Seismic structure of 15 Ma oceanic crust formed at an ultrafast spreading East Pacific Rise: Evidence for kilometer‐scale fracturing from dipping reflectors

Abstract

A grid of seismic reflection data from 15 Ma Cocos Plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading (∼200 mm/yr, full rate) East Pacific Rise (EPR). The data contain an unexpected quantity of bright reflectors throughout the crust, generally <5 km in length. Significant reflection dip angles (∼10–30°) are seen only on the isochron profiles, and several reflectors have a confirmed isochron dip orientation. Certain upper crustal reflectors (UCRs) appear to form a single network with isochron dip. The most prominent example (apparent reflection coefficient ∼0.2) projects toward a ∼70 m disruption in the igneous seafloor, and waveform analysis demonstrates that the reflector is likely the product of a low‐velocity zone on the order of tens of meters thick. Our observations suggest that the isochron‐dipping UCRs represent small‐slip, kilometer‐scale faults/fractures, and none are primarily related to the seismic 2/3 boundary or to the sheeted dike/gabbro transition. Lower crustal reflectors (LCRs) have no preferred orientation or regular pattern on the 30 km scale of this survey. Certain LCRs may be similar to the UCRs in origin, while others may originate from near‐axis ductile flow, lithologic banding, off‐axis magmatism, or some combination thereof. Overall, reflectors in this area are abundant and complex, consistent with previous off‐axis studies, but at odds with observations of young crust (<5 Ma) near the EPR. This dichotomy may be resolved by a delay in reflector formation, through some combination of postaccretionary tectonics, magmatism and hydrothermal alteration.