...... •...... •...... •...... ••...•.•...... •...... j "'_. - .... ::::::::::::::::::::::::::::::::::::::::'"''"'"'" "'"""'"'"'"'"'t• B ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: -'"•.2'"" .... '•'""•:"'"••:.•-:.'.•.:.:.'. • metamorphicsole 87 to 76 Ma' Emplacementonto the craton C

Arabian craton

Fig. 5. Hypotheticalpalinspastic reconstruction of the genesis and emplacement of theOman ophiolite [afterBoudier and Coleman, 1981; Boudier et al., 1988;Montigny et al., 1988]. (a) Priorto 101-95 Ma: Spreadingis activein theocean basin; the two plates of oceaniccrust are shown in differentshades. (b) 95-87 Ma: Theophiolite is thrustover adjacent oceanic crustal rocks, which are accreted at amphibolite- faciesconditions to thebase of theophiolite, shown by a heavyline. (c) 87-76 Ma: The ophioliteis thrustover continental rocks, which are accreted at greenschist-faciesconditions to thebase of the ophiolite. 460 Hacker:Ridge Subduction Beneath the Oman Ophiolite

subructionzone dipping away from thecontinent. They toward the southwest),following rotation of-75 ø proposethat the intraoceanicsubruction initiated at the counterclockwise[Thomas et al., 1988]. contactbetween the youngcrust and the older(5-100 m.y. older) oceaniclithosphere into which it was intruded. This MODEL possibilitywas addressed in a previousstudy [Hacker, 1990]. In contrast,Boudier and Coleman [1981], Montignyet al. A thermomechanicalmodel developed previously [Hacker, [1988;p. 359], Emeweinet al. [1988;p. 270], Nicolaset al. 1990] to testpart of the arc tectonicmodel proposed by [1988;p. 51], Boudieret al. [1988;p.289], and Thomas et al. Lippardet al. [1986], is usedin thisstudy to testpart of the [1988;p. 318] suggestthat the ophioliteoriginated on the ridgetectonic model proposed by Boudierand Coleman northeastflank of a mid-oceanspreading center and that [1981],Boudier et al. [1988]and Montigny et al. [1988]. The intraoceanicsubruction was initiated at the ridgeaxis (Figure modelpredicts the stressfield, temperaturefield, rock type, 5). It is this "ridge" modelthat is addressedin the current anddisplacement field historiesduring the earlystages of the study. ophioliteemplacement process, and these predictions are then Accordingto the Erneweinet al. [1988] modelthe absence shownto comparefavorably with field observations.Later of arc rocksin the ophioliteseems to conflictwith the stagesof theemplacement process, which may have involved inferencethat the ophioliteformed in an oceanbasin with a gravitysliding, are not modeled.Boudier et al. [1988]and half width of -450 km [B6chennecet al., 1988]. For the Montignyet al. [1988] proposethat spreadingat a mid-ocean ophioliteto havereached the craton,the 450 km of oceanic ridgeformed the Omanophiolite (Figure 5a). Later,at ~101- lithospheremust have been subducted beneath the ophiolite, 95 Ma, subructionbegan along the spreadingaxis (Figure5b). surelyenough material could have melted to form a magmatic Basalticrocks from the underthrustplate were arc. Thereare at leasttwo waysto reconcilethese metamorphosedat amphibolite-faciesconditions and accreted observations.If thesubruction occurred at a shallowangle, to the baseof the ophiolite(Figure 5b) from 101 to 89 Ma, meltingmay have been inhibited or haveproduced liquids and then continental material was accreted to the sole at unliketypical arc [e.g., Lipman et al., 1971;Coney and greenschist-faciesconditions from 87 to 76 Ma (Figure5c). Reynolds,1977]. Alternatively,the 450 km of oceanic Theconvergence velocity was assumed tobe 50 mm yr -1 lithospheremay havebeen duplicated by thrustingbefore [Boudieret al., 1988, Figure 12], althoughthe effectsof beingsubducted beneath the ophiolite,such that the eff•tive variationsin thisrate were alsoinvestigated. The rate of width of the subductedlithosphere was muchless than 450 subductionmust have been at least 25 mm yr -1, because ~630 km. km of oceanicand continentallithosphere were subducted The suggestionthat the Omanophiolite came from the beneaththe ophiolitein ~25 m.y. Convergenceparallel and n9rtheasternside of a northwest-southeastspreading ridge is perpendicularto the ridge axis was tested. The total amount basedon the northeastwardthickening taper of the slab of oceaniclithosphere subducted beneath the ophioliteis [Lippardet al., 1986]. Otherattempts to determinewhich estimatedvery roughlyas -450 km by palinspastic flank of a spreadingcenter is representedin Oman,based on reconstructionof basinalrocks [Glennie et al., 1974;Lippard chillingdirections [Pallister, 1981; Lippardet al., 1986, et al., 1986;B6chennec et al., 1988;Cooper, 1988]. The total p. 99], the inclinationof cumuluslayering in plutonicrocks amountof continentallithosphere subducted is estimatedas [Pallisterand Hopson, 1981], andthe shearsense recorded in 180 km, from the width of the ophioliteoutcrop on the the mantle tectonite [Boudier and Coleman, 1981; Nicolas et Arabiancraton. The subductedoceanic lithosphere was a few al., 1988] yield equivocalresults. million yearsold at the time of subduction,and probably Various structural indicators have been used to infer the developedatrates > 10-20mm yr- 1 [Boudieretal., 1988], thrustingdirection during emplacement of the Oman perhapsup to 50 mm yr-1 [Pallister and Hopson, 1981; ophiolite. The structuresare not uniformfrom one massifto Nicolaset al., 1988]. The convergencevelocity was assumed anotherbut in generalindicate ridge-parallel movement in the tobe 50 mm yr-1 in thesimulations. The subducted myloniticperidotire, and ridge-orthogonalmovement in the continentallithosphere was ~720-760 m.y. old [Glennieet metamorphicsole [Boudieret al., 1988;Thomas et al., 1988]. al., 1974; Gasset al., 1990; Pallister et al., 1990]. Paleomagneticdata [Thomaset al., 1988] indicatethat the The angleof subductionin the simulations(10 ø) was taken variousmassifs of the Oman ophiolitehave not rotated to be constantwith depth;note that this is different from the significantlywith respectto one another,so thesevariations geometryassumed by Hacker [1990], where the subduction in thrustingdirection represent spatial variation in the occurredat 45 ø to a depthof ~ 17 km, and thenwas horizontal. emplacementdirection. It hasbeen suggested by Boudieret al. [1985; 1988] thatthe Paleomagneticdata also allow Thomaset al. [1988] to dip of the subductionzone can be inferredfrom the wedge suggestthat the ophiolitewas createdat a northeasttrending shapeof the Oman ophiolite. Boudieret al. [1988] statethat spreadingcenter, was rotated75 ø counterclockwisesoon field observationssuggest that the dip of the subductionzone thereafter and was then rotated 40 ø clockwise before was 2o-3ø , however,published cross sections do not concur emplacementonto the Arabiancraton. Togetherwith the with this figure. The ophioliteslab thickensnortheastward paleomagneticdata the vergencedirections indicate that the from2-4 km thickin Bahlato 10-12 km thickin Rustaq,37- Oman ophiolite always moved toward the southwest:initial 72 km away[Boudier et al., 1988],indicating a dip of 5ø-15ø. movementrecorded by the peridotirewas parallel to the ridge In the Wadi Tayin massifa thicknesschange of 3.7 km occurs (towardthe southwest),and later movementrecorded by the overa distanceof 28.6 km, indicatinga taperof 7.4ø [Boudier metamorphicsole was perpendicularto the ridge (also andMichard, 1981; Boudier et al., 1988;Figure 2a]. Hacker:Ridge Subduction Beneath the Oman Ophiolite 461

