Long‐Term Evolution of Nontransform Discontinuities at the Mid‐Atlantic Ridge, 24°N–27°30′N. Issue 10 (24th October 2019)

• Record Type: Journal Article
• Authors: Zheng, Tingting; Tucholke, Brian E.; Lin, Jian
• Is Part Of: Journal of geophysical research. Volume 124:Issue 10(2019)
• Page Start: 10023
• Page End: 10055
• Publication Date: 2019-10-24
• Subjects: Mid‐Atlantic Ridge -- nontransform discontinuity -- plate reconstruction -- propagating rift; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2019JB017648
• Languages: English
• ISSNs: 2169-9313

Abstract

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