EXTENSION IN THE NORTHERN MARIANA INNER FORE-ARC

Seismic reflection, SeaMARC II acoustic imagery, and bathymetric data across the northern Mariana island arc reveal dense, high-angle, norma l faulting of the arc and inner forearc. Faults, with throws up to 780 m, cut a thick sequence of stratified but otherwise undeformed sedime nts. The major faults display a sinuous or zigzag trace that is common ly associated with rifting. This pattern can be indicative of a single event of triaxial strain that requires two contemporaneous conjugate fault pairs in orthorhombic symmetry. Interpretation and statistical a nalyses of the data show activity since the last sedimentary depositio n and concurrent fault development of two distinct fault populations, one associated with the backarc basin's east boundary fault and the ot her with extension in the forearc. Extensional faults along the easter n side of the backarc basin, near the active arc at 22-degrees-N where active rifting is occurring, have approximately the same trend as the east boundary fault in the survey area. We estimate the direction of extensional (minimum principal) strain for the east boundary fault at 22-degrees-N to be 39-degrees +/- 1-degrees, orthogonal to the extensi onal strain direction measured in the inner forearc. The change in str ain is distinct and abrupt, occurring across a 20-km-wide region along the arc massif. Within this narrow region of transition, the intermed iate and minimum principal strain directions are nearly equal in magni tude with orientations midway between the dominant forearc and backarc basin principal strain directions. Statistical analyses show that the strain from either backarc or forearc regime is not transmitted acros s this narrow region; the two opposing strain regimes coexist virtuall y juxtaposed. The coexistence of these two orthogonal, extensional fau lt regimes within the overriding plate between the Mariana backarc bas in and inner forearc may be explained by assuming that a weak, transit ional region exists along the east boundary fault zone where stresses can be more easily relieved. This allows active basin-wide extension i n the backarc basin, while the forearc data are consistent with models that require radial forearc deformation resulting from increasing arc -trench curvature of the northern half of the Mariana island arc.