3-DIMENSIONAL MANTLE CONVECTION BENEATH A SEGMENTED SPREADING CENTER - IMPLICATIONS FOR ALONG-AXIS VARIATIONS IN CRUSTAL THICKNESS AND GRAVITY

Segmentation and along-axis variations within individual segments indi cate the inherently three-dimensional nature of mantle upwelling and m elting beneath oceanic spreading centers. Numerical convection experim ents am used to explore the effects of local buoyancy forces on upwell ing and melt production beneath a segmented spreading center. The expe riments are conducted in a region consisting of a thermally defined ri gid lithosphere and a uniform viscosity asthenosphere overlying a high er-viscosity mantle half-space. A periodic plate boundary geometry is imposed consisting of spreading segments and transform offset. Buoyanc y forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt. The relative magnitu des of the buoyant and plate-driven components of mantle flow are cont rolled by the spreading rate and mantle viscosity, with buoyant flow m ore important at lower spreading rates and viscosities. Buoyant flow b eneath the spreading axis amplifies along-axis variations in upwelling near a ridge-transform intersection, and distributes the variations a long the entire spreading axis. Buoyant flow may thus be responsible f or the more three-dimensional character of slow spreading centers. Awa y from the spreading axis, thermal buoyancy drives convective rolls th at align with the direction of plate motion and which have an along-ax is wavelength controlled by the prescribed thickness of the asthenosph ere. However, the position and stability of rolls are influenced by th e segmentation geometry. In cases where the spreading center geometry does not allow a stable configuration of rolls, the flow is time-depen dent. Along-axis variations in upwelling cause variations in melt prod uction, which, imply large variations in crustal thickness that domina te the surface gravity signal. The crustal thickness distributions imp lied by these numerical experiments produce bulls-eye-shaped negative mantle Bouguer anomalies centered over spreading segments, as observed at several spreading centers. The amplitude of the anomaly increases with decreasing spreading rate.