Formation of Oceanic Crust and Its Impact on Thermal Structure at Mid-Ocean Ridge

Mid-ocean ridges, also known as seafloor spreading center, are locations where new oceanic crusts are being created. It is well understood that when two plates move apart, magma beneath them will ascend and cool and solidify into new crust, but things are not so clear and easy as they seem to be. A couple of geophysical and geochemical processes are involved in this intricate interaction between crust and mantle and it is also thought to vary substantially with spreading rate. For fast spreading ridges, two competing models have been proposed to explain the lower crustal accretion. In gabbro glacier model (or conveyor belt model), all the lower crust crystallize at the shallow dike-gabbro boundary and then subside into deeper crust. In sheeted sill model, lower crust is formed by sills emplaced in situ in deep crust. As for slow spreading ridges, heat supply is too slow to support a steady-state magma chamber. Whether this leads to crystallization occurring at depth or at shallow level is unknown. With the geochemical constraints from Oman ophiolite and the Hess Deep, and the seismic observations from ridges spreading at different rate, numerical models can be built to simulate the different thermal states of different models and examine the plausibilities of each model, thus address the fundamental question that how oceanic crust is formed. At the same time, most of the previous thermodynamic models of mid-ocean ridge system only account for mantle flow and thermal structure and exclude the existence of overlying crust, which definitely simplifies the model, but inevitably leaves out the possible influence of crust. If the true mechanism of crustal accretion can be identified and a crustal accretion model can be built, a more integrated mid-ocean ridge model that include both the impacts of mantle and crust can be created and will be helpful in better understanding the origin, properties, structure and behaviors of mid-ocean ridge system. So I propose to develop more realistic high-resolution 3D numerical models of mid-ocean ridges that allows for magmatic accretion of oceanic crust.

Fig1. Two models proposed for lower crustal accretion at fast spreading ridges. (A) Gabbro glacier model; (B) Sheeted sill model.

Fig2. Two models proposed for lower crustal accretion at  slow spreading ridges.