Despite slow opening rates generally inferred for the Mariana Trough, the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north. A side-scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 ¿50'N and 13 ¿40'N and identifies the following characteristics: the ridge across-axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross-sectional area variations of 1 to 7 km2; the along-axis mantle Bouguer gravity gradient is 0.2 mGal/km; axial segmentation occurs as overlapping axes and small deviation in trend; no transform fault offsets exist despite significant changes in the trend of the spreading center. Characteristics of the surrounding basin include shallower overall depth than in the north; no well-developed frontal arc high in the southernmost trough; the close proximity of submarine arc-type volcanoes to the spreading center; and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank. These characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin. We propose that these effects are related to (1) the geometry of trench rollback in the southern trough leading to trench-parallel extension generating inward radiating extensional faults; (2) decoupling of the trench-parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin; and (3) augmentation of the spreading center's magmatic budget by arc magmatic sources contributing to its fast spreading character. Although these effects may be accentuated in the southern Mariana Trough by the geometry of trench rollback and position of the slab, which here underlies the spreading center, they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid-ocean ridges.