Variationsin InelasticFailure of SubductingContinental Lithosphere and TectonicDevelopment: Australia-Banda Arc Convergence KushTandon •, Juan M. Lorenzo2, SriWidiyantoro 3,and Geoffrey W. O'Brien4 Factorsthat controlthe geometryof forelandbasins and accretionaryprisms, back thrusting,and changesin structuralstyle duringthe continentalcollision can be linkedto variationsin EffectiveElastic Thickness (EET). A variableEET map is computedat an incipient continentalcollision (Pliocene-Recent)site in the northernAustralian continental lithosphere along the Bandaorogen. Incorpora- tion of almostall the forearcbasin within the centralTimor Island andtomogram in the easternTimor Island suggesta more rigid northernAustralian lithosphere indenting between •125ø-127 ø E longitude. Modeled flexure deflection is matchedto seafloorbathymetry and Banda orogentopography, Pliocene conti- nental shelf, and marine complete 3D Bouguer gravity anomalies.Calculated EET of the northernAustralian continental lithosphere using an elastichalf-beam model show a sharpdecrease in EET from 230-180 km on the continentalshelf (from Roti to west of Am Island) down to •40 km on the continentalslope and beneathBanda orogen (from • 121ø-132ø E longitude)favoring an hypothesisof inelasticfailure at the start of continentalsubduction. The decreasein EET along the continental slope is not entirely uniform and parallel'to the subduction boundary,especially south of centralTimor Island.In the southof centralTimor Island, the EET remainsclose to 55 km on the continentalslope and beneaththe central Timor Island. An increase of •15 km in EET calculations on the continentalslope south of centralTimor Island shiftsthe majority of present-day strainpartitioning to the forearcregion as comparedto regionsin westernTimor Island and Tanimbar Island. A pre-existingstronger region of continentallitho- spherepresent within the transferzone between Malita andValcun Graben(con- tinental shelf) is a likely causefor lesserinelastic failure in that region during subduction.Results show that continentallithosphere is inherentlystrong on the continentalshelf under normal crustal thicknessand in absenceof significant weakeningfrom past rifling. Variations in the gradientof EET changeon the continentalslope can determinethe evolutionof many featuresin continental collision tectonics. •Departmentof Geology and Energy Institute, University of Louisiana, Lafayette, Louisiana 2DepartmentofGeology and Geophysics, Louisiana State Uni- versity, Baton Rouge, Louisiana PlateBoundary Zones 3Departmentof Geophysics andMeteorology, Bandung Insti- GeodynamicsSeries 30 tute of Technology,Bandung, Indonesia Copyright2002 by theAmerican Geophysical Union 4AustralianGeological Survey Organization, GPO Box 378, 10/1029/030GD20 Canberra, Australia 341 342 INELASTIC FAILURE AND TECTONICS: TIMORE-TANIMBAR-ARU TROUGH 121 E 123 125 127 129 131 133 135 137 -4 S -6 -'- ß -8 lore • - imor •• Australian continental • - Savu •, • •, lithosphere -10• Basinz -• X•. ArafuraBasin Australi,. , />' Bona•"'-.•t-•. ," • " ': ' Oceanic• e :as-• • • Australia ' -14 Lithpspshe- Legend ---- = ExtensionalFaults :-',•= Depocenter(Paleozoic & Mesozoic) ß = ActiveVolcano o = InactiveVolcano •, = HeatFlow (mW/m 2) AP = Ashmore Platform MG = Malita Graben PS = Petrel Sub-basin SP = Sahul Platform SS = Sahul Syncline VG = Vulcan Graben Figure 1. Generaltectonic map for thenorthern Australian continental lithosphere across the Banda Arc [Hamilton, 1974;Veevers et al. 1974;Stagg, 1993; AGSO North West Shelf Study Group, 1994; Snyder et al., 1996].Heat flow valuesare from Bowin et al. [1980].The thick line with blacktriangles (point to overthrustplate) shows the subduc- tion boundarybetween the northernAustralian continental lithosphere and the Eurasianlithosphere in Timor- Tanimbar-AruTrough; north of Flores,in Alor and Wetar,it denotesthe back-thrusting.Thick black line lettered "AGSO98R07" marks the locationof seismictrack for AGSO line 98R07.The 200 rn bathymetrycontour marks the continentalshelf-slope boundary from ETOPO-5 [National Geophysical Data Center, 1988]. 