Alternating Seismic Uplift and Subsidence in the Late Holocene At

Edinburgh Research Explorer

Alternating seismic uplift and subsidence in the late Holocene at

Madang, Papua New Guinea: Evidence from raised reefs

Citation for published version: Tudhope, A, Buddemeier, RW, Chilcott, C, Berryman, KR, Fautin, DG, Jebb, M, Lipps, JH, Scoffin, TP & Shimmield, GB 2000, 'Alternating seismic uplift and subsidence in the late Holocene at Madang, Papua New Guinea: Evidence from raised reefs', Journal of Geophysical Research, vol. 105, no. B6, pp. 13797- 13807. https://doi.org/10.1029/1999JB900420

Digital Object Identifier (DOI): 10.1029/1999JB900420

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Download date: 10. Oct. 2021 JOURNAL OF GEOPHYSICALRESEARCH, VOL. 105,NO. B6, PAGES13,797-13,807, JUNE 10, 2000

Alternatingseismic uplift and subsidence in thelate Holocene at Madang,Papua New Guinea: Evidence from raised reefs AlexanderW. Tudhope,•,2 Robert W. Buddemeier,3,4Colin P. Chilcott,•Kelvin R. Berryman,5Daphne G. Fautin,6,? Matthew Jebb,8,9 Jere H. Lipps,•0Robert G. Pearce,•Terence P. Scoffin,•and Graham B. Shimmield•

Abstract.Well-preserved mid-late Holocene coral reefs are exposed inlow coastal cliffs in thevicinity of theMadang lagoon on the north coast of Papua New Guinea. Results from U/Thand •4C dating of corals,surveying, and field mapping indicate several major changes in relativesea level over this period. Specifically, there is evidence for a relativesea level fallof > 4.5 rnabout 3000 calendar years B.P., followed by relative sea level rises of- 1.5rn about2400 calendar years B.P. and > 0.5rn about 1200 calendar years B.P. and a subsequent relativesea level fall of > 3 rn sometime in thepast 1000 years. Since regional eustatic sea levelsare believed to havebeen dropping gradually over this time frame, these observed changesinrelative sea level are interpreted asreflecting alternating tectonic uplift and subsidence.Furthermore, the detailed structure and age relationships of thecoral deposits indicate thatboth uplift and subsidence occurred rapidly, most probably as coseismic events with ver- ticaldisplacements of0.5 to 4.5 m. Theseevents may be related to rupture on NW-SE trendingreverse faults which have been mapped inthe nearby Adelbert Range and possibly on NE trendingcross faults which have been inferred from seismicity. This interpretation implies a muchgreater degree of tectonicinstability and potential seismic hazard in theregion than previouslyrecognized, although the inferred coseismic vertical displacements areshown to beconsistent with present-day local seismicity. In a broadercontext, the study illustrates howdetailed analysis of verticalchanges in coralreef structure and assemblages may be used asa sensitiveindicator of changingrelative sea level, capable of resolvingcentury timescale events and reversals.

1. Introduction ment of the land, for example, due to coseismicevents or isostaticfactors [e.g., Taylor et al., 1987; Ota et al., 1993; Analysisof the elevationand age of lateQuaternary reefs Chappell et al., 1996a; Zachariasenet al., 1999]. Alterna- maybe used to gleanevidence for pastchanges in relativesea tively, when vertical movementsof the land are well known, level. Thesedata are generallyused in one of two ways. For reefal evidence may be invoked to reconstructeustatic sea regionsand time intervalsover whichglobal (eustatic) sea level, for example, due to the growth and decay of high- level is well known the evidencefrom reefs may be used to latitude ice sheetsover the late Quaternary[e.g., Chappell, determinethe magnitude,sense, and timing of verticalmove- 1974; Chappell and Shackleton, 1986; Chappell et al., 1996b]. In this study, we adopt the first of these two ap- tMarineGeoscience Unit, Department of Geology& Geophysics,proaches, presenting evidence for vertical movementsof the EdinburghUniversity, Edinburgh. land in the area of Madang on the north coastof PapuaNew 2Onsabbatical at AustralianInstitute of Marine Science,Towns- Guinea over the past 3000 years. In particular,we focus on ville, Queensland. evidencefor rapid and high-magnitudesubsidence and uplift 3NuclearChemistry Division, LawrenceLivermore National eventsin a contextof relativelyslow net tectonicuplift. Laboratory,Livermore, California. 4Nowat KansasGeological Survey, University of Kansas,Law- rence. 2. Regional Tectonic Setting 5Instituteof Geologicaland Nuclear Sciences, Lower Hurt, New Zealand. NorthernPapua New Guinea has a complextectonic set- 6CaliforniaAcademy of Sciences,Golden Gate Park, SanFran- cisco, California. ting due to rapid and obliqueconvergence of the West Pacific 7Nowat Departmentof Systematicsand Ecology, University of and AustralianPlates (Figure 1). The regionexperiences ex- Kansas, Lawrence. tensiveseismic and volcanicactivity, and alongconsiderable 8ChristensenResearch Institute, Madang, Papua New Guinea. stretchesof the mainlandcoast, net tectonicuplift has led to 9Nowat NationalBotanic Gardens, Glasnevin, Dublin. WMuseumof Paleontologyand Department of IntegrativeBiol- the subaerialexposure of Quaternaryreef sequences.These ogy, Universityof California,Berkeley. raisedreefs are particularlywell developedon the Huon Pen- •DunstaffnageMarine Laboratory, Oban, Scotland, UK insula,where averageuplift ratesof up to 3.5 mm/yr have prevailedfor at leastthe past 300,000 years [Chappell, 1974]. Copyright2000 by the AmericanGeophysical Union. The studyarea near Madang is part of the collisionzone between the Finisterre Terrane and the Australian continent Papernumber 1999JB900420. 0148-0227/00/1999JB 900420509.00 (seeAbbott [1995] for a review). This collisionbegan in

