199The Geological Society of AmericaSpecial Paper 4362008Cenozoic volcanic arc history of East Java, Indonesia: The stratigraphic record of eruptions on an active continental marginHelen R. Smyth*Robert HallGary J. NicholsSE Asia Research Group, Geology Department, Royal Holloway University of London, Egham TW20 0EX, UKABSTRACTThe stratigraphic record of volcanic arcs provides insights into their eruptive history,theformationofassociatedbasins,andthecharacterofthedeepcrust beneath them. Indian Ocean lithosphere was subducted continuously beneath Java from ca. 45 Ma, resulting in formation of a volcanic arc, although volcanic activity was not continuous for all of this period. The lower Cenozoic stratigraphic record onlandinEastJavaprovidesanexcellentopportunitytoexaminethecomplete eruptivehistoryofayoung,well-preservedvolcanicarcfrominitiationtotermi-nation.TheSouthernMountains ArcinJavawasactivefromthemiddleEocene (ca. 45 Ma)totheearlyMiocene(ca. 20 Ma),anditsactivityincludedsignicant acidicvolcanismthatwasoverlookedinpreviousstudiesofthearea.Inparticu-lar, quartz sandstones, previously considered to be terrigenous clastic sedimentary rocks derived from continental crust, are now known to be of volcanic origin. These deposits form part of the ll of the Kendeng Basin, a deep exural basin that formed inthebackarcarea,northofthearc.Datingofzirconsinthearcrocksindicates that the acidic character of the volcanism can be related to contamination of mag-mas by a fragment of Archean to Cambrian continental crust that lay beneath the arc. Activity in the Southern Mountains Arc ended in the early Miocene (ca. 20 Ma) with a phase of intense eruptions, including the Semilir event, which distributed ash over a wide area. Following the cessation of the early Cenozoic arc volcanism, there followedaperiodofvolcanicquiescence.Subsequentlyarcvolcanismresumedin thelateMiocene(ca. 1210 Ma)inthemodernSunda Arc,theaxisofwhichlies 50 km north of the older arc.Keywords: East Java, Indonesia, stratigraphic record, Cenozoic arc volcanism.*Present address: Cambridge Arctic Shelf Programme (CASP), 181A Huntingdon Road, Cambridge CB3 0DH, UKSmyth, H.R., Hall R., and Nichols, G.J., 2008, Cenozoic volcanic arc history of East Java, Indonesia: The stratigraphic record of eruptions on an active continental margin, in Draut, A.E., Clift, P.D., and Scholl, D.W., eds., Formation and Applications of the Sedimentary Record in Arc Collision Zones: Geological Society of America Special Paper 436, p. 199222, doi: 10.1130/2008.2436(10). For permission to copy, contact editing@geosociety.org. 2008 The Geological Society of America. All rights reserved.200Smyth et al.INTRODUCTIONTheislandofJavalieswithintheIndonesianarchipelago between the landmasses of Eurasia and Australia, on the margin of the Eurasian plate. The southeastern part of this plate is known asSundaland(Fig. 1),whichistheMesozoiccontinentalcore of SE Asia (e.g., van Bemmelen, 1949; Hamilton, 1979). Java is located on Sundalands southern margin.Manystudiesofvolcanicarcsarebasedonwell-exposed areasofarcsactivelongago,suchastheCretaceousAllistos ArcinBajaCalifornia(Busbyetal.,2006)andtheCambrianOrdovicianLoughNafooey ArcoftheIrishCaledonides(e.g., DrautandClift,2001).Therearefewstudiesofyoungerarcs in areas where active volcanism continues today. One reason for this is that younger arc products cover older arc sequences. Fur-thermore, many modern arcs are in tropical areas, such as Indo-nesia, the Philippines, and the Western Pacic, that are difcult to work in, commonly are not well exposed, and are remote. The modern volcanoes on Java form part of the Sunda Arc, which is well known for volcanic activity, including the famous nineteenth century eruptions of Krakatau and Tambora, and the Pleistocene eruptionofToba.However,despitealongvolcanicrecordin Java, owing to subduction of Indian Ocean lithosphere during the Cenozoic, little is known of its pre-Pleistocene arc history.Since the middle Eocene (ca. 45 Ma) there has been north-ward subduction of the Indo-Australian plate at the Java Trench tothesouthofJava(Hall,2002).Consequently,Javaisessen-tiallyavolcanicisland,andalonghistoryofarcvolcanismis recorded in its Cenozoic stratigraphy. Products of both active and ancientvolcanicarcscanbeobserved.Aneast-westtrending chain of >30 modern volcanoes, forming part of the Sunda Arc, creates the central spine of the island of Java (Fig. 2). An arc of older, Eocene to Miocene volcanoes is parallel to and south of the Sunda Arc and is known as the Southern Mountains Arc (Smyth et al., 2005).ExposuresofbasementrocksarerareinEastJava,and before this study the basement was thought to have formed dur-ing the Cretaceous, when fragments of arc and ophiolitic mate-rial were accreted to the southern margin of Sundaland (Wakita, 2000).Therearenocontinentalbasementrocksatthesurface orreportedfromdrillinginEastJava.Thebasementisoften referred to as transitional, as to the north and west in Sunda-land the basement is continental, and in the east near to Flores, it is oceanic (e.g., Hamilton, 1979).The Cenozoic stratigraphic record preserved on land in East Javaprovidesanexcellentopportunitytoexaminetheeruptive historyofayoung,well-preservedvolcanicarc.Thepresent-day arc, active since the middle to late Miocene (ca. 1210 Ma; Soeria-Atmadjaetal.,1994),isimmediatelyobvious,butthe importance of the older Southern Mountains Arc has previously beenunderestimated.Therelativelybasicvolcanicproductsof this arc, the Old Andesites (van Bemmelen, 1949), are widely known because they are well exposed and form prominent topo-graphic features. However, the acidic products of the arc, which are widespread, have been largely overlooked. As a result of the workreportedhere,abundantquartz-richsedimentaryrocksin East Java are now known to be the primary and epiclastic prod-ucts of acidic volcanism but were previously interpreted as conti-nental sediments (e.g., Harahap et al., 2003). The acidic nature of the erupted material is a reection of the character of the underly-ing crust, which as a result of new zircon dating is now thought to include a fragment of Archean continental crust.ThefullcycleofthePaleogeneSouthernMountainsArc, from initiation to termination, can be documented in the stratigra-phy of East Java. This paper reports the stages of arc development and the sequence and timing of events within the arc, determined by detailed eld investigations accompanied by provenance anal-ysis and isotopic dating. Most of the middle Miocene was marked by a period of volcanic quiescence that followed the termination ofPaleogenearcvolcanismintheearlyMiocene(ca. 20 Ma). Near the end of the middle Miocene (ca. 1210 Ma) the modern Sunda Arc began activity in its present position, 50 km north of the Southern Mountains Arc. Possible explanations for the shift in position of arc volcanism are discussed later in this paper.PRESENT STRUCTURE OF EAST JAVAEastJavacanbesubdividedintothreeparts,broadlypar-alleltotheelongationoftheisland,representing(1)theearly Cenozoic Southern Mountains Arc, (2) a deep basin north of the arc, and (3) a marine shelf north of the basin (Figs. 3 and 4). The modern arc is built mainly on top of the basin north of the early Cenozoic arc. We rst summarize the principal features of these components of Java and then go on to discuss the stratigraphy of the Southern Mountains Arc.100EINDIANOCEAN0110E10SJavaTrenchMalayaSOUTHCHINASEASumatraBorneoSUNDALANDJAVASEAJavaFigure 1. Current plate tectonic setting of western Indonesia. The mar-gin of Sundaland (Hamilton, 1979) is dened by a black dashed line. Theblackarrowsshowrelativeplatemotionofboundariesupper plate movement relative to lower (McCaffrey, 1996). The study area is enclosed by a rectangle.Cenozoic volcanic arc history of East Java, Indonesia201Southern Mountains ArcThere are few exposures of basement rocks in Java, and in EastJavatheyareknownonlyinthewesternpartofthestudy area.Baseduponthislimitedevidence,thebasementhasbeen interpretedtobearcandophioliticrocksofCretaceousage. A volcanicarcwasbuiltuponbasementrocksfromthemiddle EocenetotheMioceneinsouthernJava(Smyth,2005),which isknowntoextendfromEastJavaintoWestJava(Soeria-Atmadjaetal.,1994).Thestratigraphicthicknessofthearc 112Eo114Eo110Eo0 50 100Kilometers7So

8So112Eo114EoYogyakarta110Eo0 50 100Kilometers7So

