This paper summarises the tectonic setting and seismicity of the Huon Peninsula, based on recently published papers. Some characteristic aspects of tectonic geomorphology are also briefly reviewed. The Huon Peninsula on the northeastern Papua New Guinea, located on ca. 6° S, is a suitable place for Quaternary sea level study, because coral reef terraces as an indicator of former sea level position are well preserved as separate terraces on land, due to strong uplift on the collision zone between the Australian and Pacific plate. The plate boundary is traced on land as Ramu-Malkahm Fault Zone. The formation of the Huon-Finisterre Range is a result of the collision of these two plates during the Quaternary. Fault plane solution for the major shallow earthquakes indicates that many of them are by reverse faults, strike of which is perpendicular to the fault zone. Thus, it can be said that the north-south oriented compression has been dominated at this fault zone. A flight of coral terraces, up to ca. 1, 000 m in altitude, fringes the northeastern coast of this tectonically active Huon Peninsula. U-series dates have been obtained from the terraces younger than Terrace VII, correlated with isotope stage 5e of the last interglacial maximum. Terrace height shows the northwestward downtilting, with some irregularity, from the maximum uplift rate of 3.5 m/ka. The uplift rate since the isotope stage 5e agrees with that for the Holocene, implying that the same pattern of tectonic uplift with the uniform rate has continued during the late Quaternary. Rapid uplift of this area is a result of repeated coseismic uplift which has an meter-scale uplift and ka-scale interval. Evidence of the coseismic uplift is obtained from both Holocene terraces and late Pleistocene terraces. Some of large landslides disrupting coral terraces are also originated coseismically. Coseismic process is an important tectonic factor for the formation and emergence and also for the destruction of coral terraces. These topics, related to paleoseismicity, are discussed in detail elsewhere in another papers in this special issue. Some landforms, related to active tectonics, such as active faults, uplifted alluvial deltas, and stream system, are also briefly discussed in the paper.
A flight of coral terraces, up to ca. 1, 000 m above sea level, is well preserved on the northern coast of the Huon Peninsula, Papua New Guinea, and records the sea level and tectonic history since ca. 300 ka. The region is located near the boundary between the Western Pacific and Australian plates. The uplift rate since the last interglacial maximum (oxygen isotope stage 5e) ranges from 0.7 m/ka at the northwest end of the coast, and up to 3.5 m/ka towards the southeast end. Late Quaternary paleoseismicity is estimated from subdivided Holocene and late Pleistocene terraces, each of which records meter-scale coseismic uplift events. The study area covers about 40 km of coast. Holocene coral terraces, up to 25 m above sea level, are divided into seven levels at maximum, four to five at most sites. The highest Holocene terrace is a reef crest representing former sea level of the culmination of the sea level rise at ca. 6-6.6 ka BP. The lower terraces are regressive terraces, which record successive intermittent uplift events, probably caused by great earthquakes. Timing of the Holocene uplift events is ca. 2.5, 3.8, 4.4-5.1, and 5.4 ka BP at Kwambu-Kilasairo area on the northwestern coast, and at ca. 0.8, 1.4, 1.8, 2.5, 3.9 and 5.4 ka BP at Kanomi-Nanda area to Hubegong on the central and southern coast. Difference in the timing of the uplift events implies that there are at least two tectonic suregions on this coast. Similar small regressive terraces are also recognized from the detailed profiles of late Pleistocene terraces. For example, in the central and southern part of the study area, 15-7 extra steps occur between transgressive terrace IIIa (ca. 52 ka) and terrace II (ca. 38 ka). Thus, we infer that repeated meter-scale uplift events have occurred at least since ca. 52 ka. The apparent interval of uplift events, about 1-1.5 ka, resembles to that for Holocene. Such a meter-scale uplift with ka-scale recurrence intervals records a different earthquake deformation cycle from the centimeter-scale uplift associated with the 1992 May earthquake (M= 7.2 : Pandolfi et al., 1994). Such a small event with shorter recurrence interval is apparently not expressed geomorphologically as in the form of regressive terrace.