Moreover,gravity modeling of Shelton[1990] showsa (Figure6). Kusznir[1980], Bratt et al. [1985], andMorton wedge-shapedslab with an angle of-11 ø. Thusa subduction and Sleep[ 1985] havedeveloped models of the coolingof zonedip of 10ø waschosen for thepresent study. oceaniclithosphere that attemptto accountfor the effectsof Subductionzones formed in youngoceanic lithosphere are hydrothermalcirculation, latent heat of crystallization,and expectedto be inclinedat shallowangles because of thelow thepresence of discretemagma chambers. Their preferred densityof younglithosphere. Turcotte et al. [1977] modelsare not significantlydifferent than the modelused in calculated that several hundred kilometers of oceanic thisstudy for lithosphereages as young as 0.1 m.y., whichis lithospheremust be overriddenby materialof mantledensity the youngestage considered here. The thermalgradient before(steep) subduction can be initiatedbecause of the within the continentallithosphere was calculatedusing heat flexuralstrength of thelithosphere. Sacks [1983] showed flow and heatproduction values for the Appalachianorogen thatsubducted lithosphere <20 m.y. old is buoyantwithin of the United States[Morgan, 1984], a passivemargin for asthenosphereand will subductsubhofizontally beneath an approximatelythe sameduration as the Arabiancraton. overridingplate. Rock Types Thermal Model Forthe purpose of modeling,the pseudostratigraphy of the Calculationswere made for a crosssection 34 km thick and ophiolite(Figures 2 and6) wassimplified to a 7-kmbasaltic 100km long(Figure 6). Theupper plate ophiolite occupied layerand a 10-kmpefidotite layer. Because of smallthickness roughlythe upper fight portion of thisarea, and the lower (<100m), thepelagic sediments overlying the Oman ophiolite platesubducted matefial began to theleft and was subducted [Robertsonand Woodcock, 1983b; Lippard et al., 1986]were beneaththe upper plate. The cross section was divided into ignored.The grid nodes in thebasaltic layer were assumed to nodesspaced 1 km verticallyand 2 km horizontally.In a 1- containmineral assemblages appropriate to theirpressure and km-thickregion above and below the subduction zone the temperature:either prehnite-pumpellyite, greenschist-, or verticalnode spacing was reduced to 100m sothat more amphibolite-faciesminerals. detailed information could be collected in the fault zone. The thermalevolution was evaluatedby solvingthe Thermal Parameters conservationof energyequation with a two-dimensional alternatingdirection implicit finite differencealgofithm. Therocks were assigned adensity of3000 kg m -3, a heat Heat conduction,heats of metamorphicreactions, capacityof 1200 J kg-! K-!, anda thermalconductivity of3 J deformationalheating, radioactive heat production in K-1m-1 s-1. The rock types change through metamorphic continentalcrust, advection of heatby rock displacement reactions,which releaseor consumeheat. In the model the weretaken into account, whereas kinetic energy and adiabatic zeolite<-> greenschist and greenschist <-> amphibolite compressionwere neglected. During subduction, heat was transitionsoccur instantaneously at 250øCand 450øC, allowedto advectbut not conductthrough the endsof the respectively.A value of 2.44 x 108J m -3 was used for the model. After subduction,erosion and heat flow from the dehydrationofzeolite-facies rocks, and 1.2 x 108J m -3 for mantle were considered. dehydrationof greenschist-faciesrocks to formamphibolite. The temperaturefield at theinitiation of eachsimulation For a discussion of the selection of these values and an wascalculated after Parsons and Sclater[ 1971] anddepends evaluation of the effects of their uncertainties,see Hacker onlyon theage of thelithosphere and the pefidotite solidus [1990].

