Three-dimensional magnetic stripes require slow cooling in fast-spread lower ocean crust

Earth's magnetic field is recorded as oceanic crust cools, generating lineated magnetic anomalies that provide the pattern of polarity reversals for the past 160 million years(1). In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations(2-4), geospeedometry(5-7) and thermal modelling(8-10) of fast-spread crust yield conflicting interpretations of where and how heat is lost near the ridge, a sensitive indicator of processes of melt transport and crystallization within the crust. Here we show that the magnetic structure of magmatically robust fast-spread crust requires that crustal temperatures near the dike-gabbro transition remain at approximately 500 degrees Celsius for 0.1 million years. Near-bottom magnetization solutions over two areas, separated by approximately 8 kilometres, highlight subhorizontal polarity boundaries within 200 metres of the dike-gabbro transition that extend 7-8 kilometres off-axis. Oriented samples with multiple polarity components provide direct confirmation of a corresponding horizontal polarity boundary across an area approximately one kilometre wide, and indicate slow cooling over three polarity intervals. Our results are incompatible with deep hydrothermal cooling within a few kilometres of the axis(2,7) and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust. A record of Earth's magnetic field constructed from near-bottom magnetization observations and oriented samples provides three-dimensional imaging of magnetic stripes in fast-spread crust, and suggests slow cooling off-axis, as opposed to deep hydrothermal cooling close to the spreading ridge.

Automated summary

The study focuses on the magnetic structure of fast-spread crust, indicating the existence of subhorizontal polarity boundaries near the dike-gabbro transition. The results suggest a broad, hot axial zone in magmatically robust fast-spread ocean crust.