North Sepik region of Papua New Guinea: palaeomagnetic constraints on arc accretion and deformation

Abstract

Palaeomagnetic results from the Bewani–Torricelli Arc complex of the North Sepik region, Papua New Guinea, provide new information about latitudinal movement of the region relative to the Australian craton during the Cenozoic and about rotational deformation following accretion of the arc to the craton's northern margin. Thermal demagnetization of more than 200 samples (10 sites) from the Bliri Volcanics complex of Palaeocene to Early Miocene age and from Late Oligocene to Middle Miocene cover sediments show: (i) a primary magnetization component in Plio-Pleistocene pelagic carbonate blocks within the Bliri Volcanics complex, acquired at the equator and indicating southward transport of oceanic lithosphere to its present location at 3.4°S; (ii) a predominant overprint in tuffs and volcanics from the Bliri Volcanics complex acquired at about 15°S, probably contemporaneous with Early–Middle Miocene accretion of the Bewani–Torricelli Arc onto the northern margin of the Australian craton; (iii) a probably primary magnetization component in pre-Late Oligocene Bliri Volcanics of the Tring Block, immediately east of the main complex, acquired at about 30°S; (iv) a Late Oligocene primary magnetization, a post-Middle Miocene post-accretion overprint, and a recent overprint of mainly normal polarity in sediments of the cover sequence, providing further latitudinal control on evolution of the accretionary margin; (v) Large-scale counterclockwise rotations of 30°+ to 110°+ relative to the Australian craton of both the Bliri Volcanics complex and the cover sediments of the main body of the Bewani–Torricelli Arc, and contrasting clockwise rotations of 100°+ to 170°+ of the structurally detached Tring Block. These rotations are attributed to sinistral movements along the Tonga–Sulawesi megashear and in particular along the landward continuation of the Bismarck Fracture Zone. The latitudinal evolution of the Bewani–Torricelli Arc shows some similarity with latitude observations from the Baining Arc (Finisterre–Huon–New Ireland–New Britain), and indicates northward movement from about 30°S in the Late Eocene to about 15°S in the Early to Middle Miocene. This suggests that both arcs may represent parts of a larger east–west oriented arc complex, which was possibly located on the Pacific plate prior to accretion.