Tectonic evolution and midplate volcanism in the South Pacific

Abstract

Changes in morphology of the Marquesas Fracture Zone are correlated with small changes in Pacific-Farallon relative motion. The simple flexural signal of a locked fracture zone may be obscured by tectonic effects, and there is no evidence for the release of shear stress on the fracture zone by vertical slip after leaving the active transform. One such small change in plate motion is documented in the South ern Austral Island region of the South Pacific. A twelve degree clock wise change in Pacific-Farallon relative motion occurred around fifty million years ago. This Eocene change in spreading direction and rate is locally constrained with observations of magnetic anomalies and spreading fabric orientation. At the southeastern end of the Cook-Austral Island chain, multiple episodes of volcanism have left a diverse population of seamounts. Volume estimates from geophysical data and modeling show that one-half to two-thirds of the volcanic material is over thirty million years old, while the remainder is less than five million years old. Seismic and bathymetric data imply the presence of abyssal basalt flows in the flexural moat of the Austral Islands, probably associated with Austral Islands volcanism, which may contribute a significant amount of material to the archipelagic apron.