Impact of crustal evolution on changes of the seismic properties of the uppermost ocean crust

Abstract

One of the most important observations of oceanic crustal evolution is that the uppermost part of the igneous crust, layer 2A, appears to thin and eventually disappear as its seismic velocity doubles from approximately 2.2 to 4.5 km s−1 in the first 40 m.y. The most likely cause is decreasing porosity in extruded basalts that compose the uppermost igneous crust by filling cracks with hydrothermally generated minerals, a by‐product of off‐axis hydrothermal circulation and alteration of basalts. New insights from two recent papers on crustal hydrothermal circulation patterns and modeling of the effects of crack geometry distributions upon seismic velocities provide guidance to relate filling of cracks within the basement to seismic velocity increases. A qualitative model of layer 2A crustal evolution is outlined that satisfies many geophysical and geochemical observations, some seemingly unrelated. Somewhat surprisingly, the crustal age at which layer 2A seismic velocities reach their maximum value is primarily controlled by the integrated permeability of the overlying sediment, which in turn, controls the hydrologic regime necessary for secondary mineral precipitation. Speculative predictions can be used to verify, refine, or invalidate this dynamic model of crustal evolution. Major scientific and technical obstacles must be overcome before a definitive model can be quantified. With such a rigorous model of crustal upper crustal evolution in hand, remote geophysical sensing tools can begin to fulfill the promise of interpreting the physical state, composition, and past history of oceanic crust.