Cooling history of the Pacific lithosphere

Plate tectonics is expressed most simply in oceanic plates where a thermal boundary layer or lithosphere forms as the plate cools during its journey away from mid-ocean ridges. Based on a seismic model of the Pacific upper mantle inferred from a new compilation of seismic surface wave dispersion measurements, we show that, on average, the Pacific lithosphere has experienced a punctuated cooling history, cooling diffusively for its first 70 Ma and then reheating in the Central Pacific between ages of 70 and 100 Ma predominantly at depths between 70 and 150 km. From 100 Ma to about 135 Ma, the processes of reheating are substantially weaker than in the Central Pacific, on average, and the lithosphere undergoes a second phase of diffusive cooling. The cause of the reheating in the Central Pacific remains unclear, although thermal plumes at a number of length scales are probably important, particularly to modulate thermal boundary layer instabilities (TBI). We argue, however, that TBI forms naturally as the plate cools. We present the results of simulations that show that, with the right rheology, TBI can explain the average characteristics of the observed cooling history of the Pacific plate. To explain the variability within age ranges observed across the Pacific, however, may require additional physical processes not included in our simulations, such as variations in the conditions of formation of the lithosphere or the effects of deeper seated thermal plumes.