Mid-Cretaceous tectonic evolution of the Pacific -Phoenix -Farallon triple junction

This dissertation involves interpretation of geophysical data and physical models of plume-ridge fluid dynamics to comment on a major plate boundary reorganization that involved the triple junction joining the Pacific, Phoenix, and Farallon plates during Cretaceous time. The tectonic system examined herein involves a major ridge-ridge-ridge triple junction that was repositioned by hundreds of kilometers contemporaneously with the eruption of a nearby large igneous province named the Manihiki plateau. The proximity in time and space of these two events suggests that the volcanic episode was instrumental in relocation of the triple junction. Furthermore, the origin of the Manihiki plateau has been questioned and hypothesized to be related to volcanism at the Ontong-Java plateau, 2000 km along the Pacific-Phoenix ridge axis to the west. Geophysical observations made at sea provide valuable constraints on possible tectonic reconstructions. High resolution multibeam bathymetry and gravity measurements reveal characteristics of seafloor structure. Structures interpreted from these data provide clues to plate boundary evolution and resulting stress field changes. Data indicate that at least two minor changes in Euler pole location occurred during the opening of the Penrhyn basin. Laboratory experiments suggest that along-axis flow of plume material may not be as robust as indicated by previous numerical models. Instead, plume material may thermally erode the lithosphere into which it rises. This would certainly prevent plume material from traveling 2000 km along axis from the Ontong-Java plateau and erupting to form the Manihiki plateau. A combination of complicated seafloor structures and results from laboratory experiments suggesting thermal modification of the lithosphere by surfacing plumes leads to speculation of a short-lived microplate involved in the relocation of the Pacific-Phoenix-Farallon triple junction. Eruption of the plateau domed the thermally weakened lithosphere and cracked it, creating new plate boundaries. The new boundaries, existing in weaker crust, relieved the far-field stresses associated with plate motion more effectively than the old set of plate boundaries. The triple junction was thus displaced from its location at the Nova-Canton trough deep at 121 Ma., and relocated to the center of the rifting Manihiki plateau 1200 km to the south