A considerable amount of new information obtained since about 1965 has contributed greatly to a fuller understanding of late Cainozoic tectonics and volcanism in Papua New Guinea. The region straddles a complex zone of convergence between the major Indo-Australian and Pacific plates (estimated rates are about 9-14 cm yr^-1), and includes two, and possibly as many as four, minor plates. There are at least six - perhaps as many as ten - plate boundaries in Papua New Guinea. Most of them are zones of convergence, characterised by different components of strike-slip motion; one, and part of another, are ridge transform zones where new sea floor is being created. The Australian continent and Ontong Java Plateau reached the region during the Cainozoic, and may have had a major influence on plate kinematics in the late Cainozoic. Late Cainozoic volcanoes of Papua New Guinea are widely distributed and chemically diverse. Andesite is common, and most volcanic rocks may be classified broadly as arc-trench type; but comendites, intra-plate rhyolites, strongly undersaturated rocks, and basalts similar to those of back-arc basins, are among the rock types represented in some areas.

Improved regional seismograph coverage in the 1970s has enhanced the resolution of the seismicity of the southern highlands of Papua New Guinea. Two zones can be defined. The Southern Highlands Seismic Zone follows the Papuan Fold Belt and extends from Kerema on the Gulf of Papua through the Star Mountains region into Irian Jaya. A second zone, the Mount Hagen Seismic Zone, plunges to the north-northeast from the Southern Highlands Seismic Zone south of Mount Hagen to intersect the intermediate-depth seismicity beneath the Ramu-Markham Valley. The southern highlands earthquakes reflect continuing Pliocene-Quaternary thrust faulting in the crust of the Indo-Australian Plate , brought about by the collision of the Indo-Australian and South Bismarck Plates over the sinking Solomon Sea Plate. Relief of compressive stress is through fracturing of the continental plate margin. The zone of fracture is now manifest in the Papuan Fold Belt. Seismic risk in the Southern Highlands Seismic Zone is less than that of the north coast, New Britain, and Bougainville, but is still high. The once-a-year largest earthquake is of magnitude 4.7, the ten-year earthquake magnitude 6.2, and the 30-year earthquake magnitude 6.9, which compare with the Californian figures of 5.7, 6.9, and 7.4, respectively. Large earthquakes are expected to occur along the zone in the future and their effects could be severe.

A co-operative survey between the Soviet Academy of Sciences and the Australian Bureau of Mineral Resources during 1979 successfully measured the acceleration due to gravity using an absolute apparatus at Sydney, Hobart, Alice Springs, Darwin, and Perth in Australia, and at Port Moresby in Papua New Guinea. The measurements have a precision of about 6 micro Gal and an accuracy of about 15 microGal. Gravity ties to earlier stations allow comparisms with GAG-2 gravity meters, OVM pendulums and IGSN71 results. Gravity differences between cities are generally not significant at the 95 percent confidence level. Gravity differences at individual cities are also not significantly different from zero. The mean difference for all cities could be interpreted as having a component of secular variation of +3.3 ± 1.2 microGal/yr.

Rods criticisms of my paper can be summarised as (1) I have not come up with any revolutionary new mechanism for the emplacement of the Belt; (2) the position of the Moho in my gravity sections is not clear; and (3) the overthrusting hypothesis does not explain either the steep dips along much of the Owen Stanley Fault system or the fact that parts of it are strike-slip in character. The last criticism is by far the most significant; the character of the Owen Stanley Fault being one of the reasons for my modification of the original Thompson-Davies theory, and I suspect the main impetus behind Rods theory of emplacement. I stated in the introduction to my paper that I have adopted the overthrusting mechanism of emplacement. I make no apology for not coming up with a revolutionary new mechanism as I think the thrusting mechanism is by far the most likely. Possible methods of emplacement which I can conceive are (1) intrusion of a large body of basaltic lava high in the crust and its subsequent differentiation to produce the observed layering; (2) uplift by isostatic or some other vertically acting force; (3) thrusting in a horizontally compressive regime. The intrusive mechanism has been ruled out by Davies (1971) on chemical and other evidence. The isostatic uplift mechanism seems unlikely in view of the very high density of the rocks forming the Belt itself and also the probable high density of the Owen Stanley metamorphics. This leaves the horizontal compression mechanism of emplacement.

Thermal waters in Matupi Harbour and Sulphur Creek, Rabaul caldera have D/H and O18/O16 ratios that are indicative of a mixed source. They are the result of mixing of local meteoric waters with hot water of marine origin. The stable isotope data are grouped into distinct areas close to the meteoric water line. They suggest that the thermal systems away from the shoreline are dominated by meteoric water and that warmed sea water only enters the springs at the shoreline. Low temperature (100°C) fumarolic exhalations from Tavurvur and Rabalankaia volcanoes consist largely of recycled meteoric water. These conclusions conflict in part with those drawn from anion ratio and trace metal contents which were inferred by previous authors to be consistent with an hypothesis of modified sea water origin. We suggest that the chemistry of these acid, mineralised geothermal waters is a reflection of their later, near surface, history and does not necessarily give a correct picture of their ultimate origin. The enhanced Fe, Mn, and Zn values of the Matupi springs are a function of the leaching potential of geothermal fluids at elevated temperatures, and of the chemistry of the porous and chemically reactive rocks through which they pass.

