Search for: All records

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.

We present an analysis of geophysical data acquired along a transect of 0–62 Ma crust located on the western flank of the Mid‐Atlantic Ridge at 31°S; all crust was formed at the same ridge segment. Crustal thickness, constrained by five wide‐angle profiles, has mean values of 5.6 km at 6.6 and 15.2 Ma, 7.0 km at 30.6 Ma, 5.5 km at 49.2 Ma, and 3.6 km at 61.2 Ma. Crustal thickness is uniform along each ridge‐parallel profile (standard deviations 0.1–0.3 km), indicating uniform along‐axis magmatic accretion over lateral distances of 40–60 km. The crustal structure of 61.2 Ma crust is not only anomalously thin compared to the other profiles but also contains regions with a linear velocity gradient from seafloor to Moho, which suggests that intense fracturing may extend to the base of the thin crust. Abyssal hill root‐mean‐square heights in the study region are 57–142 m and have an inverse correlation with spreading rate. These values are lower than the average root‐mean‐square height of 196 m elsewhere on the southern Mid‐Atlantic Ridge and indicate relatively high mantle temperatures in our study area. Unsedimented or lightly sedimented basement highs are prevalent at all ages; we argue that bottom currents scour the high topography, transporting sediment into adjacent basement lows. All drillsites planned for International Ocean Discovery Program Expeditions 390 and 393 are within 1–10 km of unsedimented or lightly sedimented basement highs, which should facilitate fluid flow and continued geochemical exchange between crust and seafloor.