ALBERT

Description: A geological and geophysical study of active submarine volcanoes offshore of Epi Island and between Epi and Efate Islands, New Hebrides Arc, has delineated five large, mainly submerged, calderas over a 90-km length of the arc. These vary in diameter from 15 to 30 km, and the two northern calderas have active post-caldera basaltic cones. Karua volcano lies on the submerged northern rim of the caldera northwest of Tongoa, and has periodically erupted medium-K20 basaltic andesite in this century. Submarine cones Epia, Epib, and Epic are aligned east-west and spaced 3.5 km apart on the rim of the northernmost caldera. Rocks dredged from these cones include basalt, dacite, and cognate gabbroic inclusions with magmatic affinities similar to those of the Karua lavas. A pronounced bimodal Si02 content is noted for the large caldera volcanoes in the New Hebrides and other oceanic arcs. This bimodality has important implications for physical processes of magma differentiation within arc-volcano magma chambers. The bulk of the uppermost level of the surficial (and seafloor) dacite pumiceous pyroclastics in the area of northeast Epi and Tongoa was derived from cataclysmic eruptions of more than one of the now largely submerged calderas.

Description: New Ireland is dissected by northeasterly and northwesterly-trending normal faults, which have fragmented the island into a series of northeasterly-tilted fault blocks. An exception to this regional pattern is the Weitin Fault Zone, which appears to represent the onshore extension of a ridge-trench transform fault system. Basement in New Ireland is formed by the Eocene(?} to Oligocene Jaulu Volcanics, and the Oligocene to middle Miocene Lemau Intrusive Complex. The basement rocks are unconformably overlain by the Miocene Lelet Limestone and the Lelet's partial lateral equivalents, the Tamiu siltstone and Lossuk River beds. In northwestern New Ireland, the lower Lelet Limestone and Lossuk River beds are succeeded conformably by the middle to late Miocene Lumis River volcanics. To the south, where the Lelet Limestone has a longer age range, the formation is overlain by the late Miocene to early Pliocene Punam Limestone, and the Pliocene to earliest Pleistocene Rataman Formation. The youngest rock units on New Ireland are Pleistocene limestones (Qc) and their lateral equivalent, the Maton Conglomerate. The Jaulu Volcanics and Lemau Intrusive Complex are the products of island arc volcanism associated with subduction along the Manus-Kilinailau Trench. Uplift and erosion of the arc was followed by widespread carbonate platform sedimentation during the early Miocene (Lelet Limestone). At the same time, fine-grained elastic sediment accumulated in adjacent, deep water areas (Tamiu siltstone and Lossuk River beds). Local volcanism, accompanied by marked subsidence, ended carbonate sedimentation in northwestern New Ireland during the middle Miocene. Bathyal pyroclastic and epiclastic sediment accumulated in that area until the late Miocene or early Pliocene (Lumis River volcanics). To the south, carbonate platform sedimentation probably continued unabated until the late Miocene, when rapid regional subsidence drowned the platform. This event appears to have coincided with reversal of the New Ireland portion of the original island arc system. Bathyal pelagic deposition subsequently became established over a wide area (Punam Limestone). Regional volcanism commenced in the early Pliocene, probably in response to subduction along the newly-formed New Britain Trench. During the remainder of the Pliocene, bathyal sedimentation continued as pyroclastic and epiclastic sediment was deposited on submarine fans (Rataman Formation). Rapid regional uplift of the southwestern margin of the New Ireland Basin followed during the early Pleistocene. Coarse-grained terrigenous sediment accumulated against active fault scarps {Maton Conglomerate), while neritic carbonates were deposited nearby {Qc).