Area of Calculation

------'-•' ...... 750øC-- --'Subduction .... < ...... 1050oc.....

• peridotite layer I •' 100 km '•

Fig.6. Initialconditions ofa simulation where 2-m.y.-old lithosphere formed at50 mm yr -1 is subductedbeneath 2-m.y.-old lithosphere formed at thesame rate. Theplate on the left represents the lithospheretobe subducted beneath the plate on the right. The heavy diagonal line marks the predefined 10 ø trace of the subduction zone. 462 Hacker:Ridge Subduction Beneath the Oman Ophiolite

Mechanical Parameters type),was consideredto be the only activemode of deformationat that node. At low temperaturesand pressures, The stressand displacementfields were calculatedwith an brittle failure occursbecause power law creepstrengths are analyticalmodel usingvelocity boundaryconditions, large at low temperatures,whereas the brittle strengthis temperature-dependentpower law constitutiverelations, and smallat low pressuresand assumed to be independentof a pressure-dependentbrittle frictional slidingrelationship. temperature.Fluid pressurewas assumedto be hydrostatic. The horizontalvelocity boundarycondition specifies the rate At high temperaturesand pressures, power law creepoccurs at which materialis subductedbeneath the ophiolite. The becausethe brittle strengthis large at high pressures,whereas stressestimate is the result of the velocity boundary power law creepstrengths are low at high temperaturesand conditionand therheology of therocks. The displacement assumedto be independentof pressure.See Hacker [1990] for field defines the advection and determines where the a discussionof the selectionof these values, and an evaluation deformation occurs. of the effects of their uncertainties. The strengthof rock wascomputed as a functionof pressure,temperature, strain rate, and rock type. Threemodes Calculation Procedures of deformationwere considered:(1) brittle frictionalsliding, (2) power law creep,and (3) transitionalbehavior (Figure 7). A time stepof 0.01 m.y. wasused to ensurestability of the Whichever mode of deformationrequired the lowest stressat thermalcalculations. Each time stepwas composedof the a givennode (i.e., at a givenpressure, temperature, and rock following'

Differential Stress (MPa) 5O 100 150 o 0 0.36 -• Frictional Shdlng _'

20{]

0.2 4o{

ß

Power-Law Cree

= A • nexp(-H/RT) o ß 80g

0.6

20 1000

0.8 • ! 25 o 25 50 75 Shear Stress (MPa) Fig.7. Strengthwas determined as a functionof pressure,temperature, and rock type. At shallow depths,low pressures, and low temperatures,frictional sliding occurs; the strength was determined by a relationshipof theform: (•s = I,tan (l&) (pressuresare shown on the vertical axis, and the strain rate is 10-12 s-l). At greatdepths, high pressures, and high temperatures, power law creep occurs; the strength wasdetermined bya relationship ofthe form: [ = A(•n exp(-H/RT) or(• = ([/(Aexp(-H/RT)))1/n (temperaturesare shown on thevertical axis). A transitionalregime occurs at intermediatedepths wherethe frictional sliding and power law creep strengths exceed 200 MPa differential stress. 3•, pore pressurefraction of totalpressure; (•s, shear traction; g, friction;(•n, normal traction; [, strain rate; A, pre-exponentialconstant; (•, differentialstress; n, stressexponent; H, activationenthalpy; R, gas constant;T, absolutetemperature. Thermobarometric gradient shown applies to oceaniclithosphere of -5 m.y. age. Hacker:Ridge Subduction Beneath the OmanOphiolite 463

