The structure of 0‐ to 0.2‐m.y.‐old oceanic crust at 9°N on the East Pacific Rise from expanded spread profiles

Abstract

We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0–0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. Velocity‐depth models for these profiles have been obtained by travel time inversion in the τ‐p domain, and by x‐t forward modeling using the WKBJ and the reflectivity methods. We observe refracted arrivals that allow us to determine directly the uppermost crustal velocity structure (layer 2A). At the seafloor we find very low Vp and VS/Vp values around 2.2 km/s and ≤ 0.43. In the topmost 100–200 m of the crust, Vp remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ∼500 m below the seafloor. High attenuation values (Qp < 100) are suggested in the topmost ∼500 m of the crust. The layer 2–3 transition probably occurs within the dike unit, a few hundred meters above the dike‐gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on‐axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where Vp ≈ 3 km/s and VS = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, Vp decreases from ∼6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. Away from the axis, the transition to the LVZ is smoother, the top of the LVZ is deeper, and the LVZ is less pronounced. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1‐km‐thick Moho transition zone.