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 years1. In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations2,3,4, geospeedometry5,6,7 and thermal modelling8,9,10 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 axis2,7 and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust.
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Near-bottom magnetic data are available at https://www.ngdc.noaa.gov/trackline/request/?surveyTypes=All%20Parameters&surveyIds=SENTRY418,SENTRY419,SENTRY420,SENTRY421,SENTRY422,SENTRY423,SENTRY424,SENTRY425,SENTRY428 and bathymetry data at https://www.ngdc.noaa.gov/auvs/sentry/AT37-08_sentry_mb.html. Thermal demagnetization data for samples are archived at https://doi.org/10.7288/V4/MAGIC/17051. High-resolution bathymetry surveys of the East Pacific Rise are from https://doi.org/10.26022/IEDA/329855. Source data are provided with this paper.
Code available upon request from the corresponding author.
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We thank the AUV Sentry and ROV Jason teams, the crew of the RV Atlantis, the science party for data acquisition, and M. Gess, S. Slead and M. Mijjum for their help with sample processing. Support for this work was provided through NSF grant numbers OCE-1459387 (J.S.G.) and OCE-1459462 (M.J.C. and B.E.J.).
The authors declare no competing interests.
Peer review information Nature thanks Suzanne Carbotte and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
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Extended data figures and tables
a) Profiles shown in map view near Pito Deep (white box). b) Flowline profiles plotted as a function of distance from the spreading centre (0 km). Gray arrows indicate the locations of larger offset faults. Lower two profiles are from sea surface swath bathymetry; remaining profiles show high-resolution bathymetry from near-bottom survey
The background (grey) shows bathymetry near Pito Deep, and the black box highlights the region shown in Fig. 1. The crossover-corrected anomaly data has a root mean square misfit of 29.7 nT, and was used to generate a sea surface magnetic inversion following the process outlined in Supporting Information in Maher et al.23
The APS 544 miniature orientation sensor is placed flush against the rock face to determine strike and dip. The sample collection basket mounted in front of ROV Jason II is perpendicular to its heading and aids in video estimates of strike when flush with the rock face. Photo credit: copyright Woods Hole Oceanographic Institute, courtesy Mike Cheadle of the University of Wyoming.
Extended Data Fig. 6 Equal area plot showing a representative Monte Carlo distribution (n = 280) illustrating effect of orientation uncertainty on uniform initial remanence direction of 040°/−20°.
Black circles show sample remanence directions with 20° uncertainty in strike and dip. Open (closed) symbols are upper (lower) hemisphere. The red circle with radius of 24° indicates 1 sigma uncertainty about the mean direction.
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Maher, S.M., Gee, J.S., Cheadle, M.J. et al. Three-dimensional magnetic stripes require slow cooling in fast-spread lower ocean crust. Nature 597, 511–515 (2021). https://doi.org/10.1038/s41586-021-03831-6