Conditions and Consequences of Western Pacific Subduction Initiation at ca. 50 Ma.

Abstract:

With Izu-Bonin-Mariana (IBM) and Tonga-Kermadec (TK) subduction zones initiating at about 50 Ma, there has been discussion on the link with a coeval change in Pacific Plate motion. Although the bend in the Hawaiian-Emperor sea mounts dates to ca 50 Ma, half of the southward motion of the Emperor chain from 80 to 50 Ma is attributed to rapid hot-spot drift. Reconstructions that fit global plate circuits and hot-spot tracks have a prominent change in direction of Pacific Plate at ca 50 Ma. The tectonic conditions and early consequences of IBM and TK initiation are different, with IBM showing rapid forearc extension over about 2 Myr and TK showing proximal and distal compression over about 20 Myr. Why are IBM and TK so different and how are they related to Pacific Plate motions? We use (1) high-resolution, time-dependent models with visco-elasto-plastic rheologies and strike-slip motion and (2) high-resolution global models. We find that strike slip motion enhances subduction initiation, even without any changes in local strike. Strikes slip motion weakens the plate boundary allowing any other force to more easily trigger subduction. From the global models (Hu et al., 2020), it is not possible to trigger a rapid change in direction of Pacific Plate motion with demise of Izanagi-Pacific spreading. However, demise of intra-oceanic subduction in the north Pacific can trigger such a change. We formulated models at 50 Ma and find that the nascent slab developing since 52 Ma along the IBM forearc plays only a small role in changing Pacific Plate model. This is consistent with earlier work showing little change in subducting plate velocity when the slab initially founders during initiation. The models are more consistent with IBM initiation being triggered by changes in relative plate motions. Before 50 Ma, there was a strong component of strike slip motion between Pacific and Philippine Sea Plates, weakening the margin. With constraints on timing revealed through analysis of recent IODP Exp 371 cores, the period of compression evident across northern Zealandia is consistent with TK being induced by this change in Pacific Plate motion.