1. Determinewhere any metamorphicreactions occur. depthexposed in the metamorphicsole in Oman;whereas the Changerock types as appropriate, and add or subtractheat. "shallow accretion"paths show pressure-temperature 2. Allow heat to conduct. trajectoriesfor materialaccreted at shallowerdepths. Many 3. Calculate the differential stressin each column that otherpressure-temperature paths are possible,but the sustainsthe imposedhorizontal velocity boundary condition. maximumpressure may be constrainedby the thicknessof the 4. Computethe resultant vertical gradient in the ophioliteslab, and the maximumtemperature can not be horizontalstrain rate anddisplacement in eachcolumn. greaterthan the "deepaccretion" path shown because 5. Determinethe amountof heatgenerated by material subductedto greaterdepths is not exposedin the deformationalheating. metamorphicsole in Oman. The decompressionand cooling 6. Move materialto reflect the displacementgradient in pathsdepend on the simulateduplift history. The uplift each column. historyis not criticalin this studybecause the ophiolitewas 7. Return to step 1. lifted up and exposedso rapidly that little thermal The simulations were halted when 450 km of oceanic overprintingof the solerocks could have occurred; the lithosphereand 180 km of continentallithosphere had been ophiolitewas subaeriallyexposed within 5-10 m.y. after the subductedbeneath the ophiolite. The smalltime stepmeans greenschist-faciesmetamorphism of the sole[Woodcock and that the calculationsare implicitly self-consistent(e.g., the Robertson,1982b, p. 67]. Note thateven higher pressures correctinterplay between deformational heating and stressis might havebeen attained if the upperplate were initially effectivelymaintained by iterationon a 0.01 m.y. time step). thickerand hasbeen stmcturallythinned during subduction as suggestedby Caseyand Dewey [ 1984]. RESULTS The box in Figure 10 showsthe peak pressures and temperaturesreached during amphibolite-facies Many simulationswere conducted using the tectonic metamorphismat the baseof the Omanophiolite, as estimated historysuggested by Boudierand Coleman [1981], Boudier et from thermobarometry[Ghent and Stout, 1981; Searle, 1990]. al. [ 1988]and Montigny et al. [1988]. The influenceof model Thesepeak metamorphic conditions are achievedin the parameterswas evaluated by varyingthe valueof each simulationwith 0.1-m.y. old li:thosphereand a limit on the parameterwhile holdingall otherparameters constant. differentialstress of 100 MPa (Figure 10b). The inferred Specifically,a rangeof valuesfor lithosphereage, subduction metamorphictemperatures are almostreached in the velocity,thermal conductivity, and maximumstress were simulationwith 2-m.y. old lithosphereand a 200 MPa examined. The effectsof subductionparallel and differential stresslimit (Figure 10a). Simulationswith perpendicularto the ridgeaxis werealso considered. lower limits on the stressor with older lithospherereach Two disparatemodels with differentlithosphere age and lower peak temperatures.Figure 3 showsthat mostdated maximumstress are shownhere. In both, subductionbegins rocksin the Oman solecooled through K/Ar hornblende at a spreadingcenter; the axisof the spreadingcenter is at the closuretemperatures [-525 ø + 25øC; Harrison,1981] over a left edgeof Figures8a and9a. In Figure8 thelithosphere is 2 periodof-7 m.y. Both the simulationspresented here sustain m.y. old andthe maximumshear stress is limitedto 100 MPa. 525ø + 25øC temperaturesfor -6-9 m.y. Note that noneof In Figure9 the lithosphereis 0.1 m.y. old andthe maximum the simulationsusing a different subductiongeometry shear stress is limited to 50 MPa. presentedin Hacker [1990] producedtemperatures in accord Both simulationsproduce qualitatively similar results. with thermobarometricestimates or temperatures>500øC Note thatsteady state is not achievedbecause the amountand for >2-3 m.y., although,there too, the highesttemperatures distributionof deformationalheating are constantlychanging were sustainedlongest in the youngersubducted plate. The andthe ageof the subductedlithosphere is steadilyincreasing. simulationspresented here suggestthat very young Althoughthere initially is lessheat in the 2-m.y.-old lithospheresupporting differential stressesof the order of lithosphere,the highermaximum stress produces more 100 MPa are requiredto replicatefield observationsin Oman. deformationalheating, leading to greaterisotherm overturn Becauseof the inversenature of the approachused in this andlonger retention of heat. The lowerpanel in eachfigure study,the predictionsof the simulationsare not necessarily correspondsto when continentallithosphere begins to be uniqueand variables or processesnot consideredmay play subductedbeneath the ophiolite. important,unrecognized roles. We can,however, evaluate the A usefultest of thesesimulations is to comparetheir sensitivityof the modelsto parametersother than predictedpressure-temperature paths with pressuresand lithosphereage and maximumstress. temperaturesinferred from field studiesin Oman. Figure 10 Increasingthe subductionvelocity reduces the time showsthe pressure-temperaturepaths for variousnodes that requiredto subducta givenamount of material. beginwithin the uppermostpart of the lower plate and finish Consequently,heat advection gains importance relative to accretedto the baseof the upperplate. Bothfigures show heatconduction. Thus for a givenamount of subductionthe four pressure-temperaturepaths. The pathslabeled "0 m.y." thermalgradient between the two platesis more extreme correspondto material subductedat the initiation of whensubduction is rapidand less so whensubduction is slow. intraoceanicthrusting, whereas the "9 m.y." pathsare for For a slow subductionvelocity, heat in the upperplate is materialsubducted 9 m.y. after intraoceanicthrusting began. transferredmore rapidly into the lower plate. Initially this Becausethe subduction rate is 50 mm yr -1, these points were causesmore rapid metamorphism of the lower platebasaltic initially 450 km aparton the downgoingplate. The paths materialand morerapid downward progression of the fault labeled"deep accretion"show pressure-temperature zone (i.e., accretionto form the sole). In the long term, trajectoriesfor materialaccreted at the inferredmaximum however,thermal equilibration of the entirecolumn causes - 464 Hacker:Ridge SubductionBeneath the Oman Ophiolite

2-m.y.-old lithosphere • 100 km 150 450oC -•

•1050oc

++++ ...... 150oC..,, • •-'.';•-'.'•.'.'•-'.'•;.'.'•,

...... 50oC •._

750 ø + + + + + +

• 150øc

+ + + + + + + + '/•/U

450øC

+++++ +++++++

greenschistretrograded to sub-greenschist continental .... ,-'--;-;,6-,:-, ".-".-".-.-"•' amphibolite lithosphere retrogradedto 5 450øc- greenschist serpentinized 9m.y. i amphiboliteperidotire greenschist

Fig. 8. A time sequenceillustrating the temporaland spatialchanges in temperaturesand rock types calculatedfor 2-m.y.-oldlithosphere (vertical exaggeration 2X). The maximumdifferential stress was 200 MPa. All materialabove the fault zone(heavy line) is the ophiolite. Hacker:Ridge SubductionBeneath the Oman Ophiolite 465

0.1-m.y.-old lithosphere • 100 km • •'150oc'•250oc...... 4-+ + o 75øøc t 1050oc•z'

...... 150øC

50øC

:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:......

