Mapping of seismic layer 2A/2B boundary above the sheeted dike unit at intermediate spreading crust exposed near the Blanco Transform

Abstract

We present results from a study mapping the seismic layer 2A/2B boundary in young ocean crust located adjacent to the north wall of the Blanco Transform Fault (BTF). We review seafloor features, geochemical and petrologic data, seismic imaging, and seismic structure and conclude that the BTF seismic study region is representative of crust formed at the intermediate spreading Juan de Fuca Ridge. The mean layer 2A two‐way travel time beneath the seafloor is 0.37 ± 0.10 s for the 42 × 12 km seismic survey area. Several regions are observed where layer 2A is consistently thin or thick over lateral distances of 5–10 km, both in a ridge‐parallel and ridge‐perpendicular direction. Layer 2A thicknesses appear more variable in the ridge‐parallel (isochron) direction than the ridge‐perpendicular (flow line) direction. There is no systematic pattern of layer 2A thickness variability with distance from the BTF, nor is there a correlation between seafloor topography and layer 2A thickness. Calculated mean layer 2A thickness for the seismic study region is 485 ± 135 m assuming an interval velocity of 2.65 km/s. The imaged layer 2A/2B boundary projects on average ∼600–650 m above the top of the sheeted dike complex mapped on the adjacent BTF north wall. We argue that the geologic context of the layer 2A/2B boundary will vary as competing processes of magmatic construction and fracturing (increasing porosity) and crustal thickening, compaction, dike intrusion, and hydrothermal sealing (decreasing porosity) vary as the crust spreads laterally from the ridge axis to ridge flanks.