Tear in the subducting slab beneath the southern Mariana Arc: evidence from P-wave tomography

Seismic tomography has developed into one of the most effective and significant sources of information in modern geophysical research, specifically in understanding subduction zone dynamics. The Western Pacific convergent margin is a well studied area, but as more data is collected and technology improves further detailed observations and theories are being developed. New tomographic images of the Mariana Arc region have enhanced resolution of gradients and strong variations in wave speeds through the use a three-dimensional ray-tracing inversion algorithm. Although the images obtained from the Mariana arc show relatively low amplitudes of heterogeneity due to the limited number of seismic stations in the area, details of the geometry and morphology of the Pacific Plate subducting beneath the Philippine Sea Plate are visualized in three dimensions with unprecedented detail. The slab has an arcuate shape in depth, similar to the surface expression of the trench axis, and plunges steeply into the lower mantle in the northern and central portion. A markedly different geometry exists south of 14°N where the slab continues to have a steep dip, but is only 200 km in length. Between the two distinct morphologies is a seismic velocity anomaly that we interpret as a tear. This near vertical tear in the slab strikes E-W, is 75-200 km in depth, and has an eastern extent of 145.5°E and continues westward to approximately 143.5°E. We propose three models to explain the formation of the tear: rapid rollback of the Challenger Deep slab segment, the junction of the Pacific and Caroline plates, or the subduction of the Caroline Islands Ridge.