Abstract
The origin of Samoan volcanism in the southwest Pacific remains enigmatic. Whether mantle melting is solely caused by a mantle plume is questionable because some volcanism, here referred to as non-hotspot volcanism, defies the plume model and its linear age-progression trend. Indeed, non-hotspot volcanism occurred as far as 740 km west of the predicted Samoan hotspot after 5 Ma. Here we use fully-dynamic laboratory subduction models and a tectonic reconstruction to show that the nearby Tonga-Kermadec-Hikurangi (TKH) subduction zone induces a broad mantle upwelling around the northern slab edge that coincides with the non-hotspot volcanic activity after 5 Ma. Using published potential mantle temperatures for the ambient mantle and Samoan mantle plume, we find that two geodynamic processes can explain mantle melting responsible for intraplate volcanism in the Samoan region. We propose that before 5 Ma, the volcanism is consistent with the plume model, whereas afterwards non-hotspot volcanism resulted from interaction between the Subduction-Induced Mantle Upwelling (SIMU) and Samoan mantle plume material that propagated west from the hotspot due to the toroidal component of slab rollback-induced mantle flow. In this geodynamic scenario, the SIMU drives decompression melting in the westward-swept plume material, thus producing the non-hotpot volcanism.
Highlights
The origin of Samoan volcanism in the southwest Pacific remains enigmatic
Out of 10 subduction experiments that were performed, we report here the results of one model (Figs 3, 4 and Table S1) that best reproduces the kinematics of trench motion as determined by our tectonic reconstruction, and best approximates the present-day slab geometry in the upper mantle
The correlation starts to be less clear at 5 Ma (Fig. 5b) and the Subduction-Induced Mantle Upwelling (SIMU) is of low magnitude for older times due to very slow slab rollback (Fig. 1d), which compares with the slow trench retreat observed during phase 2 of our model (Fig. 5c). These results strongly suggest that the SIMU around the northern slab edge of the TKH subduction zone contributes to causing the intraplate volcanism in the Samoan region after 5 Ma, in particular the rejuvenated volcanism occurring west of the predicted hotspot
Summary
The origin of Samoan volcanism in the southwest Pacific remains enigmatic. Whether mantle melting is solely caused by a mantle plume is questionable because some volcanism, here referred to as nonhotspot volcanism, defies the plume model and its linear age-progression trend. Tomography models provide images of a low-velocity seismic structure in the mantle that could represent a plume originating from the core-mantle boundary below the Samoan islands[1,2,3] Such a deep mantle plume could produce a localised hotspot from which a volcano periodically develops at the surface of the moving Pacific lithosphere, thereby producing a linear, time-progressive, westerly ageing volcanic chain[4]. A particularity of the Samoan volcanism is that in general the volcanic rocks of rejuvenated origin have a distinctive isotopic geochemistry when compared with the shield lavas[9,11] This was an additional source of motivation that led to the proposal of alternative mechanisms to explain the abnormal volcanic rejuvenation throughout the chain. A first examination of our tectonic reconstruction (see Methods) shows a phase of slab rollback starting at 10 Ma and accelerating at 5 Ma, in remarkable concomitance with the rejuvenation of volcanism throughout the volcanic chain after 5 Ma (Fig. 1c,d)
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