The structural and thermal evolution of upper oceanic crust in the western South Atlantic : insights from seismic velocities and hydrothermal models

The evolution of oceanic crust plays an integral role in global heat flow, geochemical cycles, and in shaping the environmental conditions harboring the crustal biosphere. Because oceanic crust is normally buried beneath several kilometers of water and encompasses a vast area of the Earth’s rigid surface, spatially extensive and coherent geophysical data are difficult to acquire in the oceanic domain. Consequently, our current understanding of the evolution of oceanic crust is based on partially conflicting compilations of data that are acquired at different scales and using different methods. Here I present geophysical constraints from an extensive seismic dataset that continuously covers 0-70 Ma crust in the western South Atlantic. Analysis of regional seismic velocity trends in the upper crust shows a continuous increase in basement velocity to crustal ages of at least 58 Ma. This trend indicates an evolution of upper crustal velocities that lasts significantly longer than measured or predicted by previous studies. The results provide evidence for ongoing hydrothermal circulation in relatively old upper crust, which is consistent with heat flow studies. To further test this concept, I used high-resolution seismic velocity models to estimate detailed porosity and permeability distributions that constrain models of hydrothermal fluid flow at five different crustal ages. The resulting advective and conductive surface heat fluxes are consistent with both predictions of heat flux by lithospheric cooling models and measured conductive heat flux at the seafloor. Additionally, computed hydrothermal volume fluxes largely agree with global estimates for the modeled crustal ages. The models are therefore consistent with a “sealing age” of ~65 Ma, which is also inferred from a compilation of global heat flow measurements at the seafloor. Close to the Rio Grande Rise, an oceanic plateau west of the study area, a fine-scale seismic velocity model reveals multiple large fault zones penetrating at least ~1.5 km into the crust. These faults likely accommodate differential subsidence between thickened, warm oceanic plateau crust and cold oceanic crust. Modeled fluid fluxes are elevated along the interpreted fault zones and across the seafloor. Crust adjacent to oceanic plateaus may exhibit elevated levels of tectonic activity and fluid flow globally. The estimated global volume of fluid entering the ocean in this type of setting amounts to 43 km³, which is ~2% of the hydrothermal flux in the axial region. This potentially has implications for global chemical cycles, the hydration of mature oceanic crust, and the oceanic crustal biosphere.