Quantifying tectonic strain and magmatic accretion at a slow spreading ridge segment, Mid-Atlantic Ridge, 29°N

High-resolution, deep-towed side-scan sonar data are used to characterize faulting and variations in tectonic strain along a segment of the slow spreading Mid-Atlantic Ridge near 29 degrees N. Sonar data allow us to identify individual fault scarps, to measure fault widths and spacing, and to calculate horizontal fault displacements (heave) and tectonic strain. We find that over long periods of time (>1 Myr on average), tectonic strain is similar to 10% on average and does not vary significantly along axis. There is a marked asymmetry in tectonic strain that appears to be linked to asymmetric accretion along the whole segment, indicated by similar to 50% lower tectonic strain on the east flank than on the west flank. These variations in tectonic strain do not correlate directly with changes in fault spacing and heave. Fault spacing and heave increase from the center of the segment toward the end (inside corner) on the west flank and from the outside to the inside corner across the axis. These parameters remain relatively constant along the segment on the east flank and across the axis at the segment center. Tectonic strain appears to be decoupled from magmatic accretion at timescales >1 Myr, as the decrease in magma supply from the segment center toward the end (inferred from variations in crustal thickness along the axis) is not correlated with a complementary increase in tectonic strain. Instead, tectonic strain remains relatively constant along the axis at similar to 7% on the east flank and at similar to 15% on the west flank. These results indicate that variations in fault development and geometry may reflect spatial differences in the rheology of the lithosphere and not changes in tectonic strain or magma supply along axis.