1. INTRODUCTION especiallyin the downdip directionshould play an impor- tant role in determiningthe progressof the continental Local area Airy isostasy(hydrostatic equilibrium) is collision tectonics and how foreland basins evolve with generallyfound only in large-scalegeological features, time. such as Tibetan Plateau [Lyon-Caen and Molnar, 1985; EffectiveElastic Thickness (EET) measuresthe ability Burovand Diament,1992 and 1996].In Himalayasand to supportloads and transmit bending stresses regionally as othermountains chains, the flanksof the mountainranges a rigidlithosphere rather than mass compensation being in showrigid behavioras opposedto localarea Airy isostasy a local Airy isostaticmanner (zero EET) [McAdoo et al., [Lyon-Caen and Molnar, 1983 and 1985; Royden and 1978;Watts, 1983]. There has been a long-standingdebate Karner, 1983; Stewart and Watts, 1997]. This enablesa on factors that control the value of EET in the continental part of the continentalcollision margin to supportthe lithosphere,like inelasticfailure, crustal thickening, and weight of the thrust sheetsand lead to formationof fore- tectono-thermalage (the time elapsedbetween the lastrift- landbasins [Lyon-Caen and Molnar, 1983and 1985;Roy- ing eventand the startof formationof a forelandbasin) den and Kamer, 1983; Stewartand Watts, 1997]. Within [McNutt et al., 1988;Burov and Diament, 1995 and 1996; the samecontinental collisional margin, areas with similar Stewartand Watts, 1997; Poudjom Djomani et al., 1999]. convergencevector also display markedly different tec- Wattset al. [ 1995] advocatethat the highestthe valueof tonicdevelopment (Figure 1). Differencesin the rigid part EET should be found where the 'collisionhas maximum of the continentallithosphere along a collisionalmargin, shortening.Pre-collisional high EET facilitates thin- TANDON ET AL. 343 skinned, low-angle fold and thrust tectonics, and back- thrustingon a low-angle flexurally more rigid plate [Watts et al., 1995]. EET values differing by 1.5 times within a 200 kilometers range are noticed in the Himalayas and Ganga basin [Lyon-Caenand Molnar, 1985]. In the Apen- nine foreland basin system,Royden et al. [1987] showed that the differenceswithin subductionprocesses, foreland basin and thrust sheetgeometry can be related to the seg- mentationof the lithospherewith uniform EET. In the cen- tral Brazilian Shield, Stewart and Watts [1997] calculated variable EET of > 85 km on the craton that decreases to < 25 km beneath the thrust sheets. In the central Brazilian Shield [Stewart and Watts, 1987], the EET variations are not uniform and not strikeparallel to the thrusting. At a site of a Pliocene-Recent continental-arc collision, Figure 2. Uninterpretedmigrated seismic reflection AGSO line the northern Australian continental lithospherealong the 98R07 on the northern Australian continental shelf southwest of Banda orogen [Warris, 1973] one observes significant Timor Island. Age abbreviations: Plioc(ene)-Rec(ent), variationswithin different elementsof plate-boundaryin- Mioc(cene), Pal(eocene), Eo(cene), J(urassic)-K(Cretaceous), teractions,e.g., strainpartitioning, structural style, geome- Tr(iassic), and P(ermian).The seismicsection shows how the ex- tent of normal faulting changeswith time. Some faults appearto try of the foreland basin and the adjoining accretionary detach within Cretaceous units. Most reactivated faults terminate prism, and presenceof back-thrusting.In this study, we above, or near, a regional basePliocene unconformity [Boehme, attempt to find a relationshipbetween variationsin EET 1996; Curry et al., 2000] that constrainsthe end of the dominant (down dip and along the strike of the thrust sheets)and extensional stressregime to Latest Miocene/EarliestPliocene. different plate boundary observationsfound during the The locationfor AGSO line 98R07 is shownin Figure 1. juvenile stagesof continentalcollision tectonics. We per- form elastichalf-beam modelingalong 15 profiles (Figure 6) to calculate variable EET from Roti to the Kai Plateau (west of Aru Island) traversingthrough Timor and Tanim- ton and Bowin, 1981]. Regionally,the timing of termina- bar Islands(- 121ø-137ø E longitude)(Figure 1). Modeled tion of the majority of normal faults migratesfrom Late flexure from elastichalf-beam modeling is matchedto the Miocene-Early Pliocene-Recentas one moves from west seafloor, the Banda orogen, the continental shelf from of Timor Island toward Tanimbar Island [Curry et al., Pliocene, and the marine complete 3D Bouguer gravity 2000] (Figure 3). Interestingly,the strain partitioningin anomalies.Variable EET on the continentalslope accounts the forearc basin north of central and eastern Timor Island for the changein the mechanicalstrength of the continental is under compressionunlike north of western Timor and lithospheredown dip causedby inelasticyielding [McNutt Tanimbar Island [Charlton, 1997] (Figure 3). Foreland et al., 1988]. vergance has stopped at present in central and eastern Timor Island but is still active in western Timor and 2. DIFFERENT FEATURES OF CONTINENTAL Tanimbar Islands [Charlton, 1997] (Figure 3). The width COLLISION TECTONICS of the forearc basin north of central Timor Island is also the least comparedto other regions north of the Banda At the start of continental subduction, there are normal orogen (Figure 1). In the vicinity of Timor Island, the faults due to bendingstresses on the continentalshelf