Influence of normal faults and basement topography on ridge-flank hydrothermal fluid circulation

Mid-ocean ridge-flank hydrothermal fluid circulation is drawing increasing attention since about 70 per cent of the advective heat flux through oceanic crust occurs on ridge flanks. Previous numerical studies have largely considered the effect of basement relief but ignored the role of normal faults, although it is well known that normal faults are pervasive in the ridge-flank environment. This paper investigates the importance of normal faults and basement topography in controlling ridge-flank hydrothermal convection. Numerical experiments are conducted using a sophisticated finite-element software package JHU2D. A 2-D conceptual model is constructed based on a variety of in situ geophysical data recently collected around the DSDP/ODP site 504B. Four scenarios are examined in which basement topography, and active and inactive normal faults are included. Numerical simulation results indicate the active normal faults that penetrate through the sediment layer to the seafloor are a key factor governing the sea water circulation and the thermal regime within the oceanic crust. The presence of these faults challenges the previously discovered correlation among the seafloor heatflow, bathymetry and basement relief. It is the spacing between adjacent faults that determines the wavelength of the heatflow variations. Basement relief becomes important only when there are no active faults present. Fluid circulation is thus then confined mainly within the pillow lavas layer and the seafloor heatflow anomalies therefore have the same wavelength as that of the basement topography.