Geochemical data are presented for a sequence of spilitic pillow basalts (Tumu River basalts) associated with peridotites and gabbros of the Marum ophiolite complex in northern mainland Papua New Guinea. The basalts are strongly differentiated from relatively magnesian types (Mg-value = 70) to ferrobasalts (Mg-value = 30) characterised by high levels of Fe, Ti, Zr, Nb, Y. The Tumu River basalts are enriched in large ion lithophile elements such as REE, Zr, Hf, Nb, P2O5, and compare with tholeiites from oceanic islands. Major and trace elements suggest extensive fractionation involving olivine, pyroxene, and plagioclase, followed by pyroxene, plagioclase, titanomagnetite, and ilmenite. Trace-element plots are used to examine fractionation processes and to estimate abundances in the parent magma. The calculated initial concentrations are compared with abundances and abundance ratios in least fractionated enriched and depleted tholeiites. The abundances in the parent magma are used to calculate source abundances for large (20-30%) degrees of partial melting. The levels range from 2-3 times chrondites for HREE, Ti, Y, Zr, Sc, and P2O5, to 3.5-5.5 times for LREE, and are similar to those inferred for other LREE-enriched tholeiites from both oceanic and continental areas. The chemistry of the basalts therefore reflects the mantle-source composition rather than a particular tectonic setting within an ocean basin.

On 20 July 1975 a major earthquake (MS7.9) shook the northern islands of the Solomon Islands chain. Damage amounting to at least $300,000 (Australian) occurred in the southern Bougainville/Shortland Islands region, where earthquake intensities were estimated to be MMVII-VIII. A tsunami with maximum amplitude of about two metres followed the earthquake and caused further damage. The earthquake caused landsliding, liquefaction, subsidence, slumping of roads and wharfs, and damage to villages, small government and mission buildings, and to the mining installations at Panguna. Aftershock epicentres were in a roughly elliptical area of 12 500 square kilometres off the southwestern coast of Bougainville. Focal depths were in the range 30-70 km. A fault-plane solution and the pattern of aftershocks indicate that the principal earthquake was associated with underthrusting of the Solomon Sea crust beneath Bougainville, in a northeasterly direction and with a dip of about 37°. The faulting associated with the 20 July 1975 earthquake appears to be the extension of faulting associated with a 1974 earthquake series. An aseismic zone, centred at 6°S, 154°E, exists immediately northwest of the 1975 earthquake fault zone, between zones where major earthquakes have occurred since 1970. It is considered to be a likely place for a major earthquake in future.

Four distinct volcanic rock types are found above a conjectured mantle hot spot in St Andrew Strait, northern Papua New Guinea. Hypersthene-normative basalts on Baluan Island are geochemically similar in most respects to those on oceanic islands and, together with voluminous alkali-rich rhyolites on Tuluman, Lou, and Pam Islands, constitute a strongly bimodal rock suite. The rhyolites are regarded as partial melts of basaltic crust isotopically similar to the basalts of Baluan, though with lower Sr, Rb, and Ba contents. In contrast, quartz-tholeiite basalts in the Fedarb Islands are isotopically distinct from the Baluan basalts. Dacite is also present in the Fedarb Islands, but not all of its geochemical features are consistent with a derivation by crystal fractionation from Fedarb quartz tholeiite. Like Iceland, St Andrew Strait may be underlain by a hot mantle diapir that has produced basaltic magmas as well as partial melting of basaltic crust.

Eleven ammonite species, including one new taxon, are described from eleven localities in the south-central Papua New Guinea. One of them, Fauriella boissieri (Pictet), is a member of the Berriasian Tethyan fauna extended to peri-Gondwana. Three large, but fragmentary, ammonites are identified as Puzosia aff. mayoriana (dOrbigny) and Pachydesmoceras sp. B and C, suggesting a Cenomanian age. Acanthoceras rhotomagense (Brongniart), Calycoceras (Newboldiceras) asiaticum (Jimbo), and Cunningtoniceras cunningtoni (Sharpe) indicate more distinctly the Cenomanian age. C. cunningtoni is associated with Desmoceras (Pseudouhligella) aff. ezoanum Matsumoto, both occurring in generally the same area as the type locality of Chimbuites sinuosocostatus (Casey and Glaessner). From another locality a new species, Chimbuites giganteus is described. Chimbuites is regarded as an offshoot of Eopachydiscus, and therefore a member of Pachydiscidae rather than Hoplitidae. A well-preserved specimen of Romaniceras deverianum (dOrbigny) indicates a Turonian age.

This series of maps covers the whole of Australia at a scale of 1:250 000 (1cm on a map represents 2.5km on the ground) and comprises 513 maps. This is the largest scale at which published topographic maps cover the entire continent.