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

Abstract

Abstract. 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 29øN. 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 -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 -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 comer) on the west flank and from the outside to the inside comer 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-7% on the east flank and at-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.

Topographic controls on dike injection in volcanic rift zones

“…Near the segment center, the rift valley is typically shallow, formed by closely spaced faults with relatively small throw [6,35,44].…”

“…Therefore, in magma limited environments we consider such dikes to be "double-edged" blades limited at the top by brittle failure and below by viscous creep. Excellent examples of magma limited volcanic rift zones are slowspreading mid-ocean ridges, where seismic moment studies [5] and measurements of cumulative fault throw [6] suggest that ~80% of seafloor spreading is accommodated by magmatic accretion, while the remaining 20% occurs via extensional faulting.…”

Evidence for weak oceanic transform faults

“…0094-8276/02/2002GL015612 of the model space, respectively, and on both sides of each ridge axis (Figure 2b). Seismic moment studies [Solomon et al, 1988] and measurements of cumulative fault throw from the Broken Spur segment of the MAR [Escartín et al, 1999] suggest that 15 -20% of sea-floor spreading is accommodated by extensional faulting with the rest by magmatic emplacement. Therefore, in addition to the steady-state displacement field we superimpose a far-field extensional strain, e f.f.…”

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

“…Accretion in the presence of detachments is highly asymmetrical, with approximately 50% of the plate separation accommodated along a single fault along one of the ridge flanks. Overall tectonic strain at symmetrical segments is probably much lower (<20%) [25][26][27] , and distributed instead over multiple high angle normal faults that are active o several tens of km off-axis 14 . This difference in the amount of tectonic strain may be responsible for the high rates of seismicity at asymmetrical ridge sections relative to symmetrical ones.…”