Oceanic transform faults are seismically and tectonically active plate boundaries1 that leave scars—known as fracture zones—on oceanic plates that can cross entire ocean basins2. Current descriptions of plate tectonics assume transform faults to be conservative two-dimensional strike–slip boundaries1,3, at which lithosphere is neither created nor destroyed and along which the lithosphere cools and deepens as a function of the age of the plate4. However, a recent compilation of high-resolution multibeam bathymetric data from 41 oceanic transform faults and their associated fracture zones that covers all possible spreading rates shows that this assumption is incorrect. Here we show that the seafloor along transform faults is systemically deeper (by up to 1.6 kilometres) than their associated fracture zones, in contrast to expectations based on plate-cooling arguments. Accretion at intersections between oceanic ridges and transform faults seems to be strongly asymmetric: the outside corners of the intersections show shallower relief and more extensive magmatism, whereas the inside corners have deep nodal basins and seem to be magmatically starved. Three-dimensional viscoplastic numerical models show that plastic-shear failure within the deformation zone around the transform fault results in the plate boundary experiencing increasingly oblique shear at increasing depths below the seafloor. This results in extension around the inside corner, which thins the crust and lithosphere at the transform fault and is linked to deepening of the seafloor along the transform fault. Bathymetric data suggest that the thinned transform-fault crust is augmented by a second stage of magmatism as the transform fault intersects the opposing ridge axis. This makes accretion at transform-fault systems a two-stage process, fundamentally different from accretion elsewhere along mid-ocean ridges.
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Data are freely available at https://maps.ngdc.noaa.gov/viewers/bathymetry, http://www.godac.jamstec.go.jp/darwin/e and https://www.bsh.de/EN. Bathymetric grids compiled from these sources are shown in Supplementary Information and are available at https://doi.pangaea.de/10.1594/PANGAEA.924451. Additional gridded bathymetric data of transform faults are from Romanche49, Chain50 and 5° S51 in the Atlantic Ocean, Guamblin in the Pacific Ocean off Chile52 and Prince Edward in the Indian Ocean53. The geodynamic simulations were computed using the community code ASPECT, version 2.1.0 (https://aspect.geodynamics.org). Source data are provided with this paper.
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Extended data figures and tables
Transform faults and fracture zones in the Pacific and Atlantic oceans (top) and in the Indian Ocean (bottom) were studied. Numbering is as in Supplementary Table 1.
a, Clipperton transform fault at the northern East Pacific Rise. b, Vlamingh transform fault at the southeast Indian Ridge. c, Transform fault at the slow-spreading southern Mid-Atlantic Ridge near 25° 40′ S. d, Marion transform fault at the ultraslow-spreading southwest Indian Ridge. e, Atlantis II transform fault at the southwest Indian Ridge. The colour scale is 1 km deeper for e than for a–d. The right-hand column shows the ridge–transform intersections indicated by the black boxes in the left-hand column (also shown in Fig. 1, but here with actual orientation). TDZ, transform deformation zone; FZ, fracture zone.
The transform-fault system is imposed by a kinematic boundary condition at the surface (z = 0).
The (base-10) logarithmic viscosity structure of the model run shown in Fig. 3 is shown. The difference in plate age across the transform fault results in an asymmetry in strength, which causes the shear zone separating the plates to be oblique at depth. The green lines mark the plate boundary (defined as where vx = 0) at depths of 2 km, 4 km and 6 km. Grey arrows indicate the direction of mantle flow (with intensity proportional to arrow length) projected onto the transform-perpendicular slices. The shallow, low-viscosity region in the inside corner marks the brittle deformation zone.
The vertical strain rates induced by the oblique transform shear are shown. Substantial thinning occurs along the oblique shear zone. Note the high vertical strain rates in the inside-corner region. The green lines and grey arrows are as in Extended Data Fig. 4.
a, Ductile plus brittle thinning factors for the model shown in Fig. 3. b, Profile along the black dashed line in a showing the thickness of the brittle layer (green, right axis) and the cumulative ductile-plus-brittle stretching factors β (black, left axis) for the west-moving (dashed) and east-moving (solid) plate. c, Direction of material flow (arrows) and in-profile horizontal velocity (colour scale) for the same section as in b. The green line marking the base of the brittle zone from b is included for orientation. RTI, ridge–transform intersection.
Lithospheric thinning of a transect perpendicular to the fracture zone (dashed blue line in Fig. 3) is shown. Top, ductile-plus-brittle stretching factors (black line), illustrating enhanced tectonic deformation of the inside corner and reduced extension on the outside corner. The average brittle-plus-ductile stretching factor far from the transform is about 2.8 and reflects the extension related to plate creation at the spreading axes. The differential extension near the transform (inside versus outside corner) also averages to 2.8, showing that this reflects internal deformation of the plates near the transform and does not affect far-field plate motions. The red lines mark the brittle stretching factor, which is concentrated around the transform fault and especially in the inside corner. Bottom, the corresponding thinning within the top 6 km. Black bars mark the area that has been removed by brittle extension. This area is shown in Fig. 4b for a range of model parameters.
a, b, Obliquity of the transform shear zone 10 km below top of the model, as a function of its length for different spreading rates (a) and of age offset for different transform lengths (b). c–e, Depth to plate boundary versus obliquity for different transform lengths, for slip rates of 1 cm yr−1 (c), 3 cm yr−1 (d) and 6 cm yr−1 (e). Source data
Width of transform faults43 versus age offset and spreading rate are shown on the left and right, respectively.
a, Unnamed fracture zone at the western flank of the southern Mid-Atlantic Ridge. b, J-shaped volcanic ridges extending across the Marie Celeste fracture zone (FZ) on the western flank of the Central Indian Ridge. c, Unnamed fracture zone at the northern flank of the southwest Indian Ridge (SWIR), showing volcanism extending across the fault trace. Scale bars are in kilometres.
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Grevemeyer, I., Rüpke, L.H., Morgan, J.P. et al. Extensional tectonics and two-stage crustal accretion at oceanic transform faults. Nature 591, 402–407 (2021). https://doi.org/10.1038/s41586-021-03278-9