A Push in the Right Direction: Exploring the Role of Zealandia Collision in Eocene Pacific-Australia Plate Motion Changes

The Pacific Plate underwent a significant change in motion during the early Eocene. This change has been linked to plate boundary reconfiguration, particularly in relation to subduction margins. The reconfiguration also resulted in a new Pacific-Australian plate boundary section transecting Zealandia. Following the Eocene transition, the relative rotation axis was located within continental Zealandia, and it has been hypothesized that this region acted as a pivot point. Here we investigate the extent to which collision resistance along the intra-continental Zealandia margin (length ∼1,000 km) might have impacted the motion of the Pacific Plate, which is characterized by trench lengths more than an order of magnitude greater. We first highlight the relatively large radial component in the Pacific Plate absolute rotation during the period ca. 47 and 32 Ma (i.e., the spin around the plate centroid axis). We then consider how parameterized plate boundary forces impact the tangential and radial components of the net torque (i.e., the fictitious and true torque components). We show that during this period, both the Zealandia and Izu-Bonin-Marianas (IBM) margins of the Pacific Plate were well-oriented in terms of partitioning boundary normal forces into counter-clockwise (CCW) radial torques. This analysis is supported by results from recent global-scale numerical models. The role of Zealandia cannot be established unambiguously, based on our analysis, but effects can be quantified under different assumptions. Collision resistance along the Zealandia margin could plausibly constitute a “first order” effect on Eocene Pacific Plate rotation, albeit only on the radial component.