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Tectonically Controlled Origin of Three Unusual Rock Suites in the Woodlark Basin

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Tectonically Controlled Origin of Three Unusual Rock Suites in the Woodlark Basin TECTONICS, VOL. 5, NO. 7, PAGES 1145-1160, DECEMBER1986 TECTONICALLY CONTROLLED ORIGIN OF THREE UNUSUAL ROCK SUITES IN THE WOODLARK BASIN Dallas Abbott and Martin Fisk Collegeof Oceanography,Oregon State University, Corvallis,Oregon Abstract. We proposealternative mechanisms that thisformerly subductingplate is still cold for the originof threeunusual rock suites,high-Mg enoughto retainvolatiles and to remainseismically andesites,NaTi basalts,and arclike rocks, that have active. As the old plate losesvolatiles, they rise into beendredged from the Woodlark basin,southwest the mantle convection cell which feeds the Woodlark PacificOcean. We showthat the high-Mg andesites basinridge crest. Becauseridge subduction andNaTi basaltsare associated with anunusually increasesthe likelihoodof ophioliteobduction, these coolridge environment. The coolingis dueto observationsand explanationsof Woodlarkbasin increasedhydrothermal circulation, stimulated by an tectonicsare potentiallyimportant for ophiolites. unusuallyhigh crustalpermeability. The high permeabilityis mainlydue to crackingof the INTRODUCTION Woodlarkbasin lithosphere as it passesover the flexuralbulge in front of the subductionzone, The Woodlark Basin, locatedin the southwest althoughlocal processes, such as faulting in fracture Pacific Oceaneast of PapuaNew Guinea,contains zones,may also make somecontribution. This an activelyspreading ridge, that is beingsubducted increasedconvective cooling affects magma beneaththe New Georgiaisland arc to the northeast dynamicsand chemistry at theridge crest. The (Figures1 and 2). A secondlithospheric slab, a high-Mgandesites occur where the hydrothermal relic from a previousperiod of southward circulationlowers the temperatures in the upper subductionof the Pacificplate, is emplacedbeneath oceaniccrust, causing the magma chamber to sit in the island arc from the north. themantle rather than in thecrust and promoting the Althoughoceanic crust further than 150 km from interactionof basaltmagma with harzburgite.The the subductionzone is normalmid-ocean ridge basalt NaTi basalts are also the indirect result of increased (MORB), the other rocks from the Woodlark basin crustalcooling, which causes anomalously low spreadingcenter belong to threedifferent suites: (1) degreesof partialmelting of theirdepleted mantle arclikerocks, basalts through dacites with a high source.The arclikerocks are caused by interaction contentof arclikecomponent; (2) high-Mg andesites, withvolatiles from lithosphere that was emplaced andesiteswith a high magnesiumcontent, and (3) beneaththe edge of thebasin less than 6 m.y. agoby NaTi basalts,basalts with a high sodiumand a now inactive subduction zone to the north of the titaniumcontent [Perfit et al., 1986]. The high-Mg currentlyactive one. Simple thermal models indicate andesites and NaTi basalts formed at transform zones within 50 km of the trench. The arclike rocks were generatedat ridge crestswithin 150 km of the Copyright1986 trench[Taylor and Exon, 1986]. by the AmericanGeophysical Union. The threespatially restricted suites are all unusual in theirtectonic location. High-Mg andesiteshave Papernumber 6T0474. beenpreviously found in islandarc andforearc 0278-7407/86/006T-0474510.00 regions[Johnson et al., 1983; Reaganand Meijer, Abbottand Fisk: Tectonically Controlled Rock Suites •55 156 157 I KEY & DREDGE LOCATION •TRENCH AXIS • RIDGE CREST _ ol•':•'J_< 2. M.Y.OLD [7"/'/"/•> 2 M.Y. OLD I0 155 156 157 158 ø Fig. 1. The Woodlarkbasin: an areawhere lithospheric flexure may be affectinghydrothermal and volcanicactivity (map after Taylor andExon [ 1986]). The spreadingcenter marked by double dashedlines (Ghizo ridge) is eruptingisland arc-type tholerites, dacites, and andesitesand is within the flexuralbulge. Trianglesare dredge haul locations. Dredges 31 and32 are on the Simbofracture zoneand transform fault. Dredge31 recoveredhigh-Mg andesites and arclike rocks. Dredge32 recoveredNaTi basaltsand arclike rocks. Dredge26 recoverednormal MORBs [afterTaylor, 1986]. 