8SoMADURAEAST JAVA SEAMadura StraitsINDIAN OCEANNanggulanKarangsambungJiwo HillsSurabayaRembangHighMuriaKendeng HillsSouthern MountainsDieng PlateauEAST JAVAMerapiABFigure 2.EastJavalocationmap.(A)Importantlocationsandtopographicfeatures. Theblackstarmarkstheepicenterofthe27May2006 earthquake, and the currently active volcano Merapi is marked by the black triangle. (B) Digital elevation model from SRTM data (Shuttle Radar Transect Mission, NASA) of East Java, with the centers of OligoceneMiocene volcanism marked.202Smyth et al.products is >2500 m, and within this sequence andesites are well known (van Bemmelen, 1949; Soeria-Atmadja et al., 1994), but acidicvolcanicrockshavenotbeenpreviouslyreported.The zonecontainingthearcproductsis50 kmwide.TheSouthern Mountains Arcisnowupliftedandpartiallyeroded. Thestrata typically dip uniformly toward the south between 20 and 30.Kendeng BasinThe Kendeng Basin lies directly behind and to the north of the Southern Mountains Arc (Fig. 2). The deposits of the basin are poorlyexposed. Thedepocenterismarkedbyastrongnegative Bouguer gravity anomaly of more than 580 m2 (Fig. 5). The negative anomaly can be traced eastward into a negative marine free air anomaly (Sandwell and Smith, 1997), which extends from the Straits of Madura eastward to the north of Bali. At the west end of the Kendeng Basin a relatively abrupt change in the charac-ter of the anomaly is around the modern volcanoes of Merapi and the Dieng Plateau (Fig. 5). To the west, in West Java, the anomaly is not well dened, as it becomes positive (40 m2).No basement rocks are exposed or known from drilling in this region. The basin ll has an age range of middle Eocene to Mio-cene, similar to the Southern Mountains Arc (de Genevraye and Samuel, 1972). The basin is east-west oriented, at least 400 km long,paralleltotheSouthernMountains Arc,andlledwitha succession of volcaniclastic turbidites and pelagic mudstones that are reported to be at least 6 km thick (Untung and Sato, 1978). Gravity calculations indicate that the basin may contain as much as10 kmofsediment(C.J.Ebinger,2005,personalcommun.). Based on eld observations, seismic sections, and regional gravity interpretation (Smyth, 2005), the basin is estimated to have been ~100120 km wide during the early Cenozoic. It is now partially exposed at the surface in the Kendeng Fold-Thrust Belt, where there is an estimated 1030 km of shortening (de Genevraye and Samuel, 1972; Smyth, 2005); de Genevraye and Samuel (1972) report that deformation commenced in the Pliocene.110Eo7So112Eo

0Kilometers50 100114EoJava SeaMadura IslandSUNDA SHELFKENDENG BASIN8SoIndian OceanMerapiEAST JAVAFORE-ARC BASINMODERN SUNDA ARCSOUTHERN MOUNTAINSARCSectionlineABCENTRAL JAVAPROVINCEA BSOUTHERNMOUNTAINSARCKENDENG BASIN0 50 100KmSUNDA SHELFEOCENE TO MIOCENE?? ??FORE-ARCBASINFigure 3. Structure of East Java in map-and-sketch Eocene to Miocene prole, showing the three structural provincesSouthern Mountains Arc, Kendeng Basin, and the edge of the Sunda Shelfand the modern Sunda Arc building on top.Cenozoic volcanic arc history of East Java, Indonesia203Edge of Sunda ShelfTo the north of the Kendeng Basin (Figs. 2 and 4) the hills of North Java and the offshore East Java Sea constitute an area inter-preted to be the edge of the early Cenozoic Sunda Shelf (Hamil-ton, 1979). This region has been the focus of most hydrocarbon exploration.Pre-Cenozoicbasementrockssampledbydrilling are known to be ophiolitic and arc rocks, which include chert and basic volcanic and metasedimentary rocks (e.g., Hamilton, 1979). Basin development began in the Eocene. There are between 2000 and6000 mofEocenetoPlioceneshallowmarineclasticand extensivecarbonatesedimentaryrockswithinfault-controlled basins (e.g. Ardhana, 1993; Ebanks and Cook, 1993).Thesedimentarysequenceshavebeendeformedsincethe late Miocene by numerous open, east-westoriented folds; north-ward-verging,east-westorientedthrustsinterpretedtobePlio-cene in age; and ENE-WSW normal faults (Chotin et al., 1984; Hoffmann-Rothe et al., 2001).Modern-Day Volcanic ArcTheactivevolcanoesoftheSunda Arcarebuiltmainlyon theKendengBasin(Figs. 3and4)butlocallyoverlaptheedge oftheSouthernMountainsArc.Arcactivitycommencedin itspresentpositionatca. 10 MainthelateMiocene(Soeria-Atmadja et al., 1994). The volcanoes exceed 3000 m in elevation and form the central spine of the island (Fig. 2). The average com-position of the present-day volcanic products is basaltic andesite (Nicholls et al., 1980), which is much more basic than the average compositionoftheSouthernMountains Arcproducts.Mostof the active volcanoes are situated ~100 km above the subducting slab (England et al., 2004). There are also a number of unusual K-rich backarc volcanoes (Edwards et al., 1991), which occur to the north of the axis of the arc, including Muria (Fig. 2). The erup-tive and epiclastic products of the Sunda Arc cover a signicant proportion of the island, contributing to its fertile soils.STRATIGRAPHIC RECORD OF THE EARLY CENOZOIC SOUTHERN MOUNTAINS ARCThesedimentaryrocksintheSouthernMountainsArcin East Java were deposited on the basement above a poorly dated regionalunconformity(Fig. 6).Theunconformityseparates Upper Cretaceous basement rocks and the Cenozoic succession, suggesting a long period, potentially up to 30 m.y., during which upliftanderosionoccurred. Thestratigraphicsuccessionabove 110Eo7So112Eo8So0Kilometers50 100PacitanRembangSemarang114EoYogyakartaKarangsambungSurabayaJava SeaIndian OceanMuriaMerapiMadura Island(geology not shown)Intrusive rocksMiddle Miocene CarbonatesMiddle Miocene reworkedvolcaniclastic depositsOligo-Miocene volcanicsEocene Sedimentary rocksBasement rocks -Cretaceous and olderSunda ArcModern volcaniccentersKendeng BasinApproximate southernlimit of the KendengBasin (not exposed)Pliocene clastic andcarbonate rocksPost-peneplane carbonaterocksShelf edge clastic andcarbonate rocksFaultModern volcanic and alluvialdepositsFault (inferred)KENDENG BASINSOUTHERN MOUNTAINSARCDiengPlateauSUNDA SHELFSouthernMountainsArcKendengBasinModernArcSundaShelfJiwo Hills

Figure 4. Simplied geological map of East Java, showing the main geological provinces and stratigraphic units.204Smyth et al.theunconformityintheSouthernMountains ArcandtheKen-dengBasinhasbeensubdividedintothreesynthems(Smyth et al., 2005), each representing a different period of arc activity (Fig. 6). Thetermsynthemhasbeenusedbecausemuchofthe work undertaken and published in East Java has been hydrocar-bon oriented, and as a result the term sequence is often used with seismic and sequence stratigraphic implications (e.g., Catuneanu, 2006). We have therefore chosen to avoid the term sequence for themajorstratigraphicsubdivisionsandinthisstudyusedthe term synthem, dened as an unconformity-bounded stratigraphic package (Rawson et al., 2001). The three synthems are:Synthem One: records the initiation of arc volcanism and theearlystagesofarcdevelopmentduringthemiddle Eocene (ca. 45 Ma) to early Oligocene (ca. 3428 Ma).Synthem Two: records the growth and termination of arc vol-canism in the Southern Mountains Arc during the late Oli-gocene (ca. 2823 Ma) to the early Miocene (ca. 20 Ma).SynthemThree:recordswidespreadcarbonategrowth, accompaniedbytheerosionandredepositionofrocks from earlier synthems during the middle Miocene (ca. 2010 Ma), with no signicant volcanic activity.Intheaccountofthestratigraphythatfollows,thestrati-graphicageshavebeenconvertedtonumericalages,usingthe Gradstein et al. (2004) time scale.Synthem One: Initiation of the Southern Mountains ArcMostpartsoftheSouthernMountainsrocksofSynthem Onearecoveredbyyoungerdepositsandareexposedonlyat Karangsambung,Nanggulan,andtheJiwoHills(Fig. 2).Syn-them One is not exposed in the Kendeng Basin but was sampled in blocks brought to the surface by mud volcanoes.Southern Mountains ArcBasement exposures in East Java are rare and are found only in the western part of the area at Karangsambung and the Jiwo Hills (Fig. 2). The exposed basement rocks are Cretaceous in age (Parkinson et al., 1998; Wakita and Munasri, 1994) and include basalticpillowlavas,radiolariancherts,variousmetasedimen-tarylithologies,quartz-micaschist,andhigh-grademetamor-phicrocksincludingjadeite-quartz-glaucophanebearingrocks andeclogites.Quartzveinsarecommonlyobservedwithinthe basement exposures and have not been identied in the overly-ingCenozoicrocks.Thehigh-grademetamorphicrockshave beeninterpretedtoindicatesubductionzonemetamorphism (Miyazakietal.,1998).Thebasementrocksareinterpretedto represent fragments of arc and ophiolitic material accreted to the margin of Sundaland during the Late Cretaceous. Similar rocks have previously been assumed to extend beneath the rest of East Java,wherethereisnoinformationfromsurfaceexposuresor drilling (e.g., Hamilton, 1979; Parkinson et al., 1998).SynthemOneis~1000 mthickbutisexposedonlyinthe westernpartoftheSouthernMountains ArcinEastJava.The oldestsedimentaryrocksrestdirectlyonthebasementabovea regional angular unconformity. They are poorly dated uvial con-glomerates and interbedded sandstones (Figs. 7A and 8) and are at least 50 m thick, but their total thickness is difcult to assess owing to limited and patchy exposures. These are the only rocks 0 50 100Kilometers1750 to 20001500 to 17501250 to 15001000 to 1250750 to 1000500 to 750250 to 5000 to 250-250 to 0-500 to -250-750 to -500

