EVOLUTION OF BACKARC RIFTING - MARIANA TROUGH, 20-DEGREES-N-24-DEGREES-N

The Mariana Trough is an actively opening backarc basin in the western Pacific. The trough formed by extension which longitudinally split an earlier are massif creating a crescent-shaped basin between the remnant West Mariana Ridge and eastern active volcanoes of the Mariana Are. Opening increases southward from the Volcano Islands near 24 degrees N where the two arcs join. At 18 degrees N in the central Mariana Trough the basin is widest and may be opening primarily by seafloor spreading. We present a synthesis of closely spaced shipboard gravity, magnetic, and bathymetry measurements from the northern basin (20 degrees-24 degrees N), an area that undergoes a significant progressive southward increase in extension. We have identified three stages in the evolution of rifting in this area: (1) asymmetric rifting from 24 degrees N to 22 degrees 15'N where faulting and magmatism have migrated laterally td remain near the active volcanic are side of the basin, (2) a ''localization'' of rifting from 22 degrees 15'N to similar to 21 degrees N where the primary zone of rifting separates from the active volcanic are, and (3) a further concentration of rifting from 21 degrees N to 20 degrees N leading to tile formation of deep tectonic grabens near the center of the basin. This last stage may be a precursor to (or incipient) seafloor spreading. We describe a new type of magnetic lineation resolved in a three-dimensional seafloor magnetization inversion of the trough between 20 degrees and 24 degrees N. The detailed character of these lineations, their association with tectonic structures, and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting, less magnetic rifted are crust. The development of these well-defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough, although tectonic deformation appears to be more widely distributed. In addition, the geometry of the magnetization bands with respect to the remnant are border faults indicates that rifting propagated rapidly northward.