13,797 13,798 TUDHOPE ET AL.: UPLIFT AND SUBSIDENCE IN PAPUA NEW GUINEA

Miocene-Pliocenetime and is ongoing. The FinisterreTer- bert Range and at 4000 m in the FinisterreRange [Robinson rane is composedof two distinct blocks:the Adelbert block et al., 1976; Jaquesand Robinson,1980]. Thereforevertical (largelysynonymous with the AdelbertRange) and the Finis- tectonic displacementin the study area may be influenced terre block (largely synonymouswith the Finisterre Range). both by the mappedNW trendingreverse faults in the Adel- These two blocks may be separatedby a NE trending cross bert Range which trend toward the Madang lagoonand also fault which runs throughAstrolabe Bay (Figure 1) [Abbottet by postulatednorth or NE trending crossfaults such as that al., 1994], and the change in coastlinetrend at Madang (to presumedto have rupturedduring the 1970 Madang earth- approximatelyN-S, in contrastto the regional WNW-ESE quake whose epicenter was in Astrolabe Bay (Figure 1), trend) may representan offset of a once linear Adelbert- [Everingham,1975; Abers and McCaffrey, 1994]. Finisterre block. However, marked differences in the sedi- mentaryrock recordof the two blocks[Abbott, !995] and the 3. Field Area lack of an equivalentoffset of the Bismarckvolcanic arc point to additionalcomplexity. The study site lies a few kilometersnorth of the town of Abbott [1995] suggestedthat collision of the Finisterre Madang (5 ø13'S; 145ø49'E), in an area where the dominant block was orthogonalwith continentalmaterial on both sides, ESE-WNW trendof the Papuancoastline gives way to a N-S resulting in high uplift, while the collision of the Adelbert strike and where the well-developedMadang lagoonoccurs block may have been "soft" becauseof the presenceof a (Figure2). This lagoonis 13 km long and 2-4 km wide, has trappedpiece of oceaniccrust between the Adelbertblock and an averagemidlagoon depth of 30 m, and is boundedby a the continentalmargin [Milsom, 1981]. This soft collision living barrierreef whosetop lies --1 m below meanlow water has resultedin lower ratesof uplift. For example,Miocene- springtide level. Tidal flushingof the lagoonwaters occurs age Gowop limestoneis at 1600 m elevationatop the Adel- throughthree deep passes and over the reef crest. Springtide

I I 146OE 147øE Finisterre terrane Other areas STUDY AREA • WandokaiLimestone '"'"'"'"'--• Mainlyalluvium •"':--/•Leron Formation !• Wewakbeds Bismarck Sea •'• ErapComplex •-• AureTrough •;trolabeBay • GowopLimestone '•"• BenaBena terrane

^h[•andFinisterre SarawagetVolcanicsbeds OwenStanley terrane 0 40km• UndifferentiatedHighlandsrocks I •--• LangimarShearZone

^^A^h hhhhAhh hhhhhhAh I.i=! hhhhAhhhh hhhhhAhhh .,.,! hh•hhhAAhhhhhh h h•hhhhAhhhhAhA .,.,! h h h

CAROLINE 51•3km PLATE h h hhhhhhhhhh PACIFIC PLATE h h h h h h h

Direction of TH BISMARCK Pacific- Australia relative motion PLA•'I

SOUTH BISMARCK

l FBNew Guinea

Huon Gulf PLATE

Figure1. Tectonicsetting of thestudy area, north coast of PapuaNew Guinea.In theinset, the Adelbert block and the Finis- terreblock of the FinisterreTerrane are shownin heavyshading and labeled with AB andFB, respectively.In the mainfigure (basedon Abbottet al. [1994]), heavysolid lines are faultswith barbson theupper plate of thrusts,the NE andnorth trending dashedlines are seismicallyactive strike-slip faults, and the dash-dottedline in AstrolabeBay marksa postulatedNE trending crossfault that is presumedto haveruptured during the 1970 Madangearthquake [Everingham, 1975; Abers and McCaffrey, 1994]. Insetfigure is basedon Tregoninget al. [1998]. TUDHOPE ET AL.: UPLIFF AND SUBSIDENCE IN PAPUA NEW GUINEA 13,799

