Mantle structure and tectonic evolution of the region north and east of Australia
R. HALL, W. SPAKMAN, 2003. "Mantle structure and tectonic evolution of the region north and east of Australia", Evolution and Dynamics of the Australian Plate, R.R. Hillis, R.D. Müller
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Tomographic images of the mantle beneath the region extending from the Molucca Sea eastward to Tonga, and from the Australian craton north into the Pacific, reveal a number of distinctive high seismic-velocity anomalies. The anomalies can be interpreted as subducted slabs and the positions of the slabs can be compared to predictions made by tectonic models for the region. Several strong anomalies are due to present-day subduction and the slab lengths and positions are consistent with Neogene subduction at the Tonga and the New Hebrides Trenches, where the anomalies suggest rapid rollback of subduction hinges since about 10 Ma, and beneath the New Britain and Halmahera Arcs. There are several generally flat-lying deeper anomalies which are not related to present subduction. Beneath the Bird's Head and Arafura Sea is an anomaly which we interpret to be the result of north-dipping subduction beneath the Philippines-Halmahera Arc between 45 and 25 Ma. A very large anomaly, which extends from the Papuan peninsula to the New Hebrides and from the Solomon islands to the east Australian margin, is interpreted as the result of south-dipping subduction beneath the Melanesian Arc between 45 and 25 Ma. Our interpretation implies that a flat-lying slab can survive for many tens of millions of years at the bottom of the upper mantle. There is a huge anomaly in the lower mantle which extends from beneath the Gulf of Carpentaria to Papua. We suggest this is a slab subducted before 45 Ma, which may be correlated with a Cretaceous slab beneath the Australian-Antarctic Discordance or an early Cenozoic slab sub-ducted north of Australia. The anomaly is located above the position where there must have been a change in polarity in subduction at the boundary between the north- and south-dipping subduction zones north of Australia between 45 and 25 Ma. All of these have been overridden by Australia since 25 Ma. One subduction system predicted by the tectonic models, the Marumuni Arc of Papua New Guinea, is not seen on the tomographic images.