Long-Term evolution of non-transform discontinuities at the Mid-Atlantic Ridge, 24°N - 27°30?N Dataset uri icon

abstract

  • We studied the long-term evolution of non-transform discontinuities (NTDs) on the Mid-Atlantic Ridge between 24°N and 27°30’N and from 0 to ~20-25 Ma crust by analyzing 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, and they have been quasi-stable for limited periods. Fast, short-term (<2 m.y.) propagation is driven by reduced magma supply (increased extension) in the propagating ridge tip when ridge-axis offsets in NTDs are small (<~ 5 km). Slow propagation can be longer-term and may be limited by offset. Some NTDs show classic structures of rift propagation including inner and outer pseudofaults and large crustal blocks transferred from one ridge flank to the other by discontinuous, stepwise 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) The gross volume and distribution of magma supplied to the ridge axis from the rising asthenosphere, which in turn are controlled by 3D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The configuration of the lithospheric plumbing system that delivers magma to the crust in each spreading segment. (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 m.y.) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle.

publication date

  • December 2018