Results from a series of surveys across the Mariana island arc system clarify the geologic processes by which arc systems develop and test the hypothesis that the marginal basins behind the trenches are extensional in origin.

Two surveys were made to investigate the sediment-covered upper slope on the eastern flank of the frontal arc and a mid-slope basement high which separates the upper and lower sections of the trench-facing slope. The upper slope sedimentary wedge probably spans the Tertiary history of the arc system, although a Deep Sea Drilling Project hole penetrated only to lower Miocene strata. The survey of a submarine canyon which traverses the upper slope and mid-slope basement high demonstrates recent folding, faulting, and uplift of the basement high.

Pillow basalts, diabases, and metabasalts, were dredged from the ridges within the inter-arc basin (Mariana Trough). Sampling indicated that the basin fill is almost all Quaternary in age and that crustal extension and basalt extrusion has occurred along an axial high. Late Miocene coralline limestone and late Pliocene dacite pumice were dredged from the third arc, west of the inter-arc basin, indicating more than 1 km Quaternary subsidence of the ridge. Slightly west of the third arc is a chain of seamounts interpreted as andesitic volcanoes which fed the large sediment apron to the west. This apron, covering the eastern Parece Vela Basin, consists mostly of lower and middle Miocene volcaniclastics and overlies a basaltic basement similar to that of the inter-arc basin.

Tectonic activity in the island arc system appears to be discontinuous. A typical pulse is visualized as beginning with trench formation, followed by volcanism and later by extensional basin development. In addition to the Quaternary pulse, the Philippine Sea was affected by an early Miocene tectonic event which opened the Parece Vela Basin and probably a late Eocene event which opened the basin west of the Palau-Kyushu Ridge.

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