I a few meterselevation on the outer barrier reef and on top of 145ø50'E some midlagoonpatch reefs. These depositsare particularly well exposedin an actively eroding 2-3 m high cliff which occurs along much of the mainland coast and around some KEY TO SITES islands (Figure 3). In some locations there is a second, smaller and less well exposed, cliff a few tens of meters 1: Tab Island inland which stepsup a further 1-2 m. These two cliffs have beenpreviously recognized by Panzer [1933] and by Stoddart 2: Nagada Point [ 1972]. Throughoutthe rest of this paperthey are referredto 3: Gosem Island as the "lower coastal cliff" and "upper coastalcliff" respec- 4: Jais Aben tively. A detailed investigationof the age, composition,and 5: Jais Aben internal structureof the lower and better exposedof the two access road cliffs lbrms the focusfor the presentstudy. 6: Tabat Island Inland from the coastalcliffs is a low-lying plain with lim- ited exposureof the underlyingsubstrate. However, at a few 7: Wongat Island localities,reef limestonesoutcrop at elevationsranging from a 8: Mililat Harbour few meters up to about +80 m and at distancesup to 4 km 9: North Coast from the coast. All of the limestones above about + 10 m ele- site a vation are heavily recrystallized,and consequently,they have 10: North Coast yielded no material suitable for U/Th dating (J. Chappell, site b personalcommunication, 1999). Although the coral species compositionof thesemore elevatedlimestones is indicativeof a Pleistoceneage, considerationof their stateof preservation and karst developmentled Chappell (personalcommunica- BISMARCK tion, 1999) to suggestthat reefs of last-interglacialorigin are absent above an elevation of about + 15 m. If this assessment is correct, it implies averagenet tectonic uplift of less than -0.2 mm/yr in the late Quaternary. Fartherinland, theselime- stonesgive way to the FinisterreVolcanics of Lower Oligo- ceneto Lower Miocene age [Robinsonet al., 1976].

4. Materials and Methods

Holocene reef depositsexposed in the coastal cliffs and terraces were examined at six locations on the mainland coast "• 5ø1 O'S - within and north of the lagoon,on three lagoonpatch reef islands,and on one barderreef island(Figure 2). At the Jais Aben, Wongat Island, and Tab Island sites,conventional leveling techniquesusing a Wild automaticlevel were used to measurethe elevationof raisedreef depositswith respectto • Studysite the highestliving reef crestPorites corals. Thesehighest livingcorals had a characteristicflat-topped form, with living coral tissue restricted to the outer rims and sides of the coral Deepinto lagoonpass skeleton and a bare (dead)surface across the flat top. This flat-topped,or "microatoll"form is a resultof upwardrestric- tion of coralgrowth by sealevel suchthat subsequentcoral growthis only in a lateralsense [Scoffin and Stoddart,1978]. Coral microatollscan providevaluable evidence for past Madang changesin relativesea level. For example,changes in sea Town 0 2km levelover the lifetimeof an individualcolony are recorded as I patternsof increasingand/or decreasing elevation of the colony top, while changesover longertimescales may be de- Figure2. TheMadang lagoon and surrounding area, indicat- ducedfrom comparison of theelevation of thetops of ancient ingthe location of sitesinvestigated in thisstudy. (dead)colonies of differentages [Scoffin and Stoddart,1978; Taylor et al., 1987; Zachariasenet al., 1999]. At JaisAben the elevationof the top of the living coral microatollswas range is -1 m. Within the lagoon, living reefs occur as an measuredto be -0.49 m relative to mean sea level, as defined almostcontinuous fringing reef alongthe mainlandcoast and by the Australian Commonwealth Scientific and Industrial as midlagoonpatch reefs that reachto within a few metersof ResearchOrganisation (CSIRO) for the nearbytide gauge. sea level. For comparisonof elevations between sites, the absoluteele- Well preservedHolocene raisedreef depositsform a low- vation of the highestliving Porites microatoll is assumedto lying coastalplain on the mainlandand form islandsof up to be similar at each site. We conservativelyestimate the maxi- 13,800 TUDHOPE ET AL.' UPLIFT AND SUBSIDENCE IN PAPUA NEW GUINEA