750øC

Fig. 9. A time sequenceillustrating the temporaland spatialchanges in temperaturesand rock types calculatedfor 0.1-m.y.-old lithosphere(vertical exaggeration2X). The maximumdifferential stress was 100 MPa. All materialabove the fault zone (heavyline) is the ophiolite. the fault zone to cool to temperaturestoo low for subduction. Simulations with subductionrates of 50-100 dislocationcreep and resettingof hornblendeK/Ar mmyr -1 produce peak temperatures compatible with those systematics.Deformational heating is reducedduring slower inferredfrom field studies[Ghent and Stout, 1981; Searle, subduction, which also tends to reduce the lifetime of the 1990], while simulationswith subductionrates as slow as 25 system. For thesereasons the peak temperaturesobtained mmyr -1 do not. duringslower subduction are lower than those during faster Increasingthe thermal conductivity increases the rate of 466 Hacker:Ridge Subduction Beneath the Oman Ophiolite

a 2-m.y.-old .?' :iiil deepaccretion lithosphere 0.6 .<:5" ..-¾-""' S+L ..:.#::' :•.•?:"9m.y. I O..•.y.

0.4 >:.:...... :!:•::- ..:i•:."-''".:..•.':i•:'4i:"""'l::::: allow a •:•retio

,.-a:.'-:" 0.2

:•i':' •._: K/Athornblende I :•i!'""lclosure interval

0 200 400 600 800 1000 Temperature (øC)

b O.l-m.y.-old lithosphere .':..::?!11• deepaccretion• I 0.6

BS ...:.:?'PA iil S + L ?,4::' 9'm.....y. m' • " I , ii!0 m.y. I I "½-/I I

0.4 2:!::" i:.-':' ;'"""'I I i:.-'.' .? .• • shallowaccretion :i!il.-.•• •[ I ':'111,..I I

..if" ..iii-'

