Seismic images of the Transition fault and the unstable Yakutat-Pacific-North American triple junction
In southern Alaska, the Pacific plate and Yakutat terrane subduct beneath the North American plate along the Aleutian Trench and Pamplona zone, respectively, and are sliding past each other at minimal rates along the Transition fault. As the deformation front of the Pamplona zone stepped eastward during the Pliocene–Pleistocene, the Pacific–North American–Yakutat triple junction became unstable. Four recent seismic images reveal that the Transition fault changes from a single strike-slip boundary east of the deformation front to three strands that step increasingly seaward between the deformation front and the Aleutian Trench. The southern two strands deform the Pacific crust, and the outermost of these became increasingly convergent sometime since 1 Ma, as demonstrated by young growth strata. We propose that this internal deformation of the Pacific plate is an attempt to reattain stability, which can only be reached by creating a tectonic boundary collinear with the Pamplona zone. The plate reorganization will result in initiation of subduction such that a portion of former Pacific crust will become accreted to the North American plate. Such accretion events caused by triple-junction instability may be an important mechanism for transferring oceanic crust to continental margins.