mum likely error associatedwith this assumptionto be ___0.25 m. Observationswere made on the compositionand structure of corals within the exposedcliffs. In particular,the occur- rence of massivecorals with a flat-toppedor more complex microatollmorphology was documentedto help elucidatethe pastrelative sea level historyof the area. Samplesof in situ massivePorites, Goniastrea aspera and Favia favus corals werecollected for U/Th and 14Cdating. These coral samples were inspectedunder binocularmicroscope for evidenceof recrystallization,and only thosewhich appearedpristine were taken for dating. Samplescollected for U/Th dating were further examinedby quantitativeX-ray diffraction to assess mineralogy,and only corals with <1% calcite were subse- quentlydated. U/Th dating of the coral sampleswas achievedat Edin- burgh Universityby alpha spectrometryusing silicon surface barrier detectors,following the methodologyof Veeh [1966]. Completedissolution of 2-5 g of samplewas followedby ion exchangeseparation and electrolysisto prepare separateU andTh sources.A mixedspike of 232Uand 228Th was used to determineisotopic activities of the232Th, 230Th ' 238U, and 234U isotopes. Sampleswere countedfor 2-10 daysusing exclu- sive U and Th isotopedetectors to minimize the alpha recoil contamination. Background count measurementswere per- formed under identical conditions prior to the counting of each sample. U/Th ages are expressedas yearsB.P. (before 1950) with 2 sigmaerrors based only on countingstatistics. .<:. Measurementsfor •4Cdating were made by BetaAnalytic Inc. 14Cages were corrected to estimatesof calendaryears B.P. following Stuiverand Braziunas [ 1993] and $tuiver and Reimer[ 1993],by firstcorrecting for •5•3C,then assuming AR (the difference in reservoirage of the local region compared to the model ocean) to be zero. That is, the dateshave been correctedfor a marine reservoireffect which averagesabout 400 years. Although no known-age sampleswere dated in this study, Chappell and Polach [1976] reported apparent agesof 535 and 270 yearsfor two living specimens(clam and coral) from Huon Peninsulareefs. The averageof theseval- ues is almostexactly the reservoircorrection generated by the Stuiver and Reimer [1993] model, supportingits application in this case. Two sigmaranges for datesare basedon counting statisticsalone and, therefore, do not take into account any additionaluncertainty on reservoirage correction. Previ- ousstudies have indicated that coral 14C ages corrected in this way to calendaryears B.P. are close to coral U/Th ages[Bard et al., 1990, 1993]. Therefore,from this point forward,cor- rected14C ages and U/Th agesare both referred to as"calen- Figure3. (a) The present-dayreef flat andthe lower coastal dar yearsB.P.." cliff and terrace at Jais Aben. Note the evidence of active erosionof the cliff, which, combined with the relatively nar- row reefflat (-30-40 m wide),lends support to theconcept of 5. Results a majortectonic uplift event within the past few centuries. (b) Lower coastal cliff at Jais Aben. Massive Porites coral mi- At all of the coastalsites investigated (i.e., all sitesexcept croatollsof 1-2 m diameter(e.g., under hammer) are abundant the Jais Aben accessroad), a distinct sedimentarydiscontinu- in a layernear the base of thecliff. Thetops of thesecorals ity formsa boundarytoward the base of thelower cliff. Be- (e.g., arrow)form a distinctiveboundary which is present neaththis boundary,large (1-2 m lateraldiameter) massive throughoutthe field area. Thisboundary indicates the former Poritescorals are abundant,and manyof thesehave a microa- positionof sealevel. Abovethe massive coral layer the cliff toll morphology.The flat topsof thesecorals occur at, and comprisesa mixedbranching and massive coral assemblage whichindicates arapid rise in relative sealevel. Subsequent locally define, the boundary. Noindividual coralcolonies uplifthas resulted inthe present-day subaerial exposure of (branching ormassive) were observed togrow through the thesereefs. (The hammer is240 mm long.) boundaryatany of thesites, and the boundary itself is rela- TUDHOPE ET AL.' UPLIFT AND SUBSIDENCE b,'r PAPUA NEW GUINEA 13,801 tively fiat-lying,with an undulatingrelief of <0.5 m over lat- m of the cliff. Theserelationships are illustratedin Figure4. eral distancesof at least 200 m. The largest(2 m diameter) At the three surveyedsites the highestparts of the bound- flat-toppedmicroatolls indicate relatively stable sea level ary betweenthe flat-toppedmassive coral layerbelow and the (_+0.1m) over a period of-100 years during their growth mixed branchingand massivecoral assemblageabove occur (basedon averagecoral growth rates of- 10 mm/yr). In addi- at an elevation between +1.4 m and +1.2 m with respectto tion, the widespreadoccurrence of this boundarythroughout nearbyliving reef crestPorites microatolls. Visual estimates the areasuggests that the periodof relativesea level stability indicate a similar elevation for the other six sites. The eleva- mayhave lasted for considerablylonger than 100 years.That tion of the top of the lower coastalcliff rangesfrom +3.0 m at is, not all the coralsnecessarily lived at exactlythe sametime; the mainland sites down to about +2.7 m on some of the is- somemay have died and becomeembedded in an intertidal lands. The top of the second,upper, cliff is more variablein reef flat before(younger) microatolls formed at the sameele- elevation but reaches up to +5.8 m. Although poorly ex- vationon an adjacent,prograding, reef crest. posed,the nature of the mixed branchingand massivecoral Above the distinctivelayer of microatollsthe reefal depos- assemblagewithin this upper cliff (including G. aspera, its contain a higher proportionof branchingcoral (mainly which is a commonintertidal and shallowsubtidal species) is Acropora;as in situ thicketsand as rubble),less abundant suggestiveof growth in relatively shallowwater, probably<5 massivecorals (Porites and faviids), and interstitialsediment m deep. Fartherinland, at the JaisAben accessroad site, one consistingof coraland mollusc-rich sands and gravels. Some coral samplewas collectedfor •4C datingfrom a bank at of the branchingcoral materialimmediately above the bound- about+2.2 m elevation. Local high pointsin the sameforma- ary forms0.5-1.0 m high uprightthickets implying in situ tion reach up to about +2.7 m elevation. Once again, the growthinto >1.0 m waterdepth. No massivecoral microa- coral assemblageswithin this formation (includingF. favus tolls were observedin the 1-1.5 m immediately above the and G. aspera) are suggestiveof growthin relativelyshallow boundary,but microatollsdo occurin the uppermost0.5-1.0 water.

