Asymmetric characteristics of the East Pacific Rise

The East Pacific Rise (EPR) near 17°S is more complex than expected for simple spreading. As detailed by the MELT experiment and other studies, the EPR exhibits several fundamental asymmetric characteristics. The Pacific (west) side subsides slower, has more seamounts, and exhibits less shear-wave splitting than the Nazca (east) side. The S-wave arrivals are later and Rayleigh wave phase velocities are slower to the west. Also, the electrical conductivity has an abrupt transition at the rise axis, with higher resistivity to the east, suggesting depleted mantle containing very little melt. To investigate causes of the observed asymmetry, we constructed 2-D finite element models of mantle flow beneath a mid-ocean ridge.

The Pacific superswell mantle anomaly, located ~1500 km to the west likely affects the upwelling and melt production at the EPR. For instance, the superswell may feed hot asthenospheric mantle to the spreading center. A thermal anomaly west of the rise axis could create more melt on the Pacific side, thereby producing some of the observed asymmetry. Another important factor is pressure-driven mantle flow away from the hotspots, which alters the upwelling pattern and confines melting to predominantly one side. Pressure-driven flow tends to follow the base of the overlying lid (lithosphere). Since the lithosphere thins towards the axis, flow from the west has more upwelling on the western ("upwind") side, enhancing the melt production. In contrast, flow across the axis to the east will encounter an increasingly thick lithosphere forcing a downward component to the flow, decreasing the amount of upwelling and melting on the eastern side of the axis.