ms -2SUNDA SHELFSOUTHERN MOUNTAINS ARC8So110Eo112Eo114Eo7SoKENDENG BASIN250m500m750mDiengPlateauMerapiFigure 5. Bouguer gravity anomaly map of East Java. The map also shows topographic contours at 250 m intervals.OLIGOCENEEOCENE010152025303540455PLIOCENEMIOCENETECTONICSETTINGENVIRONMENTOF DEPOSITIONSEDIMENTCHARACTERTERRESTRIAL-SHELF-BASINCRETACEOUS~30 m.y. time gapEUSTATIC EVENTNO EVIDENCE OFDEFORMATIONTERRESTRIALDELTAICSHALLOW MARINEOPEN - DEEPMARINETRANSGRESSIONSUBSIDENCECRETACEOUS+ OLDERBASEMENTOLIGO-MIOCENEEXPLOSIVEVOLCANISMRESURGENCEOFVOLCANICACITIVITYSLUMPINGAPPARENT GAP IN SECTIONREWORKEDVOLCANICS+CARBONATEROCKSVOLCANICACTIVITYEARLYBASINSEQUENCENO VOLCANICACTIVITYCARBONATEPLATFORMSANDTURBIDITEFANSUPLIFTANDEROSIONEXPLOSIVEVOLUMINOUSVOLCANISMEPICLASTICREWORKING OFEXTINCT VOLCANICARC ANDWIDESPREADCARBONATEDEPOSITIONSYNTHEMTHREESYNTHEMTWOSYNTHEMONESEMILIR ERUPTIONVOLCANIC DEPOSITSIN TERRESTRIAL-SHALLOW MARINE-SLOPE. SOMEEPICLASTICREWORKING.DEPOSITION IN THEARC, INTRA-ARC,FORE-ARC ANDBACK-ARC.VOLCANICISLANDSTERRESTRIAL-MARINEKEYForaminiferal packstonesActive arc volcanismReefal limestonesErosion or non depositionSubsidenceSuper-eruptionBasal conglomeratesSiliciclastic sedimentsProximal volcanic rocksCrystal-rich volcaniclasticsVolcaniclastic turbiditesUnconformityFig.11AFig.10Fig.7CFig.7BFig.7AAge (Ma)Figure 6. Simplied stratigraphic column of the Southern Mountains Arc. The column shows the three synthems men-tioned in the text and indicates the phases of arc development recorded by the sedimentary succession.206Smyth et al.exposed onshore that contain no fresh volcanic material (Fig. 9). They are dominated by quartz grains and metamorphic and igne-ous lithic clasts (Figs. 8 and 9), including vein and metamorphic quartz, chert, phyllite, schist, metasedimentary rock, and basalt. The lithologies identied as clasts are typical of those exposed in the Cretaceous basement (described above) and are interpreted to be the product of erosion and reworking of these basement rocks. The terrestrial conglomerates and sandstones lack palynomorphs and so cannot be directly dated, but they are overlain by a succes-sion of well-dated middle Eocene strata (Lelono, 2000).ThemiddleEocenetolowerOligoceneoftheSouthern Mountains Arc is represented by a transgressive succession, from basetotop,ofcoalsandconglomerates,sandstones,siltstones, andmudstones(Fig. 8).ThemiddleEoceneageofthelower partofthesuccessionisbasedonpalynomorphsinthecoals and organic-rich mudstones (Lelono, 2000), and the occurrence ofNummulitesandDiscocyclinahigherinthesection.The lower part is at least 200 m thick and is a noncalcareous unit of well-beddedcoals,conglomerates,quartz-richsandstones,and organic-richmuds. Thecoalsandconglomeratesarerestricted tothelower50 m. Theconglomeratesarecommonlychannel-ized,are~1075 cmthick,andcontainarangeoflithicclasts similartothoseidentiedinthebasalsectiondirectlyoverly-ingthebasement.Thecoalsvarylaterallyinthicknessfrom5 to >50 cm and have an average vitrinite reectance of 0.4% Ro (Smyth, 2005). For normal and high geothermal gradients typical 01m2ABasal part of Synthem OneBLower part of Synthem One Upper part of Synthem OneBASAL TERRESTRIAL SANDSTONES +CONGLOMERATESTOPTERRESTRIAL - SHALLOW MARINESANDSTONES + ORGANIC MUDSTONESMARINE VOLCANOGENICSANDSTONES + MUDSTONESThickness(m)Increasingvolcanicmaterial012345678910Thickness(m)0.10.20.30.40.50.60.70.80.91.11.21.41.51.61.70123Sst Clay Silt CongThickness(m)Loc. 7.7395 S, 110.1887 E Loc. 7.5477 S, 109.6694 Eo, oTrough crosslaminationParallel laminationRip up clastsCross laminationChannelsConvolute laminationIrregular erosivebed baseConglomeratePlanar sharp bed baseLoc. 7.7323 S, 110.1972 ESst Clay Silt Cong Sst Clay Silt CongOrganic-rich mudsQuartz-rich sandstones+ siltstonesConglomeratesLithology Logs CLithology Logs A and BCrystal-richvolcaniclastic sandstoneTuffaceous mudstones+ siltstonesSedimentary structuresCFigure 7.Stratigraphiclogsillustrating featuresofthetypesectionsofSyn-themOne.(A)Basalconglomerates andsandstonesoftheLukuloMember, Karangsambung Formation. (B) Quartz-richsandstonesandinterbeddedorgan-ic-rich muds of the middle Eocene Kali SongoMember,NanggulanFormation. Volcaniccomponentsincreaseupsec-tion.(C)Volcanogenicsandstonesand tuffaceous mudstones of the Jetis Mem-ber, Nanggulan Formation.Cenozoic volcanic arc history of East Java, Indonesia207of arc regions this would imply very shallow burial depths of 150 mthick,withindividual bed thicknesses ranging from 5 to 50 cm. The turbidites have a diverseplanktonicforaminiferalassemblage(Fig. 8E),includ-ingHelicosphaeraeuphratis,H.reticulate,andH.wolcoxonii (P.Lunt,2002,personalcommun.),whichprovidesanageof NP18 (36.836.2 Ma) for the lower parts of the succession. The turbidites extend into the lower Oligocene, based on biostratig-raphy(P.Lunt,2002,personalcommun.).Theyaredominated by volcanic debris (Fig. 9) such as laths of plagioclase feldspar, volcaniclithicclasts,andvolcanogenicclays,andthereareno metamorphiclithicclastswithinthecoarserbeds.Theheavy mineralassemblageisdominatedbyvolcaniczircons,which yielded a weighted mean SHRIMP U-Pb age from 17 grains of 41 1.4 Ma, similar to the biostratigraphic age of the lower part of the section, suggesting some reworking.TheupperboundaryofSynthemOneisanintra-Oligo-ceneunconformity,whichisinterpretedtobetheresultofsea level change, as the sedimentary rocks directly above and below the gap have similar bedding orientations with no indication of deformation.Thiscouldbealocalsealevelchangebutcould also be a global change. Unconformities of this age that record a global intra-Oligocene sea level fall are widely known, from the Haqetal.(1987)curve(30 Ma),theMarshallParaconformity (3229 Ma) in the Canterbury Basin, New Zealand (Fulthorpe et al., 1996), and the carbonates of Baldwin County, Alabama, USA (Baum et al., 1994).WithinSynthemOnethecontributionofvolcanicmate-rialincreasesupsectionastheproportionofbasement-derived material(metamorphicquartz,metamorphiclithicfragments, andillite,chlorite,andserpentiniteclays)decreases,andthe upperEocenesedimentaryrocksarealmostentirelydominated by volcanic debris (Fig. 9A, B). The volcanic centers supplying this material cannot be separately mapped owing to their limited exposure, but they are interpreted to follow the same trend and occupythesamepositionsasthevolcanoesoftheOligoceneMiocene arc (see below).Kendeng BasinIn the Kendeng Basin there are limited exposures, and Syn-them One is not seen at the surface, but blocks of the older litholo-gies have been brought to the surface by Pleistocene and modern mudvolcanoes.Theseblocksareterrestrialandarecomposed of shallow marine sandstones and conglomerates (de Genevraye and Samuel, 1972) similar in character to the middle Eocene sed-imentary rocks in the Southern Mountains Arc.Synthem Two: Growth and Catastrophic Termination of Arc VolcanismSouthern Mountains ArcThe upper Oligocene to lower Miocene deposits of Synthem Two are primary volcanic rocks and epiclastic rocks. These rocks are exposed extensively throughout the Southern Mountains Arc (Fig. 4) and within the fold-thrust belt of the Kendeng Basin. The oldest rocks of this synthem in the Southern Mountains Arc are reworked bioclastic tuffaceous mudstones dated biostratigraphi-cally as Oligocene, NP24 (27.330 Ma; M. Fadel, 2002, personal commun.).ThroughoutthelateOligoceneandearlyMiocenethevol-canicactivityintheSouthernMountainsArcwasextensive, explosive, and intermediate to acidic in composition. The depos-itsrangefromandesitetorhyolite,withanaverageSiO2con-tent of 67 wt% (Smyth, 2005), and include thick mantling tuffs (Fig. 10A), crystal-rich tuffs, block and ash ows, pumice-lithic breccias,andesiticbreccias(Fig. 10B),siliciclavadomes,and lavaows.TheOligoceneMiocenevolcaniccenterscanbe mapped (Smyth, 2005) by the occurrence of vent proximal facies, and at least 13 centers are presently exposed (Fig. 2), which show asimilarspacingtothevolcanoesofthepresent-dayarc.The thickness of the proximal volcanic deposits ranges from 250 to >2000 m.Thick(>700 m)successionsofvolcaniclasticsedimentary rocks surround the volcanic centers. These reworked deposits are commonly interbedded with beds of unreworked mantling tuffs and volcanic breccias >1 m in thickness. The reworked volcani-clastic beds vary in thickness from 5 to >100 cm and are