Early Cainozoic stratigraphy and structure of the Gazelle Peninsula, east New Britain: An example of extensional tectonics in the New Britain arc-trench complex

Abstract

The Early Cainozoic geological history of the Gazelle Peninsula, east New Britain, Papua New Guinea, indicates that extensional tectonics have influenced sedimentation and volcanism in this part of the New Britain arc-trench complex since the earliest Miocene. Remapping of the Gazelle Peninsula has resulted in a significant revision of the Mio-Pliocene stratigraphy and structure: three new formations (Pali River Conglomerate, Bergberg Formation and Arabam Diorite) and six new structural units (Baining Mountain Horst and Graben Zone, Wide Bay Fault System, Nengmutka Caldera, Keravat Caldera, Sikut Caldera, and Warangoi Trend) are defined. Three formations (Yalam Limestone, Nengmutka Volcanics and Sinewit Formation) are redefined. The first obvious influence of the Baining Mountain Horstand Graben Zone, which runs centrally through the Gazelle Peninsiula, postdates an orogenic event at the Oligocene-Miocene boundary. By the mid-Miocene, emergent caldera centres were localized along this extensional zone, and the products of the volcanic centres, the Nengmutka Volcanics, were distributed over a 600 km2 elongate area. Extensional structures of the Gazelle Peninsula can be traced to the north and northwest towards New Ireland and beyond where similar structures form north-trending submarine ridges, which include the alkaline volcanic islands of the Tabar and Lihir Groups. The Baining Mountain Horst and Graben Zone is the westernmost portion of a larger extensional zone, which includes the St George's Channel trough, a graben separating the Gazelle Peninsula and New Ireland, and the southern New Ireland area, an emergent horst block. These extensional structures appear to be an upper-plate stress response related to an abrupt 60° flexure of the New Britain trench. The extensional structures are radially arranged with respect to the flexure of the trench. The distribution of seismic activity and Late Cainozoic volcanic centres also relates directly to the flexure of the New Britain trench. Relatively shallow calc-alkaline volcanic centres define a calc-alkaline province sympathetically flanking the trench; in turn, an outer, deeper alkaline province is recognized. This province includes the problematical Tabar-Feni chain of alkaline volcanic centres, which lie on major fractures radial to the New Britain trench, which have allowed direct tapping of the low-velocity zone of the mantle.