Evolution of the Mariana Convergent Plate Margin System

Abstract

The Mariana convergent plate margin system of the western Pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology. The system's relative geologic simplicity and the well‐exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics. Its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins. The Mariana convergent margin's long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the Philippine Sea plate. The trenchward edge of the overriding plate has a relatively sparse sediment cover. Rocks outcropping on the trench's inner slope are typical of the early formed suprasubduction zone's lithosphere and have been subjected to various processes related to its tectonic history. Pervasive forearc faulting has exposed crust and upper mantle lithosphere. Many large serpentinized peridotite seamounts are within 100 km of the trench axis. From these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids. Ocean drilling recovered suprasubduction zone lava sequences erupted since the Eocene that suggest that the forearc region remains volcanologically dynamic. Seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution. Large portions of uplifted southern forearc are exposed at the larger islands. Active volcanoes at the base of the eastern boundary fault of the Mariana Trough vary in size and composition along strike and record regional differences in source composition. Their locations along strike of the arc are controlled in part by cross‐arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin. The western boundary is the West Mariana Ridge, the western portion of the volcanic arc active prior to formation of the Mariana Trough. The trough evolved in a two‐stage extension process of rifting and subsequent seafloor spreading. The back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid‐ocean‐type spreading center south of 20°N. The change from initial rifting to true seafloor spreading is also evident across the Mariana Trough from rifted topography near the West Mariana Ridge to spreading ridges in the central to eastern basin south of 20°N. This morphologic change indicates an early stage of extension with basin‐and‐range‐type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries. The spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys. The center of extension intersects the active volcanic front differently at either end of the Mariana Trough. In the north, extension is by rifting of arc lithosphere where it intersects the arc. In the south a major strike‐slip fault extends from the trench axis across the forearc, through the volcanic arc, and into the back arc basin. Arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center. The complexity of interrelated tectonism and magmatism in this convergent margin is daunting, but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents. Comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments.