RIDGE OFFSETS, NORMAL FAULTING, AND GRAVITY-ANOMALIES OF SLOW-SPREADING RIDGES

We develop a model relating mid-ocean ridge normal faulting and crustal structure by examining recently available high-resolution gravity and multibeam bathymetric data of the Mid-Atlantic Ridge and other spreading centers. Results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major Atlantic offsets studied, especially along the inside corner side of the offsets. Individual residual gravity anomaly spikes (local peaks), which have amplitudes of up to 20 mGal and typical across-axis spacing of 10-30 km, often coincide with individual major fault scarps, suggesting crust thinned by normal faulting. Theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive, ridge-parallel low-angle faults that originate episodically at inside corners of ridge offset intersections. Fault scarp heights and the amounts of crustal thinning (as inferred from gravity anomalies) are consistently larger at inside corners than at outside corners, supporting a model in which tectonic extension near ridge offsets is asymmetric with low-angle faults occurring preferentially at inside corners. These results on spatial variations in seafloor morphology and gravity anomalies further support a three-dimensional tectonic faulting model at oceanic spreading centers with three major characteristics: (1) Low-angle faults form preferentially at inside corners, where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets; (2) low-angle faults decrease in throw toward midpoints of long ridge segments, where large low-angle faults may not be sustained by a weak lithosphere; and (3) the residual gravity peaks are statistically larger at transform than near nontransform offsets, indicating that the length of a ridge offset and its tectonic style control the development of low-angle faults.