crystal- andvolcaniclithic-richsandstonesandtuffaceousmudstones. Theycommonlycontainabundantfragmentsofcharcoal,indi-cating the presence of vegetated slopes on the terrestrial volcanic centers. Both terrestrial and shallow marine deposits have been identied;thelatterdonotcontainabundantbioclasticmate-rial but are weakly calcareous, and their upper bedding surfaces arecommonlyintenselybioturbatedwithCruziana-typefacies traces. There are slump folds and mass wasting deposits on the anks of the volcanic centers.In the Southern Mountains Arc is a record of a major erup-tion, the Semilir Eruption, toward the end of the period of arc activity. Extensive deposits of this eruption are widespread to theeastof Yogyakarta(Fig. 4).TheSemilirandNglanggran Formations(Fig. 10A,B)aretheproductsofthisevent,and theyweredepositedinashortperiod,possiblyduringone eruptive phase between 21 and 19 Ma (Smyth, 2005). Based on measured sections, the combined thickness of the two forma-tionsvariesbetween250and1100 m. Theyarewellexposed over an area of 800 km2, and the total volume of volcanic mate-rial is estimated to be at least 480 km3. The Semilir Formation (Fig. 6)isathickaccumulationofdaciticair-fall,pyroclastic surgeandowdepositsproducedbyanexplosiveeruption. Both terrestrial and shallow marine deposits have been identi-ed, indicating that the erupted material entered the sea from theanksofthevolcaniccenter.TheNglanggranFormation (Fig. 6)isaseriesofmonomictandesiticvolcanicbreccias. Theindividualbedsareupto10 mthick,haveatbasesand tops,andcanbemappedfortensofkilometers. Thebreccias contain some blocks >3 m across. They are interpreted as vent proximalfaciessuchasowbrecciasorblock-and-ashow deposits.Thesedepositsmarktheendofvolcanisminthe 210Smyth et al.Southern Mountains Arc. There is no signicant break within or between the Semilir and Nglanggran Formations, based on zircon dating and biostratigraphic dating of the overlying for-mations (Smyth, 2005).A number of quartz-rich sandstones (Fig. 10D) are in the upperpartofSynthemTwointheSouthernMountainsArc (Smyth,2005). Volcaniczirconsfromthesandstonesforma singlepopulationwithagesthatarethesameasthatofthe SemilirFormation(Smyth,2005)andarecontemporane-ouswiththeSemilirEruption. Thequartzwithinthesesand-stones is entirely of volcanic origin (Fig. 10D). Volcanic quartz grains commonly appear clear and bright in thin section, have VolcanicBasementOtherSources406020408080 60 40 20208060KebobutakJatenSOUTHERN MOUNTAINS ARCQuartz Type %Volcanic UnknownCDA BFigure 10. Details of Synthem Two. (A) Field photograph of thick, well-bedded ashes (bar is 1m). (B) Field photograph of monomict andesitic breccia; the breccia clasts commonly exceed 1 m in size (bar is 1m). (C) Basement, volcanic, and other sources are depicted in a triangular diagram, showing a selection of formations from Synthem Two. These deposits lack material of basement origin. (D) Pie chart showing the quartz types identied petrographically in a quartz-rich sand-stone in the upper part of Synthem Two. The quartz is dominated by volcanic quartz; the image is a bipyramidal grain from the Jaten Formation, Pacitan (bar is 1m).Cenozoic volcanic arc history of East Java, Indonesia211nonunduloseextinction,andaremonocrystalline(Leeder, 1982).Thesefeaturesandothercharacteristics,including well-developed crystal faces, bipyramidal shape, melt embay-ments, and melt inclusions, can be used to distinguish volcanic quartzfromothertypessuchasmetamorphic,vein,plutonic, andsedimentaryquartz. Volcaniccrystal-richsandstonescan beproducedbyprimaryeruptivemechanismsand/orsecond-ary epiclastic processes. The sandstones and other quartz-rich volcaniclasticrocksinEastJavapreviouslywereinterpreted as continental siliciclastic deposits (Harahap et al., 2003), and itisforthisreasonthatmanyoftheacidicproductsofthe SouthernMountains Archavebeenoverlooked(asdiscussed below). Here they are interpreted to have a volcanic origin and tobelargelytheproductoftheSemilirEruptionwithsome subsequent reworking.Kendeng BasinSynthem Two exposures in the Kendeng Basin are very lim-ited, occurring only in a small thrust-bound sliver. They comprise poorlylithiedGlobigerina-richtuffaceousmudstonesranging inagefromlateOligocenetoearlyMiocene(deGenevraye andSamuel,1972).Themudstonesareatleast85 mthick(de Genevraye and Samuel, 1972) but have limited surface exposure. The mudstones are volcanogenic and apparently lack continen-talterrigenousmaterial.Thevolcanicmaterialisnegrained, reworked, and distal in character, very different from that identi-edwithinthearcatthistime.TheabundanceofGlobigerina andotherplanktonicforaminifersisindicativeofpelagicsedi-mentation in an open marine setting at water depths of a few hun-dred meters or more. Thick volcanogenic turbidites are reported from wells in this area (P. Lunt, 2002, personal commun.), but no descriptionsofthestratigraphyorwellloginterpretationshave been published.Edge of the Sunda ShelfThe offshore Eocene to Pliocene successions in the East Java Sea that have been investigated during hydrocarbon exploration are not reported to contain any volcanic material (e.g., Matthews andBransden,1995).Fieldinvestigationsduringthisstudyin northeastJavahaveidentieddistalvolcanicmaterialonthe edgeoftheSundaShelfwithincarbonates,includingthintuff layers, zones rich in smectite clays, and concentrations of volca-nic quartz and zircons. This suggests that there may be more vol-canic debris offshore than recognized up to now, and that some of the clay layers may be ne air-fall ash deposits.The stratigraphic record of Synthem Two within the South-ern Mountains Arc and Kendeng Basin accounts for only vol-canic and volcaniclastic rocks at this interval. There is no evi-dence of input of material from basement or other sources. This indicates that the volcanic arc was the only source of sediment atthistime.Therearenosignicantexposuresofcarbonates within Synthem Two in the Southern Mountains Arc or Kend-eng Basin, but carbonates do occur farther to the north on the edge of the Sunda Shelf.Synthem Three: Reworking of the Southern Mountains ArcSouthern Mountains ArcIn the Southern Mountains, to the south of the OligoceneMiocenevolcaniccenters,extensivecalcareousvolcanogenic turbidites (Fig. 11) occur with Bouma divisions A, C, D, and E. The turbidites are at least 500 m thick and have been dated within nannofossil subzones NN2NN8 (Kadar, 1986) that correspond to dates between 19 and 10 Ma. Beds vary in thickness from 20 to 75 cm, and upper bedding surfaces are commonly bioturbated with traces of Cruziana facies. Flute casts indicate that the ow directionwastowardthesoutheast,andthereareslumpfolds withsoutheasterlyvergence.Thinsectionexaminationshows that volcanic crystals and lithic fragments are well rounded, and there is no fresh or unreworked volcanic debris present. Dating of zircons (see below) supports the eld and petrographic evidence that these are reworked volcaniclastic deposits, as the zircon ages are similar to the age range of rocks in Synthem Two. Synthem Three is therefore interpreted to be the product of reworking of older arc rocks of Synthem Two rather than the product of con-temporaneous volcanism.Several tuff beds have been identied at the top of the tur-bidite sequence in the Southern Mountains but are not the result of volcanic activity in the Southern Mountains Arc. The tuffs are distalair-falldepositsandyieldzirconswithagesbetween12 and 10 Ma (P.J. Hamilton, 2003, personal commun.). These tuffs are the product of the modern Sunda Arc (Soeria-Atmadja et al., 1994), and the ages mark the initiation of volcanic activity some 50 km to the north of the Southern Mountains Arc.Inadditiontotheturbidites,therearetherstwidespread carbonatesintheSouthernMountains(Fig. 11D).Thecarbon-atesrangeinagefromlateearlyMiocenetomiddleMiocene (Lokier, 2000; Smyth, 2005) and formed isolated reefs and exten-sivecarbonateplatforms,whichcanbeobservedoverstepping the deposits of Synthem Two. The limestones are at least 200 m thick and are the source of the carbonate within the volcanogenic turbidites.Kendeng BasinVolcanogenicsedimentaryrocksidentiedintheKendeng Basin include channelized volcanic lithic conglomerates, crystal-richandvolcaniclithic-richsandstones,tuffaceousmudstones, and Globigerina mudstones. A thickness of up to 400 m of rocks isexposed(Smyth,2005),andhydrocarbonexplorationshows thereisupto3000 minthebasin(deGenevrayeandSamuel, 1972). The sandstones contain Bouma divisions B, C, and E and are locally cut by the channelized conglomerates. These conglom-erates can cut as much as 20 cm into the underlying beds and are up to 1 m thick. Slump folds are common, and most verge toward thenortheast.Measurementsofscours,grooves,channelstruc-tures, utes, and slumping indicate that sediment was transported from the south toward the north. The Globigerina mudstone is similarincharactertothoseofSynthem Two. Asamplefrom thetopoftheexposedsectionyieldedabiostratigraphicage 212Smyth et al.of foraminifer subzones N8N9 (M. Fadel, 2003, personal com-mun.) corresponding to ages between 17.2 and 14.4 Ma.Edge of the Sunda ShelfOn the edge of the Sunda Shelf are a number of lower to mid-dle Miocene quartz-rich sandstones. They were previously inter-preted to have a continental provenance and to have been derived from reworking of basement rocks of Sundaland (e.g., Ardhana, 1993; Sharaf et al., 2005), but new studies (Smyth et al., 2007) have shown that these sandstones contain a signicant proportion of volcanic quartz, volcanic zircons, and reworked volcanogenic clays. Thesevolcanicparticlesareinterpretedtohavefallenas ashontotheSundaShelf,tohavebeensubsequentlyreworked and mixed with material derived from uplifted basement blocks in the East Java Sea (Bishop, 1980; van Bemmelen, 1949), and redeposited on the edge of the Sunda Shelf.INTERPRETATION OF THE EOCENE TO MIOCENE STRATIGRAPHYInitiation of the Southern Mountains ArcThe rst evidence of arc volcanism in East Java is identied asmiddleEoceneatca. 