New Hebrides-New Britain

The New Hebrides - New Britain (NHNB) SZI event evolved into the present-day subduction system that includes the New Hebrides, San Cristobal, and New Britain trenches. The Australian plate currently subducts below a former portion of itself, lying to the north-east, that today makes up the North Fiji Basin (e.g. Schellart et al., 2006). These trenches are all currently connected to each other and initiated at similar times, which is why their onset is here attributed to one single event.

It has been suggested that the onset of the NHNB subduction zone, which is related to subduction of the Australian plate below the Pacific plate, originated by a reversal in subduction polarity at around 10 Ma. While some studies favour a time period for the SZI event between 10 and 6 Ma (Chase, 1971; Auzende et al., 1988), others suggest an onset age of between 14-11 Ma (Greene et al., 1994; Schellart et al., 2006). This polarity reversal likely occurred as a result of the collision of the Ontong Java plateau with the Vitiaz trench (e.g. Greene et al., 1994; Holm et al. 2013). The Ontong Java plateau lies on the Pacific plate that was subducting below the Australian plate during the collision, prior to the polarity reversal.

The arc associated with this SZI event builds on the pre-existing arc of the south-dipping subduction of the Pacific plate (before the polarity reversal). Therefore, the oldest ages found in the New Hebrides arc (~25 Ma) are linked to the previous subduction zone (Greene et al., 1994). At around 14-11 Ma the arc activity ceased. New arc volcanism starts again at around 6 Ma, this time related to the New Hebrides subduction (Monjoret et al., 1991; Greene et al., 1994). Similarly, along the New Britain trench, the subduction of the Pacific plate below the Australian plate built an arc (43-26 Ma). Afterwards, a gap in volcanic activity is observed between 20-12 Ma. Finally, arc volcanism associated with the new subduction zone (i.e., after the polarity reversal) started again at around 10-5 Ma (Holm et al., 2013).

Timing of SZI is based on the opening of the North Fiji basin, which is interpreted as a backarc basin behind the New Hebrides subduction. Paleomagnetic data in different locations of the New Hebrides arc display a 28-52° clockwise rotation of the eastern limb of the arc since the Late Miocene in response to the opening of the North Fiji basin (Falvey, 1978; Musgrave and Firth, 1999). Magnetic seafloor anomalies identified in the North Fiji basin constrain the initial phase of basin opening to around 12-8 Ma (Auzende et al. 1988; Pelletier et al. 1993).

In the model of Müller et al. (2016), the New Hebrides - New Britain SZI event occurs at 12 Ma. This subduction zone initiated on the south side of an island arc that had a pre-existing subduction zone along its northern margin (‘Pacific Melanesia subduction zone’ in the model). The subduction zone on the north margin of the island arc, which had a south-dipping polarity (i.e., opposite to that of the New Hebrides - New Britain subduction zone), shutdown at the same time that the New Hebrides - New Britain SZ started (12 Ma). Thus, in this model, the New Hebrides - New Britain SZI can be seen as a subduction polarity inversion. The distance separating these subduction zones was ~200-350 km. In conjunction with this SZI event, the motion of the Solomon Sea plate changed at 12 Ma, but no major plate reorganisations occurred at or immediately before 12 Ma; the motion of the Australian and Pacific plates, for example, remained unchanged during this time.

The New Hebrides and the New Britain slabs are both imaged and cataloged in the Atlas of the Underworld (van der Meer et al., 2018). Both anomalies reach the upper-lower mantle boundary. However, the New Britain slab (575 km deep) seems to be shallower than the New Hebrides slab (675 km deep). Compared to a vote map, we find a consistent fast anomaly until around 800 km depth.