Landsides occur widely on the coral terraces of the tectonically active Huon Peninsula, Papua New Guinea. Large historic earthquakes in Papua New Guinea have frequently triggered large landslides. Landslide that disrupt the coral terraces at Huon Peninsula have a variety of forms including ; 1) large translational block slide features that carry essentially intact coral terrace remnants along, without large debris flows ; 2) complex slides that may begin as rockslide or deep-seated rotational slide features that continue downslope as debris flow features. In addition, small rockfall and rockslide features that commonly occur on steep valley walls and headwall (type III). Large scale landslides of types I and II occur more frequently in the southeast of the study area where uplift rate is high, up to 3.5 m/ka. Approximate ages of 26 large, late Quaternary landslides of types I and II have been obtained by radiocarbon dating of corals from immediately above or below the debris flow units that have extended and across the Holocene reef sequence, or by U-series ages of older coral terraces that have been disrupted by landslides or where terrace have developed on older landslide debris. The youngest debris flow (type I) directly overlies a coseismically uplifted terrace dated at ca. 1, 250 yr BP. At three sites, at least, an older debris flow is intercalated with reef material that was forming during a late stage of the post-glacial transgression about 7-8 ka. More than 10 large blockslides without debris flow (type I) disrupt ca. 33 ka terrace, but these are trimmed at outer edge by the ca. 6.5 ka Holocene terrace, thus providing bracketing ages. Historic precedent of large landslides generated by major earthquakes, and the close association of debris flow deposits with coseismically uplifted terraces of Holocene age lead us to propose that many, and perhaps most, o f the large scale landslides on Huon Peninsula are earthquakegenerated.
The extensive flight of terraces occurs on the northeast seaboard of Huon Peninsula, Papua New Guinea. The terraces consist of many Quaternary reef-complex limestone bodies of different ages. Fossil nonarticulated coralline algal florae were examined in the limestones forming the Pleistocene terraces : II, IIc-lower, IIIc-upper, IIIb, IIIa-lower, IIIa-middle, IIIa-upper and IIIa-upper+ from the coast to the inland area at Kanzarua ; and IV, Vb, Va, Vib, VIa, Mic and VIIb from the coast to the inland area at Kwambu. Depositional environments of these limestones are determined on the basis of comparison with the distribution of coralline algae in the present-day reef complex around Huon Peninsula, the Ryukyu Islands and other Indo-Pacific regions. Nineteen species coming under ten genera of fossil nonarticulated coralline algae have been identified. Two coralline algal assemblages are discriminated. One is characterized by the presence of Porolithon onkodes, and thought to be indicative of shallow forereef environment. This assemblage occurs on II, IIIc-lower, IIIc-upper, IIIb, IIIa-lower, upper and IIIa-upper+ of Kanzarua, and Va, VIIc, VIIb of Kwambu, implying that these terraces correspond to ancient lagoon to reef-core facies. The other is delineated by the presence of Lithothamnion australe associated with Lithothamnion, Sporolithon and Lithophyllum species. This assemblage occurs in rhodoliths of rhodolith floatstone and packstone-wackestone exposed at the cliffs of terrace IV, Vb, Va, VIb and VIa at Kwambu. It can be thought that these fossil rhodoliths were deposited in the deep forereef environment because their constituents are very similar to those of deep water rhodoliths around the Ryukyu Islands. Therefore, each rhodolith-bearing facies of terrace cliff is the deeper counterpart of reef-core facies forming some higher terraces and the antecedent sediments to the formation of its consisting terrace. At Kwambu, terraces IV, Vb, VIb and VIa are composed of the rhodolith-bearing limestones and, these terraces thought to be abrasion platforms on the older, uplifted, deep forereef deposits.
Conducted was observation of distribution of dominant nonarticulated coralline algal species growing on well-illuminated substrata in fore-reef slope and lagoon environment on Huon Peninsula. Ten species coming under four genera were identified : Porolithon onkodes (Heydrich) Foslie, Lithophyllum insipidum Adey, Townsend et Boykins, Lithophyllum pallescens (Foslie) Foslie, Lithophyllum moluccense (Foslie) Foslie, Mesophyllum erubescens (Foslie) Lemoine, Neogoniolithon conicum (Dawson) Gordon, Masaki et Akioka, Neogoniolithon fosliei (Heydrich) Setchell et Mason, Neogoniolithon frutescens (Foslie) Setchell et Mason, Neogoniolithon sp. A (in Iryu and Matsuda, 1994), and Hydrolithon reinboldii (Weber van Bosse and Foslie) Foslie. Distribution of these species is in accordance with that in the Ryukyus, Hawaii and Guam.