UPPER TERRACE 6.0 ' 6.0

o 5.0- UPPER - 5.0 . 3,030yrBP* COASTAL CLIFF 2.7m LOWERTERRACE E 4.0 - 4.0

o r• 3.0 - 3.0 ' •,-...... 1,691' 1,953' 1,875920 yryrBP BP fossilbranching c• LOWER 1.8m 2,280yrBP and massive corals ._> CLIFF COASTAL• discontinuity(= 'boundary') • 1.0 ...... •,'••'47 •',•'8• •'•; yrB P fossilPorites microatolls; - 1.0 meansealevel 1.3m 2,448yrBP -9ø 0.0 •, MODERNREEF FLAT

._> highestliving reef crest ..• Porites m icroatoll co . ,- -1.0 -1.0 •,. - KEY FOR CORAL AGES' ._o • Porites microatoll • -2.0 I> subtidal coral -2.0 dr 14Cdated corals; age corrected tocalendar years BP

I I I I I I I I I •----10 m -•

Figure4. Diagramillustrating the field and age relationships observed inthe coastal cliffs at the sites inves- tigatedin theMadang lagoon. The lower coastal cliff, with the distinct boundary toward its base, is present at all of thecoastal sites. The upper coastal cliff wasonly seen to be welldeveloped at JaisAben and at MililatHarbour. All ages are calendar years B.P. and are based on either U/Th ages or on corrected incages (Tables 1 and 2). 13,802 TUDHOPEET AL.: UPLIFI'AND SUBSDENCE IN PAPUANEW GUINEA

+1 +1 +1 +1 +1 +1 +1

+1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1

.,.•

:> .,,•

• ,-- d o o,I o,I o,I + + + + + + +

.... o o

o,,• ,.o ,.o ,.o ,.o • •

.,,•

.,,• TUDHOPE ET AL.: UPLIFF AND SUBSDENCE IN PAPUA NEW GUENEA 13,803

Table2. Age and813C Data for CoralSamples Dated by 14C

ElevationWith 14CAge, Not Respectto Living Isotope- CalendarAge and Sample Porites Corrected, 8•3C, 2 SigmaRange, (Lab No.) Location Positionin Cliffs Microatoll, m yearsB.P. %0PDB yearsB.P. CRI-01 a Wongat Island lower coastal - +2.0 2170 +_80 +0.6 2280 (Beta-29854) cliff (2420-2036)

CRI-02b JaisAben topupper - q 5.5 2820 +_90 +0.6 3030 (Beta-29855) coastalterrace (3276-2786)

CRI-03 c Jais Aben access inland - +2.2 4710 +_100 -0.8 5,450 (B eta-29856) road (5646-5241 )

CRI-05ø JaisAben toplower - +3.0 970 +_70 + 0.4 920 (Beta-29857) coastal terrace (1054-756)

Calendarages estimatedfollowing Stuiver and Braziunas [1993] and Stuiver and Reimer [1993], assumingAR (differencein reservoirage of local regioncompared to modelocean) to be zero. Two sigmaranges for calendarages are basedon countingstatistics only. All samplesappeared to bein growthposition. Radiocarbon and 8•3C measurements areby BetaAnalytic Inc. aA 1 m diameterGoniastrea aspera colony from seawardside of WongatIsland, 15 m inlandfrom low-tide line. bA0.3 m diameterG. asperacolony from seaward edge of uppercoastal terrace. CA0.25 m diameterhalf-hemispherical Favia favus colonyatop a columnarbase (total height0.35 m) growingout and upwardfrom the sideof a near-verticalbank. Sampleis locatedabout 1.5 km inlandfrom main coast,150 m eastof highwayalong Jais Aben access road. Locationis 1.7 m abovelocal tidal stream;top of bankis -2 m abovesea level, andlocal highpoints are ~2.5 m abovesea level. dA0.4 m diameterG. asperacolony excavated from seaward edge of lowercoastal terrace.

U/Th dating of four massivePorites coralsfrom beneath not possiblefor us to accuratelyconstrain relative sea level the boundaryin the lower cliff yielded ages ranging from for this periodsince no coralmicroatolls were observed in 3227 _+425 to 2448 _+217 calendaryears B.P. Five massive thisexposure. Nonetheless, the coral assemblages are sugges- coralsfrom abovethe boundaryin the lower cliff yieldedages tive of growthin relativelyshallow water, which leads us to rangingfrom 2280 (2420-2036) to 920 (1054-756) calendar speculatethat relative sea-level was probably between about 3 yearsB.P. A massivecoral from the top of the upper cliff and 6 m abovepresent. dated at 3030 (3276-2786) calendar years BP, and a coral The single•4C-based age for thetop of theupper terrace is from the inland Jais Aben accessroad site yielded an age of indistinguishablefrom two of the four U/Th agesfor corals 5450 calendaryears B.P. Full resultsof the U/Th and•4C from beneaththe boundaryat the base of the lower coastal datingprograms are presentedin Tables1 and2, respectively. cliff. Clearly,given the heightdifference and the fact that the datedcorals beneath the boundaryare microatolls,all of these coralscannot be truly of the sameage. For the coralsat the 6. Discussion and Conclusions baseof the lower cliff to be older than the coral at the top of All of the raised reefs investigatedformed since -5500 theupper terrace there would have had to havebeen a relative calendaryears B.P. Estimatesof relative sea level for Ma- sealevel rise of > 4.5 m about3000 calendaryears B.P. with dang based on the Ice-4G model of the glacial isostaticad- subsequentvertical growth of coralsup to the new level justment process(as recently describedby Peltier [1998]) withina periodof two or threehundred years (i.e., at ratesof indicatea falling sea level over this timeframe. Specifically, severaltens of mm/yr)followed by a dropin relativesea level relative eustaticsea level is estimatedby the model to have of the sameamount (i.e. >4.5 m) around2700 calendaryears been about +4.5 m at 5000 yearsB.P., falling to about+2.1 m B.P. We believe'this to be extremelyunlikely. Instead,we at 3000 yearsB.P. and +0.9 m at 1000 yearsB.P. (Figure 5). suggestthat the coralat the top of the upperterrace is older Rapid changesin observedrelative sea level at Madangwhich than all the corals dated at the base of the lower terrace and deviate significantlyfrom this modeledestimate of gradually that the relationshipsmay be explainedby a singledrop in falling eustaticsea level are likely to reflect verticalmotion of relativesea level of >4.5 m causedby a coseismicuplift event the land due to tectonic activity. This assumptionis made -3000 calendaryears BP with subsequentgrowth of coral throughoutthe restof the discussion. microatollsup to the new lower relativesea level. Thesecor- Figure 5 presentsa plot of age versuselevation of the dated als grew from foundationsconsisting of either the former corals and a relative sea level curve labeled with uplift and forereefdeposits of the upperterrace reef or an erosionalplat- subsidenceevents. For comparison,and to aid with recon- form cut back into the former upperterrace reef. In this sce- structionof trendsin relative sea level during periodsof in- nario the periodbetween -5500 and 3000 calendaryears B.P. ferred tectonic stability, the estimatedrelative sea level for may have been characterisedby net tectonic subsidence,ei- Madang due to glacial isostaticadjustment [Peltier, 1998] is ther graduallyor as one (or more) rapid events. also indicated. Although the current study focuseson evi- The mixed branchingand massivecoral sequenceon top of dencefrom the two coastalcliffs, the startingpoint for this the boundaryin the lower cliff indicatesa relative rise in sea discussionof verticalmovements of the land is the singledate level, and hence an inferred tectonic subsidence of >2.0 m. of-5500 calendaryears B.P. from a coral at +2.2 m elevation Since no coralswere observedto have grown up throughthe at the inland Jais Aben accessroad site. Unfortunately,it is boundary,it is inferred that in the sectionsexamined, these 13,804 TUDHOPE ET AL.: UPLIFr AND SUBSIDENCE IN PAPUA NEW GUINEA

