Faulting patterns near 19 degrees 30 ' S on the East Pacific Rise: Fault formation and growth at a superfast spreading center

[1] Structural mapping from high-resolution sonar and bathymetric data collected near 19 degrees 30'S on the southern East Pacific Rise (EPR) reveals multiple packets of small, closely spaced outward-dipping faults that form antithetic to larger inward-dipping faults. The mean spacing and height of inward-dipping scarps are similar to1.6 and similar to1.9 times greater than those of outward-dipping faults, and the median spacing and height of inward-dipping scarps are similar to2.0 and similar to1.7 times greater, respectively. Accordingly, inward-dipping faults are less abundant (35-45%) but accommodate more (50-55%) of the roughly similar to4.0% strain. Although previous studies have shown that EPR fault populations exhibit exponential size-frequency distributions, one-dimensional throw and spacing data from the 19 degrees 30'S region are not described well by this model. The first major (> 40 m scarp height) fault is observed within 1-4 km of the ridge crest and invariably dips inward. Further off-axis, a transition from greater inward-dipping to greater outward-dipping fault abundance constrains the minimum half width of the active plate boundary zone to be similar to5-15 km. Inspection of additional data sets from 16 degrees and 18 degrees 30'S indicates a pattern of large inward-dipping faults with associated hanging wall deformation similar to that observed in the 19 degrees 30'S region. This pattern is distinct from the horst and graben morphology observed along the fast spreading northern EPR. The pervasive hanging wall deformation observed on the flanks of the southern EPR suggests master fault geometries that are more sharply curved or kinked at depth, relative to their abyssal hill equivalents on the northern EPR. Boundary element modeling indicates the pattern and geometry of faulting observed along the southern EPR is consistent with fault nucleation above a shallow, sill-like magma chamber with internal overpressure. Since the influence of the sill on fault growth is strongly dependent on the depth of the sill, the different patterns of faulting observed along the northern and southern EPR may reflect the small difference in axial magma chamber depth between these two regions, with a slightly shallower chamber beneath the southern EPR.