1984], but have never before been found on a The factorswhich distinguish the Woodlarkbasin transform fault. NaTi basalts have been found at from othermarginal basins are the subductionof five other sites,four of which are far from very youngoceanic crest (0-5 m.y.), a rapid subducti6nzones [CAYTROUGH, 1979;Schilling convergencerate (> 10 cm/yr), anda polarityreversal et al., 1983; Cousens et al., 1984; W.G.Melson and of subduction(late Miocene) [Colemanand T. O'Heam, unpublishedcompilation of glass Kroenke, 1981; Weisselet al., 1982; Taylor, 1986; analysesof MORB, 1985]. All four of thesesites Figure2]. The presenceof ridgesubduction, the are (or were) ridge segmentsbracketed at bothends rapidconvergence rate, and the spatialrestriction of by relativelyold (10-30 m.y.)oceaniclithosphere. thedifferent rock types implies that the unusual The NaTi basaltsfrom theWoodlark basin erupted volcaniccharacter of the Woodlarkbasin may be alonga transformfault with an agecontrast of only causedby tectonicprocesses. Therefore, we 2.6 m.y., muchyounger than in the otherlocations. examinedtwo processeswhich couldrestrict Finally, the arclikerocks are commonlyrestricted to unusualrock typesto the vicinityof the trench:(1) back-arc basins and island arcs, and the Woodlark unusuallystrong cooling of the lithosphereby basin is neither. hydrothermalcirculation, which couldbe The origin of theserocks is relevantnot only in responsiblefor the high-Mg andesitesand NaTi thecontext of Woordlarkbasin petrology, but alsoto basalts,and (2) volatile lossfrom the relict the muchmore general concern of whetherterrestrial subductingplate, which couldbe responsiblefor the ophiolitesreally represent typical oceanic crustal arclike rocks. rocks[Miyashiro, 1973; Moores and Jackson, 1974; Mooreseta!., 1984]. Ridge subduction,such as is LITHOSPHERIC COOLING AND occurringin theWoodlark basin, may increasethe PLATE FLEXURE chancesof ophioliteemplacement [Moores et al., 1984]. Consequently,there may be petrologicand chemicalsimilarities between ophiolites and the The oceaniccrest created at mid-oceanridges is unusual Woodlark basin rocks. cooledby a combinationof conductionand Abbottand Fisk: TectonicallyControlled Rock Suites 1147 valuesat depthat a rate of 0.222 kbar/km. We assume this model in the discussion to follow. Lithosphericflexure is one sourceof tensile stress.Lithospheric plates bend as they enter subductionzones. This flexure produceslarge tensionalstresses in the upperhalf of the subducting oceanicplate (Figures3 and4). Thesehigh tensile stresses can cause new cracks to form on the ß.-.: ,. surfaceof the flexural bulge. Thesecracks will Young Plate /..o ß propagatedownward until the breakingstrength of the rock exceedsthe tensilestresses induced by o flexure (Table 1). These new cracksadd to cracks o- Old Plate causedby thermoelasticstresses. Increasedhydrothermal circulation produces large chemicaland thermal changes within the oceanic o crust. If flexurally inducedcracking and faulting ,60 ;$o 460 increasesthe permeabilityor the topographic Woodlark Plate Pacific Plate roughnessof an unsealedplate, hydrothermal circulationwill intensify. Althoughoceanic crest as Fig. 2. Seismicitybeneath the New Georgiaisland old as 80 m.y. is sometimeshydrothermally active arc [after Cooperand Taylor, 1985]. Both the old, [Embleyet al., 1983], theseolder, hydrothermally previouslysubducting Pacific plate (fight) andthe active areas are located in oceanic areas with presentlysubducting Woodlark basin plate (left) are unusuallylow sedimentationrates. Because clearlyvisible. The seismicdata imply thatthe older sedimentationrates near trenchesare usuallyquite subductingplate is intactand has not brokenapart. high, the sedimentcover on mostsubducting oceanic crust reaches a thickness of 200-300 meters when the subductingplate is only 0-20 m.y. old. The convection. The relativeimportance of conduction subductingplate in the Woodlarkbasin is 0-5 m.y. andconvection depends primarily on the bulk old and is coveredby lessthan 200 metersof permeabilityof the crust-sedimentsystem. sediment;thus hydrothermal circulation and Becausemarine sedimentsare usuallyseveral orders consequentlithospheric cooling should be intensified of magnitudeless permeable than basalt [Abbott et by flexurally inducedcracking. al., 1981; Andersonet al., 1985], the depositionof 200-300 metersof sedimenton top of the basalt EVIDENCE FOR INC•ASED CRACKING basementnormally causescrustal cooling to be DUE TO LITHOSPHERIC FLEXURE dominatedby conductionrather than convection [Andersonet al., 1979]. However, the intensityof Three observations indicate that new cracks form hydrothermalconvection increases with increasing at flexural bulges: normalfault earthquakes,new total availableheat, topographic roughness, and fault blocks,and pore water helium anomalies. systempermeability [Stefanson, 1983; Fehn et al., Many trencheshave reactivated fault blockson the 1983]. Thus, changingany one of thesevariables flexurallyformed seaward trench slope [Schweller canchange the amountof convectivehydrothermal and Kulm, 1978; Watts et al., 1980]. In the coolingat mid-oceanridges. Peru-Chile trench at 220-27 ø S and in the Guatemala Fracturingincreases the numberof interconnected trench at 12ø-14ø N, someof thesefault blocks are voids within oceanic crust and thus increases the alignedparallel to the trendof the trench,rather than permeabilityof the oceaniccrust. The increased perpendicularto theoriginal spreading direction permeabilitystimulates
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