The regional tectonic fabric of the East Pacific Rise from 12°50′N to 15°10′N
Authors: Edwards M. H. , Fornari D. J. , Malinverno A. , Ryan W. B. F. , Madsen John 
Published: February 2008

SeaMARC II backscatter data for the East Pacific Rise are used to create structural maps of ridge‐parallel fabric on the crest and flanks of the rise from 12°50′N to 15°10′N. The structural data are statistically analyzed to describe the spacing, density, azimuth, facing direction, and length of faults. Results of the statistical studies are compared with predictions for the width of the zone of active fault formation, models for the generation of abyssal hills, plate kinematic predictions, models of along‐strike segmentation, and observations of the asymmetric subsidence of oceanic crust on the Cocos and Pacific plates. Comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge‐parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest. Fault density data reveal that fault distributions do not resemble periodic processes. Examination of fault density and spacing data along‐strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south. Changes in along‐strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area. Additionally, fault spacing s determined from SeaMARC I and SeaMARC II backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data. Analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area, half‐graben models are better predictors of abyssal hill morphology than full‐graben models. Within the survey area, inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults. Although the Cocos and Pacific plates subside at different rates, this asymmetry is not reflected in the tectonic component of morphology. Fault azimuth is observed to vary as a function of crustal age. Although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the Pacific and Cocos plates, the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the East Pacific Rise. Finally, the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along‐strike of the axis. The differences between the northern and southern portions of the survey area are reflected in the SeaMARC II bathymetric data which depict a continuous narrow ridge crest in the southern region, and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km. This along‐strike variability, observed over distances of less than 100 km, suggests that large‐scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the East Pacific Rise.