GEODYNAMIC CONSTRAINTS ON FLOW OF SAMOAN-PLUME MANTLE INTO THE NORTHERN LAU BASIN

The northernmost portion of the Lau Basin features numerous geochemical and geophysical anomalies. These include a strong north-to-south gradient in the trace element and isotopic (Sr-Nd-Pb) enrichment of basalts erupted along the various back-arc spreading centers (BASCs) within the basin, high ³He/⁴He (up to 28 times Ra, the ratio in Earth's atmosphere) in basalts erupted along some portions of the BASCs (e.g., Rochambeau Bank), and the presence of trench parallel shear wave splitting near the Tonga arc. The presence of these anomalies has been attributed to the southward flow of mantle material from the nearby Samoan hotspot into the northern Lau Basin. We present results from a series of numerical geodynamic experiments undertaken to test the viability of this hypothesis and to constrain both the total extent and the spatial distribution of any incursion of Samoan mantle into the Lau Basin.

Mantle flow and melting were modeled within a 2-D domain corresponding to a north-south cross-section through the upper 400 km of the mantle and extending from the Pacific plate north of the Vitiaz Lineament through the northern Lau Basin, using the COMSOL Multiphysics Finite Element Modeling package. Experiments indicate that southward-flowing Samoan mantle would experience extensive melting due to adiabatic decompression as it crosses moves from beneath older, thicker Pacific lithosphere to the younger, thinner lithosphere of the Lau Basin at the Vitiaz Lineament. Experimental predictions of melt production are compared to bathymetric data from the northern Lau Basin to constrain both the physical characteristics (e.g., temperature) and spatial distribution of the flow of Samoan mantle into the Lau Basin.