A SIMPLE-MODEL FOR THE FAULT-GENERATED MORPHOLOGY OF SLOW-SPREADING MIDOCEANIC RIDGES

We postulate that fluctuations in magmatic activity at mid-oceanic rid ges perturb the horizontal least principal stress across rift-bounding normal faults, leading to alternating phases of magmatic accretion, w hich increases valley width, and tectonic extension, which results in the growth of inner rift wall topography. Fine-scale bathymetric surve ys and earthquake fault plane solutions show that active normal faults at slow-spreading ridges are moderately dipping (approximately 45 deg rees) planar features throughout the seismogenic oceanic lithosphere. A simple quantitative model that includes flexural deformation of a 10 -km-thick elastic plate by slippage on 45 degrees dipping normal fault s can match the bathymetric profiles across several slow-spreading rid ge segments. Comparison among dip distributions of normal-faulting ear thquakes at mid-ocean ridges, in the trench-outer rise region, and on continents suggests that most events from these three tectonic environ ments initiated at dips close to 45 degrees, raising unanswered questi ons about the mechanical conditions under which the faults originated.