Buoyant Decompression Melting
Spreading center segmentation and interplate volcanism due to buoyant decompression melting: Divergent plate boundaries, where tectonic plates spread apart, are the site of mantle upwelling and melting which creates the basaltic crust of the Earth's ocean basins. Since the melting temperature of mantle rock increases with pressure, melting occurs if mantle upwells too rapidly to allow cooling by heat conduction. This is the case even at spreading rates as slow as ~ 10 km/Myr. For a plate boundary that is continuous along strike, uniform upwelling due to plate spreading should create melt, and therefore crust, uniformly along the spreading axis. However in global gravity images like those shown in Figure 1, seafloor generated at slow spreading rates show regularly spaced lineations that can be interpreted as crustal thickness variations resulting from along axis variations in upwelling and melting. Along-axis crustal thickness variations created at the spreading axis are carried off the axis resulting in seafloor lineated in the spreading direction. In constrast, fast spreading results in very uniform crustal thickness along long segments of spreading axis between large offset transform faults.
Brown faculty collaborators:Don Forsyth
Other project collaborators:See list of publications above.
Figure 1: Sea surface gravity field above divergent plate boundaries spreading at different rates: (upper left) Southwest Indian Ridge ~ 8 km/Myr; (upper right) southern Mid-Atlantic Ridge ~ 18 km/Myr; (lower right) Pacific-Antarctic Ridge ~37 km/Myr. Gravity variations are a reflection of seafloor topography. From Phipps Morgan and Parmentier (1995).
Figure 2: Three-dimensional numerical model of upwelling beneath surface plates spreading at a rate of 12 km/Myr. Large white arrow shows plate spreading direction and black dashed line shows spreading axis. This model is symmetric about the spreading axis. Color shading, black contours and white contours indicate the amount of melt present, the rate of melt production, and isotherms, respectively. Solid-state rheology includes both temperature dependence and the weakening effect of intragranular water on creep rate of mantle minerals (see Braun, et al., 2000; Choblet and Parmentier, 2001).
Figure 3: Summary of numerical experiments for a range of spreading rate and mantle viscosity from Parmentier and Phipps Morgan (1990). Convective instability like that shown in Figure 2 occurs for sufficient low spreading rate and mantle viscosity.Back to Marc Parmentier's Brown Research Profile