0.2

:::[:'•i•:.-.. / .!•;,,,• ;-'/;-' closureinterval '"':ii:--'-i.-:,

• ii ill i •00 400 •00 800 •000 Temperature (øC) Fig. 10. Pressure-temperaturepaths for rocksaccreted to the metamorphicsole at differenttimes and depths.The rectanglesillustrate peak pressures and temperaturesestimated by Ghentand Stout[ 1981] and Searle[ 1990] for the amphibolite-faciesmafic rocks in Oman. Dashedvertical lines show approximateK/Ar hornblendeclosure temperature interval [Harrison, 1981]. Shadedlines show metamorphicfacies boundaries and a solidus.BS, blueschistfacies; PA, pumpellyite-actinoliteand prehnite-pumpellyitefacies; GS, greenschistfacies; AM, amphibolitefacies; GR, granulitefacies; S+L, regionof partiallymelted rocks. (a) Lithosphereis 2 m.y. old andmaximum differential stress is 200 MPa (correspondsto Figure 8). (b) Lithosphereis 0.1 m.y. old and maximumdifferential stress is 100 MPa (correspondsto Figure9). Hacker:Ridge SubductionBeneath the Oman Ophiolite 467 heatconduction. When the thermalconductivity is increased, previously[Hacker, 1990] indicatethat suchold lithosphere the subductedplate heatsup more rapidly, and the upperplate doesnot containenough heat to replicatethe peak coolsmore rapidly. Consequently,the subductedbasaltic metamorphictemperatures recorded in Oman. rocksundergo prograde metamorphism more rapidly and supportsmaller stresses when deforming. This produces DISCUSSION interestingfeedback among deformational heating, heating throughmetamorphic reactions, and thermalconduction. The invertedmetamorphic field gradientsbeneath Smallerstresses produce less deformational heating, and ophiolitesare muchsteeper than thermal gradients produced morerapid prograde metamorphism means more rapid in thermal modelsof thrustfaults. For example,Peacock consumptionof heatby dehydrationreactions, but in the [1987] modeled the thermal effects of fluid flow in simulationsthis is nearlycompensated by the increased subductionzones. Without includingdeformational heating thermalconduction. These competing effects are so evenly he wasable to produceinverted thermal gradients as steepas balancedthat it is not possibleto discriminateamong 123K km-1, but no greater. Such gradients were produced in simulationswith conductivities varying from 2 to 4 J K-1 hismodels of veryrapid subduction (100 mm yr-1) of very m-1 s-1. young(5 rn.y.-old)lithosphere. Peacock [1987] furthernoted In simple terms,increasing the transitionalstress increases thatdeformational heating of >0.25W m-2 (equivalent toa the numberof nodesthat deformby frictionalsliding or shearstress of 160 MPa actingat subductionvelocities of 50 crystalplasticity. More importantly,a higher ceiling on the mmyr-1) could generate gradients of250 K km-1.The stressincreases deformational heating, and this retardsthe simulationsin the presentstudy also evolve thermal rate at which the upperplate cools and speedsthe rate at gradientsof the order of 100K km-1. Naturalmetamorphic which the lower plate is heated. In simulationswhere the solesbeneath ophiolites have peak metamorphic temperature maximumshear stress was limitedto 50 MPa the peak gradientsofat least 1000 K km-1 [Jamieson, 1986]. Why is it metamorphictemperatures estimated from field studies that modelsfail to producethermal gradients as steepas [Ghentand Stout, 1981; Searle, 1990] could only be obtained naturalmetamorphic field gradients?The resolutionto this in simulationswith oceaniclithosphere <0.1 m.y. old. apparentparadox is that the roles of accretionand structural Increasingthe maximumshear stress limit to 100 MPa erosionduring subduction in the generationof metamorphic resultedin appropriatesimulated peak temperaturesfor field gradientshave not beentaken into account.Downward lithosphere<2 m.y. old. Deformationalheating was much accretionduring subductionresults in the constructionof a more importantin this model than in the model of Hacker metamorphicfield gradientthat is composedof a continuum [ 1990] primarilybecause the velocitydeclined monotonically of rock quanta,each of whichhad an uniquePT path. It is this in the lattermodel, leading to decreasingdeformational interactionbetween deformation and metamorphismthat heatingwith time. producesthe unusuallysteep metamorphic field gradients Paleomagnetic[Thomas et al., 1988] and structuraldata beneathophio!ites (Figure 11). [Boudieret al., 1988] suggestthat the initial movementof the Figure 11 illustratesthree end-member cases of accretion ophiolitewas parallel to the ridge and later movementwas in thrustzones (introduced by Peacock[ 1987]) and the perpendicularto the ridge. Turcotteet al. [ 1977] showed correspondingPT evolutionof the baseof the hangingwallof throughsimple calculations that oceaniclithosphere younger the thrust(i.e., in the positionof an ophiolitesole). The than6 m.y. is morebuoyant than average mantle material and assumptionis madethat the metamorphicfield gradient will not sink whenit is overlainby averagemantle, whereas approximatesthe peak metamorphictemperatures [England lithosphereolder than6 m.y. is denseenough that it will. andThompson, 1984, p. 936]. If no accretionoccurs (Figure This mayprovide an explanationof why the changefrom 1la), i.e., the subductionfault neverchanges position with ridge-parallelto ridge-orthogonalconvergence occurred. respectto the hangingwalland footwall,then the rocksin the Duringinitial convergence the spreading center was younger hangingwallsimply cool duringthrusting. The metamorphic than6 m.y. andmore buoyant than average mantle. Later, field gradientreflects the thermalgradient at the initiation whenthe spreading center was older than 6 m.y., it wasdense of faultingbecause that is when the temperatureswere enoughto sinkbeneath the overridingOman ophiolite. highest.If accretionis discontinuous(Figure 1 lb), i.e., a large Simulationsof ridge-perpendicularsubduction were also slab of the underthrustingmaterial accretes to the madeand comparedwith the ridge-parallelsimulations. The hangingwallas a singleunit, thenthe metamorphicfield differencesare small,and existing field datacannot be usedto gradientfaithfully reflectsthe invertedthermal gradient distinguishbetween the two. that was presentduring thrusting. Continuedthrusting In summary,variation of thesekey parameterssuggests resultsonly in lower temperaturesin the accretedrocks so that the ophioliteand the materialsubducted beneath it were that the metamorphicfield gradientis unaffected.This type <2 m.y. old and that shearstresses reached ~100 MPa. There of accretionis only capableof producinginverted is oneimportant reason why theridge model is preferableto metamorphicfield gradientslike invertedthermal gradients, thearc model of Lippardet al. [ 1986]:the arc model requires i.e.,~100 K km-1. A thirdtype of accretion, continuous that the lithospheresubducted beneath the ophiolitemust be accretion(Figure ! lc), is requiredto form the steepinverted older than the ophioliteitself. If the ophioliteis 180 km metamorphicfield gradientsfound beneath ophiolites. wideand formed at rates of 20-50mm yr-1, then the During continuousaccretion a columnof rock within the subductedlithosphere must have been 4-9 m.y. old at the fault zone is built up from quantaof rocksthat comefrom initiationof subduction.The simulationspresented here and progressivelygreater distances up the fault zone. 468 Hacker:Ridge Subduction Beneath the Oman Ophiolite

A No Accretion

hangingwall

subductingplate •8rad initial

ients

T

B Discontinuous Accretion

peak hangingwall

subductingplate P

c Continuous Accretion

peak

hangingwall

subductingplate

T

Fig. 11. Threeend-member types of accretionin thrustzones. The crosssections (left) showa schematic subductionzone soonafter faultinghas begun. PT diagrams(fight) illustratePT evolutionwithin the fault zone;gray lines delineate possible pressure-temperature paths, and black lines indicate initial and peakthermobarometric arrays. White, gray, and black squares in left andfight framescorrespond. (a) No accretion.Initial andpeak gradients are identical, and an invertedmetamorphic gradient does not develop. (b) Discontinuousaccretion: the subductionfault jumps downward (from old to new position),accreting a pieceof the footwallto the hangingwall.(c) Continuousaccretion: the subduction fault movesprogressively downward (from old to new via a continuumof intermediatepositions), accretingpieces of the footwall to the hangingwall. Hacker:Ridge Subduction Beneath the Oman Ophiolitc 469

