Mechanisms of normal fault development at mid‐ocean ridges

Slow spreading ridge segments are characterized not only by small, closely spaced faults that develop near the segment center but also by large, widely spaced faults that develop near the segment ends, typically at the inside corner of a ridge‐offset intersection. In this study we investigate the competing effects of stress accumulation in the lithosphere and the yield strength of the lithosphere in controlling the location of normal fault formation and direction of propagation. Seismic velocity models from the Mid‐Atlantic Ridge in the Oceanographer‐Hayes region and 29°N and the East Pacific Rise at 9°N were used to estimate the along‐axis change in dynamic Young's modulus. Corresponding thermal and rheologic models were calculated to estimate the along‐axis variation in yield strength. We then develop a thin‐plate model to calculate the predicted location of fault initiation or reactivation and the subsequent propagation direction for different combinations of linear along‐axis gradients in Young's modulus and yield strength. On the basis of this model we define two modes of normal fault development at slow spreading segments: mode C (center) faults, which develop at the segment center and propagate outward, and mode E (end) faults, which develop at the segment ends and propagate inward. Mode C faults are predicted to form at ridges where the along‐axis variation in yield strength dominates the along‐axis accumulation of stress. Conversely, mode E faults are predicted to develop at ridges where stress accumulation toward segment ends overcomes the high yield strength in these locations. In addition to the accumulation of stress caused by along‐axis gradients in Young's modulus, we illustrate that shear stresses resisting relative plate motion along a transform fault will generate higher effective stress at inside corners, possibly concentrating mode E faulting in these locations. At fast spreading ridges, where along‐axis gradients in stress and lithospheric strength are relatively small, more uniform patterns of faulting are predicted. The results of this study quantify how the interplay between the along‐axis variations in stress state and the mechanical properties of the lithosphere controls the style of fault development at mid‐ocean ridge segments.