Convective upwelling in the mantle beneath the Gulf of California

Stirrings beneath California A high-resolution Rayleigh-wave seismic tomography study of the uppermost 200 km of mantle beneath the Gulf of California has provided evidence for localized centres of buoyancy-driven upwelling in the underlying mantle. Low shear velocities, similar to those beneath the East Pacific Rise oceanic spreading centre, were found to underlie the entire length of the Gulf, but there are three concentrated locations of anomalously low velocities spaced about 250 km apart. These seismic velocity anomalies may indicate that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle. The Gulf of California is a part of the world's seafloor-spreading system surrounded by enough seismometers to provide sufficiently high horizontal resolution to address the long-standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres. Here, Rayleigh-wave tomography is used to image the shear velocity in the upper 200 kilometres or so of the mantle; the results suggest areas of dynamic upwelling. In the past six million years, Baja California has rifted obliquely apart from North America, opening up the Gulf of California 1 . Between transform faults, seafloor spreading and rifting is well established in several basins. Other than hotspot-dominated Iceland, the Gulf of California is the only part of the world’s seafloor-spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper-mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments. Such resolution is needed to address the long-standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres. Here we use Rayleigh-wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle. Low shear velocities similar to those beneath the East Pacific Rise oceanic spreading centre underlie the entire length of the Gulf, but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart. These anomalies are 40 to 90 kilometres beneath the surface, at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin 2 , 3 . We interpret these seismic velocity anomalies as indicating that partial melting t