Plate reconstructions and tomography reveal a fossil lower mantle slab below the Tasman Sea

Abstract

The Southwest Pacific region is tectonically complex and is home to numerous fossil and active subduction zones. At the Earth's surface, there remains a geological controversy regarding the polarity and continuity of fossil subduction zones in New Zealand and New Caledonia, the origin of obducted ophiolites, the presence of high-pressure metamorphism, the occurrence of widespread Cenozoic magmatism, and the potential disappearance of one or more ocean basins. This controversy can be solved by looking at the lower mantle rather then at the Earth's surface. New P-wave and S-wave mantle tomography models from the Southwest Pacific are presented, which identify a previously unrecognized lower-mantle high-velocity anomaly that cannot be linked to Pacific subduction. The anomaly is located below the Tasman Sea at~1100 km depth, strikes NW-SE and is~2200 km by 600-900 km in lateral extent. By combining relative and absolute plate motions it is demonstrated that when the geological structures at the surface are reinterpreted as a single northeast-dipping 2500-km middle Cenozoic subduction zone (the so-called New Caledonia subduction zone) the lower mantle anomaly can be accounted for, as it is found at the predicted location and depth. Discovery of the lower mantle slab anomaly thereby solves a long-standing geological controversy in the New Zealand-New Caledonia region. Finally, reconstructions and analytical calculations predict a lower mantle slab sinking velocity of~1.5 cm/yr and a lower mantle viscosity of~10 22 Pa•s.

Mantle structure and tectonic history of SE Asia

“…Several authors (e.g. Chertova et al, 2014;Schellart and Spakman, 2012;Schellart et al, 2009;van Benthem et al, 2013; have used model UU-P07 in studies of regional mantle structure. It is the successor to the BSE model of Bijwaard et al (1998) and is based on a similar treatment of travel-time data, mantle cell division and inversion approach.…”

Mantle constraints on the plate tectonic evolution of the Tonga–Kermadec–Hikurangi subduction zone and the South Fiji Basin region

“…Important constraints on the evolution of the convergent margins in the region also include relative and absolute motions of the Australian plate, Pacific plate and microplates, the timing, direction and origin of obducted ophiolite sheets in New Caledonia, Northern New Zealand and New Guinea, as well as the age, location and polarity of magmatic arcs in the region. Another important constraint is that provided by seismic tomography of mantle structure, which has recently been applied to unravel the tectonic evolution of the New Caledonia-Three Kings Ridge-Northland region (Schellart et al 2009). Mantle tomography gives insight into the present-day mantle structure, in which high-velocity anomalies are usually interpreted as subducted slab remnants (e.g.…”

Towards absolute plate motions constrained by lower-mantle slab remnants

“…cold remnants of subducted lithosphere [8][9][10][11][12][13][14][15][16] , and that slabs, once detached, sink more or less vertically in the mantle [8][9][10]18 marking the location of their former subduction zones. Geological evidence for former subduction is constituted by orogens created in plate convergence zones, often comprising the remnants of a volcanic arc.…”

“…Absolute plate motion models have often been based on assumed hotspot fixity and are well constrained only up to the Cretaceous period owing to the lack of any preserved older oceanic hotspot tracks 1,2 . These, and other models, offer no control on absolute palaeolongitude before the Cretaceous.Seismic tomography studies of the mantle have allowed for increasingly detailed correlations between deep mantle structure, mostly focused on presumed remnants of subducted plates and plate tectonic evolution [8][9][10][11][12][13][14][15][16] . This, however, has not led to strong constraints on absolute plate motion.…”