Basement rocks of the Halmahera region, eastern Indonesia: a Late Cretaceous–early Tertiary arc and fore-arc

Abstract

Halmahera is a K-shaped island located at the junction of several major arc-trench systems of the western Pacific–eastern Indonesia region. Western Halmahera is an active volcanic arc above a zone of intense seismicity which characterizes the north Molucca Sea. Eastern Halmahera has a basement of dismembered ophiolitic rocks with slices of Mesozoic and Eocene sediments overlain unconformably by Middle Oligocene and younger sedimentary and volcanic rocks. The Mesozoic and Eocene sediments reveal notable stratigraphical and petrological similarities to the Marianas fore-arc and the eastern Halmahera Basement Complex is interpreted as a pre-Oligocene fore-arc lacking an accretionary complex. There is some evidence that the pre-Oligocene volcanic arc behind this fore-arc now forms part of the basement of western Halmahera. The Mesozoic and Tertiary sediments were imbricated together with igneous and metamorphic rocks representing the deeper parts of the fore-arc during the Late Eocene plate reorganization event recognized throughout the western Pacific margins. The eastern Halmahera Basement Complex can be traced into eastern Mindanao, probably further northwards in the eastern Philippines and may be related to similar terranes within and around the present Philippine Sea Plate. In contrast, the southern part of the island of Bacan at the southwestern end of the Halmahera group has a basement of continental metamorphic rocks associated with a deformed ophiolitic complex quite different to the basement of eastern Halmahera. The metamorphic rocks are interpreted to be part of the north Australian continental margin basement which is separated from the Halmahera Basement Complex by a splay of the Sorong Fault system and the deformed ophiolite complex of Bacan is suggested to represent magmatism in the fault zone.

Evolution of the boundary between the Philippine Sea Plate and Australia: palaeomagnetic evidence from eastern Indonesia

“…Low Cretaceous palaeolatitudes for the southern part of the Philippine Sea Plate are consistent with the lithological character of the Cretaceous sequences (Hall et al, 1988a). The interpretation of their exact site of magnetisation depends upon the inferred rotation history.…”

“…Fragments of continental crust derived from the Australian margin are found within or adjacent to the Sorong Fault system and include high-grade metamorphic rocks of probable Palaeozoic or greater age and Jurassic sedimentary rocks, representing the cover to the metamorphic basement (Brouwer, 1924;Van Bemmelen, 1949;Hamilton, 1979;Hall et al, 1988aHall et al, , 1991.…”

“…2) has a basement of ophiolitic and arc rocks (Hall, 1987;Hall et al, 1988aHall et al, , b, 1991Hall et al, , 1995bHall and Nichols, 1990;Charlton et al, 1991;Nichols and Hall, 1991). The ophiolites are remnants of a Mesozoic intra-oceanic arc overlain by, and imbricated with, Upper Cretaceous-Eocene arc volcanic and sedimentary rocks.…”

“…The geological information presented here is based on fieldwork with the Indonesian Geological Research and Development Centre (GRDC) carried out in , 1987(Hall, 1987Hall et al, 1988aHall et al, , b, 1991Hall and Nichols, 1990;Nichols and Hall, 1991) and more recent, largely unpublished, results of fieldwork carried out in 1989 and 1990 as part of the Sorong Fault Zone project. A principal aim of this project has been to provide a sound stratigraphic basis, using biostratigraphic and isotopic dating, for a tectonic understanding of the region.…”

Southeast Asia's changing palaeogeography

“…The Sangihe Arc can be traced from Sulawesi northwards into the Philippines; it is constructed on Eocene oceanic crust (Evans et al 1983) and formed near to the Sundaland margin in the Early Cenozoic (Hall 2002). The modern Halmahera Arc is constructed on older arcs, of which the oldest known is an intra-oceanic arc formed in the Pacific in the Mesozoic (Hall et al 1988(Hall et al , 1995b presumably built on older oceanic crust. Before the Eocene the location of the Halmahera Arc is not well known.…”