Long‐Term Evolution of Nontransform Discontinuities at the Mid‐Atlantic Ridge, 24°N–27°30′N

Abstract

We studied long-term evolution of nontransform discontinuities (NTDs) on the Mid-Atlantic Ridge from 0-to~20-to 25-Ma crust using plate reconstructions of multibeam bathymetry, long-range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly (RMBA), and gravity-derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi-stable. Fast, short-term (<2 Myr) propagation is driven by reduced magma supply (increased tectonic extension) in the propagating ridge tip when NTD ridge-axis offsets are small (≲5 km). Propagation at larger offsets generally is slower and longer term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge-axis offset, the evolution of NTDs appears to be controlled by three factors: (1) gross volume and distribution of magma supplied to ridge segments as controlled by 3-D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma-poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long-wavelength (5-10 Myr) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle. Key Points:• Propagation of non-transform discontinuities on the Mid-Atlantic Ridge was analyzed in reconstructions of bathymetric and gravity data • Rapid and slow propagation occurs and is controlled by magma distribution within the mantle and crust of spreading segments • We attribute long-wavelength cross-axis asymmetry in gravity anomalies to asymmetric distribution of density anomalies in the upper mantle Supporting Information:• Supporting Information S1• Movie S1 • Movie S2 Figure 1. (a) Multibeam bathymetry of the study area (within the black dashed lines) superimposed on bathymetry derived from satellite gravity (Sandwell et al., 2014). Spreading segments are labeled from A to J. Nontransform discontinuities (NTDs) and the Mid-Atlantic Ridge (MAR) axis are shown by solid black lines. The labeled white lines are major chrons; their ages, top left, are based on Ogg (2012). The red dashed lines are calculated flowlines of relative plate motion, anchored where NTDs offset the present ridge axis. The white stars locate significant oceanic core complexes. The inset locates the study area in the North Atlantic (KFZ, Kane fracture zone; AFZ, Atlantis fracture zone). (b) Residual mantle Bouguer anomaly (RMBA) within the study area (Wang et al., 2015) superimposed on satellite-derived RMBA from Lin and Zhu (2015); the contour interval is ...

Marine Vertical Gravity Gradients Reveal the Global Distribution and Tectonic Significance of “Seesaw” Ridge Propagation

“…This shows these off‐axis features are isostatically compensated by locally thinned crust in agreement with Zheng et al. (2019)…”

“…However, such data are scarce, particularly away from active ridge axes. Previous studies near the ridge axis include those of, for example: Carbotte et al (1991), Fox et al (1991), and Grindlay et al (1991), who analyzed discontinuities along the South Mid-Atlantic Ridge (MAR); Sempéré et al (1993), along the north MAR; Cochran and Sempéré (1997) and Goff et al (1997) along the Southeast Indian Ridge; and more recently Dannowski et al (2018) and Zheng et al (2019), who analyzed the behavior of nontransform discontinuities (many of which we consider SSPs), along a section of the North MAR.…”

“…These features have previously been referred to as "discordant zones" (Grindlay et al, 1991) and "nontransform discontinuities" (NTDs; e.g., Hey et al, 1980;Hey et al, 1986;McKenzie, 1986;Macdonald et al, 1988;Rea, 1978) but few have been mapped off axis at high resolution using multibeam sonar (e.g., Cochran & Sempéré, 1997;Grindlay et al, 1991;Cochran and Sempere, 1997). A recent study by Zheng et al (2019) examined the long-term evolution of NTDs. They found NTDs show structures typical of rift propagation (inner and outer pseudofaults) and crustal blocks transferred between ridge flanks by discontinuous jumps of the PR tip.…”

“…They conclude that that their model of NTDs has all the fundamental features of the standard propagating rift model. Our more global analysis does not usually include such high-resolution data, so we adopt the conclusions of the Zheng et al (2019) study and assert the off-axis traces of all NTDs reflect a different style of rift propagation.…”

“…They found NTDs show structures typical of rift propagation (inner and outer pseudofaults) and crustal blocks transferred between ridge flanks by discontinuous jumps of the PR tip. The main difference between NTDs and the propagators found at higher spreading rates is that the rate of propagation is highly variable and the direction of propagation eventually reverses (e.g., Figure 1 and Figure 1 of Zheng et al, 2019). Short reversals in propagation direction have also been observed at PRs at faster spreading ridges and have been dubbed "dueling propagators" (Johnson et al, 1983;Macdonald et al, 1988Macdonald et al, , 1991Wilson, 1990).…”