Dike complexes in American Samoa

Coherent dike complexes in which dikes make up more than 50% of the total rock occur in American Samoa in two volcanoes on Tutuila and in Ofu-Olosega volcano in the Manu'a Islands, The dikes are remarkably narrow (median width 0.24-0.36 m; average width 0.46-0.56 m, based on 860 measured dikes), significantly narrower than dikes in the Koolau volcano (Oahu, Hawaii), the most closely comparable volcano for which quantitative data are available. The vesicularity of these narrow dikes indicates that they were injected at shallow depths below the land surface: depths of 200-900 m are inferred from the topography. Many dikes have prominent bands of vesicles parallel with their margins. They tend to form clusters where the weakness of the vesicular rock favored center injection of one dike up the center of another, Examples are found of center injection repeated up to eight times. Local breccias, often composed largely of dike fragments evidently brecciated in situ, occur where irregular dikes were accommodated by deforming the host rocks. We see evidence for two dike facies: (1) narrow and vesicle banded, early formed and shallow-injected ''clustered'' dikes in the dike complexes, which are cut by (2) scattered broad and almost nonvesicular deep-injected ''solitary'' dikes. Surface eruptions were fed by solitary dikes. The ENE-trending dikes and elongation of Tutuila Island contrast with the WNW trend of rift zones and island elongation elsewhere in Samoa. Tutuila is nearly in line with the North Fiji transform fracture zone suggesting that at 1.5-1.0 Ma, when Tutuila was formed, the volcanism of Tutuila was guided by a temporary extension of this fault zone into the Pacific Plate. Fault movement offset the Samoan hotspot trace by 45 km. The en-echelon arrangement of dike exposures on Tutuila is consistent with left-lateral transcurrent motion above this concealed fracture zone.