JOINT INVERSION OF SHEAR-WAVE TRAVEL TIME RESIDUALS AND GEOID AND DEPTH ANOMALIES FOR LONG-WAVELENGTH VARIATIONS IN UPPER MANTLE TEMPERATURE AND COMPOSITION ALONG THE MID-ATLANTIC RIDGE

We report measurements of SS-S differential travel time residuals for nearly 500 paths crossing the northern Mid-Atlantic Ridge. Differential travel times of such phases as SS and S with identical source and receiver have the advantage that residuals are likely to be dominated by contributions from the upper mantle near the surface bounce point of the reflected phase (SS). Under this assumption, differential SS-S travel time residuals are mapped at the SS bounce points as a means of delineating lateral variations in mantle structure. After removing the signature of lithosphere age, we find evidence for variations in SS-S residuals along the ridge at wavelengths of 1000-7000 km. These travel time anomalies correlate qualitatively with along-axis variations in bathymetry and geoid height. We formulate a joint inversion of travel time residual, geoid height, and bathymetry under the assumption that all arise from variations in upper mantle temperature or bulk composition (parameterized in terms of Mg #). The inversion employs geoid and topography kernels which depend on the mantle viscosity structure. Inversion for thermal perturbations alone provides good fits to travel time and geoid data. The fit to topography, which is likely dominated by unmodeled crustal thickness variations, is not as good. The inversions for temperature favor the presence of a thin low-viscosity layer in the upper mantle and temperature perturbations concentrated at depths less than 300 km. Compositional variations alone are unable to match the travel time and geoid or bathymetry data simultaneously. A joint inversion for temperature and composition provides good fits to both geoid and travel time anomalies. Temperature variations are +/- 50 K and compositional variations are +/- 0.5-3% Mg # for models with the temperature variations uniformly distributed over the upper-most 300 km and the compositional variations either distributed uniformly over the same interval or concentrated at shallower depths. The magnitudes of these variations are consistent with the chemistry and geothermometry of dredged peridotites along the Mid-Atlantic Ridge.