7.0

• 6.o o . POSSIBLE o ??? ?SUBS DENCE •i RELATIVE SEA LEVEL 'F:: 5.0 SUBSIDENCE CURVES (total _>2.5 m) '•. I •.. 4.0 ??? ß UPLIFT o • (_>4.5m) <.-- 3.0

'• 2.0

.? 1.0

o "' 0.0

modern living Porites microatoll

-1.0 '''' I .... I .... I''''1''''1 .... I''''1 .... I'''' I'''' I'''' I''''1 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 Age (years BP) •1• mean(or only) U/Th age of coral colony Poritescoral microatoll, 2 sigma age error O replicateU/Thages forcoral colony subtidalcoral,2 sigma ageerror •' calendar•'•Cage rsl based on glacialisostatic adjustment (Peltier, 1998)

Figure5. Plotof age(in calendaryears B.P.) against elevation of datedcorals discussed in this study. Two possiblerelative sea level curves are illustrated: The solid line with shaded regions of uncertaintyillustrates a scenariowhere subsidenceoccurs as discretecoseismic events; the dashedline indicatesa scenariowhere subsidenceis a resultof recoveryfollowing uplift. Considerationof the available evidence suggests that the coseismicsubsidence scenario is the mostprobable. The crossesindicate estimated relative sea level for Madangbased on theIce-4G model of glacialisostatic rebound as described by Peltier[1998]; thesemodel outputare used to estimateactual relative sea level change (solid line) during periods of inferredtectonic stability.See text for furtherdiscussion. Age estimates have 2 sigmaerror bars based on counting statistics only;for the UFFh ages the errors associated with single age determinations areshown bracketing the mean colonyage. Vertical (height) error bars for twoU/Th dated corals are based on an estimateof thelikely maximumerror when these corals from Wongat Island are compared to therest of theU-series-dated corals fromJais Aben (due to potentialerrors associated with using the height of thehighest living local Porites microatollas an absolutedatum). Vertical error bars for the•4C dated corals are estimates of thelikely maximumerror when these corals (which were not surveyed with leveling equipment) are compared to all other corals.

