The thermal and compositional structure of the upper mantle beneath the North American continent is investigated using a joint inversion of seismic velocities and density perturbations. The velocity data consist of a new regional shear wave velocity model of North America and the Caribbean region obtained by surface wave tomography. The density data are estimated using a relative density-to-shear velocity scaling factor computed for continents by combining regionally filtered seismic and gravity data. We express the mineralogical variations in the mantle in terms of the global volumic fraction of iron, the parameter which has the strongest influence on density and velocity. The inferred thermal and iron content anomalies are well constrained by the data and show an age dependence down to a depth of 230 ¿ 50 km. Below the North American craton, the mantle is colder than average and depleted in iron. Maximum values are found at 100 km with $overline{delta T}$ = -440 K and $overline{delta{rm Fe}}$ = -4%, relative to the average mantle. These chemical and thermal characteristics induce opposite buoyancy forces which could explain the longevity of cratonic lithosphere. In stable continental areas, the signal is of lower amplitudes ($overline{delta T}$ = -280 K and $overline{delta{rm Fe}}$ = -2.5% at 100 km). Beneath the western Cordillera, a tectonically active region, we see no significant thermal or chemical anomaly.