Synthesis of Oceanic Crustal Structure From Two‐Dimensional Seismic Profiles.

We present a new synthesis of oceanic crustal structure from two‐dimensional seismic profiles to explore differences related to spreading rate and age. Primary results are as follows: (1) Layer 2 has an average thickness of 1.84 km but is thicker for young slow‐spreading crust and thinner for young superfast‐spreading crust. At faster‐spreading rates the layer 2/3 boundary likely corresponds to the lithologic boundary between dikes and gabbros. At slow‐spreading centers, the layer 2/3 boundary is interpreted to mark a change in porosity with depth within the dikes. (2) Total crustal thickness averages 6.15 km and is similar across all spreading rates. (3) Velocities at the top of layer 2 increase rapidly from 3.0 km/s at 0 Ma to 4.6 km/s at 10.5 Ma, with a slower increase to 5.0 km/s at 170 Ma. The rapid increase in velocity at young ages is attributed to crack closure by precipitation of hydrothermal alteration products; the increase at older ages suggests that this process persists as the oceanic crust evolves. (4) There is a correlation between velocities at the top of layer 2 and sediment thickness, with velocities of 5.8–5.9 km/s associated with a sediment thickness of 4.0–4.3 km. The thick sediment may collapse large‐scale features such as lava tubes and fractures. (5) Average velocities at the top of layer 3 are lower for young slow‐spreading and intermediate‐spreading oceanic crust (6.1–6.2 km/s) than for older or faster‐spreading oceanic crust (6.5–6.7 km/s). These low velocities are likely associated with faults penetrating into the sheeted dikes. Plain Language Summary: We compile a database from studies of oceanic crust in all ocean basins. Measurements are from crust that is of different ages and formed in different environments. All oceanic crust in our study has similar features, but there are some important differences that are related to age and spreading rate (the rate at which plates spread apart at mid‐ocean ridges). At faster‐spreading rates, the boundary between upper and lower crust is positioned at the boundary between sheeted dikes (which deliver melt from a magma chamber) and gabbros (which form within a magma chamber). At slower‐spreading rates, the boundary is positioned above the magma chamber. Here the boundary must be related to differences in porosity within the dikes which are associated with faulting processes. We also find that thick sediment will change the physical properties at the top of the underlying oceanic crust, likely because lava tubes and large‐scale fractures collapse. Key Points: The layer 2/3 boundary corresponds to the dike‐gabbro boundary at fast and superfast‐spreading ratesThe layer 2/3 boundary marks a change in porosity with depth within the dikes at slow‐spreading ratesVelocities at the top of oceanic crust are correlated with age and sediment thickness [ABSTRACT FROM AUTHOR]