corals were most probablyalready dead and embeddedin a All but one of thesehad relativelyflat topsindicative of rela- reef flat at the time of the subsidence.The presenceof thick- tively stablesea level during growth. The exceptionwas a ets of in situ branchingAcropora up to 1 m in heightgrowing colonywhich had clearlytilted duringits life. Thereforethere directlyabove the boundaryis suggestiveof instantaneous(or may have beena periodof relativesea level stability~2000 extremelyrapid, e.g., >50 mrn/yr) rise in sea level to accom- calendaryears B.P., followed by anotherpulse of >0.5 m modate the unrestrictedvertical growth of these corals. A subsidence-1200 yearsB.P. to generatethe spacefor the rapid rise in sea level is further supportedby the absenceof accumulationof the upper0.5 m of the lower terrace. any corals with microatoll form in the 1-1.5 m above the The presentelevation of the top of the lowercliff indicates boundary. These features of the internal structureand com- a more recent relative sea level fall, and hence inferred net positionof the reef deposits,combined with the resultsof the tectonicuplift, of >3 m (includingany postseismicsubmer- dating, point to a subsidenceevent of 1.5-2.0 m about 2400 gence or emergence). The dating resultssuggest that this calendaryears B.P. uplift occurredwithin the past 1000 years. Considerationof Several Porites coral microatolls were observed and sam- the rate of erosion of these Holocene cliffs and the width of pled from --1.5 m above the boundaryin the lower terrace. the present-dayreef flats lendssupport to the conceptof a TUDHOPE ET AL.: UPLIFt AND SUBSIDENCE IN PAPUA NEW GUINEA 13,805 majoruplift eventin the recentpast. The averagewidth of the reef flat at the JaisAben site is approximately40 m (Figures3 • 500 and 4). This width indicates the maximum amount of lateral o erosionthat can have occurredsince the uplift event. Anec- --• 200 dotal evidencefrom local villagers and considerationof the spacing and loss due to undercuttingof coconut palms •6 100 plantedafter World War 2 (Figure 3) suggeststhat the cliff -'-' 50 hascut back on the order of 5-10 m over the past30-40 years, (.) that is, at an average rate of-0.1-0.3 m/yr. This compares :'5 20 favorablywith our own visual observationof erosionat this site over the period 1988-1996. Although theseobservations • 10 and estimatesshould not be regardedas reliable quantitative measuresof the longer-termerosion of the cliffs, they do sug- ,• 5 gestthat the major uplift event may have occurredwithin the past few centuries. Indeed, the Russian naturalist Mik- • 2 lou(c)ho-Maclay[1884, 1975], who madethe first systematic • 1 naturalhistory observations of the Madang areain the 1870s, reportedon the frequencyand intensityof earthquakesand on • 0.5 the very considerablefear that the indigenouspeople had of earthquakes.This may imply that catastrophicearthquakes • 0.2 had occurred within collective human memory of the 1870s. The two uplift eventsdescribed here are similarin ampli- ->- 0.1 tudeto late Quaternaryevents on the Huon Peninsulawhich E O.O5 were inferred to be coseismicin origin and to have a recur- 2 3 4 5 6 7 8 renceinterval of 1000-1200years [Chappell et al., 1996a; (.) ISC bodywave magnitude Ota et al., 1993]. Chappellet al. [1996a] concludedthat these1-4-m-scale events did not appearto be partof a contin- Figure 6. Plotsof discrete(histogram) and cumulative uumof upliftmagnitudes on theHuon Peninsula, but instead (squaresymbols) number of earthquakesbinned in intervals appearedto belongto a separateclass of events.They also of 0.1 unitsof InternationalSeismological Centre (ISC) body pointedout thatuplifts of 1-4 m were10-40 times greater wavemagnitude mb in a 100 km radiusregion centered on thanthe uplift at theHuon Peninsula caused by anearthquake Madangfor theperiod 1964-1993. A leastsquares regression in May 1992[Pandolfi et al., 1994]. A coseismicorigin also to thelinear portion of thecurve (solid squares) is shown,and seemsprobable for theMadang uplifts, either as single catas- givesa b valueof 1.48. Extrapolationof thisb valueto mag- trophicevents or, conceivably,as accumulationsof smaller nitude7.0 yieldsan estimateof 0.1 eventsin 30 years,or a eventsover short periods of greatlyincreased seismic activity. return time of 300 years. In contrastto the uplift eventsthe intermittenttectonic subsidenceat Madangdoes not appearto havean analogueon the Huon Peninsula. The Madang subsidencecould be the acterizedby continentalcrust in both plates;therefore the resultof ruptureon a nearbyfault, or it couldbe relatedto deformationsignal associatedwith large-earthquakecycles eitherthe pullingdown of the upperplate in a preseismic may not be well characterizedby modelsproposed for sub- lockedsubduction zone or postseismicrelaxation following a duction of oceanic crust, a point emphasizedby Cohen subductionzone earthquake. If subsidenceis relatedto rup- (1994). Therefore,we must be tentativein drawingtectonic tureon a fault, it is mostlikely thatthe regionis on the hang- implicationsfrom our observations. ingwall of onereverse fault, which leads to uplift,but on the We can use present-dayseismicity reported in the Cata- footwall of another, which leads to subsidence. Alternating logueof the InternationalSeismological Centre (ISC) to esti- uplift andsubsidence would then be theresult of alternating mate returntimes for earthquakescapable of generatingme- failure on thesetwo faults.This seemsentirely plausible, and ter-scalecoseismic displacements. Figure 6 showscumula- perhapslikely, with identificationof NW-SE strikingreverse tive and discretebody wave magnitude(mb) versus frequency faultsin the AdelbertRange that trend towardthe Madang plotsfor shallow(ISC