SeaMARC II sidescan sonar data and seismic reflection profiles of the northernmost portion of the Mariana Trough suggest a rifting rather than a seafloor-spreading origin for lithosphere within this active backarc basin of the western Pacific. Variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin. The northern Mariana Trough widens from 70 km near 23¿N to 135 km near 21¿30'N. Major active fault zones bound both sides of the backarc basin. Active rifting and volcanism are predominant in a roughly 20-km-wide active rift zone along the eastern (arc) side of the basin from 24¿N to 22¿N. The locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness, which is greatest at the volcanic front. The zone of rifting intersects the active volcanic arc at Nikko Seamount. The distinction between the mechanism for formation of the Mariana Trough proposed here and that of slow mid-ocean ridge spreading is that no true plate boundary has developed in the northernmost Mariana Trough. Rather, the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas. Volcanism both in the rift zone and throughout the basin is fault-controlled. South of 22¿N, the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached. The transition to true mid-ocean ridge spreading occurs to the south of the survey area described here. ¿ American Geophysical Union 1996