Community structures of Pleistocene coral reef assemblages are recorded in the limestone terraces along the coast of Huon Peninsula. We made topographic profiles at Kanzarua and Sialum areas, and described the litho-and biofacies as columnar sections along them. Depositional environments of the limestones are estimated on the basis of the results from the modern coral reef assemblages. Six biofacies of the Quaternaryl imestone were recognized as follows. Hermatypic Coral Facies : This facies is characterized by the autochthonous hermatypic corals that are contained, and is accompanied with algal crusts of coralline algae. The lithofacies is represented by boundstone composed of bindstone, framestone and bafflestone. Its depositional environment is estimated to be the lagoon or forereef slope shallower than 30 m. Rhodolith Facies : This facies is characterized by abundant rhodoliths and occurred on the cliffs of terraces V a, V b and IV. In this facies, tests of the larger foraminifer Cycloclypeus carpenteri are also scattered. This lithofacies is represented by packstone to wackestone. The rhodoliths are characterized by the foraminifer Acervulina inhaerens and coralline algal LS assemblage. This assemblage is identical with that of rhodoliths distribute in deep forereef and insular shelf environments at depth of 50-150 maround Okinawa, in the Ryukyu Islands. Cycloclypeus Facies : Grains are mostly composed of tests of the larger foraminifer Cycloclypeus carpenteri. This facies occurs on the cliff ofterrace V b and in one horizon around VIb. The lithofacies is represented by packstone to wackestone. Around Okinawa Island, Ryukyu Islands, Cycloclypeus is distributed from 50 to 150 m in depth. This facies is inferred to have been deposited in a deep environment, deeper than the lower limit of hermatypic corals. Solitary Coral Facies : Solitary and ahermatypic corals, such as Flabellum spp., Dendrophyllia spp., Caryophylliidae species and Diaseris sp., characterize this facies. Their matrix consists of detritus of the skeletons on reef organisms and fine micritic carbonate. The lithology is mainly packstone to wackestone. Based on the depth distribution of these coral species in the Indo-Pacific, the facies is inferred to have been distributed on the forereef slope deeper than 30 m. Halimeda Facies : Grains are mostly composed of segments of Halimeda This facies widely spreads over the lagoon of terrace VIIb. Litho-facies is represented by packstone to wackestone. Halimeda thrives from low tide level to the edge of insular shelves in tropical to subtropical region. Sediments rich in Halimeda segments are known not only from shallow lagoonal environments but also from deep reef associated shelves and banks. Detrital Facies : This facies is characterized by an absence of the mega-fossils mentioned above and consists of fine sand and mud. This facies is considered to have been deposited on the reef slope deeper than 30 m, because it is accompanied by bryozoans and solitary corals in places. Fossil corals in this region can be divided into four assemblages on the basis of their species composition and growth forms. Assemblage A : This assemblage is represented by dendritic Acropora spp., such as A. formosa. Matrix is fine sand and mud with segments of Halimeda spp. and mollusks. Lithology of the limestone is mostly bafflestone and mudstone. The assemblage is a counterpart of recent Lagoon Assemblage and indicates that the limestone was deposited in a lagoon. Assemblage B : Encrusting Montipora spp. and massive Faviidae corals are useful for supplemental diagnostics. Framestone and rudstone are the dominant lithology. The assemblage is correlated to the recent Deep Reef Slope Assemblage, and is estimated to have inhabited the reef slope from 5 to 10 m in depth.
