Extension in the northern Mariana inner forearc

Seismic reflection, SeaMARC II acoustic imagery, and bathymetric data across the northern Mariana island arc reveal dense, high‐angle, normal faulting of the arc and inner forearc. Faults, with throws up to 780 m, cut a thick sequence of stratified but otherwise undeformed sediments. The major faults display a sinuous or zigzag trace that is commonly 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 analyses of the data show activity since the last sedimentary deposition and concurrent fault development of two distinct fault populations, one associated with the backarc basin's east boundary fault and the other with extension in the forearc. Extensional faults along the eastern side of the backarc basin, near the active arc at 22°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°N to be 39°±1°, orthogonal to the extensional strain direction measured in the inner forearc. The change in strain is distinct and abrupt, occurring across a 20‐km‐wide region along the arc massif. Within this narrow region of transition, the intermediate and minimum principal strain directions are nearly equal in magnitude 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 across this narrow region; the two opposing strain regimes coexist virtually juxtaposed. The coexistence of these two orthogonal, extensional fault regimes within the overriding plate between the Mariana backarc basin and inner forearc may be explained by assuming that a weak, transitional region exists along the east boundary fault zone where stresses can be more easily relieved. This allows active basin‐wide extension in 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.