Shallow Melt Migration and the Creation of Crustal Thickness Variations at Mid-Ocean Ridges


DOEI Funded Research: 2008


It is generally assumed that crustal thickness is fairly constant at magmatically robust, fast spreading mid-ocean ridges (MORs), while slower-spreading ridges exhibit crustal thickness variations related to the intermittency of melt supply.  It is also assumed that tectonic extension at slow-spreading MORs, as manifested by normal faulting, is inversely related to melt supply and thus to crustal thickness.  However, this assumption has been challenged by recent drilling results that found thick gabbro sections in the footwalls of major, long-lived normal faults.  Numerical modeling suggests that these long-lived faults (often termed oceanic detachments) form when about 30–50% of plate separation is accommodated by magmatism and that the footwalls can accrete significant volumes of magma.  However, the spatial distribution and volume of magma, and the factors that control the flow of this fluid, are poorly understood.  We have developed numerical modeling techniques that simulate tectonic/magmatic interaction at MORs and allow us to trace magma flow in the upper mantle and shallow crust.  We propose to use these techniques to model tectonic/magmatic interactions for a variety of spreading rates, melt supplies, and shapes/depths of magmatic intrusions.  The results will be interpreted in the context of observed geological/geophysical constraints on crustal thickness variations, and they will provide important new insights into flow and emplacement of magma at MORs.