Consequently,as eachquantum of rock arrivesat its final ophiolitesole [Hacker, 1990, pp. 495 4906]based on thearc destinationwhere it accretes,the temperatureat the accretion tectonicmodel of Lippardet al. [1986]. Severalkey resultsof locationmay havechanged significantly because of the thisstudy based on theridge tectonic model of Boudierand additionaltime requiredto carry the quantadown the fault Coleman [1981] and Boudieret al. [1988] are differentfrom zone. Thusif coolingoccurs, the invertedmetamorphic field the previoussimulations and, furthermore, match the field gradientis steeperthan it would be if all the quantaaccreted observationsin Omanmore closely. Moreover,based on the together(discontinuous accretion). simulationsin this study,several predictions can be madethat The pressuresrecorded at the peak temperatures(not canbe testedby furtherwork in Oman. necessarilythe peakpressures) are distinctivefor eachtype of 1. Peakmetamorphic temperatures recorded in the accretion,as are the PT paths. For example,continuous metamorphicsole should be greaterat the leadingedge of the accretionproduces PT pathsthat do not cross(Figure 1lc), ophioliteand lower at the trailingedge, because the entire whereasdiscontinuous accretion produces crossing PT paths systemcooled with time andaccretion began at the leading (Figure 1lb). edge. Thereare no systematicmeasurements of thespatial Tethyan-typeophiolites are an extremeexample that a variationin peakmetamorphic temperatures in Oman. A metamorphicfield gradientneed not reflectany particular usefulfield studywould be to unravelthe PT historiesof thermalgradient that was presentduring the metamorphism rocksin the soleto determinethe spatialdistribution of peak [e.g.,Spear and Selverstone, 1983]. This is truefor regional temperaturesand the shapes of PT paths. If thespatial metamorphicterranes because minerals in differentportions variationpredicted by thesimulations can be found,then this of the metamorphicfield gradientmay haveformed at studyprovides an explanationof how it mighthave differenttimes. In Tethyan-typeophiolites, however, developed.Furthermore, the spatialdistribution of PT paths deformationplays an importantrole. The metamorphicfield within the solemay yield cluesabout the geometryof gradientsin the metamorphicsoles of Tethyan-type emplacement,such as the distributionof heatwithin the ophiolitesare a sequenceof rocksthat were heatedto lithospheresubducted beneath the ophioliteand the evolution differentpeak temperatures in disparateareas at different of slab thicknesses. timesand were amalgamated into a singlecolumn during 2. The amphibolite-faciesrocks in the Omanophiolite sole subduction. shouldhave been metamorphosed during a-6-9 m.y. interval, Thrustfaulting is notthe only type of faultingin which althoughthe durationof this metamorphismis relatedto the extrememetamorphic field gradients can develop. Unusual amountof oceaniclithosphere subducted beneath the metamorphicfield gradients could develop in thefootwalls ophiolite. The greatestshortcoming of the earlier of normalfault zoneswhere the heatinghangingwall accretes simulations[Hacker, 1990] was that they indicatedthat the to theuplifting, cooling footwall (the fault zone moves amphibolite-faciesrocks formed in -1-3 m.y., whereasthe upwardwith time) (Figure 12). In suchsituations, absence of K/Ar agesindicate that the amphibolite-faciesmetamorphism accretionpreserves the initial metamorphic field gradient in Omanspanned at least7 m.y. [Allemanand Peters, 1972; (Figure12a), and discontinuous accretion leads to Lanphere,1981; Monfigny et al., 1988]. Our current metamorphicfield gradientsthat mimic the thermal knowledgeof the coolingages of the solecomes solely from gradientsduring the faulting, just like in thrustfaults K/Ar dateson samplesthat are distributedsomewhat (Figure12b). Continuous accretion, however, produces the erraticallyacross the ophioliteand from sampleswhose reverseeffect of thrustfaults (Figure 12c). The metamorphic structuraland metamorphicrelationships with surrounding fieldgradient produced by continuousaccretion during rock unitsare incompletelydocumented. Moreover, the normalfaulting is lesssteep than the thermal gradient during metamorphicpetrogenesis of only a few of the datedsamples themetamorphism. This is becauseeach quantum of rock havebeen thoroughly investigated. A majorimprovement in accretesat a relativelyhigher temperature than it would our currentunderstanding would be to determinethe cooling duringdiscontinuous accretion because the time required for agesof the metamorphicsole using an isotopicsystem more thequantum to translateup thefault zone has allowed the informativethan K/Ar, such as 40Ar/39Ar. It isnecessary hangingwallto reachhigher temperatures. thatthe metamorphicpetrogeneses of the datedsamples and Somethermobarometric studies of ophiolitesindicate that their structuraland metamorphicrelationships with nearby thepeak metamorphic pressures and tempermums decrease rock unitsbe carefullycharacterized. Comprehensive spatial downwardwithin the faultzone [e.g., Jamieson, 1986] (for an distributionof samplesfrom acrossthe ophioliteexposure alternativeview seeE1-Shazly and Coleman [1990]). Pavlis are alsorequired to testthe predictionsof the simulationsin [1986]suggested that this downward decrease implies that this study. thethrust system was rapidly exhumed or erodedduring 3. Deformation within the sole should have occurred at displacement.It is clearfrom the present study that such maximumshear stresses of- 100 MPa. Molnar and England neednot be the case. The downwarddecreasing pressure and [1990] havemade somewhat analogous calculations and temperaturegradient can be formed by accretionof quantaof concludedthat thrustingalong the Main Central Thrustin materialfrom progressivelyshallower portions of the the Himalaya alsooccurred at shearstresses of-100 MPa. subduction zone. Althoughstress magnitudes at somestages of the emplacementof the Omanophiolite could possibly be IMPLICATIONS estimatedthrough microstructural paleopiezometry, such measurementshave not yet beenreported. Paleopiezometry Manyof theimplications of thisstudy are similar to those of quartzite(present in the greenschist-faciesportion of the reportedfor earliersimulations of theevolution of theOman sole) and dunite(present in the ultramafictectonite) may 470 Hacker:Ridge Subduction Beneath the Oman Ophiolite

