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.