Southwest Pacific Absolute Plate Kinematic Reconstruction Reveals Major Cenozoic Tonga-Kermadec Slab Dragging

Tectonic plates subducting at trenches having strikes oblique to the absolute subducting plate motion undergo trench-parallel slab motion through the mantle, recently defined as a form of slab dragging. We investigate here long-term slab-dragging components of the Tonga-Kermadec subduction system driven by absolute Pacific plate motion. To this end we develop a kinematic restoration of Tonga-Kermadec Trench motion placed in a mantle reference frame and compare it to tomographically imaged slabs in the mantle. Estimating Tonga-Kermadec subduction initiation is challenging because another (New Caledonia) subduction zone existed during the Paleogene between the Australia and Pacific plates. We test partitioning of plate convergence across the Paleogene New Caledonia and Tonga-Kermadec subduction zones against resulting mantle structure and show that most, if not all, Tonga-Kermadec subduction occurred after ca. 30Ma. Since then, Tonga-Kermadec subduction has accommodated 1,700 to 3,500km of subduction along the southern and northern ends of the trench, respectively. When placed in a mantle reference frame, the predominantly westward directed subduction evolved while the Tonga-Kermadec Trench underwent 1,200km of northward absolute motion. We infer that the entire Tonga-Kermadec slab was laterally transported through the mantle over 1,200km. Such slab dragging by the Pacific plate may explain observed deep-slab deformation and may also have significant effects on surface tectonics, both resulting from the resistance to slab dragging by the viscous mantle. Plain Language Summary In studying subduction zones, where one plate dives below another back into the mantle, we tend to focus on the relative motion between the two plates, normally by assuming that the upper plate is fixed relative to the underlying mantle. Plate reconstructions in an absolute mantle frame of reference show, however, that trenches often migrate parallel to their trend relative to the mantle. In this paper, we study the southwestern Pacific Tonga-Kermadec Trench and demonstrate (1) that it is likely much younger than often assumed, and accommodated most if not all of its subduction after 30 million years ago and (2) that even in this short time period, it moved as much as 1,200km northward relative to the mantle, parallel to its strike. By comparing this history with the subducted plate that is seismically imaged in the mantle, we show that the subducted Pacific lithosphere underwent major northward slab dragging through the mantle. This dragging explains enigmatic deep earthquakes and seismological observations of mantle structure. Realizing the role of slab dragging may provide novel constraints for deep-time geological reconstructions as well as natural hazards assessment. Key Points