Sedimentary, volcanic, and tectonic processes of the central Mariana Arc: Mariana Trough back‐arc basin formation and the West Mariana Ridge

We present new multichannel seismic profiles and bathymetric data from the central Marianas that image the West Mariana Ridge (WMR) remnant arc, both margins of the Mariana Trough back‐arc basin, the modern arc, and Eocene frontal‐arc high. These data reveal structure and stratigraphy related to three periods of arc volcanism and two periods of arc rifting. We interpret the boundary between accreted back‐arc basin and rifted arc crust along the Mariana Trough and support these findings with drilling results and recent seismic refraction and gravity studies. We show that with the exception of a few volcanoes behind the volcanic front that straddle the boundary between crustal types, the modern Mariana Arc is built entirely on rifted arc crust between 14 and 19°N. Our data indicate that there is more accreted back‐arc seafloor to the west of the Mariana Trough spreading axis than to the east, confirming previous evidence for an asymmetric basin. The rifted margin of the WMR remnant arc forms a stepped pattern along the western boundary of the Mariana Trough, between 15°30′ and 19°N. In this region, linear volcanic cross chains behind the WMR are aligned with the trend of Mariana Trough spreading segments, and the WMR ridges extend into the back‐arc basin along the same strike. These ridges are magmatic accommodation zones which, to the north along the Izu‐Bonin Arc, punctuate tectonic extension. For the WMR we hypothesize that rift basins are more commonly the sites where spreading segment offsets nucleate, whereas magmatic centers of spreading segments are sites where magmatism continues from arc volcanism, through rifting to back‐arc spreading. The Mariana Trough is opening nonrigidly and is characterized by two predominant abyssal hill trends, NNW–SSE in the north and N–S in the south. Between the only two basin‐crossing fracture zones at ∼15.5 and 17.5°, N–S axes propagated north at the expense of NNW axes.