42 Mawiththerstoccurrenceoftuff layers, volcanic lithic fragments, volcanic quartz, laths of plagio-clase feldspar, and volcanic zircons. Prior to this, the sedimentary rocks, which are the oldest rocks exposed on East Java, lack evi-dence of contemporaneous arc volcanism and are the only rocks 05101520Clay Silt SandC M FABCDBioturbationAsymmetrical ripplesParallel laminationPebblesBrecciaConvolute parallellaminationUpper part ofSynthem TwoSynthemThreeThickness(m)7.8727 S110.5496 EooLoc.Figure 11.StratigraphicdetailsofSyn-themThree.(A)Loggedsectiontypi-calofthevolcanogenicturbiditesof SynthemThree,SambipituFormation. (B)Fieldphotographshowingasym-metricalripplesontheupperbedding surfacesofthevolcanogenicturbidites, SambipituFormation.(C)Fieldphoto-graph of volcanogenic turbidites within theKendengBasin,KerekFormation. (D)Fieldphotographofthereefalcar-bonates typical of Synthem Three in the SouthernMountainsArc,Campurdarat Formation.Cenozoic volcanic arc history of East Java, Indonesia213exposed on East Java that contain no volcanic debris. This sug-geststhattherewasnoarcvolcanismonJavapriorto42 Ma, which also implies no subduction of the Indian-Australian plate beneath Java at this time.Growth and Development of the Southern Mountains ArcFollowing the initiation of volcanism in the Southern Moun-tainsthecontributionofvolcanicdebrisincreasedrapidly,and thecontributionofbasement-derivedmaterialdecreased.The volcanic centers probably formed an east-west chain of volcanic islandsseparatedbyinterarcbasins,muchlikethepresentIzu-Bonin-Mariana and Aleutian Islands Arcs. A narrow volcaniclas-tic shelf built up around the isolated volcanic islands, where thick sequences of volcanic and epiclastic material were deposited. To the north and directly behind the arc, the Kendeng Basin began to subside, and material was transported from the volcaniclastic shelf into the basin.No reef carbonates are preserved within Synthem Two near thearc,suggestingthatenvironmentalconditions,suchasthe inux of volcanic detritus, prevented reef growth or, alternatively, thatthesecarbonateswerenotpreservedorexposed.However, somedistancetothenorth,ontheedgeoftheSundaShelf,a largeareaofcarbonateplatformwasdevelopingatthistime; the shelf received volcanic debris as air fall, but much less than that deposited close to the arc, and the volcanic material did not substantially inhibit carbonate growth. Wilson and Lokier (2002) showed that volcanic input can inuence carbonate development closetoactivearcswithoutkillingcarbonate-producingorgan-isms and causing carbonate deposition to cease.DuringtheOligocenetoearlyMiocene,volcanicactivity alongtheSouthernMountains Arcwasatitsmostvoluminous and explosive. The volcaniclastic shelf close to the active volcanic centers increased in width and thickness. Material was fed into the Kendeng Basin, and only in the deepest part of the basin was there pelagic sedimentation without a volcanogenic component.Termination of Volcanism in the Southern Mountains ArcFollowingalongperiodofarcactivitybetween42and 18 Ma,volcanismintheSouthernMountains Arcceased.The nal stages of activity were marked by a phase of explosive vol-canism, which included a major event, the Semilir Eruption. The age range, thicknesses, area, and estimated volumes of volcanic depositsinthevicinityoftheTobavolcaniccenter(Chesner andRose,1991)aresimilartothoseoftheSemilircenter,and the deposits of the Semilir Eruption may be distributed, like the Youngest Toba Tuffs (Song et al., 2000), over large parts of SE Asia. Work is in progress to assess their distribution.Lull and Resurgence of Volcanic ActivityFollowingtheterminationofvolcanism,therewaswide-spreaderosionofthedepositsofSynthemTwoandextensive reefgrowth.Volcanogenicturbiditesbuiltoutasthickaprons northwardfromtheinactivearcintotheKendengBasin.Sedi-mentation was rapid and on an unstable northward-dipping slope, andthedepositsbegantolltheaccommodationspacewithin the basin. Material also traveled southward from the eroding arc into the Java forearc. On fault-bounded highs or in the shelter of theextinctvolcanoes,reefsandcarbonateplatformsdeveloped, marking the rst period of extensive carbonate growth within the Southern Mountains during the middle Miocene (ca. 2010 Ma).ArcvolcanismresumedinthelateMiocene,afteralullof ~8 m.y.,50 kmtothenorthoftheextinctSouthernMountains Arc at the position of the modern Sunda Arc.CHARACTER OF THE CRUST BENEATH THE SOUTHERN MOUNTAINS ARCLittle is known of the crust beneath many young arcs, espe-cially in Indonesia, and East Java is no exception. However, study of the stratigraphy of the Southern Mountains Arc has provided some new and surprising insights into the character of the deep crust beneath the arc.Asdiscussedabove,exposuresofbasementrocksinEast Java are limited and are restricted to the west of the study area. Theserockshavebeeninterpretedtobefragmentsofarcand ophiolitic material accreted to the continental margin of Sunda-land during the Late Cretaceous (Hamilton, 1979; Wakita, 2000). It has been generally considered that these rocks are typical of the basement beneath the whole of East Java. This is supported by oil company drilling on land in East Java and farther to the north in the East Java Sea, where deep wells penetrate a varied basement, including basic and acidic volcanic rocks, slaty metasedimentary rocks, quartzites, and cherts (e.g., Hamilton, 1979; Matthews and Bransden,1995).However,theabundanceofacidicvolcanism in the Southern Mountains Arc and the dating of zircons indicate that the crust beneath much of the southern part of East Java is very different, and is much older.VolcanismTheintermediatetoacidicvolcanicrocksoftheSouthern Mountains Arcrangeincompositionfromandesitetorhyolite (6077wt%SiO2),withanestimatedaverageSiO2contentof 67wt%.Inaddition,manyofthehigh-levelintrusivebodies exposedwithintheSouthernMountains Arcaregranodiorites. Theserocksareconsiderablymoreevolvedthanthebasicto intermediate eruptive products of the present-day volcanoes. The chemistry of the modern arc volcanic rocks in East Java (Hand-ley,2006;Nichollsetal.,1980; Whelleretal.,1987)indicates that they are the products of relatively primitive subduction melts, which reveal no signicant interaction with underlying crust. The differenceinchemistryoftheproductsoftheSouthernMoun-tains Arc and the Sunda Arc could be explained in a number of ways: fractional crystallization of more basic magma, interaction ofmorebasicmagmawithfelsiccrust,andpartialmeltingof 214Smyth et al.felsiccrust.Evidencefromzircons(discussedbelow)suggests that involvement of continental basement in petrogenesis is the most likely explanation.Ancient ZirconsAt the beginning of this study there were very few good dates forvolcanicrocksoftheolderarconJava(50analyses),and allpublishedageswereK-Ardates(Ben-AvrahamandEmery, 1973;Soeria-Atmadjaetal.,1994).Noacidicrockshadbeen dated.Duringthisstudy,zirconsfrom16Cenozoicsamplesof igneous(9),volcaniclastic(3),andsedimentaryrocks(4)were dated by the SHRIMP U-Pb method at Curtin University of Tech-nology, Australia,usingthemethodsdescribedinSmythetal. (2005, 2007). More than 453 spot ages were measured, and 270 of these were Cretaceous and older.Prior to SHRIMP dating, the expected age range of the zir-con samples was Cenozoic to Cretaceous. This reects the age of arc activity known from Java, and a contribution from the Cre-taceous arc