Ridge Subduction Beneath the Oman Ophiolite
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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 SUBDUCTION BENEATH THE OMAN OPHIOLITE BradleyR. Hacker Departmentof Geology,Stanford University, Stanford, California 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 continental crust center model but not with the arc model. Moreover, remainobscure. Tethyan-type ophiolites 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 lithosphere [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 dunite 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-,amphibolite-, and greenschist-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 gabbro GEOLOGY OF TIlE OMAN OPHIOLITE (0.5-6.5 km), sheeteddikes (1-1.5 km), lavas(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 orogeny [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 basalts 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, shale, 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 Proterozoic 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 craton [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 Basalt r.'.'.' Plutonic 93.5 Ma, althoughthere are two that are slightlyolder (96.9 r.'.'.' peridotite 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