The Late Neogene plate motion of North America as a consequence of the rotation of the Pacific plate: insights from modelling

Abstract

High-resolution plate motion reconstructions enable us to observe rapid plate motion changes on a geological time scale.The North American plate has changed its absolute motion between 8 and 5 million years ago (Iaffaldano and DeMets, 2016). The aim of this study is to explain this event through a reconstruction of torques associated with forces acting on the plate boundary. Two hypotheses are tested. The first hypothesis considers the motion change of the Pacific plate as the cause of the North American plate motion change. The clockwise rotation of the neighbouring Pacific plate occurred simultaneously with the motion change of North America. In the second hypothesis it is proposed that the motion change is a result of the plate boundary transformation of a diffuse to a strike slip margin in California due to the opening of the Gulf of California. In the Late Neogene the boundary in California migrated and a diffuse boundary developed to a strike slip fault. For both hypotheses the torque that the Pacific plate exerts on the North American plate is calculated. This is done for before and after the absolute motion change of North America. A numerical model of the Pacific-North America plate boundary is used. The torques are based on the strength of the lithosphere and the relative motion between the plates. The torque difference is then used to calculate the induced Euler vector change of North America. For the first hypothesis the modelled motion change of North America does not correspond with the observations from plate reconstructions. The results do not support the Pacific plate rotation as a cause of the North American motion change. The hypothesis of the tectonic evolution in California as a cause is a more plausible explanation. The direction and magnitude of the modelled Euler vector change of North America match the observations within the 68 percent confidence interval. Although some uncertainties remain, the applied method provides a good insight into the torques behind rapid plate motion changes.