Geology and Petrology of Rabaul Caldera, Papua New Guinea R. F. HEMING* Department of Geology and Geophysics, University of California, Berkeley, California 94720 ABSTRACT parison with experimental data for high- off the north coast of New Guinea and ter- alumina basalt also suggests a shallower minates at Rabaul. Only the New Britain Rabaul caldera is unusual in that it was origin. A progressive increase in Ti02, section has the classical features of an is- formed by two episodes of construction and A1203, and alkalis along the New land arc: a deep submarine trench to the collapse on an older basalt volcano. One Britain—New Guinea arc toward Rabaul at south, an inclined seismic zone dipping collapse occurred around 3,500 yr B.P. and its eastern end cannot be explained. Key northward, and a string of volcanoes on the the latest around 1,400 yr B.P. Both were words: igneous petrology, volcanology. side of the island opposite the trench. West accompanied by the eruption of daciiic of New Britain, the trench disappears, and pumiceous ash flows. Following initial col- INTRODUCTION in the vicinity of Long Island (Fig. 1), the lapse, volcanism was confined to a large Rabaul caldera, situated at the northeast- simple Benioff zone is replaced by a north- andesite volcano in the southern part of the ern tip of the island of New Britain, is the and a south-dipping zone (Denham, 1969). caldera, but a renewal of basaltic volcanism easternmost volcano of the New West of here, the volcanic arc lies well occurred at a point on the eastern ring frac- Guinea—New Britain arc (Fig. 1). Rabaul is offshore of the mainland, and the under- ture. The later collapse of the southern cone a current center of active volcanism, and lying seismic zone is located beneath New left the caldera as a large ellipse, 14 by 9 the region is often shaken by large earth- Guinea. Earthquakes of intermediate depth km, breached on the southeast side, though quakes. Vulcan (on the western side of the are frequent in the New Britain portion of more recent volcanism has built small cones caldera) and Tavurvur (on the eastern side) the arc but do not occur in the arc west of on the caldera floor. erupted simultaneously in 1878 and 1937, the island. No trench, however, is as- The rocks of the caldera cannot un- and Tavurvur was active again in 1941 and sociated with the western portion of the arc equivocally be labeled as calc-alkalic because 1942. Other cones have been built within (Fig. 1). the contemporaneous basalts are high- the caldera, the most notable being Sulphur In New Britain, the volcanic chain has alumina types and the suite shows mild iron Creek, which was reported by early mis- two pronounced breaks, one west of enrichment. This mild iron enrichment is sionaries and explorers to have erupted in Rabaul of over 100 km and another of the not found in the lavas of some parasitic vol- 1858. same length west of Talasea. The first canoes, suggesting that it is an artifact of The first description of the geology of the break, between Rabaul and Mount the magma chamber beneath the caldera. caldera was given by Fisher (1939) in his Ulawun, contains a number of north- Differences in chemistry and mineralogy report on the eruption of Vulcan in 1937, west-trending faults and a graben (Macnab, persist between lava erupted on the western which made local authorities aware of the 1970). The second break has a more un- side of the caldera and that on the east; vulnerability of Rabaul, and a volcanologi- usual character. From Mount Ulawun the chemical variation is explained by crystal cal observatory was built on the north rim volcanic chain continues along the north fractionation at low pressures. Lack of a of the caldera. coast of New Britain then abruptly passes clearly defined Benioff zone and location north along the Talasea peninsula before close to a transform fault suggest that the STRUCTURE OF THE turning west again to the Witu Islands (Fig. origin of the magma was not controlled by NEW BRITAIN ARC 1). No active volcanoes are found between partial melting near a Benioff zone. Corn- The major regional structures are de- Talasea and Langila, at the western tip of picted in Figure 1. The New Guinea—New the island, and the arc apparently is offset * Present address: Department of Geology, University Britain volcanic arc begins west of Manam about 100 km between Talasea and Mount of Auckland, Private Bag, Auckland, New Zealand. Geological Society of America Bulletin, v. 85, p. 1253-1264, 10 figs., August 1974 1253 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/8/1253/3418429/i0016-7606-85-8-1253.pdf by guest on 02 October 2021 1254 R. F. HEM1NG Figure 1. Structure of the New Guinea