A No Accretion

hangingwall

upliftingfootwall

'•• and peak gradients

B Discontinuous Accretion

hangingwall

upliftingfootwall initial gra • gradient T

c Continuous Accretion

hangingwall

upliftingfootwall / •l'pea.k • gradient

Fig. 12. Three end-membertypes of accretionin normalfault zones. PT diagramsillustrate PT evolutionwithin the fault zone. The crosssections (left) showa schematicnormal fault zone soonafter faultinghas begun. PT diagrams(right) illustratePT evolutionwithin the fault zone;gray lines defineatepossible pressure-temperature paths, and black lines indicate initial andpeak thermobarometric arrays. White, gray,and black squares in left andfight framescorrespond. (A) No accretion.Initial and peakgradients are identical. (B) Discontinuousaccretion: the faultjumps upward (from old to new position),accreting a pieceof the hangingwallto the footwall. (C) Continuousaccretion: the subductionfault movesprogressively upward (from old to new via a continuumof intermediate positions),accreting pieces of the hangingwallto the footwall. Hacker:Ridge SubductionBeneath the OmanOphiolite 471 providesome information about the stressespresent during metamorphicfield gradientsmay form in othertectonic emplacement.Paleopiezometry can be difficult[e.g., Hacker settings,such as core complexes, where metamorphism and et al., 1990], but recentinformation about [Pierce, faultingare synchronous. 1987; Pierce and Christie, 1987] and olivine [Karato, 1989] grain growthmay assistin the interpretation. 4. The oceaniclithosphere was very young,probably <2 Acknowledgments.This paperis dedicatedto Tucker m.y. old. The ageof theOman lithosphere is notyet well "Bob" Wenzel. The manuscriptbenefitted from reviewsby constrained.U/Pb datesin plagiogranitebodies [Tilton et al., R. G. Coleman, W. G. Ernst, J. Karson, J. S. Pallister, and A. K. 1981] overlapK/Ar agesof the solerocks [e.g., Montigny et E1-Shazly.Partial supportwas providedby DOE grantDE- FG03-87ER B806 to W. G. Ernst. al., 1988]. Sm-Nd dateson threegabbro samples [100 + 20, 128 + 20, and 150 + 40 Ma; McCulloch et al., 1981] have uncertaintiesthat are too large to confirmor reject this REFERENCES prediction.A much-neededconstraint on the tectonic evolutionof the Omanophiolite will comeby determining Alabaster,T., J. A. Pearce,and J. Malpas,The volcanic the crystallizationage of the gabbro,dikes, and/or lavas more stratigraphyof the Oman ophiolitecomplex, Contrib. precisely,perhaps by 40Ar/39Ar orU/Pb dating as has Mineral. Petrol., 81,168-183, 1982. successfullybeen employed in the Bay of Islandsophiolite by Alleman, F. and T. Peters,The ophiolite-radiolaritebelt of Dunningand Krogh [ 1985]. the North Oman Mountains,Eclogae. Geol. Helv., 65, The Bay of Islandsophiolite is a Tethyan-typeophiolite 657-697, 1972. exposedin the CanadianAppalachians. The inverted Archibald,D. A., andE. Farrar,K-Ar agesof metamorphicfield gradientin its solehas even higher peak from the Bay of Islandsophiolite and the Little Port temperaturesthan the Oman sole [Jamieson,1986]. Unless Complex,western Newfoundland, and their geological anotherheat sourceis involved,the simulationspresented implications.Can. J. Earth Sci., 13, 520-529, 1976. heresuggest that the Bay of Islandsophiolite also was very B6chennec,F., J. Le Metour,D. Rabu,M. Villey, andM. youngwhen intraoceanic thrusting began. Early radiometric Beurrier,The Hawasinabasin: A fragmentof a starved studiesof the ophioliteplutonic section and sole [Dallmeyer passivecontinental margin, thrust over the Arabian and Williams, 1975; Archibald and Farrar, 1976; Martinson, platformduring of the Semailnappe, 1976;Jacobsen and Wasserburg, 1979] suggestedthat-35 Tectonophysics,151,323-343, 1988. m.y. separatedthe igneousgenesis of theophiolite and its Blendinger,W., A. van Vliet, M. W. HughesClark, emplacement.More recent,higher precision U/Pb work Updoming,ritfting and continentalmargin development [Dunningand Krogh, 1985] hasnarrowed the gap to only ~11- duringthe Late Paleozoicin northernOman, Geol. Soc. 16 m.y. This is in accordwith theprediction of thisstudy that Spec.Publ. London,49, 27-37, 1990. the amphibolite-faciesmetamorphism in the solecould Boudier,F., andR. G. Coleman, Crosssection through the continuefor -6-9 m.y. afterintraoceanic thrusting of a very peridotitein the Samailophiolite, southeastern Oman youngophiolite. Mountains,J. Geophys.Res., 86, 2573-2592, 1981. Boudier,F., and A. Michard,Oman ophiolitesthe quiet CONCLUSIONS obductionof oceaniccrust, Terra Cognita,1,109-118, 1981. The productionand accretion of metamorphicrocks at the Boudier,F., andA. 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