and ophiolitic terranes interpreted to form the base-ment of East Java. However, in addition to the expected ages, an unexpected range of ArcheanCambrian grains was identied in alargenumberofsamples(Smyth,2005,2007).Thesamples containing Cretaceous ages and those containing Cambrian and older zircons occur in distinct areas of East Java.Lithology and Zircon Age RangeThe ve intrusive and four extrusive igneous rocks analyzed contain a range of Cambrian and older zircons (n = 155) but lack Cretaceouszircons.Incontrast,thethreevolcaniclasticrocks analyzed yielded a signicant number of Cretaceous zircons (n =42)butonlyoneCambrianorolderzircon. Thefourquartz-rich sandstones analyzed yielded varied zircon ages. All of these sandstones contain Cretaceous grains (n = 33). Three sandstones from the Southern Mountains and Kendeng Basin contain a range of Cambrian and older grains (n = 22). One sandstone from the edge of the Sunda Shelf contains a single JurassicTriassic grain but no older grains.Age Ranges and Spatial DistributionThe samples containing different zircon ages do have a clear geographical pattern. The Archean grains are found only in rocks of the Southern Mountains (Fig. 12B), and the igneous rocks of thisareacontainnoCretaceouszircons(Fig. 12A).Cretaceous zirconsarefoundonlyinthenorthandwestofEastJava,and these zircons have been identied only in sedimentary and vol-caniclasticrocks. ThisisinterpretedtoindicatethattheSouth-ern Mountains are underlain by material providing Cambrian to Archean zircons but no Cretaceous zircons. To the north and west of the Southern Mountains the underlying rocks do not include Cambrian to Archean material but do include Cretaceous mate-rial. Figure 12C shows the distribution of basement rocks inter-preted from the zircon ages.Ifallthezirconagesaregroupedtogether,veagepeaks can be identied (Fig. 13): a Cenozoic peak (542 Ma) recording volcanism in the Southern Mountains and Sunda Arcs, a Creta-ceouspeak(65135 Ma)consistentwiththeinterpretedageof basement, a Cambrian to Neoproterozoic peak (5001000 Ma), a Mesoproterozoic to Paleoproterozoic peak (10002500 Ma), and an Archean peak (25003200 Ma). The Cambrian and older ages were not expected, based on knowledge of the regional geology.Source of the Ancient ZirconsSundaland,thecontinentalcoreofSEAsia,istheclosest and most obvious source of the very old zircons (Figs. 1 and 13). Rocks that could have provided abundant old zircons include gran-ites of SW Borneo (Hamilton, 1979), the Malay Peninsula (Liew and Page, 1985), the Thai-Malay Tin Belt (Cobbing et al., 1986), andSumatra(Imtihanah,2000;McCourtetal.,1996)(Fig. 13). However,intheseareastherearenoknown Archeanrocks,nor is there any indication that they are underlain by Archean crust. Geochemicalandisotopicstudiessuggestabasementnoolder than Proterozoic in areas of Sundaland, such as the Thai-Malay peninsula, that have been studied (e.g., Liew and Page, 1985). The closest area with extensive granites is the Schwaner Mountains of SW Borneo. Paleogene sedimentary rocks of north Borneo contain debris eroded mainly from the Schwaner granites and the Malay Tin Belt, including detrital zircons (van Hattum et al., 2006). The zircon populations are dominated by Cretaceous zircons with age peaks different from those of East Java; there are abundant Perm-ianTriassic zircons, rare in East Java; and Precambrian zircons are mainly Paleoproterozoic with very rare Archean grains. The differencesinzirconagesruleoutaSchwanerorThai-Malay provenance, and other granite sources in Sundaland are even more distant and paleogeographically unlikely.The largest and closest area of continental crust of Archean age is Australia, which formed part of Gondwana until the Cre-taceous. It has been suggested that small Gondwana continental fragmentscollidedwiththeeastSundalandmargininSulawesi during the Cretaceous (e.g., Wakita et al., 1996; van Leeuwen and Muhardjo, 2005), although the source of the fragments was not identied.ThereisevidenceofLateJurassicEarlyCretaceous rifting of continental fragments from the northwest and west Aus-tralian margins (Mller et al., 2000) during continental breakup preceding the separation of India from Gondwana. One of these continentalfragmentscouldhavecollidedwiththeSundaland margin.Bergmanetal.(1996)speculatedthattherecouldbe old continental crust beneath west Sulawesi, on the basis of lead isotopiccompositionsofNeogeneplutonicrocks,whichwere suggestedtohavehadan Australianorigin.Basementrocksin western Australia are dated as Proterozoic and Archean. Detrital zircons from young sediments of the Perth Basin, derived from the erosion of western Australian basement, have been well stud-iedanddated(CawoodandNemchin,2000;Pelletal.,1997; SircombeandFreeman,1999). ThezirconpopulationsinPerth Cenozoic volcanic arc history of East Java, Indonesia215Basin sedimentary rocks are remarkably similar in age to those of the Southern Mountains in East Java. Both areas yield zircons with 42002500 Ma ages (predominantly 32002500 Ma) simi-lartothe Archean YilgarnandPilbaraBlocks,20001600 Ma and13001000 MaagessimilartothePaleoproterozoicand Mesoproterozoic Capricorn and Albany-Fraser orogenic belts, and 800500 Ma ages similar to the Neoproterozoic Pinjara orogenic belt (Fig. 14). A west Australian origin is a geographically simple andplausibleexplanationforthePrecambrianzirconsofEast Java, but how did they become incorporated in igneous rocks?Inherited population includesNO Cretaceous zirconsCretaceous zirconsNO Archean zirconsInherited population containsArchean zircons0 50 100Edge of Sunda shelf: Nature of the basementuncertain;potentially arc and ophiolitic fragmentsaccreted during the Cretaceous.KENDENG BASIN: Nature of the basementuncertain; potentially arc and ophioliticfragments accreted during the CretaceousCENTRAL JAVA:AccretedCretaceousmetamorphosedarc and ophioliticcrust.SOUTHERN MOUNTAINS ARC:Underlain by a fragment of continentalcrust of Gondwana origin0 50 100KilometersKilometers0 50 100Kilometers7 So8 So7 So8 So7 So8 So110 Eo112 Eo110 Eo112 Eo110 Eo112 Eo

ABCFigure 12.Distributionofsamplesthat containedzirconpopulationsof(A) Cretaceousages,and(B) Archeanages (Smyth, 2005). (C) Interpreted character of the basement terranes in East Java.216Smyth et al.Incorporation of Zircons into East Java Igneous RocksZirconsarehighlyrefractoryandarewellknownforsur-viving repeated episodes of melting (e.g., Hanchar and Hoskin, 2003). AncientzirconscouldhavebeenintroducedintoPaleo-gene magmas of the Southern Mountains Arc either by subduc-tion of Gondwana-derived sediments or crust, or by passage of magmas through a previously emplaced Gondwana-origin conti-nental fragment at depth beneath the Southern Mountains.Subduction of Gondwana SedimentSediment from western Australia deposited on the Indo-Aus-tralian plate could have been carried north to the Java Trench as Australia moved north. However, even today a fan similar in size tothemodernBengalFanwouldberequiredtobringmaterial from AustraliatothesiteofsubductionattheJava Trench,and there is no evidence of a sediment supply and distribution system like this in the past. The huge Bengal Fan is fed by erosion from the Himalayas, but there is no evidence for a comparable orogenic belt in Australia during Cenozoic times. During the early Ceno-zoic, Australia lay much farther south of its current position (Hall, 2002), and there is little sedimentary cover today on the Indian-Australian plate south of Java (Masson et al., 1990; Kopp, 2002) in what would have been more proximal parts of such a fan.Itisunlikelythatmaterialcouldhavebeenintroduced intotheJavaTrenchbyaxialtransportfromtheHimalayas viatheBengalFan,orfromtheBirdsHeadmicrocontinent, NewGuinea.Ifsedimentwasproducedbytheerosionofthe Himalayas, it would require transport via the Bengal Fan to the Java Trench and total transport distances >4000 km. Although mud-richmaterialisconsideredtohavereachednorthern SumatrainthedistalpartsoftheBengalFan(Currayetal., 2002), there were topographic barriers at the Investigator Frac-tureZoneandtheNinetyEastRidge,andeventhenafurther 2000 km of along-trench transport. Today there is no material from the Bengal Fan reported in the Java Trench off the shore of East Java (e.g., Masson et al., 1990) and no signicant sedi-mentcoverontheIndianplatesouthofJava(Kopp,2002).It isequallyunlikelythatsedimentfromtheHimalayanregion could have traveled southeastward from Indochina across Sun-daland,bypassingnumerousbasinsandassociatedstructural highs(HallandMorley,2004)toenterEastJava. Thiswould require transport distances >3500 km.The basement rocks of the Birds Head microcontinent were notavailableforerosionduringmostoftheCenozoic,andthe