region. Bathymétrie contours are in meters with depths below 6,000 m depicted by a horizontal line pattern, and below 8,000 m, by dots. Bathymetry from Australian Department of National Development map (1970); seismic data from Denham (196i>); plate boundaries and relative motions of crustal plates are those suggested by LePichon (1968) and Johnson and Molnar (1972). Langila. Dacite and rhyolite are common in GEOLOGIC SETTING OF Fisher, 1944). Similar rocks are exposed on the volcanoes of the arc east of Talasea, but THE CALDERA southern New Ireland (Fig. 1), where again west of that point, andesite is the most silice- active northwest-trending faults control the ous rock. The geology of the area surrounding structure (French, 1966). Rabaul caldera lies close to a postulated Rabaul caldera is typical of young, Xenoliths provide few clues as to the na- junction between three crustal plates and circum-Pacific volcanic arcs. West of ture of the basement ber.eath Rabaul. Some only a few kilometers west of a transform Rabaul, the Baining Mountains (Fig. 1) diorite blocks are found, and a gabbro from fault separating the Pacific and Bismarck consist of Tertiary flows, volcanic sedi- near Kabakada (Fig. 4) resembles leu- Sea plates. The simple model of trench, in- ments, and limestones; all are intruded by cogabbro from the Baining Mountains clined seismic zone, and volcanic arc is not rocks of varied composition including (described by Macnab, 1970). West of apparent near the Rabaul caldera. Shallow leucogabbro, adanellite, and granite, Rabaul, the volcanic rocks interfinger with seismic events of large magnitude are con- which Macnab (1970) considers to be unconsolidated sediments that may con- centrated along St. George's Channel, while equivalents of the calc-alkalic volcanic tinue some distance beneath the Rabaul earthquakes of intermediate foci scatter suite. The range is cut by northwest- volcanics. Miocene limestone exposed in north and west of Rabaul but are concen- trending faults, the largest of which, the the Rembarr Range (Fig. 4) most probably trated mostly along large faults. Baining fault, is still active (Macnab, 1970; forms an isolated fault block. Uplifted Pleis- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/8/1253/3418429/i0016-7606-85-8-1253.pdf by guest on 02 October 2021 GEOLOGY AND PETROLOGY OF RABAUL CALDERA, PAPUA NEW GUINEA 1255 tocene reef limestone is found around Watom Island and at Tawui Point. Although there is no direct indication of the composition of the basement beneath the caldera, circumstantial evidence sug- gests that it may be an extension of that found in the Baining Range, with the possi- ble addition of a veneer of unconsolidated sediments. STRUCTURE OF THE CALDERA The Rabaul caldera is elliptical, measur- ing 14 km from north to south and 9 km from east to west, with a breach 5 km wide in the southeast wall (Fig. 2). Two well-preserved composite cones rise above the eastern wall (Fig. 4): The Mother (Kombiu) and South Daughter (Turan- guna). The northern wall is formed by the deeply dissected cone of North Daughter (Tovanumber). Straddling the eastern wall is the breached volcano Palangaigia and within it the low cone of Rabalanakaia (Figs. 2, 4). Within the caldera lie the active volcanoes of Tavurvur and Vulcan, the fissure of Sulphur Creek and its associated explosion craters, and the two oddly shaped Beehives that are the remnants of another ash cone. The outer slopes of the caldera decrease gradually to the south and west, though in detail the terrain is highly dissected. South- southwest of Rabaul is Mount Varzin, the remnants of a satellite volcano; 11 km to the northwest is another satellite, Watom Island (Fig. 4). The bathymetry of the caldera floor is shown in Figure 2. The northern part is es- sentially a long, narrow trough that plunges southward into the basin of Karavia Bay, which reaches a depth of 295 m. Remnants of the caldera wall are seen at the seaward entrance to the caldera, and east of them are two deep valleys that extend into the deep trench beneath St. George's Channel. The dominant feature in the area is the el- liptical fault of the caldera ring fra.cture (Fig. 4), which is partially masked by exten- sive pumice ash deposits. Where it is well exposed, between South Daughter and The Mother and east of North Daughter, it is arcuate and nearly vertical. Figure 3 is a copy of seismic reflection records made on the R.V. Mahi in Karavia Bay, with the steep escarpment of the caldera ring fault clearly shown. Displacement took pls.ee on Figure 2. Topography and bathymetry of the Rabaul caldera. Contours in meters. Bathymétrie contours are at a number of concentric arcuate faults.