Recent coral reefs are well-developed along the coast of Huon Peninsula. Most of them are of the fringing type, although reefs with narrow lagoons are found at Sialum and Gitua. We studied the hermatypic coral assemblages with SCUBA at the reef of Sialum and the fringing reef of Hubegong about 30 km southeast of Sialum. Sialum reef has a typical lagoon and reef crest with a narrow channel across the crest. The lagoon is about 3 km in length, and about 200 m in width. Its bottom is flat and shallower than 12 m. The crest is partly emerged up to a few meters at the central part of the barrier, while the other parts are almost at mean sea level. A reef slope has a groove and spur structure, and inclines steeply with several terraces to a depth of 20m. No lagoon and reef crest are developed on the Hubegong coast. We made topographic profiles on the reef slope from 0 to 21 m in depth. The slope inclines at an angle of about 25°. Small terraces were recognized at depths of 3 m and 15 m. Groove and spur morphology is not formed in this area. Hermatypic corals are very abundant in the shallower part (0-5 m), but rare in the deeper. Coarse-grained detritus and boulders derived from the coral patches cover the deeper slope. Hermatypic corals disappear at depths deeper than 30 m. Forty-three coral genera (including sub-genera) and 80 species were identified at Sialum and Hubegong. These numbers are not as large as might be expected for typical Indo-Pacific reef assemblages. This is due not only to the poorness of the actual assemblage but also to shortness of time available. Species identified in this region are similar to those of typical IndoPacific assemblages. Coral coverage is very high in the coral thicket but almost 0% on the bottom of the lagoon and on the reef crest. It exceeds 30% in the shallower forereef and decreases to 0% in the deeper part. We define 4 assemblages on the basis of the transect survey and other data, as follows. Lagoon Assemblage : The assemblage is distributed inside of the lagoon from 0 to 10 m in depth. It is characterized by the presence of A. formosa, A. pulchra, M. digitata, and P. cylindrica. Shallow Reef Slope Assemblage : It is distributed on the shallow part of the reef slope from 0 to 3 m in depth, and is defined by such species as A. hyacinthus, A. monticulosa, and A. humilis. Middle Reef Slope Assemblage : The habitat of this assemblage ranges from 3 to 5m in depth on the reef slope. A. palifera is the diagnostic species. Deep Reef Slope Assemblage : This assemblage inhabits the deeper reef slope from a depth of 5 to 30 m. As L. scabra, the representative of the assemblage is rare, massive Faviidae corals are more conveniently used for the assemblage definition.
Total number of fifty-four Pleistocene corals collected during the 1988 and 1992 international expeditions, were dated by the α-spectrometric 230Th/234U method, and the similar dates reported previously were critically reevaluated after the five-step screening by the following criteria : (1) The sample has no evidence of recrystallization of coral skeleton and/or cementation with the secondary calcite or aragonite ; (2) Skeletal textures have not changed through their diagenetic history ; (3) The sample should be free of the initial 230Th ; (4) The uranium concentration should be the same (2 to 4 ppm) as those in the present-day counterparts ; and (5) The initial 234U/238U activity ratio should be consistent with that in the present sea water, 1.144±0.002 (Chen et al., 1986). Most of the α-spectrometric dates evaluated to be fully reliable, were consistent with the TIMS (thermal ionization mass-spectrometer) 230Th/234U date of the same sample. One of the most salient observations is that many corals occurring in the Pleistocene terraces of Huon Peninsula have not necessarily preserved the closed system for uranium isotopes, even though they are composed of 100% aragonite without any textural alteration of original skeleton. For the reason, only half of α-spectrometric 230Th/234U dates were proven to be reliable in this study. On the basis of the precise α-spectrometric 230Th/234U dating, ages of terraces correlated with oxygen isotope stage 3 (Shackleton and Opdyke, 1973) become a little older than the former assignment (e.g., Bloom et al., 1974). Terrace IV of Chappell (1974) was assigned to ca. 73 ka ; and Terrace IIIa, IIIa middle, IIIIa lower, IIIb, IIIc lower, to ca. 52 ka, ca. 51 ka, ca. 46 ka, ca. 45 ka, ca. 44 ka, respectively. Ages of Terrace II b (ca. 38 ka) and II c (ca. 33 ka) are newly revised to ca. 41 ka of Terrace II a. Such increase of the age-values for Terraces II to IV works to lower the paleo sealevel estimated by Chappell and Shackleton (1986) to that from the oxygen isotopic records (Shackleton, 1987). Three α-spectrometric and one TIMS 230Th/234U dates corresponding to the last interglacial maximum were obtained from the basal part of coral limestone beneath Terrace VIb. This observation indicates decidedly that lower Pleistocene terraces such as Terraces VI to N are underlain by the limestone formed during the isotope stage 5e.
A major discrepancy exists between the late Quaternary sea level changes derived from raised coral reef terraces at Huon Peninsula in Papua New Guinea and from oxygen isotopes in deep sea cores. The two methods agree closely from 120 ka to 80 ka and from 20 ka to the present. However, oxygen isotopes from benthic forams indicate lower sea levels by 20 to 40 m during the later stages of the last glacial cycle, from 70 to 30 ka. New, high precision U-series age measurements and revised stratigraphic data for Huon Peninsula terraces that formed during this interval give similar sea levels to those based on planktonic and benthic δ18O data. The record of ice volume changes through the last glacial cycle now appears to be reasonably complete.