depth<60 kin) earthquakeswithin 100 lagoonand possible NE strikingcross faults in AstrolabeBay km of Madangfor the period1964-1993. This 100 km radius (Figure 1). is the region within which a large (say, m• 7.0 or greater) We preferthe interpretation that subsidence at Madang has thrustearthquake may be expectedto significantlyinfluence beencoseismic in responseto nearbyfault rupture. The alter- verticaldisplacement in our field area. The familiar Guten- nativeexplanation, that subsidence is eitherthe pullingdown berg-Richterlinear relationshipis observedin the cumulative of the upperplate in a preseismiclocked subduction zone or data betweenm• 4.5 and m• 6.0. Below this rangethe data set postseismicrelaxation following a subductionzone earth- is inferredto be incomplete,and the nonlinearityabove m• 6.0 quake[Thatcher and Rundle,1979; Savage, 1983; Sato and implies a magnitudelimit abovewhich either earthquakesdo Matsu'ura, 1993; Cohen, 1994], is not supportedby the ob- not occur in this region or our time period is insufficient servedrelative sea level stabilityfor a hundred,and perhaps (causingeither a deficit or excessat large magnitudes;see severalhundred, years after the inferredmajor uplift eventat below). The cumulativedata between thesemagnitude limits ~3000 calendaryears B.P. or by the inferredinstantaneous or give a seismicb value of 1.48. veryrapid subsidence around 2400 calendaryears B.P. How- We know that earthquakesof mb 7.0 have occurred in ever,the northernPapua New Guineacollision zone is char- northern Papua New Guinea, although there is no seismic 13,806 TUDHOPE ET AL.: UPLIFI' AND SUBSDENCE IN PAPUA NEW GUINEA evidence of larger magnitudes. Since seismicmoment, and wetherWilson are gratefullyacknowledged for field assistance,and hence displacement,is dominated by the largest-magnitude J. Darr and A. Bulcock are acknowledgedfor helpful informationon earthquakesin a region, we chooseto estimatethe returntime local geology. Ken Ridgewayof CSIRO, Hobart, kindly provided informationon the Madang tide gaugeelevation. Dick Peltier gener- of an earthquakeof mr,7.0. Extrapolationof our b value line ouslyprovided us with outputfrom the Ice-4G modelof relativesea to mr,7.0 infers 0.1 earthquakesof this magnitudeor abovein level run for Madang. This manuscriptbenefitted significantly from 30 years,giving a return time of 300 years. It is well known the suggestionsof Kerry Sieh, Yehuda Bock, and another,anony- that ISC magnitudesare subject to various sourcesof bias. mous,reviewer. Finally, thanksto JohnChappell for his enthusiasm for sharingideas. However, use of maximum-likelihoodbody wave magnitudes computedby the method of Lilwall [1987] to reducethese biasesyields a similar return time. References This estimationof returntime for seismicevents capable of producingmeter-scale uplift and subsidencenecessarily em- Abbott, L.D., Neogene tectonic reconstructionof the Adelbert- bodiesseveral assumptions, each of which is open to conjec- Finisterre-NewBritain collision,northern Papua New Guinea,J. ture. The mostimportant of theseare that the maximumbody SoutheastAsian Earth Sci., 11, 33-51, 1995. Abbott, L.D., E.A. Silver, and J. Galewsky,Structural evolution of a wave magnitudeachieved in this regionis >7.0 and that thrust modernarc-continent collision in Papua New Guinea, Tectonics, earthquakesdominate the seismicityin the 100 km radius 13, 1007-1034, 1994. zone of interest. Furthermore, the seismic b value of 1.48 is Abers,G.A., and R. McCaffrey,Active arc-continent collision: high, which may imply a deficit of large-magnitudeearth- Earthquakes,gravity anomalies,and fault kinematicsin the Huon- Finisterrecollision zone, PapuaNew Guinea, Tectonics,13, 227- quakesduring the sampling interval. If the true b value is 245, 1994. smaller (i.e., in the usual range of 0.8-1.2), then our return Bard, E., B. Hamelin, R.G. Fairbanks, and A. Zindler, Calibration of time estimateof 300 years is too high. 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Miklouho-Maclay, N., On volcanic activity on the islandsnear the context the Madang reefs illustrate that intermittentreversal north-eastcoast of New Guinea and evidenceof rising of the of a generaltectonic trend can exert a profoundcontrol on the Maclay-coastin New Guinea, Proc. Linn. Soc. N. S. W., 9, 963- sedimentaryarchitecture, evolution, and, therefore,facies of 967, 1884. the accumulatingdeposit. Mikloucho-Maclay, N., New Guinea Diaries 1871-1883, Kristen Press,Madang, Papua New Guinea, 1975. Milsom, J., Neogenethrust emplacement from a frontal arc in New Acknowledgments. This work was financially supportedby the Guinea, in Thrust and Nappe Tectonics,edited by K.R. McClay U.K. Natural EnvironmentResearch Council grantsGR3 09961 and andN.J. Price,Geol. Soc. Spec.Pub., 9, 417-426, 1981. GR3 8475 (A.W.T. and G.B.S.), U.S. Departmentof Energy Law- Ota, Y., J. Chappell, R. Kelley, N. Yonekura, E. Matsumoto,T. Ni- rence Livermore National Laboratory contract W-7405-Eng-48 shimura,and J. Head,Holocene reef terracesand co-seismic uplift (R.W.B.), NSF grant EAR 84-08001 (J.H.L.) and the Christensen of Huon Peninsula,Papua New Guinea. Ouat. Res.,40, 177-188, Research Institute (Fellowships to A.W.T., R.W.B., D.G.F. and 1993. J.H.L.). We thank ChristensenResearch Institute for logisticsupport Pandolfi, J.M., M.R. Best, and S.P. Murray, Co-seismicevent of and the local landownersfor accessto their land and for help in the May 15, 1992, Huon Peninsula,Papua New Guinea: Comparison field. Tim Fletcher, T. Frohm, John Mizeu, Jim Smith, and Meri- with Quaternarytectonic history, Geology,22, 239-240, 1994. TUDHOPE ET AL.: UPLIFF AND SUBSIDENCE IN PAPUA NEW GUINEA 13,807

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