Tectonic Processes and the History of the Mariana Arc: A Synthesis of the Results of Deep Sea Drilli

DSDP Leg 60 drilled 10 sites on an east-west transect at 18°N across the tectonically active trench, volcanic arc, and extensional back-arc basin on the Mariana arc system. The analysis of these cores, augmented by considerable geological and geophysical information collected in preparation for the drilling, comprises the most complete data set presently available in a zone of convergence of two ocean lithospheric plates. No significant accretion of Pacific plate sediment or crustal rocks is found on the overriding Mariana arc (Philippine plate). The arc igneous basement, which has now been sampled from the trench slope break to within a few tens of kilometers from the volcanic arc, is of early Oligocene or Eocene age and is geochemically identified as being of arc origin. Apparently exclusively tensional deformation (expressed as high-angle faulting) and vertical movement producing net subsidence occur on both sides of the trench and increase with proximity to the trench axis. Only sparse sediments are observed in the trench, on the inner trench wall, and on much of the eastern part of the fore-arc. Collectively, these features suggest that, although some periods of accretion may have occurred in the past, the Mariana fore-arc at 18°N is subject to overall tectonic erosion by the subducting plate. Arc volcanism has probably been continuous since the Eocene, but initial back-arc basin rifting has occurred at or near the arc volcanoes, producing drastic subsidence and subsequent eruption in deep water with limited lateral transport of volcanic material. Apparent fore-arc volcanoes occur close to the trench axis where the subducting ocean plate is only 15 to 30 km deep. If these features are volcanic and autochthonous, their magma source must be either from deeper in the subduction zone (requiring considerable lateral transport of the magma) or in the oceanic plate—or they must be older features that were once above a much deeper subducting slab. Back-arc spreading in the Mariana Trough at 18°N apparently began with rifting somewhat east of the axis of arc volcanism in the latest Miocene, is approximately symmetric, and has continued at a half-rate of less than 2.15 cm/y, to present. The present Mariana arc volcanoes are thus built over thin crust that is physiographically part of the back arc basin and is either stretched, thinned, and subsided arc crust or was generated at the back-arc spreading center. Backarc spreading has ridge/transform/ridge geometry, earthquake distribution, crustal structure, geomorphology, and MORB-type rocks that all mimic—yet are subtly and consistently different from—mature, large, deep ocean spreading centers. The Mariana Trough may, however, be similar to other less well-known regions where the relatively thick island arc or continental crust is initially rifting. The results of IPOD active margin drilling off Japan, the Marianas, Mexico, and Guatemala suggest that there may be systematic relationships between subduction and plate motion geometry, back-arc spreading, magma types, vertical tectonics, and the occurrence of fore-arc accretion versus tectonic erosion.