Studies of melt migration and crustal accretion at spreading centers based on seismic imaging and models of mantle flow

Two fundamentally different approaches, numerical simulation of mantle flow and seismic data processing, are applied to address three fundamental geological questions based on key geophysical observations at spreading centers: How can crust form over a narrow region at a ridge? Are there any relationships among spreading rate, mantle temperature and crustal thickness? What is the shape and location of magma chambers?; Active flow in the mantle beneath mid-ocean ridges driven by thermal, melt and compositional buoyancy is investigated using numerical methods to simulate mantle flow, melting, and melt migration. The calculations employ a two-dimensional Cartesian geometry with flow confined to a vertical plane parallel to the plate motion. This numerical model can be used to predict the distribution of melt in the subridge mantle, the width over which most melt is delivered to the crust, and the thickness of crust. Calculations using a range of parameters show that buoyancy effects can lead to a region of mantle upwelling and melting that is as narrow as the observed zone of oceanic crustal accretion.; A series of numerical experiments based on this model demonstrate that crust formed at slow spreading rates is more sensitive to perturbations in mantle temperature than crust formed at fast rates, so that the range of calculated thicknesses is much greater for crust formed at slower rates. The predicted behavior is caused by the interplay between mantle flow driven by plate separation and that driven by thermal, compositional and melt related buoyancy. The results are broadly consistent with global compilations of oceanic crustal thickness that show larger variation in crustal thickness at slower spreading rates than at faster rates.; Conventional seismic processing techniques are based on assuming the earth to be composed of plane horizontal layers. Very often the most interesting objects we wish to image, are complex structures lying beneath a rough irregular interface which include steep dips and strong lateral velocity contrasts. Wave-equation datuming (WED) originally derived by Berryhill (1979, 1984) using the Kirchhoff integral solution to the scalar wave equation, is effective in correcting distorting effects on images caused by rugged interfaces. Using the concept of superposition of "plane" waves a new approach to WED is described based on the well-known Radon transform. This algorithm is accurate for cases of complex water-bottom topography, and much more efficient than the Kirchhoff integral method. The method is derived for the two dimensional case but is easily extendable to three dimensions.; Data from mid-ocean ridges (MORs) in the Pacific and Atlantic, and from the continental margin setting have been processed to demonstrate the application of the method. One result that is demonstrated, is that the axial magma chamber (AMC) beneath the very fast spreading (Southern East Pacific Rise) displays a top that is narrow (500 m-950 m wide) and with no spreading rate dependence, and has an averaged depth of the shallowest oceanic magma reservoir (1150 m) imaged to date and correlated with spreading rate. These results support many important geophysical constraints at ridges, for example upwelling melt is concentrated at ridge axis within an narrow zone of volcanism, and show interesting magma distribution patterns under different circumstances.