BirdsHeadwasthesiteofdepositionofthickmarinecarbon-atesoftheNewGuinealimestones(Fraseretal.,1993;Pieters et al., 1983). Even had the Birds Head been supplying large vol-umesofsediment,along-trenchtransportdistances>2000 km wouldhavebeenrequiredformaterialtoenterthesubduction zone beneath Java. This is equivalent to material from southern Mexicoornorthern WashingtonStateenteringthe Allistos Arc 120E 110E 100E10S0SulawesiBruneiSabahKalimantanSarawakBali LombokJavaCELEBESSEAJAVASEAINDIANOCEANFLORESSEAMakassarStraitSunda TrenchJava TrenchSUNDASHELFWest JavaBasinEast JavaSea BasinMeratusMountainsKarimunjawa ArchSchwanerMountainsExtent of continentalcrust of Sundaland(Hamilton, 1979)Granites of Sundaland(Cobbing, 2005;van Hattum, 2005)MalayPeninsulaBorneoMalino MetamorphicComplexSumatraFigure 13. Extent of area interpreted to be underlain by continental crust, and locations of granitic bodies that may have been available for erosion during the Cenozoic. The NE-SWtrending Karimunjawa Arch, which was a signicant topo-graphic barrier throughout the Cenozoic, is also denoted.Cenozoic volcanic arc history of East Java, Indonesia217(Busby et al., 2006) of Baja California, or for northern Norway or Alabama to have been sources for the Lough Nafooey Arc (Draut and Clift, 2001) of the Irish Caledonides.Subduction of a Gondwana FragmentSubductionofcontinentalcrustisevenlesslikely.Buoy-ancyforcesmakesubductionofcontinentalcrustdifcult,and evenifthesewereovercome,thefragmentwouldhavebeen requiredtosupplymaterialtothearcfromthemiddleEocene to the early Miocene, a period of ~20 m.y. The fragment of crust eitherremainedstationarybeneaththeSouthernMountainsor was 1500 km in length, based on the present subduction rate of 75 mm/year(McCaffrey,1996),oronCenozoicplatetectonic reconstructions (Hall, 2002).403530252015105500 1000 1500 2000 2500 3000NumbersofgrainsAge (Ma)PINJARAOROGENCAPRICORNOROGENALBANY-FRASEROROGENYILGARN CRATON1000-1300 Ma 1600-2000 Ma 2500-4200 Ma 500-800 MaB Range of U-Pb SHRIMP ages from placerdeposits in Western Australia (Bruguier et al, 1999;Sircombe and Freeman, 1999) and some of theprobable source regions. N = 260A0510152025303540CRETACEOUSBASEMENT500 1000 1500 2000 2500 3000NumbersofgrainsCENOZOICVOLCANISMRange of U-Pb SHRIMP zircon ages from theSouthern Mountains Arc, East Java (Smyth,2005). N = 453NEOPROT MESOPROT PALEOPROTEROZOIC ARCHEAN PHANEROZOICFigure 14.ComparisonoftheU-Pb SHRIMPagesof(A)EastJava(Smyth, 2005), and (B) Western Australia (Bruguier et al., 1999; Sircombe and Freeman, 1999). Revised from Smyth et al. (2005).218Smyth et al.Accretion of a Gondwana FragmentThemostlikelyexplanationisthatcontinentalcrustwas already present beneath the Southern Mountains Arc when sub-duction began in the middle Eocene. We suggest that this was a western Australiarifted continental fragment that collided with themarginofSundalandduringtheLateCretaceousandmay have terminated the Cretaceous phase of subduction. Subduction wasrenewedinthemiddleEocene,andCenozoicmeltswere contaminatedbyinteractionwithcontinentalcrustbeneaththe SouthernMountains.Thedistributionofpre-Cenozoiczircons indicates that north of the Southern Mountains there is no conti-nental basement, and the deep crust is arc or ophiolitic, perhaps representing material accreted during Cretaceous subduction or underplatedduringcollision.Thecontinentalfragmentcould have been quite large and may be traceable into Sulawesi. Zircon dating and geochemical evidence (van Leeuwen and Muhardjo, 2005)showthattheMalinoMetamorphicComplexofNW Sulawesi (Fig. 13) contains zircons with ages up to 3500 Ma.DISCUSSIONThe stratigraphic record in East Java provides insights into the nature of the deep crust beneath the arc, the history of Indian-Australian plate subduction, and the contribution of the volcanic arctobasinformation.Italsoposessomequestionsaboutthe continuity and location of volcanic activity, which have relevance for other arcs.Deep Character of the East Java Volcanic ArcLittle is known about the deep crust beneath the young arcs of the Western Pacic and Indonesia. There have been few seis-mic refraction studies and few xenolith studies, and the deep crust is almost invariably covered by younger volcanic and sedimen-tary rocks. In East Java the old zircons in the Eocene to Miocene arc products provide evidence of the character of the deep crust. The insight gained was completely unexpected. When this study began, it was thought that the entire region was underlain by Cre-taceous arc and ophiolitic fragments, as suggested by Hamilton (1979). ThedistributionofagesshowsthatnorthoftheSouth-ern Mountains there is indeed crust of this character, as indicated bythesmallexposuresofbasement,butbeneaththeSouthern Mountains themselves there is a fragment of continental crust of Gondwana character and western Australian origin.Thepresenceofacontinentalfragmentbeneaththearc accountsfortheunusuallyacidiccharacterofarcvolcanismin the Eocene to early Miocene arc. The acidic products of this arc havebeenoverlookedpartlybecausetheresistantOld Andes-ites are topographically so much more obvious, although acidic volcanic and minor intrusive rocks are well exposed in many parts of the Southern Mountains. However, the main reason why the acidic products of the Eocene to early Miocene arc were missed by earlier studies is because they were erupted explosively and dispersed as volcanic ash, the ash was reworked into sediments, and the processes of eruption and tropical weathering combined to remove the unstable volcanic constituents, leaving well-sorted quartz-rich sandstones. These were previously interpreted as hav-ing been eroded from a continental Sundaland source, but careful examination of the sandstones reveals abundant evidence of their volcanicoriginonthebasisoftextures,lightmineralconstitu-ents, quartz character, clay mineralogy, and zircon character and ages (Smyth, 2005).Age of SubductionThe character of the oldest parts of the East Java stratigraphic successions indicates that Cenozoic volcanic activity began in the middle Eocene (ca. 45 Ma). There is little evidence to support the common assumption (e.g., Hall, 2002; Heine et al., 2004; Met-calfe, 1996; Scotese et al., 1988) that subduction continued from the Mesozoic into the Cenozoic without signicant interruption. ThestratigraphyofEastJavaindicatesthatoldersubduction probably terminated in the Late Cretaceous with the collision of a continental fragment rifted from western Australia. We suggest that the continental fragment extended from East Java to North Sulawesi. Ophiolites were emplaced from Java to North Borneo duringthiscollision.Theoldestsedimentaryrocksabovethe basement in East Java are of uncertain age but are middle Eocene or older and lack volcanic debris. There is little evidence for lat-estCretaceoustoearlyEocenevolcanicactivityinmostofthe SundalandmarginbetweenSumatraandSulawesi.Subduction resumedinthemiddleEocenewhenAustraliabegantomove northward rapidly after 45 Ma (Hall, 2002; Mller et al., 2000; Schellart et al., 2006), and has continued to the present day.Formation of the Kendeng BasinWhen volcanic activity resumed in the middle Eocene a basin began to form directly north of the arc. Most of the investigations of sedimentary basins on land in East Java and in the East Java Sea (Fig. 1) have been carried out as part of hydrocarbon explora-tion activity, and little attention has been given to areas close to themodernarc. TheSundaShelfbasinsaretypically>100 km fromthearcandarecharacterizedbymanyfeaturestypicalof fault-controlled basins; explanations for their origin have there-foreinterpretedthemastypesofbackarcbasinsresultingfrom subductionrollback,riftandthermalsag,orstrike-slipfaulting (e.g., Hamilton, 1979; Matthews and Bransden, 1995).The Kendeng Basin is not a typical backarc basin. Backarc basins (Taylor and Karner, 1983) may form by trench rollback, causing generation of oceanic crust (Dewey, 1980; Karig, 1971), rifting of continental crust (Kobayashi, 1985), gravitational col-lapseinthewakeofarc-continentcollision(Cliftetal.,2003), or by the trapping of old oceanic crust behind an arc (Scholl et al., 1986). Retroarc basins (Dickinson, 1974) form as compres-sive foreland basins behind a volcanic arc. Busby and Ingersoll (1995) dened backarc basins as oceanic basins behind intraoce-anicmagmaticarcs,andcontinentalbasinsbehindcontinental Cenozoic volcanic arc history of East Java, Indonesia219marginarcsthatlackforelandfoldandthrustbelts.Marsaglia (1995)identiedsimilarcategoriesofbackarcbasinswiththe addition of boundary basins resulting from extension along plate boundarieswithtranslationalcomponents.BusbyandIngersoll (1995) claim that the Sunda Shelf basins of Indonesia are non-extensionalbackarcbasinsformedinaneutralstrainregime, although the basis for this assertion is not clear, because there is abundant evidence for rifting of many of these basins (e.g., Cole and Crittenden, 1997; Hall and Morley, 2004).There is no evidence for either newly formed oceanic crust under the Kendeng Basin or for strike-slip faulting. The basin is much closer to the arc than other backarc basins and most of the backarc basins of the Sunda Shelf. The late Cenozoic deforma-tionintheregionledto~1030 kmofcontractionattheouter edge of the basin, so throughout the early Cenozoic this basin was very close to the arc. There was extension in the Sunda Shelf from theEocene,butnowherewasoceaniccrustformed.TheKend-eng Basin is an asymmetrical depression, the deepest part of the basin lies directly behind the arc, and the Eocene to lower Mio-cene volcanic and sedimentary sequence thins toward the edge of the Sunda Shelf (Waltham et al., this volume). The basin ll was derivedmainlyfromtheSouthernMountains Arconthesouth sideofthebasin.Thebasinbegantoformatthesametimeas arc activity began, and its subsidence history is closely linked to activity in the volcanic arc. There are no seismic lines crossing the basin, and it is not well exposed at the surface, so we cannot assess the role of faulting in the basin development, but it does not have the typical synrift-postrift stratigraphy of many other Sunda Shelf basins.Thereisnoevidenceofcompressionalloadinghaving contributed to basin formation. Thrusting in East Java occurred in the late Neogene long after the Kendeng Basin had formed. The close relationship between volcanic activity and basin subsidence suggests that the two are linked, and we suggest that the load of thevolcanicarcwasthemajorcauseofbasinsubsidence.The contribution of volcanic arc loading to basin formation in Indone-sia is discussed in greater detail by Waltham et al. (this volume).Movement of the Volcanic ArcActivityintheSouthernMountains Arcterminatedduring theearlyMiocene(ca. 20 Ma). Afteraperiodwithlittlemag-matism,anewepisodeofarcvolcanismbeganinEastJavaat ca. 1210 Ma in a new location. The Miocene to Holocene arc is parallel to the Southern Mountains Arc but ~50 km north of it. A signicant reduction in volcanic activity in the middle Miocene isawell-knownfeatureoftheSunda ArcfromJavaeastward, althoughsubductionwascontinuousduringthisperiod(Hall, 2002). Vigorous volcanic activity resumed at the end of the mid-dleMioceneintheSundaandBanda Arcsatca. 10 Ma(Hall, 2002; Macpherson and Hall, 2002).Macpherson and Hall (1999, 2002) suggest that the decline in volcanic activity resulted from northward advance of the subduc-tion hinge. Hinge advance prevented replenishment of the mantle wedgebyfertilemantle,andconsequentlysubduction-induced melting was inhibited as the wedge became depleted. The cause of hinge advance was the result of the collision of Australia with eastern Indonesia, and the counterclockwise rotation of the Bor-neo-Java region this collision induced (Hall, 2002). Arc volcanism resumed when hinge advance ceased and the mantle wedge was replenished by fertile mantle (Macpherson and Hall, 2002).Why the volcanic arc moved to its new position 50 km north of the Southern Mountains Arc is uncertain. There is evidence of thrusting and contraction in Java in the late Neogene, but its tim-ing is not precisely known; this is part of our current research. It is possible that contraction was linked to large-scale arc dynam-ics, such as coupling of the subduction and overriding slab. There is evidence of an old consolidated accreted material in the Java forearcthatactsasabackstoptotheactiveaccretionaryprism (Kopp et al., 2001). This suggests two phases of forearc develop-ment, and these may be linked to the two phases of arc volcanism on land in East Java. It is also possible that when the subduction hinge advanced and the mantle wedge was not being replenished, one result was a rigid, stronger overriding lithospheric plate that later failed when hinge advance ceased and weaker, warmer man-tle replenished the wedge. Another control could have been the nature of the crust beneath the arc. The zircon ages indicate that there was a change in basement type north of the Southern Moun-tainsandthattheboundarybetweencontinentalandophiolitic crust at depth may have been a preexisting structural weakness that inuenced the position at which melts could rise when arc activity resumed.Other possibilities for explaining arc migration include sub-duction erosion and a change in dip of the subducting slab. Sub-duction erosion seems unlikely, as the width of the arc-trench gap today is >300 km, and because marine data support active accre-tion at that time (e.g., Kopp et al., 2001). A change in subduction anglecannotberuledout,butitisnotablethattodaytheslab dip has increased to a very high angle after the slab descended to100 km,asseeninseismicitydata(Englandetal.,2004). A lower, not a higher, angle would be expected if the arc had moved north owing to the change in dip of the slab.The jump in position of the volcanic arc is a feature of other Indonesianarcs(e.g.,Halmahera Arc;NicholsandHall,1991) and reects the changing stresses at the plate boundary over time. We prefer an explanation that links the change in position of the arc to plate reorganization in the region. However, it is difcult to test different hypotheses. Comparison of arc development to the east and west of East Java, from Bali eastward, and toward West Java and Sumatra, might provide insights, but unfortunately even lessisknownofarchistoryintheseregionsthaninEastJava. ThisEastJavastudyshowsthatourunderstandingofarctec-tonics is still incomplete and that studies of arc stratigraphy are essential if our knowledge of arc dynamics is to be improved.CONCLUSIONSThe case study of early Cenozoic arc volcanism in East Java showstheimportanceofexaminingarcstratigraphy.Insights 220Smyth et al.have been gained into the eruptive history, formation of a deep basin behind the arc, and the character of the deep crust.TheearlyCenozoicstratigraphyofEastJavaprovides arecordofacycleofarcactivityfrominitiationinthemid-dleEocene(ca. 42 Ma)toterminationintheearlyMiocene (ca. 20 Ma). The Kendeng Basin, directly behind the Southern Mountains Arc, contains >6 km of volcaniclastic and sedimen-tary rocks. The basin is not a typical backarc basin, and its sub-sidence history is linked to volcanic activity within the South-ern Mountains Arc.ThenalstageofvolcanicactivityintheSouthernMoun-tains Arc is marked by the Semilir Eruption (ca. 20 Ma), which distributed ash over a wide area and may be comparable to the Pleistocene eruption of Toba in Sumatra. Following this phase of major eruptions, there was a lull in volcanic activity during the middleMiocene,followedbyresumptioninarcactivitytothe north of the Southern Mountains Arc, along the axis of the mod-ernSundaArcduringthelateMiocene.Themechanismsthat resulted in the decline in volcanism, and northward movement in arc axis, are not yet understood and show that our understanding of arcs is still incomplete.The stratigraphic record of volcanic arcs can provide insights into the character of the deep crust. The entire East Java region waspreviouslythoughttobeunderlainbyCretaceousarcand ophioliticfragments,butArcheantoCambrianzirconswithin acidicproductsoftheSouthernMountainsArcpointtothe occurrenceofacontinentalcrustofGondwanancharacterand western Australianoriginbeneaththeoldarc.Thiscontinental fragment is thought to have collided with Sundaland during the Cretaceous and is interpreted to have terminated the Cretaceous phaseofsubduction.Theextentofthisfragmentisnotknown but may be traceable into Sulawesi. The use of inherited zircons to determine the character of the deep crust may be applicable in other arcs.ACKNOWLEDGMENTSThisprojectwasfundedbytheSEAsiaResearchGroupat Royal Holloway University, supported by an oil company con-sortium.ZirconswereanalyzedusingtheSHRIMPfacilityat CurtinUniversityofTechnology,Perth,Australia.Financial assistance for SHRIMP dating was provided by the University of London Central Research Fund and CSIRO, Australia. The analyseswereundertakenbyJosephHamilton(CSIROand University of the West Indies) and Pete Kinny (Curtin Univer-sityofTechnology).WearegratefultoLIPI(LembagaIlmu Pengetahuan Indonesia) for eld-work permissions. We wish to thank Eko Budi Lelono, Peter Lunt, Theo van Leeuwen, Moyra Wilson,ColinMacpherson,HeatherHandley,CindyEbinger, andDaveWaltham.MarcelleBouDagher-FadelofUniversity College London provided biostratigraphic analyses. 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